Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short-circuits at high rates of charge, is one of the greatest barriers to realising high energy density all-solidstate lithium anode batteries. Utilising in-situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li6PS5Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical "pothole"-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear, i.e. the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode and therefore before a short-circuit occurs.

Accepted Article This article is protected by copyright. All rights reserved The investigation of several parameters during fibre push-out micromechanical tests on the interfacial shear strength (ISS) of the BN interphase in SiC f /SiC ceramic matrix composites (CMC) was undertaken to optimise experimental work. The SiC f /SiC composites-candidate materials for jet engine components-were manufactured with varying fibre types and interlayer thicknesses. Experimental parameters explored included analysing the effect of sample thickness on the success rate of micromechanical tests, the effect of fibre local environment whether at tow-level (intra-tow variability in ISS) or CMC architecture-level (inter-tow variability), the effect of nanoindenter flat-punch tip size & the effect of the interphase thickness itself. Over 1000 fibre push-outs were performed and analysed in this work-with data presented as cumulative distribution functions to compare and contrast samples. It was found that the ISS measured was strongly and statistically influenced by the underlying fibre roughness (interphase adherence), as well as its local fibre environment (e.g. number of nearest neighbours) only if the thickness of the interphase itself surpassed a threshold of 200 nm. Finally for thinner interphases, limited value was added to the CMC as the ISS measured were high and there was no effect from any local environment.

Push-out tests were performed on SiC-SiC fiber composites with single-and multi-layered pyrolytic carbon fibermatrix interphases. It is shown that experimental scatter is significant and a large number of tests is necessary in order to obtain statistically relevant values of interfacial shear strength. A difference between different regions of an individual fiber tow is observed, linked to a local porosity. It is shown that interfacial debonding occurs along the boundary between the fiber and the first carbon layer, regardless of the structure of the interphase, and thus interfacial shear strength is not directly linked to the structure of the interphase. KeywordsCeramic matrix composites (CMC); interfaces; push-out testing.Silicon carbide (SiC) attracts considerable interest as a candidate material for aerospace [1] and nuclear [2,3] applications due to its high temperature strength, creep resistance [4], oxidation resistance [5], and radiation tolerance [6]. In particular, it is considered for its use in aero-engines [7,8,9] and novel accident-tolerant core (ATC) and accident-tolerant fuel (ATF) concepts [10]. In order to overcome its inherent brittleness [11], while retaining beneficial chemical and nuclear properties, it is suggested to be used in the form of a continuous SiC fiberreinforced SiC matrix (SiC-SiC) composite [12].Pseudo-ductile behaviour is introduced via crack deflections and crack bridging, which occur at fiber-matrix interphase [13,14]. Therefore, the fracture properties of a composite are largely determined by the properties of its interphase, which needs to have low-toughness (promoting crack deflection), and high sliding friction coefficient (increasing energy absorption during fiber sliding) [15,16,17]. Various methods have been suggested for the characterization of these interphases, including micropillar compression [18], microcantilever fracture [19], fiber pull-out [20,21] and fiber push-out [22]. These methods determine different interfacial parameters, but in terms of experimental methodology they can be divided into two major groups -the ones that require manufacturing of each individual micro-specimen prior to testing (micropillar, microcantilever and pull-out testing) and the ones that allow for multiple tests on multiple fibers using a single prepared sample, thus potentially providing much higher number of tests and much better statistics (fiber push-out).Push-out is a well-established method for measuring the interfacial properties. In this method, a thin sample (typically in the range of 50 -100 µm) is manufactured in which the fibers are running in the direction as close to normal to the surface as possible. Load is applied to an individual fiber using a nanoindenter, until debonding occurs and the fiber is displaced. The technique provides the values of interfacial shear strength (ISS) and interfacial sliding friction coefficient.Push-out testing is conceptually a rather simple technique. However, a review of the available literature indicates that there exist several commo...

Traditional fibre push-outs for the evaluation of interfacial properties in long fibre ceramic matrix composites present their limitations—solutions for which are addressed in this work by introducing the novel trench push-out test. The trench push-out makes use of a FIB milling system and an SEM in-situ nanoindenter to probe a fibre pushed into a trench underneath, allowing in-situ observations to be directly correlated with micromechanical events. SiCf/BN/SiC composites—candidate material for turbine engines—were used as model materials in this work. Different fibre types (Hi-Nicalon and Tyranno type SA3) were coated with BN interphases, presenting mean interfacial shear stresses of 14 ± 7 MPa and 20 ± 2 MPa, respectively, during fibre sliding. The micromechanical technique enabled visualisation of how defects in the interphase (voids, inclusions & milled notches) or in the fibre (surface asperities, non-uniform coatings) affected the variability of interfacial property measurement. Graphic abstract

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