Papers

AVS2007 Session SE-TuA: Naturally and Artificially Nanolaminated Coatings

Tuesday, October 16, 2007 1:40 PM in Room 617

Tuesday Afternoon

Start	Invited?	Item
1:40 PM		SE-TuA-1 Optical Characterization of ZnO/Ag/ZnO Multilayer Films with Ag Layer Deposited by High Power Pulsed Magnetron Sputtering J. Li, S.R. Kirkpatrick, S.L. Rohde (University of Nebraska-Lincoln) ZnO/Ag/ZnO multilayer films can be used in Low-E glass, solar cells and flat panel displays. Thin silver layers sandwiched between layers ZnO have been deposited by High Power Pulsed Magnetron Sputtering (HPPMS). By varying the deposition conditions during HPPMS, a thin, dense and uniform silver layer with low resistivity, high transmittance in visible light range have been obtained. The optical properties of ZnO/Ag/ZnO multilayer films with silver layer deposited by HPPMS have been studied using spectroscopic ellipsometry and infrared spectroscopic ellipsometry. The results have also been compared with multilayer films with silver layers deposited by conventional DC magnetron sputtering. For ZnO/Ag/ZnO multilayer films, the Ag-HPPMS multilayer films showed lower resistivity, lower IR transmittance than DC magnetron sputtered films. The optical properties of ZnO/Ag/ZnO multilayer films with different silver layer thickness deposited by HPPMS have also been investigated, with respect to their potential in an array of optical applications.
2:00 PM		SE-TuA-2 Synthesis and Elastic Properties of MAX Phases J.M. Schneider, D. Music (RWTH Aachen University, Germany) M_n+1AX_n phases (space group P6₃/mmc), where M is a transition metal, A is mostly IIIA or IVA group element, X is either C or N and n = 1–3, can be referred to as nanolaminates, where MX layers are interleaved with A layers. We have investigated the valence electron concentration induced changes in the elastic properties of M₂AlC phases (M = Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) using ab initio calculations.^1,2 In terms of bulk moduli [1], we have suggested that M₂AlC phases can be classified into two groups based on the coupling between MC and Al layers: M₂AlC phases with M = VB and VIB are strongly coupled, while M₂AlC with M = IVB are weakly coupled. In terms of shearing,² we have proposed that these phases can also be classified into two groups: one group comprises M = VB and VIB, where the C₄₄ values are independent of the corresponding MC. The other group includes M = IIIB and IVB, where the C₄₄ shows a linear dependency with the corresponding MC. This may be understood based on the electronic structure: shear resistant bands are filled in M₂AlC phases with M = VB and VIB, while they are not completely filled when M = IIIB and IVB. These classification proposals exhibit identical critical valence electron concentration values for the group boundary. Experimental efforts have been dedicated towards exploring the correlation between the valence electron concentration, constitution, and the elastic properties of M₂AlC phases (M = Ti, V, Cr). Ti₂AlC can be deposited onto sapphire substrates at a growth temperature of 800 °C using a compound target and an additional source of Ti.³ V₂AlC was grown at a substrate temperature of 850 °C. We report that 450 °C is sufficient to grow crystalline Cr₂AlC thin films. This is the lowest deposition temperature ever reported for a MAX phase and is significantly lower than the crystallization temperature of an amorphous Cr₂AlC thin film based on our differential scanning calorimetry data.⁴ ¹D. Music, Z. Sun, R. Ahuja, J.M. Schneider, Phys. Rev. B 73 (2006) 134117. ²D. Music, Z. Sun, A.A. Voevodin, J.M. Schneider, Solid State Commun. 139 (2006) 139. ³C. Walter, C. Martinez, T. El-Raghy, J.M. Schneider, Steel Research Int. 76 (2005) 225. ⁴C. Walter, D.P. Sigumonrong, T. El-Raghy, J.M. Schneider, Thin Solid Films 515 (2006) 389.
