Papers

ICMCTF2005 Session E3/F1: Mechanical Properties and Adhesion

Tuesday, May 3, 2005 8:30 AM in Room California

Tuesday Morning

Start	Invited?	Item
8:30 AM		E3/F1-1 Stress Induced Delamination Methods for the Study of Adhesion of Pt Thin Films to Si Substrates W.D. Nix (Standord University) Adhesion of Pt films to Si substrates with a native oxide has been investigated using two methods of quantitative adhesion characterization. The nanoindentation induced delamination method uses an impression to store compressive strain in an overlayer film in order to induce delamination at the Pt/SiO₂ interface. Likewise, the telephone cord delamination method involves sputtering on a thick compressively stressed overlayer onto the Pt/SiO₂ films to induce telephone cord delamination patterns in the Pt film. Crack energy release rates and interface toughnesses can be calculated from the dimensions of the circular blisters or the telephone cords using currently available models. Focused ion beam (FIB) observations show that the nanoindentation method is difficult to implement due to extensive crack formation in the substrate beneath the indentation impression, causing interface toughnesses from this test to be gross overestimates. The telephone cord measurements, by comparison, give realistic interface toughnesses, allowing us to show that decreasing the argon pressure during Pt sputtering significantly increases the adhesion of the films to the substrate. Four-point bending experiments on the same interfaces give results which compare favorably with the results using the telephone cord delamination method. The mechanism for the increased toughness with decreased argon sputtering pressure has also been investigated. Results from XPS analyses show that a more chemically diffuse interface is produced at low argon sputtering pressures, likely caused by the highly energetic neutral argon atoms striking the growing surface. This may be responsible for the increased toughness at low argon pressures.
9:10 AM		E3/F1-3 The use of Raman Spectroscopy to Identify Strain and Strain Relaxation in Strained Si/SiGe Structures. P. Dobrosz, S.J. Bull (University of Newcastle, United Kingdom) SiGe alloy films have attracted interest in recent years because of their importance for high-speed microelectronic devices. In order to achieve such speed increases devices are made from silicon with a tensile strain introduced by epitaxial deposition onto a SiGe alloy with a larger lattice parameter. In this study strained silicon layers grown on a SiGe virtual substrate layers have been investigated by Raman spectroscopy to determine their strain and strain relaxation. The Raman spectra of epitaxial Si1-xGex alloys (x = 0.1, 0.15, 0.2, 0.25 and 0.3) covered with a thin (~10nm) Si cap layers were analysed with the aim of accurately detecting and separating the Si cap peak to determine its position which is related to macrostrain. However, the signal obtained from the Si cap layer is very weak and shows considerable overlap with the Si in SiGe peak. In order to find the Si cap peak position two methods were performed. The first method is to obtain a difference spectrum before and after selectively etching the Si cap and the second method involves peak-fitting and deconvolution by software. Both of these methods show very good agreement, however the peak fitting technique is a faster and non-destructive technique and is suitable for analyzing devices at various stages of processing. The generation of strain as a function of germanium content of the virtual substrate and the relaxation of this strain at defects such as dislocations will be discussed.
9:30 AM		E3/F1-4 Response to Loading and Stiffness of Coated Substrates Indented by Spheres I. Pane, E. Blank (Ecole Polytechnique Federal de Lausanne, Switzerland) Simulations of spherical indentation of a substrate coated with a protective layer have been performed in the elastic deformation range. They provide key responses that can be used to examine the load resistance of a coated substrate. The key responses result in diagrams indicating whether yielding or fracture occurs first. In this kind of diagram the ratio of coating fracture strength to substrate yield is related to the contact radius in indentation. As it is difficult to determine the contact radius experimentally, the latter has been found numerically and has also been approximated analytically. The analytical approximation plays an important role as it identifies the general loading condition for various sphere radii. This approximation can be extended to predicting the unloading curve which allows the elastic properties of a coated substrate to be determined.
