ICMCTF2012 Session AP: Symposium A Poster Session

Severe operating conditions promoting corrosion or oxidation may result to the damage and further ruin of the considered materials. Structural materials that are used in such aggressive environments are consequently usually protected by passive thin films or coatings. In the case of native thermally-grown oxides, only few studies are available in the literature concerning the coupling effect between their ageing and the mechanical behaviour of their associated substrate.

In this context, the influence of thermally-grown oxide films on the mechanical properties of polycrystalline nickel substrates has been investigated using a dynamical resonant method and instrumented indentation tests. Being a major component of superalloys used in the hottest parts of turbojets, nickel appears as a model material for oxidation due to the growth of only one form of oxide (NiO). Oxide scales have been developed at various temperatures and times to study the effect of thicknesses and microstructures on the mechanical behavior.

The dependence of the nickel elastic constants on temperature below the Curie temperature (631 K) is characterized by an anomalous behavior, resulting from its ferromagnetic character. On one hand, it is shown that the NiO layer that represents less than 3% of the total thickness of the specimens affects however significantly the elastic constants of the oxidized material below the Curie temperature, compared to those measured for the substrate in the same temperature range. These experimental results suggest that the internal stresses developed during oxidation play a key role in the competition between the magnetostrictive and elastic expansions. On the other hand, it is believed that these unexplored magnetoelastic coupling of the nickel substrate explains the scattering of the elastic modulus measurements of NiO reported in the literature.

To avoid indium diffusion across UBM, different routes have been investigated: (i) variation of residual stresses in W-Ti thin film barrier, and (ii) use of new refractory metal like tantalum and its nitride.

It is well known that refractory metal film deposited under the condition of limited atomic mobility have columnar microstructure. It is unfavorable since indium diffusion occurs along the grain boundaries. Thus, tailoring the columns microstructure from dense under compression stress state to porous under tension stress state has been explored since this should improve the performance of the barrier. We indeed report on the beneficial influence of the working pressure employed during physical deposition (PVD) on barrier performance.

On the other hand, to develop a Ta-based barrier, it is mandatory to obtain the stable phase of Ta (bcc α-Ta), for which deposition conditions have been be optimized. Bi-layer TaN / Ta are shown to be a potential diffusion barrier system. TaN underlayer is shown to promote α-Ta growth and improve barrier performance.

The aim of this work is the improvement of the mechanical and electrochemical behavior of 4140 steel substrate using HfN/VN multilayered system as a protective coating. We have grown HfN/VN multilayered via reactive r.f. magnetron sputtering technique in which was varied systematically the bilayer period (Λ), and the bilayer number (n), maintaining constant the total thickness of the coatings (~1.2 μm). The coatings were characterized by X-ray diffraction (XRD), X-ray photo electron spectroscopy (XPS), electron microscopy assisted with selected area electron diffraction. The mechanical properties were analyzed by nanoindentation method. The electrochemical properties were studied by Electrochemical Impedance Spectroscopy and Tafel curves. XRD results showed a preferential growth in the face-centered cubic (111) crystal structure for [HfN/VN]n multilayered coatings. The best improvement of the mechanical behavior was obtained when the bilayer period (Λ) was 15 nm (n = 80), yielding the highest hardness (37 GPa) and elastic modulus (351 GPa). The values for the hardness and elastic modulus are 1.48 and 1.32 times greater than the coating with n = 1, respectively. The enhancement effects in multilayer coatings could be attributed to different mechanisms for layer formation with nanometric thickness due to the Hall-Petch effect. The maximum corrosion resistance was obtained for coating with (Λ) equal to 15 nm, corresponding to n = 80 bilayered. The polarization resistance and corrosion rate were around 112.19 kOhm cm² and 3.66x10^-3 mm/year, these values were 98 % and 99 % better than those showed by the uncoated 4140 steel substrate (0.65 kOhm and 31.13 mm/year), respectively. With this idea, HfN/VN multilayered have been designed and deposited on Si(100) and AISI 4140 steel substrates with bilayer periods (Λ) in a broad range, from nanometers to hundreds of nanometers, in order to study the microstructural evolution with decreasing bilayer thickness and their related mechanical and electrochemical properties in with aim to find novel industrial applications.

Keywords: Multilayer coatings, Magnetron sputtering, Mechanical properties.

