Formation of Y3Al5O12–Al2O3 eutectic microstructure with off-eutectic composition

Homogeneous-eutectic microstructure of Y₃Al₅O₁₂–Al₂O₃ system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y₂O₃ and Al₂O₃ having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y₃Al₅O₁₂ and Al₂O₃ phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al₂O₃), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed.     

Hot isostatic pressing of SiC/Si3N4 composite with rare earth oxide additions

SiC/Si₃N₄ composites with rare earth oxide additions have been prepared by glass encapsulated hot isostatic pressing at 1850 °C and 200 MPa pressure. Mechanical properties and microstructures of the sintered samples have been studied. It is shown that different molar ratios of La₂O₃ to Y₂O₃ and the total amount of La₂O₃ and Y₂O₃ additions can affect the mechanical properties significantly. With 3 wt% La₂O₃ + Y₂O₃ additions, lower La₂O₃/Y₂O₃ molar ratio exhibits higher bending strength and median fracture toughness, but relatively lower Vickers hardness. For addition of 6 wt% La₂O₃ + Y₂O₃, the higher bending strength, Vickers hardness and fracture toughness correspond to a certain La₂O₃/Y₂O₃ molar ratio of 1.5, 1.0 and 0.5, respectively. SEM observation shows that the SiC matrix composite with fine grain size and homogeneous microstructure can be obtained.     

Polysilazane derived micro/nano Si3N4/SiC composites

The pyrolised polysilazanes poly(hydridomethyl)silazane NCP 200 and poly(urea)silazane CERASET derived Si–C–N amorphous powders were used for preparation of micro/nano Si₃N₄/SiC composites by hot pressing. Y₂O₃–Al₂O₃ and Y₂O₃–Yb₂O₃ were used, as sintering aids. The resulting ceramic composites of all compositions were dense and polycrystalline with fine microstructure of average grain size <1 μm of both Si₃N₄ and SiC phases. The fine SiC nano-inclusions were identified within the Si₃N₄ micrograins. Phase composition of both composites consist of , β modifications of Si₃N₄ and SiC. High weight loss was observed during the hot pressing cycle, 12 and 19 wt.% for NCP 200 and CERASET precursors, respectively. The fracture toughness of both nanocomposites (NCP 2000 and CERASET derived) was not different. Indentation method measured values are from 5 to 6 MPa m^1/2, with respect to the sintering additive system. Fracture toughness is slightly sensitive to the SiC content of the nanocomposite. Hardness increases with the content of SiC in the nanocomposite. The highest hardness was achieved for pyrolysed CERASET precursor with 2 wt.% Y₂O₃ and 6 wt.% Yb₂O₃, HV 23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives.     

Yb2O3 and Y2O3 co-doped zirconia ceramics

A suspension stabilizer-coating technique was employed to prepare x mol% Yb₂O₃ (x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y₂O₃ co-doped ZrO₂ powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb₂O₃ and Y₂O₃ co-doped zirconia ceramics. Fully dense ZrO₂ ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb₂O₃ content and the sintering parameters. Polycrystalline tetragonal ZrO₂ (TZP) and fully stabilised cubic ZrO₂ (FSZ) were achieved in the 1.0 mol% Y₂O₃ stabilised ceramic, co-doped with 1.0 mol% Yb₂O₃ and 4.0 mol% Yb₂O₃, respectively. The amount of stabilizer needed to form cubic ZrO₂ phase in the Yb₂O₃ and Y₂O₃ co-doped ZrO₂ ceramics was lower than that of single phase Y₂O₃-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m^1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb₂O₃ and 1.0 mol% Y₂O₃ ceramic at 1450 °C for 1 h.     

Sintering of Si3N4 with liquid in the system Ce2O3---AIN---SiO2

Liquid phase sintering of Si₃N₄ with melts from the system Ce₂O₃---AIN---SiO₂ has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si₃N₄ with two melt compositions selected from outside the glass forming region yields fully dense Si₃N₄. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si₃N₄ sintered with liquids in the system Ce₂O₃---AIN---SiO₂ were found to be inferior to those of liquids selected from Y₂O₃---AIN---SiO₂, but superior to those selected from the system MgO---AIN---SiO₂.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF₂ and YF₃ as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂. Liquid CaYAlO₄ mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F₂ at a relatively low temperature results in homogeneous YF₃ distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity.     

