Sintering And Mechanical Properties Of Y³⁺ Doped Al₂O₃ Nanocrystalline Ceramic

The Al₂O₃ ceramic possess excellent physical properties,such as high strength,corrosion resistance,and wear resistance,etc.However,the dislocation movement is restricted due to the lack of a slip system capable of plastic deformation attribute to its inherent atomic arrangement.Therefore,micro-cracks are easily generated under external load and eventually brittle fracture occurs with their rapid expansion,which limits the application of Al₂O₃ ceramic in a wide range of fields.The concept of nanocrystalline ceramic brings dawn to solve the brittleness problem of Al₂O₃ ceramic.Although the research results have shown that dense Al₂O₃ nanocrystalline ceramic with small grain size（34 nm）can be successfully prepared by two-step sintering methods using dispersed,fine and equiaxedα-Al₂O₃ nanoparticles as raw materials,since the two-step sintering method requires heating the green compact to a higher temperature（i.e.,the first step sintering）to obtain a critical relative density.The grain growth during the first-step sintering cannot be avoided,which limits the preparation of Al₂O₃ nanocrystalline ceramic with smaller grain sizes under pressureless conditions.Experimental result had shown that sintering aids can inhibit the growth of micron sized Al₂O₃ particles.Although the effect of sintering aids on the growth of nano-sizedα-Al₂O₃ particles has not been reported,it is speculated that the addition of sintering aids will also inhibit the growth of Al₂O₃ green compact during the first step sintering according to its inhibition mechanism（pinning effect）.Combined with the two-step sintering,it is possible to prepare dense Al₂O₃ nanocrystalline ceramic with finer grain size（<30 nm）.Therefore,in this paper,Y³⁺with low solubility of Al₂O₃ was selected as sintering aid,systematically studied the sintering behavior of Y³⁺dopedα-Al₂O₃nanoparticles and explored suitable two-step sintering parameters,prepared fine-grained Al₂O₃ nanocrystalline ceramic,and its mechanical properties were analyzed.High-purity and dispersedα-Al₂O₃ nanoparticles with an average particle size of4.6 nm were firstly obtained by mechanical ball milling,followed by acid corrosion and centrifuge separation.Then,α-Al₂O₃ nanopowders with different Y（NO₃）₃ contents were obtained by mixing Y（NO₃）₃ solution withα-Al₂O₃ nanoparticles.The green compacts were formed by uniaxial dry pressing.After sintering,Al₂O₃ nanocrystalline ceramic with Y³⁺contents of 0.12 wt%、0.78 wt%、1.18 wt%and 2.36 wt%can be obtained.The sintering behavior ofα-Al₂O₃ nanoparticles with different Y³⁺contents was studied by constant-heating-rate sintering without holding.The results show that the addition of Y³⁺can inhibit both the grain growth and densification of Al₂O₃ during sintering,and the inhibition effect is more significant with the increase of Y³⁺content.The distribution of Y element in Al₂O₃ nanocrystalline ceramic was characterized by TEM-EDS Mapping,and it was found that Y segregated to the grain boundary and aggregated at the triple-line boundary after sintering.Kinetic analysis showed that Y³⁺doped Al₂O₃ nanocrystalline ceramic with lower grain boundary mobility and grain boundary diffusivity,which indicated that Y³⁺segregated to the grain boundary simultaneously hindered grain boundary migration and inhibited grain boundary diffusion of Al and O atoms.The two-step sintering parameters of Al₂O₃ nanocrystalline ceramic with different Y³⁺contents were explored in detail.For the sample doped with 1.18 wt%Y³⁺,Al₂O₃nanocrystalline ceramic with mean grain size of 25 nm and relative density of 99.1%was prepared at T₁=1200℃without holding and then holding at T₂=1150℃for 40 h.The microhardness and fracture toughness of Al₂O₃ nanocrystalline ceramic doped with different Y³⁺contents were measured using the indentation method,and it was found that:For 0.12 wt%Y³⁺doped,the microhardness and fracture toughness of the Al₂O₃nanocrystalline ceramic with an average grain size of 34 nm are 14.7±0.6 GPa and4.72±0.23 MPa·m^1/2,respectively,the fracture toughness of nanocrystalline Al₂O₃ceramics is significantly higher than that of micron/submicron Al₂O₃ ceramics.In addition,the research results that Al₂O₃ nanocrystalline ceramic with different Y³⁺contents all exhibit inverse Hall-Petch phenomenon,and fracture toughness increases with decreasing grain size.Combined with the cross-sectional SEM images of Al₂O₃nanocrystalline ceramic with different grain sizes,the reasons for the inverse Hall-Petch phenomenon and the correlation between fracture toughness and grain size were analyzed.The effect of Y³⁺content on the microhardness and fracture toughness of Al₂O₃ nanocrystalline ceramic were discussed.