Hardness testing measures a material’s strength by determining resistance to penetration. The test is extremely useful in material selection because it provides a hardness value which indicates how easily a material can be machined and how well the material will wear. Hardness testing is also valuable in making decisions about treatments and coatings.
The hardness test is usually performed using test machines equipped with an indentor that is forced into the test material over a certain amount of time. LTI provides a number of hardness tests that use the indentation method:
- Rockwell, Brinell and Superficial Rockwell hardness testing are performed at Laboratory Testing Inc. on castings, forgings and other metal products where the samples are relatively large because the tests produce a large visible indentation.
- The Shore Durometer Test is available to measure the hardness of polymeric materials.
- Vickers and Knoop microhardness testing are offered by our Metallurgical Laboratory to measure small samples or small regions in a sample. These methods can measure surface or coating hardness on carburized or case-hardened parts, as well as surface conditions such as grinding burns or decarburization.
The shape of the indentor varies by type of hardness test and includes cone, ball and pyramid shapes. Each test machine also uses a different force or load application system and records an indentation hardness value in kilograms-force according to their individual hardness scales. Conversion charts comparing scales are available on LTI’s website under Resources.
The experienced technicians at LTI perform microhardness and hardness testing according to ASTM and other required specifications for the type of material and its application. All specimens required for microhardness and hardness testing can be prepared at LTI.
The Test Process
Brinell – The Brinell hardness test can be applied to almost any metallic material and is the method most commonly used to test castings and forgings that have a grain structure too coarse for other hardness testing methods. During the test, a carbide ball indenter is pressed into the sample with accurately controlled force for a specific amount of time. When removed, the material has a round indent that is measured to calculate material hardness according to a formula.
Rockwell – In addition to a Rockwell hardness test, there is a Superficial Rockwell. For each test, a minor load is applied to either a diamond cone or a steel ball indenter positioned on the test material’s surface to establish a zero reference position. Next, a major load is applied for a specified amount of time, leaving the minor load applied upon release. The Rockwell hardness number will be the difference in depth between the zero reference position and the indent due to the major load.
The choice of indenter is dependent upon the characteristics of the test material. The Rockwell hardness test applies larger minor and major load values than the Superficial Rockwell, yet both tests offer three different major load options. More than thirty different scales are used between Rockwell and Superficial Rockwell hardness testing due to the various choices and combinations of tests, indenters and major loads.
Knoop – Microhardness tests are used on very small parts and material features that are unable to be tested by the other methods and employs a test load of 1000 grams or less. The Knoop test is performed like Brinell hardness by applying controlled force for a specific amount of time to an indenter in a rhombus-shape. The impression is measured microscopically and is used along with the test load to calculate the hardness value on the Knoop scale.
Vickers – Vickers hardness testing can be done on both the micro and macro scales (some Vickers testers have a maximum test load of up to 50 kilograms). These hardness tests are also performed by applying controlled force for a specific amount of time to an indenter, which in this case is a square-based diamond pyramid. The impression measurement and test load are used in the appropriate formula to calculate the Vickers hardness value. Like Brinell and Knoop, this method has one scale that covers its entire hardness range.
Shore Durometer Test – Hardness testing of polymeric materials, including plastics and rubbers, is performed by the Shore Durometer test according to ASTM D2240. Like the hardness tests for metal, this method determines a material’s hardness value or resistance to indentation by penetration of an indenter into the test sample. Because the flexibility of polymers varies, LTI is equipped with various indenters for use in testing different types of materials from elastomers to rigid plastics.
- Brinell Hardness Testing – typically used with castings, forgings and thicker samples
- Rockwell & Superficial Rockwell Hardness Testing – used to assist in determining the grade of metal
- Vickers Hardness/Microhardness Testing – load weights up to 50 kilograms
- Knoop Microhardness Testing – load weights up to 1000 grams
- Shore Durometer Hardness Testing – testing of elastomers and plastics