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 Springof 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￾ing 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 char￾acteristic 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 degra￾dation under the high temperature. The thermo-oxidation degradation of the grease fol￾lowed 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