Effect of elastic modulus mismatch on the contact crack initiation in hard ceramic coating layer

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch,E _c /E _s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials.     

含多粗糙峰涂层等效应力的有限元分析

研究刚性平面与含粗糙峰涂层在二维与三维模型下的弹性接触问题,采用有限元法分析涂层弹性模量比、涂层厚度、粗糙峰间距、刚性平面压下深度对涂层粗糙峰表面、涂层/基体界面分布及基体等效应力分布的影响。计算结果表明压下深度对三维涂层粗糙峰表面最大应力的影响最大,涂层厚度、涂层/基体弹性模量比、粗糙峰间距的变化对应力值影响逐渐减小;增大涂层厚度、减小压下深度和粗糙峰间距、降低弹性模量比会使得三维接触模型最大等效应力值显著降低;增加涂层粗糙峰数和涂层厚度、同时降低涂层弹性模量有助于提高涂层/基体界面结合强度。相对于二维接触模型来说三维接触模型在粗糙峰表面的等效应力增大,造成这种变化的主要原因是由于涂层表面粗糙峰之间的等效应力叠加引起的。该研究为涂层粗糙峰及涂层/基体界面强度的应力分析提供依据。     

Finite element modelling of brittle fracture of thick coatings under normal and tangential loading

This paper addresses coating fracture in hard brittle coatings subjected to combined normal and tangential loads through a finite element based methodology. The coating is modelled as an elastic layer perfectly bonded to an elastic substrate with a pre-microcrack, assumed to initiate at the contact trailing edge due to high tensile stress. The predicted results are consistent with previously published coating fracture results. The model predicts a significant effect of coating thickness on crack propagation for coatings with large elastic mismatch and the final propagated crack profile is predicted to depend on friction coefficient, coating fracture toughness and sliding displacement.     

Lateral Contact Stiffness and the Elastic Foundation

A model for the lateral contact stiffness for an elastic foundation was developed. The model was evaluated using a low force and low contact pressure microtribometer capable of performing indentation and reciprocated sliding experiments. The slope of lateral force versus the lateral displacement was used to fit the shear modulus. When complementary elastic indentation measurements are made to determine the composite modulus of the elastic foundation, there is sufficient data to fit elastic modulus, shear modulus, and Poisson ratio for these thin films. Using these models, the elastic properties for a thin (~65 μm) vertically aligned multiwall carbon nanotube film were evaluated. The experiments were performed with a silicon nitride indenter (radius = 1.6 mm) over a range in loads from 100 to 800 μN. The resulting values of the elastic modulus, shear modulus, and Poisson ratio were E = 429 kPa, G = 156 kPa, and ν = 0.37, respectively.     

Multipoint indentation for material identification in three-dimensional observation based on serial sectioning

Three-dimensional (3D) observations of internal structures are important for evaluating material properties. Serial sectioning with destructive processes is traditionally employed as a 3D observation method. Identifying the boundaries of elements in microscope images and measuring the mechanical properties of each element are required for the evaluation of the mechanical properties of composite materials. This study provides a system for measuring the local hardness and elastic modulus by conducting indentation tests during serial sectioning processes. An automatic serial sectioning observation was performed during a combination process of precision cutting in high-speed milling with a single-crystal diamond tool and microscopic observation. A Vickers indenter was attached to a tool spindle table, and indentation tests were conducted under a displacement control process at submicron spatial resolution. The indentation modulus was obtained by analyzing the force–displacement profile measured during the unload process. The scale effects relating to the indentation depth in the measurements of the indentation modulus were confirmed for an Al alloy sample measured in this system. This study focused on the identification of components by using hardness information measured under the same indentation depth on a two-dimensional flat surface after precision cutting of the material. Three types of metal wires (1 mm diameter) embedded in plastic resin were used in the experiment. The hardness distributions on the serial sectioning surfaces were measured, and the values measured at each wire area on 3D positions were used for the identification of their material properties. This serial sectioning observation creates a 3D microstructural model including not only microscopic images, but also hardness and elastic modulus information for the identification of components in the microscopic area.     

