Tool coating is ubiquitous. The variations are dizzying. And, we’re at an inflection point in the technology in the U.S. So, whether you’re a tool user or a tool manufacturer, it’s a good time to dive into this topic.
Chemical vapor deposition (CVD) was the first widely used technique for coating cutting tools and it still has its applications. As Gary Lake, president of CemeCon, a coating service provider and manufacturer of coating equipment based in Horseheads, N.Y., explained, the process vaporizes a metal in a heated crucible under high pressure and high temperature and, combined with a gas stream, deposits this material on the cutting tool, where it breaks down to form the coating. This approach produces a relatively thick tool coating with excellent adhesion, but there are several significant disadvantages.
First, creating the necessary chemical reaction typically requires heating the tool substrate to over 800°C (1,472°F), which restricts the choice of the carbide grade that can be coated. That, in turn, means in most cases only indexable inserts can be coated, while round solid-carbide cutting tools cannot. The materials that can be deposited with CVD are also limited to aluminum oxide (Al2O3), titanium carbide, titanium nitride and combinations thereof (TiC, TiN, and TiCN) and (just recently) aluminum titanium nitride (AlTiN). That’s because of the limited availability of the precursor gases. Lake added that differences in thermal expansion of the substrate during the coating process can lead to some degree of tensile stress in the coating during cooling, which can cause cracking in the coating.
What’s more, Lake pointed out, the by-product of this process is “a toxic chlorine gas which has to be scrubbed.” This fact alone is leading many companies away from CVD, including cutting tool manufacturer Horn USA, Franklin, Tenn. On the other hand, while Horn uses very little CVD for its product line, Training and Technical Specialist Edwin Tonne said it’s “still relevant due to its superior adhesion relative to physical vapor deposition and its ability to apply a very thick layer for situations in which you have a lot of diffusive wear from the chips and a lot of heat. I don’t think CVD will go away anytime soon.” However, he added that it’s not a good choice for applications requiring a sharp cutting edge because “in a CVD process the coating will dog bone around the edge, potentially making it duller.”
For all these reasons, physical vapor deposition (PVD) has taken over for most tools, such that Lake estimated only 30 percent are coated with CVD today. Most of those, he added, are “the alpha form of aluminum oxide, which forms at about 800°C [1,472°F] and is a very useful product for machining many different materials.”
Chief among them are steel and cast iron. An exception is the new Tiger·tec Gold insert from Walter USA, Waukesha, Wis. Walter uses what it describes as an “innovative ultra-low pressure” CVD process to coat WKP35G milling grade carbide with a special titanium aluminum nitride (TiAlN). Product Manager Sarang Garud reported that this combination is “a significant improvement over the medium-temperature TiCN aluminum oxide CVD coatings” generally employed for milling.
“The process creates an extremely smooth coating surface with reduced friction and excellent layer adhesion to the carbide substrate that retains higher toughness,” he said. “This leads to vastly improved tool life, sometimes more than 75 percent better than existing optimized processes.” Beyond these examples, there is another area in which CVD technology really shines: diamond.
An Aero Engineer’s Best Friend
It turns out that you can coat a cutting tool with diamond using the CVD process without producing the toxins of the traditional metallic process. As CemeCon’s Lake explained, it’s CVD because it uses heat to stimulate a chemical reaction, but you’re “cracking methane and hydrogen to deposit a pure diamond film.” And the temperatures required to coat tungsten carbide are typically below the sintering temperature of the carbide, removing that problem, “though they are higher than the sintering temperature of most hardened steels.” With a micro hardness of up to 10,000 Vickers, these tools are extremely wear resistant. That, and the fact that diamond has excellent heat conductivity, explains why the biggest market is in cutting carbon fiber reinforced plastics (CFRP) and glass fiber reinforced plastics (GFRP) in the aerospace market. Lake said cutting graphite for moldmaking is probably the second biggest application and diamond-coated tools are also useful for cutting abrasive aluminum alloys and ceramics.
A diamond coating can be produced by either plasma deposition or the hot filament method, and the latter was developed by CemeCon more than 20 years ago. Since then, it has gone on to create products with diamond mono layers, in which crystals of uniform size fill the entire layer, and patented multi-layer technology that “alternates different crystal sizes to make it stronger and less porous, like plywood,” as Lake put it.
CemeCon can also tweak the size of the crystals in the coating to suit the application. For example, its AeroSpeed coating has a high preponderance of nano-crystalline diamond, making it extremely smooth for a smoother cut, Lake said. That’s because the focus is on the finish of the workpiece as opposed to wear resistance.
“But nano-crystals do not have the same functional wear resistance as larger crystals,” he explained. So CemeCon MultiSpeed and FiberSpeed products have a thicker coating of larger crystals for better wear resistance for applications where that’s the focus. They’re all awesome, though, according to CemeCon. In one study, a 5.6 mm/12.55 mm carbide countersink drill coated with
AeroSpeed made 980 holes in CFRP versus the 100 holes of an uncoated countersink.
