[en] Thin films in the W-C system were prepared by magnetron sputtering of W with coevaporated C₆₀ as carbon source. Epitaxial deposition of different W-C phases is demonstrated. In addition, nanocrystalline tungsten carbide film growth is also observed. At low C₆₀/W ratios, epitaxial growth of α-W with a solid solution of carbon was obtained on MgO(001) and Al₂O₃(001) at 400 deg. C. The carbon content in these films (10-20 at.%) was at least an order of magnitude higher than the maximum equilibrium solubility and gives rise to an extreme hardening effect. Nanoindentation measurements showed that the hardness of these films increased with the carbon content and values as high as 35 GPa were observed. At high C₆₀/W ratios, films of the cubic β-WC_1-x (x=0-0.6) phase were deposited with a nanocrystalline microstructure. Films with a grain size <30 A were obtained and the hardness of these films varied from 14 to 24 GPa. At intermediate C₆₀/W ratios, epitaxial films of hexagonal W₂C were deposited on MgO(111) at 400 deg. C. Polycrystalline phase mixtures were obtained on other substrates and hexagonal WC could be deposited as minority phase at 800 deg. C

[en] The susceptibility of four MAX phases (Ti₂AlC, Cr₂AlC, Ti₃AlC₂, and Ti₃SiC₂) to high-temperature thermal dissociation in vacuum has been investigated using in-situ neutron diffraction. In high vacuum, these phases decomposed above 1400⁰C through the sublimation of M and A elements, forming a surface coating of MC. The apparent activation energies for the decomposition of sintered Ti₃SiC₂, Ti₃AlC₂, and Ti₂AlC were determined to be 179.3, -71.9, and 85.7 kJ mol^-1, respectively. The spontaneous release of Ti₂AlC and TiC from de-intercalation during decomposition of Ti₃AlC₂ resulted in a negative activation energy.

[en] Thin films of the M_n+1AX_n-phases Ti₂AlC and Ti₃AlC₂ have been deposited by dc magnetron sputtering. In agreement with the Ti-Si-C system, the MAX-phase nucleation is strongly temperature dependent. At 900 deg. C epitaxial films of Ti₂AlC and Ti₃AlC₂ were grown, but at 700 deg. C only a cubic (Ti,Al)C phase was formed. In addition, a perovskite carbide, Ti₃AlC was grown at 800 deg. C. A bulk resistivity of 0.51 μΩ m, 0.44 μΩ m, and 1.4 μΩ m was measured for the Ti₃AlC₂, Ti₂AlC, and Ti₃AlC films deposited at 900 deg. C, respectively. By nanoindentation the hardness and Young's module was determined for an epitaxial Ti₃AlC₂ film to 20 GPa and 260 GPa, respectively

[en] We report on the synthesis and characterization of epitaxial single-crystalline Ti₃SiC₂ films (M_n+1AX_n-phase). Two original deposition techniques are described, (i) magnetron sputtering from Ti₃SiC₂ compound target and (ii) sputtering from individual titanium and silicon targets with co-evaporated C₆₀ as carbon source. Epitaxial Ti₃SiC₂ films of single-crystal quality were grown at 900 deg. C with both techniques. Epitaxial TiC(111) deposited in situ on MgO(111) by Ti sputtering using C₆₀ as carbon source was used to nucleate the Ti₃SiC₂ films. The epitaxial relationship was found to be Ti₃SiC₂(0001)//TiC(111)//MgO(111) with the in-plane orientation Ti₃SiC₂[100]//TiC[101]//MgO[101]

[en] Titanium silicon carbide (Ti₃SiC₂) possesses a unique combination of properties of both metals and ceramics, for it is thermally shock resistant, thermally and electrically conductive, damage tolerant, lightweight, highly oxidation resistant, elastically stiff, and mechanically machinable. In this paper, the effect of high vacuum annealing on the phase stability and phase transitions of Ti₃SiC₂/TiC/TiSi₂ composites at up to 1550 deg. C was studied using in-situ neutron diffraction. The role of TiC and TiSi₂ on the thermal stability of Ti₃SiC₂ during vacuum annealing is discussed. TiC reacts with TiSi₂ between 1400-1450 deg. C to form Ti₃SiC₂. Above 1400 deg. C, decomposition of Ti₃SiC₂ into TiC commenced and the rate increased with increased temperature and dwell time. Furthermore, the activation energy for the formation and decomposition of Ti₃SiC₂ was determined.

[en] Thin films of M_n+1AX_n layered compounds in the Ti-Si-C system were deposited on MgO(111) and Al₂O₃(0001) substrates held at 900 deg. C using dc magnetron sputtering from elemental targets of Ti, Si, and C. We report on single-crystal and epitaxial deposition of Ti₃SiC₂ (the previously reported MAX phase in the Ti-Si-C system), a previously unknown MAX phase Ti₄SiC₃ and another type of structure having the stoichiometry of Ti₅Si₂C₃ and Ti₇Si₂C₅. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 M layers between each A layer. In addition, epitaxial films of Ti₅Si₃C_x were deposited and Ti₅Si₄ is also observed. First-principles calculations, based on density functional theory (DFT) of Ti_n+1SiC_n for n=1,2,3,4 and the observed intergrown Ti₅Si₂C₃ and Ti₇Si₂C₅ structures show that the calculated difference in cohesive energy between the MAX phases reported here and competing phases (TiC, Ti₃SiC₂, TiSi₂, and Ti₅Si₃) are very small. This suggests that the observed Ti₅Si₂C₃ and Ti₇Si₂C₅ structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a Si layer in the TiC matrix is independent of how close the Si layers are stacked. Hardness and electrical properties can be related to the number of Si layers per Ti layer. This opens up for designed thin film structures the possibility to tune properties

[en] Ta₄AlC₃, a new member of the M _n+1AX _n-phase family, has been synthesized and characterized (n = 1-3; M = early transition metal; A A-group element; and X = C and/or N). Phase determination by Rietveld refinement of synchrotron X-ray diffraction data shows that Ta₄AlC₃ belongs to the P6₃/mmc space group with a and c lattice parameters of 3.10884 ± 0.00004 A and 24.0776 ± 0.0004 A, respectively. This is shown to be the α-polymorph of Ta₄AlC₃, with the same structure as Ti₄AlN₃. Lattice imaging by high-resolution transmission electron microscopy demonstrates the characteristic MAX-phase stacking of α-Ta₄AlC₃. Three modes of mechanical deformation of α-Ta₄AlC₃ are observed: lattice bending, kinking and delamination