Fabrication and Properties of Low-Shrinkage Reaction-Bonded Mullite

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al₂O₃ (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al₂O₃ were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al₂O₃ and SiO₂. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.     

Processing, Mechanical Properties, and Oxidation Behavior of Silicon Oxynitride Ceramics

Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si₃N₄+ SiO₂ mixture with 3 mol% CeO₂. The Ce₂O₃/SiO₂ ratio of intergranular phase (liquid phase) increased as the formation of Si₂N₂O proceeded. The intergranular liquid remained as a glass on cooling until the Ce₂O₃/SiO₂ ratio exceeded a certain value, at which point the liquid crystallized. There were great differences in thermal and mechanical properties and oxideation behavior between the specimen containing intergranular glassy phase and the one containing intergranular crystalline phase (Ce₅(SiO₄)₃N–Ce_4.67(SiO₄)₃O). The specimen containing the intergranular glassy phase showed excellent hightemperature strength and oxidation resistance.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

High-Temperature Mixed Oxidation of Nitride-Bonded Silicon Carbide in Oxidizing Gas Mixtures Containing 2% Cl2

Nitride-bonded silicon carbide ceramics have lower processing costs than many other SiC-based ceramics and adequate properties for use as high-temperature heat exchangers in oxidizing environments. Silicon nitride has much better resistance to attack by chlorine at temperatures above 900°C than silicon carbide. When nitride-bonded silicon carbide ceramics are exposed to gas mixtures containing 2% Cl₂ and small amounts of oxygen in this temperature range, the SiC is selectively chlorinated, leaving behind a porous matrix of silicon nitride. The rate of corrosion is controlled by a combination of interfacial kinetics at the surfaces of the SiC grains and transport of volatile species through the silicon nitride skeleton. In more oxidizing environments, the rate of chlorination is suppressed by the formation of a protective SiO₂ film. In highly oxidizing environments at temperatures in excess of 1200°C, the formation of volatile chloride reaction products at the interface between the SiC and the passivating SiO₂ layer causes bubbles to form in the SiO₂, which accelerates the oxidation.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Grain-Boundary Viscosity of Polycrystalline Silicon Carbides

Internal friction experiments were conducted on three SiC polycrystalline materials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron microscopy (HREM). Observations revealed a common glass-film structure at grain boundaries of two SiC materials, which contained different amounts of SiO₂ glass. Additional segregation of residual graphite and SiO₂ glass was found at triple pockets, whose size was strongly dependent on the amount of SiO₂ in the material. The grain boundaries of a third material, processed with B and C addition, were typically directly bonded without any residual glass phase. Internal friction data of the three SiC materials were collected up to similar/congruent2200°C. The damping curves as a function of temperature of the SiO₂-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential-like background. In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhibited sliding. Frequency-shift analysis of the internal friction peak in the SiO₂-containing materials enabled the determination of the intergranular film viscosity as a function of temperature.     

Reactions of Silicon-Based Ceramics in Mixed Oxidation Chlorination Environments

The reaction of silicon-based ceramics with 2% Cl₂/Ar and 1% Cl₂/1% to 20% O₂/Ar at 950 °C was studied with thermogravimetric analysis and high-pressure mass spectrometry. Pure Si, SiO₂, several types of SiC, and Si₃N₄ were examined. The primary corrosion products were SiCl₄( g ) and SiO₂( s ) with smaller amounts of volatile silicon oxychlorides. The reactions appear to occur by chlorine penetration of the SiO₂ layer, and gas-phase diffusion of the silicon chlorides away from the sample appears to be rate limiting. Pure SiO₂ shows very little reaction with Cl₂. SiC with excess Si is more reactive than the other materials with Cl₂, whereas SiC with excess carbon is more reactive than the other materials with Cl₂/O₂. Si₃N₄ shows very little reaction with Cl₂. These diferences are explained on the basis of thermodynamic and microstructural factors.     

Strengthening and Prevention of Oxidation of Aluminum Nitride by Formation of a Silica Layer on the Surface

A silica (SiO₂) layer was deposited on the surface of an AlN ceramic in order to increase the strength and to prevent the high-temperature oxidation of the material. The layer was formed on the surface by exposing coupons to the atmosphere downstream of a bed of SiC powder in a flowing H₂–0.1% H₂O atmosphere at 1450°C. A reaction between the SiC powder and H₂O in the H₂ gas resulted in the generation of SiO₂"smoke" in the product gas stream. Part of the SiO₂ smoke was subsequently deposited on the surface of the AlN specimen to form a dense and uniform SiO₂ layer. The strength of AlN was improved by about 20% apparently because of blunting of surface defects by SiO₂. More importantly, the layer was very effective in protecting the AlN from the oxidation at elevated temperatures, through the inhibition of transport of oxidants to the sample surface.     

Fabrication of Low-Shrinkage, Porous Silicon Nitride Ceramics by Addition of a Small Amount of Carbon

Successful net-shape sintering offers a significant advantage for producing large or complicated products. Porous Si₃N₄ ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1–5 vol%) of two types, with different mean particle sizes (13 nm and 5 μm), were added to α-Si₃N₄−5 wt% Y₂O₃ powders. SiC nanoparticles formed through reaction of the added carbon with SiO₂ on the Si₃N₄ surface or with the Si₃N₄ particles themselves. Such reaction-formed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si₃N₄ porous ceramics with a high porosity of 50%–60%, a very small linear shrinkage of ∼2%–3%, and a strength of ∼100 MPa were obtained.     

