Propagation Of Indentation Cracks Research Articles

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.

A study on intermetallic compound formation in Ag–Al system and evaluation of its mechanical properties by micro-indentation

In electronic packaging in recent years, silver (Ag) alloy wires have been widely adopted in wire-bond processes. The bond pads on the device chips are mostly aluminum (Al). Thus, the bonding interface is mainly Ag–Al. In recent Ag alloy wire-bond publications, it is unclear what intermetallic phases form at the interface. In this research, experiments were designed to understand the Ag–Al intermetallic compound (IMC) formation in the Ag–Al system and evaluate its mechanical properties. First, the Ag–Al alloys with compositions from 19 to 43 at.% Al were evaluated to identify the phase equilibrium and crystal structure of the Ag–Al intermetallic phases. Microstructures and phase compositions of the designed Ag–Al alloys are presented. To further study the intermetallic compound formation at the Ag/Al interface, the interfacial reaction of the Ag/Al joints at 200 °C was investigated. The µ-Ag3Al and δ-Ag2Al IMC were identified to form at the Ag/Al interface and stabilize after long-time annealing at 200 °C. At last, deformation and fracture behaviors of the bulk µ-Ag3Al and δ-Ag2Al were analyzed by the micro-indentation. The measured results reveal that µ-Ag3Al exhibits significantly higher hardness and lower fracture toughness as compared to δ-Ag2Al. Indentation crack propagation in µ-Ag3Al demonstrates the fracture characteristics of brittle materials. In the case of δ-Ag2Al, the presence of slip bands exhibits the ductility of δ-Ag2Al to endure plastic deformation prior to fracture. The effect of the mechanical properties of the µ-Ag3Al and δ-Ag2Al IMC on the Ag–Al joint reliability is discussed. New information obtained in this research is important for future study of the joint reliability and failure mechanism of Ag–Al wire bonds.

Failure mechanisms of Al2O3 with different glass phase content at cryogenic temperatures

The effects of glass phase and temperature on the mechanical properties of 99%, 92%, 85% and 75% Al2O3 are investigated. The in-situ XRD results revealed that the lattice constants of Al2O3 single crystals were reduced when temperature decreased from room temperature (RT) to 30K. The results of mechanical properties were analyzed by Weibull equation. It can be concluded that for the samples containing same content of glass phase, the bending strength and fracture toughness increased as temperature decreased from RT to 77K. While at the same test temperature, the mechanical properties degraded when the glass phase content increased. The statistical analysis of indentation crack propagation paths of 99% Al2O3 indicated that the proportion of transgranular fracture was increased at 77K.

Microindentation fracture behavior of laser surface modified alumina ceramic

A high energy density continuous wave Nd:YAG laser is used for modifying the surfaces of porous alumina ceramic compacts. The increased fracture toughness of the modified surface region at higher laser fluences is associated with increased densification and coarsening of grains in the resolidified layer. The coarsening of grains offers increased resistance to the initiation and propagation of indentation cracks, as indicated by the transition in the fracture mode from predominantly mixed to predominantly intergranular with increasing laser fluence.

Sintering Behavior of Gehlenite, Part II. Microstructure and Mechanical Properties

A self‐toughened gehlenite (2CaO·Al2O3·SiO2or “C2AS”) ceramic with randomly distributed platelet grains was prepared by the organic steric entrapment (PVA) route. The gehlenite ceramic had a density of 2.698–2.875 g/cm3, corresponding to a relative density of 90%–96%. The platelet gehlenite grains had an average thickness of 3.6±0.8 μm and a width of 12.9±3.7 μm, respectively, with an average aspect ratio of 3.6. The three‐point bending strength, fracture toughness, and Young's modulus attained were 142.1±12.1 MPa, 2.32±0.12 MPa·m1/2, and 108±6.8 GPa, respectively. Fractography as well as Vickers indentation crack propagation profiles showed that crack deflection, crack blunting, and pinning effects due to the randomly distributed platelet grains were considered to be responsible for the good mechanical properties of the gehlenite ceramic.

