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httpdxdoiorg101595205651317X695064 Johnson Matthey Technol Rev 2017 61 (2) 93ndash110

JOHNSON MATTHEY TECHNOLOGY REVIEW wwwtechnologymattheycom

Iridium Coating Processes Properties and Application Part II Analysis of the coatings against high-temperature oxidation and corrosion

Wang-ping Wu School of Mechanical Engineering Institute of Energy Chemical Equipment and Jiangsu Key Laboratory of Materials Surface Science and Technology Changzhou University Changzhou 213164 PR China

Zhao-feng Chen International Laboratory for Insulation and Energy Efficiency Materials College of Material Science and Technology Nanjing University of Aeronautics and Astronautics Nanjing 210016 PR China

Email wwp314163com

Iridium as a barrier coating is an important area of high-temperature application In Part I the introduction was presented and the different deposition processes were reviewed (1) This paper Part II describes the texture and structure evolution mechanical properties growth mechanisms and applications of Ir coatings The mechanisms of micropore formation after high-temperature treatment are also investigated in some detail

1 Texture Structure Evolution and Mechanical Properties of Iridium Coatings 11 Texture Evolution

The crystal orientation of the coating can influence its structure and further affect the properties and hence the feasibility of the coating for use in extreme

environments (2 3) For fcc metals films formed by MOCVD PVD and electrodeposition present a lt111gt orientation In general the orientation of Ir coatings formed by DCMS RFMS or MOCVD is the (111) crystal plane The (111) orientation is most likely favoured because it is the most closely packed arrangement with the lowest surface energy The orientation of Ir films created by RFMS deposited on a yttria stabilised zirconia film depended on the deposition rate (4 5) When the deposition rate was low an epitaxial (100) Ir film was formed A highly (111)-orientated Ir fi lm was grown at higher deposition rates

Murakami Yano and Sodeoka (6) suggested that an Ir coating formed by magnetron sputtering was strongly orientated to the (220) direction whilst a coating formed by electron beam PVD did not exhibit such tendencies The orientation of the coating depended on the substrate temperature the partial pressure ratio of argon to the entire total deposition atmosphere gas the deposition rate and the thickness of the coating (7 8) Wessling et al (9 10) observed that the crystallographic texture of the Ir film changed from a lt220gt to a lt111gt orientation with increasing incident energy The lt220gt-orientated films were rougher with lower densities and stronger void formation The coating with lt111gt texture was smoother and denser than that with lt110gt texture regardless of the grain size With increasing current density the surface roughness of the electrodeposited Ir coating increased while the grain size decreased and the preferred orientations changed in the order lt111gtrarrlt110gtrarrlt311gt (11) With increasing deposition temperature the surface roughness decreased while

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httpdxdoiorg101595205651317X695064 Johnson Matthey Technol Rev 2017 61 (2)

the grain size increased and the preferred orientation changed in the order lt111gtrarrlt110gtrarrlt111gt The substrate was composed of polycrystalline Nb (Figure 1(a)) (110) (200) (211) and (220) Nb diffraction peaks appeared The preferential growth orientation could be determined by a texture coefficient (TC(hkl)) which was calculated using Equation (i) (13)

Ihkl I0(hkl)TC(hkl) = (i)n1n[ i=1Ii(hkl) I0(hkl)]

TC(hkl) is the texture coefficient of the (hkl) plane I(hkl)

is the measured intensity of (hkl) plane I0(hkl) is the corresponding recorded intensity in the Joint Committee on Powder Diffraction Standards (JCPDS) data fi le and n is the number of preferred growth directions of Nb or Ir According to Equation (i) and the JCPDS Card (No 35-0789) TC(110) TC(200) TC(211) and TC(220) of Nb were 0346 2703 0463 and 0519 respectively It was indicated that the substrate had a preferred (200) orientation The Ir coating had a dominant (220) orientation in the XRD pattern (see Figure 1(b)) The diffraction intensities of the other peaks were weak The epitaxial growth of the (220)-orientated Ir coating did not occur on the (200)-orientated Nb substrate

In Figure 2 the red blue and green colours represent Ir (001) Ir (111) and Ir (101) crystal faces respectively The orientation distribution map shows that the vast majority of colour on the coating surface was green indicating that the DGP Ir coating had a preferred (101) orientation The coating was mainly composed of submicrometre-sized grains of 02ndash03 μm (Figure 3)

Figure 4 shows the maximum value distributed in the centre of the (101) pole figure This indicates that the preferred orientation of the coating was a (101) texture This result was in agreement with that of the XRD pattern (Figure 1)

15 m

Colour coded map type inverse pole fi gure [001] Iridium

The (220)-textured coating was due to the deposition of initial nuclei with preferred growth on the substrate surface The crystal orientation of the DGP Ir coating was further investigated by an electron backscatter diffraction (EBSD) technique (3)

Fig 2 Orientation distribution map for Ir grains according

111

101001

TD

RD

Inte

nsity

arb

itrar

y un

its

20 30 40 50 60 70 80 90

2 ordm

(b)

(a)

Nb(

110)

Nb(

220)

Nb(

211)

Ir(

220)

Nb(

200)

Fig 1 X-Ray diffraction (XRD) patterns of (a) Nb substrate and (b) Ir coating formed by a DGP process (Reproduced with permission of Elsevier (12))

to the inverse pole figure (Reproduced with permission of Elsevier (12))

01 015 02 025 03 035 04 045

Grain size (diameter) m

Are

a fra

ctio

n

025

02

015

01

005

0

Fig 3 Grain size distribution map of the surface of a DGP Ir coating (Reproduced with permission of Elsevier (12))



111

101001

TD

RDRD

TD

TD

RD

Texture name Harmonic L = 16 HW = 50 Calculation method Harmonic series expansion Series rank (I) 16 Gaussian smoothing 5ordm Sample symmetry Triclinic

Max = 5340 4039 3055 2311 1748 1322 1000 0756

Fig 4 Pole fi gures of 001 101 and 111 textures for the surface of a DGP Ir coating (Reproduced with permission of Elsevier (12))

The lt110gt-orientated Ir coating formed by DGP was an interesting phenomenon which can be related to the density of broken metallic bonds and the relative surface energy of the dominant crystal faces (14) Because the atom density of each crystal face was different the density of broken metallic bonds varied when different crystal faces were split The (111) crystal face has the highest atom density and interplanar spacing compared with other crystal faces in the fcc crystal structure (Table I) meaning that the lowest energy is needed to pull the (111) crystal face apart The density of broken metallic bonds in the (111) (100) and (110) crystal faces were 3464 andash2 4 andash2 and 4242 andash2

(where a denotes the lattice parameter) respectively If the (111) crystal face energy was assumed to be 1 then the (100) crystal face energy and (110) crystal face energy would be 1154 and 1223 respectively

According to the principle of crystal growth the preferred growth direction usually has the fastest growing speed From surface energy considerations the (111) orientation was expected to be preferred in the fcc structure The surface energies S111 S100

and S110 for fcc Ir were 259 J mndash2 295 J mndash2 and 319 J mndash2 (15) respectively The lt111gt orientation is typical for the growth of fcc metals since it presents the lowest surface energy and the highest atomic density Thompson and Carel (16) suggested that energetic constraints such as surface and interface energy minimisation and strain energy minimisation could lead to texture selection The lt111gt-textured grain was probably the result of surface and interface energy minimisation during coating formation and growth The lt110gt-textured grain was the result of strain energy minimisation during grain growth The lt111gt-orientated Ir coating exhibited excellent diffusion barrier properties against oxygen (17) On high-energy face lt111gt Ir the adatoms were likely to diffuse quickly to a cluster edge and expand the cluster laterally On low energy face lt110gt Ir diffusion was limited and growth proceeded in the vertical direction Due to developing a height advantage and the resulting shadowing more particle fl ux was intercepted which led to faster growth of lt110gt Ir grains (9) At low adatom mobilities lt110gt-orientated grains could outgrow lt111gt-orientated grains The lt110gt-orientated Ir coating was rough with low densities and void formation Vigorous bombardment from high-energy ions prevents the formation of low energy (100) an d (111) crystal faces Only high-energy (110) crystal faces are formed and grow under these conditions resulting in the preferred (110) orientation

