Revealing true porosity in WC-Co thermal spray coatings
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Technical Note
Revealing True Porosity in WC-Co Thermal Spray Coatings D.J. Nolan and M. Samandi (Submitted 15 November 1996; in revised form 28 February 1997) The principles underlying composite material behavior during metallographic preparation of coating cross-sections are generally not well understood. This study of the effect of extended fine polishing on apparent porosity shows that adequate polishing times, using a fine abrasive (3 ~tm) and low force, are required to remove prior deformation in the section surface and to reveal the true porosity of the underlying composite material. Insufficient polishing times can result in considerable underestimation of porosity. A model is described which proposes that the deformation induced in the material during grinding and polishing, even at low applied force, results in smearing of material into voids that exist in the plane of the section.
Keywords HVOFcoatings, metallography,porosity, WC-Cocomposites
1. Introduction Though much progress has been made in understanding the specific requirements for the proper metallographic preparation of composite thermal spray coatings, there are fundamental principles that are still poorly understood throughout the industry. One firmly established misconception that requires serious consideration is the tendency for producers to accept the metallographic preparation that provides them with the most dense or "best" looking coating, regarding evidence of porosity as being due to "particle pullout." The problem stems from the structural inhomogeneity of the composite materials, which can be composed of materials of quite different mechanical properties. For example, in the case of a WC-Co composite coating, the difference in properties of the component phases is considerable. Tungsten monocarbide is a very brittle material with a hardness between 1300 and 2200 Vickers hardness (HV), depending on crystallographic direction (Ref 1). The cobalt binder material is much more ductile and likely to have hardness below 300 HV, taking into account the hardening effect of the dissolved tungsten and carbon and the deformation induced during spraying. A further complication is the hardness of the coated substrate material (usually mild or stainless steel), which is usually less than 250 HV (Ref 2). Given the structural complexity of this system, great care must be taken in preparing cross sections to achieve a true representation of the microstructure. Traditional methods of grinding and polishing, incorporating SiC abrasive papers, are generally not recommended for WCCo coating layers due to low cutting and cost efficiency and smearing of the coating material into pores (Ref 3). Other reported disadvantages include poor edge retention and significant hard particle pullout (Ref 4). Bonded diamond grinding disks have produced better results with better edge retention and DJ. Nolan and M. Samandi, Department of Materials Engineering, University of Wollongong, North Wollongong 2500, Australia.
422--Volume 6(4) December 1997
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