Failure mode maps in the thin film scratch adhesion test

The scratch test has been used to assess thin coating adhesion for some time now. In this test a diamond indenter is drawn across the coated surface under an increasing load (either stepwise or continuous) until at some load, termed the critical load, L_c a well-defined failure event occurs; if this failure event represents coating detachment then the critical load can be used as a qualitative measure of coating-substrate adhesion. However, it is well known that a range of possible failure modes can occur and only some of these are dependent on adhesion. Other failure modes which depend on plastic deformation and fracture within the coating, rather than any adhesive failure at the coating substrate interface, may be just as useful in the assessment of coating quality particularly for tribological applications. In this paper the load regimes in which the main adhesion-related failure modes (spallation and buckling) occur as a function of coating thickness will be presented for thermally grown oxide and sputtered nitride coatings. The origin of the observed failure modes and the use of the scratch test to assess coating/substrate adhesion in a more quantitative fashion is discussed in the light of these observations.     

Mechanical characterization of nanostructured TiB2 coatings using microscratch techniques

TiB₂-based nanostructured coatings were fabricated on high-speed steel by magnetron sputtering technique. Mechanical characterization of the resultant coating-substrate systems, such as coating adhesion, friction and scratch resistance, was conducted by microscratch technique. The linearly increasing load mode of microscratch test was studied to determine the most effective and informative testing conditions and to determine the critical load (Lc) for coating failure. The mode of failure was examined by high resolution SEM and AFM. In order to gain a better understanding of the scratch behaviour during the test, a three-dimensional finite element (FE) model was developed to simulate the scratch process. The developed FE model was able to demonstrate the elastic and plastic behaviour of the coating and substrate around the contact area during scratch test. Good agreement has been observed between the FE analysis results and experimental investigations.     

Tribological analysis of fracture conditions in thin surface coatings by 3D FEM modelling and stress simulations

A tribological analysis of deformations and stresses generated and their influence on crack generation and surface fracture in a coated surface loaded by a sliding sphere in dry conditions is presented. A three-dimensional finite element method (3D FEM) model has been developed for calculating the first principal stress distribution in the scratch tester contact of a diamond spherical tip with 200 μm radius sliding with increasing load on a 2 μm thick titanium nitride coated steel surface. The model is comprehensive in that sense that it considers elastic, plastic and fracture behaviour of the surfaces. The hard coating will be stretched and accumulates high tensile stresses. At the same time, it is carrying part of the load and thus reducing the compressional stresses in the substrate under the sliding tip. The first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. The fracture toughness of the coating is calculated by identifying from a scratch test experiment the location of the first cracks and the crack density and using this as input data.     

Viscoelastic and elastic–plastic behaviors of amorphous polymeric surfaces during scratch

In this study, we propose an analysis of the residual groove after contact between a spherical indenter and an amorphous polymeric surface (polymethylmethacrylate, PMMA) in scratch experiments. The geometrical shape of the residual groove was mathematically described using an exponential decay law. Finite element modeling (FEM) of scratch tests was compared to the corresponding experimental results. Assuming a two-segment simplified constitutive law with linear elastic behavior followed by linear strain hardening, the friction at the interface between the indenter and the material was modeled with a Coulomb's friction coefficient varying from 0 to 0.4, for computed ratios a/R between 0.1 and 0.4. The FEM results for elastic–plastic contact indicate that the shape of the residual groove is directly related to the plastic strain field in the deformation beneath the indenter during scratching. It is shown that the dimensions of the plastically deformed volume and the plastic strain gradient both depend on the ratio a/R and also on the friction coefficient.     

金属基/陶瓷复合双涂层的正接触应力分析

以球形压头为模型，采用I—deasCAD／CAE软件对Hertz弹性接触状态下金属基／陶瓷双涂层系统的应力分布情况进行了理论建模，并基于模型计算了在不同的涂层厚度／接触半宽度比和外涂层／过渡层／基体弹性模量比情况下的应力分布情况。双层系统具有相同的厚度和不同的弹性模量。论述了无涂层弹性半空间体的有限元分析结果与经典的Hertz接触力学解析解结果的一致性，计算结果有助于工程中陶瓷涂层的设计与应用。     

