An oscillatory pressure sintering of zirconia powder: Densification trajectories and mechanical properties

The densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds.     

Fabrication and mechanical properties of large-scale SiC impeller via vacuum gelcasting and pressureless sintering

Impeller is the key part of centrifugal pump to convey liquid. And combined with the engineering practice, we fabricated two types of open and semi-open SiC ceramic impellers with diameter of 100 mm and 160 mm via gelcasting and pressureless sintering. B4C and C were used as the sintering additives and sprayed drying granulation method was adopted for the starting SiC powder processing. Given the size and shape complexity of impeller, the optimized pH value, dispersant content, and solid-loading content for the fabrication of the SiC impellers were determined to be 10-11, 0.8 wt%, and 40 vol%, respectively. For the open impeller, blade height, cover board thickness, drying shrinkage, and sintering shrinkage are 10.5 mm, 4 mm, 10.3%, and 21.3%, respectively. For the semi-open ones, the corresponding parameters are 41 mm, 10 mm, 10.3%, and 21.3%, respectively. The average density of the two types of impellers is 3.15 g/cm3, and the mechanical properties of both impellers, including average hardness, flexural strength, compressive strength, and fracture toughness are 2478 HV, 436 MPa, 2093 MPa, and 3.17 MPa·m1/2, respectively. All variation coefficient values are smaller than 5%, which indicates a good uniformity in densities and mechanical properties of both the impellers.     

Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering

The influence of sintering temperature on the microstructure and mechanical properties of Al₂O₃?20 wt% ZrO₂ composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m^1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Enhanced toughness and reliability of Si3N4-SiCw composites under oscillatory pressure sintering

We reported an oscillatory pressure sintering (OPS) process to consolidate Si₃N₄-SiCw composites. For comparison, the composites were also prepared by hot pressing (HP) method. The specimen by OPS process reveals an accelerated rate of grain growth in the c axial direction and thus an increased average aspect ratio compared with the specimen by HP process. The oscillatory pressure also performs a positive impact on the Si₃N₄-SiCw composites as the bulk density of the OPS specimen increases to 3.270?g?cm^?3 accompanied with higher fracture strength and hardness of 1133?MPa and 16.1?GPa, respectively, compared with those of the HP specimen. Significantly, the increased fracture toughness and Weibull modulus found in the OPS specimen indicate toughening effects and material reliability are improved aided by the oscillatory pressure. Current results suggest OPS to be a promising technique for preparing highly densified Si₃N₄-SiCw composites with enhanced mechanical properties.     

The tribological properties of SiC whisker-reinforced polyetheretherketone

SiC whisker-reinforced polyetheretherketone (PEEK) composites with different filler proportions were made into block specimens by compression molding. The friction and wear properties of the composites were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block under ambient conditions. The morphologies of the wear traces and wear debris were observed by scanning electron microscopy (SEM). It was found that SiC whisker-reinforced PEEK exhibited considerably lower friction coefficient compared with pure PEEK, while SiC whisker as a filler at a content of 1.25 to 2.5 wt % was very effective in reducing the wear rate of PEEK. Especially, the lowest wear rate was obtained with the composite containing 1.25 wt % SiC whisker. The SEM pictures of the wear traces indicated that PEEK composites undertook abrasive wear that was enhanced with increasing SiC whisker content, while for the frictional couple of carbon steel ring/composite block (reinforced with 1.25 wt % filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was also supposed that the differences in the content of SiC whisker as filler could cause the differences in the wear mechanisms of SiC whisker-reinforced PEEK composites. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2341–2347, 1998     

An oscillatory pressure sintering of zirconia powder: Rapid densification with limited grain growth