Tribological and physical-mechanical properties of protective coatings from Ni-Cr-B-Si-Fe/WC-Co-Cr before and after fission with a plasma jet

A new type of coating is developed, which is a mechanical mixture of two different powders, namely, Ni-Cr-B-Si-Fe (PG-19N-01) and WC-Co (hard alloy). After the coatings from this mixture are deposited, their surface layer is fused with a plasma jet using an eroding electrode made of W. The additional treatment of the coatings with the plasma jet yields new phases and causes the redistribution of elements in the layer 45–60 μm deep; the percentage ratio of the phases WC, α-CoCr, Co, and Ni, as well as Cr₃Ni₂ + γ-(Fe, Ni) appearing during coating deposition also changes. The redistribution of elements occurs in the upper coating layer owing to fusion with the plasma jet. These processes yield variations in the physical-mechanical properties of the coatings, such as the hardness and elastic modulus; the coating wear rate decreases severalfold. It is found that with increasing load applied to the Berkovich pyramid the elastic modulus of the coating drops from 240 (at an indentation depth of 50 nm) to 175 GPa (at 150 nm). The elastic modulus of the substrate rises from 25 to 42 GPa. The coating hardness calculated from the loading-unloading curves is 15.3 to 10.6 GPa under increased load applied to the indentor. Specimens covered with the coating fused with the plasma jet in three passes demonstrate the lowest material wear.     

Determining the elastic modulus of a hard coating from the loading curve in indentation

A method is proposed for calculating the elastic modulus of a thin hard coating from the loading curve in indentation, on the basis of modified Hertz theory. The results may be used in assessing the properties of hardened surfaces.     

Fatigue crack initiation potential from defects in terms of local stress analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.     

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

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

Method of estimating elastic properties of topocomposite based on the results of instrumental indentation

A new experimentally calculated method of determining the elastic modulus of the material of a thin coating by the loading curve of the penetration diagram of pyramidal die at the instrumental indentation into the topocomposite surface. The theoretical dependence of the change in the effective elastic characteristic of the topocomposite surface on the coating thickness was confirmed.     

Tribological and mechanical investigation of SiO2 and Si3N4 coatings on fused silica and borosilicate glass

Amorphous SiO₂ and Si₃N₄ plasma‐enhanced chemical vapour deposited (PECVD) coatings were deposited on two different substrate materials (fused silica and borosilicate glass), with three coating thicknesses (0.1, 0.5, and 1.0 μm). The mechanical properties (hardness and elastic modulus) were determined by depth‐sensing indentation, with loads from 700 mN down to 0.1 mN. Tribological behaviour was studied in instrumented oscillating sliding tests at room temperature with a ball‐on‐flat arrangement, in which the coated disc was tested against an alumina ball, at a load of 1 N. Interpretation of the measurement of hardness and modulus of the coatings has to take into consideration the influence of layer thickness and the effect of the substrate. Tensile film stress and crack generation were only observed for Si₃N₄ on fused silica above a threshold thickness. Friction and wear measurements show that the coating has an effect on friction, while wear is affected by the thin coatings only for a short running‐in phase. The morphology of the wear scars indicates that the coatings have good adhesion. Despite crack generation, delamination effects were not observed. Indentation patterns similarly showed excellent lateral homogeneity of the mechanical properties over the entire film surface, and indicated that load‐displacement curves may be used to characterise the system.     

The influence of complex surface geometry on contact damage in curved brittle coatings

The effects of complex geometry on contact damage in bi-layer systems composed of curved brittle coating layers on compliant polymeric substrate is investigated. Previous studies of this problem utilise relatively simple flat or singly curved (domed) model structures. It is not known the extent to which conclusions driven from such observations may extended to more complex (practical) geometry. Glass plates of 1000 μm thick are used as representative of the brittle coating layer, and epoxy filler under layer as representative of under layer support. A series of doubly curved specimens (having curvatures of 4 and 8 mm) are produced to allow investigation of the influence of complex curvature on the evolution of damage. The specimens are tested by indentation with spheres of 4 mm radius loaded along the convex axis of symmetry. For comparison, some specimens loaded parallel to the axis of symmetry but off-centre. The study explores the influence of supporting geometries on the conditions to initiate and propagate subsurface “radial” cracks, which are believed to be responsible for catastrophic failure of brittle-coating-based structures in certain applications, such as dental crowns. It is demonstrated that critical loads for initiation of radial cracks and the subsequent crack propagation are insensitive to complex geometry, so that simple monotonic indentation “axis and/or off-axis loading” with minimum geometrical complication “flat, simply curved” remains an appropriate route to study the evolution of radial cracks in practical brittle coating structures.     

