Structure and Tribological Properties of AlCrN + CrCN Coating

The paper presents results of the investigation of the AlCrN and AlCrN + CrCN coatings, deposited by arc evaporation method on the austenitic steel substrate. Topography studies performed with the use of AFM showed that the roughness value was 24 nm for AlCrN and 14 nm for CrCN. Chemical analysis carried out with energy dispersive X-rays spectroscopy confirmed the chemical composition of the coatings. Transmission electron microscopy (TEM) investigations showed a column structure of AlCrN and CrCN layers with a width in the range of 10–200 nm. Tribological properties analyzed using a scratch test and ball-on-disc method showed a good adhesion of the coatings to the substrate (LC2 is 40 and 46 N for AlCrN and AlCrN + CrCN, respectively) and its high wear resistance; the use of an additional CrCN layers caused a decrease in the friction coefficient by 19%. Structure modification of the AlCrN + CrCN coating system and the related improvement in its mechanical and tribological properties allowed increasing the lifetime of the coated elements. The original approach was to produce a dual-layer coating, consisting of an internal, hard AlCrN layer and an external layer of CrCN, providing good tribological properties, as well as an appropriate forming of the transition zone between the layers.


Introduction
Chromium-based ceramic coatings, e.g., CrN, currently consist of a group of wear-resistant coatings. They can be successfully used as an alternative to TiN layers, especially in hot work processes. CrN shows better high-temperature resistance than TiN and also performs well in sliding wear and corrosive environments' applications [1][2][3]. Various two-component and three-component nitride coatings can be used to increase the lifespan of tools and machine elements working in different, often extreme, or highly corrosive environments exposed to abrasive conditions and environments [4,5]. These coatings have gained several applications thanks to their properties (i.e., high hardness, corrosion, and wear resistance). In recent years, the area of nitride research on the three-component coatings Cr-X-N (X: Ti, Si, Al, O, C) has been intensively developing [2,5,6].

Materials and Methods
The investigations were carried out on the X6CrNiMoTi17-12-2 austenitic steel substrate (cylindrical discs with 30-mm base diameter and 5-mm thickness) covered by AlCrN and AlCrN + CrCN coatings. Many parts from austenitic steel are widely used in the petrol, chemical, food, or biomedical industries. Their low hardness and poor wear resistance limit potential applications; therefore, the use of PVD coatings to increase their mechanical and tribological properties seems appropriate.
Before the coatings' production process, the samples were ground and polished to obtain the roughness R a ≤ 0.02 µm. Subsequently, the samples were chemically cleaned using a multistage washing and rinsing process in washing and degreasing baths. The samples prepared in this way were placed in the working chamber on a rotating holder.
Both AlCrN and AlCrN + CrCN coatings were deposited using rotating and planar arc technology in the π1511 system (PLATIT Company, Selzach, Switzerland). Cathodes containing pure Al and Cr metals were used to produce the coatings. After evacuating the vacuum chamber to pressure 5 × 10 −5 Pa, the substrate was heated to a temperature of 450 • C. Next, the substrates were ion-etched by argon (150 sccm) at the bias voltage of 600/400 V for 20 min. To ensure the adhesion increase of the coating, an additional CrN adhesive layer with a thickness of about 200 nm was applied. After that, the main AlCrN layer was deposited using nitrogen (N 2 ) as a gas precursor. The deposition process of AlCrN was carried out using the arc current of 250 and 235 A for Cr and Al target, respectively. The substrate bias voltage was −60 V. The CrCN top layer was deposited using nitrogen (N 2 ) and acetylene (C 2 H 2 ) as a gas precursor; the arc current for Cr target was 250 A and the substrate bias voltage was −35 V. To obtain appropriate properties, the technological conditions of the coating deposition process were determined based on microhardness and tribological resistance tests (in the case of the CrCN layer). Process parameters are presented in Table 1. The cross-section structure of the investigated coatings was analyzed by scanning electron microscope (SUPRA 35, Zeiss, Oberkochen, Germany).
The surface topography of the coatings was analyzed by atomic force microscope (Park System XE-100, Suwon, Korea). The areas of coatings dimensioned 5 µm × 5 µm were scanned during the measurement, and N = 512 × 512 measurements of specimen height were made, where numbers refer, respectively, to the number of scanned lines and measuring points for each of them. The distance between lines and measuring points is constant and the same. The tests also enabled determining the roughness of the layers' surface.
