Effectiveness of polymer coatings on reducing indention damage in glass

Polymer coatings are widely used to protect glass from indentation damage. A model for the strength degradation that occurs when a sharp indenter penetrates through the coating is developed by accounting for the indentation load shared by the coating and substrate. This model accounts for the additional load supported by the coating due to the pile-up of coating material underneath the indenter. The model predicts the strength degradation as a function of indentation load, coating and substrate hardnesses, and coating thickness. Comparison of the model to experimental data for a wide range of polymer coatings (two epoxies, epoxy acrylate, and urethane acrylate) on soda-lime glass substrates shows good agreement.     

Damage resistance of SiC and Si3N4 coatings with control of microstructure and thickness

We investigated the contact damage and indentation stress–strain behavior of silicon carbide (SiC) coatings and binary coatings consisting of SiC and silicon nitride (Si₃N₄), synthesized on graphite substrates with porosities of 10 and 13% by a solid–vapor reaction, in order to determine the coatings’ damage resistance. The coating thickness was affected by the porosity of the substrate. The coatings on the substrate with 13% porosity showed a graded interface structure below the top dense layer. The SiC coatings were thicker than the SiC/Si₃N₄ composite coatings. The SiC coatings made the substrates hard, and SiC-coated substrates exhibited higher stress–strain curves than the substrates alone, but the SiC/Si₃N₄ composite coatings appeared unaffected. The coating thickness played an important role in limiting the effect of damage. The hardness values of the SiC coatings were higher than those of the substrates and the SiC/Si₃N₄ coatings. These corresponded well with the indentation stress–strain curves. The values of each coating showed saturated points depending on the applied load. This indicated that the substrate itself influenced the damage resistance of the coatings because of the layered structure of a harder coating with a softer substrate. The coatings enhanced contact damage and transmitted the damage to the substrates at a high load of P = 2000 N. Both coatings showed an extensive subsurface damage, independent of the porosity of the substrate. In cyclic indentation tests, the contact diameters linearly increased with the number of cycles and depended on the porosity of the substrate, showing smaller contact diameters by coating the substrate.     

Evaluation of the elastic-plastic properties of SiCN coating system by finite element simulations

The elastic properties of SiCN coating on substrates can be evaluated by nano-indentation test, however, it is challenging for experiments to evaluate the plastic performance of SiCN coating. Finite element (FE) is a numerical method for investigating in-depth mechanical behavior of various structures. In this paper, a contact model between Berkovich indenter and SiCN/Si system is established by FE method. The stress-strain behavior of SiCN coating is obtained by comparing the calculated P-h curves with experimental results. The indentation depth dependent elastic modulus and hardness of the SiCN coating are calculated from the P-h curves and are close to the experimental data. When the indentation depth is in excess of 10% of the coating thickness, the mechanical properties of SiCN coating tend to be influenced by the Si substrate, which also consists with experiments. The proposed approach provides an efficient tool to predict the mechanical properties of SiCN coating.     

Laser Zone Melting and microstructure of waveguide coatings obtained on soda-lime glass

This study presents a Laser Zone Melting method with potential for producing planar waveguides at large scale, based on the surface coupling of two chemically compatible glass layers which exhibit distinct indices of refraction. The method is based on a recent patent, particularly applicable to process glass and ceramics with low thermal shock resistance. Glass coatings containing 76.24% by weight PbO are thus here reported, as obtained by this method on commercial soda-lime planar glass substrates. Their higher indices of refraction (1.58 vs 1.52 for commercial soda-lime glass) result in attractive waveguiding potential, as demonstrated with measurements using focused light from a He-Ne laser beam. Scanning and transmission electron microscopy studies reveal excellent integration and compatibility between the observed coatings and substrates, where diffusion in the proximity of the interface was studied by EDS analysis. Crystalline phases have not been found within the coating, or within the substrate, as concluded from the absence of Bragg-peaks in XRD experiments.     

