Vickers and Knoop hardness test methods are defined in the following standards: ASTM E384 and ISO 6507. It is recommended that operators have a current copy of the relevant standards, which are updated regularly. In the Vickers hardness test, a load is applied smoothly, without impact, forcing a diamond indenter into the test piece, as illustrated in Figure 23.6. The Vickers diamond produces a square based pyramidal shape with a depth of indentation of about 1/7th of the diagonal length. The indenter is held in place for 10 to 15 seconds and then fully unloaded. The physical quality of the indenter and the accuracy of the applied load (which are defined in E 384) must be controlled in order to get the correct results. The two impression diagonals are measured, usually to the nearest 0.1μm, and averaged. The Vickers hardness (HV) is calculated using:
Figure 23.6. Schematic of the Vickers indenter and the shape of an impression.
HV = 1854.4L / d2
Where the load L is in gf and the average diagonal d is in μm (this produces hardness number units of gf/μm2 although the equivalent units kgf/mm2 are preferred; in practice the numbers are reported without indication of the units). Tables or automatic electronic or imaging measurements are a more common and convenient way to generate Vickers hardness numbers.
Since the 1960s, the standard symbol for Vickers hardness per ASTM E 2 and E384, has been HV. This should be used in preference to the older, obsolete symbols DPN or VPN. Rigorous application of the SI system results in hardness units expressed not in the standard, understandable kgf/mm2 values but in GPa units that are meaningless to most engineers and technicians. ASTM recommends a ‘soft” metric approach in this.
In the Vickers hardness test, it is assumed that elastic recovery does not occur once the load is removed. However, elastic recovery does occur, and sometimes its influence is quite pronounced. Generally, the impression (Figure 23.7) appears to be square, and the two diagonals have similar lengths. As with the Brinell test, the Vickers hardness number is calculated based on the surface area of the indent rather than the projected area. If the impression shape is distorted due to elastic recovery (very common in anisotropic materials), Figure 23.8, should the hard- ness be based on the average of the two diagonals? It is possible to calculate the Vickers hardness based on the projected area of the impression, which can be measured by image analysis. While rigorous studies of this problem are scant in the literature, the diagonal measurement is the preferred approach even for distorted indents, at this time. The Vickers hardness test has two distinct force ranges, “Microindentation Vickers” (10 – 1000g) and “Macroindentation Vickers” (1 – 100kg), to cover all testing requirements. The indenter is the same for both ranges; therefore Vickers hardness values are continuous over the total range of hardness for met- als (typically HV100 – HV1000). Because the shape of the Vickers indentation is geometrically similar at all test loads, the HV value is constant, within statistical precision, over a very wide test load range as long as the test specimen is reasonably homogeneous.
Microindentation Hardness Testing
Microindentation hardness testing, more commonly (but incorrectly) called microhardness testing, is widely used to study fine scale changes in hardness. Although the term “microhardness” is generally understood by its users, the word implies that the hardness is extremely low, which is not the case. The applied load and the resulting indent size are small relative to bulk tests, but the same hardness number obtained. Consequently, ASTM Committee E-4 on Metallography recommends use of the term “microindentation hardness testing” which could be given the acronym MHT. The only difference between standard Vickers testing and micro-indentation testing is the lower loads (<1kg) used. The indents made in the material are therefore smaller, and this means that more localized areas of the material can be assessed.
For example, heat treaters have utilized the technique for many years to evaluate the success of surface hardening treatments or to detect and assess decarburization, by measuring hardness at finely spaced intervals from the specimen surface.
Metallographers and failure analysts use the method for a host of purposes including evaluation of homogeneity, characterization of weldments, as an aid to phase identification, or simply to determine the hardness of specimens too small for traditional bulk indentation tests.
For additional information on Vickers hardness testing and metallographic hardness testing, refer to the Buehler SumMet Guide.
