Research On Large-scale Advanced Alumina Matrix Structural Ceramic Guideway Material And Its Wear Behaviors

As a vitally important innovation in this dissertation, diopside and Al-Ti-B master alloys are introduced in pressureless sintered alumina matrix ceramic materials to improve their performances. The author develops two series of new composites, which are Al₂O₃/diopside (AD) and Al₂O₃/Al-Ti-B/diopside (ABD), respectively, by using the technology of vacuum slip casting, isostatic cool pressing, liquid sintering, reactive sintering, and temperature gradient pressureless sintering in turn. The mechanical properties, microstructures and wear behaviors of the new fabricated composites are discussed and analyzed. The toughening mechanisms and wear mechanisms are researched as well as the grain refining performance of diopside and Al-Ti-B master alloys towards alumina matrix ceramic materials. Sintering kinetics models of large-scale advanced alumina matrix structural ceramic guideway materials are presented.The design objectives of large-scale advanced alumina matrix structural ceramic guideway materials and selecting principles for additives are proposed. Diopside and Al-Ti-B master alloys are chosen as additives. Gibbs free energy of the reactions taken place between additives and alumina are calculated in order to analyze the chemical compatibility of large-scale advanced alumina matrix structural ceramic guideway materials. The compositions of the new fabricated composites are analyzed by using the technology of X-ray diffraction analysis. The maximum volume content of additives are predicted as well as the sintering temperature and holding time of large-scale advanced alumina matrix structural ceramic guideway materials.Vacuum slip casting and isostatic cool pressing are used to shape the guideway green bodies The technology of liquid sintering, reactive sintering, and temperature gradient pressureless sintering are used to fabricate large-scale advanced alumina matrix structural ceramic guideway materials. The sintering processing parameters to prepare large-scale advanced alumina matrix structural ceramic guideway products are optimized. The experimental results, obtained by testing mechanical properties of typical specimens, indicate that by using sintering processing parameters of 1520°C and 140min, composite with introduction of 3 vol.% diopside shows better comprehensive performances, the hardness, bending strength and fracture toughness of the composite reach 15.57 GPa, 417 MPa and 5.2 MPa·m^1/2, respectively. At the sinter temperature of 1520℃and with the holding time of 140min, composite with addition of 4 vol.% Al-Ti-B master alloys and 6 vol.% diopside shows better comprehensive performances, the hardness, bending strength and fracture toughness of which reach 16.02 GPa, 370 MPa and 5.11 MPa·m^1/2, respectively.Microstructures and toughening mechanisms of pressureless sintered large-scale advanced alumina matrix structural ceramic guideway materials are studied by using scanning electron microscopy (SEM) technology. The influences of sintering processing parameters and diopside content on microstructures of AD and ABD composites are discussed and analyzed. It can be discovered from the investigation that, the grain size of large-scale advanced alumina matrix structural ceramic guideway materials increases with increasing the sintering temperature. The grain size, however, retains unchanged when given an extended holding time and the fracture mode changes with increasing the holding time. The grain shapes of pure alumina are regular and almost circular. There appear interspaces or cavities among the grains owing to incomplete developing of alumina grains. The grain boundaries of pure alumina are observable and the fracture mode is mainly intergranular failure, most likely resulting from the interfacial weakness. There are significant microstructural differences among ceramic guideway materials and pure alumina. Evidently, AD and ABD composites show a finer and more homogeneous distribution of alumina grains and additive particles compared with pure alumina, which indicates the introduction of diopside and Al-Ti-B master alloys may restrain the growth of alumina grains. The grain shapes of ceramic guideway materials are irregular and almost like an anvil horn. Interspaces or cavities almost are almost eliminated. The introduction of diopside and Al-Ti-B master alloys changes the fracture mode of large-scale advanced alumina matrix structural ceramic guideway materials, provides liquid phases in the sintering process, decreases air holes or interspaces in bodies, increases the binding energy of grain boundaries in virtue of interface reactions with alumina. The toughening mechanisms of AD composites are grain-refining effect, crack deflection, crack inflection and crack branch. The toughening mechanisms of ABD composites are grain-refining effect, pin of finely particles to cracks, deflection and torsion of cracks, microcrack toughening and divarication of cracks.The refining performances of diopside and Al-Ti-B master alloys are discussed and analyzed. MgO will react with alumina and produce MgO·Al₂O₃, which will enwrap alumina grains and restrain alumina grain boundaries form moving. Small amount of MgO·Al₂O₃ (less than 6 vol.%) can be beneficial to refine alumina grains effectively, while superfluous MgO·Al₂O₃ will bring on the abnormal grain growth of alumina and increased degree of porosity. The grain refining performance of Al-Ti-B master alloys maybe mainly attribute to its good grain refining performance towards Al. Before being nitrided, small grain size of Al may surround alumina grains and prevent them form growing with increasing the sintering temperature. At the end of the sintering process, Al is almost entirely transformed into A1N. A1N and TiN, which is produced by in situ synthesis and has a grains size of nanometer, distribute evenly over advanced alumina matrix structural ceramic guideway materials and contribute to the refining effect.Physical model about liquid sintering and reactive sintering of large-scale advanced alumina matrix structural ceramic guideway materials are proposed. The sintering densification behaviors are investigated. The apparent activated energy of large-scale advanced alumina matrix structural ceramic guideway materials is calculated by using the method of least squares. Results show that the densification of large-scale advanced alumina matrix structural ceramic guideway materials may be controlled by diffusion of ion in liquid phase. The sintering kinetic equations are deduced according to the influences of sintering temperature and holding time on linear shrinkage rate of the new fabricated composites. The sintering mechanisms are analyzed by comparing the characteristic exponent n. It is indicated that the main sintering mechanism of pure alumina may be volume diffusion (n=2.5), and that of AD and ABD composites may be volume diffusion and grain boundary diffusion (2.5-15m³/N·m while that of alumina matrix ceramic composites toughened by Al-Ti-B master alloys and diopside (AD and ABD composites) is in the order of 10^-16m³/N·m. Abrasive wear may occur during dry sliding tests of pure alumina, and the wear mechanisms of which may be brittle fracture and grain pull-out. The dominant wear mechanisms of pressureless sintered large-scale advanced alumina matrix structural ceramic guideway materials may be mechanical interlocking and plastic deformation combined with a little micro-fracture and grain pull-out.