Tech Talk
What is the difference between a CVD and a PVD coating?
More than 90% of all tool coatings are applied using either the Physical Vapor Deposition (PVD) or the Chemical Vapor Deposition (CVD) process. Each process offers the tool user advantages under specific conditions. By following a few simple guidelines, the user can determine which process is most likely to maximize production for a given application.
PVD is the most widely recognized coating process. Simply put, PVD is a line-of-sight process. This means that the hard coating material is transferred from a source to the tool, traveling in a straight line. PVD coatings usually are applied as a single layer of titanium nitride (TiN), titanium carbonitride (TiCN), or titanium aluminum nitride (TiAlN). PVD coatings have two distinct advantages over CVD coatings. First, the coatings’ 3µm to 5µm thickness works very well on sharp-edge tools, where a thicker coating might have trouble adhering and cause a dulling effect. Second, PVD is done at relatively low temperatures; therefore, it can be used on HSS and tool steels without adversely altering the steel’s underlying properties.
During the CVD process, the coating is virtually grown on the surface of the tool. Coatings applied with the process have three distinct advantages over PVD coatings. First, the CVD process will generally result in a better bond with the substrate, and better adhesion can result in increased performance. Second, the CVD process allows for much thicker coatings, which means more protection for the cutting edge and a potentially longer tool life. Third, tool coaters can apply a variety of multilayer coatings with the CVD process. By combining several different coating layers, the coater can take advantage of each layer’s unique strength. The end result is a coating that will make it possible for users to achieve a greater increase in production than they could achieve with any one layer by itself. The first CVD coatings were single layers of titanium carbide (TiC) about 5µm thick. Today’s CVD coatings range in thickness from 3µm to more than 14µm. CVD coatings are typically applied in multiple layers, combining TiC, TiCN, TiN, and aluminum oxide (Al2O3) in a variety of ways to create the finished product. CVD is now being used successfully in turning, milling, threading, grooving, extrusion, and many other applications. Historically, tool coaters and shops have chosen to use the PVD process to coat sharp edges on such tools as indexable inserts, form tools, drills, and endmills because the thin-film coating produced by PVD worked best for these applications. But advancements in the CVD coating process have made more choices available. With more precise automated control of the CVD process, coaters can apply a much thinner coating with consistency and repeatability. This has made it practical for several companies to offer a thin-film CVD coating for sharp-edge tools. Thin-film CVD coatings have proven to be effective on a variety of carbide cutting tools, including standard inserts, top-notch inserts, form tools, threading-and-grooving tools, and many other styles. Researchers also have tested these coatings on carbide drills and endmills, but their preliminary findings show that thin-film CVD coatings do not boost performance in these applications. Therefore, PVD may still be the coating of choice for drilling and endmilling. PVD continues to be the first choice for coating tight-tolerance steel as well.