Carbon reduction reaction in the Y2O3–SiO2 glass system at high temperature

In order to assess the role of carbon with respect to the grain boundary chemistry of Si₃N₄-based ceramics model experiments were performed. Y₂O₃–SiO₂ glass systems with various amount of carbon (from 1 to 30 wt.%) were prepared by high-temperature treatment in a graphite furnace. High carbon activity of the furnace atmosphere was observed. EDX analysis proved the formation of SiC by the carbothermal reduction of SiO₂ either in the melt or in the solid state. The melting temperature of the Y₂O₃–SiO₂ system is strongly dependent on the amount of reduced SiO₂. XRD analysis of the products documented the presence of Y₂Si₂O₇, Y₂SiO₅ and Y₂O₃ crystalline phases in that order with an increasing amount of free C in the starting mixture. The reduction of Y₂O₃ was not confirmed.     

Selection of Materials for Use at Temperatures above 1500°C in Oxidizing Atmospheres

The purpose of this work was to select materials for applications in oxidizing atmospheres at temperatures of a minimum of 1500°C. Several requirements had to be fulfilled, e.g. long-term oxidation resistance, low vapour pressure, high creep resistance and low gas permeability. Another important point was to avoid reactions and sintering in material combinations with silica forming materials. The investigation was based on a detailed literature screening, followed by compatibility tests at 1600°C in air with potential materials (Al₂O₃, ZrO₂, CeO₂, La₂O₃, Y₂O₃, HfO₂, MgO·Al₂O₃) and Mosi₂ as an example of a silica-forming material. The result of these experiments was, that only Y₂O₃ and HfO₂ did not show severe reactions in contact with MoSi₂. Since most of the materials available did not fulfill all the requirements, some new materials were investigated: CIPed MoSi₂ with different additives (SiC, ZrB₂) and recrystallized SiC (RSiC) with a polymer (polysiloxane) derived MoSi₂-filled coating. The oxidation behaviour of these materials was evaluated by continuous thermogravimetric measurements at 1500°C over a period of 100 h in air and by detailed postexperimental investigations.     

Preparation, sintering, microstructure, and thermal stability of Y2O3- and CeO2-doped tetragonal zirconia ceramics

Yttria- and ceria-doped tetragonal zirconia polycrystals ((Y, Ce)-TZP) with compositions 2·5 mol% YO_1·5-4 mol% CeO₂---ZrO₂, 4 mol% YO_1·5-4 mol% CeO₂---ZrO₂, and 2·5 mol% YO_1·5-5·5 mol% CeO₂---ZrO₂ were prepared from zirconia sols obtained hy hydrolysis of ZrOCl₂ solution, and their sintering, microstructure and thermal stability were studied. Sintered bodies with 99% TD were obtained by firing at 1400°C for 2 h in air. The grain size of (Y, Ce)-TZP increased with decreasing Y₂O₃ content in Y₂O₃---CeO₂---ZrO₂. (Y, Ce)-TZP was resistant to tetragonal-to-monoclinic (t → m) phase transformation during low temperature ageing as compared with 3Y-TZP.     

Mechanical properties of Al2O3 + ZrO2 + nano-SiCp ceramic composites

The mechanical properties of Al₂O₃ matrix composites reinforced by ZrO₂(2 mol% Y₂O₃) and nanometre scale SiC dispersions have been investigated. It is shown that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂(2 mol% Y₂O₃) and nano-SiC particles. The maximum flexural strength and fracture toughness of the composites are 945 MPa and 7.3 MPam^1/2, respectively. The reinforcing effect of both t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) and nano-SiC particles appears to be synergetic.     

Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

An attempt was made to prepare various F-doped β-, O-, X-, and -SiAlONs from a mixture of Si₃N₄, SiO₂, Al₂O₃, AlN, or Y₂O₃ using AlF₃ or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and -SiAlONs by X-ray diffraction. When AlF₃ was used, a single phase ceramic (O-SiAlON) was produced from a mixture of -Si₃N₄ and SiO₂ at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of -Si₃N₄, AlN, and Y₂O₃ with AlF₃ produced β-/Y--SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of -Si₃N₄ and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from -Si₃N₄ and SiO₂ at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y--SiAlON mixture showed the growth of O- and β-SiAlON and Y--SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.     

