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。     

Effect of Flaw State on the Strength of Brittle Coatings on Soft Substrates

A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ , using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated-load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack-wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome.     

Modelling the strengthening of glass using epoxy based coatings

Glass strength can be increased by applying epoxy based surface coatings. A number of models have been presented in the literature to explain the strengthening afforded by these coatings but until now there has been no clear evidence to definitively support one model over another. In this work, finite element models (FEM) of four-point bending test specimens have been developed. These models have been used to study the strength of cracked uncoated and surface coated specimens in order to identify the strengthening mechanism. The FEM results showed that full filling of the crack using epoxy coating is sufficient to heal the crack if the coating inside the crack is ideally glued to the crack surfaces. It is also shown that under these circumstances the coating modulus is relatively unimportant parameter. FEA results for partially filled cracks show that increasing the filled percentage increases the strengthening. Fractographic analysis of the 10 kg indented and coated samples showed that the fracture surfaces do not follow the median crack symmetric plane and that fracture started from another plane when coated properly, however the fracture surface of these samples still starts from the indentation site. On the other hand, fractographic analysis of the 1 kg indented and properly coated samples showed that the samples failed from their edges which indicate that the crack was overcome. The finite element results show that the diamond imprint resulting from the Vickers indentation play an important role in this type of fracture.     

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.     

Controlled Flaws in Ceramics: A Comparison of Knoop and Vickers Indentation

Application of indentation fracture analysis to Knoop and Vickers indentation is examined, with particular emphasis on determining the limitations of the point force representation for the residual stress field. Deviation from the point force approximation is insignificant for crack-size/plastic-zone-size ratios 1.3. The Vickers deformation/fracture configuration in brittle materials invariably conforms to this requirement, whereas the Knoop configuration does not (except at very high indentation loads). However, stable crack growth during a failure test extends the crack sufficiently that the strength degradation for both types of indentation is well described by the point force approximation.     

Measurement of Residual Stresses in Coatings on Brittle Substrati by Indentation Fracture

A method for evaluating stresses in coatings on brittle substrates by indentation is described. The basis for evaluations is fracture mechanics model of the radial crack system in the Vickers geometary, incorpeorating the effects of a thin surface demonstrate the methodology. The crack size on these coated specimens are found to be considerebly small than those on uncoated controls, indicating substantial (∼50 MPa) in-plane expansions observed after applying the coatings to already indented sufaces, are found to make an unexpectedly large contribution to the fracture susceptibility. The procedure offers a simple means for quantifying the mechanical integrity of coating configuration for ceramic components.     

Anisotropic mechanical properties and contact damage in air-plasma-sprayed thermal barrier coatings with bond coating nature and thermal exposure condition

The anisotropic mechanical properties and contact damage of air-plasma-sprayed (APS) zirconia-based thermal barrier coatings (TBCs) have been investigated using Vickers and Hertzian indentation tests as functions of the nature of the bond coating and the degree of thermal exposure. The hardness values of the TBC systems are dependent on the applied load at relatively low loads, and became saturated at a load of 30 N, independent of the nature of the bond coating or the degree of exposure. The values of the top coating obtained on the top surface from the Vickers indentation tests were higher than those on the sectional plane, indicating that there is an anisotropic strain behavior due to the microstructure. The regions near to the interface of the top coating and the thermally grown oxide (TGO) layer show higher values after thermal exposure, whereas the values of the APS bond coating increased and the indentation values of the high-velocity oxygen fuel (HVOF) sprayed bond coating slightly decreased after thermal exposure, owing to resintering and element deficiency during thermal exposure, respectively. In contact damage tests, the TBC system with the HVOF bond coating showed less damage than the TBC system with the APS bond coating. The shape of the damage was different between the two systems. After thermal exposure, the damage was reduced in both TBC systems, and the cracking or delamination formed at the regions near to the interface of the top coating and the TGO layer in both TBC systems.     

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.     

Effect of Tangential Loading on Critical Conditions for Radial Cracking in Brittle Coatings

Results of Hertzian contact tests investigating the effects of superposed tangential loads on the critical conditions for radial cracking at the undersurfaces of brittle coatings on compliant substrates are reported. It is demonstrated that these effects are secondary, so that conventional normal indentation remains an appropriate test procedure for characterizing this highly deleterious mode of coating fracture under a wide range of complex loading conditions.     

Densification effects on the fracture in fused silica under Vickers indentation

This study investigates the effects of densification on the deformation and fracture in fused silica under Vickers indentation by both the finite element analysis (FEA) and experimental tests. A refined elliptical constitutive model was used, which enables us to investigate the effects of the evolution of yield stress under pure shear and elastic properties with densification. The densification distribution was predicted and compared with experiments. The plastic deformation and indentation stress fields were used to analyze the initiation and morphology of various crack types. The formation mechanism of borderline cracks was revealed for the first time. This study reveals that the asymmetry of the densification distribution and elastic-plastic boundary significantly influences the cracking behavior. Under the Vickers indentation, conical cracks have the largest penetration depth. When these cracks emerge from a region far from the impression, they extend with constant radii to form circles on the sample surface. Otherwise, they tend to be initiated at the centers of the indenter-material contact edges before propagating towards the impression corners with increasing radii. Therefore, the borderline cracks consisting of successive partial conical cracks can form at a low load and makes them the first type of crack to appear.     

