Coatings Targeted to Specific Materials
Steel, cast iron, heat-resistant alloys head the list for PVD, CVD
By Jim Lorincz
PVD, CVD-coated, or uncoated—which cutting tool choice is the best for the application at hand?
Chemical vapor deposition (CVD) coating is a mature technology tracing its origins to the 1960s and widespread use for turning steel and cast iron, especially in automotive applications. Physical vapor deposition (PVD) coating technology is of more recent origin, tracing its development to the 1990s and growth to the emergence of titanium, Inconel, heat-resistant superalloys, nickel-based alloys, and other difficult-to-machine materials for aerospace, energy, medical, and chemical industry applications. Thick CVD coatings with titanium carbonitride (TiCN) and aluminum oxide (Al2O2) provide the wear resistance needed for steel and cast iron turning and interrupted cuts.
Picking the right coating type must take into account both coating processes and materials. "Chemical vapor deposition [CVD] and physical vapor deposition [PVD] are the two main coating processes for carbide inserts, each one providing interesting features and benefits. For example, CVD coatings are thick [typically 9–20 µm] and highly wear-resistant, making them well-suited for steel and cast iron machining as well as being widely used in turning operations," said Don Graham, manager of education and technical services, Seco Tools Inc. (Troy, MI). "Unfortunately, however, such thick coatings can compromise edge toughness. PVD coatings are thin [typically 2–3 µm] yet tougher and typically smoother than CVD coatings. Consequently, they are useful for machining materials, such as superalloys, titanium alloys, and difficult-to-machine stainless steels that typically notch or chip cutting edges."
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 feature 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-surfaces & coatings, Sandvik Coromant (Stockholm, Sweden).
"PVD and CVD are not competing techniques or processes. 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 optimized residual stress and hardness properties that increase wear resistance.
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.
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, randomly oriented. The same is true in our coating. The smaller the grain size, the harder the material and the more wear resistant 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."
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, meaning all of the Inconel 718, Renes, and Waspaloys etc. We have to plan the kind of tooling materials that will be needed. Titanium aluminide is abrasive, stiff, tough, and doesn’t conduct heat. Its machinability rating is very low. For example, if you made one part out of soft steel and it took one tool to make a part; it would take 20 tools to machine the part out of titanium aluminide, compared with 6 to 7 tools to machine the part from Inconel 718," said Graham.
Preferred Orientation of Crystals
"When a customer has a problem, we look at the wear mode and our stable of technology solutions," said Kurt Ludeking, product manager-turning, Walter USA LLC (Waukesha, WI). "We use a lot more micrograin substrates for their balance of toughness and hardness due to resistance to deformation. The primary role for coatings is wear resistance, but coatings also play an important role in edge toughness. Through careful design of the coating system, the substrate, and post-coating processes, edge toughness has been increased significantly with our Tiger-tec Silver turning inserts for steel," said Ludeking.
"For steel turning, we’ve changed the aluminum oxide coating to include preferred orientation of the crystals which significantly improves crater wear resistance and heat resistance that can lead to deformation of the cutting edge. Any time you have heavy depth of cut or interrupted cuts especially, chipping of the coating is a big issue. By changing the technology that we apply to the coating and the post treatment that we do, we’ve been able to increase compressive stresses and increase the chipping resistance on these new inserts," said Ludeking. "I’m talking turning here, but this technology was first implemented in milling. We’ve used the same preferred orientation technology, the blasting process, and the edge geometry in milling to improve our results, achieving gains of 30–40% in tool life and 15–30% increases in speed."
While the coating and substrate have the largest impact on tool life and application area, the edge geometry and form of the chip-breaking features are also critical. "By optimizing the edge geometry and chip flow through the chip breaking area, Walter Valenite has made additional improvements in tool life for a variety of applications," said Ludeking. "Examples include Tiger-tec Silver Turning for steel, high-temperature alloy NMS/NRS geometries with PVD Al2O3. New coatings technologies in both CVD and PVD have extended tool life significantly. For PVD coatings, this has also extended their use into new areas such as milling hardened steels and grooving in cast iron with grades WHH15 Tiger-tec Silver PVD.
"Uncoated inserts are used mainly for nonferrous applications such as aluminum, brass, and softer alloys, though not as much for stainless steels anymore," said Ludeking. "Uncoated inserts are really for those applications that require a very sharp edge. Whenever you use a coating, you can’t make the edge super sharp. Coatings just don’t stick to it, and also carbide polishes better than a coating and polished carbide works better for aluminum, brass, and sticky alloys."
