Study of the generation of residual stresses within yttriated zirconia coatings made by plasma spraying on metallic substrates.

Summary This study is devoted to the generation of stresses within yttriated zirconia coatings made by plasma spraying on stainless steel, cast iron and aluminum alloy metallic substrates. The experimental approach is articulated around the development of a device for in-situ measurement of stresses by the sag method operating in an industrial projection environment. This device allows to record the temperature and the curvature of the sample continuously. The differences of behavior related to the nature of the substrate, the speed of illumination as well as the temperature of projection were studied. For each condition, the thermal, quenching and residual stresses were measured, as well as the mechanical properties of the coating and its adhesion. When the spraying temperature is increased, the microstructural observations show an improvement of the contact quality, resulting in an increase of the quenching stress and of the mechanical properties of the coating. The influence of the kinematics is less obvious. Even if generally, a slowing down of the lighting speed induces a decrease of the mechanical properties of the coating and of the quenching stress, the microstructural observations do not clearly show this trend. Finally, the influence of the nature of the substrate seems to be decisive. The in-situ recordings indicate that during the spraying on steel, the stresses on the part are minimal. Under these conditions, the coating is only slightly damaged and the mechanical properties as well as the hardening stresses are maximal. Microscopic observations show a low level of porosity and/or cracking within the coating. Generally speaking, the average residual stress in the coating sprayed on steel or aluminum decreases with temperature, from tension to compression. On the other hand, in the case of cast iron, this residual stress is constant, of the order of 25mpa in tension. These evolutions could be confirmed by measurements resulting from the diffractometric and incremental hole methods. The latter technique also allowed, in the case of aluminum substrates, to highlight a tensile peak in the deposit near the interface. The amplitude of this peak decreases when steel and cast iron substrates are used. This property is therefore in agreement with the measured substrate-coating adhesion values, which appear minimal in the case of aluminum substrates.