2:40 PM		SE-TuA-4 Deposition of M_n+1AX_n(n=1-3)Phase Coatings by Magnetron Sputtering from Compound Targets and High Velocity Oxy-Fuel Spraying H. Högberg (Linköping University, Sweden) Magnetron sputtering has proven to be a favorable route for synthesis of films of the ternary carbides or nitrides referred to as the M_n+1AX_n(n=1-3) phases. Sputtering from elemental targets has enabled the growth of several materials systems; predominately carbidic phases with Ti as the carbide-forming transition metal (M) and with either Al, Si, Ge or Sn as the A-element, but with extensions to other metals as in the V-Ge-C system or to nitride based systems as in the Ti-Al-N system. The majority of the films have been deposited on Al₂O₃(0001) substrates to promote epitaxial growth at temperatures typically above 700 °C, but for V₂GeC as low as 450 °C. In addition to deposition of the known phases in each of the studied systems this type of process also enables the growth of new phases such as Ti₃SnC₂ and so-called intergrown structures in the Ti-Si-C and Ti-Ge-C systems. Using sputtering from compound targets of Ti₃SiC₂ and Ti₂AlC in either Ar or Ar/N₂ plasmas we are currently investigating the growth conditions for the respective phases as well as potentially quaternary Ti-Si-CN and Ti-Al-CN phases. The studies show that sputtering of the compound targets in pure Ar plasmas favors epitaxial growth conditions for Ti₃SiC₂ and Ti₂AlC when the sources are co-sputtered with Ti. The as-deposited coatings consist predominately of MAX phase, but with intergrown layers of TiC. This behavior is attributed to higher carbon content in the deposited films compared to the target composition. Addition of N₂ during growth of both systems results in films of lower crystalline quality, and with preferential nucleation of TiC or TiCN for higher N₂ flows. XPS shows that up to 30 at. % N₂ is incorporated in the Ti-Al-CN films, and that the content of nitrogen in the deposited films scales with the amount of nitrogen available in the plasma. For deposition of thick (appr. 200 µm) Ti₂AlC coatings, we have applied high velocity oxy-fuel (HVOF) spraying. Characterization shows that dense and well-adherent coatings can be deposited on steel. Structural analysis shows that the HVOF coatings consist of Ti₂AlC (the powder phase) together with Ti-Al melting phases and residual TiC and Ti₃AlC₂. Vickers indentation experiments and scanning electron microscopy imaging of the indented area showed a hardness of appr. 6 GPa, with concentric cracks at the indent.
4:00 PM		SE-TuA-8 Effect of Swift Heavy Ion Irradiation on the Hardness of Chromium Nanorods R. Nagar (Indian Institute of Technology Delhi, India); K. Sai, D. Gall (Rensselaer Polytechnic Institute); D. Jain (UGC-DAE, CSR, India); B.R. Mehta, J.P. Singh (Indian Institute of Technology Delhi, India) This presentation discusses the use of ion irradiation to controllably tailor the hardness of Cr columnar thin films. Regular arrays of slanted Cr rods, 2 µm long and 250 nm wide, were grown by glancing angle dc magnetron sputter deposition on patterned Si(100) substrates. The patterns consist of 500-nm-diameter polystyrene spheres that self-assemble to form hexagonal close-packed monolayers. The Cr rod arrays were irradiated with 100 MeV Ag⁺⁸ ions at three different fluences of 10¹³, 5x10¹³ and 10¹⁴ ions/cm², while maintaining the sample at a constant temperature of 80K. The ion-irradiation induced defect formation is dominated by electronic energy losses, with a very small contribution (approximately 0.5%) from the nuclear energy losses and negligible Ag-implantation. The average nanohardness of pristine Cr rods, as determined using a Berkovich diamond tip attached to an atomic force microscope, was found to be 0.6 GPa. Irradiation of the rods with 10¹³ ions/cm² does not lead to a measurable change in the hardness. However, for the fluence of 10¹⁴ ions/cm², the hardness increases to about 4 GPa, leading to an about eight-fold increase. The fluence-dependent hardness in these Cr rods is attributed to the ion-irradiation induced defect formation that may lead to dislocation pinning which is particularly effective due to the nanoscale dimensions of the Cr rods. These results are very promising as they demonstrate the use of swift heavy ion irradiation to tune the hardness of nanorod coatings.