Showing papers in "Journal of The European Ceramic Society in 1990"

Boron carbide ― a comprehensive review

Abstract: Boron carbide, which has a high melting point, outstanding hardness, good mechanical properties, low specific weight, great resistance to chemical agents and high neutron absorption cross-section (10BxC, x>4) is currently used in high-technology industries—fast-breeders, lightweight armors and high-temperature thermoelectric conversion. The contents of this review are: (1) introduction; (2) preparations—industrial preparative routes, powders, sintering (additives, pressureless, hot pressing, HIP); laboratory methods of synthesis (CVD, PVD, plasma, crystal growth); (3) analytical characterization; (4) phase diagram—a peritectic, nearly pure boron, and a wide phase homogeneity range (B4C-B10·5C); (5) rhombohedral crystal structure—a comprehensive model of the whole solid solution is proposed; (6) chemical properties; (7) physical properties—density, mechanical (strength, hardness, toughness) and thermo-electrical properties; (8) main industrial applications; (9) conclusion.

The toughening of alumina with nickel inclusions

Abstract: Brittle solids can be toughened by the introduction of ductile metallic inclusions. In the present study, the mechanical properties and oxidation resistance of Al2O3/Ni composites are investigated. The oxidation resistance of the ceramic/metal composite (8 × 10−11 g2 cm−4 s−1 at 1300° C) is comparable to that of many silicon nitrides. The fracture toughness of the composite containing 13 vol.% nickel is twice that of alumina alone. The square of the toughness enhancement for composites containing various amounts of nickel exhibits a linear relationship with the product of volume fraction and inclusion size, as predicted in theoretical models. For the alumina/nickel composite system, it is demonstrated that dissolved oxygen in the nickel increases the yield strength of nickel and enhances the toughness of the composite.

Crack velocity functions and thresholds in brittle solids

Abstract: A unifying treatment of environment-sensitive crack velocity functions for intrinsically brittle solids is presented. The formalism is soundly based on the concept of thermal activation barriers, but is phenomenological in that it does not attempt to identify the explicit underlying physical and chemical processes responsible for these barriers. Equations prescribing the v-G (crack velocity versus mechanical energy release rate) characteristics at specified chemical concentrations (partial pressures) and temperatures are thereby presented. These equations incorporate the familiar three velocity regions into a composite function: region I, chemically assisted fluctuations over stress-enhanced energy barriers; region III, similar but in the absence of environmental species; region II, a connecting flow-limited transport branch. In addition, the equations include provision for healing and repropagation branches in unloading-reloading cycles. Central to the argument is the assertion that zero-velocity thresholds are quiescent configurations, defined by appropriate Dupre work of adhesion terms, W. These W terms serve as reference baselines for the entire v-G function, such that changes in chemical concentration (relative humidity) or interface type (virgin versus healed) may be considered in terms of simple curve shifts along the G axis. Data for selected brittle solids, principally mica but also glass and sapphire, in moist environments are used to illustrate the formalism.

Wear mechanisms of ceramic cutting tools when machining ferrous and non-ferrous alloys

Abstract: Three ceramic cutting tool grades CC 670 (based on alumina and silicon carbide whiskers) and CC 680 (based on silicon nitride) were used for the evaluation of tool life and wear mechanisms when machining a heat-resistant alloy, Inconel 718. Tool life for both grades was determined by flank wear and depth-of-cut (DOC) notch wear except at high feed rates. Flank wear was determined by chemical interaction with the workpiece material for both cutting tool materials tested. DOC notch wear was mainly determined by seizure and pull-out of tool material fragments. Comparative tests were also performed on steel SS 2541 (similar to AISI 4340) with grade CC 670 where both flank and crater wear were found to limit tool life. The main reason for the rather good wear resistance of silicon carbide reinforced alumina in Inconel 718 in contrast to its behaviour in steel was the formation of magnesia-based protective coatings.

Joining of ceramics demonstrated by the example of SiC/Ti

Abstract: The reaction behaviour of titanium sputtered on silicon carbide was investigated in a series of tests. It was ascertained that, in the temperature range between 1250 and 1500° C, mainly the ternary phase Ti3SiC2 is formed (>90%). In joining experiments of silicon carbide with 1–3 μm thick titanium layers at 1450° C at compaction pressures between 5 and 30 MPa the formation of the ternary phase leads to a high joining strength. The achieved bending strength (σM ≈ 286 MPa; m ≈ 10) is comparable to that of the starting materials (σM ≈ 303 MPa; m ≈ 15).

