Static Indentation Hardness Testing for Coals

Abstract

Methods most widely used in determining the hardness of materials are static indentation methods.These involve the formation of a permanent indentation in the surface of the material to be examined, the hardness being determined by the load and the size of indentatation formed.
Experiments have been made to find the conditions of these testing methods for coals .In testing for coals with diamond pyramid method, using either the ordinary Vickers or microhardness tester, the load used is generally above 2kg and above 200gr, respectively .In these tests, the duration of the load is an important factor.With the Vickers tester, the full load is usually applied for 1.4 seconds.With the microhardness tester, the time of application is 30 seconds.
The micro-Vickers and Knoop hardness values are almost identical with the ordinary Vickers hardness numbers.In addition, the Knoop indenter often produces satisfactory indentations in the materials such as anthracite, semi coke and coke which are hardly indentable by the Vickers pyramid indenter or dy spherical indenter.