Wear Evolution And Micro-fracture Behavior Of Polycrystalline CBN Brazed Wheels

Polycrystalline CBN abrasive grains are composed by ultrafine microcrystalline CBN grits and AlN bond material.Due to the uniuqes microstructure,the polycrystalline CBN grains are theoretically more prone to micro-fracture,which can sharpen themselves by removing dull micro-grits and generating new cutting edges.However,the current research work on self-sharpening phenomenon of polycrystalline CBN grains still focuses on experimental investigation.The quantitative characterization of grain wear situation needs improvement and the micro-fracture mechanism of polycrystalline CBN grains is not revealed.For the abovementioned reason,experiments are conducted to quantitatively describe the wear evolution of single polycrystalline CBN grains and grinding wheels,and the effect of polycrystalline CBN grain's microstructure on fracture process is also investigated by FEA method.By synthetical consideration,the micro-fracture mechanism of polycrystalline CBN grains and its influence on the grinding performance variation are analyzed.The main work and conclusions are as follows:(1)The wear process of polycrystalline CBN grain is divided according to its wear state,and the effects of grinding wheel speed and single grain cutting thickness on grain fracture are also revealed.The result shows that its wear process contains the initial stage,the stable stage and the rapid stage.Polycrystalline CBN grains can keep a stable sharpness in a longer term than monocrystalline CBN grains.The grinding speed imcreasement will enlarge the wear rate,but the sharpness-maintained phenomenon due to grain fracture will be more pronounced.A large single grain cutting thickness can raise material removal amount while a little single grain cutting thickness can enhance the sliding and ploughing effects,which will both increase the grinding load and accelerate the grain wear.(2)The main modes and internal causes of polycrystalline CBN grain's fracture are investigated by combining the experimental and simulation results,and the effect of microstructure on its fractrue is also analysed.The result shows that the main fracture modes of polycrystalline CBN grain include micro-fracture,inner crack propagation and occasional macro-fracture.The tensile stress is the dominating influential factor during the fracture process.The microstructure of polycrystalline CBN grians can lead to stress concentration accelerating the micro-fracture,so that dull grits can be removed and the sharpness is maintained.Micro-fracture changes the grain surface topography,forming a cycle of dulling-sharpening and keeping a stable grinding performance,which will also influence the material removal process.(3)The influence of grinding parameters on the wheel's grinding performance is researched.Moreover,the wear features of both polycrystalline and monocrystalline CBN wheels are compared and the different grinding performance variations are analyzed.According to this,the advantages of polycrystalline CBN wheels are improved.It shows that the grinding speed increasement will reduce the grinding load but aggravate the vibration.A lower grinding depth and infeed speed can both lead to a less grinding load and a higher grinding quality.The wear evolution of polycrystalline CBN wheels contains two stages: during the first stage,micro-fracture is the major mode,which inhibites the formation of wear platform and adhesion and implements self-sharpening;in the second stage,grain fracture loses the self-sharpening function but becomes a failure process.By comparison,polycrystalline CBN wheels have a better self-sharpening ability and can keep a stable grinding performance in a longer term.