Coprecipitation of yttrium and aluminium hydroxide for preparation of yttrium aluminium garnet

Abstract: Coprecipitation of yttrium and aluminium hydroxide for the preparation of pure yttrium aluminium garnet (YAG) powder with small grain size is the subject of this study. Starting materials are sulphates and chlorides of yttrium and aluminium. To obtain pure YAG (Y3Al5O12), the pH during flocculation of the precursor must be chosen carefully. The presence of water increases the degree of agglomeration. To minimize agglomeration, the influence of dispersion liquids has been studied, leading to optimized conditions for precipitation.

On the microhardness of silicon nitride and sialon ceramics

Abstract: Influences of relative density, grain size, Young's modulus, flexural strength and fracture toughness on microhardness characteristics of hot-pressed silicon nitride, sintered silicon nitride, reaction-sintered sialon and liquid-phase sintered sialon have been discussed. Three new semi-empirical equations have been proposed to correlate microhardness to relative density. Indentation size effects on microhardness measurement have also been discussed.

Densification and thermal conductivity of low-sintering-temperature AlN materials

Abstract: The aim of the study was the sintering of aluminium nitride at rather low temperatures (≤1650°C). Various sintering aids were studied, in particular AlSi- and Ca-based oxides. CaCO3 and 3CaO-3SiO2-Al2O3 are the most efficient additives, because quantities as low as 0·5 wt% are enough to yield sintered AlN substrates with no open porosity. TEM observations showed that low concentrations of CaCO3 or CaSiO3 lead to location of the secondary phases at the triple points, whereas high concentrations lead to wetting of the grains. Thermal diffusivity of sintered materials was determined by a laser flash method. The highest conductivity (70 W m−1 K−1) is obtained with CaCO3 additions. These additions promote liquid formation, which cleans the surface of the AlN grains from oxygen and thereby prevents oxygen from entering the AlN lattice. The CaAl2O4 compound exhibits a lower thermal conductivity than the CaAl4O7 and Ca3Al10O18 compounds. The Ca2SiAl2O7 compound and the 27R AlN polytype have a strong negative influence on the thermal conductivity of sintered materials.

Microstructure and phase development in NiMn2O4 spinel ceramics during isothermal sintering

Abstract: Studies of the sintering behaviour of NiMn2O4 semiconducting ceramics at 1100°C under oxygen atmosphere show the advantage of a powder prepared by thermal decomposition of oxalate mixed crystals NiMn2 (C2O4)3. 6H2O at 450°C for the formation of dense ceramics with homogeneous microstructure. The microstructure of the semiconductor ceramics was established by image analysis. SEM and EDX indicate a phase separation related to partial oxygen loss as observed by thermogravimetry and redox analytic measurements. Reoxidation to a single-phase homogeneous microstructure by annealing at 800°C is possible only in porous samples. NiMn2O4 is not stable in oxygen at 1100°C. The spinel decomposes into NiO and a Mn-rich spinel matrix NixIIMn1−xIIMn2IIIO4. For producing a reproducible semiconducting ceramics it is necessary to stimulate sintering by separation of NiO. A one-phase spinel is obtained by reoxidation of long duration.

The influence of different oxides on the formation of Si2N2O from SiO2 and Si3N4

Abstract: Si2N2O powder has been produced by reacting SiO2 and Si3N4 in the presence of a liquid phase obtained by the addition of Al2O3, Y2O3, and MgO. The phase changes occurring during the reaction have been quantified by X-ray diffraction. The reaction has been studied at different temperatures and periods of time. The grain size of the starting materials has a large influence on the conversion to Si2N2O, and, by using a fine-grained SiO2, a substantial conversion to Si2N2O was obtained within 1 h at 1500°C for the Y2O3-containing material.

Structure and ionic conductivity of solid solutions in the system 0•9{(ZrO2)1−x−(CeO2)x}−0•1(Y2O3)

Abstract: Solid solutions in the ternary system ZrO 2 -CeO 2 -Y 2 O 3 , by virtue of their high-temperature stability and good electrical conductivity, have potential application in many advanced technologies such as the magnetohydrodynamic (MHD) generator. The investigations reported here include studies on the structure, electrical conductivity and oxygen ion transport number of solid solutions in the system 0·9{(ZrO 2 ) 1 − x -(CeO 2 ) x }-0·1(Y 2 O 3 ) in the temperature range 1000–1600 K in static air. It is found that electrical conduction proceeds by transport of oxygen ions through the network of oxygen vacancies in the lattice. The variation of conductivity with composition has been analysed in relation to the changes in lattice parameter and scattering of oxygen ions.

