Fabrication of graded alumina by spark plasma sintering

Abstract

Functionally graded materials (FGMs) attract considerable interest in materials science and industry, since their composition or morphology gradually changes along their length, width, or height, which provides new approach for the development of multifunctional materials. In this paper, we studied the fabrication of a gradient microstructure in alumina (Al2O3) by spark plasma sintering (SPS). During the SPS process, the applied asymmetric graphite tool configuration causes a large temperature gradient, which results in a gradually changing morphology in Al2O3 ceramics. The local temperatures were quantitatively measured through extra thermocouples during SPS processes with various asymmetric configurations. In the most asymmetric configuration, a maximum vertical temperature difference of 225 °C was detected within the sample treated at a sintering temperature of 1300 °C and a pressure of 25 MPa applied 200 °C·min−1 heating rate. The microstructure investigations demonstrated the morphology gradient in the ceramic: one part of the Al2O3 exhibited fine, nanostructured morphology with large open and permeable pores, whereas the other part was solid without pores. Our investigations show that a gradient Al2O3 ceramic can be produced with a single-step SPS process, which offers new directions in FGMs research. With an asymmetric sintering configuration and the sintering conditions, the structure of the ceramic, such as porosity, can be designed according to the requirements of the application area.

Functionally Graded Al2O3–CTZ Ceramics Fabricated by Spark Plasma Sintering

“…Gradient properties were created in the ceramic sample in two ways: (1) making up the bulk from layers of different composition, which leads to a chemical gradient, as well as (2) establishing a temperature gradient in the ceramic body during sintering, which results in a porosity gradient. The temperature gradient was generated by the asymmetric graphite configuration of the sample during the SPS sintering process, similarly to our previous study [ 42 ]. In Figure 2 a–c, we illustrated the temperatures at the top (T 1 ) and the bottom (T 2 ) of the samples, recorded by the thermocouples as a function of pyrometer temperature for different sintering temperatures.…”

“…The hardness of the constituent layers with different compositions show a gradual trend at each sintering temperature. Interestingly, the greatest hardness is exhibited by the 50–50 wt% mixed layer, which contains the least Al 2 O 3 , even though, theoretically, Al 2 O 3 is harder than CZT [ 42 ]. This phenomenon suggests that (i) hardness is basically determined by the microstructure and not only by the composition, and (ii) the porosity of the fourth layers is significantly higher than that of first layers.…”

“…The gel was filtered and washed with distilled water until chlorine ions (Cl − ) could not be detected with a 0.03 M silver nitrate (AgNO 3 ) solution. Finally, the gel was dried in an oven at 90 °C in air for 24 h, then calcined at 700 °C for 2 h. Nanosized Al 2 O 3 was prepared by hydrothermal synthesis similar to CTZ; this route of synthesis is detailed in our previous study [ 42 ].…”

“…The latter results in gradient properties through the cross section of the material in terms of either phase composition [ 39 ], chemical composition [ 40 ], or porosity distribution [ 41 ]. In our previous study [ 42 ], we also fabricated alumina with gradient porosity by creating a temperature gradient within the sample during SPS, using asymmetric graphite mold configuration.…”

Preparation and Characterization of Fibrous Alumina and Zirconia Toughened Alumina Ceramics with Gradient Porosity

“…Several manufacturing methods have been elaborated to produce porous structures including gel-casting process [ 6 ], organic foam technique, freeze casting [ 7 , 8 ], pore-forming agent method [ 9 , 10 ], or partial sintering by spark plasma sintering (SPS), employing appropriate sintering conditions, e.g., uniaxial pressure or graphite tool configuration [ 11 ]. Generally, each has its own merits and limitations.…”