Indentation Cracks Research Articles

Comparison of fracture toughness testing by the single edge v-notch beam and the surface crack in flexure method on silicon nitride

This work is intended to compare the SEVNB and the SCF methods for fracture toughness testing. Both of the test methods are applied to a silicon nitride. Both methods are standardized and both have some minor shortcomings with respect to practicability and match of the theoretical description of the test situation with the actual test situation. SEVNB is easy to perform, but uses a notch of known depth (+ small micro-cracks of unknown size in front of a notch) instead of a crack, SCF uses a presumably residual stress-free indentation crack which is sometimes difficult to measure, not always semi-elliptical, as assumed to apply the evaluation formula, and may be accompanied by lateral cracks. Both methods will be analysed with respect to possible experimental errors. Variations of the SCF method are applied in order to examine how pre-cracks can be introduced which can be detected on the fracture surface. The geometry of the SCF-pre-crack is varied by applying different loads and by removing different amounts of material. The geometry of the Knoop pre-cracks system is investigated. The results help to identify conditions under which the SCF method can be used successfully to measure the fracture toughness of silicon nitride ceramics.

Crack propagation during Vickers indentation of zirconia ceramics

A quarter finite element model of 3 mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramics undergoing Vickers indentation was established to simulate the evolution of stress and the propagation of cracks inside a sample. The indentation experiment was carried out on the Micro Vickers Hardness Tester. The results of the geometric characteristic parameters, such as the indentation diagonal half-length a, the crack length c and the maximum indent depth hm, from the indentation simulation and experiment were similar. The types of indentation cracks under various loads were determined according to the Lawn-Evan model, which exactly correspond to the simulation results. In addition, the propagation of indentation cracks was discussed based on the maximum principal stress contour plots at various stages, and the conclusions were verified by the indentation analysis model proposed by Yoffe. As a result, the model developed in this paper can be used in indentation studies to solve the related problem.

Influence of acid leaching surface treatment on indentation cracking of soda lime silicate glass

Past work has shown that water or acid soaking treatments can increase the mechanical strength of soda lime silicate (SLS) glasses. In this work, we show that acid leaching treatments result in an increase in the apparent crack resistance of the acid-leached surface of SLS glass during indentation. Vickers indentation tests in controlled environments show a humidity dependence of radial cracking, suggesting that the transport of water through the leached layer plays a critical role in the propagation of cracks to the glass surface. Molecular dynamics simulations with reactive force fields indicate that the leached surface layer can undergo pressure-induced mechanochemical reactions during indentation, which increases the bridging oxygen connectivity in the silica network of the leached layer. Such structural changes can hinder transport of water molecules from the environment to the subsurface crack tip. Based on experimental observations and simulation results, a new hypothesis is proposed that mechanochemical restructuring in the leached layer in response to the applied load may lower the transport kinetics of molecular water to critical flaws, resulting in an apparent enhancement in the crack resistance of the acid-leached surface of SLS glass.

Indentation cracking in silicate glasses is directed by shear flow, not by densification

Over the past decades, constitutive relations have been developed to compute the mechanical response of silicate glasses at the continuum length scale. They are now reliable enough that we can calculate indentation induced stress and strain fields and examine the impact of material parameters on indentation response, and especially hardness, pile-up and stress fields. In contrast to a presently widespread assumption in the literature, we show that (shear) flow stress is the primary determinant of these properties, and that densification plays a secondary role in the indentation response of all the silicate glasses. This result applies even for large values of the densification at saturation because of the high ratio between effective volumetric yield stress (i.e. yield pressure) and flow stress. It is well-known that, depending upon composition, silicate glasses exhibit very different sensitivities to indentation cracking, although all other standard mechanical properties remain quite similar. We point out that material damage incurred through plastic shear flow, and especially shear flow instability and localization may well control crack initiation, which would resolve the paradox. Shear flow instability and damage has not been quantitatively investigated in detail in silicate glasses as yet, neither experimentally nor theoretically. However, we believe it is key to an in depth understanding of cracking resistance in silicate glasses.

