Whisker Bridging Research Articles

Design and evaluation of alkali-activated slag-calcined coal gangue cement and the shrinkage control by calcium carbonate whisker

The rapid solidification and significant shrinkage of alkali-activated slag cement are key issues that restrict its practical engineering application. This paper explores the composition design of alkali-activated cements (AAC) with slow coagulation and shrinkage reduction. This paper adjusts the proportion of slag (SG) and calcined coal gangue powder (CCGP), regulates the content of Na2O·2.4SiO2-NaOH-Na2CO3 ternary activator components, and uses calcium carbonate whiskers (CW) instead of SG to prepare AAC with slow coagulation and shrinkage reduction. When the activator is composed of (66 %) Na2O·2.4 SiO2-(2 %) NaOH-(32 %) Na2CO3 mixed composition, the CO32- and Ca2+ in the system react preferentially to produce CaCO3. The decrease of Ca2+ concentration delays the formation of C-(A)-S-H, resulting in the initial and final setting time of AAC reaching 334 min and 346 min, respectively. When the CW substitution rate is 3 %, the 28d compressive strength of AAC reaches 102.2 MPa. The 49d drying shrinkage of the AAC is 7020μm/m and the 49d autogenous shrinkage is 2387μm/m. This is attributed to the combined effect of enhancement mechanisms such as whisker bridging, crack deflection, whisker pullout, whisker–matrix coalition pullout and whisker breakage. This study establishes the relationship between the composition and performance of high performance AAC, which is significance for the resource utilization of coal gangue and the practical engineering application of AAC.

Synthesis, growth mechanism, and performance of in-situ catalytic growth Al4SiC4 whiskers in Al2O3–SiC–C refractories

Ceramic whiskers exhibit excellent performance due to their special structure, making them widely used in the field of refractories. In this work, a novel Al4SiC4 whisker was designed to improve the mechanical properties and thermal shock resistance of Al2O3–SiC–C (ASC) refractories. By introducing nickel nitrate as a catalyst, the Al4SiC4 whiskers with a special "stalactite" structure could be directly grown in situ at the operating temperature of ASC (1450 °C). Combining the experimental results and first-principles calculations, the AlNi2Si alloy formed during the heat treatment process acted as catalyst sites, AlO, SiO and CO were easily absorbed on the Ni55 surface. Subsequently, the Al–O, Si–O and C–O bonds were broken and reacted with each other to grow Al4SiC4 whiskers according to the vapor-liquid-solid mechanism. As expected, the incorporation of Al4SiC4 whiskers resulted in a 57.4 % increase in cold compressive strength (CCS) and a 47.2 % increase in residual strength ratio (RSR) of the ASC refractories due to the strengthening mechanism of whisker bridging. This study presents a novel research strategy for enhancing the properties of refractories.

Combination of direct ink writing and reaction bonded for rapid fabrication of SiCw/SiC composites

Silicon carbide ceramic matrix composites are widely used in aerospace field due to their advantages of high temperature resistance, high strength and corrosion resistance. However, its application is greatly limited because of the difficulty in preparing complex shape structures by traditional machining methods. Here, a new strategy for preparing SiC w /SiC complex structure by combining direct ink writing with reaction bonding is proposed. A water-based slurry consisting of silicon carbide, carbon powder and silicon carbide whisker was developed. The influence laws of C content and SiC w content in slurry on sintering properties of direct-written samples were studied. The reaction bonding mechanism and whisker reinforcing and toughening mechanism were analyzed by means of microstructure and phase composition. The results show that the slurry exhibits shear thinning behavior with stress yield point, and its flow behavior and plasticity meet the requirements of direct writing. When the carbon content is 6.4 wt%, the maximum flexural strength is 239.3 MPa. When 15 wt% SiC w was added, the flexural strength of the composite reached 301.6 MPa, and when 20 wt% SiC w was added, the fracture toughness of the composite reached 4.02 MPa m 1/2 , which was increased by 26% and 18% compared with single-phase SiC, respectively. The reinforcing and toughening mechanisms of the whiskers mainly include whisker pullout, crack deflection and whisker bridging. After direct ink writing and reaction bonded, the whole process shows good near net forming ability. 3D printed SiC w /SiC composites have great application prospects in aerospace field.

