3.1. Microstructure and morphology

SEM micrographs of the film fracture cross-sections are shown in Figure 4 for the TiAlN monolayer with a thickness of approximately 3 μm and the TiAlN/SiNx and TiAlN/CNx multilayer coatings with a total thickness of approximately 3.6 μm [7]. TiAlN has a columnar morphology, as shown in Figure 4a. The introduction of SiNx into the coating system changed the fracture morphology of TiAlN from a columnar microstructure to a fine-grained structure

Figure 4. Cross-sectional image of TiAlN monolayer, and TiAlN/SiNx and TiAlN/CNx multilayer coatings.

of TiAlN/SiNx, as shown in Figure 4b. This effect could be attributed to growth of the primary nuclei on the top layer (Figure 4b). This result indicates that growth of crystals was blocked periodically by the development of the surface covering layer, which covered the whole surface of the crystals and suppressed grain growth [4, 7, 23]. Figure 4c shows a SEM image of a fracture cross-section of a TiAlN/CNx coating. The TiAlN/CNx also showed a fine-grained structure owing to the introduction of CNx into the coating system [7].

25 3C with the use of a pin-on-disc tribometer with a counterpart composed of SUS304 steel, placed horizontally on a turntable. The wear test was performed at a load of 0.5 N and a linear speed of 100 mm/s for a total sliding time of 600 s (corresponding to a sliding distance of 60 m). The frictional coefficients were calculated by measuring the frictional force from the wear scar area. In the SRV tests, the two test specimens, namely balls and discs, were installed in the test chamber and pressed together. As shown in Figure 3, the upper specimen was oscillated over the lower specimen at pre-programmed frequency, stroke, load, and temperature settings. In this study, the test was conducted with the use of an AISI440C ball indenter (SUS440C, 6.0 mm diameter) without lubricant under a 10 N load, with the use of a 500-μm stroke, a 50-Hz frequency, and 30,000 revolutions at room temperature and atmospheric pressure (30–45% humidity). The wear profiles of the coatings were measured by the SRV test.

SEM micrographs of the film fracture cross-sections are shown in Figure 4 for the TiAlN monolayer with a thickness of approximately 3 μm and the TiAlN/SiNx and TiAlN/CNx multilayer coatings with a total thickness of approximately 3.6 μm [7]. TiAlN has a columnar morphology, as shown in Figure 4a. The introduction of SiNx into the coating system changed the fracture morphology of TiAlN from a columnar microstructure to a fine-grained structure

Figure 4. Cross-sectional image of TiAlN monolayer, and TiAlN/SiNx and TiAlN/CNx multilayer coatings.

3. Result and discussion

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3.1. Microstructure and morphology

Figure 5 shows TEM images of the microstructure of a TiAlN/SiNx multilayer film. The TiAlN/ SiNx was formed by alternation of the TiAlN and SiNx layers at a rotation speed of 3 revolution per minute (rpm) [4]. A nanolayered structure composed of sequentially alternating TiAlN and SiNx layers was confirmed [4]. High resolution TEM images of the TiAlN/SiNx nanolayer cross-section exhibited a bilayer period of 6–8 nm and nanometer-sized grains. The white arrows in the figure indicate the film growth direction. The film morphology showed a dense columnar structure.

Figure 6 shows a TEM image of the microstructure of a TiAlN/CNx+CNx multilayer film. As shown in area I (Figure 6a, marked by an arrow), the bright dots indicate the presence of a CNx top layer phase with a uniform amorphous structure. In areas II and III of Figure 6a (indicated by arrows), micro-diffraction patterns featured both individual spots and continuous rings that corresponded to the superposition of individual diffraction patterns of TiAlN and CNx [7]. Figure 6b shows that the film morphology was fine-grained and that the growth directions of the TiAlN and CNx layers alternated, as indicated by the dark and bright layers, respectively. Figure 6c shows that the TiAlN nanolayers in the TiAlN/CNx coating were approximately 5-nm thick and separated by a matrix of amorphous carbon [4]. This result suggests that the TiAlN/CNx multilayer had a modulated structure with a periodicity of approximately 7 nm, indicating that the nanolayered structure was composed of sequentially alternating TiAlN/CNx, owing to the rotation speed of 3 rpm [4, 7, 23, 24].

Figure 5. Cross-sectional TEM images of TiAlN/SiN coating, observed at scale of (a) 500 nm and (b) 7 nm.

