Indentation Cracks Research Articles

Surface toughness of silicon nitride bioceramics: II, Comparison with commercial oxide materials

Raman microprobe-assisted indentation, a micromechanics method validated in a companion paper, was used to compare the surface toughening behaviors of silicon nitride (Si3N4) and alumina-based bioceramics employed in joint arthroplasty (i.e., monolithic alumina, Al2O3, and yttria-stabilized zirconia (ZrO2)-toughened alumina, ZTA). Quantitative assessments of microscopic stress fields both ahead and behind the tip of Vickers indentation cracks propagated under increasing indentation loads were systematically made using a Raman microprobe with spatial resolution on the order of a single micrometer. Concurrently, crack opening displacement (COD) profiles were monitored on the same microcracks screened by Raman spectroscopy. The Raman eye clearly visualized different mechanisms operative in toughening Si3N4 and ZTA bioceramics (i.e., crack-face bridging and ZrO2 polymorphic transformation, respectively) as compared to the brittle behavior of monolithic Al2O3. Moreover, emphasis was placed on assessing not only the effectiveness but also the durability of such toughening effects when the biomaterials were aged in a hydrothermal environment. A significant degree of embrittlement at the biomaterial surface was recorded in the transformation-toughened ZTA, with the surface toughness reduced by exposure to the hydrothermal environment. Conversely, the Si3N4 biomaterial experienced a surface toughness value independent of hydrothermal attack. Crack-face bridging thus appears to be a durable surface toughening mechanism for biomaterials in joint arthroplasty.

Sequential Indentation Tests to Investigate the Influence of Confining Stress on Rock Breakage by Tunnel Boring Machine Cutter in a Biaxial State

The influence of confining stress on rock breakage by a tunnel boring machine cutter was investigated by conducting sequential indentation tests in a biaxial state. Combined with morphology measurements of breaking grooves and an analysis of surface and internal crack propagation between nicks, the effects of maximum confining stress and minimum stress on indentation efficiency, crack propagation and chip formation were investigated. Indentation tests and morphology measurements show that increasing a maximum confining stress will result in increased consumed energy in indentations, enlarged groove volumes and promoted indentation efficiency when the corresponding minimum confining stress is fixed. The energy consumed in indentations will increase with increase in minimum confining stress, however, because of the decreased groove volumes as the minimum confining stress increases, the efficiency will decrease. Observations of surface crack propagation show that more intensive fractures will be induced as the maximum confining stress increases, whereas the opposite occurs for an increase of minimum confining stress. An observation of the middle section, cracks and chips shows that as the maximum confining stress increases, chips tend to form in deeper parts when the minimum confining stress is fixed, whereas they tend to formed in shallower parts as the minimum confining stress increases when the maximum confining stress is fixed.

Cracking in Brittle Coatings during Nanoindentation

The FIB/SEM investigations of the microstructure changes in the hard brittle W-C based coating deposited on softer steel substrate after nanoindentation tests revealed that a set of approximately equidistant circular cracks forms in the coating in a sink-in zone around the indent and single cracks appear under the indenter tip. Finite element modeling (FEM) indicated development and concentration of the highest principal tensile stresses in the sink-in zone and in the zone below the indenter, which are considered to be the reason for the experimentally observed cracking. The distance from the indenter tip to the first circular crack combined with the calibration curve obtained from the FEM of the location of tensile stress maxima in sink-in zone can be used as a simple method for the determination of the strength of the studied coatings.

Direct observation of indentation deformation and cracking of silicate glasses

Abstract

Study on Microstructure and Fracture Toughness of TaC Ceramic Coating on HT300

Tantalum carbide (TaC) ceramic coating was produced on grey cast iron by the combined process of casting and heat treatment. The microstructure, micro-hardness, and fracture toughness of the TaC ceramic coating were investigated by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and indentation technique. The results show that TaC ceramic coating can be divided into two apparent regions, and the density of products gradually decrease from the surface to the matrix. The main phases of the ceramic coating are only TaC, α-Fe, and Fe3C. TEM examination revealed a nanostructure ceramic, in which 200~300 nm TaC grains are cemented by iron binder. The nanoindentation hardness of TaC ceramic coating vary in the range of 14~21 GPa. A close examination of the indentation crack paths of TaC samples revealed that the TaC ceramic coating have extensive crack deflection with a great amount of intergranular fracture.

Effects of the Modification of Processing Parameters on Mechanical Properties of HVOF Cr2C3-25NiCr Coatings

The present work shows that some mechanical properties of Cr2C3-25NiCr coatings processed by HVOF thermal spraying such as elastic modulus, hardness, fracture toughness, and wear resistance are closely linked to selected processing parameters as well as to chemistry, structure, and morphology of the feedstock material. Optimization of processing parameters was based on oxygen concentration measurements inside the flame, studied by a contour plot, and curvature data recorded during spraying. The increase in velocity of in-flight particles strongly affected the interfacial fracture toughness of the coatings, as a maximum KIC of 3.71 MPa m 1/2 was reached when particle velocity increased to ~765 m/s. The micro-hardness of Cr2C3-25NiCr coatings was widely varied by changing the HVOF spray conditions, i.e., the amount of reinforcing phases and inter-splat adhesion were quite sensitive to any modification of the processing parameters. It was also found that coatings having a high inter-splat adhesion and/or low degradation of reinforcing hard phases showed reduced weight loss during erosive wear tests. Finally, from the experimental evidence reported in this work, a correlation between the elastic bending modulus of coatings and indentation crack length was also found.

Prediction of residual stress and deformation of enameled steel

The firing process to bond an enamel frit onto a steel substrate is conducted under a high-temperature condition of approximately 830°ýC for 3min, which leads to the thermal deformation of the enameled steel product. This thermal deformation of the enameled steel is one of the main problems with the firing process. The thermal deformation is also affected by the residual stress of the enamel coating, which is generated by the mismatch between the thermal expansion coefficient of the enamel and that of the steel substrate. The objective of this study was to predict the deformation and residual stress of enameled steel using an analytical model and finite element analysis. To verify the results from the prediction methods, firing and cooling experiments were performed using the same conditions as such methods, and the experimental results were compared to those of the prediction methods. The residual stress was measured using a crack indentation technique based on a pyramidal diamond indenter. The magnitude of the residual stress could be determined by comparing the crack lengths in the unstressed and stressed specimens. The results of the prediction methods were in good agreement with the experimental results with respect to the deformation and residual stress.

Driving force for indentation cracking in glass: composition, pressure and temperature dependence.

The occurrence of damage at the surface of glass parts caused by sharp contact loading is a major issue for glass makers, suppliers and end-users. Yet, it is still a poorly understood problem from the viewpoints both of glass science and solid mechanics. Different microcracking patterns are observed at indentation sites depending on the glass composition and indentation cracks may form during both the loading and the unloading stages. Besides, we do not know much about the fracture toughness of glass and its composition dependence, so that setting a criterion for crack initiation and predicting the extent of the damage yet remain out of reach. In this study, by comparison of the behaviour of glasses from very different chemical systems and by identifying experimentally the individual contributions of the different rheological processes leading to the formation of the imprint--namely elasticity, densification and shear flow--we obtain a fairly straightforward prediction of the type and extent of the microcracks which will most likely form, depending on the physical properties of the glass. Finally, some guidelines to reduce the driving force for microcracking are proposed in the light of the effects of composition, temperature and pressure, and the areas for further research are briefly discussed.

Crack Healing in Ti2Al0.5Sn0.5C–Al2O3 Composites

Oxidation induced crack healing of Al2O3 composites loaded with a MAX phase based repair filler (Ti2Al0.5Sn0.5C) was examined. The fracture strength of 20 vol% repair filler loaded composites containing artificial indent cracks recovered fully to the level of the virgin material upon isothermal annealing in air atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM‐EBSD analysis of crack microstructure indicates two different oxidation reaction regimes to govern the crack filling: near the surface SnO2, TiO2, and Al2O3 were formed whereas deeply inside the cracks Al2O3 and TiO2 and metallic Sn were detected. The presence of elemental Sn was attributed to partial oxidation of aluminum and titanium which lowered the local oxygen concentration below a threshold value required for Sn oxidation to SnO2. Thus, Ti2Al0.5Sn0.5C may represent an efficient repair filler system to trigger oxidation induced crack healing in ceramic composites at temperatures below 1000°C.

