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Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method
Superlattices and Microstructures xxx (2017) 1e12
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Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method Nikolay Petkov a, *, Totka Bakalova b, c, Tetiana Cholakova a, nek d, Martin Kormunda d, Hristo Bahchedzhiev a, Petr Louda c, Petr Rysa d b , Pavel Kejzlar Pavla Capkova a
Central Laboratory of Applied Physics, Bulgarian Academy of Sciences, 61, St. Peterburg Blvd., 4000 Plovdiv, Bulgaria Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, Studentska 2, 461 17 Liberec, Czech Republic c Faculty of Mechanical Engineering, Department of Material Science, Technical University of Liberec, Studentska 2, 461 17 Liberec, Czech Republic d Jan Evangelista Purkyne University, Faculty of Science, Ceske mladeze 8, 40096 Usti nad Labem, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2017 Accepted 9 May 2017 Available online xxx
AlSiN coatings were prepared by the cathodic arc deposition method at a temperature of 400 C and pressure of 2.6 Pa. The chemical composition, determined by SEM/EDS analysis, shows that the AlSiN coating presented here has a stoichiometry structure (Al40Si9N51). XPS and XRD analyses indicated AlN in both cubic and hexagonal modifications in the coating, and that the coating has a not fully completed nanocomposite structure. Nanoindentation measurements indicate nanohardness and elastic modulus of 39 GPa and 389 GPa, respectively. The coating has a very good adhesion strength with an average critical load of 28.3N (first cohesive failure) and 62.3N (first adhesion failure). The estimated wear rate and coefficient of friction of the coating are 27.2 106 mm3 N1 m1 and 0.7, respectively (using the friction pair AlSiN/Al2O3). © 2017 Elsevier Ltd. All rights reserved.
Keywords: AlSiN thin films Cathodic arc deposition Mechanical properties XRD and XPS analysis Tribology and wear
1. Introduction Nitrides of the materials from group III are optically transparent in the visible range of light. Among them, AlN shows good optical and promising mechanical properties and also good corrosion/oxidation resistance. AlN and Si3N4 are expected to be immiscible [1e3] in an AlSiN system. The formation of a two-phase nc-AlN/a-SiNx nanocomposite coating is expected if Al and Si are co-deposited in a reactive atmosphere containing nitrogen. They contribute to an additional increase in hardness, thermal stability and oxidation resistance. However, the influence of these elements on the structure and mechanical properties of nitride coatings when present simultaneously in the coating is not quite clear. Si-doped AlN deposited at a high temperature is extensively studied, but the AleSieN ternary system has not been studied in detail until now. When both a-
* Corresponding author. E-mail addresses: [email protected] (N. Petkov), [email protected] (T. Bakalova), [email protected] (T. Cholakova), [email protected] (H. Bahchedzhiev), [email protected] (P. Louda), [email protected] (P. Rys anek), [email protected] (M. Kormunda), [email protected] (P. Capkov a), [email protected] (P. Kejzlar). http://dx.doi.org/10.1016/j.spmi.2017.05.022 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
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Si3N4 and AlN materials are deposited together in the form of a thin film using PVD techniques, the composite AlSiN thin film is known to achieve high hardness (>30 GPa) owing to the nanocomposite nature of the film [4]. Musil et al. [5] reported the properties of AlSiN coatings with a low (~5 at.%) and high (~40 at.%) Si concentration, reactively sputtered at a temperature of 500 C, using a closed magnetic field dual magnetron system operated in an AC pulse mode. Pelisson et al. [6] also reported an investigation of the microstructure and mechanical properties of AlSiN coatings with Si, ranging in a wide interval from 0 to 23 at. %. The aim of their study was to prepare hard, optically transparent coatings with a hardness exceeding that of Al2O3. Later, the same authors [7] described the chemical state evolution of the elements in AlSiN nanostructured films with increasing Si content. Few studies are published on AlSiN coatings deposited by cathodic arc PVD techniques. Chi-Lung Chang et al. [4] obtained AlSiN coatings using a dual cathode source AlSi (12 at.% Si) with a short straight-duct filter in a modified cathode arc evaporation system, at a temperature of 450 C with maximum hardness of 35 GPa. A.P. Laskovnev et al. [8] reported the properties of AlSiN films with Si content from 19 to 27 at.%. They established that the coatings have poor adhesion to the substrate due to the large number of droplets. Generally, AlSiN coatings are identified as promising optically transparent layers [9e13]. Furthermore, such coatings arouse interest due to the fact that they possess a similar composition to the aluminium alloys widely used in the engineering industry. The study of AlSiN coatings will provide new information, which may be used to modify these industrially important alloys. This article presents a study of structure, mechanical and tribological properties of AlSiN coating obtained by the cathodic arc deposition process at a temperature of 400 C and pressure of 2.6 Pa. 2. Experimental procedure A pure aluminium cathode with 18% silicon is used for deposition of the coating in an N2 atmosphere. Tool steel discs (EN ISO HS 6-5-2) with diameters of ∅20 mm 5 mm are used as a substrate. Their nominal composition is [in wt %]: 0.9 C; 0.31 Mn; 0.34 Si; 0.026 P; 0.0005 S; 4.43 Cr; 4.78 Mo; 5.93 W; 1.79 V and 0.65 Co. The discs are hardened to 64e65 HRC and polished to a roughness (Sa) of 0.01 mm. Prior to the coating deposition, several steps are taken to improve the coating adhesion. Firstly, the tool steel substrates are cleaned in an alkaline solution in an ultrasonic bath for 5 min, and then rinsed in de-ionized water and dried with boiling ethanol and hot air. Immediately after the cleaning process, the substrates are placed into a vacuum chamber and mounted on a substrate holder rotating at a speed of 12 rpm during deposition. Finally, they are cleaned in a glow discharge of argon plasma, followed by surface cleaning with metal (Ti) ions to remove any traces of surface contamination and the native oxide layer. The technological conditions for deposition of the AlSiN coating are as follows: vacuum chamber pressure of 2.6 Pa, cathodic arc current of 130 A, negative bias voltage of 40 V and temperature in vacuum chamber of 400 C. Ti base contact sublayers are deposited to improve the adhesion of the main layer (AlSiN) to the substrate. Surface roughness is evaluated by two methods e a mechanical profilometer Dektak™ XT, which comes into direct contact with the surface of the sample tip and an atomic force microscope (AFM). The AFM (JPK Nanowizard 3) is operated in the contact mode; obtained data are processed in JPK SPM Data Processing 5.0.63 SW. The mechanical properties (nanohardness and elastic modulus) are investigated using a CSM Instruments nanohardness tester with an integrated optical microscope. Indentation is made on the desired area using a triangular Berkovich diamond indenter prism. Standard and multi-cycling progressive load and in depth control nanoindentation methods are used to evaluate the nanohardness properties of the AlSiN coatings. The hardness is calculated using the Oliver-Pharr method [14]. Coating adhesion is evaluated using a CETR UMI Multi-Specimen Test System. The scratch test is performed using a progressive load from 2 to 100 N and a speed of 10 mm/min (according to the EN1071-3:2005 standard). Failure events are detected by direct microscopic observation of the scratch tracks and by the use of acoustic emission and/or friction force measurement. The loads that characterize the adhesion are: LC1 (first cohesive failure), LC2 (first adhesion failure) and LC3 (load at which more than 50% of the coating from the scratch area is removed from the substrate). The tribology properties of the coatings are estimated by a CETR UMI Multi-Specimen Test System from Bruker at room temperature and humidity of 44 ± 2% and under dry friction. The basis of the tribological measurements is the “Ball-on-Disc” testing method. A Zeiss AXIO Imager M2 light optical microscope and a Dektak XT™ mechanical profilometer are used to evaluate the wear of the friction pairs. The mechanical profilometer is used to evaluate the profile of the grooves after the tribological experiment, as well as the width and depth of the ploughed profile. The optical microscope is employed to evaluate the spot size on the balls, at magnifications from 25 to 1000. The contrast of the images is obtained based on the individual structural reflexivity, different phases or due to the surface topography, and the intensity of reflected light is proportional to cos4, where 4 is the angle between the sample surface and the incident light beam [15]. A Zeiss Ultra Plus scanning electron microscope (SEM) equipped with an Oxford X-Max 20 energy dispersive spectrometer (EDS) is used for the local chemical analysis of the coatings. The analyses are performed at an accelerating voltage of 10 kV to minimize the influence of the substrate on the quant results due to primary electron penetration through the deposited coatings. AZtec 2.4 software is used for the quantification. X-ray photoelectron spectra (XPS) are recorded using a Phoibos 100 (Specs) hemispherical analyser operated in the FAT mode. The spectral line AlKa and MgKa are used to study the surface chemistry. High resolution spectra for pass energy 10 eV Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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Fig. 1. AFM scanned surface topography of 50 50 mm2 in 2D and 3D, and a graph of surface roughness on the diagonal.
