Coating of cutting tools
Materials Engineering Dept.
College of Engineering
University of Diyala, Iraq
Today’s needs in terms of machining by chip removal are becoming more demanding as a result of the use of advanced cutting parameters. Hence the need to achieve high performance tools capable of withstanding heavy removal, or high feed. For this purpose, the tool must be built with the best raw material and in strict compliance with the maximum precision and tighter tolerances. But all this may not be enough if we do not take into account a proper surface finishing of the tool combined with the best coating.
Methods for Coating Cutting Tools
There are two primary processes in place for coating cutting tools: CVD (Chemical Vapor Deposition) and PVD (Physical Vapor Deposition). Each of the methods has its own advantages and disadvantages. CVD coating was the original and most commonly used coating method for many years. The CVD method involves heating up the substrate within a chemical reactor and exposing the substrate to a gas stream. The gases break down on the hot substrate surface, forming a coating layer. In general, the CVD method requires temperatures around 1,000 degrees C.
A common coating uses the three gases TiCL4 (titanium tetrachloride), H2 (hydrogen), and N2 (nitrogen) to produce TiN (titanium nitride) + HCl (Hydrogen Chloride). The HCl is a bi-product of the process and must be disposed of according to strict environmental regulations. The advantages of the CVD method include optimal layer adhesion, as well as consistent layer distribution. The disadvantages of the CVD method are high temperatures affecting the substrate, few suitable materials for coating as the coating material is fed in a gaseous form, and long cycle times
PVD coating is the newer of the tool coating methods and is becoming increasingly popular in the industry. The PVD method involves transporting laminate material in a vacuum from a source via a transport space to the substrate. The laminate material is vaporized using either thermal or electrical energy from the power source, which then allows the vaporized material to adhere to the substrate.
The advantage of the PVD process is the range of suitable materials for coating, relatively low-operating temperatures, around 450° C, allowing for coating of sharp cutting edges. The disadvantages are that coating of internal surfaces is difficult (coating requires a line of sight from the laminate material to the substrate) and the surface requirements of the substrate are much higher
Primary Coating Methods
With PVD, there are two primary technologies used to coat the different substrates: the arc method (arc discharge) and the sputtering method (cathodic sputtering). Both methods share one additional advantage; the coating chambers are relatively easy to construct. The arc method involves an electrical power source (much like a lightning bolt) hitting the laminate material and transforming this material from a solid to a liquid to a gaseous phase. The advantage of this process is high layer rates (in relation to sputtering). However, since the laminate material is in all three phases (solid, liquid and gas) the potential for droplets (minute liquid particles) occurs. These droplets do not achieve the gaseous state. The sputtering method involves a thermal energy source, which transforms the solid laminate immediately to a gaseous state. No droplets occur as the material skips the liquid phase. However, the lower layer rates (in relation to arc), results in longer cycle times.
Hard Material Coatings
Most hard materials (coating is a hard material) consist of a metal and a metalloid. Some examples of familiar coatings are TiN (titanium nitride), TiCN (titanium carbo nitride), TiAlN (titanium aluminum nitride), AlTiN (aluminum titanium nitride) and AlCrN (aluminum chromium nitride). The Periodic Table of Elements shows the inventory of metals and the metalloids that are potential candidates for coatings. During the coating process, the smaller metalloid—in the case of TiN, the nitrogen or N—lodges itself in the lattice vacancies of the metal Titanium (Ti). When switching to TiCN, the carbon (C) partially replaces some of the Nitrogen (N). Following the same logic, the metals and metalloids required for the other sample coatings can be determined. This is one of the advantages of the PVD process. Since the metal is solid in the PVD chamber (CVD introduces in a gaseous phase), almost any metal is usable for coating. Of course, not all metals are beneficial, but they are available for use