Residual stresses, adhesion and thermal sprayed coatings

Most coatings are applied with a specific aim in mind, such as improving the base material resistance to corrosion and wear, or providing a thermal barrier against high temperatures. These aims can obviously be achieved if the coating is properly bonded to the substrate. One important factor which contributes to failure of plasma sprayed coatings is the residual stress produced during its manufacture. This paper provides a review of the results on residual stresses obtained in different coating systems (NiCrAlY metallic bonding layer, a Cr ₂O ₃ ceramic layer, and a WC-Co cermet layer). To study the effect of the residual stresses on the properties of the coatings (particularly on the adhesion of coatings to their substrates), two techniques are used: a Vickers indentation test is performed at the substrate/coating interface (in order to determine, by a simplified model based on mechanics concepts, 'an interfacial toughness') and four point bending test assisted by an acoustic emission system.