Material hardness and hardness testing


Material hardness and hardness testing

Material hardness is the property of the material which enables it to resist plastic deformation, usually by penetration or by indentation. The term of hardness is also referred to stiffness or temper, or to resistance to bending, scratching, abrasion, or cutting. It is the property of a material, which gives it the ability to resist being permanently, deformed when a load is applied. The greater the hardness of the material, the greater the resistance it has to deformation.

Hardness has been variously defined as resistance to local penetration, scratching, machining, wear or abrasion, and yielding. The multiplicity of definitions, and corresponding multiplicity of hardness measuring instruments, together with the lack of a fundamental definition, indicates that hardness may not be a fundamental property of a material, but rather a composite one including yield strength, work hardening, true tensile strength, modulus of elasticity, and others.

In mineralogy, hardness is normally described as the resistance of a material to being scratched by another material. The ability of materials to resist scratching by another material can be ranked by referring to the Mohs scale which assesses relative hardness of the materials.

In metallurgy hardness is defined as the ability of a material to resist plastic deformation. It is sometimes known as indentation hardness which is the resistance of a material to indentation. The usual type of hardness test is where a pointed or rounded indenter is pressed into a surface of the material under a substantially static load. Hardness measurement can be carried out at macro scale, micro scale or nano scale according to the forces applied and displacements obtained.

Measurement of the macro hardness of the material is a quick and simple method of finding mechanical property data for the bulk material from a small sample. It is also widely used for the quality control of surface treatment processes. However, the macro indentation depth is too large relative to the surface features when the surface coatings or surface properties of importance to friction and wear processes for materials are of concern.  Macro hardness measurements are highly variable and do not identify individual surface features where materials have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking. In such cases micro hardness measurements are appropriate.

Micro hardness is the hardness of a material as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of the material under 15 grams to 1 kg load. In micro hardness testing the indentation is usually so small that it is to be measured with a microscope. Micro hardness is capable of determining hardness of different micro constituents within a material structure, or measuring steep hardness gradients such as those encountered in case hardening. Conversions from micro hardness values to tensile strength and other hardness scales (e.g. Rockwell) are available for many metals and alloys. Micro indenters work by pressing a tip into a sample and continuously measuring the applied load, penetration depth and cycle time.

Nano-indentation tests measure hardness by indenting using very small indentation forces (of the order of 1 nano Newton) and measuring the depth of the indention which is made. These tests are based on new technology which allows precise measurement and control of the indenting forces and precise measurement of the indentation depths. By measuring the depth of the indentation, progressive levels of forcing are measurable on the same sample. This allows the determination of the maximum indentation load which is possible before the hardness is compromised and the film is no longer within the testing range. This also allows a check to be completed to determine if the hardness remains constant even after an indentation has been made.

There are various mechanisms and methods which have been designed to complete nano indentation hardness tests. One method of force application is using a coil and magnet assembly on a loading column to drive the indenter downward. This method uses a capacitance displacement gauge. Such gauges detect displacements of 0.2 Nm to 0.3 Nm (nanometer) at the time of force application. The loading column is suspended by springs, which damps external motion and allows the load to be released slightly to recover the elastic portion of deformation before measuring the indentation depth. Another method of nano indentation uses a long range piezo driver and an elastic element. When the indenter is moved downward by the piezo driver, the elastic element resists the movement and establishes a force. This force is measurable by knowing the distance which the indenter has moved downward after touching the film surface. An LVDT (linear variable differential transform) records the position of the shaft, thereby measuring the indentation depth and the spring force applied at one time.

Tests for hardness measurements

There are several types of hardness tests which are used with accuracy. Since the definitions of metallurgical ultimate strength and hardness are rather similar, it can generally be assumed that a strong metal is also a hard metal. These hardness tests measure the hardness of the metal by determining the resistance of the metal to the penetration of a non-deformable ball or cone. The tests determine the depth which such a ball or cone sinks into the metal, under a given load, within a specific period of time. The most common hardness test methods used these days are (i) Rockwell hardness test, (ii) Brinell hardness test, (iii) Vickers hardness test, (iv) Knoop hardness test, and (v) Shore hardness test.