On the relation between fracture toughness and crack resistance in oxide glasses

The resistances of glasses to crack initiation and crack growth are both important parameters for evaluating their mechanical performance. However, it is important to distinguish them as they refer to different mechanical phenomena. In this work, we study the difference in resistance to crack initiation and growth by determining the load required to induce indentation cracking (named crack resistance, CR) and the fracture toughness (KIc), respectively, for six oxide glasses with a wide range of CR values from as low as 0.05 N to above 100 N. These new data as well as existing literature data suggest that there is no systematic relation between CR and KIc, and as such, we conclude that these two terms should not be used interchangeably. We discuss the origin of the lacking relation by considering the competing effects of glass structure, rigidity, and atmospheric humidity on crack initiation and growth.

Residual polymerization stresses in human premolars generated with Class II composite restorations

The objective of this study was to assess the influence of filling techniques on residual polymerization stresses in resin composite restorations of the tooth. Flat planes were ground in buccal enamel surfaces of extracted human premolars, followed by preparing Class II cavities. Indentation cracks were introduced in the planes and crack lengths were measured mesio-distally (x-direction) and cervico-incisally (y-direction). Cavities were filled with a resin composite and an adhesive using three methods; one with bulk filling and two with differing incremental filling techniques. The x- and y-tensile stresses were calculated from crack lengths measured repeatedly over 360 min after filling. Elastic modulus and polymerization shrinkage of the composite were also measured. Filling technique and time after fillings were statistically significant only for the y-stress. The incremental techniques generated smaller stresses than the bulk filling. The stresses developed for 60 min after filling, while the modulus and the shrinkage stopped developing within 10 min and 2 min after irradiation, respectively. The incremental technique, in which the proximal portion of the cavity was filled first, was effective in decreasing the residual tensile stress generated by the polymerization of resin composite.

A critical evaluation of indentation crack lengths in air

AbstractAn extensive overview is presented of Vickers indentation crack lengths in ceramics in air. Measurement of such crack lengths is one of the most common and powerful assessments of the fracture properties of ceramics and the overview provides a critical evaluation of observed behavior as functions of material type and indentation load, and an extensive basis for comparison of results from new materials and analyses. The overview considers single crystals, polycrystals, transforming materials, glasses, and multiphase materials, including cermets, glass‐ceramics, and tooth enamel. The coverage extends over structural and electronic ceramics, including oxides, carbides, nitrides, and titanates. The data are presented in a single format for ease of interpretation in terms of idealized indentation fracture and for inter‐material comparisons; most data are unique to this work, but the results of selected studies from the published literature are included. The overview considers the precision and accuracy of crack length measurements and demonstrates a simple quantitative evaluation and ranking scheme for ceramic fracture based on load‐adjusted crack length and cracking susceptibility. Indentation hardness and cracking threshold are also determined and related to the susceptibility. Material toughness is related to cracking susceptibility by fracture mechanics analyses: typical crack length measurements in air are shown to provide estimates of inert toughness with a relative uncertainty of ±50%.

Input power effect on microstructure and mechanical properties of Mo[sbnd]Si[sbnd]N multilayer coatings

The MoSiN single layer and multilayer films were fabricated by a reactive radio frequency, r.f., magnetron sputtering system with pure Mo and Si targets. The Ar/N2 flow ratio was fixed at 12/8, while r.f. input powers for Mo and Si were 135 W and 0 to 150 W, respectively. The corresponding Si contents in the coating ranged from 0 to approximately 20 at.%. The microstructure of the MoSiN single layers evolved from Mo2N (111) crystalline to amorphous feature with input powers and related Si doping amounts. The MoSiN multilayer coatings were fabricated with crystalline MoN, crystalline and amorphous MoSiN layers with sequential stacking of any two building layers. The multilayer film made of MoN and crystalline MoSiN layers showed an obvious crystalline microstructure, while the amorphous MoSiN layer incorporated multilayer MoSiN systems effectively suppressed the continuity of the columnar growth. The correlation between mechanical properties, including hardness, Young's modulus, indentation cracking, and scratch behavior, were investigated and discussed in terms of film microstructure evolution.