are used for the quantification, and survey spectra are recorded at 40 eV for qualitative analyses. The sample composition is calculated from selected high resolution spectra and corresponding RSF from the CasaXPS library. Please note that the RSFs depend on photon energy. The CeC component of the C 1s peak is set to a binding energy (BE) of 284.5 eV. The 2p doubles in the spectra are fitted with a theoretical fixed area ratio of 2:1. The surface cleaning is performed in-situ in an XPS instrument by Arþ ion at 2.5 keV by a Tectra Ion Etch Sputter Gun set to a current of 15 mA in magnetron at a pressure of 2 103 Pa for 30 s. X-ray powder diffraction measurement is carried out on an X’Pert Panalytical X-ray diffractometer using CuKa radiation with a linear detector. The measurement is performed in an asymmetrical arrangement using a fixed low incidence angle of u ¼ 0.5 , as usual for measurement of thin layers in the angle range of 2q ¼ 20e80 . Phase composition is investigated using the Highscore program. In addition to the phase composition, attention is paid to the analysis of the preferred orientation of the crystallites, because the texture significantly influences the mechanical and tribological properties. 3. Results 3.1. Surface morphology Three scans are made by the AFM and Dektak ™ XT mechanical profilometer (according to the ISO 25178 standard) over different areas of 50 50 mm2 and 100 100 mm2, respectively. Fig. 1 shows 2D and 3D images of the scanned surface coating obtained by AFM analysis and a graph of the surface roughness on the diagonal of the scanned area. The result is given in Table 1 and indicates that the coating has an average surface roughness (Sa) of 0.065 mm and a root mean square roughness (Sq) of 0.091 mm. The average values of the surface parameters measured using the profilometer are given in Table 2, where: Sz is the maximum height (height between the lowest recesses and the highest projection); Sv is the maximum depth of the recess
Table 1 Parameters of AlSiN coating surface morphology according to AFM analysis. Scans Scan 1 Scan 2 Scan 3
1 2 1 2 1 2
Sa [mm]
Average [mm]
Sq [mm]
Average [mm]
0.072 0.046 0.051 0.090 0.066 0.061
0.059 ± 0.013
0.097 0.064 0.069 0.137 0.097 0.080
0.080 ± 0.017
0.071 ± 0.019 0.064 ± 0.003
0.103 ± 0.034 0.089 ± 0.009
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Table 2 Parameters of AlSiN coating surface morphology according to mechanical profilometer analysis. Parameters
Sa [mm]
Sz [mm]
Sq [mm]
Sv [mm]
Sp [mm]
Scan 1 Scan 2 Scan 3 Average
0.116 0.102 0.130 0.116 ± 0.011
2.567 2.830 2.162 2.520 ± 0.275
0.191 0.166 0.189 0.182 ± 0.011
0.371 0.995 0.744 0.703 ± 0.256
2.196 1.834 1.418 1.816 ± 0.318
(height between the lowest recesses and the median plane); and Sp is the maximum height of the protrusion (height between the median plane and the highest projection). The parameters of the profilometer measurements are: stylus type with radius of 2 mm; load of 4.9 102mN; and range of 6.5 mm. Cathodic arc technology has considerable potential for the tool coating industry, primarily because of its basic simplicity, easy implementation and economical operation under industrial production conditions. One of the disadvantages of this method is a poor coating structure (droplet formation and columnar growth). The shortcomings of this method are shown by the different surface roughness values in the evaluation of the mechanical profilometer and AFM.
3.2. Mechanical properties The coating thickness is measured by a Calotest using a steel ball with a diameter of 30 mm and a diamond paste with 0.1 mm monocrystalline diamond grains. The measured thickness is 2060 nm, at a deposition time of 160 min. Fig. 2 shows the track obtained by the Calotest. The observed light iridescence in light optical microscopy can be explained with a transparent outer (AlSiN) layer and reflexive Ti layer. The thickness of the transparent layer is near to the wave length of visible light. Therefore, the image presented in Fig. 2 is observed due to the diffraction of light, which may be attributed to the periodically formed different phases of the coating (Figs. 12 and 13 in the discussion section). The adhesion and toughness of the coatings are evaluated by a microscratch test technique, using a Rockwell diamond indenter with a 200 mm tip radius. Scratch adhesion tracks are analysed using an optical microscope coupled with a CCD camera. Three measurements are taken at different places and the average values of the measured critical loads (LC1 and LC2) are given in Table 3. The friction force Ft and acoustic emission as a function of the applied load are recorded during the scratch testing. The critical load LC1 is a point on the graph where the first crack (cohesive failure) of the coating is observed and the corresponding AE signal is recorded (Fig. 3, yellow line). LC2 is a point on the graph (Fig. 3, green line) where the damage becomes continuous and complete delaminations of the coating are achieved. This is accompanied by a sharp increase in the coefficient of friction (CoF). The coating adhesion scratch tests disclose the cohesion and adhesion properties of the tested coating. In virtue of the tests carried out, the critical load corresponding to a load leading to the appearance of the first crack LC1 lies within a range of 27e29 N, and LC2 lies within the range of 54e69 N (Table 3).
Fig. 2. The optical microscopy image of the AlSiN coating obtained by the Calotest.
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Table 3 The adhesion properties.
Scratch 1 Scratch 2 Scratch 3 Average
LC1[N]
LC2[N]
29 29 27 28.3 ± 0.9
54 64 69 62.3 ± 6.2
Fig. 3. Optical micrograph of the scratch track in the AlSiN coating and scratch test results of the friction force, CoF and AE.
Nanohardness is measured with a Bercovich diamond indenter prism. The maximum penetration depth during the nanohardness measurement is no more than 10% of the coating thickness. The measured nanohardness (H) and corresponding value of the Young modulus (E) are 38.9 GPa and 389 GPa, respectively. The calculated plasticity index [16,17] is 0.1.
3.3. Tribological properties Tribological tests are performed in order to investigate the wear processes between different sliding pairs. The tests are conducted using an Al2O3 ceramic ball and a steel ball ISO 683/13 (X105CrMo17), both with a 6.35 mm diameter. The steel ball is much softer (700 HV) than the AlSiN coating (39 GPa), while the Al2O3 ceramic ball has a hardness of 2100 HV. The CoF between the counterpart (ball) and the coating is determined during the measurement. A set of cycles was made for each counterpart material. 3.3.1. Coefficient of friction Tribological testing is conducted according to the ASTM G99-95 standard. A load 10N and rotation speed of the table 60 RPM are used during the evaluation of the CoF. The experiments are performed on a predefined 25-m path (almost 995 cycles). Fig. 4 depicts the CoF behaviour using both the Al2O3 counterpart ceramic ball and the ISO 683/13 steel ball. At the beginning of the experiment, the measured values of the CoF between the ceramic ball and the steel ball and the coating were 0.2 and 0.34, respectively. After the first 15 cycles, an increase of the CoF values to 0.25 (for the Al2O3 counterpart) and 0.49 (for the steel counterpart) was observed. After 30 cycles of rotation of the counterpart on the coating surface, values of 0.64 for the Al2O3 counterpart and 0.67 for the steel counterpart were registered. These values remained unchanged to the end of the experiment. The change in the CoF values (for the Al2O3 counterpart) is caused by the separation of the particles from the AlSiN coating. These particles are hard and abrasive and deteriorate the friction. When a steel ball is used as the counterpart, the CoF increases after a few meters, reaching a value as high as 0.85. This change occurs more rapidly because the particles of the counterpart, which have a lower hardness than the coating, separate during the experiment. This leads to an increase in CoF and at the same time a small deviation of the measured values in comparison with the ceramic counterpart. A conformance test of the mean values is performed using the ANOVA statistical method for processing data from the tribological measurements (ANOVA stands for analysis of variance). More than 650 values were processed for each group of results. According to the ANOVA, the average CoF and variance are 0.744 ± 0.044 and 0.858 ± 0.014 when the Al2O3 ball and steel ball are used as counterparts, respectively. We are able to calculate the effectiveness of the CoF reduction based on the average values. There is an average reduction of approximately 13.2% when a counterpart made of Al2O3 is used compared to the average value of CoF when a steel ball counterpart is used. The increase in the friction coefficient (friction pairs AlSiN e steel Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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Fig. 4. Friction coefficient of AlSiN coating against steel and Al2O3 ceramic balls at room temperature as a function of sliding distance.
ball) can be explained by the presence of steel particles from the counterpartat the point of contact during the tribological experiment. 3.3.2. Wear and wear rate The wear of the friction pairs is evaluated according to the EN 1071-13:2010 standard. Circular tracks are left on the flat disc sample (AlSiN sample) after the test. A Dektak XT™ mechanical profilometer is used to measure the transverse profile at four points with intervals of 90 and the cross-sectional area. Circular grinding traces are also established at each point. The amount of coating wear (VAlSiN) is calculated according to Equation (1) from the cross-sectional area of the circular machining track, and the results are described in Table 4.
