Hardness is a material’s quality to withstand localised deformation. It may be especially important when looking for a suitable material for an environment that includes little particles that can induce material wear. Soft materials suffer indentations while hard ones resist to any change in shape.<\/p>\n
Hardness should be viewed in context with other material properties<\/a> like strength, elasticity, etc. For example, many hard materials tend to be brittle, limiting their use-cases.<\/p>\n In this article, we will take a closer look at what material hardness is, how it is measured and how to compare the different units.<\/p>\n Hardness is the measure of a material\u2019s resistance to localised permanent deformation. Permanent deformation is also called plastic deformation. While elastic deformation means that a material changes its shape only during the application of force, a resulting plastic deformation means that the material will not return to its original shape.<\/p>\n Some materials are naturally hard. For example, tungsten is an incredibly hard metal that finds use as an alloying element in tool steels<\/a>. This makes sure that this group of steels can resist wear even at high temperatures during cutting operations<\/a>.<\/p>\n Cemented carbide<\/a>, which finds much use in milling cutters<\/a>, also often includes tungsten. These replaceable cutting tool bits lengthen the lifetime of cutting tools considerably.<\/p>\n On the other hand, some materials, including metals, are soft to the point that renders them useless for many applications. Pure gold is so soft that scratching or bending it does not need much effort. Therefore, adding other metals like silver, copper and aluminium<\/a> is essential to improve its hardness.<\/p>\n With some materials, heat treatment<\/a> is a possibility to induce greater surface hardness while maintaining the other qualities of the metal in its core. Machine shafts often undergo this process to guarantee a longer lifespan.<\/p>\n An engineer must also consider the ratio of hardness when creating a product design concept<\/a>. For example, in a bearing and shaft fit, the bearing has to be softer because they are easier to replace. With constant movement, one part has to wear and the choice is up to the engineer.<\/p>\n Materials behave differently under different types of loading. For example, a metal that can take a huge one-time impact extremely well may not act the same during continuous loading.<\/p>\n Hardness testing must be carried out for each case so that a well-informed choice can be made for the application.<\/p>\n The three types of hardness are scratch, rebound, and indentation hardness<\/strong>. Measuring each type of hardness requires a different set of tools. Also, the same material will have different hardness values for each of the above-mentioned types.<\/p>\n This hardness type refers to the resistance to permanent deformation when subjecting a material to a continuous load.<\/p>\n Indentation hardness is what engineers and metallurgists usually refer to when they talk about hardness<\/em>. Measuring its value is of primary interest as continuous loading is the most common form of loading metals are subjected to.<\/p>\n This type of hardness refers to a material’s ability to resist scratches on the surface. Scratches are narrow continuous indentations in the upper layer due to contact with a sharp, harder material.<\/p>\n Scratch testing is also commonly used for brittle materials such as ceramics as they do not undergo significant plastic deformation. It is important to consider scratch hardness as some material applications are highly sensitive to scoring<\/a>.<\/p>\n Consider the case of an engine cylinder liner as an example. Scratching or scoring can take place due to various reasons. The liner\u2019s surface comes in contact with a variety of metals such as piston rings, and foreign particles in fuel or lubrication oil. Sometimes improper seating of liner can contribute to it.<\/p>\n The abrasive particles can cause scratches that ultimately degrade the engine’s performance and cost more in maintenance, spare parts, and fuel consumption in the long run.<\/p>\n At the design stage, the right metal selection considers the hardness of the materials it will come into contact with. The hardness of the liner must be greater than the materials it will interact with. This helps to avert many possible issues.\u00a0<\/p>\n Rebound hardness has more to do with elastic hardness than plastic hardness. The material absorbs the energy on impact and returns it to the indenter.<\/p>\n An indenter is a reference material used for hardness testing. Dynamic hardness is usually measured by dropping a diamond-tipped hammer on the test piece and recording the hammer\u2019s bounce after it strikes the surface.<\/p>\n The closer the height to the original dropping height, the higher the value for rebound hardness.<\/p>\n What Is Material Hardness?<\/h2>\n
Types of Hardness<\/h2>\n
Indentation Hardness<\/h3>\n
Scratch Hardness<\/h3>\n
Rebound or Dynamic Hardness<\/h3>\n
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