Drilling Process has its Own Parameters
In the case of drilling, it’s important to understand the process to be able to design appropriate tools. "An example of this is drilling. While the coating is important for wear resistance in the cutting zone, it isn’t helpful in the drill flutes," said Ludeking. "Even the smoothest coating is rough enough to create additional friction and slow down chip flow, which is especially important in difficult-to-machine materials. By masking the flutes from coating and polishing the carbide surface in the flutes, chip flow is enhanced. Here also geometry plays an important role—not only the drill point geometry, but also the flute geometry has a major influence on drill performance, for example with our Xtreme DM drill."
For drilling applications, Guhring Inc. (Brookfield, WI) has developed a new high-penetration rate drill for nickel alloys. The RT100HF features a new point geometry that helps dissipate heat, a common enough problem when machining nickel alloys. The drill features a double margin flute design for stability in the cut and an improved surface finish. The RT100HF incorporates a new ultra-hard surface coating called the nano-Si. This silicon-based PVD coating is said to be 57% harder than traditional TiAlN-based coatings with the same heat-resistant properties.
Nickel-based heat-resistant alloys are noted for their extremely poor machinability. According to OSG TP & Die Inc. (Glendale Hts., IL), they are approximately ten times more difficult to machine than stainless steels. Due to their superior heat resistance, toughness, and ductile characteristics, they are commonly used in turbine blades of jet engines and other aircraft-related components where a high level of heat resistance is required. In recent years, however, nickel-based alloy applications have become more widespread for such applications as automotive components and household combustion equipment.
To meet industry demand, OSG has developed a total solution for machining nickel-based heat-resistant alloys, the EXOPRO-WWHO-Ni 3D and 5D carbide drill series. The drills are engineered with sharper cutting edges for machining treated nickel-based alloys. The sharp cutting edges are designed to suppress the generation of heat during machining and promote stable creation of cutting chips. A weak twist shape is used to attain high tool rigidity for machining high-precision holes. The drills are able to produce small cutting chips to enable trouble-free chip evacuation and stable machining. The OSG’s WXS coating enables high-speed machining with internally fed water-soluble coolant for longer tool life.
Products for Machining Tough Materials
Higher-temperature strength materials require high cutting forces when machining. To meet the requirements for turning tough alloys, both in higher speeds and feeds for faster turnaround times and longer tool life (more parts per tool), KCU10 Beyond grade from Kennametal Inc. (Latrobe, PA) for OD and ID turning, grooving, plunging, undercutting, and threading and KCU25 Beyond grade for threading, grooving, cutoff, and other turning applications both benefit from new PVD coating technology. Special surface treatments improve machining performance in high-temperature materials increasing in many cases, speeds, feeds, or depth of cut while reducing related wear issues like depth-of-cut notching. Beyond KCU10 also features a dual-layer coating applications with a top layer of AlSiTiN atop a second layer of AlTiN. The boundary between the two layers serves to help deflect microcracks.
A new family of turning inserts from Iscar Metals Inc. (Arlington, TX) brings SumoTec performance and durability to CVD and PVD carbide grades. "The SumoTec special treatment smooths the coated surface and reduces imperfections left from the coating process. The post coating treatment reduces the friction between the tool and the workpiece materials. The end result is a lower temperature, improved resistance against edge chipping, reduced built-up edge, and better all around edge life reliability in a broad range of material applications," said Randy Hudgins, Iscar national product manager-turning. SumoTec carbide grades include IC8150, IC8250, and IC8350 for mild steel and alloy steel, IC5005 and IC5010 for cast iron, IC6015 and IC6025 for stainless steel, IC806 for superalloys, and IC807 for general-purpose applications.
Emuge Corp. has introduced Top Cut, a new series of high-performance carbide end mills that features a new variable helix flute technology, and a newly developed variant of the high heat-resistant TiAlN coating. Top Cut end mills are made of micrograin carbide with proprietary relief ground cutting edges for performing roughing as well as finish-cutting operations. Emuge offers a wide range of thin film wear-resistant coatings, including titanium nitride (TiN), titanium carbonitride (TiCN), and chromium nitride (CrN) for its taps, end mills, and carbide thread mills. ME
This article was first published in the January 2013 edition of Manufacturing Engineering magazine. Click here for PDF.