FIB Milling and Characterization of CrC Coatings on Tool Steel Substrate R.M. Mineva, M. Ilievab, J. Kettlea, G. Laleva, S.S. Dimova, I. Dermendjievb, R. Shishkovb aManufacturing Engineering Center, Cardiff University, Cardiff, CF24 3AA, UK

aDepartment of Materials Science and Technology, Rousse University, Bulgaria Abstract

In micro tools, the base of a die should be ductile and the surface layer that will undergo processing should have a good machining response to various tool making processes. At the same time the resulting working surfaces of the tooling cavities should be hard, having low roughness, low wettability and high erosion resistance. To achieve such diverse properties, nano-crystalline CrC coatings deposited onto 12% Cr tool steel were investigated in this research. To verify the properties of this coating various metallographic techniques were applied. In particular, the corrosion resistance was studied by means of potentiodynamic anodic polarization. A STEM analysis of the structure was performed on samples prepared with Focused Ion Beam (FIB). The mechanical properties and grain size distribution were determined and statistically analysed. In addition, X-ray diffraction and scanning electron microscopy were used in studying the surface properties of this coating.

To investigate the response of the CrC coatings to FIB milling a series of rectangular trenches were produced using FIB/SEM cross-beam system. Especially, the effects of the ion beam current, exposure time and ion fluence on the sputtering yield and roughness of the produced micro structures were studied. Some essential parameter’ windows for performing FIB milling with relatively high sputtering rates, higher than 1 µm/min, and at the same time achieving the best possible surface integrity were determined during the experiments. Keywords: CrC, PVD, FIB, micro-tooling, STEM analyses

1. Introduction

In recent years a variety of hard coatings such as CrC, CrN, CrN(C), TiN, TiC, TiN(C) have been investigated for a variety of practical applications [1]. In this coatings part of the N atoms might be replaced by C. Their structure and composition are characterised by a complex of very high or even extreme properties useful for tooling application such as: hardness, wear resistance, scratch resistance, corrosion resistance, and low adhesion to the glass, polymers or metals. Although these coatings are broadly used for improving the cutting tools’ performance their potential in manufacturing micro cavities for injection moulding, thermal imprinting and micro-forming is underutilised. A short overview of some promising PVD/CVD coatings suitable for micro tooling application is provided below.

Hard wear resistant coatings based on Ti and Cr are usually deposited using PVD and sometimes CVD methods [2]. In order to obtain a multilayered structure with enhanced functional characteristics various new schemes has been used, for example magnetron sputtering and evaporation by DC discharge.

By means of alternating the reactive gasses multilayered Ti/TiN/TiC coatings on stainless steel could be obtained [3]. Using PECVD in vacuum Cr/N/Cr/CrN coatings [2] on different tool steels (AISI H13, 5Cr1Mo1V, AISI and K340 8Cr2Mo1V) was deposited. Also Cr/Cr3C6 coatings on M2 hardened steel [4] and nano-structured Cr/CrN coatings on galvanised 1010 mild steel was deposited using magnetron sputtering [5].

By using magnetron sputtering of two targets TiN/CN coatings on Si substrates [6], Ti/TiN/CrN coatings on Ti6-Al-4V alloy were obtained. The same idea could be applied in DC discharge evaporation where two cathodes of different materials could be used. By applying this approach multilayers of TiN/ZrN

[7], TiN/CrN [8], TiN/(Ti,Al)N [9] were deposited. The combination of PVD and PECVD enables the deposition of various multilayered TiN/CrN/Ti/Cr/TiN/CrN coatings [10, 11].

The coatings based on Ti possess very high hardness (2300-2500 HV0.05). Single layered TiN coatings are relatively cheap and could be used in various machining processes. The coatings based on Ti are characterised by high density and adhesion as well as high wear resistance. Multiple layered Ti carbide and nitride coatings have higher toughness and hardness than single layered. They are very suitable for application in highly loaded tools in metal forming, stamping, extrusion and cold working.

Compared with Ti based coatings the Cr based coatings possess lower hardness (up to 2200 HV0.05) but exhibit good anticorrosion properties. Single and multiple layered chromium coatings are used when apart from high hardness and wear resistance a good thermal and corrosion protection of the base is desired. In this case a compromise between the mechanical characteristics and protective properties of the coating is required.

Multilayered CrN/CrC and TiN/TiC coatings are also used to cover functional tool surfaces [12, 13] in production of lenses and other optical elements and reflectors. Cr-C coatings of the tooling surface demonstrate very good non-adhesive properties to the product material and are suitable for glass and plastic moulding when low surface roughness of the tooling cavity is important and the easy of de-moulding is crucial. CrC and CrN coatings provide reliable protection of the tool when corrosive gases are released during the polymer processing.

Although the usage of the hard and corrosion resistive Ti and Cr based coatings in conventional manufacturing is relatively well studied not much information is available on their usage in micro-

manufacturing. Some data regarding the usage of TiN/AlTiN, TiN/CrN and CrN/AlTiN multilayered coatings in micro machining were reported in [14-16].