Indentation cracking and deformation mechanism of sodium aluminoborosilicate glasses

AbstractDeveloping less brittle oxide glasses is a grand challenge in the field of glass science and technology, as it would pave the way toward new glass applications and limit the overall raw material usage and energy consumption. However, in order to achieve this goal, more insight into the correlation between the chemical composition and material properties is required. In this work, we focus on the mechanical properties of quaternary sodium aluminoborosilicate glasses, wherein systematic changes in glass chemistry yield different resistances to indentation crack initiation. We discuss the origin of the composition dependence of indentation cracking based on an evaluation of the deformation mechanism taking place during the indentation event. To this end, we use a simple metric, the extent of indent side length recovery upon annealing, to quantify the extent of reversible volume deformation. Finally, we also compare the compositional trend in crack initiation resistance to that in crack growth resistance (fracture toughness), showing no simple correlation among the two.

Micro-nano multilayer structure design and solid particle erosion resistance performance of CrAlNx/CrAlN coating

In this work, the CrAlNx/CrAlN micro-nano multi-layer structure coating was designed and prepared on TC11 titanium alloy by Arc Ion Plating (AIP). In addition, the monolayer CrN and CrAlN coatings was prepared for comparison. The effects of CrN and CrAlN monolayer, CrAlNx/CrAlN multilayer coatings on the micro-structure and phase composition were investigated by Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), and Energy-dispersive X-ray micro-analysis (EDS). The mechanical properties of coatings, including nano-hardness, adhesion strength and fracture toughness were studied by means of nano-indentation, micro-scratch and indentation cracking (CSEM Instruments). It was found that the residual stress of CrN coating was −1.341 GPa, and that of CrAlN coating was −1.569 GPa, while that of CrAlNx/CrAlN multilayer coating was only −0.932 GPa significantly lower than that of CrN and CrAlN coatings. In addition, as the multilayer structure of CrAlNx/CrAlN (x is the atomic percentage, x < 1) is composed of nano-scale lamellar CrAlNx (L), thin columnar structure CrAlNx (C), and micro-nano-scale columnar structure CrAlN, the unique multilayer structure makes the highest fracture toughness (8.7 ± 0.5 MPa m1/2). The minimum erosion rate was achieved for CrAlNx/CrAlN coatings under the condition of 1.265 ± 0.035 μm/g at 90° and 0.568 ± 0.022 μm/g at 30°, respectively. These results indicate that the CrAlNx/CrAlN multilayer coating has the excellent solid particle erosion resistance.

Fracture toughness determination of fused silica by cube corner indentation cracking and pillar splitting

In this paper the applicability of the pillar splitting technique for fracture toughness determination on anomalous behaving bulk fused silica glass is explored. The results are compared to conventional cube corner indentation cracking analyzed using the Lawn, Evans and Marshall model (JACerS, 63 (1980) 574). The experimental analysis is supported by constitutive Finite Element Analysis with cohesive zones to determine adequate gauge factors to correlate the load instability upon splitting to the fracture toughness Kc. The role of densification on pillar splitting was critically examined.The results show a fragmentation of the micro pillar into three parts, a failure pattern as proposed by Sebastiani et al. (Philos. Mag., 95 (2014) 1928). Therefore, the applicability of pillar splitting to (anomalous) glasses is confirmed. Cohesive zone FEA delivered the gauge factors required for fracture toughness calculation. The influence of densification on those factors, however, was found to be small for indentation cracking and negligible for pillar splitting. With the corresponding set of gauge factors fracture toughness values in good accordance with literature could be determined. Inside the SEM, moreover, electron beam irradiation has been found to enhance the fracture properties of fused silica.

