J Fail. Anal. and Preven. (2011)11: 158-166 DOI10.1007/11668-0109422-z TECHNICAL ARTICLE-PEER-REVIEWED Fatigue Failure Analysis of a Grease-Lubricated Roller bearing from an electric motor Zhi-Qiang Yu. Zhen-Guo Yang Submitted: 16 August 2010/in revised form: 30 November 2010/ Published online: 22 December 2010 C ASM International 2010 Abstract The grease-lubricated roller bearing of an elec- a roller bearing [1-4], failures of lubricating grease may be tric motor that drove a supply blower suddenly failed during the predominant cause of failure. The failed greases gen- operation. In order to identify the causes of the failure, a erally suffered from physical, chemical, and thermal variety of characterizations were carried out. The failed degradations during bearing operation [5-8], and the deg- surfaces of the bearing were observed visually and micro- radation brought about the loss of lubricating capacities, copically, and the characteristics of the lubricating grease especially under high temperatures and high velocity con were also investigated. Results showed that the surface of the ditions. Physical changes are commonly involved with the inner ring of the bearing contained contact fatigue damage, increase of oil separation, the loss of thickener structure, ind was covered with a multitude of debris and contact and the reduction of base oil content [9]. Chemical changes fatigue pits. What's more, the lubricating grease was sub- are mainly due to oxidation of base oil and thickener and jected to severe thermally induced degradation due to high loss of antioxidant additives, which will consequently service temperature, which consequently resulted in the increase the amount of acidic species and high-viscosity decrease of the lubricating capacity of the grease. Thus, the products. Komatsuzaki et al. [10] showed that the loss of lubricant film in the roller/raceway contacts was not formed base oil was generally caused by the evaporation of volatile effectively and the lubrication of the roller bearing was poor. oxidation products and was the predominant factor con- As a result, serious local wear as well as contact fatigue trolling the lubrication life of grease in cylindrical roller damage were brought about on the roller and raceway and the bearings wear finally led to the failure of the bearing most common failure event that results fror lubricating greases in rolling-element bearings is surface Keywords Bearing failure. Contact fatigue damage contact fatigue [11]. According to this failure mode, cracks Greases lubrication Lubricant degradation usually initiate at or near the contact surfaces, and subse quently form microscopic pits, which will then act as the stress concentration sites for further damages 3, 11] during Introduction continuing bearing operation. What's more, such stress concentration sites on the contact surfaces interact with Roller bearings are significant components of supply other pre-existing defects ing handling damage blower motors and play a critical role in normal operation surface inclusions, and dents to formed solid particles of the blower, although many factors can lead to failures of entrapped in the lubrication fluid [3]. These particles will accelerate the initiation of cracks and promote additional This article will present a failure analysis of one grease lubricated bearing that was sealed at the driving end of a Z.-Q.Yu()·Z.G.Yang Department of Materials Science, Fudan University, supply blower motor that had a rotation speed of 990 r/min Shanghai China in the electric power unit. The bearing was a cylindrical e-mail:yuzhiqiang@fudan.edu.cn roller bearing with 18 rolling elements. The face material
TECHNICAL ARTICLE—PEER-REVIEWED Fatigue Failure Analysis of a Grease-Lubricated Roller Bearing from an Electric Motor Zhi-Qiang Yu · Zhen-Guo Yang Submitted: 16 August 2010 / in revised form: 30 November 2010 / Published online: 22 December 2010 © ASM International 2010 Abstract The grease-lubricated roller bearing of an electric motor that drove a supply blower suddenly failed during operation. In order to identify the causes of the failure, a variety of characterizations were carried out. The failed surfaces of the bearing were observed visually and microscopically, and the characteristics of the lubricating grease were also investigated. Results showed that the surface of the inner ring of the bearing contained contact fatigue damage, and was covered with a multitude of debris and contact fatigue pits. What’s more, the lubricating grease was subjected to severe thermally induced degradation due to high service temperature, which consequently resulted in the decrease of the lubricating capacity of the grease. Thus, the lubricant film in the roller/raceway contacts was not formed effectively and the lubrication of the roller bearing was poor. As a result, serious local wear as well as contact fatigue damage were brought about on the roller and raceway and the wear finally led to the failure of the bearing. Keywords Bearing failure · Contact fatigue damage · Greases lubrication · Lubricant degradation Introduction Roller bearings are significant components of supply blower motors and play a critical role in normal operation of the blower, although many factors can lead to failures of a roller bearing [1–4], failures of lubricating grease may be the predominant cause of failure. The failed greases generally suffered from physical, chemical, and thermal degradations during bearing operation [5–8], and the degradation brought about the loss of lubricating capacities, especially under high temperatures and high velocity conditions. Physical changes are commonly involved with the increase of oil separation, the loss of thickener structure, and the reduction of base oil content [9]. Chemical changes are mainly due to oxidation of base oil and thickener and loss of antioxidant additives, which will consequently increase the amount of acidic species and high-viscosity products. Komatsuzaki et al. [10] showed that the loss of base oil was generally caused by the evaporation of volatile oxidation products and was the predominant factor controlling the lubrication life of grease in cylindrical roller bearings. The most common failure event