Comparison of hydrothermal corrosion behavior of SiC with Al2O3 and Al2O3 + Y2O3 sintering additives

Fully dense SiC bulks with Al₂O₃ and Al₂O₃ + Y₂O₃ sintering additives were prepared by spark plasma sintering and the effect of sintering additives on the hydrothermal corrosion behavior of SiC bulks was investigated in the static autoclave at 400°C/10.3 MPa. The SiC specimen with Al₂O₃ sintering additive exhibited a higher weight loss and followed a linear law. However, the SiC specimen with Al₂O₃ + Y₂O₃ additive exhibited a lower weight loss and followed a parabolic law, indicating that the corrosion kinetic and mechanism were different for these two SiC bulks. Further examination revealed that, a deposited layer was formed on the surface of SiC specimen with Al₂O₃ + Y₂O₃ sintering additive after corrosion, which can effectively protect the SiC specimen from further corrosion, and thereby improved the corrosion resistance of the SiC specimen with Al₂O₃ + Y₂O₃ sintering additive.     

Effect of CaF2 on sintering behavior and thermal shock resistance of Y2O3 materials

Y₂O₃ materials have become a popular candidate for preparing refractory crucibles for ultra-pure high-temperature alloy melting in recent years. However, its difficulty in sintering and poor thermal shock resistance limited its industrial application. The effect of CaF₂ on the densification microstructure, mechanical properties, and thermal shock resistance of Y₂O₃ materials was investigated in this paper. The main purpose of this study was to optimize the amount of CaF₂ added in the preparation of Y₂O₃ materials to improve its thermal shock resistance and get better mechanical properties. The mechanism of the densification process of CaF₂-doped Y₂O₃ materials was analyzed by phase analysis and microstructure. The results showed that successive doping of large Ca²⁺ ions caused more lattice distortion in the Y₂O₃ materials, and the diffusion rate of Y³⁺ was increased, thus enhanced grain boundary diffusion and promoted sintering densification in the Y₂O₃ materials. Meanwhile, the addition of CaF₂ also significantly reduced the apparent porosity and enhanced the mechanical properties of the materials. The improvement of these properties was attributed to the increased relative density of CaF₂-doped Y₂O₃ materials and the high sintering activity of CaF₂. In addition, crack deflections effectively improved the thermal shock resistance of the materials. The residual flexural strength ratio of Y₂O₃ materials doped with 1 wt % CaF₂ was increased by 21.2% after thermal shock test compared with undoped specimens.     

Influence of Y2O3 on densification,flexural strength and heat shock resistance of cordierite-based composite ceramics

In order to fabricate a heat transfer ceramic-based pipeline for concentrated solar power, rare earth Y₂O₃ was utilized as a modifying agent to improve the physico-chemistry properties of the cordierite-based composite ceramics. The influences of the sintering temperature and Y₂O₃ additive on the densification, flexural strength, and thermostability were investigated. The research results indicate that the densification degree of the composite ceramics gradually increases with elevated temperature, and the initial sintering temperature decreases with the addition of Y₂O₃. In addition, the flexural strength and heat shock resistance of the ceramic materials were improved with the addition of Y₂O₃. In particular, a sample containing 7 wt% Y₂O₃ (sample E4) sintered at 1360 °C showed the best properties with a relative density of 92.49%, a flexural strength of 126.81 MPa, and strength loss rate of -7.74% after 30 heat shock cycles. X-ray diffraction and scanning electron microscopy analysis showed that parts of Y³⁺ ions dissolving into high-temperature liquid phases could reduce liquid viscosity to accelerate grain crystallization and pore elimination. The second phase of yttrium silicate properly impeded the generation of β-spodumene with lower strength during the heat shock process. Overall, a cordierite-based composite ceramic with low porosity was obtained with high mechanical strength and heat shock resistance and can be regarded as a highly potential material for solar heat transfer pipelines.     