VAlSiN ¼
prðS1 þ S2 þ S3 þ S4 Þ 2
;
(1)
where: r is the radius of the circular machining track [mm]; S1 to S4 are the cross-sectional areas at four points, with intervals of 90 , spaced evenly along the circular machining tracks [mm2]. The amount of wear on the balls is calculated by Equation (2), and the results are described in Table 4. Fig. 5 shows the destroying integrity of the counterpart after the tribological test.
Vball ¼
pA3 B 32D
;
(2)
where: Vball is the amount of wear on the balls [mm3]; A is the smallest diameter of the machining track [mm]; B is the diameter in the direction perpendicular to the smallest diameter [mm]; and D is the diameter of the ball [mm]. The wear rate of the coatings and the counterparts is calculated by Equation (3) [18]:
wear rate ¼
wear volume load sliding distance
(3)
Table 4 shows the wear and wear rate of the AlSiN coating in contact with the steel and Al2O3 counterparts, and the wear and wear rate of the balls. The wear of the Al2O3 counterpart is 93.2% less in comparison with the steel counterpart. The AlSiN coating is very abrasive and a large amount of material from the steel counterpart was deposited on the coating surface during the tribological experiment. Moreover, no distortion of the coating was observed. In this case, it is not correct to talk about the wear of the AlSiN coating but the area of influence of the counterpart on the coating. In this case, Table 4 does not specify any data for the wear and wear rate of the AlSiN coating. The experiment with the steel counterpart was conducted in order to determine its wear rate. The wear rate of the AlSiN coating is determined using only the friction pair AlSiN/Al2O3. Table 4 Wear and wear rate of the AiSiN coating and counterparts. Type of balls
Wear volume VAlSiN [103 m3]
Coating wear rate [106 mm3 N1 m1]
Wear volume Vball [103 mm3]
Ball wear rate [106 mm3 N1 m1]
Steel ISO 683/13 Ceramic Al2O3
e 6.8
e 27.2
5.6 0.4
22.4 1.5
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Fig. 5. Destruction of the integrity of the Al2O3 (left) and the steel (right) counterparts at the contact with the AlSiN coating.
3.4. Chemical composition and structure analysis A Zeiss Ultra Plus scanning electron microscope equipped with an Oxford X-Max 20 energy dispersive spectrometer was used for the local chemical analysis of the coatings. The analyses were performed at an accelerating voltage of 10 kV, working distance of 8.5 mm, live time of 100 s, energy range of 10 keV, number of channels of 4096, and with pulse pile up correction. AZtec 2.4 software was used for the quantification. Before the EDS analysis, the sample surfaces were cleaned using Ar-plasma at 0.5 kV for 60 min at a pressure of 106 Pa. The average values of the determined composition of AlSiN coating by SEM/EDS (in at. %) are 50.5 N; 40.3 Al and 9.2 Si. 3.4.1. XPS analysis The XPS study shows that the surface has significant carbon contamination, probably due to sample storage, see Fig. 6. The argon ion sputtering reduced the carbon content to half on the surface. At the same time, the oxygen content is increased, possibly due to the presence of oxides and several oxynitrides. The compositions show systematically higher concentrations of C, Al and O when the Mg anode is used. Therefore, it can be assumed that these elements are located in higher amounts in the top few nm because Al anode photons have a high energy, so the analytic volume is deeper. This trend remained when the mainly carbon contamination was removed by Arþ sputtering. The high-resolution spectra are not influenced by the sputtering procedure and the carbon was not completely removed. The oxygen-rich surface is also supported by the XRD data, where no oxide rich phases are detected. Therefore, the XPS spectra of Al 2p (see Fig. 7) are composed of two sets of doubles. The first set, the full line, has Al 2p3/ 2 at approximately BE 73 eV and Al 2p1/2 at BE 74 eV, and the AlN contribution [19e21] may be approximately 60% in this form. The second set, the dashed line, has Al 2p3/2 at approximately BE 73.3 eV and Al 2p1/2 at BE 74.3 eV and the AleOH [19,21] or native AleO [20] contribution may be approximately 40% of Al. The high-resolution spectra of Si 2p in Fig. 8 show the presence of Al KLL in the spectra taken by the Al anode. The XPS spectra of Si 2p, see Fig. 8, are composed of two sets of doubles. The first set, the full line, has Si 2p3/2 at approximately BE 99 eV and Si 2p1/2 at BE 100 eV, and the SieSi and SieAl contributions (metals) [19,20] may be approximately 5%. The second
Fig. 6. Survey spectra measured on samples before and after sputtering.
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Fig. 7. High resolution XPS spectra of Al 2p measured by the Al anode before sputtering.
set, the dashed line, has Si 2p3/2 at approximately BE 101 eV and Si 2p1/2 at BE 102 eV and the SieN [19] contribution may be approximately 95% of the Si in this form. Native SiOx or SiO2 are not identified but their presence can be overlapped in the SieN contributions as described in the literature on AlSiON [19]. The spectra of N 1s show one broad component at approximately BE 396.6 eV (Fig. 9) identified as AleN, similarly to those presented for AlSiN [19] and AlN [20,21]. 3.4.2. XRD analysis The elemental composition suggests the possible presence of the following phases: AlN, SiN, AlSiN. The X-ray diffraction pattern is presented in Fig. 10. The XRD pattern shows reflections corresponding to AlN in both modifications: cubic (space group F-43 m) and hexagonal (space group P63mc), besides very weak reflections on the background coming from the steel substrate. The reflections belonging to cubic AlN are marked by red lines, and reflections of the hexagonal form by black lines. Reflections corresponding to the SiN were not found in the diffraction pattern. A very weak and broad reflection at 2q ¼ 28.44 corresponds to the 111 reflection of the Si phase (cubic, space group Fd-3m). This result is consistent with the XPS measurements. The XPS investigation revealed the SieSi metallic bonds for ~15% of Si atoms indicating the presence of Si crystallites. The large broadening of the 111 Si peak profile (with a half-width FWHM of approximately 2 in 2q) indicates a very small Si crystallite size, which is lower than 10 nm. As the profile broadening increases with the diffraction angle, Si reflections corresponding to higher angles cannot be observed. XPS measurement also revealed the SieN bonds; however, the SiN reflections were not found in the X-ray diffraction pattern. This means that in the AlN the Al positions are randomly substituted by Si atoms, creating mixed AlSi nitride.
Fig. 8. High resolution XPS spectra of Si 2p measured by the Al anode and Mg anode before sputtering.
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Fig. 9. High resolution XPS spectra of N 1s measured by the Al anode before sputtering.
Another structural feature in the thin films is the preferred orientation of the crystallites. By analysing the intensity ratio of both the cubic and hexagonal phases of AlSiN, we can conclude that there is a crystallographic plane of 111 cubic form, which is identical to the 001 hexagonal form of AlSiN, which is preferentially oriented parallel to the sample surface. This crystallographic plane is illustrated in Fig. 11. 4. Discussion At the beginning of the study an AlSiN coating was deposited with thickness of 0.5 mm for 40 min. During the measurement of its thickness by the Calotest it was observed that the coating looked like it had multilayers (Fig. 12), but grew as a monolayer. This led us to perform the chemical analysis longitudinally in this sector. The composition of the coatings was analysed using an X-ray energy dispersive spectrometer attached to a SEM system. The EDS analysis (Fig. 13) shows that there are no unexpected elements. So, the observed light iridescence in the optical microscope (Fig. 12) can be explained with a transparent (AlSiN) outer layer and a reflexive Ti layer. The thickness of the transparent layer is near to the wavelength of visible light. Therefore, the image presented in Fig. 12 is observed due to the diffraction of light, which may be attributed to the different periodically formed phases of the coating. The coating shows high nanohardness (H ¼ 39.5 GPa), higher than the values presented in the literature (25 GPa [5,9], 30 GPa [23], 32 GPa [11], and 35 GPa [4]), and good adhesion to the substrate (average values are LC1 ¼ 30.3 ± 1.7 N and LC2 ¼ 40.3 ± 1.2 N). So, we decided to prepare samples with a higher thickness to analyse the structure and tribology properties of the AiSiN coating deposited at these technological parameters (see Section 2). The study showed that the observed phenomenon in the AlSiN coating is again recorded with a thick AlSiN coating, i.e. the coating thickness does not influence the observed interference phenomenon.
