Due to the ease and speed of the microindentation test, it has been widely applied to measure the mechanical properties of materials like metals, alloys, polymers, and ceramics. This test is ideal for evaluating the hardness of very small/thin samples, complex shapes, individual phases of a material, and surface coatings/platings, as well as identifying depths of case-hardened layers, homogeneity, weldments, decarburization or carburization, grind burns, and surface contamination. ASTM E384-22: Standard Test Method For Microindentation Hardness Of Materials details the specifications for the test method of the microindentation hardness of materials.
Microindentation Hardness Tests
Microindentation tests are utilized to evaluate and quantify hardness variations of material occurring over a small distance—on a microscopic scale. To determine these differences requires a very small physical indentation that is typically made with a square-based pyramid indenter (Vickers hardness scale) or an elongated, rhombohedral-shaped indenter (Koop hardness scale) at very low test forces. Microindentation hardness testers must be carefully constructed to accurately apply the test forces on the selected test load exactly at the desired location; they must have a high quality optical system, such as a light microscope equipped with a filar type eyepiece or a video image and computer software, to precisely measure the diagonal (or diagonals) of the small indents after load removal.
What is ASTM E384-22?
The test method in ASTM E384-22 covers determination of the microindentation hardness of materials, which involves using a calibrated machine to force a diamond indenter of specific geometry into the surface of the material being evaluated. This test method is largely applicable to heat treaters, metallographers, metallurgists, material engineers, and failure analysists.
What Does the Hardness Test Method in ASTM E384-22 Cover?
- Microindentation tests made with Knoop indenter (rhombic-based pyramidal shaped diamond indenter with edge angles) and Vickers indenter (a square-based pyramidal-shaped diamond indenter with face angles of 136°) under test forces in the range from 9.8 × 10-3 to 9.8 N (1 to 1000 gf).
- An analysis of the possible sources of errors that can occur during microindentation testing and how these factors affect the precision, bias, repeatability, and reproducibility of test results.
- Requirements for direct verification and calibration of the testing machine.
- Requirements for the manufacture and calibration of Vickers and Knoop reference hardness test blocks.
Vickers and Koop Hardness Test
Because microindentation hardness tests reveal hardness variations that commonly exist within most materials, a single test value may not be representative of the bulk hardness. ASTM E384-22 specifies that Vickers tests at 1000 gf can be utilized for determination of the bulk hardness, or Koop tests, an alternative to the Vickers test for very thin layers, can determine bulk hardness when conducted at forces from 100 gf to 500 gf. A Koop or Vickers hardness number is then computed using the test load, the impression length, and a shape factor for the indenter type used for the test.
What Is the Purpose of the Hardness Test?
Hardness is the property of a material that allows it to resist permanent distortion, bending, penetration, indentation, and scratching. Hardness tests are useful for materials evaluation, quality control of manufacturing processes, and research and development efforts. Hardness can be correlated to tensile strength for many metals and alloys, and it is an indicator of machinability, wear resistance, toughness, durability, and ductility. These tests are therefore useful to learn about the physical properties of a material, expected service behavior, and the shelf-life of a product.
ASTM E384-22: Standard Test Method For Microindentation Hardness Of Materials is available on the ANSI Webstore.