X-ray diffraction studies of the coatings were carried out on the X Pert PRO system made by Panalytical Company (Almelo, The Netherlands) using filter radiation of a cobalt lamp. The phase identification of the tested coating was carried out in grazing incidence geometry (GIXRD).
The microstructure of the examined coatings was observed by the ultrahigh-resolution scanning/transmission electron microscope (TITAN 80-300, FEI, Hillsboro, OR, USA). The thin lamellas for TEM observations were prepared by FIB technique using a Hitachi FB-2100 device (Tokyo, Japan) with gallium ions.
The adhesion of the coatings was investigated by the scratch-test method on the Revetest device (CSM Instrument, Peseux, Switzerland) by moving the diamond indenter Rockwell C on the sample's surface with the gradually increasing load in the range of 0-100 N. The indenter's sliding speed was 10 mm/min and the load increase rate was 100 N/min. The critical load, leading to the loss of the coating adhesion to the substrate material, was determined by observations of the damage developed in the scratch test on a light microscope as well as the values of the acoustic emission (AE) and recorded friction force (F t ).
Tribological properties of the coatings were determined with the use of the ball-on-disc test. The test was performed on the THT (CSM Instruments) device. The following parameters were applied: normal load 10 N, sliding distance 1000 m, sliding speed 0.2 m/s, temperature 23 • C (±1 • C), counterpart type Al 2 O 3 ball, counterpart diameter 6 mm.

Results and Discussion
The examined coatings show a compact structure and adhere tightly to the steel substrate. Visible defects or delaminations were not found. The main AlCrN layer shows a porous and columnar structure (Figure 1a), while the top CrCN layer is characterized by a dense structure. The boundary lines both in between the AlCrN and CrCN layers as well as between the CrN adhesion layer and substrate are well visible on the scanning electron microscope (SEM) image (Figure 1b). To identify the phase composition of the investigated coating, an X-ray diffraction (XRD) phase analysis was performed. Due to the overlapping of reflections from the substrate and coating material and their intensity, which made the analysis of the obtained results difficult, the grazing incidence geometry (GIXRD) diffraction technique was used ( Figure 2). The obtained results were analyzed using the X′Pert Data computer program, and then the phases were identified based on JCPDS cards. In the angular range 2θ from 20° to 100°, the diffraction patterns contain four reflections that can be identified as (111), (200), (220), and (311) for the CrN (fcc) phase. The obtained reflections can be appropriately assigned to the following interplanar spacings: 0.238, 0.206, 0.145, and 0.123 nm for the CrN phase.
Moreover, some shifts of the reflexes toward higher or lower reflection angles found on the diffraction patterns may indicate the presence of texture or internal stresses. The observations of the analyzed coatings and substrate surface topography using atomic force microscopy ( Figures 3-5) revealed a characteristic surface topography with a significant heterogeneity connected with occurrences of the microdroplets of various sizes, which is related with the essence of the employed PVD process for depositing the coatings. To identify the phase composition of the investigated coating, an X-ray diffraction (XRD) phase analysis was performed. Due to the overlapping of reflections from the substrate and coating material and their intensity, which made the analysis of the obtained results difficult, the grazing incidence geometry (GIXRD) diffraction technique was used ( Figure 2). The obtained results were analyzed using the X Pert Data computer program, and then the phases were identified based on JCPDS cards. In the angular range 2θ from 20 • to 100 • , the diffraction patterns contain four reflections that can be identified as  To identify the phase composition of the investigated coating, an X-ray diffraction (XRD) phase analysis was performed. Due to the overlapping of reflections from the substrate and coating material and their intensity, which made the analysis of the obtained results difficult, the grazing incidence geometry (GIXRD) diffraction technique was used ( Figure 2). The obtained results were analyzed using the X′Pert Data computer program, and then the phases were identified based on JCPDS cards. In the angular range 2θ from 20° to 100°, the diffraction patterns contain four reflections that can be identified as (111), (200), (220), and (311) for the CrN (fcc) phase. The obtained reflections can be appropriately assigned to the following interplanar spacings: 0.238, 0.206, 0.145, and 0.123 nm for the CrN phase.
Moreover, some shifts of the reflexes toward higher or lower reflection angles found on the diffraction patterns may indicate the presence of texture or internal stresses. The observations of the analyzed coatings and substrate surface topography using atomic force microscopy (Figures 3-5) revealed a characteristic surface topography with a significant heterogeneity connected with occurrences of the microdroplets of various sizes, which is related with the essence of the employed PVD process for depositing the coatings. Moreover, some shifts of the reflexes toward higher or lower reflection angles found on the diffraction patterns may indicate the presence of texture or internal stresses.
