Kinetics analysis of mullite formation reaction at high temperatures

A detailed kinetics analysis was performed on the mullite (3Al₂O₃·2SiO₂) formation reaction occurring at high temperatures ranging from ∼1600 to 1800°C. The counterdiffusion model of Al³⁺ and Si⁴⁺ fluxes was used to derive the kinetics equation of the mullite formation reaction. From the parabolic kinetics between the thickness of the mullite layer and time, the reaction rate constant (k) for the mullite formation was determined to be a function of the average diffusion coefficient of Si⁴⁺ ions. This kinetics equation can be used to estimate the mullite formation at any temperature. By substituting previous experimental data into the present kinetics equation, average diffusion coefficient values of Si⁴⁺ ions in the mullite layer were calculated and these values are in good agreement with the diffusion coefficient values calculated using Aksay’s interdiffusion equation for mullite formation. The activation energy values for the diffusion of the Si⁴⁺ ions were estimated to range from 730 to 780 kJ/mol, which are close to those obtained from previous diffusion and creep experiments.     

Effects of SiC interfacial coating on mechanical properties of carbon fiber needled felt reinforced sol-derived Al2O3 composites

3D carbon fiber needled felt and polycarbosilane-derived SiC coating were selected as reinforcement and interfacial coating, respectively, and the sol−impregnation−drying−heating (SIDH) route was used to fabricate C/Al₂O₃ composites. The effects of SiC interfacial coating on the mechanical properties, oxidation resistance and thermal shock resistance of C/Al₂O₃ composites were investigated. It is found that the fracture toughness of C/Al₂O₃ composites was remarkably superior to that of monolithic Al₂O₃ ceramics. The introduction of SiC interfacial coating obviously improved the strengths of C/Al₂O₃ composites although the fracture work diminished to some extent. Owing to the tight bonding between SiC coating and carbon fiber, the C/SiC/Al₂O₃ composites showed much better oxidation and thermal shock resistance over C/Al₂O₃ composites under static air.     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.     

Preparation and mechanical properties of Al2O3–15wt.%ZrO2 composites

Low-temperature-sinterable high purity α-alumina powder was mixed with Zr(OH)₄ gel synthesized by a precipitation method. The resulting gel mixture was calcined at 600 °C for 2 h. The Al₂O₃–15wt.%ZrO₂ composites were sintered for 2 h in air in the temperature range between 1350 and 1500 °C. Nearly full densification and the maximum bending strength of 932 MPa were achieved for the Al₂O₃–15wt.%ZrO₂ composites sintered at 1425 °C, whereas the highest fracture toughness of 8.5 MPa m^1/2 was obtained after sintering at 1475 °C.     

Effects of Al2O3 content on densification,microstructure, mechanical properties and oxidation resistance of TaC-Al2O3 composites

TaC-Al₂O₃ composites were prepared by hot pressing. Influence of Al₂O₃ content ranging from 10 to 40 vol. % on densification, phase composition, microstructure, mechanical properties and oxidation behavior of the TaC-Al₂O₃ composites was investigated. With 30 and 40 vol. % Al₂O₃ addition a closed porosity was achieved. The Al₂O₃ particles were uniformly distributed among TaC grains retarding grain growth and resulting in refined microstructures with grains below 2 μm in size. The most densified composite with 40 vol. % Al₂O₃ addition exhibited good mechanical properties with a Vickers’ hardness of 17.8 GPa, a flexural strength of 485 MPa and a fracture toughness of 5.4 MPa·m^1/2. After holding at 700°C for 3 h in air, the dense 30 and 40 vol. % Al₂O₃ compositions showed hardly noticeable and mainly surface oxidation, whereas less densified TaC-Al₂O₃ composites with 10 and 20 vol. % Al₂O₃ content and with open porosity were disintegrated to powders.     

Phase reaction and sintering behavior of a Al2O3–20wt%AlN–5wt%Y2O3 system

Two major problems exist in the processing of AlN. The first is the difficulty in achieving full densification even at relatively high sintering temperatures. The second is the formation of the spinel phase, AlON. Pure AlN sintered at temperatures up to 2000°C have produced no more than 90–93% densification in the former case, while AlN rich ternary systems (AlN–Al₂O₃-sintering agent) have resulted in the detrimental formation of AlON well before full densification can occur. This paper reports on the phase reaction and sintering behavior of a ternary Al₂O₃–AlN–Y₂O₃ system near the critical temperature range of 1600–1700°C, in a carbo-thermal reduction furnace in a fully nitrogen environment. Full densification (>98%) for AlN without the formation of AlON was achieved by sintering an initial Al₂O₃ rich ternary system (Al₂O₃–20wt%AlN–5wt% Y₂O₃) at a relatively low temperature of 1680°C. Formation of the AlON was delayed until 1700°C, at which a stoichiometric γ-AlON (Al₃O₃N) with spinel type structure was obtained. Thermal conductivity values for a sintered substrate obtained with low oxygen content within the AlN matrix reached 125 W m⁻¹ K⁻¹.     

