Hardness is a fundamental material property defined as the resistance a solid exhibits to permanent shape change when a force is applied. Because this resistance manifests in several ways, a single test cannot capture the full spectrum of a material’s mechanical behavior. Measuring hardness requires selecting the appropriate technique based on the material’s nature and the desired information, differentiating between simple identification and precise engineering quantification.
Defining Hardness in Materials Science
Materials scientists categorize mechanical hardness into three primary types, each representing a different failure mechanism. Scratch hardness describes a material’s resistance to surface abrasion and cutting, primarily relevant for geological study and mineral identification. This resistance relates to the material’s ability to prevent the dislodging or fracture of its surface atoms when a sharper object is dragged across it.
Indentation hardness measures the resistance to permanent plastic deformation under a localized, concentrated compressive load. When a stiff indenter is pressed into a surface, the material deforms. The size or depth of the resulting permanent impression is measured to calculate a hardness value. This type of measurement is widely used in manufacturing and engineering because it provides quantifiable data related to a material’s strength and wear characteristics.
Dynamic or rebound hardness is the third type, which assesses the energy lost when an object impacts a surface. A harder material absorbs less energy from the impact and causes the object to rebound higher. Understanding these distinctions is important because a material that performs well in one category may not necessarily resist scratching or dynamic impact with the same efficacy.
The Comparative Mohs Scale
The Mohs scale of mineral hardness is a qualitative, relative system used primarily in geology to determine a mineral’s scratch resistance. Developed in 1812 by German mineralogist Friedrich Mohs, the scale arranges ten common minerals in order of their ability to scratch one another. This test is non-absolute, meaning the difference in hardness between ratings is not proportional.
The scale begins with:
Talc (1)
Gypsum (2)
Calcite (3)
Fluorite (4)
Apatite (5)
Orthoclase (6)
Quartz (7)
Topaz (8)
Corundum (9)
Diamond (10)
The test is simple: if an unknown mineral can be scratched by quartz but cannot scratch it in return, its Mohs hardness is between 6 and 7.
Because the Mohs scale is based on an ordinal ranking, it lacks the precision required for modern engineering specifications. For instance, diamond (10) is vastly harder than corundum (9) in absolute terms, a difference not reflected by the single integer jump. Despite this lack of proportionality, the scale remains an accessible and efficient field tool for the quick identification of minerals based on their surface integrity.
Standardized Indentation Testing Methods
For manufactured materials like metals and ceramics, hardness is measured using standardized indentation tests that provide precise, quantitative values. These methods involve applying a specific force, or load, to an indenter of a defined geometry for a set period, and then measuring the dimensions of the resulting impression. Adherence to strict standards, such as those set by ASTM, ensures the reliability and comparability of the resulting hardness numbers.
Brinell Hardness Test
The Brinell hardness test utilizes a large, hardened steel or tungsten carbide ball indenter, typically with a diameter of 10 millimeters. A predetermined load, sometimes up to 3,000 kilograms-force (kgf), is applied to the material’s surface. The test is classified as an optical method because the diameter of the circular impression is measured afterward using a specialized microscope, and this diameter is used to calculate the Brinell Hardness Number (HBW). This method creates a large impression, which averages out variations in materials with coarse or non-uniform microstructures, making it suitable for testing castings and forgings.
Rockwell Hardness Test
Rockwell testing is the most widely adopted method in manufacturing due to its speed and direct readout. Instead of measuring the impression size, the Rockwell test measures the depth of penetration. The process begins by applying a minor load to seat the indenter, establishing a datum position. A major load is then applied and subsequently removed, leaving the minor load in place. The final Rockwell Hardness value is derived from the difference in the depth of penetration between the minor and major loads, providing a result without the need for secondary optical measurement.
Vickers and Knoop Tests
The Vickers and Knoop tests use diamond indenters, which allows them to test very hard materials, including ceramics, and are often employed for microhardness applications.
The Vickers test uses a square-based diamond pyramid indenter, creating a precise, geometric impression. The hardness value (HV) is calculated by measuring the diagonals of the square indentation under a microscope and relating this area to the applied load. Because the indenter shape is consistent across all loads, Vickers provides a continuous hardness scale suitable for a wide variety of materials, including thin sections and case-hardened components.
The Knoop test is similar to Vickers but uses a rhombic-based diamond indenter that creates an elongated, narrow impression. This shape is advantageous for measuring the hardness of brittle materials, thin coatings, or areas with restricted space, as the force is spread over a greater length. Like Vickers, Knoop is an optical method where the length of the long diagonal is measured to determine the Knoop Hardness Number (HK).
Dynamic and Specialized Measurement Techniques
Beyond static indentation, specialized methods are used for materials that are either too large for benchtop testing or are non-metallic, such as polymers and rubbers. Dynamic or rebound hardness tests are preferred for large, in-situ components like engine blocks and bridge structures.
Leeb Rebound Test
The Leeb rebound test, developed in 1975, involves projecting an impact body with a carbide ball tip onto the surface of the material. The Leeb hardness value (HL) is determined by measuring the velocities of the impact body immediately before and after it strikes the surface. A harder surface absorbs less kinetic energy from the impact, causing the body to rebound at a higher velocity. The ratio of the rebound velocity to the impact velocity, multiplied by 1000, gives the final Leeb number. This method is valued for its portability and its ability to provide a quick, non-destructive test in the field.
Shore Durometer Test
For non-metallic materials like plastics, elastomers, and soft rubber, the Shore Durometer test is the standard. The instrument, called a durometer, measures the material’s resistance to indentation by a spring-loaded indenter foot. The result is a dimensionless number between 0 and 100, with a higher number indicating greater resistance and thus a harder material.
The durometer test uses different scales depending on the material’s stiffness. Shore A is the most common scale, used for softer materials like flexible rubber and elastomers. Shore D employs a sharper indenter and a greater force to measure harder materials, such as rigid plastics and hard vulcanized rubber. The choice of scale ensures the measurement is relevant to the material’s intended application.