Enhancement Of Fracture Toughness Of Al₂O₃-Based Ceramics From Cellulose Nanofibrils

As a high-performance structural ceramic,alumina ceramics are widely used in the materials industry and have potential applications covering high-speed cutting tools,dental implants,chemical and electrical insulators,wear parts and various coatings.These applications stem from their high hardness,chemical inertness and high electrical insulation.However,their application is significantly affected by material brittleness.The fracture toughness of the material reflects the resistance of the ceramic to crack initiation and propagation.Therefore,increasing the fracture toughness is very important for achieving a wider application of alumina ceramics.In this paper,the renewable resources cellulose nanofibrils were used as fibril.By regulating the alumina sintering system,the cellulose nanofibrils can be used as a carbon template for retaining the fiber morphology,and the silicon carbide fiber is formed in situ in the alumina ceramic by the carbothermal reduction reaction and the gas-solid growth mechanism.The fiber dispersion of cellulose nanofibrils under different molding methods and its influence on the mechanical properties of alumina ceramics were studied,and the mechanism of action was explored.Theα-Al₂O₃ was used as the matrix material,and the cellulose nanofibrils powders were used as the precursor fiber to toughen alumina ceramic.The cellulose nanofibrils suspension was used as the compression molding binder to prepare the alumina ceramic by the argon atmosphere sintering process.The phase composition and microstructure of the samples were characterized by various material testing techniques.The effects of the addition amount of cellulose nanofibrils on the physical properties,phase composition and microstructure of alumina ceramics were studied.The results show that the KIC of the sample without cellulose nanofibrils was 4.22MPa·m^1/2;this value increased to 7.15 MPa·m^1/2 on addition of 2.5 wt%cellulose nanofibrils,which is a 69.43%increase.The cellulose nanofibrils were pyrolyzed inside the alumina ceramic matrix to form amorphous carbon fibers with a diameter of 20 to 100 nm.The toughening mechanism of cellulose nanofibrils is that SiO₂ in alumina matrix reacts with amorphous carbon fibers in situ to form silicon carbide fibers.In the process of internal crack propagation of alumina ceramics,in-situ fibers are formed in crystals and play the role of bridge effect between the interfaces.The bending strength of the alumina ceramics gradually decreases with the addition of the cellulose nanofibrils content.The decrease is due to the reduced overall density of the material after the addition of the cellulose nanofibrils.After pyrolysis at high temperature,organic matters in the cellulose nanofibrils were discharged,leaving pores,which lowers the density of the material.The growth process of in-situ formed SiC fibers followed a gas-solid（VS）mechanism.The amorphous carbon fiber after pyrolysis of cellulose nanofibrils was used as the carbon template,SiO₂ was used as the silicon source,and the inert gas was used as the shielding gas.The silicon carbide fiber having the（111）plane preferential growth was synthesized in the alumina ceramic through the carbothermal reduction process.In order to improve the dispersibility of cellulose nanofibrils in alumina matrix,cellulose nanofibrils suspension was used as the only additive in alumina injection molding process.The effects of the addition amount of cellulose nanofibrils on the rheological properties and physical properties of alumina ceramic slurry were investigated by means of IR,Zeta potential,viscosity and rheology.The results show that there are a large number of hydroxyl anion groups on the surface of cellulose nanofibrils.By changing the zeta potential and steric hindrance of alumina surface,the slurry stability in alumina injection molding was improved.As the content of cellulose nanofibrils increases,the viscosity of the slurry increases significantly.The addition of cellulose nanofibrils results in a fluid type of alumina slurry between the plastic and pseudoplastic fluids due to the spatial network organization of the cellulose nanofibrils.When the solid content is 50 vol%and the pH value of the solution is 9.5,the fracture toughness is 5.80 MPa·m^1/2 on addition of 0.15 wt%cellulose nanofibrils,which is a 28.90%increase.The bending strength of alumina ceramics was not reduced as compared to that which was not added.The addition of cellulose nanofibrils can be used as a stable dispersing agent for alumina slurry to uniformly disperse ceramic powder.It can also be used as an additive to form silicon carbide fiber toughened alumina ceramic by high temperature sintering process.