Papers

ICMCTF2005 Session B10: Surface Processing and Modeling

Friday, May 6, 2005 8:30 AM in Room Golden West

Friday Morning

Start	Invited?	Item
8:30 AM		B10-1 Process Efficiency of Plasma Electrolytic Oxidation of Aluminium in Pulsed Unipolar Current Mode L.O. Snizhko (Ukrainian State University for Chemical Engineering, Ukraine); A.L. Yerokhin, T. Pilkington, A. Leyland, A. Matthews (The University of Sheffield, United Kingdom) Oxide ceramic coatings can be produced on aluminium alloys by high voltage plasma electrolytic treatment in an aqueous alkaline electrolyte involving a plasma discharge at the anodic metal surface. The plasma electrolytic oxidation (PEO) process may be operated to produce high temperature oxide phases resulting in an oxide ceramic layer that has high microhardness and is both electrically insulating and corrosion resistant. Low process efficiency is one obstacle to wider industrial application of the PEO process in spite of the relatively "green" nature of the process chemistry. Consequently the complex relationship between the coating properties and the electrical regime employed in a given electrolyte should be determined to establish an optimum oxide growth rate for a required composition. Pulsed current regimes are one route that may be employed to produce high temperature alumina phases in a PEO coating whilst moderating the average power consumption. This work reports the use of a pulsed unipolar current regime used to produce PEO coatings on an aluminium alloy H30 in a simple potassium hydroxide alkaline electrolyte of pH10-11. Electrical parameters varied in the experimental design were the duty cycle, varied between 0.2-0.8, and the frequency, varied in the range DC-5 kHz. The efficiency of the coating process has been assessed by measuring the coating growth rate, mass change due to the treated oxide surface layer and the quantity of gasses evolved during treatment. The oxide ceramic phases present in the PEO coating have been characterised by XRD analysis and the morphology by SEM. Results are compared to oxide ceramic layers produced under DC galvanostatic parameters.
8:50 AM		B10-2 Effects of Passivating Additions to the Alkaline Electrolyte on the Process Efficiency of Plasma Electrolytic Oxidation of Al A.L. Yerokhin (The University of Sheffield, United Kingdom); L.O. Snizhko (Ukrainian State University for Chemical Engineering, Ukraine); T. Pilkington (The University of Sheffield, United Kingdom); D.O. Mysnyakin, N.L. Gurevina (Ukrainian State University of Chemical Technology, Ukraine); A. Leyland, A. Matthews (The University of Sheffield, United Kingdom) Process efficiency is the major issue in Plasma Electrolytic Oxidation (PEO), since high voltages applied allow several collateral processes to complement the main oxidation process on the metal surface, including anodic dissolution and gas oxygen evolution. Additions of certain passivating substances to alkaline solutions are well known to inhibit the process of anodic dissolution and have been widely used in corrosion protection of metals (in particular, of aluminium alloys) as well as in practice of conventional anodising. However the effect of such additions at high anodic potentials, where plasma discharge occurs at the surface of growing oxide film, is studied insufficiently. This work provides a systematical study of the influence of passivating additions, affecting various stages of anodic dissolution process, on the oxide current yield during PEO of Al in DC mode. The oxidation treatment was carried out at a constant current density 10 A dm^-2 for 30 min in 1 g l^-1 KOH solution with 0.01 to 0.2 M of a passivating substance added. The passivating additions studied included NaAlO₂, Na₂SiO₃, Na₃PO₄, KMnO₄ and K₂Cr₂O₇. Oxide film growth efficiency was evaluated from measurements of the film thickness, sample mass loss and volume of anodic gas evolved. The film morphology and phase composition was studied by means of SEM, EDX and XRD. As a result of the work, the concentration ranges are found for the above substances to provide substantial increase in the process efficiency of plasma electrolytic oxidation.
