Structure and Wear resistance of Duplex Nitride Coatings on VT9 Titanium Alloy

Abstract

The combination of high specific strength and low weight of titanium alloys (titanium weighs almost half as much as steel) makes them indispensable in the aviation, automotive, and chemical industries, as well as in medicine. In recent years, there has been a trend toward using titanium not only in high-performance automobiles but also in the mass production of components for electric and hybrid vehicles. Ukraine has some of the world's largest titanium ore deposits (specifically, the Irshanske and Samotkanske deposits). Therefore, titanium production is of paramount importance for Ukraine.

Titanium alloys VT6 (Ti-6Al-4V) and VT9 (Ti-6Al-3Mo-2Cr) are among the most popular titanium alloys worldwide. While possessing high strength, they nevertheless have one significant drawback: low wear resistance (susceptibility to scratching and abrasive wear due to friction). Known surface hardening methods, such as nitriding and oxidation, form a thin phase film of titanium nitride or oxide, 2…3 microns thick, on the surface of the titanium alloy. This film transitions into a 20…30 micron thick layer hardened by interstitial elements (nitrogen or oxygen). However, this thin hardened layer also cannot significantly increase the wear resistance of the titanium alloy, especially under high specific loads. To improve the wear resistance of titanium alloys, physical vapor deposition (PVD) of a 2…20 µm-thick titanium nitride layer onto the surface is widely used. Even in this case, such thin coatings cannot provide significant wear resistance to structural components made of titanium alloys operating under increased specific loads. In recent years, there has been a trend toward increasing the wear resistance of titanium alloys through the use of thicker wear-resistant coatings formed by various methods (duplex coatings), which can be used at higher specific loads.

Nitriding of the VT9 titanium alloy was carried out in a reaction chamber with nitrogen at a partial gas pressure of 10⁵ Pa and a temperature of 850 °C for 4 hours. Physical vapor deposition (VPD) of titanium nitride and zirconium nitride with a thickness of 30 μm on the surface of the VT9 alloy was carried out using a Bulat-3 installation at an operating pressure of 6.65×10⁻³ Pa and a voltage of 800…1000 V. The specimen surfaces were pre-cleaned by discharge at a voltage of 1000…1500 V for 3 min.

To determine the tendency of the coatings to chip, their surfaces were indented using a diamond indenter with a conical surface, using a Rockwell tester, under a load of 100 kg.Conclusions. The duplex structure of the resulting coating consists of a diffusion-nitrided layer and a physical vapor deposition (PVD). The nitrided and PVD layers had approximately the same thickness (30 µm) and the total thickness of the two-layer coating was 60…70 µm. The microhardness of zirconium nitride-based PVD coatings was significantly lower than that of titanium nitride-based coatings. For example, the maximum hardness of a ZrN PVD coating was 2465 HV_0.1, while for TiN it was 2707 HV_0.1. The microhardness of PVD coatings deposited onto pre-nitrided titanium specimens were significantly higher than that of specimens without a nitrided underlayer. Thus, the maximum microhardness of PVD coatings increases by 50…150 HV_0.1.