Phillip W. Centers

The Role of Oxide and Sulfide Additions in Solid Lubricant Compacts

A study of antimony trioxide-molybdenum disulfide solid lubricating compacts found no evidence to support hypotheses that the tribologically beneficial role of antimony trioxide in those compacts results from sacrificial oxidation of antimony trioxide, direct retardation of molybdenum disulfide oxidation, or formation of eutectic of antimony and molybdenum oxides. Data do support a hypothesis that the role of antimony trioxide is to plastically deform at asperity flash temperatures permitting molybdenum disulfide crystallites attain and retain a preferred orientation to improve overall tribological condition. Studies of other and sulfides as bulk additions in lubricating compacts support the concept. Additional confirmation is provided by the finding that lower test temperatures are required to obtain the beneficial effect of antimony oxide additions to graphite as compared with additions to MoS2; evidently, higher flash temperatures are experienced by the graphite compact as shown by calculations. Excell...

Tribological performance of MoS2 compacts containing MoO3, Sb2O3 OR MoO3 and Sb2O3

Selected oxides and sulfides have been previously investigated as bulk additives to enhance the tribological performance of MoS2 in air from ambient temperature to about 315 °C. Sb2O3, in particular, has been identified as a superior additive for high temperature aerospace applications. In experiments designed to investigate a hypothesis that the superior tribological performance of MoS2-Sb2O3 formulations results from formation of an Sb2O3-MoO3 eutectic, compacts of MoS2 with MoO3, Sb2O3 or MoO3 and Sb2O3, were prepared and tribologically evaluated at 316 °C. Formulations containing Sb2O3 or MoO3 resulted in higher friction but much less wear than MoS2 compacts, with the wear volume of MoS2-MoO3 compacts at about half that of compacts containing Sb2O3. Data from compacts containing mixtures of MoO3 and Sb2O3 indicate no synergism between the additives, as would be expected if an eutectic of MoO3-Sb3O3 yields any significant benefits in MoS2-Sb2O3 formulations as hypothesized previously. In fact, X-ray diffraction and differential thermal analysis data indicated no evidence of eutectic formation. Alternatively, it is hypothesized that these soft oxides, owing to their low shear strength, permit MoS2 crystallites to retain a preferred orientation achieved during asperity contact, improving the overall tribological performance. The protective role of MoO3 in MoS2 compacts is anomalous in that MoO3, a product of the oxidation to be avoided in high temperature applications of MoS2, is tribologically beneficial in compacts with MoS2 in such applications.

Laboratory evaluation of the on-line ferrograph☆

Abstract A laboratory evaluation of the on-line Ferrograph for fluids containing well-characterized debris indicates that the wear concentration, as measured by the instrument, is unaffected by variation in fluid pressure (0.21–0.65 MPa (absolute); 30–80 lbf in −2 (gauge)) and temperature (65–99 °C) within the range examined, that the difference between L , the concentration of large debris particles, and S , the concentration of small debris particles, describes the severity of wear and that changes in debris concentration at or near the level of 1 ppm can be detected. Correlation of on-line and analytical ferrographic data was apparent; such correlation was not found for spectrometric oil analysis, colorimetric iron analysis or particle count data. Start-up and low fluid temperature problems encountered with the unit may be avoided by sensor insulation and preheating. Additional field evaluations are required to determine suitability for many potential applications.

Tribological performance of MoS2 compacts containing sulfur, Sb2S3 or Sb2S4

Abstract The friction coefficient, wear volume and wear surfaces of solid lubricant compacts of MoS 2 formulated with Sb 2 S 4 , Sb 2 S 3 , different amounts of sulfur, and no additive, were assessed. Wear and friction were determined in dry air from room temperature to 589 K using a pin-on-disk apparatus. Comparatively, Sb 2 S 4 acts more beneficially than other low shear strength materials, which have been hypothesized to soften at asperity contact temperatures, allowing MoS 2 crystallites to attain and retain a preferred tribological orientation. Small amounts of sulfur, which may be a decomposition product of Sb 2 S 4 , improve the tribological performance of MoS 2 at 547 and 589 K and also appear to act beneficially in combination wth Sb 2 S 3 . The mechanism by which the additives improve the performance of MoS 2 is unknown; however, it is hypothesized that the superior performance of Sb 2 S 4 as an additive to MoS 2 may result from several contributory factors, including its low shear strength and, perhaps, its ability to provide sulfur, which also improves MoS 2 performance, as a decomposition product at higher test temperatures.