Table I Atomic Structure (Hard Sphere Model) and Surface Energy (Shkl) and (hkl) Plane of Iridium with Monoatomic fcc Structure

hkl 111 100 110

hkl crystal planes

a

a

a

a

Surface density 23a2 2a2 14a2

Surface energy Shkl J mndash2 259 295 319



The glow discharge and the vigorous bombardment therefore make the (100) and (111) diffraction peaks weak Due to their lower atomic density along the growth direction (110)-textured grains grew faster than (111)-textured grains with the same atomic deposition rate and finally won the competition (9 10)

When the deposition of the Ir coating began the substrate temperature was relatively low resulting in a low surface mobility of the deposited atoms lt110gt grains grew faster than lt111gt grains when the deposited atoms had a low surface mobility For fcc metals incident energetic particles favour the development of lt110gt over the lt111gt texture due to the effect of channeling and thereby less localised radiation damage in the lt110gt grains For the DGP process the Ir layer formed suffered from the vigorous high-energy ion bombardment and sputtered ions So a lt110gt-textured Ir coating was easily obtained by a DGP process

12 Structure Evolution

Polycrystalline coatings are usually formed through the nucleation of isolated crystals on the surface of a substrate Moreover the microstructures and properties of the coatings depend on grain nucleation growth coalescence and film thickness growth (18ndash21) Figure 5 shows the scanning electron microscope (SEM) micrographs of a DGP Ir coating on a Nb substrate The surface of the coating consisted of many small aggregates There were no microcracks or pores on the coating surface (Figure 5(a)) The chemical composition of the coating consisted only of Ir as demonstrated by the energy-dispersive X-ray spectroscopy (EDS) pattern The fracture surface of the coating consisted of columnar grains (Figure 5(b))

The columnar grains extended through the entire coating thickness The coating thickness was 6ndash7 μm

The growth and coalescence processes are part of the coating formation The islands growth occurred until they touched each other to form a continuous coating The coalescence process was important for the design of coatings with special properties After solid-like coalescence of two islands there might be a grain boundary between them or they might form a boundary-free island Surface energies and super saturation controlled the coalescence and growth process through material transport by surface and bulk diffusions From Figures 6(a) to 6(c) the rough surface for the thin coating evolved into a smooth surface for the thick coating The relatively rough surface consisted of some large aggregated particles (Figure 6(a)) The coating consisted of some hillock-like particles but the surface was relatively smooth (Figure 6(b)) Compared with Figures 6(a) and 6(b) the structure evolution of the coating was due to the low substrate temperature and the shadowing effect which resulted in low mobility of the adatoms and the limitation of adatom diffusivity The high concentration of Ir atoms in the boundary layer led to their agglomeration The agglomerated clusters and the sputtered Ir particles were deposited to form hillock-like particles (Figure 6(b)) For Figures 6(a) and 6(b) the coating belonged to the growth and coalescence processes Figure 6(c) shows that the surface of the coating was uniform and dense and the surface was composed of small fi ne grains The surface morphology was typical for the resputtering of the coating (Figure 6(d)) The surface was homogeneous and dense The adsorption and diffusion of the Ir atoms were accelerated due to the high substrate temperature with increasing bias voltage When the bias voltage

Fig 5 SEM micrographs of (a) surface and (b) fracture surface of an Ir coating on a Nb substrate (Reproduced with permission of Elsevier (12))

10 m 1 m

Ir

2 4 6 8 10 12

655 524 393 262 131

0

(a) (b)



10 m 10 m 10 m 10 m

(a) (b) (c) (d)

Growth and coalescence Thickness growth

Fig 6 SEM images of the surface of a DGP Ir coating on Mo substrates (a) Us = ndash100 V (b) Us = ndash200 V (c) Us = ndash300 V (d) Us = ndash400 V (Reproduced with permission of Elsevier (22))

(Us) was changed from ndash100 V to ndash400 V the coating became thick The bias voltage not only affected the substrate temperature but also the deposition rate of the coating (23) The microstructure of the coating was influenced by its increasing thickness

The microstructure of the coating could affect its properties and hence its possible applications The homologous temperature TSTM defined as the ratio of the substrate temperature TS to the melting point of thin coating material TM is one of the main parameters The substrate temperature plays a key role in determining the adatom surface mobility and the bulk diffusion rates The structure zone models (21 24) classified the microstructure of metallic coatings into three different zones as a function of TSTM bull Zone I (TSTM lt 03) is the columnar structure

with lots of pores and open columnar boundaries This structure is promoted by a high gas pressure or a low homologous temperature Shadowing dominates

bull Zone II (03 lt TSTM lt 05) consists of columnar grains separated by distinct and intercrystalline boundaries The grain sizes increase with TSTM

and may extend through the coating thickness at high TSTM Surface diffusion is significant

bull Zone III (05 lt TSTM lt1) consists of equiaxed grains with a bright surface The grain diameters increase with TSTM and the bulk diffusion of adatoms is predominant

Generally RFMS and DCMS produce a columnar grained Ir coating However a DCMS Ir coating deposited at 25ordmC had a rough surface contained irregularities and had a porous columnar structure due to the effect of shadowing (25) A RFMS Ir coating

deposited at 800ordmC had a dense columnar structure RFMS Ir coatings deposited at less than 100ordmC show an lsquoorange-peel-likersquo surface topography which consists of densely packed sub-100 nm grains (26) A MOCVD Ir thin coating presents compact columnar and coarse grains An electrodeposited Ir coating had large columnar grains as well as large equiaxed grains PLD Ir films deposited at 300ordmC and 500ordmC have a polycrystalline structure with nanometre grain size A DGP Ir coating deposited at 800ndash1000ordmC showed a columnar grain with sub-micrometre sized grains For the DGP process the columnar Ir coating should belong to the Zone II structure The microstructure of Zone II was in agreement with that of the coating The columnar grain resulted from the deposited atoms having sufficient surface mobility to diffuse and to increase the grain size However the Ir coating had many nanovoids at the interface ascribed to low mobility of the deposited atoms and the high deposition rate of the coating

13 Mechanical Properties

The deposition temperature could enhance the grain growth of the Ir coating However the grain size influences the mechanical properties of the coating such as its hardness and electrical resistivity The microhardness of Ir products fabricated by different processes is shown in Table II According to the Hall-Petch relationship (29 30) polycrystalline material displays an increase in hardness with decreasing grain size Vickers hardness (Hv) can be related to the grain size (D) by Equation (ii)

Hv = H0 + kHDndash12 (ii)



Table II Hardness of Iridium Fabricated by Different Processing Methods (26ndash28)

MagnetronProcess method DGP Electrodeposition CVD CastingSputtering

Hardness GPa 21 95 442 298 210-24

Sub-Grain size Nanometre Micrometre Micrometre Millimetremicrometre

where H0 and kH are constants The highest hardness was found in the magnetron sputtered Ir films owing to their fine crystal grain size Kuppusami et al (31) found that the hardness of an Ir coating formed by DCMS was ~225 GPa Hagen et al (32) reported the mechanical properties of Ir coatings with a titanium sub-layer deposited by RFMS The obtained values agreed well with data from Kuppusami Murakami and Ohmura (31) The heat-treated Ir coatings after indentation showed low hardness values compared with the as-deposited Ir coating (33 34)

The electrical resistivity of the coatings mainly depends on the grain size (35) El Khakani Chaker and Drogoff (36) pointed out that the resistivity of an Ir film was highly influenced by the energetic particle bombardment conditions during film growth The resistivity of Ir films measured by four-point resistivity tests decreased with increasing substrate temperature (37) which was attributed to the grain size When the RFMS deposition rate of an Ir film was increased its crystalline quality decreased while the resistivity increased (38)