反应烧结SiC陶瓷脆性去除特征及刻划力波动行为

主要研究不同加工深度及压头形状刻划条件下反应烧结碳化硅(RB-SiC)陶瓷脆性去除特征和刻划力波动行为之间的关系。采用半径分别为400nm的金刚石玻氏压头以及8.7μm的圆锥压头进行恒切深刻划,并利用扫描电子显微镜对刻划后的SiC陶瓷表面进行测量。最后,通过Daubechies小波进行横向力和切向力信号分解,并结合划痕表面损伤形式,给出不同细节信号及近似信号与加工损伤的联系。实验结果表明:对于圆锥压头,随着加工深度的增大,表面形貌为塑性挤出、微破碎和大面积表面破碎共存的形式。此外,在脆性断裂去除情况下,随着压头尖端半径的减小,破碎程度增加且刻划力信号能量由低频段逐渐扩散到整个频域。同时低频段的能量逐渐占据主要地位。不同程度的表面微破碎及边缘微破碎对刻划力细节信号分量贡献较大。反应烧结碳化硅结构本身差异以及缺陷引起的大面积断裂是刻划力波动能量的主要来源,而且随着加工深度的增大而增大。     

An overview of the potential of quantitative coating adhesion measurement by scratch testing

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 paper 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.     

Progressive and constant load scratch testing of single- and multi-layered composite coatings

The development of effective coating systems that offer high protection against erosion–corrosion is of utmost importance in several industrial sectors. Such a coating class is typically designed to provide an effective barrier against aggressive environments combined with cathodic protection. The adherence to the substrate ensures full performance and reliability of the coatings during service. In the present investigation, the scratch response of single- and multi-layered composite coatings made from superimposed layers of a modified phenyl–methyl silicone resin and of aluminum–magnesium (Al–Mg 4.5 wt%) metallic powders was investigated. The applied loads, the contact conditions between the indenter and coating surface and the sliding speeds were analyzed, and empirical models were developed accordingly. The scratch response of the composite coatings was mapped according to the testing conditions, thus providing a useful instrument for designers and practitioners.     

The influence of strain hardening of polymers on the piling-up phenomenon in scratch tests: Experiments and numerical modelling

The aim of this study was to relate the scratching behaviour of polymers to their mechanical properties. A thermosetting resin (CR39) and a thermoplastic polymer (PMMA) were studied using a microscratch tester allowing in situ observation of the contact area. These two polymers exhibit different elastic and viscoplastic properties, the main difference being the large ability of CR39 to strain harden, whereas PMMA softens. A spherical indenter was used to vary the level of deformation imposed on the samples. The response was initially elastic, then viscoelastic and finally mainly viscoplastic with increasing penetration of the indenter into the material. The two polymers displayed the same response for small levels of deformation, while at larger strains PMMA showed more pronounced plastic behaviour. The origin of this difference in behaviour was investigated by means of a three dimensional finite element analysis. The rheology of PMMA and CR39 was simplified and modelled by assuming linear elastic behaviour and a viscoplastic law taking into account their strain hardening capacity at high strains. Strain hardening was found to be a key factor to correctly model the material flow around the indenter. The response of the polymers was governed by the ratio between the plastic and elastic strains involved in the deformation in the contact region. In first approximation, the representative strain was imposed mainly by the geometry of the indenter, while the elastic deformation was controlled by the mechanical properties of the material, a larger strain hardening leading to a greater elastic deformation and a lower plastic strain thus a better scratch resistance of the specimen.     

On the effect of substrate deformation at scratching of soft thin film composites

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.     

Finite Element Analysis of Multilayered and Functionally Gradient Tribological Coatings with Measured Material Properties

A model has been developed to study the stress distribution in Ti_{1 ? x}C_x multilayered functionally gradient (FG) coatings, with a top coating of diamond-like carbon (DLC), on 440C stainless steel substrates. Using the finite element method, these gradient coatings were assumed as a series of perfectly bonded layers with unique material properties and layer thickness. In addition, a matrix of nanoindentation experiments were performed to measure material properties of each Ti_{1 ? x}C_x layer on separate coating blocks. The yield strength of the coating materials was then determined by coupling the finite element analysis model in connection with the nanoindentation technique. Once developed, this model was used to examine the threshold of plasticity and identify the plastic deformation zone inside the multilayered coatings and substrate. This work shows how the multilayered FG Ti/TiC/DLC coating system improves the coating integrity under heavy loading conditions.     