Sintering and sliding wear studies of B4C-SiC composites

Dense boron carbide (B₄C) – silicon carbide (SiC) composites were obtained by spark plasma sintering technique at 1800°C with 3 wt% and 6 wt% aluminium oxide (Al₂O₃) additives. Addition of sintering additives results in formation of aluminium silicate (Al₂SiO₅) liquid phase which accelerates sintering kinetics and helps in obtaining high density ~ 99%. Microstructures reveal uniformly distributed SiC particles in B₄C matrix. Increase in alumina from 3 wt% to 6 wt% results in decrease in hardness from 35.1 ± 0.8 to 33.7 ± 0.9 GPa, and increase in fracture toughness from 5.9 ± 0.4 to 6.5 ± 0.4 MPam^0.5. Using a ball-on-disk tribo tester under dry unlubricated conditions at 5, 10 or 15 N load, influence of alumina content on friction and wear properties of B₄C-SiC composites was investigated against SiC counterbody with a linear speed of 0.08 m/s for 60 min. The coefficient of friction (COF) increased from 0.25 to 0.65 with load, and the influence of alumina on frictional behaviour appeared to be negligible. With increase in load, wear volume of the composites increased from 7.5 × 10⁻² mm³ to 16.1 × 10⁻² mm³ for B₄C-10 wt% SiC - 3 wt% Al₂O₃ and from 4.7 × 10⁻² mm³ to 14.8 × 10⁻² mm³ for B₄C-10 wt% SiC - 6 wt% Al₂O₃ composites. Microcracking, abrasion and pull-outs contributed as major wear mechanisms of composites in selected wear conditions. The relation between wear behaviour and mechanical properties of sintered composites is discussed.     

Aging behavior of Al2O3 short fiber reinforced Al-Cu alloy composites

Al2O3 short fiber reinforced AI-Cu composites containing 1%, 3%, 5% and 7% Cu were fabricated by a squeeze casting technique. The as-cast Al2O3/Al-Cu composites were solution treated at 535 ℃ and then aged at 170, 190 and 210 ℃, respectively. Age hardening behavior of the Al2O3/Al-Cu composites was analyzed by measuring the hardness of the samples at different aging temperatures and aging time. Microstructures of the composites were observed by transmission electron microscope（TEM）. The results indicate that the hardness of the Al2O3/Al-Cu composites containing 7% Cu is much higher than that containing 1%-5% Cu because of the large amount of CuAl2 precipitant in the Al2O3/Al-Cu composite. With the increase of Cu content from 1% to 7%, the time needed for the appearance of peak hardness shortened, indicating that the addition of Cu can accelerate the kinetic of CuAl2 precipitation in the Al2O3/Al-Cu composites. The Al2O3/Al-Cu composite containing 7% Cu shows the highest increment of hardness by aging treatment. Therefore, in order to get a higher peak hardness, the Al2O3/Al-Cu composites need more Cu addition as compared with the un-reinforced Al-Cu alloys.     

Transmission electron microscopy study of interfacial microstructure formed by reacting Al–Mg alloy with mullite at high temperature

Transmission electron microscopy (TEM) has been used to study the interfacial microstructure formed by reacting Al–Mg alloy with mullite (Al₆Si₂O₁₃) at high temperature (>900°C). The TEM study was used in order to understand the strong effect of Mg addition on the nature of the reaction between Al and mullite used to form Al/Al₂O₃ composite. After reaction at 1050°C, the formation of a layered structure between the Al–1% Mg alloy and mullite was observed. An alloy layer with a much higher concentration of Mg than the starting alloy was found present next to the initial mullite surface. Between the alloy layer and mullite, a dense and continuous layer made of small MgAl₂O₄ (spinel) and Si particles was present. The layer apparently stopped further reaction between Al–Mg alloy and mullite by preventing transport of the metals to the reaction front and the Si reaction product away from the reaction front. The microstructure resulting from the initial reaction indicated the reaction proceeded by replacing Si atoms with Al and Mg atoms on mullite {210} lattice planes and forming MgAl₂O₄ {311} lattice planes simultaneously.     

