Microhardness Testing

Microhardness testing is a useful tool for the microstructural analysis of a finished surface. Information such as phase identification and fracture toughness data can be determined. Hardness is defined as the resistance to penetration by an indenter, and the Knoop and Vickers measurements are the most common.

A Knoop hardness number (HK) is obtained by:

where P is the load in kilogram force, Ap is the projected area of indentation in millimeters, and d is the length of the longest diagonal in millimeters.

The Vickers hardness number (HV) is obtained by:

where P is the load in kilogram force, As is the surface area of indentation in millimeters, and d is the length of the longest diagonal in millimeters.

Surface preparation is typically required before hardness testing to ensure that accurate reproducible results can be obtained. The surface finish used depends on the load to be applied to the indenter. For microhardness testing (1 to 1000 gf), a final polish is recommended.

Microhardness testing is a useful technique in characterizing different phases, and it has also been used to determine the fracture toughness in brittle materials. Fracture toughness is determined by initiating a controlled crack, then breaking the specimen, or by measuring crack lengths and applying fracture toughness equations (Ref 8, 9, 10, 11).

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