Creep deformation of polycrystalline mullite

Abstract: Creep tests for polycrystalline mullites (3Al 2 O 3 .2SiO 2 ; average grain sizes, d =1·4 and 2·1 μm) were carried out in air at temperatures between 1365 and 1480°C. The dependence of steady-state strain rates, ϵ, on stress and grain size was given by ϵασ/d 2·5 at lower temperatures ( −1 ). The strain rate apparently increased at higher stresses where fracture often occurred during creep testing within small beam deflections. The SEM examination of fractured specimens revealed two types of fracture mode: slow crack growth pattern observed on the fracture surface of specimens with the small grain size, and cavitation at grain boundaries (intergranular separation) observed on the tensile surface of specimens with the larger grain size.

Fatigue of silicon nitride ceramics under cyclic loading

Abstract: Cyclic fatigue behavior of normally sintered and reaction-bonded silicon nitride materials was investigated from the microstructural point of view. It has been found that in any silicon nitride material the crack propagates by cyclic loading even under the condition of K max ISCC , and that surface upheavals (microscopic lifting of the surface) and different type of cracks are produced in the region near the cyclically propagating cracks. It is supposed that asperity-contacts of crack surfaces are closely related to the fatigue damage behavior.

Cyclic fatigue behavior of Si3N4 ceramics

Abstract: Tension-compression fatigue tests and alternating cantilever bending fatigue tests were carried out to clarify the fatigue behavior of sintered silicon nitride (Si 3 N 4 ). Cyclic fatigue at room temperature depended mainly on the number of cycles accumulated rather than on time. An alternating load accelerated the fatigue process much more than a pulsating load. The modified Goodman line agreed well with the results of the fatigue test at room temperture. The design methodology proposed for metal fatigue would be applicable to the sintered Si 3 N 4 used in this study. At high temperatures, however, the cyclic fatigue depended on frequency. At high temperatures, the lower the frequency, the less the fatigue resistance. A failure diagram was constructed to show effects of mean stress and stress amplitude on the fatigue strength. An equation expressing allowable stress under cyclic loading was proposed. Microstructural observations suggest that fatigue of sintered Si 3 N 4 at room temperature is caused by microcracks at the grain boundaries. The progress of cyclic fatigue at high temperatures is caused by slow crack growth and creep deformation.

Deposition of ZnO films on polycrystalline alumina substrates by spray pyrolysis

Abstract: Zinc oxide films have been grown on polycrystalline alumina substrates by a spray pyrolysis method. Film growth was carried out between 200 and 500°C. The concentration of the starting solution ranged between 0·03 and 0·5 mol/litre. The effect of temperature and concentration on the film morphology, growth rate, homogeneity and crystallographic orientation is discussed. The experimental conditions for a good substrate covering are described.

Sialon ceramics made with mixtures of Y2O3Nd2O3 as sintering aids

Abstract: Sialon ceramics have been prepared by pressureless sintering at 1775 and 1825°C, using mixtures of Y 2 O 3 Nd 2 O 3 as sintering aids. It was found that at 1775°C less dense materials were obtained, but by raising the sintering temperature to 1825°C fully dense materials could be produced, even when pure neodymia was used. The hardness of the Nd-sialon ceramics was found to be slightly lower than for the corresponding Y-sialon ceramics, but the fracture toughness was approximately the same. It was also observed that the amount of intergranular phase increased when yttria was replaced by neodymia.

Preparation of sialons by the nitrided pressureless sintering (NPS) technique

Abstract: The nitrided pressureless sintering (NPS) technique has been used to prepare sialon compositions in the O′-β′ field, along the Si 3 N 4 -Al 2 O 3 join. Samples were fully nitrided in less than 4h at 1370°C, conditional upon achieving sufficiently high surface areas during milling. TGA and XRD analyses indicate that the nitridation mechanism involves the early formation of yttrium silicate phases during the prenitridation step which are in turn consumed below the nitridation temperature, probably in favour of a partial Si 3 N 4 -SiO 2 -Y 2 O 3 -Al 2 O 3 liquid phase. Yttrium aluminium garnet (YAG), whose volume fraction increases with time at temperature and with increasing level of Al addition, is formed promptly under these nitridation conditions. Sintering of the fully nitrided materials in nitrogen was carried out in the temperature range 1500–1830°C, with specimens achieving maximum density by 1600°C. XRD analysis reveals that the sole product of sintering low alumina compositions was β′-sialon.

