EVALUATION OF ADHESION, HARDNESS AND MICROSTRUCTURE OF CrN e CrAlN COATINGS DEPOSITED BY HIGH POWER IMPULSE MAGNETRON SPUTTERING

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1 EVALUATION OF ADHESION, HARDNESS AND MICROSTRUCTURE OF e CrAlN COATINGS DEPOSITED BY HIGH POWER IMPULSE MAGNETRON SPUTTERING Bruno César Noronha Marques de Castilho, Mechanical Engineering - Materials, bruno_castilho@usp.br Monica Costa Rodrigues Guimarães, Material Engineering - Metals, monirodrigues@usp.br Carlos Cunha, HiPIMS Technician, carlaostz@gmail.com Wagner Rafael Correr, Characterization Techniques Specialist, wcorrer@gmail.com Paulo Mordente, Surface Engineering, paulo.mordente@br.mahle.com Fernando Alvarez, Surface Engineering, alvarez@ifi.unicamp.br Haroldo Cavalcanti Pinto, Metallurgical Engineering, haroldo@sc.usp.br Abstract. Chromium nitride films have been investigated due to its excellent tribological properties characterized by high wear, oxidation and corrosion resistances. In the present study, approximately 20µm thick and CrAlN coatings were deposited in the form of monolayers on AISI 304 stainless steel substrates using High Power Impulse Magnetron Sputtering (HiPIMS). X-Ray Diffraction (XRD) was used to evaluate the microstructure and phase composition of the coatings. The surface morphology of the coatings was studied using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Energy Dispersive X-Ray Spectroscopy (EDX) was used in both coatings to determine chemical composition. The mechanical properties were characterized using nanoindentation and scratch techniques. The XRD data showed that the and CrAlN coatings exhibited FCC structure. Nanoindentation showed higher hardness for CrAlN coating which also exhibited better adhesion to the substrate. Keywords: HiPIMS, hard coatings,, CrAlN, characterization 1. INTRODUCTION Hard coatings with high chemical and thermal stability are a suitable solution for increasing the lifetime of tools and components commonly employed in many different industrial applications, such as engine components (Rodriguez et al., 2002; Jakubeczyova et al., 2015). Physically vapor deposited coatings are used in several applications such as medical devices and processing industry (cutting tools, dies, etc.). Magnetron Sputtering is a well-established Physical Vapor Deposition (PVD) technique used to deposit metallic as well as compound thin films (Ehiasarian et al., 2002). High Power Impulse Magnetron Sputtering (HiPIMS) is a technique that provides high ionization degree of the sputtered material, a considerably large fraction of ionized metal particles created by the dense plasma generated in front of the target. It can be operated at low pressures and without droplets generation (Mishra et al., 2010). The main advantage of using HiPIMS technology over conventional processes is that it promotes a better coating quality, since it works with lower deposition rates, which allows high density, resistance to oxidation and higher hardness as well as excellent adhesion. Despite of a larger number of parameters involved when compared to other commercial processes, the HiPIMS physical deposition method yields compounds with high purity and structural control, both at the atomic level as at the nanoscale (Helmersson et al., 2006). The precise adjustment of the process parameters allows the deposition of monolayers, multilayer, nanocomposites, functionalized nanostructures and particles for various applications. Currently, the transition metal nitrides, such as, and CrAlN, have been widely studied for applications requiring low friction coefficient and high resistance to wear, corrosion and oxidation. The coatings of those materials deposited by HiPIMS show a fully dense microstructure and a smooth surface, excellent corrosion protection capabilities and high adhesion (Terrat et al., 1991; Ehiasarian et al., 2004). The aim of the present study is to characterize monolayer coatings of and CrAlN deposited on stainless steel by HiPIMS and to compare their structural and mechanical properties. Techniques for the characterization will be X-Ray Diffraction, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Nanoindentation and Scratch tests. 2. EXPERIMENTAL DETAILS 2.1. Materials and CrAlN coatings were deposited on AISI 304 austenitic stainless steel substrate by HiPIMS. The 30mm diameter substrates were grinded and polished to a mirror finish (15, 9, 3 and 1µm diamond paste followed by 0.05µm silica) and they were ultrasonically cleansed in acetone. The ion etching and depositions were performed in a high vacuum

