In ASTM C730, the Knoop Indentation Hardness Test is used to determine the Knoop Indentation Hardness of glass. It determines the mechanical hardness of very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. The test is valid for a wide range of test forces and very few samples are required.
In ASTM C730, a pyramidal diamond point is pressed into the polished surface of the test sample with a known load, usually 100g, for a specified dwell time, and the dimension of the resulting indentation is measured using a microscope.
The indenter leaves a tiny four-sided impression of about 0.01 to 0.1 mm in size. The length of the impression is approximately seven times the width, and the depth is 1/30 the length. Hence, after measuring only the length of the longest side, the area of the indentation can be calculated. The area of the indentation and the weight of the load are then used to calculate Knoop Indentation Hardness.
The Knoop indentation hardness test is used in ASTM C720 to test a wide variety of glasses, from television faceplates to polished plate glass. In general, the accuracy of the test will depend on the smoothness of the surface.
For ASTM C730, the surface of the sample should be smooth and polished. The sample should be over ten times as thick as the indentation depth but not so thick as to affect the hardness of the glass. In general, if specimens are at least 0.10 mm thick, the hardness will not be affected by variations in the thickness.
For ASTM C730:
- Knoop Indentation Hardness (HK) is calculated by
HK = 14.229(F/D2)
F = the applied load
D2 = the area of the indentation
2. The Knoop hardness number (KHN) is calculated by:
KHN = (P/A)
P = load
A = projected area of the indentation
ASTM C730 is used to determine the Knoop indentation hardness of glass and to verify the Knoop hardness testing machine by using standard glasses.