Low-temperature joining of SiC ceramics using NITE phase with Al2O3-Ho2O3 additive

In order to avoid the property degradation resulting from high-temperature joining process, nano-infiltrated transient eutectoid (NITE) phase with the Al₂O₃-Ho₂O₃ as the joining adhesives was adopted to join silicon carbide (SiC) ceramics with the attempts to lower down the joining temperature. The liquid-phase-sintered silicon carbide (LPS-SiC) specimens were joined at 1500-1800°C by spark plasma sintering (SPS) under the pressure of 20 MPa. The results of the shear test and microstructure observation showed that the joining process could be finished at a relatively lower temperature (1700°C) compared to other NITE-phase joining. In contrast to the shear strength of 186.4 MPa derived from the SiC substrate, the joint exhibited the shear strength of 157.8 MPa with the fully densified interlayer.     

Gelcasting of silicon carbide ceramics using phenolic resin and furfuryl alcohol as the gel former

A new non-aqueous gelcasting system of phenolic resin and furfuryl alcohol combined with a curing catalyst was developed for casting of reaction bonded silicon carbide ceramics. This gelling system could be carried out in air, and the surface exfoliation phenomenon that seems inherent to the acrylamide gelcasting system could also be eliminated. Polymerization of the premix solutions and rheological properties of the non-aqueous silicon carbide suspensions were studied. After curing and subsequent pyrolysis of the concentrated silicon carbide suspension, homogenous silicon carbide/carbon green body with a relatively high strength of about 18 MPa could be formed. Dense complex-shaped SiC ceramic parts with flexure strength of 300±20 MPa and fracture toughness of 3.87±0.19 MPa m^1/2 can be successfully produced after reaction sintering at 1700 °C for 30 min under vacuum.     

Water sorption properties of hardened cement paste cured or stored at elevated temperatures

Structural changes take place when mature hardened cement paste cured at room temperature is heated. BET V_m values were found to decrease as a function of the time the specimens had been stored in water either at 60°C or at 100°C. The decrease is greatest at 100°C. The complete water adsorption isotherms show that heating also makes the pore structure more coarse. The development of BET surface areas for pastes cured near 100°C depend strongly on the temperature history before the curing temperature is reached.     

Creep and Stress–Strain Behavior after Creep for SiC Fiber Reinforced, Melt-Infiltrated SiC Matrix Composites

Silicon carbide fiber (Hi-Nicalon Type S, Nippon Carbon) reinforced silicon carbide matrix composites containing melt-infiltrated silicon were subjected to creep at 1315°C at three different stress conditions. For the specimens that did not rupture after 100 h of tensile creep, fast-fracture experiments were performed immediately following the creep test at the creep temperature (1315°C) or after cooling to room temperature. All specimens demonstrated excellent creep resistance and compared well to the creep behavior published in the literature on similar composite systems. Tensile results on the after-creep specimens showed that the matrix cracking stress actually increased, which is attributed to stress redistribution between composite constituents during tensile creep.     

Preparation of carbon material with SiC-concentration gradient by silicon impregnation and its oxidation behaviour

A carbon material with a SiC-concentration gradient was prepared by a silicon impregnation process. In order to prepare the carbon material with a SiC-concentration gradient without any adhesion of metallic silicon, the weight of silicon powder per physical surface area of the carbon substrate was in the range of 3·9-4·1 g cm⁻² and then heating the carbon substrate in silicon powder at 1450°C for 3 h, but the amount of silicon powder beyond this range resulted in the adhesion of metallic silicon. The concentration of SiC along the thickness in the sample obtained decreased rapidly up to 0·5 mm and gradually in a range of 0·5-1·0 mm. A remarkable improvement in the oxidation resistance was observed on the sample with a SiC-concentration gradient at 1400°C, which was due to the formation of a protective film of SiO₂ glass on the surface. In the results of the thermal shock test in the sample, no destructions and no cracking of the samples were observed.     