Toughening mechanisms of Al2O3-matrix composites with SiC whiskers

Whisker reinforced ceramic tool is currently used in the field of high-speed machining of difficult-to-machine materials due to its excellent high temperature mechanical properties. However, the toughening contribution values of crack deflection, whisker bridging and whisker pull-out effect in the whisker toughening model still depend on experimental measurement, which are time-consuming and laborious. In this paper, the scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS) are used to observe the number of main toughening models on the whole crack under different loads (98 N, 196 N and 294 N). The weights of crack deflection, whisker bridging and whisker pull-out effect toughening mechanisms are calculated by analytic hierarchy process (AHP), entropy method and synthetic method. The model parameters are optimized. According to the relation between the areas of whisker cross section and crack cross section, the spacing size between the nearest neighbor whisker centers of crack deflection model is calculated. The variation law of the toughening contribution value of whisker bridging and whisker pull-out effect with the change of angles between crack propagation path and whisker axis (ACW) is calculated by simulation. The ACWs with the highest contribution of whisker toughening mechanism to fracture toughness are obtained. Finally, the toughening contribution values of the three toughening models are determined. It provides a reference for the prediction of fracture toughness of whisker toughened ceramic tools.

Preparation and toughening mechanism of Mo2NiB2‐based cermets with SiC whiskers

AbstractMo2NiB2 is a kind of cermet with excellent mechanical properties, stable chemical properties, and excellent corrosion resistance and is often used in wear‐resistant application fields, such as injection molding machine parts, can making tools and hot copper extruding dies. The brittleness of Mo2NiB2‐based cermets limits their wide application. Mo2NiB2‐based cermets were prepared by the vacuum sintering method, the effect of SiC whiskers (SiCw) on the microstructure and mechanical properties of cermets was investigated, and the toughening mechanism of SiCw on cermets was further discussed. The results indicate that with increasing SiCw content, the indentation fracture toughness (KIC), transverse fracture strength (TRS), and Vickers hardness (HV) of the cermets first increase and then decrease. The HV, TRS, and indentation fracture toughness of Mo2NiB2‐based cermets with 0.5 wt% SiCw are 1 113 HV, 1 620 MPa, and 27.97 MPa·m1/2, respectively, which are 16.8%, 22.7%, and 25% higher than those without SiCw. In the sliding friction tests, Mo2NiB2‐based cermets with 0.5 wt% SiCw have the smallest friction coefficient, low wear rate, and high wear resistance. SEM observation and analysis of the crack path and fracture surface showed that the toughening mechanism is whisker bridging, crack deflection, microcrack toughening, and whisker pull‐out. The results indicate that the addition of 0.5 wt% SiCw can effectively improve the mechanical properties of Mo2NiB2‐based cermets and further expand the application space of Mo2NiB2‐based cermets.

Oxidation and thermal cycling behavior of c-AlPO4 and SiC whisker co-modified mullite deposited on SiC-C/SiC composites

A dense and uniform c-AlPO4 and SiC whisker co-modified mullite coating was successfully prepared on SiC-coated C/SiC composites with a thickness of about 70 μm by hydrothermal electrophoretic deposition. The thermal cycling performance of the coating samples was evaluated by thermally cycling between 1773 K and room temperature in a corundum tube furnace. Results show that the addition of SiC whisker (SiCw-mullite coating) significantly improved the thermal cycling resistance of single mullite outer layer on account of the toughening mechanism of SiC whiskers, such as whisker bridging and crack deflection. Then, the addition of c-AlPO4 (c-AlPO4-SiCw-mullite coating, termed ASM coating) further improved the thermal cycling resistance on account of the healing and bonding performances of c-AlPO4 at high temperature. After 40 thermal cycles, the weight loss of ASM coating on SiC-coated C/SiC composites was 4.5% compared to 5.24% for the SiCw-mullite coating. The retention rate of compressive strength of ASM coating on SiC-coated C/SiC matrix was 80.66% compared to 73.65% for the SiCw-mullite coating.

