ICMCTF1998 Session F1/E4: Mechanical Characterization - Mechanical Properties and Adhesion

Tuesday, April 28, 1998 8:50 AM in Room San Diego

Tuesday Morning

Start	Invited?	Item
8:50 AM		F1/E4-2 Comparison of Experimental and Calculated Depth Sensing Indentation Curves for Samples with a Thin Coating - Summary of VAMAS Results S.R.J. Saunders (National Physics Laboratory, United Kingdom); D.T. Smith (National Institute of Standards & Technology); N.J. Jennett (National Physical Laboratory, United Kingdom); T. Yoshida (University of Tokyo, Japan) The results of an international project on the measurement of hardness and Young's modulus of thin coatings using depth sensing indentation are summarized. The work focused on the thickness range where during indentation a significant interaction with the substrate occurred, so that in order to obtain a uique value for the coating properties it is necessary to use either analytical or numerical modelling methods. The project was organised within the VAMAS programme aimed at promoting improved test methods for later incorporation into international standards. Two coating systems were examined (alumina deposited onto aluminium and aluminium on sapphire) with the thickness range from 0.1 to 5.0 microns. A number of different test parameters were investigated experimentally including maximum load, loading rate, indenter geometry, hold time at maximum load and repeat loading, as well as, of course, coating thickness and type. The project was designed to include a wide range of different instrument types, from AFM-based instruments through to the micro indentation hardness instruments. Where possible a single test parameter was investigated by at least three different instrument types. The modelling work was carried out using state-of-the-art models, even though in many cases a complete description of the elastic/plastic deformation was not possible. The experimental results showed a wide scatter, but in general, where it could be demonstrated that calibration procedures were adequate, reasonable agreement was found considering the wide range of instrument types investigated. Modelling results were also varied but insights into indentation have been gained from the current study and it has been possible to produce draft procedures for deconvoluting the experimental data to derive the coating properties. A useful outcome of the work, to date, is that it is possible to determine experimentally the point at which plastic deformation of the substrate begins.
9:10 AM	Invited	F1/E4-3 Indentation of Graded Materials: Theory , Experiments and Applications A.E. Giannakopoulos (MIT) In this work, we consider micro- and macro-indentation of materials whose elastic and plastic properties are graded monotonically as a function of position. Firstly, theoretical and computational results are shown for the normal indentation, by a sphere or sharp indenter, of a substrate whose Young's modulus increases or decreases smoothly beneath the indented surface. Simple closed form solutions are presented for the determination of continuous force-depth curves for graded materials subjected to normal indentation. The theoretical predictions are shown to be fully consistent with experimental observations in materials with controlled elastic gradients. It is also demonstrated both theoretically and experimentally that controlled elastic gradients of surface coatings could be used to fully suppress indentation cracking. We then direct attention to the effects of gradients in plastic properties on the indentation response. A hardness index for graded materials is then proposed whereby the indentation response of materials with spatially varying microstructure and properties could be uniquely characterized. A number of practical applications of the developments of this work are also addressed.
9:50 AM		F1/E4-5 Crack Formation Mechanisms During Micro and Macro Indentation of Coatings - Observations and Numerical Simulations N.B. Thomsen (Danfoss A/S, Denmark); A.C. Fischer-Cripps (University of Technology, Sydney, Australia); M.V. Swain (CSIRO, Australia) The continuing increase in the use of advanced coating systems in engineering applications demands a much better understanding of the failure mechanisms and require improved experimental characterisation techniques. For quality control purposes simple, quick and reliable characterisation methods for mechanical properties are highly desirable. Today the most widely used method for extracting information about mechanical properties is indentation. In the present study crack formation is investigated on both micro and macro scale using spherical indenter tips. Depth sensing indentation by the UMIS-system is used for micro scale and Rockwell indentation on the macro scale. The predominant driving force for coating failure and crack formation during indentation is plastic deformation of the underlying substrate. The aim is to relate the mechanisms creating both delamination and cohesive cracking on both scales with fracture mechanical models in order to quantitatively determine coating fracture properties. During the study a number of different coating systems were investigated including DLC's and PA-CVD/TiN. The effects of coating thickness and processing parameters as well as substrate hardness were included. A non-linear elastic-plastic finite element model of the coating systems loaded with a spherical indenter is applied to simulated stress and displacement distributions of both indenter and specimen material which cause the onset of cracking. A range of crack formation mechanisms and corresponding stress and displacement distributions are presented. These results are related to fracture mechanical approaches used to predict intrinsic fracture properties of the coatings systems. Although the study is in its initial phase very interesting and promising results have been obtained.
