Performance of fluid and solid lubricants in spacecraft applications

Results of a series of lubricant screening and performance tests are described that provide criteria for the selection of proper Lubrication for precision spacecraft mechanisms, such as reaction wheels, gyroscopes, solar array drives, and gimbals. An eccentric, thrust—bearing apparatus was used for rapid screening and relative ranking of lubricants, and results are compared to life test data for a series of oils. Synthetic hydrocarbon oils and greases are recommended for the first three types of mechanism, while newly developed, high density, oriented, MoS₂, films are recommended for gimbals.     

聚全氟甲乙醚苯酰胺抗氧抗腐作用机理研究

采用量子化学方法计算了全氟甲乙醚苯酰胺（化合物I）与聚全氟甲乙醚氧自由基（RiO.)的结合能以及与Fe原子的化学吸附作用能,探讨了该化合物作为抗氧抗腐添加剂的作用机理,结果表明,RiO.自动结合于苯环的C7原子上,生成C－O键和稳定的加成产物,其结合能为－417．33kJ/mol;化合物I能与Fe原子形成较稳定的络合物,其化学吸附作用能为－105．02kJ/mol,合成了聚全氟甲乙醚苯酰胺抗氧抗腐添加剂（PFPA）,并进行了UV和IR光谱分析,PFPA分子中苯环的UV和C＝0的IR特征吸收峰分别为240．7nm和1713．9cm^-1。抗氧抗腐性能测试结果表明,加入PFPA添加剂,能使氟醚润滑油的失重较空白样减少近7倍,使Al2O3仙化下的热分解温度提高19度,研究表明,聚全氟甲乙醚苯酰胺的最低轨道（LOMO）与氧自由基的单占分子轨道 （SOMO）相互作用,形成亲核加成产物,阻止氧自由基RfO．的链锁反应和酸性产物的生成,添加剂的最高占有分子轨道（HOMO）的与Fe原子的LOMO相互作用形成稳定的化学吸附态和油膜,阻止金属催化分解并保护金属。     

Surface diffusion and flow activation energies of perfluoropolyalkylether

Surface diffusion of perfluoropolyalkylether (PFPE) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silicon wafers was investigated over the temperature range of 25 to 50°C using scanning microellipsometry. Zdol exhibits a much lower mobility and a distinctly different thickness profile as compared to Z15. The activation energy for surface diffusion of Zdol is higher than that of Z15, reflecting the stronger affinity of its hydroxyl end groups for the substrate. The viscosity flow activation energy E ^* is compared with that of surface diffusion E _d ^* yielding E _d ^* E ^* for Z15, and E _d ^* 1.5E ^* for ZOn leave from Korea Insitute of Science and Technology, PO Box 131, Cheongryang, Seoul, Korea 305-701.     

Effect of temperature on tribological performance of a silicon nitride ball material with a linear perfluoropolyalkylether

The effect of temperature in rolling contact performance of a hot isostatically pressed (HIP) silicon nitride ball material with a linear perfluoropolyalkylether (PFPAE) was studied using a ball-on-rod type rolling contact fatigue tester. The test temperature ranged from ambient to 343°C for a period of 24 h at a stress of 5.5 GPa using thin dense chrome (TDC)-coated T-15 bearing races. The lubricant and its decomposition products, specifically acid fluoride and acids, attacked Si₃N₄ balls at all test temperatures resulting in corrosion pitting. The presence of metal fluoride on all the Si₃N₄, transferred from the races, was detected by X-ray photon spectroscopy (XPS). The thickness of the oxide layer formed on the balls, as determined by Auger electron spectroscopy (AES) increased with temperature. The changes in physical properties of post-test lubricant showed that the lubricant was stable at temperatures up to 288°C. The change in viscosity was constant up to 288°C and with a significant change above 288°C. The FTIR analysis of 316 and 343°C post-test lubricant showed the presence of carboxylic acid. The total acid number (TAN) increased linearly up to 288°C and accelerated at 316 and 343°C. The study indicates that the use of Si₃N₄ balls with a linear PFPAE results in an incompetent tribo system.     

Desorption Kinetics of Polyether Lubricants from Surfaces

Desorption or evaporation is one of the mechanisms for loss of perfluoropolyalkylether (PFPE) lubricants from the surfaces of data storage media. One approach to minimizing PFPE loss to desorption is the use of lubricants with increasing molecular weight or increasing average chain length. In order to understand the effects of chain length on the lubricant evaporation kinetics we have studied the desorption kinetics of monolayer films of oligomeric ethers with varying chain length adsorbed on the surface of graphite. The desorption pre-exponents, v, and desorption barriers, E _des , have been measured for poly(ethylene glycol) dimethyl ethers, CH₃O(CH₂CH₂O) _m CH₃, with m=1,2,3,4,8 and 10. These are models for the PFPE known as Fomblin Z, which has a structure CF₃O(CF₂CF₂O) _x (CF₂O) _y CF₃. The results show that the desorption pre-exponents are independent of chain length and have an average value of v=10^18.7±0.3 s^–1. The E _des for the poly(ethylene glycol) dimethyl ethers vary non-linearly with chain length and can be fit with a power law expression of the form E _des =a+bN , where N is the total number of atoms in the oligomer backbone (N=3m+3) and the scaling exponent has a value of 1/2. This non-linear dependence of E _des on chain length has also been observed in recent studies of the desorption kinetics of straight chain alkanes from graphite. A desorption mechanism is described that explains the non-linearity of E _des for the poly(ethylene glycol) dimethyl ethers. The implication for the lifetime of lubricants on data storage media is that the long chain PFPE lubricants desorb more rapidly than one might expect based on simple linear scaling of the E _des of lower molecular weight PFPEs.     

