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Mechanical Behavior study of a coated system by computer simulation of the scratch test
Abstract
One way to evaluate a coated system is through the scratch test. The results obtained depend of the variables including mechanical properties and geometry of indenter, loading, displacement, material properties in the system as hardness, elastic modulus, microstructure, roughness surface, thickness, among others, which are indicated in ASTM C1624 / 05. This paper analyzes through scratchtest simulation, the effect of the indenter geometry (conical and spherical), the loading (20 N and 50 N), the thickness coating (2,1 µm and 4,6 µm) and the friction coefficient values (0,3 and 0,5) in the stresses and plastic deformation behavior at the surface of a coated system. The results suggest that the coefficient of friction has a high importance in the mechanical performance of the coated system.
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Material deformations and the influence of coating thickness and elastic modulus were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress, strain, and displacement computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a diamond ball sliding with increasing load on a thin titanium nitride (TiN) coating on a flat steel substrate. The ball was modelled as rigid, the coating was linearly elastic, and the steel substrate was elastic – plastic, taking into account strain hardening effects. It was shown that a thin TiN ceramic coating on a steel substrate has only a very slight effect on friction and on the plastic deformations (i.e., the groove formation) in the surface, but changes considerably the stress pattern at the surface. The stress simulations showed how a thicker hard coating on a soft substrate has a better load-carrying capacity that a thinner one. Higher tensile stresses at the coating/substrate interface increase the risk for interface cracks and delamination of the thicker coating. A stiffer hard coating on a soft substrate has a better load-carrying capacity than a more elastic one. The stiffer coating will accommodate higher tensile stresses with the same indentation depth compared to a more elastic one. The results show that much more attention should be given to optimizing the elastic properties of the coating than previously has been done. In many cases, it can be much more effective to improve the wear resistance of the coated surface by focusing on the elastic modulus of the coating than changing the coating thickness. D 2005 Elsevier B.V. All rights reserved.
The scratch test has been used to assess the adhesion of thin hard coatings for some time now and is a useful tool for coating development or quality assurance. However, the test is influenced by a number of intrinsic and extrinsic factors which are not adhesion-related and the results of the test are usually regarded as only semi-quantitative. The stress state around a moving indenter scratching a coating/substrate system is very complex and it is difficult to determine the stresses which lead to detachment. Furthermore the interfacial defect state responsible for failure is unknown. However, by a careful analysis of the observed failure modes in the scratch test (not all of which are related to adhesion) it is possible to identify adhesive failures and in some cases these occur in regions where the stress state is relatively simple and quantification can be attempted.Ideally engineers would like a material parameter (such as work of adhesion or interfacial toughness) which can be used in an appropriate model of the coating-substrate system stress state to determine if detachment will occur under the loading conditions experienced in service. This data is not usually available and the development of such models must be seen as a long term goal. In modern indentation and scratch systems the work of friction (or indentation) can be directly measured and the relationship between this parameter and adhesive failure can be demonstrated in some cases. This chapter reviews the main adhesion-related failure modes and the stresses responsible for them and indicates where quantification is possible illustrating this with results from hard coatings on steel, thermally grown oxide scales and optical coatings on glass. The use of empirical calibration studies, directly measured work of friction and quantification by finite element methods is discussed.
Today the scratch test is an important tool, widely used for the development of some industrial sectors (anti-scratch coating, glass industries, automobile industries,...). Surface mechanics uses this process to characterise the local and interfacial behaviour of material (wear, friction and adherence). In this work, an aluminium alloy, used for the transportation of a granular material, is studied. A comparison of experimental, numerical and analytical results is discussed. (c) 2007 Elsevier B.V All rights reserved.
