ICMCTF2006 Session B9: Max Phases and other Nanolaminated Coatings

Thursday, May 4, 2006 1:30 PM in Room Royal Palm 1-3

Thursday Afternoon

Start	Invited?	Item
1:30 PM	Invited	B9-1 Applications and Industrial Implementation of MAX-Phases J.P. Palmquist (Kanthal AB, Sweden) During the last years the MAX-phases have attracted worldwide interest from universities and companies due to their unique combination of useful properties. The M_n+1AX_n -phases, where n=1, 2, or 3, M is an early transition metal, A is an A-group element (mostly IIIA and IVA), and X is either C or N, represent a new class of machinable high-temperature engineering materials. Like their corresponding binary carbides and nitrides, they are elastically stiff, have relatively low thermal expansion coefficents, good thermal and electrical conductivities and are corrosion resistant. However, mechanically they are relatively soft and readily machinable, thermal shock resistant and damage tolerant. Moreover, some phases shows good fatigue, creep and oxidation resistance and can be used in air, vacuum and inert atmospheres up to 1400°C. Kanthal AB (Sweden) and 3-ONE-2 LLC (USA) have taken the lead on these compounds and are now offering components made from two of these phases under the trade name of Maxthal 312 (Ti₃SiC₂) and Maxthal 211 (Ti₂AlC). This presentation will focus on the potential industrial applications for MAX-phases as engineering material for corrosive and/or high-temperature environments. Furthermore, great interests have been shown for MAX-phase coatings. Thin films sputtered from Maxthal targets can for example be used in electrical contacts, with excellent wear and arc-resistant behavior combined with very low contact resistance. Thicker coatings made by thermal spraying techniques can be used as protective coatings in various applications.
2:10 PM		B9-3 Synthesis and Elastic Properties of Cr₂AlC Thin Films by Magnetron Sputtering D.P. Sigumonrong, C. Walter, T. El-Raghy, J.M. Schneider (RWTH Aachen University, Germany) MAX phases are a group of materials with fascinating properties and large application potential. Prerequisite for the application is a solid understanding of the process-structure-property relationship. One open question in this context is the effect of ion energy during magnetron sputtering on the structure and properties of the synthesized thin films. The main motivation of this work is to investigate how the material-plasma interactions may be utilized for tailoring the elastic properties of Cr₂AlC. Approximately one micrometer thick Cr₂AlC films were synthesized by magnetron sputtering at floating substrate potential (V_s = -12 V) while the substrate temperature (T_s) was varied from 450°C to 850°C. It was found that the lowest substrate temperature for deposition of polycrystalline Cr₂AlC is 450°C. Differential scanning calorimetry experiments indicate that crystallization temperature of an amorphous film is around 620°C. This is an indication for surface diffusion based mass transport during sputtering at 450°C substrate temperature while bulk diffusion is required for the phase transformation during calorimetry. The study of material-plasma interaction was performed at 450°C by varying the ion energy through changes of the substrate bias potential from -12 V to -310 V. It was found that at V_s ≥ -310, the MAX phase forms. The overall phase purity is approximately 97%. No significant change in the chemical composition of the films grown at 450°C ≤ T_s ≤ 850°C at floating potential, and at -240 V ≤ V_s ≤ -12 V at T_s = 450°C was detected. The elastic modulus of the films grown at T_s = 450°C at floating potential is approximately 60% of the calculated value. Increasing the substrate bias potential from -40 V to -240 V results in the formation of films with elastic properties very similar to the calculated ones.