For different deposition processes and conditions the deposited Ir coating has different growth modes and textures leading to differences in the microstructure The microstructure and growth modes of coatings play an important role in their physical and mechanical properties The adhesive force of the coating on the substrate is a key value and affects the application of the coating in extreme environments A general rule of thumb says (39) that a critical load of 30 N in scratch testing with a Rockwell C diamond tip is generally sufficient for tooling applications Chen et al (40) reported that the adhesive force of DGP Ir coatings on refractory metals were measured by a scratch tester at about 50ndash60 N Thus the DGP Ir coating adhered well to the refractory metals which was due to the strong metallurgical bond and the formation of a transition layer at the interface The adhesion forces of a RFMS Ir coating (26) on stainless steel silica (SiO2) and tungsten carbide (WC) substrates were 100 mN 125 mN and 165 mN respectively The adhesive forces

of MOCVD Ir coatings (35) on TiO2SiO2Si SiO2Si and Si substrates were 15 N 9 N and 5 N respectively probably due to the fact that the lattice constant of Ir is closer to that of TiO2 than to those of SiO2 and Si Good adhesion between an electrodeposited Ir coating and a Re substrate was due to the excellent match between their thermal expansions and good solid solubility between the two elements (27)

2 Growth Mechanism of Iridium Coatings

For a PVD system if the species changes from vapour to solid at a pressure p a free enthalpy change is involved (41 42) Equation (iii)

ΔG = nΔμ = nkT ln(pp0) = nkT ln(η) (iii)

where p0 is the equilibrium vapour pressure and η is the degree of speciesrsquo saturation The formation of the coating by a vapour deposition process is characterised by the formation and growth of nuclei Depending on the interaction energies of the substrate atoms and the sputtered atoms the following three growth modes can occur (43 44) (a) Layer by layer or Frankndashvan der Merwe (FM) (b) Island or VolmerndashWeber (VW) (c) Layer plus island or StranskindashKrastanov (SK)

The growth modes of the coating were in terms of surface energies (γ) Wetting angle (θ) of a nucleus on a substrate surface was described by Youngrsquos equation (45) Equation (iv)

γS = γSF + γF middot cosθ (iv)

where γS is the surface energy of the substrate γF

is the surface energy of the coating and γSF was the interface energy VW growth (θ gt 0) required that γS lt γSF + γF whereas FM growth (θ = 0) required that γS gt γSF + γF SK growth occurred because the interface energy increased with film thickness Gerfin et al (46) suggested that the growth mechanism of an Ir fi lm by MOCVD followed the VW mode The growth mode of a DGP Ir coating on a graphite substrate presented a conical grain structure formed via the SK mode due to



poor wetting between Ir and carbon (47) The interface bond between the Ir coating and the graphite substrate was the only mechanical locking effect However the interface bond between an Ir coating and a metallic substrate is an ionic bond which could help to improve the adhesion of the coating to the substrate In Figure 7 the growth mode of a DGP Ir coating was VW resulting in a columnar grain structure Usually continued growth of the layer after initial island (VW) growth occurred by columnar growth Therefore the coating growth probably began with island clusters which grew three dimensionally

The growth mode of the coating was controlled not only by interface energies but also by saturation The degree of speciesrsquo saturation and the growth kinetics of the coating were responsible for the difference in the coating structure The degree of speciesrsquo saturation played a decisive role in nucleation on the substrate which can be explained as follows (42) For three-dimensional (3D) nucleation a number N of atoms formed the nucleus Equation (v)

ΔG3D = NkT ln(p0p) + N23Y = minusNΔμ3D + N23Y (v)

where Y is the contribution of the interface energy 3D nucleation for the coating required supersaturation The faster growth rate of the coating deposited at a stable substrate temperature resulting in supersaturation conditions might cause a SK growth mode to be followed (42) If the deposition continues the nucleation centres become more numerous and then coalescence of two-dimensional (2D) islands occurs As the size of the 2D islands increases the coverage of the substrate surface is enlarged The growth process of the fi lm was controlled by migration re-evaporation nucleation coalescence and thickness growth (see Figure 7)

21 Metal-Organic Chemical Vapour Deposition of Iridium Coating

The growth mechanism of Ir coatings formed using MOCVD depends not only on the deposition parameters and the substrates but precursor chemistry also plays a key role Sun et al (48) investigated the effects of both oxygen and substrate on Ir film growth by MOCVD Without oxygen the Ir film contained carbon Oxygen removed carbon and also prevented carbon incorporation from other reactive gas components Oxygen could also control the deposition rate of the film and had a significant impact on the morphology of the film Garcia and Goto (49) and Goto Vargas and Hirai (50) investigated the MOCVD of Ir coating using Ir(acac)3 complex with and without the addition of oxygen and found that the Ir coating consisted of Ir clusters with diameter 1ndash4 nm surrounded by amorphous carbon The carbon composition was about 20 wt The addition of oxygen was effective to suppress carbon incorporation into the Ir coating A highly pure Ir coating containing less than 1 wt carbon and further oxygen could be obtained by controlling the oxygen flow rate to avoid the formation of IrO2 A change of deposition rate at a turning point around 600ordmC was observed corresponding to the change of activation energy from 160 kJ molndash1 to 24 kJ molndash1 Such changes are normally linked to the change of the CVD rate controlling step from a surface chemical reaction to a gaseous diffusion process with increasing temperature

Semyannikov and coworkers (51) conducted a set of studies on the deposition of Ir using Ir(CO)2(acac) The compound decomposed in vacuum and in hydrogen with the formation of the following major products

Single atom arrives Migration

Re-evaporation

Collision and combination Nucleation

Island growth

Island shape

Coalescence Film

Fig 7 Thin film growth process



acetylacetone (H(acac)) carbon monoxide carbon dioxide and acetyl (C2H3O) Thermal decomposition of Ir(CO)2(acac) in the presence of oxygen was accompanied by the formation of water vapour The appearance of columnar structures in the Ir coating was observed on substrates of different natures (metals and oxides) (52) At the initial stages of growth a coating with compact structure was deposited then the growth mechanism changed and a layer with a columnar structure was formed Furthermore a thin layer with high carbon concentration was formed on the growing metal surface which led to structural changes as the process continued and resulted in the formation of coatings with a non-uniform layered structure The growth rate of an Ir film on a titanium carbon nitride (TiCN) substrate was significantly higher than that on a SiO2 substrate The Ir film on SiO2 was rough due to an initial 3D growth mode on isolated islands

Vargas et al (53) showed that an Ir film could be epitaxially grown by MOCVD on sapphire single crystal substrates using Ir-acetylacetonate precursors Gerfin et al (46) reported that the growth rate of a MOCVD Ir film depended on the substrate temperature Two growth regimes could be distinguished ie the kinetically controlled (Tsublt350ordmC) and the mass-flow-controlled region The Ir film consisted of islands thus the growth mechanism of the Ir film followed the VW mode Gelfond and coworkers (54) studied an Ir film deposited with Ir(acac)3 on SiO2 and found a transition layer of IrSiO The Ir films consisted of randomly orientated closely spaced particles of different shape and size