Processing and Surface Flaw Tolerance of Alumina Bilayers

Alumina bilayers of different relative thicknesses and densities were produced with a strong interface using a modified gel-casting technique. Tolerance to surface damage is examined using biaxial flexure of disks damaged with a single Vickers indentation at various loads. The greatest surface flaw tolerance is seen in bilayers consisting of a thin porous layer on the tensile surface coupled to a thick dense layer. Here, the modulus mismatch causes redistribution of the applied stress, and fracture initiates at the internal porous-dense interface rather than at the surface from the introduced indentation flaw.     

Crack Suppression in Strongly Bonded Homogeneous/Heterogeneous Laminates: A Study on Glass/Glass-Ceramic Bilayers

A study is made of a glass/glass-ceramic bilayer as a model homogeneous/heterogeneous laminate. The underlying objective is microstructural design of ceramic layer systems with optimum mechanical properties, alternating hard layers, for wear resistance, with tough layers, for fracture resistance. Mica flakes in the glass-ceramic layer inhibit the propagation of well-developed intrusive cracks, by bridging; these same flakes render the structure susceptible to distributed damage, by providing discrete weakness at the microstructural level. A major distinguishing feature of the bilayer design is the incorporation of a strong interface, so that cracks are inhibited by the underlayer rather than deflected between the layers. Vickers and Hertzian indentation tests on specimen cross sections demonstrate the capacity of the glass-ceramic layer to arrest radial and cone cracks penetrating from the adjacent glass layer. Additional Hertzian tests on the outer surfaces of glass layers in a coating/substrate configuration show diffuse damage accumulation in the glass-ceramic substrate layers. This diffuse damage absorbs energy and shields cone cracks in the glass from the applied loading. Implications concerening the design of damage-tolerant laminate structures are discussed.     

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.     

Experimental Measurement of Residual Stress Field around Sharp Indentation in Glass

A new technique is developed to measure the residual stress field around Vickers indentations in glass and ceramics. This technique uses a small indentation as a microprobe to measure the residual stress at a specific position near a large indentation. The approach is based on the observation that the crack lengths of the small indentation are changed under the influence of the residual stress field created by the large indentation. A simple fracture mechanics model is derived to calculate the residual stress from the measurement of the changes of the crack lengths of the small indentation. The results show that the residual stress around Vickers indentations is a nonequal biaxial field; both tensile and compressive stresses exist around a sharp indentation and decrease as the distance from the center of indentation increases. This technique can be easily extended to many other cases of residual stress in ceramics and composites.     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.     

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.     

Knoop and Vickers Indentation in Ceramics Analyzed as a Three-Dimensional Fracture

A three-dimensional fracture analysis is applied to the Knoop and Vickers indentation fracture of ceramics. A brief discussion of the accuracy of the analysis applied to model the step load on the crack face caused by the residual stresses is given. A study is made of the effect of the elongated plastic zone in Knoop indentation on the unloaded radial fracture. It is shown that for small indentation loads the published experimental data can be verified by varying the depth reached by the semielliptical plastic zone with given surface length. An analysis and interpretation of the interaction between the two halfpenny-shaped radial cracks induced by Vickers indentation is also given.     

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.     

Analysis of the mechanical properties of TiN/Ti multilayer coatings using indentation under a broad load range

In this study, physical vapor deposition was used to prepare TiN/Ti multilayer coatings as well as the corresponding monolithic coatings for comparison. Nanoindentation using a large load range (5–4800 mN) and finite element method (FEM) simulations were conducted to investigate the influence of various multilayer structures on the mechanical behavior of multilayer coatings. The nanoindentation results show that the TiN/Ti multilayer coating has the maximum hardness and Young's modulus while retaining good crack resistance and fracture toughness. The FEM results show that increasing the number of layers in the multilayer coatings reduced the hardness and Young's modulus as well as the maximum stress, while it increased the equivalent plastic strain. As the layer thickness ratio increased, both the hardness and Young's modulus gradually increased, and the stress in the coating reached its maximum at the highest thickness ratio. In addition, to consider the effect of the indentation depth on the coating, the influence of the number of layers and the layer thickness ratio on the multilayer coating is combined into the indentation response of the multilayer coating. Therefore, we establish an expression describing the relationship between the number of layers and the ratio of the layer thickness to the mechanical properties of TiN/Ti multilayer coatings.     

Effect of substrate roughness on the contact damage of DLC coatings

The effect of substrate roughness on the contact damage of diamond-like carbon (DLC) coatings has been investigated. The coatings were deposited on steel substrates of varying roughness by plasma-enhanced chemical vapour deposition (PEVCD) and subjected to nanoindentation. At high loads, pop-ins in the load-displacement curves were observed, associated with the formation of ring cracks initiated in the coating surface. The subsurface deformation and fracture of the coatings were inspected using focused ion beam (FIB) microscopy. It was found that the mean roughness of the coating have been increased relative to their substrates, except for the very rough substrate, in which the coating roughness was relatively decreased. The rougher substrates were found to lead to an increase in the load required to induce pop-ins in the nanoindentation load-displacement curves.