ICMCTF2003 Session F1/E3-1: Mechanical Properties and Adhesion

Tuesday, April 29, 2003 8:30 AM in Room Royal Palm 4-6

Tuesday Morning

Time Period TuM Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2003 Schedule

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8:30 AM F1/E3-1-1 Plasticity Size Effects at Small Length Scales
A. Bushby (Queen Mary, University of London, United Kingdom)
Relationships between size, strain gradients and plastic relaxation of stresses have long been proposed and have as long been experimentally ambiguous and theoretically controversial. The ‘hardness size effect’ has been the archetypal example of plastic deformation at small length scales, in which the resistance to plastic deformation increases as the size of the contact in an indentation test is reduced. In conventional plasticity theory with no length scale, this should not happen. The diverse experimental reports and the many theoretical attempts to explain the phenomenon have resulted in an extensive and often controversial literature. The deformation behaviour of materials under contact loading is of prime technological importance, and a criterion for yielding is important as a design tool. As technology moves increasingly to smaller scales in thin films, nanostructured materials and micro-electro-mechanical systems, existing yield criteria fail rather badly. The ability to incorporate known internal stresses, and to vary the stress and thickness of individual layers in a semiconductor superlattice, is a very powerful tool, opening up new possibilities for investigations that cannot be achieved by varying external stresses on a specimen that is sensibly homogeneous. In this way, from the initial yield stress of single-crystal strained-layer superlattices under indentation, we demonstrate a new criterion, of which the key feature is that it is to be averaged over a finite volume.
9:10 AM F1/E3-1-3 A New Technique to Determine the Elastoplastic Properties of Thin Metallic Films Using Sharp Indenters
JL Bucaille, S. Stauss, J. Michler (EMPA, Switzerland)
In recent years, nanoindentation has established itself as one of the most convenient techniques to assess the mechanical properties of thin films, i.e. their Young's modulus and their hardness using the measurement of the force penetration (F-h) curve during the loading and unloading. However, the mechanical understanding of the indentation process itself, which involves several nonlinearities (inelastic material behaviour, large and non-homogeneous deformations), is very intricate. We have conducted a two dimensional finite element analysis using different sharp indenters. For any included indenter angle, we have constructed dimensionless functions relating the characteristic parameters of the F-h curves measured in indentation to the elastoplastic parameters, in particular the yield strength and the strain hardening coefficient. We have shown that the use of two or more indenters allows to determine the plastic properties of metals at large strains with a good accuracy. Nanoindentation tests were performed on galvanically grown nickel films. We have determined several points on the true stress-true strain diagram in the plastic regime. These values are in good agreement with the true stress-true strain curve measured in micro-tension tests. The application to metallic thin films is discussed.
9:30 AM F1/E3-1-4 Plastic Deformation of Nano-structured Metals in Thin Film Form
Q. Wei, T. Jiao, K.T. Ramesh, E. Ma (The Johns Hopkins University)
Nano-structured materials (NSMs) have drawn tremendous interest in the materials science and engineering community. One of the unique properties of nano-structured metals lies in its plastic deformation mechanisms. Apart from the controversial inverse Hall-Petch relations reported for NSMs, one of the mostly debated subjects is how plastic deformation takes place when the grain size of the materials is reduced to nano-meter range (for example, a few nano-meters). Various deformation mechanisms have been simulated, yet without sufficient experimental validation. To the best of the author's knowledge, the reported experimental studies on the plastic deformation of NSMs lack the consistency that is indispensable for the construction of constitutive laws of NSMs. The inconsistent experimental results also owe to the technical difficulty in processing high quality, volume flaw-free NSMs. Considering this situation, we have developed a technique that can be used to study the plastic deformation of NSMs in thin film form. The experimental technique is based on a modified torsional Kolsky bar technique. We have investigated the plastic deformation behavior of NSMs such as electrodeposited Ni, Cu, Al, etc. The mechanical testing experimental results are combined with post mortem microstructural analysis to reveal the underlying plastic deformation mechanisms for these NS metals.
9:50 AM F1/E3-1-5 Additional Considerations Regarding the Computation of the Composite Hardness of Coated Systems
E.S. Puchi-Cabrera (Universidad Central de Venezuela)