22 Atomic Layer Deposition of Iridium Coating

Song et al (55) found that a hybrid ALD Ir fi lm followed a 3D island growth pattern Hybrid ALD could increase the rate of formation of the Ir seed layer on the substrate surface Knapas and Ritala (56) reported the reaction mechanism in two ALD processes using Ir(acac)3 as a precursor Ir(acac)3-O2 and Ir(acac)3-O3-H2 processes at 300ordmC and 195ordmC respectively For the first process the reaction of Ir(acac)3 occurred upon its adsorption as the adsorbed oxygen atoms combusted part of the ligands during the Ir(acac)3 pulse For the second process the adsorption of Ir(acac)3 was molecular on the film surface The formation mechanism of the ALD Ir film was self-limiting (57) Growth proceeded in a FM mode in one monolayer or a fraction of a monolayer The Ir film was deposited during one growth cycle When the precursor dose was high enough to saturate

the precursor adsorption and possibly the surface reactions the film growth became self-limiting The growth rate was constant over the whole surface Due to the self-limiting growth mechanism the ALD Ir films had conformality and uniformity over a large area The growth and mechanism of an ALD Ir film from Ir(acac)3

followed two steps a nucleation step and steady-state growth The formation of Ir nuclei on the substrate surface was crucial for the growth of the Ir fi lm The common oxygen-based Ir ALD processes rely on the Ir surface to catalytically dissociate molecular oxygen to form reactive atomic oxygen for fi lm growth The first metallic nuclei were most likely formed by some minor decomposition of the Ir complex precursor Then these first nuclei catalysed the growth of the Ir film

Christensen and Elam (58) studied the mechanism of ALD Ir film formation using an Ir(acac)3-O2 mixture and proposed a two-step mechanism for Ir ALD The first step was the reaction of an Ir(acac)3 precursor with adsorbed oxygen species on the ALD Ir fi lm surface releasing one or two of the acetylacetonate ligands through ligand exchange and ~01 ligand through combustion The second step was that the remaining acetylacetonate ligands were released by combustion during the subsequent oxygen exposure and the film surface was repopulated with oxygen species Knapas and Ritala (56) presented the growth mechanism of an ALD Ir film from an Ir(acac)3 precursor In contrast with previous work (58) three chemical systems with the corresponding chemical reactions based on the Ir(acac)3 precursor were synthesised Equations (vi) to (viii) (a) Ir(acac)3-O2 system at 225ordmC (Ir deposition)

Ir(acac)3 (g) + 1725O2 (g) rarr Ir (s) + 15CO2 (g) + 105H2O (g) (vi)

(b) Ir(acac)3-O3-H2 system at 195ordmC (Ir deposition)

Ir(acac)3 (g) rarr Ir(acac)3 (ads) + 545O (g) + 20H2(g) rarr Ir (s) + 15CO2 (g) + 305H2O (g) (vii)

(c) Ir(acac)3-O3 system at 195ordmC (IrO2 deposition)

Ir(acac)3 (g) rarr Ir(acac)3 (ads) + 365O (g) rarr IrO2 (s)+ 15CO2 (g) + 105H2O (g) (viii)

23 Physical Vapour Deposition of Iridium Coating

Chang Wei and Chen (59) used the embedded atom method (EAM) potential to study the structures



adsorption energies binding energies migration paths and energy barriers of Ir adatoms and small clusters on Ir (100) (110) and (111) surfaces and found that the barrier for single adatom diffusion was lowest on the (111) sur face higher on the (110) surface and highest on the (100) surface The exchange mechanisms of adatom diffusion on (100) and (110) surfaces were energetically favoured On all three Ir surfaces Ir2

dimers with nearest neighbour spacing were the most stable Houmlrmann et al (5) studied the growth and structural properties of Ir films deposited by electron beam evaporation on the substrate surface At 950ordmC the growth of the Ir fi lm proceeded via 3D nucleation coalescence of the isolated islands and subsequent layer-by-layer growth finally resulting in fl at Ir films Wessling et al (9 10) studied the effect of process parameters such as power bias voltage pressure and target-to-substrate distance on the structure of the Ir film Growth of porous Ir films with high specific surface area delivered low surface mobility of the deposited atoms

24 Electrodeposition of Iridium Coatings

The structure evolution of the Ir coating with increasing current density is related to crystallisation kinetics and mass transfer during the electrodeposition process Toenshoff et al (60) electrodeposited an Ir coating from a chloride electrolyte at 570ordmC and a deposition rate of ~20 μm hndash1 The electrodeposited Ir coating had a columnar structure and high ductility Zhu et al (11) studied the effects of current density and temperature on the structure of an Ir coating When the deposition temperature was increased the nucleation rate decreased but the growth rate increased thus the macroscopic compactness of the coating decreased with increasing grain size Huang et al (34) reported the laminar structure of an Ir coating which resulted from using high and low current densities alternately and suggested a growth mechanism for this coating A two-segment cathodic current was used and coarse Ir grains were found to be deposited on the top side under the lower current density while ultra-fine Ir grains were applied on the bottom side by the higher current density segment Higher current density would result in larger over-potential in the electrode promoting nucleation of Ir grains in the coating Conversely a small over-potential in the electrode promoted growth of the Ir grains which resulted in a thick and columnar grain structure in the Ir coating Therefore a laminar columnar structure of Ir coating was obtained by the

repeated two-segment cathodic current The thickness of each layer was determined by the quantum of electrons transferring at the cathode in each two-segment current cycle

25 Double Glow Plasma Deposition of Iridium Coatings

For the DGP process the bias voltage not only affected the deposition rate but also the substrate temperature Under the same deposition conditions the Ir coating exhibited (220) orientation regardless of the substrate The substrates affected the growth rate of the Ir coating as well as the surface topography The structure of the Ir coating was influenced by bias voltage gas pressure and the effect of the substrate An Ir coating formed by DGP on a Ti substrate was composed of irregular compacted columnar grains with many nanovoids at the interface (see Figure 8) The DGP Ir coating exhibited a lt110gt texture on different substrates due to the formation of initial nuclei with preferred growth on the substrate surface and a channelling effect during the deposition process

When only the substrate bias voltage was applied the whole surface of the substrate was sputtered out (Figure 9(a)) In this case the glow discharge caused the substrate to be bombarded with positive ions of the inert atmosphere and such ion bombardment heated the surface of the substrate to an elevated temperature Another effect of the glow discharge was to clean and activate the substrate surface When the target bias voltage was applied the substrate and the target were co-sputtered out (Figure 9(b)) This resulted in the formation of a transition zone between the coating and the substrate (Figure 9(c)) Because of the large difference between the target and the substrate bias voltages the sputtered substrate and coating atoms

Coating

Interface

Substrate 200 nm

Fig 8 Field emission scanning electron microscope (FESEM) image of the interface between an Ir coating and a Ti substrate (Reproduced with permission (61))



(a)

(b)

(c)

(d)

Substrate atom

Coating atom

Fig 9 Overview of grain structure evolution during deposition of a polycrystalline Ir coating by DGP (a) sputtered substrate atoms (b) co-sputtered coating and substrate atoms nucleation process (c) growth and coalescence to form a continuous film (d) thickness growth (Reproduced with permission (61))

were formed on the substrate surface with time and temperature But the target bias voltage was higher than substrate bias voltage resulting in a high sputtering rate for the target The substrate atoms were suppressed by the intense coating atoms and formed a mixed boundary layer on the substrate surface A pure Ir coating was obtained (Figure 9(d))

The growth mechanism of the Ir coating by DGP depended on a kinetic adsorption and diffusion process with nucleation coalescence and thickness growth At the beginning of the deposition process the growth of the coating was mainly controlled by the nucleation rate (61) Due to the low substrate temperature resulting in low mobility of the deposited atoms some nanovoids were present at the interface During the deposition process the substrate temperature was increased and then kept steady After this the growth of the coating was governed by the growth rate A higher substrate temperature provided enough energy for surface mobility of adatoms

26 Pulsed Laser Deposition of Iridium Coating

Aziz (62) reviewed the fundamentals of growth morphology evolution in PLD in two prototypical growth modes metal-on-insulator island growth and semiconductor homoepitaxy In metal-on-insulator film

growth the same morphology progression occurred equiaxed islands elongated islands percolating metal film and hole filling The kinetic freezing model involving the competition between island-island coalescence and deposition driven island-island impingement explained the morphological transitions in both thermal deposition and PLD For low temperatures the high island density of PLD dominated the morphology evolution and PLD films reached percolation sooner than thermally deposited fi lms As the temperature was increased the PLD percolation thickness approached and then exceeded the thermal percolation thickness It indicated the increasing importance of an energetic effect This effect appeared to be kinetic energy induced adatom-vacancy pair creation which had the net effect of moving atoms upward resulting in a vertical shape transition of the islands Taller islands coalesced more rapidly thereby delaying the point of the elongation transition where coalescence was overwhelmed by impingement

