The tribological behavior of Ni–Cu–Ag-based PVD coatings for hybrid bearings under different lubrication conditions

In a cryogenic environment, components like bearings with interacting surfaces in relative motion (tribosystems) often generate undesired heat and experience high wear. Because the properties of conventional bearing materials like stainless steel cannot be applied to this temperature range, the PVD coating based on metal–metal pairs with better frictional properties must be employed. To test the suitability of the Ni–Cu–Ag-based PVD coatings of hybrid bearings for liquid rocket engine turbopumps and to obtain reliable coating material data in the extreme environment, the tribological behaviors of coatings under the cryogenic fluid (liquid oxygen and liquid nitrogen) and water lubricated conditions are studied, respectively. In the paper, the specimens are in a vibrocryotribometer with the ball-on-plane contact type, and various running conditions in terms of lubricants, contacting loading, and contacting velocity are examined. The simulated experiment for testing the actual tribological performance of Ni–Cu–Ag-based PVD coatings for hybrid bearings is tested. The results of the tests indicate that the coatings can be suitable for cryogenic tribosystems of turbopumps. In the cryogenic environment, the volume wear rate of coatings increases rapidly with the contacting loading, but 15 min later, the volume wear volume of coatings turns into 2.5–15×10⁻⁴ mm³. Besides, under the liquid oxygen condition in simulating the liquid rocket engine turbopumps environment, the friction coefficients are 0.03–0.1.     

The Performance of PS400 Subjected to Sliding Contact at Temperatures from 260 to 927°C

Adequate high-temperature lubrication between loaded surfaces in sliding contact can be one of the most challenging tribological problems confronting today's designers. In an attempt to provide a possible solution a test program was initiated to evaluate PS400, a recently patented, high-temperature solid lubricant coating. Made from nickel–molybdenum–aluminum, chrome oxide, silver, and barium fluoride–calcium fluoride, PS400 is a variant of the earlier coating, PS304, but is formulated for higher density, smoother surface texture, and greater dimensional stability. It was initially developed to minimize the start–stop wear in foil air bearings but is expected to perform well in other high-temperature applications where sliding friction and wear are a concern, such as variable inlet guide vanes and process control valve stems. To better define its operational capabilities, a series of tests was conducted to study the behavior of PS400 under reciprocating sliding contact at temperatures from 260 to 927°C. The tests were performed on stationary, uncoated cobalt-based superalloy bushings loaded against reciprocating PS400-coated shaft specimens in a flat-on-cylinder configuration at Hertz contact pressures from 14.1 to 20.1 MPa. For tests conducted below 927°C, friction coefficients ranged from 0.37 to 0.84 with wear factors on the order of 10^?5 and 10^?6 at the high temperatures but substantially increased at the lowest temperature. Data collected at 927°C were limited because the coating was found to be dimensionally unstable at this temperature.     

Analysis of Foil Journal Bearings with Backing Springs

Gas-lubricated foil journal bearings are simple, in construction, lightweight and well suited for high-temperature applications in turbomachinery. Hearing stiffness is governed primarily by the foil flexural stiffness. The bearing consists essentially of thin overlapping circular metal foils, one end of which is cantilevered to the bearing housing and. the other end rests on an adjacent foil.

An analysis of gas-lubricated foil bearings is presented with a specific type of backing spring used under the foils to control bearing preload, and stiffness. The backing spring acts like, an elastic foundation tinder the foil and radically changes the hydrodynamic pressure distribution generated in the gas film. The pressure distribution is obtained by simultaneously solving the compressible Reynolds equation and. the elasticity equations governing the compliant bearing surface, consisting of foils and backing springs. An iterative scheme is used, to obtain pressure distributions for heavily loaded cases, involving extensive computation, because of the sensitivity of pressure solution to small changes in film thickness distributions attributable to the compliant bearing surface. Pressure distribution, film thickness, bearing load capacity, iterative solution convergence characteristics and bearing power dissipation are presented as a function of journal eccentricity.     

Journal Design Considerations for Turbomachine Shafts Supported on Foil Air Bearings

Foil air bearings can offer substantial improvements over traditional rolling element bearings in many applications and are attractive as a replacement to enable the development of advanced oil-free turbomachinery. In the course of rigorous testing of foil journal bearings at NASA Glenn Research Center, shaft failure was repeatedly encountered at high ambient temperature and rotational speed, with moderate radial load. The cause of failure is determined to be excessive non-uniform shaft growth, which increases localized viscous heating in the gas film and eventually leads to a high-speed rub and destruction of the bearing and journal. Centrifugal loading of imbalance correction weights and axial temperature gradients within the journal due to the hydrodynamic nature of the foil bearings, determined by experiment and finite element analysis, are shown to be responsible for the non-uniform growth. Qualitative journal design guidance is given to aid in failure prevention.     

