ICMCTF1999 Session F1/E4-2: Mechanical Properties and Adhesion

Thursday, April 15, 1999 1:30 PM in Room San Diego

Thursday Afternoon

Start	Invited?	Item
1:30 PM		F1/E4-2-1 Stress and Oxidation in CuNi Thin Films W. Brueckner, S. Baunack (Institute of Solid State and Materials Research Dresden, Germany) CuNi films are interesting for resistive and thermoelectrical applications. A heat treatment (e.g. under Ar atmosphere) is used for microstructural stabilization. During annealing (i) a surfacial oxidation occurs and (ii) a high tensile film stress develops being critical for stability and reliability. In this paper, the correlation between stress evolution and oxidation is considered. The stress evolution was investigated on a 400 nm thick Cu_0.57Ni_0.42Mn_0.01 film on oxidized silicon during a thermal cycle to 550 °C using a laser-optical substrate-curvature technique. Cycle-stop-prepared samples were used to clarify the correlation between stress development, microstructural evolution, and oxidation. The microstructure was studied by transmission electron microscopy. Oxidation was investigated by Auger electron spectroscopy and resistance measurements. The grain morphology is columnar with a medium diameter of 20 nm and is nearly unaffected by annealing. The degree of oxidation depends on the ambient atmosphere. Whereas in Ar the oxide layer grows with increasing temperature, only a thin oxide is formed in a N₂/H₂ atmosphere. By comparing the results, the oxidation-induced stress contribution under Ar could be separated. In the stress curve, the striking feature associated with oxidation is a tensile stress component of about 500 MPa around 350 °C. At this temperature, a surfacial NiO layer of some nanometers thickness is formed. The stress component is semiquantitatively explained by grain-boundary diffusion of Ni to the surface and the induced contraction within CuNi due to Ni lost. With progressive oxidation (1) a CuO-NiO double layer forms, growing without essential contributions of diffusion within CuNi and, therefore, without oxidation-induced stress development and (2) the tensile stress relaxes presumably by plastic deformation (diffusional flow, mechanical twinning) as already observed in unoxidized CuNi films.
1:50 PM		F1/E4-2-2 Internal Stresses, Young's Modulus and Practical Adhesion of Organic Coatings Applied onto 5754 Aluminum Alloy A.A. Roche (INSA de Lyon, FRANCE); J. Bouchet, P. Hamelin (IUT A Génie Civil de Lyon I, FRANCE) It is well known that when epoxy resins are applied onto metallic substrates and cured, internal stresses are developed within the organic layer. These stresses reduce the adhesive strength and occasionally induce cracks in the coated materials. Mechanical properties (residual stresses, Young's modulus and practical adhesion) of organic layers (DGEBA DER 332 epoxy resin and IPD hardener) with different thicknesses were measured. The coatings were deposited on aluminum alloy (5754) after different treatments (degreasing, chemical etching and anodizing). Either bi-layer (with a perfect interface with an null thickness) or tri-layer models (with an real interphase between the substrate and the organic coating) were considered to evaluate residual stresses and Young's modulus of the interphase and the remaining coating having the bulk properties. These different models where determined by using one and two dimensional approaches based on the plate and curve beam theories. Thin, thick coating, interphase and bulk properties (Tg, conversion and interphase thickness) were determined by using differential thermal analysis (DSC) and FTIR spectroscopy. Young's modulus, curvature radii of coated samples and practical adhesion were determined by 3 point flexure tests. The locus of the failure initiation was determined by scanning electron microscopy (SEM) and electron microprobe (EMPA). Results show the limitation domain of the bi-layer model when the coating/substrate interphase is formed. For the same coating formulation and the same curing conditions, residual stresses, interphase Young modulus, practical adhesion, conversion and Tg of coatings depend on the nature of the substrate treatment and on the coating thickness. A gradient of residual stresses and modulus were observed according to the different surface treatment. When adhesive failure was observed, the adherence increases when internal stresses within the interphase decrease.
2:10 PM		F1/E4-2-3 Splitting Hard Films off the Substrate V. Cherepanov, V. Necula, L. Druker (Chessen Group Inc., Canada) Ultra hard films deposited on steel substrates are usually compressed. Therefore, they tend to swell and surge. If this happens, the coating breaks. We study the conditions for the splitting of a compressed thin film off a substrate. The force that tears down the film from the substrate is due to the compression (negative tension) in the film. The origin of the compression is twofold: (i) the deposited films are formed compressed and (ii) the thermal expansion causes additional compression when the films cool down. The force that affixes the film to the substrate is the surface tension. We found the critical tension for the splitting instability. The compression stronger than critical leads to the film rising. We studied the following problem: the substrate is semi-infinite, and it fills the space with z<0. A solid film with the thickness h and with the surface tension coefficient between the film and substrate γ, is affixed to the substrate. We found that the film splits off when absolute value of the negative the in-plane tension exceeds 6γ/h. Note that the tension depends on temperature because of the difference in thermal expansion of the film and substrate. Actually, hard coatings consist of many layers of materials with different elastic properties. We found a condition for the splitting instability of a layered structure on a substrate surface, which is a generalization of the above criterion. In this case, splitting can occur between any pair of layers or between the substrate and the first layer. We developed software, which computes the critical tension for a layered coating. This allowed us to optimize the coating composition with respect to the applied tension.