Fabrication and mechanical properties of ZrO2–Al2O3–SiC(w) composites by oscillatory pressure sintering

Tetragonal zirconia polycrystalline (TZP) ceramics have been considered as the important structural materials in many key technology fields, but some of their mechanical properties do not meet the requirement under the extreme environmental conditions. In this work, SiC whiskers and Al2O3 particles were used as reinforcement additives to synergistically toughen TZP ceramics, and a newly developed oscillatory pressure sintering (OPS) technique was employed to fabricate the new zirconia-based ceramics. For comparison, the traditional hot pressing (HP) method was also used to prepare these composites. The results showed that with 15 vol% SiC whiskers addition, the toughening mechanisms such as whisker bridging, whisker pulling-out, crack deflection, and crack branching appeared apparently, thereby improving fracture toughness of such composites. In contrast to HP, OPS can eliminate defects, improve densification, obtain more uniform microstructure, and significantly enhance mechanical properties of the composites.

Fabrication and thermal shock behavior of Si3N4 whiskers toughened γ-Y2Si2O7 coating on porous Si3N4 ceramics

In order to prevent water from entering the porous Si3N4 ceramics, α-Si3N4 whiskers toughened γ-Y2Si2O7 coatings were prepared by a liquid infiltration and filling method. Effects of α-Si3N4 whiskers content on the microstructure, waterproof performance, mechanical properties and thermal shock behavior of the ceramic coatings were systematically studied in this paper. Results indicated that the addition of whiskers could effectively improve the performance of the coatings under thermal shock due to the toughening mechanism of whiskers including crack deflection, whisker bridging, whisker pulling out and other energy dissipation processes. The coating with a α-Si3N4 whisker/γ-Y2Si2O7 mass ratio of 1/9 possessed a flat, dense and defect-free surface microstructure. After five thermal cycles under a thermal shock temperature difference of 1200 °C, water absorption of the coating increased slightly from ~3.2% to ~6.0%. The whisker reinforced coating is promising to be used for high temperature wave-transmitting materials in long-time service.

Crack tolerant silicon carbide ceramics prepared by liquid-phase assisted oscillatory pressure sintering

Silicon carbide ceramics were prepared by liquid-phase assisted oscillatory pressure sintering (OPS) with graphene and in-situ synthesized SiC whisker as the reinforcements. The effects of sintering temperature on the densification, morphology and mechanical performances of the SiCp-SiCw-graphene ceramics were investigated. In the temperature range from 1700 to 1800 °C, the densification rate of SiCp-SiCw-graphene ceramics was accelerated, ascribing to the reduction in viscosity of the glassy phase. At 1800 °C, the flexural strength and fracture toughness of the OPS ceramics corresponded to 697 MPa and 5.8 MPa m1/2, respectively, which were higher than that of the hot-pressed ceramics under the same temperature conditions. Multiphase toughening mechanisms, such as whisker bridging and pullout, graphene bridging and delamination, were considered as the primary mechanisms. This work demonstrates an effective strategy to prepare silicon carbide ceramics at low sintering temperature.

Fabrication of ZrO2 whisker modified ZrO2 ceramics by oscillatory pressure sintering

Zirconia ceramics were prepared by oscillatory pressure sintering (OPS) with zirconia whisker as the reinforcement. The effects of whisker fraction on the microstructure, densification and mechanical properties of the ceramics were investigated. By applying an oscillatory pressure ranging from 25 to 35 MPa to the powder mixtures, the relative density of the OPS zirconia reached approximately 99.7% at whisker fraction of 15 wt%. Because of the accelerated densification by grain boundary sliding and diffusional creep, the average grain size of the OPS zirconia was 0.9 μm, which was much finer than that of the pressure-less sintered zirconia (1.5 μm). The peak values of flexural strength and fracture toughness of the OPS zirconia corresponded to 1376 MPa and 6.7 MPa m1/2, respectively. Whisker bridging, crack deflections were considered as the primary toughening mechanisms. This study provides an approach to prepare zirconia ceramics with uniform, fine grains and excellent fracture strength and toughness.

