Processing effects on residual stress and related properties of thermally sprayed coatings

Residual stress is an inherent feature of thermally sprayed coatings, originating from the high-temperature production process. It affects the deposit integrity, properties and performance. Therefore, it is necessary to develop basic understanding of the stress evolution and the key factors that affect its magnitude, sign and gradients. Correlation of stress to processing parameters and deposit properties is essential for process optimization and control of the deposit properties. The following aspects were addressed in this work: identification of stress origins, development of accurate and reliable stress measurement techniques and effects of key processing parameters on stress, deposit structure and relevant properties.;Four representative types of materials of practical interest were used in these studies: two ceramics---yttria-stabilized zirconia and alumina---and two metals---nickel alloys and molybdenum.;Several methods for stress determination were used: x-ray diffraction, neutron diffraction and in-situ curvature method. For the latter, a new instrument was designed and built, with several improvements over previous concepts. The methods were found to be complementary, in the nature of information provided by each of them.;The evolution of stress was studied on both splat and coating level. The effects of coating thickness were observed as the deposit was built up. The effects of the following processing parameters were studied for plasma spraying: deposition temperature, particle energy and deposition rate. The quenching and thermal stress contributions to the residual stress were also separated.;Deposition temperature was found to be the most influential processing factor affecting the stress level and deposit properties. Different coating materials have different proportion of quenching and thermal stress, depending on their thermal expansivity, brittle/ductile nature and deposition temperature. Since for most material combinations, the thermal stress is compressive, while the quenching stress is tensile, one can vary the stress over a considerable range---from tensile to compressive---by varying the deposition temperature within reasonably achievable limits. In combination with other deposition parameters, one can optimize the deposit stress and other properties simultaneously.