A modified shear test for adhesion evaluation of thermal sprayed coating

A modified shear test has been proposed to evaluate the adhesion strength of thermal sprayed coating. The modified shear test piece was cut to possess a semicircular notch at the corner of the protruded step. The effectiveness of the notch introduction to the test piece was investigated by means of photoelasticity observation using shear test pieces cut out of an epoxy-resin plate, finite element method (FEM) and practical shear testing of thermal sprayed coating. The specimen for the shear testing was prepared from a 75 Cr3C2/25 NiCr (w/w) cermet coating sprayed onto a mild steel by use of a high velocity oxy-fuel process. The photoelasticity observation and the calculation by FEM revealed that the introduction of a notch of a proper size reduced the stress concentration at the corner of the protruded step of a shear test piece. The adhesion strength evaluated from the shear test pieces with a notch increased with increasing the notch radius up to 0.3 mm owing to the reduction of the stress concentration. This means that the introduction of the semicircular notch of a proper size is valid for the evaluation of the adhesion strength of thermal sprayed coatings. The calculation by FEM has shown that the introduction of the notch is more effective for the coatings with larger elastic moduli compared with the substrate.