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

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

Vickers indentation hardness of stoichiometric and reduced single crystal TiO2 (rutile) from 25 to 800 C

The indentation microhardness of stoichiometric and reduced single crystal rutile (TiO2) from 25 to 800 C is presented in this paper. The results serve two main purposes. One is to assess the effect of rutile's stoichiometry on its hardness. The other is to test recently suggested theory on solid lubrication with sub Stoichiometric rutile in an effort to better understand shear controlled phenomenon. Microhardness was measured using a Vickers diamond indentor on both vacuum and hydrogen reduced single crystal rutile from 25 to 800 C. The results indicate that stoichiometry and temperature have a pronounced effect on rutile's hardness. The measured effects lend support to theory on solid lubrication by enhanced crystallographic slip and suggest that solid lubricant materials may be produced by careful atomic level tailoring (stoichiometry control)

University Homepage Affordances: The Influence of Hyperlinks on Perceptions of Source Credibility

The technology affordances of university website homepages were evaluated to inform the development of prototypical examples of accessible public university and exclusive private university homepages. Affordances are characteristic of the environment that, when perceived, afford or provide opportunities for action (Gibson, 1986). In addition, affordances, such as hyperlinks, also prompt heuristic processes that lead to judgments that are based on peripheral cues rather than substantive information. Integrating the MAIN model (Sundar, 2008) and the Two-Factor Theory (Herzberg, 1966; Zhang & Von Dran, 2000), eye tracking and survey methodology were used to assess differences in perception and credibility judgments of the prototypes developed to represent the website homepages of accessible and exclusive universities. A content analysis was used to assess hyperlinks and other design features of the website homepages of the 10 most accessible and 10 most exclusive Ohio universities. Consistent with prior research, results indicated relatively little variation among the hyperlinks and design elements of university website homepages. The features were used to develop prototypes representative of the two types of university homepages. Those prototypes served as the manipulated independent variable in an experiment and, although the manipulation was correctly perceived, the differences were not statistically significant. Correspondingly, the credibility measures, although consistent with theoretical predictions, were not statistically significant based on the type of prototype viewed. This study thus did not provide evidence of a relationship between number of hyperlinks and credibility. Perceived hygiene and motivator factors however, were significantly positively associated with credibility, consistent with two-factor theory. Additionally, prior experience, particularly with university websites, was associated with the extent to which credibility assessments were not neutral. Participants with more experience were significantly more likely to make non-neutral assessments of credibility than those with less experience, results which are consistent with theory underlying the MAIN model and provide evidence of heuristic processing. This work provides evidence that two-factor theory complements the MAIN model, with potential theoretical and practical benefits. Universities can apply them to develop websites that better meet with user expectations and are thus perceived more favorably

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