A novel oscillatory pressure sintering (OPS) process is reported to prepare high‐quality ceramics. The oscillatory pressure was applied at three stages (initial, intermediate, and final) during sintering process of zirconia ceramics for the first time. The microstructure of the samples prepared by OPS develops in a more homogeneous manner, leading to a higher final density, a smaller average grain size, and a narrower distribution of grain sizes compared with the samples prepared by conventional pressureless sintering (PS) and hot‐pressing (HP) processes. Remarkably, the OPS samples was obtained at relatively lower heating temperature and less soaking time for 1300°C and 0.5 hours than the samples prepared by other two techniques at 1450°C and 1 hour. The current results suggest that OPS is an effective technique for preparing high‐quality zirconia ceramics with low heating temperature and short sintering time, thus, it obviously reduces cost.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Contact-mechanical properties at pre-creep temperatures of fine-grained graphene/SiC composites prepared in situ by spark-plasma sintering

Hertzian indentation was used to investigate for the first time the elastic–plastic behavior and contact damage evolution with temperature (in the 25–850 °C range) in air of different in situ grown graphene/SiC composites. Three SiC starting powders differing in polytypes and particle sizes were liquid-phase densified by spark-plasma sintering, thus producing ceramic composites that contain about 4 vol.% of epitaxial grown graphene flakes at the grain boundaries and have interesting differences in their microstructure. The general trend in the contact mechanical behavior is qualitatively similar among the different composites, with the elastic modulus, yield stress, and critical loads for the onset of quasi-plasticity and ring/cone cracking all decreasing with increasing temperature, with the differences at the quantitative level being explained in terms of the particular microstructure of each graphene/SiC composite.     

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

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w 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 SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). 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.     

Enhanced properties of pure alumina ceramics by oscillatory pressure sintering

A novel oscillatory pressure sintering (OPS) process to consolidate high-quality pure alumina ceramics is reported. The microstructure of the ceramics prepared by OPS develops into a higher final density, a smaller and a narrower distribution of grain sizes compared with those prepared by conventional pressureless sintering (PS) and hot-pressing (HP) processes. Enhanced mechanical properties of alumina ceramics were investigated by OPS process. The bending strength, hardness and elastic modulus of the OPS specimen reached about 546 MPa, 19.1 GPa and 374 GPa, respectively, i.e values significantly higher than that of the specimens by PS and HP. XRD analysis indicates the strengthening of atomic bonds aided by oscillatory pressure. The results suggest OPS to be an effective technique for preparing high-quality pure alumina ceramics.     

A novel approach to improve flexural strength of Al2O3‐20 wt% ZrO2 composites by oscillatory pressure sintering

In this communication, Al₂O₃‐20 wt% ZrO₂ composites (ZTA) were prepared by a novel oscillatory pressure sintering (OPS) process. An almost perfectly full density of 99.94% and a relatively high flexural strength of 1145 MPa can be achieved for the OPS‐fabricated specimen, which were higher than those prepared by pressureless sintering (98.24% and 682 MPa) and hot pressing (99.53% and 990 MPa). The enhancement of flexural strength of the OPS‐fabricated specimen was correlated with its smaller flaw size and higher fracture surface energy. It was suggested that OPS was an effective approach to fabricate ZTA composites with ultrahigh density and high strength.     

Si-rich pressureless sintering of the gelcasted Ti3SiC2 bulk ceramic

The Si-rich pressureless sintering was used to fabricate the Ti₃SiC₂ bulk ceramic. The results show that the optimized Ti₃SiC₂ suspension could be prepared at the absolute value of zeta potential, pH level, PAA-NH₄ dosage, and solid loading of 62.1 mV, 11, 2.0 wt%, and 50 vol%, respectively. The channels existing in the Si-free sintered body facilitated the reactants and products to diffuse to the interior and out of the Ti₃SiC₂ matrix, thereby forming the porous reaction layer of TiC-Ti₃SiC₂. The co-effects of the channels and the reaction layer of TiC-Ti₃SiC₂ severely lowered mechanical properties of the Si-free sintered Ti₃SiC₂ ceramic. On the contrary, the Si-rich sintering method isolated the volatile carbon and established a closed Si-rich atmosphere to sinter the green Ti₃SiC₂ cylinder. The porosity, density, fracture toughness, hardness, and flexural strength of the Si-rich sintered Ti₃SiC₂ ceramic reached 0.74 vol%, 4.36 g/cm³, 5.49 MPa·m^1/2, 4.03 GPa, and 383 MPa, respectively.     

