Tungsten carbide is harder than sapphire and retains a superior edge after extensive use. Tungsten carbide is not a metal alloy but the product of powder metallurgy; very fine grains of two materials are mixed and fused under high temperature and pressure.
While the fine knife edge is vulnerable to chipping under lateral forces, this composite material is exceptionally strong in compression, which is the dominant force applied during sectioning.
The hardness of tungsten carbide contributes greatly to its durability when sectioning. Durability depends on many factors including types of sample being cut, frequency of use and care taken in handling the blades.
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- Thin sectioning ability - Because of the nature of tungsten carbide, there is a minimum section thickness that can be obtained. At grain boundaries the edge is microscopically discontinuous. The size of these discontinuities makes smooth sections below 1µm seemingly impossible as in these thinnest of sections minor knife marks are more apparent. Since contrast in sections below 1µm is very low, thicker sections are preferred for most light microscopy applications.
- Embedding materials - Tissues are usually embedded in plastic resins like Histocryl, LR White, Spurr's, glycol-methacrylate or methyl-methacrylate. Resins do not diminish blade life. Curiously, wax is destructive to these extremely hard edges. It seems that wax can pluck particles from the very fine edge and, however soft the wax, it quickly blunts a tungsten carbide edge.
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Tungsten Carbide products are the result of a powder metallurgy process which primarily uses tungsten carbide and cobalt metal powders. Typically, compositions of mixes will range from 4% cobalt to 30% cobalt.
The chief reason for choosing to use tungsten carbide is to take advantage of the high hardness which these materials exhibit thus retarding the wear rate of individual components. Unfortunately, the penalty attached to high hardness is a lack of toughness or strength. Fortunately, by choosing compositions with higher cobalt contents, strength can be achieved alongside hardness.
Choose low cobalt content for applications where the component will not be expected to experience impact, achieve high hardness, high wear resistance.
Choose high cobalt content if the application involves shock or impact and achieve greater wear resistance than most other materials can offer, combined with the ability to resist damage.
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