Sliding wear resistant properties of C.P. titanium and SP-700 alloy surface-hardened by Ar-5%CO gas were evaluated using a counterpart material of a bearing steel, JIS SUJ-2 steel and Nishihara type of sliding wear testing machine. In the latter, two disk specimens were rotated at different rotating speeds under a given compressive applied load, yielding a sliding ratio of 20 %. Wear tests were repeated intermittently for several times, and a respective test time period in each series of wear tests was primarily varied. The mass loss in both disks was measured after each test. Wear resistance of annealed C.P. titanium without surface hardening was inferior to that of annealed SUJ-2 steel, but surface hardened C.P. titanium resulted in superior wear resistance over quench-tempered SUJ-2 steel with a hardness of 720 in Hv. Observation results of worn surfaces in both disks indicate that preferential wear occurred in the convex region of a furrow-like pattern formed by a lathe machining, resulting in a reduction of surface roughness values with wear progress. When a respective test time period was extended to 21.6 ks, adhesive wear took place between worn surfaces in both specimens, and the mass loss ratio in titanium disk increased at a much higher rate compared with that of a respective test time period of less than 14.4 ks. The steel debris torn off from worn surface of SUJ-2 steel disk was observed to adhere to the worn surface in surface hardened C.P. titanium disk. Wear resistant property of surface hardened SP-700 alloy was also superior to quench-tempered SUJ-2 steel.KEY WORDS: surface hardening; sliding wear resistance; C.P. titanium; SP-700 alloy; SUJ-2 steel; worn surface; adhesive wear; surface roughness.
© 2008 ISIJimproved by the increase of hardness in base alloys or in the surface layer of metals or alloys. The former is basically performed by combination of alloy designing and heat treatment, but it is not always useful in titanium materials because the attainable maximum hardness in titanium alloys is very limited compared with steels. The latter is achieved by application of various surface engineering technologies, which have been mostly studied in titanium materials. In fact, various surface engineering technologies such as anodizing, shot peening, nickel or hard chromium plating, various diffusion treatments or plasma spray have been investigated for improvement of wear resistance of titanium or its alloys. 3,8,9) Among them, surface hardening using oxygen gas, which is a diffusion treatment, has been most widely studied in titanium materials, and the wear resistant property of C.P. titanium or Ti-6%Al-4%V alloys surface-hardened by this method was evaluated.10-13) Thermal oxidation for surface hardening adopted in these studies was mostly conducted at 1 173 K, which accompanied with marked oxidation. To solve this problem, Dong et al. developed the oxygen boost diffusion process for the deepcase hardening of titanium alloys, where the oxide layer formed during thermal oxidation was utilized f...