Advantages and disadvantages of the Knoop method

The Knoop method has the following advantages

• The Knoop method can be used with any and all materials and test specimens, from soft to hard, as the procedure covers the entire hardness range.
• There is only one type of indenter, which can be used for all Knoop methods.
• Evaluation is more precise than the Vickers method, because the Knoop measurement diagonal (longitudinal diagonal) is longer for any given indentation depth.
• The test is non-destructive, and there is only very minor damage to the specimen surface (less than that with Vickers, because both the indentation depth and the risk of crack formation at the indent edge in glass and ceramics is lower than with Vickers).
• It is particularly suitable for testing small, longish components and very thin layers as well as brittle materials (glass and ceramics) for which no other method is appropriate.

The Knoop method has the following disadvantages

• The surface quality of the specimen must be good, because the indent is measured optically. This means that the test location should be prepared, otherwise precise evaluation is difficult.
• The process is rather slow (compared with the Rockwell method). The test cycle takes somewhere between 30 and 60 seconds, not including the time taken to prepare the specimen.
• Due to the need to conduct optical indent evaluation, Knoop hardness testers must be equipped with an optical system, which makes them more expensive to purchase than Rockwell testers.
• Use of this method is not very common in Europe.