Larry S. Helmick

The effect of humidity on the wear behavior of bearing steels with RfO(n-C3F6O)xRf perfluoropolyalkylether fluids and formulations©

A reciprocating tribometer and four-ball tribometer were used under controlled environmental conditions to determine the wear behavior of bearing steels with a linear perfluoropolyalkylether (PFPAE) lubricant which does not contain difluoroacetal groups. Formulated lubricants (base oil plus additive) containing potential antiwear additives were also tested and results compared to those for the unformulated base oil. Using the reciprocating tribometer, wear of M-50 steel in air was studied under sliding boundary lubrication conditions at 50°and 150°C with relative humidity ranging from 1 to 95%. Wear was observed to decrease sharply as humidity increased from 1 to 5%, then did not change significantly as humidity increased to 95%. These results indicate that the high wear previously observed in a PFPAE fluid at low humidities was not due to the presence of difluoroacetal groups, but may be characteristic of PFPAE fluids. Reduced wear observed at low humidities in screening tests with several formulated lub...

The Decomposition Reaction Path of a Linear PFPAE Under Tribological Conditions

It has previously been shown using 19F nuclear magnetic resonance (NMR) spectroscopy that a linear perfluoropolyalkyl ether (PFPAE) fluid undergoes decomposition by a disproportionation reaction mechanism to form acid fluorides when heated in the presence of aluminum chloride. It is shown in this study that the same linear PFPAE fluid decomposes by the same reaction mechanism to form the same acid fluorides when heated at 345°C in the presence of dry air and bearing steels. Then, it is shown that these acid fluorides readily undergo hydrolysis to carboxylic acids when they are exposed to atmospheric moisture. Finally, carboxylic acids were detected in highly stressed linear PFPAE fluids exposed to the atmosphere after recovery from fatigue testing experiments. Therefore, it is concluded that under real tribological conditions, linear PFPAE fluids initially decompose in the presence of bearing steels by a disproportionation reaction mechanism to form acid fluorides which subsequently react with atmospheric moisture to form carboxylic acids.

The Effect of Additives on the Wear Behavior of Bearing Steels with RfO(CFfO)x(CF2CF2O)y(CF2CF2CF2O)zRf Perfluoropolyalkylether Fluids

In previous research, high wear of M-50 bearing steel in low humidity air environments was observed for RfO(CF2O)x (CF2CF2O)y(CF2CF2CF2O)zRf base fluid in a reciprocating tribometer. In this work, the effect of humidity on wear was observed with a four-ball tribometer using the same base fluid in an oxygen-deficient environment with 52100 bearing steel. In low humidity atmospheres, air was found to decrease wear compared to nitrogen in the four-ball wear tribometer. Next, the effects of additives soluble in the base fluid on the wear behavior of M-50 and 52100 steel were studied at low humidity using both a reciprocating tribometer as well as a four-ball tribometer. An attempt was made to adjust conditions of the four-ball test to allow its use for routine screening of additives; however, under the conditions attempted, the four-ball tribometer, while improved in repeatability for petfluoropolyalkylethers, was not as discriminating as the reciprocating tribometer. All of the additives were found to be eff...

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.

Determination of the Thermal Stability of Fluids by Tensimetry: Instrumentation and Procedure

A computerized tensimeter and experimental procedure for determination of the thermal decomposition temperature (Td) of perfluoroalkylethers have been developed and tested. Both the apparatus and the procedure are described in detail. Results of testing with bis (2-ethylhexyl) phthalate and trimethylolpropane triheptanoate demonstrate that the reciprocal of the decomposition temperature is a linear function of the logarithm of the gas volume/heated liquid volume ratio. The Td obtained for each compound at a gas volume/heated liquid volume ratio of one was similar to the value previously reported using an isoteniscope technique. Results of testing with a polymer of hexafluoropropylene oxide demonstrate that this instrument and procedure can be used to determine the Td of perfluoroalkylethers. Presented at the 35th STLE/ASME Tribology Conference in Fort Lauderdale, Florida October 16–19, 1989

Thermal-Oxidative Pretreatment and Evaluation of Poly(hexafluoropropene Oxide) Fluids

Two commercial poly(hexafluoropropene oxide) fluids were thermally pretreated at 343 C in pure oxygen. IR and nuclear magnetic resonance(NMR) spectra indicate that this pretreatment was effective in removing hydrogen end-capped impurities. A decrease in the quantity of volatile material produced during thermal oxidative decomposition and increase in the thermal decomposition temperature indicated improvement in the stability of the fluids. However, this pretreatment failed to render the fluids completely stable in oxidizing atmospheres at 316 C in the presence of metal alloys.

Effects of Linear PFPAE Decomposition Products on the Rolling Contact Fatigue Performance of M50

In bench tribology tests, the influence of lubricant decomposition products on the lubricated system and on lubricant performance are often overlooked, primarily because testing is performed in vented, or open, systems. However, the lubricant in a gas turbine engine might be expected to more closely follow the principles of interaction that are found in unvented, or closed, systems. Results reported here comparing vented and unvented bearing housings in rolling contact fatigue (RCF) tests with a linear perfluoropolyalkylether (PFPAE) lubricant and VIM-VAR M50 steel at 316°C and a stress of 4.8 GPa clearly show that significantly more wear, corrosion, and fluid breakdown occur in the closed system than in the open system. Under these conditions, PFPAEs catalytically decompose to corrosive products. These corrosive products are partially vented in the open system, but retained in the closed system, causing more extensive corrosion of bearing materials, and significantly affecting the lubricants tribological performance. Post test lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricants physical and chemical properties. Changes were more severe in the closed system. The nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra also showed significantly more carboxylic acid buildup in the stressed fluid from the closed system. The films formed in the tribojunction were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Organic films formed in the open system reduced wear while inorganic films formed in the closed system resulted in higher wear. Consequently, we conclude that more attention needs to be given to the effect of decomposition products during bench type tribological testing of high temperature lubricants.

A study of boundary lubrication thin filmsproduced from a perfluoropolyalkylether fluid on M-50 surfaces.II. Humidity effect and topography studies

Tribology experiments under controlled humidity were conductedusing a Cameron-Plint tribometer with an M-50 cylinder slidingon an M-50 flat specimen in the presence of a linearperfluoropolyalkylether fluid. The tests were conducted at 100,70, 59, 10, 7 and 5% relative humidities. The surface thin filmsformed on the M-50 flat specimen were studied by grazing-angle-microscope/FTIR technique. A thicker surface filmof RfCOOH was observed for the tests conducted atrelative humidities equal to or larger than 15%. A monodentatemodel discussed in the preceding paper was adopted to be thesurface film configuration. For the lower humidity tests, anultra-thin surface film of RfCOOH-type was observed.An asymmetrical bridging model was proposed to account for thisobservation. In addition, theνasym,OCO band shifted to lower wavenumbers as the relative humidity of the controlled environmentwas decreased. These band shifts at different humidities warrantthe use of the multilayer model over the metal soap model toaccount for the experimentalobservation. Topographical and frictional image studies on thecylinder wear scars tested at 100, 98, 70, 59, 15 and 5% relativehumidities have been carried out using atomic force andfrictional force microscopies. Residual surface features werefound to be present on those specimens tested at 100, 98, 70, 59and 15% relative humidities. These findings are complementarywith the results obtained from the M-50 flat specimens studiedby GAM/FTIR technique.