Fig. 10. X-ray diffraction pattern of the AlSiN coating deposited onto the HSS substrate.
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Fig. 11. Crystallographic plane preferentially oriented parallel to the sample surface, pink Al(Si) atoms, dark blue nitrogen atoms. Some Al atoms are randomly substituted by Si. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The chemical analysis made by SEM/EDS showed that the AlSiN coating present here has a stoichiometry structure (Al40Si9N51). As it can be seen, the ratio between the Al and Si in the coating is the same as in the cathode used. Generally, an AlN coating obtained by PVD methods has hexagonal crystallites. XRD analysis shows that the addition of Si resulted in the formation of AlN in both modifications: cubic and hexagonal. The presence of approximately 95% of Si in form of silicon nitride and SiN reflection were not found in the X-ray diffraction pattern. In addition, SieN bonding almost converted to Si3N4 [22] when the content reached 9.0 at. % and an amorphous silicon nitride (a-Si3N4) is usually positioned at 101.8 eV, which leads us to conclude that the AlSiN coating presented here has a nanocomposite structure. The nanocomposite is not fully completed due to the presence of Si crystallites and the solid solution of AlSiN, in which a small number of Al atoms are substituted by Si atoms. Moreover, this leads to an increase in the hardness of the AlN [24] from 28 GPa to 39 GPa, which has an influence over the tribological properties. It is well known that the cathodic arc deposition (CAD) process forms a large quantity of droplets (Fig. 14). The surface morphology analyses confirmed this. The average surface morphology values measured by the profilometer are two times higher than those measured by AFM. This is due to the fact that the profilometer scans an area more than four times larger than the AFM as well as the presence of the droplets. Due to the presence of the droplets, the increase in thickness should lead to an increase in surface roughness. The surface roughness has a direct influence over the CoF behaviour (see Fig. 4). When the counterpart is made from a soft material (like the steel ball) this coating surface acts as an abrasive during the dry friction. When the counterpart is made from a hard material (like Al2O3) this roughness affects the oscillations of the CoF behaviour. It is known that the layers deposited by this method have residual stresses, whereby the stresses increase with an increase in thickness. The deposited AlSiN coating has a very good adhesion strength, which has a slight dependence on the thickness. No information was found in the literature concerning investigation of AlSiN coating adhesion.
Fig. 12. A segment of the optical microscopy image of the AlSiN coating obtained by the ball-cratering test.
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Fig. 13. The measurement line and distribution of the elements from the top of the coating to the substrate.
Fig. 14. Surface morphology of the AlSiN coating.
Different wear mechanisms were found for the coating sliding against different counterparts. For the coating sliding against steel balls, the adhesive wear plays a major role in the wear process. Due to the much higher hardness of the film than the steel ball and the absence of the adhered layer on the ball, material was removed and transferred from the ball to the wear track and accumulated to form a transferred layer. On the other hand, the coating sliding against an Al2O3 ball showed a totally different wear mechanism. During the wear test the generated wear debris was not transferred to the wear track but to the ball. The estimated wear rate and coefficient of friction of the coating using the friction pair AlSiN/Al2O3 are 27.2 106 mm3N1m1 and 0.7, respectively.
5. Conclusions It was found that the AlSiN coating has very good adhesion to the base material, tool steel (EN ISO HS 6-5-2) and SEM/EDS chemical analysis determined that the AlSiN coating presented here has a stoichiometry structure - Al40Si9N51. XPS and XRD analyses showed that in the coating indicated AlN in both modifications: cubic and hexagonal; the coating has a nanocomposite structure but not fully completed. High nanohardness and highly abrasive propertiesare detected at the specific deposition parameters. This was observed in the interaction of the coating with the steel ball ISO 683/13, where the wear of the steel is very intense. The wear rate and CoF of the coating are determined using the Al2O3 ceramic ball, because in this case the wear rate of the counterpart is less than the coating. Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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This study shows that the coating thickness has no significant influence over the nanohardness of the coating or its tribological properties. However, the coating thickness is important in determining the wear rate when the coating is applied in the processing of metals. In summary, the AlSiN coating presented in this article is very suitable for covering cutting tools for machining and cutting of steel parts. The coating is suitable for application on temperature sensitive steels and aluminium alloys. Acknowledgement The results of project LO1201 were obtained through the financial support of the Ministry of Education, Youth and Sports in the framework of targeted support within the “National Programme for Sustainability I” and the OPR & DI project “Centre for Nanomaterials, Advanced Technologies and Innovation (CZ.1.05/2.1.00/01.0005)” and with the support of the Institutional Endowment for the Long Term Conceptual Development of Research Institutes at the TUL ME. The authors acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2015073. We would like to thank Craig Hampson for his help with English language correction. References n, L. Karlsson, L. Hultman, Influence of Si on the microstructure of arc evaporated (Ti,Si)N thin films; evidence for cubic solid [1] A. Flink, T. Larsson, J. Sjole solutions and their thermal stability, Surf. Coat. Technol. 200 (2005) 1535e1542. [2] F. Weitzer, K. Remschnig, J.C. Schuster, P. Rogl, Phase equilibria and structural chemistry in the ternary systems M-Si-N and M-B-N (M¼ Al, Cu, Zn, Ag, Cd, In, Sn, Sb, Au, Tl, Pb, Bi), J. Mater. Res. 5 (10) (1990) 2152e2159. [3] A. Mazel a, P. Marti, F. Henry, B. Armas, R. Bonnet, M. Loubradou, Nanostructure and local chemical composition of A1N-Si3N4 layers grown by LPCVD, Thin Solid Films 304 (1997) 256e266. [4] Chi-Lung Chang, Chi-Song Huang, Effect of bias voltage on microstructure, mechanical and wear properties of AleSieN coatings deposited by cathodic arc evaporation, Thin Solid Films 519 (2011) 4923e4927. sek, P. Zeman, R. Cerstvý, [5] J. Musil, M. Sa D. Herman, J.G. Han, V. Satava, Properties of magnetron sputtered AleSieN thin films with a low and high Si content, Surf. Coat. Technol. 202 (2008) 3485e3493. [6] A. Pelisson, M. Parlinska-Wojtan, H.J. Hug, J. Patscheider, Microstructure and mechanical properties of AleSieN transparent hard coatings deposited by magnetron sputtering, Surf. Coat. Technol. 202 (2007) 884e889. lisson-Schecker, Hans Josef Hug, Jo €rg Patscheider, Complex phase compositions in nanostructured coatings as evidenced by photoelectron [7] Aude Pe spectroscopy: the case of AleSieN hard coatings, J. Appl. Phys. 108 (2010) 023508, 1e8. [8] A.P. Laskovnev, A.T. Volochko, G.V. Markov, V.V. Uglov, D.P. Rusalsky, P.N. Misuno, Structure and phase composition of ion-plasma Al-Si-N coatings, Proc. Natl. Acad. Sci. Belarus (1) (2012) 17e21. [9] H. Liu, W. Tang, D. Hui, L. Hei, F. Lu, Characterization of (Al, Si)N films deposited by balanced magnetron sputtering, Thin Solid Films 517 (2009) 5988e5993. €rg Patscheider, Comparison of AleSieN nanocomposite coatings deposited by HIPIMS [10] Erik Lewin, Daniel Loch, Alex Montagne, Arutiun P. Ehiasarian, Jo and DC magnetron sputtering, Surf. Coat. Technol. 232 (2013) 680e689. lisson-Schecker, Hans Josef Hug, Jo €rg Patscheider, Morphology, microstructure evolution and optical properties of AleSieN nanocomposite [11] Aude Pe coatings, Surf. Coat. Technol. 257 (2014) 114e120. lisson-Schecker, Hans Josef Hug, Bogdan Rutkowski, Jo €rg Patscheider, AlN/Si3N4 multilayers as an interface [12] Magdalena Parlinska-Wojtan, Aude Pe model system for Al1-xSixN/Si3N4 nanocomposite thin films, Surf. Coat. Technol. 261 (2015) 418e425. [13] S.K. Mishra, Swati Kumari, Soni, Development of hard and optically transparent Al-Si-N nanocomposite coatings, Surf. Interface Anal. (2016), http://dx. doi.org/10.1002/sia.5954. [14] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology, J. Mater. Res. 19 (1) (2004) 3e20. [15] T. Bakalova, P. Louda, L. Voleský, K. Bor uvkov a, L. Svobodov a,Nanoadditives SiO2 and TiO2 in Process Fluids. In: Manufacturing technology. J. E. Purkyne University in Usti nad Labem, 15, No 4. pp. 502e508 (2015) ISSN 1213-2489, [16] A. Leyland, A. Matthews, On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behaviour, Wear 246 (2000) 1e11. [17] J. Musil, Flexible hard nanocomposite coatings, RSC Adv. 5 (2015) 60482e60495, http://dx.doi.org/10.1039/C5RA09586G. [18] K. Singh, P.K. Limaye, N.L. Soni, A.K. Grover, R.G. Agrawal, A.K. Suri, Wear studies of (TieAl)N coatings deposited by reactive magnetron sputtering, Wear 258 (2005) 1813e1824. [19] L. Rebouta, A. Sousa, M. Andritschky, F. Cerqueira, C.J. Tavares, P. Santilli, K. Pischow, Solar selective absorbing coatings based on AlSiN/AlSiON/AlSiOy layers, Appl. Surf. Sci. 356 (2015) 203e212. [20] B. Vincent Crist, Handbook of Monochromatic XPS Spectra, XPS International, 2004. [21] Pei-Yin Lin, Jr-Yu Chen, Yu-Chang Chen, Li Chang, Effect of growth temperature on formation of amorphous nitride interlayer between AlN and Si(111), Jpn. J. Appl. Phys. 52 (8S) (2013), http://dx.doi.org/10.7567/JJAP.52.08JB20. [22] S.U. Kang, L.I.U. DaMeng, S.H.A.O. TianMin, Microstructure and mechanical properties of TiAlSiNnano-composite coatings deposited by ion beam assisted deposition, Sci. China, Technol. Sci. 58 (10) (2015) 1682e1688. [23] A. Pelisson, M. Palinska-Wojta, H.J. Hug, J. Patscheider, Microstructure and mechanical properties of AleSieN transparent hard coatings deposited by magnetron sputtering, Surf. Coat. Technol. 202 (2007) 884e889. [24] N. Petkov, T. Bakalova, T. Cholakova, L. Volesky, P. Louda, Mechanical and tribological properties of AlN hard films deposited by reactive magnetron sputtering, J. Tech. Univ. Sofia Plovdiv Branch Bulg. 22 (2016) 141e144.
