Neutron Diffraction Residual Stress Measurements in Suspension HVOF Sprayed Al2O3 and YSZ Coatings

During the thermal spray process, residual strain (or stresses) are formed within the coating/substrate due to phenomena resulting from processing conditions. The nature and magnitude of residual stresses in these coatings impact on their overall qualities and failure mechanisms. This investigation thus involves a nondestructive neutron diffraction residual stress evaluation of suspension high-velocity oxy-fuel (S-HVOF) thermal sprayed alumina and YSZ coatings onto 304 stainless steel substrates. S-HVOF spray is a high deposition efficiency process to deposit coatings from the submicron or nanometric feedstock. Neutron diffraction measurements were performed at the UK ISIS facility, using ENGIN-X pulsed neutron diffractometer to obtain a thorough thickness residual stress profile. The Z-scanning method was used to avoid pseudo-strains in the neutron diffraction measurements whereby the incident neutron beam/gauge volume was partially submerged and traversed vertically through the coating thickness. The residual stress in the alumina coating was compressive across its thickness while the stress changed from tensile to compressive in the YSZ coatings. The residual stress measurements were complemented by lab based x-ray diffraction residual stress measurement techniques. Microhardness and nanoindention of the coatings were also performed to gain a comprehensive understanding of the stresses in SHVOF sprayed ceramic coatings.