A MODIFIED SHEAR TEST FOR ADHESION EVALUATION OF THERMAL SPRAYED COATING

A modified shear test has been proposed to evaluate the adhesion stren gth of thermal sprayed coating. The modified shear test piece was cut to possess a semicircular notch at the corner of the protruded step. T he effectiveness of the notch introduction to the test piece was inves tigated by means of photoelasticity observation using shear test piece s cut out of an epoxy-resin plate, finite element method (FEM) and pra ctical shear testing of thermal sprayed coating. The specimen for the shear testing was prepared from a 75 Cr3C2/25 NiCr (w/w) cermet coatin g 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 c oncentration at the corner of the protruded step of a shear test piece . The adhesion strength evaluated from the shear test pieces with a no tch increased with increasing the notch radius up to 0.3 mm owing to t he reduction of the stress concentration. This means that the introduc tion of the semicircular notch of a proper size is valid for the evalu ation of the adhesion strength of thermal sprayed coatings. The calcul ation by FEM has shown that the introduction of the notch is more effe ctive for the coatings with larger elastic moduli compared with the su bstrate. (C) 1998 Elsevier Science S.A. All rights reserved.