Indentation Cracks Research Articles

Microstructure and indentation damage resistance of ZrB2‐20 vol.%SiC ipo‐eutectic composites

AbstractZirconium diboride with 20 vol.% silicon carbide bulk composites were fabricated using directionally solidification (DS) and also by spark plasma sintering (SPS) of crushed DS ingots. During the DS the cooling front aligned the c‐axis of ZrB2 grains and its Lotgering factor of f(00l) was high as 0.98. The Vickers hardness was anisotropic and it was high as 17.6 GPa along the c‐axis and 15.3 GPa when measured in an orthogonal direction. On both surfaces, even when using 100 N indentation load, no cracks were observed, suggesting a very high resistance to crack propagation. Such anomalous behavior was attributed to the hierarchical structure of DS sample where the ZrB2 phase was under strong compression and the SiC phase was in tension. In the SPSed sample, the microstructure was isotropic respect to the direction of applied pressure. Indentation cracks appeared around the indent corners but not emanated from the diagonals, confirming high damage resistance.

Fracture toughness, fracture energy and slow crack growth of glass as investigated by the Single-Edge Precracked Beam (SEPB) and Chevron-Notched Beam (CNB) methods

We show that the Single-Edge Precracked Beam (SEPB) test is not only suitable to the determination of the fracture toughness (KIc) of glass, but also offers a unique opportunity to assess the slow crack growth behavior in a single experiment lasting for few minutes. Besides, we found that it is possible to get either a stable or an unstable final fracture regime (pre-cracked specimen) depending on the testing parameters, and that the unstable case is preferable for the estimation of KIc. The "pop-in" precrack was found mostly to close completely once the load was suppressed on the bridge-flexure device. This led to a reopening event on the loading curves. It is noteworthy that all these original observations were made possible thanks to the design of a very stiff testing apparatus (6.7 MN m−1) allowing for a cross-head speed as small as 0.01 μm s−1. Results obtained on four grades of commercially available glasses are compared to those stemming from Vickers indentation cracking and chevron notched experiments.

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.

Indentation cracking behaviour and structures of nanophase separation of glasses

Author(s): Cheng, S; Song, C; Ercius, P; Cionea, C; Hosemann, P | Abstract: Correlative microscopy is used to compare the performance and nanophase structures of soda-lime-silica, fused silica and borosilicate glasses using Vickers indentation crack analysis and the transmission electron microscopy (TEM) Fresnel contrast method. It is found that the observed indentation cracking behaviour is correlated to the nanophase separation structure of these glasses. The so-called normal cracking behaviour of soda-lime-silica glass is influenced by its spinodal nanophase separation; while the anomalous cracking behaviour of fused silica is due to the uniform single phase structure. Borosilicate glass has a droplet nanophase separation and shows intermediate cracking behaviour. These results indicate that in order to produce low britleness glasses it is important to control nanophase separation structure of a glass.

Effect of brittle scratches on transmission of optical glass and its induced light intensification during the chemical etching

Chemical etching experiments are carried out to expose and change the morphology of multiple scratches and single scratches in a set of K9 glasses, during which the transmission of some glasses is measured. The results show that brittle scratches have an influence on the transmission, and the transmission decreases at first and then increases obviously with the increase of etching time for the glass with a high density of cracks. The scattered field distribution around trailing indent crack(s) and the transmission of the glass with a scratch etched deeply (adjacent cracks have coalesced with each other) are simulated under different parameters. Using finite-difference time-domain (FDTD) algorithm, the light intensification effect around two single-evolving scratches and two adjacent cracks with different parameters are simulated, respectively. The results indicate that the position with the maximum light intensity changes for the glass with a scratch and its light intensity enhancement factors (LIEFs) will generally decrease as the etching process progresses. Compared with the morphology evolution of a scratch during the etching process, the changing mechanisms of transmission and LIEFs are revealed. This work will contribute to the improvement of service performance of optical glass by chemical etching process.