Permeability of the porous Al2O3 ceramic with bimodal pore size distribution

Porous Al₂O₃ ceramics with bimodal pore size distribution were fabricated by partial sintering with monodispersed PMMA micro balls as pore agent. The porosity of the fabricated porous Al₂O₃ is increased with content of the pore agent increase, the bulk density and bending strength are decreased, accordingly. Relations between pressure drop and flow velocity of the air through the porous Al₂O₃ fit the Forchheimer's equation well for compressible fluid. Due to pores introduced by the pore agent, the Darcy permeability and inertial permeability of the porous Al₂O₃ are increased obviously. For given flow velocity, with increase of the PMMA content, the Forchheimer's number of the fluid through the porous Al₂O₃ is decreased, which results in decrease of the inertial resistance ratio to the total pressure drop. The porous Al₂O₃ ceramics with pores introduced by the monodispersed PMMA micro balls show higher permeability while the filtration selectivity is not deteriorated.     

Effect of Y2O3 and ZnO co-doping on the densification and properties of magnesium aluminum spinel

The effects of Y₂O₃ and ZnO co-doping on the densification and properties of magnesium aluminum spinel were investigated. The physical phase composition, microstructure, elemental distribution, densification, apparent porosity, particle size distribution and average corrosion depth of the specimens were investigated by XRD, SEM-EDS, Archimedes drainage method, Nano Measurer 1.2 software, and molten salt corrosion tests. The results showed that after co-doping with Y₂O₃ and ZnO, the cations in the sintering aids could dissolve into the spinel structure, forming solid solution ZnAl₂O₄, second phase Y₃Al₅O₁₂ and Al₂Y₄O₉, which inhibited the abnormal grain growth and made the grain distribution more uniform, thus promoting the densification of the samples. The best co-doping amount of Y₂O₃ and ZnO was 1 wt% Y₂O₃-3 wt.% ZnO, the relative density of the sample was 99.3%, the apparent porosity was 0.021%, and the grain size was the most uniform (6.52 μm). After the sample was placed into the aluminum electrolytic molten salt electrolyte for 1 h, and it was found that the sample doped with 1 wt% Y₂O₃-3 wt.% ZnO had the minimum average corrosion depth (131.9 μm) and the best corrosion resistance.     

Effects of different particle size of spinel and Y2O3 additive of the periclase-spinel refractory on the sintering densification and corrosion resistance to copper smelting slag

In order to analyze the sintering densification and copper smelting slag corrosion resistance of periclase-spinel refractories, the periclase-spinel refractories were prepared with fused magnesia, magnesia-rich spinel, industrial alumina, and yttrium oxide as the main raw materials. The different particle sizes of spinel in material and with or without Y₂O₃ additive were studied. The study demonstrated that: (1) The different particle sizes of spinel in periclase-spinel refractories can result in different effects. Adding particle spinel to the refractory can improve the strength and corrosion resistance of the periclase-spinel refractories. The addition of spinel and magnesia powders in the matrix resulted in cracks due to the great difference of coefficient of thermal expansion between magnesia and spinel. The reduction in bulk density and strength of the material decreased slag penetration resistance because of its poor sintering properties. While adding the alumina in the matrix can further fill the crack and prevent slag penetration by the volume expansion of in-situ reaction to form spinel. (2) The periclase-spinel refractories can be reacted with Cu slag to form a Mg₂FeO₄ insulating layer as the iron ion becomes oxidized. Adding Y₂O₃ in periclase-spinel refractories can result in grain boundary phase reconstruction, which can promote sintering densification, improve the slag physical infiltration resistance, and improve the chemical corrosion resistance of materials.     

Effect of Y2O3 on the physical properties and biocompatibility of β–SiAlON ceramics

To investigate the effects of Y₂O₃ on the physical properties and biocompatibility of β–SiAlON ceramics, β–SiAlON ceramics were prepared with Al, Si, and α–Al₂O₃ powders using a direct nitriding technique. As a sintering additive, Y₂O₃ helps lower the sintering temperature and forms β–SiAlON ceramics. In this study, the physical and biological properties of the prepared ceramics were investigated to evaluate their use as bone-repairing material. Experiments revealed that the main crystal composition of the sample was Si₄Al₂O₂N₆, containing small amount of additional phases Y₃Al₅O₁₂ with increasing content of Y₂O₃. The porosity and compressive strength initially decrease and then increase to their initial values, whereas the bulk density exhibits the opposite trend with an increased proportion of Y₂O₃. The proliferation of osteoblastic and angiogenic cells demonstrates that β–SiAlON and Y₃Al₅O₁₂ have good biocompatibility; however, the sample porosity has a slight effect on the cell proliferation rate. This implies that in human tissues, bone-repairing speed can be adjusted by modifying the sample porosity or material surface roughness. Therefore, Y₂O₃ can be added to β–SiAlON ceramics to regulate their microstructure, physical properties, and biological properties for tissue engineering applications.     

