Alumina is usually the product of aluminum burning in oxygen. At room temperature, the hardness of the crystal is about 9 (this hardness is second only to diamond), commonly known as corundum. This substance is the main component of ruby and white jade that we are familiar with.

Testing hardness with a hardness tester is a common method in modern times, and the most common testing method is Rockwell hardness test. Rockwell hardness tester measures the hardness of metal by using the depth of diamond penetrating metal. The greater the penetration depth, the smaller the hardness. The formula for calculating Rockwell hardness is HR = (k-h)/C. In the above formula, k is a constant, the diamond indenter K=0.2MM, and the hardened steel ball indenter k = 0.26 mm; H is the indentation depth of the sample after releasing the main load; C is also a constant, generally c = 0.002 mm It can be seen that the shallower the indentation, the greater the HR value and the higher the hardness of the material. Generally, the codes HRA, HRB and HRC are used to indicate the hardness of materials, where HRC indicates that the test load is1471.1n (150kg-f), and the pressure is tested with a diamond cone with the top angle of 120 degrees. In short, the higher the HRC value, the higher the hardness and wear resistance, but the greater the brittleness. I hope my answer is helpful to LZ.