Abstract

Structural ceramic materials are usually strong or tough, although both properties are required for high temperature applications. During the past decades, intensive research has been carried out to engineer the ceramic’s microstructure, which allowed the development of advanced materials with improved toughness (due to the action of different toughening mechanism) while maintaining their strength. Other approach consisted on mimicking the structure of natural materials, as these can be strong and tough (despite the fact that they are comprised by brittle constituents). One example is the mother of pearl shell (Abalone Nacre) with approximately 95 % of calcium carbonate platelets imbedded in 5 % of protein layers. This material presents strength and toughness 20 times higher than its constituents and do not follow the rule mixture of properties. This behaviour is only possible due to the nacre structure which presents high level of organization from nano to micro scale. Considering these aspects, this work addresses the design of alumina refractories with nacre-like structure, based on the use of the Magnetically Assisted Slip Casting (MASC) technique and the Transient Liquid phase sintering (TL) procedure. Cold and hot mechanical tests were carried out to evaluate the prepared samples. The nacre-like refractories (NLR) had a structure comprised by alumina platelets with aluminium borate as a second phase. The latter played two important roles: (i) stabilized the structure, and (ii) enhanced the mechanical properties by increasing the bond strength between the platelets. The NLR showed average flexural strength of 672 MPa, fracture toughness of 11 MPa.m^1/2 at room temperature and stable crack propagation. Crack branching, deflection and bridging were the main toughening mechanisms responsible for this improved behavior. The samples evaluated at 1200°C presented high mechanical strength (280 MPa) and fracture toughness of 6 MPa.m^1/2. The same toughening mechanisms observed at room temperature as well as stable crack propagation (throughout the entire tested range of the load versus displacement curve) were identified in the measurements carried out at high temperature (1200°C). When compared to Ceramic Matrix Composites (CMCs), the prepared NLR presented twice the specific strength of these ceramics in the temperature range of 750-1200°C. Therefore, due to their enhanced performance, nacre-like refractories can be considered as novel structural materials for specific high temperature applications.