| dc.description.abstract | Alumina-forming materials, such as FeCrAl-based alloys, Ni, Co or Fe based superalloys, have been long studied because of their excellent oxidation resistance and promising applications at high temperatures. Recently, alumina forming ceramics (carbides, nitrides and borides) with nanolayered structures, gained a lot of attention as a good candidate material for high temperature applications because of their excellent oxidation resistance, good mechanical properties, and thermal stability at much higher temperatures than alumina forming metallic alloys. Some of the most interesting alumina forming ceramics belongs to families of nanolayered ternary carbides and nitrides with a generalized formula Mvn+1AXvn (n=1,2,3) (MAX), and nanolayered ternary borides with a generalized formula (MB)v2zAvx(MBv2)vy (z = 1-2, x=1-2 and y=0-2) (MAB), with A element being Al.
In this dissertation work, alumina-forming mechanism of MAX phases, namely Tiv2AlC is studied. The results of this study indicate that Al deficient of approximately 4% in Tiv2AlC leads to the breakaway oxidation, i.e. formation of Alv2Ov3+TiO2 oxide, instead of protective Al2O3 oxide layer. The outcomes of this study expected to be contributing towards the research and development of advanced Tiv2AlC MAX phase with good oxidation resistance behavior. A thin layer of Tiv2AlC was successfully diffusion bonded onto Ti alloy Ti6242 at 800 °C by Pulsed Electric Current Sintering (PECS) to evaluate feasibility of using Ti2AlC as environmental barrier coating (EBC) for high-temperature Ti-alloys.
Crv2AlC is another MAX phase with the potential of forming protective alumina oxide layer when exposed to high temperature in oxidizing environment. The results in this study show that formation of Crv7Cv3 sub-layer can be avoided in Cr2AlC samples free of chromium-carbide impurities or in samples with excess Al. In addition, we have shown that small amounts of impurities in starting Cr powders (such as Fe) as well as absence of Cr7C3 sub-layer leads to the wrinkling of alumina scale and its premature spallation. Although MoAlB has been proposed as good candidate material for high temperature applications because it forms stable and protective alumina oxide layer, its mechanical properties at high temperatures have not been studied in the past. In this study a quasi-static and cyclic compression testing at room and high temperatures were conducted on MoAlB. The results show that MoAlB goes through a brittle-to-plastic transition (BPT) at around 800 °C, similar to that observed in MAX phases. While posttesting microstructural observation showed only a few bends and kinks of individual grains, extensive microcracking was observed | en |