Al2O3-ZrO2 eutectic ceramic via ultrasonic-assisted laser engineered net shaping

Abstract Al2O3-ZrO2 eutectic ceramic structures of thin wall and cylinder with average eutectic spacing 60–70 nm were prepared by ultrasonic-assisted laser engineered net shaping (LENS). Effect of ultrasonic power on micro-topography was investigated. X-ray diffractometer analysis and scanning electron microscopy observation were carried out for sample phase composition and micro-topography respectively. The friction and wear properties were tested by microcomputer control universal friction and wear tester. Thermal stability and microhardness were measured by synchronous thermal analyzer and vivtorinox hardness tester respectively, and surface fracture toughness was calculated by indentation crack length. The results show that with the increase of ultrasonic power from 20 to 110 W, micro-topography is cellular eutectic, cellular coexisting with dendritic eutectic, dendritic eutectic, dendritic coexisting with cellular eutectic in turn. Eutectic spacing is 60.29 nm while ultrasonic power is 120 W, grain boundary and colony organization are eliminated completely. When the ultrasonic power changes in 20–160 W, the eutectic spacing shows a tendency to decrease first and then increase. Surface fracture toughness increases to 7.67 ± 0.2 MPa m1/2. The reasons for improvement of fracture toughness were analyzed from the aspects of refined eutectic spacing and residual compressive stress. Friction coefficient is small and has no big fluctuation. Wear mechanism exhibits intergranular brittle fracture and exfoliation wear.

Chemical tempering of soda lime silicate glasses by ion exchange process for the improvement of surface and bulk mechanical strength

The aim of this work is improving mechanical properties of soda-lime glass by imparting compression stress on the surface through ion exchange and investigate the effect of process parameters on chemical strengthening. KNO3 salt bath was used for ionic exchange at varying temperatures and durations. Treated samples were investigated for their optical transmittance, surface potassium ionic concentrations and concentration profiles from surface to bulk using EDS and line scan EDS techniques. Effect of process temperature and time on the amount of mechanical strengthening was determined through the bending strengths (four-point), scratch resistances and indentation behaviors as micro hardness, crack formation load limit and cracking probability. Results were evaluated regarding ionic concentration variations. Transmittance decreased by 2% after the treatment. Surface potassium concentration increased with increasing temperature and was increased to become saturated in a short period. Potassium concentrations decreased from surface to bulk, and diffusion reached to deeper layers with increasing time and temperature. Ionic exchange improved micro hardness by 8–17%, threshold loads for indentation crack initiation 2–10 times, cracking probability down to 50%, strength up to 3.5–4 times and scratch resistance 2–4 times. Crack types changed between radial, median and lateral; depending on the degree of chemical tempering.

Critical evaluation of indentation fracture toughness measurements with Vickers indenter on yttria-stabilized zirconia dental ceramics

The purpose of this study was to investigate and analyze fracture toughness (KIc) of yttria stabilized tetragonal zirconia (Y-TZP) dental ceramics by the Vickers indentation fracture test. In order to determine fracture toughness, the Vickers indenter was used under the load of 294.20 N (HV30). The cracks, which occur from the corners of a Vickers indentation, were measured and used for fracture toughness determination, through five mathematical models according to (I) Anstis, (II) Evans and Charles, (III) Tanaka, (IV) Niihara, Morena and Hasselman and (V) Lankford. Morphology of indentation cracking was determined by scanning electron microscope. The most adequate model for determination of fracture toughness (KIc) of yttria stabilized tetragonal zirconia dental ceramics by the Vickers indentation fracture test is Lankford model.

Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS)

