Composites with eutectic microstructure by hot pressing of Al2O3–Y2O3 glass microspheres

In the work we describe the use of Al2O3–Y2O3 glass microspheres with high alumina contents prepared by flame synthesis as a precursor for materials with binary eutectic microstructure and high hardness. XRD amorphous microspheres with eutectic composition in a pseudobinary system Al2O3–Y3Al5O12 (YAG) were prepared by feeding a crystalline powder precursor of respective composition into methane-oxygen flame. The microspheres with the mean diameter ≈10 μm were hot-pressed in vacuum under various conditions (temperatures 840-1600°C, pressure 30 or 40 MPa, isothermal dwell 0-120 min). Hot pressing at 1600 °C without isothermal dwell yielded fully dense bulk materials with fine two phase microstructure with Al2O3 and YAG phases percolating at submicrometre level and with hardness 15 GPa. Extension of the isothermal dwell resulted in undesirable coarsening of the resulting microstructure

Effect of sintering temperature on phase evolution, microstructure, and mechanical properties of La2Ce2O7/40 wt.% YSZ composite ceramics

In this work, La2Ce2O7/40 wt% YSZ (LC40Z) mixed powders were used to fabricate composite bulk ceramics with their potential use as a material for high-performance thermal barrier coatings (TBCs). For this purpose, the effect of sintering temperature on the chemical reactivity of LC and YSZ was investigated via hot-pressing at the temperatures of 1300, 1400 and 1500 °C. X-ray diffraction analysis (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) were utilized to characterize the phase and microstructure evolution in both LC40Z powder mixtures and composite bulk ceramics. Results showed that solid-solution reactions occurred between the YSZ and LC during hot-pressing of the LC40Z powder mixtures, indicated by a new phase of La2Ce0·2Zr1·8O7 (LCZ) observed in the XRD patterns. The detailed analysis of Raman spectra confirmed the gradual transition from fluorite LC to pyrochlore LCZ structure, which was demonstrated by the appearance of characteristic pyrochlore bands. All composites densified by hot-pressing exhibited a high relative density above 95%. The average grain size of the LC40Z composites increased significantly with increasing sintering temperature, while gradual pore-healing was observed. The associated mechanical properties of LC40Z ceramics were also reported. The Vickers hardness values increased with increasing sintering temperature, which is consistent with the microstructure evolution and relative density variations. The highest hardness, with a value of 10.99 ± 0.23 GPa, was achieved for the composite hot-pressed at 1500 °C. The fracture toughness results showed the same dependence on sintering temperature. The fracture toughness increased from 1.97 ± 0.15 to 2.4 ± 0.14 MPa m1/2, indicating that the mechanical properties of the LC40Z composites can be tailored by changing the sintering temperature during hot-pressing

Study of thermal behaviour of yttrium-aluminate glasses by DSC and high temperature XRD analysis

Yttrium-aluminate glasses with high alumina content are potential candidates for various applications ranging from transparent ballistic protections, through infrared transparent windows, to hosts for rare earth elements in materials used for solid state lasers. These glasses are interesting because of their optical and mechanical properties and high chemical and thermal stability. Preparation of bulk Al2O3-Y2O3 glasses is very time and energy consuming and prepared materials are very expensive. Different ways of mentioned bulk glasses preparation were reported, however the combination of flame synthesis and subsequent HP sintering of glass microspheres published by Rosenflanz seems to be very promising from technological and economic point of view. In this work the glass microspheres with different alumina content were prepared by flame synthesis. The morphology of prepared particles was studied by optical microscopy and SEM, XRD, DSC and high temperature XRD analysis were performed in order to determine of optimal conditions for hot-press sintering of glass microbeads

The preparation of bulk aluminate glasses by hot pressing of glass microspheres

In this work the yttrium-aluminate glasses with different composition A60Y40M (76.8 mol.% Al2O3), A63Y37M (81.8 mol.% Al2O3), and YAG (62.6 mol.% Al2O3) were prepared by flame synthesis in the form of glass microspheres and characterized by OM, SEM, XRD, HT XRD and DSC analysis. On the basis of the results of HT XRD and DSC analysis, the individual crystallisation experiments were performed. The morphology of crystallized microspheres was studied by SEM and the content of individual crystalline phases was evaluated. The microspheres with the eutectic composition A60Y40M were hot-pressed in vacuum under various conditions (temperatures 900-1600°C, pressure 80 MPa, isothermal dwell time 0-30 min). The highest hardness 17.7 GPa and fine grained microstructure was observed for the samples hot pressed at 1300°C with isothermall dwell time of 30 min

Y3Al5O12-α-Al2O3 composites with fine-grained microstructure by hot pressing of Al2O3-Y2O3 glass microspheres

Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al2O3 and 23.2 mol. % Y2O3 were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 degrees C, 1300 degrees C and 1600 degrees C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 +/- 0.7 GPa and an indentation fracture toughness 4.9 +/- 0.3 MPa.m(1/2) was obtained by sintering at 1600 degrees C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of alpha-Al2O3 grains in the mu m-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples