Effect of Powder Oxidation on Microstructures and Mechanical Properties of Cold-Sprayed Nickel Coatings and Improvement by Post-spray Heat Treatment

This study investigated the effect of powder pre-oxidation on the microstructures and mechanical properties of cold-sprayed nickel coatings. The artificially pre-oxidized nickel powders at 200, 300 and 400 °C for 5 h show the resulting oxygen contents of 0.27, 0.36 and 0.41 wt.%, as compared to 0.21 wt.% in the feedstock powder. Microstructurally, the higher oxygen contents of the impact particles significantly increased in both the number and size of the pores in the as-sprayed coatings by using the pe-oxidized powders, as a result of the porosities of 0.7, 1.5 and 3.3% compared to 0.4% by using the as-atomized powder (natural oxidation condition). Mechanically, the increased oxygen contents of powders result in the reduced properties for the as-sprayed Ni coatings, as the microhardness of 263.2 HV_0.1, 245.3 HV_0.1 and 236.3 HV_0.1 and the tensile strength of 94, 76 and 61 MPa by using oxidized powders compared to those of 289.2 HV_0.1 and 208 MPa by using natural oxidation powder. In addition, post-spray heat treatment at 800 °C for 2 h effectively reduces the small-sized pores and nonbonded particle-particle boundaries within the coatings, which is attributed to a combination effect of annealing twins and dislocation slip during heat treatment. As a result, the microhardness significantly decreased to 135.3 HV_0.1, 126.7 HV_0.1, 124.5 HV_0.1 and 114.7 HV_0.1, while the tensile strength is increased to 210, 166, 133 and 117 MPa, respectively.