DFM measurement showed that the TiAlN coating had a Sy value of 211 nm. The Sy value for the TiAlN/SiNx was approximately 180 nm. Thus, the Sy of the TiAlN/SiNx coating decreased

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As shown in Figure 8, the average grain diameters for TiAlN + CNx (Figure 8a), TIAlN/ CNx + TiAlN (Figure 8b), and TiAlN/CNx + CNx (Figure 8c) films were 168, 175, and 161 nm, respectively [7]. The Sy values for the TiAlN + CNx, TIAlN/CNx + TiAlN, and TiAlN/ CNx + CNx films were 31.8, 46.9, and 28.2 nm, respectively. The mean grain diameters and Sy value (roughness) of the multilayer TiAlN/CNx + TiAlN and TiAlN/CNx + CNx films were smaller than that of the TiAlN film owing to the introduction of the CNx layer [7]. As a result, the average grain diameters of the TiAlN and TiAlN/CNx film decreased owing to the deposition of the CNx top layer. The TiAlN and TiAlN/CNx films consisted of an arrangement of globular grains with fine spaces and intervals, which became filled by CNx during the

Figure 8. Surface topography map and cross-sectional image profiles of the TiAlN + CNx (a), TiAlN/CNx + TiAlN top (b)

together with the grain diameter.

and TiAlN/CNx + CNx top films (c).

Figure 6. Cross-sectional TEM images of the TiAlN/CNx + CNx coating observed at scales of (a) 1000 nm, (b) 50 nm, and (c) 5 nm.

The surface morphology and roughness of the films were observed by AFM in DFM and analyzed by collecting three-dimensional surface profile data. The mean grain diameters of the samples were determined by averaging three or more results obtained from a 3 3 μm area. As shown in Figure 7, the average grain diameters for the TiAlN (Figure 7a) and TiAlN/ SiNx (Figure 7b) films were 342 and 274 nm, respectively. In Figure 7, we compared the TiAlN and TiAlN/SiNx films, to show that the average grain diameter of TiAlN/SiNx was smaller than that of TiAlN. Hence, the introduction of the SiNx layer contributed to a decrease in the grain diameter [4]. The roughness of the TiAlN and TiAlN/SiNx films was determined by measuring the grain diameter and surface peak-peak height (Sy) with the AFM, where Sy is defined as the height difference between the highest and lowest peaks in the topology [4]. The

Figure 7. Surface topography map and cross-sectional image profiles of the TiAlN (a) and TiAlN/SiNx (b) films.

DFM measurement showed that the TiAlN coating had a Sy value of 211 nm. The Sy value for the TiAlN/SiNx was approximately 180 nm. Thus, the Sy of the TiAlN/SiNx coating decreased together with the grain diameter.

As shown in Figure 8, the average grain diameters for TiAlN + CNx (Figure 8a), TIAlN/ CNx + TiAlN (Figure 8b), and TiAlN/CNx + CNx (Figure 8c) films were 168, 175, and 161 nm, respectively [7]. The Sy values for the TiAlN + CNx, TIAlN/CNx + TiAlN, and TiAlN/ CNx + CNx films were 31.8, 46.9, and 28.2 nm, respectively. The mean grain diameters and Sy value (roughness) of the multilayer TiAlN/CNx + TiAlN and TiAlN/CNx + CNx films were smaller than that of the TiAlN film owing to the introduction of the CNx layer [7]. As a result, the average grain diameters of the TiAlN and TiAlN/CNx film decreased owing to the deposition of the CNx top layer. The TiAlN and TiAlN/CNx films consisted of an arrangement of globular grains with fine spaces and intervals, which became filled by CNx during the

The surface morphology and roughness of the films were observed by AFM in DFM and analyzed by collecting three-dimensional surface profile data. The mean grain diameters of the samples were determined by averaging three or more results obtained from a 3 3 μm area. As shown in Figure 7, the average grain diameters for the TiAlN (Figure 7a) and TiAlN/ SiNx (Figure 7b) films were 342 and 274 nm, respectively. In Figure 7, we compared the TiAlN and TiAlN/SiNx films, to show that the average grain diameter of TiAlN/SiNx was smaller than that of TiAlN. Hence, the introduction of the SiNx layer contributed to a decrease in the grain diameter [4]. The roughness of the TiAlN and TiAlN/SiNx films was determined by measuring the grain diameter and surface peak-peak height (Sy) with the AFM, where Sy is defined as the height difference between the highest and lowest peaks in the topology [4]. The

Figure 7. Surface topography map and cross-sectional image profiles of the TiAlN (a) and TiAlN/SiNx (b) films.

Figure 6. Cross-sectional TEM images of the TiAlN/CNx + CNx coating observed at scales of (a) 1000 nm, (b) 50 nm, and

(c) 5 nm.