2:30 PM		F1/E4-2-4 Evolution of the Microstructure and Mechanical Properties of TiNx Thin Films Through the Thickness O.R. Shojaei (Idem, Switzerland); A. Karimi, J.L. Martin (Ecole Polytechnique Fédérale de Lausanne, Switzerland) Magnetron sputtered TiN_x thin films exhibit a heterogeneous microstructure through the film thickness. The morphology consists of fine cristallites close to the interface with growing column sizes towards the surface. The microstructural changes can modify physical and mechanical properties of these coatings. To study such modifications, various TiN_x films deposited onto Si substrates containing a layer of SiN were subjected to the bulge test and depth sensing nanoindentation measurements. Both techniques revealed decreasing values of elastic modulus and hardness with increasing thickness. Scanning Electron Microscopy and Atomic Force Microscopy indicated a more compact layer with fine cristallites structures and hemispherical topography in the lower section (30-50nm) of the film followed by a columnar structure with larger grains and defects growing towards the surface resulting in a rough faceted cubic structure. X-ray diffraction analysis and electrical resistivity measurements confirmed the increasing grain size with higher thicknesses. The related stress, measured both by the sin²(psi) method performed on TiN_x free standing membranes and the Bulge test shows the same trend through the thickness. In this paper mechanical tests analysis are described and the results achieved by diffrent methods are compared. The relationship between mechanical properties and grain boudary effect are discussed together with the comparison of Bulge test and nanoindentation measurements in such non isotropic structures.
2:50 PM		F1/E4-2-5 Substrate Straining Tests of TiN Coatings on Tool Steel Substrates T.W. Coyle, Z. Zheng (University of Toronto, Canada) TiN coatings on high speed tool steel substrates were tested in four-point flexure to characterize the coating strength and adherence. Two thicknesses of the PVD coatings were arc deposited onto the AISI M2 steel substrates, the surfaces of which were either ground (220 grit) or polished (1µm diamond). The straining was accomplished using a small four point bend fixture which allowed the surface to be examined under load by SEM. The onset of cracking in the coating and the crack density are reported as a function strain for the four combinations of coating thickness and substrate surface roughness.
3:10 PM		F1/E4-2-6 Critical Loads and Frictional Force Measurements in the Industrial Scratch Testing of TiN on M2 tool Steel. K.M. Dyrda, M. Sayer (Queen's University, Canada) A robust scratch tester has been developed to evaluate PVD TiN coatings on M2 tool steel by plots of the effective coefficient of friction for indenter movement, µ_eff = F_horizontal/F_vertical, as a function of the vertical force applied to a Rockwell C diamond indenter. A clear transition between different slopes in such a graph is correlated with the critical load for coating failure as identified by microscopic observations. The variation in µ_eff as a function of vertical load is modeled in terms of the ploughing force and residual stress within the coating, and the shearing force due to adhesion between the coating and the substrate. Results for a range of coatings on different substrates will also be presented to demonstrate the versatility of the unit for industrial testing.
3:30 PM		F1/E4-2-7 Comparison of the Failure Mechanisms During Scratch Testing of Chromium Nitride and Titanium Nitride Coatings Deposited by Cathodic Arc Evaporation A.M. Peters, I. Reimanis, J.J. Moore, B. Mishra (Colorado School of Mines) Because of the availability of commercial scratch testing machines and the minimal time required for data acquisition, scratch testing is one of the most widely performed tests for thin films. Nonetheless, some debate regarding the information one can obtain using this test still exists. This paper examines some of the useful information that can be achieved using the friction-monitored scratch test on chromium nitride and titanium nitride thin films. Particular attention is paid to the failure mechanisms of both Cr-N and Ti-N thin films. The idea of a critical load, L_c, is discussed, including a comparison of the methods used by several researchers to determine it. A brief introduction to the method and some examples of spalling, chipping, cracking, buckling and wear failures follow.
