Hardness is resistance of material to plastic bend caused by indentation. Occasionally hardness Applies to resistance of material to scratching or abrasion. In various cases relatively quick and minimal hardness test could substitute tensile test. Resistance may be measured from a small sample of material without Breaking it.
Hardness is one of the most commonly measured Characteristics of a ceramic. Its value helps to characterize resistance to deformation, densification, and fracture.
Resistance is important for cutting tools, wear and abrasion-resistant parts, prosthetic hip-joint balls and sockets, optical lens glasses, ballistic armor, molds and dies, valves, and seals. In fact, many ceramic specifications list minimum hardness requirements.
The hardness of ceramic supplies is a property which is of distinguished magnitude as it relates to the capability of the material to withstand penetration of the surface through a combination of brittle fracture and plastic flow.
Often, hardness is directly equated to wear resistance. This is a mistaken notion with many metallic components and is absolutely an incorrect selection criterion with regards to engineering ceramic materials.
The world standards for regular ceramic resistance have varying requirements for control of loading rate during the indentation cycle. A literature analysis suggests that loading rate may affect measured hardness in several instances. In view of the uncertainty over this problem, extra experiments over a range of indentation loading rates were achieved on a steel, sintered silicon carbide, and an aluminum oxynitride.
Contemporary standard methods for static resistance measurements limit the rate of load application, but there is extensive variability in the requirements. The requirements are limitations to the rate of indenter motion, the loading cycle time, or a vague statement to prevent impact.
Ceramics are commonly more brittle than metals and can have analogous stiffness (modulus of elasticity) and similar strength, particularly in compression. But in a tensile test they are likely to fail at a much lower applied stress. This is because the surfaces of ceramics nearly always contain minute cracks ("Griffith cracks"), which magnify the applied stress.
The oldest method of testing hardness was to use a hard file on the test piece and see how difficult it is to remove material. The next method is called the Rockwell hardness test. This is the most widely used test in the seal business. Resistance is read on two separate scales. The most accepted is the "C" scale that uses a diamond cone. The less accepted "B" scale utilizes a ball analogous to that used in the Brinell 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 kilograms, by the actual surface area of the indentation, in square millimeters.The consequence is a pressure measurement, but the units are rarely confirmed.
Vickers Hardness test It is the standard method for measuring the resistance of metals and ceramics, particularly those with exceptionally durable surfaces: 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 read from a conversion table.
The relative microhardness of a material is determined by the Knoop indentation test. In this test, a pyramid-shaped diamond indenter with apical angles of 130 and 172 30is pressed against a material. Making a thombohedral impression with one diagonal seven times longer than the other. The hardness of the material is determined by the depth to which the Knoop indenter penetrates.
The shore scleroscope measures resistance 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 specimen to be tested, and the hardness quantity depends on the height to which the hammer rebounds; the harder the material, the higher the rebound. 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 should not be used alone for product design specifications.
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Article Added on Wednesday, January 20, 2010
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