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 SpringThermogravimetric 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 indi￾cated 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-contain￾ing degradation products were formed on the bearing raceway. The contents of grease were changed, and lubri￾cating 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 for￾mation 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 specifica￾tions 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