Mechanical Properties and Adhesion

Wednesday, April 12, 2000 8:30 AM in Room San Diego

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8:30 AM F1/E4-3-1 Mechanical Properties of Composites and Mixtures with Bichalcogenides as Main Constituent
A. Karimi (Swiss Federal Institute of Technology (EPFL), Switzerland); A. Rozen, E. Bergmann (Geneva School of Engineering, Switzerland)
Thin films (0.1 - 6 microns) were deposited from several targets, that were mixtures of molybdenum disulfide and niobium diselenide with various metals, hard materials and polymers. Substrates were high speed steel samples. A subcoat of titanium nitride or c-MoTiN was used for most of the coatings. Chemical composition, structure and morphology were analysed by standard methods: EDX, WDX, GDOS, DRX, LOM,SEM, AFM. Microhardness was measured by Vickers indentation on thick coatings. Adhesion was verified by Rockwell cone indentation. The mechanical properties were then measured by nanoindentation using differential loading. The procedure allows the determination of hardness and elastic modulus as a function of depth. From the discontinuities in the load-penetration recordings, qualitative conclusions can be drawn with respect to ductility. The results show, that the mechanical properties of the coatings can be varied systematically and in a large range by forming mixtures or composites. Some coatings were also tested in drilling and surface milling with end mills. The most promising coatings were used in a hobbing operation
8:50 AM F1/E4-3-2 Surface Profiling of Coated Stents
D. Marton (University of Texas Health Science Center at San Antonio, Department of Radiology); A. Watson, C. Simon, E.A. Sprague, J.C. Palmaz (University of Texas Health Science Center at San Antonio)
Stents are widely used in humans to keep blood vessels open. Many stents are made of stainless steel and other alloys and coated with various materials, including gold, a-SiC:H, and polymers in order to improve patency and/or radio opacity. During the placement of stents, their diameter is increased significantly, often by as much as 300%. This expansion is commonly associated with significant plastic deformation of the stent. The changes in the surface topography that result from such deformation may be important for the stent's performance and were subject of this study. Three different stents were investigated before and after expansion using surface profilometry, AFM, and spectroscopical methods. The results indicate that the quality of the stent coatings should be carefully controlled and post-expansion studies must be included in the characterization of stents.
9:10 AM F1/E4-3-3 Thickness Dependence of Mechanical Properties of on-Wafer Low-k Ultrathin Polytetrafluoroethylene Films
J.G. Wang, F.G. Shi (University of California, Irvine); T.G. Nieh (Lawrence Livermore National Laboratory); B. Zhao, M. Brongo (Conexant Systems Inc.); S. Qu, T. Rosenmayer (W. L. Gore & Associates, Inc.)
Elastic modulus and hardness and their respective thickness dependence of on-wafer low-k ultrathin dielectric polytetrafluoroethylene films are investigated using the dynamic contact module (DCM) in a frequency-specific depth-sensing indentation. The experimentally observed modulus and hardness of the ultrathin polytetrafluoroethylene films are thickness dependent: both the modulus and hardness decrease with increasing thickness sharply for the thickness being less than 500 nm. However, the modulus and hardness become independent of the thickness and equal to about 2.3 GPa and 58 MPa respectively for the thickness larger than about 500 nm. Mechanisms responsible for the observed thickness dependence of modulus and hardness are discussed.
9:30 AM F1/E4-3-4 Mechanical Characterization of Ceramic Coatings
S.-J. Wang, M.E. Graham, A. Madan, R.A. Hoffman (Northwestern University)
Direct measurements of the strength and toughness of ceramic hard coatings and of the coating/substrate interface have been a challenge in the mechanical characterization of coatings. From the perspective of coating design and lifetime prediction, a reliable technique is needed. To meet this requirement, a multistrain cracking-buckling specimen (MSCB) has been developed. The MSCB specimen developed by Wang and his colleagues, allows simultaneous measurement of the tensile cracking behavior and the compressive buckling and spalling behavior of the coating. The coating is deposited in multiple strips onto one lateral face of a substrate flexure-test beam. Plastic deformation of the substrate is induced by a four-point bending test. The strains transferred to the coating strips through the elastic/plastic deformation of the substrate cause tensile cracks to form and debonding and buckling to propagate along the interfaces. Measurements of the critical stresses for cracking, buckling, and spalling, and of the crack saturation strain allow determination of the toughness of the coating and the coating/substrate interface as well as a determination of the tensile strength of the coating and the shear strength of the interface. Examples of measurements, using this technique for some monolithic and composite nitride hard coatings on M2 steel substrates will be presented.
9:50 AM F1/E4-3-5 Determination of Mechanical Properties of Thick Coatings Produced by the Plasma Spray Method
K.-D. Bouzakis, A. Lontos, N. Vidakis, K. David (Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Dept., Aristoteles University of Thessaloniki, 54006, Greece); V. Kechagias (KELIM S.A., Thessaloniki, Greece)
The surface enhancement of conventional constructive materials that are being used in tribosystems with the aid of the plasma spray method is a well-established practice. This flexible and economically efficient procedure enables superior tribological properties for machine elements that are designed for demanding properties and is widely used for new parts as well as for repair purposes. This method as well as relative ones, such as the plasma transferred arc (PTA), are mature processes that offer monolayer or multilayer thicker coatings in a wide variation of mechanical properties. The deposition strategy and the applied conditions are responsible for the discrete and distinguished properties of this coating type. The present paper illustrates a contest of two plasma spray deposited coatings, by means of well established analytical and experimental tools that offer the ensemble of the film bulk mechanical properties and may be used to characterize quantitatively the quality of such coatings. Hereby, a relatively hard film, i.e. the two-lateral Al 1031 one and the softer Ni-5%Al one are examined with respect to their static, fatigue and creep properties, illustrating different behaviour considering also other deposition parameters, such as porosity, deposition temperature, grain size etc.