Fabrication of super flux and high thermal shock resistance ceramic membrane support

Ceramic membranes play an important role in high temperature gas-solid filtration. However, the thermal stability of the ceramic support at high temperatures has always been a problem. In this study, porous fused silica ceramic supports were fabricated with hexagonal boron nitride as a sintering aid. The results shown that hexagonal boron nitride could inhibit the crystallization of fused silica ceramic particles at high temperature and act as a sintering addictive to reduce firing temperature. The obtained supports have an average pore size of 72?µm, an open porosity of 42%, a bending strength of 16.5?MPa, a Weibull modulus of 8.67 and a gas permeability of 4.23?×?10⁵ m³/(m² h bar). The bending strength of the support remained 16?MPa after 30 cold-hot cycles, exhibiting high thermal shock resistance. After corrosion in 20?vol% H₂SO₄ solution for 8?h, the weight and the bending strength of the support were diminished by 0.6% and 24.32%, respectively. So, the ceramic support showed good acid corrosion resistance.     

Synthesis and characterization of reaction-bonded calcium alumino-titanate-bauxite-SiC composite refractories in a reducing atmosphere

To take full advantage of the excellent properties of CA₆ present in calcium alumino-titanate (CAT) and reduce the formation of the low melting point phase (anorthite), CAT-bauxite-SiC composite refractories were fabricated under buried sintering in order to achieve low thermal expansion, superior high-temperature performance, and increased alkali resistance. Furthermore, the corrosion mechanism of K vapor was investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results show that CA₆ present in CAT can be partially retained and the hot strength of CAT-bauxite-SiC composites slowly decreases when the amount of CAT added is less than 21.6?wt%. The cold strength and bulk density decrease with the CAT content, and the residual ratio of MOR firstly decreases and subsequently increases with the CAT content. For the specimens with CAT additions, 43.2?wt% CAT results in the highest volume expansion at high temperatures. It is proposed that the corrosion mechanism of CAT aggregates under buried sintering is as follows: 1) K vapors penetrate into the CAT with high CA₆ content through the lamellar CA₆ gap and deposit on the inner regions of CAT; and 2) K vapors react with corundum and anorthite present in CAT and cause the microstructural destruction of CAT due to a decrease in the amount of the Al₂O₃-CaO-SiO₂ liquid phase in the CAT. The alkali resistance of the CAT-bauxite-SiC composites decreases as the CAT content increases, which is attributed to poor sintering densification and high apparent porosity.     

Effect of Y2O3 on acid resistance of alumina ceramic

This work aimed to improve the acid resistance of an alumina ceramic. Acid corrosion of alumina ceramic composed of Al₂O₃-CaCO₃-SiO₂-MgO-Y₂O₃ (ACSMY) was investigated in a hydrochloric-hydrofluoric acid solution at 65 °C for 30 min. The effect of Y₂O₃ content on sintering temperature, density, and acid solubility were discussed. The composition and microstructure of this material were analyzed. The acid solubilities of minor crystal phases (Y₃Al₅O₁₂, CaAl₁₂O₁₉, Ca₂Al₂SiO₇, and CaAl₂Si₂O₈) and the effect of them on acid resistance of this alumina ceramic were studied. The results showed that Y₂O₃ additive can enhance density and change the type of phases. Phases with good acid resistance and dense structure lead to a crust formed on the surface of ceramic during acid corrosion. The crust can effectively protect the interior structure of the sample from acid solution, and then improve the acid resistance of the material.     

Effects of nano-alumina content on the formation of interconnected pores in porous purging plug materials

The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al₂O₃ in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ₀ exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.     

