The Influence of Loading Rate and Hold Time on the Nano-mechanical Properties of γ-TiAl and Plasma Mo-Si-Ti Coating

Mo-Si-Ti coating was prepared on the surface of γ-TiAl by means of the double-glow plasma surface alloying technique at different soaking times. The coating had good surface quality and was primarily composed of (Ti, Mo) 5 Si 3 , TiSi 2 , TiSi, Ti 5 Si 3 , Ti 5 Si 4 , and MoSi 2 . It was observed that the thickness of the deposition layer increased with increasing holding time. However, the thickness of the diffusion layer did not increase further when the holding time exceeded 2 h. The hardness and elastic modulus of the Mo-Si-Ti coating first increased and then decreased with the increase in soaking time. Low crystallite size and a hard phase in the Mo-Si-Ti coating prepared at a soaking time of 2 h were responsible for its high hardness of 26.3 GPa. The results of the nanoindentation test with loading rates of 100-5000 μN/s and a holding time of 10 s showed that the loading rate did not affect the elastic modulus of γ-TiAl and Mo-Si-Ti coating. On top of that, γ-TiAl exhibited strain-hardening behavior with an increase in loading rate, and the Mo-Si-Ti coating exhibited strain-hardening behavior at low loading rates and strain-softening behavior at high loading rates. The results of the nanoindentation test with loading rates of 100-5000 μN/s and holding times of 2-10 s showed that the hardness of γ-TiAl and the Mo-Si-Ti coating varied with those parameters because of the difference in the plastic energy accumulation and release time. When the loading time was short, γ-TiAl exhibited strain-hardening behavior at low loading rates and strain-softening behavior at high loading rates. However, the Mo-Si-Ti coating exhibited strain-hardening behavior at all loading rates. The strain-hardening behavior of γ-TiAl was observed at all loading rates when the retention time was long. However, the Mo-Si-Ti coating exhibited strain-hardening behavior at low loading rates and strain-softening behavior at high loading rates.