Avoid Microindentation Hardness Testing at Low Loads!

摘要

For many years, ASTM E384, Standard Test Method for Microindentation Hardness of Materials, has stated that the load range for microindentation hardness testing with both Knoop and Vickers indenters is 1 gf to 1,000 gf. But, is it realistic to consider using very low-test forces when the indents are measured with a light optical microscope? ASTM E384 also states that tests that produce a Knoop long diagonal or a Vickers mean diagonal of less than 20 urn should be avoided as the precision in measuring such small indents is poor. The standard recommends using loads no lower than 25 gf. This article shows that the Knoop test exhibits better measurement precision at loads of 200 gf and below because the long diagonal is 2.7 times greater in length than the Vickers mean diagonal length for the same specimen tested at identical loads. Knoop, however, does not produce constant hardness values as the load changes. Most microindentation hardness testers provide the user with a 10× objective used to find the area of interest for testing and one measurement objective, with 40× magnification being the most common. A few testers have offered 50× or 60× objectives to measure the indents. It is rare to find a tester with a multiple objective turret. If one can use a 100× objective with a numerical aperture of 0.95-while obtaining adequate image contrast-indents as small as 14.7 μm in length could be measured. But the challenge is to obtain acceptable image contrast at 1,000× magnification so that the indent tips can be clearly seen. Realistically, a minimum diagonal length of 20 μm is a more realistic limit. These two ASTM standards should eliminate recommending the use of test loads less than 50 gf for Vickers and less than 20 gf for Knoop.