Mechanics of the indentation test and its use to assess the adhesion of polymeric coatings

The indentation test provides a simple means by which the adhesion of coatings can be qualitatively assessed. On the way to establishing a quantitative measurement of the adhesion strength of coatings and films, it is important that the mechanics of this test are clearly understood. To investigate the influence of factors such as the coating thickness, the indenter radius, and friction during the test, numerical simulations of the indentation of a typical polymeric coating, polymethylmethacrylate (PMMA), bonded to a rigid substrate were conducted by using the finite element method. The stress generated during the indentation test were obtained by employing an accurate constitutive model of the elastic-viscoplastic behaviour of the polymeric coating under consideration. The results of this analysis illustrate the effects of the factors mentioned above on the deformation of the coating during indentation, its confinement, and interfacial shear, and the normal, shear, and hoop stress distributions occurring during indentation. These results provide insight into the possible failure mechanisms operative during the indentation of thin coatings and the important effects of the coating thickness during such tests.     

Nanoindentation-induced delamination of submicron polymeric coatings

An elastic model is developed to estimate the interfacial strength between a submicron surface coating and a compliant substrate. The analysis uses a shear-lag model and assumes the plane-stress state in the surface coating. The critical indentation load for the indentation-induced delamination of the coating from the substrate increases with the third power of the indentation depth and is a linear function of the reciprocal of the coating thickness. The indentation-induced delamination of SR399 ultrathin surface coatings over acrylic substrate has been evaluated, using the nanonindentation technique for coating thicknesses of 47, 125, 220 and 3000 nm. For the submicron coatings, the dependence of the critical indentation load on the coating thickness supports the elastic model. The interfacial strength is found to be 46.9 MPa. In contrast, the polymeric coating of 3000 nm displays multiple “excursions” in the loading curve, and the critical indentation load is a linear function of the indentation depth.     

Surface damage resistance and yielding of chemically strengthened silicate glasses: From normal indentation to scratch loading

We report on surface elasticity, plastic deformation and crack initiation of chemically strengthened soda-lime silicate and sodium aluminosilicate glasses during lateral indentation and scratch testing. Instrumented indentation using a normal indenter set-up corroborated previous findings on the effects of chemical strengthening on surface Young's modulus, hardness, and indentation cracking. Using lateral indentation in the elastic-plastic regime, we find a pronounced increase in the scratch hardness as a result of chemical strengthening, manifest in higher work of deformation required for creating the scratch groove. Thereby, the glass composition is found to play a stronger role than the absolute magnitude of surface compressive stress. Using a blunt conical stylus for instrumented scratch testing reveals three distinct modes of scratch-induced surface fracture, which occur during scratching or after unloading. Occasional micro-cracking caused by pre-existing surface flaws at low scratching load can be completely suppressed through chemical strengthening. The intrinsic defect resistance to microcracking is reduced as a result of ion stuffing, depending on the initial glass composition, whereas the resistance to abrasive yielding is enhanced by several hundred MPa.     

A study on the elastic modulus of silicone duplex or bi-layer coatings using micro-indentation

A Fischerscope continuous microindenter with a spherical indenter was used to obtain maximum indentation load and depth data for a 2.2 mm sheet of RTV11 (a silicone elastomer), a 1.6 mm sheet of J501 (an elastomer containing 60% silicone and 40% butyl acrylate styrene) as well as six duplex elastomeric coatings. The duplex coatings consisted of RTV11 top coat and J501 bond coat. The Waters’s empirical relationship was used to determine the modulus of elasticity E for the RTV11 and J501 sheets. The Waters’s relationship was then used to determine the equivalent modulus, E_c, for duplex coatings from maximum indentation load versus elastic indentation depth data. The values of E_c as determined from the Waters’s model (and experimental data) were in good agreement with the values obtained by an equivalent stiffness method. By being able to determine E_c from the equivalent stiffness method and using this value in the Waters’s model, one may determine the load versus elastic depth of indentation for duplex coatings.     