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Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method Nikolay Petkov a, *, Totka Bakalova b, c, Tetiana Cholakova a, nek d, Martin Kormunda d, Hristo Bahchedzhiev a, Petr Louda c, Petr Rysa d b , Pavel Kejzlar Pavla Capkova a
Central Laboratory of Applied Physics, Bulgarian Academy of Sciences, 61, St. Peterburg Blvd., 4000 Plovdiv, Bulgaria Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, Studentska 2, 461 17 Liberec, Czech Republic c Faculty of Mechanical Engineering, Department of Material Science, Technical University of Liberec, Studentska 2, 461 17 Liberec, Czech Republic d Jan Evangelista Purkyne University, Faculty of Science, Ceske mladeze 8, 40096 Usti nad Labem, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2017 Accepted 9 May 2017 Available online xxx
AlSiN coatings were prepared by the cathodic arc deposition method at a temperature of 400 C and pressure of 2.6 Pa. The chemical composition, determined by SEM/EDS analysis, shows that the AlSiN coating presented here has a stoichiometry structure (Al40Si9N51). XPS and XRD analyses indicated AlN in both cubic and hexagonal modifications in the coating, and that the coating has a not fully completed nanocomposite structure. Nanoindentation measurements indicate nanohardness and elastic modulus of 39 GPa and 389 GPa, respectively. The coating has a very good adhesion strength with an average critical load of 28.3N (first cohesive failure) and 62.3N (first adhesion failure). The estimated wear rate and coefficient of friction of the coating are 27.2 106 mm3 N1 m1 and 0.7, respectively (using the friction pair AlSiN/Al2O3). © 2017 Elsevier Ltd. All rights reserved.
Keywords: AlSiN thin films Cathodic arc deposition Mechanical properties XRD and XPS analysis Tribology and wear
1. Introduction Nitrides of the materials from group III are optically transparent in the visible range of light. Among them, AlN shows good optical and promising mechanical properties and also good corrosion/oxidation resistance. AlN and Si3N4 are expected to be immiscible [1e3] in an AlSiN system. The formation of a two-phase nc-AlN/a-SiNx nanocomposite coating is expected if Al and Si are co-deposited in a reactive atmosphere containing nitrogen. They contribute to an additional increase in hardness, thermal stability and oxidation resistance. However, the influence of these elements on the structure and mechanical properties of nitride coatings when present simultaneously in the coating is not quite clear. Si-doped AlN deposited at a high temperature is extensively studied, but the AleSieN ternary system has not been studied in detail until now. When both a-
* Corresponding author. E-mail addresses: [email protected] (N. Petkov), [email protected] (T. Bakalova), [email protected] (T. Cholakova), [email protected] (H. Bahchedzhiev), [email protected] (P. Louda), [email protected] (P. Rys anek), [email protected] (M. Kormunda), [email protected] (P. Capkov a), [email protected] (P. Kejzlar). http://dx.doi.org/10.1016/j.spmi.2017.05.022 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
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Si3N4 and AlN materials are deposited together in the form of a thin film using PVD techniques, the composite AlSiN thin film is known to achieve high hardness (>30 GPa) owing to the nanocomposite nature of the film [4]. Musil et al. [5] reported the properties of AlSiN coatings with a low (~5 at.%) and high (~40 at.%) Si concentration, reactively sputtered at a temperature of 500 C, using a closed magnetic field dual magnetron system operated in an AC pulse mode. Pelisson et al. [6] also reported an investigation of the microstructure and mechanical properties of AlSiN coatings with Si, ranging in a wide interval from 0 to 23 at. %. The aim of their study was to prepare hard, optically transparent coatings with a hardness exceeding that of Al2O3. Later, the same authors [7] described the chemical state evolution of the elements in AlSiN nanostructured films with increasing Si content. Few studies are published on AlSiN coatings deposited by cathodic arc PVD techniques. Chi-Lung Chang et al. [4] obtained AlSiN coatings using a dual cathode source AlSi (12 at.% Si) with a short straight-duct filter in a modified cathode arc evaporation system, at a temperature of 450 C with maximum hardness of 35 GPa. A.P. Laskovnev et al. [8] reported the properties of AlSiN films with Si content from 19 to 27 at.%. They established that the coatings have poor adhesion to the substrate due to the large number of droplets. Generally, AlSiN coatings are identified as promising optically transparent layers [9e13]. Furthermore, such coatings arouse interest due to the fact that they possess a similar composition to the aluminium alloys widely used in the engineering industry. The study of AlSiN coatings will provide new information, which may be used to modify these industrially important alloys. This article presents a study of structure, mechanical and tribological properties of AlSiN coating obtained by the cathodic arc deposition process at a temperature of 400 C and pressure of 2.6 Pa. 2. Experimental procedure A pure aluminium cathode with 18% silicon is used for deposition of the coating in an N2 atmosphere. Tool steel discs (EN ISO HS 6-5-2) with diameters of ∅20 mm 5 mm are used as a substrate. Their nominal composition is [in wt %]: 0.9 C; 0.31 Mn; 0.34 Si; 0.026 P; 0.0005 S; 4.43 Cr; 4.78 Mo; 5.93 W; 1.79 V and 0.65 Co. The discs are hardened to 64e65 HRC and polished to a roughness (Sa) of 0.01 mm. Prior to the coating deposition, several steps are taken to improve the coating adhesion. Firstly, the tool steel substrates are cleaned in an alkaline solution in an ultrasonic bath for 5 min, and then rinsed in de-ionized water and dried with boiling ethanol and hot air. Immediately after the cleaning process, the substrates are placed into a vacuum chamber and mounted on a substrate holder rotating at a speed of 12 rpm during deposition. Finally, they are cleaned in a glow discharge of argon plasma, followed by surface cleaning with metal (Ti) ions to remove any traces of surface contamination and the native oxide layer. The technological conditions for deposition of the AlSiN coating are as follows: vacuum chamber pressure of 2.6 Pa, cathodic arc current of 130 A, negative bias voltage of 40 V and temperature in vacuum chamber of 400 C. Ti base contact sublayers are deposited to improve the adhesion of the main layer (AlSiN) to the substrate. Surface roughness is evaluated by two methods e a mechanical profilometer Dektak™ XT, which comes into direct contact with the surface of the sample tip and an atomic force microscope (AFM). The AFM (JPK Nanowizard 3) is operated in the contact mode; obtained data are processed in JPK SPM Data Processing 5.0.63 SW. The mechanical properties (nanohardness and elastic modulus) are investigated using a CSM Instruments nanohardness tester with an integrated optical microscope. Indentation is made on the desired area using a triangular Berkovich diamond indenter prism. Standard and multi-cycling progressive load and in depth control nanoindentation methods are used to evaluate the nanohardness properties of the AlSiN coatings. The hardness is calculated using the Oliver-Pharr method [14]. Coating adhesion is evaluated using a CETR UMI Multi-Specimen Test System. The scratch test is performed using a progressive load from 2 to 100 N and a speed of 10 mm/min (according to the EN1071-3:2005 standard). Failure events are detected by direct microscopic observation of the scratch tracks and by the use of acoustic emission and/or friction force measurement. The loads that characterize the adhesion are: LC1 (first cohesive failure), LC2 (first adhesion failure) and LC3 (load at which more than 50% of the coating from the scratch area is removed from the substrate). The tribology properties of the coatings are estimated by a CETR UMI Multi-Specimen Test System from Bruker at room temperature and humidity of 44 ± 2% and under dry friction. The basis of the tribological measurements is the “Ball-on-Disc” testing method. A Zeiss AXIO Imager M2 light optical microscope and a Dektak XT™ mechanical profilometer are used to evaluate the wear of the friction pairs. The mechanical profilometer is used to evaluate the profile of the grooves after the tribological experiment, as well as the width and depth of the ploughed profile. The optical microscope is employed to evaluate the spot size on the balls, at magnifications from 25 to 1000. The contrast of the images is obtained based on the individual structural reflexivity, different phases or due to the surface topography, and the intensity of reflected light is proportional to cos4, where 4 is the angle between the sample surface and the incident light beam [15]. A Zeiss Ultra Plus scanning electron microscope (SEM) equipped with an Oxford X-Max 20 energy dispersive spectrometer (EDS) is used for the local chemical analysis of the coatings. The analyses are performed at an accelerating voltage of 10 kV to minimize the influence of the substrate on the quant results due to primary electron penetration through the deposited coatings. AZtec 2.4 software is used for the quantification. X-ray photoelectron spectra (XPS) are recorded using a Phoibos 100 (Specs) hemispherical analyser operated in the FAT mode. The spectral line AlKa and MgKa are used to study the surface chemistry. High resolution spectra for pass energy 10 eV Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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Fig. 1. AFM scanned surface topography of 50 50 mm2 in 2D and 3D, and a graph of surface roughness on the diagonal.
