Influence of the Torch to Substrate Velocity and Resulting Temperature on the Residual Stresses in Alumina Plasma Sprayed Coatings

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 stress formation during spraying and upon cooling with fixed or rotating substrates. The beam is fixed onto a pair of knife edges by springs. The knife edges are disposed on a water-cooled rotating cylindrical substrate holder and the beam substrate (2 x 15 x 100 mm3) is parallel to the holder axis. The torch is moved back and forth parallel to the holder axis and the beam temperature is recorded by a thermocouple spot welded to it and also by an IR pyrometer. The influence of beam temperature for a given torch/substrate velocity on the residual stresses is studied for alumina and zirconia coatings. With fixed substrates a sharp increase of the residual stresses related to coating microstructure exists for a transition temperature around 600°C. It seems to correspond to a columnar growth throughout the layered splats. The effect of the torch to substrate velocity and so the pass thickness is studied too.