Rockwell hardness test

Rockwell hardness test is a hardness measurement method which is based on the net increase in depth of impression as a load is applied. Hardness numbers have no units and are normally given in some scales such as the A, B, C, R, L, M, E and K scales. The higher the number in the scales means the harder is the material. Rockwell hardness test is the most used and versatile of the hardness tests.

In the Rockwell method of hardness testing, the depth of penetration of an indenter under certain arbitrary test conditions is determined. The indenter is either a steel ball of some specified diameter or a spherical diamond tipped cone called ‘Brale’ which is of 120 degree angle and 0.2 mm tip radius. The type of indenter and the test load determine the hardness scale (A, B, C, etc.).

A minor load of 10 kg is first applied, which causes an initial penetration and holds the indenter in place. Then, the dial is set to zero and the major load is applied. Upon removal of the major load, the depth reading is taken while the minor load is still on. The hardness number may then be read directly from the scale.

The hardness of ceramic substrates can be determined by the Rockwell hardness test. This test measures the difference in depth caused by two different forces, using a dial gauge. The Rockwell hardness value is determined for the load applied, the diameter of the indenter, and the indentation depth by using standard hardness conversion tables.

The Rockwell hardness testing machine to measure the hardness of metal measures resistance to penetration like the Brinell test, but in this case, the depth of the impression is measured rather than the diametric area. With the Rockwell testing machine, the hardness is indicated directly on the scale attached to the machine. This dial like scale is really a depth gauge, graduated in special units.

For soft materials such as soft steel, copper and aluminum alloys, a 1.6 mm diameter steel ball is used with a 100 kg load and the hardness is read on the ‘B’ scale. In testing of harder materials such as steels and cast irons, a 120 degree diamond cone is used with up to a load of 150 kg and the hardness is read on the ‘C’ scale.

Rockwell testing machine uses two loads with one is applied directly after the other. The first load (known as the minor load) of 10 kg is applied to the sample to help seat the indenter and remove the effects, in the test, of any surface irregularities. The purpose of the minor load is to create a uniformly shaped surface for the application of the major load. The difference in the depth of the indentation between the minor and major loads provides the Rockwell hardness number.

There are several Rockwell scales other than the B and C scales, which are known as the common scales. The other scales also use a letter for the scale symbol prefix, and many use a different sized steel ball indenter. A properly used Rockwell designation has the hardness number followed by ‘HR’ (Hardness Rockwell), which is followed by another letter which indicates the specific Rockwell scale. For example, 60 HRB indicates that the specimen has a hardness reading of 60 on the B scale.

There is another Rockwell testing machine known as the ‘Rockwell superficial hardness testing machine’. The working of this machine is similar to the standard Rockwell testing machine, but it is used to test thin strip, or lightly carburized surfaces, small parts or parts that might collapse under the conditions of the regular test. This machine uses a reduced minor load, just 3 kg, and has the major load reduced to either 15 kg or 45 kg depending on the indenter, which are the same ones used for the common scales. Using the 1.6 mm diameter, steel ball indenter, a ‘T’ is added (meaning thin sheet testing) to the superficial hardness designation. For example, a superficial Rockwell hardness is 15T-25, which indicates the superficial hardness as 25, with a load of 15 kg using the steel ball. If the 120 degree diamond cone is used instead, then ‘T’ is replaced with ‘N’.

The standardized a set of scales (ranges) for Rockwell hardness testing under use is normally designated by a letter.  This set is given below.

  • A scale – It is used for cemented carbides, thin steel, and shallow case hardened steel.
  • B scale – It is used for copper alloys, soft steels, aluminum alloys, and malleable iron, etc.
  • C scale – It is used for steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel and other materials harder than the harness value of B100.
  • D scale – it is used for thin steel, medium case hardened steel, and pearlitic malleable cast iron.
  • E scale – It is used for cast iron, aluminum and magnesium alloys, and bearing metals.
  • F scale – It is used for annealed copper alloys and thin soft sheet metals.
  • G scale – It is used for phosphor bronze, beryllium copper, and malleable irons.
  • H scale – It is used for aluminum, zinc, and lead.
  • K, L, M, P, R, S, and V scales – They are used for bearing metals and other very soft or thin materials, including plastics.

Brinell hardness test

Brinell hardness is determined by forcing a hard steel or carbide sphere of a specified diameter under a specified load into the surface of a material and measuring the diameter of the indentation left after the test. The Brinell hardness number, or simply the Brinell number, is obtained by dividing the load used, in kg, by the actual surface area of the indentation, in sq mm. The result is a pressure measurement, but the units are normally not stated.

