Hardness is a property of the material to determine whether it can work in a specific application, and there are several types of hardness tests to measure the ability. In this article, we’ll get straight into the hardness in mechanical engineering, introduce different types of material hardness and common hardness scales.
Hardness refers to the ability of a material to resist local plastic deformation caused by mechanical indentation or wear. Plastic deformation also called permanent deformation, means that the material will not return to its original shape. Hardness depends on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity. The local resistance of solid to the invasion of external objects is an index to compare the hardness and softness of various materials. Different materials usually have different hardness, common hard materials are ceramics, diamond and hard metal such as titanium, while wood and common plastics are softer. Cemented carbide, which is a commonly used material of CNC milling cutters, is a hard material.
Hardness can be assessed by a number of techniques, there are mainly three types of hardness measurements: scratch hardness, indentation hardness, and rebound hardness.
1. Indentation Hardness: measures the ability to withstand surface indentation and the ability of the measured sample to resist deformation under the constant load of a sharp object. The higher the indentation hardness, the greater ability to not have any resulting deformation from applied compression. Indentation hardness mainly used in engineering and metallurgical fields. The traditional indentation hardness test method is using hard indenters of defined geometries and sizes to press into the material under a particular force, and record the deformation parameters like indentation depth. Common indentation hardness scales are Brinell, Rockwell and Vickers.
2. Scratch Hardness: measures the resistance of a sample to permanent plastic deformation due to friction from a sharp object. Materials that are less impacted by scratching will have higher scratch hardness. The most common scratch hardness test is the Mohs scale for mineralogy. The principle is that objects made of harder materials will scratch objects made of softer materials. The method is to find the hardest material that the given material can scratch, or the softest material that can scratch the given material. When measuring coating hardness, scratch hardness refers to the force required to cut the film onto the substrate.
3. Rebound Hardness: also known as dynamic hardness, measures the “bounce” height of the diamond hammer when it falls from a fixed height to the material. A material with higher rebound hardness will lead to a higher bounce when the hammer is dropped. This hardness is related to the elasticity of the material under test. The measuring device of rebound hardness is called sclerometer.
Different hardness scales include Rockwell C & B (metals), Brinell (ball indenter, metals), Vickers (diamond-shaped indenter), Knoop (diamond-shaped micro-indenter), Meyer (rarely used), Shore A & D (rubber & softer plastics), and Mohs (minerals). The international unit of hardness is N/mm². The unit Pascal is also used for hardness. The different types of hardness have different measuring scales. For scratch, indentation, and rebound hardness, the measurement methods vary, including Brinell, Rockwell, Knoop, etc. Since these units are derived from these measurement methods, they are not suitable for direct comparison. However, there are hardness conversion tables equivalence between the different scales. The similarity is that in all of these scales, a harder material will have a higher value.
Common hardness units:
– Rockwell hardness (HRB, HRC, etc.): based on the indentation hardness, measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). Due to the different indenters or loads, there are several alternative scales, such as HRA, HRB, HRC, HRD, etc., the most commonly used are HRB and HRC scales. They both express hardness as an arbitrary dimensionless number, HRB is usually used for aluminum, brass, and soft steels, and HRC is used for harder steels.
– Brinell hardness (HB): measures the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. BHN stands for Brinell Hardness Number, which is the same meaning as HB. The Brinell hardness test is commonly used to determine the hardness of materials like metals and alloys.
– Vickers hardness (HV): calculate the hardness by optically measuring the diagonal lengths of the impression left by the indenter, then converts the measurements to HV using a table or formula. The Vickers test is easier to use than other hardness tests and can be used for all metals, one of the widest scales among hardness tests. The unit of Vickers hardness is known as Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH).
– Knoop hardness (HK): calculated by measuring the indentation produced by a diamond tip that is pressed onto the surface of the sample. It is used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes.
– Shore A & D: Shore hardness is a measure of the resistance of a material to the penetration of a needle under a defined spring force. The letter A is used for flexible types and the letter D for rigid types. Shore A hardness scale measures the hardness of flexible mold rubbers that range in hardness from very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility. Shore D hardness is specified for harder elastomer measurements using a needle that ends with a 30°point angle and is not blunted.
– Mohs: one of the most important tests for identifying mineral specimens, measures a mineral’s resistance to scratching.