Preparation And Sintering Performance Under High Pressure Of Nanoporous ?-Al₂O₃ Powder

Alumina?Al₂O₃?is one of the most commonly used ceramic materials in production and life with good mechanical properties,high temperature resistance and abundant resources on the earth.At the same time,like other ceramic materials,alumina has disadvantages such as poor plastic deformation ability and poor fracture toughness,which has attracted scientists to improve its properties.In this work,a porous nano-alumina powder was prepared by a sol-gel method,and a high-pressure sintering method was used to obtain the alumina block with nano-grain sizes which has significantly improved hardness and fracture toughness.In the process of preparing alumina powder by sol-gel method,aluminum nitrate nonahydrate and citric acid were used as raw materials.Ammonia was added to adjust the pH during the stirring process to form a sol system.The gel obtained after aging was heat-treated at 200?firstly and then grounded by planetary ball milling.Finally the precursor is heated into?-Al₂O₃ powder with good dispersibility.The particle size of the alumina powder obtained by this method is between 80 and 150 nm with pores of 20 to 50 nm distributed inside the crystal grains.On the basis of the above work,the porous nano-scale?-Al₂O₃ powder was used to perform powder sintering experiments on a six-face high-pressure sintering equipment.The results show that porous powder will be broken under a certain high pressure,accompanied with a certain degree of plastic deformation.So that some dislocations defects and internal stress of lattice distortion are accumulated in the grain.At a certain sintering pressure and temperature,new crystal grains are formed by some broken crystals as the core,and the pores in the crystal grains are gradually disappeared.In this experiment,the density,hardness,fracture toughness and other properties of the Al₂O₃ bulk prepared under different pressures and sintering temperatures were explored.The transparent dense Al₂O₃ bulk was prepared with a grain size of about 80 nm and a Vickers hardness of 26.6 GPa.It is worth mentioning that the Al₂O₃ bulk with fracture toughness of 7.9 MPa·m^1/2 and Vickers hardness of 20.1 GPa was prepared under a experimental conditions of 6 GPa and800?for 30min.The fracture toughness level was 2?3 times above common Al₂O₃ bulk.Through the analysis of microstructure and morphology,it is found that there are many irregular-sized pores inside the crystal grains.These pores release much energy when the crack expands,thus preventing the crack further expanding.At the same time,a large number of defects such as dislocations inside the grains enhance the strength of the bulk to a certain extent,making up for the adverse effect of pores on the hardness,and maintaining the bulk's hardness at a high level.