Effect of heating rate on the mechanical properties and microstructure of Ti(C,N)-based cermets

An appropriate heating rate in the sintering process is crucial to obtain the Ti(C,N)-based cermets with superior properties. In this paper, Ti(C,N)-based cermets were sintered to investigate the influence of heating rate on the mechanical properties and microstructure of the cermet materials. The transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (KIC) were tested. The microstructure, indention crack, fracture morphology and phase composition of the cermets were also studied by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results reveal that the heating rate has a great influence on the mechanical properties and microstructure of Ti(C,N)-based cermets. The cermets sintered at the heating rate of 3°C/min between 1300°C and 1430°C have the optimum comprehensive mechanical properties with a transverse rupture strength of 1605±107MPa, a hardness of 12.02±0.25GPa and a fracture toughness of 10.73±0.40MPam1/2. The heating rate can affect the reaction among the constituents of Ti(C,N)-based cermets and then influence the elements distribution in the core–rim microstructures and the lattice parameter of Ti(C,N) phase. When the heating rate is between 2°C/min and 5°C/min, the lower the heating rate is, the coarser the Ti(C,N) grains become. A higher heating rate is detrimental to the formation of core–rim microstructures, and a lower heating rate can result in grain coarsening and inhomogeneous microstructure. The observation of indention cracks and fracture surfaces show that the intergranular cracks and intergranular fractures mainly occur in the cermets with larger binder mean free path and medium grains. While the cleavage fractures appear more in the cermets with grain coarsening, and the transgranular fractures exist more in the cermets with non-fully developed fine grains.

Fast heating thermal shock test for β-SiAlON with molten metals as heating medium

A novel fast heating thermal shock test (FHTST) was carried out to evaluate the thermal shock behaviour of pressureless sintered β-SiAlON. Molten nonferrous metals, including molten Sn and Al, were used as the heating medium for the FHTST. The retained strength and Vickers crack propagation of the samples after the FHTST were evaluated. For comparison, a quenching thermal shock test (QTST) with room temperature water was also carried out. The differences between the FHTST and QTST for the β-SiAlON ceramic were investigated. For the FHTST with molten metal, the retained strength showed no decrease and Vickers cracks did not propagate with a temperature difference (ΔT) up to 900°C. However, for the QTST with room temperature water, the retained strength reduced sharply at the ΔT larger than 600°C and the indentation cracks propagated significantly. The finite element method (FEM) calculation showed that the differences between FHTST and QTST results could be caused by the different stress status (compressive stress or tensile stress) on the surface. FHTST with molten metal would be a suitable thermal shock evaluating method for the ceramics used in nonferrous metals industry.

Fabrication and mechanical properties of Si3N4/(W, Ti)C/Co graded nano-composite ceramic tool materials

Si3N4/(W, Ti)C/Co nano-composite ceramic tool materials with multilayered structure were fabricated by hot pressing technology. The composites without Co were used as the surface layers and the composites containing Co were used as the inner layers. The flexural strength, fracture toughness, and hardness of the composites were tested, and the fracture surfaces and indention cracks were observed. The experimental results showed that the flexural strength and fracture toughness were largely increased by the graded structure and the high hardness was still maintained. The five-layer graded nano-composites, with a thickness ratio of 0.2, which were sintered under a pressure of 30MPa at 1700°C in vacuum condition for 45min, had the optimum comprehensive mechanical properties with a flexural strength of 992MPa, a hardness of 17.83GPa, and a fracture toughness of 10.5MPa·m1/2. The formation of elongated β-Si3N4 grains and the residual compressive stress in the surface layer induced by the graded structure contributed to the improved properties. A synergistic effect of intergranular and transgranular fracture, crack bridging, and deflection contributed to the strengthening and toughening mechanisms.