are used for the quantification, and survey spectra are recorded at 40 eV for qualitative analyses. The sample composition is calculated from selected high resolution spectra and corresponding RSF from the CasaXPS library. Please note that the RSFs depend on photon energy. The CeC component of the C 1s peak is set to a binding energy (BE) of 284.5 eV. The 2p doubles in the spectra are fitted with a theoretical fixed area ratio of 2:1. The surface cleaning is performed in-situ in an XPS instrument by Arþ ion at 2.5 keV by a Tectra Ion Etch Sputter Gun set to a current of 15 mA in magnetron at a pressure of 2 103 Pa for 30 s. X-ray powder diffraction measurement is carried out on an X’Pert Panalytical X-ray diffractometer using CuKa radiation with a linear detector. The measurement is performed in an asymmetrical arrangement using a fixed low incidence angle of u ¼ 0.5 , as usual for measurement of thin layers in the angle range of 2q ¼ 20e80 . Phase composition is investigated using the Highscore program. In addition to the phase composition, attention is paid to the analysis of the preferred orientation of the crystallites, because the texture significantly influences the mechanical and tribological properties. 3. Results 3.1. Surface morphology Three scans are made by the AFM and Dektak ™ XT mechanical profilometer (according to the ISO 25178 standard) over different areas of 50 50 mm2 and 100 100 mm2, respectively. Fig. 1 shows 2D and 3D images of the scanned surface coating obtained by AFM analysis and a graph of the surface roughness on the diagonal of the scanned area. The result is given in Table 1 and indicates that the coating has an average surface roughness (Sa) of 0.065 mm and a root mean square roughness (Sq) of 0.091 mm. The average values of the surface parameters measured using the profilometer are given in Table 2, where: Sz is the maximum height (height between the lowest recesses and the highest projection); Sv is the maximum depth of the recess
Table 1 Parameters of AlSiN coating surface morphology according to AFM analysis. Scans Scan 1 Scan 2 Scan 3
1 2 1 2 1 2
Sa [mm]
Average [mm]
Sq [mm]
Average [mm]
0.072 0.046 0.051 0.090 0.066 0.061
0.059 ± 0.013
0.097 0.064 0.069 0.137 0.097 0.080
0.080 ± 0.017
0.071 ± 0.019 0.064 ± 0.003
0.103 ± 0.034 0.089 ± 0.009
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Table 2 Parameters of AlSiN coating surface morphology according to mechanical profilometer analysis. Parameters
Sa [mm]
Sz [mm]
Sq [mm]
Sv [mm]
Sp [mm]
Scan 1 Scan 2 Scan 3 Average
0.116 0.102 0.130 0.116 ± 0.011
2.567 2.830 2.162 2.520 ± 0.275
0.191 0.166 0.189 0.182 ± 0.011
0.371 0.995 0.744 0.703 ± 0.256
2.196 1.834 1.418 1.816 ± 0.318
(height between the lowest recesses and the median plane); and Sp is the maximum height of the protrusion (height between the median plane and the highest projection). The parameters of the profilometer measurements are: stylus type with radius of 2 mm; load of 4.9 102mN; and range of 6.5 mm. Cathodic arc technology has considerable potential for the tool coating industry, primarily because of its basic simplicity, easy implementation and economical operation under industrial production conditions. One of the disadvantages of this method is a poor coating structure (droplet formation and columnar growth). The shortcomings of this method are shown by the different surface roughness values in the evaluation of the mechanical profilometer and AFM.
3.2. Mechanical properties The coating thickness is measured by a Calotest using a steel ball with a diameter of 30 mm and a diamond paste with 0.1 mm monocrystalline diamond grains. The measured thickness is 2060 nm, at a deposition time of 160 min. Fig. 2 shows the track obtained by the Calotest. The observed light iridescence in light optical microscopy can be explained with a transparent outer (AlSiN) layer and reflexive Ti layer. The thickness of the transparent layer is near to the wave length of visible light. Therefore, the image presented in Fig. 2 is observed due to the diffraction of light, which may be attributed to the periodically formed different phases of the coating (Figs. 12 and 13 in the discussion section). The adhesion and toughness of the coatings are evaluated by a microscratch test technique, using a Rockwell diamond indenter with a 200 mm tip radius. Scratch adhesion tracks are analysed using an optical microscope coupled with a CCD camera. Three measurements are taken at different places and the average values of the measured critical loads (LC1 and LC2) are given in Table 3. The friction force Ft and acoustic emission as a function of the applied load are recorded during the scratch testing. The critical load LC1 is a point on the graph where the first crack (cohesive failure) of the coating is observed and the corresponding AE signal is recorded (Fig. 3, yellow line). LC2 is a point on the graph (Fig. 3, green line) where the damage becomes continuous and complete delaminations of the coating are achieved. This is accompanied by a sharp increase in the coefficient of friction (CoF). The coating adhesion scratch tests disclose the cohesion and adhesion properties of the tested coating. In virtue of the tests carried out, the critical load corresponding to a load leading to the appearance of the first crack LC1 lies within a range of 27e29 N, and LC2 lies within the range of 54e69 N (Table 3).
Fig. 2. The optical microscopy image of the AlSiN coating obtained by the Calotest.
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Table 3 The adhesion properties.
Scratch 1 Scratch 2 Scratch 3 Average
LC1[N]
LC2[N]
29 29 27 28.3 ± 0.9
54 64 69 62.3 ± 6.2
Fig. 3. Optical micrograph of the scratch track in the AlSiN coating and scratch test results of the friction force, CoF and AE.
Nanohardness is measured with a Bercovich diamond indenter prism. The maximum penetration depth during the nanohardness measurement is no more than 10% of the coating thickness. The measured nanohardness (H) and corresponding value of the Young modulus (E) are 38.9 GPa and 389 GPa, respectively. The calculated plasticity index [16,17] is 0.1.
