Hardness tests are used to determine the hardness of metals, rocks and minerals.
Hardness tests measure a material's resistance to scratching or abrasion, cutting, or permanent deformation caused by indentation under pressure. These tests could more precisely be said to test the materials for "give" (yield strength), tensile strength, or elasticity rather than for actual hardness. The Mohs hardness test is used to test minerals and gemstones. One of three scales --- the Rockwell, Brinell, or Vickers hardness tests --- is used to test metal strength.
The Rockwell Hardness Scale
The Rockwell Hardness test examines a metal's resistance to bending, shape change or penetration when force is applied. A non-deformable ball or cone is pressed into the metal under varying degrees of pressure either to see if the ball or cone will damage the metal, or to find out how much pressure it takes to damage the metal. The results are compared to the Rockwell Hardness Scale, which lists 15 letters of the alphabet (A to H, K to M, P to S, and V). These letters correspond to specific indentation tools, a load or pressure amount, and are assigned an arbitrary numerical value. The numerical values increase with the hardness of the metal.
THe Brinell Hardness Test
Like the Rockwell Hardness Scale, the Brinell Hardness test use the non-deformable balls or cones to look for a metal's resistance to bending, shape change or penetration when force is applied. The Brinell test doesn't use a scale. It determines the metal's hardness by using a microscope to measure the diameter of the indentation, then dividing the force applied during the test by the diameter's surface area.
The Vickers Hardness Test
The Vickers Hardness test uses a ball or cone to test for metal hardness in the same manner as both the Rockwell Hardness Scale and the Brinell Hardness Test. The Vickers Test performs two indentations at an angle of 136 degrees, applied for 10 to 15 seconds. The two indentations are then examined under a microscope to determine the area of indentation. The results of the two indentations are averaged.
The Mohs Hardness Scale
The Mohs Hardness Test examines the resistance of minerals and gemstones to scratching and abrasion and compares their hardness relative to a scale of minerals and gemstones with a known hardness value. The scale ranges from talc --- a soft mineral talc with a value of one --- to the hardest substance known -- the diamond -- which has a hardness value of 10. As with the Rockwell Hardness Scale, the numerical values are arbitrary. The difference in hardness between each numerical value on the Mohs Scale is also arbitrary. The difference between one and two on this scale is small as compared to the much greater difference in hardness between 9 and 10.
Tags: Hardness Scale, Rockwell Hardness, Rockwell Hardness Scale, ball cone, Brinell Hardness, force applied, minerals gemstones