Gradient MoSi2-borosilicate glass high emissivity coating with enhanced contact and impact damage resistance

High emissivity coatings on fibrous insulation tiles played an important role in thermal protection systems and thereby intrigued many researchers; however, there was little emphasis on the mechanical properties of the coatings. In this study, a gradient MoSi₂-borosilicate glass coating with a dense surface layer and a porous interlayer was designed for mullite fibrous ceramics. Mechanical properties and structure parameters of the coating layers and the substrate were studied. The gradient coating was compared with a monolayer dense coating of the same composition and same surface density in contact damage resistance, impact resistance and emissivity. Compared with monolayer dense coating coated substrates, the gradient coating coated ones exhibited two times higher load bearing capacity in Hertzian indentation test at the same displacement of 1?mm; they appeared to be stiffer and harder at constant load of 20?N, and showed better impact resistance at impact energy range of 0.25–0.75?J in the falling weight test; besides, fatal radial cracks were not observed in gradient coatings after the tests. In addition, the gradient coating had higher emissivity (0.838) than the monolayer dense coating (0.816) because of the significant absorptivity increase and reflectivity decrease by small gradual slopes in the rough surface.     

Adhesion of diamond coatings on steel and copper with a titanium interlayer

Adherent diamond coatings on steel and copper were obtained by using a titanium interlayer. The adhesion of the coatings was evaluated by scratch tests and micro-indentation tests. The diamond coating on steel exhibited a much higher critical load than on copper, as revealed by the scratch tests. However, an observation on the back of the scratch-delaminated film and on the corresponding substrate surface showed that the detachment occurred between the diamond film and the titanium interlayer. Therefore, the difference in the critical scratch load is due mainly to a substrate effect, making it difficult to compare the adhesion of different coatings.On the other hand, Knoop indentation tests showed interesting results: a small indentation load causes round spallation in the film with no observable crack. An exponential sink-in deformation under the indentation is proposed, y=−a exp(−bx). The coating adhesion is considered to be equivalent to the deformation stress at the edge of the spallation zone. The adhesion of diamond coatings on steel and copper with a titanium interlayer is evaluated quantitatively using this model. Furthermore, a thermal quench method is proposed to estimate the coating adhesion. The results found are in agreement with the indentation model.     

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3 GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8 GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10 GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.     

Cracking analysis of HVOF coatings under Vickers indentation

The fracture strength of five HVOF coatings, which are made of hard metals, Tribaloy alloy, and superalloys, respectively, coated on 1018 low carbon steel substrate, is studied under Vickers indentation, associated with FEA stress computation. The cross sections of the coating specimens are examined on a Hitachi Model S-570 scanning electron microscope (SEM), which investigates the quality and measures the geometry of the coatings. The mechanical properties of the coatings and the substrate are determined in the cross sections using the nano-indentation technique. The cracking behavior of the coatings under different indentation loads is investigated using a Vickers hardness tester. Three-dimensional finite element analysis (FEA) simulation of the Vickers indentation test is conducted to determine the stress fields in the coating/substrate systems in order to understand the fracture mechanisms of the coatings under the indentation loads using the ABAQUS software package. The FEA stress results are in good agreement with the experimental observation of Vickers indentation.     

Analytical estimation of fracture toughness for circumferential cracks in thin hard films on ductile substrates

In this study, the fracture toughness of circumferential crack caused by indentation effect of a rigid indenter on a thin and elastic coating deposited on the elastic substrate was calculated. In the coating and substrate, the analytical solution of displacement and stress field was used. The complete adhesion was considered for the coating on the substrate. The location of maximum circumferential stress was investigated using the analytical calculation of the stress and it was found that this place was located at a distance away from the center of the indenter. Then, the stress intensity factor and energy release rate for plane strain state was determined, and consequently, the energy release rate for a channel crack was calculated. Finally, the fracture toughness was calculated with energy release rate curves for plane strain crack and crack channeling. This method was used to calculate the fracture toughness of TiN/TiCrN ceramic multilayer coating which was deposited on the GTD450 substrate using the Cathodic Arc PVD method. To validate the results, the analytically calculated crack radius was compared with the experimental crack radius in the fracture load and the difference between the radiuses was in the acceptable range.     

