Mechanisms of Adhesion of Tungsten Coating to Mild Steel Substrate

Abstract

This paper describes the measurements of adhesive strength and metallographic observations of coating films obtained by means of wire explosion spraying, together with some considerations on the mechanisms producing the adhesive bonds on mild steel substrate. The sprayed materials were tungsten, molybdenum and piano wire. The critical shearing stresses of adhesion measured by the shearing test specially designed, ranges in 17∼26 kg/mm² in the case of tungsten and molybdenum and about 25 kg/mm² in the case of high carbon steel coated on the substrate of mild steel. These values are about 5 times as large as those obtained by flame spraying. The mechanism of adhesion of coating to mild steel substrate is studied in the case of tungsten coating. The high strength of adhesion is attributed to the formation of welded zone between coating and substrate which is observed by electron microscope and electron probe X-ray microanalyser (EPMA). The mechanisms are explained by means of high impinging velocity and high temperature of sprayed particles which are characteristics of wire explosion spraying. Theoretical considerations attempt to explain the mechanisms of formation of the structures and the adhesion properties.