弹性箔片轴承的研究进展*

近年来,弹性箔片轴承在高速透平机械的应用中展现出巨大的潜力。为获取轴承高承载力、减少轴承启停过程的摩擦与磨损、提高轴承的高速稳定性,研究人员开展了大量的理论、实验与应用研究。从弹性箔片轴承的结构特性、涂层特性和建模方法3个方面对气体弹性箔片轴承的研究进展进行综述。首先从轴承结构优化的角度,介绍箔片轴承结构改进的发展历程,以及具有新颖结构设计的多叶搭接式弹性动压箔片轴承及其优点;然后从减少轴承启、停阶段摩擦力,提高弹性箔片轴承使用寿命的角度出发,比较分析了应用广泛的多种涂层,总结国内外新型润滑剂的适用条件及减摩效果;最后从轴承建模角度,介绍经典的简单弹性模型、考虑箔间摩擦力与分离等影响因素的系列复杂模型,为后续模型构建与性能分析提供依据。展望弹性箔片轴承在高精尖等领域的前景,并对弹性箔片轴承的研究方向提出了建议。     

Development of Surface Coatings for Air-Lubricated,Compliant Journal Bearings to 650°C

Surface coatings for an air-lubricated, compliant journal for an automotive gas turbine engine were tested to find those capable of withstanding temperatures of either 540°C (1000°F) or 650°C (1200°F). Also, the coatings have to be capable of surviving the start-stop sliding contact cycles prior to rotor lift-off and at touchdown. Selected coating combinations were tested in start-stop tests at 14 kPa (2 psi) loading for 2000 cycles at room and maximum temperatures. Specific coating recommendations are: Cdo and graphite on foil versus chrome carbide on journal up to 370°C (700°F); NASA PS-120 (Tribaloy 400, silver, and CaF₂) on journal versus uncoated foil up to 540°C (1000°F); and chemcially adherent Cr₂O₃ on journal and foil up to 650°C (1200°F). The chemically adherent Cr₂O₃ coating system was further tested successfully at 35 kPa (5 psi) loading for 2000 start-stop cycles.     

Structural Stiffness and Coulomb Damping in Compliant Foil Journal Bearings: Theoretical Considerations

Compliant foil bearings operate on either gas or liquid, which makes them very attractive for use in extreme environments such as in high-temperature aircraft turbine engines and cryogenic turbopumps. However, a lack of analytical models to predict the dynamic characteristics of foil bearings forces the bearing designer to rely on prototype testing, which is time-consuming and expensive. In this paper, the authors present a theoretical model to predict the structural stiffness and damping coefficients of the bump foil strip in a journal bearing or damper. Stiffness is calculated based on the perturbation of the journal center with respect to its static equilibrium position. The equivalent viscous damping coefficients are determined based on the area of a closed hysteresis loop of the journal center motion. The authors found, theoretically, that the energy dissipated from this loop was mostly contributed by the frictional motion between contact surfaces. In addition, the source and mechanism of the nonlinear behavior of the bump foil strips were examined. With the introduction of this enhanced model, the analytical tools are now available for the design of compliant foil bearings.     

Assessment of thermal effects on the levitation speed of bump foil bearings made of low cost spring steel

The demand for higher speeds and torque capacity from micro turbines, heat pumps and turbochargers has necessitated the development of high temperature resistant foil bearings. This paper focuses on investigating the effects of successive thermal cycles on the levitation speed of bump foil bearings made up of low cost spring steel. Bump foil bearings were designed for high stiffness of 1.64 MN/m². Rotor dynamic analysis indicated highest frequency of 4790.5 Hz corresponding to second flexure mode of rotor bearing system up to which it remained stable. The bump foil bearing fabrication procedure was established and rotor was tested under suitably designed bearing rig. The orbital analysis indicated that levitation speed decreased with increase in temperature. During second and third thermal cycle, at lower rotor speeds drastic variations in amplitude of vibrations and uneven waveforms were indicative of unbalance condition of rotor. With further increase in rotor speeds, the rotor - bump foil bearing system attained the balanced state indicative of safe design.     

Wear properties of DLC-coated steel rollers running with highly contaminated lubrication

The presence of hard contaminants in lubrication can lead to the premature failure of rolling bearings. To reduce the negative effect of such contaminants, hard carbon-based coatings (diamond-like carbon; DLC) can be applied to the surfaces of steel bearings. DLC coatings generate a low friction and a high sliding wear resistance to enhance the tribological properties and improve the durability of running components. This work explores the merits of DLC coatings for use in very demanding applications, such as in highly contaminated environments. The wear properties of DLC-coated bearing rollers were evaluated by comparing them with uncoated rollers. The degree of wear found on the coated rollers was serious, especially under relatively high contaminant concentrations. The three-body abrasive wear produced a relatively coarse scoring of the coating surface, which caused the corresponding disc to suffer more damage than the disc running against an uncoated roller under the same operating conditions. The results indicate that supposedly wear-protective coatings cause even more damage to running surfaces once they have been broken up by hard contaminants, and highlight the importance of keeping the bearing coating intact. In practise, it is important to eliminate contaminants from the lubricant of rolling bearings, in particular for bearings with a DLC anti-wear coating.     

