Double Cantilever Beam Technique Research Articles

Vacuum crack growth in alkali silicate glasses

Crack growth velocity in alkali silicate glasses was measured in vacuum across 10 orders of magnitude with double cantilever beam technique. Measured and literature crack growth data were compared with calculated intrinsic fracture toughness data obtained from Young's moduli and the theoretical fracture surface energy estimated from chemical bond energies. Data analysis reveals significant deviations from this intrinsic brittle fracture behavior. These deviations do not follow simple compositional trends. Two opposing processes may explain this finding: a decrease in the apparent fracture surface energy due to stress-induced chemical changes at the crack tip and its increase due to energy dissipation during fracture.

Manufacturing of Optimized Ge Substrates Using a Covalent Bonding Process

Germanium is an interesting material for applications ranging from nanoelectronics to solar cells, near infrared detectors, gamma spectroscopy detectors or optical components [1]. Previous studies demonstrated the interest of thick germanium-on-insulator (GeOI) substrates for the manufacturing of advanced photonics devices [2].Two methods are commonly used to manufacture GeOI substrates for photonic applications. First, a rather thick, slightly tensily strained Germanium layer can be grown on top of a 200mm Si(001) wafer and the top part transferred, thanks to the Smart CutTM approach, onto oxidized Si. The threading dislocation density in that Ge layer will be of the order of a few 106 cm-2, then, which could be a drawback for future applications. To manufacture GeOI with, on top, a higher quality Ge film, another approach is to start from bulk Ge, which has a much better crystalline quality [3, 4]. The drawback of this approach is that the thin Ge film on top of the GeOI substrate does not have any internal strain, then. In order to fabricate high quality strained GeOI substrates, in the end, one could start from Ge films coming from bulk Ge donors and strain while bonding them on top of 200mm Si(001) substrates.Covalent bonding is an interesting candidate in order to obtain high quality Ge films on Si while managing, at the same time, the internal stress. Indeed, stress can be tailored with the proper temperature control during the bonding process itself. The EVG®Combond® equipment used in this study enables to bond at “high” temperature Si/Ge heterostructures with a very high bond strength. In this paper, the bonding process performed will be described and characterized.A bonding at temperatures higher than Room Temperature is adopted to generate tensily strained Ge films. The choice of the right temperature depends on two criteria, however: (i) have the highest stress possible in the future Ge film and (ii) allow the Si/Ge heterostructure with thick substrates to withstand the stress without breaking at room temperature just after bonding. Point (i) requires high temperatures, while low temperatures are better for point (ii); a tradeoff will thus have to be identified. Simulations were performed in order to evaluate the bow of the structure after bonding and will be presented in the paper.Si/Ge heterostructures were fabricated at 100°C. A very high bond strength (> 5 j/m2) was measured using the classical double cantilever beam (DCB) technique on an equivalent Si/Ge/Si structure. Indeed, the bow with a Si/Ge heterostructure was too important to allow a correct bond strength measurement. Thus, an epitaxy of a few µm of Ge was performed on top of a Si wafer and the Si/Ge stack bonded to another Si wafer with the same covalent bonding process prior to measurements of the bonding interface strength. Bow in temperature of the Si/Ge heterostructure was also measured. A heating ramp and then a cool down phase were applied to the bonded pair in an atmospheric pressure chamber with a curvature radius measurement done with a laser beam. The bonded pair reached an equilibrium point with a bow of 0 µm for a temperature close to 100°C, in line with the bonding temperature adopted (figure 1).Using this newly qualified bonding process, a strained Ge film was transferred onto an Si wafer. The process flow will be described in the paper, as well as the characterization of the thin Ge film transferred at the end of the process. The quality of the Ge film will be characterized using a SP2 equipment in haze mode. Transmission Electron Microscopy will be performed to observe the amorphous zones at the bonding interface, and Focused-Ion-Beam Scanning Electron Microscopy will yield the thickness of the Ge film transferred. Figure 1: Bow measurement in temperature of a Si/Ge heterostructure, bonded using a covalent bonding process at 100°C [1] R. King et al., ‘Pathways to 40% efficient concentrator photovoltaics’. 20th European Photovoltaic Solar Energy Conf., Barcelona, Spain, 2005, p. 118[2] V. Reboud et al., “Structural and optical properties of 200 mm germanium-on- insulator (GeOI) substrates for silicon photonics applications,” Proc. SPIE 936714, 1–6 (2015). [3] F. Letertre et al., ‘Germanium-on-insulator (GEOI) structure realised by the Smart CutTM technology’, Mater. Res. Soc. Proc., 2004, 809, (B4.4), p. 153[4] C. Deguet et al., ‘200 mm germanium-on-insulator (GEOI) structures realized from epitaxial wafers using the smart cut technology’, Electrochem. Soc. Proc., 2005, 2005-06, p. 78 Figure 1