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al₂O₃·2SiO₂) or mullite + BSAS ((1− x )BaO· x SrO·Al₂O₃·2SiO₂, 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.     

Effects of High Water-Vapor Pressure on Oxidation of Silicon Carbide at 1200°C

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H₂O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO₂ scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO₂ layer, which remained constant in thickness with time as the crystalline SiO₂ continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO₂ volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO₂ to porous, nonprotective SiO₂.     

Corrosion of Mullite by Molten Salts

The interaction of molten salts of different Na₂O activities and mullite is examined with furnace and burner tests. The more-acidic molten salts form small amounts of Al₂O₃; the more-basic molten salts form various Na₂O–Al₂O₃–SiO₂ compounds. The results are interpreted using the Na₂O–Al₂O₃–SiO₂ ternary phase diagram, and some possible diffusion paths are discussed. The generally higher melting points of Na₂O–Al₂O₃–SiO₂ compounds lead to better behavior of mullite in molten salts, as compared to SiO₂-protected ceramics such as SiC. Mullite-coated SiC is discussed, and the corrosion behavior is evaluated.     

Nanoindentation of a Polymer-Derived Amorphous Silicon Carbonitride Ceramic

Nanoindentation has revealed significant scatter of the microscale hardness and elastic modulus of an amorphous SiCN ceramic, because of structural inhomogeneities (nanopores and clusters of free carbon within the material). As a consequence of the common feature, an amorphous nature, SiCN, in regard to its mechanical properties, resembles SiO₂ glass more than SiC or Si₃N₄. However, because of the stronger Si—C and Si—N covalent bonding, SiCN is harder and stiffer than SiO₂. The mean hardness—13 ± 2 GPa, measured at a load of 250 mN for SiCN—is approximately half that of polycrystalline Si₃N₄ (24.9 ± 0.6 GPa) but higher than that of SiO₂ glass (8.9 ± 0.04 GPa). The elastic modulus of the SiCN, measured at a load of 250 mN, is 121 ± 10 GPa.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Hot Isostatic Pressing and High-Temperature Strength of Silicon Nitride-Silica Ceramics

Dense Si₃N₄ ceramics containing 2.5 and 20 wt% SiO₂ were hot isostatically pressed by a glass-encapsulation method. Their fracture strengths showed no degradation up to 1400°C, despite the presence of a large amount of SiO₂ glass. Moreover, in the case of Si₃N₄-20 wt% SiO₂, plastic deformation of as much as 2.5% was observed during bending tests at the strain rate ε = 1.5 × 10⁻⁴/s. The α- to β-phase-transformation rate decreased with increasing SiO₂ content.     

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

The effects of glass additions on the properties of (Zr,Sn)TiO₄ as a microwave dielectric material were investigated. The (Zr,Sn)TiO₄ ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GH_z. Several glasses, including SiO₂, B₂O₃, 5ZnO–2B₂O₃, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO₄ sintered with ZnO-B₂O₃–SiO₂ (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO₄ sintered at the same temperature. A 5-wt% addition of SiO₂, to (Zr,Sn)TiO₄, when sintered at 1200°C, gave the best Q : Q = 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.     

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO₂ doping on the sintering behavior, microstructure, and dielectric properties of BaTiO₃-based ceramics was investigated. Silica was added to (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) powder prepared using the solid-state method. SiO₂-doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO₅) existed in samples when SiO₂ was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO₂ additives increased. It was found that BCTZ ceramics sintered with SiO₂ are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO₂ addition (3% and 5%) due to higher formation of BaTiSiO₅. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO₅ phase on the dielectric properties of BCTZ ceramics, the BaTiSiO₅ composition was synthesized from individual BaCO₃, TiO₂, and SiO₂ powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO₅, in the sintered ceramics.     

Etching of Silicon Carbide Materials at Elevated Temperatures in a Nitrogen-Based Gas

Two sintered SiC-based materials were heat-treated for 150 h at 1300°C in a nitrogen-based gas (1.2% H₂, 0.6% CO) at a total pressure of 130 Pa. Sintered SiC samples were also preoxidized and then exposed to this gas under the same conditions to evaluate the protective nature of an SiO₂ scale. In this atmosphere, SiO gas and cyanogens are predicted to form, rather than SiO₂. Experimental studies confirmed that etching of sintered SiC occurs. Preoxidation does not provide protection from etching, because of the rapid removal of SiO₂ by H₂ as H₂O and SiO.     

Corrosion of Silicon Carbide in Gases and Alkaline Melts

The corrosion behavior of sintered SiC in gaseous environments and alkaline melts was investigated at 900°C. In oxidizing atmospheres such as normally exist in a gas turbine, SiC forms a dense coherent surface film of SiO₂ which is not corroded by thin layers of condensed sodium sulfate. However, under some conditions, especially when very low oxygen pressures are maintained at the SiC surface or when basic salt melts or slags containing carbonaceous material are present, rapid corrosion of the ceramic can occur. On the other hand, SiC is inert in pure N₂, H₂, or H₂-H₂S mixtures at 900°C. These different modes of behavior are discussed in the context of possible high- temperature applications of SiC ceramics.