Potential of the Sinter-HIP-technique for the Development of High-temperature Resistant Si3N4-ceramics

Silicon nitride ceramics were densified with the sintering additives Y₂O₃ and SiO₂ by a two-step sinter-HIP-process. Three compositions with additive contents between 2 and 7 wt% Y₂O₃ were prepared to study the influence of the processing conditions on the mechanical properties. The minimum additive content required for nearly complete densification (>98.5%) was only 2 wt% Y₂O₃. However, densification was limited to certain Y₂O₃/SiO₂ ratios. The additive-rich samples revealed a mean strength at room temperature up to 800 MPa which degrades at 1400°C. The material with only 2 wt% Y₂O₃ has a room temperature strength of ∼500 MPa, but no strength degradation up to 1400°C. The lower strength correlates with a pronounced increase in brittleness with a decreasing additive content indicated by a fracture toughness of only 2·5 MPam^1/2 for composition 2/0. The investigated materials exhibit a relatively high creep resistance at 1400°C with creep rates down to 1·5×10⁻⁹ s⁻¹.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.     

Permeability and corrosion resistance of porous SiOC-bonded SiC ceramics prepared by the preceramic polymer

Porous SiC ceramics have been used in high temperature flue gas filtration fields because of their excellent properties such as high strength, high temperature resistance, corrosion resistance, and long service time. This work reports the porous SiOC-bonded SiC ceramics prepared at low temperature. The properties of porous SiC ceramics were first investigated with silicone resin content from 10 to 25 wt%, and then the effects of different pore-forming agent contents on the behaviors of porous SiC ceramics were discussed by adjusting poly (methyl methacrylate) PMMA microbeads from 5 to 20 wt%. The prepared porous SiC ceramics showed apparent porosity from 17.3% to 57.7%, compressive strength from 6 to 216 MPa, and Darcy permeability k₁ ranging from 7.02 × 10⁻¹⁴ to 1.45 × 10⁻¹² m². The corrosion behavior of porous SiC ceramics was investigated in acidic and alkaline media. The porous SiC ceramics showed better corrosion resistance in acidic solutions.     

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Mullite fiber was used to fabricate ZrO₂-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y₂O₃ on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO₂ grains. Prepared porous fibrous ZrO₂-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO₂. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y₂O₃ reacted with Al₂O₃ and SiO₂ to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength.     

Zirconia-strengthened yttria ceramics for plasma chamber applications

Yttria (Y₂O₃) is the most popular lining material for the inner walls of plasma chambers, owing to its well-known resistance against corrosion by reactive fluorinated plasma. Yttria has a relatively lower toughness and strength compared to alumina (Al₂O₃) and zirconia (ZrO₂). The present work aims to explore the strengthening and toughening of yttria by doping with zirconia, while retaining its good corrosion resistance. Ceramics with zirconia to yttria molar ratios of 2:8 and 5:5 (named 20ZY and 50ZY, respectively) were fabricated by pressure sintering in Ar atmosphere. The corrosion of the prepared ceramics in reactive fluorinated plasma (C₄F₈/CHF₃/Ar) was investigated and compared with that of yttria as a control. The results indicated that 20ZY exhibited excellent corrosion resistance, comparable to that of yttria, while the fracture toughness and flexural strength showed increases of 87% and 44%, respectively. 50ZY exhibited a further improvement in fracture toughness and flexural strength, but at the price of a much lower resistance against plasma corrosion. A percolation model was proposed to interpret the observed plasma corrosion behaviors.     

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 2: Sintering behaviour and micro-mechanical properties

This second part of the report deals with, how the sintering additives Y₂O₃, Al₂O₃, and MgO influence the sintering behaviour of SRBSN. Paraffin-based feedstocks with varying sintering aid compositions and silicon grain size were used for moulding macro- and micro-scale samples. It was observed that compositions with smaller Si grain size (with correspondingly high SiO₂ content) and containing Al₂O₃ as sintering additive exhibit higher shrinkage and lower residual porosity when sintered at 1700–1800 °C after nitridation. The mechanical properties determined for micro-scale samples were obtained by three-point bending tests, with the resulting characteristic strength values σ₀ ranging from 500 MPa up to 1200 MPa. Surprisingly residual porosity did not play the role of a strength limiting factor; rather it was observed that the presence of crystalline secondary phases – mainly Y₂Si₃O₃N₄ – was responsible for reducing the micro-bending strength. As micro-samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si₃N₄ and volatilization of SiO₂ which is considered to be responsible for the occurrence of secondary phases preferred at the sample surface. The powder bed condition was also found to play a prominent role in the development of the secondary phases during liquid phase sintering.     