Crack growth in borate and silicate glasses: Stress-corrosion susceptibility and hydrolytic resistance

A double cantilever beam technique in air equipped with ultrasound modulation was used to measure the crack velocity v in borate and silicate glasses. In all glasses v and the stress intensity KI followed the empirical correlation v ~ KIn. Indicated by its smallest KI at v = 1 µm s − 1, KI* = 0.27 MPa m0.5, the silicoborate glass containing 70 mol% B2O3 was found most susceptible to stress-corrosion enhanced crack growth. Contrarily, the sodium calcium magnesium silicate glass appeared least susceptible with KI* = 0.57 MPa m0.5. No clear correlation is evident between KI*, reflecting the stress-corrosion susceptibility, and the hydrolytic resistance for all glasses under study, but values of n obtained from the present study and taken from previous literature for 35 glasses tend to decrease with increasing network modifier ion fraction. Energy dissipation during stress-corrosion enhanced crack propagation is assumed to cause this trend.

Improvement of Mode I Interlaminar Fracture Toughness of Stitched Glass/Epoxy Composites

This study examines the improvement of Interlaminar Fracture Toughness (IFT) of multilayered 3D glass/epoxy textile composites when through thickness reinforcement is introduced. Three stitching techniques have been examined: Modified Lockstitch (ISO-301), Single-yarn Orthogonal-stitch (ISO-205) and Double-yarn Orthogonal-stitch (ISO-205). It was found that the use of class ISO-205 manual-type stitched reinforcement significantly enhanced the Mode I-IFT, GIC measured using a Double Cantilever Beam technique. Furthermore, in every case, the use of class ISO-205 stitching and high stitch densities offer a significant improvement of 74.5 % on Mode I-IFT against interlaminar delamination.