Effect of sintering temperature on microstructure and mechanical properties of boron nitride whisker reinforced fused silica composites

Novel BN whisker reinforced SiO2 matrix composites were achieved via hot-pressing at various temperatures, oriented to an enhanced mechanical properties of fused silica. Near full dense BNW/SiO2 composite sintered at 1350 °C presents optimal mechanical properties with bending strength of 124.8 ± 3.4 MPa and fracture toughness of 1.76 ± 0.32 MPa m1/2. The improved bending strength is mainly ascribed to the strengthening effects of BNW and crystallized nanosized α-cristobalite. Indentation crack paths indicate that the crack deflection and interface debonding are the main toughening mechanisms when BNW/SiO2 composite is dense. The desirable dielectric properties (dielectric constant of 3.93 and loss tangent of 0.46 × 10−3), the tiny mean thermal expansion coefficient (1.40 × 10−6 K−1) and thermal diffusivity (1.16 mm2/s) suggest that the BNW/SiO2 composite is definitely a promising high temperature electromagnetic wave transparent material.

Ferroelastic domain switching and R‐curve behavior in lead zirconate titanate (Zr/Ti = 52/48)‐based ferroelectric ceramics

AbstractIn this work, ferroelastic domain switching and R‐curve behavior in lead zirconate titanate (Nb/Ce co‐doped Pb(Zr0.52Ti0.48)O3, ab. PZT‐NC)‐based ferroelectric ceramics were investigated, using the indentation‐strength‐in‐bending (ISB) method. Firstly, Vickers indentation test examined the notable fracture anisotropy of PZT‐NC ceramics between the poling direction and its perpendicular direction, and the crack open displacement (COD) profiles in the two directions were also theoretically calculated from the indentation fracture mechanics. And then two kinds of ferroelastic domain switching modes (in‐plane and out‐of‐plane) were used for explaining such anisotropic propagation behavior of indentation cracks. The subsequent three‐point bending test illustrated the dependence of fracture strength on indentation load and the rising crack growth resistance curves (R‐curves) in two directions. The resulted R‐curves were fitted by the Hill's type Growth Function successfully, giving the reasonable values of crack extension exponential (n), plateau fracture toughness (Kmax), and initial fracture toughness (Kini). The in‐plane ferroelastic domain switching was identified as a more significant toughening mechanism for PZT‐NC ceramics than the out‐of‐plane switching due to more switchable domains.

Direct observation of the displacement field and microcracking in a glass by means of X-ray tomography during in situ Vickers indentation experiment

The actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27% of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is ≈ 14 GPa and is in agreement with the intrinsic glass strength.

Microstructure and mechanical properties of a SiC containing advanced structural ceramics

