Papers

ICMCTF2009 Session B5-1: Properties and Characterization of Hard Coatings and Surfaces

Tuesday, April 28, 2009 8:00 AM in Room Golden West

Tuesday Morning

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8:00 AM		B5-1-1 Contribution of an In Situ Microscopic Approach for a Better Understanding of the Oxidation Phenomena of TiSiN Nanocomposite Films Deposited on Steel P. Steyer (Laboratoire MATEIS, France); A. Mège-Revil, G. Thollet (INSA de Lyon, France); R. Chiriac (UCBL-LMI, France); C. Sigala, C. Esnouf (INSA de Lyon, France) Lifetime of TiN coatings is limited by its poor resistance at high temperatures. Often, an addition of silicon enhances both mechanical and physico-chemical properties, owing to the formation of a nanocomposite structure. In this study, pure Ti and TiSi (80/20) targets were arc-evaporated to produce hard, single-layered coatings. Magnetron sputtered SiN_x films were also synthesized for a comparison purpose. The nanocomposite structure was validated by nanohardness measurements and confirmed by XRD and TEM. Through thermogravimetric experiments it is shown that in isothermal and dynamic conditions, the refractory character inherent to SiN_x governs the oxidation behaviour of TiSiN. However, in thermal cycling conditions TiSiN greatly resists while SiN_x does not withstand temperature variations. The aim of this study is to understand the role of SiN_x on the oxidation of the TiSiN nanocomposite film. For this purpose, an in situ approach of the oxidation phenomena is detail ed, based on experiments performed in a SEM operating in environmental conditions up to 1000°C. The "scenario" of attack is described: oxidation first initiates at coating defects (open pores and droplets), then extends throughout the surface. On the other hand, the poor thermal fatigue resistance of SiN_x is also evidenced at the microscopic scale by the whole flaking of the film during the cooling step. This strong propensity to cracking would be the consequence of the huge difference of thermo-mechanical behaviours between the film and the substrate. The nanocomposite structure thus allows to combine the refractory nature of SiN_x matrix with the beneficial thermo-mechanical effect due to TiN nanograins.
8:40 AM		B5-1-3 Synthesis and Oxidation Resistance of TiAlSiN and Multilayer TiAlSiN/CrAlN Coating N. Fukumoto, H. Ezura, T. Suzuki (Keio University, Japan) Si incorporation to conventional nitride coatings have become a popular research in the area of hard coatings for cutting tools. The coatings with Si-incorporated show high hardness, high temperature oxidation resistance. This nanocomposite structure of crystal phase and tissue-like amorphous Si rich phase is the cause for the superior characteristics. Multilayer coatings are similarly widely studied and show unique characteristics compared to monolayer coatings. The choice of the two alternating layers and the multilayer period becomes an important factor for the coatings characteristics. For both types of coatings, the oxidation resistance is a demanding factor for usage in cutting tools and therefore the oxidation behavior needs to be clarified. Here we report on the investigation into the oxidation resistant of Si-incorporated TiAlN monolayer and TiAlSiN/CrAlN multilayer coating. The coating was deposited by cathodic arc ion plating (AIP) with Ti₆₅Al₂₅Si₁₀ target at 300°C and bias voltage of 100 V. XRD and HR-TEM results showed that a cubic structured coating with a nanocomposite structure was formed for TiAlSiN coating. The existence of Si-rich amorphous phase at grain boundary was confirmed by EDX. The coating showed a high hardness of 38 GPa. The oxidation resistance was investigated by annealing the samples in air. Cross-sectional SEM observation and an elemental depth profile analysis were done by glow discharge optical emission spectroscopy (GD-OES). TiAlSiN coating showed good oxidation resistance with Ti,Si-rich oxide layer operating as diffusion barrier and nitride coatings still existed after 900°C. TiAlSiN multilayer coating with Cr₄₀Al₆₀N was also deposited by AIP. The substrate was rotated at rotational speeds of 5.0 and 10.0 rpm to form two samples of different multilayer periods. Changes in characterization by forming a multilayer structure were observed by XRD, nanoindentation, and HR-TEM. XRD results showed that a single cubic structured TiAlSiN/CrAlN multilayer coating was formed. From HR-TEM observation the multilayer periods were 4.3 and 9.8 nm. The multilayer coatings were also annealed in air to investigate its oxidation resistance by XRD, SEM, and GD-OES and compared with monolayer coatings.
