Whisker Pull-out Research Articles

Effect of TiC whisker addition on properties of YG10F alloy

The cemented carbide material (YG10F) with different additions of TiC whisker (0%, 0.3%, 0.6%, mass fraction) was prepared by different techniques. The effect of TiC whisker addition on the density, microhardness and toughness of the experimental material was investigated. The results indicate that after the wet-milling for 8 h and sintering in vacuum at 1 440 °C, the toughness of YG10F is remarkably improved and meanwhile higher microhardness is obtained by 0.3% TiC whisker addition. Preliminary analysis suggests that the main toughening mechanism of TiC whisker in YG10F is whisker pull-out and bridging phenomena.

Fabrication and mechanical properties of ZrB2–SiCw ceramic matrix composite

A ZrB2 based ceramic matrix composite containing 20 vol.% SiC whisker was fabricated at 1800 °C for 1 h under a uniaxial load of 40 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that mechanical properties were improved by introduction of SiC whisker, especially for the fracture toughness. The fracture toughness was ∼5.97±0.3 MPa m0.5, which was much higher than that of monolithic ZrB2 and SiC particle reinforced ZrB2 composite. The toughening mechanisms were whisker bridging, crack deflection around whiskers as well as whisker pull-out and fracture. The results presented here point to a potential method for improving fracture toughness of ultra-high temperature ceramic.

Microstructure and properties of tetrapod-like ZnO whiskers reinforced Al matrix composite

Aluminum matrix composites reinforced by one-dimension structure whiskers have a wide range of applications. The paper is concerned with ZnO whiskers with peculiar tetrapod shape which were introduced to 66-1 Al alloys. 25 vol.% tetrapod-like ZnO whiskers (ZnO w) reinforced 66-1 Al matrix composites were fabricated by a squeeze casting process. The tetrapod-like ZnO w are well dispersed in the Al matrix. There are dimples, a few whisker and whisker pull-out in fracture surfaces of the composites, showing a mixture of brittle failure and ductile failure. The prepared composites possess higher strength, hardness and density than the 66-1 Al matrix alloy due to the addition of tetrapod-like ZnO w as reinforcement, and isotropic properties which is superior to other whisker reinforced Al matrix composites can be achieved owing to the peculiar structure.

Processing and tensile properties of hydroxyapatite-whisker-reinforced polyetheretherketone

Polyetheretherketone (PEEK) was reinforced with 0–50 vol% hydroxyapatite (HA) whiskers using a novel powder processing and compression molding technique which enabled uniform mixing at high whisker content. Texture analysis showed that viscous flow during compression molding produced a preferred orientation of whiskers along the specimen tensile axis. Consequently, the elastic modulus or ultimate tensile strength of HA-whisker-reinforced PEEK was able to be tailored to mimic human cortical bone. PEEK reinforced with 40 and 50 vol% HA whiskers exhibited elastic moduli of 17 and 23 GPa, respectively. Elastic constants were measured using ultrasonic wave propagation and revealed an orthotropic anisotropy also similar to that measured in human cortical bone. PEEK reinforced with 10 and 20 vol% HA whiskers exhibited an ultimate tensile strength of 90 and 75 MPa, respectively. Tensile specimen fracture surfaces showed evidence of brittle failure in both reinforced and un-reinforced PEEK. Whisker pullout was observed with PEEK adhered to HA whiskers, suggesting a relatively strong interface between the PEEK matrix and HA whisker reinforcements.

Influence of SiC whisker morphology and nature of SiC/Al 2O 3 interface on thermomechanical properties of SiC reinforced Al 2O 3 composites

Thermomechanical properties of a 35 vol.% SiC whiskers/Al 2O 3 matrix composite were investigated as a function of whisker surface quality. Two batches of SiC whiskers (Tateho-SCW-1-S) were studied. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy and whisker morphology, as determined by SEM or TEM, was correlated to the thermomechanical properties of the composites. The surface oxygen content of the whiskers was shown to strongly affect the composite thermomechanical properties. High oxygen surface content appears to affect the whisker/matrix interfacial bonding thus decreasing the amount of crack deflection, whisker pullout and whisker bridging which are required to reach high fracture toughness values.