The fracture toughness of SiC-AlN structural ceramics can be improved by obtaining fine grain size distribution of both SiC and AlN phases and a weak SiC-AlN interfacial solid solution layer which helps in crack deflection. In order to achieve this, a SiC containing ceramics i.e. 55wt.%SiC+45wt.%AlN is prepared by spark plasma sintering (1850 °C/50 MPa/5 min.). High density (96.6%) with a very fine grain microstructure comprising of intimate mixture of both the SiC and AlN phases are obtained in the product. Vol% & wt% of SiC & AlN phases estimated through the EDS mapping and Rietveld analysis are found to be 54 vol% & 46 vol%, and 54.2 wt% & 45.8 wt% respectively. Formation of solid solution layer between hexagonal AlN (a = 3.1114 Å, c = 4.9792 Å) and hexagonal SiC (a = 3.081 Å, c = 5.031 Å) phase is confirmed indirectly by change in AlN lattice parameter and directly by EDS mapping. Electron probe micro analysis (EPMA) study reveals that, the solid solution between AlN and SiC phase is formed by mainly diffusion of SiC in AlN phase. Young modulus and hardness of the SPS product are similar to those reported in the literature. The product shows higher indentation fracture toughness (6.7 ± 0.9 MPa.m0.5 and 4.8 ± 0.6 MPa.m0.5 obtained using Median and Palmqvist crack model respectively) than the fracture toughness of a well known ultra-high temperature structural material i.e. ZrB2–20SiC (3.9 ± 1.1 MPa.m1/2). The indentation cracks are found to initiate at the AlN-SiC interface (Hv1263–1641) and are found to propagate along either the AlN phase (Hv1434–1720) or AlN-SiC interface. The cracks are found to stop when these encounter either SiC phase (Hv1896–2476) or porosity. The higher value of fracture toughness may be attributed to the mismatch in coefficient of thermal expansion (CTE) of SiC and AlN phase, fine grain size distribution, secondary crack generation and a weak AlN-SiC interfacial solid solution layer which helps in crack deflection. This product may find potential application as high temperature structural material since SiC containing materials exhibits outstanding high temperature oxidation and corrosion resistance.

Low‐temperature oxidation‐induced crack healing in Ti2Al0.5Sn0.5C–Al2O3 composites

AbstractThe oxidation‐induced crack healing of an Al2O3 composite loaded with various volume fractions of Ti2Al0.5Sn0.5C repair filler particles was investigated by annealing in air at a relatively low temperature of 700°C. After annealing a composite with 20 vol.% repair fillers (with a particle size of ~5.6 µm) for 48 hours, artificial indentation cracks prepared on the surface, as well as pores near the surface, were completely healed by filling with condensed oxidation products. Additionally, minor fraction of metallic Sn was detected. A complementary study by X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy dispersive X‐ray spectroscopy revealed that nano‐sized oxidation products (SnO2, TiO2, and α‐Al2O3 phase) were formed as major crack‐filling species. After healing, the strength recovery of the Al2O3 composites was significantly improved in the composites loaded with more than 10 vol.% repair fillers and achieved 107% at 700 for 48 hours.

Performance of Hybrid Powder-Suspension Axial Plasma Sprayed Al2O3-YSZ Coatings in Bovine Serum Solution.

Ceramic coatings on metallic implants are a promising alternative to conventional implants due to their ability to offer superior wear resistance. The present work investigates the sliding wear behavior under bovine serum solution and indentation crack growth resistance of four coatings, namely (1) conventional powder-derived alumina coating (Ap), (2) suspension-derived alumina coating (As), (3) composite Al2O3—20wt % Yittria stabilized Zirconia (YSZ) coating (AsYs) deposited using a mixed suspension, and (4) powder Al2O3—suspension YSZ hybrid composite coating ApYs developed by axial feeding plasma spraying, respectively. The indentation crack growth resistance of the hybrid coating was superior due to the inclusion of distributed fine YSZ particles along with coarser alumina splats. Enhanced wear resistance was observed for the powder derived Ap and the hybrid ApYs coatings, whereas the suspension sprayed As and AsYs coatings significantly deteriorated due to extensive pitting.

Multi-walled carbon nanotube-reinforced boron carbide matrix composites fabricated via ultra-high-pressure sintering

To well preserve the fine structure of MWCNTs in the fabrication of ceramic matrix composites, an ultra-high-pressure sintering method is employed here. The effects of the MWCNTs on the microstructure and properties of the composites are investigated. The B4C-MWCNTs composites are prepared under the pressure of 4.5 GPa at a low sintering temperature of 1300 °C for 10 min. The results indicate that the densification method is beneficial to protect MWCNTs from destruction and create a good interfacial combination between the MWCNTs and the matrix. The indentation crack length of the B4C-MWCNTs composites decreases and the electrical conductivity increases continuously with the increase of MWCNTs additive amount, reaching the optimum values at 5 vol% MWCNTs addition. The microstructural observations indicate that the reinforcement mechanism of the composites is mainly the crack branching, crack deflection, bridging, and pullout of MWCNTs. This finding provides a promising approach for producing more robust ceramics with CNTs reinforcing.