that results from failed lubricating greases in rolling-element bearings is surface contact fatigue [11]. According to this failure mode, cracks usually initiate at or near the contact surfaces, and subsequently form microscopic pits, which will then act as the stress concentration sites for further damages [3, 11] during continuing bearing operation. What’s more, such stress concentration sites on the contact surfaces interact with other pre-existing defects including handling damage, surface inclusions, and dents to formed solid particles entrapped in the lubrication fluid [3]. These particles will accelerate the initiation of cracks and promote additional debris production [12]. This article will present a failure analysis of one greaselubricated bearing that was sealed at the driving end of a supply blower motor that had a rotation speed of 990 r/min in the electric power unit. The bearing was a cylindrical roller bearing with 18 rolling elements. The face material Z.-Q. Yu (&) · Z.-G. Yang Department of Materials Science, Fudan University, Shanghai, China e-mail: yuzhiqiang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2011) 11:158–166 DOI 10.1007/s11668-010-9422-z
J Fail. Anal. and Preven. (2011)11: 158-166 of the bearing was GCr15 bearing steel, while the cage material was an alloy of copper and zinc. The lubricating grease was lithium based containing MoS2 particles. Dur its operation, the bearing suddenly failed when its operation temperature exceeded the warning limit of 70C. After that, the lubricating grease which was found on the side of the raceway of the detached failed bearing was agglomerated, semisolid, and heavily. Meanwhile, the raceway surface of the inner ring of the bearing showed the signs of contact fatigue and wear. Thus, in order to identify the causes of the failure, the lubricating grease used in the failed bearing was collected and then inspected by Fourier transform infrared spectroscopy(FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis(TGA), while the micromorphologies and chemical compositions of the wear faces were examined by scanning electron microscopy Fig. I Dismounted samples of failure bea (SEM) and energy dispersive spectroscopy(EDS). Based on the analysis and relevant discussion, failure prevention methodologies for similar grease-lubricated roller bearing FT-IR Analysis were developed Figure 2 shows the FT-IR spectra of used and fresh grease samples. The fresh grease spectrum(Fig. 2a) shows char acteristic absorbance peaks of carboxylate stretch at Investigation Methods 1597 cm and hydroxyl at 3441 cm due to the presence of hydroxystearate thickener. The bands at 1460 and FT-IR with KBr discs, XRD with Co Ka radiation, and 1377 cm were assigned to Ch vibrations from the base TGA under N2 purge were applied to investigate the oil [9]. On contrast, the intense carbonyl(c=O) band at structural and thermal characteristics of the greases. 1709 cm occurred in the infrared spectrum of the used Besides that, microstructures of the greases and the wear grease(Fig. 2b), which was from the oxidation of base oil marks on the bearing inner-ring surface were observed by and thickener in the greases. In addition, that the thickener SEM. Chemical compositions of the bearing material were peaks were reduced to a broad ill-defined band indicates and the material hardness(HRC) was also measured determined by phe that the sample of used grease was mainly composed of base oil and carbonyl-containing degradation products Hence, it can be concluded that the gre raceway contact suffered heavily thermo-oxidation degra- Observation Results and Analysis dation under the high temperature The thermo-oxidation degradation of the grease fol- logy of the detached lowed the free radicals reaction mechanism [13], seen in bearing. It is obvious that the raceway surfaces of the bearing inner ring were worn heavily and there were large grease in the initial phase of the oxidation reaction with the number of pits and debris on the surface. The inner-ring temperature increase during the bearing rolling. In general, urfaces were a red-brown color and appeared polished the reaction speed was very slow. which may the result of direct contact between the rollers rH-Ar'+H and the raceway during operation. Also, the lubricating grease was found to be solidified and pushed out of the where RH denotes the base oil and thickener in grease, R and H were free radicals of the alkyl and hydrogen Characterizations of the Lubricating Grease The reaction between the alkyl free radicals and oxon) d quickly to generate peroxide groups( after alkyl radicals formation during the chain propagation Samples of used greases from the failed bearing were Successively, the reaction occurred between the peroxide identified by FT-IR, XRD, SEM with EDS, and TGA, and groups and Rh(the base oil and thickener) by direct was then compared with the fresh greases abstracting hydrogen atoms from RH and then generated Spring
of the bearing was GCr15 bearing steel, while the cage material was an alloy of copper and zinc. The lubricating grease was lithium based containing MoS2 particles. During its operation, the bearing suddenly failed when its operation temperature exceeded the warning limit of 70 °C. After that, the lubricating grease which was found on the side of the raceway of the detached failed bearing was agglomerated, semisolid, and heavily. Meanwhile, the raceway surface of the inner ring of the bearing showed the signs of contact fatigue and wear. Thus, in order to identify the causes of the failure, the lubricating grease used in the failed bearing was collected and then inspected by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), while the micromorphologies and chemical compositions of the wear faces were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Based on the analysis and relevant discussion, failure prevention methodologies for similar grease-lubricated roller bearing were developed. Investigation Methods FT-IR with KBr discs, XRD with Co Kα radiation, and TGA under N2 purge were applied to investigate the structural and thermal characteristics of the greases. Besides that, microstructures of the greases and the wear marks on the bearing inner-ring