Super Hard, Very Tough
In Nanocomposite coatings find new applications in more durable tools and turbine blades
By Ronghua Wei, Ph.D., N. Sastry Cheruvu, Ph.D., and Kent E. Coulter, Ph.D.
Nanotechnology addresses the development, manipulation and use of materials and devices on the scale of roughly 1-100 nanometers, and the study of phenomena that occur at this scale. A nanometer is so very small, one-billionth of a meter, that it is difficult to visualize. There are 25 million nanometers in an inch. A nanometer is to a meter what a marble is to the Earth or how much a hair grows in a second.
Nanoscale science and engineering offer both challenges and opportunities for new scientific understanding and potential technological advances. Since the concept was first introduced in 1959 by physicist Richard Feynman in a talk called “There’s Plenty of Room at the Bottom,” scientists have predicted that manipulating matter at the atomic scale will change the nature of almost every human-made product, from medicines to electronics, optics, data storage, energy and national security.
Scientists at Southwest Research Institute (SwRI) are making room for nanotechnology at the top, so to speak, applying nanoscale coatings to equipment surfaces to improve overall durability. Nanostructured thin-film coatings are producing highly wear-resistant surfaces for components from manufacturing tools to aerospace turbines.
SwRI has more than 25 years of experience in surface modification technologies. More than 2,000 square feet of facilities are dedicated to ion beam and plasma-based modification and coating of advanced materials. Using its full complement of ion deposition, physical vapor deposition (PVD) and chemical vapor deposition (CVD) technologies, as well as its 10 vacuum coating chambers, SwRI’s Surface Engineering Group addresses clients’ coating needs through controlled engineering of surface properties.
Material surface erosion
In the nanostructured thin-film application, the SwRI team first identified a problem: material erosion. The solution was a new technique for creating thicker nanocoatings.
Solid particle erosion affects key components of modern machinery in various applications such as compressor blades and vanes of turbine engines, turbine blades of advanced aircraft engines, and impellers of fluid pumps. Coatings technology generally tries to improve the surface wear of various components. Tribology is the science and technology of interacting surfaces in relative motion and includes studies of friction, lubrication and wear. At SwRI, tribologists and materials scientists investigate using hard coatings to minimize damage to these components, allowing conventional materials to continue to be used, protected by a hard coating.
For coatings tribology, the traditional position on wear resistance is that the harder and stiffer a material is, the better. This position has fueled interest in developing thin-diamond, cubic-boron-nitride and carbon-nitride coatings, including several under development at SwRI. While hardness is important, coatings elasticity and toughness are equally important for protection against abrasion, impact and erosive wear. Coatings should match the flexibility of their substrate to minimize stress under applied loading. They also should move in concert with the substrate without cracking or delaminating, but ceramic coatings are typically three to four times more rigid than the substrate. By manipulating microstructural characteristics of coating materials such as phase distribution and grain size, the SwRI team can control crack propagation in coatings, which is important in improving contact yield pressure and fracture toughness.
When toughness or durability is considered, plastics are more durable than ceramics in many applications. By combining the properties of various materials to create a new material, age-old wear problems can be addressed. For example, an ideal coating would combine the hardness of ceramics, the toughness of plastics and metal, and some flexibility allowing it to follow the lead of the substrate under loads.
A powerful promise of nanotechnology is the potential to manipulate the very building blocks of matter, allowing materials customization that enables the material-on-demand concept. By manipulating the arrangement of atoms and molecules, as well as the strength or order of their bonding, at least in principle, one can induce desirable changes in a material, or better yet, custom-tailor a substance. The concept is not new. In the field of polymers, for example, scientists change the position of certain molecules in a chain to create new polymers with desirable properties.
However, nanotechnology is distinct from polymer science because miniaturization induces an effect such that the properties of materials at the nanoscale differ from those of the bulk material. By reducing the size of the grains and changing the composition of materials, scientists can induce desirable properties.
