2.1. Sample preparation

harsh machining environments without lubrication or under water-lubricated conditions. Tribological applications of thin films to cutting tools have considerably extended tool life and enabled the realization of dry machining and high-speed machining of hardened materials [3, 4]. Films of diamond-like carbon (DLC) and transition-metal and carbon nitrides (CNx), such as titanium aluminum nitride (TiAlN) and CNx are the most widely used coatings for tribological applications, such as for forming and cutting tools. Thus, it is desirable to improve the tribological properties of these films to enhance their performance in tool applications [5].

In this chapter, topics related to evaluation of the surface morphology and tribological properties of thin films are reviewed. Our recent results on the morphology and tribological properties of TiAlN monolayer, and TiAlN/SiNx and TiAlN/CNx multilayer coatings deposited on cemented carbide cutting tools and silicon wafer substrates by reactive magnetron sputtering deposition are referred in [4, 6, 7]. First, nanoscale TiAlN/SiNx multilayer films were deposited to improve the hardness of TiAlN; we found that the mechanical properties of the multilayer film were considerably improved compared with those of the monolayer film [4]. The introduction of a SiNx layer led to the formation of hard coatings owing to suppression of the TiAlN grain growth, grain refinement, and a decrease in surface roughness. A decrease in the grain diameter and associated decrease in surface roughness likely led to improved mechanical and tribological properties of the coatings [4]. However, the wear performance of the TiAlN/SiNx coating under ambient or high-temperature conditions showed negligible improvement because of its high friction coefficient. Second, CNx is an important tribological material and CNx thin films feature attractive properties such as improved hardness and elasticity and a lower friction coefficient [8, 9]. CNx thin films are currently being extensively studied for their potential tribological applications owing to their favorable mechanical and tribological properties [10]. These materials have already found applications as protective overcoatings for hard discs and read/write heads [11, 12]; furthermore, such materials are of interest in the field of nanotechnology owing to their high wear resistance and low friction properties [11, 12]. It has been found that amorphous CNx films exhibit good wear resistance with a low coefficient of friction (COF) in the range of 0.07–0.3, which is dependent on the N/C ratio and the fraction of the sp3-bonded carbon in the film [8, 9]. Therefore, attention has been paid for combining low COF CNx and hard nitrides together by forming composites or multilayer coatings to achieve the desired mechanical and tribological properties, for example, TiN:CNx composite films [13,

Several studies have indicated that multilayer coatings can exhibit high hardness and fracture resistance with low compressive stress through control of the parameters of the layered structure [5, 17]. The tribological behavior of carbon-based thin films is also strongly influenced by their chemical composition, polycrystalline structure, and surface morphology [18]. However, there remain uncertainties regarding the effects of the deposition of (Ti,Al)N, TiAlN/SiNx, TiAlN/CNx, and CNx coatings on the surface morphology, microstructure, and tribological properties of these coatings. The potential for low friction coefficients and high resistance to abrasive wear are important characteristics for high-speed and hard material cutting applications. In investigations that have aimed to increase the wear resistance and tribological properties of TiAlSiN coatings for wet cutting applications, improved tribological properties have been achieved through the incorporation of chromium (5–10 at%) into PVD TiAlSiN coatings [19–22].

14], and CNx:TiN multilayer coatings [15, 16].

78 Lubrication - Tribology, Lubricants and Additives

Figure 1 shows a schematic illustration of the multi-target DC reactive magnetron sputtering equipment used in this experiment. The equipment consisted of four independent target holders and DC power was applied to both the target holders and the substrate holder. As shown in Figure 2, in this study we prepared: a TiAlN monolayer (TiAlN), a TiAlN/SiNx multilayer, and a TiAlN monolayer with a CNx top layer (TiAlN+CNx), a TiAlN/CNx multilayer with a top TiAlN layer (TiAlN/CNx+TiAlN), and a TiAlN/CNx multilayer with a CNx top layer (TiAlN/CNx+CNx) [4, 6, 7]. All coatings were prepared on polished Si wafers and cemented carbide tools by DC magnetron sputtering from TiAl alloy (50/50 at%, 99.99% purity), carbon (99.99% purity), and Si (99.99% purity) targets. Alternating deposition of TiAlN and SiNx or CNx layers was applied to realize the TiAlN/SiNx and TiAlN/CNx multilayer coatings [4, 7]. Full details of the deposition process have been previously reported [7]. The coating process consisted of three steps such as heating, cleaning by ion bombardment, and multi-nanolayer coating. The system base pressure was maintained at approximately 3.5 <sup>10</sup><sup>3</sup> Torr to produce enough Ar ions for ion bombardment cleaning of the substrate [4]; the substrate was heated at a power of 7000 W for 30 min prior to deposition and the substrate temperature was maintained at ~420C for the deposition. During the ion bombardment

2.2. Morphology and microstructure observations

the mean grain diameter parameters.

Figure 3. Swing type friction tester (SRV).

2.3. Mechanical and tribological properties evaluation

The microstructures of the coatings were evaluated by TEM and SEM cross-sectional imaging. AFM was used to observe the surface of the TiAlN, SiNx, TiAlN/SiNx, TiAlN+CNx, TiAlN/ CNx+TiAlN, and TiAlN/CNx+CNx coatings [4, 7]. The morphological characteristics of the coatings were measured with the use of AFM in dynamic friction mode (DFM) with a carbon nanotube tip having a radius of approximately 44 nm. The AFM system (Digital Instruments Nanoscope III, Hysitron Inc.) was used. Calculations were performed within the scanning probe image processor (SPIP) software, which is a standard program for processing AFM data at the nanoscale. The grain diameter and surface roughness of the coatings were determined by scanning an area 3 3 μm with the AFM. To investigate the effects of grain diameter on the surface morphology and the boundaries between the grains, simulations were performed to calculate the mean grain diameter and surface roughness of the scanning area. The coating surfaces were characterized with the use of the roughness analysis module; the values for surface roughness, average roughness (calculated by Sa: distance between peaks), and peakpeak roughness (calculated by Sy: height difference between the highest and lowest peaks in the image), were obtained by analyzing the images and cross-sectional profiles and measuring

Surface Morphology and Tribological Properties of Nanoscale (Ti, Al, Si, C)N Multilayer Coatings Deposited by…

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Mechanical properties, including hardness and adhesion strength were measured by Vickers hardness and scratch testing (CSM Instruments SA). The load used for hardness measurements was 0.025 N. A scratch tester was used to apply an increasing load with a spherical diamond indenter having a radius of approximately 0.2 mm. The critical load Lc, determined by acoustic emission (AE) observations of the scratch, was used as a quantitative measurement. Full details

The tribological properties were evaluated from pin-on-disc friction and SRV testing methods. The pin-on-disc wear test was performed at an air humidity of 50 10% and a temperature of

of the methods used in the hardness and scratch tests have been previously reported [4].

Figure 1. Schematic of DC sputtering equipment.

Figure 2. Samples of mono- and multilayer coatings.

cleaning process, Ar ions were directed at the substrate with a substrate bias of 500 V. Subsequently, the multi-component multi-nanolayer films were coated with a gas mixture of Ar (220 ml) and N2 (160 ml). On the basis of preliminary experiments, the optimal deposition parameters were determined and multilayer coatings were fabricated. To compare the frictional properties of the monolayer and multilayer coatings, the total film thickness of the TiAlN, or TiAlN/CNx layers was set to be 3 μm. The CNx top layer was approximately 0.5 μm. For multilayer coatings, the layer period was set to be approximately 7 nm and the thickness of the TiAlN layer was approximately 6 nm [4, 7].
