Effect of oxides on wear resistance and surface roughness of ferrous coated layers fabricated by atmospheric plasma spraying

The objective of this study is to investigate the correlation of microstructure, wear resistance, and surface roughness in ferrous coated layers applicable to cylinder bores. Four kinds of ferrous spray powders, two of which were STS 316 steel powders and the others were blend powders of ferrous powders mixed with 20 wt.% Al2O3–ZrO2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coated layers showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coated layers, while Al2O3–ZrO2 oxides were mainly formed in the martensitic matrix for the blend coated layers. The wear test results revealed that the blend coated layers showed the better wear resistance than the STS 316 coated layers because they contained a number of hard Al2O3–ZrO2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during the wear process. The STS 316 coated layers showed the slightly worse wear resistance than the blend coated layers, but they showed the excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material and the surface roughness, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides.