Hardness is a fundamental property of materials, representing their resistance to permanent deformation from scratching, indentation, or abrasion. This characteristic is important for understanding how a material will perform. Quantifying hardness helps in selecting appropriate materials for diverse applications, from engineering to geology.
What is Hardness and Why Measure It?
Hardness is not a single property, but rather a measure of resistance to different types of localized permanent deformation, such as from penetration or scratching. It can refer to resistance against continuous pressure, surface scratches, or general wear.
Measuring hardness helps in quality control during manufacturing processes, ensuring products meet specific material standards. It also guides material selection for particular uses, such as choosing metals for tools or construction components. This assessment predicts how well materials will perform and endure in their intended environments.
Scratch Hardness: The Mohs Scale
The Mohs scale of mineral hardness is a qualitative, ordinal scale ranging from 1 to 10. It characterizes a mineral’s scratch resistance based on the principle that a harder material can scratch a softer one. This scale, introduced in 1812 by German mineralogist Friedrich Mohs, uses ten reference minerals.
To determine a material’s Mohs hardness, it is tested against these standard minerals. If a material is scratched by one mineral but not another, its hardness falls between those two points. For example, a material scratched by apatite (5) but not fluorite (4) has a Mohs hardness between 4 and 5. The Mohs scale is commonly used in geology and gemology for field identification.
Indentation Hardness Testing
Indentation hardness testing involves applying a known force to a specific indenter and measuring the resulting impression. These methods provide quantitative hardness values, widely used across industries. The size or depth of the indentation indicates the material’s resistance to permanent deformation.
The Rockwell hardness test measures the depth of permanent indentation. It uses either a diamond cone or a steel ball indenter, with different scales for various materials. A minor load is first applied, followed by a major load, and the hardness value is derived from the net increase in indentation depth. This test is valued for its speed, reliability, and suitability for various metals, including thin and case-hardened steel.
The Brinell hardness test uses a large ball indenter, typically 2.5 or 10 mm in diameter, made of tungsten carbide. A specified load, often between 187.5 and 3000 kgf, is applied for a set time. The diameter of the resulting indentation is measured, and the Brinell hardness number is calculated from this diameter and the applied load. This method is suitable for materials with rough surfaces, such as forgings and castings, as the large indenter averages out irregularities.
The Vickers hardness test uses a square-based pyramidal diamond indenter. This indenter creates a very small indentation, making the Vickers test suitable for a wide range of materials, including thin sections, small parts, metals, ceramics, and polymers. The diagonals of the square indentation are measured, and the Vickers hardness value is calculated from these measurements and the applied load. This method is often used for microhardness testing and evaluating case hardening depths.
Specialized Hardness Measurement
Beyond scratch and indentation methods, specialized tests cater to materials with unique properties. Shore hardness, also known as durometer hardness, measures the resistance of softer materials to indentation. This test is used for polymers, rubbers, and plastics.
A durometer device, with a spring-loaded indenter, presses into the material’s surface. The depth of the indentation is measured, and a higher number on the Shore scale indicates greater resistance to indentation and a harder material. Different Shore scales, such as Shore A and Shore D, are used depending on the material’s stiffness, with Shore A for softer materials and Shore D for harder plastics.