Plain ceramics

Inherently high compressive strength, chemical stability and hot hardness of the ceramics led to powder metallurgical production of index able ceramic tool inserts since 1950. Alumina (Al2O3) is preferred to silicon nitride (Si3N4) for higher hardness and chemical stability. Si3N4is tougher but again more difficult to process. The plain ceramic tools are brittle in nature and hence had limited applications.

Basically three types of ceramic tool bits are available in the market;

• Plain alumina with traces of additives – these white or pink sintered inserts are cold pressed and are used mainly for machining cast iron and similar materials at speeds 200 to 250 m/min

• Alumina; with or without additives – hot pressed, black colour, hard and strong – used for machining steels and cast iron at VC= 150 to 250 m/min

• Carbide ceramic (Al2O3+ 30% TiC) cold or hot pressed, black colour, quite strong and enough tough – used for machining hard cast irons and plain and alloy steels at 150 to 200 m/min.

However, the use of those brittle plain ceramic tools, until their strength and toughness could be substantially improved since 1970, gradually decreased for being restricted to

• Uninterrupted machining of soft cast irons and steels only

• Relatively high cutting velocity but only in a narrow range (200 ~ 300 m/min)

• requiring very rigid machine tools

Advent of coated carbide capable of machining cast iron and steels at high velocity made the then ceramics almost obsolete.