Ultrasonic Velocity Measurements in Alumina-zirconia Ceramic Composite System

Among novel structural materials, composites are attractive because they combine the properties of two o more individual components. As an important example, in alumina-zirconia ceramic composite system, the incorporation of α-Al2O3 into stabilized zirconia increases significantly the mechanical and thermal properties. In this work an alumina-zirconia ceramic composite have been prepared with different α-Al2O3 contents from 10 to 95 wt% in order to investigate the enhancement in mechanical properties of this composite. The different composition were monitored by precise ultrasonic velocity measurements. In order to find out the factors affecting the variation in wave velocity, the ceramic composite have been characterized by X-ray diffraction and scanning electron microscopy. It was found that the ultrasound velocity changed considerably with respect to the ceramic composite composition. Mainly, we studied the behavior of the physical material property hardness, an important parameter of the ceramic composite mechanical properties, with respect to the variation in the longitudinal and shear wave velocities. Shear wave velocities exhibited a stronger interaction with microstructural and substructural features as compared to that of longitudinal waves. In particular, this phenomema was observed for the highest α-Al2O3 content composite. Interestingly, an excellent correlation between ultrasonic velocity measurements and ceramic composites hardness was observed. Introduction: The study of ultrasonic wave propagation in metals and composites gives information on the microstructure, mechanical and physical properties of the material. The quantitative assessment of microstructural changes and properties can be carried out through the measurement of ultrasonic parameters such as attenuation and velocity . The ultrasonic wave velocity depends on the elastic constants and density of the body while ultrasonic wave attenuation depends on microstructure and crystalline defects . The wave propagation in a composite is affected by different material parameters such as density, stiffness, chemical composition and microstructural features . Hing et al. determined the elastic properties of ZrO2Al2O3 ceramic composite system from ultrasonic velocity measurements and found that the wave velocity increases up to a maximum for about 3 wt% of unstabilized ZrO2 dispersed in Al2O3 matrix and decreases monotonically thereafter . The increase in moduli, shown by an increased in the ultrasonic velocity, is attributed to phase transformation of the unstabilized ZrO2 from tetragonal to a monoclinic phase, which leads to a toughening and strengthening effect [3] .The ZrO2-Al2O3 ceramic composites display different properties depending on the raw materials, chemical composition and preparation route. The development of diverse methods for fabricating transformation-toughness ceramics such as ZrO2-Al2O3, mulliteZrO2, Si3N4ZrO2 and others, has received significant interest recently. The ZrO2-Al2O3 ceramic composite have been studied most widely among them. It has been observed that the microstructure of the matrix material and the zirconia particles dispersed in the alumina matrix are so important in order to produce optimally tough transformation-toughened composite materials that increase the mechanical and thermal properties of the composite . In this work, we investigate the influence of α-Al2O3 seeding on the sinterability of ZrO2-Al2O3 and monitored by precise ultrasonic velocity measurements. The ceramic composite is characterized by X-ray diffraction and scanning electron microscopy in order to find out the factors affecting the variation of the longitudinal and shear wave velocity. Results: Homogeneous sols of pseudoboehmite and ZrO2(Y2O3) were prepared by a mecanochemical treatment employing HNO3 as a peptisizing agent. Pseudoboehmite with average formula Al4O3(OH)6 and specific surface area 227 m/g was obtained from a basic aluminum sulfate derived from alunite mineral (U.G. Process) . The basic salts of such process were completely hydrolyzed in an aqueous ammonia medium at 70-80°C at pH 9-10. Suspensions were prepared with pseudoboehmite sols seeded with 2.5 mass% α-Al2O3 of 0.20 μm (Taimicron, TM10). Tetragonal zirconia powders (TOSOH, TZ-3YS) with average particle size of 0.26 μm were added in adequate proportions for each composition. The suspensions were ultrasonically stirred and thereafter spray dried using a YAMATO Mini-spray dryer ADL31. In this way, mixtures of TZ-3YS and α-Al2O3 seeded pseudoboehmite with compositions of 100, 90, 70, 50, 30, 15 and 0 mass % of ZrO2 were prepared. The mixed powder was compacted into half cylindrical shaped 40×20×10 mm samples by isostatic compression at 200 MPa. The samples were presintered at 1250°C and finally sintered at 1550 °C for 1h . The samples were identified with the letters SDI followed by a number that indicates the Al2O3 content except the sample that contains 100 % wt of ZrO2 as shown in Table 1. The density of the sintered samples was measured by the Archimedes method. The theoretical density was estimated with the rule of mixtures. The microstructure and phases content of the sintered samples were determinated by scanning electron microscopy (SEM, Jeol mod. 6400) and X-ray diffraction (XRD, Phillips 5000) respectively. Sample ID Composition wt% ZrO2-Y2O3/Al2O3 ZTY 100/0