Mechanical Modeling of Scratch Behavior of Polymeric Coatings on Hard and Soft Substrates

An ASTM standard scratch test is utilized to study the scratch behavior of polymeric coatings on soft and hard substrates. Depending on the different combination of polymeric coatings and substrates utilized, various damage modes can take place, which include coating delamination, transverse cracking, and buckling failure. A soft coating on a hard substrate will give rise to an entirely different scratch damage pattern from those of a hard coating on a soft substrate. The stress and strain responses of scratch on polymeric coating are analyzed using three-dimensional finite element (FE) simulation. The analysis provides mechanistic insights for the observed polymer coating deformation mechanisms and failure modes. Usefulness of the ASTM scratch method and FE modeling to evaluate polymer coating scratch behavior is discussed.     

器件韧性镀膜微划痕破坏机理研究

对硅基体上之韧性镀膜（铝膜）的粘结强度及破坏机理进行微划痕实验及理论研究,从实验中观察出该体系的破坏特征,进而测量出微划前水平驱动力、划痕深度随划前水平位移并伴随着界面脱胶发生的变化规律,针对微刈痕破坏特征,建立了双粘聚力模型,并对由微划痕引起的界面弹塑性脱胶进行了数值模拟,给出界面脱胶时能量释放率随其他材料参数变化的理论预测曲线,并将预测值与文中的铝／硅实验结果及与文献中关于铂／氧化镍的实验结果进行对比,达到基本符合。通过以对韧性薄膜／脆性基体的微划痕实验研究和理论分析,获得如下主要结论：（1）韧性膜的微划痕破坏特征为,当划刀尖端接近界面时,将突然发生薄膜测界面的脱胶现象,并在界面附近脆性基一侧形成界面裂纹并扩展;微划痕的水平驱动力表征了整个薄膜脱胶体系的能量释放率;薄膜或涂层材料的塑性变形对微划痕过程有较强的抑制作用。（2）界面的分离强度和材料的剪切强度对微划痕过程有重要的影响。（3）划痕刀片的几何特征角对刻痕水平驱动力影响不大。     

On the possibility of evaluating the resistance of materials to wear by ploughing using a scratch method

Although the plastic strain that a material can withstand in sliding contact is an important mechanical characteristic determining its resistance to ploughing wear, this material property could not be measured so far. In this study, a scratch test to measure the critical plastic strain is explored. A hard spherical indenter scratches the tested surface under progressive loading to induce an increasing plastic strain on the surface. When the plastic strain exceeds the limit of plastic deformation on the surface, micro-fracture takes place at the ridges of the scratch groove. The critical load to initiate this micro-fracture is detected in the scratch test and then the critical plastic strain is calculated. A measure of the resistance of a material to wear by ploughing, this critical plastic strain to micro-fracture has been used both for evaluating the cohesion of coatings and for screening wear-resistant materials.     

Two-body abrasive wear of electroless nickel composite coatings

The two-body abrasive wear of electroless nickel (EN), EN-silicon carbide, and EN-alumina composite coatings have been investigated using a scratch test with a diamond indenter. The coatings were heat treated at temperatures of 100–500° C. The hardness of the coatings increased with heat treatment temperature from 500 HV100 for the as-deposited condition to 1008 HV100 when fully hardened. Scratch testing showed that the as-deposited coating had scratch tracks with a high degree of plasticity, signs of microploughing and tensile cracking and was characterised as a ductile failure. On the other hand, the heat-treated coatings showed chipping and cracking on the edge of the scratch tracks, failing in a brittle manner. The heat-treated EN-silicon carbide coatings, however, exhibited no cracking nor chipping, believed to be due to its higher fracture toughness than the other heat-treated coatings, attributable to its lower phosphorus content. The volume of material removed from the silicon carbide scratch track was 1/3 of the volume removed from the steel substrate at a 20 N load, and showed the best wear/ scratch resistance of any of the coatings tested.     

