Effect of intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered α-SiC

The effect was investigated of the intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered (PLPS) α-SiC densified with 10 vol.% 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, or Yb) additives. It was found that the sliding-wear behaviour of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Most importantly, the sliding-wear resistance of PLPS SiC is found to increase with decreasing size of the RE³⁺ cation in the rare-earth oxide additive, with a lower susceptibility to mild and severe wear and a delayed transition to severe wear. Underlying this effect is likely the hardening of the intergranular phase resulting from the increase in the field strength of the RE³⁺-O²⁻ bonds as the size of the RE³⁺ cation decreases. Tailoring the intergranular phase chemistry via the selection of RE₂O₃ sintering additives with cations as small as possible thus emerges as a potentially interesting approach to improving the sliding-wear resistance of PLPS SiC ceramics.     

Oxidation resistance of SIC-AlN ceramics coated by oxidation-assisted-pack cementation process

Liquid-phase-pressureless-sintered SiC-AlN-Y₂O₃ composites were coated by means of modified pack cementation process (OXPAC, OXidation-assisted-PAck Cementation) using rare-earth oxides, RE₂O₃ (RE = Sc, Er, Sm, Lu, Ho), as reactive powders. The coatings, composed by the oxidation products of SiC, AlN and rare-earth silicates, were adherent to the substrate, without porosity and with a thickness of 10 μm. The oxidation resistance of the coated SiC-AlN-Y₂O₃ ceramics was also investigated at 1500 °C for a period of 200 h. The coated samples showed specific weight gain lower than the uncoated and pre-oxidised samples. Furthermore, the specific weight gain linearly increased with the rare-earth cationic radius.     

Oxidation behaviour of nano-sized SiC particulate reinforced Alon composites

Silicon carbide (SiC)-aluminium oxynitride (Alon) ceramic composites exhibited improved mechanical properties, but the high temperature oxidation behaviour was unknown. The aim of this investigation was to identify oxidation characteristics and kinetics of 8 wt% SiC-Alon composites over a temperature range between 700 °C and 1200 °C in air. The Alon matrix and SiC particles near the surface were oxidized to form Al₂O₃ and SiO₂, respectively. The starting oxidation temperature of Alon was observed to be about 1000 °C. While the addition of nano-sized SiC particles resulted in a reduced starting oxidation temperature due to the large cumulative surface area and high total surface energy, the oxidation resistance at higher temperatures of 1100 °C and 1200 °C was remarkably enhanced. The oxidation kinetics changed from a linear weight gain for pure Alon into a logarithmic weight gain for the composites due to the formation of a dense protective oxidation layer arising from the presence of SiO₂.     

Oxidation behaviour of pressureless liquid-phase-sintered α-SiC with additions of 5Al2O3 + 3RE2O3 (RE = La,Nd, Y,Er, Tm,or Yb)

The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the sintering additives of 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, Y, Er, Tm, or Yb) by thermogravimetry experiments in oxygen at 1075–1400 °C for up to 22 h. It was found that the oxidation is in all cases passive and protective, with kinetics governed by the arctan-rate law. This is because the PLPS SiC ceramics develop oxide scales having no cracks or open porosity and accordingly prevent the parent material from direct contact with oxygen. In addition, these oxide scales crystallize gradually during the exposure to the oxidizing atmosphere with the attendant reduction in the amorphous cross-section available for oxygen diffusion. It was also found that the rate-limiting mechanism of the oxidation is outward diffusion of RE³⁺ cations from the intergranular phase into the oxide scale, and that the activation energy of the oxidation increases with increasing size of the RE³⁺ cation. It was also observed that the oxidation of PLPS SiC increases with increasing size of the RE³⁺ cation, an effect that is especially marked for cation sizes above 0.9 Å because the oxidation rate becomes several orders of magnitude faster. This trend is attributable to the oxide scales being more crystalline, and containing crystals that are more refractory and amorphous residual phases that are more viscous as the size of the RE³⁺ cation decreases. Finally, implications for the design of PLPS SiC ceramics with superior oxidation resistance are discussed.     

Effect of the sintering additive content on the non-protective oxidation behaviour of pressureless liquid-phase-sintered α-SiC in air

The long-term oxidation resistance of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the content of sintering additive (in particular, YAG) at 1500 °C in air. It is shown that, regardless of the vol.% YAG, the oxidation is passive at that high temperature, with a kinetics given by the paralinear-rate law. This is because the oxide scales grow due to oxidation of the SiC grains, but recede due to the formation of a eutectic phase and to the carbothermal reduction of YAG. It is also shown that the oxidation resistance of PLPS SiC decreases markedly with increasing vol.% YAG, an effect that is especially marked above 7.3 vol.% YAG where a change in oxidation behaviour occurs. Thus, while up to 7.3 vol.% YAG the PLPS SiC ceramics gain mass during the entire oxidation process (500 h) because the oxide scales are at least semi-protective, from 11.1 vol.% YAG onwards the PLPS SiC ceramics first gain mass and then lose mass linearly over oxidizing time because the oxide scales are non-protective. Finally, implications for the design of PLPS SiC ceramics that can tolerate prolonged exposures at high temperatures in air are discussed.     