surface were observed by SEM. Chemical compositions of the bearing material were determined by photoelectric direct reading spectrometry and the material hardness (HRC) was also measured. Observation Results and Analysis Figure 1 displays the external morphology of the detached bearing. It is obvious that the raceway surfaces of the bearing inner ring were worn heavily and there were large number of pits and debris on the surface. The inner-ring surfaces were a red-brown color and appeared polished which may the result of direct contact between the rollers and the raceway during operation. Also, the lubricating grease was found to be solidified and pushed out of the bearing track. Characterizations of the Lubricating Grease Samples of used greases from the failed bearing were identified by FT-IR, XRD, SEM with EDS, and TGA, and was then compared with the fresh greases. FT-IR Analysis Figure 2 shows the FT-IR spectra of used and fresh grease samples. The fresh grease spectrum (Fig. 2a) shows characteristic absorbance peaks of carboxylate stretch at 1597 cm−1 and hydroxyl at 3441 cm−1 due to the presence of hydroxystearate thickener. The bands at 1460 and 1377 cm−1 were assigned to CH vibrations from the base oil [9]. On contrast, the intense carbonyl (C=O) band at 1709 cm−1 occurred in the infrared spectrum of the used grease (Fig. 2b), which was from the oxidation of base oil and thickener in the greases. In addition, that the thickener peaks were reduced to a broad ill-defined band indicates that the sample of used grease was mainly composed of base oil and carbonyl-containing degradation products. Hence, it can be concluded that the grease in the roller/ raceway contact suffered heavily thermo-oxidation degradation under the high temperature. The thermo-oxidation degradation of the grease followed the free radicals reaction mechanism [13], seen in Eq. 1. The alkyl free radicals (R• ) were formed in the grease in the initial phase of the oxidation reaction with the temperature increase during the bearing rolling. In general, the reaction speed was very slow. RH ! D R þ H ðEq 1Þ where RH denotes the base oil and thickener in grease, R• and H• were free radicals of the alkyl and hydrogen, respectively. The reaction between the alkyl free radicals and oxygen gas occurred quickly to generate peroxide groups (ROO• ) after alkyl radicals formation during the chain propagation. Successively, the reaction occurred between the peroxide groups and RH (the base oil and thickener) by direct abstracting hydrogen atoms from RH and then generated Fig. 1 Dismounted samples of failure bearing J Fail. Anal. and Preven. (2011) 11:158–166 159 123
J Fail. Anal and Preven.(2011)11: 158-16 Fig 2 FT-IR spectra of (a)fresh and(b) used grease 46050 m223763135 597 103802 1n1837 1377.2 146096 2924.12 (a)00300302000020008001001400120010090060040 1000 177449154107 129952 7130 13T09 1460.17 285463 292449 400003600320029002400200018001600140012001000 the hydrogen peroxide and secondary alkyl free radicals, as R"+h-Rh (Eq4) shown in reactions(2)and(3). In this way, the reactions R·+R·→R-R would not cease until the termination of the chain occurred via reaction with the hydrogen free radical, and/or mutual When the temperature increased the hydrogen peroxide coupling, as shown in reactions(4)and (5), respectively. (ROOH) decomposed into alkyl-oxygen radicals(RO) R+O→ROO (Eg 2)and hydroxyl radicals(HO), as shown in reaction (6) ould fur ROO·+RH→ROOH+R (Eq 3) produce the final alkyl radicals (R,), as shown in
the hydrogen peroxide and secondary alkyl free radicals, as shown in reactions (2) and (3). In this way, the reactions would not cease until the termination of the chain occurred via reaction with the hydrogen free radical, and/or mutual coupling, as shown in reactions (4) and (5), respectively. R þ O2 ! ROO ðEq 2Þ ROO þ RH ! ROOH þ R ðEq 3Þ R þ H ! RH ðEq 4Þ R þ R ! R R ðEq 5Þ When the temperature increased the hydrogen peroxide (ROOH) decomposed into alkyl-oxygen radicals (RO• ) and hydroxyl radicals (HO• ), as shown in reaction (6), below. These groups could further react with RH to produce the final alkyl radicals (R• ), as shown in Fig. 2 FT-IR spectra of (a) fresh and (b) used grease sample 160 J Fail. Anal. and Preven. (2011) 11:158–166 123
J Fail. Anal. and Preven. (2011)11: 158-166 reactions(7) and(8), which would return to the above XRD Analysis and SEM Observation chain propagation again. As a result, thermal-oxidative he lubricating rease was repeatedly Figure 3a and b shows the XRD patterns of the fresh and decreased used grease sample, respectively. From the X-ray analysis, ROOH→→RO·+HO° (Eq 6)the patterns. The strongest diffraction peak from the fresh HO+RH→H2O+RO (Eq 7) sample, as shown in Fig. 3a, appeared at about 20=14 RO·+RH→ROH+R cq 8) corresponding planar spacing d=0.63 nm, another strong diffraction peak presents at about 20= 26, corresponding Thus, during the chain termination the content of the planar spacing d=0.34 nm. According to PDF cards, they arbonyl-containing oxidation products including acidic could be attributed to(100)and (101)crystalline planes of species and high-viscosity products increased in the the molybdenum disulfide(Mos2). In addition, some lubricating grease and heavily deteriorated the lubricating diffraction peaks of MoSz also appeared in thi properties of the grease profile. This means that the fresh grease was a According to Xue et al. [14], the intensiy egree of the observation and EDS analysis of the fresh grease confirmed of C=0 peaks lubricating grease containing MoS2. Actually, SEM was directly proportional to the degradation grease.As illustrated in Fig. 2b, the C=0 peak was sharp this fact as well. Figure 4a and b shows SEM and EDS and the peak intensity was high(compared with the fresh results of the fresh grease sample, respectively. As shown grease, see Fig. 2a). Consequently, it could be concluded in Fig. 4a, there were some even particulates(arrows)with that large amounts of compounds containing C=0 group a size of around 5 um dispersing in a fine microstructure of were produced in the used greases, i.e., the greases in the the greases. The EDS results indicated that those particu roller/raceway contacts were oxidized and degraded lates consisted of molybdenum (Mo) and sulfur (S) heavily. elements, namely MoS2. In comparison, the intensity of ig. 3 X-ray analysis of (a) fresh and (b)used grease 6000 0.000 40.000 4000 30 Spring
