Image

"By creating a coating with a smoother surface, less friction occurs and the chips flow across the surface of the insert better," said Graham. "We do this by orienting the crystal structure of the aluminum oxide layer parallel with the insert's surface."

In the future look for continued development of harder, more wear-resistant PVD coatings that can run at higher speeds, getting closer to the performance of CVD coatings in that respect.

The coating type is chosen based on a number of variables, but the most important factors are the workpiece's material properties and the application range that the insert is designed for. For steels, good crater wear and abrasive wear resistance are critical, so aluminum oxide (Al2O3) is almost always a component of coatings intended for steel machining.

Coating type is chosen based on a number of variables, but the most important factors are the workpiece’s material properties and the application. Image courtesy of Seco Tools.

It is this thermal barrier that is important in high-speed operations in which plastic deformation is the key concern.

The Fume Eliminator 860 (FE860) is a portable, on-torch fume extraction solution designed for enhanced functionality, performance and safety. The FE860 can be carried throughout your work area and connects directly to the welding torch to capture fume at the source - the most effective method.

The substrate is the foundation for the coating. Having the correct foundation enables the coating to be supported and bonded in the best possible way and enhances the overall performance of the tool.

"These trends will continue to move the industry toward the ideal cutting tool—hard, wear-resistant, and with excellent edge toughness—enabling higher-productivity machining across the whole spectrum of materials," said Ludeking.

Inserts with a combination of coatings are very common in turning operations, so these tools can be used in a wider variety of materials and conditions.

"Manufacturers can design coatings that combine the best properties of the different coatings and provide excellent resistance to several wear modes," said Ludeking.

These high temperatures within the cutting zone can enter the cutting edge, leading to plastic deformation of the tool, which reduces tool life. Applying a coating to the carbide insert improves wear resistance, allowing a single insert to remove more material. In most cases, higher machining speeds also can be achieved.

While the coating's makeup itself is important, it also needs to be applied to the correct substrate to get the maximum benefits.

For example, carbon and alloy steels generate high temperatures during the turning process. They will also react chemically with the carbide insert. The tool's coating must be able to resist the heat and allow the chips to flow smoothly to reduce crater wear and flank wear. The different properties of other materials result in different wear modes, which the coating needs to resist.

Like any metal cutting operation, the best place to start is by examining the workpiece material. The type of material being turned plays a key role in determining whether a coated or uncoated carbide insert is needed.

"About 80 percent of inserts used today are coated carbide grades. They give machinists the ability to increase metal removal rates and cutting speeds, while increasing tool life well beyond what is possible with an uncoated insert," said Graham.

Chemical vapor deposition (CVD) and physical vapor deposition (PVD) are the two main coating processes applied to carbide inserts. CVD coatings are the thicker (often 5 to 20 microns) of the two and are very wear-resistant. This makes them suitable for steels and cast irons.

"By layering the coatings we can tailor a product to tackle a specific problem or material," said Graham.

Keep up to date with the latest news, events, and technology for all things metal from our pair of monthly magazines written specifically for Canadian manufacturers!

Image

"Because each layer has its own function, such as adding toughness, lubricity, or wear resistance, combining them together gives us the best of each."

"These properties make PVD coatings very useful for workpiece materials that are tough, sticky, or work-harden easily like stainless, high-temperature alloys, and nonferrous alloys," said Ludeking.

Image

"A turning operation, because it's typically a continuous cut, creates a lot of heat," explained Don Graham, manager of education and technical services for Seco Tools. "This makes it essential that a coated tool be used."

"Selecting the correct coating type means taking both coating processes and all of the coating materials into consideration," said Graham.

"The workpiece material properties tell us quite a bit about what the machining conditions will be like at the cutting edge," said Kurt Ludeking, product manager for Walter Tools. "The coating must be chosen to resist the wear modes that will operate during machining and the high temperatures created during cutting."

"Since aluminum is a 'soft' material, BUE is a major concern," said Ludeking. "Preventing this requires an extremely smooth surface, as well as a very sharp cutting edge. Because nonferrous materials don't generate the high temperatures at the cutting edge, uncoated inserts are still effective on these materials. Coated inserts will still have a significant speed and tool life advantage, although it is smaller in nonferrous alloys. "

Texturing enables engineers to control the growth of the Al2O3 crystal structure, aligning it in such a manner that it creates a much smoother surface. This engineering of the coating at the atomic level achieves three goals:

"These properties make them much more challenging for the coating because there are additional wear modes to protect against," said Ludeking.

The PVD coating process provides a smooth coating that adheres well to sharp edges. Because PVDs are thinner than CVD coatings, and because they are applied at lower temperatures, they don't affect the toughness of the tool the way CVD coatings do.

Easily access valuable industry resources now with full access to the digital edition of Canadian Metalworking.

The increasing use of difficult-to-machine materials continues to place pressure on tooling manufacturers to develop new products that can withstand the heat generated from the cutting process, while providing longer tool life.

As with CVD, the PVD coating type is chosen based on the intended application and the wear modes that will be created during the machining process. Each coating has unique hardness, chemical, and physical characteristics that make it appropriate for different material classes.

Easily access valuable industry resources now with full access to the digital edition of Canadian Fabricating & Welding.

Superalloys (nickel-, cobalt-, and iron-based) are hard, very tough, and generate even higher cutting temperatures at the same cutting speeds as steels. Because they also work-harden very easily, they tend to create depth-of-cut notching, said Ludeking.

Textured coating typically has an inner TiCN base layer, which adheres to the substrate and creates strength at the cutting edge, and a top layer of Al2O3 that forms a thermal barrier, permitting higher cutting speeds.

"The CVD coating process depends on chemical reactions, and the coatings are bonded to the substrate both chemically and physically," explained Ludeking. "This makes it possible to create relatively thick coatings that provide excellent crater and flank wear resistance, as well as an excellent thermal barrier protecting the substrate."

Also expected is the further development of CVD processes that take advantage of preferred orientation of the coating grains, known as texturing, for higher wear and heat resistance, combined with postcoating processes that make the coatings tougher and more resistant to chipping.

Titanium, another high-temperature material, has most of the same properties as the superalloys, except that crater wear tends to be the most common wear mode, and BUE tends to be more common as well.

Using steel as an example, a multilayer coating provides crater wear, flank wear, and plastic deformation resistance far better than TiN, TiCN, or Al2O3 could provide on their own.

"Fortunately for end users, cutting tool manufacturers match the many technical details of substrates and coatings into highly effective products, so all that is required is to choose the correct grade for the application," said Ludeking.

This structural change creates a coating that has better mechanical and thermal properties, as well as better wear resistance and toughness, said Graham. Seco's textured coating, for example, has an inner TiCN base layer, which adheres to the substrate and creates strength at the cutting edge, and a top layer of Al2O3 that forms a thermal barrier, permitting higher cutting speeds.

PVD coatings are thinner (often 1.5 to 4 microns) than CVD but are tougher and smoother. This makes them the correct choice for difficult-to-cut materials.