The thinnest of thin layers can make a big difference in tool life and performance.
In the early days of tungsten carbide tools, before coatings, tool manufacturers realized the tools would last longer and resist cratering if they put a little bit of titanium carbide (TiC) in the mix when making the tool. This had the desired effect, but the more TiC that was added, the weaker and more brittle the tool became. Then someone hit on the idea of applying a thin layer of TiC to the surface of the tool. It worked. This was in about 1970, said Don Graham, manager, turning products and education services, Seco Tools,Inc., Troy, Mich.
A year or so later, tooling companies started using a titanium nitride (TiN) coating, and in 1973, aluminum oxide began to be used.
Graham described the properties of these three coatings:
- TiC gives abrasion resistance and prevents the chip from dissolving the tool material, leaving craters.
- TiN prevents a built-up edge, where the workpiece material sticks to the cutting edge. This spoils the surface finish and also, when the buildup is dislodged, it pulls away part of the coating and maybe the cutting edge. This coating is the familiar gold-colored one.
- Aluminum oxide provides resistance to heat in two ways. First, it is an outstanding thermal insulator; second, and more important, it is stable to very high temperatures.
These coatings are applied by a chemical vapor deposition (CVD) process, Graham said. The tools are placed in a chamber. At 950 – 1100 degrees C, gases are pumped into the chamber, where they react and deposit a thin layer of material on each tool.