Strength Distribution in Commercial Silicon Carbide Materials

Four types of commercial silicon carbide samples from different sources were characterized in terms of baseline strength and strength distribution (reliability). Four-point flexural strength of each material was determined on 30 test bars, 5.1 by 0.64 by 0.32 cm, for a reliable estimate of the Weibull modulus values. The results show that the average strength of these sintered silicon carbide samples ranged from 380 to 482 MPa (55 to 69 ksi) at room temperature and 307 to 470 MPa (45 to 68 ksi) at 1370°C (2500°F). Considerable variations in strength were found among specimens of each material. Baseline Weibull modulus values ranged from 8 to 11 at room temperature and 7 to 11 at 1370°C (2500°F). The strength scatter clearly reflected flaw variability, which must be minimized to improve reliability in sintered silicon carbide materials.     

Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: I. Experimental study

The uniaxial tensile and creep behaviour of an alumina fibre-reinforced silicon carbide composite is studied. The damage mechanisms during tensile loading are identified on the basis of the elastic response and in-situ morphological analysis. Tensile tests show that the composite presents a pseudoductile behaviour due to matrix microcracking and fibre-matrix debonding. Temperature induces changes in the tensile behaviour because of variations in load transfer conditions and in the axial residual stress borne by the fibres and the matrix. The creep curves at 1100°C under vacuum present an extended tertiary part, especially at low creep stress. The unloading-reloading loops periodically performed during creep show a progressive decrease in longitudinal stiffness. Progressive interface debonding during creep is invoked to explain: (i) the strain rate increase during tertiary creep, (ii) the decrease of the elastic modulus and (iii) the large fibre pull-out observed on the creep fracture surface. The different creep rupture modes at low and high stresses are related to the capability of the remaining intact fibres to support the overload failure of the first fibres.     

Potential of SiC as a heat exchanger material in combined cycle plant

The short and long term mechanical properties of a sintered silicon carbide intended as a heat exchanger material have been investigated. The short term strength shows an acceptable scatter characterised by a Weibull modulus of seven from room temperature up to 1400°C. In the time-dependent regime failure occurs by subcritical crack growth from surface located inherent defects at high stresses. Below a threshold stress oxidation blunting of these surface defects occurs and causes a transition from subritical crack growth to diffusion creep as life-limiting mechanism. Unlike other ceramics, the threshold stress for subcritical crack growth falls within the low probability range of fast fracture. Failure mechanism maps presenting the life-limiting mechanisms of the investigated sintered silicon carbide over a range of stresses and temperatures are presented.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

The influence of reaction parameters on the free Si and C contents in the synthesis of nano-sized SiC

Uniform nano-sized beta-silicon carbide (β-SiC) powder was synthesized from the reaction of silicon (Si) and carbon black (C). Mixed Si and C-black powder were pressed into pellets and the influence of four parameters, temperature (1250, 1300 and 1350 °C), heating rate (20 and 50 °C/min), soaking time (1 and 3 h) and atmosphere (vacuum and argon), were tested. It was found that higher temperatures, higher heating rates and longer soaking times in a vacuum system lead to lower free Si content in the SiC powder created. Temperature was the parameter with the greatest influence on the Si content of the SiC powder. This study also found that the Si–C reaction occurs through gas–solid (SiO–C) and solid–solid (Si–C) reactions that occur simultaneously.     

Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite

This paper deals with the microstructure and mechanical properties of reaction bonded silicon carbide reinforced with random chopped carbon fibers of 3 mm length. The composites were fabricated by dispersing chopped carbon fibers into bimodal SiC/C suspension, forming green body through slip casting, and then reaction sintering at 1700 °C. The effect of the chopped fiber fraction on microstructure and mechanical properties was evaluated. A significant increase of fracture toughness was obtained as the carbon fiber fraction approaches 30 vol.%. The chopped fibers had reacted with liquid silicon during reaction sintering, so little fiber pullout was observed. Crack deflection and bridging is the predominant mechanism for the composite toughening.     

