ICMCTF2007 Session B7: Max Phases and other Nanolaminated Coatings

Wednesday, April 25, 2007 1:30 PM in Room Royal Palm 1-3

Wednesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	B7-1 Elastic Properties and Electronic Structure of Nanolaminates D. Music, J.M. Schneider (RWTH Aachen University, Germany) MAX phases (space group P6₃/mmc) exhibit a combination of ceramic and metallic properties due to their nanolaminated structure: layers of carbides (or nitrides) are interleaved with layers of A elements. In terms of electronic structure, this structural description may be expressed by layers of high electron density interleaved with layers of low electron density. Based on the coupling between these layers, M₂AC phases can be classified into two groups, while this is not the case for M₂AN phases. We show that interleaved high/low electron density layers and the coupling thereof is not limited to MAX phases only. Hexagonal phases, such as M₂SbP phases (M = Ti, Zr, Hf, space group P6₃/mmc), Y_n+1Co_3n+5B_2n, (n = 1, 2, 3, inf, space group P6/mmm), and W₂B₅-based phases (space group P6₃/mmc) exhibit also a nanolaminated structure. The bonding is of covalent-ionic nature with the presence of metallic character. A similar nanolaminated structure can be found in cubic phases, for example in RM₃X (space group Pm-3m), where R and M are transition metals and X is B or N. For these cubic perovskites, we show that the coupling between MR and MX layers in RM₃X can be switched from predominantly covalent-ionic to metallic in character by varying the valence electron concentration. Based on the similarity in electron density distribution between the here investigated compounds and the MAX phases, we propose that alternating covalent-ionic and metallic bonding may give rise to similar properties as observed for MAX phases.
2:10 PM		B7-3 An Investigation of MAX Phases by Thin Film Combinatorial Methods T.H. Scabarozi (Drexel University); J.D. Hettinger, W.M. Tambussi (Rowan University); M.W. Barsoum (Drexel University); S.E. Lofland (Rowan University) We have used combinatorial methods on a variety of parameters to investigate the phase diagram of MAX phase materials via rf sputtering. We have set up temperature gradients of up to 300 °C across 2-inch sapphire wafers to investigate phase formation and epitaxy. We have also used co-sputtering of up to 4 targets to look at phases and solubility, which we have studied by scanning Raman, electron backscattering diffraction and x-ray diffraction. Tribological properties, such as friction and wear, were studied with a nanoindenter. Electronic transport was also investigated. We give several examples to show the utility of the combinatorial technique. This work was supported by NSF DMR-0503711.
2:30 PM		B7-4 Synthesis and Characterization of MAX-Phase Thin Films in the V-Ge/Si-C Systems by DC Magnetron Sputtering O. Wilhelmsson (Uppsala University, Sweden); P. Eklund, H. Högberg, L. Hultman (Linköping University, Sweden); U. Jansson (Uppsala University, Sweden) MAX phases (where M is an early transition metal, A is an element from group 13-14 and X is carbon or nitrogen) belong to an interesting group of ternary carbides and nitrides with an intriguing mixture of metallic and ceramic properties. During synthesis, the MAX phases form in competition with their monocarbide/nitride (MX) relatives. Depending on the chemistry of the M-A-X system and on kinetic constraints during deposition, a solid solution, i.e., (M,A)X or M(A,X), may form. The presence of such solid solutions will influence the stability of the MX and can, e.g., accelerate segregation of X or precipitation of A and MA compounds. In this study, we have deposited ternary carbides in the V-Ge-C and V-Si-C systems by dc magnetron sputtering from elemental targets. We have deposited V₂GeC MAX-phase thin films. These are grown epitaxially on Al₂O₃(000l) at about 850°C. There are indications that previously unknown V₃GeC₂ has also formed in our thin films . In the V-Si-C system, however, only the binary equilibrium phases of VC_x and V_xSi_y were observed. As determined from XRD, XPS, and TEM, the MAX-phase growth in the V-Ge-C system was retained down to deposition temperatures of 450-500°C. Compared to our previous results in the Ti-Al-C system,¹ the deposition temperature is considerably lower (about 700°C for Ti₂AlC), but it is comparable to what has been achieved for Cr₂AlC.² In the Ti-Al-C system, growth of the solid solution (Ti,Al)C was observed at low temperatures. In the V-Ge-C system, however, Ge segregated and crystallized. The explanation to the difference in A-element behavior is found in the chemistry of the two M-A-X systems. ¹Wilhelmsson et al. J. Cryst. Growth 291 290 (2006) ² Walter et al Thin Solid Films 515 389 (2006) .
