Two-stage SPS Sintering Preparation Of Ultrafine-grained Sialon-based Ceramics And Study On The Microstructure And Properties

Sialon ceramics have high hardness,excellent room temperature and high temperature strength,and good creep resistance,oxidation resistance and thermal shock resistance.It has been widely used in transportation,petrochemical industry,metal smelting,aerospace,and other fields.However,in the traditional liquid phase sintering process,the densification and strengthening-toughening of Sialon-based ceramics were carried out simultaneously,and the hierarchical control cannot be achieved,which makes it difficult to have high hardness and high toughness at the same time,thus hindering the engineering application of ceramic materials.In this paper,a new process of two-stage liquid phase sintering preparation with inorganic oxide and alloy powder as sintering aids was proposed.In order to obtain Sialon-based ceramics with excellent comprehensive properties,the“melting point gradient”of composite sintering aids was used to realize the hierarchical regulation of densification and strengthening-toughening of Sialon-based ceramics by spark plasma sintering technology.Ultrafine-grained O’-Sialon-based ceramics and ultrafine-grainedβ-Sialon-based ceramics were prepared by two-stage liquid phase sintering using Si₃N₄ and AlN as powder matrix,Y₂O₃-Al₂O₃/GH4169 superalloy composite powder and Y₂O₃-Al₂O₃/Ti-22Al-25Nb alloy composite powder as additives,respectively.For Y₂O₃-Al₂O₃/GH4169 superalloy composite powder additives,the two-stage sintering temperatures were 950°C～1150°C and 1150°C～1250°C,respectively.The material was densified in the first stage and formed ultrafine-grained sintered clusters under the liquid phase diffusion of GH4169 superalloy powder,and further densified and toughened in the second stage.For Y₂O₃-Al₂O₃/Ti-22Al-25Nb alloy composite powder additives,the two-stage sintering temperatures are 1125°C～1215°C and 1240°C～1425°C,respectively.The material was densified in the first stage.In the second stage,Ti-22Al-25Nb alloy powder participated in strengthening-toughening by liquid phase diffusion.In the fracture process of ultrafine-grained O’-Sialon-based ceramics prepared by two-stage sintering,there was a crack deflection toughening mechanism dominated by ultrafine-grained sintered clusters.The crack propagated around the edge of the ultrafine-grained sintered clusters with strong bonding force for multiple deflections,thereby improving the fracture toughness of O’-Sialon-based ceramics.With the increase of sintering holding time,the fracture toughness of ultrafine-grained O’-Sialon-based ceramics was excellent,ranging from 5.79 to 8.52 MPa?m^1/2.In the fracture process of ultrafine-grainedβ-Sialon-based ceramics prepared by two-stage sintering,there was a composite toughening mechanism of diffusion toughening dominated by Ti-22Al-25Nb alloy powder and particle dispersion toughening dominated by Ti N phase.With the increase of sintering temperature,the fracture toughness of ultrafine-grainedβ-Sialon-based ceramics was in the range of 6.11～7.29 MPa?m^1/2.Ultrafine-grained O’-Sialon-based ceramics achieved superplastic compression deformation of square billet at low temperature and high strain rate.The superplastic deformation mechanism was the viscous flow of grains caused by the softening and melting of the intercrystalline glass phase at low temperature.Based on the elastic-power exponential strengthening material model,a finite element model of Vickers indentation of ultrafine-grained Sialon-based ceramics was established.The distribution of displacement field,stress field,and strain field in the indentation deformation zone of Sialon-based ceramics was analyzed.The room temperature plastic parameters of ultrafine-grained Sialon-based ceramics were determined by the combination of finite element simulation technology and Vickers hardness test,and the method of establishing the relationship between room temperature plastic stress-strain curves of ultrafine-grained Sialon-based ceramics was proposed.