reactions (7) and (8), which would return to the above chain propagation again. As a result, thermal-oxidative stability of the lubricating grease was repeatedly decreased. ROOH ! D RO þ HO ðEq 6Þ HO þ RH ! H2O þ RO ðEq 7Þ RO þ RH ! ROH þ R ðEq 8Þ Thus, during the chain termination the content of the carbonyl-containing oxidation products including acidic species and high-viscosity products increased in the lubricating grease and heavily deteriorated the lubricating properties of the grease. According to Xue et al. [14], the intensity of C=O peaks was directly proportional to the degradation degree of the grease. As illustrated in Fig. 2b, the C=O peak was sharp and the peak intensity was high (compared with the fresh grease, see Fig. 2a). Consequently, it could be concluded that large amounts of compounds containing C=O group were produced in the used greases, i.e., the greases in the roller/raceway contacts were oxidized and degraded heavily. XRD Analysis and SEM Observation Figure 3a and b shows the XRD patterns of the fresh and used grease sample, respectively. From the X-ray analysis, it is clear that there were significant differences between the patterns. The strongest diffraction peak from the fresh sample, as shown in Fig. 3a, appeared at about 2θ = 14°, corresponding planar spacing d = 0.63 nm, another strong diffraction peak presents at about 2θ = 26°, corresponding planar spacing d = 0.34 nm. According to PDF cards, they could be attributed to (100) and (101) crystalline planes of the molybdenum disulfide (MoS2). In addition, some weak diffraction peaks of MoS2 also appeared in this XRD profile. This means that the fresh grease was a lithium lubricating grease containing MoS2. Actually, SEM observation and EDS analysis of the fresh grease confirmed this fact as well. Figure 4a and b shows SEM and EDS results of the fresh grease sample, respectively. As shown in Fig. 4a, there were some even particulates (arrows) with a size of around 5 μm dispersing in a fine microstructure of the greases. The EDS results indicated that those particulates consisted of molybdenum (Mo) and sulfur (S) elements, namely MoS2. In comparison, the intensity of Fig. 3 X-ray analysis of (a) fresh and (b) used grease sample J Fail. Anal. and Preven. (2011) 11:158–166 161 123
J Fail. Anal and Preven.(2011)11: 158-16 ZEKE 1,9818从m 26/NoU/85 ) C 1200 0 1800 SMo 600 SMo (b) Fig 5 SEM and EDS of the used grease sample (a)SEM micrograph ig. 4 SEM and EDS of the fresh grease sample. (a)SEM micro- and (b) EDS analysis )EDS analysis containing copper, would further accelerate the oxidation MoS2 diffraction peaks was decreased significantly in XRD of the base oil and thickener, and consequently degraded atterns of the used grease sample, as shown in Fig. 3b grease[9] [9]. Actually, the metallic elements Cu, Zn, Fe it, the strongest diffraction peak(20= 42, d=0.21 nm) etc. in the used grease were mainly derived from the could be attributed to(110) crystalline plane of the Cu-Zn bearing (ring or roller)and the cage materials (Cu, Zn intermetallic (identified by PDF cards ). At the same time, alloy ). That is to say, the roller bearings had suffered a the weak diffraction peaks of Fe Cr2O4 (about 20= 36, serious extent of wear. With regard to MoS2 diffraction d=0.25 nm, 20= 56, d=0.16 nm, etc. also existed in peaks being weakened in XRD curve of the used grease Fig. 3b. It revealed that the used grease sample was a sample, it suggested that the contents of Mos2 particulates omplex mixture of compounds like MoS2, Cu-Zn inter- in the used grease had decreased since the content of met metallic, FeCr2O4, and so on. Figure 5a shows sEM debris increased in the grease. Thus, the relative value of icrograph of the used greases. Compared to the micro- the Mos, content was reduced. In addition, another factor graph of the fresh grease(Fig. 4a), it is obvious that may be that part of the MoS2 particulates had been disso- were a large amount of irregular particulates in the ciated from the grease and agglomerated on the raceways greases. The EDS analysis of different particulates showed due to loss of the grease network structure induced by that these particulates mainly contained elements like Cu, thermal degradation/oxidation Zn, Fe, Mo, S, etc(see Fig 5b). It can be inferred that the The above analysis demonstrated that the phase com- uneven particulates were resulted from agglomeration of ponents of the used grease samples were quite different different compounds made from these elements, which with that of fresh ones. The structure of the grease in was consistent with above XRD results of the used grease. bearing had significantly changed during the bearing The presence of metallic particulates, particularly those operation
MoS2 diffraction peaks was decreased significantly in XRD patterns of the used grease sample, as shown in Fig. 3b. In it, the strongest diffraction peak (2θ = 42°, d = 0.21 nm) could be attributed to (110) crystalline plane of the Cu–Zn intermetallic (identified by PDF cards). At the same time, the weak diffraction peaks of FeCr2O4 (about 2θ = 36°, d = 0.25 nm, 2θ = 56°, d = 0.16 nm, etc.) also existed in Fig. 3b. It revealed that the used grease sample was a complex mixture of compounds like MoS2, Cu–Zn intermetallic, FeCr2O4, and so on. Figure 5a shows SEM micrograph of the used greases. Compared to the micrograph of the fresh grease (Fig. 4a), it is obvious that there were a large amount of irregular particulates in the used greases. The EDS analysis of different particulates showed that these particulates mainly contained elements like Cu, Zn, Fe, Mo, S, etc. (see Fig. 5b). It can be inferred that the uneven particulates were resulted from agglomeration of different compounds made from these elements, which was consistent with above XRD results of the used grease. The presence of metallic particulates, particularly those containing copper, would further accelerate the oxidation of the base oil and thickener, and consequently degraded the grease [9]. Actually, the metallic