The observations of the analyzed coatings and substrate surface topography using atomic force microscopy (Figures 3-5) revealed a characteristic surface topography with a significant heterogeneity connected with occurrences of the microdroplets of various sizes, which is related with the essence of the employed PVD process for depositing the coatings.
The observations of the investigation of the coatings' surface topography using AFM revealed a varied topography of the coatings. In the case of AlCrN coating, there were large, individual, single, droplet-shaped microparticles. However, in the case of AlCrN + CrCN coating, there were numerous small microparticles with an almost spherical shape. Additionally, an analysis of the surface development was carried out, determining the ratio of the actual area measured for a given sample to its rectangular projection on the XY plane. As a result of the investigations, it was found that the surface of the AlCrN coating showed an increase in surface development by about 8.7%, while in the case of the AlCrN + CrCN coating the surface development was 2.2%. The data mentioned above showed that the CrCN layer reduced the surface inhomogeneity of the AlCrN coating ( Table 2). The measurements that were taken also allowed determining the layers' surface roughness by the root mean square (RMS) and the layers' surface roughness (R a ). The AlCrN + CrCN coating shows significantly lower roughness.        For analysis of the structure of the AlCrN and AlCrN + CrCN coating produced by arc PVD method on the austenitic steel substrate, STEM and TEM modes were used. Figure 6 shows the STEM-BF image of the AlCrN + CrCN coating on which the substrate (X6CrNiMoTi17-12-2), the AlCrN layer, and the CrCN layer are marked. The individual layers are deposited uniformly and correctly adhere to each other and the substrate material. Profile analysis results (Figure 6b) confirmed the assumed chemical composition and additionally showed the occurrence of three layers in the coating: the interlayer located directly at the substrate, with a thickness of approximately 200 nm, the main AlCrN layer, and an external CrCN layer. The chemical composition of the substrate was confirmed by the analysis of the energy dispersion X-rays spectroscopy (Figure 6c).  The interlayer located directly next to the substrate contains Cr and N. It shows a crystalline structure, which was confirmed both by bright field imaging (diffraction contrast) and nanodiffraction pattern identified as CrN in [001] direction ( Figure 7). Crystallites have a column structure; their width is about 10-50 nm.  The external layer ( Figure 9) with a thickness of approximately 500 nm does not contain Al. Its chemical composition includes Cr and N (also the chemical composition of Cr, N, and C is possible). The column structure (Figure 9a,b) is similar to that in the main layer. Electron diffraction allows identification of the CrN or CrCN phase in the direction [011] (Figure 9d).
It has been reported earlier that nanocrystalline chromium-based coatings demonstrated good tribological and mechanical properties as well as oxidation resistance and thermal stability compared to coatings used on an industrial scale [27]. The appropriate form of the structure and properties of the coating mentioned above occurred as a result of optimization of deposition conditions and choice of the coating material. HRTEM examination of the AlCrN and CrCN showed that the coatings The external layer ( Figure 9) with a thickness of approximately 500 nm does not contain Al. Its chemical composition includes Cr and N (also the chemical composition of Cr, N, and C is possible). The column structure (Figure 9a,b) is similar to that in the main layer. Electron diffraction allows identification of the CrN or CrCN phase in the direction [011] (Figure 9d).
It has been reported earlier that nanocrystalline chromium-based coatings demonstrated good tribological and mechanical properties as well as oxidation resistance and thermal stability compared to coatings used on an industrial scale [27]. The appropriate form of the structure and properties of the coating mentioned above occurred as a result of optimization of deposition conditions and choice of the coating material. HRTEM examination of the AlCrN and CrCN showed that the coatings consisted of fine crystallites (Figures 8c and 9c), which corresponds to the results achieved by earlier investigations [17,28,29].
