| Al2O3-ZrO2(3Y) composite ceramic is very attractive as a structural material with long-term service under medium and high temperature for applications of aerospace, jet aircraft engine, and gas turbine system applications due to its low density, excellent mechanical properties, oxidation resistance, and high temperature performance. With the development of aeronautics and astronautics techniques, the requirements for preparation and formation process of Al2O3-ZrO2(3Y) improve gradually. However, due to its particular characters of special structure and chemical bond, it always needs high temperature (above 0.5-0.8 Tm) and low forming strain rate (about 10-4-10-5 s-1) during sintering, superplastic deformation and diffusion bonding processes, which limits the engineering application in ceramics’components. Present investigation focuses on using TiO2 as the modification additions to reduce the production temperature and improve the deformation efficiency of Al2O3-ZrO2(3Y) ceramics.The sintering, superplastic deformation and bonding behaviors of the Al2O3-ZrO2(3Y) composite ceramics doped with four different amounts of TiO2 (the four TiO2 contents are 0 wt.%,1 wt.%,4 wt.%and 8 wt.%, respectively) were systematically investigated. Moreover, the microstructure evolutions, especially, the relationships between TiO2 contents and grain boundary features were examined. The main purpose of this paper is to clarify the effect mechanisms of TiO2 on the sintering, superplastic deformation and diffusion bonding of Al2O3-ZrO2(3Y) composite ceramics. The primary conclusions are as follows:By doping TiO2, the optimal sintering temperature of Al2O3-ZrO2(3 Y) composite ceramics decreases from 1450℃ to 1300℃, meanwhile, the hardness and fracture toughness of the doped specimens still retain as much as 85% and 97% of the undoped counterpart, respectively. SEM-EDS results demonstrate that a large amount of Ti cations segregated along grain boundaries. Therefore, three grain boundary features occurred in Al2O3-ZrO2(3Y) ceramics doped with different amount of TiO2, namely, "clean" grain boundary, thin grain boundary liquid phase (1.5-2 nm thickness), and thick grain boundary liquid phase (50-150 nm thickness). From the viewpoints of solid solution chemical bond and grain boundary feature, this paper discusses the enhancement mechanisms of TiO2 dopants on the densification of Al2O3-ZrO2(3Y) ceramics.The superplastic deformability of Al2O3-ZrO2(3Y) ceramics can be also improved by doping TiO2 additions. It was found out that the strain rate of Al2O3-ZrO2(3Y) ceramics were heightened more than two magnitude orders. After doping 8 wt.%TiO2, the strain rate of Al2O3-ZrO2(3Y) increased from 10-5s-1～10-4 s-1 to 10-3 s-1-10-2 s-1 at 1400℃, and it increased from 10-7 s-1-10-5 s-1 to 10-5 s-1～10-3 s-1 at 1300℃. Besides that, the strain hardening during deformation was effectively eliminated by doping TiO2. The results indicate that the enhancement of superplasticity and inhibition level of strain hardening is closely related to TiO2 concentration. After deformation, the texture of elongated alumina grains was developed but its intensity decreases with increasing TiO2 concentration. Moreover, three types of grain boundaries are also observed in the corresponding deformed TiO2-doped specimens, which means that grain boundary phases exist throughout the deformation process, and consequently they relax the stress concentration, improve the cation diffusion via the liquid phases, and prevent the cavity nucleation during deformation. With the increase of TiO2 concentration, the stress exponent and deformation activation energy gradually decrease from ～3 to ～2 and from 705-749 kJ/mol to 589-605 kJ/mol, respectively. These superplastic characteristic values and microstructure evolutions prove that the superplastic deformation mechanism transforms from texture-controlled grain boundary sliding accompanied by Coble’s diffusion into grain boundary sliding accompanied by amorphous phases.The microscopic processes and shear strength for the superplastic diffusion bonding of Al2O3-ZrO2(3Y) ceramic doped with and without TiO2 at 5MPa under 1250～1300℃ were examined. The joint quality of Al2O3-ZrO2(3Y) ceramics is enhanced by TiO2 doping. After doping TiO2, the shear strength of Al2O3-ZrO2(3Y) joint bonded at 1300℃×5 MPa increases from 18.8 MPa to 183.9 MPa. The dominant bonding mechanisms before and after doping TiO2 are diffusion and cooperative grain boundary sliding associated with superplastic deformation, respectively. Based on the positive effects of TiO2 on superplastic deformation and diffusion bonding, the large-strain superplastic deformation (e> 0.8) and diffusion bonding were completed simultaneously in one process at low temperature of 1300 ℃. No cavity and crack occurs in the joint after superplastic diffusion bonding, and even the resultant interface is hard to be identified. The bonding strength of this joint is 198.7 MPa, which is almost equal to matrix material’s strength of 203.9 MPa. |
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