9:50 AM		E3/F1-6 Material Properties Extracted from Indentation Experiments by Inverse Finite Element Calculations S. Stauss (Swiss Federal Institute of Technology Lausanne, EPFLd, Switzerland); J. Michler (Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland); E. Blank (Ecole Polytechnique Federal de Lausanne, Switzerland) Instrumented indentation testing has established itself as a versatile technique to assess mechanical properties of small structures, devices and thin films. In this paper the conditions for back-calculation of the stress-strain behaviour of thin films from the load displacement curve have been investigated. Inverse finite element calculations controlled by an automated fitting algorithm are used to extract relevant material parameters such as elastic modulus, yield stress and hardening coefficient. Contrary to earlier papers dealing with yield and strain hardening based on semi-empirical analysis of indentation curves, the present approach, which does not rely on empirical parameters, can also be applied to layered structures. It turns out that experimental load-penetration curves can be numerically reproduced without the correct set of material parameters of the constitutive model, i.e. there is no unambiguous relationship between load-displacement data and the underlying stress-strain curve. In order to obtain unambiguous results, supplementary information must be taken into account for the fitting procedure. This additional information can be obtained by a more differentiated interpretation of the load-penetration curve or it may be taken from independent measurements, such as post-indentation surface topography measurements or preliminary estimations of material parameters based on semi-empirical indentation techniques. The approach presented here has been validated on bulk Cu and Al alloys and coatings.
10:10 AM		E3/F1-7 Atomic Level Modeling of the Interface Bonding Between the Aluminum and Silicon Surfaces V.M. Stoilov, L. Inci, A.M. Alpas (University of Windsor, Canada) The tribological properties and mechanical behavior of near surface regions of materials with second phase precipitates, such as AlSi alloys, remains controversial. Although it is commonly accepted that the intrinsic near-surface deformation behavior of these materials arises from the interplay between the polycrystalline matrix(Al) and second phase inclusions(Si), little is known about the specific deformation and debonding mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model Al-Si near-surface microstructures. Modified embedded atom method (MEAM) potential and the Verlet integration algorithm have been used. The interface has been modeled in two-dimensional structure, and MD simulations were carried out using about 100000 atoms. The evolution of the subsurface microstructure of aluminum-silicon system has been investigated to develop a constitutive relationship (stress-strain, traction-displacement) for the incoherent aluminum-silicon interfaces. Furthermore, the debonding, fracture of the Si inclusions, the influence of the local temperature and the deformation rate for these systems have been studied.
10:30 AM		E3/F1-8 Developments in Nano-Impact Testing: A Comparison of Static and Dynamic Measurements of Hardness and Fracture Toughness N.M. Jennett, G. Aldrich-Smith, J.W. Nunn (National Physical Laboratory, United Kingdom) Nanoindentation is now commonly used to make quasi-static measurements of the modulus and hardness of small volumes of materials. Indeed it is one of the few methods available to measure elastic and plastic properties at small length and volume scales. The NanoTest^TM (MicroMaterials Ltd, Wrexham, UK) has a pendulum-based design, which makes it easily possible to use the instrument as a nano-impact tester. The high resolution depth sensing capability of instrumented (nano)indentation testing enables direct quantification of the velocities of impact and rebound and, therefore, the energy absorbed by the sample during an impact. Materials with a wide range of properties have been tested (including ceramics, metals and polymers) over a range of impact velocities. Damage has been inspected by various optical, electron and scanned probe microscopies in order to relate the energy absorbed to the damage created. This paper discusses the practical aspects of deriving measurements of dynamic hardness and fracture toughness from nano-impact testing. Calibration of the instrument for effective mass and damping is described and estimates of dynamic compliance (instrumental energy losses) made. The study compares the dynamic results with those obtained from quasi-static methods such as indentation and Palmqvist cracking.
10:50 AM		E3/F1-9 Impact Test on PVD-Coatings and on Various Substrates at Elevated Temperatures K.-D. Bouzakis (Aristoteles University of Thessaloniki, Greece); E. Lili, A. Sampris, N. Michailidis, G. Maliaris, S. Kompogiannis (Laboratory for Machine Tools and Manufacturing Engineering, Greece) Impact tests are commonly performed at room temperatures, excluding substrate's and coating's material properties modifications effects, occurring at higher temperatures, on the film failure. Since coatings applied to cutting tools experience high stresses at elevated operational temperatures, impact test investigations on coated cutting inserts at increased temperatures, up to 400°C, were performed. By means of a developed heating system, implemented in an electromagnetic impact tester, the specimens are heated up to a desired temperature, remaining stable during the test, through a PID controller. In order to avoid an overheating of the impact tester's structural parts and measuring instruments, an air-cooling system was applied. Substrate and coating mechanical properties alterations, occurring during the impact test at elevated temperatures, leading to material's deformation behavior modifications were considered, in order to elucidate the related coating failure mechanisms. The conducted investigations revealed coating fractures and removals at lower impact loads, in comparison to the corresponding ones at room temperature. These effects induced by substrate's material plastic flow activation during the successive impacts, were elucidated by means of a FEM supported simulation of the impact test, considering the modified material properties at increased temperatures.
11:10 AM		E3/F1-10 Thin Film Stress Measurement by Instrumented Optical Fibre Displacement Sensor S. Chowdhury, M.T. Laugier (University of Limerick, Ireland) This work describes a simple stress measurement instrument based on the bending beam method together with a sensitive non-contact fibre optical displacement sensor. The fibre optical displacement sensor is interfaced to a computer and a Labview programme enables film stress to be determined from changes in the radius of curvature of the film-substrate system in real time. The stress measurement instrument was tested for two different kinds of thin films, hard amorphous carbon nitride (CN) and soft copper (Cu) films on silicon substrate deposited by RF magnetron sputtering. Residual stress was found compressive (-0.83 GPa to -0.44 GPa) for CN films and tensile (121 MPa) for Cu films. Stress developed in CN thin films deposited at substrate temperatures in the range 50-550oC was also examined and it was found that stress in CN films decreased from 0.83 GPa to 0.44 GPa with the increase of substrate temperature.
11:30 AM		E3/F1-11 Processing of Chromium Tungsten Nitride Hard Coatings for Glass Molding C.H. Lin, J.G. Duh (National Tsing Hua University, Taiwan) The CrWN coatings were fabricated on WC and stainless steel substrates, which were frequently used as glass molding die materials, by ion beam assisted deposition (IBAD). Hardness and phase formation phenomenon of the CrWN coating after annealing at elevated temperatures were analyzed by nanoindentation and X-ray diffraction, respectively, to investigate thermal stability of the coatings. In the CrWN coating, hardness of the deposit was raised to 25 GPa with 4 at.% of W doping. The CrWN coatings exhibited an increase of 25 % in hardness as compared to CrN. Superior mechanical properties after annealing at temperatures for glass molding were revealed. To estimate the feasibility of applying the CrWN coatings in the glass molding process, surface roughness of molded glass was also measured. After hot pressing against the CrWN/WC assembly, roughness of the glass was controlled within 10 nm, which was satisfactory for surface condition requirements with practical optical accuracy.
11:50 AM		E3/F1-12 Deposition of AlN on Substrates by Reactive Magnetron Sputteing U. Figueroa (ITESM-CEM, Mexico); O. Salas, J. Oseguera (ITESM, Mexico) AlN films were deposited on Al based substrates by reactive magnetron sputtering to investigate their potential for tribological applications. The main processing variables were working pressure, and adhesion layer. The resulting films were analyzed by Scanning Electron Microscopy + Energy Dispersive x-ray Microanalysis, x-ray diffractometry, microhardness and scratch and wear testing. The reactive atmosphere was studied by Optical Emission Spectroscopy. Low working pressures and Ti and Al adhesion layers showed the best results in terms of tribological properties. The effect of residual stresses on adhesion and low hardness of the substrate were identified as some of the critical problems in these Al-AlN deposition systems for tribological applications.