Residual Stress in HVOF Thermal Sprayed Coatings(1st Report). In-situ Measurement of Stress Generation during HVOF Thermal Spraying.

Generation of residual stresses in high velocity oxy-fuel (HVOF) thermal sprayed coatings was studied by measuring the curvature change of substrates in-situ during spraying. 316L stainless steel, Hastelloy C, and WC-12%Co powders were sprayed by a HVOF gun powered by the combustion of kerosene and oxygen. It was found that the substrate curvature changed through 3 stages : 1) abrupt change at the onset of spraying, 2) continuous change during spraying, and 3) continuous smooth change during cooling after spraying. They correspond to 1) generation of stress within the surface layer of the substrate, 2) within the coating, and 3) superposition of macroscopic thermal stress. The stress during spraying was compressive and could be as much as 400 MPa, depending on the powder material and spray conditions. SEM observation of splats collected on polished substrates revealed that the majority of HVOF particles were not fully molten before impact. Therefore, HVOF sprayed coatings consisted of much thicker and plastically deformed particles than plasma sprayed coatings. The microhardness of the surface layer of the substrates beneath the HVOF coatings was found to be increased significantly whereas no such change was observed for plasma sprayed coatings. These results indicate that the plastic deformation caused by impingement of poorly molten particles at high velocity is the cause of compressive stress during HVOF thermal spraying.