The purposes of the present study are firstly to establish a reliable experimental method to evaluate the adhesion strength and secondly to reveal the relationship between the adhesion strength and the residual stress in the coatings. SUS304 stainless steel coatings were deposited on low carbon steel substrates by HVOF spraying. Two types of samples were fabricated. One is a cylindrical shape for the adhesion test and the other has a rectangular dimension for residual stress evaluation. The tensile adhesion tests were carried out by modifying the conventional test method. Circumferential interface cracks were introduced by putting carbon thin layer on the substrate before spraying, and the edge of the substrate was masked to prevent undesirable deformation during spraying. The interface fracture toughness was estimated from the load at failure and the width of carbon area in radial direction (corresponding to the crack length) by finite element analysis. The residual stress was evaluated by measuring the relaxed strain at the substrate surface for reduction of the coating thickness. Thinning of the coatings was performed by polishing. It was possible to evaluate the adhesion strength of SUS304 steel coatings with good reproducibility for the samples deposited under various spray conditions. The correlations between adhesion strength and residual stress will be discussed.

A new method for evaluating the tensile adhesive strength of thermal sprayed coatings has been developed using a test specimen that incorporates an artificially introduced circumferential crack to control the stress intensity factor at the crack tip along the interface between the coating and the substrate. FEM-analysis is carried out to calculate a correction factor and the stress intensity factor for the test specimen. When the results of tensile test are sorted out using the stress intensity factor at which failure occurred, constant values should be obtained regardless of changes in geometry such as crack length and the diameter of test specimen. The method was first applied to air plasma spray (APS) coatings and then to HVOF sprayed coatings. Effects of substrate preparation such as surface roughness and preheating temperature on the resultant coating adhesion were studied experimentally.