Dynamic characteristics of a flexible rotor system supported by a viscoelastic foil bearing (VEFB)

Foil bearings have been considered as an alternative to traditional bearings with the increasing need for high-speed, high-temperature turbomachinery. However, the lack of adequate load capacity and sufficient damping capacity is a key technical hurdle to super-bending-critical operation as well as widespread use of foil bearings in turbomachinery such as turbopumps, turbocompressors and turbochargers. A new foil bearing, ViscoElastic Foil Bearing (VEFB) is suggested in this paper. The super-bending-critical operation of the conventional bump foil bearing and the VEFB is examined, as well as the structural dynamic characteristics. The structural dynamic test results show that the equivalent viscous damping of the VEFB is much larger than that of the bump bearing and that the structural dynamic stiffness of the VEFB is comparable or larger than that of the bump bearing. The results of super-bending-critical operation of the VEFB indicate that the enhanced structural damping of the viscoelastic foil dramatically reduces the vibration near the bending critical speed. With the help of increased damping resulting from the viscoelasticity, suppression of the nonsynchronous orbit is possible beyond the bending critical speed.     

Load Capacity Estimation of Foil Air Journal Bearings for Oil-Free Turbomachinery Applications

This paper introduces a simple “Rule of Thumb” (ROT) method to estimate the load capacity of foil air journal bearings, which are self-acting compliant-surface hydrodynamic bearings being considered for Oil-Free turbomachinery applications such as gas turbine engines. The ROT is based on first principles and data available in the literature and it relates bearing load capacity to the bearing size and speed through an empirically based load capacity coefficient, D. It is shown that load capacity is a linear function of bearing surface velocity and bearing projected area. Furthermore, it was found that the load capacity coefficient, D, is related to the design features of the bearing compliant members and operating conditions (speed and ambient temperature). Early bearing designs with basic or “first generation” compliant support elements have relatively low load capacity. More advanced bearings, in which the compliance of the support structure is tailored, have load capacities up to five times those of simpler designs. The ROT enables simplified load capacity estimation for foil air journal bearings and can guide development of new Oil-Free turbomachinery systems.     

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10^?4 mm³/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.     

Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery

The performance and durability of advanced, high temperature foil air bearings are evaluated under a wide range (10 to 50 kPa) of loads at temperatures from 25° to 650 °C. The bearings are made from uncoated nickel based superalloy foils. The foil surface experiences sliding contact with the shaft during initial start/stop operation. To reduce friction and wear, the solid lubricant coating, PS304, is applied to the shaft by plasma spraying. PS304 is a NiCr based Cr₂O₃ coating with silver and barium fluoride/calcium fluoride solid lubricant additions.

The results show that the bearings provide lives well in excess of 30,000 cycles under all of the conditions tested. Several bearings exhibited lives in excess of 100,000 cycles. Wear is a linear function of the bearing load. The excellent performance measured in this study suggests that these bearings and the PS304 coating are well suited for advanced high temperature, oil-free turbomachinery applications.     

弹性箔片动压径向气体轴承起飞转速和承载能力的实验研究

在建立的气体轴承性能测试实验台上对新型弹性箔片气体动压径向轴承进行了起飞转速和承载能力的实验研究,并分别通过摩擦力矩和径向位移响应频谱两种方法分析了轴承的起飞转速。结果表明:轴承起飞后摩擦力矩逐渐减小并趋于稳定,且径向位移响应频谱图上只有较大的低倍频分量出现,高倍频分量和其它频率分量要小得多;两种分析方法得到的轴承起飞转速基本吻合;载荷越大,轴承和转子中心的偏心距越大;由于箔片弹性变形使卸载过程存在能量损失,同样载荷下卸载时的偏心距比加载时大。     

On the Fluid Flow and Thermal Analysis of a Compliant Surface Foil Bearing and Seal

Foil gas bearings have been applied successfully to a wide range of high-speed rotating machinery such as air cycle machines (ACMs) and auxiliary power units (APUs). The performance of these bearings are based on the high pressure gas in a very thin layer between the journal and the bearing governed by the Reynolds equations. Generation of heat in these bearings especially at high journal rotating speed and high loads or at high ambient temperature directly affect their performance. Thermal and fluid flow analysis of an advanced compliant foil journal bearing/seal are presented. The side flow (known as leakage) and the approximate temperatures are the results of this analysis. The result of preliminary analysis shows that the major portion of the heat is carried through conduction and using the modified Couette flow approximation for the present working fluid, air, helped in analysis of the temperature magnitude, which can be related to the gas viscosity behavior and thin gas film thicknesses.     