9:00 AM		B5-1-4 Non-Linear Finite Element Modeling of Ultra-Hard Nanocomposites: The Ratio of Hardness to Yield Strength, Origin of the Ultra-Hardness and the Limits of Nanoindentation to its Measurement M.G.J. Veprek-Heijman (Technical University Munich, Germany); R.G. Veprek (Integrated Systems Laboratory, ETH, Switzerland); A.S. Argon, D.M. Parks (Massachusetts Institute of Technology); S. Veprek (Technical University Munich, Germany) Using a non-linear constitutive material model for super- (H ≥ 40 GPa) and ultra-hard (H ≥ 80 GPa) materials that accounts for the pressure enhancement of elastic moduli and of flow strength¹, we show that the ratio of the hardness to yield strength (H/Y) amounts to about 2.3 to 2.9 for the super- and ultra-hard nanocomposites, even though the ratio of hardness to Young's modulus (H/E) is relatively high. These results are briefly discussed in terms of the expanding cavity model and the elastic-plastic transition. It is shown that a well defined elastic-plastic transition and the pressure enhancements are responsible for the different mechanical behavior of these materials, compared to that of softer ones having a similarly high H/E ratio but a lower H/Y, such as elastomers. These results lend strong support to our recent explanation of the origin of hardness enhancement in the ultra-hard nanocomposites². We further present the effect of plastic deformation and resultant blunting of the diamond indenter, and discuss the limitations to the measurement on ultra-hard materials by means of nano-indentation. ¹R. G. Veprek et al., Mater. Sci. Eng. A 448(2007)366. ² S. Veprek et al., Phil. Mag. Lett. 87(2007)955.
9:20 AM		B5-1-5 Secondary Ion Mass Spectrometry and Auger Electron Spectroscopy Used at nm Range Depth Resolutions and Tens of nm Range Lateral Resolution for Ultimate Characterization of Nanostructured Hard Coatings D. Duday, P. Choquet, V. Valle, J.N. Audinot, J. Guillot, V. Hody, H.N. Migeon (CRP-Gabriel Lippmann, Luxembourg) The latest developments in the field of Secondary Ion Mass Spectrometry (SIMS) and Auger Electron Spectroscopy (AES) have provided powerful instruments for the characterisation of nanostructured hard coatings like nanocomposite or superlattice coatings. SIMS can now achieve a depth resolution in the nm range, which allows an excellent chemical characterisation of any chemical element including H even at trace concentrations in, for example, superlattice coatings. AES achieves a spatial resolution able to image small features of some 20nm like grains in hard coatings. The SIMS technique is also able to detect small quantities of contaminants (ppm or less) or isotopes with a spatial resolution of 50nm. Then, the influence of low concentrations of contaminants (<0.1at.%) on hard coating mechanical properties can be studied with SIMS. For a-C/M_xN nanocomposite film, SIMS also allows to compare the H content and repartition in different coatings of close composition and in some cases, using standard samples, it is possible to quantify the H content in coatings. The detection of isotopes by SIMS allows to determine the origin of elements in coatings e.g. O coming from coating or from oxidant atmosphere during oxidation resistance tests. The good spatial resolution of Auger and SIMS can also be used to study the chemistry inside the wear tracks. In this presentation, SIMS and Auger techniques, associated to TEM, will be used to analyse several nanostructured hard coatings before and after oxidation or wear tests with the objective to focus on new relationships between chemical composition, microstructure and wear or oxidation resistance.
10:00 AM		B5-1-7 Solid Solution Nanocomposite Cr-Si-N Thin Films L. Castaldi (Empa, Switzerland); V. Shklover (ETH Zürich, Switzerland); P. Schwaller (Empa, Switzerland); R. Sanjines (EPFL, Switzerland); J. Patscheider (Empa, Switzerland) Cr-Si-N thin films were deposited by closed field reactive unbalanced magnetron sputtering onto cemented carbide and Si substrates in order to investigate the relationship between their nanostructure and mechanical properties. The systematic variation of the Si and Cr target power ratio allowed to adjust the Si atomic concentration Si/(Si+Cr) between 0 and 100 at. %. Structural analysis by X-ray powder diffraction (XRD) showed a shift of the cubic CrN diffraction peaks up to Si/(Si+Cr) ~ 7 % and a progressive decrease of the crystallite size with increasing Si/(Si+Cr). XPS analysis suggest the formation of solid solution Cr(Si)N up to Si/(Si+Cr) ~ 7 % and a segregation of X-ray amorphous SiN_x at higher Si/(Si+Cr) values, causing the formation of a nanocomposite consisting of a solid solution of Cr(Si)N and SiN_x. A maximum of hardness, strain to failure and resistance to plastic deformation was obtained in the region Si/(Si+Cr) ~ between 12 and 35 %, which is distinctively above the percolation threshold found for nanocomposites with (almost) zero solubility for Si. Optical and conductivity measurements were performed as well to investigate the relationship between Si/(Si+Cr) and the percolation behavior of the solid solution nanocomposite coatings.