The Brinell hardness test uses a desk top machine to press a 10 mm diameter, hardened steel ball into the surface of the test sample. The machine applies a load of 500 kg for soft metals such as copper, brass and thin stock. A 1500 kg load is used for aluminum, and a 3000 kg load is used for materials such as iron and steel. The load is usually applied for 10 seconds to 15 seconds. After the impression is made, the diameter of the resulting round impression is measured. The measurement is done to +/- 0.05 mm using a low magnification portable microscope. The hardness is calculated by dividing the load by the area of the curved surface of the indention. The area of a hemispherical surface is arrived at by multiplying the square of the diameter by 3.14159 and then dividing by 2. Usually for ready reference, a calibrated chart is provided, so with the diameter of the indentation the corresponding hardness number can be known. A well-structured Brinell hardness number reveals the test conditions, and looks like this, “80 HB 10/500/30” which means that a Brinell hardness of 80 is obtained by  a 10 mm diameter hardened steel ball with a 500 kg load applied for a period of 30 seconds. On tests of extremely hard metals, a tungsten carbide ball is used in place of the steel ball. Among the three hardness tests namely Rockwell, Brinell, and Vickers, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which usually more accurately account for multiple grain structures and any irregularities in the uniformity of the material.

The Brinell hardness test was one of the most widely used hardness tests during World War II for the measuring armour plate hardness, The test was usually conducted by pressing a tungsten carbide sphere 10 mm in diameter into the test surface for 10 seconds with a load of 3,000 kg, then measuring the diameter of the resulting depression.

The BHN is calculated according to the following formula as shown in Fig 1.  In this Fig, BHN is the Brinell hardness number, F is the imposed load in kg, D is the diameter of the spherical indenter in mm, and Di is the diameter of the resulting indenter impression in mm.

calculation-of-brinell-hardness-number

Fig 1 Calculation for Brinell hardness number

Several BHN tests are usually carried out over an area of the material sample. On a typical material sample, each test results in a slightly different number. This is due to not only the minor variations in quality of the material (even homogenous material is not absolutely uniform) but also because the test relies on careful measurement of the diameter of the depression. Small errors in this measurement leads to small variations in BHN values. As a result, BHN is usually quoted as a range of values (e.g. 220 to 250 or 220 -250) rather than as a single value. The BHN of face hardened sample uses a back slash ‘\’ to separate the value of the face hardened surface from the value of the rear face. For example, a BHN of 550\350 – 380 indicates the face hardened surface has a hardness of 550 and the rear face has a hardness of 350 to 380.

The Brinell hardness test method given above is called ‘HB 10/3000 WC’. Other types of hardness tests use different materials for the sphere and/or different loads. Since softer materials deform at high BHN, hence tungsten carbide is used to measure the hardness of steels with high hardness. When the BHN values are higher than 650, the tungsten carbide ball begins to flatten out and the BHN values indicate a greater difference in hardness than the actual harness is, while BHN value of above740 the ball flattens out so badly that it cannot be used.

When there are widely different values for quoted BHN then the Poldi hardness testing machine can be used in place of the Brinell hardness testing machine. The Poldi hardness test is less accurate than the Brinell hardness test but can be used in the field. The Poldi hardness test has the advantage that the testing unit is portable, so measurements can be carried out in the field. The Poldi portable unit relies on a hammer blow impression in a standardized sample.

In the standard test method for determining the Brinell hardness of metallic materials, the load applied is usually 3,000 kg, 1,500 kg, or 500 kg, so that the diameter of the indentation is in the range 2.5 mm to 6 mm. The load is applied steadily without a jerk. The full test load is applied for 10 seconds to 15 seconds. Two diameters of impression at right angles are measured, and the mean diameter is used as a basis for calculating the Brinell hardness number (BHN), which is done using the conversion table given in the standard.

Vickers hardness test

Vickers hardness is a measure of the hardness of a material, calculated from the size of an impression produced under load by a pyramid shaped diamond indenter. Devised in the 1920s by engineers at Vickers limited in the United Kingdom, the diamond pyramid hardness test, as it also became known, permitted the establishment of a continuous scale of comparable numbers that accurately reflected the wide range of hardnesses found in steels.

