MAX Phase Thin Films : Unique Multifunctional Ceramics with the Elements Ti, Si, Ge, Sn, and C
Sammanfattning: Mn+1AXn phases are ternary carbides or nitrides (X) consisting of an early transition metal (M), and (A)- group element (group III-V). They combine ceramic and metallic properties with high oxidation and thermal shock resistance as well as low resistivity. Depending on stoichiometry, they can be classified as 211 (n=1), 312 (n=2), and 413 (n=3) phases. The main purpose of this Thesis is to present the synthesis by epitaxial growth of Tin+1ACn (A: Si, Ge, Sn; n=1-3) thin solid films and to report on the material’s intrinsic mechanical and electrical properties. DC magnetron sputtering of MAX-phase carbides from three individual elemental targets is presented as an original and successful deposition method. The emphasis is on the archetypical Ti3SiC2, but I also demonstrate growth of a wide range of other single-crystal Tin+1ACn thin films, including Ti2GeC, Ti3GeC2, Ti2SnC, previously available only in bulk form, as well as completely new phases of Ti4SiC3, Ti4GeC3, and Ti3SnC2, together with some intergrown 523 (211+312) and 725 (312+413) structures.A combination of x-ray diffraction (XRD), transmission electron micrcoscopy (TEM) analysis, x-ray photoelectron spectroscopy, elastic recoil detection analysis, and Rutherford backscattering spectrometry of the films reveal single-phase and epitaxial growth of Tin+1SiCn(0001) (n = 2, 3) and Ti2GeC MAX phases at substrate temperatures (TS) above 700 to 1000 °C. For TS = 500 – 700 °C, Si is accommodated at twin boundaries between TiC(111) planes. Depositions at TS = RT – 350 °C yield nc-TiC/SiC nanocomposite films or TiC growth with substitutionally incorporated Si due to kinetic constraints. Vacuum-annealing with in situ XRD measurements of the films between 800 – 1400 °C revealed a thermal stability of up to ~1000 °C. A MAX-phase decomposition model is presented within this Thesis. It starts by Si out-diffusion and evaporation from the surface between ~1000 – 1100 °C and is accompanied by any O uptake and SiO evaporation. Subsequently, the free Ti3C2 slabs relax and undergo detwinning. The decomposition process is ended by TiC0.67 formation by C redistribution and recrystallization with void formation.The mechanical response to deformation was tested on Ti3SiC2(0001) films using nanoindentation. Small applied normal forces yielding a minimum on plastic deformation reveal hardness values of up to 24 GPa, which decrease with larger indentation depths. Young’s moduli between 320 and 343 GPa were measured. Atomic force microscopy (AFM) surface imaging and Focused Ion Beam cross-sectional TEM studies confirm that mechanical deformation in this ductile ceramic takes place by kink formation and delamination along basal planes, due to edge dislocation pile-ups forming the kink boundaries resulting in local deformation-energy dissipation. Friction measurements yield a friction coefficient (?) of 0.1 for normal loads of FN = 100-200 ?N. ? increases to 0.8 with increased FN up to 0.24 N, as delamination and kinking are introduced accompanied by third-body abrasion as shown by scanning electron microscopy. By comparing electrical resistivity values obtained by four-point probe measurements, it is found that all studied MAX-phase thin film systems exhibit good conduction properties.
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