Reinforced or whiskered, ceramics use extremely fine-grained silicon-carbide crystals that are called “whiskers” because they resemble small hairs under a microscope to reinforce and toughen basic ceramic compounds.
In ceramic tool materials, single-crystal silicon carbide whiskers, on the order of one micron in diameter and 0.003937 in. (100 microns) in length, are intertwined within the alumina-matrix structure. These whiskers have a tensile strength of about 1 million psi and dramatically improve the fracture toughness of the tool material. They also effectively block and prevent propagation of cracks.
Reinforced ceramics work differently from other cutting materials. With reinforced ceramic cutting tools, the objective in machining is to generate high temperatures ahead of the cutting tool to soften or plasticize the workpiece material. That facilitates the removal of material and a reduction in cutting forces. The ideal cutting temperature in nickel alloy is about 1,800 degrees F, for example.
Cutting with ceramic inserts requires high surface speed and balanced feedrates. High speed is necessary to generate the high temperature in the shear zone and to ensure that the heat propagates into the chip-forming zone immediately ahead of the cutter. When cutting speeds are too slow, insufficient heat is generated to soften the material in this zone, and the cutting forces are raised and insert failure occurs.
A strategy for using ceramic inserts is to program fewer, but deeper cuts that bury the insert deep in the workpiece. This moves the notch formation further up the face of the insert to an area that has a larger, stronger cross section. Ramping cuts should be programmed to accommodate these tools and fixed depths of cut should be avoided to spread wear over a larger section of the insert.
When machining interrupted cuts with reinforced ceramics, it is important to keep the speed of the cutter high. A rule of thumb is to estimate the percentage of voids in the workpiece surface and increase cutting speed by that percentage. This increase in surface speed offsets the loss of heat generation created by the voids.
Whiskered ceramics work best on hard ferrous materials and difficult-to-machine nickel-base alloys, including Inconel, Waspoloy and Hastelloy. They do not work well on ferrous alloys below Rc 42 hardness because of the chemical reaction that occurs between iron and the carbon that is part of the silicon carbide reinforcing material.
REF: http://www.cutting-tool.americanmachinist.com/BDEList.aspx
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