Recent advances in the chromium nitride PVD process for ... - CiteSeerX

Journal of Materials Processing Technology 167 (2005) 415–421

Recent advances in the chromium nitride PVD process for forming and machining surface protection G.G. Fuentes a,∗ , R. Rodriguez a , J.C. Avelar-Batista b , J. Housden b , F. Montalá c , L.J. Carreras c , A.B. Cristóbal d , J.J. Damborenea d , T.J. Tate d,e a

c

Centre of Advanced Surface Engineering-AIN, San Cosme y San Damian s/n, E-31191 Pamplona, Spain b Tecvac Ltd., Buckingway Business Park, Swavesey, Cambridge CB4 5UG, UK Grup TTC. Recubrimientos Avanzados Avinguda Can Rosés, nau 8. Pol. Ind. Can Rosés 08191 Rub´ı. Barcelona, Spain d Centro Nacional de Investigaciones Metal´ urgicas CENIM, Avenida Gregorio del Amo 8, E-28040 Madrid, Spain e IC Consultants Ltd., Exhibition Road, London SW7 2QA, UK

Abstract This paper reports on the tribological properties of commercial Cr–N physical vapour deposition (PVD) processes for enhanced mechanical protection of forming and machining tools. The study is carried out on conventional tooling materials as substrates, i.e. high speed steels (M2) and cemented tungsten carbide. Electron beam (EB), magnetron sputtering (MS) and cathodic arc (CA) commercial Cr–N PVD processes from three different EU coating centers/companies are evaluated in terms of their tribological behaviour by standard characterization techniques for material properties: surface hardness and roughness, film adherence strength and wear resistance under sliding conditions. The results are discussed on the basis of their current applications and their influence on tool performance for manufacturing processes. Moreover, film tribological performance is correlated with their microstructural properties as obtained by electron microscopy and X-ray diffraction. In terms of surface hardness and adherence strength, CA Cr–N seems to outperform EB and MS coatings. However, EB and MS exhibit enhanced surface finish which might be more appropriate for some applications. © 2005 Published by Elsevier B.V. Keywords: Tribology; Surface treatments; Cr–N coatings

1. Introduction Contact interactions between sliding materials under load conditions represent critical phenomena for quality and performance assurance of forming and machining manufacturing processes [1–3]. Adequate tribological properties between tools and working materials determine not only the maximal durability of tooling service but also the manufacturing quality of the final component. The mechanical and tribological properties of forming and machining tools influence material plastic flow in extrusion, surface finishing in stamping and moulding, cutting speed and precision in machining, or maintenance of the working material properties without undesired local modifications due to overheating or stress [1–4]. ∗

Corresponding author. Tel.: +34 948 421 101; fax: +34 948 421 100. E-mail address: [email protected] (G.G. Fuentes).

0924-0136/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2005.06.011

For certain applications, advanced surface treatments provide additional protection on forming/machining tools in terms of greater surface hardness, adequate friction and improved wear resistance. They can limit the appearance of defects characteristics in forming (lapping and bending), and in machining (chipping, cracking, overheating, etc). Physical vapour deposition (PVD) of chromium nitride and its alloys with aluminium are emerging as a good alternative to other conventional surface treatments for tool protection [5–8]. PVD Cr–N has good hardness and toughness, which allows deposition of films as thick as 20 ␮m without significant performance loss. Its friction coefficient (in sliding contact with 100Cr6 steel) is comparable to that of TiN, although it can be improved by optimising the growth parameters [8]. Cr–N shows high chemical stability making its use appropriate for hot working applications, as its oxidation temperature is about 700 ◦ C [9]. In addition, its characteristic

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G.G. Fuentes et al. / Journal of Materials Processing Technology 167 (2005) 415–421

silver colour has extended its applications towards the decorative sector. In fact, sectors such as the automotive or the ceramic industry are considering Cr–N as a feasible alternative to galvanic hard chromium. There are however certain drawbacks which limit its applications in high added value tools. Its excellent chemical stability makes it difficult to remove the coating once the treated tools need to be recoated, due to wear of the original Cr–N layer. Different initiatives are presently running in order to overcome these limitations [10]. In this paper, the authors report on the tribological performance of different PVD Cr–N coatings used and commercialized by different EU centers/companies. Three PVD processes for Cr–N deposition, electron beam, magnetron sputtering and cathodic arc, are compared in terms of film microstructure, surface hardness and film adherence strength, and wear resistance under contact sliding conditions. In addition, the study compares coating performance on typical forming/machining materials such as high speed steel M2 (hardened/tempered, 62HRC), and cemented carbide (K10 graded, Co and Ni as binder material). Results evidence the good overall tribological properties of the different coatings.