Synthesis of Al 2O 3/TiCN-0.2%Y 2O 3 Composite by Hot Pressing

Al 2O 3/TiCN composites were synthesized by hot pressing. The influences of components and HP temperature on mechanical properties, such as bending strength, breaking tenacity and Vickers hardness were investigated. The results showed that the mechanical properties of Al 2O 3/TiCN composite increased with temperature when hot pressing temperature is below 1650 °C. The mechanical properties reached their maximums when the composites were sintered at 1650 °C for 30 min under hot pressing pressure of 35 MPa, the value of bending strength, breaking tenacity and Vickers hardness was 1015 MPa, 6.89 MPa · m 1/2, and 20.82 MPa, respectively. When hot pressing temperature was above 1650 °C, density decreased because of decomposition with increased temperature, and mechanical properties dropped because of rapid growth of grains in size at high temperature. Microstructure analysis showed that the addition of Y 2O 3 led to the formation of YAG phase so as to inhibit the growth of crystals. This helped to improve breaking tenacity of the composites. TiCN particles with diameters of 1 μm dispersed at Al 2O 3 grain boundaries, inhibited grain growth and enhanced mechanical properties of the composites. SEM study of the propagation of indentation cracks showed that the bridge linking behavior between matrix and strengthening phase might lead to the formation of the coexisted field of crack deflection, branching and bridge linking. The mechanism of this phenomenon was that the addition of Y 2O 3 improved the dispersion of TiCN particles so as to enhance the tenacity of the composites. The breaking tenacity was changed from 5.94 to 6.89 MPa · m 1/2.

In situ observation of relativity between domain and indentation crack propagating in barium titanate single crystal

Instant electric field-induced 90° domain switching and propagation of indentation crack in a poled BaTiO 3 single crystal, as well as sustained field-induced indentation crack propagation and 90° domain switching in an unpoled BaTiO 3 single crystal have been investigated in situ using optical microscopy. The results show that the critical instant field for appearing 90° domain switching is less than that for propagation of indentation crack normal to the instant field, and there is no direct relation between crack propagating and domain switching. Delayed propagation of unloaded indentation crack in atmosphere under sustained field, which is less than the critical field for instant propagation of crack, can occur and result in enlargement of the indentation-induced 90° domain switching zones.

Environment-Assistant Cracking of Silicon Single Crystals

Stress corrosion cracking in humid air and hydrogen-induced cracking under cathodic charging with hydrogen of silicon single crystals have been investigated by using a constant deflection sample with indentation cracks. The results show that there is no stress corrosion cracking in humid air under constant deflection. However, hydrogen-induced delayed propagation of indentation cracks occurs during dynamic charging with hydrogen at a current density of 140 mA/cm 2 in 0.5 mol/L H 2 SO 4 + 0.25 g/L As 2 O 3 solution. The normalized threshold stress intensity factor for hydrogen-induced cracking of silicon single crystals is K IH = 0.9 K IC .

Combined effect of electric field and residual stress on propagation of indentation cracks in a PZT-5H ferroelectric ceramic

The combined effect of electric field and residual stress on propagation of unloaded indentation cracks in a PZT-5 ceramic has been studied. The results show that residual stress itself is too small to induce delayed propagation of the indentation cracks in silicon oil. If applied constant electric field is larger than 0.2 kV/cm, the combined effect of electric field and residual stress can cause delayed propagation of the indentation crack after passing an incubation time in silicon oil, but the crack will arrest after propagating for 10–30 μm because of decrease of the resultant stress intensity factor induced by the field and residual stress with increasing the crack length. The threshold electric field for delayed propagation of the indentation crack in silicon oil is E DP = 0.2 kV/cm. If the applied electric field is larger than 5.25 kV/cm, combined effect of the electric field and residual stress can cause instant propagation of the indentation crack, and under sustained electric field, the crack which has propagated instantly can propagate continuously, until arrest at last. The critical electric field for instant propagation of the indentation crack is E P= 5.25 kV/cm. If the applied electric field is larger than 12.6 kV/cm, the microcracks induced by the electric field initiate everywhere, grow and connect in a smooth specimen, resulting in delayed failure, even without residual stress. The threshold electric field for delayed failure of a smooth specimen in silicon oil is E DF = 12.6 kV/cm and the critical electric field for instant failure is E F = 19.1 kV/cm.

