Hardness is one of those concepts that has many definitions in many contexts. Challenging tasks can be hard, water can be hard, and materials can be hard. In precision measurement, hardness most often refers to the level of resistance held by a material. In other words, a hard material will be more resistant to deformation than a soft material. Deformation of material can occur through scratching, cutting, bending, indentation, or any other permanent effect. Metals in particular are often described by their hardness, and the level of hardness required for a particular metal will depend on the job. Hardness is the measurement of how resistant a solid matter is to any of these potential outcomes of an applied force.
Hardness is defined by three types of measurement: indentation, scratch, and rebound. Each type of hardness measurement is calculated using a specialized scale. Understanding the various ways in which hardness can be measured is important. Fully knowing the language around hardness and therefore the way in which a particular material is defined as more or less hard will assist you in choosing the right materials for a job. We will outline each type of hardness for you, as well as how to measure each one. Whether you work in engineering, physics, construction, chemistry, electricity or any other field that utilizes the principles of precision measurement, knowing the hardness of the materials you are working with will have an impact on the success of your project.
Indentation hardness is the resistance of a material to compression. When weight is applied to a softer material then the object responsible for the additional force will leave a mark or imprint. In some circumstances you might want a softer material that will easily form around another material when force is applied. However, in other instances having a harder material that is resistant to indentation is necessary, perhaps when a part needs to withstand constant force from a corresponding sharp object or part. Indentation hardness is measured by the Rockwell, Vickers, Shore, or Brinell scale. Each of these scales measures the dimension and size of the created indentation using a standardized source.
Scratch hardness refers to the degree of resistance of a material to fracture or abrasion from the application of a sharp object. In general, a material that has higher scratch hardness will always be able to scratch a material that has lower scratch hardness. The harder a material is in this case, the higher tolerance the material will have to the friction caused by an outside object. The most commonly used scale to measure scratch hardness is the Mohs scale. The Mohs scale is an ordinal scale depicting the ability of one material to resist scratches from another material. The most well-known application of the Mohs scale is in classification of minerals, such as diamonds.
Rebound hardness is the degree of bounce resulting from one material being dropped upon another material. Typically, rebound hardness is determined by dropping a diamond-tipped hammer on the material in question and measuring the level of bounce that results. The harder a material is, the higher the hammer will rebound. The Leeb rebound hardness test uses the amount of energy loss to define a quotient of hardness to a particular material.
Generally, the hardness of a material will increase with decreasing particle size because the density of the molecules increases, making the material more resistant to damage. A higher level of hardness is not always better, for the required hardness of a material depends largely on the application it is being used for. It is important to understand the different types of hardness, how to measure them, and what level of each is necessary for a particular precision measurement task.