2. Experiments 2.1. Materials and coating process Cr–N films (2–3 ␮m thick) were deposited on hardened and polished AISI M2 steel discs (60HRC, 29.5 mm × 0.5 mm) and polished tungsten carbide (K10, nominal composition: 92.7 wt% WC; 1.0 wt% TaC; 0.3 wt% Cr2 C3 ; 6.0 wt% Co) discs (29.5 mm × 4.8 mm). Three extended industrial PVD techniques were used for the comparative study: electron beam (EB), magnetron sputtering (MS) and cathodic arc (CA). EB evaporation was carried out using a Tecvac IP70L PVD coater. MS was performed using a Metaplas Ionon MZR 323 equipped with a 400 mm × 500 mm capacity vacuum chamber. High purity Cr magnetron targets of 90 cm × 25 cm were used. In the case of the CA process, the coatings were performed using a home-made vacuum chamber equipped with four operative Cr cathodes. All processes included a 5–10 min substrate etching process in order to remove surface contamination to improve coating adherence.

2.2. Coating characterization techniques Films structure were characterized using scanning electron microscopy (Cambridge Stereoscan 250 Mk2 scanning electron microscope) in: (a) top and (b) fracture crosssection surfaces. Additional microstructure analysis were performed by X-ray diffraction in the Bragg–Brentano configuration (θ–2θ). Film stoichiometry was assessed using a 1000 RF Jovin–Yvon glow discharge spectrometer, equipped with a 35 channel hemispherical monochromator. Surface roughness measurements were made with a Wyco RST 500 optical profilometer, using vertical scanning and phase shift interferometry to a vertical resolution better than 10 nm. Mechanical and tribological characterizations were carried out using a combined set of techniques. Universal hardness (HU) was measured using Vickers ultra-microhardness tester (Fischerscope H100XY VP) set to a final load of 10 mN. Tribological tests were measured in a Falex Isc-320pc tribometer, under ambient conditions in a pin-on-disc configuration, using cemented WC counterballs (1/8 in.) and an applied load of 5 N during 20,000 cycles. All tests were done at a constant sliding speed of 10 cm/s. The volume loss during the sliding cycles was measured using a Wyco RST 500 optical profilometer to obtain 3D topographical images of the wear tracks. Finally, film adhesion tests were carried out in a Scratch Tester CSEM A02-143b equipped with optical and acoustic analysis. The tests are carried out using a Rockwell diamond tip (200 ␮m diameter) and a final load of 50 N at a constantly increasing rate of 50 N/min.

3. Results 3.1. Previous characterization of the films Film stoichiometries, as obtained by GDOES are gathered in Table 1 as a function of the PVD process. MS and EB processes reveal N concentrations around 30 at%, whereas in the case of CA, the N concentration is 40 at%. XRD analyses indicate that Cr–N films show preferentially the CrN cubic phase. Moreover, MS and EB show small diffraction peaks, which can be assigned to the hexagonal ␤-Cr2 N lattice structure. In the case of CA only cubic CrN was observed with strong (0 0 2) texture [11].

Table 1 Chemical composition of Cr–N processed by EB, MS and CA as measured by GDOES Lattice structurea by XRD

PVD process

Composition by GDOES [at%] Cr

N

O

Electron beam Magnetron sputtering Cathodic arc

70–75 65–70 65–60

25–30 30–35 35–40

2–3 2–3 1–2

Crystalline phases detected by XRD. a In EB and MS, c-CrN phase is dominant over ␤-Cr N. 2

c-CrN + ␤-Cr2 N c-CrN + ␤-Cr2 N c-CrN


The Cr–N equilibrium phase diagram [12] indicates that for N concentrations below 35–40 at% only the hexagonal Cr2 N phase can be present, whereas a mixture of h-Cr2 N and c-CrN is expected above this threshold. The fact that EB and MS films exhibited both h-Cr2 N and c-CrN phases might be an indication that the GD is under-estimating the total N concentration. Analogously, CA films show only cCrN structure, in agreement with the fact that they present the largest N concentration (Table 1).