elements Cu, Zn, Fe, etc. in the used grease were mainly derived from the bearing (ring or roller) and the cage materials (Cu, Zn alloy). That is to say, the roller bearings had suffered a serious extent of wear. With regard to MoS2 diffraction peaks being weakened in XRD curve of the used grease sample, it suggested that the contents of MoS2 particulates in the used grease had decreased since the content of metal debris increased in the grease. Thus, the relative value of the MoS2 content was reduced. In addition, another factor may be that part of the MoS2 particulates had been dissociated from the grease and agglomerated on the raceways due to loss of the grease network structure induced by thermal degradation/oxidation. The above analysis demonstrated that the phase components of the used grease samples were quite different with that of fresh ones. The structure of the grease in bearing had significantly changed during the bearing operation. Fig. 4 SEM and EDS of the fresh grease sample. (a) SEM micrograph and (b) EDS analysis Fig. 5 SEM and EDS of the used grease sample. (a) SEM micrograph and (b) EDS analysis 162 J Fail. Anal. and Preven. (2011) 11:158–166 123
J Fail. Anal. and Preven. (2011)11: 158-166 Thermogravimetric Analysis According to above analysis, the greases used in bearing suffered heavily thermo-oxidation degradation due to higl The thermal stability of the fresh and used grease sample temperature during the bearing operation. A large amount under ambient conditions was analyzed by TGA in order to of low-molecular compounds including carbonyl-contain- further verify the above results. The TGA of the grease ing degradation products were formed on the bearing were carried out under N2 with a heating rate of 20C/min raceway. The contents of grease were changed, and lubri 50 to 900 C. Figure 6a and b show the weight loss cating capacity of the grease were deteriorated curves of the fresh and used grease samples, respectively. Consequently, the lubricant film in the roller/raceway The fresh grease showed a simple degradation step starting contact could not be formed effectively, which would result at around 210C and ending at 783C with a weight loss in contact fatigue wear of the counterfaces and the for- by 98.55%, i.e., the grease was basically completely mation of contact debris. Thus, by forcing the bearing to decomposed. While for the used greases, as shown in run under poor lubricating conditions, serious friction and Fig 6b, the thermal decomposition started at temperature wear were resulted of 1166C with a decrease by 93 4C compared with that of the fresh grease, which is attributed to low-molecular compounds resulted from the thermal degradation of the Chemical Composition and Microscopic Features used grease during the bearing operation. This showed that Analysis of the Inner-Ring Surface the thermal stability of the used greases was lower than that In order to identify the features of the fracture process of of the fresh one. In addition there was still a residue of 60.6%(wt %)at final degradation temperature around the surfaces of the bearing the inner ring were examined 887C though the temperature was shifted up to 104C higher as compared to fresh s. Then. the residue Chemical Composition and Hardness analysis onstituent was examined by ICP-AES. The results indi- cated that its compositions were copper(around 36 wt%). The chemical composition of the bearing materials was zinc (27.6 wt%), iron(10.2 wt%), and their oxides. This determined by photoelectric direct reading spectrometry further confirmed the above XRD and EDS analysis results The results were shown in Table I It can be seen that the ease composition of the materials corresponded to the specified composition range. On the pieces cut out from the inner ring, the hardness(HRC)at different orientations was determined. The results showed that the distribution of the X1=089 hardness values of the failed bearing inner-ring material ranged from 61 to 62(HRC)and met the 60-64 hRC range 60 requirement. These results demonstrate that the chemical 50 Delta Y=985504% composition and the average hardness of the bearing materials were qualified and consistent with the specifica Y2-15121% tions for GCr15 bearing steel. 3645℃ 48.96100200300400500600700794 Microstructure Observation and Analysis The wear indentations on the raceway of the bearing inner ring were detected using SEM and EDS. Figure 7a shows =116.642 the surface of the wear zones was covered with a multitude of irregular pits. In order to further analyze the cause of ts, a magnified image of a single pit was observed and the DaY=394667% composition within the pit was investigated as shown in Fig. 7b and c. It can be m Fig. 7b that the formed by the removal of metal from the surface with no obvious surface plastic deformation. The pit had sides perpendicular to the contact surface and exhibited (6 ) s 9 10 20 30 1 m perature o 0 o 9 shape. A number o cracks can be bserved at and near the Fig6 Thermogravimetric analysis of(a)fresh and(b)used grease of surface rolling contact fatigue. The main chemical composition within the pit shows the presence of iron and Spring
Thermogravimetric Analysis The thermal stability of the fresh and used grease sample under ambient conditions was analyzed by TGA in order to further verify the above results. The TGA of the grease were carried out under N2 with a heating rate of 20 °C/min from 50 to 900 °C. Figure 6a and b show the weight loss curves of the fresh and used grease samples, respectively. The fresh grease showed a simple degradation step starting at around 210 °C and ending at 783 °C with a weight loss by 98.55%, i.e., the grease was basically completely decomposed. While for the used greases, as shown in Fig. 6b, the thermal decomposition started at temperature of 116.6 °C with a decrease by 93.4 °C compared with that of the fresh grease, which is attributed to low-molecular compounds resulted from the thermal degradation of the used grease during the bearing operation. This showed that the thermal stability of the used greases was lower than that of the fresh one. In addition, there was still a residue of 60.6% (wt.