In situ observation of fracture behavior of in situ TiBw/Ti composites

Abstract This paper presents the results of an in situ study of crack nucleation and growth in an in situ TiB w /Ti composite. The crack/microstructure interactions are studied along with the underlying crack-tip shielding by crack bridging and whisker pull-out. The reinforcement/matrix interface is also shown to retard crack growth. Final fracture is shown to occur by quasi-cleavage fracture mode.

The mechanical properties of polyurethane elastomer reinforced and toughened by CaSO4-whisker

CaSO4 whisker reinforcing and toughening mechanisms for polyurethane elastomer were studied. The effects of dispersity of CaSO4 whisker and interfacial bonding state on reinforcement and toughness were discussed. The microanalyses showed that CaSO4 whisker reinforcing mechanism for polyurethane elastomer mainly was load transferring and its toughening mechanism involved crack deflection and whisker pullout. The results indicated that composites with 5%–10% CaSO4 whisker exhibited the best mechanical properties. Good bonding interface was formed between whisker and matrix after the surface of CaSO4 whisker was treated by silane coupling agent. The fairly improved strength and toughness are attributed to the better interfacial bonding state.

Structural and Mechanical Properties of Particulate, Whisker and Unidirectional Bi-2223/Ag Composites

In order to obtain high Tc superconducting materials with improved mechanical properties, Bi1,5Pb0,5Sr2Ca2Cu3Ox / Ag-composites with 3 different filler morphologies were successfully synthesised. It is shown that Ag-addition improves Bi-2223 grain growth and reduces sample-porosity. An alignment of the ceramic phase along the surface of the Ag was observed and the results of TEM, SEM, DTA and EDX mapping are discussed, conclusions concerning the alignation-mechanisms based upon these observations are compared with popular models described in literature. Composites exhibit increased ductility and isostrain conditions are met, Little effect on the mechanical strength is observed but the mismatch in thermal expansion coefficient induces a residual stress field at the filler matrix interface and allows toughening mechanisms to appear, resulting in a clear post-peak behaviour. SEM pictures of whisker debonding, crack bridging and whisker pullout, all demonstrating a suitable filler-matrix interface, are included. The role of the filler morphology and the sequence of the processing steps on the toughening efficiency will be discussed.

Dental resin composites containing silica-fused whiskers—effects of whisker-to-silica ratio on fracture toughness and indentation properties

Dental resin composites need to be strengthened in order to improve their performance in large stress-bearing applications such as crowns and multiple-unit restorations. Recently, silica-fused ceramic whiskers were used to reinforce dental composites, and the whisker-to-silica ratio was found to be a key microstructural parameter that determined the composite strength. The aim of this study was to further investigate the effects of whisker-to-silica ratio on the fracture toughness, elastic modulus, hardness and brittleness of the composite. Silica particles and silicon carbide whiskers were mixed at whisker : silica mass ratios of 0 : 1, 1 : 5, 1 : 2, 1 : 1, 2 : 1, 5 : 1, and 1 : 0. Each mixture was thermally fused, silanized and combined with a dental resin at a filler mass percentage of 60%. Fracture toughness was measured with a single-edge notched beam method. Elastic modulus and hardness were measured with a nano-indentation system. Whisker : silica ratio had significant effects on composite properties. The composite toughness (mean±SD; n=9) at whisker : silica=2 : 1 was (2.47±0.28) MPa m 1/2, significantly higher than (1.02±0.23) at whisker : silica=0 : 1, (1.13±0.19) of a prosthetic composite control, and (0.95±0.11) of an inlay/onlay composite control (Tukey's at family confidence coefficient=0.95). Elastic modulus increased monotonically and hardness plateaued with increasing the whisker : silica ratio. Increasing the whisker : silica ratio also decreased the composite brittleness, which became about 1/3 of that of the inlay/onlay control. Electron microscopy revealed relatively flat fracture surfaces for the controls, but much rougher ones for the whisker composites, with fracture steps and whisker pullout contributing to toughness. The whiskers appeared to be well-bonded with the matrix, probably due to the fused silica producing rough whisker surfaces. Reinforcement with silica-fused whiskers resulted in novel dental composites that possessed fracture toughness two times higher than, and brittleness less than half of current dental composites.