Broader Coating Options
As mentioned above, PVD is now the preferred method for metallic coatings and offers a wider choice of materials and thicknesses than CVD, with safer chemistry. Each of the current processes use an energy source to vaporize the metals in a target, combine them with a gas, and drive them to the surface of the cutting tool. Because PVD usually operates below 600°C (1,112°F), it can deposit on all carbide grades, making the technique suitable for solid round tools. In many cases the coating can even be produced below 500°C (932°F), enabling the PVD coating of HSS tools.
Furthermore, metastable coatings like aluminum chromium nitride (AlCrN) and nano-composite coatings like titanium silicon nitride (TiSiN) can be produced with this method. Another important advantage of PVD is the possibility to introduce, control, and design compressive stress in the coating, which is beneficial for many cutting applications, particularly for interrupted cutting.
Lake and others report that AlCrN based coatings can deliver an increase in hardness and oxidation resistance, leading to improved cutting performance. It’s also reported that for some specific applications, alloying with vanadium, chromium, boron, carbon or oxygen might lead to improved performance due to better tribological and mechanical properties as well as enhanced oxidation resistance and stability at high temperatures. Finally, PVD also makes it possible to coat tools with ceramics.
According to Lake, the cathodic arc process is probably the most common PVD technique today. “You literally strike an electrical arc on the face of a target, it runs around randomly, and as it does it creates a vapor along with some molten particles, because there’s a liquid phase associated with this process,” he said. With a deposition rate of 1.2-1.5 µm/hr, cathodic arc is a relatively fast method, but some of the liquid droplets may reach the surface of the tool. The second mainstream method, and CemeCon’s core technology, is sputtering, which uses thermal energy to convert the metal directly into a gas. The deposition rate is 0.8-0.9 µm/hr “on a good day,” as Lake put it, but the surface quality and coating control are improved.
The distinction can be critical for some applications, like a solid round tool such as an end mill. “A nominal coating of 3 µm is most common,“ said Lake. “For materials in which maintaining a sharp cutting edge is advantageous, a coating thickness of 1.5-2.5 µm, or a nominal of 2 µm, is desirable. That’s relatively easy to do with the sputtering process, but the arc process creates a field that preferentially deposits coating on sharp edges.”
Sphinx micro tools from BIG Kaiser Precision Tooling Inc., Hoffman Estates, Illinois, are another good example. Product Manager Cory Cetkovic said that of the roughly 6,000 parts in its catalog, 90 percent are smaller than 3 mm in diameter and they coat standard tools down to 0.2 mm in diameter and even smaller specials. So, only a precise PVD coating can “retain a sharp cutting edge, reduce the cutting forces, and do a good job of breaking the chip,” he said.
Cetkovic added that BIG Kaiser is using an intriguing double-layer concept that combines the performance benefits of two different coatings. “For example, in micro end mills we’re having a lot of success with a base layer of aluminum chromium silicon nano-composite, which is extremely heat- and scratch-resistant, has a low coating temperature, a relatively low coefficient of friction (0.3-0.4), and high micro hardness, in the 4,000 Vickers range. Once we have this hard, tough coating, we apply a low temperature molybdenum-based coating with a coefficient of friction of less than 0.1. This improves chip flow and the surface finish of the resulting hole, and also reduces tool wear. We’re taking two coatings that are essentially complete opposites and layering them to obtain the benefits of both.”
A similar example in Sphinx micro drills uses a base layer of aluminum titanium chromium nitride (AlTiCrN), a modern coating with high toughness and hardness suitable for machining stainless steel, hardened steel, and superalloys. In both examples the two layers total 3-7 µm thick (1-4 µm each) depending on the tool diameter and application. The tolerance on the total thickness is 2-3 µm.
Applying multiple layers can also inhibit the formation and propagation of micro cracks, which might otherwise occur in some applications. As is the case with a multi-layer diamond coating, the interfaces between PVD coating layers can help deflect cracks. This reduces the penetration depth of the cracks and results in significantly better tool life, said CemeCon’s Lake. Even well established coatings like TiAlN and AlCrN have been improved, and the trend is to increase aluminum content in the coatings to improve wear resistance.
High Energy in PVD Technology
High Power Impulse Magnetron Sputtering (HiPIMS) is widely regarded as the most exciting coating advancement in recent years. You could argue that it’s just another way to generate the vapor phase in PVD. But Lake referred to HiPIMS as “probably the only innovative change in the industry that I’ve seen in my 30 years of doing this.” Not only does it use a different energy source than cathodic arc and traditional sputtering, it delivers much higher energy than the original sputtering technique. The result is almost 100 percent ionization of the metal portion of the vapor, explained Lake, versus “about 70 percent in cathodic arc and 5 to 10 percent in traditional sputtering. The high level of ionization provides improved structures and ways to use polarity to improve the adhesion of the coating.” The resulting benefit is a coating that is both stronger and smoother. And in CemeCon’s patented approach, HiPIMS also delivers a deposition rate of 2 µm/hr, which is roughly 33 percent faster than the cathodic arc process.
