Voting for coating: Turning Performance

Cutting tool materials have seen only a few significant advancements in the past 100 years or so, such as the introduction of high-speed steel in the 1800s and cemented carbide in the middle of the 20th century. Today, cemented carbide still dominates the market for basic turning and milling while HSS continues to rule the tap and drill markets.

It is difficult to identify other products or technologies of that age that remain applicable in modern manufacturing. Technology that old normally has been obsolete for decades. Contemporary machining has seen the introduction of advanced materials like ceramic, polycrystalline diamond and cubic boron nitride, but these materials do not have the same broad range of applications as cemented carbide and HSS.

So how does a century-old technology remain viable at present-day machine shops? There are a few reasons. Cutting tool manufacturers have altered chemistry to improve performance and mitigate weaknesses. In other cases, manufacturers have employed advanced grinding techniques for solid tools and have improved pressed geometry for inserts. Both approaches are effective. And when combined, they offer countless combinations for creating cutting tools. These combinations routinely are amended and presented to the market with catchy names as the latest and greatest cutting tool advancements.

The application of coatings greatly enhances the qualities of the cutting tool by improving lubricity, increasing hardness and insulating the tool from elevated temperatures.

The third and probably most effective advancement has been the development of coatings that are applied to cutting tools. Working to keep up with demand for improved productivity and performance, cutting tool manufacturers started applying coatings to tools in the 1960s. Application of coatings was a step change in cutting tool technology, much like the shift from HSS to cemented carbide.

Coatings are applied using one of two processes: chemical vapor deposition or physical vapor deposition.

CVD processes are performed under very high temperatures, and the coating materials react chemically with the material of the cutting tool. Because they are bonded through chemical reactions, CVD coatings have strong adhesion to the tool. Thus, they can be applied in thick layers. CVD coatings tend to be stronger, and the thicker layers provide an excellent heat barrier to the cutting edge.

PVD coatings are applied at lower temperatures than CVD and do not bond through chemical reactions. Rather, PVD coatings are deposited on the surface of the tool, forming a protective layer. Lower application temperatures preserve the mechanical properties of sensitive cutting tool materials. PVD coatings are applied in thinner layers and, unlike CVD, do not reduce the keenness of the cutting edge.

The application of coatings greatly enhances the qualities of the cutting tool by improving lubricity, increasing hardness and insulating the tool from elevated temperatures. These improvements ultimately translate into increased tool life and higher removal rates.