10:30 AM F1/E4-3-7 A New Method of Determining Strength and Fracture Toughness of Thin Hard Coatings
G. Jaeger, I. Endler, M. Heilmaier, A. Leonhardt (Institute for Solid State and Materials Research Dresden, Germany)
A method is presented for measuring strength and fracture toughness of thin films. For that purpose a bulk steel substrate is pre-cracked by means of fatigue loading for enabling partly or complete separation of the specimen prior to the coating process. After coating the thin film covers the crack ideally, if the former fatigue crack in the substrate is exactly closed. At the location of the fatigue crack line the thin film can be considered as free-standing. During bend loading the part of the film bridging the opening crack is strained until final breakage occurs. This rupture firstly occurs at the front face of the specimen followed by successive crack growth on both sides. The load acting solely on the film can be derived by eliminating the load influence of the substrate through calculating the difference between the two load-displacement curves corresponding to the intact and the damaged film, respectively. Specimens with completely separated substrate allow direct determination of the film properties. The film strength can be calculated from maximum load at film rupture on the front face of the specimen. Fracture toughness is calculated from the film load and the corresponding compliance change during successive crack growth on the specimen sides. The method is applied successfully to hard coatings of the system Ti-Al-C-N on S6-5-2 steel substrate. While values of film strengths have been determined for partly and completely separated substrates, fracture toughness could be obtained only in the case of partly separated specimens due to the brittleness of the investigated films. Unstable crack growth in the film on the sides caused the fully separated specimens to fail abruptly. For the investigated hard coatings TiN, TiCN and TiAlN the film strength is determined to be 490 MPa, 410MPa, 140 MPa, respectively. The fracture toughness amounts to 9,9 MPa@sr@m for TiN, 9,0 MPa@sr@m for TiCN and 4,2 MPa@sr@m for TiAlN.
10:50 AM F1/E4-3-8 Analysis of Cracking of Thin Coatings on a Metal Substrate
C. Xie, X. Li, W. Tong (Yale University)
A ductile metallic substrate with a brittle ceramic thin film coating is one of most common coating-substrate structures encountered in engineering applications. Fracture behavior of the thin film coating and the adhesion between the coating and substrate are among the major considerations in evaluating the integrity and quality of such coating-substrate systems. A reliable and consistent measurement of these properties is also critical in improving the thin film processing technologies. A nonlinear finite element analysis of the coating-substrate system subjected to uniaxial tension will be presented in this talk. In particular, a cohesive interface model is used to characterize the adhesion between the coating and the substrate. The coating is assumed to be elastic while the substrate is allowed to deform elastic-plastically. Both cracking and decohesion of the thin film are assessed. The numerical results are compared with previous empirical and analytical solutions and with experiments on an anodic oxide-aluminum system.
11:10 AM F1/E4-3-9 Qualitatively Study of Beta Silicon Carbide Residual Stress by Raman Microscopy
Y.M. Lu (Kung Shan Institute of Technology, Taiwan,R.O.C); I.C. Leu (Kung Shan Institute of Technology, Taiwan,R.O.C.)
Silicon carbide was chemically vapour deposited on isotropic pyrolytic graphite substrates by using methyltrichlorosilane as the source gas and hydrogen as the carrier gas.Raman microscopy was used to qualitatively determined the residual stress existing in CVD beta silicon carbide.The results showed that residual stress of CVD beta silicon carbide not only depends on the deposition temperature but also on the other reaction parameters,such as precursor concentration,carrier gas flow rate.Raman microscopy study results were tried to compare with the wafer central deflection method which had made previously,and explained by the SEM photographs and TEM observations.The residual stress of CVD beta silicon carbide is always compressive and can be relaxed by recovery and recrystallization processes because intrinsic stress is the predominant stress in the films.
11:30 AM F1/E4-3-10 Tribological and Mechanical Properties of a TiC/VC Modulated Multilayer Hard Coating
L. Kolodziejczyk (University of LODZ, Poland); S. Fouvry, P. Kapsa (Ecole Centrale de Lyon, CNRS, France); B. Wendler (University of LODZ, Poland)
To improve the wear resistance for cutting applications, hard coating have to combine several characteristics such as a good corrosion resistance, a low friction coefficient an high hardness and above all a strong adhesion on the substrate. Multilayer carbide coatings associating high corrosion resistance and elevate hardness can be a solution to these requirements. Co-deposition difficulties and weak adhesion between layers and substrate have until now limited such possibility. A modulated technology was recently introduced which consist to perform first metallic PVD multilayers (Ti/V) and successively generating the carbides by carbon diffusion from the high speed steel substrate. Less time and energy consuming, such a coating technology allows the optimisation of any multilayer structure whereas the carbon diffusion process strongly improves the adhesion. The purpose of this study is to analyse the microstructural aspects, the hardness and the tribological properties of such typical multilayers structure. Constant thickness coating (6 ┬Ám) consisting from 2 to 200 superimposed monolayers have been investigated under severe dry sliding conditions. Both gross slip Fretting and Pin on disk conditions have been investigated for similar pressure and sliding speed conditions. It concludes to similar tendencies which permits to optimise the number of monolayers in terms of wear resistance. It was also shown that the wear kinetics appears higher on pin on disk than on the fretting situations. To quantify such aspect an energy approach is applied which consists in relating the interfacial dissipated energy with the wear volume. It is shown that the energy wear coefficients which can be extrapolated from this approach strongly depend on the third body kinematic through the interface.