10:10 AM		F1/E4-6 Strength of HVOF Coating-substrate Interfaces C.M. Nygards, K. Ravi-Chandar, K.W. White (University of Houston) A spherical indentation method has been used to analyze thermally sprayed HVOF-coatings on steel substrates. The coatings crack when the indenter is pushed down in the coating, and the substrate deforms plastically. Upon unloading cone cracks form at the surface, and upward extending cracks at the coating/substrate interface. On unloading delamination occurs between the coating and the substrate. The microsstructure of the coating, and the thresholds for cracking and delamination has been used to form an FEM-model. The model is used to calculate the strength of the interface.
10:30 AM		F1/E4-7 Measurement of the Micro Mechanical Properties of Sol-Gel Tio₂ Films A.K. Jamting (CSIRO, Australia); J.M. Bell (Queensland University of Technology, Australia); A.C. Fischer-Cripps (University of Technology, Australia); M.V. Swain (CSIRO, Australia) The interest in the use of sol-gel derived films in tribological applications during recent years has increased the interest in the mechanical properties of these films. In this study we have utilised a number of different techniques in order to understand the behaviour of sol-gel derived TiO₂ films during mechanical testing. The commonly used ultra micro indentation technique, which have been used to measure elastic modulus and hardness, has also been complemented by a bi-axial bending technique and residual stress measurements to fully determine the mechanical properties of the thin films. The results from both indentation and bending tests have been used with a finite element analysis technique to determine the stress distribution, and determine modes of failure of the films. The films have subsequently been analysed using RBS, XRD, SEM and AFM in order to determine the structure of the films and to look for evidence of fracture to correlate with the modelling results. A comparison of the behaviour of the sol-gel TiO₂ films to PVD deposited films has also been carried out.
10:50 AM		F1/E4-8 Load Carrying Capacity of Aluminium O Wanstrand, M. Larsson, A. Kassman Rudolphi, S. Hogmark (Uppsala University, Sweden) By the introduction of a hard coating it is possible to increase the load carrying capacity of aluminium (Al). To avoid plastic deformation of the soft Al it is necessary to have a coating of a sufficient thickness. The thickness of the coating is determined by the contact conditions e.g. load, contact radius, coating hardness and modulus. In this investigation the plastic deformation of thin foil coated Al was studied. The composites were indented by a hard sphere. The depth of the residual impression on the Al substrate was determined using surface profilometry. Nickel, molybdenum and tungsten foils were used and the indentor was a cemented carbide ball. Influence of foil thickness, ball size and indentation load on the deformations were investigated.
11:10 AM		F1/E4-9 Comparative Scratch Testing of Hard and Soft Composite Coatings K. Dyrda, M. Sayer (Queen's University, Canada); D. Barrow (Datec Corporation, Canada) Novel coatings have been created using a sol gel composite technique in which powders are added to a sol gel host. The coatings are applied by spray or dip coating and fired at temperatures up to 650C. Some applications are competitive with coatings such as TiN on tool steel. This paper presents a comparative assessment of coating adhesion and mechanical properties for hard TiN and softer composite coatings using scratch testing. The coatings are processed to simulate service conditions such as high temperature cycles in various gaseous atmospheres. Scratch testing is performed with variable test parameters including loading rate, scratch speed and indenter wear. The conditions for optimum measurements are discussed.
11:30 AM		F1/E4-10 Mechanical Properties of W and W-C Thin Films: Young's Modulus, Toughness and Adhesion E. Harry, A. Rouzaud, P. Juliet (French Atomic Energy Commission, France); M. Ignat (ENSEEG, France) This paper aims at investigating in depth basic mechanical propeties of W and W© single layer coatings, in order to develop the optimum W/W© multilayer stucture for mechanical applications. Thin films are characterized by three point bending test to estimate their Young modulus when the experiment is performed in elastic domain of materials. This device is also equiped with an acoustic emission detector allowing to detect film cracking and to determine its fracture resistance using Evan’s model. The adhesion of film on substrate is analysed with a microtensile device adapted to a SEM. A theoretical investigation is developed in terms of stress release in craked layer using Mezin’s approach. Elastic modulus of W and W© thin films are found to be very close to the W bulk value. The measured mode I fracture resistances are reasonnably good and ranged from 1 to 2.5 MPa.m^½ for W layers and from 0.5 to 0.7 MPa.m^½ for W© coatings. W and W© layers behaviours are significantly different and might be correlated to films morphology. Adhesion of these coatings is also investigated by shear-lag model with Mezin’s development. In this development, a way to characterize adhesion is to consider the aspect ratio of the cracked coating, i.e the ratio of the crack spacing to the film thickness. A very good agreement is found between the theoretical computed ratio and the experimental one, pointing out the excellent adhesion of the film to its substrate.
11:50 AM		F1/E4-11 Micro/nanomechanical Characterization of Ceramic Films for Microdevices X. Li, B. Bhushan (The Ohio State University) Amorphous carbon coating are employed to protect magnetic coatings in thin-film magnetic recording disks and heads. These coatings have to be thin (5-20 nm) in order to minimize loss of read back signal amplitude. Nanomechanical characterization of ultra thin amorphous carbon coatings of 5, 10, 20 nm thickness, deposited by ion beam deposition technique on silicon was studied. Hardness, elastic modulus and nanoscratch resistance of these coatings were measured using a nanoindenter. The scratch damage was observed by atomic force microscopy. The effect of interlayer on the mechanical properties of these coatings was studied. The protection mechanisms of these coatings to the silicon substrate are discussed