The ASTM C1326 test is a microhardness test that uses a pointed, pyramidal diamond shaped, rhombic-based indent, pressed into a test ceramic specimen. This uses a predefined load, which produces a permanent indentation on specimens. This indentation is used to provide a relative resistance of the specimen to penetration.
ASTM C1326 test method explains the measurement of Knoop indentation hardness of advanced ceramics. This test is specifically used for thin sheets or very brittle materials, where a small indentation is made for testing purposes. The applied force or the predetermined load in ASTM C1326 is used to determine Knoop hardness that basically reflects upon the material’s resistance to penetration by the pointed, pyramidal diamond shaped Knoop indenter. Keeping the same load, Knoop indenters are typically 2.8 times longer and shallower than Vickers indentations. This helps in making easier-to-read indentations.
In the ASTM C1326 test, the elastic springback of the narrow diagonal is considered to be negligible, and the ratio of the applied load with the projected indentation area on the surface of the ceramic specimen gives Knoop hardness. By measuring Knoop hardness for a broad range of predetermined loads, full characterization can be obtained. Knoop hardness of ceramics is inversely proportional to the indentation size or the indentation force. The nature of the curve obtained is termed as indentation size effect. The test method involved in ASTM C1326 makes indentation without cracking. The amount of applied load should be such that the hardness reading stays in the plateau region of the indentation size effect curve without exceeding the limit of cracking.
Ceramic specimen in ASTM C1326 is of standard size.
At the time of development of the test procedures in ASTM C1326, the amount of force applied was expressed in units of grams-force (gf) and kilograms-force (kgf). However, even though the standard system specifies Newton (N) is the International system of Units of force, continued usage of gf and kgf are occasionally observed.