Thermodynamic calculations of Si-C-O fiber stability in ceramic matrix composites

Abstract: Thermochemical fiber degradation reactions of multiphase Si-C-O fibers in inert (Ar) and oxidic (air) atmosphere as well as single-phase C and SiC fibers in various carbide, nitride and oxide matrices were calculated in the temperature range 1000–2000° C. The calculations indicate a greater stability of the single-phase SiC as compared to the C fiber in all matrices. The C fiber is fairly stable in AlN and Al 2 O 3 matrix. The silicon carbide fiber is stable in AlN and Si 3 N 4 as well as in Al 2 O 3 and SiO 2 . Above critical temperatures metal carbides are formed at the fiber-matrix interface. An attempt was made to define boundary conditions for the fabrication of fiber-reinforced ceramic matrix composites.

Weakest-link failure predictions for ceramics using finite element post-processing

Abstract: Weakest-link failure prediction models according to Weibull, Stanley and Lamon have been implemented in a post-processing program supplementing a finite element modelling code. Failure predictions for equibiaxial loading from uniaxial bending test data reveal significant differences between the various formulations. The choice of a particular failure model for a particular material therefore has to be validated by additional information, e.g. by fractography or micrography.

Synthesis of high purity AlN by nitridation of Li-doped Al-melt

Abstract: The isothermal nitridation of 2·3 wt% Li-doped Al-melt at 1273 K was investigated. Li promotes complete conversion of the Al to AlN and acts as a ‘getter’ reducing the oxygen impurities in the nitrogen atmosphere by forming LiAlO2 and Li2O. Thus, highpurity AlN powder could be obtained, containing low oxygen contamination (0·80 ± 0·02 wt%).

Sintering and electrical properties of CaO-doped Y2O3

Abstract: The sinterability and electrical properties of CaO-doped Y2O3 were studied as a function of the CaO concentration. Density increased with the CaO concentration with the maximum density being observed in the sample containing 1 mol% CaO. The electrical conductivity of CaO-doped Y2O3 varied according to the amount of the water vapor and oxygen pressures. Transport numbers of the electron hole, proton and oxygen were calculated from electromotive force measurements. These results confirmed that a decrease in electrical conductivity as the water vapor pressure increases is attributed to the decrease in hole conductivity.

Wear behaviour of partially stabilized zirconia at high sliding speed

Abstract: The wear behaviour of three partially stabilized zirconias (PSZs) against steel was investigated on a pin-on-disk wear machine under dry conditions (from 5 m/s to 50 m/s, at 5 N load). In order to maintain plane-on-plane contact, the ceramic pins (dia. = 5 mm) were ground on the wear machine before the wear tests. Particular behaviour of PSZ ceramics was found with extremely heavy wear only in a special range of sliding speed between 10 m/s and 20 m/s and much lower wear was found outside this range. The heavy wear was accompanied by a phase transformation (tetragonal-cubic), which was identified by analysing the wear debris by means of XRD. Delamination mechanism was observed by analysing the worn surfaces of the pin and the wear debris with SEM attached to an EDAX system. The wear resistance of Mg-PSZ was generally better than that of Y-PSZ under the present conditions, especially at high sliding speeds (> 20 m/s).

Evaluation of subcritical crack extension under constant loading

Abstract: The determination of subcritical crack growth data is necessary for lifetime predictions of ceramic components. In this context the problem of measuring subcritical crack growth with natural cracks and macro-cracks is considered. It can be stated that the crack growth law for small natural cracks cannot be derived from measurements carried out with macro-cracks. For the determination of the v-K I relation with specimens containing natural flaws different methods are applied. Especially, a lifetime method developed by the authors allows the determination of crack growth rates down to 1 × 10 −12 ms −1 , Resulting v-K I curves are reported for hot-pressed silicon nitride, Al 2 O 3 and glass.

Thermal expansion and equation of state for ceramic injection moulding suspensions

Abstract: In order to model the solidification of ceramic suspensions in the injection moulding operation, linear and cubical thermal expansion coefficients for the suspensions were obtained, and the equation of state was derived. The relevance of the law of mixtures to the prediction of these properties was investigated. The accuracy with which group contribution theory can predict the thermal properties of the organic vehicle was evaluated. Such predictions permit the numerical modelling of solidification to be executed with the minimal experimentation for diverse suspensions.

