Papers

ICMCTF2008 Session B1-2: Sputtering Coatings and Technologies

Thursday, May 1, 2008 1:30 PM in Room Golden West

Thursday Afternoon

Start	Invited?	Item
1:30 PM		B1-2-1 Oxidation Behavior, Microstructure Evolution and Thermal Stability in Nanostructure CrN/AlN Multilayer Hard Coating S.K. Tien, C.H. Lin, Y.Z. Tsai, J.G. Duh (National Tsing-Hua University, Taiwan) Surface modification engineering is the technology to deposit a foreign material onto the surface of interest to improve specific desired properties. In this study, nanostructured CrN/AlN multilayer coatings with different modulation periods were fabricated by RF magnetron sputtering technique. The hardness of as-deposited CrN/AlN coating with 4nm modulation was 28.2GPa, which was 60% higher than that estimated by rule of mixture. The hardness enhancement was caused by the specific coherent interfaces between cubic CrN and metastable cubic AlN. The hardness degradation ratio of CrN/AlN coating with modulation period of 4nm was only 8.1% at 700^oC, which was superior to that of CrN coating. Microstructure of CrN/AlN coatings exhibited a dense columnar structure and the surface roughness of multilayer coating retained below 5nm after annealing at elevated temperature. After heat treatment at 800°C for 1 hr, only one oxide layer smaller than 50nm in thickness was found in the annealed CrN/AlN coating with 4nm. This Al₂O₃-Cr₂O₃ solid solution was still existed even after heat treatment at 950^oC for 1 hr. It was demonstrated that the CrN/AlN coating exhibited superior thermal stability compared to the TiN/AlN coating at elevated temperature. The oxidation resistance behaviors and mechanisms of CrN/AlN with different modulation periods and TiN/AlN multilayer coatings were discussed and proposed. It was concluded that mechanical properties and thermal stability of CrN/AlN multilayer coating with 4nm were much superior to that of CrN, AlN, and TiN/AlN coatings. A promising nanostructured hard coating candidate was then developed.
2:10 PM		B1-2-3 Oxidation Tuning of Cr-Al-N Coatings by Incorporation of Yttrium F. Rovere, P.H. Mayrhofer (Montanuniversität Leoben, Austria); R. Braun (German Aerospace Center (DLR), Germany); J.M. Schneider (RWTH Aachen University, Germany) Cr-Al-N hard coatings exhibit considerable resistance to oxidation due to the formation of stable and dense Cr₂O₃/Al₂O₃ mixed oxides. Further optimization can be achieved by alloying reactive elements (RE), such as yttrium (Y), which promote densification and improved adherence of the oxide scales formed. This widens their application potential to future high temperature fields of advanced machining, automotive and aerospatial industry, where temperatures of 1000 °C are reached and exceeded. Here we use magnetron sputtered Cr-Al-N coatings with Al/Cr ratios close to 1.2 and Y-contents of 0, 1, 2, and 4 at%, corresponding to YN mole fractions of 0, 2, 4, and 8% to study the impact of yttrium incorporation on the oxidation resistance. Thermo gravimetric analysis in synthetic air is performed under isothermal (10 h at 900, 1000 and 1100 °C) and dynamic (T_max = 1400 °C) conditions. Furthermore, cyclic oxidation tests from room temperature (RT) to 900 and RT to 950 °C in ambient atmosphere (1000 one-hour-cycles) are conducted for coated γ-TiAl specimens. Structural and morphological studies of the oxidized films are performed by X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. Based on our results we can conclude that all films studied form Cr₂O₃/Al₂O₃ mixed oxide scales. Coatings with 2 mol% YN show considerably lower weight gain associated with an improved oxidation resistance compared to Y-free Cr-Al-N films due to reactive element effects (keying of the oxide scale by oxygen inward diffusion as rate limiting process, suppression of interfacial void growth due to Y-rich precipitates at the coating/scale interface). With higher yttrium content however, these effects are inefficient as porous, non protective oxide scales are formed. This can be explained by the Y-induced retarded transformation of γ-Al₂O₃ into the stable and dense α-Al₂O₃. The combination of these effects results in the observed improved oxidation resistance of 1 at% Y (2 mol% YN) alloyed Cr-Al-N coatings.