Corrosion of a Dense, Low-additive Si3N4 in High Temperature Combustion Gases

The oxidation/corrosion resistance of silicon nitride is determined by a number of factors, most notably the amount and chemical composition of the grain boundary phases produced as a result of adding oxides to promote densification during sintering/hot pressing. The intergranular material may be very active during high temperature exposure, producing a deleterious effect on the oxidation/corrosion processes. A method for producing silicon nitride with low levels of additives (only 0·59% Al₂O₃ and 1·22% Y₂O₃) and the benefits in terms of corrosion behaviour are described in this paper. The results of tests in combustion gases, where the concentration of sulphur in the fuel and contaminants in the ingested air have particular effects on the rate of corrosion, are reported.     

Effect of sintering additives on microstructures and mechanical properties of short-carbon-fiber-reinforced SiC composites prepared by precursor pyrolysis–hot pressing

Short-carbon-fiber-reinforced SiC composites were prepared by precursor pyrolysis–hot pressing with MgO–Al₂O₃–Y₂O₃ as sintering additives. The effects of the amount of sintering additives on microstructure and mechanical properties of the composites were investigated. The results showed that the composites could be densified at a relatively low temperature of 1800 °C via the liquid-phase sintering mechanism and the composite density and mechanical properties improved with the amount of additives. The amorphous interphase in the composites with more additive content, not only avoided the direct contact of the fibers with matrix, but also improved the fiber–matrix bonding. It proved that the fiber–matrix interphase characteristics played a key role in controlling mechanical properties of the composites.     

Conductivity of Na5+xYAlxSi4-xO12 ceramics

A series of ceramics samples, Na_5+xYAl_xSi_4-xO₁₂, has been prepared by a solid state reaction with the starting materials of SiO₂, Y₂O₃, Al₂O₃ and Na₂CO₃. Their crystalline structure and morphology have been studied by the determination of XRD, IR, TG, DTA and SEM. Their conductivity has been measured by means of the complex impedance method. The dependence of the conductivity and density of the samples on the amount of the added Al₂O₃ and the reaction between the conductivity and the temperature have been discussed. When x = 0, the density of the sintering sample is 90% T.D., and the conductivity is 1·48 x 10⁻¹ (ωcm)⁻¹ at 300°C; when x = 0·1, the density is up to 97% T.D., and the conductivity up to 1·74 x 10⁻¹ (ω cm)⁻¹ at 300°C.     

Oxidation behavior of hot-pressed Si3N4 with Re2O3 (Re=Y, Yb, Er, La)

Four different β-Si₃N₄ ceramics with silicon oxynitrides [Y₁₀(SiO₄)₆N₂, Yb₄Si₂N₂O₇, Er₂Si₃N₄O₃, and La₁₀(SiO₄)₆N₂, respectively] as secondary phases have been fabricated by hot-pressing the Si₃N₄–Re₄Si₂N₂O₇ (Re=Y, Yb, Er, and La) compositions at 1820°C for 2 h under a pressure of 25 MPa. The oxidation behavior of the hot-pressed ceramics was characterized and compared with that of the ceramics fabricated from Si₃N₄–Re₂Si₂O₇ compositions. All Si₃N₄ ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C and the oxidation products of the ceramics were SiO₂ and Re₂Si₂O₇. The Si₃N₄–Re₄Si₂N₂O₇ compositions showed inferior oxidation resistance to those from Si₃N₄–Re₂Si₂O₇ compositions, owing to the incompatibility of the secondary phases of those ceramics with SiO₂, the oxidation product of Si₃N₄. Si₃N₄ ceramics from a Si₃N₄–Er₄Si ₂N₂O₇ composition showed the best oxidation resistance of 0·198 mg cm⁻² after oxidation at 1400°C for 192 h in air among the compositions investigated herein.     

