Study On Preparation And Thermal Shock Resistance Of ZTA Composite Ceramics

Zirconia Toughened Alumina?ZTA ceramics?ceramics are widely used in engineering ceramics materials for excellent properties such as high strength,high toughness,corrosion resistance,wear resistance and high cost-effective.However,the poor thermal shock resistance of ZTA ceramics limits their application in the field of rapid cooling.According to thermal shock resistance theory of ceramic materials,the thermal shock resistance of ZTA ceramics can be improved by reducing the thermal expansion coefficient and elastic modulus and improving strength and toughness and thermal conductivity of the materials.In this paper,Cordierite,Nano-Al₂O₃ powders and SiC powders were introduced to reduce the thermal expansion coefficient,enhance the strength and the toughness and thermal conductivity of ZTA ceramics and improve thermal shock resistance.Firstly,effects of sintering aids content of SiO₂,Ca O and TiO₂ and sintering temperature on the properties of ZTA ceramics?the mass ratio of Al₂O₃ to Zr O₂ was7:3?were studied.Phase composition and microstructure of the ceramics were analyzed by XRD and SEM,thermal shock resistance experiments were carried out at300?¹000?.The results show that with increase of sintering temperature,peak strength of t-ZrO₂ decreases,while peak strength of m-ZrO₂ gradually increases.High sintering temperature would lead to abnormal growth of t-ZrO₂ grains and bring about phase transition easier.Effect of a small content of sintering aids and low sintering temperature on promoting sintering is slight,due to little growth of crystal grain and high porosity.However,when content of sintering aids is large and sintering temperature is high,a lot of glass phases are generated and crystal grains grow abnormally,which damage mechanical properties of ZTA ceramics.When SiO₂-CaO-TiO₂ is 4 wt%,sintering temperature is 1580?for 4 h,ZTA ceramics with high density,uniform grain size and low porosity can be obtained,fracture modes are intergranular fracture and transgranular fracture.relative density,flexural strength,fracture toughness and hardness of ZTA ceramics are 96.7%,399.4 MPa,5.3MPa·m¹/2/2 and 7.2 GPa,respectively.When thermal shock temperature is up to 800?,thermal shock strength retention rate slumps,only 32.4%,which indicates that critical thermal shock temperature of ZTA ceramics is 800?.Secondly,in order to improve thermal shock resistance of ZTA ceramics,Cordierite?2MgO·2Al₂O₃·5SiO₂?with small thermal expansion coefficient was added to ZTA ceramics.In this section,effects of Cordierite content and sintering temperature on properties of ZTA ceramics were studied.When Cordierite content is3 wt%and sintering temperature is 1480?for 4 h,the results display that relative density,bending strength and fracture toughness of composites are 93.3%,346.4 MPa and 3.15 MPa·m^1/2,respectively.Cordierite with low melting point is beneficial to sintering of ZTA ceramics.However,when Cordierite content gets more,it has undesirable impact on mechanical properties because of Cordierite's loose structure and poor mechanical properties.Moreover,thermal expansion coefficient of ZTA ceramics measured from room temperature to 800?drops gradually,at 5 wt%,?=6.46×10^-6/?.It causes that thermal shock strength retention rate of composites jumps gradually,the maximum value is 59.6%.When Cordierite content exceeds 3wt%,strength retention rate grows little,but mechanical properties decline serious,so optimal value of Cordierite is 3 wt%.Thirdly,Nano-Al₂O₃ powders were applied to replace Micro-Al₂O₃ powder in ZTA composites,and effects of Nano-Al₂O₃ powders content on the properties of samples was studied at 1480?for 4 h.The results demonstrate that relative density,flexural strength,fracture toughness and thermal shock strength retention rate of ZTA ceramics goes up firstly and then falls along with the increasing content of Nano-powder,which are 95%,393.7 MPa,3.98 MPa·m¹/2/2 and 64.7%at 15 wt%Nano-powder.SEM micrographs show that Nano-powder material is seriously heaped and it is difficult to disperse uniformly by ball milling alone in ZTA ceramics.In order to solve the serious agglomeration problem of Nano-Al₂O₃ particles,a high-power ultrasonic cell dispersion instrument was uesd to disperse Nano raw material.Effect of ultrasonic dispersion time on size distribution and specific surface area of aggregates and properties of composite materials were studied.The results indicate that the average size of powders is the smallest and specific surface area is largest after 30 min ultrasonic treatment,average aggregate size is 100.6 nm and specific surface area is 73.26 m²/g.At the same time,flexural strength and fracture toughness of ZTA ceramics added 15 wt%treated Nano-Al₂O₃ can reach 436 MPa,4.6 MPa·m^1/2,excellent mechanical properties significantly better thermal shock resistance of ZTA ceramics,and strength retention rate rises to 73.6%.Finally,SiC was introduced into the ZTA/Cordierite/Nano-Al₂O₃ composite ceramics.Effects of SiC content and sintering modes on properties of ZTA composite ceramics were studied.XRD,SEM and EDS spectrums display that SiC powders are mostly oxidized to SiO₂ and gas after sintering at 1480?for 4 h on pressureless sintering,flexural strength and thermal shock strength retention rate of ZTA composite ceramics decline continually.In order to suppress SiC oxidation,hot pressing sintering with faster sintering speed,lower sintering temperature and in argon was used.XRD and EDS spectrums demonstrate that SiC powders aren't oxidized after hot pressing sintering at 1450?,30 MPa for 4 h.When SiC content is7 wt%,ZTA ceramics with high relative density,excellent mechanical and thermal shock resistance properties can be obtained,which are 98.3%,503.6 MPa and 86.5%.