Anomalous wear behavior of MoS2 films in moderate vacuum and dry nitrogen

440C steel thrust ball bearing races lubricated with 1 m thick sputtered films of MoS₂ were tested in the unidirectional and oscillatory modes against bare steel balls in moderate (10^–4–10^–5 Pa 10^–6–10^–7 Torr) vacuum and in 1 atmosphere of 99.999% pure ( 1 ppm water) N₂ in the same unbaked environmental chamber. Over 90% of the residual gases in the chamber vacuum consisted of H₂O vapor. The bearings operated in N₂ showed substantially longer lives compared to the specimens tested in vacuum. Scanning electron microscope (SEM) tribometry was also performed on an MoS₂ film powder-burnished onto a 440C flat. This flat was repeatedly oscillated against bare, hemispherical-tipped 440C pins on fresh wear tracks in the same type of N₂ and column vacuum of ~10^–3 Pa 10^–5 Torr itself containing over 90% residual H₂O. The SEM-generated results on the burnished film confirmed the same, atmosphere-dependent difference in wear life observed with the sputtered layers. Varying the moisture content of the burnished flat and its immediate environment by cryosorption predictably manipulated the coefficient of friction and wear life of MoS₂. The various possible causes of this perplexing phenomenon are reviewed, and a plausible hypothesis is offered attributing the unexpected wear life reduction to the physico-chemical consequences of residual H₂O hydrogen-bonding to the oxidized and/or hydrated edge and basal plane sites of MoS₂ in moderate vacuum. The site-specific sorption of water is severely hindered in 1 atm N₂ by the gas molecules disrupting the H-bonding mechanism.     

Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part II. SEM tribometry of silicon in vacuum and hydrogen

Scanning electron microscope (SEM) tribometric data on polycrystalline silicon (poly-Si) vs. poly-Si, Si(100) vs. Si(100) and Si(111) vs. Si(111) interfaces, obtained in Torr and in 0.2 Torr partial pressure of hydrogen gas ( ) from room temperature to 850^°C, were performed under standard and much slower thermal ramping rates. The friction data were analyzed per the methodology described in part I of this paper series. The results indicate a highly beneficial friction- and wear-reducing regime within a relatively narrow thermal region. This desirable region coincides with some chemisorption of excited species of molecular hydrogen just before the mass thermal desorption of surface hydrides. These data represent the tribochemical equivalent of a method routinely used in electronics, whereby deep electron traps (dangling Si bonds) are passivated by baking in molecular hydrogen. The also exerts a moderating influence on the size of the friction noise at all test temperatures. However, the general level of friction beyond the beneficial thermal region is high. In parallel, the general wear rate of Si representative of the entire range of standard thermal ramping in both atmospheric environments is in the extremely high 10^-12m³/(N m) range. Operating strictly in the beneficial, low-friction thermal regime resulted in a several orders-of-magnitude reduction in the wear rate over those measured under standard thermal ramping conditions. Although the results confirm previous findings that Si is not a good material of construction for miniaturized moving mechanical assemblies (e.g., microbearings and gears), there seems to be some limited possibility of gas-phase lubrication of Si micromechanisms with rarefied hydrogen at surface temperatures between 100 and 300^°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some Topographical and Tribological Characteristics of a CaF2/BaF2 Eutectic-Containing Porous Nichrome Alloy Self-Lubricating Composite

Scanning electron photomicrography, x-ray energy dispersion spectroscopy and metallographic sectioning techniques were combined to determine some topographical and tribological characteristics of a high temperature, self-lubricating composite. This bearing material consists of a porous nichrome alloy matrix, vacuum impregnated with a CaF₂-BaF₂ eutectic. The results indicate that a specific, multistep post-machining treatment of a composite part may be advantageous for effective high temperature service. Unique topographical and compositional changes of the sliding surfaces of the composite were observed after short term, low load use at a 1,200 F temperature in a helium environment. The data, as represented by high magnification photomicrographs and x-ray spectra, can be used as inspection tools (visual standards) for examining parts machined from the subject bearing material.     

