Hardness, as applied to most materials, particularly metals, has been defined in many ways by physicists, metallurgists and engineers. These definitions range from a “conglomeration of properties of a material more or less related to each other” to the commonly accepted idea of “resistance to permanent indentation.” These varied definitions point out the complexity of truly defining hardness so that it satisfies all the criteria associated with the characteristics of hardness.

Before considering the widely used indentation method of hardness testing, mention should be made of other concepts of hardness determination, such as resistance to scratching, wear or abrasion and cutting, in addition to rebound and the measurement of rebound velocity.

The general categories or types of hardness tests are as follows:

Static Indentation Test

As the most commonly used type of static indentation test, a ball, diamond cone or a specific shaped diamond is forced into the material being tested. The relationship of the test force to the area or depth of the indentation is the measure of hardness. The Rockwell, Brinell, Knoop, Vickers and Ultrasonic hardness tests are of this type. This test shows how materials deform in their plastic range after having first passed through their elastic limit and acquiring a permanent set.

Scratch Test

One material is capable of scratching another. The Moh’s scale and Bierbaum scratch are of this type. Moh’s scale, developed in 1822, is still used today for classifying minerals. The scale consists of 10 original minerals numbered arbitrarily from one to 10 as follows:

Diamond 10

Apatite 5

Corundum 9

Fluorite 4

Topaz 8

Calcite 3

Quartz 7

Gypsum 2

Orthoclase

6 Talc 1

Dynamic Hardness Test

The dynamic hardness of a material is the measure of rebound or velocity of the rebound produced by a rapidly falling mass that includes an indenter. The most commonly used test method employs the Scleroscope tester, invented in 1906 by A.F. Shore and still used today. In this test, a diamond-tipped hammer inside a graduated glass tube is allowed to fall under its own weight from a fixed height on to the test specimen. The resulting rebound is measured from the graduated glass scale. A variation on this test is done by measuring the return velocity of the hammer instead of the height of rebound. The Leeb test is a modern electronic version of the Scleroscope. It uses a carbide ball hammer that is spring rather than gravity powered. An electronic sensor measures the velocity of the hammer as it travels toward and away from the surface of the sample.

Abrasive and Cutting Hardness

There are two types-one relating to a material's resistance to wear and the other to the cutting ability. The abrasive mode usually requires the specimen to be mounted on a rotating table and is subjected to a wearing action of an abrasive wheel applied at a specific pressure. The cutting mode is an indication of the workability of metals and involves variables such as sharpness, speed and amount of pressure applied to the cutting tool. Abrasion of materials has been the subject of many investigations through the years and is associated to hardness in a more comparative manner than other methods of testing.

Because of the many variables associated with this testing, such as friction, lubrication, speed of test, surface conditions and other factors, no one method is recommended for determining abrasive hardness. Wear is closely related to friction and lubrication and these two factors are distinct phenomena. As a result, test procedures and interpretation varies with this type of testing. The development of a wear tester gives the material engineer and scientists an invaluable source for determining ranking wear resistance for the purpose of optimizing material selection or developing a given application.

Electromagnetic (Eddy Current)

This test uses the principle of electro-magnetism. An electric current is applied to a coil, producing an electro-magnetic field in the coil, which builds and collapses with the frequency of the current. The test specimen of a conducting material is placed within this field and generates its own alternating current called eddy current. By using devices such as a magnetic bridge system, an eddy current tester can distinguish variations in mass, shape, conductivity, permeability, hardness and alloy content to that of a known material by the relative amounts and types of eddy current produced. An experienced operator can determine, to some degree, case depth hardness, but not as accurately as through indentation hardness. Also, this type of testing is extremely fast-restricted only by how fast the test specimen can be passed through the coil-making it suitable for in-line, high volume sorting of ferrous metals.

Ed Tobolski is a hardness product manager at Instron (Norwood, MA). For more information, call (800) 473-7838, e-mail [email protected] or visit www.instron.com.