HYBRID CERAMIC BEARINGS FOR DIFFICULT APPUCATIONS

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York - N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Safely or of its Divisions or Sections, or printed Wits publications. Discussion is printed only it the paper is published in an ASME Journal. Authorization to photocopy material tor internal or personal use under circumstance not taffing within the fair use provisions of the Copyright Act Is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page Is paid directly to the CCC, 27 Congress Street Salem MA Requests for special permission or bulk reproduction WWI be addressed to the ASME Technical Publishing Department Copyright by ASME All Rights Resented Printed in U.S.A HYBRID CERAMIC BEARINGS FOR DIFFICULT APPUCATIONS Michael M. Dezzani and Philip K. Pearson The Torrington Company Torrington, Connecticut ABSTRACT The Torrington Company under contract from the Advanced Research Projects Agency (ARPA) has developed a hybrid bearing with improved properties for difficult applications. M50 and M50 NiL steel rings were nitrided to produce rolling contact raceway surfaces with hardnesses near Rockwell C 70. Rings were assembled with NBD-200 silicon nitride balls. Full scale bearing tests were run under conditions that included 150 C temperature, surface flaws created by hard particle contamination, partial EHD lubrication, and the sliding action of balls rururing under thrust loading. The hybrid bearings had longer life than all steel bearings and demonstrated resistance to the surface peeling mode of failure initiation. Higher strength of the rolling contact surfaces, high residual compressive stresses in the nitrided layers, and a more favorable action in ceramic to steel rolling contact are discucw1 as the reasons for improved performance of the hybrid over all steel bearings. INTRODUCTION This paper describes the manufacturing and testing of a hybrid bearing development for use in difficult applications. Silicon nitride has a long history of demonstrating good properties for rolling contact fatigue. It also has very high hardness and resistance to high temperature and corrosion damage that make it suitable for extreme application conditions. In theory all ceramic bearings would be desirable, but their use has been limited by problems of ring manufacturing as well as application problems arising from differences in thermal expansion properties. The most common use for ceramics today is in hybrid bearings using silicon nitride rolling elements and steel rings. These perform well in applications such as machine tool spindles where conditions are predictable, clean, and controlled; but their use in more demanding applications is limited by the properties of the steel rings. The need for an improved hybrid bearing is to withstand conditions of higher temperatures, poor lubrication, contamination, and poor rolling with sliding. These are the conditions that cause failures in service and make surface initiated fatigue the most frequent mode of failure in bearings today. HYBRID BEARING DEVELOPMENT The hybrid bearings for this development used Norton Advanced Ceramics NBD-200 hot isostatically pressed silicon nitride balls. The balls were finished to Grade 5 tolerances. Rings were manufactured from both M50 (4.0 Cr, 4.2 Mo, 1.0 V, 0.8 C) and M50 NiL (4.0 Cr, 4.2 Mo, 1.3 V, 3.5 Ni, 0.13 C) steels. Nitriding was chosen to improve the surface and near surface zones of the steel rings. Nitriding increases the hardness to be closer to that of the silicon nitride. Nitriding is a diffusion process which becomes part of the microstructure and, therefore, not subject to delamination as hard coatings might be. Nitrided zones are thicker than applied coatings. Nitriding temperatures are close to the tempering temperatures for M50 and M50 NiL steels. Therefore, the high bearing hardness below the nitrided layer is not changed significantly. The nitriding process used for the bearing rings was ferritic nitrocarburizing; that is, a hydrocarbon gas was added to the nitriding atmosphere. Rings were nitrided in a fluidized bed furnace for 32 hours Presented at the International Gas Turbine and Aeroengine Congress & Exposition Houston, Texas - June 5-8, 1995 This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 30, 1995

