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How do the Knoop and Vickers hardness tests measure hardness?

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Angle grinder — abrasive Worker using an angle grinder to smooth a piece of metal.

Both the Vickers and Knoop hardness tests measure a material's resistance to indentation by pressing a diamond-tipped device into its surface under a specific load.1

Vickers Hardness Test

The Vickers test uses a square-based pyramid diamond indenter with opposite sides meeting at a 136° angle. It is primarily designed for metals, applying loads up to 120 kilograms-force. The resulting square impression is measured with a microscope, and the hardness number (HV) is calculated based on the load and the area of the indentation.21

Knoop Hardness Test

The Knoop test employs an elongated four-sided pyramid indenter, which allows for lower indentation pressures. This makes it suitable for testing brittle materials like glass and ceramics without causing damage. Because the impression is elongated—with a length approximately seven times its width—the hardness (HK) can be determined by measuring only the length of the longest side.31

Both tests provide quantitative data essential for evaluating abrasive materials, ensuring they are harder than the surfaces they are intended to grind or polish.1