Assessment of adhesion between thin film and silicon based on a scratch test

Thin film coatings are commonly utilized to prevent wear, modify surface properties, and manipulate the frictional behavior of various mechanical systems. The behavior of a coating has a direct effect on the life as well as performance of the system. However, the coating itself is subject to damage, and the quality of the coating is related to the adhesion characteristics between the coating and the substrate. Therefore, a quantitative assessment of the adhesion properties of thin film is important to guarantee the reliability of not only the thin film but also the mechanical system. In this study, ramp loading scratch tests were performed to assess the adhesion characteristics of Ag and ZnO thin films coated on a silicon wafer. Silver thin film, deposited by sputtering, and ZnO thin film, fabricated by a sol-gel method, were used as scratch specimens. Scratch tests using a diamond tip were performed with a continuously increasing normal force. During the scratch test, the normal and frictional forces were monitored to assess the integrity of the film. The Benjamin and Weaver model commonly used for obtaining the horizontal force during the scratching of films coated on a substrate showed large discrepancies with the experimental results. In this work, the model was modified with a plowing term to minimize the difference between the experimental and theoretical results. Using the modified model, the experimental results could be predicted with an accuracy of about 10%.     

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.     

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。     

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.     

Finite element modeling of the scratch response of a coated time-dependent solid

Scratch tests is one of the most efficient tests to investigate the mechanical resistance of coated and uncoated surfaces. Nevertheless, the complexity of material and interface makes difficult the comprehension of this test. For that purpose, efficient computational modeling is required. In this paper, we present a remeshing procedure specially developed for the computational modeling of scratch tests of coated materials. This procedure allows to perform scratch tests with high ratio penetration depth over layer thickness. Then, it is used to investigate the influence of the scratching velocity on the scratch behavior of a polymer substrate coated with a hard elastic coating. The substrate is considered as an elastic–viscoplastic solids and the coating follows a linear elastic behavior. First macroscopic results such as material deformation, scratch hardness and apparent friction coefficient are presented. Then the stress distribution in the film and at the coating/substrate interface are analyzed regarding the cohesive or interfacial failure of the system. A simple interfacial failure criterion is also proposed.     

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.     

Critical Load Analysis for a Layered Half-Space under Sliding Indentation

Analyses are presented for cracking or delaminating problems of thin coatings on dissimilar thick substrate materials under a normal and a tangential force. Based on the energy principle associated with the consideration of substrate deformation, an analytical expression has been proposed to predict the critical load of an adherent stiff thin coating on a compliant substrate. The critical load has been shown to depend on the hardness, the coating thickness, the surface energy of adhesion, the coefficient of friction, contact areas of a scratch track, and elastic moduli of coatings and substrates. Experimental observations of microscratch testing on a CrN coating deposited on a soft coating, labeled as SC-I, over a Cu/Zn alloy substrate have shown that buckling and cracking in a semicircular arc ahead of an indenter are the predominant failure modes, thereby confirming the assumption of the theoretical calculation. Using Beuth's solution, a simple fracture model describing the cracking of thin coatings has been developed over a range of practical elastic mismatches and applied to solve cracking problems of compliant coatings in this work. Microscratch results from another soft coating, labeled as SC-II, on a Cu/Zn alloy substrate have revealed that cracks first occur along the edge of the circular contact area in the rear of the indenter due to the tensile stress. The stress formulae of Hamilton and Bower and Fleck are therefore introduced into this model to compute the critical load required to initiate a crack in the coating and the stress intensity factor of the coating.

     

Three-dimensional finite element modelling of the scratch test for a TiN coated titanium alloy substrate

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.     

Scratching maps for polymers

The scratching technique has gained interest in recent times due to its varied applications to a number of engineering materials, especially for the evaluation of surface scratch resistance of plastics. Scratching provides a convenient and reliable means to investigate the mechanical properties of organic polymers under various contact conditions. The scratch hardness method is widely adopted to provide a first-order evaluation of the relative scratch resistance of materials for comparison purposes. The method also allows the identification and the assessment of the surface deformation processes and maps defining the scratch deformation modes as a function of contact conditions may be generated. These scratching maps may present experimental results in terms of the deformation mechanism, the scratch hardness and the friction coefficient. This paper primarily provides a review of the application of scratching maps for polymers. Results for the scratch hardness and the deformation mechanisms for a poly(methylmethacrylate) (PMMA), a poly(tetrafluoroethylene) (PTFE) and an ultra-high molecular weight poly(ethylene) (UHMWPE) are presented. The PTFE system is also described following the effects of γ-irradiation; radiation produces a marked reduction in toughness. The scratches were produced on the polymer surfaces by cones and spheres of various size under a number of contact conditions (e,g, applied normal load, strain, scratch velocity, etc.). SEM imaging and laser profilometry are used for the study of the deformation mechanisms and the measurements of the scratch profiles. It is shown that polymers exhibit a wide range of scratch deformation characteristics and that the deformation mechanism is determined by the most efficient energy dissipation process for the particular external constraints.     

