Scope:

The Knoop indentation hardness is determined in ASTM C849 to characterize ceramic whitewares. Attempts have been made to determine tensile strength, grinding speeds, and other hardness scales using Knoop indentation hardness but to no avail. Such calculations are limited in scope and should be used with caution, except for special cases where a reliable basis for the conversion has been obtained by comparison tests.

Test Procedure:

In ASTM C849 testing, 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 can be used 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. 

Video 01: Knoop hardness test

Specimen size:

For ASTM C849, 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.

Data:

Knoop indentation hardness (HK) and the Knoop hardness number (KHN) are reported in this ASTM C849 test. 

Conclusion:

The Knoop indentation hardness is determined in ASTM C849 to characterize ceramic whitewares.