Through-the-thickness residual stress analysis by neutron diffraction in inoxidizable martensitic steel samples with and without tungsten carbide coating

Abstract

The residual stress (RS) induced in the substrate by thermal spray-coating plays a significant role in the behaviour of coated materials. The characterisation of High-Velocity Oxy-Fuel (HVOF) coatings through RS determination justifies various cases of failure or bad performance due to the delamination of coating which is not easily interpretable through more traditional microstructural analyses and mechanical tests. Two 13Cr4Ni inoxidizable martensitic steel samples, one having a thermal spray (WC–12% Co) HVOF coated surface, the other a shoot peened surface, were analyzed by neutron diffraction (ND) in order to determine the real RS depth-profile and investigate the effects of the thermal process on the considered materials. The stresses resulted in the range from − 80 to 50 MPa for the uncoated sample, while for the coated sample they increased gradually from a slight compressive status in correspondence of the uncoated surface, to greatest values of ~ 250 MPa in correspondence of the coating. A traditional X-ray investigation on the coated surface measured compressive RS values σ = − 550 ± 40 MPa, showing that ND investigation is very suitable for critical applications, where a much greater stress situation could be present at a diverse depth. The results achieved yielded trends that can be used in monitoring of the coating characteristics, in particular adhesion, thereby confirming the relevance of thermal stresses induced by HVOF coating processes.