Tribology and Lubrication Technology November 2012 : Page 28
gineering techniques were compared to determine which best enhance the tribological performance of a Ti64 alloy and a 60Ni–40Ti alloy. 13 Test treatments included diffusion, hard coatings (TiN and CrN), a soft coating (Cu–Ni–In), titani-um-matrix TiB2 in situ -formed composite and shot peening with AISI 52100 bearing steel. 14 Diffusion treatments included oxygen diffusion, nitriding and carburizing. In addition to studying the effects of indi-vidual surface engineering approaches, some were combined in an attempt to maximize their effects but at the same time retain the mechanical properties of the titanium alloy result-ing from proper heat treatment. Both dry and lubricated fric-tion and wear tests were conducted. Lubricated tests were performed in engine-conditioned diesel engine oil and the following conclusions were drawn: 1. The difference between passing and failing was dis-tinct—the materials and surface treatments clearly fell into a specific category. The successful treatments yielded friction coefficients less than 0.15 while the treatments that failed had friction coefficients greater than 0.26. 2. CrN-coated Ti64 specimens had the least amount of wear, followed closely by DLC-coated Ti64 specimens. 3. Nitrided and OD-treated specimens yielded the lowest friction coefficients and their wear was also compara-ble to those of CrN and DLC-coated specimens. 4. For some treatments (such as the carburized surfaces) test-to-test repeatability was an issue. Transitions to severe wear were not repeatable, and the issue of con-sistent behavior was a cause for rejection of that par-ticular treatment. 5. Various wear modes were exhibited by the materials tested with three-body abrasive wear, pitting or coat-ing fracture and adhesive wear being the most com-mon. 6. The ratio of the wear of the ball specimen to the flat specimen differed from one material couple to anoth-er, and must be considered in the context of the tribo-system. 7. Tests in lubricants of other types may produce differ-ent rankings, since the lubricant used in lubricated tests contained soot, acids and possible engine debris. Titanium alloys are currently used and successfully lubri-cated in limited aerospace applications—one proven meth-odology is a mitigation strategy for fretting wear. Hager ex-plains, “Typically the surfaces are thermal sprayed with a soft metallic coating and then a bonded solid lubricant is applied 13 Figure 5 | Researchers at NASA’s Jet Propulsion Laboratory and the California Institute of Technology developed an amplifier for boost-ing electrical signals from space. It is made of a superconducting nio-bium titanium nitride coiled into a double spiral 16 millimeters in di-ameter. (Courtesy of NASA/JPL-Caltech) on the surface of the thermal sprayed coating. The metallic coating is often an aluminum bronze alloy or a copper nickel indium alloy. The surface solid lubricants are often molybde-num disulfide in an epoxy binder or graphite in a silicate binder.” conclUsion With the processing issue largely under control, attention continues to turn toward solving the remaining lubrication issue. The ORNL studies and other studies have proven that effective lubricants for titanium alloys do exist. The chal-lenge is translating these findings into mainstream engineer-ing and manufacturing processes that will enable titanium alloys to transform the trucking, automotive and many other industries. “There is no universal surface treatment or coating that fits in all systems,” Qu says. “Each system has to be tested and analyzed case-by-case.” Jean Van Rensselar heads her own communications/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at firstname.lastname@example.org. D.G. Bansal, O.L. Eryilmaz and P.J. Blau (2011), “Surface Engineering to Improve the Durability and Lubricity of Ti–6Al–4V Alloy,” Wear , 271 (9-10) pp. 2006-2015. 14 Shot peening is a cold-working process that entails bombarding a surface with shot pellets at a force sufficient to create deformation. 28 (PASCAL) Science quiz: What high-level computer language was named after a French mathematician and philosopher?