Tribological properties of alumina-boria-silicate fabric from 25 to 850 C

Demanding tribological properties are required of the materials used for the sliding seal between the sidewalls and the lower wall of the variable area hypersonic engine. Temperatures range from room temperature and below to operating temperatures of 1000 C in an environment of air, hydrogen, and water vapor. Candidate sealing materials for this application are an alumina-boria-silicate, ceramic, fabric rope sliding against the engine walls which may be made from copper- or nickel-based alloys. Using a pin-on-disk tribometer, the friction and wear properties of some of these potential materials and possible lubrication methods are evaluated. The ceramic fabric rope displayed unacceptably high friction coefficients (0.6 to 1.3) and, thus, requires lubrication. Sputtered thin films of gold, silver, and CaF2 reduced the friction by a factor of two. Sprayed coatings of boride nitride did not effectively lubricate the fabric. Static heat treatment tests at 950 C indicate that the fabric is chemically attacked by large quantities of silver, CaF2, and boron nitride. Sputtered films or powder impregnation of the fabric with gold may provide adequate lubrication up to 1000 C without showing any chemical attack

Tribological characteristics of silicon carbide whisker-reinforced alumina at elevated temperatures

The enhanced fracture toughness of whisker reinforced ceramics makes them attractive candidates for sliding components of advanced hear engines. Examples include piston rings and valve stems for Stirling engines and other low heat rejection devices. However, the tribological behavior of whisker reinforced ceramics is largely unknown. This is especially true for the applications described where use temperatures can vary from below ambient to well over 1000 C. An experimental research program to identify the dominant wear mechanism(s) for a silicon carbide whisker reinforced alumina composite, SiCw-Al2O3 is described. In addition, a wear mechanism model is developed to explain and corroborate the experimental results and to provide insight for material improvement

Composition optimization of chromium carbide based solid lubricant coatings for foil gas bearings at temperatures to 650 C

A test program to determine the optimum composition of chromium carbide based solid lubricant coatings for compliant gas bearings is described. The friction and wear properties of the coatings are evaluated using a foil gas bearing test apparatus. The various coatings were prepared by powder blending, then plasma sprayed onto Inconel 718 test journals and diamond ground to the desired coating thickness and surface finish. The journals were operated against preoxidized nickel-chromium alloy foils. The test bearings were subjected to repeated start/stop cycles under a 14 kPa (2 psi) bearing unit load. The bearings were tested for 9000 start/stop cycles or until the specimen wear reached a predetermined failure level. In general, the addition of silver and eutectic to the chromium carbide base stock significantly reduced foil wear and increased journal coating wear. The optimum coating composition, PS212 (70 wt% metal bonded Cr3C2, 15 wt% Ag, 15% BaF2/CaF2 eutectic), reduced foil wear by a factor of two and displayed coating wear well within acceptable limits. The load capacity of the bearing using the plasma-sprayed coating prior to and after a run-in period was ascertained and compared to polished Inconel 718 specimens

Tribological evaluation of an Al2O3-SiO2 ceramic fiber candidate for high temperature sliding seals

A test program to determine the relative sliding durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications as described. This work represents the first reporting of the sliding durability of this material system. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated by the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperatures from 25 C to 900 C, loads from 1.3 to 21.2 Newtons, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter, elastic modulus, and a pretest fiber heat treatment on friction and wear were measured. In most cases, wear increased with temperature. Friction ranged from about 0.36 at 500 C and low velocity (0.025 m/s) to over 1.1 at 900 C and high velocity (0.25 m/s). The pretest fiber heat treatment, which caused significant durability reductions for alumina-boria-silica ceramic fibers tested previously, had little effect on the alumina-silica fibers tested here. These results indicate that the alumina-silica (Al2O3-SiO2) fiber is a good candidate material system for high temperature sliding seal applications

Method of making carbide/fluoride/silver composites

A composition containing 30 to 70 percent chromium carbide, 5 to 20 percent soft noble metal, 5 to 20 percent metal fluorides, and 20 to 60 percent metal binder is used in a powdered metallurgy process for the production of self-lubricating components, such as bearings. The use of the material allows the self-lubricating bearing to maintain its low friction properties over an extended range of operating temperatures

Technical Development Path for Foil Gas Bearings

Foil gas bearings are in widespread commercial use in air cycle machines, turbocompressors and microturbine generators and are emerging in more challenging applications such as turbochargers, auxiliary power units and propulsion gas turbines. Though not well known, foil bearing technology is well over fifty years old. Recent technological developments indicate that their full potential has yet to be realized. This paper investigates the key technological developments that have characterized foil bearing advances. It is expected that a better understanding of foil gas bearing development path will aid in future development and progress towards more advanced applications

A new test machine for measuring friction and wear in controlled atmospheres to 1200 C

This paper describes a new high temperature friction and wear test apparatus (tribometer) at NASA Lewis Research Center, Cleveland, Ohio. The tribometer can be used as a pin-on-disk or pin-on-ring configuration and is specially designed to measure the tribological properties of ceramics and high temperature metallic alloys from room temperature to 1200 C. Sliding mode can be selected to be either unidirectional at velocities up to 22 m/sec or oscillating at frequencies up 4.5 Hz and amplitudes up to + or - 60 deg. The test atmosphere is established by a controlled flow rate of a purge gas. All components within the test chamber are compatible with oxidizing, inert, or reducing gases

Industry Needs Fulfilled by Patented NASA PS300 Solid Lubricant Technology

In 1999, the NASA Glenn Research Center was awarded a patent (#5866518) for a new high-temperature solid lubricant coating material, PS300. A combination of wear-resistant metals and ceramics with solid lubricant additives, PS300 reduces friction and wear in sliding contacts from below ambient to over 650 C. This lubricant is an outgrowth of over three decades of high-temperature tribological research and was specifically developed as a shaft lubricant to protect foil air bearings used in Oil-Free turbomachinery, like gas turbines. Foil bearings are lubricated by air at high speeds but experience sliding and wear during initial startup and shut down when a lubricating film of air has not yet developed. PS300 shaft coatings have successfully lubricated foil bearings for over 100 000 cycles without wearing out

The application of a computer data acquisition system for a new high temperature tribometer

The two data acquisition computer programs are described which were developed for a high temperature friction and wear test apparatus, a tribometer. The raw data produced by the tribometer and the methods used to sample that data are explained. In addition, the instrumentation and computer hardware and software are presented. Also shown is how computer data acquisition was applied to increase convenience and productivity on a high temperature tribometer

Transition of PS300 Solid Lubricant Coating Technology to Field Aided by Demonstration on Key Substrate Alloys

PS300 is a high-temperature solid lubricant coating originally developed to lubricate nickel-based superalloy shafts operating against foil air bearings in Oil-Free turbomachinery applications. PS300 is a plasma-spray-deposited coating developed at the NASA Glenn Research Center. It is available for non-exclusive licensing and has found applications in aerospace and industry. PS300 reduces friction and wear from below room temperature to over 650 C in both oxidizing and reducing environments. Early development centered on coating nickel-based shafts for use in turbomachinery. Potential industrial and aerospace customers, however, expressed interest in using the coating on a wide variety of substrates including steels, stainless steels, and nonferrous alloys like aluminum and titanium. To support this interest, a research program was carried out at Glenn in which nine different substrate candidate materials were evaluated for suitability with the PS300 coating. The materials were first coated with PS300 and then tested for coating strength and adhesion both before and after exposure to high-temperature air