Dense silicon carbonitride ceramics by pyrolysis of cross-linked and warm pressed polysilazane powders

This study reports on the pyrolysis and densifaction behavior of cross-linked poly(hydridomethylsilazane) powders. The influence of the cross-linking procedure such as temperature and annealing time of the polymer powders on the compaction behavior under cold and warm pressing conditions is discussed. The degree of cross-linking is determined by thermal mechanical analysis (TMA). In addition to particle sliding which is assumed to be the compaction mechanism obtained by cold-pressing, the polymer powder consolidates by plastic deformation applying warm-pressing. A continuous 3-dimensional polysilazane network is formed after a dwelling time under these conditions. Pyrolysis of the cross-linked and compacted polysilazane powder in argon at 1100°C gives crack-free amorphous silicon carbonitride Si_3+xC_x+yN₄ with compositions ranging from x=1·47 and y=0·88 for cold pressed samples to x=1·47 and y=1·86 for warm pressed materials. The residual open porosity is significantly reduced from 10–15 vol% in the cold pressed specimens to 1·3–5 vol% by the warm pressing procedure. The weight loss during pyrolysis between room temperature and 1300°C is about 5 wt% lower than that for cold pressed specimens. This result is explained by a reduced methane evolution during the polymer-to-ceramic conversion and is in accordance with the enhanced carbon content of the warm pressed material.     

Degradation of PZT-4D hard piezoceramics under moderate heating

The changes produced in the dielectric permittivity, the transverse piezoelectric coefficient, the k₃₁ coupling factor, the s₁₁^E elastic compliance and the mechanical quality factor of PZT-4D hard piezoceramics by heating have been investigated. The ceramics were then repoled, and the reversible and irreversible components of the changes quantified. The results showed that depolarisation began at 150°C. A high level of poling was retained even after heating at 300°C (d₃₁=−83×10⁻¹² C N⁻¹ and k₃₁=0.225), only 20°C below the transition temperature. However, a significant irreversible degradation of the mechanical quality factor, Q_m, occurred at a temperature as low as 100°C. Experiments on thinned specimens showed that the degradation of Q_m took place in the Ag doped layer produced by the electrodes. Indentation surface cracks were also introduced into the ceramics to investigate the behaviour of cracks during the thermal treatments.     

Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: II. Modelling of tertiary creep

The tertiary creep of an alumina fibre-reinforced silicon carbide composite is modelled on the basis of the damage mechanisms activated during tensile creep tests carried out under vacuum at 1100°C. Progressive fibre-matrix debonding induced by the difference between the radial creep strain of the fibres and that of the matrix mantle is used to explain the axial creep behaviour. Fibre failure and the subsequent stress redistribution are also taken into account. The modelling approach successfully describes: (i) the time evolution of the creep rate, (ii) the decrease of the elastic modulus, (iii) the failure mode after tertiary creep and (iv) the stress dependence of the creep rate in the secondary stage and of the time to rupture of the composite. It is shown that a conventional creep stress exponent cannot be determined in this and similar composite materials because of the stress dependence of the damage accumulated in the composite before the secondary stage is reached.     

High Temperature Deformation of Precursor-derived Amorphous Si–B–C–N Ceramics

The deformation of amorphous Si–B–C–N ceramics derived from polymeric boron-containing polysilylcarbodi-imide was investigated at temperatures between 1500 and 1700°C in nitrogen atmosphere. The composition of the as-thermolysed ceramic was determined to be SiB_0·41±0·11C_3·39±0·44N_2·42±0·31O_0·22±0·14 and no remarkable change was observed after testing at 1600°C in nitrogen atmosphere. Under uniaxial compression a true strain of more than 30% was achieved without visible damage. By compression at constant strain rate a maximum densification of 26% was achieved. Besides densification non-Newtonian viscous flow was observed with an apparent activation energy of 0·8 MJ mol⁻¹ and an apparent activation volume between 2·5 and 0·2 nm³. A free volume in the same order of a few cubic nanometers was assumed. At 1600°C the amorphous ceramic starts to crystallize with a volume fraction of crystalline phase less than 10% after several hours. The apparent increase in stress during deformation was explained by changes in sample size, porosity and intrinsic structure due to a decrease in free volume.     

