Influences Of H-BN On The Comprehensive Properties Of Low Carbon Al₂O₃-C Refractories

Al₂O₃-C（Al₂O₃-Zr O₂-C）slide gate is significant refractories for continuous casting system.During past decades,low carbon containing Al₂O₃-C refractories with excellent mechanical properties and thermal shock resistance are developed through designing the nano structure in the matrix,including the addition of nano carbon sources or nano oxides,and introduction of B₄C to catalyze in-situ formed nano carbon,However,the service conditions of slide gates are harsh,and thus,it is required further improving their oxidation resistance,and thermal shock resistance.To response the above problems,in this thesis,hexagonal boron nitride（h-BN）combined with nano carbon source B₄C was introduced into Al₂O₃-C refractories to improve their oxidation resistance and thermal shock resistance,due to its laminated structure similar with graphite,good oxidation resistance,and corrosion resistance to hot metal or melted slag.However,it still needs to determine if h-BN can substitute graphite to develop the toughening effect of lamellar structure or whether h-BN influences the comprehensive properties of Al₂O₃-C refractories;then it need to be determined the influence of the content of h-BN and different sized h-BN on the mechanical properties,and thermal shock resistance,and oxidation resistance;Finally,based on these researches,the transition metal catalyst is introduced,and the structure and properties evolution laws are determined to reveal the toughening and strengthening mechanism of multiple phases enhance the low carbon Al₂O₃-C refractories.Based on these studies mentioned previously,these conclusions can be drawn:1.The introduction of h-BN can improve the comprehensive of Al₂O₃-C refractories,including toughness,thermal shock resistance,and oxidation resistance.After treated at1400℃,the graphite would transform into graphite-Si C heterostructure,which is beneficial to enhance the strength of Al₂O₃-C refractories,and h-BN showed no structure evolution,and thus the formation of ceramics whiskers was retarded due to that h-BN replaced graphite completely.Therefore,the CMOR and HMOR of h-BN containing specimens were slightly lower than that of graphite containing specimens.The COMR of specimen was reduced to35.51±0.44 MPa,when the addition amount of h-BN was 2 wt%.Noteworthily,h-BN retained its own laminated structure,which enable h-BN can develop the sliding mechanism,consequently,the toughness and thermal shock resistance of Al₂O₃-C refractories was improved.Meanwhile,h-BN possessed better oxidation resistance than the nano carbon sources,and B₂O₃ was formed after oxidation of h-BN,and B₂O₃ would further react with Al₂O₃ to form aluminum borate,consequently the pores were filled effectively,the air penetration was inhabited,therefore,the oxidation resistance was improved obviously.When the addition amount of h-BN was 2 wt%,the residual intact layer ratio still reached80.1%and showed no obvious oxidation layer after being oxide for 5 hours.2.The mechanical property and thermal shock resistance can be adjusted with introducing different sized and content of h-BN.When the graphite was partially replaced by 0.5 wt.%10μm sized h-BN,the Al₂O₃-C refractories treated at 1400℃showed the best mechanical property,CMOR 42.63±3.10 MPa,HMOR 14.12±0.67 MPa.In addition,the thermal shock resistance of this batch of specimen was improved.This was mainly attributed the introduction of h-BN would induce more crack propagated along the interface between aggregate and matrix,which is feasible to the dissipation of thermal stress.Considering about the different sized h-BN,the introduction of h-BN with larger size would facilitate the in-situ growth of Si C whiskers,and thus the mechanical properties was enhanced.However,introducing the smaller sized h-BN could lead to more crack propagation along the interface between aggregate and matrix and prolong the crack propagation path,which enhanced the thermal shock resistance of the Al₂O₃-C refractories.In summary,0.1μm h-BN containing specimen showed the highest residual ratio 42.5%,and1μm h-BN containing specimen possessed the highest residual strength 15.67 MPa.The oxidation resistance was improved with increase of the particle size of h-BN.3.Nickel acetate could facilitate the in-situ growth of numerous MWCNTs,and the CMOR of specimens treated at intermediate temperature was increased.After treated at1200℃,the CMOR of specimen GB_Ni（introducing h-BN and graphite collaboratively）and specimen BN_Ni（introducing h-BN individually）were 36.73±3.51 MPa and 32.22±3.73MPa,respectively.Compared with specimens treated at 1400℃,the specimens treated at1200℃possessed higher HMOR,and the HMOR of specimen BN_Ni reached 23.60 MPa.Moreover,specimens treated at 1200℃possessed better toughness and thermal shock resistance than that of specimens treated at 1400℃,which should be attributed to the formation of a large number of MWCNTs,when specimens were treated at 1200℃.The simultaneous introduction of h-BN and nickel acetate is a promising method to develop ultra-low carbon Al₂O₃-C refractories with lower firing temperature.