Enhanced mechanical and tribological properties of oscillatory pressure sintered WC–GNPs composites

It remains as a challenge to develop binderless WC ceramics that integrate high mechanical properties and low friction wear. Here, we report the preparation of strong and tough WC ceramics with low wear rate by adding graphene nanoplatelets (GNPs) and using oscillatory pressure sintering (OPS) process. The introduced GNPs lead to the formation of nearly fully dense composites with the aid of an oscillatory pressure. The OPS-prepared WC–0.3-wt% GNPs composites reached a high flexural strength, hardness, and fracture toughness, being up to 1420 MPa, 24.9 GPa, and 6.89 MPa m^1/2, respectively. Moreover, a low friction wear rate of 3.17 × 10⁻⁷ mm³ N⁻¹ m⁻¹ is achieved for such composites, which can be ascribed to the formation of a friction lubrication film during dry sliding friction process and their higher mechanical properties.     

Mechanical properties and tribological performance of alumina matrix composites reinforced with graphene-family materials

This paper introduce the formation of alumina matrix composites reinforced with multilayered graphene, graphene oxide and nickel-phosphorus coated multilayered graphene. The powder metallurgy technique followed by the Spark Plasma Sintering (SPS) method were utilized to fabricate the specimens. The influence of graphene-family material additions on microstructure was investigated, and correlated with measurements of mechanical properties. The emphasis of the research has been placed on the tribological performance conducted with the use of the ball-on-disc method under loads of 10 N and 30 N. Both the wear tracks of composites and the corresponding counterparts were carefully analysed, to evaluate the combined influence of mechanical properties and tribofilm formation on the measured wear rates. All results were compared to pure alumina as a reference specimen.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Mechanical and tribological properties of epoxy modified by liquid carboxyl terminated poly(butadiene-co-acrylonitrile) rubber

Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin was modified using liquid carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) rubber. The liquid CTBN contents used ranged from 2.5 to 20 parts per hundred parts of resin (phr). Mechanical properties of the modified resins were evaluated and the microstructures of the fracture surfaces were examined using SEM technique. The changes in storage modulus and the glass transition temperature were also evaluated using dynamic mechanical analysis (DMA). The tribological tests were performed using a ball-on-disc tribometer. The worn surfaces and the ball counter-mates after tribological tests were investigated using optical microscope technique. The results revealed the influence of liquid CTBN content on mechanical and tribological properties, and also microstructure of the modified epoxy resins. Impact resistance increased whereas the storage modulus and the hardness decreased when the CTBN rubber was introduced to the epoxy network. The coefficient of friction of the CTBN-modified epoxy was lower than that of the neat epoxy. The CTBN content of lower than 10 phr was recommended for improving the wear resistance of epoxy resin. Changes in tribological properties of the CTBN-modified epoxy correspond well to those in mechanical changes, especially the toughness properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of short carbon fiber content on the mechanical properties of TiB2-based composites prepared by spark plasma sintering

In this study, TiB₂-30 vol% SiC composites containing 0, 5, 10, and 15 vol% short carbon fibers (C_f) were produced by spark plasma sintering (SPS). The effect of carbon fiber content on microstructure, density, and mechanical properties (micro-hardness and flexural strength) of the fabricated composites was studied. Scanning electron microscopy (SEM) results indicated that the fibers were uniformly dispersed in the TiB₂–SiC matrix using wet ball milling before SPS process. Fully dense TiB₂–SiC–C_f composites were achieved by SPS process at 1900°C for 10 min under 30 MPa. With the addition of fibers, the relative density of the composites did not change considerably. Mechanical tests revealed that microhardness was reduced about 19% by the incorporation of carbon fibers, whereas the flexural strength improved significantly. However, the flexural strength diminished by adding carbon fibers above to critical value (5 vol%) due to residual thermal stresses, nonhomogeneous structure and graphitization of carbon fibers. It was found that the composite with 5 vol% C_f had the highest flexural strength (482 MPa), which was enhanced by 20% compared with the TiB₂–SiC composite.     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.