Weight reduction in schizophrenics by molindone

The weight-reducing property of molindone, a recently introduced antipsychotic drug, was tested in 9 hospitalized chronic schizophrenic patients. There was an average weight loss of 7.6 kg after 3 months on molindone; most of the loss occurred during the first month. The mechanism producing this weight loss is uncertain, but a central anorexigenic effect may be an important factor.     

Crystal-structure-controlled tribological behavior ofcarbon-graphite seal materials in partial pressures of helium andhydrogen. I. Specimen characterization and fundamentalconsiderations

Tribology Letters - Three commercially available carbon-graphite cryogenic materials(EC-5, M234 and P5N) that are used as seals in rocket turbopumpassemblies have been characterized as part of a...     

Polyimide Composite Retainer Materials Optimized for Minimal Wear and Film Transfer

To reduce torque and torque noise, the designers of sputtered MoS₂ film-lubricated precision ball bearings must avoid the use of self-lubricating composite retainers which exhibit high wear and film transfer rates. To develop an essentially benign ball separator with minimal, ball pocket wear and polymer transfer film formation, the tribological behavior of two chemically homologous polyimides were optimized by blending with solid lubricant additives, as guided by a Taguchi design of experiments approach. The study employed an L9 fractional factorial lest matrix design with analysis of variance calculations. A fluorinated and a non-fluorinated version of a commercially available polyimide were compounded with two types of powdered solid lubricants, using three levels of filler content. The results of flat-on-flat oscillatory wear tests performed with these composites sliding against 440C bearing steel were analyzed and compared with the wear rates of two commercially available polymeric composite retainer materials. The results of the optimization study indicated that the composite with the lowest wear consisted of the fluorinated Polyimide B filled with 7.5%, by volume, of molybdenum disulfide (MoS₂). Its wear rate was substantially below that of the two commercial retainer materials. Analysis of variance calculations showed that all three factors, i.e., polyimide type, filler type, and filler content, proved to be significant in reducing the wear of these retainer candidates.     

Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part III. SEM tribometry of polycrystalline diamond in vacuum and hydrogen

In this part III of a multi-part paper series, the results of additional SEM tribometric experiments are described, performed with polished, mostly C(100)-oriented polycrystalline CVD diamond film [PCD_C(100) vs. PCD_C(100)] counterfaces sliding in Torr and in 0.1–0.3 Torr partial pressures of pure hydrogen gas. These tests were completed under a 28 g (0.27 N) normal load, under standard and slow thermal ramping conditions at temperatures ranging from room temperature to 1000^°C. The friction data were examined per the computer logging and analysis techniques described in part I. The treatment of the data is similar to that of Si in part II: the maximum and the average coefficients of friction (MAX.COF and COF) and their ratios (the friction noise FN) are employed to measure possible lubricative interaction of the diamond surfaces with rarefied hydrogen. The results indicate that excited species of molecular hydrogen enter into tribothermally catalyzed reactions not only with Si but with PCD_C(100) surfaces as well. Similar to the behavior of Si, the most beneficial friction-reducing regime occurs in a temperature range just before the thermal desorption of adsorbates. The general magnitudes of MAX.COF, COF and the FN are significantly lower than those of the Si crystallinities, in both vacuum and . The wear rate of the PCD_C(100) film characteristic of the standard thermal ramping test procedure performed mostly in is around , in good agreement with the wear rate previously measured in vacuum for unpolished, fine-cauliflowered diamond films. The data indicate that smooth polycrystalline diamond is a significantly better bearing material for miniaturized moving mechanical assembly applications than Si. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crystal-structure-controlled tribological behavior ofcarbon-graphite seal materials in partial pressures of helium andhydrogen. III. Cryogenic (100 K) wear tests

