Hardness

The hardness of ceramic materials is a property which is of high significance as it relates to the ability of the material to withstand penetration of the surface through a combination of brittle fracture and plastic flow.

Often, hardness of ceramic material, as with other materials is directly equated to wear resistance. This is a mistaken concept with many metallic components and is definitely an incorrect selection criterion with regards to engineering ceramic materials.

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Wear behaviour of ceramic materials is complex and is dependent upon many variables, of which ceramic hardness is an important variable but not the only significant variable.

For example, in many wear environments, such as the erosive wear behaviour of oxide engineering ceramics, it is the ratio of fracture toughness to hardness of ceramics which is found to be of most significance in determining the wear performance.

In many wear environments, a much & “softer” material such as a zirconia can outperform & “harder” materials such as aluminas or silicon carbide.

Ceramic hardness measurements in engineering are generally measured using a Vickers hardness test. In this test a pyramidal diamond indenter is pressed into a polished surface under known loading conditions and the size of the indentation is related to the hardness of the material.

It should also be noted that the hardness value quoted for any material is a function of the type of test conducted and the loading conditions employed. Lighter loads typically provide higher hardness values.

Typically in a Vickers Hardness test, the notation HV10 or HV20 relates to the applied load in kg (in this case 10 or 20 kg respectively).

Other factors that need to be taken into account when interpreting data for hardness of ceramics are the amount of porosity in the surface, the grain size of the micro structure and the effects of grain boundary phases.

Some typical ceramic hardness values are provided below: