8. Preparation of Apparatus
8.1 Verification of Load—Most of the machines available
for Knoop hardness testing use a loaded beam. This beam
should be tested for zero load. An indentation should not be
visible with zero load, but the indenter should contact the
sample. A visible indentation should be obtained with a load of
0.1 gf. Other methods of verifying the load application are
given in Practice E 4.
8.2 Verification by Standard Glasses—Table 1 gives the
Knoop hardness of several National Institute of Standards and
Technology (NIST) standard glasses. Knoop hardness measurement
on a piece of one of these glasses that has been
ground and polished within the last 24 h should agree with the
value in the table 65 %. Tests should be made using 100 gf.
9. Procedure
9.1 Specimen Placement—Place the specimen on the stage
of the machine in such a way so that the specimen will not be
able to rock or shift during the measurement.
9.2 Specimen Leveling:
9.2.1 The surface of the specimen being tested must lie in a
plane normal to the axis of the indenter. Fig. 2 shows an
indentation as it will appear through the microscope with five
points labeled. To level the specimen, make a test indentation
using a 100-gf load.
9.2.2 The following minimum specifications must be met:
OA 5 OB 6 5 % (1)
OC 5 OD 6 5 %
9.2.3 Leveling the specimen to meet these specifications is