Mechanical and thermal properties of Si–C–N material from polyvinylsilazane

Commercial polyvinylsilazane was crosslinked and then crushed to powder. The powder was compacted by cold isostatic pressing at 630 MPa and pyrolysed at 1050 °C in flowing argon. Crack-free Si–C–N material was obtained. Bulk density of the material was 1.95 Mg m–3. Open porosity was 9.6%. The material was amorphous as a result of X-ray diffraction analysis. Elastic modulus measured by pulse–echo method was 105 GPa. Vicker's hardness calculated from indentation at 98 MPa was 6.1 GPa. Fracture toughness measured by indentation fracture method was 2.1 MPa m1/2. Average bending strength was 118 MPa. The material shrank 1.9% in length during heating up to 1400 °C in nitrogen. The thermal expansion coefficient of the material heat treated up to 1400 °C increase from 3.08 × 10–6 °C–1 at 100 °C to 3.96 × 10–6 °C–1 at 1200 °C.     

Sinterability of magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 m; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.     

Production of Si3N4 by carbothermal reduction and nitridation of sepiolite

A study was undertaken to determine the optimum parameters for production of Si3N4 powders by carbothermal reduction and nitridation (CTRN) of Turkish sepiolite. Test samples were prepared by mixing 99% purity –325 mesh carbon black and –100 mesh leached brown sepiolite with C/SiO2 molar ratios of 1.5; 3; 4; 5, and 7.5. Prepared samples were subjected to the CTRN process at temperatures of 1200, 1300, 1400 and 1450 °C for 2, 4, 8, and 16 h. The CTRN process was conducted in an atmosphere controlled tube furnace in a nitrogen flow of 4.5 cm3/min. All products were examined by XRD and SEM-EDX to determine the transformation, morphology and chemical composition. The results showed that the best Si3N4 transformation occurred at 1400 °C for 16 h with C/SiO2 molar ratio of 4.     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Preparation of micro-coiled SiC and TiC fibres by vapour phase metallizing of micro-coiled carbon fibres

Micro-coiled fibres of SiC and TiC were prepared by the vapour phase metallizing of micro-coiled carbon fibres with full preservation of the coiling morphology of the source coiled carbon fibres, and their preparation conditions and bulk electrical resistivity were examined. The SiC_1,0 coils were obtained at 1400 °C for 2 h, and TiC_1.0 coils were obtained at 1100–1200 °C for 1.5 h. The bulk resistivity of the coiled TiC fibres sharply decreased with the bulk density and was 10^–2 S^–1 cm at 1.4 g cm^–3.     

Mechanochemical Synthesis of Magnesium Aluminate Spinel in Oxide-Hydroxide Systems

This work presents results of the mechanochemical pretreatments of binary mixtures of hydroxides and oxides of Mg and Al powders (activated in the energy-intensive planetary ball mill for 10 h). The effect of mechanical activation on the conversion to MA spinel and on the solid-state synthesis in the temperature range 600–1200°C has been investigated. Structural changes, as a consequence of milling, were followed by X-ray diffraction analysis and the degree of conversion to spinel was determined by chemical analysis, by leaching of the products of synthesis. The aim of the intensive milling of powdery mixture has been to achieve so-called "soft" mechanical synthesis or, at least, of a partial chemical reaction, and thus, to initiate the high-temperature solid-state synthesis of MA spinel at a lower temperature than 1400°C. Pure MA spinel was prepared by synthesis of these mechanochemically activated powders at a low temperature as 1000°C. The obtained results indicate that high-energy ball milling can effectively decrease the synthesis temperature.     

Synthesis of boron nitride from triammoniadecaborane and hydrazine under pressure

Boron nitride (BN) of low crystallinity was synthesized from triammoniadecaborane (TAD) and hydrazine at 125 MPa below 650° C. TAD itself was pyrolysed at 600° C and 125 MPa to form a mixture of amorphous boron and boron nitride containing BH and NH bonds. The infrared spectrum of the pyrolysed product of TAD itself at 600° C and 125 MPa showed the BNB absorption at 800cm^–1 due to the formation of B₃N₃ structures. The X-ray diffraction (XRD) of the reaction product from TAD and hydrazine at 600° C had broad diffractions centred at 2=25.5° and 43.0° (CuK). The BH absorption at 2500cm^–1 decreased in intensity on increasing the N/B ratio from 0.3 to 0.85, and disappeared finally at a ratio of N/B=1.3. The reaction product at 125 MPa had a porous structure. The electron diffraction of the specimen changed from faint rings to spots on circular rings after heat treatment at 800° C for 10 h. The heat-treated specimen, however, did not give sharp reflections corresponding to hexagonal BN in the XRD profile. BN of low crystallinity was transferred to cubic BN at 1200° C and 6.5 GPa in a 90% yield, which was higher than that of well-crystallized BN in the presence of AIN.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Effect of NiO and/or TiO2 mullite formation and microstructure from gels

