Investigation Of Spark Plasma Sintering And Mechanical Properties Of Al₂O₃/YSZ And Si₃N₄ Ceramics

Yttrium stabilized zirconia and silicon nitride ceramics are advanced structural ceramic materials.Zirconium oxide ceramics is widely used in dental materials,human bones and cutting tools,etc due to its phase transformation toughening and excellent mechanical properties.Silicon nitride ceramics are widely used in cutting tools,engine parts and metal forming molds due to their low density,high strength,wear resistance,corrosion resistance and thermal shock resistance.However these materials have the common inherent brittleness and sintering inertness of ceramics.It is an important task for ceramics research to search for fabrication method of the high-density ceramic and further to improve its toughness.Spark plasma sintering is characteristized by heating with pulsed DC current directly passes through the graphite die.SPS provides fast heating,low sintering temperature,short sintering time and high density with negligible grain growth,which is beneficial to improve the strength and toughness of materials.SPS is a novel sintering technique for ceramic materials.In this work,the spark plasma sintering of Al₂O₃/YSZ?3 mol%Y₂O₃?composite powder prepared by mechanical mixing method and chemical coprecipitation method was studied based on the spark plasma sintering?SPS?process of commercial yttria-stabilized zirconia ceramics;Three kinds of systems Al₂O₃-YSZ,Al₂O₃-AlN-Yb₂O₃ and Al₂O₃-AlN-Yb₂O₃ were used as sintering additives to perform spark plasma sintering on Si₃N₄ ceramics and their phase,microstructure and mechanical properties were studied.Finally,the effect of TiC_0.3N_0.7 on the?-Sialon performance of sintering with Al₂O₃-AlN-Yb₂O₃ as additives was studied.The following conclusions were obtained:?1?The investigation of spark plasma sintering on the 25 Al₂O₃/ZrO₂?in wt.%?composite powders prepared by mixing or chemical co-precipitation method shows that the composite powder prepared by mixing is a mixture of yttrium-stabilized zirconia tetragonal phase T-YSZ and?-Al₂O₃ phase before and after sintering.No Al atom was found in the solid solution of T-YSZ.The pores exist around the?-Al₂O₃ grains due to its fast growth.The best mechanical properties were obtained in the sample SPSed at 1300?,for 10 min under pressure of 20 MPa,with Vickers hardness of 15.14 GPa and fracture toughness of 4.27 MPa m^1/2.However,The composite powders prepared by co-precipitation method,consist of a single tetragonal phase only,All Al atoms are dissolved in the T-YSZ phase.A small amount of?-Al₂O₃ phase precipitated after SPS sintering,which distributed in the T-YSZ matrix uniformly,leading to improvement of the fracture toughness.The optimum mechanical properties was obtained in the sample SPSed at 1200?for 10 min under pressure of 20MPa with Vickers hardness of 14.75 GPa and fracture toughness of 5.40 MPa m^1/2.?2?The?-Si₃N₄ ceramic was investigated by spark plasma sintering using?-Si₃N₄ as raw material and 6.0 Al₂O₃-6.0 YSZ?in wt.%?as sintering additives.A fully dense?-Si₃N₄ceramic with excellent mechanical properties and interlock microstructures of elongated grains was obtained at 1600?for 15 min under 45 MPa,with the Vickers hardness of 16.15GPa and the fracture toughness of 6.75 MPa m^1/2.?3?A high performance?-Sialon ceramic was prepared by two-step SPS sintering without pressure and with pressure using Al₂O₃,AlN and Yb₂O₃ as sintering additives in this work to avoid the reaction between the sample and the graphite mold under high temperature and high pressure.In the first stage:the mixed powders after ball milling,cold isostatic pressing and then SPS sintering to obtain a relative high density of 3.23 g/cm³,the complete phase transformation of?-Si₃N₄ to?-SiAlON with interlock microstructures of elongated grains and relative high hardness of 15.46 GPa and and fracture toughness of 6.47 MPa m^1/2.With pressure sintering in the second stage,the density of the sample further increases to 3.25g/cm³,the hardness reaches to 16.13 GPa.The thermal conductivity decreases as the additive content increases and the thermal conductivity at room temperature is 16.9 W m^-1 K^-1..?4?The?-Sialon ceramics with TiC_0.3N_0.7 were prepared by two-step spark plasma sintering with Al₂O₃,AlN and Yb₂O₃ as additives.The effect of TiC_0.3N_0.7 on the phase,microstructure and properties of?-Sialon ceramics was studied.The experimental results show that the phase of the sample is almost?-Sialon and trace TiC_0.3N_0.7 through the first step of without pressure and second step of sintering.A small amount of transition phase Yb₂Si₃N₂O₅,Yb₄Si₂N₂O₇ appears in the middle process.The phase of TiC_0.3N_0.7 is uniformly distributed in the?-Sialon matrix,which hinders the crack propagation and improves the fracture toughness of the material.The fracture toughness of?-Sialon ceramics with 1.0TiC_0.3N_0.7?in wt.%?was added reached 7.67 MPa m^1/2,which was increase 18.5%higher than the 6.47 MPa m^1/2 when TiC_0.3N_0.7 was not added.The Vickers hardness is 15.73 GPa and thermal conductivity at room temperature is 13.6 W m^-1 K^-1.