Adhesion Energy and Bonding Wave Velocity Measurements

Direct bonding is now widely used in microelectronics for SOI elaboration or 3D integration. One of its main characterizations is the adherence energy, also called bonding energy, which estimates the strength of the bonded structure and its evolution with the different process parameters (annealing temperature, plasma activation…). The double cantilever beam (DCB) technique described by Maszara et al. [1] is the most common technic to measure the adherence energy. During the bonding another key characterization, the adhesion energy, deserves to be studied. This quantity gives information about the attractive forces involved when contacting the two surfaces and helps to understand the bonding wave velocity. Rieutord et al. [2] suggested an analytical model based on air flow dynamics between the two wafers and showed an analytical relation between the adhesion energy and the bonding wave velocity. In this work, a simple measurement apparatus derived from the DCB technique has been developed to measure the adhesion energy and the bonding wave velocity. Beams cut in <001> bonded silicon wafers are opened at the bonding interface by a blade insertion; then the blade is suddenly partially removed allowing the structure to be bonded again. Recording multiple infra-red pictures one can follow the rebonding wave propagation and measure its velocity. The blade allows an easy and long debonded area in order to be sure to detect a stable rebonding wave speed. As a part of the blade is not totally removed, one can also determine the remaining debonded length when the wave propagation stops due to the known blade thickness. The energy associated to this length and calculated using El Zein [3] equation is the adhesion energy. In this experiment, upper and lower substrates of the bonded structure are allowed to be deformed. We also check that measurements performed on 2 cm wide beams are equivalent to larger beams in order to prevent any fluid edge effect as described by Radisson et al. [4]. So according to these considerations, we assume that the apparatus well describe the Rieutord’s hypothesis of infinite wide beams free to deform in the perpendicular direction and we can then compare our measures to his analytical predictions. We will show in this paper a very good correlation between the adhesion energy and the bonding wave speed. The surface energy has been varied by changing the surface roughness. Surprisingly, an interesting evolution of the adhesion energy after the blade stop will be presented. During 90 minutes, a slow increase of the bonding energy is observed at room temperature and 50% Relative Humidity. A mechanism will be proposed to explain this long time evolution mainly based on a slow kinetics of capillary condensation between the rough surfaces [5]. We will then discuss the choice of the energy adhesion value and show that long time values combined to measured velocities well agree with Rieutord’s model (cf. figure). REFERENCES [1] W. P. Maszara, G. Goetz, A. Caviglia and J. B. McKitterick, “Bonding of silicon wafers for silicon on insulator”, J. Appl. Phys., 64 p 4943 (1988) [2] F. Rieutord, B. Bataillou, and H. Moriceau, “Dynamics of a bonding front”, Physical Review Letters, vol. 94, pp. 236101, (2005). [3] M. S. El-Zein and K. L. Reijsnider, “Evaluation of GIC of a DCB Specimen Using an Anisotropic Solution”, Journal of Compo. Tech. & Research, 10(4), pp.151-155, (1988) [4] D. Radisson, “Direct bonding on patterned surfaces”, PhD thesis, Grenoble Univ., France (2014) [5] F. Fournel, C. Martin-Cocher, D. Radisson, V. Larrey, E. Beche, C. Morales, P. A. Delean, F. Rieutord, and H. Moriceau, “Water Stress Corrosion in Bonded Structures”, ECS Journal of Solid State Science and Technology, 4 (5) P124-P130 (2015) Figure 1

Reliable four-point flexion test and model for die-to-wafer direct bonding

For many years, wafer-to-wafer (W2W) direct bonding has been very developed particularly in terms of bonding energy measurement and bonding mechanism comprehension. Nowadays, die-to-wafer (D2W) direct bonding has gained significant attention, for instance, in photonics and microelectro-mechanics, which supposes controlled and reliable fabrication processes. So, whatever the stuck materials may be, it is not obvious whether bonded D2W structures have the same bonding strength as bonded W2W ones, because of possible edge effects of dies. For that reason, it has been strongly required to develop a bonding energy measurement technique which is suitable for D2W structures. In this paper, both D2W- and W2W-type standard SiO2-to-SiO2 direct bonding samples are fabricated from the same full-wafer bonding. Modifications of the four-point flexion test (4PT) technique and applications for measuring D2W direct bonding energies are reported. Thus, the comparison between the modified 4PT and the double-cantilever beam techniques is drawn, also considering possible impacts of the conditions of measures such as the water stress corrosion at the debonding interface and the friction error at the loading contact points. Finally, reliability of a modified technique and a new model established for measuring D2W direct bonding energies is demonstrated.

