The definition of hardness is the resistance of the material opposing to localized deformation. This term can easily apply to several types of deformation, whether from indentation, cutting, bending, or scratching. When it comes to metals, ceramics, and most polymers, deformation is considered plastic deformation of its surface. When we talk about elastomers and specific polymers, the definition of hardness is the resistance instead of elastic deformation of the surface. It doesn’t exist a fundamental description that could indicate the hardness is an intrinsic property of a material, yet a composite one that brings significant contributions from the yield strength, work hardening, modulus, true tensile strength, and some other factors. Engineers take hardness measurements to check the quality of materials. These measurements are usually quick and seem to be considered nondestructive tests because the marks or indentations taken during the test are typically low-stress areas. 

A large variety of methods can be used to determine the hardness of a substance. A couple of the most popular ones are detailed in the following lines. 

Mohs Hardness Test

Everybody should know that one of the oldest ways of measuring the hardness of material was devised by the German mineralogist Friedrich Mohs in 1812. The test was named after him, and it involves observing if the material’s surface can be scratched by a known substance or a specifically defined hardness. The goal was to give numerical values to the physical property, so the minerals were ranked along the Mohs scale. This scale contains ten minerals that received arbitrary hardness values. If you want to accurately gauge the hardness of industrial materials, including steel or ceramics, this is not the perfect fit. Mohs hardness test greatly facilitates the identification of several minerals in the field, but it’s not suitable for steel or ceramics. When it comes to engineering materials, one can use a variety of instruments that, over the years, were developed to provide a precise measure of hardness level. Many usually apply a load and then measure the depth or size of the resulting indentation. In conclusion, hardness can be measured on macro, micro, and nanoscale. 

Brinell Hardness Test

One should know that the oldest of the hardness test methods is the Brinell hardness test, which can be used on engineering materials. This test was invented by Dr. J. A. Brinell in Sweden back in 1900. It does use a desktop machine to apply a specific load to a hardened sphere of a specified diameter. The Brinell hardness number also called the Brinell number, is a value obtained by dividing the weight used (kilograms) by the measured surface area of the indentation (square millimeters). This test is usually used to determine the hardness metal of forgings and the casting, which has a prominent grain structure. This test can measure over a reasonably large area less affected by the entire course grain structure of the materials that are both the Rockwell and the Vickers tests. 

One can test various materials with the Brinell test by varying the test load and indenter ball size. This test is usually conducted on iron and steel castings in the USA using a 3000 kg test force combined with a 10 mm diameter ball. One can use a 1500 kg load for aluminum castings, a 500 kg load for copper, brass, and thin stock, and a 5- or 10-mm diameter ball. The same test is conducted in Europe using a more extensive range of forces and ball diameters. The general idea is to use a 1 mm carbide ball and a 1 kg load to test small parts. People often refer to these load tests as baby Brinell tests. Both the Brinell hardness number and the test conditions should be reported. For example, when the value stated looks like “60 HB 10/1500/30”, the Brinell hardness is 60 using a 10mm diameter ball and a 1500 kg load applied for 30 seconds. 

Rockwell Hardness Test

This test is usually conducted using a machine that applies a specific load and then measures the depth of the resulting impression on a surface. For example, the indenter may be a spherical diamond-tipped cone of 120° angle and a 0.2 mm tip radius called a brale or a steel ball of specified diameter. First, the tester will apply a 10 kg load that will cause a reduced initial penetration to seat the indenter and remove the effects of surface irregularities that may appear. After this point, the dial is set to zero, and the tester applies the first load. The depth reading is taken once the significant weight is removed, keeping the minor load on. This is when the hardness number can be read directly from the scale. One should remember that the indenter and the test load can determine the hardness scale used – A, B, C, etc. 

When it comes to soft materials, including copper alloys, aluminum alloys, or mild steel, one can use a 1/16″ diameter steel ball with a 100-kilogram load while the hardness value can be read on the “B” scale. However, if you want to test some more rigid materials such as cast iron and many steel alloys, one must use a 120 degrees diamond cone with a maximum of 150-kilogram load while the hardness can be read on the “C” scale. “C” scales are the standard used scales. The correct Rockwell value will contain the “HR” letters upon the hardness number and the scaling letter. For example, 100 HRC states that the material has a hardness reading of 100 on the C scale. 

A -Cemented carbides, shallow case-hardened steel, and thin steel 

B -Copper alloys, aluminum alloys, soft steels, malleable iron, etc.