The scratch-test investigation is the most frequently performed test to examine the characteristics of the cohesive and adhesive strength of coatings. Therefore, the values of the critical load characterizing the adhesion of the coatings to the substrate material were determined with a linearly increasing load by scratch-test method (Figures 10 and 11) and by in situ optical metallographic observation (Figures 12 and 13) [30][31][32][33]. The first critical load L C1 corresponds to the first coating damage, which is usually related to cohesive failures associated with the cracking of material within the coating (Figures 12a and 13a). The second critical load L C2 corresponds to the first adhesion-related failure, which is connected with the start of complete delamination of the coating (Figures 12b and 13b). The examined coatings show high values of critical load. The first critical load L C1 occurred at 19 and 13 N, while the second critical load L C2 occurred at 40 and 46 N for AlCrN and AlCrN + CrCN coating, respectively. In order to determine the nature of the damage formed during the scratch test, optical observations of the scratches were made. Under load below the critical load, the first symptoms of damage of the coatings were small bilateral chipping on the edge of the scratch track. At critical load, chipping occurred, combined with local delamination of the coatings.   To determine the tribological properties of the investigation coatings, an abrasion test under dry slide friction conditions was carried out by the ball-on-disc method. Three tests were performed for each of the tested coatings. Figures 14 and 15 present the graph of friction coefficient µ changes obtained during wear tests in relation to the sliding distance. To determine the tribological properties of the investigation coatings, an abrasion test under dry slide friction conditions was carried out by the ball-on-disc method. Three tests were performed for each of the tested coatings. Figures 14 and 15 present the graph of friction coefficient µ changes obtained during wear tests in relation to the sliding distance. To determine the tribological properties of the investigation coatings, an abrasion test under dry slide friction conditions was carried out by the ball-on-disc method. Three tests were performed for each of the tested coatings. Figures 14 and 15 present the graph of friction coefficient µ changes obtained during wear tests in relation to the sliding distance. To determine the tribological properties of the investigation coatings, an abrasion test under dry slide friction conditions was carried out by the ball-on-disc method. Three tests were performed for each of the tested coatings. Figures 14 and 15 present the graph of friction coefficient µ changes obtained during wear tests in relation to the sliding distance.  The initial transition state was unstable, during which the friction coefficient decreased, then rose with the sliding distance increase, and stabilized, which in the analyzed cases occurred after a distance of about 100 m. In the case of the AlCrN coating, a slight increase of the friction coefficient during the sliding distance was visible. The addition of CrCN as the top layer to AlCrN caused a reduction of the friction coefficient. It was related to the addition of carbon contained in the CrCN layer. It created a transfer layer to facilitate sliding by reducing the contact surface between the coating and the ball [34]. The values of the friction coefficient for AlCrN were consistent with the literature [35,36], similarly as in the case of AlCrN + CrCN, where the results corresponded to the tribological properties of the CrCN layer [2,20,37,38]. However, as shown by studies conducted by Warcholinski and others [15] on CrCN coatings, this coefficient may be reduced by changing the percentage of carbon in the coating. An additional analysis of the weight loss of the tested coatings (Table 3), as well as the measurements of the width of the abrasion trace (Figure 16), showed that the coatings had good wear resistance. The weight loss and abrasion trace width measurements of the AlCrN coating made based on the ball-on-disc test were 0.011 g and 711-720 µ m, respectively. In the case of the AlCrN + CrCN   The initial transition state was unstable, during which the friction coefficient decreased, then rose with the sliding distance increase, and stabilized, which in the analyzed cases occurred after a distance of about 100 m. In the case of the AlCrN coating, a slight increase of the friction coefficient during the sliding distance was visible. The addition of CrCN as the top layer to AlCrN caused a reduction of the friction coefficient. It was related to the addition of carbon contained in the CrCN layer. It created a transfer layer to facilitate sliding by reducing the contact surface between the coating and the ball [34]. The values of the friction coefficient for AlCrN were consistent with the literature [35,36], similarly as in the case of AlCrN + CrCN, where the results corresponded to the tribological properties of the CrCN layer [2,20,37,38]. However, as shown by studies conducted by Warcholinski and others [15] on CrCN coatings, this coefficient may be reduced by changing the percentage of carbon in the coating. An additional analysis of the weight loss of the tested coatings (Table 3), as well as the measurements of the width of the abrasion trace (Figure 16), showed that the coatings had good wear resistance. The weight loss and abrasion trace width measurements of the AlCrN coating made based on the ball-on-disc test were 0.011 g and 711-720 µ m, respectively. In the case of the AlCrN + CrCN The initial transition state was unstable, during which the friction coefficient decreased, then rose with the sliding distance increase, and stabilized, which in the analyzed cases occurred after a distance of about 100 m. In the case of the AlCrN coating, a slight increase of the friction coefficient during the sliding distance was visible. The addition of CrCN as the top layer to AlCrN caused a reduction of the friction coefficient. It was related to the addition of carbon contained in the CrCN layer. It created a transfer layer to facilitate sliding by reducing the contact surface between the coating and the ball [34]. The values of the friction coefficient for AlCrN were consistent with the literature [35,36], similarly as in the case of AlCrN + CrCN, where the results corresponded to the tribological properties of the CrCN layer [2,20,37,38]. However, as shown by studies conducted by Warcholinski and others [15] on CrCN coatings, this coefficient may be reduced by changing the percentage of carbon in the coating. An additional analysis of the weight loss of the tested coatings (Table 3), as well as the measurements of the width of the abrasion trace (Figure 16), showed that the coatings had good wear resistance.
