Cutting Tools Thinner PVD Coatings for Sharp Edges
“PVD coatings are typically thinner coatings which are very suitable for solid tools, i.e. solid drills, solid-carbide end mills and taps where you want to have sharp edges. Another fea- ture of the PVD coating is that you have compressive stresses that really keep the coating together, producing a much tougher coating,” said Helen Blomqvist, senior manager-sur- faces & coatings, Sandvik Coromant (Stockholm, Sweden).
Workpiece Materials are a Good Place to Start “With today’s vast array of advanced coating processes and coating materials, it’s not always easy to determine the best insert grade for your application. The best place to start is with the workpiece material because the type of material being machined plays a key role in determining whether a coated or uncoated carbide insert is needed,” said Seco’s Graham. “Coated carbide inserts are a must for working with ferrous materials such as iron, cast iron, steel, or stainless steel. When machining superalloys, you’ll want to use a coated insert most of the time, especially when cutting alloys with medium-to-high machinability ratings. Titanium alloys also benefit with coatings, especially when not using high- pressure coolant,” said Graham.
Walter’s Tiger-tec Silver turning inserts feature preferred ori- entation of the crystals to improve deformation and crater wear resistance and chipping resistance for steel turning.
“PVD and CVD are not competing techniques or process- es. They are complementary and now overlap to some degree, in deposition temperature but also in the materials used. Aerospace applications involve machining difficult-to-machine materials, titanium, Inconel, and high-temperature alloys. Usually, in these areas PVD coatings are principally used with some complementary use of CVD coatings,” said Blomqvist. Sandvik Coromant grades GC4234 and GC3234 for replaceable head drills feature a tough micrograin cemented carbide with a PVD coating made with new technology. The multilayered TiAlN coatings in each grade are engineered for the specific requirements of the process. GC4234 for ISO P (steel) drilling features low-residual stresses that enhance edge line chipping resistance for crater and flank wear resistance; GC3234 for ISO K (cast iron) drilling features op- timized residual stress and hardness properties that increase wear resistance.
Seco has had good success with a nano layer coating, which isn’t particularly new, but alternates layers of titanium aluminum nitride and titanium silicon nitride and keeps the grain size of the coating very small. “By grain size, I mean the individual elements that make up the coating,” said Graham. “All metallic materials have grains, like cement blocks, ran- domly oriented. The same is true in our coating. The smaller the grain size, the harder the material and the more wear re- sistant the coating. We’ve had good success with our TH1000 turning grade for hard materials and machining superalloys at high speeds in combination with our Jetstream high-pressure coolant toolholder systems. We can machine Inconel 718 at 600 SFM and get good tool life. We’re also experimenting with the use of aluminum oxide coatings at higher speeds on superalloys. Aluminum oxide coatings don’t work very well at low speeds, meaning 100 or 200 SFM [30.5 or 61 m/min], but if you can get the speed up to 600 or 700 SFM [183 or 213.4 m/min] which we can with Jetstream, we get very good tool life with these products.”
“PVD coatings typically have much thinner coatings which are very suitable for solid
tools, solid drills, solid-carbide end mills, and taps where you want to have sharp edges.”
One material that isn’t common yet, but that is coming is titanium aluminide. It’s a lightweight metal with excellent high-temperature properties. “The material has the potential to replace much of the nickel-based materials in a jet engine,