Mechanical and Fracture Properties Evolution of Gnathostome Dental Enamel and Orthodentine

During gnathostome diversification modifications to dental form and mineralized tissues (enamel, dentines, and cementum) facilitated the exploitation of novel food resources, leading to the occupation of an unprecedented variety of ecological trophic niches. Generally, it has been assumed that the intra‐tissue biomechanics of these constituents had little bearing on whole‐tooth functionality, aside from mammalian enamel in occluding dentitions. Many mammals, for example, possess teeth that self‐wear to a functional topography with a diversity of derived tissues—some which possess unique mechanical attributes to resist wear and fracture. Here we formally test the hypothesis that gnathostome dental tissue material properties were static prior to the cladogenesis of Mammalia. Hardness (a proxy for wear resistance) and elastic modulus (proxy for structural rigidity and relevant to whole tooth rigidity) were tested using two standardized material science techniques, microindentation and nanoindentation, as well as a novel approach for quantifying fracture propagation patterns from indentation cracks. These data were analyzed in an ecological context using modern phylogenetic comparative methods. The results show these material properties to be highly variable within and between major groups of gnathostomes, with enamel and orthodentine hardnesses ranging from ~2.1 gigapascals (GPa) to ~6.1 GPa and ~0.5 GPa to ~1.5 Gpa, respectively. Elastic modulus values measured spanned ~34.6 GPa to ~112.5 Gpa in enamel, and from ~15.6 GPa to ~33.4 GPa in orthodentine. Lissamphibia display the lowest hardness and elastic modulus values of both enamel and orthodentine while the higher hardness and elastic modulus values are found in specific taxa from chondrichthyans, mammals, and some squamates. Aside from enamel hardness, there is no significant relationship between most material properties and assigned dietary classes. An ancillary goal of this work is also to glean initial insights about how dental attributes for non‐mammalian and mammalian taxa more generally may contribute to whole‐tooth form, function, performance, and diet. Clade‐specific complex fracture patterns in the enamels of mammals and chondrichthyans, for example, show that gnathostome lineages independently evolved traits that function to control fracture and minimize damage associated with catastrophic failure. Overall, this study suggests that selection operated at the tissue level primarily in enamel to bring about shifts in whole‐tooth functionality across Gnathostomata.Support or Funding InformationNSF EAR 0959029 awarded to Gregory M EricksonSigma Xi GIAR awarded to David Ian KayThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effects of ion irradiation on chemical and mechanical properties of magnetron sputtered amorphous SiOC

Amorphous silicon oxycarbide (SiOC) films, fabricated by magnetron sputtering, were irradiated at room temperature with 3.5 MeV Fe ions to damage levels of 10, 20, and 50 displacements per atom (dpa). Irradiation-induced changes in the nature of the atomic bonds were studied using Raman spectroscopy and X-ray photoelectron spectroscopy. The chemical composition of the surfaces of the films remained relatively unchanged after ion irradiation. Surface topography of the films was studied by atomic force microscopy and it was found that ordered topographic patterns were formed on the surface of the irradiated films. Deformation behavior of the films was studied by a combination of nanoscratch experiments and in-situ scanning probe microscopy. No signs of cracks or chipping were observed around the scratches on the as-deposited and irradiated films. Furthermore, microindentation experiments with a Vickers indenter were performed on selected films and the impressions were studied by scanning electron microscopy. The film irradiated to 20 dpa showed an increased resistance to indentation cracking compared to the as-deposited film. The study demonstrates that amorphous SiOC, with desirable structural stability and mechanical properties, can be a promising candidate for irradiation tolerant materials.