The thermal spray coating industry relies extensively on the use of microhardness as a means of assessing coating quality. Indeed, apart from optical microscopy, it is the only technique in widespread use, and is of particular importance for characterising wear resistant coatings. The attraction of the technique is clear: It is a well-established laboratory procedure using readily available standard equipment. The technique has a history of application going back decades. The underlying scientific principles are simple, giving the operator a sense of security. Quantitative data are generated in a relatively short time. Necessary surface preparation is done anyway for optical microscopic examination. The data produced are sufficiently repeatable for conclusions based on it to have validity. There is sufficient reproducibility between different operators, that comparisons between materials tested by different laboratories or by different personnel at the same laboratory is possible. There is some utility to the technique. That is, that the statistics obtained correlate with some desired material property. Although the experimental work detailed below was performed on a WC Co sample, the inter-person repeatability is a function of indent size, and not of material tested per se. This can clearly be seen by examination of the reproducibility charts given in the appendix of the ASTM Standard for microhardness testing. Examination of these charts demonstrates that reproducibility deteriorates with decreasing load and increasing hardness of the material. There is thus a direct correlation between degree of reproducibility and indent size. The charts were produced from a round-robin conducted by the personnel of standards laboratories, working with samples of ferrous and nonferrous metals; materials that deform plastically to produce more idealised indents than those generally encountered in thermal spray coating materials. It follows from these charts that microhardness measurement as commonly conducted on hard materials such as cemented carbides, with loads of 300 g and less, result in the measurements being performed off-scale with respect to reproducibility. The low loads used in the thermal spray coatings industry, and indeed the use of microhardness rather than one of the macrohardness tools are largely due to geometrical considerations in consequence of the coating thickness. The standard test method for cemented carbides when geometry is not a consideration, is the Rockwell A scale, which uses a 60-kg load w4x. Microhardness was traditionally used for research purposes, such as showing anisotropy in single crystals, measuring the relative hardness of different grains within a composite structure or cast iron, and for following the depth of case-hardened layers. For these applications, the exact values generated are of less importance. The operator dependence demonstrated here show that the absolute levels of reproducibility in indentation measurement make the quantitative data obtained subjective on the operator to an unacceptable level. This challenges the widespread use of microhardness statistics for quality control purposes in the thermal spray industry, and the usefulness of the reported values as found in the literature. It should be stressed, however, that this paper does not relate to the issue of the reliability of microhardness testing as an empirical means for comparing and ranking different samples, when the same operator performs all measurements on the same equipment, and a sufficiently large, statistically representative sampling of microhardness indentations is analysed using correct statistical techniques. Alternative methods of measuring microhardness indentations, and alternative techniques for quantifying the hardness of these and similar materials are discussed.
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