2 at 525 C (975 F) using an atmosphere flow of 600 cubic feet per hour ammonia, 300 cfh nitrogen, and 200 cfh propane. The sequence of manufacturing operations for the rings was soft machining, carburizing (M50 NIL only), hardening, tempering, rough grinding of all surfaces, Striding, finish grinding, and polishing of raceways. The raceway finish of the inner rings was 0.08 Am, the outers 0.12 to 0.16 Ian Ra. The compound or white layer on nitrided surfaces causes poor rolling contact fatigue life, and all of this was removed from the raceway surfaces by finish grinding. The hardnesses and residual stresses at and below the raceway surfaces are shown in Figures 1 and 2, respectively. BEARING TEST PROCEDURES The bearing test procedures were designed to simulate conditions found in aircraft turbine main shaft bearings; however, features of this test address the conditions that cause failures in many types of bearings and applications. Temperature of operation is high. Contamination creates surface flaws. Thin films provide only partial elastohydrodynamic (EHD) lubrication. Thrust operation causes ball spin which creates sliding in the rolling contact zones. Tests were carried out using 40 mm bore angular contact thrust bearings with nine one-half inch balls and bronze retainers. Tests were run at 10,000 rpm with oil corresponding to MIL-L-7808J at 150 C (300 F). The calculated END film thickness was Am (3 Ain). The thrust load of the hybrid bearings was 7500 N (1700 lbs), and the maximum compressive contact stress was 2.62 GPa (380 ksi). The test results were compared to previous tests of all steel bearings run at equivalent contact stress and also at equivalent load. For the 2.62 GPa contact stress condition, all steel bearings were run under 11,800 N (2700 lbs) load. At equivalent loading of 7500 N, all steel bearings develop a lower contact stress of 2.28 GPa (330 ksi). The bearings were first run for 15 minutes in oil which had been contaminated with 2.5 ppm of 20 Am aluminum oxide. The bearings were disnsembled, washed, and the rings were examined for surface damage. The bearings were then reassembled and run in different rigs with clean oil. Details of this test and previous test results were reported by Averbach et. al. in references (1,2,3). TEST RESULTS The bearing life results are listed in Tables I through IV. Three types of all steel bearings used M50 steel balls in M50 steel rings, in M50 NiL steel rings, and in M50 NIL steel rings with TDC coating. TDC is a thin dense chromium coating with hardness near Rockwell C 70, applied in a manner to have compressive residual stresses for good rolling contact performance. The hybrid bearings showed superior performance at the equivalent contact stress condition. They were also superior at equivalent loading, a condition under which the all steel bearings were running at a lower contact stress. None of the ten nitrided M50 hybrids failed within the 2000 hour suspension time for the test. The nitrided M50 NiL hybrid bearings had three failures, but the L10 life of this hybrid combination was longer than the life of any of the all steel bearings, even though the best of the all steel bearings had the very hard TDC surface on M50 NiL rings. The absence of nitride,' M50 hybrid failures prevented the calculation of an Lb O life for this group. Tables II and IV show statistical calculations of 90 percent lower confidence limits, assuming a Weibull slope of 1.5 for