PACS: 61.05.c, 62.20.Qp

Acknowledgements

This research was supported by "El patrimonio Autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas" under contract RC-No. 275-2011. Moreover, the authors acknowledge the Serveis Científico-Técnics of the Universitat de Barcelona for TEM analysis.

Oxygen incorporation in Cr₂AlC was studied by a combination of ab initio calculations and combinatorial magnetron sputtering. Cr, Al and C targets were sputtered in Ar and Ar+O₂ atmosphere, leading to three different oxygen contents in the thin films. From X-ray stress analysis it was determined how lattice parameters of Cr₂AlC are influenced by the oxygen content.

The energy of formation was calculated for Ti₂AlC_1-xO_x, V₂AlC_1-xO_x and Cr₂AlC_1-xO_x with O occupying interstitial and substitutional sites. For Cr₂AlC_1-xO_x the energy difference between interstitial and substitutional incorporation of oxygen is -0.7 eV/oxygen atom, while it is +2.6 and +1.2 eV/oxygen atom for Ti₂AlC_1-xO_x and V₂AlC_1-xO_x, respectively. This indicates that oxygen does not substitute carbon, as observed for Ti₂AlC_1-xO_x [2], but is incorporated interstitially in the Al-layer of Cr₂AlC_1‑xO_x, even in the presence of carbon vacancies.

Experimentally, an increase of the a lattice parameter of +0.18% was observed while the c lattice parameter was hardly affected. These experimentally determined volume changes are in good agreement with the predictions from ab initio calculations on interstitial incorporation of oxygen, corroborating the notion of oxygen interstitials in Cr₂AlC. A solubility limit of 3.5 at% of oxygen was observed under the deposition conditions studied within this work. An increase of oxygen content above this limit led to the formation of an x-ray amorphous phase.

[1]: D.E. Hajas et al., Surf. Coat. Technol. (2011), doi:10.1016/j.surfcoat.2011.03.086.

[2]: J. Rosen et al., Appl. Phys. Let. 92 (2008), 064102.

The Cr-B-N coatings are promised due to their high hardness, wear- and corrosion resistance. It is well known that introduction of Si and Al to different hard coatings (CrN, TiN, and Ti-B-N) improves the tribological characteristics and oxidation resistance. The aim of present work is to study the structure and properties of Cr-Al-Si-B-(N) coatings deposited by magnetron sputtering of composite targets.

Targets were fabricated using the method of self-propagating high-temperature synthesis (SHS) according to the follow combustion reaction X(Cr+2B)+(100-X)(4Al+3C+Si₃N₄) (X=15, 30, and 40). Main advantages of SHS are follow: self-purification of the final product from impurities (adsorbed and dissolved) result in high combustion temperature and combustion rate; achievement of controlled residual porosity of ceramics due to hot pressing of products follow by high-temperature process of combustion; synthesis of metastable supersaturated solid solutions.Targets composition represented by CrB, Cr₅Si₃, and Cr₄Al₁₁ phases.

Magnetron sputtering was performed either in an atmosphere of argon or reactively in a gaseous mixture of argon and nitrogen. During magnetron sputtering, the substrate temperature and bias voltage were kept constant in the range of 300-500⁰C and -500-0 V, respectively. The silicon and alumina plates, nickel alloy and cemented carbide disks were used as the substrates. The structure, chemical and phase composition of coatings were studied by means of X-ray diffraction, transmission and scanning electron microscopy, Raman spectroscopy, and glow discharge optical emission spectroscopy. The coatings were characterised in terms of their hardness, elastic modulus, elastic recovery, adhesion strength, friction coefficient, wear rate, oxidation resistance, thermal stability, and diffusion barrier properties.

The first experiments demonstrated relatively high hardness till 40 GPa, Young’s modulus below 270 GPa, elastic recovery up to 50%, oxidation resistance and good diffusion barrier properties at the temperature high than 1100⁰C.

Turbocharged gasoline and diesel engines are of significant interest due to their capacity to allow smaller displacement automotive engines with improved fuel economy and lower CO₂ emissions. As exhaust temperatures continue to increase, oxidation resistance and mechanical properties of exhaust manifolds and turbocharger components will become problematic for some alloys of interest. This study compared the 850 and 950^oC cyclic oxidation behavior in air plus 10 vol.% water vapor (simulated exhaust gas) of IN713C, HR230, MM247, IN939, CW6MC, and two Ti-~48%Al alloys. The room temperature tensile properties of selected alloys were evaluated before and after oxidation at 850° and 950°C by using miniature tensile bars as oxidation specimens. Additionally, selected Ni-based alloys were aluminized via chemical vapor deposition and/or slurry processes, and the resultant coating microstructures and 950^oC oxidation behaviors were compared. The TiAl alloys oxidized more rapidly than the Ni-base alloys or aluminide coatings after 100, 1-h cycles and scale spallation was observed at 950°C. However, only minor changes in ductility and tensile strength for TiAl were observed. Larger decreases in ductility were observed for 713C. Longer exposures are in progress.