Chen et al (42 63) studied the epitaxial growth of Pt and Ir films by PLD grown on MgO-buffered Si(100) substrate It was found that both Pt and Ir fi lms featured remarkable atomic-scale smooth surfaces and had the same epitaxial relationship with substrates Different from the non-compact surface morphology of Pt film the morphology of Ir film offered a rectangular grain shape and the grains were arrayed regularly and compactly The difference in the surface morphology of both electrode films was due to the degree of speciesrsquo saturation Hence the fast growth rate of Pt film resulted in a supersaturation condition which might cause a SK growth mode rather than a FM mode due to its 3D nucleation In contrast the slow growth rate of the Ir film resulted in a subsaturation condition which led to the FM mode because of its 2D nucleation

3 Micropore Formation Mechanism in Iridium Coatings after High-Temperature Treatment

In our previous publications (40 64ndash72) the thermal stability and oxidation resistance of DGP Ir coatings on refractory materials were studied After high-temperature treatment at 1400ordmC under an inert atmosphere micropores and microbubbles appeared on the coating surface (68) After high-temperature treatment at 2000ordmC under an oxyacetylene flame micropores were still formed on the Ir coating surface and these could provide pathways for oxygen to attack the substrates (65 67 70 71 73) To date although



Reed Biaglow and Schneider (74) claimed that CVD is the only established process for the fabrication of Ir-coated Re combustion chambers operated at 1800~2200ordmC some publications (12 27 68 75 76) suggest that micropores are created on the surface after high-temperature treatment

Micropores diffused outward in a MOCVD Ir coating after heat treatment (77) and appeared in an electrodeposited Ir coating on a Re substrate at the interface of the coating and substrate after thermal resistance testing (27) Huang et al (78) ascertained that micropores in an electrodeposited Ir coating on a Re substrate were found at the grain boundaries and were concentrated to penetrating holes with the growth of Ir grains which resulted in disastrous failure of the coating The micropores in the Ir coating moved outwards after thermal cycling due to the underdense as-deposited coating (75 79) Hu (80) suggested that micropores appeared on the surface of an Ir coating after high-temperature oxidation Micropores were formed in Ir coatings formed by various deposition processes after high-temperature treatment in oxygen or inert environments

A polycrystalline Ir coating consisted of a large number of individual grains separated by boundaries A grain boundary is the interface between two grains in a polycrystalline material Corrosion and oxidation occurred preferentially at the grain boundaries The mean misorientation angles of grains on the surface and cross-section of the coating were 386ordm and 456ordm respectively (Figure 10) The defect density in the

coating was shown to be higher because a high-angle boundary was imperfectly packed compared to the normal lattice

After heat treatment equiaxed grains with indistinct grain boundaries were formed Some large grains and many pores appeared on the coating as seen in Figure 11(a) There are two main reasons for this pore formation On the one hand it may be attributed to the recrystallisation of the Ir crystals (72) After heat treatment equiaxed grains replace the columnar grains in the coating The removal of Ir atoms during heat treatment results in the creation of vacancies which accumulate to form micropores A moving grain boundary might also be expected to be a suitable sink for vacancies to form micropores On the other hand the pores could result from the aggregation of inner micropores between grain boundaries In Figure 11(b) some micropores and microbubbles appear at the surface and at the interface The size and quantity of micropores are distributed in a gradient along the cross-section of the coating This may be because the grain size became larger after heat treatment the micropores in the grain boundaries diffused outward and became larger in the coating The micropore formation at the interface between the coating and the substrate was attributed to the Kirkendall effect (80) owing to different diffusion rates for Ir and Mo at the interface between the coating and the substrate In Figure 12(a) the as-prepared coating was composed only of Ir No new phase was formed A new Ir215Mo85 phase was formed in the Ir coating after heat treatment (Figure 12(b))

(a) (b)

10 20 30 40 50 60

Misorientation angle ordm

Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line

Fig 10 Misorientation angle distribution images of grains on (a) surface (b) cross-section of the as-deposited Ir coating



Fig 11 SEM images of (a) surface and (b) cross-section of the Ir coating after heat treatment at 1400ordmC for 15 h in an argon atmosphere

10 m

(a) (b)

20 m

which may be due to a reaction at the Mo-Ir interface The diffraction peaks of the Ir coating became sharper and narrower indicating recrystallisation and grain growth in the coating According to the Mo-Ir phase diagram (81) four intermetallic phases (ie IrMo Ir3Mo ε IrMo3) can exist at 1200ndash1475ordmC with Ir able to dissolve over 20 at Mo at 1400ordmC The intermetallic compound Ir215Mo85 (717 at Ir) belonged to the Ir3Mo phase with hexagonal D019 structure type A solid solution of Mo in Ir at 1400ordmC is realisable Mo reacted with Ir alloy to produce two distinct intermetallic layers IrMo and Ir3Mo at high temperature (82 83) The Ir3Mo phase was found at high temperature as reported by Chen et al (84) In Figure 12(b) the IrMo phase was not formed possibly because the heat treatment temperature and time were insufficient

The DGP monolayer Ir coatings were ablated by an oxyacetylene torch with a flame temperature of ~2000ordmC for 35 s The surface of the as-ablated coating was composed of equiaxed grains with distinct grain boundaries (Figure 13(a)) Some micropores appear on the coating surface In Figure 13(b) some micropores appear as fractures on the external surface of the coating These micropores could provide tunnels for oxygen diffusion resulting in the oxidation consumption of the substrate which would weaken the Ir-substrate bond

and eventually cause the failure of the coating Under inert gas protection formation of micropores could occur for the following reasons First the micropores may partly result from grain growth of the Ir at high temperature and represent pre-existing intercolumnar defects arising from the original as-deposited growth habit of the Ir coating Second they may result from the aggregation of inner defects such as vacancies voids or grain boundaries Subsequently these inner defects in the as-deposited coating aggregate together

Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt

Fig 12 XRD patterns of the Ir coating on a Mo substrate (a) before (b) after heat treatment at 1400ordmC for 15 h in an argon atmosphere



20 m

(a)

10 m

(b)

Fig 13 SEM images of the as-ablated Ir coating on Mo substrate (a) coating surface (b) fracture surface

resulting in the formation of micropores However different conditions apply for ablated Ir coatings in an abla ting or oxidising environment Ablation processing is an erosive phenomenon which can be interpreted as the combined effect of thermochemical ablation with thermophysical and thermomechanical attacks from high temperature pressure and velocity of the combustion flame

Since Ir did not form a condensed oxide micropores appeared on the surface under severe conditions The surface of the ablated monolayer coating presented imperfections including pores bulges and cracks after exposure to the flame however the Ir coating kept sufficient adhesion to limit the weight loss of the Ir-coated Mo specimen The deposited monolayer Ir coating had a preferred (220) orientation A previous publication (85) showed that a lt110gt-textured Ir coating was easily obtained by DGP The lt110gt-textured Ir coating was not dense however a lt111gt-textured Ir coating was dense (101) Some pinholes were present in the monolayer Ir coating however the multilayer Ir

coating could support high-temperature ablation After ablation some micropores appeared on the surface (See Figure 14(a)) The surface porosities of multilayer and monolayer after ablation were about 055 and 2 respectively Therefore the porosity on the surface of the multilayer coating was less than that of the monolayer In Figure 14(b) the three layers appear on the fracture surface of the coating The multilayer Ir coating was debonded from the substrate by the released gas The grains became large aggregates then grain recrystallisation and growth occurred The interface between adjacent layers became relatively unclear due to the interdiffusion effect The diameter of micropores was about 01ndash06 μm Many pinholes in the Ir coating were aggregated to form micropores and diffused outward but were reduced after heat treatment (75)

