Malcolm K. Stanford

Test Evolution and Oil-Free Engine Experience of a High Temperature Foil Air Bearing Coating

During the start-up and shut-down of a turbomachine supported on compliant foil bearings, before the bearings have full development of the hydrodynamic gas film, sliding occurs between the rotor and the bearing foils. Traditional solid lubricants (e.g., graphite, Teflon®) readily solve this problem at low temperature. High temperature operation, however, has been a key obstacle. Without a suitable high temperature coating, foil air bearing use is limited to about 300°C (570°F). In oil-free gas turbines, a hot section bearing presents a very aggressive environment for these coatings. A NASA developed coating, PS304, represents one tribological approach to this challenge. In this paper, the use of PS304 as a rotor coating operating against a hot foil gas bearing is reviewed and discussed. During the course of several long term, high cycle, engine tests, which included two coating related failures, the PS304 technology evolved and improved. For instance, a post deposition thermal treatment to improve dimensional stability, and improvements to the deposition process to enhance strength resulted from the engine evaluations. Largely because of this work, the bearing/coating combination has been successfully demonstrated at over 500°C (930°F) in an oil-free gas turbine for over 2500 hours and 2900 start-stop cycles without damage or loss of performance when properly applied. Ongoing testing at Glenn Research Center as part of a long term program is over 3500 hours and 150 cycles.Copyright

Resilient and Corrosion-proof Rolling Element Bearings Made from Superelastic Ni-Ti Alloys for Aerospace Mechanism Applications

Mechanical components (bearings, gears, mechanisms) typically utilize hardened construction materials to minimize wear and attain long life. In such components, loaded contact points (e.g., meshing gear teeth, bearing balls-raceway contacts) experience high contact stresses. The combination of high hardness and high elastic modulus often leads to damaging contact stress and denting, particularly during transient overload events such as shock impacts that occur during the launching of space vehicles or the landing of aircraft. In this webinar, Dr. DellaCorte will introduce the results of a research project that employs a superelastic alloy, Ni-Ti for rolling element bearing applications. Bearings and components made from such alloys can alleviate many problems encountered in advanced aerospace applications and may solve many terrestrial applications as well

Effects of humidity on the flow characteristics of a composite plasma spray powder

The effects of environmental humidity on the flow characteristics of a multicomponent (composite) plasma spray powder have been investigated. Angular and spherical BaF2−CaF2 powder was fabricated by comminution and by atomization, respectively. The fluorides were blended with nichrome, chromia, and silver powders to produce a composite plasma spray feedstock. The tap density, apparent density, and angle of repose were measured at 50% relative humidity (RH). The flow of the powder was studied from 2 to 100% RH. The results suggest that the feedstock flow is only slightly degraded with increasing humidity below 66% RH and is more affected above 66% RH. There was no flow above 90% RH except with narrower particle size distributions of the angular fluorides, which allowed flow up to 95% RH. These results offer guidance that enhances the commercial potential for this material system.

Dimensional Stability, Microstructure, and Cohesion Strength of Composite Solid Lubricant Coatings after Heat Treatment

PS304 is a plasma spray deposited coating composed of Ni-Cr, Cr 2 O 3 , Ag, and BaF 2 -CaF 2 that has been developed for reduction of friction and wear at high temperatures. Physical properties of the coating after varying heat treatments and using varying weight percentages of chromium in the Ni-Cr constituent have been investigated. Dimensional stability was reduced with increasing heat treatment temperature and the coating growth rate decreased with time. The dimensional stability was also reduced as the Cr content in the Ni-Cr constituent increased from 10 to 30 wt%. Coating growth was due to oxidation of the Ni-Cr constituent. The optimum cohesion strength for PS304 was essentially the same after heat treatment at 650 or 725°C for 100 h. Further investigation indicated that heat treatment at 725°C for 100 h increases the dimensional stability in a high temperature service environment.

Post Irradiation Evaluation of Thermal Control Coatings and Solid Lubricants to Support Fission Surface Power Systems

The development of a nuclear power system for space missions, such as the Jupiter Icy Moons Orbiter or a lunar outpost, requires substantially more compact reactor design than conventional terrestrial systems. In order to minimize shielding requirements and hence system weight, the radiation tolerance of component materials within the power conversion and heat rejection systems must be defined. Two classes of coatings, thermal control paints and solid lubricants, were identified as material systems for which limited radiation hardness information was available. Screening studies were designed to explore candidate coatings under a predominately fast neutron spectrum. The Ohio State Research Reactor Facility staff performed irradiation in a well characterized, mixed energy spectrum and performed post irradiation analysis of representative coatings for thermal control and solid lubricant applications. Thermal control paints were evaluated for 1 MeV equivalent fluences from 1013 to 1015 n/cm2. No optical degr...

Microstructure, Physical Properties, and Tribological Characteristics of Composite Solid Lubricant Coatings with Gas Atomized BaF2-CaF2

PS304 is a NASA-developed composite solid lubricant for friction and wear reduction at high temperatures. The microstructure, physical properties, and tribological performance of PS304 using standard comminuted BaF2-CaF2 and novel gas atomized BaF2-CaF2 were compared. Two atomized BaF2-CaF2 particle size distributions were used. Overall, feedstock powder with atomized BaF2-CaF2 flowed more freely than standard PS304 feedstock. The cohesion strength of coatings with atomized BaF2-CaF2 was lower than that of standard PS304, while the hardness of all of the studied coatings was essentially the same (approximately 58 HRA). Pin wear was lower against the new coatings and the coefficients of friction were essentially the same for all coatings. These results indicate that atomized BaF2-CaF2 can be substituted for comminuted BaF2-CaF2, providing a high-volume BaF2-CaF2 fabrication technique to reduce the cost of PS304 and improve its commercial availability.

Design Considerations for Resilient Rolling Element Bearings Made From Low Modulus Superelastic Materials

Nickel-titanium based superelastic materials are emerging as candidates for rolling element-bearing applications [1]. When properly prepared, these unique intermetallics are hard, exhibit excellent tribological properties and are intrinsically corrosion immune [2]. In addition, recent investigations have revealed that, unlike traditional bearing steels, superelastics can endure much higher levels of recoverable elastic strain during compressive deformation [3]. This behavior enables bearings that are more resilient to load induced damage such as raceway denting and from the ingestion of hard particles. Despite these positive attributes, these alloys differ significantly from conventional steels and these differences must be carefully considered to achieve successful applications. These differences include reduced elastic modulus, high hardness and enhanced resistance to indentation loads. The current paper introduces nickel-titanium based superelastic bearing materials and compares their properties to current bearing materials. General bearing design practices and manufacturing processes are also examined to identify and explore the challenges and opportunities for making resilient rolling element bearings utilizing this new class of superelastic materials.Copyright