Invited: Water Stress Corrosion in Bonded Structures

Direct bonding is now a well-known technique to join two flat surfaces without any additional material. This technique is used in many application and especially in SOI (Silicon on insulator) elaboration or in some backside imager manufacturing process which is now almost a mass production process. Direct bonding mechanism study is then very important in order to clearly understand this bonding behaviour. Especially in the case of silicon or silicon dioxide hydrophilic surface, the role of water is essential. Water could lead to lots of different reactions at the bonding interface and especially the water stress corrosion could play an important role in the direct bonding mechanism.After having reviewed the different silicon dioxide water stress corrosion in the literature [1,2], its influence in the direct bonding area will be pointed out. The direct bonding energy measurement, G, using the double cantilever (DCB) beam technique is for instance highly affected by the water stress corrosion [3, 4, 5, 6]. In addition to the direct bonding energy, this DCB technique is also very interesting to allow improvement of the direct bonding mechanism. Indeed, as the water stress corrosion only appears with siloxane bonds, it could be an interesting characterization of the classical silicon direct bonding reaction:Si-OH+Si-OH => Si-O-Si+H2OWe will then show that siloxane bonds could appear already at room temperature and not only during the classical post bonding annealing process using for instance plasma activated bonding by changing the relative humidity of the characterization atmosphere. This water stress corrosion is also put into evidence during adhesion bonding energy and subsequent G measurement in which the hysteresis cycle which appears between these two measurements shows that specific siloxane bonds appear after the bonding wave propagation [7, 8]. Moreover, they put in evidence the effect of the atmosphere humidity in such hysteresis cycle.In addition to this, the water stress corrosion could be used to put into evidence the remaining water at the bonded interface [9]. Indeed in specific dioxide/dioxide bonding, Ventosa et al. [10] show that the water could remain at the interface until post bonding annealing treatment of 400°C during 2 hours and we will show that this can be seen by the water stress corrosion which takes place even in anhydrous atmosphere for this bonding type. Thus, the stress corrosion study in anhydrous atmosphere can deeply participate to the elaboration of a direct bonding mechanism.Moreover, one could think that the water stress corrosion plays an active role in the direct bonding evolution. Indeed, in other silicon dioxide processes, as CMP for instance, the water stress corrosion helps to hydrolyze the surface and to remove the polished material. The water stress corrosion might help the surface asperities to deform in order to increase the bonding area and thus the bonding energy. Furthermore, as the water stress corrosion rate depends on the temperature [11], it could also participate to the bonding energy evolution versus annealing temperature. The direct bonding mechanism could then be seen as a mechanical model of asperity deformation greatly helped by the trapped water at the bonding interface.Besides the bonding energy evolution, we will also discuss how the water stress corrosion study of bonded structure could be very interesting for the characterization of bonded interface chemical resistance. Indeed a good relation is seen between the chemical interface resistance and the water stress corrosion sensitivity [12].The water stress corrosion study in bonded structure and especially in silicon or silicon dioxide bonding is then very fruitful and should deserve to be extended to other type of bonding interface and materials. REFERENCES [1] S.M. Wiederhorn, et al., J.Mater.Sci. (17) 3460 (1982).[2] M. Ciccotti, J.Phys.D (42) 214006 (2009)[3] O. Vallin, et al., Mat.Sci.andEng. R50 109 (2005).[4] T. Martini, et al.,Soc. 144(1) 354 (1997).[5] Y. Bertholet, et al., Sens.Actua.A 110 157 (2004).[6] F. Fournel, et al., J.Apl.Phys. (111) 104907 (2012).[7] D. S. Grierson et al., ECS Trans. 33(4) 573 (2010).[8] D. Radisson, to be published.[9] C. Martin-Cocher, to be published.[10] C. Ventosa, et al., J.Appl.Phys. 104 (2008).[11] S. N. Crichton et al., J.Am.Ceram.Soc. 82(11) 3097 (1999).[12] T. Suni, et al., ECS Trans. (19) 70 (2003).

Surface modification of nanofibrillated cellulose films by atmospheric pressure dielectric barrier discharge

A dielectric barrier discharge in a gas mixture of tetrafluoromethane (CF4) and O2 was used for tailoring the surface properties of nanofibrillated cellulose (NFC) films. The surface chemical composition of plasma-modified NFC was characterized by means of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, while surface morphology was illustrated by atomic force microscopy. Wettability was characterized through the static sessile drop method. The adhesion between NFC and polylactide (PLA) laminated films was tested by the double cantilever beam technique. As a result of atmospheric pressure plasma treatment, the water contact angle of NFC films increased and the values were comparable with those of PLA films. On the other hand, surface chemical characterization revealed inhomogeneity of the plasma treatment and limited improvement in adhesion between NFC and PLA films. Further research in this direction is required in order to enhance the uniformity of the plasma treatment results.