Ultrasonic testing for flaw detection in ceramics

Abstract: An advanced high frequency ultrasonic imaging system has been developed for detecting fine flaws in ceramics. This system consists of a computer for data and image processing, a high precision xyz-scanner with an immersion transducer, a high frequency (5–150 MHz) pulser, a receiver, and a peak detector. By using the imaging system with high frequency from 50 to 100 MHz, a 10-μm tungsten wire embedded 1·5 mm under the surface in SiC ceramic is detected. Some image processing techniques are used to enhance the ultrasonic image and detect fine flaws with low reflection echo levels.

Susceptibility to agglomeration of fine PLZT powders prepared from nitrate solutions

Abstract: PLZT powder which has various degrees of agglomeration was prepared from an aqueous nitrate solution by three hydrolysis methods. They were fine, chemically homogeneous and amorphous. Their particle morphologies, however, were very different due to the different physico-chemical precipitation conditions. Addition of hydrogen peroxide, and a liquid-gas interface reaction were the most effective methods for producing a weakly agglomerated powder by hydrolysis.

Multiaxial strength and stress rupture of hot pressed silicon nitride

Abstract: The equibiaxial static fatigue resistance of hot pressed silicon nitride was measured at high temperature. A concentric ring scheme was used to load disks at up to 1300°C in air. Equibiaxial lifetimes were substantially shorter than uniaxial lifetimes, primarily a consequence of a drastically reduced biaxial strength. The static fatigue trends were similar in uniaxial and biaxial loading. The ring-on-ring scheme is not suitable for stress rupture testing if creep deformations are present or cracks grow large without going critical.

Sensitivity of SnO2 semiconductor gas sensors to CO in the presence of high concentrations of water vapour

Abstract: Amongst future uses of semiconductor gas sensors, one important application could be the measurement of emission gases for the control of combustion processes. In such measurements, water vapour is always present as a factor that can cause interference. Combustion gases, depending on the fuel, can contain up to 20% by volume of water vapour. Therefore, it is important to elucidate the influence exercised by water vapour on the sensor. In this study, the sensitivities of SnO 2 -based gas sensors to CO and water vapour were investigated in the laboratory. The concentration of water vapour was varied in the range of 0 to 19·9% by volume. The sensors tested were thick film sensors prepared in the laboratory as well as commercial TGS 812 sensors. Water vapour and CO were found to have a synergistic effect on the conductance of TGS sensor but not on the conductance of the thick film sensor. Due to the absence of the synergistic effect between CO and water vapour the thick film sensor is much less sensitive to the variation of the concentration of water vapour in the range of a few percent to 20% by volume than the TGS 812 sensor.

Preparation studies of amorphous Al2TiO5

Abstract: An amorphous form of Al2TiO5 containing only ∼0·5% of volatile constituents is prepared by the sol-gel route by hydrolysis of Al(OCH2CH2OC2H5)3/Ti(OCH2CH2OC2H5)4 mixtures in 2-ethoxy-ethanol followed by evaporation of the solvent and stepwise heating up to 650°C. Only a small part is transformed into the crystalline state on annealing at 650°C, which is far below the formation temperature of Al2TiO5 (1280°C). In contrast to amorphous Mg2TiO4, decomposition takes place on annealing at low temperature. At 700°C, TiO2 (rutile) is formed, followed by sudden crystallization of α-Al2O3 at ∼930°C, measured by differential scanning calorimetry. Al2TiO5 and Mg2TiO4 are characterized by an endothermic formation enthalpy, indicating the role of the entropy term, which appears to be positive. Reaction sintering takes place more effectively starting from amorphous Al2TiO5.

Surface segregation in ceramic materials during cooling or under a temperature gradient

Abstract: This paper concerns the dynamic segregation of impurities in iono-covalent semiconducting compounds subject to large temperature changes. Simultaneously to the temperature gradient a chemical potential gradient takes place in these compounds. From the expression of the flux of mobile species (vacancies and cations in the considered materials) it has been shown that segregation (or precipitation) effects must be observed near surfaces. These effects are directly related to the mobilities of the different cations. The predicted effects are consistent with segregation layer thicknesses found experimentally in ceramic materials after cooling. This analysis confirms the fact that in ceramic materials the dynamic segregation effects near surfaces during cooling must be taken into account in the interpretation of segregation results observed at room temperature. These changes in composition near surfaces can have important technological consequences, for example in ceramic powder preparation, and consequently in powder sintering or in the aging of ceramics subject to large temperature changes at high temperature.