2 chamber (Plasma-HIPIMS-250) system, base pressure around 10-3 torr with 200mmX100mm pure Chromium (99.5%) and Chromium Aluminum (99.5%) targets. Substrates were placed at the substrate holder at a distance of 65mm from the target. An interlayer of Cr was used for the coating and a CrAl for the CrAlN coating to provide better adhesion and to reduce residual stress Coating process Firstly, the chamber was heated up to 300 C at a pressure of 1.0torr. The other parameters are described in Tab. (1). The chamber pressure was maintained at 4.0mtorr during the deposition process. The flow rate of Argon and Nitrogen was 40 and 50sccm, respectively. Table 1. Process parameters for two samples. Ion Etching Bas e Layer Depos ition Coating Depos ition Sample T Hipims Time (h) Bias (V) Atmos phere Time (h) Bias (V) Atmos phere T ( C) Time (h) Bias (V) Atmos phere ( C) (W) 02: Ar 01:30 80 Ar :30-80 Ar / N CrAlN 02: Ar 01:30 80 Ar :30-80 Ar / N Characterization The crystalline structure of the films was investigated by X-Ray Diffraction (XRD) using a MRD-XL Diffractometer (PANalytical, the Netherlands), with a Bragg-Brentano configuration (θ-2θ) with parallel beam geometry. The X-Ray parallel beam of 3x3mm 2 was produced by a Co-Kα radiation ( Å, 40kV and 40mA), the diffractograms were obtained in a 2θ range from 34 to 125, step size 0.05 and at 10s per step. The XRD analysis was also utilized for the evaluation of preferred orientation. A Field-Emission Scanning Electron Microscope (FEG-SEM) Inspect F-50 (FEI, The Netherlands) was used to characterize the morphology at the top and the cross-section of the coatings. The composition of the coatings was measured by X-Ray Energy Dispersive spectrometer (Apollo X SDD, EDAX, USA). The surface roughness was calculated from AFM images obtained in a Nanosurf FlexAFM (Nanosurf, Switzerland). A PB 1000 nanoindenter (Nanovea, USA) equipped with a Berkovich diamond indenter was used to measure hardness. For each sample, twenty points separated by a distance of 15µm were measured with a load of 150mN at the top and 50mN at the cross-section and the average hardness and standard deviation were calculated. To evaluate tribological properties of the coatings scratch tests were carried out using the same Nanovea PB The tests were performed with conical diamond indenter (200µm radios) using progressive load from 0N to 100N at a loading rate of 100N/min and the scratch length of 4mm. 3. RESULTS AND DISCUSSION 3.1. EDX Analysis It is possible to observe through the analysis of the Tab. (2) that the chemical composition is close to the expected stoichiometry. The coating presented nearly 50-50at%, which is the desired proportion of Cr and N. For the CrAlN coating, the composition was expected to be 50at% of Nitrogen and 25at% of Chromium and Aluminum. Once again the values were close to the expected ones, with 46.79at% of Nitrogen, 26.83at% of Chromium and 26.83at% of Aluminum. This indicates a correct adjustment between the flow rate of Nitrogen and the sputtering rate of the metals. Table 2. Results of EDX analysis. Chemical composition [at%] Cr Al N CrAlN XRD analysis Figure (1) illustrates the XRD patterns of the coating (a) and CrAlN (b). (002) peak is dominant for the and CrAlN coatings, which indicates that the specimens have a strong, preferred orientation in this plane. The crystalline structure of is FCC, the space group is Fm-3m and the strain-free lattice parameter is 4.144Å (JCPDS # ).

3 CrAlN coating presents a shift in the 2θ value which can be attributed to a decrease in lattice parameter of CrAlN due to the substitution of some atoms of Cr by Al atoms in the lattice (Kimura et al., 2003). Intensity (a.u) (002) (111) (022) (113) θ 2θ XRD - C θ ( ) 3.3. Microstructure Figure 1. XRD analysis of a) and b) CrAlN coatings. Figure (2) shows SEM images of (a and c) and CrAlN (b and d) from the top and the cross-section. The microstructure of CrAlN coating on substrate surface from top view shows smaller grains than. No growth defects (droplet defects) can be observed on the film surface and both shows a dense microstructure. Cross-section images (Fig.2 (c), (d)) show columnar structure. It is also possible to observe at the cross-section the presence of a layer between the base layer and the substrate. That is believed to be due the formation of a residual Cr coating during the ion etching process, probably due to the long period in which this process occurred. a) c) b) d) Figure 2. SEM images of surface morphology of a) and b) CrAlN, and the cross-section of c) and d) CrAlN.