Investigation of scratch resistance of thin carbon overcoat media using micro scratch testing

This paper investigates the scratch resistance of thin carbon overcoat media using a novel micro scratch testing method. Ramp loading was applied to a carbide blade, scratching along a rotating hard disk surface and the load corresponding to the penetration of the carbon overcoat, referred to as the critical load, was used as the measure of scratch resistance. The contact electric resistance (CER) at the cutting point of the blade/media interface was monitored through the scratch process and the critical load was determined when the CER dropped to zero. It has been found that CER monitoring is a very accurate and reliable way of determining the critical load. Scratches were characterized using scanning electronic microscopy (SEM) and it has been found that at the critical point the carbon overcoat was delaminated, peeled off, or plowed away. The micro scratch testing has shown to have the sensitivity to differentiate the effect of carbon thickness, carbon type, lubricant, and substrate type on media scratch resistance.     

Substrate Effects on the Micro/Nanomechanical Properties of TiN Coatings

Nanoindentation and nanoscratch tests were performed for titanium nitride (TiN) coatings on different tool steel substrates to investigate the indentation/scratch induced deformation behavior of the coatings and the adhesion of the coating–substrate interfaces and their tribological property. In this work, TiN coatings with a thickness of about 500 nm were grown on GT35, 9Cr18 and 40CrNiMo steels using vacuum magnetic-filtering arc plasma deposition. In the nanoindentation tests, the hardness and modulus curves for TiN/GT35 reduced the slowest around the film thickness 500 nm with the increase of indentation depth, followed by TiN/9Cr18 and TiN/40CrNiMo. Improving adhesion properties of coating and substrate can decrease the differences of internal stress field. The scratch tests showed that the scratch response was controlled by plastic deformation in the substrate. The substrate plays an important role in determining the mechanical properties and wear resistance of such coatings. TiN/GT35 exhibited the best load-carrying capacity and scratch/wear resistance. As a consequence, GT35 is the best substrate for TiN coatings of the substrate materials tested.     

Surface and subsurface damages and magnetic recording pattern degradation induced by indentation and scratching

Magnetic recording pattern degradation due to head–disk impact and scratching are simulated by static indentation and scratch testing, respectively, on a pre-recorded thin film magnetic recording disk. Different magnitude of controlled stresses were used to induce stress and physical damage to the magnetic recording disk resulting in erasure and distortion of the magnetic recording pattern. Both nanoindentation and scratching resulted in the elastic–plastic deformation of the multilayer coating of the magnetic recording disk but in different relative magnitude and types of in-plane stresses (which are effective in causing magnetization changes). For residual indentation and scratch depths of the order of the magnetic disk coating thickness, magnetization changes in the recording pattern were observed even though the protective carbon overcoat was not damaged. Large magnetic pattern distortion and erasure results where cracks and pileups, and delamination and buckling damages were observed for deeper indentation marks and scratch grooves, respectively.     

Micro-scale abrasive wear testing of duplex and non-duplex (single-layered) PVD (Ti,Al)N, TiN and Cr–N coatings

A micro-scale abrasive wear test, based on ball-cratering, has been used to evaluate the wear resistance of duplex and non-duplex (Ti,Al)N, TiN and Cr–N coatings. The term duplex is used here when plasma nitriding is followed by PVD coating. Coatings without the plasma nitriding stage are termed single-layered. Coating properties were evaluated by surface profilometry, hardness and scratch testing. All duplex coatings showed higher micro-abrasive wear resistance than their single-layered counterparts, with the duplex (Ti,Al)N coating achieving the best performance. After a certain number of ball revolutions, the coating material became worn through, exposing the substrate material. After this point, the presence of a hard nitrided case diminished the scratching action of the SiC abrasive particles. The experimental results also indicate that the choice of the PVD coating plays an important role in improving the micro-abrasive wear resistance. Apart from single-layered and duplex Cr–N coatings, all the other coating systems provided a higher micro-abrasive wear resistance than the uncoated substrate (hardened AISI H13 steel). The poor abrasive wear resistance recorded for the single-layered and duplex Cr–N coatings could be attributed to the hardness of the Cr–N being much lower than that of the SiC abrasive particles, which caused tearing of the coating with subsequent delamination. The wear pattern observed was found to change from surfaces characterised by grooves (uncoated substrate, single-layered TiN and Cr–N systems and duplex Cr–N system) to surfaces which exhibited multiply indented surfaces (single-layered and duplex (Ti,Al)N systems), indicating a transition between wear mechanisms. This transition was found to be dependent on the ratio between the hardness of the SiC abrasive particles and surface (coating) or subsurface hardness. By decreasing this ratio, the ability of the SiC abrasive particles to scratch the composite surface was reduced and the resistance to micro-scale abrasion was improved.     

