hard coatings: state-of-the-art and applications györgy...

Hard coatings: state-of-the-art and applications

György Radnóczi

Research Institute for Technical Physics and Materials Science, H1525 Budapest, P.O. Box 49, Budapest, Hungary

INTRODUCTION

Hardness values of some bulk materials (GPa):Diamond 80-100 Cubic BN 50 Si3N4 17Steels 2FCC metals ~1

Coatings: 1-10 µm thick layers20-50 GPa hardness

Hardness: TiN: 27 GPa, CNx: 20 GPa, DLC: 20-25 GPaOther properties: friction coef. (<0.2, ideal: 0.01), wear (mass/time), elasticity (10%), in some cases 50-80%thoughness, plasticity,thermal stability (1000oC), enviromental stability (water, vacuum, oxygen)

I. Hard PVD ceramic coatings –Single metal nitride PVD coatings e.g. TiN, CrN, ZrNThe first generation of hard PVD coatings were single metal nitrides such as TiN, CrN and ZrN. They have been exploited commercially since the middle of the 80’s in cutting applications because of their higher hardness compared to high speed steel and cemented carbide and for decorative purposes because of their attractive appearance. TiN has a distinctive yellow-gold colour, CrN looks unsurprisingly like chrome and ZrN has a green-gold colour. ZrCN is used to simulate gold in decorative applications such as watch cases.

These PVD coatings are still available and in many applications are the best option.

However their temperature resistance is insufficient for applications such as high speed machining. TiNfor example decomposes at 450 °C.

Therefore the next step in the development of hard PVD coatings was the improvement of the temperature resistance to make the coatings more suitable for applications such as high speed machining and general high temperature wear protection.

A standard TiN coating has hardness of 25-30 GPa

II: Hard PVD ceramic coatings –Alloy coatings improve oxidation resistance, e.g. TiAlN

This improvement in temperature resistance was achieved by introducing other elements such as Cr, Al or Y, as well as C into the TiN lattice.

bTiN

bTiAlCN

r=Gb/τ

r

G; shear modulusb; Burgers vectorτ; shear stress

Ordered alloysDisordered alloys

http://www.pvd-coatings.co.uk/

Material Hardness GPa

TiN 27Ti0.5Al0.5N 28TiAlVN 30Ti0.75Al0.25N 31

III. Hard PVD ceramic coatings –The development of superlattices

Further improvement to the properties of hard PVD coatings was achieved in the third generation of hard PVD coating development through the deposition of multilayers and superlattices.Multilayers become superlattices when the period of the different layers is less than 10 nm. Multilayered coatings of materials with similar crystal structures tend to form columnar crystals built of alternating epitaxial lamellae which extend through the whole coating, provided that the thickness of the individual lamellae is sufficiently thin, typically 5–25 nm. Such coatings are referred to as superlattice coatings.

One of the first examples of superlattice coatings was obtained by combining TiN/VN and TiN/NbN. Several authors have shown that this type ofmultilayered coating structure can improve the hardness as well as the toughness, compared to single layers of the same materials. By selecting a suitable combination of materials for themultilayered structure it is possible to improve the resistance against wear, corrosion, oxidation, etc.

Supermodulus effect: the hardness of superlatticesdepend on the period and shows a maximum at superlattice period of about 5-10 nm.

TiN-Nb0.4V0.6N superlattice

Vickers microhardness of TiN/V1-xNbx N superlatticeswith x=0, 0.4 and 1 as a function of wavelength.

M. Shinn, L. Hultman, S. A. Barnett: J. Mater. Res. 7 (1991) 901

Har

dnes

sG

Pa

60

50

40

30

20

IV. Hard PVD ceramic coatings –The recent development of nanocomposite coatings

PRINCIPLESA nanocomposite coating consists of at least two phases: a nanocrystalline phase and an amorphous phase, or two nanocrystalline phases. The basic idea for the design of nanocomposites is based on the thermodynamically driven segregation (inmiscibility of the components) in binary (ternary, quaternary) systems. The phase separation leads to the spontaneous self-organization of a stable nanoscale structure. This generic concept has recently led to the development ofnanocomposite PVD coatings. These PVD coatings have nm sized grains and exhibit enhanced yield strength, hardness and toughness properties as a result of the well-known Hall-Petch effect.

