Whiskers toughen ceramics for hardest cuts.

Author:Smith, Keith H.
 
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Outstanding hardness of whisker-reinforced ceramic inserts has made possible the machining of many aerospace materials previously workable only by grinding. Greatest savings to date have been shown in the heat-treated alloy steels, die steel, weld overlays, hard facings, and hard irons.

Though first introduced to machine aerospace nickel-based alloys, whisker-reinforced ceramic inserts are also demonstrating their ability to turn hard materials other than nickel alloys in the 45-65 Rc range.

Machining speeds using the whisker-reinforced ceramic WG-300 material can be increased up to eight times those of uncoated tungsten carbide tools and four times those of coated carbide tools.

The secret of whisker-reinforced ceramics is their ability to withstand high temperatures while maintaining strength and hardness. Heat generated in the shear zone ahead of the tool has been traditionally thought of as a negative factor in machining and associated with heat-related failure of cemented carbide cutting tools, unless, of course, cutting speed was reduced to a point where carbide inserts would give acceptable life.

Ceramic inserts dramatically affect the mechanism for chip formation, which can be likened to the sideways slide of a deck of cards caused by the rake face of the tool. The chip is formed first by grain boundary distortion in front and below the shear plane, followed by grain boundary dislocation. This results in a chip which is always thicker than the layer of material being removed.

A large amount of shear stress is required to cause plastic deformation and shear to occur in the shear zone. This results in the generation of significant quantities of heat. In fact, as much as 80% of the heat generated during cutting is produced in this way. The other 20% comes from the sliding of the chip over the tool rake face and the contact of the flank of the tool with the workpiece.

Most of the heat generated during metal cutting is dissipated by the chip carrying it away. As cutting speeds increase, however, the process reaches a point at which heat generated in the shear zone cannot be conducted away during the very short time in which the metal passes through this zone, and the heat attacks the insert.

Because whisker-reinforced ceramic inserts can withstand high temperatures while maintaining strength and hardness, there is an optimum cutting speed--outside the range of carbide tools--where the heat generated lessens the cutting forces by softening the metal and...

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