9.3 Magnitude of Test Load—Atest load of 1000 gf (9.8 N)
is specified. If cracks develop at this load, measurements
within 500- or 250-gf (4.9- or 2.45-N) loads may be made
although the Knoop indentation hardness does vary with load.
Table 1 gives an indication of the magnitude of this variation to
be expected. In all cases, the load actually used should be
reported.
9.4 Application of Test Load:
9.4.1 Start the machine smoothly. The rate of indenter
motion before contact with the specimen shall be 0.20 6 0.05
mm/min. If the machine is loaded by an electrical system or a
dash-pot lever system, it should be mounted on shock absorbers
which damp out all vibrations by the time the indenter
touches the specimen. If the specimen is handloaded, take
extreme care to see that the loading rate never goes higher than
0.25 mm/min.
9.4.2 The indenter should remain in contact with the specimen
between 20 and 30 s. After the indenter has been in
contact with the specimen for the required dwell time, carefully
raise it off the sample to avoid a vibration impact at this time.
9.5 Spacing of Indentations—Allow a distance of at least
three times the short diagonal between indentations.
9.6 Number of Indentations—The number of indentations
will vary with the type of specimen. For example, if one is
investigating the hardness gradient in a sample, he will make a
series of indentations and plot the KHN as a function of
distance. In the usual test, one has a piece of ceramic
whiteware that is fairly homogeneous and he is trying to obtain
a mean KHN for that specimen. In this case, it is recommended
that at least ten indentations be made and that both the mean
KHN and the standard deviation be reported. The standard