Veprek at al.: J. Vac. Sci. Technol. A 21 (2003) 532

-combine hard crystalline grains (e.g. carbide) and amorphous DLC or CNxwith high elastic modulus to achieve correspondingly high hardness and elastic recovery;

-maintain nanocrystalline grain sizes at the 10~20 nm level to facilitate grain boundary sliding and restrict initial crack formation;

-separate the nanograins by an amorphous matrix of thickness above 2 nm to prevent interaction of atomic planes in the adjustednanocrystalline grains, but less than 10 nm to restrict the path of a straight crack.

Superhard coating materials are defined by hardness values that exceed 40 GPa. For this purpose:

N.B. Dahotre, S. Nayak / Surface & Coatings Technology 194 (2005) 58–67

Hall-Petch relation:

H= Ho+Kyd-1/2

The main mechanisms of low deformability can be summarized as follows:

Hinder dislocation generationHinder dislocation movementIncrease the blocking power of grain

boundariesHinder GB slidingHinder crack generationHinder crack propagation

The strength of (super)hard coatings is due either to structural properties or the incorporated internal stresses during growth.

These examples show, that superhardness is due to structural effects, since neither the grain size, nor the plastic hardness changes until the temperature of re-crystallisation (around 1100 oC) is reached. The best stabilizer of microstructure in a-Si+N4 GB phase of about 1 nm in thickness.

S. Veprek and M. Jilek: Pure Appl. Chem., Vol. 74, No. 3, pp. 475–481, 2002.

Ti1-xAlxN/a-Si3N4 Nc-TiN/a-Si3N4/a-TiSi2

Tr

Example of annealing behaviour of a (Ti1-xAlx)N/a-Si3N4 sample deposited on cemented carbidesubstrate at a temperature of about 300 °C;

Ternary nc-TiN/a-Si3N4/a-TiSi2nanocomposite deposited on steelsubstrate; total Si-content 8 at. %, 3.5 % Si3N4, 4.1 % TiSix. The thickness of the coating is 20 µm.

Decrease of the hardness and no change of the crystallite size is observed upon annealing of the ZrN/Ni coatings above 400 °C. Simultaneously, a decrease of the high compressive stress in the coatings is observed.

(P. Karvankova, H. Männling, Ch. Eggs, S. Veprek. Surf. Coat. Technol. 146–147, 146 (2001).

ZrN/Ni

Hardness due to internal stresses

Trel

Jörg Patscheider,

MRS Bulletin, March, 2003, p. 180

Silicon Nitride Fraction (at%)

Nan

ohar

dnes

s (G

Pa)

0 50 10025

30

35

40

45nc-TiN/a-Si3N4

a-Si3N4TiN

1)

2)

3)

1) Decrease of TiNgrain size due to Si3N4 addition.

2) Secondary nucleation of TiN, formation of ncTiN, sharp, thin phase boundaries. Deformation by GB sliding.

3) Thick, deformable Si3N4 between TiNgrains.

nanocomposites

CSIRO Australia

Secondaryelectron FIB images of cross-sections througha) TiN, b) TiN-5% Si, c) TiAlVN-6%Si, films.a) b)

c)

K. Sedláčková, P. Lobotka, I. Vávra, G.RadnócziAccepted in Carbon

50 nm

CNM23G. Radnoczi et al.:Surface and Coatings Technology 180 –181 (2004) 331–334

Hexagoanal Ni/Ni3CGraphite-like C

C+ Ni, Ts= 200oC

C+Ni800oC/SiO2 substrate

Graphene sheets

CNM27

fcc Ni

Gy. J. Kovács, G. Sáfrán, O. Geszti, T. Ujvári*, I. Bertóti*, G. RadnócziSurface and Coatings Technology 180-181, (2004) 331-334.

