Common Problems in Microhardness Testing

Microhardness testers are delicate instruments. Extremely light forces (typically from 10 to 1,000 g.) must be accurately applied, and the resultant impressions (some as small as 10 microns) must be precisely measured under high magnification. And so there are a number of problems inherent to all of them.

Three Basic Problems

Microhardness testing problems can be separated into three categories: accuracy, repeatability and correlation, and can be traced to five main causes: machine, operator, environment, sample preparation, and calibration.

Microhardness Testers

Microhardness testers use dead weights to create force. Unlike Rockwell hardness testers, these light loading devices (10 to 2,000gf) stack the dead weights directly on top of the indenter. This eliminates the error of magnification and a lot of other negatives, such as knife-edges and hanging weights. Other units utilize screw drives to apply force, and load cells to control the amount of force applied. These types have their own set of repeatability and durability issues. Generally, these force application systems are robust. However, issues of indenter stroke can create erroneous loads.

With most machines, load application is done in two speeds: a “fast speed” to bring the indenter close to the test piece and a “slow” speed to contact the work and apply the load. The “stroke” of the indenter is usually set with a measuring device. Once this “indenter tip to test surface” distance is set, the high power objective is focused on the test surface. Once properly focused, the operator is assured the work piece is contacted at the proper speed, and that impacting of the load has not occurred. It takes approximately 30 seconds for an instrument to make an impression, considering ASTM E384 standard dwell time of 15 seconds. This is part of the “slow” problem mentioned previously.

Alignment of the indenter with the objectives is critical when measuring case depths or just trying to accurately place an impression on a specific spot. Although the accuracy of the hardness value is not affected by this error, if the operator is measuring effective case depth, the distance from the edge of the sample may be wrong and result in an erroneous measurement.

Also, if the operator is attempting to make an impression on a particular grain, or in the center of a thin coating, misalignment can make this difficult, if not impossible, to accomplish. Knocking the indenter or the objective with the sample can cause misalignment, so care must be taken when loading samples or rotating the turret.