Hardness tests are performed on myriad materials and at a variety of scales, such as Rockwell HRC, HRB and superficial scales. Microindentation, formally referred to as microhardness testing, is a method for measuring the hardness of a material using microscopic scale indentations.
Microindentation tests are the most difficult hardness tests to do automatically. Because of the light test forces frequently used for microindentation testing, the instruments used to apply the force must be more delicate and more precise than traditional Rockwell or Brinell testers. Light test forces are often required because of the size of the part. Gears for watches, for example, are too small for any other test method.
When doing a case-depth analysis, the indents many times have to be close together to detect the hardness change. In that case, microindentation tests are the only ones that will fit. The light test forces produce small impressions—the average indent ranges from 100 to 200 microns, or about 0.004 to 0.008 inch, around the size of a human hair.
The process that most requires microindentation testing is case-depth analysis of case hardened processes. Parts such as gears and bearings are heat treated so that the surface is hard enough for wear resistance, while the core material is softer for toughness. The heat-treating process typically creates a case hardness of HRC 55 approximately 0.05 inches thick. It is critical that the heat treater be able to verify that the correct levels of hard surface and soft core have been achieved. Typically this is done by performing a series of tests starting from the surface and progressing to the core of the sample. By correctly spacing the tests and plotting the results, it is possible to see and calculate the hardness shift that indicates the case depth.
For example, manufacturers of crankshafts for diesel truck engines often guarantee that their crankshafts can be reground two or three times during the useful life of the crank. When a crank is new and held in the engine block by soft bearings, the clearance between the crank and the bearing is small and critical. As the engine is used, the surface wears away until the clearance becomes too large. The engine then either has to be scrapped or rebuilt. When the engine is rebuilt, the crank’s worn surface is ground to a known smaller diameter—first regrinds are normally 0.02 inch smaller than the original. When they are assembled back in the engine block, they are fitted with undersized soft bearings to restore the clearance to its original amount.
To regrind a crankshaft, the case depth has to be correct. If the case depth of the crank is too little, the soft inner core can be exposed during a regrind. When the engine is put back in service, the soft surface will wear excessively and could have a catastrophic failure, destroying the engine. If the case depth is too hard or too deep the crank can fail because of fatigue, another catastrophic failure.
Modern Technology
Technology has made microindentation testing easier to do and with more accurate results. Digital testers were one of the first advancements, eliminating the need to manually calculate hardness values. Then, as personal computers (PCs) increasingly populated the work place, semiautomatic, preprogrammed, auto-traversing systems were developed to free the operator from the process of moving the sample between each test. A motorized stage moved the part to the desired position, and then the PC plotted the results and automatically calculated the case depth while printing a test report in the desired format.Because PC processing speeds and storage capacities increased, image analysis began to be used to automatically measure indent sizes.
With the advent of digital cameras, it became relatively easy to measure things using the camera. All digital cameras have pixel arrays. Each pixel can be either on or off. If a black and white image is projected on the pixel array, the pixels in the dark areas will be off and the light ones will be on. By counting the number off, the size of the dark spot on the image can be determined. The size of an indent determines the hardness value.
Lighting is needed to see the indent, and the intensity of the lighting can have a large effect on which pixel is on or off. The reflection of the surface also can change the lighting drastically. In a manual situation the operator would simply make necessary adjustments to suit his eye. Early image analysis systems did not allow for adjustments to be adequately made.
Although the initial systems suffered from pixel size limitations, as well as lighting problems, large pixels caused low resolution, and light variations caused shading errors. The latest generation of image analysis systems has overcome the drawbacks of the older units. They provide a reliable means for performing repetitive, high-quality testing, while eliminating the need for costly manual intervention. An automated turret dispels the inefficiencies of manual rotation. Samples can be set up and, at the click of the mouse, a complete indentation and measurement analysis can be automatically run.
The fully integrated autofocus mechanism of today’s microindentation systems operates quickly and accurately to allow the image analysis system to identify and record indent size and hardness measurement. All of this, coupled with flexible user-friendly software, gives the operator extensive capabilities in generating test patterns, completing the analysis and generating reports. An operator only has to locate the sample in the tester, specify the test pattern and press the start button.
Also designed to assist today’s user, a large number of predetermined patterns can be stored in the memory and recalled. The computer programs are easy to use, so that one-off tests can still be done efficiently without penalty.
After training, an operator can go between a case-depth study and a single test without missing a beat. The computer can automatically set up the tester by programming test force, indenter selection and optics magnification to suit predetermined test parameters. Because of this, the operator can frequently save 3 to 5 minutes per setup, even when doing one test.
Technology has finally caught up with microindentation testing, and productivity has increased because of the implementation of automation. Most automatic applications run all day, in as many as three shifts per day. The actual test time is shorter by about 10%, because the test report is generated automatically. A big savings results from freeing the operator to do other tasks while the tests are run. This could result in 50% of his time for other jobs.
Microindentation is no longer the most difficult or time-consuming hardness test performed. The return on investment is hardly microscopic—depending on usage, but 1 year is normal, and the ability to perform precise, delicate hardness tests ensures even the smallest parts and components will deliver large-scale performance. Q
Tech Tips
- The process that most requires microindentation testing is case-depth analysis of case hardened processes.
- The instruments used to apply the force must be more delicate and precise than traditional testers.
- Because PC processing speeds and storage capacities increased, image analysis began to be used to automatically measure indent sizes.