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400 500 600 700 800

Hard

ness

(GPa

)

Temperature (Celsius)

"C-Ni""CNx-Ni"

Ni

CNx

E=100-130 GPa E=60-100 GPaFCC cubic cryst. phaseHexagonal cryst. phase

C/CNx 1-2 nm C/CNx 2-5 nm 20-50 nm

Internal stresses: not measurable


Ni: 210 GPa

40 50 60 70 80 90

100 110 120 130 140 150

0 100 200 300 400 500 600 700 800

Elas

tic M

odul

us (G

Pa)

Temperature (Celsius)

"C-Ni""CNx-Ni"


0.20

0.25

0.30

0.35

0.40

0.45

0 100 200 300 400 500 600 700 800 900

Tempereature (0C )

Fric

tion

coef

ficie

nt

C-Ni

CNx-Ni

FN = 3mN

A ’Sharp Rockwell’ head was used for friction tests with 1 micron/sec velocity and 3 mN load.

NANOCOMP –P. B. Barna et al. ResearchResearch InstituteInstitute forfor TechnicalTechnical PhysicsPhysics andand MaterialsMaterials ScienceScience -- MFAMFAKonkolyKonkoly--Thege 29Thege 29--33 , H33 , H--1121 Budapest 1121 Budapest www.mfa.kfki.huwww.mfa.kfki.hu

TiAlN(C) coating with increasing Carbon content

An objective in the development of self-lubricating wear-resistant coatings is to make use of lubricious phases such as graphite, amorphous carbon or MoS2 incorporated into coatings. A series of (Ti,Al)(N,C) coatings with different carbon contents (0-28 at.%) have been deposited by reactive magnetron sputtering of TiAl in a mixture of Ar, N2 and CH4 gases.

NANOCOMP – P. B. Barna et al. ResearchResearch InstituteInstitute forfor TechnicalTechnical PhysicsPhysics andand MaterialsMaterials ScienceScience -- MFAMFAKonkolyKonkoly--Thege 29Thege 29--33 , H33 , H--1121 Budapest 1121 Budapest www.mfa.kfki.huwww.mfa.kfki.hu

Localization of 3D Carbon inclusions by HRTEM

TiAlNC 28 at.% C

C appears to be incorporated into theTiAlNC crystal lattice

C is present between the lamellae of TiAlNC crystal

A.Kovács, B.Veisz, P.B.Barna, G.Radnóczi, and M.StüberEMC abstract

FFT filtered HRTEM image (a) with simulated image of the transition between WC substrate and TiAlNCcoatings. (b) the atomic view of the interface from the direction of the normal of the interface.

A critical feature of the industrially applied coatings is the durability, which depends much on the properties of the substrate-coating interface.

WC

TiAlCN

V. Hard PVD ceramic coatings –Amorphous hard coatings, DLC and CNx

DLC film growth- by energetic particles – subplantation, 10 eV

Compressive stress develops 1-10 GPa

Relaxations during growth: above 100 oCduring annealing above 400 oC in oxygen or air

above 600 oC in vacuum

Corresponding to the subplantation process the general structure of ta-C films consists of three zones:

1) a transition zone as an intermixing of the surface substrate elements with carbon,

2) the real DLC layer with dominating sp³ bonds and 3) a more graphitic thin surface layer.

As a well-proven rule of thumb, the hardness lies above one tenth of the Young’s modulus: H > E/10.The Young’s modulus is directly determined by the stiffness and the density of the atomic bonds and correlates with the fraction of sp³ bonds. Correspondingly, the Young’s modulus varies for amorphous carbon films over an extreme range from < 100 GPa up to > 700 GPa (for perfect diamond polycrystals E=1143 GPa).

B. Schultrich, H.-J. Scheibe, V. WeihnachtFraunhofer Institute for Materialand Beam Technology IWS, Dresden, Germany

steel

5 µm

80%sp3 mainly sp2

Mass Selective IBMass Selective IB 120eVLaser ArcPulsed High current ArcFiltered Laser Arc

Research Institute for Technical Physics and Materials Science of the Hungarian Academy of Sciences

29

SAMPLE 21.1 V509, 540Kx54 V512Si: 5 at%, C:70 at%, N: 25 at%Ts= 350 Co

5 nm

Anisotropicatomic structure, no columnarmorphology.


