Because the action of machine tools is constant, cutting and grinding away material at high RPMs to shape workpieces, carbide cutting tools often are selected instead of hardened steel, to retain a sharp cutting edge and prolong tool life. However, when highly abrasive materials like carbon-fiber reinforced polymer/glass-fiber reinforced polymer sandwich materials (CFRP/GFRP), graphite, aluminum alloys, and ceramics are machined — even carbide tools may be prone to rapid wear.
In those cases, further hardening of cutting tools with various specialty coatings can significantly prolong wear-resistance and service life. For cutting tools that are extremely expensive, this not only reduces costs but speeds overall cycle time too. For applying to cutting tools like taps, reamers, end mills, drills, inserts, counter sinks, and others, these coatings come in a variety of styles from physical vapor deposition (PVD) coatings up to proprietary diamond coatings.
Need for greater abrasion protection — In a growing number of industries, including automotive and aerospace, manufacturers are emphasizing design and weight reduction. Consequently, designers increasingly are using composite fiber reinforced plastics in many parts. However, these composites are exceedingly rough on cutting tools.
“The problem with the carbon and graphite fibers is that they are very high-strength and extremely abrasive,” explained Volker Derflinger, a senior manager for Oerlikon Balzers, which has been producing specialized coatings for components and tools for over 30 years. “For cutting tools to withstand heavy wear, a specialty coating with a very high resistance to abrasion is needed.”
In the automotive and other industries that require stronger, lighter materials, aluminum-silicon alloys are used, too. However, the higher the silicon content, the more abrasive becomes the material.
“With aluminum-silicon alloys, there are very hard silicon particles embedded in the aluminum,” Derflinger said. “When you have to cut the material, the silicon content is extremely abrasive and can rip up the carbide tool. Even tooling with typical protective hard coatings can degrade very quickly.”
High-performance coatings — When it comes to machining very abrasive materials, uncoated carbide tools experience accelerated wear. To increase tool life, high-performance coatings can provide a vital protective barrier. According to Derflinger, the ideal coating would have qualities such as a very hard, protective surface that maintains the sharp cutting edges, allowing clean and precise cuts while speeding production time.
PVD coatings. One alternative that is increasingly being adopted in these industries are strong, non-hazardous, Physical Vapor Deposition (PVD) coatings. PVD describes a variety of vacuum-deposition methods that can produce thin coatings. PVD typically is used to coat tools and components at relatively low temperatures of 150-500°C, which avoids altering the fundamental material properties.
Among the PVD options are several carbon-based coatings that provide a unique combination of extreme surface hardness and low-friction coefficient properties. One example, BALINIT Hard Carbon, by Oerlikon Balzers is used to coat tools for machining nonferrous materials, including aluminum alloys with up to 12% silicon content (AlSi-12). The coating is ideal for CFRP and GFRP materials, if the volume of fibers is not too high.
“The hard carbon coating works on CFRP and GFRP, but only when the fiber content is on the lower side,” Derflinger reported. “The more fiber content, the more abrasive the material is and then you will need an even harder coating.”
The BALINIT hard-carbon coating is thin, smooth, and has high hardness (40-50 GPa) that makes the coating well suited to applications that require superior wear protection. In addition, the thin, smooth application helps to maintain sharper cutting edges.
For example, at a Malaysian manufacturer producing HDD aluminum alloy baseplates, coated-carbide end-mill tooling exhibited less abrasive wear and produced 95% more parts, with 55% lower production costs than untreated tooling.
The combination of coating hardness along with a low-friction coefficient also can dramatically improve production even with dry machining. In an application involving CFRP and thermoplastics workpieces, for instance, a hard carbon-coated countersink tool produced 180% more parts than an uncoated tool. In another comparison, a coated carbide end mill doubled the parts produced with dry machining, compared to an untreated tool using lubricant.
Diamond coatings. When carbon content in composites or silicon content in aluminum alloys becomes too high, cutting tools typically require a diamond coating to prevent premature wear. Traditionally, polycrystalline diamond (PCD) cutting tools have been used in such instances. PCD is a composite of diamond particles sintered together with a metallic binder. Diamond is the hardest of all materials, and therefore the most abrasion-resistant, too.
