3.3. Evaluation of tribological properties

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

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

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

The results from the characterization of the TiAlN, TiAlN/SiNx, TiAlN + CNx, TiAlN/ CNx + TiAlN, and TiAlN/CNx + CNx films are summarized in Table 1 [4, 7]. The hardness measured for TiAlN was 2590 HV. The hardness values of the multilayer TiAlN/SiNx, TiAlN/ CNx + TiAlN, and TiAlN/CNx + CNx films were higher than those of the monolayer TiAlN and TiAlN + CNx films. The increase in hardness could be attributed to the introduction of a large number of TiAlN/SiNx and TiAlN/CNx interfaces in the cases of the TiAlN/SiNx and TiAlN/CNx films, respectively [4, 7]. The high hardness of the multilayer coatings is related to the role of interfaces as effective obstacles to lattice dislocation slip, which is the dominant deformation mechanism in microscale composite coatings. Owing to these interfacial and nanoscale effects, conventional lattice dislocation slip was prevented in the nanostructure

Table 1. Vickers microhardness, critical load, grain size and roughness (Sy) of monolayer and multilayer films.

columnar to a fine-grained morphology [25].

86 Lubrication - Tribology, Lubricants and Additives

to contribute to hardening of materials [25].

tribological properties of the films [4, 7].

3.2. Evaluation of mechanical properties

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