An improved specimen geometry for ASTM C633-79 to estimate bond strengths of thermal spray coatings

ASTM Standard C633-79, “Standard Test Method for Adhesion or Cohesive Strength of Flame-Sprayed Coatings,” is widely used in industry and research for evaluating bond strengths of thermal spray coatings. Tests are conducted by applying the coating to the end of a cylindrical test specimen 25.4 mm (1 in.) in diameter by 25.4 mm (1 in.) long. The coating surface is bonded to an uncoated cylinder of the same material and geometry. The force required to pull the cylinders apart is recorded. The bond strength is calculated by dividing the maximum force by the area of the 1 -in. diameter cylinder assuming that the stress is uniform over the area where the debonding occurs.A combination of finite-element stress analysis and experimental stress analysis using strain gages was used to evaluate the stresses at the interface between the coating and substrate. Finite-element analysis of the standard specimen geometry shows that the maximum stress at the coating interface can be 25 % higher than the average stress. An elongated specimen was selected, constructed, and evaluated to produce the uniform stress distribution assumed by ASTM C633-79. Strain gage measurements and epoxy tensile tests have verified that the bond strengths measured with the elongated specimen provide better estimates of bond strengths than tests with the standard specimen.