7. Test Specimens
7.1 The Knoop indentation hardness test is adaptable to a
wide variety of ceramic whiteware specimens. In general, the
accuracy of the test will depend on the smoothness of the
surface and, whenever possible, ground and polished specimens
should be used. The back of the specimen shall be fixed
so that the specimen cannot rock or shift during the test.
7.1.1 Thickness—As long as the specimen is over ten times
as thick as the indentation depth, this will not affect the test. In
general, if specimens are at least 0.10 mm thick, the hardness
will not be affected by variations in the thickness.
7.1.2 Surface Finish—As pointed out above, the accuracy
of the test depends on the surface finish. However, if one is
investigating a surface coating or treatment, he cannot grind
and polish the sample. Experience has shown that six indentations
on a ground and polished surface of glass will reproduce
within 61 %. Six indentations on an “as-received”
surface may be as bad as 610 %. Ground and polished surfaces
should be used. If this is not possible, the number of indentations
should be increased.
7.1.3 Radius of Curvature—The KHN obtained will be
affected even when the curvature is only in the direction of the
short diagonal. Care should be used when relating KHN values
obtained on curved surfaces to those obtained on polished flat surface