%) at final degradation temperature around 887 °C though the temperature was shifted up to 104 °C higher as compared to fresh greases. Then, the residue constituent was examined by ICP-AES. The results indicated that its compositions were copper (around 36 wt.%), zinc (27.6 wt.%), iron (10.2 wt.%), and their oxides. This further confirmed the above XRD and EDS analysis results of the used greases. According to above analysis, the greases used in bearing suffered heavily thermo-oxidation degradation due to high temperature during the bearing operation. A large amount of low-molecular compounds including carbonyl-containing degradation products were formed on the bearing raceway. The contents of grease were changed, and lubricating capacity of the grease were deteriorated. Consequently, the lubricant film in the roller/raceway contact could not be formed effectively, which would result in contact fatigue wear of the counterfaces and the formation of contact debris. Thus, by forcing the bearing to run under poor lubricating conditions, serious friction and wear were resulted. Chemical Composition and Microscopic Features Analysis of the Inner-Ring Surface In order to identify the features of the fracture process of the surfaces of the bearing the inner ring were examined. Chemical Composition and Hardness Analysis The chemical composition of the bearing materials was determined by photoelectric direct reading spectrometry. The results were shown in Table 1. It can be seen that the composition of the materials corresponded to the specified composition range. On the pieces cut out from the inner ring, the hardness (HRC) at different orientations was determined. The results showed that the distribution of the hardness values of the failed bearing inner-ring material ranged from 61 to 62 (HRC) and met the 60–64 HRC range requirement. These results demonstrate that the chemical composition and the average hardness of the bearing materials were qualified and consistent with the specifications for GCr15 bearing steel. Microstructure Observation and Analysis The wear indentations on the raceway of the bearing inner ring were detected using SEM and EDS. Figure 7a shows the surface of the wear zones was covered with a multitude of irregular pits. In order to further analyze the cause of pits, a magnified image of a single pit was observed and the composition within the pit was investigated as shown in Fig. 7b and c. It can be seen from Fig. 7b that the pit was formed by the removal of metal from the surface with no obvious surface plastic deformation. The pit had sides perpendicular to the contact surface and exhibited irregular shape. A number of cracks can be observed at and near the contact surface, which can be classified as the failure mode of surface rolling contact fatigue. The main chemical composition within the pit shows the presence of iron and Fig. 6 Thermogravimetric analysis of (a) fresh and (b) used grease sample J Fail. Anal. and Preven. (2011) 11:158–166 163 123
J Fail. Anal and Preven.(2011)11: 158-16 Table 1 Chemical composition of the bearing inner-ring material (wt %) C Mn 0.963 0.582 1064 0.017 0.008 Specified 0.95-105 0.40-0.6 0.95-1.20 1.30-1.65 corrosion, and was just resulted from wear due to poo lubrication Eichler et al. [15] stated that a bearing running under a well-lubricated condition benefits from a lubricant film which could completely separates the two counterfaces Namely, there was hydrodynamic lubrication between the asperities and the betula g. However, if there is, or is likely to be contact between asperities, then the bearing is said to be running in the boundary lubrication regime. From the analysis in the Characterizations of the Lubricating Grease"section, it is 100pm inferred that the chemical compositions of used grease were changed due to thermo-oxidation degradation, which led to a loss of lubricating capacity during the bearing operation. Consequently, the lubricant film in the roller raceway contact was not formed effectively, and thus could not contribute to an effective separation of the contacting surfaces. Whenever two curved surfaces were in contact under load, the contact began to occur along a very small circular or elliptical area and resulted in the rolling contact fatigue with continued bearing operation Due to the damage of used grease structure, MoS2 parti cles were aggregated in"sludge"on the damaged raceway surface or in the pits. Figure 8a shows the SEM micrograph 5 AgoE(q) of partial pits contained black particles, and Fig &b shows its EDS result. It can be seen that the composition of particle in the pits was mainly molybdenum and sulfur, while the 1200 presence of iron and chromium was mainly resulted from metallic wear debris from the matrix material of the bearing e formation of contact fatigue pits were accompanied by the occurrence of the wear debris. The bearing steel debris oxidized and formed the distinct red powder [ 16], which can in return cause abrasive wear. Figure 9a shows 600 the SEM micrograph of the red-brown discolored zone on the outer perimeter of the inner ring of the bearing, and 300 Fig. 9b shows the EDS results of the particles on the sur- face. As shown in Fig. 9a, there were large amounts of particulates on the discolored surface of the raceway with the parallel bands pattern. EDS analysis shows that the chemical composition of particulates was mainly iron and Fig. 7 SEMand EDS of wear pits of outerperimeter of bearing inner ring. oxygen(see Fig 9b, namely, iron oxides Fe2O3(red-brown a)Iregular pits. (b)magnified image of a single pit, and(e) EDS of pit particulates). The bands show the position where the bearing surface was subjected to sliding and thus wear. chromium(see Fig. 7c), which was consistent with that of These particulates further acted as stress concentration sites the bearing materials itself. It is further illustrated that and accelerated the initiation of surface cracks. Under formation of the pits had nothing to do with any kinds of rolling and rolling-sliding contact fatigue, flaking occurred
