Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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i T S S e Renewed Interest in Triboglide Due to its Excellent Wear and Friction Properties 5 T riboglide is a high temperature wear resistant lubricant and a derivative of NASA's 200 series coating system. After its collaborative development in the 1990s, a thermal spray specification data sheet was developed that covers a family of thermal sprayed high velocity oxygen fuel (HVOF) coatings consisting of a chrome-carbide matrix with a nickel binder and a eutectic calcium-barium fluoride solid lubricant. The coating was rated for use between 800°-1300°F for sliding type use and its composition contains 70 wt% hard CrC phase in 18 wt% Ni binder with a 12 wt% eutectic CaF2-BaF2 component. During development, parametric studies were conducted, where changes in spraying distance, angle, powder particle size, feed rates, and test temperature were correlated with tribological performance using various percentages of solid lubricants. Initial ring-on-ring wear tests done by EG&G Pressure Science, Beltsville, Md., on Triboglide, Inconel 718, and NiCrAlY indicate that Triboglide showed no signs of microcracking while Inconel and NiCrAlY showed smearing and microabrasion. Tests show that the HVOF technique is preferred for applying Triboglide coatings. With renewed interest from companies like Honeywell and Boeing, Plasma Technology Inc. (PTI) is reexamining Triboglide as a hard, wear-resistant, and low friction lubricious coating for applications such as air foil bearings, actuators, and wear components where oil-based lubrication is scarce. Rotor shafts, actuators, and other components that operate in high temperature and high speed environments tend to wear faster due to rapid degradation of lubricant oil, thus creating a need for solid lubrication that does not compromise on its high temperature wear and friction coefficient. The HVOF thermal spray process is similar to the combustion spray process, except that HVOF was developed to produce extremely high spray velocity. A number of HVOF guns such as JetKote, JP 5000 or 8000, and Diamond Jet, use different methods to achieve high velocity spraying. Jet Kote uses a simpler system comprised of a high-pressure combustion nozzle and air cap. Fuel gas (hydrogen, propane, or propylene) and oxygen are supplied at high pressure, and combustion occurs outside the nozzle but within an air cap supplied with compressed air. The compressed air pinches and accelerates the flame and acts as a coolant for the HVOF gun. Powder is fed at high pressure axially from the center of the nozzle. Fig. 1 — Photomicrograph of PS 304 coating on a titanium shaft. HVOF CrC-NiCr Triboglide Triboglide + Ag NASA PS 304 Fig. 2 — Cross-section photos of different Triboglide coatings and NASA PS 304 coating. Another method (JP 5000) consists of a high-pressure water-cooled HVOF combustion chamber and long nozzle. Fuel (kerosene, acetylene, propylene, and hydrogen) and oxygen are fed into the chamber; combustion produces a hot high-pressure flame, which is forced down a nozzle to increase its velocity. Powder may be fed axially into the HVOF combustion chamber under high pressure or fed through the side nozzle where the pressure is lower. HVOF coatings are very dense, strong, and feature low residual tensile stress or, in some cases, compressive stress, which enables thicker coatings to be applied than previously possible with the other processes. The very high kinetic energy of particles striking the substrate surface does not require particles to be fully molten to form high quality HVOF coatings. HVOF has become the quality standard for carbide and cermet materials, as gas velocities over 6000 ft/s allow particle velocities greater than 3000 ft/s. The result is a coating that approaches theoretical density with bond strengths above 10,000 psi. PTI conducted extensive wear testing of Triboglide coatings and compared them to NASA's PS 304 class coatings. ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 65

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