The following questions and comments have been asked by readers of our recent series of blogs on Hardness Testing (Parts 1-8). Special thanks to Debbie Aliya (Aliya Analytical, www.itothen.com), George Vander Voort (Vander Voort consulting, www.georgevandervoort.com) and Alan Stone (Aston Metallurgical Services Company, www.aston-met.com) for their contributions to this subject.
Comments & Questions – (Part Seven, Microhardness Testing Tips)
Applied load: It affects microhardness numbers (unlike Rockwell scales).
Comment 1: It is important to remember that ASTM E140 only recognizes microhardness conversions with minimum loading of 500 grams.
Comment 2: I think that the above mentioned hardness testing book says that Vickers can be converted from 300-gram loads and Knoop you are correct at 500 grams.
Vibration dampening: A microindentation tester is a delicate instrument. Make sure that there is no vibration or other environmental conditions present that could affect the results!
Comment 1: Closing or slamming of doors, even resting and removing one's elbows from the bench on which the tester is located can cause problems.
Comment 2: Testers are even more sensitive to vibration as the test load is decreased.
Test result interpretation: Make certain that you understand the test results.
Comment 1: Knoop and Vickers test results are considered test force independent. For test forces below 500 grams, however, the results can vary significantly.
Comment 2: Compared to 500-gram results (for example), Knoop hardness at 25 grams 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.
Comment 3: Interesting. I have always used mainly Knoop so did not realize that the Vickers went the other way.
Response: A partial answer as to why test forces below 500 grams have significant variation is that the application of a smaller load often results in measurement errors.
Testing thin parts: Make sure that the thickness of the sample is at least 10 times the depth of the indent.
Comment: What else can contribute to inaccurate readings?
Response: Believe it or not, the absence of common sense when conducting hardness testing is the major cause of inaccurate data. This applies to the hardness testing of parts that fail to meet the 10 times the depth of indentation rule.