The hardness values of seven silicon nitride ceramics were measured at an applied load of 2.45 N for both Vickers and
Knoop indenter geometries. It was found that the measured Knoop hardness, HK, is generally lower than the corresponding
Vickers hardness, HV. There is a strong correlation between (HV/HK) and (a/d), where a and d are the measured lengths of
the minor and the major diagonals of the Knoop indentation, respectively. Such a correlation indicates that the difference
between HV and HK may be attributed to the elastic recovery occurring at the indentation.
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