4 Table (3) presents the thickness values of the coating measured with the SEM images at the cross-sections displayed in Fig. (2). The difference between the sum of the thickness of the base layer and the coating and the total value is due to the previously discussed layer formed during the ion etching process (nearly 0.5µm). Table 3. Measured values of coating thickness. Thickness (µm) Base Layer Coating Total CrAlN The surface morphologies of and CrAlN were studied using AFM. In Fig. (3) (a) and (b) are shown the 3D AFM images. The root mean square (RMS) roughness values of the as-deposited and CrAlN coatings are 0.239µm and 0.158µm. The average roughness (R A) shows 0.184µm and 0.120µm for and CrAlN respectively. The value for the CrAlN coating is smaller than for the coating and it is related to the fact that the CrAlN coating has thinner columnar grains when compared to the. Figure 3. AFM Images of the surface of (a) and CrAlN (b) films. Area of 30µm x 30µm Mechanical and tribological properties Nanohardness The nanohardness tests were carried out for the and CrAlN monolayer coatings on the sample at the cross-section and at the top using a Berkovich diamond indenter. Twenty indentations were made in each sample and values reported herein represent the average and the standard deviation. The maximum applied load at the cross-section was 50mN, and CrAlN coatings displayed hardness values of (12.0 ± 1.5)GPa and (13.0 ± 2.8)GPa, respectively. The measurements at the top were made with a loading of 150mN and the measured values of hardness were (12 ± 1.2)GPa and (16.5 ± 2.0)GPa. The difference between the hardness values from the top and the cross-section for the CrAlN indicates the anisotropy of the coating Scratch Three scratch tests were performed 2mm apart for both and CrAlN coatings. Critical load (L c) measurements for adhesion strength were 34.5N and 46N respectively. Both coatings failed by delamination as can be observed from Fig.(4), which indicates adhesion failure. Figure 4. Scratch test on (a) and (b) CrAlN coatings.

5 4. CONCLUSIONS In this work, coatings of and CrAlN were deposited using the HiPIMS technique. The results show higher value of nanohardness for the CrAlN coating, which is expected, according to the literature. Both coatings showed lower nanohardness values than expected and this may be justified due to the deposition parameters, which need to be further studied to produce highly compacted CrAlN, which will yield higher hardness in the CrAlN coating. The adhesion of the CrAlN was higher than the and further studies of residual stresses should be made to help understanding its influence in those results. Both films assume a well-defined densely packed columnar structure with low surface roughness, which is desirable for the current application. Further studies will broaden the range of the deposition parameters. 5. REFERENCES EHIASARIAN, A. P. et al. Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique. Thin Solid Films, v. 457, n. 2, p , Jun EHIASARIAN, A. P. et al. Influence of high power densities on the composition of pulsed magnetron plasmas. Vacuum, v. 65, n. 2, p , Apr HELMERSSON, U. et al. Ionized physical vapor deposition (IPVD): A review of technology and applications. Thin Solid Films, v. 513, n. 1-2, p. 1-24, Aug JAKUBECZYOVA, D. et al. Tribological Tests of Modern Coatings. International Journal of Electrochemical Science, v. 10, n. 9, p , Sep KIMURA, A. et al. Anisotropic lattice expansion and shrinkage of hexagonal TiAlN and CrAlN films. Surface & Coatings Technology, v. 169, p , Jun MISHRA, B. et al. Advances in Thin Film Technology through the Application of Modulated Pulse Power Sputtering. In: CHANDRA, T.;WANDERKA, N., et al (Ed.). Thermec 2009, Pts 1-4, v , p (Materials Science Forum). RODRIGUEZ, R. J. et al. Tribological behaviour of hard coatings deposited by arc-evaporation PVD. Vacuum, v. 67, n. 3-4, p , Sep TERRAT, J. P. et al. Structure and Mechanical Properties of Reactively Sputtered Chromium Nitrides. Surface & Coatings Technology, v. 45, n. 1-3, p , May ACKNOWLEDGEMENT To MAHLE Metal Leve, BNDES, CAPES and CNPq. 7. LIABILITY OF INFORMATIONS The authors are solely responsible for the information included in this paper.