SELF-HEALING BEHAVIOUR OF MAX PHASE CERAMICS STUDIED BY COMPUTED X-RAY TOMOGRAPHY

Abstract

MAX phase ceramics are promising candidates as high temperature materials. The self-healing capabilities of these materials may further improve their reliability and reduce maintenance costs during service. However, previous two dimensional research on their healing behaviour is either inadequate or sometimes even biased, because crack healing is essentially a three dimensional process. In this study, the self-healing behaviour of two MAX phase ceramics: Ti2AlC and Cr2AlC were investigated in three dimensions using high resolution synchrotron and laboratory X-ray tomography.The Ti2AlC showed remarkable healing ability of repeatedly repairing cracks at 1150 °C for three healing cycles with each healing cycle lasting less than 66 minutes. The healing kinetics of Ti2AlC was revealed to have a strong dependence on crack location, which deviates from a previously proposed uniform healing model. The Ti2AlC maintains its healing kinetics when the post-healing crack propagation follows a different growth path to the original crack. While a decreased healing kinetics was observed if the crack grows through a previous healed zone. The strength recovery is closely related to the healing percentage, where a full strength recovery was achieved with a healing percentage of over 90 %. In comparison, the Cr2AlC showed a much slower healing kinetics, with a parabolic constant of 4.3×10-3 µm2•s-1. The healing process of Cr2AlC was revealed to be more or less independent of crack location, where crack tip is always healed first. The crack gap is filled by purely Al2O3 after healing without the formation of Cr2O3. The composition of the healed area varies along the crack. A Cr7C3 sublayer beneath the Al2O3 layer was found at the healed zone of crack root, while missing in the healed zone at crack tip. The influence of impurity Cr particles on healing kinetics was investigated through a correlative microscopy study, combining X-ray tomographic slices with SEM images. The existence of Cr particles was found to accelerate the healing rate. However, this was always accompanied by the formation of large pores, which may be detrimental to the strength recovery after healing.

Date of Award	1 Aug 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Paul Mummery (Supervisor)