Various of toolmaking processes, such as micro-milling, EDM, laser ablation and ion beam milling, can be employed in fabricating 3D tooling cavities. This study investigates the FIB machining response of CrC coatings in the context of a possible use for this micro-technology for manufacture of cavities for micro-injection moulding and thermal imprinting. Simultaneously, FIB processing was employed to determine more accurately the structure and properties of the tooling surfaces.

2. PVD coating experiment

Cr/C coatings have been deposited by magnetron DC sputtering in a vacuum furnace with electrical graphite heating.

In this research, an analogue of the D2 (X153CrMoV12, BS EN ISO 4957:2000) tool steel was chosen as a substrate material for the experiments. The typical composition of the alloy was: 1.45-1.7% C; 11-12.5%Cr; 0.4-0.6%Mo; 0.15-0.3%V. This class of ledeburitic chromium steels has a very high resistance to abrasive and adhesive impact of the hard carbides in the structure. It is characterised by good toughness, low dimensional change, high pressure resistance, secondary hardening, hence very suitable as a base material for a subsequent nitriding or coating (CVD, PVD). The steel is normally subjected to quenching in oil or martempering (austenitisation followed by step quenching) which reduces the risk for deformation and cracking. After tempering it is suitable for moulding abrasive and reactive polymers. The Cr/C coating is intended to increase the corrosion and wear resistance while preserving the toughness of the tool.

A first ion etching for cleaning the surface was performed in Ar. After the cleaning of the substrates the CrC coatings were deposited. The PVD parameters are shown in Table 1.

Table 1 Ion etching and coating conditions

Cleaning temperature Tc=500oC Time of cleaning tc=10 min The Ar gas flow rate (standard cubic centimetres per minute) GAr=28.6 sccmPressure in the vacuum chamber during the cleaning Pc=15 Pa Pressure during the deposition of coating Pd = 0.45 Pa Temperature during the deposition Td = 450oC Time for deposition td=120 min The Ar gas flow rate GAr=11.4 sccm The CH4 gas flow rate GCH4=7 sccm Discharge voltage U=460V Discharge current I=7A Distance between the target and substrate Lt-s=70 mm

The flow rate presented in Table 1 assumes a standard temperature of 20 °C and pressure of 101.325 kPa. The target that was used consisted of sintered Cr with 99.998% purity.

After the deposition the coatings was slowly cooled to the ambient temperature within the furnace.

15001700190021002300250027002900310033003500

40 50 60 70 80 902Θ°

I

Cr7C3 [321]

Cr7C3

[102]

Cr7C3

[421]Fe [110]

Cr7C3

[312]

Cr7C3 [801]Fe [200]

Fig.1 X ray diffraction patterns of Cr/C coatings onto

the tool steel substrate.

3. Sample characterization

The X-ray diffraction studies (DRON2 Fe-Kα radiation) showed (Fig.1) that the prevailing stoichiometric composition of the coating was Cr7C3.

The coatings’ adhesion was investigated using a scratch test (CSEM ‘REVETEST’) which revealed 43 N critical scratching load (typical for CrC coatings [17]).

A 3% aqueous solution of NaCl open to air was used as an aggressive environment for evaluating the coatings corrosion behaviour. This standard solution was selected because chlorine ions from plastic decomposition are expected to attack the tooling material during the moulding process. First the open circuit potential has been measured for 120 min. Next test was the potentiodynamic polarization measurement. The potentiodynamic curves have been recorded by a potentiostat OH – 405 RADELKIS in a three – electrode cell consisting of the studied sample, a reference saturated calomel electrode (SCE) and a Pt counter electrode. The sweep rate of the potential was 60 mVmin-1 and the applied voltage was in the range from -0.6 V to +1.2 V.

The free potential (-28/-430 mV for CrC/D2), corrosion potential (-160/-400 mV for CrC/D2) and cathodic current peak (0.8*10-3/3.0*10-3 mA/cm2 for CrC/D2) has been measured and showed that for coated samples the values were in order of magnitude different than those for D2. These values were typical for the CrC which proves its low porosity and high protective properties for the steel substrate.

The thickness of the coatings was measured using OMNIMET software on DIC contrast micrographs and was 30 µm (+/- 1.5 µm). The samples were prepared using nital and picral reagents to reveal the grains (Fig. 2). The microstructure of the steel substrate consisted of bainite, pearlite, and carbides as well as Cr reach ferrite and corresponds to micro hardness (Vickers) of MHV0.025 = 240 (+/-15) versus MHV0.025 = 1400 (+/- 65) for the coating.

FIB milling for TEM sample preparation (Fig.3) was used to prepare 78 nm thick lamellae allowing for in-situ STEM (Scanning Transmission Electron Microscopy) analysis (Fig.4). The structure morphology was analysed using the OMNIMET image processing software. STEM micrographs showed grain structure which was statistically analysed. The grain size distribution is presented in Fig.5. 60% of the grains

were in the range 6 to 12 nm and the average grain size was 10nm.

Fig.2 DIC (differential interference contrast) picture of the CrC coating onto D2 tool steel.

