Here are some suggestions for successful microhardness testing, including some common errors that often occur.

1. The optical measurement of the indent is the most common source of errors.
     a. Make sure the magnification is correct. The image should fill from 50-75% of the total field of view.
     b. The indent is actually slightly larger than you can see due to the limitations of the microscope. Practice by reading the calibration indents on certified test blocks.
     c. Adjust the optics for maximum contrast.
     d. Avoid vibration.

2. The sample surface preparation is important.
     a. The smaller the indent, the better the required surface finish.
     b. Make sure that there is no dirt or other foreign material on the top or bottom surface of the sample or the indenter.
     c. Make certain that the surface preparation does not affect the hardness of the test point.

3. Sample mounting is important.
     a. The sample must be mounted so that the test point is perpendicular to the indenter.
     b. The sample must not move or deflect during the test force application. Avoid excessively large loads too close to the edge of the sample (springboard effect).
     c. If measuring near the edge of the sample, the type of mounting material may influence the readings.

4. Verify the tester calibration frequently using certified traceable test blocks.

5. Make certain that the test force application and dwell times are correct for the samples you are testing.
     a. Materials that exhibit significant cold-flow characteristics may require a longer dwell time. Check the test specification.
     b. Location of the indentation with respect to the edge of the sample and proper spacing of the indentations must follow the requirements specified in ASTM E384.

6. A microindentation tester is a delicate instrument. Make sure that there is no vibration or other environmental conditions present that could affect the results.

7. Make certain that you understand the test results. Knoop and Vickers test results are considered test force independent. However, for test forces below 500 g, the results can vary significantly. For example, compared to 500 g results, Knoop hardness at 25 g can go up by 200 points while the Vickers value could go down by 50 points on the same sample, due solely to the test force difference.

8. Remember that conversion to Rockwell “C” values is only valid when using 500-gf loads. Refer to ASTM E140.

9. When testing thin parts, make sure that the thickness of the sample is at least 10 times the depth of the indent.

10. Always follow the appropriate ASTM requirement, typically ASTM E384, ASTM E92 or ISO 6507.

 

We will present our final thoughts on this subject next time in Part 8.

 

References

  1. Stone, Alan and Daniel H. Herring, “Practical Considerations for Successful Hardness Testing,” Industrial Heating, April 2006.
  2. Lysaght, Vincent E., and DeBellis, Anthony, Hardness Testing Handbook, American Chain and Cable Company, 1969.
  3. Wilson Instruments Division, Instron Corporation, Norwood, MA (www.wilsoninstruments.com)
  4. Midea, S., “Brinell Testing,” HOT TOPICS in Heat Treatment & Metallurgy, Vol. 2 No. 12, December 2004.
  5. Midea, S., “The Basics of Microindentation Hardness Testing,” HOT TOPICS in Heat Treatment & Metallurgy, Vol. 1 No. 2, December 2003.
  6. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).