X-ray Residual Stress Measurement in Metallic and Ceramic Plasma Sprayed Coatings

Processing-induced residual stresses play an important role in the production and performance of thermally sprayed coatings. Their precise determination is a key to influence the coating properties by modification of process variables and to understand the processing-structure-property relationship. Among various methods for residual stress measurement, x-ray diffraction holds a specific position as being nondestructive, phase distinctive, localized, and applicable for real parts. The sin² ψ method is commonly applied for bulk materials as well as coatings. However, the results are often reported without sufficient experimental details and the method is used in its simplified form without justification of certain assumptions. In this investigation, the sin² ψ x-ray diffraction method was used to measure residual macrostress in plasma sprayed metallic (nickel, NiCrAlY, and molybdenum) and ceramic (ZrO₂ + 8% Y₂O₃) coatings. Reproducibility of the method was tested and the assumptions allowing its use are discussed and experimentally verified. For nickel coatings, a comparison with hole drilling and neutron diffraction measurements is presented. The influence of processing factors such as deposition temperature and coating thickness is studied and the results are discussed.