Abstract Periodic banded structure has great influence on the microstructure and fracture toughness of eutectic ceramic produced by LENS system. Formation Mechanism of periodic banded structure and influence regularity from laser power, scanning speed, feeding rate and feeding proportion were studied. X-ray diffractometer analysis and scanning electron microscopy observation were carried out for sample phase composition and microstructure respectively. Microhardness was measured by vivtorinox hardness tester, and surface fracture toughness was calculated by indentation crack length. The results show that periodic banded structure is divided into organizational coarsening zone and divorced eutectic zone. Due to the influence of upper cladding layer heat, the eutectic structure in the interlayer bonding zone is obviously coarsening in organization coarsening zone. When laser power is lower, Al 2 O 3 phase takes the lead in nucleation and grows up quickly, forming a discrete block structure of Al 2 O 3 particles in divorced eutectic area. When laser power is higher, ZrO 2 phase takes the lead in nucleation and forms a discrete block structure of ZrO 2 particles. Al 2 O 3 -ZrO 2 ceramics with the shape of thin-walled and cylinder were manufactured under the optimized process conditions, whose independent nucleation disappears and the thickness of periodic banded structure reduces to 10 µm. The eutectic spacing of prototype structure reaches 120–130 nm, has uniform microstructure and good surface morphology. The average micro-hardness is 18.59 GPa. Surface fracture toughness increases to 6.52 MPa m 1/2 .

Statistical Analysis of Vickers Indentation Fracture Toughness of Y-TZP Ceramics

The purpose of this study was to investigate and analyse the fracture toughness of yttria stabilized tetragonal zirconia polycrystal (Y- TZP) dental ceramics by means of the Vickers indentation technique. Fracture toughness was analysed under the following loads: 29.42 N, 49.03 N, 196.13 N and 294.20 N. Cracks, developed from the vertices of a Vickers indentation, were measured and used for determination of fracture toughness using nine different models. It was found that the fracture toughness depends on the indentation load, morphology of indentation cracking and the applied model of fracture resistance. The set of fracture toughness data was analysed using the two-parameter Weibull statistics. The Weibull modulus and scale parameter were determined by the linear regression method. It was concluded that the Weibull distribution can describe fracture toughness of Y-TZP ceramics properly.

Rapid synthesis, electrical, and mechanical properties of polycrystalline Fe2AlB2 bulk from elemental powders

AbstractPolycrystalline Fe2AlB2 bulk including minor Al2O3 is synthesized by reactive hot pressing from Fe, Al, and B powders at 1200°C and 30 MPa for 30 minutes, with a relative density of 96%. The present approach enables a markedly reduced holding time compared with previous studies. The derived Fe2AlB2 shows an electrical resistivity of 2.27±0.01 μΩ·m, Vickers hardness of 10.2±0.2 GPa, flexural strength of 232±25 MPa, compressive strength of 2101±202 MPa, fracture toughness of 5.4±0.2 MPa·m1/2 and work of fracture of 117±12 J/m2. No dominant indentation cracks are observed, indicating that Fe2AlB2 may be quite damage tolerant. Interestingly, a noncatastrophic failure is present in the SENB test, with a high work of fracture. The energy‐absorbing mechanisms in inhibiting crack formation are delamination and pullout of Fe2AlB2 grains.

Microstructure, fracture, electrical properties and machinability of SiC-TiNbC composites

Three SiC based composites with 30, 40 and 50% of additives (Ti and NbC with ratio of 9:16) have been prepared by hot pressing without other sintering additives. The microstructure, porosity, and chemical composition were studied using SEM/EDS. Local mechanical properties such as hardness and elastic modulus of individual components of the composite were investigated by nanoindentation. Hardness and fracture toughness of the composites were evaluated by means of Vickers macroindentation. Indentation cracks were observed and their propagation was analyzed. It was shown that the present phases were distributed uniformly. The materials with 40wt% and 50wt% TiNbC were almost fully dense with porosity lower than 1%. The individual constituents shown similar elasticity modulus (550–590GPa). Indentation fracture toughness was comparable in all materials, between 2.7–3.0MPa.m1/2. Cracks in SiC were mostly straight, transgranular. In other places they propagated both trans- and intergarnularly, often followed SiC/TiNbC and TiNbC/TiNbC grain boundaries. The four-point bending strength was 435MPa for 30% TiNbC and is comparable in all materials within the error of measurement. These results suggest much lower cohesive strength of TiNbC grain boundaries. Electrical conductivity increased with increasing amount of TiNbC and in all materials was more than 1000S/m. Consequently, all composites were EDM machinable, the surfaces of the cut were intensively oxidized.

