1. Introduction
Titanium alloy has the advantage of high specific strength, excellent corrosion resistance, strength retention at high temperatures and good biological adaptability. It has been widely used in various industries, such as aerospace and biomedical engineering. Titanium alloys can be divided into α-phase or near-α-phase, α + β-phase and β-phase or near-β-phase according to different microstructures. Among them, Ti6Al4V is the most widely used α + β phase titanium alloy and has excellent comprehensive properties. However, these excellent properties also bring difficulties to its processing. With the development of metal cutting technology, more efficient, environmentally friendly and cost-saving cutting methods are being developed. Among them, the coating can not only effectively inhibit the occurrence of mechanical wear, adhesive wear, diffusion wear, oxidation wear and other phenomena, but also reduce the friction coefficient and delay temperature diffusion. Ultimately, long-lasting, efficient and high-quality processing is achieved. Therefore, research on coating becomes particularly important. At present, the research on coating technology has mainly been focused on physical vapor deposition (PVD) coating and chemical vapor deposition (CVD) coating, which have developed from single layer to multi-layer and even composite coating. Coating thickness has also been increasing, from micron scale to nano scale development. Coating material research includes binary TiN, TiC coatings, ternary TiCN, TiAlN coatings, multi-component Cr series, Zr series, B series, Ta series, diamond and diamond-like coatings, etc.
Regarding the research on titanium alloy cutting tool coating materials, Jawaid et al. used the PVD method to coat with TiN coating and process TC4 [
1]. They found that the adhesive wear of the rake face caused the coating to delaminate and peel off. They also observed frictional wear and diffusion wear. The hardness and oxidation resistance of TiN coatings increase with the addition of Al element [
2,
3]. In high-speed, high-efficiency machining, the oxidation resistance temperature of TiAlN coatings is still too low (<800 °C), which restricts the applications of cutting tools [
4]. An et al. used CVD and PVD methods to coat the surface of milling inserts with Ti(C, N)/Al
2O
3/TiN and (Ti, Al)N/TiN coatings, respectively, and used them to process Ti-6242S and Ti-555 [
5]. For titanium alloys, it was found that adhesive wear and diffusion wear occurred during the processing of coated tools, and PVD-(Ti, Al)N/TiN coated tools had the longest processing life. Kuram et al. used the PVD method to coat single-layer TiCN, AlTiN and TiAlN materials and double-layer TiCN/TiN and AlTiN/TiN materials on the surface of cemented carbide tools and conducted TC4 high-speed milling tests to analyze the relationship between coating materials and the number of layers [
6]. In terms of the influence on tool life, it was found that friction, peeling, and adhesive wear mainly occur on the flank surface of the tool, accompanied by the generation of mechanical cracks. Coating peeling, adhesive wear, and pit wear mainly occur on the rake surface. The coating can effectively reduce the wear rate of the tool. Multi-layer coatings prepared via the PVD method will delaminate during processing, which reduces the wear resistance of the tool. The single-layer TiCN coating has higher hardness, lower wear rate and excellent surface quality. Yi et al. added B and Ta elements to the traditional AlTiN coating material. They found that the coating doped with rare metals improved the hardness, oxidation resistance and bonding strength of the tool surface [
7]. Niu et al. found that the choice of coating technology is mainly affected by the material being processed. PVD-coated TiN/TiAlN tools are more suitable for milling TC6 and TC17. In the processing of TC11 titanium alloy, CVD-coated TiN/Al
2O
3/TiCN tool performance is even better [
8]. Biksa et al. used (Al, Ti)N-WN, (Al, Ti)N-MoN, (Al, Ti)N-CrN, (Al, Ti)N-VN and (Al, Ti)N-NbN coatings when processing Ti6Al4V alloy. They found that (Al, Ti)N-VN coated tools have the highest tool life parameters and the best wear mechanism [
9].
In the research on the coating of endmills for titanium alloy processing, the application of TiCN and TiCrN coatings prepared using PVD technology is relatively mature, but the pursuit for enhanced tool performance remains persistent. In their research on new coating technologies for titanium alloy cutting, Srinivasan et al. used hot filament chemical vapor deposition (HF-CVD) technology for a double-layer diamond coating on the carbide insert substrate to enable the tool to have an excellent performance in Ti6Al4V cutting [
10]. Thepsonthi et al. coated the surface of micro endmills with cBN coating. Compared with uncoated endmills, the coated tools achieved lower cutting temperatures and lower wear rates in the cutting of Ti6Al4V [
11]. Caliskan et al. coated the cemented carbide milling inserts base with a TiAlN coating and then attached an aCN diamond-like carbon (DLC) coating on it. Their results showed that the aCN/TiAlN coated tool had better adhesion and lower friction coefficient, resulting in a smoother surface processing quality and a longer cutting life [
12]. Volosova et al. used the PVD method to coat TiN-Al/TiN, TiN-AlTiN/SiN and CrTiN-AlTiN-AlTiCrN/SiN nanocomposite coatings on cemented carbide endmills, and conducted Ti6Al4V milling experiments. They found that CrTiN-AlTiN-AlTiCrN/SiN nanocomposite coating tools had better wear resistance and could obtain a longer cutting life [
13].
Traditional coatings, such as AlCrN coating, TiN + Al2O3 coating and TiC + Al2O3 coating etc., have long been used in titanium alloy cutting. However, with the improvement of machine tool performance, traditional coatings can no longer meet current processing needs, and Si-based and Zr-based coatings have high hardness, high heat resistance and good elastic–plastic characteristics. These properties are particularly important under high-temperature intermittent processing conditions. However, at present, it is still difficult to obtain accurate information on the affinity of the Ti and Al elemental content of the substrate layer to the base coating layer, especially the affinity under high temperature conditions. The interaction mechanism between the Si-based and Zr-based functional layers and titanium alloys is still a hot topic in research. This study prepared TiAlN/TiSiN and TiAlN/TiSiN/ZrN coatings; the Si-based coating was represented by TiAlN/TiSiN, and Zr-based coating was represented by TiAlN/TiSiN/ZrN. First, the TiAlN base layer was analyzed to obtain the effect of different Ti:Al ratios on bonding force. Then, functional layer coating was performed based on the optimal base layer and was analyzed to obtain Si-based and Zr-based coatings. The mechanical properties and oxidation resistance were also analyzed. Finally, the influence of different coatings on tool cutting performance was verified through cutting experiments to obtain the optimal coating structure for titanium alloy milling.