Hard facts

Theory of hardness testing

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The Knoop method represents an alternative to the Vickers method. The two methods differ from each other in the following aspects:

  • With an identical test load, the Knoop diamond indenter penetrates the specimen only half as deep as the Vickers diamond indenter. Due to this shallow indentation depth, the Knoop method is particularly suitable for testing very thin layers (e.g. aluminium foil).
  • Due the long and narrow (elongated) test indent, the Knoop method is best suited for use with small, longish test specimens, whilst the Vickers method is better for small, rounded specimens (square indentation).
  • The Knoop method is particularly suitable for testing very hard and brittle materials (glass, ceramics), where the Vickers indentation would lead to cracking (measurement accuracy).

For any given indentation depth, the Knoop diagonal (the longitudinal diagonal) is around three times as long as the arithmetic mean of the Vickers diagonals, which means that the Knoop method provides higher measurement precision, especially in conjunction with very low test loads.

Comparison between Knoop and Vickers methods