Ambient and elevated temperature mechanical properties of hot-pressed fused silica matrix composite

Abstract The ambient and elevated temperature mechanical properties of two kinds of hot-pressed fused silica matrix composites, SiO 2 +5 vol.% Si 3 N 4 and SiO 2 +5 vol.% Si 3 N 4 + 10 vol.% C f , were investigated. Si 3 N 4 additions greatly enhanced the ambient strength and fracture toughness, while, further incorporation of chopped carbon fibers only but sharply increased the fracture toughness value from 1.22 to 2.4 MPa m 1/2 . The strength of the two composites synchronously exhibited anomalous gains at certain elevated temperature range especially from 1000 to 1200 °C, and reached their maximum values at 1000 °C, 168.9 and 130.6 MPa, which were 77.0 and 77.4% higher than their ambient strength, respectively. The two composites exhibited catastrophic fracture even at 1000 °C, but manifested prominent plastic deformation at 1200 °C and usually no fracture occurred during the strength test. Vickers’ indentation crack propagation behavior, combined with fractographs studies, suggested that toughening from carbon fiber was attributed primarily to the fiber bridging, pull-out and crack deflection.

The sub-critical indentation fracture process in soda-lime-silica glass

The sub-critical propagation of indentation cracks in soda-lime silicate glass was investigated. Vickers indentations were produced using a force of 9.8 N, and both as-indented and annealed samples were studied. Indented bars were subjected to various flexural loading and unloading cycles in a water environment. Fractographical analysis revealed that the radial crack propagation process could be divided into two successive stages. Initially, the radial crack growth is strongly influenced by interaction with the lateral crack, which obstructs crack growth. This “pinning” effect of the lateral crack leads to bowing of the radial crack front into a complex shape. In the second stage, the radial cracks escape from the lateral cracks interaction and assume a semi-elliptical shape. Measurement of radial crack speed from the markings left on the fracture surface allowed the crack shape factor, ψ and the indentation-residual stress factor, χ, to be calibrated as a function of the crack size, c. The shape factor was found to be a decreasing function of c during the first stage of propagation, then it abruptly increases when radial cracks escape from lateral crack interaction. For further crack extension, ψ decreases again, assuming absolute values larger than those measured during the first stage. Conversely, the residual stress factor is constant during the first phase of radial crack growth while it suddenly decreases and becomes zero for larger c. The implications of the trend in ψ( c) and ψ( c) on the stable growth and on the time-to-failure predictions in static fatigue tests are discussed.

Chemical stability, microstructure and mechanical behavior of LaPO 4-containing ceramics

The use of LaPO 4 as a weak interface in composites for high temperature applications was investigated using tape-cast laminates and fiber model systems. Three laminates were fabricated with LaPO 4 as one component and Al 2O 3, Y 3Al 5O 12 or LaAl 11O 18 as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems (Al 2O 3 fiber/LaPO 4 coating/Al 2O 3 matrix and Y 3Al 5O 12 fiber/LaPO 4 coating/Al 2O 3 matrix) were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y 3Al 5O 12 fiber-reinforced composites with LaPO 4 coatings have potential as high temperature materials in oxidizing environments.

Flat R‐Curve from Stable Propagation of Indentation Cracks in Coarse‐Grained Alumina

The fracture toughness of coarse‐grained A12O3, known for pronounced “Iong”‐crack R‐curve behavior, was studied in the “short”‐crack regime utilizing the stable propagation of indentation cracks in bending. A combination of in situ microscopic crack growth observations and mechanical testing enabled measurement of crack extension curves. They reflect the contributions of residual indentation stress intensity and applied bending stress intensity on the total crack driving stress intensity and allow determination of the residual stress factor χ and the toughness KR. The results indicate that χ depends on indentation load and AR is surprisingly constant rather than increasing. To resolve the latter contradiction with long‐crack R‐curve behavior, combined short/long‐crack fracture tests were performed with the same specimens. Starting with stable indentation crack growth and continuing with stable long‐crack extension, the previous toughness results were confirmed, i.e., constant toughness from indentation cracks and increasing toughness from long cracks. The influence of crack‐opening behavior on bridging‐controlled R‐curve toughening can qualitatively explain the observed discrepancies.