In Figure 15 the multilayer Ir coating may be thick enough to protect the substrate from oxidation at high temperature The multilayer Ir coating could inhibit the formation of cracks or micropores extended through

5 m 5 m

(a) (b) Fig 14 SEM images of (a) surface (b) fracture surface of the multilayer Ir coating on Nb substrate after ablation at 2000ordmC for 40 s



Multilayer

Substrate

(a)

(b)

(c)

Ir(g) IrO2(g) IrO3(g) O2 infi ltrates coating through pinholes

Nb2O5 gaseous release

Fig 15 Schematic diagram of failure of multilayer Ir coating during the ablation process (a) coating structure (b) the duration of the process (c) coating failure (Reproduced with permission of Springer (70))

the thickness of the coating (Figure 15(b)) Although the surface of the deposited Ir coating was dense Ir was not expected to form a protective oxide coating because of the formation of the volatile IrO3 The evaporation of the Ir and its oxide resulted in the formation of micropores These pinholes or micropores on the surface of the coating acted as pathways for oxygen to penetrate the coating Once oxygen reached a reaction site on the Nb substrate it quickly reacted with Nb to form an oxide Tuffias Melden and Harding (86) reported that the outward diffusion of Nb was the major factor contributing to interdiffusion This diffusion resulted in void formation which compromised the structural integrity of the coating Because the Nb substrate ignited with oxygen in the quickly flowing

flame masses of gas were released from the surface of the as-coated specimen The released gases severely influenced the thermal stability of the multilayer Ir coating (Figure 15(c)) An intermediate layer of Re or W could be employed to prevent interdiffusion of the coating and the substrate (86) The underdense multilayer Ir coating and the interdiffusion between the coating and the substrate resulted in the failure of the multilayer Ir coating

At about 1100ordmC Ir does not form a condensed oxide due to the formation of the volatile oxide (IrO3) Figure 16 The as-deposited coating was composed of columnar grains with high-angle boundaries Numerous grain boundaries gave preferred sites for the onset of oxidation during high-temperature treatment Therefore the Ir grain boundaries in the coating could easily react with oxygen and formed gaseous IrO3 Although the quantity of volatile Ir oxide was low during the temperature rise and cooling periods the grain boundaries provided tunnels for oxygen diffusion and oxidation of the Ir grains then occurred at the interface The gaseous IrO3 was formed at the grain boundaries resulting in the formation of micropores For the high-angle boundary of the columnar grains (Figure 10) the coating had a higher energy state due to a combination of high interfacial and surface energies A columnar-to-equiaxed grain transition occurred resulting in a more stable atomic arrangement which minimised the total energy in the coating The coating configuration must have sufficient energy to overcome an activation energy barrier During high-temperature treatment recrystallisation occurred There was an energetic driving force for grain boundary formation which eliminated the energies of the free surfaces of the two contacting columnar grains in exchange for the lower energy of a new grain boundary After high-temperature

O2

O2

IrO3 Columnar grains

Micropores

Equiaxed grains

Fig 16 Micropore formation mechanism in an Ir coating after high-temperature treatment in an oxidising environment (Reproduced with permission of Elsevier (72))

Voids High-temperature

treatment Initial nuclei



treatment the micropores in the coating by rapid surface or bulk diffusion mechanisms reduced the high surface energy associated with the coating confi guration (79) The micropores in the coating diffused outward after high-temperature treatment due to the extrusion effect under grain growth Although the surface of a pure Ir coating produced some pinholes and micropores after high-temperature treatment ceramic coatings may protect the Ir coating to some extent An as-coated specimen was laid on an Al2O3 firebrick Because the ablation temperature was higher than the melting point of Al2O3 one small portion of the firebrick located at the surface of the Ir coated specimen was melted The melted Al2O3 flowed on the surface of the coating which was then partially covered by melted Al2O3 (see Figure 17) The surface of the Ir coating appeared as many volcano-like micropores The coating seemed to be melted and many micropores were found However the remaining surface was dense because the melted Al2O3 filled in the pores on the surface of the coating Al2O3 could therefore be used as an outer layer for oxidation protection because of its attractive properties such as low vapour pressure low oxygen diffusion coefficient good corrosion resistance and oxidation stability (87 88) Therefore Al2O3 as a top layer could seal micropores on the surface of the coating and inhibit the evaporation of the Ir Even if the ceramic oxide layer cracked during rapid cooling the evaporation of Ir would be minimised and orders-of-magnitude less oxygen would be permitted to attack the coating

4 Conclusion and Outlook

Re-Ir material is currently of particular interest for engineering applications In China a short nozzle

of Re-Ir has been tested and the temperature of combustion chamber wall reached 2090ordmC for 300 s without failure (89) There are a number of processes available to produce thin or thick Ir coatings on various substrates (as shown in Part I (1)) with varying results as to the quality of the as-deposited coatings The texture of the coating affects its microstructure and mechanical properties The texture evolution of the coating is controlled by the deposition parameters and conditions A lt111gt-textured Ir coating is dense which could provide an effective diffusion barrier against oxygen The growth mechanism of Ir coatings produced using different deposition processes is different and the process should therefore be selected according to the requirements of the intended application For vapour and solution deposition processes the formation of the coatings is controlled by migration nucleation coalescence growth and thickness growth After high-temperature treatment micropores were formed on the surface and the fracture surface of the Ir coating Micropores may supply tunnels for oxygen to attack the substrate Grain boundaries may act as sources and sinks for vacancies in a high-temperature environment Inner defects such as grain boundaries and vacancies accumulated in the coating during high-temperature treatment result in the formation of micropores Evaporation of volatile Ir oxide occurred preferentially at the grain boundaries again resulting in micropore formation The columnar-to-equiaxed grain transition resulted in a more stable microcrystalline arrangement The micropores in the Ir coating diffused outward after high-temperature treatment due to the extrusion effect under grain growth Therefore a dense and thick Ir coating could improve the life of engineering components in very high-temperature environments

Fig 17 SEM images of the surface of multilayer DGP Ir coating after oxidising ablation at 2200ordmC for 90 s (Reproduced with permission of Elsevier (71))

100 m 5 m

5 m

Al2O3

Ir coating



Furthermore a ceramic oxide layer on the surface of an Ir coating may minimise the evaporation and oxidation of the coating

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (Grant Number 50872055 E020703) and the Natural Science Foundation of Jiangsu Province (Grant Number BK20150260) The authors wish to thank the referees for their helpful suggestions and Editors Ms Sara Coles and Ming Chung for the editing

References

1 W-P Wu and Z-F Chen Johnson Matthey Technol Rev 2017 61 (1) 16

2 J-P Zhao X Wang Z-Y Chen S-Q Yang T-S Shi and X-H Liu J Phys D Appl Phys 1997 30 (1) 5

3 S Eroglu and B Gallois J Phys IV France 1993 3 C3-177

4 H Osamura lsquoDevelopment of Long Life and High Ignitability Iridium Spark Plugrsquo Seoul 2000 FISITA World Automotive Congress Seoul South Korea 12thndash15th June 2000 F2000A144

5 F Houmlrmann H-Y Peng Th Bauer Q Li M Schreck Y Lifshitz S-T Lee and B Stritzker Surf Sci 2002 513 (3) 525

6 H Murakami T Yano and S Sodeoka Mater Trans 2004 45 (9) 2886

7 M Hecq and A Hecq J Vac Sci Technol 1981 18 (2) 219

8 D-Y Park D-S Lee H-J Woo D-I Chun and E-J Yoon Tong Yang Cement Corp US Patent 6025205 2000

9 B Wessling W Mokwa and U Schnakenberg J Electrochem Soc 2008 155 (5) F61

10 B Wessling D Luumlsebrink W Mokwa and U Schnakenberg J Electrochem Soc 2008 155 (5) F66

11 L Zhu S Bai H Zhang and Y Ye Appl Surf Sci 2013 265 537

12 W-P Wu Z-F Chen X-W Cheng and Y-W Wang Nucl Instr Meth Phys Res Sect B Beam Int Mater Atoms 2013 307 315

13 L-B Wang Z-F Chen P-Z Zhang W-P Wu and Y Zhang J Coat Technol Res 2009 6 (4) 517

14 Z Zhao C Wang M Li L Wang and L Kong Appl

Surf Sci 2006 252 (12) 4257

15 V G Beshenkov L A Fomin D V Irzhak V A Marchenko V I Nikolaichik and A G Znamenskii Thin Solid Films 2012 520 (23) 6888