2:50 PM		B7-5 Nucleation and Growth of Magnetron Sputtered M₂AlN (M: Ti, V, Cr) MAX Phase Thin Films M. Beckers, C. Höglund (Linköping University, Sweden); N. Schell (ROBL-CRG at ESRF, Grenoble, France); L. Hultman (Linköping University, Sweden) MAX phase ternary transition metal carbides have already found their way into industrial use due to their inimitable combination of metallic and ceramic properties. Current thin film research explores fundamental material properties as well as potential applications, but is also mainly focused on carbide alloys, as carbon is easily provided by solid state deposition sources. However, we have recently shown that also phase-pure Ti₂AlN as a nitride MAX phase can be synthesized as thin films by reactive co-sputtering from elemental as well as alloy targets. In this study we show that Ti₂AlN(0001) forms readily on lattice-matched substrates for a narrow range of nitrogen partial pressures. For conditions of increasing nitrogen supersaturation, however, an initial substoichiometric NaCl-structure Ti_1-xAl_xN(111) layer forms which during spinodal decomposition provides nucleation sites for Ti₂AlN. Furthermore, we compare and contrast the effect of nitrogen super-stoichiometry for M₂AlN formation when substituting Ti by Cr and V. The films were deposited by reactive sputtering from elemental Ti (Cr, V) and Al targets between 690°C and 800°C substrate temperatures onto single crystal MgO(111) and Al₂O₃ (0001) substrates. The nucleation and growth was investigated by in-situ x-ray diffraction during deposition. The as deposited films were characterized by transmission electron microscopy, Rutherford backscattering spectroscopy, and resistivity measurements.
3:10 PM	Invited	B7-6 Properties of MAX Phases, Characterisations and First Principle Prediction G. Hug (ONERA-CNRS, France) In the last years first principle electronic structure calculations based on the Density Functional Theory (DFT) have been successfully applied to describe the bonding and to understand the structural stability of the MAX phases. In addition the DFT provides a direct prediction of the elastic properties that are in general in good agreement with experiments. The literature on this topic is now sufficiently rich and general trends can be drawn : the MAX phases exist in a variety of compositions and it is possible to understand their structural stability directly from ab initio. Several teams have also directly compared the theoretical predictions with spectroscopic experiments including X-ray photoemission (XPS), X-ray absorption (XAS), inelastic X-ray scattering or Electron Energy Loss (EELS) spectroscopies. These techniques provide a direct measurement of the electronic density of states and can therefore be used to confirm the DFT calculations. However, one has to take care of the specificity of the technique that is used, since the measured quantity often stems from an excited state. By using the appropriate theory for the excited state, a correct link can be made between theory and experiment. After showing that reliable DFT calculations can be routinely made, the next step consist in predicting macroscopic properties. For example, it has been shown that the effect of temperature can be added to the DFT to calculate ternary phase diagrams and ascertain the limits of stabilities of different phases. We will show also that transport properties can be predicted directly from DFT using Botlzman theory with adequate approximations. The state of the art in this domain will be reviewed and, in conclusion, the perspective for the future of theoretical modeling of the MAX phases will be commented.