A 2000-year perspective on indentation crack resistance and brittleness of glass

Data on micromechanical properties of ancient and medieval glass fragments and glasses of early modern and today's manufacturing were compiled to offer a tentative look on changes in the brittleness of glass over the past 2000years. Chronology of glass making was based on glass chemistry using rare earth element (REE) pattern, fluxing agents and provenance analysis while cracking and brittleness were evaluated from Vickers indentation in the course of the mean crack initiation load CR and the crack length-to-half diagonal ratio c/a. A structural analysis revealed that CR increases and c/a decreases with increasing molar volume i.e. decreasing atomic packing fraction, which allows us to classify periods of high and low crack-resistant and brittle glasses along the time line of historic glass technology.

Nanoindentation and fracture toughness of nanostructured zirconia/multi-walled carbon nanotube composites

Multi-walled carbon nanotubes (MWCNTs)/3mol% yttria-doped tetragonal zirconia (3Y-TZP) composites were produced using spark plasma sintering (SPS) with MWCNT content ranging within 0–2wt%. In the present paper, it was shown that the addition of MWCNTs results in a refinement of the composites microstructure. Moreover, nanoindentation tests were performed in order to monitor the change in elastic modulus and hardness with MWCNT content and it was found that both properties decrease with the addition of MWCNT content. A novel method was used to measure the true fracture toughness of the composites by producing a shallow surface sharp notch machined by ultra-short pulsed laser ablation on the surface of beam specimens. The true fracture toughness obtained on this laser machined single edge V-notch beam (SEVNB) specimens tested in four point bending was compared to the indentation fracture toughness measured using a Vickers indenter. It was found that the indentation fracture toughness increases with increasing MWCNT content, while the true fracture toughness determined with SEVNB was practically independent of the composition. Finally, it was concluded that the increase in the resistance to indentation cracking of the composites with respect to 3Y-TZP matrix cannot be associated to higher true fracture toughness. The results were discussed in terms of transformation toughening, damage induced in front of the notch tip, microstructure of the composites, and fracture toughness of 3Y-TZP.

Subcritical Crack Growth in Unpoled and Poled PZT Ceramics under Applied Static and Oscillating Electric Fields. I. Experimental Observation

The effects of both electric field and mechanical stress on the subcritical crack growth (SCG) behavior of lead zirconate titanate (PZT) ceramics are investigated. In Part I, the growth of indented crack in unpoled and poled soft PZT ceramics subjected to different static and oscillating electric fields is studied. The experimental results indicated a faster crack growth rate initially followed by a slower growth rate with time. The SCG behaviors along directions perpendicular and parallel to the applied electric fields are different and depend on the field intensity, field polarity, frequency, and wave-form of the applied oscillating fields. It is suggested that the crack driving forces for the SCG behavior in PZT ceramics are due to the residual stress from indentation and the application of the electric field.

Comparison of Ti(C,N)-based cermets processed by hot-pressing sintering and conventional pressureless sintering

A suitable sintering method is important to obtain the Ti(C,N)-based cermets with superior properties. In this paper, Ti(C,N)-based cermets were fabricated by hot-pressing sintering (HP) and conventional pressureless sintering (PLS) technology, respectively, to investigate the influence of different sintering methods on the microstructure and mechanical properties of cermets materials. The microstructure, fracture morphology, indention cracks and phase composition were observed and detected using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (KIC) were also measured. The results reveal that all of the Ti(C,N)-based cermets exhibit core–rim microstructures with black cores, white cores and grey rims embedded into metal binder phases. The grain size of the samples fabricated by HP is much finer and the structure is more compact than those fabricated by PLS, while there exist pores in the HP sintered samples. The sintering process has no influence on the phase composition of cermets, but affects the phase content and crystallinity. The samples fabricated by PLS present higher transverse rupture strength, fracture toughness and density than samples fabricated by HP. However, the HP sintered samples possess a higher hardness.