16 C V Thompson and R Carel Mater Sci Eng B 1995 32 (3) 211

17 J Goswami C-G Wang P Majhi Y-W Shin and S K Dey J Mater Res 2001 16 (8) 2192

18 C V Thompson Ann Rev Mater Sci 2000 30 159


20 I V Markov ldquoCrystal Growth for Beginners Fundamentals of Nucleation Crystal Growth and Epitaxyrdquo 2nd Edn World Scientific Publishing Co Pte Ltd Singapore 2003

21 C R M Grovenor H T G Hentzell and D A Smith Acta Metall 1984 32 (5) 773

22 W-P Wu Z-F Chen X Lin B-B Li and X-N Cong Vacuum 2011 86 (4) 429

23 K Mumtaz J Echigoya T Hirai and Y Shindo Mater Sci Eng A 1993 167 (1ndash2) 187

24 H Levinstein J Appl Phys 1949 20 (4) 306

25 S M Sabol B T Randall J D Edington C J Larkin and B J Close ldquoBarrier Coatings for Refractory Metals and Superalloysrdquo B-MT-(SPME)-25 TRN US0603658 Bettis Atomic Power Laboratory (BAPL) Pennsylvania USA 2006 pp 1ndash28

26 H-U Kim D-H Cha H-J Kim and J-H Kim Int J Prec Eng Manuf 2009 10 (3) 19

27 L-A Zhu S-X Bai and H Zhang Surf Coat Technol 2011 206 (6) 1351

28 W-P Wu X Lin Z-F Chen Z Chen X-N Cong T-Z Xu and J-L Qiu Plasma Chem Plasma Proc 2011 31 (3) 465

29 S-C Tjong and H Chen Mater Sci Eng R Reports 2004 45 (1ndash2) 1

30 T Hanamura and H Qiu lsquoUltra-Fine Grained Steel Relationship Between Grain Size and Tensile Propertiesrsquo in ldquoAnalysis of Fracture Toughness Mechanism in Ultra-Fine-Grained Steelsrdquo NIMS Monographs Springer Japan 2014 pp 9ndash25

31 P Kuppusami H Murakami and T Ohmura J Vac Sci Technol A 2004 22 (4) 1208

32 J Hagen F Burmeister A Fromm P Manns and G Kleer Plasma Process Polym 2009 6 (S1) 678

33 H-J Li H Xue Q-G Fu Y-L Zhang X-H Shi and K-Z Li J Inorg Mater 2010 25 337

34 Y-L Huang S-X Bai H Zhang and Y-C Ye Int J Refract Metals Hard Mater 2015 50 204

35 Y Ritterhaus T Hurrsquoyeva M Lisker and E P Burte



Chem Vap Deposition 2007 13 (12) 698

36 M A El Khakani M Chaker and B Le Drogoff J Vac Sci Technol A 1998 16 (2) 885

37 Y-S Gong C-B Wang Q Shen and L-M Zhang Vacuum 2008 82 (6) 594

38 T D Khoa S Horii and S Horita Thin Solid Films 2002 419 (1ndash2) 88

39 S Hogmark S Jacobson and M Larsson Wear 2000 246 (1ndash2) 20

40 Z-F Chen W-P Wu X-N Cong and L-B Wang Adv Mater Res 2011 314ndash316 214

41 J A Venables ldquoIntroduction to Surface and Thin Film Processesrdquo Cambridge University Press Cambridge UK 2000

42 T-L Chen X-M Li S Zhang and X Zhang Appl Phys A 2005 80 (1) 73

43 E Bauer and J H van der Merwe Phys Rev B 1986 33 (6) 3657

44 J A Venables G D T Spiller and M Hanbucken Rep Prog Phys 1984 47 (4) 399

45 N Kaiser Appl Optics 2002 41 (16) 3053

46 T Gerfin W J Haumllg F Atamny and K-H Dahmen Thin Solid Films 1994 241 (1ndash2) 352

47 Z-F Chen W-P Wu L-B Wang and Y Zhang Surf Eng 2011 27 (4) 242

48 Y-M Sun J P Endle K Smith S Whaley R Mahaffy J G Ekerdt J M White and R L Hance Thin Solid Films 1999 346 (1ndash2) 100

49 J R V Garcia and T Goto Mater Trans 2003 44 (9) 1717

50 T Goto R Vargas and T Hirai J Phys IV France 1993 3 (C3) 297

51 P P Semyannikov N B Morozova K V Zherikova S V Trubin I K Igumenov and N V Gelfond ECS Trans 2009 25 (8) 887

52 V Yu Vasilyev N B Morozova T V Basova I K Igumenov and A Hassan RSC Adv 2015 5 (41) 32034

53 R Vargas T Goto W Zhang and T Hirai Appl Phys Lett 1994 65 (9) 1094

54 N V Gelfond I K Igumenov A I Boronin V I Bukhtiyarov M Yu Smirnov I P Prosvirin and R I Kwon Surf Sci 1992 275 (3) 323

55 S-I Song J-H Lee B-H Choi H-K Lee DshyC Shin and J-W Lee Surf Coat Technol 2012 211 14

56 K Knapas and M Ritala Chem Mater 2011 23 (11) 2766

57 T Aaltonen M Ritala V Sammelselg and M Leskelauml

J Electrochem Soc 2004 151 (8) G489

58 S T Christensen and J W Elam Chem Mater 2010 22 (8) 2517

59 C-M Chang C-M Wei and S-P Chen Phys Rev B 1996 54 (23) 17083

60 D A Toenshoff R D Lanam J Ragaini A Shchetkovskiy and A Smirnov lsquoIridium Coated Rhenium Rocket Chambers Produced by Electroformingrsquo 36th AIAAASMESAEASEE Joint Propulsion Conference and Exhibit Las Vegas USA 24thndash28th July 2000

61 W-P Wu and Z-F Chen Acta Metall Sin (Engl Lett) 2012 25 (6) 469

62 M J Aziz Appl Phys A 2008 93 (3) 579

63 T Chen X-M Li and X Zhang J Cryst Growth 2004 267 (1ndash2) 80

64 Z-F Chen W-P Wu and X-N Cong J Mater Sci Technol 2014 30 (3) 268

65 W-P Wu Z-F Chen X N Cong and L-B Wang Rare Metal Mater Eng 2013 42 (2) 435 (In Chinese)

66 Z-W Zhang Z-H Xu J-M Wang W-P Wu and ZshyF Chen J Mater Eng Perf 2012 21 (10) 2085

67 W-P Wu and Z-F Chen J Wuhan Univ Technol-Mater Sci Ed 2012 27 (4) 652

68 W-P Wu Z-F Chen and X Lin Adv Mater Res 2011 189ndash193 688

69 W-P Wu Z-F Chen H Cheng L-B Wang and Y Zhang Appl Surf Sci 2011 257 (16) 7295

70 W-P Wu Z-F Chen and L-B Wang Prot Met Phys Chem Surf 2015 51 (4) 607

71 W-P Wu J-J Jiang and Z-F Chen Acta Astronaut 2016 123 1

72 W-P Wu and Z-F Chen Surf Interface Anal 2016 48 (6) 353

73 J-M Wang Z-W Zhang Z-H Xu X Lin W-P Wu and Z-F Chen Corros Eng Sci Technol 2011 46 (6) 732

74 B D Reed J A Biaglow and S J Schneider Mater Manuf Proc 1998 13 (5) 757

75 K Mumtaz J Echigoya H Enoki T Hirai and Y Shindo J Mater Sci 1995 30 (2) 465

76 D H Lowndes D B Geohegan A A Puretzky D P Norton and C M Rouleau Science 1996 273 (5277) 898

77 W-B Yang L-T Zhang Y-F Hua and L-F Cheng Int J Refract Metals Hard Mater 2009 27 (1) 33

78 Y-L Huang S-X Bai H Zhang and Y-C Ye Appl Surf Sci 2015 328 436

79 K Mumtaz J Echigoya and M Taya J Mater Sci



1993 28 (20) 5521

80 C-Y Hu lsquoStudy on the CVD Iridium Coated Rhenium Compositersquo PhD thesis Central South University Changsha China Dissertations from CNKI 2002 (in Chinese)