Microstructure and thermal shock resistance of AlBOw- and BNw-whisker-modified thermal barrier coatings

To improve the crack propagation resistance of YSZ thermal barrier coatings during the thermal cycle, three kinds of thermal barrier coatings were prepared by atmospheric plasma spraying: YSZ, AlBOw-modified YSZ and BNW-modified YSZ. SEM, EDS and XRD were used to analyse the morphology, composition and phase composition of the sprayed powder and coating section. The phase structures of the YSZ, YSZ+AlBOw and YSZ+BNw coatings were t' phase. The cross-section of the coating presents a layered structure with pores inside. The porosity values of the YSZ, YSZ+AlBOw and YSZ+BNw coatings are 10.33%, 14.17% and 12.52%, respectively. The thermal shock resistance of three groups of coatings after 5 min at 1000 °C was analysed. The failure behaviour of the coatings after several thermal cycles was studied. The results show that the thermal shock resistance of the coatings with AlBOw is slightly lower than that of the YSZ coatings. The thermal shock resistance of the BNw coatings is 62.2% higher than that of the YSZ coatings. The whisker inhibits the crack propagation and prolongs the life of the coatings via crack deflection, whisker pull-out and whisker bridging.

A composite structural high-temperature-resistant adhesive based on in-situ grown mullite whiskers

The effect of whisker growing catalyzed by different amounts of AlF3 on the various properties of an epoxy/silicone resin-based high-temperature-resistant adhesive was comprehensively investigated. Although the addition of AlF3 promoted the oxidation reactions, accelerated the premature formation of mullite whiskers, increased the production of SiC ceramic, and reduced the thermal expansion of bonding layer, the excessive use of AlF3 induced many more large holes in the bonding layer, caused the structural heterogeneity, and made mullite whiskers overgrow, thus finally led to the weaker mechanical properties of adhesives. The best dosage of AlF3 in this work was 3 % (mass fraction), a uniform composite structure that whisker weaved adhesive frame kept steady from 1100 °C to 1500 °C, and an impressive bonding strength of 63 MPa was achieved at 1100 °C, 80 % higher than that of the blank sample. Under the synergistic effect of whisker repair, whisker bridging, whisker drawing and crack deflection, a multi-stage fracture phenomenon happened during the shear test, and the fracture displacement of 1100 °C-calcined adhesive with 3 % AlF3 increased by 61 %, compared with that of the black sample.

Erosion resistance and toughening mechanism of AlBO and BNw whiskers modified thermal barrier coatings

In order to improve the erosion resistance and toughness of thermal barrier coatings, YSZ coatings, 20 vol%-AlBOw whisker modified YSZ coatings and 20 vol%-BNw whisker modified YSZ coatings were prepared by plasma spraying. The cross-section structure, morphology, composition and phase composition of coatings and powders were analyzed by SEM, EDS and XRD. The erosion resistance of three coatings was tested by self-made erosion tester. The crack growth resistance of coatings was tested and calculated by Vickers hardness tester. The results show that the density of the coating is improved by adding AlBOw and BNw. The whisker inhibits the crack propagation through the mechanisms of crack deflection, whisker pull-out and whisker bridging. The addition of whisker improves the erosion resistance of YSZ coating by 8.17% and 13.94%, which can effectively improve the service life of thermal barrier coating.

Hybrid effect, mechanical properties and enhancement mechanism of oil-well cement stone with multiscale silicon carbide whisker

Cement-based materials typically have a relatively high compressive strength and a relatively low tensile strength. In this paper, multiscale silicon carbide whiskers (SW) were used to improve mechanical properties of oil-well cement stone. The structure and properties of whisker cement stone were analyzed via mechanical testing and cement sheath integrity. The results demonstrated that multiscale SW mixing showed a positive hybrid effect and could effectively improve the compressive strength, the flexural strength, and the splitting tensile strength of oil-well cement stone by 8.96%, 137.44% and 27.96%. The modulus and porosity of the whisker cement stone significantly reduced. The filling effect, whisker pinning, the pullout, and the whisker bridging were regarded as effective mechanisms for the addition of SW when improving the mechanical properties and microstructures of oil-well cement stone.

Microstructure and properties of SiCw/SiC composites prepared by gel-casting combined with precursor infiltration and pyrolysis

Near net-shaped SiCw/SiC composites with higher fracture toughness and isotropic mechanical properties were successfully fabricated by gel-casting combined with precursor infiltration and pyrolysis (PIP). In the gel-casting process, the effects of dispersant dosage, pH value, and solid loading on the rheology of slurries were investigated in detail. A well-dispersed slurry was obtained when the pH was 11 and dispersant dosage was 1.2wt%. As the solid loading increased from 28 to 40vol%, the flexural strength of green bodies and SiCw/SiC composites increased from 62.34 and 260.38 to 80.21MPa and 308.29MPa, respectively, and the fracture toughness of SiCw/SiC composites increased from 4.56 to 5.32MPam1/2. However, when the solid loading reached 44vol%, uneven pores were observed in the green bodies. The highest density and mechanical properties of composites were obtained when the solid loading was 40vol%. The toughening mechanisms involved were whisker bridging, whisker pull-out and crack deflection.