10:20 AM		B5-1-8 Microstructure and Erosion Resistance of Nanocomposite Ti-Si-C-N Coatings Deposited using HMDSN Precursor R. Wei, C. Rincon, E. Langa (Southwest Research Institute) Thick nanocomposite Ti-Si-C-N coatings (20-30µm) were deposited on Ti-6Al-4V substrates by magnetron sputtering of Ti in a gas mixture of Ar, N₂ and HMDSN (hexamethyldisilazane) under various deposition conditions. Microstructure and composition of the coatings were studied using SEM, XRD and EDS. It has been identified that the Si concentration of these coating varies from 0% (TiN) to 16 at% (Ti-Si-C-N), while the structure of these coatings is similar to the nanocomposite Ti-Si-N coatings and consists of nanocrystalline TiC0.3N0.7 and/or TiN in an amorphous matrix of SiC_xN_y with the grain size of 5 to100 nm, depending on the coating preparation process. These coatings exhibit excellent adhesion when subjected to Rc indentation tests at 150kG load. The mechanical properties of the coatings were studied using nano-indentation and micro-indentation techniques. The microhardness of these coatings varies from 1200 Hv (25g) to 3400, while the nano-hardness v aries from 10 to 26 GPa. Both the microhardness and nanohardness are lower than those of similar coatings prepared using TMS (trimethylsilane). However, the erosion test using a micro sand erosion tester at both 30° and 90° incident angles shows that these coating have very high erosion resistance up to a few hundred times improvement has been observed. There are a few advantage of using the HMDSN precursor to prepare the Ti-Si-C-N coatings over conventional magnetron sputtered deposition of Ti-Si-N coatings including composition uniformity, precursor handling safety and high deposition rate. The coating can be applied to protect gas turbine compressor blades from solid particle erosion and steam turbine blades from liquid droplet erosion, as well as other mechanical components that experience severe abrasion.
10:40 AM		B5-1-10 Mechanical and Tribological Properties of Nano-Scale Multilayered TiN/CrN Coatings M. Konchady, S. Yarmolenko, D. Pai, J. Sankar (North Carolina A&T State University) Multilayer TiN/CrN coatings with varying bilayer periods have been deposited on silicon (100) and 316 stainless steel substrates using reactive magnetron sputtering technique. Structural properties of the coatings were analyzed by X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). Multilayer coatings have shown predominant <100> orientation with superlattice structure formation below 20 nm bilayer period on silicon substrates. Nanohardness measurements, fracture indentation and scratch tests were performed to characterize the mechanical properties. Multilayered coatings, in general, have shown superior mechanical properties to both homogenous TiN and CrN coatings with nano-hardness values reaching 32 GPa in the nanometer range which is 40 % higher than the predicted value from rule of mixtures. Tribological behavior of the coatings was evaluated on micro-tribometer using reciprocating pin-on flat configuration with WC ball as counterface. Single-layer TiN coatings deposited using the same technique is used as a reference.
11:00 AM		B5-1-11 The Effects of Interlayer Grading and Pulsed Biasing on Chromium Nitride Films J.A. Freeman, P.J. Kelly, G. West (Manchester Metropolitan University, United Kingdom); N.M. Renevier (University of Central Lancashire, United Kingdom); J.W. Bradley (University of Liverpool, Untied Kingdom) Although chromium nitride (CrN) coatings are an industry standard for hard wear-resistant thin films, methods of improving /modifying film properties are still being sought. Here we present the results of a study into the effects of interlayer grading, and the use of pulsed biasing (100-350 kHz) during both the sputter cleaning and deposition stages on the structural and tribological properties of CrN films. Pulsed biasing is known to significantly increase the ion current drawn at the substrate, compared to continuous DC biasing, which can modify film properties. And the use of a graded Cr-to-CrN interlayer can reduce interface stresses and, thereby, improve coating-to-substrate adhesion. Coatings were deposited reactively in a closed field unbalanced magnetron sputtering system. The substrates used were tool steel and aluminium coupons. The films were characterised by SEM, EDX and XRD and the tribological properties were measured by scratch testing, wear testing and n anoindentation. The preliminary results suggest both approaches can significantly improve the tribological properties of CrN films.