Experimental study of mechanical properties and scratch resistance of ultra-thin diamond-like-carbon (DLC) coatings deposited on glass

To improve the mechanical and tribological properties of glass, an ultra-thin diamond-like-carbon (DLC) coating of 2.2 nm thick was deposited on the surface of glass, using the linear ion beam deposition technique. The coated glass showed significant improvement in scratch resistance against severe damage, such as cracking, delamination, and chipping. To understand the mechanism, an experimental study was carried out. It was found that the major contribution to the significantly improved scratch resistance attributable to the compressive stress, which was yielded during the deposition process and resides in the coating, as well as the top layer of glass substrate.     

新型多功能膜/基结合力测定仪的研制

采用改进划痕法原理及切向力检测临界载荷方法，研制了一种新型多功能膜／基结合力测定仪。该仪器结构新颖，实用简便，不仅能测定金属膜层与基体的结合强度，还能对金属膜层进行模拟单颗粒磨损试验。试验表明，该仪器具有较高的可靠性。     

Theoretical study of adhesion energy measurement for film/substrate interface using pressurized blister test: Energy release rate

In this paper, a new loading method able to subtly control the crack driving force for pressurized blister test was proposed. A theoretical study of the adhesion energy measurement for film/substrate interface using this loading method was presented. Problems considered include solving the exact volume under a circular blister and the elastic strain energy stored in a thin blistering film, determining the work done by the poured colored liquid as external force to the system and the elastic strain energy stored in the compressed air. A new formula of energy release rate was finally presented. A comparison between the work presented here and the existing work was made.     

Grey theory applied to evaluate the tribological performances of the a-C:H(N) coating films prepared by differing the nitrogen content and the film thickness

The purpose of this paper is to study the tribology performances of the aC:H(N) films by using a nanotester under different scratch loads and velocities. From the measurements of the friction coefficient and wear volume, the tribological performances including wear resistance and friction coefficients were evaluated for the hydrogenated amorphous carbon films prepared by differing film thickness and nitrogen volume friction in the gas mixture of (C₂H₂+N₂). Taguchi experimental design and the grey relational analysis were used to investigate the influence of specimen parameters (film’s thickness, nitrogen content in the film), and operating conditions in tribological tests (scratch load and scratch velocity) on the friction coefficients and the wear volume arising in the specimens with different coating films. It is found that the wear volume of thin film is increased by increasing either the nitrogen volume fraction or film thickness. Moreover, the optimal combination of the testing parameters was also determined in the use of the present model.     

AZO薄膜理想表面形貌的制备与加工工艺优化

非晶硅薄膜太阳能电池主要采用掺氟氧化锡(FTO)导电玻璃作为基板,但FTO薄膜雾度较低、表面形貌无法优化,导致无法得到较优的陷光结构,从而限制了太阳能电池的转换效率。为了进一步提升太阳能电池的转换效率,探讨了替代型的掺铝氧化锌(AZO)薄膜,通过优化前段磁控溅射镀膜工艺和后段湿化学蚀刻工艺,用以平衡AZO薄膜的光电性能和雾度,从而获得具有理想表面形貌的AZO导电玻璃,使其成为理想的非晶硅薄膜太阳能电池的基板材料。实验表明,经工艺优化后制作的AZO导电玻璃可提升光电转换效率。     

Factors Influencing the Abrasion Resistance of Thin-Film Magnetic Media

Abrasion testing was performed on DC sputtered hydrogenated carbon films on commercial thin-film disks processed under several conditions. Four film characteristics were found to influence the abrasion resistance of the disk, i.e., roughness, film adhesion, carbon toughness, and coefficient of friction. It was found that both fine-scale surface topography from the sputtered layer and substrate texture degraded abrasion resistance. Excellent abrasion resistance was observed for carbon films as thin as 10 nm on polished substrates when the magnetic film topography was minimized. Film adhesion was degraded by exposure to temperature and humidity. Hydrogen incorporation into the carbon films reduced film hardness which degraded abrasion resistance.     

Adhesion and abrasive wear resistance of TiN deposited on electrical discharge machined WC–Co cemented carbides

Electrical discharge machining (EDM) is a non-traditional machining method extensively used to manufacture complex geometries of hard and brittle materials such as WC–Co cemented carbides (CC). Although the thermal action of the EDM process is known to yield a relatively poor surface integrity in these materials, it may be minimized through the implementation of multi-step sequential EDM and post-EDM surface treatments. Particularly, hard coating application has been demonstrated to be effective for decreasing the EDM-induced mechanical degradation. However, additional studies are required on such coating–EDMed substrate systems to determine other crucial properties in terms of applications, e.g. adhesion and micro-scale wear behaviour. In this work the adhesion strength and the microabrasive wear resistance of TiN deposited on EDMed substrates have been evaluated by means of scratch and crater grinder testing, respectively. The results indicate that both critical load for decohesion of the coating from the substrate and coating specific wear rate increase with finer-executed EDM, reaching values close to those measured for a TiN coating deposited on a ground and polished substrate.