Temperature dependence of crack wake bridging stresses in a SiC-whisker-reinforced alumina

The crack face bridging behavior of a SiC-whisker-reinforced alumina composite was characterized between room temperature and 1400°C in air. The bridging relations of the material were deconvoluted from the chevron-notched specimen R-curves by employing the fracture mechanics weight function method and assuming that pullout bridging was the dominant bridging mechanism. The results indicated that the maximum bridging stress decreased linearly with increasing temperature while the distribution of the bridging stresses appeared to shift towards larger crack opening displacements. The former trend was found to be in good agreement with literature data on the temperature dependence of residual stresses measured in similar composites, while the latter agreed with the observation of an increasing whisker pullout length in such composites with increasing temperature. In agreement with previous studies of this material, some crack-tip toughening due to a microcrack damage zone formed around the crack-tip by diffusional cavitation could be observed at T>1000°C. Although the material exhibited toughening by both a microcrack zone and a bridging zone between 1000°C and 1300°C, no influence from the microcrack zone on the bridging stresses was detected.

Whisker-reinforced bioactive composites containing calcium phosphate cement fillers: effects of filler ratio and surface treatments on mechanical properties.

Calcium phosphate cement (CPC) sets to form microporous solid hydroxyapatite with excellent osteoconductivity, but its brittleness and low strength prohibit use in stress-bearing locations. The aim of this study was to incorporate prehardened CPC particles and ceramic whiskers in a resin matrix to improve the strength and fracture resistance, and to investigate the effects of key microstructural variables on composite mechanical properties. Two types of whiskers were used: silicon nitride, and silicon carbide. The whiskers were surface-treated by fusing with silica and by silanization. The CPC particle fillers were either silanized or not silanized. Seven mass ratios of whisker-silica/CPC were mixed: 0:1 (no whisker-silica), 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0 (no CPC). Each powder was blended with a bisphenol-a-glycidyl methacrylate-based resin to harden in 2 x 2 x 25 mm molds by two-part chemical curing. The specimens were tested in three-point flexure to measure strength, work-of-fracture (toughness), and elastic modulus. Two-way analysis of variance was used to analyze the data, and scanning electron microscopy was used to examine specimen fracture surfaces. The whisker-silica/CPC ratio had significant effects on composite properties (p < 0.001). When this ratio was increased from 0:1 to 1:0, the strength was increased by about three times, work-of-fracture by five times, and modulus by two times. Whisker surface treatments and CPC filler silanization also had significant effects (p < 0.001) on composite properties. Scanning electron microscopy revealed rough fracture surfaces for the whisker composites with steps and whisker pullout. Resin remnants were observed on the surfaces of the pulled-out whiskers, indicating strong whisker-matrix bonding. In conclusion, incorporating highly osteoconductive CPC fillers and ceramic whiskers yielded composites with substantially improved mechanical properties compared with composites filled with CPC particles without whiskers. The composite properties were determined by whisker-to-CPC ratio and filler surface treatments.