the nitrided M50. Differences in surface behavior accompanied the differences in life, and these observations support the conclusions to be drawn from the life results. The all steel bearing failures began with a process of peeling. Peeling is the growth of shallow areas of surface distress less than 100 Am (0.004 in) deep. The failure process begins when the already thin lubrication film becomes even thinner as it spreads out at the surface flaws caused by the contamination. Partial EHD conditions allow metal to metal contact at the edges of the flaws. The coefficient of friction rises to increase stress at the surfaces. The failure process ends when peeling creates a larger area of surface distress from which a macro spall develops. An example of this from an M50 all steel bearing is shown in Figure 3. The TDC coated raceways followed the same failure process, with the difference being that the harder TDC surface made the initial contamination flaws smaller and the rate of peeling slower, leading to the longer life of the TDC coated raceways. The nitrided raceways with silicon nitride balls were free of peeling damage. They had many flaws because the aluminum oxide contamination was harder than the nitrided surfaces, but none of these developed into peeled areas. Nitrided raceways are shown in Figure 4. 2

3 Rockwell C Hardness 80 0M50 ar M50 NIL O Q025 Q inches O 0.25 Q Depth Figure 1: Hardness Profiles, M50 and M50 NiL Steel Bearing Rings 1.25 mm ksi Residual Stress MPa awls , Inches mm Depth Figure 2: Residual Stress, Nitrided M50 and M50 NiL Steel Bearing Rings 3

4 Table 1: Summary of Results for 208 Bearing Tests, Hours Maximum Contact Stress 380 ksi - (Fl) Failed Inner Ring, (FB) Failed Ball, (S) Suspended Hybrid Nitrided M50 Hybrid Nitrided M50 NiL M50 M50 NIL With TDC 2034 (S) 797 (Fl) 68 (Fl) 434 (Fl) 2034 (S) 1299 (Fl) 147 (Fl) 486 (Fl) 2038 (S) 1882 (Fl) 310 (Fl) 722 (F1) 2038 (S) 2001 (S) 316 (Fl) 695 (FB) 2020 (S) 2016 (5) 334 (Fl) 1100 (FB) 2020 (S) 2022 (S) 746 (F1) 1504 (S) 2006 (S) 2032 (S) 1127 (Fl) 1535 (5) 2038 (S) 2032 (S) 1500 (S) 1535 (S) 2004 (S) 2001(5).1500 (S) 1544 (S) 2004 (S) 2174 (S) 1500 (S) 1587 (S) 1684 (S) Table 2: Summary of Weibull Analysis for 208 Bearing Tests, Maximum Contact Stress 380 ksi 90% Lower Weibull Bearing Material L10, hours Confidence Slope L10, hours Hybrid Nitrided M (estimated) Hybrid Nitrided M50 NIL M50 NIL. & TDC Rings M * No Failures, therefore an life could not be determined Table 3: Summary of Results for 208 Bearing Tests, Hours Approximately 1700 Pounds Thrust Load - (Fl) Failed Inner Ring, (FB) Failed Ball, (S) Suspended Hybrid Hybrid M50 NiL Nitrided M50 Nitrided M50 NiL M50 M50 NIL With TDC 2034 (S) 797 (Fl) 429 (F1) 270 (FI) 374 (F1) 2034 (S) 1299 (Fl) 445 (Fl) 317 (Fl) 901 (Fl) 2038 (5) 1882 (Fl) 502 (Fl) 326 (Fl) 1082 (Fl) 2038 (S) 2001(5) 635 (Fl) 677 (Fl) 2213 (S) 2020 (S) 2016 (S) 666 (Fl) 779 (Fl) 2213 (S) 2020 (S) 2022 (S) 1390 (Fl) 998 (F1) 2278 (S) 2006 (S) 2032 (S) 1916 (Fl) 1372 (Fl) 2278 (S) 2038 (S) 2032 (5) 2010 (Fl) 1500 (Fl) 2395 (5) 2004 (S) 2001(S) 2095 (S) 2715 (S) 3571 (S) 2004 (S) 2174 (S) 2095 (S) 2890 (S) 3571 (5) Table 4: Summary of Weibull Analysis for 208 Bearing Tests, Approx Pounds Thrust Load 90% Lower Weibull Bearing Material L10, hours Confidence Slope L10, hours _. Hybrid Nitrided M (estimate) Hybrid Nitrided M50 NIL M M50 NiL M50 NIL & TDC Rings * No Failures, therefore an L 10 life could not be determined Downloaded From: on 11/25/2018 Terms 4 of Use:

5 aea Ii "ta.c=1. "" -r r-rd1/4 a- cr-ar a-- Figure 3: Progression of damage in M50 raceway, all steel bearing. Upper, initial 15 minute damage. Middle, after running 22 hours. Lower, macro spall developing at 68 hours. Figure 4: Nitrided raceways in hybrid bearings. Upper, M50 after initial 15 minute damage. Middle, M50 after running 208 hours. Lower, M50 NiL spall after 797 hours.

