Microfabrication of Al2O3/ZrO2 bi-phase ceramics with continuous fibrillar microstructure by co-extrusion

Abstract

In the last two decades, new materials have been required for lightweight rigid structures with potential for high temperature operations [1, 2]. The evolution of intermetallic alloys and monolithic ceramics has been encouraged by the needs of the aerospace industries and the realization that the more conventional alloy systems have reached a development limit in relation to high temperature stability and deformation resistance. The technical limitations of these monolithic ceramics were due either to their low fracture toughness or a high temperature ceiling dictated by oxidation and creep-cavitation initiated by sintering liquid residues. Continuous fiber-reinforced ceramic matrix composites (CMCs) are the best solution for high-risk engineering components in which high specific stiffness and high temperature tolerance are required (rocket nozzles, engine flaps, hot structures in aircraft and engines ext.) [3–5]. However, the current limitations for CMCs are the lack of a commercial fiber of reasonable cost and a thermal stability above 1200 ◦C and the lack of a flexible fabrication route in which the matrix achieves neartheoretical density within a woven fiber preform. Therefore, this work presents an innovative processing route while is designed to overcome these limitations while retaining a high proportion of the beneficial properties of both monolithic ceramics and CMCs. Co-extrusion can be defined as the passing of two or more pastes through the same die to manufacture a green body of constant cross sectional area. It has been widely used recently to produce multilayer ceramics [6], multilayer tubes [7], alumina and PbO-containing ferroic ceramics [8], zirconia and stainless steel metalceramic pipes [9] and fine-scale alumina, mullite and ZTA components [10–14]. One of the main advantages of using a co-extrusion technique is the significant reductions in the number of processing steps. It has also been proven that deliberately introduced weak interfaces in a laminar structure suppress the catastrophic failure and increase the fracture toughness and work of fracture by the operation of debonding and crack deflecting mechanisms [15]. The main objective of the present work is to demonstrate the feasibility of forming multiphase aligned fibrillar microstructures from nano-size sol particle precursors using an innovative multiple co-extrusion process. To achieve this, two sol-derived high solidsloading pastes of alumina and zirconia were coextruded in parallel (with the presence of a weak zirconia interface), and layed-up in a closed-packed linear array to form a heterogeneous macro-plug for subsequent extrusion assuming that the flow properties of the chemically different pastes are similar. Boehmite (γ -AlOOH) sol (Remal A20, Remet corp., USA) was used as the alumina source. The sol had average particle size and solids-loading of 40 nm and 20 wt%, respectively. The received sol was stable at a pH value of 4. The received boehmite sol was seeded with 2 wt% of the total mass using ultrafine (α-Al2O3 (30 nm, BDH Chemical, UK, high purity polishing powder) powders. The flow chart for the seeding process, followed by paste preparation is given in Fig. 1. To seed the boehmite sol, the seeding powder was first dispersed in distilled water and then added into boehmite sol. 1 wt% glycerol and celacol were also added in order to minimize the surface roughness of the extrudate and increase the green strength, respectively. 1 wt% celacol was first dissolved in water at 85 ◦C and then added to the seeded sol in order keep it plastically deformable during the multiple extrusion stage. The seeded sol was first stirred magnetically for 10 h and then ultrasonic agitation was employed at 15 kHz for 3 h for further dispersion of any particle agglomerates which might be present [16]. The final sol composition i.e., boehmite + 2 wt% seeding powder + 1 wt% glycerol + 1 wt% celacol was ball-mixed for 2 days using high purity zirconia balls. The mixed seeded sol was then vacuum filtered in order to obtain a gel structure. The resulting soft white gel was further compacted using pressure filtration apparatus to squeeze out the excess water, and obtain an extrudable paste. For the preparation of zirconia paste, zirconia sol was prepared using ultrafine and high purity zirconia powders (average grain size 30 nm, VP zirconia, Degussa Ltd., Germany) with the addition of 3 mol% yttria. Kinetically stable and well dispersed zirconia sol having 20 wt% solids-loading was prepared by the addition of a small amount of zirconia to the water, while the suspension was magnetically stirred. The best pH value in order to obtain the maximum stability was found to be 8.5 and this pH value was maintained using ammonia. 1 wt% glycerol and celacol were added to the prepared zirconia sol. 1 wt% cyclohexanone (C6H10O)