9:10 AM		B10-3 Residual Stresses in Plasma Electrolytic Oxidation Coatings on Al Alloy Produced by Pulsed Unipolar Current R.H.U Khan, A.L. Yerokhin, T. Pilkington, A. Leyland (The University of Sheffield, United Kingdom); L.O. Snizhko (Ukrainian State University for Chemical Engineering, Ukraine); A. Matthews (The University of Sheffield, United Kingdom) Plasma Electrolytic Oxidation (PEO) of aluminium can be used to form thick alumina coatings, consisting predominantly of alpha and gamma phases. Residual stresses have a significant effect on the quality and performance of the PEO coatings, e.g. on hardness, adhesion, wear resistance and fatigue crack propagation. Therefore, it is important to understand and forecast the development of stresses which occur as a result of the coating process. In the present work, residual stresses were studied in oxide ceramic coatings produced by PEO on BS Al 6082 alloy. The coatings were obtained from a dilute alkaline electrolyte using a pulsed unipolar voltage mode with frequency and duty cycle varied within the ranges of 50 to 5000 Hz and 0.2 to 0.8, respectively. The current density and total charge were maintained at 10 A dm^-2 and 1.8 kC dm^-2, respectively. The stresses attributed to α-Al₂O₃ constituent of the coating were examined using X-ray diffraction Sin²ψ method. The coating thickness and morphology were studied using scanning electron microscopy and elastic modulus evaluated from nanohardness measurements. It is found that compressive normal and shear stresses were present in the coatings, which indicate the appearance of stress gradient across the coating thickness. Depending on the treatment parameters, internal normal stresses in the coating varied from -111±19 to -818±47 MPa and shear stress varied from -81±5 to -336±21 MPa. Correlations between internal stress and coating thickness, morphology and phase composition are discussed. PEO regimes favorable for production of the coatings with minimal stress are recommended.
9:30 AM		B10-4 Synthesis and Characterization of Hard Metal Coatings by Electro Plasma Technology P. Gupta (CAP Technologies, LLC, LBTC, Louisiana State University); G. Tenhundfeld, E.O. Daigle (Louisiana State University); L. Cato (Concurrent Technologies Corporation); J. Cannon (Benet Laboratories) Electro plasma technology (EPT) is a cathodic atmospheric plasma process and has shown a great promise in deposition of metal coatings. EPT coatings have exhibited excellent adhesion to substrate along with high deposition rates. Cr coatings have been used in many applications due to their superior corrosion resistance, mechanical and tribological properties. There is a need for alternative material due to formation of Cr (VI) in chromium acid solution is hazardous and causes problems of toxic waste disposal. Ta and Mo are one of the refractory materials that have gained some attention. The present study involves synthesis of Mo coating on 4330V steel and Inconel 718 substrate by using EPT. Zn-Mo coatings were also deposited. Surface morphology and roughness, hardness, structure, composition, tribological and corrosion response of coatings were characterized. An attempt has been made to develop an overall understanding of the processing-structure-property relationship of the EPT coatings.
10:10 AM		B10-6 Microstructure and Properties of Ceramic Coating Formed on 2214 Aluminium Alloy by Micro-Arc Oxidation E. Tillous, T. Toll-Duchanoy (Institut Nationale Polytechnique de Lorraine, France); E. Bauer-Grosse (Ecole des Mines, France); L. Dujardin, A. Viola (Messier-Bugatti, France) A ceramic coating was obtained on a 2214 aluminium alloy by micro-arc oxidation using the CERATRONIC process1. Optical microscopy, scanning electron microscopy with energy dispersive spectrometry (EDS), electron probe microscopy analysis with wave-length dispersive spectrometry (WDS) and X-ray diffraction were used to investigate its microstructure from the free surface to the metal/oxide interface. The microhardness along the depth of the coating was also measured. The results show that the micro-arc oxidized coating on the Al alloy is composed of two distinct regions: a porous external layer and a dense internal layer. The porous layer is composed of nearly pure Al₂O₃ pavements more or less embedded into a γ-Al₂O₃ matrix containing itself electrolyte species and elements of the alloy. The dense layer is made of a γ-Al₂O₃ - α-Al₂O₃ mixture with an increasing proportion of α-Al₂O₃ towards the metal/oxide interface. Inside this layer, the hardness is high, evolving from 1250 to 2000 HV0.01 with the increasing α-Al₂O₃ proportion. Concerning the external porous layer, it was too difficult to measure the hardness outside the pavements due to the high porosity but the hardness value of the pavements is close to the dense layer one. Those characterizations will allow us to understand the MAO coating mechanism. 1. J. Beauvir, Patent WO 01/81658A1 (2001).
10:30 AM		B10-7 Materials Informatics for the Design of Novel Coatings L.R. Zhao, K. Chen, Q. Yang (National Research Council, Canada); J.R. Rodgers (Toth Information Systems, Inc., Canada) A novel approach based on materials informatics towards design of novel coatings is presented. Rational selection of coating material constituents to achieve desired properties and performance remains to be a scientific challenge. Although the experimental trial-and-error approach is widely used, modeling-based methods such as material informatics for materials selection need to be explored and developed. In this study elastic coefficients and moduli of several titanium-based binary and ternary nitride systems were derived from density functional theory calculation. The calculated elastic properties were then correlated to the intrinsic hardness as well as the brittle/ductile trend of the materials using established physical and engineering criteria. To demonstrate the effectiveness of this process, coatings with selected material constituents from the calculations were synthesized using the unbalanced magnetron sputtering technique and evaluated by nanoindentation. The results indicate that it is possible to guide the design of coatings with target properties from materials informatics.