Quantitative Adhesion Properties and Interfacial Behavior of SiCN Barrier Layer and Cu Film

Adhesion of the SiCN barrier layer and Cu film interface is one of the important characteristics that reflect the interfacial structure. NH3 plasma treatment of the Cu surface is a well-known way of improving adhesion. The results of X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) depth profiles indicate that the plasma treatment imparts a difference to the formation of the interface with SiCN. Adhesion properties are regarded as fracture energies measured by double cantilever beam (DCB) and 4-point bending (4PB) techniques. The influence of the NH3 plasma treatment of the Cu surface on adhesion is quantitatively discussed. The treated samples showed approximately twice the fracture energy of the non-treated samples. After 4PB and DCB measurements, fracture surfaces were investigated by XPS and atomic force microscopy (AFM). The formation of a N-Cu chemical bond on the Cu surface was enhanced as a result of removing oxygen by the plasma treatment. N-Cu chemical bonding contributes substantially to better SiCN/Cu interfacial adhesion.

Effects of ultraviolet radiation on ultra-low-dielectric constant thin film fracture properties

The effects of ultraviolet (UV) radiation on ultra-low-kdielectric (ULK) thin film fracture toughness were studied. This work discusses both critical and subcritical crack growth behavior under different environments. The critical fracture toughness was measured as a function of applied phase angle by using the four-point bend flexure and mixed-mode double cantilever beam techniques. Results of critical fracture toughness obtained under different loading configurations and phase angles were found to increase with the UV treatment time. In contrast, mode I subcritical fracture toughness thresholds and the crack propagation velocity appeared to be insensitive to UV curing processes. This study revealed that subcritical fracture toughness values reduced with the moisture concentration in the environment.

Validation of a novel approach to determination of threshold for stress corrosion cracking ( KISCC)

Determination of threshold stress intensity for stress corrosion cracking ( K ISCC), using circumferential notch tensile (CNT) specimens has been developed recently. This has also been claimed to be a novel and considerably cost-advantageous approach in comparison with the traditional methods, viz., compact tension (CT) and double cantilever beam (DCB) techniques for determination of K ISCC. However, the limited first K ISCC data, generated using CNT specimens have been queried for the lack of their comparison with the data generated by other techniques. This work presented here provides the first comparative results and validation for use of the novel CNT testing for generation of K ISCC data.

Critical and Sub-critical Debonding in Nano-clustering Porous Low-k Films

AbstractVery few porous low-k dielectric materials meet the basic requirements for integration into the back end of the line (BEOL) metallization. According to the International Technology Roadmap for Semiconductors, 2005, candidates for the 45 nm node need a k&lt;2.2 and a minimum adhesion strength of 5 J/m2. Recently, a low-k dielectric material was developed, called nano-clustered silica (NCS). It is a spin-on glass with k&lt;2.3. NCS is constitutively porous, with a micro- and mesopore size of ~2.8 nm. The first reported adhesion strength of this material was 10+ J/m2. We investigated the nature of the adhesive strength of NCS by critical and sub-critical fracture and Fourier Transform IR Spectroscopy (FTIR). The four-point bend technique and a mixed-mode double cantilever beam technique were employed. The sub-critical crack growth studies were performed in humid environments and ambient temperatures. Different post-treatments were used on NCS to achieve different molecular structure, as measured with FTIR. A correlation between molecular structure and critical adhesion energy was found. Atomistic parameters were calculated from the sub-critical crack growth data. A dependency of fracture behavior on post-treatment and, therefore, structure was observed.

Interface structure and adhesion of wafer-bonded GaN/GaN and GaN/AlGaN semiconductors

Material integrations of GaN/GaN and Al0.25Ga0.75N/GaN semiconductors through wafer bonding technology were reported in this work. The wafer surface and interface microstructures were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy. The interface adhesion (bonding strength) was estimated based upon the interface fracture energy γo measured by double-cantilever beam technique. The interface adhesion properties of several different wafer-bonded III-V semiconductors were also compared. By comparing the atomic chemical bond energy Eo with the measured interface fracture energy γo, the bondability of a few major III-V semiconductors was analyzed.

