Papers

ICMCTF2008 Session B6-1: Hard and Multifunctional Nano-Structured Coatings

Monday, April 28, 2008 10:00 AM in Room Golden West

Monday Morning

Start	Invited?	Item
10:00 AM		B6-1-1 Self-Organization of Structure Defects in Super-Hard Nanocomposites D. Rafaja (TU Bergakademie Freiberg, Germany) The development and self-organization of microstructure features and microstructure defects like phase constitution, preferred orientation of crystallites, formation of semi-coherent phase boundaries and formation of defect networks are assumed to influence the mechanical properties of the super-hard nanocomposites substantially. This interplay between the microstructure of the super-hard nanocomposites and their mechanical properties is already being used in numerous applications. This contribution will show the existence of the self-organization phenomena in super-hard Ti-Al-Si-N, Cr-Al-Si-N, Zr-Al-Si-N and BN nanocomposites and illustrates the effect of the self-organization on the hardness of the nanocomposites in particular. The microstructure of the nanocomposites will be described in terms of the phase composition, spatial distribution of individual phases, size and preferred orientation of crystallites, mutual orientation of neighboring crystallites, degree of the partial coherence of neighboring crystallites, morphology of the internal interfaces, local interactions of crystallites at the internal interfaces, and the kind, the density and the ordering of the self-organized microstructural defects. The central experimental methods used for the microstructure analysis are the X-ray diffraction (XRD) and the transmission electron microscopy with high-resolution (HRTEM). In order to be able to use XRD for the local analysis of the interface phenomena in nanocomposites, some methodical development was necessary. The most important results of the modified XRD theory and its use for the study of self-organization phenomena in super-hard nanocomposites will be presented.
10:40 AM		B6-1-3 Combined Effects of Wear and Oxidation of Mixed Arc-Evaporated / Magnetron Sputtered CrSiN Coatings Deposited on Steel A. Mège-Revil, P. Steyer (INSA de Lyon, France); V. Rachpech, J.F. Pierson (Ecole des Mines de Nancy - LSGS, France); J. Fontaine, M. Guibert (Ecole Centrale de Lyon - LTDS, France); C. Esnouf (INSA de Lyon, France) A decade ago, the increasing need for tools able to withstand severe machining led to the development of nanocomposite films. The first generation of TiSiN gave high performance in terms of durability¹. Promising results can also be expected using more refractory nitrides like Cr_xN. The aim of this study is to characterize the behavior of coated steel at high temperature in static conditions and in wear conditions (tribo-oxidation). Cr-Si-N films (either CrN- or Cr₂N-based) result from the combination of the arc-evaporation of a Cr target with the magnetron sputtering of a Si target. Structural investigation of the coatings were conducted by XRD and MET, evidencing a nanocomposite structure. An optimum CrN/SiN_x coating was found for a 6 at. % of Si doping, leading to the desired nanocomposite structure and causing a hardening at 40 GPa as observed by nanoindentation. The oxidation behavior was characterized in air by isothermal (973 to 1148 K), dynamic (1 K/min) and thermal fatigue tests (10 cycle runs: 300-1073-300 K). The influence of oxidative conditions on wear was investigated with a temperature-controlled reciprocating ball-on-flat tribometer at 300, 423 and 573 K. The tribological tests were made more discriminant for the films by coating the balls, which rubbed on a bare steel plate². The oxidation resistance of CrN/SiN_x is greatly enhanced in comparison with CrN whereas Cr2N-based films are very similar. Oxidation activation energy, critical oxidation temperature and resistance to thermal cycling are all improved with the CrN/SiN_x nanocomposite coating. Oxidation and tribological behavior are discussed in light of the nanoscale structure and of the silicon enrichment. ¹P. Steyer, A. Mege, D. Pech, C. Mendibide, J. Fontaine, J.F. Pierson, C. Esnouf, P. Goudeau, "Influence of the nanostructuration of PVD hard TiN-based films on the durability of coated steel", Surf. Coat. Technol. (2007) in press. ²C. Mendibide, P. Steyer, J. Fontaine, P. Goudeau, "Improvement of the tribological behaviour of PVD nanostratified TiN/CrN coatings - An explanation", Surf. Coat. Technol. 201 (2006) 4119.
