It is generally known that plasma sprayed coatings exhibit rather a low strength thanks to their characteristic microstructure with porosity and microcracks. To determine the role of varying types of deformation in different parts of the coatings profile, 1.8 mm thick chromia coatings on a steel substrate were made using WSP plasma spray. Stress gradients were then measured "in-situ" on a four-point bending device during bend deformation of the coated beam using neutron diffraction. Only compressive loading in coatings increased the resulting stresses. From the plot of stresses vs. applied strain (linear dependence) the Young’s moduli of the substrate and the coating were determined. Both values agree well with those obtained by mechanical testing. In general it is believed that, in a simplified way, interlamellar voids (cohesion defects) roughly parallel to the substrate decrease the tensile strength in the direction perpendicular to the substrate, while intralamellar cracks, roughly perpendicular to the substrate, significantly affect the Young’s modulus of coating. It is supposed that the tensile deformation of coatings opens the cracks and no stress increase is observed. On the other hand the applied compressive deformation in the coating closes the intralamellar microcracks and the internal stresses increase, as suggested by obtained results.

Properties of thermally sprayed coatings, including residual stress, are controlled by various parameters of the spraying process. This study is focused on three thermal spraying techniques with significantly different particle temperatures and velocities. These are plasma spraying, twin wire arc spraying and high velocity oxy-fuel spraying. For each method, in-flight particle diagnostics was performed. Through-thickness residual stress profiles in Ni+5%A1 coatings on steel substrates were determined nondestructively by neutron diffraction. The stresses range from high tensile in the plasma sprayed coating to compressive in the HVOF one. Various stress generation mechanisms, including splat quenching, peening, and thermal mismatch, are discussed with respect to process parameters and material properties.

Neutron diffraction is a promising tool for the investigation of residual stresses in thermally sprayed coatings. In principle, the neutron diffraction method has several distinct advantages over other methods. 1) It is possible to perform triaxial stress analysis throughout the thickness of both the coating and the substrate without material removal. 2) The stress can be determined in all phases of a multi-phase coating. 3) Repeated measurements can be performed on mechanically or thermally fatigued specimens. 4) Stress concentrations and shape/edge effects in actual parts can be located. In this paper, these unique capabilities will be reviewed first. In the second part of the paper it will be shown how the analysis of these coatings differs from experimental analysis of bulk materials. Finally, the analysis of the stress distribution in plasma sprayed NiCrAlY and ZrO 2 +Y 2 O 3 thermal barrier coatings in as-sprayed and annealed states will be presented and discussed.