2:30 PM		B9-4 Deposition and Characterization of DC Magnetron Sputtered Ti-Al-C and Ti-(Al_1-xSi_x)-C Thin Films O. Wilhelmsson, E. Lewin (Uppsala University, Sweden); J. Emmerlich, P.E. Eklund, H.H. Högberg, L. Hultman (Linköping University, Sweden); U. Jansson (Uppsala University, Sweden) Ternary carbides are an interesting group of materials with a high potential in thin film applications. This group of compounds includes the so-called MAX-phases (M: transition metal; A: A-group element; X: carbon or nitrogen) with the general formula M_n+1AX_n (n = 1-3). The structure can be described as a nanolaminate with slabs of monocarbide or mononitride (MX) interleaved with square planar layers of the A-element. This gives rise to an unique mixture of metallic and ceramic properties, for example, electrical and thermal conductivity, high ductility and oxidation resistance. Another example of ternary carbides is the intermetallic non-oxide perovskites with the general formula Me¹₃Me²C. This group of materials has been reported to be superconductive and have interesting mechanical properties. In this study we report on a systematic investigation on ternary carbides within the Ti-Al-C system. We have deposited thin films with MAX-phase structure of Ti₂AlC(000l) and Ti₃AlC₂(000l) above 800°C. At lower deposition temperatures a cubic (Ti,Al)C was formed, that exhibits interesting mechanical and lubricating properties. Additionally, the non-oxide perovskite Ti₃AlC was deposited at temperatures 300-800°C. The influence of Si substitution of Al in the MAX-phases, i.e. Ti(Al_1-xSi_x)C, has also been investigated. All films have been characterized by XRD, XPS and high-resolution TEM. Mechanical and electrical properties of these materials will also be discussed.
2:50 PM		B9-5 Si - the Weak Link in Ti₃SiC₂ MAX Phases; Thermal Stability and High-Temperature Friction Measurements of (0001)-Oriented Thin Films J. Emmerlich (Linköping University, Sweden); M. Rester (Academy of Science, Austria); O. Wilhelmsson (Uppsala University, Sweden); G. Gassner (University of Leoben, Austria); H. Hoegberg (Linköping University, Sweden); U. Jansson (Uppsala University, Sweden); L. Hultman (Linköping University, Sweden) M_n+1AX_n phases (M=transition metal; A=A-group element; X=C and/or N; n=1-3) are multifunctional materials which bridge ceramics and metals. In the Ti₃SiC₂ model system Ti₃C₂ layers (isostructural to their binary carbides) are interleaved with weakly bonded atomic planes of Si acting as mirror planes thus constituting a nanolaminate structure. In this study Ti₃SiC₂(0001) thin films were epitaxially grown on Al₂O₃(0001) substrates at 850°C by DC magnetron sputtering from elemental targets of Ti, C and Si. Their thermal stability in the temperature range of RT to 1400°C was investigated using an evacuated furnace with in-situ XRD instrumentation. The Ti₃SiC₂ MAX phase was found to decompose at ~1200°C into TiC. XPS and ERDA measurements showed a Si depleted film, i.e. Si as the weakest bonded element with the interface to vacuum segregates to the free surface and evaporates. The thin film nature with short diffusion distances compared to bulk materials explains the large discrepancy to a decomposition temperature of ≥2000°C stated for the latter. The loss of Si accompanied by a volume reduction resulted in porous and oxygen rich films, which was corroborated by TEM investigations. High temperature friction measurements on Ti₃SiC₂(0001) thin films at 700°C exhibited a much lower coefficient of friction of ~0.4 compared to ~0.75 at RT. Initial results of Raman investigations point to TiO₂ formation to be responsible for the decrease in the coefficient of friction at 700°C.