So what’s the downside? Why hasn’t HiPIMS taken over? One reason is process complexity. As Horn’s Tonne explained, “In a standard PVD coating process you may have 20 parameters to manage. With HiPIMS you may have well over 100. But if you manage those correctly, it gives you a very high level of control over the process so you get a more consistent coating across the board.”
This is another way of saying, as Lake did, that HiPIMS has only recently been truly commercialized, and while we’re still learning, we now have well defined inputs and settings to deliver a variety of consistent coatings. Cost isn’t much of a factor. CemeCon prices HiPIMS technology 10-15% higher than traditional PVD equipment. So, depending on your workflow and how you calculate costs, HiPIMS may actually work out to be cheaper, given its higher deposition rate. Lake observed that adoption of HiPIMS by major tool manufacturers in Asia and Europe is well ahead of the United States.
“U.S. companies were locked into the arc process for a variety of reasons. But the risk of adoption is quite low because you can use this technology to produce your legacy products if you need to and still have the opportunity to introduce something new,” said Lake. “We’ve had more major companies purchase this equipment for inclusion in their product development phase in the last two years than we’ve seen in the last 20.”
HiPIMS Applications Abound
All the tool manufacturers we spoke with for this article are on board in a big way. Shane Schirmer, application and sales engineer for Horn USA, raved about the “smoother, more uniform surface” created by HiPIMS, “which also creates a sharper edge.”
He also pointed to HiPIMS’s ability to build more accurate layers. This fits nicely with Horn’s mantra of achieving consistency in everything it does, proving out a process and automating it.
Among its latest HiPIMS coated products are HP35 (which refers to a 3-µm thick coating on Horn’s proprietary “5” substrate) and HP36, which work well in materials up to 70 Rockwell in hardness.
“They work well in high-temp superalloys and also in the canning industry, where they’re getting away from CBN-tipped inserts. These new carbide inserts are more cost effective and work just as well,” reported Schirmer. “They are also effective in induction hardened materials that are too soft for CBN but too hard for standard carbide options.” He also referred to a new HiPIMS coating (EG35) doubling or even tripling tool life with no other changes to the speeds and feeds. Horn is now offering the EG coating throughout its insert lines.
Walter USA’s Garud pointed to the automotive industry trend to use more aluminum and an overall increase in stainless steel, titanium alloys, and Inconel alloys in all industries, especially in medical devices and aerospace parts. “Demand for ISO turning inserts with an extremely smooth surface finish with a tough substrate is quite high now,” he said. “For such applications, especially in finishing operations where low built-up edge (BUE), low work hardening and high precision are required, Walter uses the HiPIMS coating process.”
He added that the PVD process inherently enables an “up-sharp” tool geometry (i.e., a non-honed edge), but “because of the HiPIMS process the edges have a very high coating integrity with a very low tendency to flake off. Combined with a smooth, near-mirror finish that prevents BUE, the overall tool life is enhanced.” Walter offers specific HiPIMS coated products for aluminum alloys, Inconel (including 55+ Rc applications like Inconel 718), titanium, and stainless alloys.
Another excellent example is CemeCon’s “silicon-doped” InoxaCon product, a TiAlSiN coating deposited in layers of either 1.5 or 3 µm. With a maximum operating temperature of 1,100ºC (2,012ºF), Lake described it as a “truly remarkable product that is taking on difficult-to-cut materials like stainless steels, titaniums, and nickel-based alloys like Renés and giving performance improvements of 20-50 percent over previously applied coatings.” In one case (an 8 mm-diameter solid-carbide end mill cutting 1.4301 stainless), the InoxaCon coated tool exhibited 26 µm of wear versus 97 µm on an AlCrN-coated tool over the same time frame.
Finally there’s titanium diboride (TiB2), which is unique to CemeCon. The company developed it in the late ‘90s as a liner for aluminum die castings (because aluminum doesn’t stick to it) and realized it had potential in cutting tools. As Lake explained, “the reason even carbide tools fail when machining aluminum is because aluminum from the substrate of the workpiece transfers to the tool and the tool galls, binds, and breaks. A thin film of titanium diboride will resist transfer and gives much longer life in non-ferrous materials. We even see some benefit in titanium.”
Lake said a standard sputtering system would yield a TiB2 coating with a hardness of around 4,000 Vickers and there would be fairly high stress in the coating (which causes the coating to pull away from the tool substrate). A HiPIMS system using the same target delivers a hardness of 5,000 Vickers (25 percent harder) and less stress. The lower stress allows for a thicker coating if that’s desirable, though for non-ferrous applications CemeCon would recommend a coating of only 1.5 to 2 µm to maintain a very sharp edge. “You end up getting better adhesion, a different structure as measured by hardness, and less stress, which gives you a wider range of thicknesses to choose from. That’s the difference between what standard sputtering and HiPIMS bring to the market.”