ICMCTF2008 Session E2-3: Mechanical Properties and Adhesion

Friday, May 2, 2008 8:00 AM in Room California

Friday Morning

Start	Invited?	Item
8:00 AM	Invited	E2-3-1 Competition Between Gradual Wear and Fracture Mechanics Based Wear L.M. Keer, K. Zhou, D. Epstein (Northwestern University); S. Liu, D. Hua (Caterpillar Inc.) Coatings used in many industrial applications are subject to wear caused by high, concentrated loads, insufficient lubrication, surface roughness, and relative sliding. Therefore, it is of significance to study the mechanisms of coating wear in order to improve the reliability and durability of coatings. It has been observed that two types of wear processes, gradual polishing wear and facture-based wear. take place simultaneously in coatings under cyclic rolling-sliding contact.¹The present work is aimed to develop a model that accounts for the two wear patterns and the competition between them, with the ultimate goal of developing a predictive model of coating wear. A coating fracture simulation tool that incorporates a rough contact solver² serves as a starting point for our fracture-based wear model. A 2D version of the code has been developed with stress-controlled crack kinking. In the simulation, it is assumed that there are pre-existing micro-cracks within the coating. Depending on the stress field at the crack tips, such initial cracks can propagate in different directions under cyclic contact load. The simulation results have demonstrated that both tips of a crack may grow to reach the coating surface thus chipping out a particle from the coating under certain circumstances. A few parameters have been found to be crucial to the occurrence of chipping, e.g., the depth of the initial crack from the surface, the orientation of the crack, the relative position of the crack to the major roughness asperities, the loading pattern, and the cyclic number of the loading. The Archard-type polishing wear model characterizes the steady removal of small particles form the coating surface and therefore is applicable to the gradual wear modeling of coatings.³ In the mixed model incorporating the fracture-based wear model into the Archard-type gradual model, the chipping and gradual wear compete to modify the surface and therefore change the pressure distribution between the contacting surfaces. The surface evolution can be traced over the whole course of loading cycles. ¹Mercer, C. at al. 2003. Surface & Coatings Technology 173: 122-129. ²Polonsky, I. A. and Keer, L. M. 2002. ASME Journal of Tribology 124 (1): 14-19. ³Chekina, O. G., Keer, L. M., Liang H. 1998. Journal of the Electrochemical Society 145 (6): 2100-2106.
8:40 AM		E2-3-3 Thermal Evolution of Hard Ti-B-N Based Coatings Deposited on Stainless Steel C. Paternoster, A. Fabrizi (Marche Politechnic University, Italy); Ph.V. Kiryukhantsev-Korneev (State Technological University/Moscow Institute of Steel and Alloys, Russia); R. Cecchini, S. Spigarelli (Marche Politechnic University, Italy); D.V. Shtansky (State Technological University/Moscow Institute of Steel and Alloys, Russia) Surface properties modification in thermomechanical stressed materials is an important way to improve the performances in extreme environments. An effective way to enhance superficial properties of material is to coat them with appropriate films. The use of thin films in different industrial fields (not only for metal working, machining, die-casting but also as in industrial plants and everyday applications, such as lightning or cutlery) is raising interest for the wide range of material composition and properties available, and it is still growing. Hard Ti-B-N based coatings, with the addition of Cr or Al and Si, deposited through Ion Implantation magnetron sputtering on AISI 304 stainless steel have been investigated in the as-received condition and after different kinds of thermal treatment (thermocycling at 900° and 800°C in vacuum and thermocycling at 850° and 750°C in air for 6 hours, thermal treatments in air and in vacuum at 900°C for different durations). Their mechanical properties have been studied through Nanoindentation and Scratch Test, to have information related to hardness, Young modulus and the adhesion to the substrate. TEM and XRD investigations were aimed at defining the microstructure and the present phases after the thermal treatment procedures while AFM and SEM have been used to investigate the topology of the coatings. Models for the thermal evolution of the system coating + substrate after thermal treatment are proposed.
9:00 AM		E2-3-6 Mechanical Properties and Deformation of CrN and Cr/CrN Coatings During Nanoindentation: Experiments and Theoretical Calculations R. Daniel, C. Mitterer (University of Leoben, Austria); M. Herrmann, F. Richter (Chemnitz University of Technology, Germany) Mechanical properties of thin transition metal nitride coatings significantly depend on the coating structure, stress level and also on the coating/substrate combination. In this study an effect of grain size, residual stress and substrate material on hardness, Young’s modulus and deformation mechanism of the CrN coatings was systematically studied by means of nanoindentation using Berkovich, cube corner and spherical diamond indenters. In order to understand the deformation mechanism that operates in the coating while it is mechanically loaded, the microstructural features observed in the indented regions by scanning electron microscopy are correlated with experimental load-displacement curves. A strong effect of brittle silicon and ductile steel substrates on plastic deformation of the coatings is discussed in details. In addition, a possible improvement of the coating resistance due to plastic deformation by introducing a Cr interlayer with various thicknesses between 100 nm and 2.9 µm was investigated. For this purpose, nanoindentation experiments with spherical indenters with various radii were performed showing an enhancement of hardness and resistance to damage of the Cr/CrN coating system at certain stress and layer thickness combinations. This fact is related to a positive effect of an interlayer on the growth of the CrN top layer and mechanical response of the coating system during indentation. The deformation behaviour of both CrN and a dual layer Cr/CrN coating systems are compared. The experimental observations are combined with theoretical calculations of the von Mises stress distribution under the applied load.
