Page 131 - Chalcogenide Glasses for Infrared Optics

108 Cha pte r F o u r

4.3 Physical Properties Important for Optical Use

4.3.1 Hardness
Hardness is a property important in fabrication of a glass. To pro-
duce good optical surfaces, some degree of hardness is required. It
has already been mentioned that some physical properties are
interdependent. As shown in Chap. 2, the higher the softening
point of a glass, the harder its surface. A photograph of the Leitz
Miniload Hardness Tester used by AMI is shown in Fig. 4.13. The
instrument has an arm with a Knoop pyramid diamond tip that is
slowly pressed into the surface of the specimen under a specified
load determined by added weights. The load weights may vary
from 15 to 2000 g depending upon the material to be tested. For
these soft materials, AMI usually uses the 50-g load. The diamond
penetrates the surface, leaving a distinctive long diagonal mark

FIGURE 4.13 Photograph of the Leitz Miniload Hardness Tester instrument
used by AMI to measure Knoop hardness of chalcogenide glasses.