Fatigue fracture behaviour of PEEK: 2. Effects of thickness and temperature

Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly(ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100°C. In all the thickness experiments and in the experiments with temperatures of 39 and 50°C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a ‘brittle’ manner. In all other experiments both ‘brittle’ and ‘ductile’ modes of fracture are observed. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.     

Effect of the substrate temperature on the physical characteristics of amorphous carbon films deposited by d.c. magnetron sputtering

The growth rate, composition, electrical resistivity, mass density, refractive index and microstructure of amorphous carbon (a-C) films prepared by direct current (d.c.) magnetron sputtering were investigated as functions of the substrate temperature (50–350°C). The hydrogen content determined by elastic recoil detection analysis (ERDA) and the electrical resistivity of films were found to be dependent on both the base pressure in the deposition chamber and substrate temperature. For films deposited below 200°C, the hydrogen content was less than 2 at.% and the substrate temperature was the only parameter which affected their electrical resistivity. The electrical resistivity decreased from 0.2 to 0.03 Ωcm as the substrate temperature increased from 50 to 200°C. The mass density of films evaluated from Rutherford backscattering (RBS) data and film thickness decreased from 2.2 to 1.4 g cm⁻³ with increasing substrate temperature. A linear relationship between the refractive index and the mass density of a-C films was clearly established. From the optical measurements, the decrease in mass density was correlated to an increase in porosity of films with increasing substrate temperature. The decrease in electrical resistivity with increasing substrate temperature was attributed to a graphitization of a-C films. This modification of the microstructure of a-C films as the deposition temperature was varied from 50 to 350°C was observed by examination of the cross-section of samples by transmission electron microscopy and Raman spectroscopic analyses of a-C films.     

Thermal evolution and crystallisation of polydimethylsiloxane–zirconia nanocomposites prepared by the sol–gel method

Polydimethylsiloxane–zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in different molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO₂ content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400°C and the structural evolution of samples treated at different temperatures has been followed by X-ray diffraction, transmission electron microscopy, infrared and ²⁹Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N₂ sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600°C of high surface area materials with pore sizes below 3 nm. Samples are amorphous up to 800°C. At 1000°C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO₂ content. At 1400°C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO₂ phases are present with ZrO₂ average crystallite dimensions never exceeding 20 nm, for ZrO₂ content ≤50 mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. ©     

Rejuvenation and physical ageing of a polycarbonate film subjected to finite tensile strains

Differential storage and loss tensile moduli, E′ and E″, were determined intermittently at 10 Hz on specimens of an annealed polycarbonate film during stress relaxation at static tensile strains from 1.2 to 6.25% at 50°C. It was found that E′ and 1/E″ decrease when a static strain is applied but thereafter they increase progressively with time. These changes, which increase with the applied strain until it becomes 4%, are attributed primarily to a rejuvenation of a specimen (an increase in segmental mobility) followed by physical ageing (a progressive decrease in segmental mobility). Measurements at a static strain of 3% at six temperatures from 30°C to 130°C showed, among other things, that the rates of increase of E′ and decrease of E″ are sensibly independent of temperature up to 110°C.     

Influence of temperature on the structure of SiC coatings prepared by dynamic ion mixing

We deposited silicon carbide films, 0.5 μm and 0.86 μm thick at room temperature (RT) and 750 °C on (100) silicon wafers and TA6V substrates. An SiC target was sputtered with a 1.2 keV Ar⁺ ion beam delivered by a Kaufman-type ion source, and the growing films were continuously bombarded with a beam of 160 keV Ar⁺ ions. The microstructural state of the films was investigated by complementary techniques: transmission electron microscopy (TEM), high-resolution TEM, glancing X-ray diffraction (GXRD) and Fourier transform infrared spectroscopy (FTIR). All these characterization methods show that the bombardment of the growing films induces important structural changes. The SiC films prepared at RT without mixing are amorphous, whereas those deposited by dynamic ion mixing (DIM) at RT exhibit the beginning of crystallization of the β-SiC phase. At 750 °C the films prepared by DIM are formed of nanocrystallized grains of the cubic β-SiC phase.