Application of the glass-ceramic process for the fabrication of whisker reinforced celsian-composites

The glass-ceramic process has been investigated as an alternative route to the common ceramic process for the formation of whisker reinforced ceramic composites. In the BaSiAlTiO system composites have been prepared with celsian (BaAl2Si2O8) as the matrix and hollandite (Ba1.23Al2.46Ti5.54O16) as the reinforcement. Different starting materials have been subjected to a thermal treatment, and the product samples have been studied with respect to their microstructures and the crystalline phases present. By annealing pellets of a finely powdered, quenched melt containing Ba, Si, Al and O, to which crystalline TiO2 was added, a complete transformation of hexacelsian, formed in a first step, into celsian was achieved at 1000°C. In the celsian matrix, needle-shaped hollandite crystals precipitate, which are distributed homogeneously over the matrix phase. In these dense composites, typical reinforcement mechanisms, such as crack deflection and whisker pull-out have been observed.

Effect of whisker surface treatment on the mechanical properties of 20vol%SiCw/BAS glass-ceramic composites

20vol%SiCw/BAS (barium aluminosilicate) composites with as-received, acid-leached and heat-treated whiskers were fabricated by a hot-pressing technique and their mechanical properties were evaluated. The whiskers used in these composites were characterized using Auger electron spectrometry (AES) and scanning electron microscopy (SEM), and the observations were correlated with the mechanical properties. The results show that the oxygen content of the whisker surfaces was reduced by the hydrogen fluoride (HF) acid-leaching treatment and the surface geometry of whiskers was smoothed by the heat-treatment in an inert gas atmosphere. The corresponding composites show an obvious increase (18% and 31%, respectively) in toughness compared with the composites reinforced with as-received whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, crack bridging and whisker pullout for the composites with surface treated whiskers.

Effect of loading velocity and testing temperature on the fracture toughness of a SiCw/6061Al alloy composite

Abstract The effects of loading velocity and testing temperature on the fracture toughness of a SiC whisker reinforced 6061 aluminum alloy composite were investigated. A precracked three-point bend specimen configuration was selected for fracture toughness measurement, with tests being conducted at loading velocities of 10 −2 –10 m s −1 , and from room temperature to 473 K. The results showed that the fracture toughness increases with increasing loading velocity, but, the difference with respect to room temperature is small, because the fracture toughness decreases slowly with the increase of testing temperature. The composite material failed mainly by whisker pull-out and whisker breaking.

Dental composite resins containing silica-fused ceramic single-crystalline whiskers with various filler levels.

Currently available direct-filling composite resins are susceptible to fracture and hence are not recommended for use in large stress-bearing posterior restorations involving cusps. The glass fillers in composites provide only limited reinforcement because of the brittleness and low strength of glass. The aim of the present study was to use ceramic single-crystalline whiskers as fillers to reinforce composites, and to investigate the effect of whisker filler level on composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers, thereby improving retention in the matrix. The composite flexural strength, elastic modulus, hardness, and degree of polymerization conversion were measured as a function of whisker filler mass fraction, which ranged from 0% to 70%. Selected composites were polished simulating clinical procedures, and the surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean +/- SD; n = 6) of 196+/-10 MPa, significantly higher than 83+/-14 MPa of a microfill and 120+/-16 MPa of a hybrid composite control (family confidence coefficient = 0.95; Tukey's multiple comparison). The composite modulus and hardness increased monotonically with filler level. The flexural strength first increased, then plateaued with increasing filler level. The degree of conversion decreased with increasing filler level. The whisker composite had a polished surface roughness similar to that of a conventional hybrid composite (p>0.1; Student's t). To conclude, ceramic whisker reinforcement can significantly improve the mechanical properties of composite resins; the whisker filler level plays a key role in determining composite properties; and the reinforcement mechanisms appear to be crack pinning by whiskers and friction from whisker pullout resisting crack propagation.

