Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.
Knoop test methods are defined in ASTM E384
Knoop Test Method
Knoop testing is done with a rhombic-based pyramidal diamond indenter that forms an elongated diamond shaped indent.
- The indenter is pressed into the sample by an accurately controlled test force
- The force is maintained for a specific dwell time, normally 10 - 15 seconds.
- After the dwell time is complete, the indenter is removed leaving an elongated diamond shaped indent in the sample.
- The size of the indent is determined optically by measuring the longest diagonal of the diamond shaped indent.
- The Knoop hardness number is a function of the test force divided by the projected area of the indent. The diagonal is used in the following formula to calculate the Knoop hardness.
HK = Constant X test force / indent diagonal squared
The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows:
Where the 450 is the calculated hardness and 0.5 is the test force in kg.
Because of the wide test force range, the Knoop test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.
- The elongated diamond indenter and low test forces allows testing very small parts or material features not capable if being tested any other way.
- One scale covers the entire hardness range.
- Test results a mainly test force independent over 100g.
- A wide range of test forces to suit every application.
- The main drawback of the Knoop test is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement.
- Slow. Testing can take 30 seconds not counting the sample preparation time.