Graphite and PMMA as pore formers for thermoplastic extrusion of porous 3Y-TZP oxygen transport membrane supports

A gas permeable porous support is a crucial part of an asymmetric oxygen transport membrane (OTM). Here, we develop feedstocks for thermoplastic extrusion of tubular, porous 3Y-TZP (partially stabilized zirconia polycrystals, (Y₂O₃)_0.03(ZrO₂)_0.97)) ceramics, using graphite and/or polymethyl methacrylate (PMMA) as pore formers. The influence of pore former content and type, 3Y-TZP particle size and support sintering temperature on the microstructure, porosity and gas permeability were studied. Using at least 40 vol% pore former, consisting of graphite and PMMA in the volume ratio 2:1, tubes with gas permeability exceeding the target of 10⁻¹⁴ m² are obtained. In the temperature range 1250–1400 °C the support gas permeability is insensitive to the sintering temperature, and the feedstocks shrink more than 15% during sintering, making them ideal for co-sintering with functional OTM layers. This demonstrates the suitability of thermoplastic extrusion for fabrication of porous 3Y-TZP OTM supports, or for other technologies requiring porous ceramics.     

Rapid densification mechanism and properties of h-BN/ZrO2 composites with oxide additives by spark plasma sintering

Large size high density h-BN/ZrO₂ composites (ø = 110 mm, h = 15 mm) were rapidly prepared by spark plasma sintering (SPS) with sintering cycle 20 min. The effects of additives on the mechanical properties, microstructure evolution, and corrosion resistance of the h-BN/ZrO₂ composites were studied. The Al₂O₃, MgO, SiO₂, La₂O₃ and ZrO₂ rapidly formed heterogeneous eutectic crystals under the action of SPS. The low eutectic compounds significantly promoted the diffusion of h-BN or ZrO₂, also increased the density. The flexural strength of h-BN/ZrO₂ composites could reach 196.31 MPa, and the apparent porosity was only 0.42 %. The additives are combined with zirconia to form a high viscosity eutectic, whose corrosion resistance to molten steel is obviously better than that of single ZrO₂. The corrosion depth of h-BN/ZrO₂ composites was only 114 µm after corroded in molten steel at 1550 °C for 80 min. The comprehensive properties of h-BN/ZrO₂ composites were obviously improved by appropriate additives.     

Evaluation of 316L stainless mill additives on microstructure and alkali corrosive behavior of enamel coatings

Excellent chemical stability, good corrosion resistance, and compositional controllability make vitrified enamel promising coating materials. The corrosion resistance of the [–Si–O–]/[–Si–O–Al–]-based enamel coatings decreases in alkaline environments, whereas can be improved by incorporating metal oxides, such as ZrO₂ and SnO₂. The introduction of these oxides results in an increase in the sintering temperature and a decrease in the toughness of the coatings. It was observed that the corrosion resistance of the coatings in an alkaline medium could be improved using metal powder-based additives without increasing the sintering temperature and affecting the mechanical properties of the coatings. We used powdered 316L stainless steel as the mill additive, and the effects of this additive on the structure and alkali corrosion resistance of the vitrified enamel coatings were studied. Results indicate that the addition of an appropriate quantity of the additive (4 wt.%) improves the extent of gas consumption during the enamel sintering, decreases the degree of porosity and the size of the pores. Furthermore, the mechanical properties of the skeleton formed between the pores could be improved, because blocklike corrosion products are not peeled off, which increases the corrosion resistance of the coatings. The corrosion products, such as Ca-based compounds, block the corrosion holes, resulting in a decrease in the corrosion rate also.