Investigation of Cr-Al composite coatings fabricated on pure Ti substrate via mechanical alloying method: Effects of Cr-Al ratio and milling time on coating, and oxidation behavior of coating

Cr-Al composite coatings were successfully fabricated on pure Ti substrate using mechanical alloying method. The coating consisted of an inner composite layer of coarse Cr particles and an outer composite layer of refined and highly homogenized microstructure. Effects of both Cr-Al ratio and milling time on the preparation of composite coating were investigated. The formation process of the coating was discussed. The high-temperature oxidation resistance was tested in air atmosphere at 850 °C using the cyclic method. The mass gain of the substrate was reduced during oxidation because of the as-prepared coating. The as-prepared coating could largely decrease the mass gain during oxidation. The surface morphology and the phase compositions of the oxidized coating provided clear evidences that the oxide was corundum-type α-Al2O3, which formed a dense and oxidation resistant layer and protected the inner coating from severe oxidation. The AlCr2 intermetallic phase was detected in the oxidized coating. The formation of α-Al2O3 was attributed to the pre-formation of Cr2O3, which was a structural template and stimulated the growth of crystalline α-Al2O3. The cross-section of the oxidized coating exhibited both densification and oxidation of the coating. The oxidation process of the as-synthesized coating was elucidated. The coating was considered to be protective, which could effectively improve the high-temperature oxidation resistance of the substrate due to the densification of coating and the formation of dense protective oxide films in and on the coating at high temperature.