Real time simultaneous in-line wear and lubricant condition monitoring

Abstract An in-line complete oil breakdown rate analyzer (COBRA) and quantitative debris monitor (QDM R ) were installed in the oil system of an aircraft turbine engine lubricant simulator. The in-line COBRA readings correlated well with those of a standard laboratory model COBRA and total acid number of the deteriorating lubricant. Increases in QDM R particle counts corresponded with increases in trace iron content which reflected abnormal wear occurring in the simulator. Analytical and direct reading ferrographic data confirmed the rapid increase and eventual modest decrease in wear debris concentration. After test completion it was determined that a mainshaft bearing had experienced abnormal wear. Thus for the first time successful capability for real time simultaneous in-line wear and lubricant condition monitoring was demonstrated.

Sublimation-controlled oxidation of antimony trioxide

Abstract Previous investigators noted that a mixture of dimorphic Sb 2 O 3 oxidizes to orthorhombic Sb 2 O 4 more rapidly than each dimorph separately. It was hypothesized that Sb 2 O 3 (c) sublimes to low-energy nucleation sites on Sb 2 O 3 (o) to oxidize. In this study thermodynamic calculations indicated that the driving force for oxidizing a sublimed species is much greater than that for nonsublimed Sb 2 O 3 . The hypothesis was validated experimentally using small-particle-size (10–20 μm) Sb 2 O 3 to achieve appreciable sublimation rates. It was found that the oxidation rates of both Sb 2 O 3 (c) and Sb 2 O 3 (o) are linearly correlated to the equilibrium sublimation pressure of each dimorph. Also, the apparent activation energy of oxidation was determined to be 42.6–45.9 kcal/mole, approximating the enthalpy of sublimation for Sb 2 O 3 (∼43–47 kcal/mole). These observations strongly support the hypothesis that Sb 2 O 3 oxidation is sublimation controlled.

Effect of Polydimethylsiloxane Concentration on Ester Foaming Tendency

A recently formulated hypothesis relates the viscosity of polydimethylsiloxane (PDMS) additives to neutral, profoamant, or antifoamant performance in esters. At ten parts-per-million of PDMS addition, very low viscosity PDMS additives are hypothesized as neutral or ineffectual, due to solubility in the ester. Intermediate viscosity PDMS additives are profoamant due to increased surface activity attributed to more limited solubility of the partially miscible PDMS in esters, and Lewis acid-base interaction of the siloxaneester couple. High viscosity dispersed particulate polydimethylsiloxanes are antifoamant, inducing bubble collapse by a Griffith flaw mechanism. The foaming tendencies of an ester are related here to varying concentrations of PDMS of intermediate viscosities, i.e., 500, 1000, and 5000 mm2s−1 (cSt), which are similarly neutral, profoamant, and antifoamant as concentration is increased. The hypothesis proposed is supported by the observations reported and provides a basis for a more complete ...

Modeling and Prediction of Foaming Tendencies of Polydimethylsiloxane-Polyol Ester Mixtures

An algorithm modeling the effects of polydimethylsiloxane (PDMS) on polyol ester foaming is developed. The model predicts foaming based upon PDMS viscosity, concentration and temperature. Model data are compared to laboratory results. Significant factors that influence foaming tendencies of PDMS-polyol ester systems are described. Applicability and limitations of the model are identified. Presented at the 49th Annual Meeting in Pittsburgh, Pennsylvania May 1–5, 1994

An improved method of ferrographic quantitation

Abstract Previous investigators assumed that ferrographic area coverage data were linearly related to the quantity of sample analyzed. However, theoretical examination of ferrographic deposition of wear debris and treatment of published data confirm that coverage varies with sample quantity by a power law relationship. Imrpved normalization and wear trending procedures based upon power curve fitting of area covered measurements vs. sample quantities are developed. The new procedures should increase confidence and reliability in both ferrographic measurement and wear severity index trending.

Microcomputer Simulation of Lubricant Degradation in Turbine Engines Using Laboratory Data

A FORTRAN, MS-DOS operating system algorithm was constructed and implemented to predict ester-based lubricant behavior in a turbine engine. Input data consists of selected lubricant flow rates, bulk lubricant and bearing temperatures, and laboratory generated data. Execution of the program uses lubricant property data as a function of time at several temperatures to produce mathematical functions describing total acid number, viscosity change and evaporation as a function of temperature at selected times. Static coker data are employed to estimate coking at the hot end bearing cover. The program output data are predicted values of critical lubricant properties as a function of engine hours, reflecting lubricant additions to the system because of evaporative and seal losses and also degradation predicted to occur during cool-down cycles. Computer-generated data are compared with actual engine data; the benefits and limitations of the program are identified. Presented at the 43rd Annual Meeting in Cleveland...