It is a standard method for measuring the hardness of metals, mainly those with extremely hard surfaces. In this method of the hardness testing the surface is subjected to a standard pressure for a standard length of time by means of a pyramid shaped diamond. The diagonal of the resulting indention is measured under a microscope and the Vickers hardness value is read from a conversion table.

The indenter employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex at an angle of 136 degrees. The diamond is pressed into the surface of the material at loads ranging up to around 120 kg, and the size of the impression (normally less than 0.5 mm) is measured with the aid of a calibrated microscope. The Vickers number (HV) is calculated using the formula HV = 1.854(F/D2), where with F is the applied load (measured in kg) and D2 the area of the indentation (measured in square millimeters). The applied load is usually specified when HV is mentioned.

The Vickers test is reliable for measuring the hardness of metals, and also used on ceramic materials. The Vickers testing method is similar to the Brinell test. Rather than using the steel ball type indenter of the Brinell test and have to calculate the hemispherical area of impression, the Vickers testing machine uses a penetrator which is square in shape, but tipped on one corner so it has the appearance of a diamond of the playing card. The Vickers indenter is a 136 degrees square based diamond cone. The diamond material of the indenter has an advantage over other indenters because it does not deform with time and use. The impression left by the Vickers penetrator is a dark square on a light background. The Vickers impression is more easily ‘read’ for area size than the circular impression of the Brinell method. Like the Brinell test, the Vickers number is determined by dividing the load by the surface area of the indentation (H = P/A). The load varies from 1 kg to 120 kg.

To perform the Vickers test, the specimen is placed on an anvil which has a screw threaded base. The anvil is turned raising it by the screw threads until it is close to the point of the indenter. With start lever activated, the load is slowly applied to the indenter. The load is released and the anvil with the specimen is lowered. The operation of applying and removing the load is controlled automatically.

Several loadings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness machines. A filar microscope is swung over the specimen to measure the square indentation to a tolerance of plus or minus 1/1000 of a millimeter. Measurements taken across the diagonals to determine the area are averaged. The correct Vickers designation is the number followed “HV” (Hardness Vickers). The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments. Although thoroughly adaptable and very precise for testing the softest and hardest of materials, under varying loads, the Vickers machine is a floor standing unit that is rather more expensive than the Brinell or Rockwell hardness testing machines.

Knoop hardness test

The relative micro hardness of a material is determined by the Knoop indentation test. This test method was devised in 1939 by F. Knoop and colleagues at the National Bureau of Standards in the United States. By using lower indentation pressures than the Vickers hardness test, which had been designed for measuring metals, the Knoop test allowed the hardness testing of brittle materials such as glass and ceramics.

Knoop hardness test is a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known load, for a specified dwell time, and the resulting indentation is measured using a microscope. The hardness of the material is determined by the depth to which the Knoop indenter penetrates.

The diamond indenter employed in the Knoop test is in the shape of an elongated four-sided pyramid. The geometry of this indenter is an extended pyramid with the length to width ratio of 7:1 and respective face angles of 172 degrees for the long edge and 130 degrees for the short edge. Pressed into the material under loads which are often less than one kg, the indenter leaves a four sided impression of around 0.01 mm to 0.1 mm in size. The length of the impression is about seven times the width, and the depth is 1/30 of the length. Given such dimensions, the area of the impression under load can be calculated after measuring only the length of the longest side with the aid of a calibrated microscope. The final Knoop hardness (HK) is derived from the formula HK = 14.229(F/D2) where F is the applied load (measured in kg) and D2 the area of the indentation (measured in square millimeters). Knoop hardness numbers are often cited in conjunction with specific load values.

It is the standard test methods for indentation hardness of organic coatings. In this test, Knoop hardness determinations are made at 23 +/- 2 deg C temperature and 50 +/- 5 % relative humidity. The samples are equilibrated under these conditions for at least 24 hours. They are then rigidly attached to the movable stage so that the surface to be measured is normal to the direction of the indentation. The apparatus is preset to apply a 25 grams load. The time the indenter is in contact with the sample is to be 18 +/- 0.5 seconds. The length of the long diagonal of the impression is measured with the filar micrometer eyepiece. The procedure is repeated until at least five impressions have been made at widely spaced locations. The Knoop hardness number is then calculated by the formula given above.

