Calibration of Microhardness Tester

Most microhardness testers lead a charmed life in comparison to Rockwell and Brinell testers. They usually wind up in laboratory environments, free from the dirt and oil that can plague other hardness testers. Consequently, microhardness testers tend to have long useful lives, with little to go wrong between calibrations.

Fortunately, most microhardness testers are very consistent in their ability to apply force, and, with the exception of load cell units, there rarely is an issue when it comes to calibration.

While measuring systems in micro-hardness testers vary widely, ranging from micrometer heads on an ocular, to optical encoders attached to a digital readout, they have one thing in common. They depend on being at a specific magnification, or distance from the objective (the ocular slides in/out to increase/decrease magnification). If not careful, measuring accuracy can be compromised.

To directly verify the measuring system, a stage micrometer is placed on the anvil, and the instrument’s measuring lines are place at a known distance on the stage micrometer. This distance is then compared to the unit’s measuring system. If they agree, all is well. If not, the ocular is moved in or out to increase or decrease the measuring distance.

Microhardness testers usually are indirectly verified against standard hardness blocks. As the resultant impressions are extremely small, these standard blocks can appear to last forever. But, if improperly handled, blocks can become scratched making them difficult to read, emphasizing the need to check multiple standard hardness blocks.

Since microhardness testers’ impressions reside in very thin sections of the test surface, it is important that test blocks are calibrated in the forces that are typically used. Test block values should only be considered at the forces at which they were calibrated. This means that it is inappropriate to use Rockwell blocks and convert them to Vickers or Knoop values.

Properly used and maintained, microhardness testers are invaluable tools. Taking the time to review the recommendations made in this article will lead to more credible results and better process control.