30

Plan-view high-resolution electron micrographs of structures forSi-C-N films grown at 350 oC, corresponding to region I (a) and

region III (b). T. Berlind, N. Hellgren, M.P. Johansson, L. Hultman: Surf. and Coatings Technology 141 (2001) 145-155


31

5 nm

JEOL 3010 (300kV)

Si31.1,Si: 7.5at%, C: 68 at%, N:28At%

Dd

Radnóczi1, G. Sáfrán1, Zs. Czigány1, T. Berlind2, L. Hultman3:Structure of DC sputtered Si-C-N thin filmsThin Solid Films 440 (2003) 41-44

Linköping, Newcastle, BudapestNew Fullerene-like Materials: HPRNCT-2002-00209.

CNx, by IBAD

CNx on Si

CNx on Ti CNx, by IBADLinköping, Newcastle, BudapestNew Fullerene-like Materials: HPRNCT-2002-00209.

VI. Hard PVD ceramic coatings –Low friction coatings

Attention recently has focussed on the development of hard, wear resistant, low friction coatings such as Graphit-iC™, MoST™ and Dymon-iC™, with the automotive industry again the driving force. Coating materials: graphite, MoS2, WS2, oxides, minerals in nanocomposites with nitrides.The coefficient of friction must be below 0.1, sometimes close to 0.01.

http://www.pvd-coatings.co.uk

Cr, C+Cr,

Yang S, Li X., Renevier NM., Teer DG. Surface and Coatings Technology 2001;142-144; 85-93

The friction behavior of Diamor® is studied by a fretting tribometer (ball on disk) in comparison to classic hard coating (TiN, CrN). No lubrication was used. This result demonstrates the improvement in the tribological properties in comparison to classic hard coatings, which can be obtained with DLC films under this testing condition.

„the real DLC layer with dominating sp³ bonds and

a more graphitic thin surface layer”

B. Schultrich, H.-J. Scheibe, V. WeihnachtFraunhofer Institute for Material and Beam Technology IWS, Dresden, Germany

Friction behavior ofDiamor® and nitridefilms on steel against100Cr6 ball.

VII. Hard PVD „ceramic” coatings –Metal-metal nanocomposites

In many prospective applications other than cutting tools, where the substrate stiffness and strength may be comparatively low, ultrahard ceramic films (which tend also to exhibit high stiffness) are rarely the ideal solution. Hard, stiff ceramic films are fundamentally mismatched to these soft, low elastic modulus materials. As has been demonstrated previously, by bearing designers and polymer tribologists, a bulk materials approach to wear which utilises both the hardness (H) and the elastic modulus (E) of the material gives more predictable results in ranking wear performance of engineering coatings. In this respect, a high H/E ratio is desirable, since this implies a longer ‘elastic strain to failure’ (ie. improved resilience) for the material. It is often metallic films which (if sufficiently hard) give superior results, due primarily to a combination of resilience and toughness more closely matched to the substrate material properties.In essence, such coatings tend inherently to be more ‘damage tolerant’.

Properties: brittle (TiN, diamond and other ceramic materials), elastic (CNx, some nanocomposites, e.g. CNx-Ni)plastic (tough, soft, like Sn-Al)

“On the significance of the H/E ratio in wear control : A nanocomposite approach to optimised tribological behaviour.”A.Leyland & A.Matthews Wear 246 (2000) 1.

To fit the properties of the substrate and coating, for softer materials metallic coatings can be better.

Thin Film Physics Laboratory

Cu-Ag

350A Cu + 440A Ag6x10-6 mbarroom temperature

F. Misjak, G. Radnóczi

Load controlled mode 1mN fixed load.Loading and unloading rate 0,02mN/s.

