1. Introduction

Currently, thin hard physical vapor deposition (PVD) coatings are widely used to improve the tribological performance of forming tools, cutting tools, and machine elements [1]. In these applications, the surface morphology and tribology of the coated part are the most important factors influencing the tool and equipment performance [2]. Cutting tools might be used in

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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].

The wet (water soluble fluid) machining performance of single layer TiAlN, and multilayer TiAlSiN and TiAlCrSiN coatings were assessed when drilling into a carbon steel workpiece (S50C, 50-53HRC). The objective of the present study was to assess the performance of 6-mm diameter WC–Co drills (OSG Corporation, Japan). The critical wear regions of the drills were examined metallographically with the use of a scanning electron microscope (SEM) to identify wear mechanisms acting at the cutting edges of the single layer TiAlN and multilayer TiAlCrSiN

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

http://dx.doi.org/10.5772/intechopen.73141

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This chapter is focused on the influence of morphology on the tribological properties of TiAlN, TiAlN/SiNx, and TiAlN/CNx multilayer coatings with and without a CNx top layer deposited on Si(100) and cemented carbide tool steel (WC) substrates. We also compare the performance of the multilayer coatings with that of a TiAlN monolayer with and without a CNx top layer. To study the friction behavior of these films, in relation to their various structures and surface morphologies resulting from the deposition parameters, we measured the microstructure and surface morphology of the films by transmission electron microscope (TEM) and SEM imaging together with atomic force microscope (AFM) measurements. Vickers hardness, pin-on-disc friction, and high-frequency linear-oscillation (SRV) friction testing were also used to study the tribological properties and wear resistance of such coatings. Furthermore, we compared the tribological properties of the multilayer TiAlCrSiN and TiAlSiN coatings with those of a single layer TiAlN coating to evaluate their possible application to surfaces of cutting tools. The machining performance of the single layer TiAlN, multilayer TiAlSiN, and TiAlCrSiN coated

drills were investigated in the drilling of carbon steel (S50C, hardness 50HRC).

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

coated drills.

2. Experimental methods

2.1. Sample preparation

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

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]. The wet (water soluble fluid) machining performance of single layer TiAlN, and multilayer TiAlSiN and TiAlCrSiN coatings were assessed when drilling into a carbon steel workpiece (S50C, 50-53HRC). The objective of the present study was to assess the performance of 6-mm diameter WC–Co drills (OSG Corporation, Japan). The critical wear regions of the drills were examined metallographically with the use of a scanning electron microscope (SEM) to identify wear mechanisms acting at the cutting edges of the single layer TiAlN and multilayer TiAlCrSiN coated drills.

This chapter is focused on the influence of morphology on the tribological properties of TiAlN, TiAlN/SiNx, and TiAlN/CNx multilayer coatings with and without a CNx top layer deposited on Si(100) and cemented carbide tool steel (WC) substrates. We also compare the performance of the multilayer coatings with that of a TiAlN monolayer with and without a CNx top layer. To study the friction behavior of these films, in relation to their various structures and surface morphologies resulting from the deposition parameters, we measured the microstructure and surface morphology of the films by transmission electron microscope (TEM) and SEM imaging together with atomic force microscope (AFM) measurements. Vickers hardness, pin-on-disc friction, and high-frequency linear-oscillation (SRV) friction testing were also used to study the tribological properties and wear resistance of such coatings. Furthermore, we compared the tribological properties of the multilayer TiAlCrSiN and TiAlSiN coatings with those of a single layer TiAlN coating to evaluate their possible application to surfaces of cutting tools. The machining performance of the single layer TiAlN, multilayer TiAlSiN, and TiAlCrSiN coated drills were investigated in the drilling of carbon steel (S50C, hardness 50HRC).