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiCw)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650°C. The influence of SiCw on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiCw reached a peak value of 5.6MPam1/2 at 15wt% whiskers, which is higher than that of the RBSC with as-received SiCw (fracture toughness of 3.4MPam1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.

Thermal cycling behavior and oxidation resistance of SiC whisker-toughened-mullite/SiC coated carbon/carbon composites in burner rig tests

An oxidation protective SiC whisker-toughened mullite coating was prepared on SiC coated carbon/carbon (C/C) composites by a novel sol–gel technique. The thermal cycling and oxidation behavior of the coated C/C coupons was investigated by thermally cycling between room temperature and 1450±55°C on a jet fuel burner rig facility. Results show that incorporating proper amounts of SiC whiskers can significantly improve the thermal cycling resistance of the mullite coating due to the energy dissipation mechanism induced by the whisker bridging and crack deflection.

Preparation and characterization of continuous alumina based fiber reinforced with orientated mullite whisker

Single crystal mullite whiskers were directly added into the precursor sols to synthesis the orientated whisker-reinforced continuous alumina based fibrous matrix composite. The solid content and spinnable viscosity range were enhanced but the zeta potential was reduced by the addition of the mullite whiskers into the sol. The Al oligomers content increased while monomer and Al13 decreased with the increase of mullite whisker. After sintering at 1250°C, the obtained compact nano grains fibers were reinforced with accordant axial direction whiskers which were orientated by the axial force derived from the spinning process. The phase composition and compact structure were not affected by the presence of the mullite whiskers, while the continuous spinning length became low. Although tensile strength of single composite fiber was decreased at a high mullite whisker content due to the low densification and cracks of composite fiber, whisker bridging and pullout during fiber fracture may have increased the strength at a mullite whisker content of 1wt.% to 642.6MPa.

Hydroxyapatite whisker reinforced 63s glass scaffolds for bone tissue engineering.

Bioactive glass (BG) is widely used for bone tissue engineering. However, poor mechanical properties are the major shortcomings. In the study, hydroxyapatite nanowhisker (HANw) was used as a reinforcement to improve the mechanical properties. 63s glass/HANw scaffolds were successfully fabricated by selective laser sintering (SLS). It was found that the optimal compressive strength and fracture toughness were achieved when 10 wt.% HANw was added. This led to 36% increase in compressive strength and 83% increase in fracture toughness, respectively, compared with pure 63s glass scaffolds. Different reinforcement mechanisms were analyzed based on the microstructure investigation. Whisker bridging and whisker pulling-out were efficient in absorbing crack propagating energy, resulting in the improvement of the mechanical properties. Moreover, bioactivity and biocompatibility of the scaffolds were evaluated in vitro. The results showed that composite scaffolds with 10 wt.% HANw exhibited good apatite-forming ability and cellular affinity.

Characterization of mechanical behavior and mechanism of calcium carbonate whisker-reinforced cement mortar

In order to reinforce cement mortar, a new kind of micro-fibrous material, calcium carbonate whisker (CaCO3 whisker), was incorporated in this study. Microstructure, mechanical properties and reinforcing mechanism of this composite were characterized. It was found that the addition of CaCO3 whisker improved not only the compressive and flexural strength of cement mortar, but also the load–deflection curves and work of fracture. Further work using mercury intrusion porosimetry tests confirmed the filler effect and the refining of the pore distribution of whiskers in cement mortar. Scanning electron microscopy showed that the microscopic mechanism primarily consists of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. These mechanisms are related to the matrix strength. As compared to the strong matrix, the weak matrix that was modified with CaCO3 whisker achieved the highest increase in strength and toughness of the cement mortar. This is likely attributed to the crack deflection mechanism, which is weakened by the strong interfacial bonding between the CaCO3 whisker and cement matrix in the stronger mortar matrix.