Processing and properties of Nextel™ 720 fiber reinforced alumina-zirconia ceramic matrix composites

The continuous Nextel? 720 fiber-reinforced zirconia/alumina ceramic matrix composites (CMCs) were prepared by slurry infiltration process and precursor infiltration pyrolysis (PIP) process. The introduction of submicron zirconia powders into the aqueous slurry was optimized to offer comprehensively good sintering activity, high thermal resistance and good mechanical properties for the CMCs. Meanwhile, the zirconia and alumina preceramic polymers were used to strengthen the porous ceramic matrix through the PIP process. The final CMC sample achieved a high flexural strength of 200 MPa after one infiltration cycle of alumina preceramic polymer and thermal treatment at 1150 °C for 2 h. The flexural strength retention of the improved CMC sample was 104% and 89% respectively after thermal exposure at 1100 °C and 1200 °C for 24 h.     

Mechanical characterisation and hydrothermal degradation of Al2O3-15 vol% ZrO2 nanocomposites consolidated by two-step sintering

The objective of this work was to study two-step sintering as a means of controlling the microstructure of Al₂O₃ matrix nanocomposites containing nanometric inclusions of ZrO₂ 15% by volume and evaluate its hydrothermal degradation as function of time and its mechanical properties. Powders of Al₂O₃ and ZrO₂ were prepared and compacted by means of isostatic pressing and sintering in different heating cycles. The results showed that two-step sintering allowed a more efficient microstructural control than single-step sintering, resulting in good mechanical properties. The studied nanocomposites showed excellent resistance to hydrothermal degradation compared to commercial ZrO₂ and ZrO₂ TZ-3Y-E.     

A novel method for fabricating brick-mortar structured alumina-zirconia ceramics with high toughness

The present work reports a novel and simple approach to prepare alumina-zirconia composites with superior toughness. Alumina microspheres were innovatively used as the raw materials, followed by coating zirconia and hot-pressing sintering to fabricate alumina-zirconia ceramics. The resultant ceramics are given a unique brick-mortar microstructure, in which the zirconia “mortar” layers continuously distribute around the alumina “brick” matrix, leading to outstanding fracture toughness of 7.34 MPa·m^1/2 and high strength of 635.84 MPa when prepared with zirconia contents of 10 wt%. The major explanation could be ascribed to that crack tips in sintered samples tend to propagate along the zirconia “mortar” layer, accompanied by deflection and branching, which effectively improve the fracture toughness of composites. The uniformity and integrity of the brick-mortar structure could be well tuned by varying the amount of zirconia. This method has reference significance for the preparation of high toughness alumina-based multiphase ceramics.     

Influence of different chemical treatments on the surface of Al2O3/ZrO2 nanocomposites during biomimetic coating

The objective of this study is to evaluate the influence of different chemical surface treatments (H₃PO₄, HNO₃, and NaOH) in the formation of calcium phosphate phases on the surface of Al₂O₃/ZrO₂ (5 vol%) nanocomposite. For this purpose, Al₂O₃/ZrO₂ samples were shaped, calcined at 400 °C, sintered at 1500 °C, subjected to different chemical treatments, and biomimetically coated from 14 to 21 days. Surface characterization was performed by scanning electron microscopy, atomic force microscopy, confocal microscopy, X-ray diffraction, and infrared spectroscopy. It was observed that the preliminary chemical treatment favored the formation of particular calcium phosphate phases of interest, such as α-TCP (alpha-tricalcium phosphate), β-TCP (beta-tricalcium phosphate), and HA (hydroxyapatite). The differences among the percentages of the phases formed affected the homogeneity of calcium phosphate distribution within the nanocomposites as well as the roughness of the formed layer, effectively contributing to adhesion, proliferation, and desired cell biofixation on bone implant.     

Microstructure and Young’s modulus evolution during re-sintering of partially sintered alumina-zirconia composites (ATZ ceramics)

Partially and fully sintered alumina-zirconia composites (ATZ ceramics), with porosities decreasing from 53.5 to 1 %, have been prepared by uniaxial pressing and firing at 1000–1500 °C and characterized by the Archimedes method and mercury intrusion porosimetry. Young’s modulus has been measured via the impulse excitation technique at room temperature, resulting in an almost exponential porosity dependence (which is unusual for partially sintered ceramics in which the microstructure is dominated by concave pore surfaces), and at elevated temperatures up to 1500 °C during heating and cooling, resulting in a temperature master curve with a low-temperature inflection point around 200 °C (accompanied by a damping maximum). Both results confirm previous findings for zirconia and are typical for zirconia-containing ceramics. When the original firing temperature is exceeded, sintering and densification continues, albeit with a temperature lag when the sintering activity (specific surface area) is reduced as a consequence of previous firing.     

Thermal conductivity of porous Alumina-20 wt% zirconia ceramic composites

Effect of porosity and temperature on thermal conductivity of the porous Alumina-20 wt% Zirconia (3 mol.% Y₂O₃) ceramic composites with and without niobia were investigated. The ceramic powders were synthesized by the sol-gel route using alkoxide precursors. The porosity in the composites was maintained in the range of 9.5–65 vol% using starch as a space holder material. After processing, samples were compacted uniaxially and sintered at 1873 K for 3 h. The thermal conductivity of porous ceramic composites with and without niobia dopant was measured at three different temperatures of 300, 473, and 673 K using laser-flash technique. The thermal conductivity of the samples was reduced with increasing temperature and porosity. At temperature of 300 K, the thermal conductivity value of 11 W/m.K was obtained for the undoped sample S0 with 17 vol% residual porosity, dropped to 2 W/mK for the sample S40 containing 65 vol% porosity, and for the same sample it was further reduced to the lowest value of 0.68 W/m.K at 673 K. The measured conductivity values were used to determine the grain boundary thermal resistance value (R) of the samples which exhibited an ascending trend with the porosity. The obtained thermal conductivity for the different porous composites was verified and formulated with the Maxwell-Eucken and Ticha models. The results showed that the experimentally measured conductivity values follow a descending order with the models while at the higher-porosity level (57–65 vol%), it fits well with the Ticha equation with only 9% and 4.6% deviation for undoped and doped samples, respectively. Results also revealed that the addition of niobia significantly reduced thermal conductivity at the lower porosity levels, but at higher porosity level the effect of porosity was more dominant.     

Effects of SiC,SiO2 and CNTs nanoadditives on the properties of porous alumina-zirconia ceramics produced by a hybrid freeze casting-space holder method

Highly porous alumina-zirconia ceramics were produced by adding space-holder materials during freeze casting. To increase the strength of porous ceramics, different amounts of nanoadditives (silicon carbide-SiC, silica-SiO₂, and multi-wall carbon nanotubes-CNTs) were added. Space-holder materials were removed by preheating, and solid samples were produced by sintering. Up to 68% porosity was achieved when 40% space-holder was added to the solid load of slurry. Wall thicknesses between pores were more uniform and thinner when nanoadditives were added. Compressive tests revealed that SiC nanoparticles increased the strength more than other nanoadditives, and this was attributed to formation of an alumina-SiC phase and a uniform distribution of SiC nanoparticles. Results indicated that by including 20% space-holder materials and 15% SiC nanoparticles, the density decreases by 33.8% while maintaining a compressive strength of 132.5 MPa and porosity of 43.4%. Relatively low thermal conductivities, less than 3.5 W/K-m, were measured for samples with SiC nanoparticles.