Shore hardness test

Shore hardness is a measure of the resistance of material to indentation by 3 spring-loaded indenters. The higher is the number, the greater is the resistance. The Durometer scale was defined by Albert Ferdinand Shore, who developed a device to measure Shore hardness in the 1920s. The term Durometer is often used to refer to the measurement as well as the instrument itself. Durometer is typically used as a measure of hardness in polymers, elastomers, and rubbers.

The shore scleroscope measures hardness in terms of the elasticity of the material. A diamond-tipped hammer in a graduated glass tube is allowed to fall from a known height on the sample to be tested, and the hardness number depends on the height to which the hammer rebounds. The harder is the material, the higher is the rebound.

The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as ‘Durometer hardness’. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the hardness reading can change over time. Hence the indentation time is sometimes reported along with the hardness number. The results obtained from this test are a useful measure of relative resistance to indentation of various grades of polymers. However, the Shore Durometer hardness test does not serve well as a predictor of other properties such as strength or resistance to scratches, abrasion, or wear, and is not to be used alone for product design specifications.

Shore hardness, using either the Shore A or Shore D scale, is the preferred method for rubbers / elastomers and is also generally used for ‘softer’ plastics such as polyolefins, fluoropolymers, and vinyls. The Shore A scale is used for ‘softer’ rubbers while the Shore D scale is used for ‘harder’ ones. The shore A hardness is the relative hardness of elastic materials such as rubber or soft plastics can be determined with an instrument called a Shore A Durometer. If the indenter completely penetrates the sample, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 results. The reading is dimensionless.

Mohs hardness test

Mohs hardness is defined by how well a substance will resist scratching by another substance. It is rough measure of the resistance of a smooth surface to scratching or abrasion, expressed in terms of a scale devised by the German mineralogist Friedrich Mohs in 1812. The Mohs hardness of a mineral is determined by observing whether its surface is scratched by a substance of known or defined hardness. To give numerical values to this physical property, minerals are ranked along the Mohs scale, which is composed of 10 minerals that have been given arbitrary hardness values. Mohs scale is given in Tab 1.

Tab 1 Mohs hardness scale
Mineral Hardness
Talc 1
Gypsum 2
Calcite 3
Fluorite 4
Apatite 5
Orthoclase 6
Quartz 7
Topaz 8
Corundum 9
Diamond 10

As is indicated by the ranking in the scale, if a mineral is scratched by orthoclase but not by apatite, its Mohs hardness is between 5 and 6. In the determination procedure it is necessary to be certain that a scratch is actually made and not just a ‘chalk’ mark that will rub off. If the species being tested is fine-grained, friable, or pulverulent, the test may only loosen grains without testing individual mineral surfaces; thus certain textures or aggregate forms can hinder or prevent a true hardness determination. For this reason the Mohs test, while greatly facilitating the identification of minerals in the field, is not suitable for accurately gauging the hardness of industrial materials such as steel or ceramics. Another disadvantage of the Mohs scale is that it is not linear; that is, each increment of one in the scale does not indicate a proportional increase in hardness. For instance, the progression from calcite to fluorite (from 3 to 4 on the Mohs scale) reflects an increase in hardness of approximately 25 percent; the progression from corundum to diamond, on the other hand (9 to 10 on the Mohs scale), reflects a hardness increase of more than 300 percent. A comparison of Mohs hardness with the Knoop hardness is given in Fig 2.

comparison-of-mohs-hardness-and-knoop-hardness

Fig 2 Comparison of Mohs hardness with Knoop hardness

Barcol hardness test

Barcol hardness is a method by which a hardness value obtained by measuring the resistance to penetration of a sharp steel point under a spring load. The instrument, called the Barcol impressor, gives a direct reading on a 0 to 100 scale. The hardness value is often used as a measure of the degree of cure of a plastic.

Barcol hardness test method is used to determine the hardness of both reinforced and non-reinforced rigid plastics. The sample is placed under the indenter of the Barcol hardness tester and a uniform pressure is applied to the sample until the dial indication reaches a maximum. The depth of the penetration is converted into absolute Barcol numbers.

Barcol hardness values are also used to determine degree of cure of resin. Resin is considered cured when it has a hardness value greater than or equal to 90 % of the surface hardness value.