Preparation Of Y-TZP/MgAl₂O₄ Ceramic Composite And Research On Its High-Strain-Rate Superplasticity

Ceramics, which have low-density, high strength and hardness, resistance to elevated temperatures, corrosion, wearable and abrasion, are used specially and widely industrial fields. But the difficultly, high-cost and low-reliability appear in machining for its inherent brittleness, which impede the industrial application in some important fields.From 1980s to now, researchers have made great efforts on improving the superplasticity of ceramics, emphasizing on four kinds of ceramics: oxide ceramics, nitride ceramics, carbide ceramics and glass ceramics. But the low strain rates and concurrent cavitations occurred during deformation of ceramics, which block the application of the super plastic ceramics. High-strain-rate superplasticity (HSRS) is usually referred to as the demonstration of ductility at a strain rate of 10^-2 s^-1 or higher, HSRS in ceramic systems was observed in recent years, which have a lot of advantages during the ceramics manufacture, for example, good precision forming, machinability, reliability, material utilization and productivity.In this study, Y-TZP/MgAl₂O₄ ceramics composites were preparated by one-step and multi-step synthesizing processes. Compared with the latter, the one-step synthesizing process has simpler techniques and better symmetrical phases. Furthermore, we discuss the superplasticity of this ceramics during the low, medium and high strain rate at high temperatures and deformation mechanism of the HSRS ceramics.In the multi-step synthesizing processes, Y-TZP powder is produced by alcohol-aqueous salt solution and MgAl₂O₄ powder is produced by chemical coprecipition method, the size of particles are 15 nm and 95 nm, respectively. The mechanical impurity of these powders are molded and sintered at 1580℃. The size of Y-TZP/MgAl₂O₄ composite powder of produced by one-step synthesizing processes is 71 nm. With she same molding, the sintered compact is prepared at 1500℃.The crystal phases of Y-TZP/MgAl₂O₄ nano-powders and ceramics were investigated by X-ray diffraction (XRD). The morphology, size and distribution of particle were observed by a scanning electron microscope (SEM) and BET. Results show that: Y-TZP phase kept unchanging after MgAl₂O₄ phase was introduced during the one-step synthesizing processes, Y-TZP phase and MgA1₂O₄ phase have excellent association area.Compared with the multi-step synthesizing processes, Y-TZP/MgAl₂O₄ ceramics prepared by one-step have good mechanical performances: bending strength and fracture toughness reached 647 MPa and 8.75 MPa-m^1/2, respectively, and the microhardness was 1335.We also researched the superplastic deformation behavior at high temperature, maximum tensile elongation reaches 415% at a strain rate of 2.2x10^-2 s^-1 at 1550℃. The strain rate sensitivity of m increase with increasing the strain rate, the maximum value is 0.888, which indicates the excellent ability of the superplasitic deformation at the high temperature. At last, the intragranular dislocation, twin crystal, subgrain boundary and diffusion were observed by HRTEM and EDX. And proposal the deformation mechanism of the HSRS ceramics: grain boundary sliding is first, grain boundary diffusion creep accommodation and the intragranular dislocation creep accommodation are necessary during the deformation.