El objetivo de este trabajo es estudiar a partir de simulaciones el comportamiento mecánico de un sistema sustrato/recubrimiento. Se analizan los esfuerzos de contacto y la deformación elástica, al aplicar una carga normal a la superficie de un sistema compuesto por una película delgada resistente al desgaste de Carburo de Tungsteno (WC) y un sustrato de acero inoxidable. El análisis se basa en simulaciones utilizando el método Monte Carlo a partir del algoritmo de Metrópolis; el fenómeno fue simulado a partir de una estructura cristalina centrada en las caras fcc, tanto para el recubrimiento como para el sustrato, asumiendo el plano de deformación uniaxial en el eje z. Los resultados fueron obtenidos para diferentes valores de carga normal aplicada a la superficie del recubrimiento, obteniendo así la curva Esfuerzo vs Deformación del sistema. A partir de la curva se calculó el módulo de Young, obteniéndose un valor de 600 GPa, la cual concuerda con resultados experimentales.
This research focuses on simulate and to analyze using Finite Element Method, the stresses behavior obtained in a system (substrate+coating) as a result of the contact with a rigid particle under normal load and sliding along the surface of the system, which has roughness Ra of 0,36?m. The results show that in the scratch test simulation, the roughness of coated system must be considered contrary to the literature simulations of this field, since this affects the stresses and deformations behavior during contact and sliding. Thus, to increase surface roughness peaks greater possibility of inducing stress cracking.
Evaluation of wear mechanisms of thick thermal sprayed cermet coatings is a challenging endeavor given the numerous process-induced structural and chemical changes as well as presence of residual stresses. In an effort to understand the damage processes under contact load and their sensitivity to process induced microstructural attributes, controlled scratch testing was used. Detailed assessment of the resultant damage zone provided repeatable cracking patterns that are categorized as (i) Localized collapsing of material, (ii) angular cracks, (iii) primary semi-circular and developed semi-circular cracks and iv) splat delamination. A correlation was established linking observed damage mechanisms to process induced microstructural descriptions including role of spray particle conditions and residual stresses. Quantitative correlations between delamination load for cracking and process induced variable including particle properties as described by the non-dimensional melting index concept as well as residual stresses were established. Melting index captures the combined effect of particle thermal and kinetic history and thus coating porosity and process induced decarburization. The results highlight the critical role of coating density and stress evolution during coating formation. The research points to scratch testing as a powerful evaluation method to characterize contact response of thick thermal spray cermet coatings including operative mechanisms.
The proper design of wear resistant coatings applied to cutting tools comprises the optimization of the mechanical properties (Young's modulus, yield strength, adhesion, intrinsic stresses, fracture, fretting etc.) of the coating-tool system. The goal is to find material and structural solutions which keep the resulting stress–strain field under typical application conditions below the stability limits of the system. Based on nanoindentation measurements obtained from the coating-tool system which should be optimized, a scratch test is dimensioned with respect to load range and indenter geometry. The measured data from this “Physical Scratch Test” are used to simulate spatial stress profiles and to calculate the von Mises stress characteristics and the maximum normal stresses in the scratch direction. In a further step, the simulations are used to suggest scratch parameters for a “Fine Tuned Scratch Test” which increase the sensitivity of the test for specific depth regions in the coating-tool architecture and allow improved and more sensitive investigations of critical interfaces, transition layers and surface-near substrate regions. The tests were performed at PVD coated inserts (nitrides and oxides) and compared with the results obtained from cutting tests.
The scratch resistance of coatings and the adhesion between coating and substrate are usually determined in model experiments performed with sharp diamond indenters. These common methods as described for example in EN 1071-3 often fail for the combination of hard coatings on soft substrates due to very small critical loads or no detectable failure modes at all. Hence in the industry a lot of highly subjective and provisional test methods are used. On the one hand, quantitative comparability is difficult with common methods since defects already occur at very small loads for ductile and relative soft substrate materials like plastics. On the other hand, wear of the indenter in contact with hard coatings like pure diamond coatings requires its cost-intensive replacement.