Wear experiments were performed at cryogenic (~100 K)temperatures with the highly graphitic EC-5, the amorphous M234and the turbostratic P5N carbon-graphite seal materialspreviously evaluated in parts I and II of this paper series. Theexperiments were run in ~1 atm of 99.999% pure He and in a 85%N₂+15% H₂ gas mixture, against polished-SiC counterfaces. The wear rates (W) of thematerials were in the order ofW_EC-5>W_M234>W_P5N. All materialswore less in P_{H 2} than inP_He. Considering the magnitude of therespective wear rates, the W_M234 was the mostresponsive and the W_EC-5 the least responsive to thewear-reducing effects of H₂. The cryotester wear dataconfirmed the part II room-to-high-temperature SEM tribometertest results in that the surface chemical behavior of acarbon-graphite seal vs. -SiC tribosystemmust be taken into account in determining tribologicalperformance as a function of test atmosphere and temperature.     

Re(de)construction-induced friction signatures of polished polycrystalline diamond films in vacuum and hydrogen

Pin-on-flat SEM tribometry was performed with polished, mostly C(100)-textured and acid-cleaned polycrystalline CVD diamond films heated to 950°C then cooled to room temperature. Testing in~1.33×10^-3 Pa = 1× 10^-5 Torr vacuum was followed by similar experimentation in 13 to 40 Pa (0.1to 0.3 Torr) partial pressures of 99.999%-pure H₂. In vacuum, all tests showed the characteristic step function-with-trough coefficient of friction (COF) signatures previously hypothesized as footprints of wear- and thermal desorption-induced generation, re(de)construction and passivation of the danglingσ bonds on the interacting surfaces. In hydrogen, all wear tracks exhibited stepfunction-like COF curves caused by adsorbate de(re)sorption on heating and cooling. A distinct re(de)construction COF trough obtained at the highest temperatures could be duplicated during repeated sliding in the same track on a large number (but not all) of the wear paths. The repeatable, incremental reduction in COF at the onset of heating and its substantial reduction on final cooling are attributed to tribocatalytically enhanced dissociative chemisorption of molecular hydrogen. The wear rates of the polished diamondon the pin tip, as controlled by the progressively reduced unit stresses caused by the enlargement of the wear scar, are between3.9×10^-16 and 2.6×10^-16m³/(N m) in P_{H 2}, in good agreement with previous data. This revised version was published online in June 2006 with corrections to the Cover Date.     

Magnéli phases of anion-deficient rutile as lubricious oxides. Part II. Tribological behavior of Cu-doped polycrystalline rutile (TinO2n−1)

As a follow-up to the work described in part I of this paper series, a preliminary study was conducted with polycrystalline rutile Ti_O−x to render its friction and wear performance independent of the environment. The main goal was to confine the most tribologically desirable oxygen stoichiometries of the crystallographic shear-induced Magnéli phases (CSMP) of rutile by doping with cations similar in size and polarizability to the Ti⁴⁺, but with lower valences. The resultant chemical expulsion of oxygen from the rutile lattice was intended to generate CSMP free of friction and wear variations caused by changes in the thermal–atmospheric environment. Copper, iron, cobalt and nickel ions were tried as dopants introduced as their stable oxides via a simple ball-milling, hot-pressing and annealing procedure, but only a (Ti + Cu)O_1.80 model blend resulted in a desired reaction. A portion of the copper entered the lattice to form a new titanium–copper CSMP, resembling the well-known V₃Ti₆O₁₇ catalyst equivalent to an undoped rutile CSMP with an O/Ti ratio of 1.89. Although the shear behavior of this new, wear-resistant compound was in accordance with predictions, its friction is higher and more variable than desired for a wide environmental regime lubricant. The preparation technique was only sufficient to demonstrate that oxygen vacancy-induced creation of low-friction CSMP may be possible by doping; it does not appear to be useful for formulating practical, rutile-based lubricious oxides. This revised version was published online in August 2006 with corrections to the Cover Date.