Polymeric and colloidal gels with a constant molar ratio of (Al+Ni and/or Ti)/Si=3/1 and various (Al/Ni and/or Ti) ratios (up to 21.42 mol% NiO+TiO2) were prepared and used to study the effect of the precursor chemical homogeneity on mullite formation processes and the resulting microstructure. Both kinds of gel precursors were preheated at 750°C for 3 h in order to obtain appropriate gel-derived glasses for further thermal processing. After annealing for several time periods at temperatures between 750 and 1500°C, differences in crystallization pathways were observed. Polymeric gels crystallized Al–Si and NiAl2O4 spinels from the amorphous form at temperatures in the range between 900 and 1000°C, depending on the amount of aluminium substitution. Mullite formation was initiated at temperatures between 1100 and 1200°C, except for the higher substituted 3:2 mullite in which it was produced at 1000°C. In constrast, -Al2O3 and NiAl2O4 spinel were the first crystalline phases identified at 750°C in specimens from colloidal gels, whereas mullite was formed at temperatures higher than 1200°C. In specimens with high substitution, mullite was observed at lower temperatures. Although the sequences of reaction from either kind of gel were rather different, mainly at low temperatures (as could be inferred from the chemical homogeneity attained in both gel-derived glasses), the final set of crystalline phases after long annealing at 1400°C was quite similar. Differences in the microstructure of specimens from either type of gel precursor after annealing at 1400°C concerned the size of mullite particles and the presence of secondary phases in specimens derived from colloidal precursors. © 1998 Kluwer Academic Publishers     

Polysilacyclobutasilazanes: pre-ceramic polymers for the preparation of sintered silicon carbide monoliths

A family of pre-ceramic polymers based upon silacyclobutasilazanes was prepared. Upon heating to 200–250 °C the polymers crosslink to intractable resins through a ring-opening polymerization of the silacyclobutyl group. In an inert atmosphere the polymers convert to Si-C-N-O chars upon pyrolysis to 1200 °C. At higher temperatures (> 1400 °C) the Si-C-N-O chars loose nitrogen and carbon monoxide to give stable chars containing only silicon carbide and carbon. The polymers were used as binders in press- and-sinter and transfer-moulding applications to give silicon carbide monoliths with sintered densities above 3.13 g cm^–3 and four-point flexural strengths above 70 kpsi (483 MPa).     

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y₂O₃-ZrO₂ (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y₂O₃ stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y₂O₃ in the t-zirconia began to decrease as predicted by the Y₂O₃-ZrO₂ phase diagram. The c-ZrO₂ phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO₂, which has a higher thermal conductivity than tetragonal or cubic zirconia.     

Sliding wear of self-mated Al2O3-SiC whiskerreinforced composites at 23–1200°C

Microstructural changes occurring during sliding wear of self-mated Al₂O₃-SiC whiskerreinforced composites were studied using optical, scanning electron microscopy and transmission electron microscopy. Pin-on-disc specimens were slid in air at 2.7 m s^–1 sliding velocity under a 26.5 N load for 1 h. Wear tests were conducted at 23, 600, 800 and 1200°C. Mild wear with a wear factor of 2.4 x 10^–7–1.5 x 10^–6 mm³ N^–1 m^–1 was experienced at all test temperatures. The composite showed evidence of wear by fatigue mechanisms at 800°C and below. Tribochemical reaction (SiC oxidation and reaction of SiO₂ and Al₂O₃) leads to intergranular failure at 1200°C. Distinct microstructural differences existing at each test temperature are reported.Resident Research Associate at NASA Lewis Research Center.     