Nanoscale coatings, such as titanium nitride (TiN) coatings applied to tool inserts or diamond-like carbon layers applied to high-end computer hard disks, are very thin (less than 10 microns) and work well. However, attempts to thicken these coatings to 100 microns for use in more abrasive environments have proven intractable. These thicker coatings tend to exhibit internal stress that tears them apart. An analogy would be the paint on a house: If it is too thin, it will not provide the needed protection. Beyond a certain thickness, however, it also does not perform properly.
The SwRI team has developed some super-hard, very tough and extremely erosion-resistant nanocomposite coatings for deposition on turbine blades, cutting tools, food processing equipment and other components that improve wear resistance by orders of magnitude over the base substrate materials. The coatings consist of thick layers of titanium, silicon, carbon and nitrogen (Ti-Si-C-N) with a structure composed of titanium carbonitride (TiCN) nanocrystals 4-7 nanometers thick in a matrix of amorphous, glass-like silicon carbonitride. Like bathroom tiles, the TiCN crystals, grouted in place by the silicon carbonitride, are stronger and perform better than either material alone. These coatings also exhibit much greater toughness than single-phased, micron- to submicron-grain-sized TiN. As a result, they exhibit a seven- to 10-fold increase in erosion resistance over “state-of-the-art” TiN-based commercial coatings. In precision tooling applications, these nanocomposite coatings offer a significantly more durable protection without changing the discernible geometry of the instrument.
SwRI developed a plasma-enhanced magnetron sputtering (PEMS) technology to produce the thickest nanocomposite coatings ever reported. Evaluations conducted at SwRI and by outside organizations indicate that the coatings exhibit superior performance, an improvement of approximately an order of magnitude against solid particle erosion and heavy load wear over other state-of-the-art coatings.
SwRI designed and built prototype PEMS deposition systems based on magnetron sputter deposition, a physical vapor deposition process that uses filaments to generate large volumes of enhanced plasma. The plasma density in the deposition systems is greatly enhanced because the global plasma is generated in the entire vacuum chamber, not just in the front of the magnetron, increasing by up to 25 times the ion flux generated by magnetron plasma alone. The enhanced ion bombardment greatly increases the film density while reducing the grain size, resulting in greatly improved overall tribological performance.
The coatings obtained using the PEMS process are nanocomposites composed of titanium nanocrystals from 4-7 nanometers across in an amorphous, glassy matrix composed of silicon, carbon and nitrogen, or in its chemical notation, Ti-Si-C-N. The total silicon content is less than 3.5 percent. The coating exhibits hardness as high as 40 gigapascals (GPa), compared to 2-3 GPa for stainless steel and 100 GPa for diamond. Erosion resistance is up to 100 times higher than the uncoated material. SwRI has developed the PEMS process parameters to control microstructure, adhesion and erosion resistance of materials.
The SwRI nanocomposite coatings have performed exceptionally well in a variety of internal and external performance tests. SwRI evaluated the erosion resistance of various nitrides and the SwRI-developed nanocomposite coatings outperformed all other coatings, including popular TiN-based coatings, in erosion resistance. In independent tests, one laboratory found that the SwRI nanocomposite coatings outperformed the state-of-the-art hard coatings submitted by commercial companies by seven to 10 times. Another independent laboratory evaluated composite coating samples at temperatures up to 800° F to simulate aerospace engine compressor environments, and the SwRI-developed nanocomposite coatings reduced material erosion by 50-90 percent in this application. In military comparisons of helicopter rotor blades with nanocomposite coatings and existing rotor blade protection materials, TiSiCN coatings clearly outperformed other coatings in erosion protection.
The SwRI-developed PEMS technology and its resulting nanocomposite coatings have generated interest from a wide range of industries for numerous applications, including aerospace engines, gas and steam turbines, oil and gas equipment, and food processing equipment. With funding from industry, SwRI has brought this technology to near-maturity. Treated components have been deployed in various industrial settings, resulting in cost savings from reduced tool replacement and downtime.
The SwRI team is operating two PEMS systems: a smaller, 30-cubic-inch chamber with two magnetrons used for process development and a larger, one-cubic-meter chamber outfitted with four large rectangular magnetrons for prototype demonstration and small-quantity production. The process and coatings technology is proven. The next phase is scaling up the process for commercialization of the technology on a larger scale.
Published in the Spring 2008 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.