Research On WC Materials Toughened By Carbon Nanotube/Ultrafine Porous Boron Nitride Nanofiber

WC possesses many advantages such as high hardness and high corrosion resistance.However,it also has a shortcoming of low fracture toughness.After addition of metallic binder like Co,the composites with overall balance between hardness and fracture toughness could be used in the field of cutting/mining tools and wearproof parts.However,metallic binder could lower corrosion-resistance and red harness.Therefore,research on WC materials without metallic binder has been employed.Usually,ceramic parti cl e/fiber or other carbon nano materials like carbon nanotube(CNT)with high Young's modulus are considered as toughening phase instead of metal referring to ceramics toughening.Researches on WC-Co materials toughened by CNT have been reported,and proper addition of CNT could be helpful on fracture toughness.However,sintering temperature for the binderless WC materials is higher,and CNT is transformed to graphite under high sintering temperature easily.So,it's necessary to investigate the transformation of CNT and its influence on mechanical properties.For this issue,WC-0.1 wt.%CNT composite was sintered at 1600?1900? using spark plasma sintering(SPS).Based on analysis results of phase composition and microstructure,it's revealed that the addition of CNT was helpful to suppressing formation of W2C.According to Raman spectroscopy analysis,chemical and structure characteristics of CNT could be maintained until sintering temperature was 1900?.When sintering temperature met 1900?,it's observed that G' peak changed,which indicated structural variation of CNT.Meanwhile,graphite phase was examined easily.On the other hand,Best hardness(22.81 ±0.81 GPa)and fracture toughness(8.95±0.38 MPa·m1/2)appeared at the sintering temperature of 1700?.When sintering temperature was going up,the hardness and fracture toughness lowered,which could be elucidated as transformation from CNT to graphite.Because of shortcoming of CNT on chemical stability under high sintering temperature,boron nitride nanotube(BNNT)could be an alternative in view of similarity of carbon and boron nitride materials.In recent years,scientists in functional materials are working on other boron nitride materials like ultrafine porous boron nitride nanofiber(UPBNNF)with similar performance of BNNT that possesses far high cost on synthesis.In this dissertation,WC-x wt.%UPBNNF(x=0.05,0.075,0.1,and 0.125)composites were SPS prepared as well as WC.All specimens with UPBNNF were better than pure WC on hardness and fracture toughness.WC0.05 wt.%UPBNNF and WC-0.075 wt.%UPBNNF possessed best fracture toughness((9.46±0.63 MPa·m1/2)and best hardness(28.79±0.47 GPa),respectively.Necking appeared on UPBNNF under tension in the images of crack paths,which indicated the ends of the fiber was pinned.It could be explained that turbostratic structure of UPBNNF led to deformation to consume crack energy,which is similar to high tensile strength carbon fiber with lower elastic modulus.Therefore,"pinning and stretching" could be summarized as the toughening mechanism.Meanwhile,nanomechanical properties were employed on WC and WC-UPBNNF materials.it's concluded that the properties of WC and WC-UPBNNF materials are similar.Owing to toughening effect of UPBNNF in WC matrix,it's valuable to study the effects of UPBNNF on WC containing other ceramics as toughening phases.According to published papers on WC-Si3N4 composites,the optimized WC-10 wt.%Si3N4 composite with addition of x wt.%UPBNNF(x=0,0.01,0.05,0.10,and 0.15)were prepared by SPS.It's discovered that the addition of UPBNNF resulted in elevated hardness and fracture toughness.Meanwhile,the addition of UPBNNF also led to other fracture modes of Si3N4 such as sliding and tearing modes besides opening mode;the cracks of Si3N4 grain were partial comparing with traditional through crack.It's demonstrated the fracture mode of Si3N4 is transformed from catastrophic fracture to controllable fracture,which also indicated higher shear-bearing capacity.And then Si3N4 with gradual fracture and bridging simultaneously consumed energy resulting in toughening WC matrix.The fracture mechanical phenomena of WC-Si3N4 with addition of UPBNNF could be attributed to internal liquid phase in the process of Si3N4,and the liquid phase containing rearrangement of atoms was important to form fine nanostructure between Si3N4 and nano pores of UPBNNF.In addition,nanoindentation was employed on WC-10 wt.%Si3N4 specimens without and with addition of 0.1 wt.%UPBNNF.It's displayed that the groups of indentation modulus and hardness curves were more ordered in shallow surface(0?30 nm),which could be resulted from stress variation in surface after addition of UPBNNF.Finally,WC-8 wt.%ZrO2-x UPBNNF(x=0,0.05,0.10 and 0.15 wt.%)composites were SPS sintered for further exploring effects of UPBNNF on mechanical properties of WC composites.With addition of 0.05 wt.%UPBNNF,the composite owned balanced overall mechanical properties.The higher hardness and fracture toughness could be attributed to super plasticity of ZrO2 under high sintering temperature led to formation of fine nanostructure between ZrO2 and nano pores of UPBNNF.