3:50 PM		B7-8 Microstructural Investigation of Thermally Sprayed MAX Phase Coatings M. Sonestedt (Chalmers University of Technology, Sweden); J. Frodelius (Linköping University, Sweden); S. Björklund (University West, Sweden); J.-P. Palmquist (R&D Kanthal AB, Sweden); H. Högberg, L. Hultman (Linköping University, Sweden); K. Stiller (Chalmers University of Technology, Sweden) MAX phase materials belong to a group of ternary ceramics. They can be described as nanolaminates of metal (M) carbides or nitrides (X) separated by a single layer of A-atoms. This structure, with a relatively weak bonding between the M and A atoms compared to that of M-X bonds, gives rise to a previously unforeseen combination of metallic and ceramic properties, which makes these materials highly functional. For instance, MAX phases possess high thermal and electric conductivity like metals and can be machined, but like ceramics they have good oxidation resistance. The unusual properties of these materials have attracted a great deal of technological interest and inspired researchers. So far, MAX phase materials have been produced in bulk form or as thin films by sputtering. For many new applications alternative deposition processes are necessary. Therefore, the aim of our work is to develop a new process to obtain MAX-phase coatings using thermal spraying. As a first step, we have produced coatings using the deposition of Maxthal 211 (Ti₂AlC) powder by high velocity oxy fuel (HVOF) spraying. This is a relatively cold process but with high kinetic energy. X-ray diffraction (XRD) of the depostied coatings verify that the MAX phase survive the coating process and forms dense and thick films. Further microstructural studies were performed with both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The obtained results will be presented and discussed.
4:10 PM		B7-9 Reactive Chemical Vapor Deposition of Ti₃SiC₂ Layers on SiC Substrates S. Jacques, H. Fakih, O. Dezellus, M.-P. Berthet, F. Bosselet, M. Sacerdote-Peronnet, J.-C. Viala (University of Lyon, France) Ti₃SiC₂ thin coatings were deposited at 1100°C on SiC substrates by reactive chemical vapor deposition (RCVD). The method consists in producing the MAX-phase layer from a reaction between SiC and a H₂/TiCl₄ gaseous mixture during a short deposition time. For long deposition times exceeding 15 minutes, the simultaneous growth of additional Ti₅Si₃C_x and TiC sub-layers occurs from solid state diffusion when the initial Ti₃SiC₂ sub-layer thickness exceeds about 1 µm. But by repeating alternative short SiC/Ti₃SiC₂ deposition and consumption sequences and tailoring the duration of each sequence it was possible to process graded multi-layered coatings (from thick SiC and thin Ti₃SiC₂ sub-layers to thicker Ti₃SiC₂ sub-layers without SiC). In each case, the coating microstructure and texture were characterized.
4:30 PM		B7-10 Effect of N Stoichiometry and Sc Alloying on the Growth and Properties of Quaternary Ti:Sc-Al-N MAX Phase Thin Films C. Höglund, M. Beckers, J. Birch, L. Hultman (Linköping University, Sweden) MAX phases are attracting scientific and technological interest due to their unique physical properties. Furthermore ScN has been reported to be a semiconductor¹, hence a Ti₂AlN MAX phase doped with Sc is an ideal model alloy to study the influence of M element doping on crystallographic structure, film growth and electronic properties. Here we present results on pure Ti-Al-N and doped Ti:Sc-Al-N thin films deposited by reactive magnetron sputtering onto single crystal MgO(111) substrates, capped with 100-150 nm thick epitaxial TiN(111) seed layers. The films were co-deposited from elemental Ti, Al and Sc targets at temperatures between 600°C and 900°C. The as-deposited films were characterized by Rutherford backscattering spectroscopy, x-ray diffraction, transmission electron microscopy, nanoindentation and resistivity measurements. Upon growth of ternary Ti-Al-N films we observe the formation of the Ti₂AlN(0001) MAX phase at Ti:Al power ratios of 3.5:1, substrate temperatures of 850°C and a nitrogen partial pressure of 0.24 mTorr. Lower nitrogen partial pressures with concurrent nitrogen depletion lead to phase mixtures with intergrown intermetallics. Nitrogen addition promotes growth of spinodally decomposing nitrogen-substoichiometric cubic Ti_1-xAl_xN_y(111). Upon substitution of Ti by Sc, under similar growth conditions, the hypothetical MAX phase Sc₂AlN is not observed to nucleate. Instead, we obtain phase mixtures of cubic Sc_1-xAl_xN and Al₂Sc. The solubility limit of Sc in new quaternary (Ti:Sc)₂AlN MAX phase alloys will also be reported. ¹ D. Gall, M. Städele, K. Järrendahl, I. Petrov, P. Desjardins, R. T. Haasch, T.-Y. Lee, and J. E. Greene, Phys. Rev. B 63, 125119, 2001 .