Figure 1. Typical microstructure of WC-17Co HVOF-sprayed coating showing WC grains and binding metal matrix (Scanning Electron Microscope image).
For many years, thermal spray coatings have been employed for improved thermal, wear and corrosion protection in many industrial domains. Thermal spray coatings are routinely used in many applications including aircraft engines, power plant turbines and on pulp rolls in the paper industry, where high temperature damage or extensive wear occur. Plasma spraying (water or gas stabilized), wire arc, high velocity oxy-fuel (HVOF), detonation gun and flame spray are the most common deposition methods of thermal spray coatings.
In order to ensure optimum function of coatings, the associations between the mechanisms of coating formation and its mechanical properties should be known. Although the mechanisms for formation of these coatings have been extensively studied, obtaining comprehensive information on mechanical properties continues to be difficult due to the heterogeneity nature of the coating (Figure 1).
So far, 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 disregards the strongly heterogeneous structure containing a softer binding matrix and hard particles. The situation is even more difficult for cohesion and adhesion testing because one of the few standardized tests is tensile testing, by brazing or gluing a pair of samples and then pulling them apart. This method not only makes the evaluation of coating adhesion relatively difficult but it is also restricted by the tensile strength of the braze/glue.
This article presents a research project that deals with the latest methods for testing the mechanical properties of thermal spray coatings using instrumented indentation and scratch testing. The results of low load indentation and scratch tests on HVOF coatings are presented.
Thermal Spray Coatings: Heterogeneous Material
A key factor for determining the indentation parameters is the analysis of the grain size of thermal spray coatings. The properties of the individual splats or grains can be measured at very small loads of a few milinewtons, and when the load is increased a larger volume is involved which shows a ‘composite’ value of mechanical properties. This composite value ignores the material’s heterogeneity and until recently, this type of measurement has been routinely carried out. Yet, to better understand the relationship between the coating functionality and the deposition parameters, it is important to know the material’s properties on different scales. Measurement of these properties has been possible only lately using instrumented indentation and advanced automated matrix measurements.
In addition, due to nonstop recording of indentation depth and force, it is possible to calculate several other key characteristics of the material (besides hardness) such as elastic modulus and the plastic and elastic part of the indentation work. This provides a better insight about the elastic-plastic behavior of the coating, which can be closely associated with its wear and failure resistance.
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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.