Annealing effects on microstructure and mechanical properties of chromium oxide coatings

Reactive radio frequency magnetron sputter-deposited chromium oxide coatings were annealed at different temperatures and times. The influence of annealing temperature on the microstructure, surface morphology and mechanical properties was examined by X-ray diffraction, nanoindentation, pin-on-disc wear and scratch tests, respectively. X-ray results show that the chromium oxide sputtered at room temperature in low oxygen flux is primarily amorphous. Annealing below 400 °C did not cause much change, while annealing at higher temperature of 500 °C caused a significant change in microstructure and mechanical properties. Hardness increased from 12.3 GPa to 26 GPa, and the wearability improved with higher annealing temperature due to the formation of crystalline Cr₂O₃ phase, which occurs at 470 °C. Annealing time had little effect on mechanical properties and microstructure, although coating surface roughness increased with a longer annealing time. Coating adhesion was improved by annealing, due to residual stress relief and possible interfacial interdiffusion.