Structural characteristics and mechanism of action of antioxidant anticorrosive additives for perfluoropolyalkylether fluids

The combination energies of perfluoropolyalkyletherphenylamide (PFPEA) with perfluoropolyalkylether oxygen radicals (R_fO) and with Fe atoms have been calculated by quantum chemical methods. The values are ‐417.33 and ‐105.02 kJ/mol, respectively. The structural characteristics and the mechanism of action of an antioxidant anticorrosive additive have been explored. It was found that the highest occupied molecular orbital (HOMO) of the additive is a π molecular orbital and it has the properties of a typical electron donor. The HOMO of the PFPEA additive reacts with the lowest unoccupied molecular orbital (LUMO) of an Fe atom and forms a coordinate bond and chemically adsorbed film on the metal surface. The film inhibits catalytic degradation and protects the metal surface. The additive is also capable of accepting the electron of R_fO. The R_fO radical combines automatically with the C₇ atom of the benzene ring to terminate the R_fO decomposition reaction. The PFPEA additive has been synthesized and analysed by UV and IR. Measurement of antioxidant anticorrosive properties indicates that, in the presence of the synthesised additive, the weight loss of a perfluoropolyether lubricant is reduced by about 7 times and the degradation temperature is increased by about 19°C compared with the base oil without the additive. The experimental results are consistent with the conclusions from the theoretical study.     

Effect of humidity on wear of M-50 steel with a branched perfluoropolyalkylether lubricant

Humidity is an uncontrolled variable in many tribology tests. This report shows that changes in relative humidity can affect wear for all perfluoropolyalkylethers regardless of their molecular structure. Using a Cameron–Plint tribometer under controlled environmental conditions, wear of M-50 steel with a Krytox (a branched perfluoropolyalkylether) lubricant was studied under boundary lubrication conditions at 150^°C in air with relative humidity ranging from 1 to 95%. Both wear and friction decreased sharply as humidity was increased from 1 to 20%, then were constant as the humidity was increased to 95%. Thus, wear is highly dependent on humidity when relative humidity is less than 20%. The similar effect of humidity on wear previously observed for Fomblin Z, a linear perfluoropolyalkylether containing difluoroacetal groups, and Demnum S, a linear perfluoropolyalkylether which does not contain difluoroacetal groups, indicates that this may be a general property of all commercially available perfluoropolyalkylethers, branched as well as linear ones. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stabilization of a linear perfluoropolyalkylether fluid

The thermo‐oxidative stability of a linear perfluoropolyalkylether fluid has been improved by stressing the fluid at 371°C while bubbling air or nitrogen in an overboard configuration. It is believed that the minor amounts of less stable species present in the fluid, possibly hydrogen‐containing species, are degraded and removed during the process leaving the residual fluid with enhanced stability. Two antioxidation/corrosion additives, a substituted triphenyl phosphine and a substituted diphenyl ether, were formulated with the stressed fluid, and their behavior was compared with the same formulations in unstressed fluid. While the phosphine formulation in stressed fluid showed better stability than its formulation in unstressed fluid, no such effect was noticed with the diphenyl ether additive formulations. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of thermal stressing on perfluoropolyalkylethers at elevated temperatures

Fultz et al. have reported that the thermo‐oxidative properties of linear PFPAEs can be improved by stressing the fluid at elevated temperature (371°C) in the presence of air. A study of M‐50 steel coupons exposed to unstressed and stressed linear PFPAE fluids at 260 °C and 330 °C each reveal complex surface layers. For the coupon exposed to the unstressed fluid at 260 °C, a subsurface layer is observed between the iron oxide and iron substrate that has been characterized as being composed of FeF₂. In contrast, the coupon exposed to the stressed fluid has a marked increase in the iron oxide thickness ∼2–3 times) when compared to the unstressed sample and shows no evidence of a buried fluorine‐containing layer. An increase in temperature (330 °C) in the stressed fluid O–C test was required to form a subsurface FeF₂ layer. It is proposed that the elimination of the fluorine layer found on the M‐50 substrate increases the upper temperature limit found from the oxidation–corrosion studies. The increase in the oxide layer thickness implies that the FeF₂ layer found in the unstressed sample acts like a diffusion barrier which inhibits the outward movement of Fe⁰ and the decreased rate of iron oxide growth. This revised version was published online in June 2006 with corrections to the Cover Date.