2:30 PM		B1-2-4 Effect of Extra Nitrogen Additions in the Preparation of CrN & AlN Films on H13 by PVD Process A. Rojo (ITESM-TOL, Mexico); J. Oseguera, O. Salas, J. Acosta (ITESM-CEM, Mexico) This study presents the relationship found between the development of the CrN & AlN films by the traditional way of adding the argon-nitrogen mixture and a different way of adding separately extra nitrogen near of the substrate. It was found a relationship between microstructure-processing for the development of CrN and AlN films where the extra nitrogen added is an important factor. Fluent Software was used to model the flow pattern generated by these two forms of introducing the gases to the reaction chamber. The reactive atmosphere and the process conditions were monitoring by Langmuir probe measurement and optical emission spectroscopy (OES). The relative phase composition and microstructure were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray microanalysis. All the films analyzed for this study were deposited on H13 tool steel as substrate by reactive magnetron sputtering during a PVD process.
2:50 PM		B1-2-6 Pulsed Bias Sputtering of Crystalline Chromia and Alumina Films M. Audronis, A. Matthews, A. Leyland (University of Sheffield, United Kingdom) Dielectric oxides are important technological materials with a large variety of applications. Reactive physical vapour deposition of crystalline coatings of such non-conducting oxides (e.g. chromia and alumina) is desirable, but often problematic. It is usually found that the required crystalline structure and stoichiometry of coatings cannot be easily achieved. Methods such as substrate heating, radio frequency (RF) plasma enhancement and application of a negative substrate bias, are often used to improve the structure and stoichiometry of such films. However, the necessity to heat the substrates can complicate the deposition process, and limit the range of substrates that can be coated. RF substrate biasing (or other substrate biasing technologies) are complicated and/or expensive, whilst direct current biasing of insulating coatings generally causes damage by arcing. In this paper we report on biased sputter deposition of oxide coatings (Cr₂O₃, Al₂O₃) using "dual-frequency" (2F) pulsed-DC magnetron sputter deposition configuration (in which the target and substrate bias are pulsed at frequencies of 130 and 250 kHz, respectively). Crystalline Cr₂O₃ coatings, for example, were produced at substrate temperatures as low as 90 °C. Al₂O₃ coatings containing some crystalline constituents (k@and/or θ phases) were obtained at a substrate temperature in a range of 124-158 °C. The results suggest that generating optimal ion bombardment conditions at the growing film surface during deposition is a critical factor in defining the structure of such coatings. Too low or too high energy ion bombardment can result in amorphous coatings, while a window of optimal ion energies exists within which crystalline coatings can be deposited at relatively lower substrate temperatures. It is hypothesized that the effectiveness of the 2F-pulsed-DC processing configuration, in contrast to 1F/synchronised pulsed-DC configuration, is due to the fact that it allows the whole range of charged species (positive ions, negative ions and electrons) abundant in such discharges to be deployed in beneficially modifying coating growth conditions at the film surface.
3:10 PM		B1-2-7 Magnetron Sputtering and Characterization of α-Alumina Coatings on WC/Co Substrates T.I. Selinder, M. Elfwing (Sandvik Tooling, Sweden); E. Wallin, U. Helmersson (Linköping University, Sweden); H. Blomqvist (Sandvik Tooling AB, Sweden) Hard coatings deposited by physical vapor deposition (PVD), e.g., magnetron sputtering are frequently used for reducing the wear of metal cutting tools. α-alumina is a desirable material as it offers high temperature stability. Typically the coating is grown by metal target sputtering in reactive gas environment, which is known to cause problems when insulating coatings are deposited. So far, pure α-alumina coatings have been grown by chemical vapor deposition and PVD methods, only at very high substrate temperatures. In this paper is described the reactive sputtering of crystalline α-alumina coatings on cemented carbide cutting tool bits. Pure Al was used as target material, sputtering was done in an argon/oxygen gas mixture, and the substrates were kept at 600°C. Recent advances in high power impulse magnetron sputtering were effective in eliminating problems with insulating layers on the target. Moreover, hysteresis effects with respect to oxygen gas flow were alleviated, which enabled stable growth at a high deposition rate. High power impulses with peak power in excess of 200 Wcm^-2, and high repetition frequency, created an intense plasma, helpful in nucleating the α-phase. X-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) showed that stoichiometric crystalline alumina was formed on WC/Co. XRD was further used for residual stress analysis of coatings grown at different substrate bias values. TEM studies showed the detailed microstructure and growth mode of the film in cross section. Technological testing and the commercial use of PVD alumina coatings for tooling applications are addressed.
