Thin Film Synthesis of New Nanolaminated Ternary Carbides

Sammanfattning: Ternary transition metal carbides with inherently nanolaminated crystal structure are a class of materials with typically higher damage tolerance, better machinability and lower brittleness compared to the binary counterparts, yet retaining their satisfactory electrical and thermal conductivity. Their interesting properties can be related to the laminated structure. Though studies of their properties based on calculations and bulk materials have suggested potential thin film applications, such as high temperature hard coatings and electrical contacts, a relatively small number of these phases have been synthesized as thin films.  Investigation of thin film deposition of these inherently nanolaminated materials further the understanding of their phase formation and crystal growth.Motivated by predicted superconductivity and thermoelectric properties of molybdenum carbides and related layered molybdenum compounds, nanolaminated materials in the Mo-Ga-C ternary system were studied. Apart from the previously reported Mo2GaC, a new layered carbide, Mo2Ga2C, was synthesized in both thin film and bulk form with a postulated crystal structure related to Mo2GaC. The proposed structure was further validated by first principles calculations, showing higher stability compared to other crystal structure as well as other competing phases. The calculated lattice parameters were consistent with values from Rietveld analysis of X-ray and neutron diffraction patterns. In addition, both scanning transmission electron microscopy and X-ray photoelectron spectroscopy showed experimental evidence of the close structural-chemical relation between Mo2Ga2C and Mo2GaC.Driven by a need of high temperature protective coatings in nuclear applications, Zr-based nanolaminated carbides have become more attractive. In this work, another nanolaminated carbide, Zr2Al3C4, was synthesized in thin film form by pulsed cathodic arc deposition. Formation of the Zr2Al3C4 phase and its competing phases was studied with X-ray diffraction of thin films deposited with varying incoming flux compositions, temperatures and substrate materials. On 4H-SiC(001) substrates, highly phase-pure epitaxial Zr2Al3C4 films were formed, whereas depositions on Al2O3(001) substrates resulted in competing phases. A growth behavior similar to that of nanolaminated Mn+1AXn phases (M is a group 3-7 transition metal; A is commonly a group 13-14 element; X is C or N; n = 1 - 3) was observed, despite the structuraland chemical differences between Zr2Al3C4 and MAX phases.