C -Steel, titanium, pearlitic malleable iron, hard cast irons, deep case-hardened steel, and other materials harder than B 100

D -Thin steel and medium case-hardened steel and pearlitic malleable iron

E -Cast iron, bearing metals, aluminum, and magnesium alloys,

F -Annealed copper alloys, thin, soft sheet metals

G -Phosphor bronze, malleable irons, beryllium copper,

H -Aluminum, lead, zinc technique 

K, L, M, P, R, S, V -Bearing metals and other very soft or thin materials, including plastics.

Rockwell Superficial Hardness Test

This test is often used to test thin materials, some lightly carburized steel surfaces, or even some parts that keep bending or crushing under a regular test. This test needs the same items as the Rockwell tester, yet the loads are much reduced. Usually, the examiner uses a 3-kilogram load while the significant weight is between 15 and 45. This test requires a 1/16″ diameter steel ball indenter; a “T” is added to the superficial hardness designation. For example, when one reads 23 HR15T, a hardness value of 23, 15 kilograms load, and a steel ball. 

Vickers and Knoop Microhardness Tests

This type of test is more than a modification of the Brinell test used to measure the hardness of the thin film coatings or the surface hardness of case-hardened parts. One small diamond pyramid is pressed into the sample under reduced loads during these tests than those used for the Brinell test. The main difference between the Vickers and the Knoop Tests is just the shape of the diamond pyramid indenter. This test requires a square pyramidal indenter prone to cracking brittle materials. More than that, the test requires a rhombic-based pyramidal indenter. The indenter was developed to produce longer and shallower indentations. The Knoop indentations are 2.8 times longer than the Vickers indentations for the same load. 

The load varies from 10g to 1000 g, and this low amount can create a small indent that will then be measured under a microscope. Due to such low indents, one has to take hard coatings like TiN at very high magnification. Therefore, the surface usually needs to be polished. The diagonals of the impression will be carefully measured, and the values will be used to obtain a hardness number called VHN, usually from a lookup table or chart. This test can characterize some sturdy materials, yet the hardness will be measured over a reduced region. 

The hardness values will be expressed as 2600 HK25 or HK25, which means that the hardness is 2600 Knoop at a 26-gram force load. Also, one should know that the Knoop and Vickers hardness values will differ slightly. However, these values will be close enough for hard coatings to be within the measurement error, which can be used interchangeably. 

Scleroscope and Rebound Hardness Tests

This type of test is an ancient one that involves dropping the diamond-tipped hammer inside a glass tube under its weight force from a fixed height onto the test specimen. This height will be measured on a graduated scale. This scale will be arbitrarily chosen, consisting of Shore units divided into 100 parts. Each part represents the average rebound from pure hardened high-carbon steel. Also, the scale will continue higher than the 100 units to include the metals with a higher hardness. The Shore Scleroscope will measure the hardness of a material in terms of material elasticity. The hardness value will depend on the height to which the hammer will rebound. The harder the material, the higher the rebound. 

The Rebound Hardness Test Method is nothing but a recent advancement that builds on the Scleroscope. Many electronic instruments on the market can measure the impact body’s energy loss. This type of device typically uses a spring to accelerate tungsten, spherical carbide-tipped mass, towards the surface of your test object. When the weight contacts the surface, it has specific kinetic energy so that the impact will produce an indentation on the surface. This indentation will take some power from the impact body. The impact body will lose more energy and rebound velocity, that’s less when a larger indentation is produced on softer material. One will measure the rates of the impact body before and after impact and the loss of velocity related to Brinell, Rockwell, or any other typical hardness value. 

Durometer Hardness Test

The Durometer is the instrument engineers use to measure the indentation hardness of elastomers/rubbers and soft plastics, including vinyl, polyolefin, and fluoropolymer. This device usually uses a calibrated spring that applies pressure to the indenter foot. The indenter foot form is either a cone or a sphere. An indicating device measures the depth of indentation. Durometers usually come in a vast diversity of models, and by far, the most popular model is the Model A; it’s currently used to measure softer materials. In comparison, Model D is explicitly used for more rigid materials.  

Barcol Hardness Test

This type of test aims to obtain a hardness value by merely measuring the penetration amount of a sharp steel point under a spring load. The specimen must be placed under this device indenter, while a uniform pressure is usually applied until the dial indication reaches the maximum point. This test is generally used to measure the hardness of reinforced and non-reinforced rigid plastics, and it can determine the value of the cure of resins and plastics.