Indentation behavior of polymer coatings on glass

For relatively soft polymer coatings on soda-lime glass substrates the indentation load increases substantially when the indenter penetrates into the glass substrate since the glass can now directly support some of the indenter load. A model for the indentation load-depth behavior 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 indentation behavior as a function of 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.     

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.     

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.     

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。     

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.     

Effect of Crystallites on Surface Damage and Fracture Behavior of a Glass-Ceramic

A study was conducted of the effect of crystallization on the fracture toughness, strength, and resistance to surface damage of glass-ceramic materials with a range of microstructures obtained by different heat treatments. The hardness indentation method was used as a quantitative tool to simulate mechanical surface damage. In the uncrystallized glass and in the glass-ceramic heat-treated to result in a uniform fine-grained structure, crack size increased monotonically with indentation load. In contrast, in the glass-ceramics heat-treated to result in a microstructure consisting of larger crystallites (a few micrometers) contained within a fine-grained matrix, a discontinuity in the crack size vs load curve presented evidence for crack-pinning at crack sizes which were a small multiple of the intercrystallite spacing. At the position of crack-pinning, the fracture toughness showed a discontinuous increase with increasing crack size that was attributed to crack deflection. The strength of the glass and fine-grained glass-ceramic measured in biaxial flexure decreased monotonically with indentation load. The strength at low values of indenter load of the glass-ceramic heat-treated to yield the coarser crystallites within the fine-grained matrix was independent of indentation load, indicating stable crack propagation prior to fast fracture. At the higher values of indenter load, the coarse-grained glass-ceramics exhibited a monotonic decrease in strength with increasing indentation load. The results of this study indicate that the strengthening observed on crystallization of a glass can be attributed to a combination of a decrease in flaw size achieved at a given mechanical surface treatment, an increase in fracture toughness, and a modification in the mode of crack propagation.     

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.     

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.     

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.     

Determination of epoxy coating wet-adhesive strength using a standardized ASTM/ISO scratch test

A new approach for evaluating the wet-adhesive strength of epoxy-based coatings was carried out based on a recently standardized ASTM/ISO scratch test. A linearly increasing scratch normal load was applied during scratch to induce progressively increased delamination stress at the coating and steel substrate interface. Thus, the applied critical load to cause coating debonding can be experimentally determined. To find out the corresponding stress magnitude to incur coating debonding, finite element methods (FEM) modeling was conducted to analyze the stress fields around the scratch tip during scratching. The wet-adhesive strength is then quantitatively determined. Based on the above methodology, investigation on a set of model coating systems suggests that the critical load for coating delamination is significantly influenced by water exposure time, coating thickness, and substrate surface roughness. By combining the standardized scratch tests and FEM modeling, the proposed approach is found to be effective for quantitative assessment of epoxy coating wet-adhesive strength and for the development of high performance protective coatings for various industrial applications.     

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.     

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.     

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.     

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.     

Effects of Grain Size of Hot-Pressed Silicon Nitride on Contact Damage Morphology and Residual Strength

Effects of the grain size of hot-pressed Si₃N₄ on contact damage morphology and residual strength were studied using the elastic/plastic indentation method with a spherical indenter. The contact damage, initially formed with increasing indentation load, was Hertzian cracks in the Si₃N₄ consisting of fine grains (mean grain size: 0.2 μm). In the coarser-grained Si₃N₄ (mean grain size: 0.8 μm), there was a damage texture, consisting of grain-sized microcracks. The residual strength was degraded at a load slightly higher than the critical load for contact damage formation. The strength degradation was not caused by contact damage but the residual stress formed around the impressions.     

Hertzian contact damage in a highly porous silicon nitride ceramic

Hertzian indentation tests were performed to evaluate the contact damage behavior of a highly porous Si₃N₄ ceramic. Using a bonded-interface technique, the Hertzian contact damage patterns were examined. As a result of intragranular microfracture under Hertzian contact, a distributed subsurface damage region is developed beneath the indenter. It was found that the damage region extends progressively with increasing contact load. In strength tests, failures were observed to originate from Hertzian indentation sites, giving rise to a gradual strength degradation.     

Strength Degradation of Thermally Tempered Glass Plates

An earlier theory of contact-induced strength degradation of brittle materials is extended to include plates in residual surface compression. The scale of the strength-controlling flaw is predicted by indentation fracture mechanics, with the modifying effect of the residual field incorporated into both indentation and strength equations. Experimental verification of the predictions is obtained from diamond-pyramid indentation tests on thermally tempered glass plates. As with untempered plates, the theory accounts for the load dependence of the strength loss; it also explains the insensitivity of the degradation characteristics to initial flaw distribution and identifies toughness as the controlling material parameter. Most significant, however, is the demonstration that surface strengthening can produce dramatic improvements in degradation resistance. The possibility of obtaining all parameters necessary for a complete degradation analysis of a given tempered inaterial entirely by routine indentation/strength testing is discussed.     

Effect of Aging on the Mechanical Properties of UV Curable Optical Fiber Coatings

Abstract

A dual-layer urethane acrylate UV-cured coating is widely used to protect optical fibers because of its well-balanced mechanical properties, weathering resistance and rapid curing. The long-term mechanical behavior of fiber coatings is important for the reliability of optical fibers. Long-term exposure of UV-cured polyether urethane acrylate films was carried out in dry air and in water at elevated temperatures. Tensile testing was performed to reveal changes in mechanical properties and dynamic mechanical analysis to determine both the glass transition temperature and the crosslink density. The equilibrium swelling allowed assessment of the crosslink density. Tensile testing and strip force measurements were performed on virgin and aged optical fibers. Initially the fracture strengths of the secondary coatings increased under all aging conditions indicating post-curing reactions and the possible loss of uncrosslinked species. Aging under wet conditions led at a later stage to hydrolytic degradation of the network and to a decrease in the fracture stress. The equilibrium swelling and equilibrium modulus measurements showed good correlation with the changes in strength. The primary coatings showed a decrease in mechanical strength after only 2–4 weeks under all conditions.