The present communication re-examines the basis of a previous model that has been published recently by the author [1], for the computation of the composite hardness of coated systems as a function of the relative indentation depth, applicable to monolayer films. The results that were initially derived from simple geometrical considerations are compared with those that are obtained taking into account the volumes of the plastic zones under in the film and substrate. Such comparisons are conducted on coated systems which include electroless Ni-P (EN) on a 6063 aluminum alloy and EN and an under stoichiometric TiNx deposit on a 7075-T6 aluminum alloy. Furthermore, the consideration of the validity of a linear law of mixtures is also re-examined and the results obtained for the coated systems mentioned above are compared with those derived by assuming that the composite hardness could also be given by a harmonic law of mixtures, rather than a simple linear law. Finally, an extension of the models examined is proposed in order to compute the absolute hardness of the different compounds that comprise multi-layer films, such as, TiAlN (multilayer) and TiAlN+WC/C deposited on different steel substrates.

[1] E. S. Puchi-Cabrera, Surf. Coat. Technol., 160 (2002) 177-186.

10:10 AM F1/E3-1-6 Nanoindentation with Spherical Indenters: Finite Element Studies of Deformation in the Elastic=Plastic Transition Regime*
G.M. Pharr, Y.J. Park (University of Tennessee and Oak Ridge National Laboratory)

Recent experimental studies have shown that nanoindentation load-displacement data obtained with spherical indenters are influenced by stress in the specimen in a manner that may be useful for residual stress measurement. However, the influences are significant only in the elastic-plastic transition, a deformation regime that has largely been ignored in previous theoretical and experimental treatments. In this study, finite element simulation techniques were used to examine spherical indentation in the elastic-plastic transition and characterize the development of the constraint factor (the ratio of the hardness to the flow stress) with penetration depth for a wide variety of materials with different yield strengths and work hardening behaviors. The simulations show that the initial portion of the transition at small penetration depths is essentially independent of the work hardening behavior, apparently because deformation in the subsurface plastic zone is constrained by surrounding elastic material. This is a useful result in that it implies that some material properties may be measured from the indentation load-displacement data without a knowledge of the work hardening behavior. At a depth approximately equal to that at which the plastic zone that escapes to the free surface and plasticity is no longer elastically constrained, the constraint factor diverges and the indentation behavior depends on the work hardening characteristics of the material. Influences of friction between the indenter and specimen were also examined. Implications for mechanical property measurement by nanoindentation with spherical indenters are discussed.

* Research at the Oak Ridge National Laboratory SHaRE Collaborative Research Center was sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.

10:50 AM F1/E3-1-8 A Modified Blister Test to Study the Adhesion of Ceramic Coatings Based on Local Helium Ion Implantation
R. Escobar Galindo, A. van Veen (Delft University of Technology, The Netherlands); J.H. Evans (United Kingdom); H. Schut (Delft University of Technology, The Netherlands); J.Th.M. De Hosson (University of Groningen, The Netherlands)

A modified blister test has been developed based on helium ion implantation into the metal substrate prior to the ceramic coating deposition. After a post-deposition annealing blisters are formed by agglomeration of the implanted gas at the ceramic-metal interface. In this way implantation, followed by annealing, can be used to increase the pressure in the blister in a controlled way and induce decohesion. When a microsieve with a periodical pattern of circular pores is used during the implantation, the control over the initial blister size is assured. Two different microsieves were employed in this work with pore diameter of 1.5 and 3 microm respectively. The distance between the center of neighbor pores is twice the pore diameter. Decohesion is non-destructively monitored by means of Positron Beam Analysis (PBA) and Environmental Scanning Electron Microscopy (ESEM). A relation between blistering parameters (for example radius, height) and the adhesion parameters such as the work of adhesion can be obtained.