chromium (see Fig. 7c), which was consistent with that of the bearing materials itself. It is further illustrated that formation of the pits had nothing to do with any kinds of corrosion, and was just resulted from wear due to poor lubrication. Eichler et al. [15] stated that a bearing running under a well-lubricated condition benefits from a lubricant film which could completely separates the two counterfaces. Namely, there was no contact operating under elastohydrodynamic lubrication between the asperities and the bearing. However, if there is, or is likely to be contact between asperities, then the bearing is said to be running in the boundary lubrication regime. From the analysis in the “Characterizations of the Lubricating Grease” section, it is inferred that the chemical compositions of used grease were changed due to thermo-oxidation degradation, which led to a loss of lubricating capacity during the bearing operation. Consequently, the lubricant film in the roller/ raceway contact was not formed effectively, and thus could not contribute to an effective separation of the contacting surfaces. Whenever two curved surfaces were in contact under load, the contact began to occur along a very small circular or elliptical area and resulted in the rolling contact fatigue with continued bearing operation. Due to the damage of used grease structure, MoS2 particles were aggregated in “sludge” on the damaged raceway surface or in the pits. Figure 8a shows the SEM micrograph of partial pits contained black particles, and Fig. 8b shows its EDS result. It can be seen that the composition of particle in the pits was mainly molybdenum and sulfur, while the presence of iron and chromium was mainly resulted from metallic wear debris from the matrix material of the bearing. The formation of contact fatigue pits were accompanied by the occurrence of the wear debris. The bearing steel debris oxidized and formed the distinct red powder [16], which can in return cause abrasive wear. Figure 9a shows the SEM micrograph of the red-brown discolored zone on the outer perimeter of the inner ring of the bearing, and Fig. 9b shows the EDS results of the particles on the surface. As shown in Fig. 9a, there were large amounts of particulates on the discolored surface of the raceway with the parallel bands pattern. EDS analysis shows that the chemical composition of particulates was mainly iron and oxygen (see Fig. 9b, namely, iron oxides Fe2O3 (red-brown particulates). The bands show the position where the bearing surface was subjected to sliding and thus wear. These particulates further acted as stress concentration sites and accelerated the initiation of surface cracks. Under rolling and rolling-sliding contact fatigue, flaking occurred Table 1 Chemical composition of the bearing inner-ring material (wt.%) C Si Mn P S Cr 0.963 0.582 1.064 0.017 0.008 1.464 Specified 0.95–1.05 0.40–0.65 0.95–1.20 ≤0.027 ≤0.02 1.30–1.65 Fig. 7 SEM and EDS of wear pits of outer perimeter of bearinginner ring. (a) Irregular pits, (b) magnified image of a single pit, and (c) EDS of pit 164 J Fail. Anal. and Preven. (2011) 11:158–166 123
J Fail. Anal. and Preven. (2011)11: 158-166 0透以起 003.004005006.007008009.00 Fig8 SEM and EDS of partial pits contained black particles (a)Pits Fig.9 SEM and EDS of red-brown discolored zone on the outer ontained black particles and(b) EDS of black particles perimeter of bearing inner ring(a)particulates and bands(b) EDS of as a progression of the pits [3], and led to the formation of large, irregular-shaped pits which caused rapid failure of resulted in direct friction between the two counterfaces and oc AO e of surface contact fatigue damage 3. Due to damage of the grease structure, MoS2 particu- Conclusions and Remedial Measures lates under rolling and rolling-sliding contact were aggregated by"sludge"on the raceways. The wear 1. The generation of cracks at or near the contact surface debris and pits acted as stress concentration sites. These and presence of flaky wear particles and geometric inhomogeneities led to highly localized shaped pits provided conclusive evidences fo stresses, rapid crack ion. and the formation of contact fatigue. This evidence was found contact fatigue pits continued operation, the detailed electron microscopic investigations of the atigue progress cau flaking. This resulte damaged surface of the inner ring of the bearing. The the formation of large, irregular pits and the accumu- dominant mode of the bearing failure was surface lation of debris which cause rapid deterioration and contact fatigue between the rollers and the rac failure of the bearing. 2. During operation of the bearing, the lubricatin 4. To extend the lifetime of the bearing, probably the best suffered heavily thermo-oxidation degradation due countermeasure is to replace the lubricating greases and temperature. The chemical compositions of greases were hoose temperature resistant, antioxidant lubricating changed, which led to a loss of lubricating capacity and grease such as RD-l, which is a high-temperature failure of the greases. The lubricating film in the roller/ mposite grease. In addition, shortening the cycle of raceway contact cannot be effectively formed, which lubricant replenishment in the track of the bearing is Spring
as a progression of the pits [3], and led to the formation of large, irregular-shaped pits which caused rapid failure of the bearing. Conclusions and Remedial Measures 1. The generation of cracks at or near the contact surface and presence of flaky wear particles and irregularshaped pits provided conclusive evidences for surface contact fatigue. This evidence was found through detailed electron microscopic investigations of the damaged surface of the inner ring of the bearing. The dominant mode of the bearing failure was surface contact fatigue between the rollers and the raceways. 2. During operation of the bearing, the lubricating greases suffered heavily thermo-oxidation degradation due to high temperature. The chemical compositions of greases were changed, which led to a loss of lubricating capacity and failure of the greases. The lubricating film in the roller/ raceway contact cannot be effectively formed, which resulted in direct contact friction between the two counterfaces and occurrence of surface contact fatigue damage. 