As a cutting tool material, PCD has good wear-resistance but it lacks chemical stability at high temperatures and dissolves easily in iron. So, PCD-coated tools usually are limited to cutting materials such as high-silicon aluminum, metal matrix composites (MMC), and CFRPs.
In addition, PCD tools are geometrically limited in structure, and may be too rough or unrefined for the optimal machining of the wide range of nonferrous materials.
Finally, PCD cutting tools can be quite expensive. As an alternative, plasma-assisted chemical vapor deposition (PACVD) can be used to apply crystalline diamond structures in varying thicknesses and roughnesses. This can be highly advantageous for machining fiber-reinforced plastics, graphite, nonferrous materials, and ceramics. The diamond coating extends tool life while also improving cutting quality and surface finish.
With the PACVD coating process, a carbide cutting tool is sequentially coated by two different gasses in a heated vacuum container, assisted by plasma. Each alternating cycle that builds the atomic layer on the surface and the number of cycles thus controls the thickness of the final coating.
“As a cost effective, high-performance alternative, specialized PACVD-based diamond coatings can increase the service life of the tool,” Derflinger stated.
Derflinger noted that standard PVD-applied metal-doped carbon coatings have a hardness of approximately 15 GPa, whereas a “diamond-like” carbon coatings range from 20 to 50 GPa. In comparison, a diamond coating reaches a hardness of 80 to 100 GPa.
The PACVD process allows the diamond coating to be applied at varying thicknesses (6 μm to 12 μm), which can be customized to suit the application.
“Within any cutting process, the coating is constantly being removed. The thicker the coating, the longer it takes to wear it off,” Derflinger explained. “Once you are into the carbide, the wear is accelerated further. So, a thicker coating normally gives a longer tool life, which then lowers manufacturing costs.”
In terms of satisfying such qualities, BALINIT Diamond Micro and Nano coatings are examples of PACVD-based diamond coatings formulated specifically according to the needs of a wide range of highly-abrasive, nonferrous materials. While both are well suited to machining GFRP, CFRP, and ceramics, the rougher “micro” formulation is ideal for graphite.
With this approach to machining, carbide tools can be coated to enable cutting of graphite workpieces with better quality and substantially greater speeds. Combined with significantly extended tool life, this allows even sophisticated workpieces and fine structures to be produced with a single cutter in a single clamping, which can help to eliminate cost-intensive reworking procedures.
When it comes to machining aluminum alloys, including those with high silicon concentrations (AlSi-17 or higher) and ceramic particles, the “nano” diamond coating can replace more expensive PCD tools.
In an application where a Duralcan composite workpiece comprised of ceramic particle-reinforced aluminum materials was drilled, a PACVD-based, diamond-coated cutting tool drilled 20 times more holes compared to even very hard “diamond-like” carbon coatings.
The “nano” coating also works well with abrasive CFRP and GFRP materials. In one example, production significantly increased when a tool manufacturer drilled holes in CFRP/Al composite workpieces with the PACVD-based diamond coating. Compared to about 60 holes drilled with untreated tooling, approximately 380 holes were drilled with the PACVD-based diamond coating.
For instance, in an application drilling Duralcan composite workpieces, comprised of ceramic particle-reinforced aluminum materials, a PACVD-based diamond coated cutting tool drilled 20 times more holes compared to even very hard “diamond-like” carbon coatings.
Finally, in the machining of ceramics, typically for the dental industry, both PACVD-based diamond coatings can substantially boost production and extend tool life while improving the surface quality of the workpiece. As an example, when a micro ball-nose end-mill tool was used to machine a zirconium oxide workpiece for a dental application, a PACVD-based diamond coating produced about 900 finished parts, compared to about 100 parts for an untreated tool.
The bottom line — Some manufacturers may be inclined to use uncoated carbide cutting tools or traditional coatings because of their familiarity with such methods. However, those that take advantage of the superior capabilities of high-performance hard-carbon PVD and diamond PACVD coatings will significantly harden carbide cutting tools and improve part quality. This will enhance production and tool service life, which also improves the bottom line.
“Even if the cutting tool is expensive, you can put a hard coating on it and you will get a much better performance out of it,” said Derflinger. “That is why in the future more and more tools are going to have specialty coatings.”