C/Al2O3复合材料的固体粒子冲蚀行为与磨擦磨损性能研究

以溶胶浸渍热处理技术路线制备的碳纤维布叠层缝合预制件增强Al₂O₃(C/Al₂O₃)复合材料为对象,以刚玉粉为介质,研究了复合材料的固体粒子冲蚀行为,按照GB5763-2008规定的条件研究了复合材料的磨擦磨损性能。室温下,复合材料冲蚀率随着冲击角度与送粉量的增大而增加;温度升高,由于机械冲击和热冲击的双重作用,冲蚀率显著变大。在GB5763-2008规定的条件下,C/Al₂O₃复合材料具有稳定的摩擦系数和很低的磨损率。结合微观形貌分析,探讨了复合材料的冲蚀与磨损机理。得益于连续碳纤维的补强增韧作用,即使基体致密度低于单体Al2O3陶瓷,C/Al₂O₃复合材料在冲蚀和磨损时不会发生脆性断裂,使用安全性优于单体Al₂O₃陶瓷。     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

In-situ sintering reaction of Al2O3–LaAl11O18–ZrO2 composite

Al₂O₃–LaAl₁₁O₁₈–ZrO₂ composites were prepared by in situ sintering reaction of different proportions of Al₂O₃ and La₂Zr₂O₇. The studied batches were uniaxially pressed and pressureless sintered at 1600 °C up to 1725 °C for 1 h. Phase composition study reveals that the only present phases are alumina, lanthanum hexaluminates and zirconia. No other intermediate phases are present. Rodlike LaAl₁₁O₁₈ was observed in the sintered bodies containing more than 25 wt.% LaAl₁₁O₁₈. The effect of rodlike particles on the densification and mechanical behavior was discussed. It was found that increasing the LaAl₁₁O₁₈ content more than 25 wt.% enhances the fracture toughness, but reduces both the bending strength and the hardness of the sintered composites.     

Mullite/molybdenum ceramic–metal composites

Dense (>98 th%) homogeneous mullite/Mo (32 vol.%) composites with two different Mo average grain sizes (1.4 and 3 μm) have been obtained at 1650°C in vacuum and in reducing condition. Depending on the Mo grain size and processing atmosphere, the K_IC ranges from 4 to 7 MPa m^1/2 and σ_f from 370 to 530 MPa. The MoO₂–2SiO₂·3Al₂O₃–Mo system was found to be compatible in solid state, and a solid solution of ≈4 wt% of MoO₂ in mullite at 1650°C was detected. A solid state dewetting of MoO₂ from the surface of the Mo particle takes place during sintering. It was found that the absence of MoO₂ in the mullite/Mo composites by processing in reducing conditions increases the strength of the metal/ceramic interface and the plasticity of the Mo metal particles, thus strengthening the composite by a crack bridging mechanism. As a result, the K_IC and the σ_f values of the ceramic–metal composite were found to be ≈4 times and ≈2 times higher than the ones corresponding to the mullite matrix.     

Reactive synthesis of alumina-boron nitride composites

The reactions of aluminum borates (9Al₂O₃ · 2B₂O₃ and 2Al₂O₃ · B₂O₃) with aluminum nitride (AlN) have been used as a new chemical route to synthesize alumina-boron nitride (Al₂O₃–BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200 °C and completed at around 1500 °C. Soaking at temperatures higher than 1800 °C resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al₂O₃ matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al₂O₃ network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young’s modulus and high strain tolerance.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Effects of heat treatment on phase contents and mechanical properties of infiltrated B4C/2024Al composites

The B₄C/2024Al composites were successfully produced by pressureless infiltration method, and the effects of heat treatment on phase content and mechanical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical properties testing. The results show that phases of B₄C/2024Al composites include B₄C, Al, Al₃BC, AlB₂ and Al₂Cu. The phase species remain unchanged; however, the phase content of the composites changes significantly after heat treatment at the temperature of 660, 700, 800 or 900 °C for 12, 24 or 36 h. It is found that the heat treatment results in not only considerable enhancement in hardness, but also reduction in bending strength of the composites. Heat treatment at 800 °C for 36 h does best to hardness of the composites, while at 700 °C for 36 h it is the most beneficial to their comprehensive mechanical properties.     