Contact-resonance atomic force microscopy for nanoscale elastic property measurements: Spectroscopy and imaging

Quantitative measurements of the elastic modulus of nanosize systems and nanostructured materials are provided with great accuracy and precision by contact-resonance atomic force microscopy (CR-AFM). As an example of measuring the elastic modulus of nanosize entities, we used the CR-AFM technique to measure the out-of-plane indentation modulus of tellurium nanowires. A size-dependence of the indentation modulus was observed for the investigated tellurium nanowires with diameters in the range 20–150 nm. Over this diameter range, the elastic modulus of the outer layers of the tellurium nanowires experienced significant enhancement due to a pronounced surface stiffening effect. Quantitative estimations for the elastic moduli of the outer and inner parts of tellurium nanowires of reduced diameter are made with a core–shell structure model. Besides localized elastic modulus measurements, we have also developed a unique CR-AFM imaging capability to map the elastic modulus over a micrometer-scale area. We used this CR-AFM capability to construct indentation modulus maps at the junction between two adjacent facets of a tellurium microcrystal. The clear contrast observed in the elastic moduli of the two facets indicates the different surface crystallography of these facets.     

Influence of counterbody material on wear of ta-C coatings under fretting conditions at elevated temperatures

The high temperature tribological performance of tetrahedral amorphous carbon coatings has been analyzed at elevated temperatures up to 250 °C in air against three different counterbody materials—steel 100Cr6, α-alumina and silicon nitride. The results show that the counterbody material influences the friction and wear behavior and therefore coating life time strongly. This effect is well known for these coatings at room temperature under dry environmental conditions, equivalent to conditions above 100 °C when water molecules desorb from the surface. However, the sharp difference in tribological performance between silicon nitride on the one hand and alumina and steel on the other hand cannot be understood in this context. Analyzing the friction behavior during the running-in phase, it is evident that only alumina and steel form a stable interface with constant low friction and relatively low wear rates. Silicon nitride forms an unstable interface with fluctuating COF and relatively high wear rates due to its own inherent tendency to tribo-oxidation.     

Subsurface crack mechanisms under indentation loading

A linear elastic fracture mechanics analysis of plane-strain indentation of a homogeneous half-space with a subsurface horizontal crack was performed using the finite element method. Stress intensity factor results obtained for an infinite plate with a central crack subjected to far-field tension and a half-space with a frictionless subsurface horizontal crack under a moving surface point load are shown to be in good agreement with corresponding analytical results. Crack mechanism maps illustrating the occurrence of separation, forward and backward slip, stick, and separation at the crack interface are presented for different indentation load positions and crack face friction coefficients. Results for the stresses in the vicinity of the crack tips and the mode I and mode II stress intensity factors are given for different indentation positions, crack face friction coefficients, and both concentrated and distributed surface normal tractions. Although indentation produces a predominantly shear and compressive stress field, mode I loading conditions are shown to occur for certain indentation positions. However, the magnitude of the mode I stress intensity factor is significantly smaller than that of mode II, suggesting that in-plane shear mode crack growth is most likely to occur in the absence of microstructural defects. The significance of crack face friction and sharpness of the indenter on the subsurface shear mode crack propagation rate is interpreted in terms of the mode II stress intensity factor range and material behavior.     

Frictional and adhesive behavior of organic–inorganic hybrid coatings on surgical grade stainless steel using nano-scratching technique

Because of their mechanical properties, metals are the most widely used materials as orthopaedic implants. However they cannot provide a natural bond with the mineralized bone and they also release metallic particles due to degradation or tribologic events. One way to improve the metallic implants performance is to apply protective organic–inorganic sol–gel coatings. In this work, stainless steel substrates are coated with films made by a sol–gel technique from organosilane precursors.Although mechanical properties of similar films have been studied, there is no information about adhesion, friction or deformation processes of silica-based hybrid films to stainless steel substrates.Hybrid coatings with higher amount of inorganic components (called TMH) have almost no elastic response and the debris due to chipping or delamination does not persist into the indentation trace. With the film with high content of organic compounds was found elastic recovery in early stages of loading and there is evidence of pile-up at the edges of the trace with higher load applied. After the unloading the film has a persistent deformation and is removed due to the asynchronic recovery of the film and the substrate. The combined two-film coating shows a lot of debris in the trace. This is an unusual but possible behavior of polymeric coatings and could be attributed to different recoveries between the first inorganic layer (called TEOS–MTES), the substrate and the upper TMH film. This fact produces delamination and crack formation in the TEOS–MTES coating, inducing tensile efforts, and finally the upper film is pulled-off.     

Surface stresses in coated steel surfaces—influence of a bond layer on surface fracture

Thin hard coatings in the thickness range of only a few micrometers deposited by physical vapour deposition (PVD) on components or tools can improve the friction and wear properties by several orders of magnitude. A 2 μm thick TiN (E=300 GPa) coating on a high-speed steel substrate with a bond layer at the interface between the coating and the substrate was modelled by micro-level three-dimensional finite-element method (3D FEM) in order to optimise a coated surface with regard to coating fracture. Both compliant low modulus (E=100 GPa) and stiff high modulus (E=500 GPa) bond layers at the coating/substrate interface of 200 and 500 nm thickness were investigated. First principal stresses were simulated for scratch test geometry in the load range of 7.5-15 N. Very high stress concentrations of above 5700 MPa tensile stresses were observed in the bond layer just behind the contact zone for the stiffer bond layer. The stiff bond layer generated 5 times higher tensile stress maxima compared to the compliant bond layer. There was approximately 3.5 times larger strain in the compliant bond layer compared to the stiff bond layer. The general coating design advice based on this exercise is that when a bond layer is used e.g. for coating/substrate adhesion improvement should the bond layer be less stiff than the coating not to generate high and critical tensile stresses. The thickness of the bond layer may vary and is not critical with respect to generated stresses in the surface.     

纳米压痕法测试电刷镀镍镀层的硬度和弹性模量

介绍一种新型的多功能纳米材料性能测试仪,阐述仪器工作原理和压痕数据分析法。应用该仪器对钢基体上的电刷镀镍镀层的硬度、弹性模量以及抗压痕形变能力等微观力学性能进行测试,并研究镀层力学性能随镀层厚度的分布规律。结果显示,镀层表层和亚表层存在硬度和弹性模量较高的区域,镀层内部存在缺陷区域的硬度和弹性模量降低,在镀层与基体的界面处存在过渡区。除此以外,整个镀层的力学性能较为一致,没有明显的梯度分布,其平均硬度和弹性模量分别为7.02GPa和183GPa,其中硬度为钢基体的2.9倍。在试验载荷下镍镀层具有比钢更好的抗压痕形变能力。为改进镀层工艺、开发新型体系材料以及扩大镀层应用领域提供了有效数据。     

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

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

Boundary element numerical analysis of elastic indentation of a sphere into a bi-layer material

The initial part of load/penetration plot in depth sensing spherical indentation (nano-indentation) is analysed. The results of a numerical study using a specifically developed simulation tool based on the boundary element method are presented.They reveal that the usual linear relationship between the indentation depth and the square of the contact radius for homogeneous materials is also valid in the case of a bi-layer material.It is also shown that the elastic response of the bi-layer material is specific of the coating alone only if the indentation depth, h, is less than 2% of the coating thickness, t.For higher indentation depths (10%>h/t>1%), the macro-elastic response of the composite is seen to saturate with an overall elastic response still containing a contribution of roughly 10% of the coating modulus.The hypothesis of indentation limit h/t<10% of bi-layer material insuring coating-specific response for penetration hardness (Bückle's rule) is roughly two orders of magnitude high when applied by default to the corresponding coating-specific elastic response.