FRACTURE-MECHANICS APPROACHES TO COATING STRENGTH EVALUATION

The incidence of mechanical failure of flame sprayed molybdenum coatin g on piston rings prompted development of reliable methods of characte rising and improving bond strengths. A method of using the ASTM specim en with a small modification, to estimate fracture toughness propertie s of the coating is proposed. Flame spraying process parameters are op timised to produce coatings of high adhesive strength using Taguchi te chniques by using the modified ASTM specimen. Experiments are conducte d using four point bend specimen, to study the coating behaviour under complex stress conditions. The effect of coating thickness on the fra cture resistance of the interface is studied. Coating strength is char acterised in terms of fracture mechanics parameters fracture toughness and J(crit.); using modified ASTM and four point bend specimens, resp ectively. The J(crit.) values are determined by virtual crack extensio n technique, in conjunction with finite element analysis solutions. Th eoretical investigations by finite element analysis are conducted on t he test specimens to gain information about the stresses responsible f or failure initiation and the failure initiation zones. Copyright (C) 1996 Elsevier Science Ltd