Contact damage evolution in a diamond-like carbon (DLC) coating on a stainless steel substrate

A diamond-like carbon thin film was coated onto a stainless steel substrate using plasma assisted chemical vapour deposition (PACVD). Instrumented indentation and scratching were used, supported by focused ion beam (FIB) microscopy, to explore deformation and fracture behaviours of this coating system. The formation and growth of ring and radial cracks in the coating, as well as plastic flow in the ductile substrate, were observed to be the predominant deformation processes for this coating system. Lateral cracking occurred at the interface of the coating/substrate following indentation, but in the middle of the coating following scratching. No evidence of plastic flow within the coating was observed. Coating deformation is, therefore, controlled by its fracture energy. An indentation-energy-based model was applied to evaluate the fracture toughness of the coating.     

The effects of thin compressive films on indentation fracture toughness measurements

Thin compressive films have been shown to decrease the lengths of radial cracks produced by a Vickers indentation in a variety of non-metallic materials. The intrinsic stress of submicrometre thick films deposited by reactive ion beam sputtering was measured by a cantilever technique. The change in the apparent indentation fracture toughness of the coated material was correlated with film thickness and stress, indentation load, and the nature of the substrate.     

Nanoindentation-induced deformation behaviour of diamond-like carbon coatings on silicon substrates

The deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates induced by indentation has been investigated. DLC coatings, deposited by a plasma-assisted chemical vapour deposition technique, were subjected to nanoindentation over a range of maximum loads from 100 mN to 300 mN. The resulting load-displacement plots displayed pop-ins for maximum loads of 200 mN and above, with no distinct pop-out for any of the loads studied. Compressive deformation of the coating, up to a strain of ∼ 9%, was observed. The coating-substrate composite was devoid of cracks at lower loads, but at the maximum load of 300 mN, ring cracks in the coating and a median crack in the substrate were observed. Furthermore, cracking, {111} slip and localized phase transformations were observed in the silicon substrate. The onset of these structural changes was correlated to the mechanical behaviour during indentation.     

火焰喷涂NiCrBSi涂层的纳米力学性能

为揭示热喷涂涂层在不同尺度下的力学性能,在45钢基体上制备了平均厚度为750μm的火焰喷涂NiCrBSi涂层,利用纳米压痕技术研究了不同压痕深度下涂层表/截面力学性能、弹塑性和压痕变形行为。结果表明:涂层表/截面力学性能均呈现明显的尺寸效应,硬度、弹性模量、弹塑性随压入深度增加不断降低。涂层表面表现出高弹性,其压痕弹性功与总压痕功的比值ηIT在500nm深度内达到52%,而涂层截面为40%;涂层截面具有高硬度和高模量,其纳米硬度和弹性模量在2000nm深度内比涂层表面分别高28%和33%。涂层压痕变形表现为理想塑性、凹陷、凸起和裂纹等多种特征,随着压入深度增加,涂层表/截面弹塑性差异逐渐降低,并在2500nm深度同时下降到35%。涂层单一薄层结构在不同方向具有相同的硬度和弹性模量,但随压入深度增大,压头包含的涂层体积增大,相邻薄层,特别是孔隙、裂纹、层间边界等缺陷对涂层性能的影响逐渐增强,导致涂层表/截面硬度和弹性模量的差异性随压痕深度增加不断降低。     

Deep reactive ion etching and focused ion beam combination for nanotip fabrication

We have studied the fabrication of high-aspect ratio silicon tips by a combination of deep reactive ion etching and focused ion beam. The reactive ion etching is used to obtain so-called “rocket tips” which can be fabricated with a high aspect ratio. The rocket tips are further processed by using a focused ion beam to obtain nanotips at their apex. Typical results obtained are nanotips with a basis radius of 200 nm and a height of 2.5 μm, with an apex radius of 5 nm, located on top of a 3 μm wide and 9 μm high silicon column. The process would allow however obtaining column heights of several tens of microns.     

In-situ investigation on the deformation and damage behaviour of diamond-like carbon coated thin films under uniaxial loading

In the present work, the failure behaviour of diamond-like carbon (DLC) coatings on thin steel substrate under uniaxial tensile loading is analyzed in-situ in scanning electron microscopy as well as ex-situ using focused ion beam cross section and transmission electron microscopy. Aim of the work is to find correlations between the failure behaviour of the coating system, the adhesion and the stress-strain behaviour of a DLC coating system under tensile loadings conditions. Therefore thin amorphous DLC films were coated onto thin stainless steel foils using a plasma assisted chemical vapour deposition technique. It is found from the in-situ investigations that at increasing strains cracks were formed in the coating, with decreasing spacing at higher strains. By comparing uncoated steels foils with coated systems the stress-strain behaviour of a DLC coating was determined. The DLC coating, although already strongly cracked, bears loads up to a total strain of 15%. Cross section analyses with a focused ion beam and microscopy techniques supported these investigations. During straining the formation of two deformation bands adjacent to the Cr adhesion layer was observed. This deformation bands also indicate a high interfacial adhesion.     

The investigation of lateral damage effects of ion-implanted layers by back-scattering techniques

Using a particle beam of small diameter the back-scattering technique can be applied to the microanalysis of small geometries. By moving a sample in steps of 5 μm with respect to the beam, lateral properties of the surface layers can be investigated. Particle beams of He⁺ ions with diameters of 5–100 μm have been used.We found that the width of a damaged area which was defined by SiO₂ masking during ion implantation was smaller than the width of the window in the mask itself.A wire for masking was placed at a distance of 1 mm from the surface of the sample. In this case lateral annealing of the implanted area was observed, which might be due to an effect of the scanning ion beam.     

Contact damage in TiN coatings on steel

Damage mechanisms beneath Vickers indentations are examined on 5 μm TiN film deposited on stainless steel substrate as a function of load. Prominent mode of cracking includes surface edge cracks and subsurface inclined cracks. No interfacial delaminations were noted at the TiN/steel interface. The tangential traction, radial stress and shear stress distribution around an axisymmetric indentation field are used to assess the driving force for crack propagation.     

Deformation and fracture of germanium single crystals

Diamond-pyramid indentations, made with loads of 1 to 100 g at room temperature on {111}, {110} and {112} faces of germanium crystals, were studied by scanning electron microscopy. The Vickers-hardness number, which was highest for {111} faces, increased with the square root of the load by a factor of about 10 on any given surface, ranging from values as low as 60 to about 850, the latter corresponding to the VHN reported in the literature. The lengths of major cracks, generally seen on the surface of the crystal as an extension of the indentation diagonals, were found to be proportional tod-d ₀, whered ₀, equal to about 3 μm, represents a lower limiting value of the indentation diagonald, below which cracks do not seem to form. The increase of the VHN and of the crack-lengths with load is explained in terms of a tentative model involving the formation and the stress-induced growth of laminar “plastic” patches in a volume surrounding the indentation.     

Cone cracks around Vickers indentations in fused silica glass

Surface and subsurface deformation and cracking around Vickers identations in fused silica have been studied. The indentations were sectioned by making the Vickers indents on and near the tip of a pre-existing crack. The characteristic median and lateral cracks around the impression and shallow cracks within the surface of the indentation were observed on the specimen surface. The subsurface deformation showed compacted or densified zones, devoid of any flow line rosettes. The dominant cracks, similar to the Hertzian cone cracks observed around purely elastic spherical indentations, occurred outside the compacted zones. These cone cracks make angles of 30 to 40° with the specimen surface. Multiple cone cracks with shallower angles often formed outside the major cone cracks. It has been suggested that the expansion of the boundary of the compacted zone as the indenter load is increased can cause median cracks during loading while the mismatch of strain at this boundary may give rise to lateral cracks during unloading.     

Finite element analysis of vickers indentation cracking processes in brittle solids using elements exhibiting cohesive post-failure behaviour

The cracking processes during the indentation test of brittle solids is simulated by means of the finite element method (FEM) using elements exhibiting cohesive post-failure behaviour and alumina as the model material. The results show that at low indentation loads, median cracks could nucleate at full loading but Palmqvist cracks only nucleate in the unloading stage and that they may not join up even after full unloading. Such cracks are stable as they are embedded in a region of high hydrostatic compression throughout the indentation test. At high indentation loads, both median and Palmqvist cracks nucleate early during the loading stage and coalesce to form a half-penny crack on further loading. Although the cracks are embedded in a region of high hydrostatic compression during loading, an annular tensile region eventually develops in between the cracked material beneath the indenter and the surrounding uncracked material during the unloading stage of the macro-indentation. This not only provides the driving force for existing cracks to grow but also new crack systems to form. The present work shows that for brittle solids with negligible plastic deformation, the mismatch in elastic recovery between the cracked and uncracked bodies on unloading plays an important role in indentation fracture processes.     

Relationship between fatigue limit and Vickers hardness in steels

In this work, a method to predict the fatigue limit by using Vickers hardness measurements is proposed. Tests carried out in small regions of different annealed, quenched and quenched-tempered alloy steels allowed an improvement of the empirical Murakami-Endo's equation.In this method, the plastic deformation caused by the indentation is assumed to be the defect from which the process of initiation and propagation of cracks originate, analogously to small cracks.Fatigue limits for four kinds of steels in different metallurgical states (annealed, quenched and quenched-tempered) were estimated in two different ways, and the obtained values were compared to the experimental ones. A good correlation between Vickers hardness and the fatigue limits estimated by direct plastic deformation zone measurements using optical microscopy was envisaged.     

Phases analysis and impact of phases on fracture mechanism of AZ61‐SiC composite

Phase composition of AZ61‐SiC composite with 5 wt.% of nanosized silicon carbide reinforcement was analysed and failure mechanism by in situ tensile test in scanning electron microscope was observed. Microstructure of the experimental materials was heterogeneous with grain size of 15 μm. Based on the quantitative analysis of composite, besides, silicon carbide strengthened particles added externally into the matrix magnesium silicide, magnesium oxide, and aluminium/manganese particles formed in situ were found in the matrix. In situ tensile test in scanning electron microscope has shown that reinforcing particles substantially influenced failure mechanism. Large, brittle magnesium silicide particles (size of 40 μm–50 μm) cracked during tensile deformation and at the same time, as a result of different physical properties, decohesion of the matrix and smaller aluminium/manganese, silicon carbide and magnesium oxide particles (size of 5 μm–10 μm, 10 μm and 50 nm respectively) occurred. Reinforcing particles and brittle secondary phases driven micro voids and their coalescence was found as a major cause of large cracks formation. Subsequently the increase of stress caused the cracks propagation by the coalescence of fractured particles and decohesively release smaller dispersed particles. The fracture propagated at approximately 90° angle to the direction of the tensile load direction. Fracture surface had feature of transcrystalline and intercrystalline failure.     

Behaviour of small cracks during their propagation from Vickers indentations in coarse-grain steel: An experimental investigation

In this investigation we look at the influence of the local residual stresses caused by Vickers-pyramid indenting on the initiation and early propagation of small cracks from indentations in coarse-grain martensitic steel. The size of these indentations is comparable to the grain size. Specimens with and without a local residual stress field were tested on a rotary bending machine. A focused ion beam and a scanning electron microscope were used to reveal the influence of those stresses on the location of the cracks’ initiation and the mechanism of the small-crack propagation. The existing local residual stresses assist in the initiation of two cracks at a level lower than the fatigue limit. The early small-crack propagation is gradually obstructed by the residual stress-field configuration until the cracks become non-propagating cracks. At levels higher than the fatigue limit, both cracks succeed in breaking through the compressive stressed domain and link together. From that moment the crack begins to behave as a long crack, penetrating outside the indentation into the tensile-stressed domains.     

Dynamic shear plugging of beams and plates with an advancing crack

The perforation response of a thin or intermediately thick beam/plate struck by a rigid, heavy, flat-nosed projectile traveling at a high velocity is studied analytically. Based on a newly developed expression relating indentation depth to crack length, a shear-plugging model including two stages: indentation and crack growth, is proposed. In this model, shear resistance in the ligament decreases as the cracks propagate through the target thickness. The closed-form solutions for residual velocities, plastic energy, shear zone width, crack propagation speeds, and temperature rise are derived. A coupled shear–tension solution that takes into account global deformation of the plate is also developed. The coupled solution improves prediction for the case with the initial impact velocity near the ballistic limit. Comparisons with experimental results presented in the open literature are made showing rather good correlation, which validates the accuracy of the present solution.     

Effect of microstructure on micromechanical performance of dry cortical bone tissues

The mechanical properties of bone depend on composition and structure. Previous studies have focused on macroscopic fracture behavior of bone. In the present study, we performed microindentation studies to understand the deformation properties and microcrack–microstructure interactions of dry cortical bone. Dry cortical bone tissues from lamb femurs were tested using Vickers indentation with loads of 0.245–9.8 N. We examined the effect of bone microstructure on deformation and crack propagation using scanning electron microscopy (SEM). The results showed the significant effect of cortical bone microstructure on indentation deformation and microcrack propagation. The indentation deformation of the dry cortical bone was basically plastic at any applied load with a pronounced viscoelastic recovery, in particular at lower loads. More microcracks up to a length of approximately 20 μm occurred when the applied load was increased. At loads of 4.9 N and higher, most microcracks were found to develop from the boundaries of haversian canals, osteocyte lacunae and canaliculi. Some microcracks propagated from the parallel direction of the longitudinal interstitial lamellae. At loads 0.45 N and lower, no visible microcracks were observed.     

On borehole indentor (BHI) measurements and analysis

In situ measurements on stress–indentation curves conducted with the National Research Council (NRC), Canada and the Arctic and Antarctic Research Institute (ARRI), Russia borehole indentors (BHI) are analyzed and classified. This establishes harmony with laboratory observations on stress–strain diagrams and some compatibility with the recommendations (ISO/DIS 19906) on estimating uniaxial ice strengths from BHI strengths. The analysis is devoted mainly to consider the influence of local ice conditions and the indentation rates on the pressure–indentation curves. Simultaneous records of the acoustic emission (AE) detected by accelerometers installed on the ice surface, within 1.5 m of the indentation plate, indicated that major cracks are nucleated at the ice/plate interface. Microstructural analysis of the indented ice confirmed this important conclusion in addition to revealing recrystallization as well as healing activities in the indented ice. An attempt, with extremely limited success, has been made in applying conventional ice failure criteria for predicting the observed stress–indentation curves. Phenomenologically, however, a power-law between the indentation-rate and upper-yield strength exhibits the same rate sensitivity (about 3) usually obtained for strain-rate dependence of uniaxial strengths. Numerical solutions of the rate-sensitive indentation processes must be developed (as has successfully been achieved for uniaxial tests) on microstructure-property based mathematical (rheological) model that includes the effects of the rate-dependent kinetics of deformation, microcracking and crack-enhanced creep. Premature brittle fractures are contact problems and modeling must consider the nucleation of cracks in ice at the ice/plate contact surface.     

Fracture processes of hybrid fiber-reinforced mortar

Subregion Scanning Computer Vision (SSCV), a digital image based method for measuring surface deformation is used to examine the role of the fibers in the fracture process of mortars reinforced with hybrid blends of microfiber (less than 22 μm in diameter) and macrofiber (500 μm in diameter). Closely-spaced microfibers interact with cracks at the microstructural level and hamper the widening of coalesced microcracks, thus encouraging the growth of multiple cracks. The microfibers improved pre-peak mechanical performance and strength by delaying the formation of a through-specimen macrocrack. Macrofibers were most effective at bridging macrocracks and imparting ductility to the composite due to their geometry and greater length. Compared to mortar reinforced with a single fiber type, an increase in strength and toughness was seen with a blend of steel macrofibers and either steel or PVA microfibers. Finally, based on the crack topography observed, the reduction in water permeability of cracked mortar achieved with hybrid fiber-reinforcement, measured directly in a parallel study, was governed by multiple crack development.     

ESIS TC 6 ROUND ROBIN ON FRACTURE TOUGHNESS

Abstract— The Technical Committee "Ceramics" (TC 6) of the European Structural Integrity Society (ESIS) organized a round robin relating to the fracture toughness of ceramic materials at room temperature. Five materials were tested with five testing methods by eighteen laboratories. The five testing methods were: chevron notched beam in four point bending, direct measurement of the cracks emanating from a Vickers indentation, indentation strength by four-point bending, single edge precracked beam in four-point bending, and single edge notched beam in four-point bending. The results of the round robin performed in the period 1993 to 1994 are presented and discussed.     

Flaking failures originating from microholes of bearings under rolling contact fatigue

Rolling contact fatigue tests were carried out using plates with microholes (diameter was about 100 μm and depth was about 140 μm) under three different loads (maximum values of Hertzian stress were about 3250, 3550 and 3840 MPa, respectively) and the surface cracks initiating from those holes were observed. It was found that there is a threshold value of maximum Hertzian stress whether surface cracks originate from microholes or not, and its value is between 3250 and 3550 MPa. However, flaking failures occurred even when the stress values were lower than the threshold value. In order to investigate the relation between the flaking failures and the cracks, sectional observations of the subsurface cracks were made before and after the surface layer separations. From these observations, it was found that the subsurface cracks caused the flaking failures even when the maximum value of Hertzian stress was lower than the threshold value of surface crack initiation.