Non-ferrous engineering alloys (eg. titanium and aluminium alloys; nickel alloys and austenitic stainless steels) are increasingly used in the aerospace, automotive, chemical processing, and biomedical industries, owing to combinations of desirable functional properties such as corrosion resistance, high strength-to-weight ratio, biocompatibility, toughness and durability in extreme environments. However, it is well known that such materials exhibit poor tribological properties – especially under conditions of sliding wear and/or abrasion.

Nanocomposite coatings comprising a hard nanocrystalline phase embedded in an amorphous matrix have been found to exhibit improved tribological properties over a range of varying conditions. This study investigates the wear characteristic of Zr-Al-Ni nanocomposite films prepared by reactive magnetron sputtering. The sliding and abrasive wear behaviour of different coating-substrate systems was studied using reciprocating ball-on-plate sliding and slurry microabrasion wear testing, respectively, in different ambient environments. SEM, (with EDX) and XRD evaluations were conducted to determine coating thickness/morphology and chemical/phase composition. Hardness and elastic modulus were also determined by instrumented micro- and nano-indentation measurements.

The CrAlSiN thin films with various Si contents were deposited by a magnetron sputtering system. In this study, effects of silicon contents on the microstructure, mechanical properties and oxidation resistance of CrAlSiN films were investigated. The crystalline structure of thin film was determined by a glancing angle X-ray diffractometer (GA-XRD). The surface and cross-sectional morphologies of thin films were examined by a field emission scanning electron microscopy (FE-SEM). The hardness and Young's modulus of thin film were evaluated by a nanoindenter. The adhesion of coatings was determined by the scratch tester and Rockwell-C hardness tester, respectively. For the oxidation resistance evaluation, the CrAlSiN thin films were annealed at 700, 800, and 1000 ^oC for 100 hrs in air, respectively. It was found that the microstructure and mechanical properties of CrAlSiN thin films were affected by the silicon content. The oxidation resistance of CrAlSiN coating was enhanced as the silicon content increased. The mechanism for the oxidation resistance improvement of coatings was also proposed in this work.

ON MACHINING OF HARDENED AISI D2 STEEL WITH COATED TOOLS

Wilmar Mattes, mattes@catolicasc.org.br; Carlos Eduardo Viana, ceviana@catolicasc.org.br

This paper describes an experimental investigation on machining of a difficult-to cut material, AISI D2 steel of hardness 65 HRC with three tool coatings (AlCr, TiAlN and TiAlN + AlCrN). It was found that the most feasible feeds and speeds fall in the ranges 0.08–0.20 mm/rev and 80–120 m/min, respectively and that most of the tested coatings tools reached the end of life mainly due to flank wear. The highest acceptable values of tool life and volume of material removal were obtained at the lowest speed tested (70 m/min), indicating that this speed is more suitable for machining the selected tool/work material combination. While the highest feed used resulted in the highest volume of material removal, lower feeds resulted in higher tool life values. It was also found that the most appropriate feeds for this type of hardened steel are 0.14 mm/rev for finishing operations and 0.20 mm/rev for roughing operations. The best results were obtained with tools coated with TiAlN + AlCrN for the parameters of lower flank wear, better surface quality and dimensional.

Thermal barrier coatings applied on turbine blades and vanes provide reduction of temperature on these components, which in turn leads to increasing their lifetimes. Producing TBCs of appropriate thickness and uniform coating material distribution on the surface of a component is of critical importance in meeting their intended performance and durability requirements. This paper presents results of plasma sprayed (APS) thermal barrier coatings investigation using 3D optical white-light scanning method. The purpose of this investigation was non-destructive evaluation of thickness and distribution of TBCs on jet engine turbine vanes, as well as oxidation-induced degradation related to thermally grown oxide (TGO) formation underneath the ceramic top coat. Obtained results provide information concerning coating material distribution on the surface and in cross sections of the turbine vanes. Methodological aspects and final results are discussed and analyzed.