Light intensification effect of trailing indent crack in fused silica subsurface

A finite-difference time-domain algorithm was applied to solve Maxwell’s equations to obtain the redistribution of an electromagnetic plane wave in the vicinity of a trailing indent crack (TIC). The roles of five geometrical parameters playing in light intensification were calculated numerically under the irradiation of a 355-nm normal incidence laser. The results show that the light intensity enhancements between the nearest neighbor pits were remarkable, which may lead to damage. The calculated results reveal that the light intensity enhancement factor (LIEF) can be up to 11.2 when TIC is on the rear-surface. With the increase of the length as well as the depth of pits, LIEF increased. Conversely, with the increase of the axis of pits, LIEF gradually declined to a stable status. It was observed that there exists an optima width or gap, which enables LIEF to be increased dramatically and then decreased gently. By comparison, results suggest that the worst cases occur when the depth and the length are both very large, especially if the width equals to 2 l and the gap equals the width. This work provides a recommended theoretical criterion for defect inspection and classification.

Low temperature fracture toughness of PMMA and crack-tip conditions under flat-tipped cylindrical indenter

Fracture toughness of a PMMA (poly(methyl methacrylate)) plate was measured at −20°C, −40°C and −80°C, using a newly-developed flat-tip indentation method, and compared with the room temperature results. A flat-tipped cylindrical indenter with a sharp edge, made from hardened steel, was used for the experiments. For comparison, indentation damage and crack patterns from a tungsten-carbide tip with a microscopically-chipped edge were also examined. The zigzag imprint front shows indentation cracking can indeed be generated under a flat-tipped indenter, as implied by the theoretical singular stress field at the contact edge of a sharp-edged and flat-tip indenter. Finite element analysis (FEA) simulations were used to confirm the theoretical fracture toughness equations for pure elastic stress fields, and were used to determine the elastic stress field at the indentation crack-tip following the theoretically-determined cracking angle to illustrate the nature of the mixed-mode elastic fracture. However, as shown by optical cross-section observations, plastic damage in PMMA at the contact edge alters the cracking/damage pattern.

Finite element design and fabrication of Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool material

Based on the idea of a functionally gradient material, the gradient self-lubricating ceramic tools were designed to solve the problem that the antifriction property and wear resistance of self-lubricating ceramic tools cannot be simultaneously considered. Physical, composition distribution, and micromechanical models were established. The finite element method was used to analyze the residual thermal stress of symmetrically distributed gradient ceramic tools with different compositional distribution exponent (n) values. When n=1.8, the stress gradient was small and the lowest value of the maximum Von Mises stress was obtained. An Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool material with good mechanical properties was then fabricated by hot-pressing technique according to the optimum design results. Residual stresses were calculated by indentation crack method. Results showed that in the gradient self-lubricating ceramic tool material, compressive stresses were formed in the surface layers, whereas tensile stresses were formed in the middle layer, which were consistent with FEM-calculated findings.

Evolution of Indentation Induced Cracks on WC Ceramic under Cavitation Loading in Water

For engineering ceramics, cracks produced in preparation process, i.e. sintering and surface finishing, have a profound impact on the mechanical strength and wear. Under cavitation loading, the flaws on the surface and subsurface are preferentially eroded. In this paper, artificial cracks are introduced on WC ceramic by indentation of a Vickers indenter at a load of 10 kgf. Five positions on the cavitation eroded area are selected. The average length of the cracks is 71.6 mm. The cavitation erosion tests are terminated at 1, 3, and 5 min to reveal the evolution of cracks. A single pattern for the evolution of cracks is recognized. Two new cracks at the tip of indentation cracks first nucleate and then propagate (accompanied by pullout of grains). The angle between the two new cracks ranges from 30° to 45°. The evolution of adjunct two groups of cracks gives birth to local breakout finally. Steps which are provided limited resistance to breakout are observed. The nucleation and propagation of the four indentation cracks do not always occur simultaneously.