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3.2. Microstructure Fig. 1a–c shows SEM micrographs of three CrN coatings (2–3 ␮m) as deposited on HSS substrates by: EB (a), MS (b) and CA (c), respectively, top surface (right) and fracture cross-section (left). SEM pictures exhibit different film surface and cross-sectional microstructure depending on the deposition technique. As-deposited EB films show that its microstructure is composed of small grains (0.5 ␮m size

Fig. 1. SEM micrographs of: (a) EB, (b) MS and (c) CA, Cr–N coatings on HSS in fracture cross-section (left) and top surface (right).

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approximately) embedded in a more dense structure through the film (Fig. 1a, left). MS films show a dense microstructure with a well-defined coating/substrate interface (Fig. 1b, left). For the CA films the fracture cross-section also reveals a dense microstructure as observed for MS films. Examination of the top surface morphology of different films reveals that both EB and MS present low roughness, resembling the morphology of the uncoated substrate. Conversely, CA films shows characteristic micro-droplets on the surface. 3.3. Mechanical and tribological characterization Fig. 2 shows, respectively, (a) universal hardness at 10 mN final load HU, (b) surface roughness, Rq , and (c) wear coefficient, k, for the three CrN PVD processed surfaces; HSS (black bars) and cemented carbide (grey bars). Data corresponding to the pristine substrate are also included for com-

Fig. 2. (a) Universal hardness (HU), (b) surface roughness (Rq ) (quadratic standard deviation) and (c) wear coefficient (k) of EB, MS and CA, Cr–N coatings as labelled, deposited on HSS (black) and cemented carbide (grey). References for pristine HSS and cemented carbide are shown.

parison. Universal hardness represents the inherent resistance of the material to be elastically plus plastically deformed. This parameter represents a critical issue for the surface quality assessment of a forming/machining tool prior to use. The magnitude of HU determines decisively the abrasive wear resistance of the tool surface and therefore its service life. Analysis of HU reveals that the surface hardness of the HSS increases from 7400 N/mm2 up to around 12,000 N/mm2 for CrN (all cases). In the case of the cemented carbides, the surface HU decreases from 14,000 N/mm2 to 12,500 N/mm2 for CA and MS, whereas HU keeps constant for the EB films. HU values observed in this work are in good agreement with data reported in the literature for CrN based films [6,7,10]. Surface roughness measurements are depicted in Fig. 2b) for the three PVD processes on HSS and WC, together with Rq for polished substrates (tool). Surface roughness is an important parameter, which determines, on one hand, a good material flow in forming processes and, on the other hand, improved lubrication in dry or wet machining processes, limiting overheating of the sliding zone between tool and machining material. Root mean squared surface roughness, Rq , represents the mean value of the surface height profile (Ra ) inversely weighted by the difference between each surface point height. This means that Rq is a good approximation of Ra , with reduced contributions from large surface defects, for instance voids or micro-droplets. Results derived from Fig. 2b suggest that MS process provides the best surface finish with very low Rq values of 18–22 nm. EB deposited films show larger Rq values of 20–25 nm, while largest Rq is observed for the CA films, with values up to 35–40 nm. This large value for the Rq of CA films correspond to the deposition of micro-droplets from the cathodes. Roughness values are in agreement with SEM examination (Fig. 1) of the top surfaces of the PVD films, in which droplet defects are visible on CA coatings. Wear is the most critical tribological parameter for forming and machining processes. Tool wear, in its different regimes (severe, mild), dictates the performance and quality of the forming/machining operation. Excessive tool wear not only limits the tool life, but also is detrimental for the process quality. Wear coefficients, k, are represented in Fig. 2c for the three PVD CrN processes and for the reference material. Wear coefficients, k, represent the volume loss of the coated surface (tool) normalized by the sliding distance and the applied load. Fig. 2c provides evidence of significant improvement of wear resistance of the tested surface with respect to the uncoated substrates. k as measured directly on HSS substrates is around 8 × 10−16 m3 /(N m), decreasing down to 2.5 × 10−16 m3 /(N m) for EB and MS CrN films and down to 1 × 10−16 m3 /(N m) in the case of the CA coatings, representing approximately an order of magnitude improvement in surface performance. A better insight of the effects of the CrN coatings on the wear coefficients of the HSS substrates can be obtained from the wear tracks after 20,000 sliding cycles of test. The results are represented in Fig. 3 for: (a) pristine HSS, (b) EB, (c) MS and (d) CA deposited Cr–N