3:10 PM		B9-6 Nucleation, Growth, and Phase Stability of Magnetron Sputtered Ti₂AlN MAX Phase Thin Films M. Beckers, N. Schell, A. Mücklich, R.M.S. Martins, W. Möller (Forschungszentrum Rossendorf, Germany); L. Hultman (Linkoping University, Sweden) Thin films of the M_n+1AX_n phase Ti₂AlN were deposited by reactive co-sputtering from elemental Ti and Al targets at 690°C and 800°C. Single crystal MgO(111), MgO(100) and Al₂O₃(0001) - with and without a 10 nm thick deposited cubic (Ti_0.63Al_0.37)N seed layer - served as substrates. The nucleation, growth, and phase stability was characterized by in-situ/ex-situ x-ray diffraction and transmission electron microscopy. Depositions with seed layer onto MgO(111) at 690°C result in a tilted basal-plane growth, with Ti₂AlN{10-12}<-12-10>// MgO{111}<110>. This orientational relationship is persistent for growth onto MgO(100). An increased substrate temperature of 800°C leads to basal plane growth onto MgO(111) and Al₂O₃(0001) with Ti₂AlN{0001}<-12-10>// MgO{111}<110> and Ti₂AlN{0001}<-12-10>// Al₂O₃{0001}<-12-10>. The different formation can be understood in terms of constrained Ti₂AlN nucleation due to kinetic limitation. The thermodynamically favourable basal plane growth, which needs elemental partitioning of the incoming Ti and Al flux, is prevented by lower substrate temperatures. That in turn leads to specific interfacial adaptation and hence titled basal plane growth. Deposition onto MgO(111) at 690°C without deliberate cubic seed layer lead to a self organized (Ti_1-xAl_x)N interfacial structure with concurrent spinodal decomposition. The resulting rough interface morphology does not allow epitaxial growth and hence leads to a non-textured final Ti₂AlN film. Post-deposition annealing of only 50 nm thick, basal plane textured films grown on MgO and Al₂O₃ in vacuum reveals Ti₂AlN MAX phase decomposition already at temperatures of 1050°C. Both films show major loss of Al, with subsequent formation of intermediate Ti-Al-N phases. In the case of MgO substrate the decay is further promoted by interfacial Ti-Al-Mg-O spinel formation.
3:30 PM		B9-7 Stability of Interface Relative to Preferred Orientation of Fe/Ti Multilayers Submitted to Thermal Annealing T. Chen (Dalian University of Technology, PR China); Z.L. Wu, B.S. Cao (Dalian Univeristy of Technology, PR China); M.K. Lei (Dalian University of Technology, PR China) The Fe/Ti nanometer-scale multilayers of nominal bilayer thickness of 10.0-50.0 nm with alternating Fe and Ti sublayers thickness ratio of 1:1 are deposited by direct current magnetron sputtering. The Fe/Ti multilayers were in situ submitted to thermal vacuum annealing at the temperatures ranged from 250 to 600 for 1 h. The stability of interface in the Fe/Ti nanometer-scale multilayers is investigated using Rutherford backscattering spectrometry (RBS), wide angle x-ray diffraction (WAXRD), differential scanning calorimetry (DSC), and high resolution transmission electron microscopy (HRTEM), respectively. The intermetallics FeTi is formed at the interface between Fe and Ti sublayers for the modulation wavelengths of Fe/Ti nanometer-scale multilayers during the thermal annealing. The formation of the Fe-Ti solid solution, and the nucleation and growth of the intermetallics FeTi are characterized in the Fe/Ti nanometer-scale multilayers with the bilayer thickness of 10.0-50.0 nm, which is dependent on the thermal annealing temperature relative to the modulation wavelengths of Fe/Ti nanometer-scale multilayers. The formation mechanism of the intermetallics FeTi in the Fe/Ti nanometer-scale multilayers was discussed on the basis of thermodynamics and diffusion theory.
3:50 PM		B9-8 Electronic Structure and Elastic Properties of Nanolaminated Phases 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 this notion of interleaving of high/low electron density layers and the coupling thereof is not limited to MAX phases only. M₂SbP phases (space group P6₃/mmc), where M = Ti, Zr, and Hf, exhibit also a nanolaminated structure where MP layers are interleaved with Sb. The bonding is of covalent-ionic nature with the presence of metallic character. This notion also holds for many nanolaminated borides: RM₃B (space group Pm-3m) and W₂B₅-based phases (space group P6₃/mmc). For the cubic perovskites with the general formula of RM₃B (R and M are rare earth and 4d metals, respectively), we show that the coupling between MR and MB layers in RM₃B can be switched from predominantly covalent-ionic to metallic in character by varying the population of the M d-shells. W₂B₅, Mo₂B₅, Re₂B₅, and Os₂B₅ reveal that B forms covalent-ionic chains interleaved with a transition metal and that the electron density between these layers is low. It is reasonable to assume that alternating covalent-ionic and metallic bonding in the compounds discussed may give rise to similar properties as observed for MAX phases.