Hardness profile

0,5

1,5

2,5

3,5

4,5

5,5

6,5

0 10 20 30 40 50 60 70 80 90 100

Distance (x10 micron)

Hard

ness

(GPa

)

Ezüst Réz

Fix load 1mN

1.5GPa

Ag Cu

Ag Cu

Distance µm/10

F. Misjak, T. Ujvári, G. Radnóczi

Depth controlle, max. depth 50 nmLoading and unloading rate 0,01mN/s

Hardness profile

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

0 20 40 60 80 100

Distance (x10 micron)

Har

dnes

s (G

Pa)

Ezüst Réz

0 200 400 600 800 1000(µm)

1.5GPa

Ag Cu

F. Misjak, T. Ujvári, G. Radnóczi

Applications:

Cutting toolsBearingsPistons in car industryGearsFood and formaceptical industryImplantsDecorative coatingsHard discsCoin stamp headsGlasses

CSIRO

B. Schultrich, H.-J. Scheibe, V. WeihnachtFraunhofer Institute for Materialand Beam Technology IWS, Dresden, Germany

DLC coated components ofmachines of packagingindustry

S. Veprek and M. Jilek: Pure Appl. Chem., Vol. 74, No. 3, pp. 475–481, 2002.

SPCN

1203EDSR

(P

20-P30) without

anycoating

TiN-coated

TiAl)N

-coated

coatedw

ithnc-(Ti1-

xAlx)N/a-S

i3N4

nanocomposite

coatedw

ithnc-(Ti1-

xAlx)N/a-S

i3N4

nanocomposite

nc-(Ti1-xAlx)N

/a-Si3N

4 multilayer

nanocomposite

A variety of tools for dry and fast turning, milling, drilling are coated and successfully used by the customers. Under the conditions of the dry machining, which saves the environmentally risky coolants and saves costs, the temperature of the cutting edge and of the rake reaches 600–800 °C. This illustrates the need of thermally stable and oxidation resistant coatings.

Example of the cutting performance of various coatings. Symmetric milling of steel CK45, feed 0.23 mm/tooth, depth of cut 2 mm, cutting speed 179 m/min.

Nan

ohar

dnes

s (G

Pa)

0 50 10025

30

35

40

45 nc-TiN/a-Si3N4

a-Si3N4TiN

1)

2)

3)

Silicon Nitride Fraction (at%)Jörg Patscheider,

MRS Bulletin, March, 2003, p. 180

P.B. Barna-M. Adamik 1998

In this work: hardness, friction, conductivity, STS

SUMMARY

The phases constituting a nanocomposite can be: a nanocrystalline phase and an amorphous two nanocrystallinetwo amorphous phases.

The basic idea for the design of nanocomposites is based on the thermodynamically driven segregationin binary (ternary, quaternary) systems. The segregation leads to the spontaneous self-organization of a stable nanoscale structure. Kinetic limitation of any changes at the application conditions.

This generic concept has recently led to the development of superhard (>40GPa) and multifunction nanocomposite PVD coatings.

SUMMARY

ACKNOWLEDGMENT

Gy. J Kovács, K. Sedláckvá, Zs. Czigány, I. Bertóti, T Ujvári, O. Geszti, G. Sáfrán P. B. Barna The HPRN partners

This work was supported by theHungarian National Science Foundation(OTKA T-30424 and T-043359) and also by the EU FP5 under the contract New Fullerene-like Materials: HPRN-CT-2002-00209

THANK YOU FOR THE ATTENTION

[email protected]

Veprek, M. Jilek, Pure Appl. Chem., Vol. 74, No. 3, pp. 475–481, 2002.

ACHIEVMENTS

-Conventional hard materials are brittle and undergo fracture at a strain of 0.1 %. It was therefore promising to find in the nanocompositesan unusual combination of very high hardness of 40–100 GPa, high elastic recovery of 80–94 %, and a high resistance against crack formation.

-A recent theoretical analysis of the experimental data showed that the conventional fracture physics and mechanics when scaled down to a crystallite size of 3–5 nm can explain the mechanical properties of coatings.

-The high resistance against crack formation is due to the small stress concentration factor of 2–4 in the nanocomposites, which is orders of magnitude smaller than that of ordinary microcrystalline materials. For the same reasons, the yield stress needed to propagate a nanocrack is much higher in the nanocomposite than in a microcrystalline material even if they have the same stress intensity factor.

-The high elastic recovery of up to 94 % and high range of predominantly elastic deformation of 10 % which results in a very high elastic energy density upon indentation can be understood in terms of reversible flexing.

-Superhardness is the consequence of the formation of stable nanocomposites with a strong interface which avoids grain boundary sliding.

hard coatings: state-of-the-art and applications györgy...

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