Microhardness And Fracture Toughness Study Of ?l₂O₃ Nanocrystalline Ceramic

Alumina ceramic is widely used in machinery,electronic power,building industry,chemical industry and medical industry due to its high strength,good insulation,corrosion resistance against acids and bases,abrasion resistance.In the earth crust,Oxygen and Aluminum is the highest levels of the non-mental and mental elements,respectively,so alumina ceramic is easy to obtain and low in cost.However,the brittleness which is the fatal flaw of alumina ceramic limits its applications.Nanocrystalline microstructures are expected to give?l₂O₃ nanocrystalline ceramic with good plasticity and toughness.Hardness and fracture toughness are very important mechanical properties of ceramic materials,and these two properties parameters can affect the ceramic's applications in various fields directly.Therefore,the preparation of dense ultrafine grained alumina ceramic and study of the hardness and fracture toughness of?l₂O₃ nanocrystalline ceramic with different grain sizes are of great significance.However,due to the difficulty of preparation of nano-powders and the over growth of grains,it is difficult to prepare?l₂O₃ nanocrystalline ceramic samples.The relationship between the hardness and fracture toughness and grain size of?l₂O₃ nanocrystallline?<100 nm?ceramic have not been reported.In this paper,fully dispersed?-?l₂O₃ nanoparticles with a mean particle size of 4.2 nm,a size distribution of 2-8 nm,an average particle size of 7.2 nm and a size distribution of 2-15 nm were obtained by ball milling and hydrochloric acid etching.Two-step sintering method?the ceramic green compacts is heated at a rate of 10?C/min to a higher temperature T₁ without hold,and then cooled at the same rate to a lower temperature T₂ holding for 40 h,and finally cooled down to room temperature at 5?C/min?was applied to prepare a series of dense?l₂O₃ nanocrystalline ceramics with different grain sizes.Then,the microhardness of?l₂O₃ nanocrystalline ceramic with different grain sizes were measured by Vickers hardness tester,and the relationship between hardness and grain size was studied.The results showed that when the grain size of?l₂O₃ nanocrystalline ceramics decreased from 96 nm to 36 nm,its microhardness decreased from 17.70 GPa to 11.00GPa,which is known as inverse Hall-Petch relationship.Using indentation crack,hardness and modulus of elasticity the Fracture toughness of?l₂O₃ nanocrystalline ceramics with different grain sizes was calculated.Analysis of fracture toughness data showed that as the average grain size of?1₂O₃ nanocrystalline ceramic decreases from 96 nm to 36 nm,its fracture toughness increases from 3.02±0.21 MPa?m^1/2 to 4.12±0.16 MPa?m^1/2,increases by 36.42%.