Microstructure and Crack Propagation of Electrically Conductive SiC Based Composites

The work deals with the observation of microstructure and crack propagation from macro-indentation in a set of newly developed materials. Complex composites based on SiC matrix with 30, 40 and 50 wt. % of additives (Ti and NbC) were hot pressed at 1960 oC in air atmosphere under 30 MPa pressure for 1.5 hour. The microstructure and chemical composition were studied by SEM equipped with EDX analyzer. Hardness of the composites was evaluated by means of classic Vickers macro-indentation and fracture toughness was determined by the Anstis method. Indentation cracks were observed and their propagation was analyzed. The electrical conductivity as function of volume fraction of additives was determined.

Manufacture of iron-based, amorphous coatings with high fracture toughness

Amorphous iron-based material have excellent corrosion behaviour, show good tribological performances and exhibit interesting thermophysical properties. The deposition as a coating system by thermal spraying technology is an innovative approach to manufacture these materials. In this study, the mechanical properties of three iron-based amorphous coatings with different chromium content xCr = 0, 5 and 15 at.% are presented deposited by means of High Velocity Oxygen Fuel Spraying. For the determination of the amorphous content the linear relationship between crystallization energy and amount of amorphous structures is used. Comparing the crystallization energies of amorphous ribbons manufactured by melt spinning to those of feedstock materials and free standing coatings, assumptions regarding the amorphous contents are drawn. The results show that the amorphous content in the feedstock material is influenced by the amount of chromium content. Furthermore, the amorphous content of all coatings do not exceed those of the feedstock materials. Powder xCr = 15 at.% and the corresponding coating exhibit smallest amount of amorphous structure, presumably due to a not fully melted state of the impacting particles. The values of fracture toughness of the coatings are determined by means of indentation and subsequent measurement of the crack lengths. Furthermore, values of indentation modulus and hardness are measured and compared to each other. While length of indentation cracks decreases with increasing chromium content, an increase in indention modulus and hardness is observed. In comparison to ceramic reference YSZ and the steel reference 1.4404, all amorphous coatings show promising properties such as low indentation crack lengths and high hardness.

Vickers Indentation Cracking of Ion-Exchanged Glasses: Quasi-Static vs. Dynamic Contact

The indentation deformation and cracking responses of ion-exchanged glasses were measured using quasi-static and dynamic loading cycles. Two glass types were compared, a normal glass that deforms to a large extent by a shearing mechanism and a damage resistant glass that comparatively deforms with less shear and more densification. The quasi-static indentation cracking threshold for median/radial cracks for the ion-exchanged normal glass was determined to be 7 kilograms force (kgf), while the ion-exchanged damage resistant glass required loads exceeding 30 kgf. The increased cracking threshold of the damage resistant glass composition is attributed to the deformation mechanism, i.e. deformation with greater densification/less shear results in less subsurface damage and less residual stress. Both glass types were also subjected to dynamic indentation where the contact event time was more than 10,000 times shorter than the quasi-static condition. Under dynamic loading conditions, the cracking thresholds of the ion-exchanged normal and damage resistant glasses increased to greater than 50 kgf and greater than 150 kgf, respectively. The stress induced optical retardation was compared for quasi-static and dynamic indents made at sub-cracking threshold loads for both glasses. For indents made at the same sub-cracking threshold load in the normal glass, optical retardation mapping indicates less residual stress surrounding dynamic indents when compared to quasi-static indents. This suggests a rate dependence on the deformation mechanism in normal glasses with higher rates promoting densification in favor of shear. However, for damage resistant glass, the stress induced optical retardation is the same for indents made at both quasi-static and dynamic indentation rates.

