Formation mechanism of electrical discharge TiC-Fe composite coatings

J.W Murray,S.J Algodi,M.W Fay,P.D Brown,A.T Clare

doi:10.1016/j.jmatprotec.2016.12.011

J.W Murray, S.J Algodi + Show 3 more

Open Access

https://doi.org/10.1016/j.jmatprotec.2016.12.011

Copy DOI

Abstract

Comparison of electric discharge (ED) processed single deposit and continuum TiC-Fe cermet coatings, formed from a sacrificial powder metallurgy TiC tool electrode at negative polarity, on 304 stainless steel, provided insight into the ED coating (EDC) formation mechanism. A deposit from a single spark event was dominated by TiC, phase separated from a ∼2wt% Fe matrix, with strongly aligned grains and banded microstructure, indicative of solidification from the coating/substrate interface. Conversely, a continuum coating, subjected to ∼200 spark events per location, exhibited a more complex, banded microstructure, with a mixture of equiaxed and columnar TiC grains within a ∼30wt% Fe-based matrix, along with some concentrations of carbon from the oil dielectric. It is considered that each sparking event remelts previously solidified coating material, with or without further TiC particle incorporation, leading to gradual TiC dilution and the development of a TiC-Fe composite coating with increasing levels of substrate material forming the matrix.

Highlights

Electrical discharge coating (EDC) is a surface modification process used to produce coatings from tool electrodes, and/or powder suspended in a dielectric fluid, onto a target workpiece
We report on a cross-sectional transmission electron microscopy (TEM) investigation of a TiC-Fe cermet coating produced from a TiC tool electrode formed on a 304 stainless steel substrate using standard EDC processing parameters. 304 stainless steel is used commonly across a range of engineering applications and provides an appropriate template for the appraisal of EDC processing phenomena
This study has provided insights into the mechanisms of cermet coating development during conventional EDC processing of stainless steel using a TiC sacrificial tool electrode, through comparison of a single deposit with a continuum coating

Summary

Introduction

Electrical discharge coating (EDC) is a surface modification process used to produce coatings from tool electrodes, and/or powder suspended in a dielectric fluid, onto a target workpiece. The EDC process is thought to involve a complex mechanism of localised melting, on the scale of an individual discharge, of the workpiece along with material removal, material deposition from the tool electrode and intermixing within the near surface of the workpiece. Functional coatings are formed by the repetition of thousands or millions of individual discharges, depending on the total area of coverage. In this context, there is need for detailed microstructural investigations of EDC coatings, to gain improved understanding of electrical discharge coating formation mechanisms at the fundamental level

Results

Discussion

Conclusion