Abstract
A novel method to obtain Al2O3–ZrO2 nanocomposites is presented. It consists of the co-precipitation step of boehmite (AlO(OH)) and ZrO2, followed by microwave hydrothermal treatment at 270 °C and 60 MPa, and by calcination at 600 °C. Using this method, we obtained two nanocomposites: Al2O3–20 wt % ZrO2 and Al2O3–40 wt % ZrO2. Nanocomposites were characterized by Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Sintering behavior and thermal expansion coefficients were investigated during dilatometric tests. The sintering temperatures of the nanocomposites were 1209 °C and 1231 °C, respectively—approximately 100 °C lower than reported for such composites. We attribute the decrease of the sintering temperature to the specific nanostructure obtained using microwave hydrothermal treatment instead of conventional calcination. Microwave hydrothermal treatment resulted in a fine distribution of intermixed highly crystalline nanoparticles of boehmite and zirconia. Such intermixing prevented particle growth, which is a factor reducing sintering temperature. Further, due to reduced grain growth, stability of the θ-Al2O3 phase was extended up to 1200 °C, which enhances the sintering process as well. For the Al2O3–20 wt % ZrO2 composition, we observed stability of the zirconia tetragonal phase up to 1400 °C. We associate this stability with the mutual separation of zirconia nanoparticles in the alumina matrix.
Highlights
Alumina-toughened zirconia (ATZ) and zirconia-toughened alumina (ZTA) are important materials for high-temperature structural [1,2,3] and medical [4,5,6,7] applications due to their excellent strength and toughness, high wear resistance and temperature stability, and the fact that they are both chemically inert
The first was the co-precipitation of the precursors; The second was 20 min of Microwave hydrothermal synthesis (MHS) at t = 270 ◦ C and p = 60 atm, in order to obtain a crystalline mixture of AlO(OH) and ZrO2 ; The third was the drying of the precipitates at room temperature
The bands observed for the as-synthesized samples were identified; they are listed in Table 1 [10,36,37,46,47,48,49,50,51,52]
Summary
Alumina-toughened zirconia (ATZ) and zirconia-toughened alumina (ZTA) are important materials for high-temperature structural [1,2,3] and medical [4,5,6,7] applications due to their excellent strength and toughness, high wear resistance and temperature stability, and the fact that they are both chemically inert. In order to obtain these properties, the tetragonal metastable phase of zirconia (t-ZrO2 ) needs to remain stable at room temperature; t-ZrO2 is normally only stable at Materials 2018, 11, 829; doi:10.3390/ma11050829 www.mdpi.com/journal/materials. One method for ensuring that t-ZrO2 remains stable at room temperature is to partially stabilize it with an yttria (Y2 O3 ) dopant (YSZ) [4,5,6,8,9]. There are two ways to stabilize t-ZrO2 particles: the first is by alloying them with other materials while the other is by ensuring that their particle size does not exceed 35 nm. Stabilization of the t-ZrO2 phase requires certain special synthesis techniques to be used, especially when one desires to have a uniform zirconia particle distribution in an alumina matrix [10,11]. Recent work has shown that synthesizing Al2 O3 with ZrO2 nanopowders (up to ~40 wt % ZrO2 ) could lead to the partial stabilization of t-ZrO2 [12]
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