Characterization of microstructure and hardness of PCVD deposited hard coatings

Ti_1-xAl_xN hard coatings have been synthesized by direct current (dc) plasma-enhanced chemical vapor deposition (PCVD). Dependences of aluminum content and annealing at elevated temperatures on the microstructure and hardness of Ti_1-xAl_xN coatings were investigated. The results show that plastic hardness measured by means of indentation test increases with x increasing up to 0.83 and then decreases. XRD measurements indicate that the coatings with x < 0.83 are fcc solid solution with 3 - 10 nm grain scale. When x = 0.83, relatively soft h-AlN phase is precipitated in Ti_1-xAl_xN coating, while the coating hardness dramatically decreases. Furthermore, the nano-crystalline structure and high hardness of the coating can keep up to 900°C, which indicates the synthesized coating having good thermal stability.