11:20 AM		B5-1-12 A New Approach to the Synthesis of Adherent Hard Coatings with High Toughness A.N. Ranade (Northwestern University); L.R. Krishna (International Advanced Research Centre (ARCI), India); Y.W. Chung (Northwestern University) Traditional ceramic coatings provide abrasive wear protection because of high hardness. However, these coatings have low fracture toughness, making them susceptible to surface or internal flaws and failure under high impact loads. In addition, when deposited onto metal substrates, lower thermal expansion coefficients of ceramic coatings compared to those of metals can cause thermal stress that may result in delamination. This paper explores a new approach to the synthesis of adherent hard coatings with high toughness. The approach begins with a metal matrix identical to that of the substrate, followed by the incorporation of nanoscale hard particles to increase hardness by Orowan strengthening. Theoretical estimates indicate that incorporation of 10 vol. % of such nanoscale particles can raise the hardness by as much as 20 GPa. Since the coating matrix is identical to that of the substrate, this should result in maximum adhesion and minimum thermal stress. Furthermore, by choosing nanoscale particles whose structure is semi-coherent with the metal matrix, local stress at the particle-matrix interface may activate the motion of screw dislocations, thus preserving the high fracture toughness of the matrix. This paper will present initial results of this exploration, using Ti as the matrix and semi-coherent TiB₂ nanoparticles as the strengthening agent. Characterization tools include x-ray diffraction (structure), AFM (surface roughness), SEM/TEM (size and distribution of nanoparticles), nanoindentation (elastic modulus, hardness, and fracture toughness), and scratch testing (adhesion), as a function of nanoparticle concentration. These studies should provide a general strategy for designing adherent hard coatings with high toughness.
11:40 AM		B5-1-9 Microstructural and Mechanical Properties of Sputter Deposited TiN/SiN_x Multilayer Coatings on Si Substrate V. Chawla, R. Jayaganthan, R. Chandra (Indian Institute of Technology Roorkee, India) Titanium Nitride (TiN) has been widely used as protective coatings for bearings, gears and cutting tools due to its extreme hardness, high thermal and chemical stability. However, it oxidizes at 600°C which is of major concern for the aforementioned applications. To obviate these difficulties, TiN multilayer coatings, TiN/SiN_x, has been identified as potential material for realizing the improved oxidation resistance, ultrahigh hardness and strength properties. An interlayer, especially, ceramic layer, between TiN coatings, reduces the porosity, cutting off the paths for the corrosive solution to the substrate and thus increasing the protection against corrosion. Therefore, the development of multilayer coatings based on TiN/SiN_x with the desired microstructural morphology with good mechanical properties is gaining importance in recent times. The experimental studies pertaining to the influence of process parameters on microstructural and interfacial ch aracteristics of TiN/SiN_x coatings are limited in the literature. Therefore, the present work has been focused to investigate the microstructural and mechanical features of TiN/SiN_x multilayer coating deposited on Si substrate by DC/RF-Magnetron sputtering under different processing conditions. XRD was used to identify the formation of different phases and FE-SEM and AFM for microstructural characteristics of films formed under different process conditions. The formation of phases such as (111), (200) and (220) were observed. The equiaxed grain morphology in the TiN/SiN_x multilayer coating has been observed, which is due to activation of renucleation kinetics and periodic interruption of its growth due to the optimized sputtering conditions. HR-TEM has been used to characterize the crystal structure and orientation of the grains in the TiN/SiN_x coatings. The cross sectional TEM images of the coatings were analyzed to substantiate the morphology of TiN and SiN_x layers. The size of the TiN crystal was around 7.0 nm and SiN_x was found to be an amorphous matrix. The mechanical properties such as hardness and Young’s modulus of the TiN/SiN_x coatings were measured by nanoindentation technique. The observed hardness of the coatings was around 30 Gpa. The deformation characteristics of the coatings were explained using its microstructural characteristics such as grain size and amorphous SiN_x.