Cemented carbide

Cemented carbide is a hard material used extensively as cutting tool material, as well as other industrial applications. It consists of fine particles of carbide cemented into a composite by a binder metal. Cemented carbides commonly use tungsten carbide (WC), titanium carbide (TiC), or tantalum carbide (TaC) as the aggregate. Mentions of 'carbide' or 'tungsten carbide' in industrial contexts usually refer to these cemented composites. Most of the time, carbide cutters will leave a better surface finish on the part, and allow faster machining than high-speed steel or other tool steels. Carbide tools can withstand higher temperatures at the cutter-workpiece interface than standard high-speed steel tools (which is a principal reason for the faster machining). Carbide is usually superior for the cutting of tough materials such as carbon steel or stainless steel, as well as in situations where other cutting tools would wear away faster, such as high-quantity production runs. Cemented carbides are metal matrix composites where carbide particles act as the aggregate and a metallic binder serves as the matrix (like gravel aggregate in a matrix of cement makes concrete). Its structure is thus conceptually similar to that of a grinding wheel, except that the abrasive particles are much smaller; macroscopically, the material of a carbide cutter looks homogeneous. The process of combining the carbide particles with the binder is referred to as sintering or hot isostatic pressing (HIP). During this process, the binder eventually will be entering the liquid stage and carbide grains (much higher melting point) remain in the solid stage. As a result of this process, the binder is embedding/cementing the carbide grains and thereby creates the metal matrix composite with its distinct material properties. The naturally ductile metal binder serves to offset the characteristic brittle behavior of the carbide ceramic, thus raising its toughness and durability. By controlling various parameters, including grain size, cobalt content, dotation (e.g., alloy carbides) and carbon content, a carbide manufacturer can tailor the carbide's performance to specific applications. The first cemented carbide developed was tungsten carbide (introduced in 1927) which uses tungsten carbide particles held together by a cobalt metal binder. Since then other cemented carbides have been developed, such as titanium carbide, which is better suited for cutting steel, and tantalum carbide, which is tougher than tungsten carbide. The coefficient of thermal expansion of cemented tungsten carbide is found to vary with the amount of cobalt used as a metal binder. For 5.9% of cobalt a coefficient of 4.4 µm·m−1·K−1 is found, whereas the coefficient is around 5.0 µm·m−1·K−1 for a cobalt content of 13%. Both values are only valid from 20 °C (68 °F) to 60 °C (140 °F), but more data is available from Hidnert. Carbide is more expensive per unit than other typical tool materials, and it is more brittle, making it susceptible to chipping and breaking. To offset these problems, the carbide cutting tip itself is often in the form of a small insert for a larger tipped tool whose shank is made of another material, usually carbon tool steel. This gives the benefit of using carbide at the cutting interface without the high cost and brittleness of making the entire tool out of carbide. Most modern face mills use carbide inserts, as well as many lathe tools and endmills. In recent decades, though, solid-carbide endmills have also become more commonly used, wherever the application's characteristics make the pros (such as shorter cycle times) outweigh the cons (mentioned above). To increase the life of carbide tools, they are sometimes coated. Five such coatings are TiN (titanium nitride), TiC (titanium carbide), Ti(C)N (titanium carbide-nitride), TiAlN (titanium aluminium nitride) and AlTiN (aluminium titanium nitride). (Newer coatings, known as DLC (diamond-like carbon) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal CVD and, for certain applications, with the mechanical PVD method. However, if the deposition is performed at too high temperature, an eta phase of a Co6W6C tertiary carbide forms at the interface between the carbide and the cobalt phase, which may lead to adhesion failure of the coating.