压痕法和十字交叉法评价类金刚石硬质涂层的界面结合强度

涂层与基体界面结合强度是硬质涂层材料一个关键的性能指标。应用压痕法和十字交叉法测试了硅基/类金刚石(diamond-like carbon,DLC)涂层的界面结合强度。结果表明:利用Vickers压痕法和Hertz压痕法测量所得硅基/DLC涂层的临界载荷分别为0.981N和300N。用Vickers压痕法测量时,载荷达到临界载荷后涂层将产生环状开裂,当载荷进一步增大时,还会产生径向裂纹;对于Hertz压痕法,载荷从300N增加到800N时,涂层环状裂纹从1个增加到4个。通过采用十字交叉法测量得到硅基/DLC涂层界面拉伸强度和剪切强度分别为(8.9±2.7)MPa和(20.1±2.6)MPa,表明该涂层抗剪切性能良好,拉伸分离后界面比剪切分离后界面的均匀性更好。压痕法和十字交叉法评价硬质涂层的界面强度简单易行,结果准确,具有广泛的应用前景。     

Comparison of the adhesion of diamond films deposited on different materials

Indentation tests combined with acoustic emission spectra were used to compare the adhesion of diamond films deposited on various substrates, including Ti, Cr, Si and Ti coated Cu. We show that indentation in the diamond coatings may cause the following failure modes: (a) the substrate cracking; (b) the film cracking and localised detachment; and (c) the film delamination and the delamination propagation. Acoustic emission during indentation loading provided essential information in predicting what mode of failure occurs. Combined with the acoustic emission spectra, the indentation tests are reliable in comparing the adhesion of diamond films deposited on the same or similar substrate materials. However, the comparison of the film adhesion on very different substrates, like Cu and Ti, is not so straightforward. Acoustic emission spectra also revealed that indentation caused substrate cracking prior to the failure of the film/substrate interface for diamond coatings on Si. In this case, the indentation tests are not valid to compare the coating adhesion.     

Mechanical properties of thermally sprayed porous alumina coating by Vickers and Knoop indentation

Depending on the thermal spraying conditions, coatings obtained can present different defects, like pores, cracks and/or unmelted particles, and different surface roughnesses, that can affect the determination of the hardness and elastic modulus. The present work investigates the mechanical properties, determined by means of Knoop and Vickers indentations, of a plasma as-sprayed alumina coating, obtained with a nano-agglomerated powder sprayed using a PTF4 torch, in order to highlight how the surface defects interfere into the indentation process. As a main result, Knoop indentation compared to Vickers one gives less dispersive results (15% and 33%, respectively), that are, in addition, more representative of the coating properties. The mean values obtained are 110 ± 40 GPa for the elastic modulus and 1.75 ± 0.42 GPa for the hardness. In addition, and for the two indenter types used, multicyclic indentation has been performed because it allows a more appropriate characterization of such heterogeneous coatings due to the representation of the mechanical properties as a function of the indentation load and/or the penetration depth, leading to more reliable results according to the depth-variability of the coating microstructure.     

Acoustic Emission during Indentation Fracture

Acoustic emission measurements were made during controlled indentation of soda-lime glass. Pulses were detected at specific points during the loading or unloading cycles. The response and reproducibility were dependent upon the indenter shape. Comparison with earlier data reported in the literature showed that the acoustic activity was correlated with crack initiation events corresponding to median/ radial, lateral, or other fracture modes.     

Imaging in evaluation of polymer coatings adhesion to glass at high humidity

Adhesion reduction occurring after polymer coated glass was immersed in water was studied in a variety of UV-cured urethane acrylate coatings containing alkyloxysilane adhesion promoting additives. It was observed that water accumulated under the coating surface in drops and formed ‘blisters’ in the glass-polymer interface. A non-destructive imaging technique was developed to measure the average size of the water blisters. The size of the water blisters within the interface was correlated with the wet adhesion force measured by coating resistance to 180° peel. The force of coating resistance to 180° peel off glass surface decreased non-linearly with the increase of the average size of the water blisters. It was concluded that the decrease in adhesion between the coating and glass was a result of stretching and breaking of the silane bridging bonds and polymer fibrils by water condensing on the glass surface within the polymer-glass interface. The mathematical model relating coating wet peel resistance force with the size of the debonding produced by water accumulation was presented.     

Nanoindentation-induced deformation behaviour of tetrahedral amorphous carbon coating deposited by filtered cathodic vacuum arc

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-μm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate.