Brittle Substrate Research Articles

Flexural test as an alternative to tensile test for bond strength of resin cement to zirconia

ObjectiveThere has been a great many technical difficulties in measuring the bond strengths between brittle dental substrates and materials, especially in preparing specimens. This study evaluated the validity of the relatively easy flexural bond strength (FBS) test in measuring bond strength of resin cement to zirconia as an alternative to the cumbersome tensile bond strength (TBS) and micro-tensile bond strength (MTBS) tests. Materials and methodsThe FBS and TBS of resin cement to zirconia were measured experimentally after three surface treatments on a zirconia ceramic: air abrasion only (A), conditioning with Single Bond Universal (U) after air abrasion, and conditioning with Z-Prime Plus (Z) after air abrasion. The data were investigated using two-way analysis of variance (ANOVA), Weibull statistics, and a theoretical simulation. ResultsIn both the FBS and TBS tests, the experimental data were consistent and quantitatively similar. First, according to ANOVA, the U group showed the highest bond strengths in both tests, followed by the Z group and the A group. In each surface treatment group, the FBS was always higher than the TBS. Second, the Weibull fitting showed the same order of strength in both tests (A < Z < U) and in all surface treatment groups (FBS > TBS). Third, the theoretical ratios calculated from the Weibull moduli agreed well with the experimental ratios of the FBS to the TBS. ConclusionThe FBS test can be an alternative to the TBS and MTBS tests in measuring the bond strength of brittle resin cement to zirconia.

Quantitative investigation of abrasive grit size dependency of subsurface damages for the metal-bonded abrasives on Zerodur glass-ceramic

We present a quantitative evaluation of abrasive grit size dependency of subsurface damages on Zerodur glass-ceramic substrates induced by precision grinding. The depths of subsurface damages are evaluated for the metal-bonded grinding cup tools with four different grit sizes, D126, D64, D35, and D15, for the fixed nominal grinding process parameters. The micro-crack depth is driven by the mechanism between the abrasive grit and brittle glass, which generates median or radial cracks through the bulk material. The initiation and propagation of subsurface damages beneath the brittle glass-ceramic substrate of Zerodur are explored by the cross-sectional polishing method, which takes advantage of the optical contact bonding method without any adhesive, and are imaged by scanning electron microscopy. A novel hybrid methodology, which comprises both destructive and non-destructive subsurface evaluation tools, is developed and employed for evaluating the micro-cracks quantitively. The procedure is based on a repetitive and consecutive chemical acid etching after the surface is grounded, flat substrate polishing with a conventional polishing machine, a white light interferometer, and an image analysis. The four grit sizes, D126, D64, D35, and D15, correspond to the rough cut through to the complete precision grinding’s fine cut. The subsurface damages are measured to be 71 µm, 35 µm, 25 µm, and 7 µm from rough to fine grit sizes. The results are consistent with the theoretical model and previous work.

Microstructural Effects on the Interfacial Adhesion of Nanometer-Thick Cu Films on Glass Substrates: Implications for Microelectronic Devices.

Improving the interface stability for nanosized thin films on brittle substrates is crucial for technological applications such as microelectronics because the so-called brittle–ductile interfaces limit their overall reliability. By tuning the thin film properties, interface adhesion can be improved because of extrinsic toughening mechanisms during delamination. In this work, the influence of the film microstructure on interface adhesion was studied on a model brittle–ductile interface consisting of nanosized Cu films on brittle glass substrates. Therefore, 110 nm thin Cu films were deposited on glass substrates using magnetron sputtering. While film thickness, residual stresses, and texture of the Cu films were maintained comparable in the sputtering processes, the film microstructure was varied during deposition and via isothermal annealing, resulting in four different Cu films with bimodal grain size distributions. The interface adhesion of each Cu film was then determined using stressed Mo overlayers, which triggered Cu film delaminations in the shape of straight, spontaneous buckles. The mixed-mode adhesion energy for each film ranged from 2.35 J/m2 for the films with larger grains to 4.90 J/m2 for the films with the highest amount of nanosized grains. This surprising result could be clarified using an additional study of the buckles using focused ion beam cutting and quantification via confocal laser scanning microscopy to decouple and quantify the amount of elastic and plastic deformation stored in the buckled thin film. It could be shown that the films with smaller grains exhibit the possibility of absorbing a higher amount of energy during delamination, which explains their higher adhesion energy.

Using Methods of Analysis of Optical Images of Friction Surfaces of Ti–Al–N Based Coatings for Assessing Accumulation of Damage and Diagnostics of Failure in Tribological Tests

The processes of damage accumulation and failure in thin ceramic coatings based on the Ti–Al–N system deposited on ductile steel and brittle sitall substrates have been investigated during tribological tests under dry friction conditions. Fractal analysis was carried out to quantitatively assess damage to friction surfaces, as well as Fourier and wavelet analysis of their images obtained using optical microscopy. It is shown that the spectral analysis (Fourier and wavelet analysis) make it possible to reveal an unambiguous correlation between the quantitative parameters obtained and the rate of wear of coatings at the stage of steady wear. At the same time, the fractal dimension provided an estimate of the dynamics of accumulation of damage at the run-in stage but did not allow one to be produced at the stage of steady-state wear.

A differential strategy for measurement of a static force in a single-point diamond cutting by a force-controlled fast tool servo

This paper presents a new strategy for applying a piezoelectric (PZT) force sensor to the measurement of static force in a force sensor-integrated fast tool servo (FS-FTS) system for single-point diamond cutting on planar brittle material substrates. A conventional PZT force sensor unit composed of a PZT force sensor and a charge amplifier cannot be employed for measurement of static force due to the discharging characteristics of PZT materials. In this paper, to realize the long-term stable measurement of static cutting force, the time constant of the PZT force sensor unit is set to be infinitely large by removing the feedback resistor from the charge amplifier. The influence of bias current leakage, which arises as a result of removing the feedback resistor, is compensated through obtaining the differential output of the two PZT force sensor units having identical charge amplifiers and identical thermal characteristics. After microgroove-cutting experiments to verify the stability of the PZT force sensor unit in the conventional FS-FTS unit, a theoretical model for the PZT force sensor unit is established. Some basic experiments are then carried out to verify the feasibility of the proposed differential measurement strategy. Furthermore, a modified FS-FTS unit with a pair of PZT force sensor units based on the proposed strategy is developed, and its effectiveness is demonstrated through experiments.

Quantification of the Influence of Thin Film Microstructure on Adhesion for a Model Brittle-Ductile Interface

The influence of film microstructure on interface adhesion was studied on a model brittle-ductile interface consisting of thin Cu films on brittle glass substrates. Therefore, 110 nm thin Cu films were deposited on glass substrates using magnetron sputtering. While film thickness, residual stresses, and texture of the Cu films were maintained comparable in the sputtering processes, the film microstructure was varied during deposition and via isothermal annealing resulting in four different Cu films with bimodal grain size distributions. The interface adhesion of each Cu film was determined using stressed Mo overlayers, which triggered delamination of the Cu film from the glass substrates in the shape of straight spontaneous buckles. The model of Hutchinson and Suo for spontaneous buckles was applied to quantify the mixed mode adhesion energy for each film system ranging from 2.35 J/m² for the films with the highest amount of large grains to 4.90 J/m² for the films with the highest amount of nanosized grains. This surprising result could be clarified using an additional study of the buckles using focused ion beam cutting and quantification via confocal laser scanning microscopy to decouple and quantify the amount of elastic and plastic deformation stored in the buckled thin film. It could be shown that the films with smallest grains exhibit the possibility to absorb a higher amount of energy during delamination, which explains their improved adhesion.

The effects of blast lag in abrasive jet machined micro-channel intersections

Abrasive jet micro-machining (AJM) uses compressed air carrying abrasive solid particles to micro-machine a variety of features into surfaces. If the feature sizes are less than the size of the abrasive jet footprint, then a patterned erosion-resistant mask is used to protect the substrate material, leaving exposed areas to define the features. Previous investigations have revealed a ‘blast lag’ phenomenon in which, for the same dose of abrasive particles, narrower mask openings lead to channels that are shallower than wider ones. Blast lag occurs when using AJM on brittle substrates because of the natural tendency to rapidly form a V-shaped cross-sectional profile which inhibits abrasive particle strikes on the narrow vertex at the feature centerline. In this paper, the blast lag phenomenon is studied when using AJM to machine a network of microfluidic channels. It is found that, in some cases, differences in blast lag occurring at channel intersections and within the channels themselves, can lead to channel networks of nonuniform depth. A previously developed surface evolution model is adapted to allow prediction of the onset of blast lag in the channels and intersections and thus explain these differences. Finally, methods to eliminate the differences are discussed.

Preparation and Properties of Carbon Fiber/Carbon Nanotube Wet-Laid Composites.

In this study, carbon nanotubes (CNTs) were introduced into carbon fiber (CF) wet-laid composites as functional nano-fillers to fabricate multi-functional composites with improved mechanical, electrical, and thermal properties. It was considered that the wet-laid process was most suitable in order to introduce filler into brittle and rigid carbon fiber substrates, and we established the conditions of the process that could impart dispersibility and bonding between the fibers. We introduced polyamide 6 (PA6) short fiber, which is the same polymeric material as the stacking film, into carbon fiber and CNT mixture to enhance the binding interactions between carbon fiber and CNTs. Various types of CNT-reinforced carbon fiber wet-laid composites with PA6 short fibers were prepared, and the morphology, mechanical and electrical properties of the composites were estimated. As CNT was added to the carbon fiber nonwoven, the electrical conductivity increased by 500% but the tensile strength decreased slightly. By introducing short fibers of the same material as the matrix between CNT–CF wet-laid nonwovens, it was possible to find optimum conditions to increase the electrical conductivity while maintaining mechanical properties.

An Experimental Investigation on Pull-Off Tests Conducted on FRP Composites Applied to Brick Units

In the present study, a series of pull-off tests is performed to determine the bonding properties of fiber reinforced polymer (FRP) composites applied to a brittle substrate. Pull-off test is simple, fast and cheap and it is generally employed to evaluate the bond strength between the composite and the substrate. It is a procedure that can be easily performed on-site for checking the quality of the reinforcement adhesion.A steel composite made of a unidirectional steel fabric embedded in an organic matrix (bi-component epoxy resin) was applied to the surface of fired-clay bricks. A shallow core perpendicular to the surface was realized in the test specimen, leaving the core attached to the brick. A steel dolly was glued to the external surface of the core using epoxy adhesive. A loading device was employed to apply the tensile force until failure. The effects of moisture presence on the substrate and shape of the cores on the pull-off strength were investigated.

Substrate geology controlling different morphology, sedimentology, diagenesis and geochemistry of adjacent travertine bodies: A case study from the Sanandaj-Sirjan zone (western Iran)

A travertine fissure ridge and travertine mound are situated ~800 m from each other at the western margin of the active tectono-volcanic Sanandaj-Sirjan zone (western Iran). Despite their close proximity, the two geobodies show a difference in morphology, lithofacies, lithotypes, diagenesis and geochemistry. Petrographic analysis of the fissure ridge carbonates revealed homogeneous sparitic fabrics with dendritic structures reflecting precipitation under fast-flowing conditions from calcite supersaturated spring water. The mound carbonates are much more heterogeneous and display dominantly micritic fabrics with (mainly volcanic) clasts reflecting a lower energy flow regime and lower calcite saturation. In contrast to the limited diagenetic overprint of the fissure ridge carbonates, the more porous mound carbonates are affected by dissolution, cementation, and formation of Mn/Fe-oxide/hydroxides, attesting of a strong control on porosity by early diagenetic processes. The widespread bright luminescent calcite phases especially in sparitic fabrics within both geobodies display high Mn concentrations in calcite indicating suboxic precipitation conditions.The overlapping 87Sr/86Sr signatures and δ13C signatures point to a mixture of CO2 that dominantly originated from dissolution of the marine carbonate Qom Formation (Oligo-Miocene). The different δ18O signatures, indicate different upwelling systems caused by different substrate settings. The fissure ridge formed on a hard and brittle limestone substrate with precipitation from dominantly subsurface-sourced fluids. The mound travertine formed on top of fairly unconsolidated and fractured volcanic strata. The depleted δ18O signatures and impure micrite-dominated fabrics of the mound travertine reflect precipitation from thermal water that mixed with surrounding groundwater. This difference is also reflected in a different calculated precipitation temperature that varies between 4 and 21 °C for the mound versus 25–50 °C for the fissure ridge.Our findings show that subsurface geology exerts a major control on precipitation processes resulting in differences in travertine morphology, sedimentology, diagenesis, and geochemistry, despite that the travertine bodies occur adjacent to each other.

Non-contact polishing of single crystal diamond by ion beam etching

We propose a non-contact surface finishing method for brittle substrates by ion beam etching and we experimentally demonstrate polishing of (100) single crystal diamond surface. We model and simulate the polishing process, and verify the results experimentally by monitoring individual defects during the surface treatment. Rapid flattening of scratches and digs, as typically present on brittle substrates after mechanical polishing, is observed: trench depth is typically removed by 95% in less than 30 min. The polishing method relies on physical bombardment of the substrate surface with accelerated inert gas ions, rendering it highly versatile and applicable to a wide variety of materials.

Development of novel lapping technique for hard brittle material substrate using Electric Field assisted Polishing

Development of novel lapping technique for hard brittle material substrate using Electric Field assisted Polishing

Making Research Paper

In thermal stress cleaving with laser, the substrates of brittle material are divided by crack propagation due to the thermal stress. Therefore, distribution of the stress is essential and should be controlled properly for high accuracy of process, but boundary conditions and unexpected disturbances sometimes cause asymmetry of the distribution. This paper concerns a photoelastic observation technique to investigate the asymmetry of thermal stress distribution, which provides the isoclinic fringe pattern and the isochromatic fringe pattern around the crack propagating in the laser cleaving process for brittle material substrates. The technique can visualize the asymmetry of the distribution and some results are reported in this paper for transparent substrates.

Fundamental understanding of the deformation mechanism and corresponding behavior of RB-SiC ceramics subjected to nano-scratch in ambient temperature

To get insight into nano-scale deformation behavior and material removal mechanism of RB-SiC ceramic, nanoscratch experiments were performed using a Berkovich indenter. Structure changes in chips and subsurface deformation were characterized by means of Raman spectroscopy. The result shows that the SiC phase underwent amorphization in ductile chips, while no amorphous feature can be observed in brittle chips and substrate within scratch groove. The following estimated stress surround the indenter reveals that amorphous deformation in ductile chips is governed by tangential stress (above 95 GPa), whereas the dislocations-based substrate deformation mechanism was dominated by normal stress. In the end, the effects of normal load and scratching velocity on the scratch behavior including scratch residual depth, elastic recovery and friction coefficient that related to RB-SiC ceramic deformation mechanism were also analyzed. With the increase of normal load, the deformation mechanism transfers from ductile to brittle fracture mode and cause the decrease of elastic recovery and the increase of residual depth and friction coefficient. Furthermore, the increased high density of dislocations as a result of the increased scratching velocity give rise to the increase of scratch hardness, which finally result in the increase of elastic recovery and decrease of residual depth and friction coefficients. This study contributes a new understanding of the brittle material deformation mechanism during a nano-scale scratching process.

Analytical approach for global load-slip behaviour of FRP plates externally bonded to brittle substrates with anchors

A partial interaction procedure is developed for obtaining analytical solutions for the global load-slip behaviour of fibre-reinforced polymer plates adhesively bonded and mechanically anchored to brittle substrates. This is performed by adopting a matrix approach where the slip and slip strain at any point is given by the product of a solution matrix which is a function of: the position; the zone of solution - whether it is elastic, softening or debonding; and a coefficient vector. It is shown that the procedure can be used as a convenient research tool for extracting the material bond properties from standard experimental pull-push tests, or for use in advanced numerical simulations to develop anchorage systems for structures retrofitted with fibre-reinforced polymer (FRP) composites.

An optimized sample preparation approach for atomic resolution in situ studies of thin films.

This work provides the details of a simple and reliable method with less damage to prepare electron transparent samples for in situ studies in scanning/transmission electron microscopy. In this study, we use epitaxial VO2 thin films grown on c-Al2 O3 by pulsed laser deposition, which have a monoclinic-rutile transition at ~68°C. We employ an approach combining conventional mechanical wedge-polishing and Focused Ion beam to prepare the electron transparent samples of epitaxial VO2 thin films. The samples are first mechanically wedge-polished and ion-milled to be electron transparent. Subsequently, the thin region of VO2 films are separated from the rest of the polished sample using a focused ion beam and transferred to the in situ electron microscopy test stage. As a critical step, carbon nanotubes are used as connectors to the manipulator needle for a soft transfer process. This is done to avoid shattering of the brittle substrate film on the in situ sample support stage during the transfer process. We finally present the atomically resolved structural transition in VO2 films using this technique. This approach significantly increases the success rate of high-quality sample preparation with less damage for in situ studies of thin films and reduces the cost and instrumental/user errors associated with other techniques. The present work highlights a novel, simple, reliable approach with reduced damage to make electron transparent samples for atomic-scale insights of temperature-dependent transitions in epitaxial thin film heterostructures using in situ TEM studies.

Fracture analysis of channel cracks in flexible multilayer structure films under bending conditions

Recently, the development of various types of flexible electronic devices has received great research attention. However, brittle films, such as transparent conducting oxide electrodes and permeation barrier films, are still needed to maintain the high performance and reliability of flexible devices. In this study, we established a fracture mechanics model of brittle films on a polymer substrate under bending conditions. Using numerical simulation, we investigated the effects of a buffer layer between the brittle film and substrate on the fracture performance when the channel crack was formed in the brittle film. The mechanics model indicated that the Young’s modulus and thickness of the buffer layer were the important parameters determining the crack resistance of the film. The buffer layer with higher stiffness than the substrate enhanced the crack resistance of the brittle film as the thickness of the buffer layer increased. In contrast, the buffer layer with lower stiffness than the substrate reduced the crack resistance of the brittle film. In this case, the increase in the thickness of the buffer layer further decreased the fracture performance of the brittle film. The present study is expected to provide practical design guidance to enhance the bending durability of the brittle films and multilayer barrier films on polymer substrates.

Cleavage cracking of ductile-metal substrates induced by brittle coating fracture

Brittle coatings are often used to protect underlying ductile substrates from damage. Recent experimental observations show that the fracture of a brittle coating can cause the micro-cracking of the ductile metal substrates, threatening the safety and reliability of engineering structures. The cracking mode of the substrates is unclear and the corresponding mechanism remains poorly understood. In the present work, by performing room-temperature uniaxial tension experiments with a 10−4 s−1 straining rate, we have observed cleavage cracking in ductile-metal substrates (pure iron, AISI 1020 steel and brass) coated with WC-10%Co-4%Cr. Theoretical analysis reveals that the cleavage cracking is the result of brittle coating and its fracture, which synergically inhibit the local plastic deformation of the underlying substrate via two mechanisms. One is to restrain the dislocation nucleation and mobility near the interface of the substrate. The other is to bring a local high strain rate loading to the substrate, due to fast crack propagation in the brittle coating. The coupling of two effects leads to the nucleation of cleavage crack in the normally ductile substrates and causes significant loss in their ductility. This detrimental effect will be much more pronounced with the increase in the coating thickness. The findings shed new light on the failure mechanisms of brittle coating-metal substrate and provide guidelines in the material design of such systems.

Flexible Adhesive in Composite-to-Brick Strengthening-Experimental and Numerical Study.

This paper investigates composite-to-brick strengthening systems with flexible adhesive made of polyurethane (Carbon Fibre Reinforced Polyurethane (CFRPU) and Steel Reinforced Polyurethane (SRPU)) and epoxy resin (Carbon Fibre Reinforced Polymer (CFRP) and Steel Reinforced Polymer (SRP). The specimens were tested in a single lap shear test (SLST). LVDT displacement transducers (LVDT – Linear Variable Differential Transformer) and digital image correlation method (DIC) based measurement systems were used to measure displacements and strains. The obtained results were applied in a numerical analysis of the 3D model of the SLST specimen, with flexible adhesives modeled as a hyper-elastic model. The DIC and LVDT based systems demonstrated a good correlation. Experimental and numerical analysis confirmed that composite-to-brick strengthening systems with flexible adhesives are more effective on brittle substrates than stiff ones, as they are able to reduce stress concentrations and more evenly distribute stress along the entire bonded length, thus having a higher load carrying capacity.

Determination of interfacial fracture energy for 'pop-in' delaminations in a glass-epoxy system using the indentation method

The adhesion of coatings often controls the performance of the substrate-coating system. Certain engineering applications require an epoxy coating on a brittle substrate to protect and improve the performance of the substrate. Interfacial adhesion measurements of such systems provide a quantitative metric of that performance. Indentation experiments were performed to induce interfacial delaminations in a glass substrate-epoxy coating system. Delamination loads and radii were measured and applied to an analytical solution originally developed by Rosenfeld et al. (1990) to estimate the interfacial fracture energy of the substrate-coating adhesive bond. Results suggest an interfacial bond strength far exceeding that of similar previously studied glass-epoxy systems.