Diamond

Diamond is the hardest known (Knoop hardness ~ 8000 kg/mm2) material that can be used as a cutting tool material. It has most of the desirable properties of a cutting tool material such like high hardness, low friction, good thermal conductivity, non-adherence to most materials, and good wear resistance. However, the factors that weigh against its use are the high cost, possibility of oxidation in air, allotropic transformation to graphite above temperatures of 700^oC, very high brittleness and difficulties associated in shaping it to a suitable cutting tool form.

Natural diamond tools can be used for relatively light cuts and have an extremely high tool life, which justifies their high cost. However, a natural diamond is unreliable because of the impurities present in that and its easy cleavage. Artificial diamonds which are basically polycrystalline are extensively used in industries as a tool material because they can be formed to any given shape with substrates of cemented carbide.

They are used with a negative rake angle (- 5^o) for machining hard materials whereas, positive rake angles (15^o) can be used for soft materials such as copper. They cannot be used for machining low carbon steels, titanium, nickel, cobalt or zirconium because of the possible reaction with the work material.