The effects of rare-earth oxide additives on the densification of pressureless sintering B4C ceramics

Boron carbide ceramics used as neutron absorbing materials in fast breeder reactor were fabricated with boron carbide powders and different rare-earth oxide additives by pressureless sintering. The effects of rare-earth oxide as well as phenolic resin on densities and mechanical properties of the composites were studied. The addition of Dy₂O₃, Eu₂O₃, and Sm₂O₃ was found to be beneficial in the densification of B₄C ceramics. B₄C with 4 wt% rare-earth oxide and 18 wt% phenolic resin, exhibiting bulk density of 90–96% T.D., flexural strength of 276–358 MPa, could be prepared by pressureless sintering at 1960–2080 °C, which are capable of meeting the requirement of fast breeder reactor.     

Influence of grain size on high temperature oxidation behavior of Cr2AlC ceramics

The influence of grain size on the oxidation behavior of Cr₂AlC at 1100 °C and 1200 °C for different times was investigated using fine grained (2 μm) and coarse grained (60 μm) samples. The two materials show a good oxidation resistance owing to the formation of a dense and continuous Al₂O₃ layer. The oxidation rate of the fine grained Cr₂AlC is relatively faster than that of the coarse grained Cr₂AlC. The microstructure and phase composition of scale was characterized. After oxidation at 1100 °C and 1200 °C for long times up to 100 h, only a dense and continuous α-Al₂O₃ oxide layer formed on both the fine grained and coarse grained Cr₂AlC. However, after oxidation at 1100 °C for a relatively short 2 h period, a Cr₇C₃ compound was detected beneath the α-Al₂O₃ oxide layer on the coarse grained Cr₂AlC, yet no Cr₇C₃ was found in the fine grained Cr₂AlC. The oxidation mechanism of the fine and the coarse grained Cr₂AlC was discussed.     

Effect of Ar or N2 sintering atmosphere on the high-temperature oxidation behaviour of pressureless liquid-phase-sintered α-SiC in air

The effect of the Ar or N₂ sintering atmosphere on the oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was studied. PLPS α-SiC specimens processed under Ar or N₂ atmospheres were isothermally oxidized at 1100–1450 °C in air for up to 500 h, and their oxidation kinetics, activation energy, and rate-controlling mechanisms were compared. It was found that, regardless of the sintering atmosphere, the oxidation is passive due to the formation of oxide scales. In addition, below 1350 °C the oxidation is protective, with a kinetics that follows initially the arctan-rate law and then the parabolic-rate law. However, from 1350 °C onwards the oxidation becomes only semi-protective, with a kinetics that obeys the arctan-rate law briefly and then the paralinear-rate law. Furthermore, the activation energies and rate-controlling mechanisms are similar for the arctan and paralinear oxidations, but different for the parabolic oxidation. It was also observed that the N₂-processed material oxidizes more slowly than the Ar-processed material below 1200 °C due to a greater crystallization of its oxide scale, whereas above 1200 °C the Ar-processed material is more oxidation-resistant due to greater viscosity of its oxide liquid. Implications concerning the optimization of the processing route of PLPS SiC for high-temperature applications in air are discussed.     

Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.     

Crack-healing behavior of hot-pressed TZ3Y20A-SiC ceramics

Crack healing behavior of hot-pressed TZ3Y20A-SiC ceramics has been investigated by high-temperature oxidation. Semi-elliptical surface cracks with a length of 100 μm have been set on the tensile side of each specimen using a Vickers hardness indenter. Based on flexural strength and observations of crack appearance, the effect of crack healing is finally judged. Cracks on ZrO₂(Y₂O₃)–Al₂O₃ (TZ3Y20A) ceramics cannot be healed by heat treatment. However, for TZ3Y20A-SiC ceramics, complete crack-healing has been realized by heat treatment at 800 °C, 1000 °C and 1100 °C for 30 h, 10 h and 5 h, respectively. The crack-healing mechanism is attributed to the formation of SiO₂ caused by high temperature oxidation during heat treatment.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Oxidation of ZrC-30 vol% SiC composite in air from low to ultrahigh temperature