3.3. Tribological properties Tribological tests are performed in order to investigate the wear processes between different sliding pairs. The tests are conducted using an Al2O3 ceramic ball and a steel ball ISO 683/13 (X105CrMo17), both with a 6.35 mm diameter. The steel ball is much softer (700 HV) than the AlSiN coating (39 GPa), while the Al2O3 ceramic ball has a hardness of 2100 HV. The CoF between the counterpart (ball) and the coating is determined during the measurement. A set of cycles was made for each counterpart material. 3.3.1. Coefficient of friction Tribological testing is conducted according to the ASTM G99-95 standard. A load 10N and rotation speed of the table 60 RPM are used during the evaluation of the CoF. The experiments are performed on a predefined 25-m path (almost 995 cycles). Fig. 4 depicts the CoF behaviour using both the Al2O3 counterpart ceramic ball and the ISO 683/13 steel ball. At the beginning of the experiment, the measured values of the CoF between the ceramic ball and the steel ball and the coating were 0.2 and 0.34, respectively. After the first 15 cycles, an increase of the CoF values to 0.25 (for the Al2O3 counterpart) and 0.49 (for the steel counterpart) was observed. After 30 cycles of rotation of the counterpart on the coating surface, values of 0.64 for the Al2O3 counterpart and 0.67 for the steel counterpart were registered. These values remained unchanged to the end of the experiment. The change in the CoF values (for the Al2O3 counterpart) is caused by the separation of the particles from the AlSiN coating. These particles are hard and abrasive and deteriorate the friction. When a steel ball is used as the counterpart, the CoF increases after a few meters, reaching a value as high as 0.85. This change occurs more rapidly because the particles of the counterpart, which have a lower hardness than the coating, separate during the experiment. This leads to an increase in CoF and at the same time a small deviation of the measured values in comparison with the ceramic counterpart. A conformance test of the mean values is performed using the ANOVA statistical method for processing data from the tribological measurements (ANOVA stands for analysis of variance). More than 650 values were processed for each group of results. According to the ANOVA, the average CoF and variance are 0.744 ± 0.044 and 0.858 ± 0.014 when the Al2O3 ball and steel ball are used as counterparts, respectively. We are able to calculate the effectiveness of the CoF reduction based on the average values. There is an average reduction of approximately 13.2% when a counterpart made of Al2O3 is used compared to the average value of CoF when a steel ball counterpart is used. The increase in the friction coefficient (friction pairs AlSiN e steel Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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Fig. 4. Friction coefficient of AlSiN coating against steel and Al2O3 ceramic balls at room temperature as a function of sliding distance.
ball) can be explained by the presence of steel particles from the counterpartat the point of contact during the tribological experiment. 3.3.2. Wear and wear rate The wear of the friction pairs is evaluated according to the EN 1071-13:2010 standard. Circular tracks are left on the flat disc sample (AlSiN sample) after the test. A Dektak XT™ mechanical profilometer is used to measure the transverse profile at four points with intervals of 90 and the cross-sectional area. Circular grinding traces are also established at each point. The amount of coating wear (VAlSiN) is calculated according to Equation (1) from the cross-sectional area of the circular machining track, and the results are described in Table 4.
VAlSiN ¼
prðS1 þ S2 þ S3 þ S4 Þ 2
;
(1)
where: r is the radius of the circular machining track [mm]; S1 to S4 are the cross-sectional areas at four points, with intervals of 90 , spaced evenly along the circular machining tracks [mm2]. The amount of wear on the balls is calculated by Equation (2), and the results are described in Table 4. Fig. 5 shows the destroying integrity of the counterpart after the tribological test.
Vball ¼
pA3 B 32D
;
(2)
where: Vball is the amount of wear on the balls [mm3]; A is the smallest diameter of the machining track [mm]; B is the diameter in the direction perpendicular to the smallest diameter [mm]; and D is the diameter of the ball [mm]. The wear rate of the coatings and the counterparts is calculated by Equation (3) [18]:
wear rate ¼
wear volume load sliding distance
(3)
Table 4 shows the wear and wear rate of the AlSiN coating in contact with the steel and Al2O3 counterparts, and the wear and wear rate of the balls. The wear of the Al2O3 counterpart is 93.2% less in comparison with the steel counterpart. The AlSiN coating is very abrasive and a large amount of material from the steel counterpart was deposited on the coating surface during the tribological experiment. Moreover, no distortion of the coating was observed. In this case, it is not correct to talk about the wear of the AlSiN coating but the area of influence of the counterpart on the coating. In this case, Table 4 does not specify any data for the wear and wear rate of the AlSiN coating. The experiment with the steel counterpart was conducted in order to determine its wear rate. The wear rate of the AlSiN coating is determined using only the friction pair AlSiN/Al2O3. Table 4 Wear and wear rate of the AiSiN coating and counterparts. Type of balls
Wear volume VAlSiN [103 m3]
Coating wear rate [106 mm3 N1 m1]
Wear volume Vball [103 mm3]
Ball wear rate [106 mm3 N1 m1]
Steel ISO 683/13 Ceramic Al2O3
e 6.8
e 27.2
5.6 0.4
22.4 1.5
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Fig. 5. Destruction of the integrity of the Al2O3 (left) and the steel (right) counterparts at the contact with the AlSiN coating.
3.4. Chemical composition and structure analysis A Zeiss Ultra Plus scanning electron microscope equipped with an Oxford X-Max 20 energy dispersive spectrometer was used for the local chemical analysis of the coatings. The analyses were performed at an accelerating voltage of 10 kV, working distance of 8.5 mm, live time of 100 s, energy range of 10 keV, number of channels of 4096, and with pulse pile up correction. AZtec 2.4 software was used for the quantification. Before the EDS analysis, the sample surfaces were cleaned using Ar-plasma at 0.5 kV for 60 min at a pressure of 106 Pa. The average values of the determined composition of AlSiN coating by SEM/EDS (in at. %) are 50.5 N; 40.3 Al and 9.2 Si. 3.4.1. XPS analysis The XPS study shows that the surface has significant carbon contamination, probably due to sample storage, see Fig. 6. The argon ion sputtering reduced the carbon content to half on the surface. At the same time, the oxygen content is increased, possibly due to the presence of oxides and several oxynitrides. The compositions show systematically higher concentrations of C, Al and O when the Mg anode is used. Therefore, it can be assumed that these elements are located in higher amounts in the top few nm because Al anode photons have a high energy, so the analytic volume is deeper. This trend remained when the mainly carbon contamination was removed by Arþ sputtering. The high-resolution spectra are not influenced by the sputtering procedure and the carbon was not completely removed. The oxygen-rich surface is also supported by the XRD data, where no oxide rich phases are detected. Therefore, the XPS spectra of Al 2p (see Fig. 7) are composed of two sets of doubles. The first set, the full line, has Al 2p3/ 2 at approximately BE 73 eV and Al 2p1/2 at BE 74 eV, and the AlN contribution [19e21] may be approximately 60% in this form. The second set, the dashed line, has Al 2p3/2 at approximately BE 73.3 eV and Al 2p1/2 at BE 74.3 eV and the AleOH [19,21] or native AleO [20] contribution may be approximately 40% of Al. The high-resolution spectra of Si 2p in Fig. 8 show the presence of Al KLL in the spectra taken by the Al anode. The XPS spectra of Si 2p, see Fig. 8, are composed of two sets of doubles. The first set, the full line, has Si 2p3/2 at approximately BE 99 eV and Si 2p1/2 at BE 100 eV, and the SieSi and SieAl contributions (metals) [19,20] may be approximately 5%. The second
Fig. 6. Survey spectra measured on samples before and after sputtering.
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Fig. 7. High resolution XPS spectra of Al 2p measured by the Al anode before sputtering.
set, the dashed line, has Si 2p3/2 at approximately BE 101 eV and Si 2p1/2 at BE 102 eV and the SieN [19] contribution may be approximately 95% of the Si in this form. Native SiOx or SiO2 are not identified but their presence can be overlapped in the SieN contributions as described in the literature on AlSiON [19]. The spectra of N 1s show one broad component at approximately BE 396.6 eV (Fig. 9) identified as AleN, similarly to those presented for AlSiN [19] and AlN [20,21]. 3.4.2. XRD analysis The elemental composition suggests the possible presence of the following phases: AlN, SiN, AlSiN. The X-ray diffraction pattern is presented in Fig. 10. The XRD pattern shows reflections corresponding to AlN in both modifications: cubic (space group F-43 m) and hexagonal (space group P63mc), besides very weak reflections on the background coming from the steel substrate. The reflections belonging to cubic AlN are marked by red lines, and reflections of the hexagonal form by black lines. Reflections corresponding to the SiN were not found in the diffraction pattern. A very weak and broad reflection at 2q ¼ 28.44 corresponds to the 111 reflection of the Si phase (cubic, space group Fd-3m). This result is consistent with the XPS measurements. The XPS investigation revealed the SieSi metallic bonds for ~15% of Si atoms indicating the presence of Si crystallites. The large broadening of the 111 Si peak profile (with a half-width FWHM of approximately 2 in 2q) indicates a very small Si crystallite size, which is lower than 10 nm. As the profile broadening increases with the diffraction angle, Si reflections corresponding to higher angles cannot be observed. XPS measurement also revealed the SieN bonds; however, the SiN reflections were not found in the X-ray diffraction pattern. This means that in the AlN the Al positions are randomly substituted by Si atoms, creating mixed AlSi nitride.
