Lewis Rosado

Rolling Contact Fatigue Life and Spall Propagation of AISI M50, M50NiL, and AISI 52100, Part I: Experimental Results

This article is the first part of a three-part series that investigates the rolling contact fatigue (RCF) initiation and spall propagation characteristics of three bearing materials, namely, AISI 52100, VIM-VAR AISI M50, and VIM-VAR M50NiL steels. Although there is substantial prior work published on the rolling contact fatigue initiation of these materials, little has been published on their spall propagation characteristics after spall initiation. It is recognized that rapid spall growth can lead to catastrophic bearing failure. Hence, understanding the spall growth phase and factors that may cause accelerated growth rates is key to achieving a reliable and robust bearing design. The end goal is to identify control parameters for optimizing bearing materials for improved spall growth resistance. This first part study features the experimental results from 208-size (40 mm bore) angular-contact ball bearings endurance life tested at maximum Hertzian contact stress levels of 3.10 GPa and bearing outer race temperatures up to 131°C. Spall propagation experiments were conducted on new and life tested bearings at 2.10 and 2.41 GPa maximum contact stress. Spall propagation experiments show that all materials exhibit a rapid or critical spall growth rate after undergoing an initial low-rate spall growth period. The time-to-critical growth rate is dependent on contact stress and was swiftest in AISI 52100 steel. To better understand the underlying physics, the driving factors, and failure mechanisms, the state of stress is modeled using finite element analysis in Part II and an in-depth microstructural analysis of selected bearings is presented in Part III.

The effect of diamond-like carbon coatings on the rolling fatigue and wear of M50 steel

Abstract The rolling contact fatigue and wear characteristics of uncoated and diamond-like carbon coated VIM-VAR M50 bearing steel were investigated at room temperature and 177°C (350°F) and a Hertzian stress level of 4.8 GPa (700 ksi). The coatings were deposited via ion beam enhanced deposition and were approximately 33 nm thick. Rolling fatigue and wear tests were conducted using a ball-on-rod type tester. Results did not indicate any significant difference in the fatigue life of coated and uncoated specimens at room temperature, at 90% confidence level. However, the coating significantly improved the fatigue life (90% confidence level) at 177°C and wear resistance at both temperatures. Some correlation was noted between wear and fatigue life for the uncoated specimens at both temperatures, but none for the diamond-like carbon coated (the fatigue life was independent of wear) specimens. Scanning electron microscopy did not show any sign of coating delamination.

Rolling Contact Fatigue Life and Spall Propagation Characteristics of AISI M50, M50 NiL, and AISI 52100, Part III: Metallurgical Examination

This is the third part of a three-part series that investigates the rolling contact fatigue initiation and spall propagation characteristics of three bearing materials, namely, AISI 52100, VIM-VAR M50, and VIM-VAR M50 NiL steels. Though there is substantial prior work published on the rolling contact fatigue initiation of these materials, little is known about their spall propagation characteristics after spall initiation. In Part III, 208-size, 40-mm-bore bearings are examined for changes in appearance of the microstructure as well as residual stress as a function of depth in the circumferential direction. The correlations between the experimental results from Part I and computer modeling in Part II are made.

Evaluation of fatigue and wear characteristics of M50 steel using synthetic ester turbine engine lubricants—Part 1

The fatigue and wear characteristics of AISI VIM-VAR M50 steel were evaluated using three fully formulated synthetic ester-based lubricants. Tests were conducted using a rolling contact fatigue test rig of the ball-on-rod type at a cyclic maximum hertzian stress level of 4.83 GPa (700 klbf in−2) and temperatures of 23°C (74°F) and 177°C (350°F). The lubricants evaluated included three polyol esters conforming to the MIL-L-7808J (3.16 cSt at 100°C), MIL-L-23699 (4.93 cSt at 100°C), and DOD-L-85734 (4.93 cSt at 100°C) specifications. Although preliminary results seem to suggest that rolling contact fatigue life may be affected by the type of lubricant, the differences observed were not statistically significant. Of the three lubricants tested, MIL-L-7808J generally exhibited the highest fatigue life while DOD-L-85734 showed the lowest. According to specific film thickness calculations, all three lubricants were tested under boundary lubrication. It is believed that, due to this operating lubrication regime, wear volume per stress cycle and fatigue life were unaffected by lubricant type and only affected by temperature. Also, there was some correlation found between wear and fatigue life for all lubricants.

Rheology of perfluoropolyalkylether fluids in elastohydrodynamic lubrication

The rheological characteristics of two branched and two linear, commercial perfluoropolyalkylether (PFPAE) fluids were studied under high pressures and temperatures. The effects of branching and carbon-to-oxygen (C :O) ratio on the pressure-viscosity-temperature behavior and on the non-Newtonian behavior of these fluids were studied experimentally under high pressures and temperatures. The branching and the higher C :O ratio seemed to increase the pressure-viscosity coefficients of these fluids. The effects of the viscosity and the pressure-viscosity coefficient on the capabilities of these fluids to generate elastohydrodynamic lubrication (EHL) film thickness were studied and compared with experimental measurements. All of the fluids studied seemed to follow the Roelands viscosity model and classical EHL theory (1). The C :O ratio also influenced the temperature dependence of the limiting-shear-strength proportionality constant. The results show that for similar-viscosity fluids, the linear PFPAE with higher C :O ratio is most desirable for wide temperature use.

Evaluation of fatigue and wear characteristics of M50 steel using high temperature synthetic turbine engine lubricants. Part II

Abstract The fatigue and wear characteristics of AISI VIM-VAR M50 steel were evaluated using two high-temperature synthetic turbine engine lubricants. Rolling contact fatigue (RCF) tests were conducted on a ball-on-rod type rig at a cyclic Hertzian contact stress of 4.8 GPa (700 ksi) and temperatures of 177 °C (350 °F) and 274 °C (525 °F). The two lubricants tested included a five-ring polyphenylether (MIL-L-87100, 12.64 cSt at 100 °C) and perfluoropolyalkylether (25.51 cSt at 100 °C). RCF results were compared with those obtained with a polyolester (MIL-L-7808J) at 177 °C. Weibull analysis and analysis of variance conducted on the RCF data show significant increases in fatigue life and reductions in wear rate when compared with MIL-L-7808J lubricant at 177 °C. However, at 274 °C, fatigue life was substantially reduced and wear of M50 steel increased with both high-temperature lubricants.

Rotating Brush Seal-Experimental Performance Evaluation

In this paper, we will present the results of the initial rotational tests for the rotating intershaft brush seal. Data will be presented comparing the rotating brush seal, both single and tandem arrangements to multi-tooth labyrinth seals and also fracture ring carbon seals.