Sample Preparation Before Micro Vickers Hardness Testing

In most cases, samples are sectioned and mounted prior to micro Vickers hardness testing. The samples are then ground, sanded and polished to provide a test surface free of scratches and surface texture.

Sometimes production demands do not allow an operator to spend the time necessary to realize a proper finish. This often results in undulating surfaces, rounded edges that are difficult to discern, or surface imperfections that make it difficult to accurately measure the tips of the impression.

When etching samples, the surface of the part is chemically corroded, providing metallographic contrast. Although it helps define grain structure, heat affected zones in welds, total case depths and decarb layers, the etching of samples lessens the contrast between the test surface and the tips of microhardness impressions.

In many cases this diminished contrast can make it difficult or impossible to measure the impression. If microhardness testing is performed on an etched sample, it should only be the minimum etching required to visually discern the desired attribute.

Although most automated micro-hardness testing systems can automatically measure Vickers and Knoop impressions repeatedly on finely prepared samples, many systems are fooled by surface anomalies and changes in contrast. Systems that utilize “thresholding” techniques to determine the impression size are the most susceptible to these issues and exhibit the greatest amount of variation and error.