9:20 AM	Invited	E2-3-7 Atomic Scale Modeling of Interfacial Adhesion in Optical Coatings P.D. Bristowe (University of Cambridge, United Kingdom) This paper reviews some recent atomic scale calculations of the mechanical properties of a metal/oxide interface that is commonly found in low-emissivity optical coatings. The interface studied is Ag(111)/ZnO(0001) because observations suggest that it is one of the weakest present in a typical multilayer optical stack. An atomic model for the interface is constructed and relaxed using a first principles methodology based on density functional theory. The main quantity calculated is the ideal work of separation but the method also provides information on the strength of the interfacial bonding through the electron density distribution, bond populations and densities of states. Various atomic configurations are studied to determine the effect of changing the local coordination and terminating species at the interface. In addition the influence of residual stress, chemical alloying and hydrogen interpenetration are investigated. The results are compared to experimental measurements of the work of adhesion and microcopy observations where possible. Predictions are made which should aid the design of improved coatings. The study forms part of a larger UK research program on the modeling of functional coatings using multiscale methods and the present paper is placed in that context.
10:00 AM		E2-3-9 Atomistic Modelling of Nanoindentation of Oxide Materials I. Gheewala, E. McGee, S.D. Kenny, R. Smith (Loughborough University, United Kingdom) We will describe atomistic scale modelling using molecular dynamics of the nanoindentation of magnesium oxide and titanium dioxide in both the rutile and anatase structures. These simulations will show the role of both the surface we choose to indent and the indenter orientation. The atomistic processes taking place in these materials during the indentation process will be elucidated through the study of the activated slip systems and the phases present both during and post indentation. These findings will be connected to the features observed in the experimental work on these materials. We will also show the results of nanoscratching simulations on these materials. A coupled molecular dynamics/finite element model will be presented. Simulations of nanoindentation will be used to illustrate how this can be used to extend the length scales in our simulations, allowing a more realistic simulation of the experiments.
10:20 AM		E2-3-10 Finite Element Analysis of Adhesion Failure During Contact in Multilayer Coatings on Glass J. Chen, S.J. Bull (Newcastle University, United Kingdom) The major in-service failure mechanisms of modern solar control coatings for the architectural glass can be mechanical (e.g. scratch damage). Many of these coatings are multilayer structures of less than 100nm thickness and different coating architectures are possible (i.e. different layer materials, thickness and stacking order). For high performance solar control coatings deposited by physical vapour deposition processes the active layer is a thin silver coating (~8nm thick) surrounded by anti-reflection coatings (e.g. ZnO, SnO₂) and barrier layers (e.g. TiO_xN_y). Scratches are often found during delivery of the coated glass (called transit scratches) and it has been determined that the cause of the scratches is the polymer balls sprayed onto the glass to separate sheets while in transportation. Laboratory simulation of the transit scratches has determined that the adhesion between layers within the multilayer stack is critical in determining performance. The generation of stress during the indentation and scratch testing of multilayer coated glass has been modelled by Finite Element analysis to determine which interface is most likely to fail and the likely size of interfacial defects. The locus of failure and the calculated defect size compare favourably to experimental values and show that relatively large (diameter much greater than the coating thickness) interfacial flaws need to be present for failure to occur. The possible origins of these defects will be discussed.
10:40 AM		E2-3-12 Cohesive Zone Modeling for Delamination Evaluations of Diamond Coating Tools J. Hu, Y. Chou (The University of Alabama); R. Thompson (Vista Engineering, Inc.) Interface delamination is the primary weakness that limits the machining performance of diamond coating tools despite their superior tribological properties. Thus, quantitative characterization and modeling of coating delamination are important to the design and usage of diamond coating tools. We applied a cohesive zone model to study the interface delamination of diamond coating on a tungsten carbide substrate. The cohesive zone model is based on the traction-separation law, i.e., the traction across the interface first increases with the separation until it reaches a maximum, then rapidly decreases and vanishes. The cohesive zone model is represented by three parameters: the maximum normal strength, and the normal and shear characteristic lengths, which were determined from the tungsten-carbide fracture properties. The cohesive zone model was implemented in finite element codes to simulate the indentation process, using a spherical diamond indenter. The quasi-static structural analysis was performed to simulate the loading and unloading cycle during indentation. In addition, the material behaviors are perfect elastic and elastic-plastic with a hardening rule for the diamond coating and tungsten carbide substrate, respectively. The model was applied to evaluate coating attribute effects on the delamination size. The simulation results indicate that (1) increasing the coating elasticity will reduce the delamination size, (2) increasing the coating thickness will generally reduce the delamination size, but (3) increasing the coating residual stress magnitude will marginally increase the delamination size.