Characterization of Thermal Spray Coatings by Instrumented Indentation and Scratch Testing

Thermal spray coatings have been used for several decades for enhanced wear, corrosion and thermal protection in various industrial domains. These coatings are routinely used in power plant turbines, aircraft engines, on pulp rolls in the paper industry and in many other applications where their mechanical properties have to be known.

This application report presents new methods for testing of mechanical properties of thermal spray coatings by means of instrumented indentation and scratch testing. Namely, the results of low load indentation and scratch tests on HVOF coatings will be presented in this application report.

Introduction

Thermal spray coatings have been used for several decades for enhanced wear, corrosion and thermal protection in various industrial domains. These coatings are routinely used in power plant turbines, aircraft engines, on pulp rolls in the paper industry and in many other applications where extensive wear or high temperature damage occur. The most common deposition methods of thermal spray coatings are plasma spraying (water or gas stabilized), high velocity oxy-fuel (HVOF), wire arc, flame spray and detonation gun.

To achieve the best functionality of the coatings one needs to know the relationships between the mechanisms of coating formation and its mechanical properties. While the mechanisms of formation of such coatings have been rather well investigated, detailed information on mechanical properties still remains difficult to obtain because of the heterogeneity of the coating (Figure 1).

Typical microstructure of WC-17Co HVOF-sprayed coating showing WC grains and binding metal matrix (Scanning Electron Microscope image).

Figure 1. Typical microstructure of WC-17Co HVOF-sprayed coating showing WC grains and binding metal matrix (Scanning Electron Microscope image)

Until now mainly macro scale methods such as four-point bending or microhardness at relatively high loads have been used for measurements of mechanical properties. Such methods measure ‘composite’ properties of the coating but they ignore the strongly heterogeneous structure composed usually of hard particles and a softer binding matrix. For adhesion and cohesion testing, the situation is even more complicated as one of the few standardized test is tensile testing by gluing or brazing two samples and then pulling them apart. Not only is the evaluation of coating adhesion by this method quite difficult but it is also limited by the tensile strength of the glue or braze.

The research project presented here deals with new methods for testing of mechanical properties of thermal spray coatings by means of instrumented indentation and scratch testing. Part I of the work presents the results of low load indentation and scratch tests on HVOF coatings. Part II will discuss the results of both low and high load indentation and cyclic indentation on HVOF and plasma sprayed coatings. Selected results of high load indentation on HVOF coatings are also mentioned in this part of the work.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.

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