National Physical Laboratory

Hardness and Microhardness Mapping

Hardness is a key quality assurance test for the hardmetal and powder route material industry, because it quickly defines whether the processing schedule has achieved the required end-product characteristics. Work at NPL has focused on evaluating hardness testing on brittle materials, and in supporting broad application of the techniques available.

Standards

NPL has been influential in evaluating conventional hardness tests for metals applied to brittle materials. In the ceramic field, we have coordinated European work to validate EN 843-4 on Vickers, Knoop and Rockwell ‘N’-scale hardness testing. In the hardmetal field, we have been studying correlations between Rockwell ‘A’ (force derived from a mass of 60 kg), often used in the USA, and Vickers HV30 Tests (used in Europe). NPL has also experimented with Vickers HV100 tests for the determination of fracture toughness via the Palmqvist cracking method. We are currently evaluating a NIST HV1 hardmetal standard reference material for potential as an HV30 reference material for the hardmetal industry, since there are no metallic reference blocks available for hardness above 1000 HV30.

Depth-sensing hardness

Taking visual measurement out of the equation could be the key to improved consistency of hardness determination. Research at NPL has evaluated the technique on a wide range of hard surfaces, predominantly coating systems, and predominantly at forces less than 1 N, using nanoindentation systems. At higher forces, we have found that, the inhomogeneity of ceramic materials on the HV1 or HK2 scales where cracking is at an acceptable minimum means that the scatter of outcomes is larger than attributable to reading errors, so currently, there seems to be no advantage in moving towards this technique.

For hardmetals and other more ductile materials, we have pioneered a new system of making hardness scans across a surface, e.g. of a joint or functionally graded material. The instrument, named a ‘scanning indentation mechanical microprobe’ (SIMM) is based on conventional depth-sensing principles, but operates under stepper motor control which allows a raster of hardness indentations to made on a flat surface. The machine can be controlled to indent to a given depth, or to a given maximum force. The loading/unloading curve produced can then be analysed in an appropriate way to give either conventional hardness data, Martens’ hardness, or other parameters, which can then be plotted as a map for easy visualisation of hardness gradients.

For more information: Roger Morrell

Last Updated: 25 Apr 2012
Created: 29 Jul 2007

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