6 DISCUSSION At least three conditions may account for the better performance of the nitrided M50 and M50 NiL hybrid bearings. These are harder, dent resistant surfaces, very high residual compressive stresses to resist crack initiation and propagation, and more favorable action at the ceramic to steel rolling contact interface. Analytical studies have explained how conditions of roughness, traction, contamination, and flaws cause stresses at the surfaces to be greater than the sub-surface stresses of Hertzian contact. Olver, Cole, and Sayles have shown that contacting surfaces of normal roughness can result in stress on the order of twice that of the sub-surface maximum shear. (4) Chin has calculated the high tensile stresses that develop in the presence of individual surface flaws. (5) Fatigue begins when behavior ceases to be perfectly elastic and micro yielding takes place. Harder surfaces have higher yield properties to resist this onset of fatigue. This explains the excellent rolling contact performance of the silicon nitride itself in addition to the improved life of the nitrided steel surfaces and, to a lesser extent, the TDC coating. The high residual compressive stresses in the nitrided layer are also expected to contribute to longer life. The residual stresses in the outermost zone within pm (0.001 in) of the surface are the product of grinding and finishing and tend to be similar for all steels. It is just below this depth, the zone into which cracks must propagate for peeling as well as macro spalling, that the compressive stresses from nitriding will resist crack growth. The rolling contact action of ceramic to steel must also be considered. Rhoads, Bashyam, and Crecelius demonstrated that hybrid bearings are capable of sustained operation at reduced oil flow in high speed turbine engine bearings. (6) Insufficient lubrication film is at the beginning of the failure process in many applications. Ceramic hybrid bearings operate at lower temperatures and prevent any possibility of metal to metal contact, both beneficial features when lubrication films are thin and partial EHD conditions exist. The difference between the nitrided M50 and nitrided M50 NiL hybrid bearings in these tests may be statistical or, if real, show a benefit from the greater carbide content and harder M50 surface in resisting the micro scuffing that takes place in partial EHD conditions. CONCLUSION Hybrid bearings using silicon nitride balls and nitrided M50 and M50 NiL raceways perform better than all steel bearings in conditions of contamination, surface flaws, marginal lubrication, and traction that are the cause of many failures in service. REFERENCES 1. Averbach, B.L. and Bamberger, E.N., "Analysis of Bearing Incidents in Aircraft Gas Turbine Main.shaft Bearings", Tribology Transactions, Vol. 34, 2 pp , Averbach, B.L., Van Pelt, S.C., Pearson, P.K. and Bamberger, EN., 'Surface Initiated Spalling Fatigue in M50 and M50 NiL Bearings:, Lubrication Engineering, Vol 47, 10, pp , Averbach, B.L., Van Pelt, S.C., and Pearson, P.K., 'Initiation of Spoiling in Aircraft Gas Turbine Bearing?, American Institute of Aeronautics and Astronautics paper AIAA , 26th Joint Propulsion Conference, July Olver, A.V., Cole, S.J., and Sayles, R.S., "Contact Stresses in Nitrided Steels", 19th Leeds Lyon Symposium on Tribology, September 1992, published Elsevier, Chiu, Y.P., "The Role of Residual (Internal) Stresses on Microspalling Rolling Contact, Rolling Element Bearing Symposium '91", Orlando, Rhoads, M.A., Bashyam, M., and Crecelius, W.J., 'Large Engine Hybrid Ceramic Bearings", 94-GT-262, ASME International Gas Turbine and Aeroengine Congress, June ACKNOWLEDGMENTS The authors wish to thank the Advanced Research Projects Agency who sponsored the Improved Hybrid Bearing development and the many others who contributed to this work. The bearing test procedure and baseline all steel data were developed in collaboration with GE Aircraft Engines under contract to the Air Force Wright Research and Development Center. We are also grateful to many colleagues at The Torrington Company who were part of this effort, especially Mr. Henry Daverio for the bearing life testing, Mr. Donald Church for the laboratory work, and Dr. Jack Woodilla, Director of the Advanced Technology Center, for support and encouragement of this materials development program. 6