Formation of high-density TiN/Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> ceramic composites using preceramic polymer

The formation, microstructure and properties of high-density TiN/Ti₅Si₃ ceramic composites created by the pyrolysis of preceramic polymer with filler were investigated. Methylpolysiloxane was mixed with TiH₂ as filler and ceramic composites prepared by pyrolysis at 1200°C to 1600°C under N₂, Ar and vacuum were studied. When a specimen with 70 vol.% TiH₂ was pyrolyzed up to 1600°C in a vacuum after a preheat treatment at 850°C in a N₂ atmosphere and subsequently heat-treated at 1600°C for 1 h under Ar at a pressure of 2 MPa, a ceramic composite with full density was obtained. The microstructure of the ceramic composite was composed of TiN and Ti₅Si₃ phases. Under specific pyrolysis conditions, a ceramic composite with a density of 99.2 TD%, a Vickers hardness of 18 GPa, a fracture toughness of 3.5 MPam^1/2, a flexural strength of 270 MPa and a electrical conductivity of 6200 ohm⁻¹·cm⁻¹ was obtained.     

Microstructure and mechanical properties of Al2O3/Er3Al5O12/ZrO2 prepared by a high-frequency zone melting method

Oxide directionally solidified eutectic ceramics (DSECs) have excellent mechanical properties, oxidation resistance, and ablation resistance in an ultra-high temperature environment above 1600 °C. In this study, Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary DSECs were prepared by high-frequency induction zone melting. Their diameters were considerably greater than those of samples prepared with other processing techniques under the conditions where the microstructures were uniform. The component, microstructure, and mechanical properties in these samples were investigated. The results indicate that these DSECs were composed of only Al₂O₃, Er₃Al₅O₁₂, and ZrO₂ phases. As the solidification rate increased, both the eutectic phase size and eutectic spacing decreased continuously, and the microstructure was refined. The relationship between the eutectic spacing and solidification rate satisfied the formula λ²ν ≈ 33.8 ± 0.49. A polygonal colony structure appeared at a growth rate of 6.7 μm/s, which was associated with the uneven distribution of the temperature field. The hardness and indentation fracture resistance increased marginally with an increase in solidification rate, achieving 17.6 ± 0.8 GPa and 5.0 ± 0.5 MPa·m^1/2, respectively. The hardness of the Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary DSEC was 18.3% less than that of the Al₂O₃/Er₃Al₅O₁₂ binary DSEC owing to the reduced hardness of the ZrO₂ phase added in the eutectic composition. The ternary DSEC's indentation fracture resistance was 88.4% greater than that of the binary eutectic ceramic owing to the phase transformation toughening of the ZrO₂ phase.     

Al2O3/Mo composite and its tribological behavior against AISI201 stainless steel at elevated temperatures

Al₂O₃-reinforced molybdenum (Mo) composites were successfully prepared by powder metallurgy to improve the wear resistance of Mo components at high temperature. The reinforced Al₂O₃ particles are uniformly distributed in the Mo matrix; thus, the Al₂O₃/Mo composite is harder than monolithic Mo. The friction coefficients of both monolithic Mo and the Al₂O₃/Mo composite decrease by 37% and 42%, respectively, at 700 °C compared with those at room temperature (self-lubricating phenomenon). This phenomenon is attributed to the formation of very soft MoO₃ and FeMoO₄ metal oxides on the friction surface at high temperature. The Al₂O₃/Mo composite has better wear resistance than monolithic Mo at both room temperature and at 700 °C. The notable resistance of the composite particularly at 700 °C can be attributed to its increased hardness and the soft tribofilm forming on the worn surface.     

Über die Reaktionsfähigkeit von Al2O3 und NiAl2O4 in Chlor und Chlor-Sauerstoffgemischen bei höheren Temperaturen

Reactivity of Al₂O₃ and NiAl₂O₄ in chlorine and chlorine-oxygen mixtures at elevated temperatures To test the reaction inhibiting efficiency of Al₂O₃ or NiAl₂O₄ as a protective layer formed on Ni-Cr-Al alloys in chlorine at higher temperatures, for the present the rate of chlorination of Al₂O₃ and NiAl₂O₄ pellets has been investigated in Cl₂ and Cl₂-O₂-mixtures between 700 and 1000°C. The rate of chlorination of Al₂O₃ with 5,8 × 10^?2 mg/cm²h at 1000°C is significantly smaller than the corresponding rate of NiAl₂O₄ with 2,6 mg/cm²h at 800 °C. The rate of chlorination of Al₂O₃ is increased with increasing Cl₂ pressure and decreased with increasing O₂ pressure. As expected, for NiAl₂O₄ no oxygen pressure dependence of the chlorination rate was detected.