In this work we present the first results of this test applied to 500 nm thick Chromium Nitride (CrN) films deposited by CVD on polycrystalline copper substrates. The samples were implanted with 30 keV He+ ions up to doses of of 3, 4 and 5 x 1016 cm-2 and annealed afterwards in vacuum at 973 K for 30 minutes. Experiments on uncoated copper samples will be shown in order to explain the mechanism of helium bubble growth and swelling that causes the creation of blister in the copper surface.

11:10 AM F1/E3-1-9 Effect of Surface Roughness of D2 Tool Steels on Adhesion of TiN Coatings Deposited by PVD
C.E. Pinedo (University of Mogi das Cruzes, Brazil); A.R. Franco Jr, AP Tschiptschin (University of Sáo Paulo, Brazil); P.K. Venkovsky (Brasimet Co., Brazil)
On coating, not only the surface metallurgical characteristics are important, but also surface finishing is an important parameter for the adhesion control. In this paper, the influence of substrate surface finishing both on morphology and adhesion of TiN layers was studied. Substrate material used for this investigation was a cold work tool steel AISI D2. Before coatings the samples were quenched and tempered in vacuum chamber for a hardness range of 57-60 HRC. The heat treatment cycle used austenitizing at 1080°C and oil cooling, followed by double tempering at 540°C, two hours each. PVD Ion Plating unit assisted by an electron beam source was used for deposition of the TiN coatings and the deposition parameters were: substrate temperature: 450 - 500°C, deposition time: 70 min, nitrogen pressure: 18 x 10-4 mbar, DCP arc current: 200 A, and work piece negative bias of around 50 V. Scanning electron microscopy (SEM) and X-ray diffraction were used for characterizing morphology of the coatings. Scratch test and microabrasion calowear tests were performed for the surface tribological characteristics evaluation. It was verified that TiN layers deposited on high roughness substrates presenting low adhesion, due to the porous formation on hills and columnar growth of TiN in the valleys of such a substrates. The best results were obtained for the coatings with average roughness (Ra) around 0,25 mm.
11:30 AM F1/E3-1-10 Adhesion of Thermally Sprayed Alumina-Titania Wear Resistant Coatings Analyzed and Optimized with Neural Networks
S. Guessasma, G. Montavon, C. Coddet (LERMPS-UTBM, France)

Thermal spraying is an attractive technique of coating manufacturing offering a wide choice of materials and processes. This technique allows many problems of wear, corrosion and thermal degradation to be resolved. Alloy powder can be processed by direct injection in a plasma medium where the powder particles are heated, fused and propelled towards the material to be coated. Particles quench and flatten rapidly forming a stacking of lamella.

Alumina-13%wt titania wear resistant coatings were obtained with this process under atmospheric conditions. In this survey, coating adhesion measured locally with interfacial indentation test was studied as a function of the process operating conditions. In order to recognize the most influencing factors on adhesion and the corresponding evolutions with few experimental sets a novel technique of optimization was used. It is based on the Artificial Neural Networks (ANN) principal considering training and test procedures permitting a prediction of the property dependence on experimental conditions.

This study points out the following conclusions: Adhesion was proved to be sensitive to coating thickness, porosity and test load. It depended largely on most parameters modifying the temperature gradient between the substrate and the coating in addition to the in-flight particles thermal status. These effects were quantitatively demonstrated and predicted with optimized neural network structures.

11:50 AM F1/E3-1-11 Magnetron Sputtered TiN/SiNx Nanocomposite Coatings with High Harness and High Toughness
S. Zhang, D. Sun, Y.Q. Fu, H.J. Du (Nanyang Technological University, Singapore)
TiN/SiNx nanocomposite coatings with thickness from 500 nm to 1000 nm have been deposited by magnetron sputtering by co-sputtering with pure Ti target and Si3N4 target in the Ar/N2 atmosphere. The as-deposited coating composition is characterized by X-ray Photo Spectroscopy (XPS), the surface topography is studied by Atomic Force Microscopy (AFM), and microstructure by Transmission Electron Microscopy (TEM) and X-ray Diffractometry (XRD). The results show that the Si content has an important effect on the microstructure. Adjustment in Si content creates TiN peak shift. The hardness and toughness of the as-deposited coatings also vary with Si addition.
Time Period TuM Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2003 Schedule