Fig. 8. High resolution XPS spectra of Si 2p measured by the Al anode and Mg anode before sputtering.
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Fig. 9. High resolution XPS spectra of N 1s measured by the Al anode before sputtering.
Another structural feature in the thin films is the preferred orientation of the crystallites. By analysing the intensity ratio of both the cubic and hexagonal phases of AlSiN, we can conclude that there is a crystallographic plane of 111 cubic form, which is identical to the 001 hexagonal form of AlSiN, which is preferentially oriented parallel to the sample surface. This crystallographic plane is illustrated in Fig. 11. 4. Discussion At the beginning of the study an AlSiN coating was deposited with thickness of 0.5 mm for 40 min. During the measurement of its thickness by the Calotest it was observed that the coating looked like it had multilayers (Fig. 12), but grew as a monolayer. This led us to perform the chemical analysis longitudinally in this sector. The composition of the coatings was analysed using an X-ray energy dispersive spectrometer attached to a SEM system. The EDS analysis (Fig. 13) shows that there are no unexpected elements. So, the observed light iridescence in the optical microscope (Fig. 12) can be explained with a transparent (AlSiN) outer layer and a reflexive Ti layer. The thickness of the transparent layer is near to the wavelength of visible light. Therefore, the image presented in Fig. 12 is observed due to the diffraction of light, which may be attributed to the different periodically formed phases of the coating. The coating shows high nanohardness (H ¼ 39.5 GPa), higher than the values presented in the literature (25 GPa [5,9], 30 GPa [23], 32 GPa [11], and 35 GPa [4]), and good adhesion to the substrate (average values are LC1 ¼ 30.3 ± 1.7 N and LC2 ¼ 40.3 ± 1.2 N). So, we decided to prepare samples with a higher thickness to analyse the structure and tribology properties of the AiSiN coating deposited at these technological parameters (see Section 2). The study showed that the observed phenomenon in the AlSiN coating is again recorded with a thick AlSiN coating, i.e. the coating thickness does not influence the observed interference phenomenon.
Fig. 10. X-ray diffraction pattern of the AlSiN coating deposited onto the HSS substrate.
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Fig. 11. Crystallographic plane preferentially oriented parallel to the sample surface, pink Al(Si) atoms, dark blue nitrogen atoms. Some Al atoms are randomly substituted by Si. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The chemical analysis made by SEM/EDS showed that the AlSiN coating present here has a stoichiometry structure (Al40Si9N51). As it can be seen, the ratio between the Al and Si in the coating is the same as in the cathode used. Generally, an AlN coating obtained by PVD methods has hexagonal crystallites. XRD analysis shows that the addition of Si resulted in the formation of AlN in both modifications: cubic and hexagonal. The presence of approximately 95% of Si in form of silicon nitride and SiN reflection were not found in the X-ray diffraction pattern. In addition, SieN bonding almost converted to Si3N4 [22] when the content reached 9.0 at. % and an amorphous silicon nitride (a-Si3N4) is usually positioned at 101.8 eV, which leads us to conclude that the AlSiN coating presented here has a nanocomposite structure. The nanocomposite is not fully completed due to the presence of Si crystallites and the solid solution of AlSiN, in which a small number of Al atoms are substituted by Si atoms. Moreover, this leads to an increase in the hardness of the AlN [24] from 28 GPa to 39 GPa, which has an influence over the tribological properties. It is well known that the cathodic arc deposition (CAD) process forms a large quantity of droplets (Fig. 14). The surface morphology analyses confirmed this. The average surface morphology values measured by the profilometer are two times higher than those measured by AFM. This is due to the fact that the profilometer scans an area more than four times larger than the AFM as well as the presence of the droplets. Due to the presence of the droplets, the increase in thickness should lead to an increase in surface roughness. The surface roughness has a direct influence over the CoF behaviour (see Fig. 4). When the counterpart is made from a soft material (like the steel ball) this coating surface acts as an abrasive during the dry friction. When the counterpart is made from a hard material (like Al2O3) this roughness affects the oscillations of the CoF behaviour. It is known that the layers deposited by this method have residual stresses, whereby the stresses increase with an increase in thickness. The deposited AlSiN coating has a very good adhesion strength, which has a slight dependence on the thickness. No information was found in the literature concerning investigation of AlSiN coating adhesion.
Fig. 12. A segment of the optical microscopy image of the AlSiN coating obtained by the ball-cratering test.
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Fig. 13. The measurement line and distribution of the elements from the top of the coating to the substrate.
Fig. 14. Surface morphology of the AlSiN coating.
Different wear mechanisms were found for the coating sliding against different counterparts. For the coating sliding against steel balls, the adhesive wear plays a major role in the wear process. Due to the much higher hardness of the film than the steel ball and the absence of the adhered layer on the ball, material was removed and transferred from the ball to the wear track and accumulated to form a transferred layer. On the other hand, the coating sliding against an Al2O3 ball showed a totally different wear mechanism. During the wear test the generated wear debris was not transferred to the wear track but to the ball. The estimated wear rate and coefficient of friction of the coating using the friction pair AlSiN/Al2O3 are 27.2 106 mm3N1m1 and 0.7, respectively.
5. Conclusions It was found that the AlSiN coating has very good adhesion to the base material, tool steel (EN ISO HS 6-5-2) and SEM/EDS chemical analysis determined that the AlSiN coating presented here has a stoichiometry structure - Al40Si9N51. XPS and XRD analyses showed that in the coating indicated AlN in both modifications: cubic and hexagonal; the coating has a nanocomposite structure but not fully completed. High nanohardness and highly abrasive propertiesare detected at the specific deposition parameters. This was observed in the interaction of the coating with the steel ball ISO 683/13, where the wear of the steel is very intense. The wear rate and CoF of the coating are determined using the Al2O3 ceramic ball, because in this case the wear rate of the counterpart is less than the coating. Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022
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This study shows that the coating thickness has no significant influence over the nanohardness of the coating or its tribological properties. However, the coating thickness is important in determining the wear rate when the coating is applied in the processing of metals. In summary, the AlSiN coating presented in this article is very suitable for covering cutting tools for machining and cutting of steel parts. The coating is suitable for application on temperature sensitive steels and aluminium alloys. Acknowledgement The results of project LO1201 were obtained through the financial support of the Ministry of Education, Youth and Sports in the framework of targeted support within the “National Programme for Sustainability I” and the OPR & DI project “Centre for Nanomaterials, Advanced Technologies and Innovation (CZ.1.05/2.1.00/01.0005)” and with the support of the Institutional Endowment for the Long Term Conceptual Development of Research Institutes at the TUL ME. 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Suri, Wear studies of (TieAl)N coatings deposited by reactive magnetron sputtering, Wear 258 (2005) 1813e1824. [19] L. Rebouta, A. Sousa, M. Andritschky, F. Cerqueira, C.J. Tavares, P. Santilli, K. Pischow, Solar selective absorbing coatings based on AlSiN/AlSiON/AlSiOy layers, Appl. Surf. Sci. 356 (2015) 203e212. [20] B. Vincent Crist, Handbook of Monochromatic XPS Spectra, XPS International, 2004. [21] Pei-Yin Lin, Jr-Yu Chen, Yu-Chang Chen, Li Chang, Effect of growth temperature on formation of amorphous nitride interlayer between AlN and Si(111), Jpn. J. Appl. Phys. 52 (8S) (2013), http://dx.doi.org/10.7567/JJAP.52.08JB20. [22] S.U. Kang, L.I.U. DaMeng, S.H.A.O. TianMin, Microstructure and mechanical properties of TiAlSiNnano-composite coatings deposited by ion beam assisted deposition, Sci. China, Technol. Sci. 58 (10) (2015) 1682e1688. [23] A. Pelisson, M. Palinska-Wojta, H.J. Hug, J. 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Please cite this article in press as: N. Petkov et al., Study of surface morphology, structure, mechanical and tribological properties of an AlSiN coating obtained by the cathodic arc deposition method, Superlattices and Microstructures (2017), http://dx.doi.org/ 10.1016/j.spmi.2017.05.022