3. Due to damage of the grease structure, MoS2 particulates under rolling and rolling-sliding contact were aggregated by “sludge” on the raceways. The wear debris and pits acted as stress concentration sites. These geometric inhomogeneities led to highly localized stresses, rapid crack initiation, and the formation of contact fatigue pits. Under continued operation, the pitting/fatigue progress caused flaking. This resulted in the formation of large, irregular pits and the accumulation of debris which cause rapid deterioration and failure of the bearing. 4. To extend the lifetime of the bearing, probably the best countermeasure is to replace the lubricating greases and choose temperature resistant, antioxidant lubricating grease such as RD-1, which is a high-temperature composite grease. In addition, shortening the cycle of lubricant replenishment in the track of the bearing is Fig. 8 SEM and EDS of partial pits contained black particles. (a) Pits contained black particles and (b) EDS of black particles Fig. 9 SEM and EDS of red-brown discolored zone on the outer perimeter of bearing inner ring (a) particulates and bands (b) EDS of particulates J Fail. Anal. and Preven. (2011) 11:158–166 165 123
l66 J Fail. Anal and Preven.(2011)11: 158-16 probably the simplest approach for ensuring a bearing S, Hayano, M: Deterioration of lithium soap greas running under a well-lubricated condition all the time ctional life in ball bearings. NLGI Spokesm. 53, 246-251 8. Cann, P. M. Doner, J. P. Webster. M. N. wickstrom, V: Grease Acknowledgments This investigation was supported by Shanghai degradation in rolling element bearings. STLE Trans. 44, 399- Leading Academic Discipline Project, Project Number B113. The 404(2001)(in English) support is gratefully acknowledged. 9. Cann, P. M. Grease degradation in a bearing simulation device. Trib. Int 39, 1698-1706(2006)(in english 10. Komatsuzaki, S, Uematsu, T, Kobayashi, Y: Change of grease characteristics to the end of lubricating life. NLGI Spokesm. 63 References 22-27(2000)(in English) I1. Fernandes. P.J. L. Mcduling. C: Surface contact fatigue failures 1.zhou,ZX∴滚动轴承常见失效形式分析与正确维护( Common in gears. Eng. Fail. Anal. 4(2), 99-107(1997)(in English) failure type analysis of rolling bearing and its correct mainte- 12. ASM Metals Handbook, Failure of Rolling-Element Bearings, nance). Constr. Mach. 9, 67-71(2003)(in Chinese) Vol. ll, Failure Analysis and Prevention, 9th edn. American 2. Li, YJ, Tao, C H, Zhang, W.F., et al. Fracture analysis on cage ng. Eng. Fail. Anal. 15(6), 79 -Delisi for Metals, Metals Park, OH, pp. 490-513(1986)(in 13.Xie,F,Yu,YP,Yao,JB:脂的氧化与抑制( Oxidation of 3. Fernandes, PJ. L: Contact fatigue in rolling-element bearings grease and its inhibition). Synth. Lubr. 30(3), 49-52(2003)(in Eng. Fail. Anal.(4), 155-160( 1997)(in English) 4.Yang,WQ,Chen,YZ:减少电动机滚动轴承故障的途径14.xue,J, Zhang,JY.,Wang,C. 谱技术在轴承润滑脂 (Methods for reducing the faults in rolling contact bearing of motor). 分析上的应用( Application of spectrum technique Explos. Proof Electr. Mach. 40(2), 28-30(2005)(in Chine ng grease 7,25-29(2003)(in 5. Carre, D.J., Bauer, R, Fleischauer, P D: Chemical analysis of from spin-bearing tests. ASLE Trans. 26, 15. Eichler, J W, Matthews, A, Leyland, A, et aL. The influence of 475-480( coatings on the oil-out performance of rolling bearings. Surf. 6. Tomaru T. Ito. H. Suzuki. T: Grease-life estima- oat. Technol. 202, 1073-1077(2007) (in English) tion and rioration in sealed ball bearings. In: Proc. 16. Berthier, Y, Vincent, L, Godet, M. Fretting fatigue and fretting JSLE Int. Trib. Conf, 1985, pp. 1039-1044(in English) wear. Trib. Int. 22, 235-242(1989)(in English)
probably the simplest approach for ensuring a bearing running under a well-lubricated condition all the time. Acknowledgments This investigation was supported by Shanghai Leading Academic Discipline Project, Project Number B113. The support is gratefully acknowledged. References 1. Zhou, Z.X.: 滚动轴承常见失效形式分析与正确维护 (Common failure type analysis of rolling bearing and its correct maintenance). Constr. Mach. 9, 67–71 (2003) (in Chinese) 2. Li, Y.J., Tao, C.H., Zhang, W.F., et al.: Fracture analysis on cage rivets of a cylindrical roller bearing. Eng. Fail. Anal. 15(6), 796– 801 (2008) (in English) 3. Fernandes, P.J.L.: Contact fatigue in rolling-element bearings. Eng. Fail. Anal. (4), 155–160 (1997) (in English) 4. Yang, W.Q., Chen, Y.Z.: 减少电动机滚动轴承故障的途径 (Methods for reducing the faults in rolling contact bearing of motor). Explos. Proof Electr. Mach. 40(2), 28–30 (2005) (in Chinese) 5. Carre´, D.J., Bauer, R., Fleischauer, P.D.: Chemical analysis of hydrocarbon grease from spin-bearing tests. ASLE Trans. 26, 475–480 (1983) (in English) 6. Tomaru, M., Suzuki, T., Ito, H., Suzuki, T.: Grease-life estimation and grease deterioration in sealed ball bearings. In: Proc. JSLE Int. Trib. Conf., 1985, pp. 1039–1044 (in English) 7. Hosoya, S., Hayano, M.: Deterioration of lithium soap greases and functional life in ball bearings. NLGI Spokesm. 53, 246–251 (1989) (in English) 8. Cann, P.M., Doner, J.P., Webster, M.N., Wickstrom, V.: Grease degradation in rolling element bearings. STLE Trans. 44, 399– 404 (2001) (in English) 9. Cann, P.M.: Grease degradation in a bearing simulation device. Trib. Int. 39, 1698–1706 (2006) (in English) 10. Komatsuzaki, S., Uematsu, T., Kobayashi, Y.: Change of grease characteristics to the end of lubricating life. NLGI Spokesm. 63, 22–27 (2000) (in English) 11. Fernandes, P.J.L., Mcduling, C.: Surface contact fatigue failures in gears. Eng. Fail. Anal. 4(2), 99–107 (1997) (in English) 12. ASM Metals Handbook, Failure of Rolling-Element Bearings, Vol. 11, Failure Analysis and Prevention, 9th edn. American Society for Metals, Metals Park, OH, pp. 490–513 (1986) (in English) 13. Xie, F., Yu, Y.P., Yao, J.B.: 脂的氧化与抑制 (Oxidation of grease and its inhibition). Synth. Lubr. 30(3), 49–52 (2003) (in Chinese) 14. Xue, J., Zhang, J.Y., Wang, C.T.: 红外波谱技术在轴承润滑脂 分析上的应用 (Application of infrared spectrum technique in bearing grease analysis). Bearing 7, 25–29 (2003) (in Chinese) 15. Eichler, J.W., Matthews, A., Leyland, A., et al.: The influence of coatings on the oil-out performance of rolling bearings. Surf. Coat. Technol. 202, 1073–1077 (2007) (in English) 16. Berthier, Y., Vincent, L., Godet, M.: Fretting fatigue and fretting wear. Trib. Int. 22, 235–242 (1989) (in English) 166 J Fail. Anal. and Preven. (2011) 11:158–166 123