Features of corrosion resistance of AlN–SiC ceramics in air up to 1600°C

The kinetics and mechanism of (80% AlN + 20% SiC) and (50% AlN + 50% SiC) powders and ceramics oxidation in air up to 1600°C were studied with the aid of TG, DTA, XRD, EPMA, SEM and metallographic analysis methods. The ceramics samples were obtained by hot pressing fine-dispersion AlN and SiC powders with an average particle size of 1 μm at 1800°C for 2 h. This ensures a fine-grain material structure with a uniform distribution of phase components. It was shown that in a nonisothermal regime, a three-stage oxidation mechanism takes place. It was established that the scale formed consists of three oxide layers. In the inner layer, Al₁₀N₈O₂ oxynitride and β-SiO₂ (cristobalite) phases were observed; in the intermediate layer, β-SiAlON was found for samples with a relatively low SiC content whereas -Al₂O₃ was present in samples with a greater SiC content. The outer layer contains 3Al₂O₃.2SiO₂ (mullite) as a main phase, the latter ensuring highly protective properties of the scale. The materials investigated can be considered as having extremely high resistance to corrosion up to 1550°C.     

Thermodynamics of the Al–C–O system and properties of SiC–AlN–Al2OC composites

Based on a recent thermodynamic evaluation of the Al–C–O system, the standard Gibbs free energies of formation of both aluminium oxycarbides Al₄O₄C and Al₂OC are given, and a classical stability diagram is shown at 2100 K. Because Al₂OC is unstable below 1715 °C, the stable würtzite compound 2AlN.Al₂OC has been preferred, and formed in-situ as the second phase in SiC-based composites. Starting with commercial powders of -SiC, AlN, Al₂O₃ and Al₄C₃, dense materials are obtained by pressureless sintering (up to 2020 °C) or hot-pressing (up to 1950 °C), owing to the liquid phase from the Al₂O₃–Al₄C₃ system. The existence of a miscibility gap is shown, and the microstructures are fine grained and equiaxed. Compared with SiC–Al₂OC alloys, the hot-pressed materials with 90 wt% SiC exhibit slightly higher mechanical properties and a good retention nearly up to 1500 °C.     

Influence of chemical composition on corrosion of alumina in acids and caustic solutions

The corrosion behaviour of pure alumina, doped with different amounts of MgO, Y₂O₃, ZrO₂, Cr₂O₃, Co₃O₄ and BaO, was investigated. Corrosion media were concentrated HCl, HNO₃, HF, H₂SO₄/H₃PO₄ (1:1 mixture), HCl (1%) and NaOH (10%). The reaction conditions were 180°C (autoclave) for 168 h. The corroded specimens were investigated by SEM, the leaching solutions were analysed with ICP-OES (optical emission spectroscopy with inductively coupled plasma). The corrosion resistance was found to depend mainly on the microstructure and the composition of the grain boundary phase. Both parameters are influenced by the type and concentration of the additive, the method by which the sintering aid was added and the sintering conditions.     

Piezoelectric properties of Pb(Mn1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics with sintering aid of 2CaO–Fe2O3 compound

Since the electromechanical devices move towards enhanced power density, high mechanical quality factor (Q_m) and electromechanical coupling factor (k_p) are commonly needed for the high powered piezoelectric transformer with Q_m≥2000 and k_p=0.60. Although Pb(Mn_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMnN–PZ–PT) ceramic system has potential for piezoelectric transformer application, further improvements of Q_m and k_p are needed. Addition of 2CaO–Fe₂O₃ has been proved to have many beneficial effects on Pb(Zr,Ti)O₃ ceramics. Therefore, 2CaO–Fe₂O₃ is used as additive in order to improve the piezoelectric properties in this study. The piezoelectric properties, density and microstructures of 0.07Pb(Mn_1/3Nb_2/3)O₃–0.468PbZrO₃–0.462PbTiO₃ (PMnN–PZ–PT) piezoelectric ceramics with 2CaO–Fe₂O₃ additive sintered at 1100 and 1250 °C have been studied. When sintering temperature is 1250 °C, Q_m has the maximum 2150 with 0.3 wt.% 2CaO–Fe₂O₃ addition. The k_p more than 0.6 is observed for samples sintered at 1100 °C. The addition of 2CaO–Fe₂O₃ can significantly enhance the densification of PMnN–PZ–PT ceramics when the sintering temperature is 1250 °C. The grain growth occurred with the amount of 2CaO–Fe₂O₃ at both sintering temperatures.