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Tham khảo tài liệu 'tribology - lubricants and lubrication part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 72 Tribology - Lubricants and Lubrication Pronounced striations on the open fracture surfaces of micropits prove a significant contribution of mechanical fatigue to the crack propagation. The SEM details of Figures 47a and 47b confirm this finding. Therefore it is concluded that a variant of corrosion fatigue is the driving force behind crack growth of micropitting in gray staining. Fig. 47. SEM-SE details of the inner ring raceway of the deep groove ball bearing of Figure 46a revealing a distinct striations on a micropit fracture surface and b the same microfractographic feature on the open fracture face of another micropit The additional chemical loading is not considered in fracture mechanics simulations of micropit formation by surface initiation and subsequent propagation of fatigue cracks Fajdiga Srami 2009 . The findings discussed above however suggest that gray staining can be interpreted as corrosion rolling contact fatigue C-RCF . 5.4 Surface embrittlement in operation Although quickly obscured by subsequent overrolling damage in further operation shallow intercrystalline fractures are sporadically observed on raceway surfaces Nierlich Gegner 2006 . Illustrative examples are shown in the SEM images of Figures 48a and 48b. Fig. 48. SEM-SE images of the rolling contact surfaces of a a TRB roller and b a cam The microstructure breaks open along former austenite grain boundaries. The affected raceway is heavily smoothed by mixed friction. Figure 48a and 48b characterize the lateral Tribological Aspects of Rolling Bearing Failures 73 surface of a roller from a rig tested TRB and gray staining on the cam race tracks of a camshaft respectively. The even appearance of the separated grain boundaries points to intercrystalline cleavage fracture of embrittled surface material by frictional tensile stresses. The micropit on a raceway suffering from gray staining in Figure 49 suggests partly intercrystalline corrosion assisted crack growth. Striation-like crack arrest