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Fig. 3. Wear tracks of: (a) pristine HSS, (b) EB Cr–N, (c) MS Cr–N, and (d) CA Cr–N coatings on HSS, after 20,000 cycles of pin-on-disc sliding tests (see text for experimental conditions).

coatings. Fig. 3a exhibits a typical surface in mild–moderate wear regime in which the material is removed through the abrasive action of small particles around 1–2 ␮m during the sliding process. In fact, the optimal picture of the wear track suggest the presence of micrometric scaled particles (small fragments of removed material) remaining on top of the worn surface. Wear tracks of EB and MS CrN films show hints of mild wear, as suggested by the lack of particles on the worn area of the coatings, indicating a significant protection for the steel tool surfaces. On the other hand, observation of wear tracks on CA films after pin-on-disc sliding tests did not show any evidence of material loss after 20,000 sliding cycles. 3.4. Scratch adhesion tests The scratch test technique was carried out to characterize film adherence strength. Good coating/substrate adhesion is required to access the PVD process for enhanced tooling service. Fig. 4a–f shows optical images of scratch tracks measured at 40 N load for EB, MS and CA on HSS (left images) and cemented carbide (right images). Fig. 4 reveals that different types of adhesive layer/substrate failures appear under strong plastic deformation conditions. Below 40 N load only small conformal failures in the form of lateral fissuration show up (10–20 N not shown here). For all three Cr–N coatings, first visible adhesive failures appear at loads larger than 35–40 N, indicating an overall good coating/substrate adhesion strength regardless the deposition technique used. In particular, at 40 N (load range compared in Fig. 4), scratch tracks of EB films on HSS exhibit

small lateral and internal fissurations and first hints of lateral chipping (Fig. 4a) occurring as stress is released due to strong plastic flow. For MS films, scratch tracks show frontal deformations and lateral chipping at 35 N, which constitute evidence of adhesive type failures. In the same load range (35–40 N), CA films do not show any evidence of conformal or adhesive failures (Fig. 4e). In the case of the PVD films on cemented carbides, scratch tracks measured over the same load range do not evidence any presence of significant adhesive failures. Only CA films on cemented carbide show some hints of small layer failures of conformal type (Fig. 4f). The reason for a better film adhesion strength of CrN films on cemented carbides compared to HSS is not striking since the elastic and plastic properties of CrN closer resemble those of the cemented carbides. All these features cause the CrN film to effectively follow the deformation flow of the substrate, which occurs during forming/machining processes.

4. Discussion and concluding remarks The mechanical and tribological performance of three industrial PVD process for protective Cr–N films are compared in terms of ultra-microhardness, surface finish, wear resistance and coating/substrate adhesion. Results indicate a good overall tribological performance for all the three PVD methods. For all cases, the authors noted improvements in surface hardness (especially in the case of HSS substrates), and reductions in the wear coefficient. Surface finish was

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Fig. 4. Scratch images of: (a) EB, (c) MS and (d) CA films on HSS, and (b) EB, (d) MS and (f) CA on cemented carbide at 40 N of applied load.

also investigated and low overall roughness values were reported. CA films exhibited the poorest surface finish due to the deposition of micro-droplets from the cathode during the growth process. The three PVD film growth methods reported here show good wear resistance and film/substrate adhesion strength, making them suited as protective coatings in forming/machining applications. Closer observation of wear and scratch tests might suggest that CA films exhibit superior tribological performance with respect to that of EB and MS films, in particular on HSS substrates. This observation is reflected in the fact that a negligible wear track was produced on this coating, and, on the other hand, no hints of adhesion failures were observed in the scratch tests at the 40 N load onset. In the case of cemented carbide substrates, all three Cr–N PVD coatings exhibited comparable tribological performance. The fact that the CA films exhibited slightly better

tribological performance could be due to its compact and dense microstructure (Fig. 1c) composed by a single (0 0 2) textured cubic Cr–N. Nevertheless, CA produces a poor surface finish due to higher surface roughness. This fact could limit the use of Cr–N films deposited by CA, especially for applications in which the processed materials demand ultra-high surface finish quality. In these cases, MS or EB might be preferred alternatives.

Acknowledgements The authors are greatly indebted to: (i) the UK EUREKA Unit from the Department of Trade and Industry, for supporting the project CREST ! 2949 under the EUREKA program funding; (ii) Ministry of Industry and Technology of Spain


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