Comparison Standards for Microhardness Value
Microhardness tests measure the hardness of materials by measuring the resistance of a sharp indentation on the surface of a material to penetration of an indenter. Microhardness testing is often used to characterize the mechanical properties of metals, alloys, plastics, and coatings, as microhardness values correlate well with other common mechanical properties such as yield strength. While the most significant advantage of microhardness tests is the low amount of material needed to do the test, a significant limitation of the tests is the variability of measurements, as a few test locations may not represent the entire microstructure of a material. However, there are standards in place to help ensure accurate and fair comparisons of measurements; these standards address the treatment of the sample before and after testing, the differences between Rockwell, Knoop, Vickers, and other scale tests, and their correlation to other tests such as ultimate tensile strength.
Prior to beginning any microhardness tests, the sample should be carefully prepared for testing. It is important to ensure that the surface of the sample is clean and free of any defects or contamination, as any debris can affect the results. Additionally, the surface should be polished to a specific grade, depending on the material being tested, as this helps minimize the variability of measurements. Environmental conditions should also be taken into account; the temperature and humidity of the testing environment should be consistent and within the material’s tolerance for those conditions. This can be achieved through environmental chambers and humidity boxes, although the use of these may not be always be necessary.
The types of microhardness tests can also affect the accuracy of comparisons. For example, different Rockwell tests may have different maximum test loads and indenters; this means that a Rockwell B test may have different results than a Rockwell C test. Additionally, while Rockwell tests use a diamond indenter, Knoop and Vickers tests use differently shaped diamond indenters, so a Vickers measurement on the same material may be slightly different than a Knoop measurement. It is also important to consider other tests when comparing microhardness values; the ASTM defines “standard” ultimate tensile strength values for many metals and alloys, so these values can be used to compare different microhardness measurements.
Once all tests are completed, the results must be carefully analyzed. It is important to compare not just the average of all tests, but also the minimum and maximum values as these can give an indication of the spread of measurements. Additionally, outliers must be identified and considered; these are measurements which are significantly out of range and can have a significant effect on the average because of their size. A statistical analysis of the results can be useful in identifying any outliers or trends in the measurements.
In conclusion, comparison of microhardness values is a complex process. Careful preparation of the sample and consideration of the environment and type of microhardness tests must be taken into account for accurate results. Additionally, careful analysis of the results must be done to ensure that fluctuations or outliers are accurately identified and taken into account. By following these guidelines, accurate comparisons can be made between different microhardness tests and between microhardness and other tests, such as ultimate tensile strength.
