Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure

Abstract

One of the most creative achievements of researchers in the structural ceramics field is ceramic-matrix composite materials. Like so many materials science accomplishments, uni-directionally aligned continuous fibre reinforced ceramic composites [1–3] or laminated plate-form ceramic composites [4–12] actually partially imitate the structure of a natural material, in this case wood, either in one or two dimensions. The advantage of layered structures is that they perform the function of fibre-reinforced ceramic composites, but are much easier to fabricate. Plate-form laminated ceramic composites with a weak interface such as the SiC/graphite system, usually show a high work of fracture and fracture toughness as high as 14 MPam1/2 [4, 5]. However, the major problem associated with the plate-form laminate is that it possesses two delamination directions, which prevents the laminates from enjoying widespread applications as structural components. Therefore other structures are needed, either at the microor meso-/macro-level, to address this problem. From the meso-/macro-structure point of view, one possible solution to the intrinsic delamination problem in plate-form ceramic laminates is to once again look to a natural material and imitate the ring structure of wood in three-dimensions. This strategy reduces the potential delamination directions in a laminated ceramic material from two to zero when the structure of a laminate changes from plate form to a self-sealed ring, i.e., a highly anisotropic structure. The objective of the present work is to create a ceramic laminate by imitating the concentric cylinder tree trunk structure and to examine the delamination resistance and fracture behaviour of the structure. A simple shaping technique, a modified slip casting method, has been used to achieve a self-sealed ring structure for a variety of ceramic laminates. In our experiment, the silicon carbide/carbon system was chosen as an example to demonstrate this structure. This system has two characteristics: (1) the graphite not only happens to be a successful sintering aid for SiC but also provides a weak interface [4, 5] and (2) our previous research [4] showed that the carbon layers can be converted into porous silicon carbide layers. Hence a single-phase SiC ceramic with a better oxidation resistance can be obtained while at the same time retaining a suitably weak interface. SiC slurry was prepared by mixing 88 wt% of SiC, 8 wt% of Al2O3 and 4 wt% of Y2O3 with water with solid to liquid ratio of 30/70 by volume. The concentration of carbon in the water based graphite slurry was between 2.5 and 5% by volume. SiC/graphite laminates were slip-cast with alternate SiC and graphite layers in a plaster of Paris mould with a casting chamber 10 mm in diameter and 50 mm in length. All the laminates were fabricated in such a way that the outermost layer and core were SiC. The thickness of the SiC layer was varied from 50 to 600 μm and that of the graphite layer from 5–20 μm by adjusting the viscosity of the slurry and casting time. The number of SiC layers in the structure was varied from 20 to 25. After slip casting, the green bodies were slowly dried in air for 48 h and sintered at temperatures ranging from 1,800 to 1,850◦C for 1 h in an Ar atmosphere at 1 atm pressure. Bulk densities were measured by the Archimedes method. The theoretical density of the laminates was calculated on the basis of the rule of mixtures. Three-point bending tests on specimens with a span of 25.4 mm were conducted using an Instron machine (8502) at room temperature. The crosshead speed was 0.06 mm/min. Since ceramic rods with circular crosssections were used for the mechanical property tests, the work of fracture/failure work was used to characterize the fracture resistance of the silicon carbide/carbon laminates. The total work per unit volume during a bending test can be written as: