Papers

ICMCTF2004 Session F1/E3-1: Mechanical Properties and Adhesion

Tuesday, April 20, 2004 8:30 AM in Room California

Tuesday Morning

Start	Invited?	Item
8:30 AM		F1/E3-1-1 On the Evaluation of Stresses During Nanoindentation with Sharp Indenters N. Schwarzer (Technische Universitum a Chemnitz, Germany); G.M. Pharr (The Universtiy of Tennessee) Recent investigations of nanoindentation unloading curves [1] have shown that the pressure distribution of a sharp indenter can be approximated by a “effectively shaped” indenter during the unloading process. The geometrical form of the effective indenter shape is determined by the shape of the plastic hardness impression formed during indentation. During unloading this concept allows one to separate the stress field into a purely elastic one determined by the effective indenter and the residual stresses caused by the plastic flow. Applying a straight forward simple algorithm this concept can be used to evaluate the complete elastic field of the effectively shaped indenter and thus provide information about the real elastic indenterstresses during the unloading process. This model is applied to a variety of different materials and the resulting fields of intenterstresses are discussed. [1] G. M. Pharr, A. Bolshakov: J. Mater. Res., Vol. 17, No. 10, Oct 2002
8:50 AM		F1/E3-1-2 Measurement of Thin Film Elastic Constants by X-Ray Diffraction P.O. Renault (Université of Poitiers, France); D. Faurie, E.W. Lebourhis (Université of Poitiers, France); Ph. Goudeau, F.K. Badawi (Université de Poitiers, France) Polycrystalline thin solid films can exhibit physical, chemical and mechanical properties which can differ from their bulk counterparts. Thin films elaborated by physical vapour deposition can possess a microstructure with high volume proportion of surface and interface atoms, high defect density and preferential orientation. This particular microstructure may induce noticeable modifications of their elastic properties. Moreover, as the typical geometrical dimensions of thin films is very small, specialized testing techniques with a small probe size have to be developed to study mechanical properties. Our laboratory has developed in situ tensile testing in a X-Ray diffractometer. The measurement technique is similar to the one used to measure x-ray elastic constants on bulk materials and is based on the classical sin²ψ method which is sufficient for isotropic or quasi-isotropic polycrystalline materials. The preliminary works allowed to measure elastic coefficients, Poisson ratio and Young modulus, of a 150 nm thick tungsten thin film. However, deposited thin films are often textured and can be mechanically anisotropic. In this case, a specific method called crystallite group method has to be used. By means of suitable grain-interaction models, a measurement of elastic constants of textured thin gold films has been obtained.
9:10 AM		F1/E3-1-3 Observation of Fracture and Plastic Deformation during Nanoindentation and Nanoscratching Inside the Scanning Electron Microscope R. Rabe, J.-M. Breguet (Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland); P. Schwaller (Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland); J.-L. Bucaille (Swiss Federal Laboratories for Materials Testing and Research (EMPA)and, Switzerland); J. Patscheider, J. Michler (Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland) During nanoindentation the formation of cracks within the film as well as of pile-up or sink-in around the indent are known to affect significantly the precision of hardness and Young's modulus measurements. The crack pattern in brittle films and the amount of pile-up/sink-in in ductile films allow, however, also to estimate film fracture toughness and to gain knowledge on the film strain hardening behavior, respectively. The shape of the residual imprint and the extension of cracks at the surface after unloading are therefore often analyzed by atomic force microscopy or scanning electron microscopy. However, the contact area under load, the moment of crack initiation and the extension of the cracks during the loading/unloading cycle cannot be deduced unambiguously by analyzing only the residual imprint and the load-displacement data. We have built a miniaturized nanoindentation and nanoscratch device for use inside a scanning electron microscope. The indentation axis is operated at an inclined angle with respect to the SEM column that allows to look at the surface around the indent during indentation. Crack and pile-up formation can be directly observed with sub-micrometer resolution and linked to the load-displacement data. In a first part of the paper the basic design concepts, the calibration and the precision of the instrument will be discussed. In a second part two case studies - nanoscratching of polymer films and nanoindentation of hard nanocomposite coatings - will be presented that demonstrate the potential of the technique. Observations during nanoscratching of polymers films revealed the shape of the contact area and of the location of crack initiation with respect to the tip. In the case of nanoindentation of hard coatings, the formation of cracks during loading and unloading and the correlation of crack formation to discontinuities in the load-displacement curve is discussed.
9:50 AM		F1/E3-1-5 Evaluation of Young's Modulus of CVD Coatings by Different Techniques. C Bellan, J. Dhers (CEA, France) The silicon carbide and pyrolytic carbon thin films, deposited by chemical vapor deposition in a fluidized bed, are used in the manufacture of nuclear fuels of High Temperature gas cooled Reactors. The integrity of these coatings ensures the absence of gas coolant contamination. Therefore, it is important to be able to simulate the thermomechanical behaviour of these layers what involves the exact knowledge of the mechanical characteristics (fracture stress, Young modulus, Poisson's ratio...). The elastic modulus of the SiC and PyC thin films have been evaluated by various techniques, at ambient and hot temperature (until 1600°C). The studied mechanical tests are the four-point bend tests, the resonance, the nanoindentation.... From these tests, the most adapted techniques have been discussed and defined, according to the nature of the layer, the temperature and the method of examination. This work is original because few comparative data exist in the literature.