Delamination properties of translaminar-reinforced composites

In the paper the delamination crack propagation is investigated in 3D woven carbon and glass fiber composites. Composites were produced in 3TEX company and had volume fraction of transverse fibers equal 2–5% of total volume of fibers. The reinforcement acts as crack bridging, greatly improving material resistance to delamination. In this paper investigation of this material has been performed using double cantilever beam technique and models elaborated for large scale bridging in UD composites. The following alterations and novelties are introduced: 1) The appropriate form and shape of specimens with glued additional metallic tabs was elaborated allowing to avoid the premature break of specimen arms; 2) Original loading device was designed to transfer high load directly to crack faces, and additional measurement of crack opening was proposed (as in the case of UD composite) to find the bridging law; 3) The convenient analytical formula for G I c calculation was proposed and used; 4) Instead of traditional R curve which is geometry dependent it was proposed to characterize the delamination resistance as function of crack opening displacement. The quantitative delamination characteristics for materials investigated are presented.

High-temperature fracture toughness of SiC-Mo5Si3C composite

Silicon carbide-based composites are candidate materials for high-temperature structural applications in heat engines and heat recovery systems [1, 2]. For these applications, the ceramic components are often needed to be manufactured by processes with near-net-shape capability to minimize the requirements to perform very expensive and difficult post-machining operations [3]. Among the various fabrication processes, the melt infiltration process, which has the advantages of near-netshape capability and short processing time, is proposed to be one of the potential processes for such applications [4]. However, SiC-based composites made by conventional melt infiltration processes showed poor high-temperature mechanical properties, which are primarily limited by the low melting-point residual phases as exhibited by the reaction-bonded SiC [5–7]. Much effort has thus been made to reduce or eliminate the low-melting-point residual phases in the composites. One way to reduce the residual phases is through alloy infiltration [8–10]. Better approaches have been demonstrated by Lim et al. [11] and Shobu et al. [12], where high-melting-point MoSi2 and Mo(Si,Al)2 were directly infiltrated into the SiC performs. Both the infiltrated SiC-MoSi2 and SiC-Mo(Si,Al)2 composites exhibited significant increases (more than a 40% increase at 1400 ◦C) in fracture strength between 1200 and 1400 ◦C. In a previous report [13], a novel SiCMo5Si3C composite was fabricated by directly infiltrating high-melting-point Mo5Si3C into the SiC preforms. The composite also showed significant increases in fracture strength between 1200 and 1500 ◦C, which was attributed to the toughening effects provided by the infiltrated Mo5Si3C phases at elevated temperatures. To clarify this point, the fracture toughness of the composite is studied in the present report. The fracture toughness for ceramics can be characterized by various techniques, such as the double torsion (DT) technique [14], the double cantilever beam (DCB) technique [15], the indentation fracture (IF) technique [16], the indentation strength (IS) technique [17], the single edge precracked beam (SEPB) technique [18], the chevron notched beam (CNB) technique [19], etc. The IS method was adopted in the present study because it has proved to be efficient and economical [20]. The preparation procedure for the composite as well as the infiltration characteristics have been reported elsewhere [13, 21], so they are only briefly introduced here. Namely, a SiC preform of dimensions 40× 16× 6 (mm) was prepared from the raw SiC powder (α, circa 3.2 μm, purity 99%, Showa Denko, Japan) through a cold isostatic pressing (CIP) process. An infiltrant of the Mo45Si30C5 composition was prepared from the raw powders of MoSi2 (circa 2.93 μm, Japan New Metals), SiC (the same as above) and Mo (circa 1.3 μm, purity 99.94%, Japan New Metals). The infiltration was performed at 2100 ◦C in an induction furnace in 1 atm argon. Typically, the temperature was ramped to 2100 ◦C in 5 min from 1000 ◦C, held for 20 min, followed by a 5-min cool down to 1000 ◦C, where the power was cut off. The powder mixture was found to melt above 2000 ◦C, and spontaneous infiltration could be achieved above 2050 ◦C. Although 20 min for infiltration was employed in the present study, several minutes were found to be sufficient for a full infiltration. The infiltrated composites are relatively dense, but the relative density could only reach 95% of the theoretical value at best. The remaining porosity is attributed to the uninfiltrated areas up to 20μm and closed pores in the SiC matrix that could not be reached during the infiltration process, as exhibited by the typical microstructure of the composite in Fig. 1. Also, the volumetric shrinkage during solidification would contribute to the porosity. Further improvement in product density is very difficult. The XRD and SEM/EDS investigations confirmed that the composite is only composed of SiC and Mo5Si3C. Beams for the IS testing were cut from the infiltrated samples using a diamond saw, and ground to the nominal dimensions of 3× 1.5× 12 (mm). The tensile surface was polished with 1 μm diamond paste to remove residual stress due to machining and to produce a finish for optical microscopic observations. The edges of the prospective tensile face were slightly chamfered.

