Progress in modern applications in the aerospace or energy generation industries is increasingly determined by access to higher temperatures. Increasing effort is thus put into research of ceramics, which generally provide superior high temperature chemical and structural stability compared to their metallic counterparts. The governing factor is here the control of their intrinsic brittleness, which is required for their use in a structural environment.

One way to alleviate ceramic brittleness is to promote plastic processes. A model system in this regard are the MAX phases, ternary carbides and nitrides with a hexagonal layered crystal structure, which show a critical resolved shear stress for basal plane slip of as low as 77 MPa. A possible explanation for this unusual ductility was proposed on the basis of electronegativity differences, which promote electron density shifts and locally ease the motion of dislocations. In theory, this effect may be modified through compositional substitutions within the layers, and as such it was proposed as a design basis for other ceramic layered systems, for example with a cubic crystal structure. Yet, experimental evidence to support this approach is scarce.

The present work was aimed at further experimental investigation of the concept of modifying plasticity in layered ceramic systems by means of compositional variation between layers. This was facilitated by single crystal micromechanical testing of two model systems, the MAX phases and the cubic Th₆Mn₂₃ structure. To evaluate the effects of compositional variation a total of three MAX phases, Ti₃SiC₂, Ti₃AlC₂ and Cr₂AlC, and four Th₆Mn₂₃ phases, Al₁₆Co₇Ti₆, Al₁₆Ni₇Ti₆, Al₁₆Co₇Zr₆ and Co₁₆Zr₆Si₇, were considered. The MAX phases were tested via micro-compression at room and cryogenic temperatures, where the main aim was to assess a value for the friction stress, and the possibility of its variation with temperature. In preparation of this work, a detailed analysis of micropillar deformation was performed in Cr₂AlC using digital image correlation assisted pillar strain mapping, along with considerations of pillar size and crystal orientation, to ensure accurate assessment of the yield stress. In the Th₆Mn₂₃ structure type, exploratory testing was carried out using nanoindentation, aiming to compare changes in hardness as an estimate of plasticity. These data were supported by transmission electron microscopy, to link experimental observations to the underlying material defects.

The results of the MAX phase micropillar compression studies suggest that deformation is governed by the motion of dislocation sources, and a link was established for the extraction of afriction stress from compression experiments on this basis. This was furthermore shown to vary significantly with orientation, suggesting pronounced non-Schmid effects, which were further explained on the basis of dislocation mobility and taken into account for the design of tests on the basis of composition. Testing of different MAX phase compositions at room and cryogenic temperatures suggested an increase in strength for Cr₂AlC compared to Ti₃SiC₂ and Ti₃AlC₂, The tests at cryogenic temperatures further indicate an increase in strength with a decrease in temperature for all components, which was furthermore most pronounced for Ti₃AlC₂ and Cr₂AlC. Yet, pronounced scatter in the data was observed for both studies.

The hardness measurements in the Th₆Mn₂₃ structure confirmed a variation with layer composition, with analysis by transmission electron microscopy supporting the claim that plasticity is mediated by dislocations moving parallel to the layers. The ranking in hardness data was slightly offset with regards to what was predicted from variations in electronegativity, from which an alternative scaling on the basis of layer electronegativity was proposed.