George W. Fultz

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...

Characterization of Model Perfluoropolyalkylethers by Miniaturized Thermal Oxidative Techniques—Part I: Modified Oxidation-Corrosion Test

The metal catalyzed thermal oxidative stability of commercially available and model perfluoropolyalkylether (PFPAE) fluids was investigated by oxidation corrosion tests. Using model PFPAE fluids, prepared via direct fluorination of hydrocarbon analogues, a wider variety of specific molecular structures were available for study than previously available from more conventional PFPAE chemistry synthesis procedures. Because of a limited supply of the model fluids, a miniaturized oxidation corrosion test, using only 6 ml of fluid, was devised. This test, conducted at constant temperatures, was compared to a similar test using 20 ml of fluid on the more abundant commercial fluids and then extended to the model fluids. Details of the miniaturized test procedure and apparatus are presented. Also presented are the chemical structure-thermal oxidative stability relationships of the fluids as derived from this study. The destabilizing effect of an -OCF 2 - group is confirmed and other more subtle effects are noted.

High speed Civil transport (HSCT) hydraulic fluid development

GE Aircraft Engines required a new hydraulic fluid for engine nozzle actuation for the High-Speed Civil Transport (HSCT). Perfluoropolyalkylethers (PFPAEs), because of their nonflammability and high temperature stability, were selected early in the program. The authors have found two formulated PFPAE fluids for the application. The viscosity-temperature data, pressure-viscosity data, EHL (elastohydrodynamic lubrication) film forming ability, thermal stability data, antirust protection data and wear data were measured and the data used to select the candidates. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

The use of MIL-PRF-87257 hydraulic fluid as a switching medium for a high-power microwave switch

The article investigates the use of synthetic polyalphaolefin-based MIL-PRF-87257 hydraulic fluid as a switching dielectric medium in gigawatt-class repetitive pulsed power applications.

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...

The effect of water content on the dielectric strength of polyalphaolefin (PAO) coolants

Many variables can affect the dielectric strength of coolants and insulating fluids including particulates, water content, temperature, viscosity, and pressure. The present paper evaluates the effects of dissolved water content, adjusted in a controlled manner, on the dielectric strength and the variability of breakdown voltages in synthetic polyalphaolefin (PAO) MIL-PRF-87252 coolants obtained from three different suppliers. Additionally, the effect on dielectric strength of using nitrogen gas versus air for the removal of water is evaluated. The results indicate that the dielectric strength of the fluids decreases by up to 80% over a range of 0 to 70 ppm water content. The variability in breakdown voltage generally increases with increasing water content for all of the samples tested. These results indicate the notable effects of water at concentrations below the MIL-PRF-87252 specified acceptable limit of 50 ppm. The effect on the dielectric strength of using two different sparging gases for the removal of water, dry air or dry nitrogen, was irrelevant when compared at similar water contents.

Hydraulic System Component Storage with Military Hydraulic Fluids

In the military aerospace community, most hydraulic fluid pumps and components are currently being stored in rust inhibited fluids containing barium dinonylnaphthalene sulfonate (BSN). Fluids containing barium are hazardous waste after use, with expensive disposal, and have caused operational problems in aircraft hydraulic systems including helicopters and fighter aircraft. In this program, bearings and pistons were stored in jars containing both operational hydraulic fluids (MIL-PRF-83282, MIL-PRF-87257, and MIL-PRF-5606) and rust-inhibited hydraulic fluids containing BSN (MIL-PRF-46170 and MIL-PRF-6083). In addition, hydraulic pumps were filled with MIL-PRF-83282, MIL-PRF-87257, and MIL-PRF-46170. Hydraulic pumps were not filled with MIL-PRF-5606 or MIL-PRF-6083 because these hydraulic fluids are being phased out of military aerospace applications as operational and storage fluids, respectively. Jars, containing bearings and pistons, as well as hydraulic pumps, were stored for up to three years in a laboratory environment to determine if operational fluids would protect them from rusting during storage. After each year, the bearings, pistons, and pumps were inspected for corrosion. At the end of three years of storage, pumps were endurance tested using fresh operational fluid, MIL-PRF-83282. The bearings, pistons, and pumps showed no rusting for the duration of storage with either operational or storage fluids. The pumps stored with the operational fluids, MIL-PRF-83282 and MIL-PRF-87257, were in better condition than the pump stored with the rust-inhibited fluid. The operational hydraulic fluids, MIL-PRF-83282 and MIL-PRF-87257, provided excellent protection against rusting during storage. Manuscript contributed February 1, 2005 Review led by Paul Bessette

Multi-Purpose, Moisture-Resistant, High Load Carrying Polyalphaolefin Based Grease, MIL-PRF-32014

A lithium-soap-thickened synthetic hydrocarbon (polyalphaolefin) grease was developed in response to failure of a sodium-soap-thickened mineral oil grease on long term storage in the F-107 engine of the Cruise Missile. The new grease was heavily fortified with performance improving additives and has demonstrated excellent results in bench tests, in an engine test and in the actual application. The superior capabilities of the grease include very high speed (30,000 rpm), high load, high temperature, low water washout, excellent corrosion inhibition and excellent wear resistance. A military specification, MIL-PRF-32014, was written to procure this grease. Testing of the grease in C-5 aircraft landing gear is currently underway and new applications are being investigated. This grease is anticipated to meet most of the military applications and is being proposed as a new multipurpose grease that could significantly reduce the number of greases in the military inventory. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002

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.

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.