Advanced Ceramics with Dual Functions of Healing and Decomposition.

This study developed advanced ceramic materials with both healing and decomposition functions using a metastable product generated under superheated steam. The developed composite material comprises ZrC particles dispersed in a yttria-stabilized zirconia (YSZ) matrix. After introducing a surface crack of approximately 120 µm on the composite specimen, it showed a complete strength recovery rate after one hour of heat treatment under superheated steam at 400 °C, while it exhibited a decomposition behavior after one hour of heat treatment in air at 400 °C. The XRD analysis of the heat-treated specimens showed that the final product was monoclinic ZrO2 under both steam and air conditions. In other words, full strength recovery in superheated steam was achieved by a chain reaction involving metastable intermediate products derived from H2O, unlike the reaction in air.

Advanced Self-Healing Ceramics with Controlled Degradation and Repair by Chemical Reaction.

Controlling the chemical reaction rate concerning degradation and repair is found to be important to design advanced self-healing ceramics. The recovery and degradation behaviors of strength and stiffness were investigated by exposing aqueous solutions of different pH and calcium ion concentrations to the introduced crack on typical self-healing ceramics dispersed with alumina cement as a self-healing agent. The chemical reaction of cement undergoes the following three stages: dissolution of components such as calcium ions, formation of a gel, and formation of final products. Experimental and thermodynamic assessments revealed that even under conditions where the final products are identical (thermodynamic equilibrium), kinetic effects (excessive dissolution of components or insufficient crystal formation) result in strength degradation rather than repair. It was also suggested that the repair function could be enhanced by controlling the nucleation site of the crystals.

Finite Element Analysis of the Size Effect on Ceramic Strength.

The most prominent effect of the weakest link theory, which is used to derive the Weibull statistics of ceramic strength, is the size effect. In this study, we analyze the size effect on ceramic strength using the finite element analysis (FEA) methodology previously proposed by the authors. In the FEA methodology, the data of the microstructure distribution (i.e., relative density, size, and aspect ratio of the pore and the grain size) are considered as input parameters of a continuum damage model via a fracture mechanical model. Specifically, we examine five sizes of rectangular specimens under three types of loading conditions. Then, we simulate the fracture stresses of sets of 30 specimens under each size and loading condition and obtain the relationship between the scale parameter and effective volume using the Weibull distribution. The results suggest that the proposed FEA methodology can be applied to the analysis of the fracture probability of ceramics, including the size effect.

Suitability Evaluation of LaNi5 as Hydrogen-Storage-Alloy Actuator by In-Situ Displacement Measurement during Hydrogen Pressure Change.

The swelling ability of LaNi5 for application to hydrogen-storage-alloy (HSA) actuator is discussed through the measurement of the swelling ratio in hydrogen. The HSA actuator is driven by hydrogen pressure change causing the swelling of HSA. LaNi5 is one of the candidate materials for HSA actuators as well as palladium. Some prototypes of HSA actuators using LaNi5 have been fabricated; however, the kinetic swelling ability of LaNi5 itself has been not investigated. In this paper, the authors investigated the static and kinetic swelling ability of LaNi5 powder under hydrogen atmosphere. The results showed that the swelling ratio increased by 0.12 at the phase transition pressure. Response time decreased with an increase in the charged pressure during absorption, while it remained constant during discharge. Reaction kinetics revealed that these swelling behaviors were explained by hydrogen absorption and lattice expansion. The swelling ability of LaNi5 was also compared with that of palladium. The results show that LaNi5 swells 1.8 times more than palladium under 0.5 MPa. LaNi5 is suitable for an actuator driven repeatedly under more than the phase transition pressure. Palladium can be used for one-way-operation actuator even under 0.1 MPa since its response time during the evacuation was much longer than during the pressurization.

A Novel Design Approach for Self-Crack-Healing Structural Ceramics with 3D Networks of Healing Activator.

Self-crack-healing by oxidation of a pre-incorporated healing agent is an essential property of high-temperature structural ceramics for components with stringent safety requirements, such as turbine blades in aircraft engines. Here, we report a new approach for a self-healing design containing a 3D network of a healing activator, based on insight gained by clarifying the healing mechanism. We demonstrate that addition of a small amount of an activator, typically doped MnO localised on the fracture path, selected by appropriate thermodynamic calculation significantly accelerates healing by >6,000 times and significantly lowers the required reaction temperature. The activator on the fracture path exhibits rapid fracture-gap filling by generation of mobile supercooled melts, thus enabling efficient oxygen delivery to the healing agent. Furthermore, the activator promotes crystallisation of the melts and forms a mechanically strong healing oxide. We also clarified that the healing mechanism could be divided to the initial oxidation and additional two stages. Based on bone healing, we here named these stages as inflammation, repair, and remodelling stages, respectively. Our design strategy can be applied to develop new lightweight, self-healing ceramics suitable for use in high- or low-pressure turbine blades in aircraft engines.