Measurement of Interfacial Fracture Toughness Under Combined Mechanical and Thermal Stresses

The interfacial fracture resistance of a polyimide passivation/underfill interface was measured using an asymmetric double cantilever beam technique. A thin layer of polyimide was coated on a standard epoxy as one of the beams and an underfill was flowed over the polyimide film and cured to form the second beam. The relative contributions of the thermal and mechanical stress were systematically varied by varying the thickness ratio of the two beams. The results indicate that the effect of thermal residual stress must be taken into account in the calculation of critical strain energy release rate to obtain the interfacial fracture energy.

Subcritical crack growth in Y–Al–Si–O–N glass

Oxynitride glass (Al 2O 3 · ( 3 2 ) Y 2O 3 · ( 3 4 ) SiO 2 · ( 3 4 ) Si 3N 4) has exhibited subcritical crack growth when submerged in deionized water. Crack velocities ranging from 10 −7 to 10 −5 m/s were observed for applied stress intensities ranging from 0.67 to 0.69 MPa m 1/2, respectively, using the constant moment double cantilever beam technique. A power-law exponent of 142 ± 9 and kinetics parameter of 216 ± 14 were calculated. It was difficult to produce stable crack growth in oxynitride glass in an air environment. A commercial glass slide was used to verify the experimental technique, and the data for the glass slide was consistent with previous reports in the literature.

The toughening of alumina with iron: Effects of iron distribution on fracture toughness

Two composites have been fabricated by hot pressing powder blends of alumina with 20 volume percent of ductile iron particles. The composites differ in the shape, size and distribution of the iron particles. The fracture toughness of each composite has been obtained in situ, by testing inside a scanning electron microscope, using a double cantilever beam technique modified specifically for small ceramic specimens. Observation of the crack-particle interactions has enabled information to be gained about the toughening mechanisms occurring and hence the parameters for microstructural tailoring of these materials have been deduced. Results showed that the fracture toughness of the composites differed greatly due to the distribution of the iron throughout the microstructure, which in turn affected the type and degree of observed toughening mechanism. These material-specific toughening parameters were then used to fabricate a third alumina/iron composite with a more optimised fracture toughness.

Effect of composite layering on the fracture toughness of brittle matrix/particulate composites

Glass matrix/Ni particulate composites, ranging from 0 to 25% particulate phase, were prepared both as single-volume-fraction composites and as multi-volume-fraction layered composites. Fracture toughness (K c) measurements were made on all composites using the applied moment double cantilever beam technique. The measured toughness values for the layered composites were found to be equivalent to those of the single-volume-fraction composites. The fracture toughness measured for the layered composites was found to be dependent on the volume of composite phase tested and ultimately on the number of crack-particle interactions which occurred. R-curve like behaviour was observed in the layered composites.

Sigmoidal Indentation–Strength Characteristics of Polycrystalline Alumina

The fracture properties of a series of three polycrystailine aluminas are examined using indentationstrength and double cantilever beam techniques. The indentation‐strength response is shown to be sigmoidal with concave‐down behavior at small indentation loads and concave‐up behavior at large indentation loads. A model is developed for the general response, combining increasing toughening by ligamentary bridging at small crack lengths and increasing residual‐stress relief by lateral cracking at large indentation loads. The model is fit to the strength data and used to deconvolute the underlying toughness variation and predict the intrinsic strength of the materials. Direct measurements of toughness using the “long‐crack” double cantilever beam geometry are shown to overestimate the toughness variations effective during “short‐crack” strength tests.