10:50 AM		B10-8 Residual Stresses Measured via MD Simulation Applied to PVD DC Sputter Deposition P. Klein (Fraunhofer Institut für Techno- und Wirtschaftsmathematik, Germany); B. Gottwald (Universität Stuttgart, Germany); T. Frauenheim, C. Köhler (Universität Paderborn, Germany); A. Gemmler (Fraunhofer Institut für Produktionstechnik und Automatisierung, Germany) Normally, the development of sputtered layer systems as well as their manufacturing by deposition processes is characterised by empirical methods. Funded by the Deutsche Forschungsgemeinschaft (DFG) a simulation tool based on the molecular dynamic method for sophisticated engineering of PVD sputter deposition was developed. With this simulation it is possible to correlate coating relevant process parameters to resulting coating properties. For simulation itself, the existing MD technology was extended in two directions: 1) Development of mesoscopic boundary conditions for the MD simulation to describe the integration of the subtrate 2) Formulation of a continuously differentiable atomistic observable for the local stress tensor which is consistent with thermodynamics in the sense that its divergence is given by external forces. Input parameters for MD simulation are performed by the substrate temperature and the in situ monitored data of the particle flow. Therefore, the sputter power, Argon flow, and sputter pressure were varied during the coating experiments. The particle flow was measured by mass spectrometer with an integrated energy analyzer and quartz crystal micro balance. As material system Cu coating on Si (111) was studied. This system was used because it is extensively applied in the microelectronic industry, and potential functions, required in MD, are already known. Using this approach more detailed information about the microscopic origin of stress formation and coating morphology is obtained.
11:10 AM		B10-9 Modeling Considerations for Two-Gas Reactive Sputtering Processes. D.J. Christie, W.D. Sproul, D.C. Carter (Advanced Energy Industries, Inc.) Two-gas reactive sputtering has long offered the promise of a new range of ternary compounds. However, only recently has partial pressure regulation of both gases led to demonstrated access to the entire control surface.¹ Earlier two-gas reactive sputtering modeling work describes the characteristic process physics affecting pressure and flow.² However, practical implementation of the model implies several key considerations. It is desirable to visualize the three dimensional control response space in terms of reactive gas flows as a function of reactive gas partial pressures. This is achieved by computing the effect of the sputtering ion flux and the two reactive gas fluxes to the target, resulting in fluxes of target material and two reactively formed compounds to the chamber surfaces. This plus fluxes of the two reactive gas species results in a flux of five distinct particle species to three conceptually distinct segments of the chamber surfaces. The large number of variables in the resulting equations drives the use of automation to obtain closed form algebraic solutions. It was necessary to vary the sticking coefficients of the gases from unity to achieve a reasonable model fit to experimental data. The authors are aware of no published sticking coefficient data for reactive sputtering processes. The ad hoc model fitting approach developed to determine the sticking coefficients may provide a means of inferring sticking coefficients for use in future modeling work. Details of the model implementation are presented and the method for fitting sticking coefficients is described. Model calculations for SiN_xO_y are compared to experimental data over the available control surface. ¹ W.D. Sproul et al., Proceedings of the 46th Annual Society of Vacuum Coaters Technical Conference, 98 (2003). ² P. Carlsson et al., J. Vac. Sci. Technol. A 11, 1534 (1993).
11:30 AM		B10-10 Infrared Thermography-Assisted Detection of Surface Alteration during Plasma Cleaning and Implications on Subsequently Deposited Magnetron Sputtered Films M. Diesselberg, H.-R. Stock, H.-W. Zoch (Foundation Institute for Materials Science, Germany); R. Cremer, H.-G. Fuss (CemeCon AG, Germany) Nowadays many types of hard wear-resistant films e.g. TiAlN or a-C:H are deposited by magnetron sputtering. Commonly a plasma etching step is used to remove surface contaminants like oxides prior to the coating cycle. During etching the surface of substrates, e.g. high speed steel or aluminium, is changed due to the intensive ion bombardment. However, edge related effects can be often observed after the etching cycle e.g. re-deposition of sputtered contaminants and varying surface roughness towards the edge of the sample. Main focus of this work is the investigation of the plasma etching step using a pulsed power supply by means of in-situ infrared thermography due to the fact that changes in roughness or surface composition subsequently result in different emission ratios while etching. Scanning electron microscopy and atomic force microscopy complete these investigations. Implications on subsequently deposited TiN and TiNiN films with regard to adhesion and topography can be made. It is found that the anisotropic etching of disc shaped substrates has a strong effect especially on hillock formation during film deposition. These results illustrate the potential of thermography as a powerful tool to optimise the plasma etching of plain and complex substrates.