In the present paper scratching of soft thin film/substrate structures, using sharp conical indenters, is studied theoretically and numerically. For simplicity, but not out of necessity, the material behavior of the film as well as the substrate is described by classical elastoplasticity accounting for large deformations. Explicit material parameters are chosen in order to arrive at representative results as regards material behavior and indenter geometry. The main efforts are devoted towards an understanding of the influence from the film/substrate boundary on global scratching properties at different material combinations. Global quantities to be investigated include scratch hardness, contact area and apparent coefficient of friction at scratching. The numerical investigation is performed using the finite element method (FEM) and the numerical strategy is discussed in some detail.
Scratching of thin film/substrate structures is studied theoretically and numerically. In most cases, the material behavior
of the film as well as the substrate is described by classical elastoplasticity accounting for large deformations; further,
pressure-sensitive flow models are considered. The main efforts are devoted toward an understanding of the influence from
the film/substrate boundary on the stress distribution at scratching but for comparative reasons, scratching of homogeneous
materials are also studied and pertinent results presented. Among other things, the results are discussed in relation to delamination
initiation and growth at scratching. The numerical investigation is performed using the finite element method, and the numerical
strategy is discussed in some detail. The most important finding given by the present study is that high shear stresses are
the main driving force for delamination initiation and growth along the film/substrate interface. It was also noted that the
influence from pressure-sensitive flow on the stress fields related to delamination initiation is small, both quantitatively
and qualitatively.
KeywordsScratching-Film/substrate structures-Stress distribution-FEM analysis-Delamination
The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.
Surface coatings, like titanium nitride (TiN) and diamond-like carbon (DLC) coatings, offer high wear resistance and good friction performance for a wide range of applications. With novel advanced techniques like modelling and simulation, the performance of these coatings can be predicted under a wide range of loading conditions. This provides valuable information for the coating design and for the use of coatings in different applications. A previously developed three-dimensional finite element method (FEM) model was used for calculating the first principal stress distribution in a scratch test contact as a spherical diamond tip is moving with increased load on DLC and TiN coated high speed steel surfaces containing no residual stresses. The used three-dimensional model is comprehensive in the sense that it considers elastic and plastic behaviour of the contacting surfaces. The first cracks to appear on the surface are angular cracks on the edge of the scratch groove. This corresponds to the region of high two directional tensile stresses occurring in FEM stress simulations at the edge of the scratch groove. The coating/substrate stiffness ratio influences considerably on the coating behaviour. The TiN coating that had higher Young's modulus compared to the substrate material experienced high tensile stresses when loaded by the diamond stylus. The DLC coating that had lower stiffness compared to substrate material experienced comparatively low tensile stresses. The DLC coatings had maximum tensile stresses in the range of 700–900 MPa. The TiN coatings had tensile stresses in the range 2200–3000 MPa. The coating thickness had only minor effect on the maximum tensile stress level for the 1 and 2 μm thick coatings. The location of the experimentally observed first crack in the surface corresponds to the location of maximum tensile stress concentrations in the stress simulations and the direction of the observed crack corresponds with the stress components in the calculated stress field. The role of residual stresses is discussed.
A three-dimensional (3D) finite element (FE) simulation of a rigid Rockwell C indenter scratching a TiN/Ti-6Al-4V coating/substrate system is presented. Coulomb friction between the indenter and the surface of the coating/substrate system was considered. The material properties of the coating and substrate were assumed to be elastic–plastic following a bilinear law with isotropic strain hardening. The von Mises yield criteria was used to determine the onset of plastic deformations. The scratch depth profiles at different moving distances were studied. The distributions of the stress field at the contact surface, in the coating, and at the interface of the coating/substrate system were investigated. The finite element results can be used to explain the failure modes of coated materials at the scratch test.
Caracterización de propiedades mecánicas mediante análisis inverso del Método de los Elementos Finitos combinado con ensayo de indentación
- J Isaza
- I Mariaka
- J Ramirez