4:10 PM		B9-9 On the Heat Capacities of M₂AlC (M = Ti, V, Cr) Ternary Carbides H. Drulis, M. Drulis (Institute of Low Temperature and Structure Research PAS, Poland); S. Gupta, T. El-Raghy, M.W. Barsoum (Drexel University) The heat capacities, c_p, of bulk polycrystalline samples of Ti₂AlC, V₂AlC and Cr₂AlC in the 3-260 K temperature range are presented. The c_p and their temperature dependencies were quite similar due to the structural and chemical similarities of these compounds. Nevertheless, at all temperatures the heat capacity of Cr₂AlC was higher than the other two. The density of states at the Fermi level were 3.9 (eV unit cell)^-1, 7.5 (eV unit cell)^-1 and 14.6 (eV unit cell)^-1 for Ti₂AlC, V₂AlC and Cr₂AlC, respectively. A linear dependence was found between the number of d-electron along the row Ti, V, Cr and the density of states at the Fermi level. The results obtained for the lattice contribution in c_p are analysed using the Debye and Einstein model approximations for c_p = c_v. Good description of c_p is obtained if one assumes that nine phonon modes vibrate according to the Debye model approximation whereas remaining three of twelve modes expected for M₂AlC formula unit, fulfil an Einstein-like phonon vibration pattern. Debye temperatures, θ_D, describing acoustic phonons and Einstein temperature,θ_E, describing optical phonons contributions have been estimated for the studied compounds. The Debye temperatures are reasonably high and fall in the range of 600 K to 700 K.
4:30 PM		B9-10 Properties of MAX Phases M₂TlC (M = Ti, Zr, Hf), and M₂GaN (M = Ti, V, Cr) Using ab Initio Calculations J.A. Warner (The University of Wisconsin-Madison,); S.K.R. Patil, S.V. Khare (The University of Toledo) MAX phases have been a subject of interest recently because of their useful mechanical, electrical and thermal properties¹ . Here we have studied two groups: (i) Ti₂GaN, V₂GaN, Cr₂GaN and (ii) Ti₂TlC, Zr₂TlC, Hf₂TlC of the 211 MAX phases. We calculated the lattice parameters, bulk modulus B and local electronic density of states (LDOS) of these phases using first-principles total energy calculations within the generalized gradient approximation (GGA) to density functional theory (DFT). Core electrons were implicitly treated by ultra soft Vanderbilt type pseudopotentials. Our computed lattice structural parameters match the experimental values within 5% for all six materials. Values for B were computed to be (i) 158, 170, and 180 GPa and (ii) 125, 120, and 131 GPa for the first and second group respectively. These values suggest that Ti₂TlC, Zr₂TlC and Hf₂TlC maybe the softest of all the MAX phases explored so far. The total density of states shows that all six materials are conducting and that the Cr₂GaN is the most conducting of the first group. Amongst the second group Ti₂TlC is the most electrically conducting followed by Zr₂TlC and Hf₂TlC. The major features in LDOS are i) the hybridization of the M d orbitals with X p orbitals and (ii) M d orbitals with A p orbitals; where M denotes a transition metal (Ti, V, Cr, Zr or Hf), A denotes Ga or Tl and X denotes either nitrogen or carbon. The bulk modulus correlates well with the position of the hybridization peaks and decreases as they move towards Fermi level E_f ¹ M. W. Barsoum, Solid St. Chem. 28, 201 (2000).