Vickers and Knoop Hardness Testers

Macro & Micro Hardness

There are two unique Vickers parameters noted as either Micro hardness or Macro hardness. The Micro Vickers scale uses loads ranging from 1gf to 1000gf to measure permanent deformation. The Macro Vickers scale, however, uses loads ranging from 1kg to 100kg for harder materials.

The Micro and Macro Vickers Hardness Test is conducted according to the ASTM E384 standard. The Macro Vickers test was formally conducted according to the withdrawn ASTM E92 standard. Both Vickers Hardness tests use a pyramid diamond indenter at a specific 136° angle. The indentation is held in a given material for a specific amount of time, called dwell time, which results in a square shaped indentation left in the material. Once the indenter is removed, the diagonals of the indentations are measured to determine the Vickers Hardness value. In lieu of the small size of the indentation, an optical lens is utilized to accurately measure the diagonals of the indentation for the most accurate reading. There are many methods available for determining the Vickers hardness value ranging from a basic microscope to fully automatic CCD optical analysis with special software.

Knoop Metal Hardness Testing is an alternative to the Micro Vickers Hardness testing that was invented by the National Bureau of Standards (commonly known as NIST). Due to the rhombus-based pyramidal diamond indenter paired with micro loads (10gf to 1000gf), the Knoop Metal Hardness Test causes minimal surface deformation on your material. For this reason, Knoop hardness testing is used on softer and brittle materials.

A high powered microscope is required in order to measure the indentation to determine your Knoop Hardness value (HK). According to ASTM E384, the Knoop Hardness test needs a highly polished surface finish due to the pyramidal indenter shape which is commonly found on mounted, metallurgically prepared test samples. Due to the wide testing load range, the Knoop test is commonly applied on various metallic materials. The length to width ratio of the indenter is 7:1 and has a long edge of 170° and a short edge of 132°. Due to the design of the indenter, the Knoop hardness testing scale is used where no other hardness scale can accurately perform.

Case depth is a hardened layer added to the surroundings of metallic materials to extend the wearing life of the parts while undergoing their intended applications. Micro hardness testing is commonly used to measure the depth of the added layer to evaluate the added properties of the material. This is due to the minimal deformation left behind from a Micro Hardness test. Surface hardness, effective hardness depth and the depth profile are used to identify these added properties. A cross section of the material is taken and the core hardness is measured. By knowing the core hardness of the material, a series of indentations is often used from the edge of the specimen until the core hardness point is reached. The information is then plotted on a simple graph and the depth of the added layer is then calculated. Case depth testing (or CHD) allows for confirmation of a parts lifespan for those that have to endure a high level of stress.