3:50 PM		F1/E4-2-8 3D Boundary Element Modelling of Elastic Ball on Flat Contact on Bi-Layer Composites With Very Thin Coatings : Influence of Internal Stress R. Kouitat, J. von Stebut (Ecole des Mines, France) Boundary element modelling is a numerical tool especially well suited for the analysis of stress states in media. In a series of preceding papers we have shown this technique to be especially economic and powerful for bi-layer modelling under elastic contact loads. In the present paper we extend the 2D mechanical analysis of very thin coatings to the more complex situation of a 3D ball on flat contact. We show that the main results of the 2D case still hold ; an important stress gradient in the coat-ing and a sharp stress discontinuity at the interface. Results are given for coatings roughly twice as stiff as the substrate as well as for the inverse situation. The effect of frictional surface stress and various "model" internal stress fields is also analysed. These findings are of practical interest for general elastic contact as occurring in many lubricated friction applications of very thin films. However they also apply to the initial phase of scratch-testing where, for a given, absolute coating thickness, t, the relative thickness t/a_H (a_Hbeing the hertzian contact radius) keeps decreasing as the contact load increases.
4:10 PM		F1/E4-2-9 The Concept of an Advanced Impact Tester Supported by Evaluation Software in Characterization of Hard Layered Media K.-D. Bouzakis, N. Vidakis, K. David (Aristoteles University of Thessaloniki, Greece) The coating impact test, in combination with its Finite Elements Method (FEM) simulation, is used to characterize the fatigue behaviour of thin hard coatings as well as of multilayer ones. It may also used in case of thicker films produced by the PTA method. Successive impacts of a cemented carbide ball onto a plane coated specimen induce severe contact loads and strain superficially the layered compound. The fatigue failure mode of each specimen is classified by means of SEM observations, EDX microanalyses and profilometry. FEM simulating models of the impact test are used to determine the critical stress components, which introduce coating fatigue failure. Critical values for stress components, responsible for distinctive fatigue failure modes of the coating substrate compounds are obtained and the fatigue limits of various coatings are illustrated in general applicable Smith and Woehler diagrams. To further improve this method, an advanced impact tester supported by appropriate software facilities, able to evaluate the fatigue strength of hard coatings, was developed. In this enhanced tester the contact loads as well as the number of impacts can be readily varied so that the fatigue failure for coatings with different technological specifications and material properties can be obtained. Moreover, the data continuous acquisition as well as the real time monitoring and evaluation of the test bench were redesigned. The test results are recorded in diagrams containing the impact load versus the number of successive impacts that a coating substrate compound can withstand. Thus, through appropriate computer software, the fatigue strength of thin hard coatings can be automatically determined. The code is supported by an extended data base, which includes conducted FEM calculations that cover a wide range of coating substrate compounds, considering also a variety of technological specifications and material properties.
4:30 PM		F1/E4-2-10 Characterisation of Surface Coatings Using the Nanoscratch Technique; Appraisal and Examples N.X. Randall, R. Consiglio (CSEM Instruments, Switzerland) As the thickness of functional coatings continually decreases to satisfy the structural and protective needs of modern day applications, quantitative instrumentation has become a necessity for adequate evaluation of material properties, particularly scratch resistance and adhesion at the film-substrate interface. The Nano Scratch Tester (NST) is a new instrument overcoming the limitations of both the classical stylus scratch test (normal force range) and the atomic force microscope technique (short sliding distances), allowing scratch lengths of up to 10 mm. Tangential force and penetration depth are simultaneously measured during the scratching process, both in a multipass contact fatigue mode. For high resolution inspection of the deformed or damaged area, a scanning force microscope (SFM) is integrated into the system. Experimental results are presented for a range of different materials. The results indicate very good reproducibility and confirm the application of this new instrument for the accurate characterisation of elasticity, hardness, adhesion and mechanical integrity in coated systems where the film thickness is less than 1 µm.
4:50 PM		F1/E4-2-11 The Effects of the Various Intermittent Annealing Temperatures on the Mechanical Properties of the 45 Degree Rolled Rhenium Samples. O.S. Es-Said (Mechanical Engineering Department, Loyola Marymount University); A. Ocegueda, N. Abourialy, Q. Yue, C. Pham, W. Al Haysh, C. Soza, J. Foyos (NSF Research Experience for Undergraduates (REU) Program, Loyola Marymount University); J. Carlen (Rhenium Alloys, Inc.) The objective of this experiment was to analyze the properties of Rhenium rolled along the 45° direction. Density, microhardness, along with pore and grain structures were analyzed. The study was divided into three parts. In each of those parts, a 10-12 percent reduction in thickness along the 45° angle was applied between intermittent annealing temperatures. The annealing temperatures were: 1200 ° C, 1650 ° C, and 1800 °C. The crystallographic texture of the processes were evaluated and correlated to the mechanical properties. The study showed that the density of all tested samples remained the same. The microhardness increased with thickness reduction for all annealing temperatures. Twinning was more evident as percent reduction increased. Preferred orientation was observed in the (0002) plane as percent reduction increased.