Evaluation of the Shearing Strength of a WC-12Co Thermal Spray Coating by the Scraping Test Method

Kenji Kaneko

doi:10.3390/coatings5030278

Abstract

This paper reports on an experimental and analytical investigation conducted into efficacy of the scraping shear-test method in estimating the shearing adhesive strength of a thermally sprayed coating. It was found that the critical average shear stress, the apparent failure strength of WC-Co thermal spray coating, depends on both the dimensions of the test piece and the loading position around the interface between the coating and the substrate. More specifically, the apparent critical shear stress decreased as the height and width of the test piece increased. In addition, the apparent critical shear stress increased with increasing coating thickness and with decreasing loading point distance measured from the interface. Consequently, the real adhesive strength of thermally sprayed coating could not be ascertained from these experimental results. Furthermore, most of the failure initiation points were inside the coating, as opposed to at the interface. This fact means that the results of the tests do not indicate the interfacial adhesive strength, but rather the shear strength of the coating. Three-dimensional finite element method (FEM) analysis showed that the distributions of the shearing stress at the loading points were virtually the same at failure, regardless of the dimensions of the test piece. These results suggest that the scraping test method needs a corresponding numerical analysis of the failure mode in order to produce reliable results and is not necessarily able to estimate the interfacial adhesive strength of thermally sprayed coating.

Highlights

Sprayed coatings have been applied to various important system parts, such as gas-turbine blades, combustion chamber walls, aerospace equipment, and the boiler tubes of industrial plants, all of which are used under severe conditions
Many test methods have been proposed to estimate the adhesive strength of thermal spray coatings for the purpose of comparing the relative adhesive strengths of various coatings
The objective of this paper is to clearly investigate these problems affecting the scraping shear test and estimate the efficacy of the testing method

Summary

Introduction

Sprayed coatings have been applied to various important system parts, such as gas-turbine blades, combustion chamber walls, aerospace equipment, and the boiler tubes of industrial plants, all of which are used under severe conditions. For precise design of such coated parts, it is important to establish the adhesive strength criteria for thermally sprayed coating systems. Many test methods have been proposed to estimate the adhesive strength of thermal spray coatings for the purpose of comparing the relative adhesive strengths of various coatings. Some of these test methods include tensile tests of the coated specimen [1,2], bending tests of the coated plate/ring [3,4,5,6], scraping shear tests of the coated ring [7,8,9], and the tensile pin-test [10]. A number of researchers have conducted lifetime evaluations [14,15,16,17,18,19]

Objectives

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Results

Conclusion