Oxidation of ZrC-30 vol.% SiC is investigated in air using furnace and oxyacetylene torch. The microstructure and phase composition of oxide scales are analyzed via SEM, XRD, and Raman. At 800 and 1100 °C, SiC is embedded in the porous and cracked ZrO₂ scales, which have a single-layer structure and are almost non-protective. At 1300 and 1500 °C, the protective effect of oxide scales is enhanced by the formed SiO₂. The scales consist of two subscales, outer and inner layers, during oxidation at 1300 °C for ≥1 h, and 1500 °C for ≥15 min. The growth kinetics of both layers is analyzed. At ∼1700 °C, a new layer is observed between the outer and inner layers, which should contain less carbon than the inner layer. At ∼2100 °C, the oxide scale is porous and contains many big holes. This scale shows a single-layer structure, which mainly consists of ZrO₂.     

Oxidation resistance of a gradient self-healing coating for carbon/carbon composites

A gradient self-healing coating consisting of three layers, SiC-B₄C/SiC/SiO₂, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B₄C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO₂ as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B₄C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions.     

The effect of Al2O3 on sintering and crystallization of MgSiO3-based glass-powder compacts

The influence of Al₂O₃ (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO₃) was experimentally determined. The investigated compositions were Y_0.125Mg_0.875Si_0.875B_0.125O₃ and Y_0.125Mg_0.725Ba_0.15Si_0.875B_0.125O₃. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 °C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 °C and 1100 °C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al₂O₃ in glass batches affected the thermal properties of the glasses and resulted in MgSiO₃-based glass ceramics well sintered between 900 °C and 1100 °C. In the BaO-free MgSiO₃ glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

The effect of antioxidants on the oxidation behaviour of magnesia–carbon refractory bricks

Oxidation of carbon is the main problem in magnesia–carbon refractories. The effects of various antioxidants, Al, Si, SiC and B₄C on the oxidation resistance of magnesia–carbon bricks were investigated at temperatures of 1300 °C and 1500 °C. Carbon losses as wt.% of the bricks were calculated and oxidized areas of the bricks were examined by XRD, SEM and EDS. B₄C was found to be the most effective antioxidant at both temperatures. Magnesium–borate (Mg₃B₂O₆) compound was determined to be present by characterization studies on B₄C added specimens. Magnesium–borate, which is in liquid state above 1360 °C, had an excellent effect on the oxidation resistance of the bricks by filling up the open pores and forming a protective layer on the surface. Forsterite (Mg₂SiO₄) and spinel (MgAl₂O₄) provided similar effects on the Si and Al added specimens respectively at both temperatures. The SiC added specimens had similar phases with Si added specimens, but SiC was the least effective antioxidant at both temperatures.     

Pyrolysis synthesis of Si–B–C–N ceramics and their thermal stability

Si–B–C–N ceramics were synthesized by co-pyrolyzing hybrid polymeric precursors of polycarbosilane (PCS) and polyborazine (PBN). The pyrolysis behavior and structural evolution of the hybrid precursor, the microstructure and composition of the prepared Si–B–C–N ceramics were fully investigated. It was found that the copyrolysis of hybrid polymeric precursors in Ar led to the release of CH₄, CH₃NH₂ and CH₃CN gases at temperatures ranging from 200 to 1100 °C, and finally resulted in the formation of amorphous Si–B–C–N ceramics. In particular, the Si–B–C–N ceramics formed from the hybrid precursor with PBN/PCS mass ratio of 1 could keep amorphous state up to the annealing temperature of 1800 °C with weight change of only 2.08%. But this amorphous ceramics would decompose to form crystalline SiC, BN and Si₃N₄ at 2000 °C. Additionally, compared with PCS-derived SiC ceramics, the Si–B–C–N ceramics showed improved anti-oxidation performance up to 1300 °C due to the formation of borosilicate layers covering the ceramics.     

Fabrication and electrical properties of Pb(Co1/3Nb2/3)O3 ceramics

Pb(Co_1/3Nb_2/3)O₃ (PCN) ceramics have been produced by sintering PCN powders synthesized from lead oxide (PbO) and cobalt niobate (CoNb₂O₆) with an effective method developed for minimizing the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development, dielectric and ferroelectric properties of the sintered ceramics. From X-ray diffraction analysis, the optimum sintering temperature for the high purity PCN phase was found at approximately 1050 and 1100 °C. The densities of sintered PCN ceramics increased with increasing sintering temperature. However, it is also observed that at very high temperature the density began to decrease. PCN ceramic sintered at 1050 °C has small grain size with variation in grain shape. There is insignificant change of dielectric properties with sintering temperature. The P–E hysteresis loops observed at −70 °C are of slim-loop type with small remanent polarization values, which confirmed relaxor ferroelectric behavior of PCN ceramics.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.