RESIDUAL STRESS MEASUREMENT DURING PLASMA SPRAYING OF ALUMINA OR ZIRCONIA AND COMPARISON WITH A ID MODEL

A system, developed in the laboratory, allows to record in situ the deformation of a flat beam with a displacement sensor and so to analyse stresses formation during spraying and upon cooling with fixed or rotating substrates. For alumina coatings sprayed on XC38 forged steel substrates a systematic study of the preheating temperature and pass thickness has been performed. Preheating temperature was varied from 200°C to 700°C and pass thicknesses were either thin (3 to 25 µm) or thick (60 to 180 µm). In the latter case the splats within a pass remain in a molten state as soon as the deposited thickness is over 100-180 µm depending on spraying conditions. For thin passes the quenching stress σq increases with temperature and is maximum for 25 µm thick passes (up to 70 MPa at 600°C). For thick passes preheating temperature must be lower than 500°C and σq reaches a maximum of 55 MPa. The corresponding coating cross sections show the existence of macrocracks. The comparison of the experimental results with those calculated with a ID simplified model developed by Tsui and Clyne allows to determine, the Young's modulus Ed of the coatings. For thin passes Ed can reach 160 GPa while for thick passes it reaches its peak at about 70 GPa.