The weight loss and abrasion trace width measurements of the AlCrN coating made based on the ball-on-disc test were 0.011 g and 711-720 µm, respectively. In the case of the AlCrN + CrCN coating, the weight loss decreased (0.009 g) and the width of the abrasion trace was in the range of 424-428 µm, which proved its better wear resistance. Abrasion trace in the case of AlCrN + CrCN was much narrower than in the AlCrN. In both cases, no matter of complete coatings wear-through occurred because the maximum wear-in depths were below their thicknesses. 424-428 µ m, which proved its better wear resistance. Abrasion trace in the case of AlCrN + CrCN was much narrower than in the AlCrN. In both cases, no matter of complete coatings wear-through occurred because the maximum wear-in depths were below their thicknesses. The conducted research on the structural, mechanical, and tribological properties of the analyzed coatings produced by PVD process and the obtained investigation results indicate the suitability of the produced AlCrN + CrCN coating for new application possibilities. Thanks to the obtained nanocrystalline structure, good adhesion to the substrate, and good tribological properties, they can provide higher wear resistance compared to the coatings currently used on an industrial scale.

Conclusions
The coating system consisting of AlCrN layer and carbon-based CrCN layer was successfully deposited on the X6CrNiMoTi17-12-2 steel substrate.
Adhesion tests showed a good adhesion of the coating to the substrate. The critical load value for the AlCrN + CrCN coating was 46 N. The investigation indicated the occurrence of a transition zone between the substrate and the coating, which affected the improved adhesion.
According to the results of the ball-on-disc test, the investigated coating showed good tribological properties. The friction coefficient of the AlCrN + CrCN coating was examined under the technically dry friction conditions, and its value equaled 0.62. The use of an additional CrCN layer on the surface of the AlCrN coating caused a decrease in the friction coefficient by 19%. The beneficial tribological properties of the friction associations with the participation of the investigated AlCrN + CrCN coating indicated that it may be applied to cover selected elements of abrasive joints of machines and tools and, consequently, to enhance their efficiency and durability.
The chemical composition and structure, to a large extent, determined the suitability of the investigated layers as protective coatings. The tests of steel specimens with AlCrN + CrCN coating deposited onto their surface with TEM indicated a sharp transition zone between the substrate material and the layer. In general, no reasons were implying the epitaxial growth of the examined layers. Although coatings with better tribological properties exist, the potential of the CrCN layer is relatively poorly studied and very promising.  The conducted research on the structural, mechanical, and tribological properties of the analyzed coatings produced by PVD process and the obtained investigation results indicate the suitability of the produced AlCrN + CrCN coating for new application possibilities. Thanks to the obtained nanocrystalline structure, good adhesion to the substrate, and good tribological properties, they can provide higher wear resistance compared to the coatings currently used on an industrial scale.

Conclusions
The coating system consisting of AlCrN layer and carbon-based CrCN layer was successfully deposited on the X6CrNiMoTi17-12-2 steel substrate.
Adhesion tests showed a good adhesion of the coating to the substrate. The critical load value for the AlCrN + CrCN coating was 46 N. The investigation indicated the occurrence of a transition zone between the substrate and the coating, which affected the improved adhesion.
According to the results of the ball-on-disc test, the investigated coating showed good tribological properties. The friction coefficient of the AlCrN + CrCN coating was examined under the technically dry friction conditions, and its value equaled 0.62. The use of an additional CrCN layer on the surface of the AlCrN coating caused a decrease in the friction coefficient by 19%. The beneficial tribological properties of the friction associations with the participation of the investigated AlCrN + CrCN coating indicated that it may be applied to cover selected elements of abrasive joints of machines and tools and, consequently, to enhance their efficiency and durability.
The chemical composition and structure, to a large extent, determined the suitability of the investigated layers as protective coatings. The tests of steel specimens with AlCrN + CrCN coating deposited onto their surface with TEM indicated a sharp transition zone between the substrate material and the layer. In general, no reasons were implying the epitaxial growth of the examined layers. Although coatings with better tribological properties exist, the potential of the CrCN layer is relatively poorly studied and very promising.