Fig. 3 Stages of the TEM Lamellae preparation using

FIB in both milling and deposition mode and subsequent nano-manipulation of the prepared

lamellae (x3500).

Fig. 4 STEM image of the CrC lamellae.

Fig. 5 Grain size distribution of the Cr/C coating.

4. FIB milling of the Cr/C coating

It is considered [18,19] that refinements of the CrC grain structure during the PVD could lead to more “favourable” conditions when employing machining technologies with high specific processing energy, such as FIB milling, in comparison with the conventional polycrystalline materials. For example, when a larger number of grains forms the walls and edges of micro features, the geometrical accuracy and surface finish will be improved because the selective ‘milling’ effects

due to crystallographic changes can be attenuated. The material refinement also increases grain boundaries that could be considered crystallographicaly disordered or amorphous and lead to a more uniform material removal. In addition, Cr/C possesses lower thermal diffusivity and conductivity than the tooling metals and it is expected to be easily sputtered by accelerated Gallium (Ga) ions without any significant thermal impact.

The FIB structuring was conducted on a XB1540 Carl Zeiss FIB/SEM cross-beam system, which is equipped with a FIB column (Canion 31), and a gas injection system for gas assisted etching and FIB-induced chemical vapour deposition. Rectangular trenches were produced in this experiment to study the effects of the ion beam current, exposure time and ion fluence on the sputtering yield and roughness of the resulting surface (Fig. 6).

Fig.6 FIB milled patterns produced with different ion beam current (Iion ) and time (ts). Number of layers

NL=1

The ion fluence in nC/µm2 was calculated according to equation (1):

AtIf sion

i×

= (1),

where Iion, nA is the target ion beam current of charged Ga+ ions and A, µm2 - the target area size of features.

‘Scandium SIS’ software was used to study the produced patterns. The depth of trenches was estimated by applying tilt compensation (Fig. 7, 8). The gray scale line profiles were generated and the standard deviation (SD) of the data was calculated to judge about the surface roughness (Fig. 9). The data corresponding to very deep trenches with pronounced re-deposition and irregular shape of the bottom and those with irrelevant depth of milling (< 0.4 µm) was excluded from ion fluence analyses. Some essential processing parameters were determined during the experiments:

(1) Relatively high milling rates (>1 µm/min): Iion > 1 nA; ts > 60 s; fi > 2.3 nC/µm2;

(2) The best achievable surface quality: Iion =0.1- 0.5 nA; ts = 30-300 s; fi = 0.4- 0.5 nC/µm2;

(3) Compromise between (1)&(2): Iion ∼0.5 nA; ts = 30-300 s; fi ∼ 0.9 nC/µm2;

For most of these experiments only a single layer was milled in order to minimise the optimisation time. Increasing the number of layers (NL), whilst keeping the ion fluence constant, resulted in better surface quality however this was at the expense of the final

CrC

depth of the milled features (Fig.10) and a significant increase of milling time (due to the reduced sputtering rates).

Thus, the sputtering of the material in layers leads to a better surface integrity as a result of the reduced dwelling time over each processing pixel. Such layer-based processing results in less extensive selective sputtering due to the structural anisotropy and non-homogeneity of the material. Local redeposition is also less pronounced when milling strategies with high NL are applied.

Fig. 7 Depth of FIB milling as a function of Iion and ts.

(on x-axis: FIB current according to the legend)

0

1

2

3

4

5

4.7 2.3 2.3 1.9 0.9 0.9 0.9 0.5 0.4 0.4

Fluence, nC/µm^2

Dep

th, µ

m

Fig. 8 Depth of FIB milling as a function of fi.

0

20

40

60

80

4.7 2.3 2.3 1.9 0.9 0.9 0.9 0.5 0.4 0.4

Fluence, nC/µm^2

SD

Fig. 9 The influence of fi on surface quality.

Fig.10 FIB milled CrC with Iion =200mA, ts =120s, NL=1 (a) and NL=50 (b).

5. Conclusions

1. PVD coatings were produced in thicknesses of about 30µm suitable for micro fictionalisation of the

surface. The stoichiometric composition of the coating material was x-ray determined to be Cr7C3. The properties of the nano structured (10nm average grain size) surface layers demonstrated good mechanical properties (MHV0.025 = 1400), superb corrosion resistance and low adhesion (measured by scratch tests). 2. The experiments showed that to achieve a good surface finish with sputtering rates of approximately 1 µm/min the FIB milling of the CrC coatings should be carried out with ion fluence of around 0.9 nC/µm2 . The best processing strategy is to use a higher number of layers in order to reduce the re-depositioning effect and improve the surface quality.

Acknowledgments

The research reported in this paper is funded by the MicroBridge programme supported by the Welsh Assembly Government and the UK Department of Trade and Industry, and the EPSRC Programme “The Cardiff Innovative Manufacturing Research Centre”. Also, it was carried out within the framework of the EC FP6 Networks of Excellence “Multi-Material Micro Manufacture (4M): Technologies and Applications” and the FP6 Integrated Project “Charged Particle Nanotech” (CHARPAN).

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