81 L Brewer and R H Lamoreaux lsquoMolybdenum Physico-Chemical Properties of its Compounds and Alloyrsquo Atomic Energy Review International Atomic Energy Agency Vienna Austria 1980 7 263

82 E K Ohriner and E P George J Alloys Compd 1991 177 (2) 219

83 J C Hamilton N Y C Yang W M Clift D R Boehme K F McCarty and J E Franklin Metall Trans A 1992 23 (3) 851

84 S Chen C-Y Hu J-M Guo and J-M Yang Rare Metal Mater Eng 2005 34 (6) 916

85 J Haumlmaumllaumlinen E Puukilainen M Kemell L Costelle M Ritala and M Leskelauml Chem Mater 2009 21 (20) 4868

86 R H Tuffias G J Melden and J T Harding lsquoHigh Temperature Oxidation-Resistant Thruster Materials Phase IIrsquo NASA Contractor Report 187205 1991

87 K Mumtaz J Echigoya H Enoki T Hirai and Y Shindo J Alloys Compd 1994 209 (1ndash2) 279


89 X-H Zhang F-T Xu Z-H Jia H Q Li K-M He and D Chen Mater China 2013 32 (4) 203 (in Chinese)

The Authors

Wang-ping Wu received his doctorate in Materials Processing Engineering at Nanjing University of Aeronautics and Astronautics China in 2013 and held a Pikovsky Valazzi Scholarship at Tel Aviv University Israel where he was a Postdoctoral Fellow He is now a Senior Lecturer at the School of Mechanical Engineering in Changzhou University China His research interests are mainly directed towards the synthesis and characterisation of films and coatings of the noble metals and their alloys

Zhao-feng Chen is Professor of Materials Science and Director of the International Laboratory for Insulation and Energy Efficiency Materials at the College of Material Science and Technology at Nanjing University of Aeronautics and Astronautics His research interests include advanced insulation composite materials and the application of coatings of noble metals



the grain size increased and the preferred orientation changed in the order lt111gtrarrlt110gtrarrlt111gt The substrate was composed of polycrystalline Nb (Figure 1(a)) (110) (200) (211) and (220) Nb diffraction peaks appeared The preferential growth orientation could be determined by a texture coefficient (TC(hkl)) which was calculated using Equation (i) (13)

Ihkl I0(hkl)TC(hkl) = (i)n1n[ i=1Ii(hkl) I0(hkl)]

TC(hkl) is the texture coefficient of the (hkl) plane I(hkl)

is the measured intensity of (hkl) plane I0(hkl) is the corresponding recorded intensity in the Joint Committee on Powder Diffraction Standards (JCPDS) data fi le and n is the number of preferred growth directions of Nb or Ir According to Equation (i) and the JCPDS Card (No 35-0789) TC(110) TC(200) TC(211) and TC(220) of Nb were 0346 2703 0463 and 0519 respectively It was indicated that the substrate had a preferred (200) orientation The Ir coating had a dominant (220) orientation in the XRD pattern (see Figure 1(b)) The diffraction intensities of the other peaks were weak The epitaxial growth of the (220)-orientated Ir coating did not occur on the (200)-orientated Nb substrate

In Figure 2 the red blue and green colours represent Ir (001) Ir (111) and Ir (101) crystal faces respectively The orientation distribution map shows that the vast majority of colour on the coating surface was green indicating that the DGP Ir coating had a preferred (101) orientation The coating was mainly composed of submicrometre-sized grains of 02ndash03 μm (Figure 3)

Figure 4 shows the maximum value distributed in the centre of the (101) pole figure This indicates that the preferred orientation of the coating was a (101) texture This result was in agreement with that of the XRD pattern (Figure 1)

15 m

Colour coded map type inverse pole fi gure [001] Iridium

The (220)-textured coating was due to the deposition of initial nuclei with preferred growth on the substrate surface The crystal orientation of the DGP Ir coating was further investigated by an electron backscatter diffraction (EBSD) technique (3)

Fig 2 Orientation distribution map for Ir grains according

111

101001

TD

RD

Inte

nsity

arb

itrar

y un

its

20 30 40 50 60 70 80 90

2 ordm

(b)

(a)

Nb(

110)

Nb(

220)

Nb(

211)

Ir(

220)

Nb(

200)

Fig 1 X-Ray diffraction (XRD) patterns of (a) Nb substrate and (b) Ir coating formed by a DGP process (Reproduced with permission of Elsevier (12))

to the inverse pole figure (Reproduced with permission of Elsevier (12))

01 015 02 025 03 035 04 045

Grain size (diameter) m

Are

a fra

ctio

n

025

02

015

01

005

0

Fig 3 Grain size distribution map of the surface of a DGP Ir coating (Reproduced with permission of Elsevier (12))



111

101001

TD

RDRD

TD

TD

RD


Max = 5340 4039 3055 2311 1748 1322 1000 0756







hkl 111 100 110

hkl crystal planes

a

a

a

a












10 m 1 m

Ir

2 4 6 8 10 12

655 524 393 262 131

0

(a) (b)



10 m 10 m 10 m 10 m

(a) (b) (c) (d)


















Hardness GPa 21 95 442 298 210-24
























Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





111

101001

TD

RDRD

TD

TD

RD


Max = 5340 4039 3055 2311 1748 1322 1000 0756







hkl 111 100 110

hkl crystal planes

a

a

a

a












10 m 1 m

Ir

2 4 6 8 10 12

655 524 393 262 131

0

(a) (b)



10 m 10 m 10 m 10 m

(a) (b) (c) (d)


















Hardness GPa 21 95 442 298 210-24
























Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors












10 m 1 m

Ir

2 4 6 8 10 12

655 524 393 262 131

0

(a) (b)



10 m 10 m 10 m 10 m

(a) (b) (c) (d)


















Hardness GPa 21 95 442 298 210-24
























Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





10 m 10 m 10 m 10 m

(a) (b) (c) (d)


















Hardness GPa 21 95 442 298 210-24
























Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors







Hardness GPa 21 95 442 298 210-24
























Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors













Re-evaporation


Island growth

Island shape

Coalescence Film




























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





























Coating

Interface

Substrate 200 nm




(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





(a)

(b)

(c)

(d)

Substrate atom

Coating atom

















(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors










(a) (b)

10 20 30 40 50 60


Num

ber f

ract

ion

010

008

006

004

002

0

Fitted line

10 20 30 40 50 60


Num

ber f

ract

ion

012

010

008

006

004

002

0

Fitted line





10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors






10 m

(a) (b)

20 m




Ir

Ir215Mo85

(111

)

(222

)(3

11)

(220

)

(200

)

(a)

(b)

20 30 40 50 60 70 80 90

2 ordm

Inte

nsity

cou

nt




20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





20 m

(a)

10 m

(b)






5 m 5 m




Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





Multilayer

Substrate

(a)

(b)

(c)







O2

O2


Micropores

Equiaxed grains











100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors










100 m 5 m

5 m

Al2O3

Ir coating




Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors






Acknowledgements


References















Surf Sci 2006 252 (12) 4257








































































1993 28 (20) 5521











The Authors





















































1993 28 (20) 5521











The Authors




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