# Study on Wear Resistance of Ti-6 Al-4V Alloy Composite Coating Prepared by Laser Alloying
## 3 Results and Discussion
### Impact of Temperature Variation on Friction and Wear Properties
#### 3.3.1 Effect on Friction Coefficient  
Figure 12 shows the friction coefficient of Ti6 Al4V alloy and alloying coating at different temperatures. In Figure 12a, the friction coefficient of Ti-6 Al-4V alloy fluctuated in the range of 0.41-0.46 in the stable stage at room temperature. When the temperature rose to 350 \({}^{\circ}\)C, the friction coefficient of Ti-6 Al-4V alloy decreased to 0.40-0.42, and at 700 \({}^{\circ}\)C, the friction coefficient of Ti-6 Al-4V alloy was less than 0.40.  
Figure 11: Wear cross-section trajectory curve of alloying coating under different load.  
Figure 12: Variation curve of friction coefficient of Ti-6 Al-4V alloy and alloying coating at different temperature. (**a**) Ti-6 Al-4V alloy; (**b**) alloying coating.  
Figure 12 shows that, as the temperature increases, the friction coefficient of Ti6 Al4V alloy during friction and wear gradually decreases, because, as the temperature increases, the viscosity of the metal increases, the material softens, and the friction resistance becomes smaller, while the surface of the Ti6 Al4V alloy is oxidized, and the oxide film is separated from the wear debris formed under the load, and accumulates on the grinding surface, entering the pits or furrows of the wear scar, acting as a lubricant, and reducing the friction coefficient.  
When the temperature rose to 700 \({}^{\circ}\)C, the friction coefficient of Ti6 Al4V titanium alloy fluctuated greatly. The friction coefficient at the initial stage is large and the stability is good. As the wear time increases, the friction coefficient decreases, but after 13 min, the friction coefficient has a non-monotonic changing trend with time. Because the Ti6 Al4V titanium alloy is not fully oxidized at the initial stage, similar to the wear mechanism at 350 \({}^{\circ}\)C, but with the extension of the wear time, the Ti6 Al4V titanium alloy is severely oxidized. When friction wear is caused on the grinding ball and the oxide layer, the friction coefficient increases, and then the oxide film will squeeze and deform under the load, acting as a lubricant, and the friction coefficient will be reduced, so the fluctuation is larger.  
As shown in Figure 12b, at room temperature, the coefficient of friction of the coating was relatively stable, maintaining a range of 0.37 to 0.40. When the temperature rose to 350 \({}^{\circ}\)C, the friction coefficient of the coating was lower than the value at room temperature in the initial stage of wear. After the wear lasted for 10 min, the friction coefficient began to increase beyond the value at room temperature. Because with the prolongation of wear, the Al3 Ti and Ti5 Si3 phases on the surface of the coating are oxidized to form oxide films of Al2O3, SiO2, TiO2. The oxide film is very thin and is mainly Al2O3. Al2O3 is a brittle oxide, which breaks and flakes under the pressure of the grinding ball to form abrasive debris, and then contacts the grinding ball with the alloyed coating. Due to the interaction between the hard phase of the grinding ball and the coating surface and the abrasive debris, the friction coefficient becomes larger, which is manifested by the wear mechanism of abrasive wear, oxidative wear, and adhesive wear.