Conversion mechanism of polyborosilazane into silicon nitride-based ceramics

The pyrolysis of polyborosilazane in anhydrous ammonia has been studied up to 1200 °C through the analysis of the gas phase and the characterization of the solid residue by elemental analyses, thermogravimetric analysis, Fourier transform-infrared analysis, X-ray photoelectron spectroscopy, X-ray diffraction and ²⁹Si cross-polarization/magic angle spinning-nuclear magnetic resonance. The pyrolysis mechanism involves four main steps: (1) below 400 °C, an evaporation of residual solvent and reaction with ammonia leading to an increase of nitrogen content; (2) from 400–600 °C, reaction with ammonia leading to an increase of nitrogen content and formation of preceramic polymer-ceramics intermediate solid with a three-dimensional network along with evolution of gaseous species; (3) from 600–800 °C, completion of loss of C-H functionalities and progress of formation of an amorphous ceramic, accompanied by evolution of CH₄ and H₂; (4) from 800–1200 °C, completion of formation of an amorphous hydrogenated Si-B-O-N solid composed of SiN_4–x O_x(x=0,1,2,3) and BN_3–y O _y (y=0, 1), along with evolution of H₂     

Chemical vapour growth of HfP whiskers and their properties

Whiskers and ribbon-like single crystals of -HfP (hexagonal) have been prepared from HfCl₄+PCl₃+H₂+Ar gas mixtures at 1100–1200 °C using a metal impurity-activated chemical vapour deposition process. The growth conditions, morphology and chemical properties were examined. The 3.5–6.5 mm (average 4 mm) long HfP whiskers were obtained at 1200 °C using Si+Pt or Si+Pd mixed impurities. The HfP whiskers were very stable against oxidation up to 3 h exposure at 1000 °C and for 80 min immersion in concentrated HCl solution at 50 °C.     

Influence of annealing on properties and structure of alumina

Alumina bodies were prepared from pure alumina powder (98.9% Al₂O₃ consisting of 82% > 53m). The powder was compacted by hot-pressing at 1200° C, Compacted bodies were annealed at 1300, 1400 and 1500° C. Annealing continued at each maximum temperature for 25, 50 and 100 h. Strong bodies were obtained with maximum bulk density of 2.32 g cm^–3 and minimum apparent porosity of 30.21%. The change in sintering parameters with annealing was correlated with developed structure.     

Influence of TiC additions on the oxidation behaviour of Si3N4-based ceramics

The oxidation behaviour in air of Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids, with and without a dispersion of TiC particles, has been studied between 1200 and 1400 °C. The influence of TiC additions on the oxidation kinetics is discussed in comparison with the results obtained for the Si3N4–Y2O3–Al2O3 reference material. However, in all cases and within the experimental temperature range, the oxidation kinetics were observed to be of a parabolic type. The microstructure of the oxide scales formed, which has been characterized by scanning and transmission electron microscopy, was observed to depend on the oxidation temperature and the initial composition of the material. © 1998 Chapman & Hall     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.     

Phosphor-doped BaTiO3: Microstructure development and dielectric properties

The phosphor cation retards BaTiO₃ formation during the first reaction stage, and favours Ba₂TiO₄ formation. As a consequence, the end of the reaction is only accomplished at 1150 °C. The average particle size of the synthesized powder, after attrition milling was <1.0 m. Isopressed bars were sintered from 1200–1400 °C. Dilatometric measurements showed the existence of two sintering mechanisms; these were confirmed by means of isothermal sintering experiments. Microstructural development at low sintering temperature differs from that corresponding to higher ones. Very high permittivity values were measured in the 1325–1350 °C sintered samples. This anomalous dielectric behaviour is associated with the presence of phosphor.     

Magnetic resonance characterization of solid-state intermediates in the generation of ceramics by pyrolysis of hydridopolysilazane

Chemical intermediates produced from the pyrolysis of hydridopolysilazane (HPZ) were studied in the solid state by multinuclear nuclear magnetic resonance and electron spin resonance. When pyrolysed at temperatures of 1200°C, uncured HPZ forms a ceramic material with a composition of Si_2.2N_2.2C_1.0. A series of HPZ-derived ceramics was produced using a number of different heat-treatment temperatures, varying between 300 and 1200°C. Solid-state magnetic resonance data generated from this set of HPZ-derived ceramics elucidate important features of this complex transformation. Silicon atoms initially exist in two types of sites in the polymer,Si(Me)₃ and ()₃SiH sites. Upon pyrolysis between 300 and 400°C, the silazane cyclizes and cross-links, forming an intractable, insoluble solid. Increasing the pyrolysis temperature to between 400 and 600°C creates a matrix that is partially inorganic; at heat-treatment temperatures in this range, many of the C-H bonds of the starting polymer are cleaved. Elevating the heat-treatment temperature to between 600 and 1200°C generates a series of chemical structures with silicon in a tetrahedral site of the general form SiN_4–xC_x, where x=0, 1, 2, 3, 4. No crystalline forms of Si₃N₄ or SiC were detected in the material prepared at even the highest heat-treatment temperature of 1200 °C.