Constitutive modeling of indentation cracking in fused silica

AbstractFused silica shows three distinct regimes during nanoindentation, that is, plastic deformation, inelastic densification, and cracking. Cohesive zone FEM is used to study these regimes for different indenter geometries. In a three‐dimensional model, the median/radial cracking is considered by introducing cohesive element planes that are aligned along the indenter edges perpendicular to the indented surface. In addition to comparing indentation cracking data with experimental data, the role of densification on indentation crack growth is critically examined using a pressure independent von Mises and a pressure dependent Drucker‐Prager Cap constitutive model. The results show that the Drucker‐Prager Cap model delivers an accurate description of the elastic‐plastic deformation conditions for all examined indenter geometries. Material densification leads to shorter crack lengths and thus the approach by Lawn et al. (J Am Ceram Soc, 1980;63:574‐581) results in larger indentation‐based fracture toughness values. Once the crack was initiated its propagation is comparable for blunt indenter geometries (Berkovich), while densification leads to a slower crack propagation for sharper indenter geometries.

Surface roughness and morphology evolution of optical glass with micro-cracks during chemical etching.

Chemical etching is usually utilized to measure, reduce, and remove the subsurface micro-cracks in optical components, which makes it significant to study the surface evolution of optical components during the etching process. Etching experiments were carried out for glass with artificial cracks and micro-cracks under different etching conditions. The etching rate was obtained, which is linear with the hydrofluoric acid (HF) concentration and greatly affected by etching temperature. By measuring the surface roughness (SR) and morphology of glasses after etching, it is found that the crack width always increases with etching time, while the crack depth remains unchanged after the crack is completely exposed. Meanwhile, the SR increases sharply at first, then increases slowly, and finally decreases with the increase of etching time. Considering the influence of HF concentration, etching temperature, and the diffusion coefficient on the etching rate, simulation models were established for etching trailing indent cracks (TICs) to further analyze the evolution of SR and morphology. The simulation results were compared with the experimental ones, also indicating that the maximum SR (Ra) increases greatly with the crack's aspect ratio and the model for analyzing the crack's morphology evolution is more reasonable.

Vickers indentation fracture toughness of Y-TZP dental ceramics

The purpose of this study was to investigate and analyse the fracture toughness of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) dental ceramics by means of the Vickers indentation technique. Indentation tests are commonly used for the evaluation of the fracture toughness of brittle materials, particularly glass and ceramics, because this technique requires a small sample from which a large number of data points can be generated rapidly. However, a wide variety of equations for the evaluation of the fracture toughness by means of Vickers indentation are available. Therefore, it is important to choose the most appropriate model for the description of this ceramics' fracture behavior. In the present work, nine different fracture toughness equations are considered and the results are compared with the literature value obtained by a conventional method, single edge V-notched beam (SEVNB) test. The influence of the applied indentation load on fracture toughness was also observed. Tests were conducted under four different loading conditions (29.42N, 49.03N, 196.13N, and 294.20N). Obtained results indicate that the fracture toughness of Y-TZP dental ceramics depends on the indentation load, morphology of indentation cracking as well as on the applied model of fracture resistance.

Optimum Removal Amount of Surface with Indentation Crack in Evaluation of Fracture Toughness of Single Crystal Silicon

Optimum Removal Amount of Surface with Indentation Crack in Evaluation of Fracture Toughness of Single Crystal Silicon

Microstructure and Micro-mechanical Properties of Cu Based Amorphous Composites with Shape-Memory Crystals

Abstract Tapered Cu 50 Zr 42 Al 8 alloy with good glass-forming ability was fabricated by copper mould suction casting. Cubic AlCu 2 Zr and B2-ZrCu, thermal-induced martensitic B19'-ZrCu phases form on the glass matrix along the radial direction structure. Size effects on micro-mechanical properties and a unique crack healing behavior were studied. Micro-hardness of monolithic bulk metallic glass (BMG) displays “smaller is softer” trend, that is, the softer surface and the harder center. Whereas the lager size composites possess a softer center and a harder surface, due to the integrative action of the secondary phases. Amorphous matrix is toughened by TRIP (transition induced plasticity) effect of shape-memory phase as well as weakened and embrittled by AlCu 2 Zr crystal. After annealing, self-healing of Vickers indentation crack occurs as the thermoelastic transformation of shape-memory crystals. As loading, stress-induced martensites transform from B2 to B19', along with volume expansion. Annealed upon the reverse transformation temperature, B19' to B2 transform accompanied by a volume shrinkage, and a restoring force from the structure stress is set up to drive the crack closure.