Measurement of Stress Development during HVOF Thermal Spray

Abstract

Direct and quantitative observation of the stress generation during HVOF spray is carried out by measuring the curvature of substrates in-situ during spraying. A high pressure HVOF gun is used to spray SUS316L, Hastelloy C and WC-12%Co powder onto SUS316L substrates. The observed curvature data indicate that there are 3 regimes of stress evolution during the HVOF spray: (1) generation of compressive stress on the substrate surface at the beginning of spraying, (2) stress buildup in the coating during spraying, and (3) superposition of stress due to the mismatch in the thermal expansivity between the coating and the substrate as the specimen cools down to the room temperature after fabrication. Compressive stress ranging from 70 to 400 MPa is observed in the second regime during the HVOF spray; the value depending on the powder materials and spray conditions. Microstructural observation reveals that a significant portion of the coatings consists of poorly molten particles. Beneath the coatings formed by the HVOF process, a thin layer of increased hardness exists within the substrate.