10:30 AM		F1/E3-1-7 Evidence by AFM of Plastic Damage in Thin Films Around Buckling Blisters C. Coupeau, J. Colin, M. George, J. Grilhe (Université de Poitiers, France) Thin films and coatings improve the mechanical properties of the bulk materials, such as diamond-like carbon, and are thus of great interest in a number of new technologies. However, thin films often develop high residual stresses during the deposition process which may reach a few gigapascals in compression in the case of deposition by sputtering. Such large compressive stresses may cause the nucleation and growth of individual blisters over an initially debonded patch, resulting in interesting topographical structures, such as circular blisters, straight-sided wrinkles or telephone cord structures. The delamination of compressed thin films from hard substrates has been extensively studied in the field of continuum elasticity theory. As proposed by several authors, the wrinkle shape is a solution of the equilibrium equations of thin plates called the Foppl-von Karman equations. For one-dimensional problem, this buckled-state solution is in good agreement with AFM observations of buckling patterns. Unusual shapes have however been evidenced in the case of gold films deposited by sputtering on a Si substrate. The internal stresses have been determined by XRD method to be around 0.5 GPa in compression. The buckling patterns are characterized by well-defined spherical blisters with strong bending of the film all around the delaminated structures. The observed configuration cannot be explained in the field of elasticity, i.e. the experimental profile of the buckled film is not a solution of the Fopp-Von Karman equations. Finite-element simulations and high spatial resolution XRD have been performed to have a better idea of the stress level involved in the observed phenomenon and to consequently understand this unexpected buckling structures. Finally it is shown that the formation of buckling patterns with rings, experimentally observed by AFM, can be an interesting way of investigations plastic mechanisms taking place in thin films and coatings.
10:50 AM		F1/E3-1-8 Analysis of Indentation Fracture of a Compliant Coating on a Hard Substrate A.W. Stewart (Colorado School of Mines); G.G.W Mustoe (Advanced Coatings and Surface Engineering Laboratory); B. Mishra, J.J. Moore (Colorado School of Mines) Fracture of compliant coatings on hard substrates during indentation is studied using a non-linear finite element analysis. A metal-containing diamond-like carbon (Me-DLC) film on a hard steel substrate is the system investigated. Both the coating and substrate are assumed to be elastic-perfectly plastic. In the models, the indenter is taken to be rigid or deformable. FEM of indentation without fracture in comparison to experimental load-displacement data is used to verify the model and to predict the onset of fracture within the film/substrate system. Through-thickness cracks are modeled by introducing small surface cracks in locations where the non-fracture model predicted high surface tensile stresses. These cracks are then allowed to propagate normal to the interface during the indentation process. The fracture FEM load-displacement curves are compared to experimental data where fracture was known to occur. A fracture mechanics analysis is then used in conjunction with the load-displacement data to estimate the fracture toughness of the coating.
11:10 AM		F1/E3-1-9 A Fast Method Based on Nanoindentations and on FEM Supported Calculations to Determine Coating Elastoplastic Properties K.-D. Bouzakis, N. Michailidis (Aristoteles University of Thessaloniki, Greece) An accurate procedure to determine coatings and other materials stress-strain curves, by means of the "SSCUBONI" ("Stress Strain CUrves Based On NanoIndentation") algorithm, was introduced in recent publications. This algorithm performs a finite element method based continuous simulation of the nanoindentation, considering the actual indenter tip geometry. The accuracy of this algorithm has been proved to be very high, however it is time consuming and demands experienced users. In the present paper, the fast and fully automated procedure "NAOS"(Nanoindentations based Approach Of Stress-strain curves), taking into account nanoindentation diagrams and laws deriving from the "SSCUBONI" algorithm is introduced. The laws extracted, refer to relations between the developed stresses and the applied indentation load. Moreover they describe the effect of the indentation depth on the occurring strains during the unloading stage of nanoindentations onto elastoplastic specimens with various deformation characteristics. In this way the determination of the elasticity modulus, the yield strength and the tangent moduli of plasticity of various materials through an appropriate evaluation of nanoindentation measurement results is enabled. Characteristic results concerning various coatings elastoplastic properties, obtained by means of the developed method, will be introduced.
11:30 AM		F1/E3-1-10 Toughness Measurement of Hard Coatings by Two Steps Uniaxial Tensile Test S. Zhang, D. Sun, Y. Fu, H. Du (Nanyang Technological University, Singapore) Fracture toughness of bulk materials is a topic of classical measurements, but that of thin films, especially hard and superhard coatings, is a totally different game owing to specimen dimensions (thickness) and lack of convincing test procedure. Fracture toughness is the ability of material to resist the growth of a pre-existing crack. For thin films and coatings, however, the testing usually includes crack initiation and propagation thus the result is not the same as the classical "fracture toughness". It is recommended to simply call it "toughness" to avoid the confusion. For engineering applications, it is of vital importance that the toughness of coating be evaluated with a convincing method and procedure. This paper presents a new and relatively easy method in which two steps uniaxial tensile test is performed to characterise toughness of hard coatings. In the test, the coating/substrate system is subjected to uniaxial tensile stress until the coating fractures. After the load is removed to allow complete elastic recovering, this system is subjected to a second a second loading to the same extension of the substrate. The toughness of the coating is derived from the difference between the two subsequent loading extension curves. The onset of the fracture of the coating is determined by the loss of linearity in the load-extension curve. The crack initiation and propagation patterns are examined with scanning electron microscope (SEM). As an example, toughness of hard nc-TiN/a-SiNx nanocomposite coatings are tested with satisfactory results.