梯度覆层体受粗糙面滑动作用时的弹塑性分析

运用大限元软件分别虽对单覆层体及梯度体受多微凸体粗糙面滑动使用时产生的应力在变进行了计算和研究，着重比较两种覆层体爱相同表面载荷作用下，在基体及表面出现部分塑性变形时，表面层、基体内及表面 基体界面处的应力、应变分北度层在防止其本产生塑性变形及改善界面应力等方面比单纯层体具有明显的优点。本文的研究结果表为表南 选一览表主加工提供参考。     

A numerical/experimental study of contact damage in non-planar porcelain/metal bilayers

In this report, we investigate and visualize the effect of shape irregularity on contact damage in a brittle coating on a stiff metal substrate. Hertzian contact damage in a dental porcelain layer of thickness between 0.25 and 0.75 mm, fused onto a Ni–Cr alloy substrate in both curved and planar geometries was studied with the aid of the finite element method and experimental investigation. Three failure modes were examined with varying porcelain layer thickness: cone cracking at the upper surface of the porcelain, median or interface cracking at the layer/substrate interface and plastic deformation below the contact area in the substrate. It is shown that curvature has very little effect on the initiation of surface cone cracks in this system, but substantial effect on the initiation of interface radial cracks. In particular, curvature reduces the critical load for the onset of interface cracks.     

Mechanisms of blistering and chipping of a scratch-resistant coating

In most cases, scratching of the surface of a polymeric glass elicits brittle behavior. Industrial solutions have been successfully used to improve the scratch resistance of polymeric glasses and a common way is to coat the substrate with a thin film. However, one of the limitations of this method is the risk of cracking and chipping. The origin of the success of the coating technique is still of great research interest and further work will be required to explain the improvement in scratch resistance and predict the cracking in anti-scratch coatings. The present study contributes to these aims.

Using a single-asperity scratching device allowing in situ observation of the scratch, the fracturing of a thin (3.5 μm) nano-composite coating deposited on a viscoelastic–viscoplastic substrate (polycarbonate) was investigated under different conditions of temperature and scratching speed. Four types of fracture mechanisms were observed, depending on these two variables. The processes involved in deformation of the system were: (i) delamination (blister formation) and fracture (chipping) of the coating and (ii) viscoelastic–viscoplastic deformation of the substrate. Image analyses were performed on video sequences of the different processes leading to damage of the film. The quantitative results are discussed in terms of the damage mechanisms involved.     

Effect of substrate surface roughness on mechanical properties of diamond-like carbon coatings

Abstract

A plasma enhanced chemical vapour deposition (PECVD) amorphous carbon coating was deposited onto 100Cr6 steel substrates having varying degrees of surface roughness. The samples were subsequently evaluated to determine the correlation between substrate roughness and coating performance. The steel substrates were prepared before coating deposition to attain five different levels of roughness: (a) ground; (b) superfinished (SF); (c) polished to 1000 grit; (d) polished to 220 grit and (e) polished to a 1 μm diamond finish. The aim of the investigation was to determine the degree of finish required for good tribological performance and coating adhesion. The mechanical and tribological properties of the samples were assessed by nanoindentation, ramped load scratch testing, and pin on disk wear testing. Nanoindentation testing was used to determine the hardness of the samples and the relative contributions to the system hardness from the substrate and coating were separated using the model of Korsunsky et al. Nanoindentation testing showed that the coating hardness (when separated from the system hardness) was lower for the samples with the SF substrate than the others: the reasons for this are discussed in the light of Raman measurements on the fractions of diamond-like and graphite-like bonding in the coatings. Ramped load scratch testing was used to determine coating adhesion and the scratch test failure mode. With the exception of the samples with the ground substrate finish, studies of the friction coefficient plots during scratch testing showed little variation between the samples, and SEM imaging revealed a common failure mode of severe spallation at the scratch track border. The samples with the ground substrate showed differences in response between scratches parallel and perpendicular to the grinding direction, with scratches parallel to the grinding direction showing more severe spallation. The average critical load to failure, as determined by the point of first failure in the scanning electron microscope, was lower for the coatings on the SF substrate than the coatings on the 220 grit, 1000 grit and 1 micron finished substrates. The critical load to failure for the samples with ground substrates was lower than the other substrate surface finishes. Pin on disk wear testing of the samples against a steel ball revealed that the major effect of the varying substrate roughness was on the wear of the counterface, with rougher substrate finishes generally resulting in higher wear rates of the counterface, although the smoothest substrate finish, the micrometre finish, also resulted in higher wear. The sample whose substrate was superfinished gave least wear of the counterface and this was therefore the optimum finish for the samples when considering their performance in a tribological couple.