The effect of whisker orientation in SiC whisker-reinforced Si 3N 4 ceramic matrix composites

Si 3N 4 ceramic matrix composites reinforced by nearly unidirectionally aligned SiC whiskers have been prepared by extrusion and hot pressing. Unlike the case in traditional Si 3N 4 ceramic matrix composites reinforced by random SiC whiskers, the mechanical properties of the composites exhibit a significant dependence on whisker orientation. In the direction of whisker alignment for SiC(w)/Si 3N 4 composites, increments in bending strength and fracture toughness of 200 MPa and 3 MPa·m 1/2 are obtained respectively, compared to the values in the direction perpendicular to whisker alignment. Based on microscopic fractographic observation and micromechanics analyses, the effects of whisker orientation on toughening mechanisms are discussed. The results indicate that the whisker orientation, θ , is a decisive factor for the essential toughening mechanisms of whiskers. Only in the case of small θ and weak interface can whisker pullout occur, and whisker has maximum toughening effect. The results show that effects of whisker strengthening and toughening can be improved simultaneously through whisker oriented alignment. ©

Effects of Loading Velocity on Fracture Toughness of a SiCw/A6061 Composite at Elevated Temperatures

The influence of loading velocity on the fracture toughness of a SiC whisker reinforced 6061 aluminum alloy composite was investigated at elevated temperatures. A precracked three-point bend specimen configuration was selected for fracture toughness measurement, with tests being conducted at loading velocities of 10 -2 to 10 m.s -1 , and at temperatures up to 473 K. The results showed that the fracture toughness increases with increasing loading velocity, but the difference with respect to room temperature is small, because the fracture toughness decreases slowly with increasing testing temperature. The composite material failed mainly by whisker pull-out and whisker breaking.

Abrasive wear behaviour of SiC whisker-reinforced silicate matrix composites

The abrasive wear behaviour of SiC whisker-reinforced borosilicate and magnesium aluminosilicate (MAS) composites is investigated under different conditions. Generally, MAS composites exhibited superior wear resistance compared to borosilicate composites. The superior wear resistance of MAS composites is attributed to its higher mechanical properties. The wear resistance increased by increasing volume fraction of whiskers. Sliding distance, sliding velocity and load exhibit significant influence on the material loss. During abrasive wear, silicate matrices undergo extensive contact damage due to repeated indentation of SiC particulates/whiskers. Under repeated contact events, cracks generated at various locations of the matrix simultaneously grow and join resulting in delamination of composite material. In the composites, material removal is delayed by various toughening mechanisms like crack deflection, bridging and whisker pull-out.

Effect of loading speed on fracture behaviour of ZrO 2(Al 2O 3) matrix composites reinforced with SiC whiskers

The fracture toughnesses, fracture behaviour and toughening mechanisms of 20 vol% SiCw–ZrO 2 (2 mol% Y 2O 3), 20 vol% SiCw–ZrO 2 (6 mol% Y 2O 3), 20% SiCw–Al 2O 3 and 20 vol% SiCw–20 vol% ZrO 2 (2 mol% Y 2O 3)–Al 2O 3 composites (SiCw means SiC whisker) were investigated by three-point bending tests, XRD and scanning electron microscopy techniques. The results show that the fracture toughnesses of the composite materials increase with increasing loading speed because of the increase in the amount of dynamic t→m ZrO 2 transformation and whisker pullout effect. Whisker/matrix characteristis have a significant influence on the effect of loading speed on the fracture behaviour. 1998 Elsevier Science Limited and Techna S.r.l.

Microstructure and Mechanical Properties of Hot-Pressed TiB 2–SiC w Composites

Hot-pressed composite ceramics with 0–30 vol% SiC w have been investigated to determine the effect of composition (i.e., amount of SiC whiskers) on the microstructure and mechanical properties. The composites exhibited superior mechanical properties, especially in fracture toughness, compared to monolithic TiB 2. Fracture toughness measured by the single-edge notched beam method increased from 3.4 MPa m for the hot-pressed unreinforced TiB 2 to 7.8 MPa m for the composite containing 30 vol% SiC whiskers. The lack of increase in flexural strength of the composites with increasing volume fraction of SiC whisker can be explained by a decrease in relative density associated with SiC w agglomerates. Finite element method analysis and scanning electron microscopy observation show that the toughening mechanisms may be attributed to crack deflection around whiskers and whisker bridging due to whisker pullout.