84 Lubrication - Tribology, Lubricants and Additives

Figure 8. Surface topography map and cross-sectional image profiles of the TiAlN + CNx (a), TiAlN/CNx + TiAlN top (b) and TiAlN/CNx + CNx top films (c).

deposition of CNx as the top layer. Bonds formed between the CNx and TiAlN (or TiAlN/CNx) led to an increase in the area of boundaries [4, 7]. The morphology was related to the thickness and morphology of the top coating. The CNx top layer had a considerable effect on the surface morphology and roughness, changing the real contact area and the friction and wear behavior. Generally, the surface roughness decreased as the grain size decreased. This trend was accompanied by an improvement in the density of the morphology with a marked transition from a columnar to a fine-grained morphology [25].

coatings [4, 26, 27]. Scratch tests were conducted on the coatings and the results are shown in Table 1. The multilayer TiAlN/SiNx, TiAlN/CNx + TiAlN, and TiAlN/CNx + CNx films showed higher critical load values than the monolayer TiAlN and TiAlN + CNx films. These results suggest that the improved adhesion strength might be attributed to the interfaces of the multilayer preventing extension of fractures and the multilayer structure improving the wear

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All the mono- and multilayer systems were investigated by a reciprocating SRV friction test under dry conditions, and with water and polyalphaolefin (PAO) as a lubricating film to characterize the coating frictional properties. PAO (WO-20) made by Nissan is a lubricating oil for engines and commercially available. Since PAO has characteristics such as low pour point, high viscosity index, evaporation characteristics, low traction, etc., it was used as a lubricant film in this study. The Si wafer substrates (test discs) were coated with the monolayer and multilayer systems and tested with an AISI440C ball indenter, (SUS440C, diameter: 6.0 mm). This test provided information about the cycle number dependence of the friction coefficient, and the wear behavior of the coated substrate and of its tribological counterpart. The wear volume was deduced from the wear depth created at the counterpart and was used to quantify the counterpart wear. Optical microscopy and energy dispersive X-ray spectros-

Figure 9 shows the variation of the coefficient of friction as a function of the number of sliding cycles for the investigated mono- and multilayer systems under dry conditions at room temperature. The coefficient of friction was taken as the average of four tests. Although the SiNx, TiAlNx, and TiAlN/SiNx coatings showed similar friction coefficients (1.11, 0.93, and 0.99, respectively) in frictional contact with a steel counterpart (Figure 9a-c), the abrasive wear of the SiNx and TiAlN coating was greater than that of the TiAlN/SiNx coating. Investigations of the wear after testing were performed with an optical microscope. Corresponding optical photomicrographs and cross-sectional images of the wear marks formed on the coatings are shown in Figure 10, comparing the monolayer SiNx, TiAlN, and multilayer TiAlN/SiNx films. The average cross-sectional areas of the wear tracks were measured at three or more locations after 30,000 revolutions. For the dry conditions, the wear depth of SiNx was over 29.5 μm (Figure 10a) and that of the TiAlN was approximately 13.6 μm (Figure 10b). The TiAlN/SiNx film had a wear depth of 9.9 μm (Figure 10c) and showed better wear resistance than that of the SiNx and TiAlN films. Although the TiAlN film showed lower friction coefficients than that of the TiAlN/SiNx film (Figure 9a–c), the abrasive wear of the TiAlN film was greater than that of the TiAlN/SiNx film. The wear resistance of the TiAlN/SiNx film was enhanced owing to its nanolayer microstructure and small grain size compared with that of the TiAlN film. We believe that the decrease of the grain diameter might have caused a decrease of the surface roughness, which led to the improved tribological properties of the coating. Conversely, as shown in Figure 9c-f, the monolayer TiAlN + CNx film showed a lower friction coefficient

resistance of the coating [4].

3.3. Evaluation of tribological properties

copy were used to examine the wear of the coated substrate.

3.3.1. Frictional and wear properties under dry conditions

The introduction of SiNx or CNx onto the TiAlN monolayer and the apparent decrease in the grain size could have contributed to the small increase of the hardness for the multilayer TiAlN/SiNx and TiAlN/CNx. The first factor that we considered was the structure parameter. When SiNx or CNx was introduced onto the TiAlN, there was a decrease in the grain size, an increase in the compressive stress level, and an improvement in the coating density accompanying the transition from a columnar to fine-grained morphology. All these factors are known to contribute to hardening of materials [25].

Factors such as residual stress, morphology, phase composition, and grain size are usually taken into account as hardening mechanisms and were considered here; however, we did not identify any major changes between the ternary and binary films that could explain the observed trend in hardness. We believed that the decrease in grain diameter might have resulted in a decrease in surface roughness, which led to the improved mechanical and tribological properties of the films [4, 7].
