Use of neutron diffraction for stress measurements in thin and thick thermal sprayed coatings

Thermal spraying is a very widely used and cost-effective technique for the enhancement of surface properties which has already been exploited in numerous applications. The spectrum is vast: corrosion protection, wear-resistance and abrasion-resistance, thermal barriers, electric (dielectric) coatings, etc. For a long time stress has been recognized as a serious factor which influences the integrity and overall performance of coatings. Stress formation during thermal spraying is a complex phenomenon. Significant efforts have been made to better understand the microscopic mechanism of stress formation and to develop practical models for numerical prediction of stress distributions in coatings. Due to the high penetrating power of neutrons and spatial resolution in the millimetre and sub-millimeter range, neutron diffraction is, perhaps, the most versatile method for stress determination, and has been used extensively for experimental validation of theoretical predictions. Neutron diffraction residual stress results are presented here for coatings of different types to illustrate the capabilities of the technique: a thin (∼0.3 mm) Mo-Mo2C composite HVOF coating, several examples of millimeter-thick ceramic and metallic coatings, and thick coatings (∼10 mm) of iron made by spray forming.