3:30 PM		B1-2-8 Structure and Mechanical Properties of Reactively Sputtered Complex Oxide Films T.K. Chen, C.H. Wu, K.H. Hung, M.S. Wong (National Dong Hwa University, Taiwan) Reactively sputtered hex-element Al_xCoCrCuFeNi oxide films with varying aluminum content were prepared at a substrate temperature of 200 by sputtering an individual homogeneous alloy targets as well as by co-sputtering an alloy target and a pure Al target. X-ray diffraction and transmission electron microscopy studies show the films are nanocomposite in nature and comprise only a cubic-spinel phase and amorphous phase without accompanying other crystalline oxide phases. A small, negative deviation from Vegard’s law was observed for the spinel phase, which indicates changes in cation distribution. The film hardness and elastic modulus were measured by means of nanoindentation in which the continuous stiffness measurement (CSM) was applied. The film hardness is increased with aluminum content. The maximum hardness and modulus achieved here is 22.6 ± 1.6 and 220 GPa, respectively, for Al₂CoCrCuFeNi oxide film. After annealing at 500 for 5 hours, the hardness enhanced to 26 GPa. These hardness values are among the hardest available hard oxide materials. Further annealing at 700 and 900 for 5 hours dramatically decrease film hardness mainly due to the increase of both crystallinity and grain size that results in disappearance of the nanocomposite structure.
3:50 PM		B1-2-9 Characterisation of Alumina-Zirconia Thin Films Deposited by Reactive Dual Magnetron Sputtering D.H. Trinh (Linköping University, Sweden); T. Kubart, T. Nyberg, M. Ottosson (Uppsala University, Sweden); M. Collin, I. Reineck (Sandvik Tooling AB, Sweden); L. Hultman, H. Högberg (Linköping University, Sweden) Composite thin films of alumina and zirconia are of interest for many applications that require toughness, wear resistance and high hardness, such as cutting tools and bearings. Previous studies show that mixed alumina-zirconia composite films can be deposited with Radio Frequency (RF) sources, thereby preventing arcing on ceramic or poisoned metallic targets, but the resulting films are limited in thickness, in addition the films have been mainly deposited with laboratory-scale systems and on well-defined surfaces such as Si (100) rather than industrially relevant substrates. In this study, limitations in thickness (<500 nm) were addressed through use of reactive dual direct current (DC) magnetron sputtering of metal targets in a pilot-plant scale system to deposit the oxide films. Control of the oxygen partial pressure during deposition was used to minimise poisoning of the target surface. With this method, thicker films (~1 µm) were possible. The films were deposited on tungsten carbide cutting inserts at a temperature of 250°C. A full range of compositions were examined, from the pure individual oxides to a number of mixed compositions with both oxides. The structure of the as-deposited films was studied through a combination of x-ray diffraction and electron microscopy. The pure zirconia films contained a textured monoclinic phase. This phase persisted even when the alumina content in the films increased above the solubility limit of alumina in zirconia. Increasing the alumina content further resulted in the formation of an amorphous phase in the films. The amorphous films were subsequently annealed to achieve a crystalline structure. The implications for metal cutting with such films will be discussed.
4:10 PM		B1-2-10 Comparative Study on Ag-Doped ZnO and Cu-Doped ZnO Thin Films Grown by Simultaneous dc and RF Magnetron Sputtering D.R Sahu, J.-L. Huang (National Cheng-Kung University, Taiwan) Ag-doped ZnO and Cu doped ZnO thin films have been grown using simultaneous RF magnetron sputtering of ZnO and dc sputtering of Cu and Ag. The structural, electrical and optical properties of the films are investigated. The effect of different amount of Ag and Cu on the properties of films was investigated in various partial pressure of argon atmosphere. Several analytical tools such as spectrophotometer, x-ray diffraction, scanning electron microscope and four point probes were used to explore the causes of the changes in electrical and optical properties. It is observed that both film shows different surface morphology with preferential crystalline growth orientation along (002). More significantly Ag-doped ZnO film show low resistance and high transmittance than that of Cu-doped film of same thickness. The moderate Cu-doping in the ZnO film can improve its microstructure and electrical resistivity. ZnO film doped with 2 at% Cu shows resistivity of the 10⁸ Ohm .cm. Details comparative study on the properties of the film will be presented during presentataion.