Abstract
The incidence of mechanical failure of flame sprayed molybdenum coating on piston rings prompted development of reliable methods of characterising and improving bond strengths. A method of using the ASTM specimen with a small modification, to estimate fracture toughness properties of the coating is proposed. Flame spraying process parameters are optimised to produce coatings of high adhesive strength using Taguchi techniques by using the modified ASTM specimen. Experiments are conducted using four point bend specimen, to study the coating behaviour under complex stress conditions. The effect of coating thickness on the fracture resistance of the interface is studied. Coating strength is characterised in terms of fracture mechanics parameters fracture toughness and J crit; using modified ASTM and four point bend specimens, respectively. The J crit values are determined by virtual crack extension technique, in conjunction with finite element analysis solutions. Theoretical investigations by finite element analysis are conducted on the test specimens to gain information about the stresses responsible for failure initiation and the failure initiation zones.
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