11:00 AM		B6-1-4 Growth and Mechanical Properties of Nanocomposite Cr-Si-N Films by a Hybrid Coating System Q. Wang, K.H. Kim (Pusan National University, Korea) Abstract Cr-Si-N films were deposited on (100) silicon wafers and AISI D2 substrates by a hybrid coating system of arc ion plating (AIP) using a Cr target and D.C. magnetron sputtering technique using a Si target in N2/Ar gaseous mixture. The microstructure and mechanical properties of the Cr-Si-N films were studied by varying the sputtering current of Si target, nitrogen flow rate and substrate bias voltage at fixed arc current of Cr target. The chemical composition and microstructure changes of the coatings were investigated by electron probe microanalyzer (EPMA) and X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscope (AFM). The results showed that substrate bias voltage and nitrogen flow rate had important effects on growth of the Cr-Si-N coatings. Nanocomposite Cr-Si-N coatings consisting of CrN or Cr₂N nano crystallites in an amorphous Si₃N₄ tissue phase can be formed by enough Si addition into the coating. The nitrogen flow rate and substrate bias voltage can influence the phase composition and grain size of the CrN/Cr₂N nano crystallites in the nanocomposite. The evolution of film hardness and internal stresses were investigated with respect to the changes of chemical composition and microstructure. The corresponding mechanisms were explored.
11:20 AM		B6-1-5 Electronic Structure, Mechanical and Structural Properties of Al-Si-N Nanocomposite Coatings A. Pélisson, H.-J. Hug, J. Patscheider (Empa, Swiss Federal Laboratories for Materials Testing and Research, Switzerland) Thin films of Al-Si-N were deposited by unbalanced magnetron sputtering from pure Al and Si targets in an Ar/N₂ reactive atmosphere at 200°C and 500°C. The composition was varied from pure AlN to Al-Si-N with 23 at.% of Si. The optical transmission in the visible range of light is the order of 80% at all compositions. The microstructure was investigated by XRD and HRTEM. The films are crystalline with the hexagonal structure of AlN and show a (002) texture up to 12-16 at.% of Si. For Si contents below 6 at.% the Si atoms substitute Al atoms in the AlN lattice. Exceeding this solubility limit, a nc-Al_1-xSi_xN/a-SiN_x composite is formed. The mean crystallite size decreases from 60 nm in pure AlN to 5 nm in the nanocomposite upon silicon addition. Both solubility limit and grain size are found to be stable upon annealing for 2h at 1000°C. Nanoindentation measurements evidenced a weak hardness maximum exceeding 30 GPa at 8-12 at.% of Si that coincides with a negligible intrinsic residual stress (<0.5 GPa). The hardness enhancement over a broad composition range can be explained by two complementing mechanisms based on solid solution hardening for the solid solution, followed by grain refinement due to the formation of a nanocomposite. XPS analyses showed that changes in the chemistry of the system at different silicon contents are predominantly associated with silicon and nitrogen while aluminum remains unaffected. Multilayers of Al_1-xSi_xN/SiN_x with a bilayer thickness of about 5 nm and SiN_x thickness between 0.35 and 2 nm prepared at 200°C showed partial heteroepitaxy, illustrating that the growth of Si₃N₄ on AlN provides predominantely coherent interfaces.
11:40 AM		B6-1-6 Influence of Si and Al Content on the Oxidation Kinetics of Arc Evaporated Al-Cr-N Thin Films C. Tritremmel (University of Leoben, Austria); P.H. Mayrhofer (Montanuniversität Leoben, Austria); C. Mitterer (University of Leoben, Austria); M. Lechthaler (Oerlikon Balzers, Liechtenstein); P. Polcik (Plansee GmbH, Germany) The increasing demand on coatings for applications at high temperatures has raised the development of advanced metal nitride coatings, such as Al-Cr-N. Whereas detailed information is available in the literature on the thermal stability of Al-Cr-N coatings, only little is known on the oxidation kinetics of Al-Cr-N and Si alloyed Al-Cr-N. Here, we investigate the high temperature oxidation behavior of Al-Cr-N and Al-Cr-Si-N coatings. The coatings investigated were prepared by means of cathodic arc evaporation in N₂ atmosphere of powder metallurgically prepared Al-Cr and Al-Cr-Si targets. The influence of Al and Si on the oxidation kinetics of Al-Cr-N films was studied using coatings prepared from Al-Cr targets with 50, 60, and 70 at% Al and coatings prepared from Al-Cr-Si targets with 5, 10, and 20 at% Si having a constant Al/Cr ratio of 1. In the as-deposited state all coatings investigated exhibit a single-crystalline cubic structure. The oxidation studies are performed using combined differential scanning calorimetry and thermo-gravimetric analysis in synthetic air. By dynamical measurements up to 1500 C the onset temperature for oxidation is determined. The oxidation kinetics are investigated using isothermal measurements for 24 h in synthetic air at the onset temperature for oxidation, 1000 and 1100°C. The activation energy for the oxidation as a function of the chemical composition of the coatings is calculated by means of Arrhenius plots. The characterization of the oxidation products is conducted by X-ray diffraction, energy-dispersive X-ray analysis and cross-sectional investigations by scanning electron microscopy.