Small-Scale Thinking: Drilling Performance

Courtesy of NanoMech Nanotechnology opens up new coating possibilities, performance.

“Nanotechnology” has been a buzzword in a variety of industries for some time now. But what does it mean when it comes to coating materials for cutting tools, and what’s the current state of nanostructured tool coatings?

Defining nanoscale might be a good place to start. A consensus defines nanoscale as features on the order of 100 nanometers or smaller (a nanometer is one billionth of a meter).

For cutting tool coatings, however, it’s not that simple. Although feature sizes of nanocoating materials certainly fit the 100nm or smaller scale, development of new coating processes and materials is producing tools for difficult machining applications that, in some cases, can far exceed the performance of conventional tools in both life and machining capability.

According to Dr. Ajay Malshe, founder and chief technology officer of NanoMech, Springdale, Ark., using nanotechnology to develop tool coatings requires a different mindset than the chemical composition-based thinking that has driven materials science for the past century or longer.

“Basically, we are exploiting size as opposed to exploiting chemistry,” he said. “Conventional thinking about properties doesn’t apply at nanoscale. For tool coatings, for example, that may mean materials that are both harder and tougher than conventional materials, or it may mean creating new surface textures on the coating that can act as a kind of microchipbreaker during machining.”

CBN Coating Update

NanoMech is the parent company of Duralor, a Springdale-based company that developed and is commercializing a nanotechnology process for depositing CBN composite coatings on cutting tools. (see “Coating’s Holy Grail,” October 2008). The CBN nanocomposite coating was then in its infancy. Over the ensuing 2 years, Duralor has focused on finding applications for CBN-coated tools and working with selected customers—about a dozen, according to Duralor president Bob Reed—to commercialize CBN-coated tools.

Courtesy of Platit

Platit’s current generation of nanocomposite coatings feature an adhesion layer, a monoblock or gradient core layer and a nanocomposite top layer. Total thickness of the coating system is 2.72µm.

“We are currently focused on machining steels hardened up to 54 HRC,” Reed said. “The coating is also very good in machining of prehardened steels and powder metallurgy materials.” Reed said Duralor’s current work is mainly in uninterrupted turning applications, although the company expects to ramp up development of CBN-coated inserts and round tools for interrupted cutting in the coming months.

Developed with researchers at the University of Arkansas, Duralor’s Tufftek process consists of first electrostatically depositing submicron grains of CBN on the carbide substrate, then infiltrating the CBN grains with TiN, TiCN, TiC or other traditional tool coating materials using chemical vapor deposition. The technology is capable of depositing relatively thick (100μm or more) coatings, but a more typical thickness is less than 20μm.

Reed said the CBN-coated tools offer a significant cost advantage over PCBN-tipped tools, and, because of potential tripling or quadrupling of tool life, also are competitive on a cost basis with conventional multilayer coated inserts.

Courtesy of Rushford Hypersonic

In testing, a ¼ “-dia. jobber drill coated with Rushford Hypersonic’s HPPD SiC coating produced 1,546 holes in ½ “-thick 304 stainless steel with no coolant. Diameter of the last hole was 0.2495 “.

In addition to expanding to milling and possibly drilling applications, Duralor research is focused on using different materials to infiltrate the electrostatically deposited CBN grains. The company is working on CVD alumina as a potential matrix material, as well as on incorporating lubricants into the matrix.

“A tool coated in that way would combine CBN nanoparticles with a lubricant, such as molybdenum disfulfide, that would be supplied continuously to the cutting edge,” Malshe explained. “So our process really is a platform for combining materials and combining coating architectures in ways not possible using conventional CVD or PVD processes.”

Mach 8 and Beyond

Another nanocoating process is hypersonic plasma particle deposition (see “Flute Insurance,” October 2009).

“Basically, we disassociate molecules into their elemental state to create reactants in a plasma stream,” explained Daniel Fox, president of Rushford (Minn.) Hypersonic LLC. “We then reassemble them through a nucleation process and accelerate the reassembled molecules to speeds of more than Mach 8 (about 6,000 mph) to deposit them on the substrate.”

According to Fox, hitting the substrate at high speed causes the nanoparticles, which are 2nm to 20nm in size, to undergo a phase change and chemically bond to the substrate. The result is a thin, monolayer coating with excellent adhesion and high hardness and fracture toughness.

“Our coatings typically have a hardness of more than 37 GPa and fracture toughness of more than 3.1 MPa,” Fox said. “And, the better we can control our particle size distribution, the higher the fracture toughness and hardness. Within the next 6 months, we expect to have fracture toughness of 6 MPa and well above 50 to 60 GPa in hardness.”

Rushford is using HPPD to produce SiC coatings, but is preparing to add titanium and boron to the process. “That will allow us to produce TiC, TiN, boron-nitride and boron-carbide materials.”

Like Duralor, Rushford is working with customers to find applications for HPPD-coated tools. “We’ve been working with customers to coat a lot of HSS substrate materials mainly in the drilling area, but we are investigating turning and milling with carbide inserts as well,” Fox said.

The company is also still quantifying tool life increases and other benefits. Fox believes one of the main advantages of HPPD-coated tools will be their ability to expand dry machining applications. “The ability to run without coolant makes machining inherently greener,” he said. “That’s important in Europe right now, and it will become more important in the U.S. as well. We are working with customers to combine the coating with tool geometries and substrates optimized for dry machining.”

More Nanocomposites

NanoMech and Rushford use unconventional processes to deposit some relatively unconventional coating materials. However, the idea of combining two or more materials to form nanocomposite coatings with improved properties is also being applied with more traditional tool coating materials.