Tribological Characterisation of Air-Sprayed Epoxy-CNT Nanocomposite Coatings

Epoxy-carbon nanotube (CNT) composites are promising coating materials for wear and corrosion critical applications such as marine bearings, shafts, bolts and gears. However, there are insufficient tribological data available for design. This article described the fabrication and tribological testing of an epoxy-CNT composite coating composed of a commercial epoxy primer and commercial CNT filler. The CNT filler was pre-treated so that it was compatible with epoxy resin and was dispersed using a ball milling process. A reciprocating sliding test rig was built for the measurement of friction and wear of the coatings which were subjected to multi-pass testing using the ball-on-plate sliding geometry. The rig allowed testing with either constant or varying normal force, along with measurement of normal and tangential forces. Thus, the coefficient of friction (COF) under ramping or constant normal force could be determined. Following testing, the samples were examined using an optical microscopy to determine the severity of any galling which had taken place. The coatings were found to display encouraging properties in all aspects of testing. COF values of around 0.2 were recorded under a nominal contact pressure up to 1 GPa. This coating can be used for components which require anti-corrosion and low friction properties.     

DLC solid lubricant coatings on ball bearings for space applications

The environment of space offers special challenges for the lubrication of components in sliding and rolling mechanisms. Hydrogenated diamond-like carbon (DLC) films are being studied as solid lubricant coatings to simultaneously fulfil specifications regarding wear resistance and low friction behaviour under ambient atmosphere and in vacuum.In this paper, the tribological behaviour of highly hydrogenated DLC coatings (50 at% hydrogen) is assessed. Coating composition was optimised on flat AISI 52100 steel substrates based on ball-on-disc tribotest results in air, vacuum and dry nitrogen environments. The developed DLC coatings can be tailored to yield ultra-low friction values in vacuum (μ=0.008). The average friction coefficient range obtained in humid air, dry nitrogen and vacuum for the range of applied loads were, respectively, 0.22 to 0.27, 0.02 to 0.03, and 0.007 to 0.013.New in this work is that optimised DLC coatings were applied to ball bearings for space applications. The torque and life tests of coated pairs of angular contact bearings in air revealed that relatively high bearing torques are generated which increase with time, but the amount of coating wear generated during in-air operation appears relatively light. In vacuum, low torques are generated after a prolonged running-in period. Low-torque life exceeds that observed for MoS₂ by a factor of about two. It is concluded that, in contrast to MoS₂ coated bearings, DLC-coated bearings for space applications might therefore be capable of undergoing in-air ground testing without too much disruption of the subsequent in-space performance.     

考虑轴颈偏斜的双层径向波箔轴承静特性分析

考虑了轴颈偏斜的影响,建立了双层径向波箔轴承的理论模型。用有限差分法对控制方程进行求解得出静特性参数,分析了轴承数、偏心率、轴承柔性、长径比和轴颈偏斜对双层波箔轴承静特性的影响。结果表明,双层波箔轴承比单层波箔轴承具有更大的承载能力,比刚性表面动压轴承对轴颈偏斜具有更好的适应性。     

高速透平箔片动压气体轴承固体润滑涂层研究进展

箔片动压气体轴承广泛应用于高速透平膨胀机、微小型燃气轮机、涡轮增压机等高速透平机械中。固体润滑涂层可为箔片动压气体轴承提供启停阶段的润滑保护,对保证箔片动压气体轴承稳定性与使用寿命至关重要。本文回顾了国内外箔片动压气体轴承固体润滑涂层的研究历史与现状;结合文献资料着重分析了PS系列、Korolon系列和DLC系列的固体润滑涂层特性,并对其应用场合和优缺点进行了探讨。     

Bearing Retainer Material for Modern Jet Engines

Modern jet engines require bearing retainers that can provide reliable service under stringent and unusual conditions. In current commercial transports, these engines commonly operate at 1.5 × 10⁶ DN for 12000 hours or more between overhauls, and in military applications, the cage and bearings are required to retain its integrity when the oil is lost for up to one minute while at foil power. Standard bronze retainers with lead plate over silver plate have proven inadequate to meet these requirements. To find a more suitable retainer, candidate cage materials and coatings were evaluated in a test rig simulating engine conditions. A silver plate AMS 6415 cage material was found capable of surviving a program which comprised 50, one minute, oil cutoff cycles. An additional program was conducted to optimize the Ag-plate coating thickness which was established as being 0.001 to 0.002-inches thick. Subsequent engine evaluation of this silver plated steel retainer verified the rig test results. This material combination is now used in production engines in both commercial and military aircraft.