Structural Ceramic Materials Research Articles

A study of heat conduction in structural ceramic materials. Part III. Mathematical model of measuring cell

The process of heat propagation in a specimen is considered in an approximation of a one-dimensional heat flow with side leakages of heat. They are modelled as a function of the heat sources (sinks). The chosen stationary heating and the temperature field in the specimen are described by a nonlinear one-dimensional differential equation. The boundary conditions and the source function are determined from experimental data. The nonlinear one-dimensional differential equation is used in an implicit identification method and solved numerically; a minimum of the quality criterion is determined at each iteration step in the search procedure. The identification procedure is performed by explicit and implicit methods of solution of inverse problems of heat conduction. A numerical simulation has shown that the method of component-wise minimization is the most efficient.

Studies of heat conduction in structural ceramic materials. Part II. A study based of the solution of inverse problems of heat conduction

The process of energy transfer in refractory materials is studied under conditions of intense high-temperature heating. The coefficients of inverse problems of heat conduction (IPHC) are analyzed. The extremum coefficients of IPHC are determined and classified as explicit and implicit in accordance with the methods used for their estimation. Coefficient-type IPHC make it possible to bring the processing of results as close as possible to the measuring cell and to diminish the effect of errors of measurement on the solution of the problems and to increase simultaneously the efficiency of the experiments. A method for performing physical experiments, high-temperature heating sources, and measuring cells are described. The boundaries of the confidence interval of the instrumental error are shown. Results of single measurements of the thermal conductivity of MgO-base ceramics in an optical furnace are presented. Computation of the errors shows that the total relative instrumental error of measurement of the thermal conductivity is quite acceptable for high-temperature studies.

A Study of Heat Conduction in Structural Ceramic Materials. Part IV. The Method of Descriptive Identification of Heat Conduction

Methods of descriptive identification of heat conduction in structural ceramic materials are described. The method of descriptive regularization is applied in an algorithm of implicit identification. The optimization problem is predominantly solved for thermal conductivity as a function of the temperature in the convex part of the space of possible solutions. The main regularization criteria of the solution of the inverse optimization problem are the geometrical characteristics of the dependence of the thermal conductivity on the temperature, which is represented by a parabolic function. The position of the minimum of the dependence of the thermal conductivity on the temperature is determined by progressive displacement of the temperature range to the region of high temperatures. The results of the determination of the temperature dependence of the thermal conductivity were chosen under the assumption that the difference between the curves describing individual measurements is minimum. The results of the numerical solutions of the model problem show that the experimental data modeled in the computational process at a specified value of comparatively high intensity of normally distributed noise do not bear information about systematic errors. The computation is performed for various positions of temperature constraints and provides curves that are close to the exact solution of the problem.

A Study of Heat Conduction in Structural Ceramic Materials. Part V. Thermal Conductivity of Ceramics Based on MgO

The results of an experimental study are presented and the effectiveness of the earlier described methods is analyzed. The thermal conductivity of one and the same specimen is studied in a gas-dynamic bench and in an optical furnace in order to estimate the accuracy of the determination of systematic errors of the methods. The effect of various heat loads is determined. A comparison of the data shows that the algorithm of descriptive regularization makes it possible to reduce the effect of systematic errors on the thermal conductivity of a MgO material from 40 to 7% at the cold end and from 7 to 3% at the hot end. The residual scattering of individual curves does not exceed the range of the instrumental error of measurement, and the standard deviations reflect its value. The random nature of the residual scattering proves the earlier drawn conclusion that the effective thermal conductivity is independent of the density of the heat flow measured within 482 – 691 kW/m2 accurate to 15%.

A study of thermal conductivity of structural ceramic materials. Part I. State of research of thermal conductivity of structural materials

At the present time, experiment is a reliable method for studying the thermal conductivity of new ceramic materials and especially refractories. However, the range and possibilities of existing devices do not meet the requirements for measuring thermal conductivity, especially at a high temperature. At very high thermal loads under the conditions of formation of surface columnar crystal structures, thermoelastic stresses, disturbances in vibration of the elementary lattice, and other phenomena, the thermal conductivity can be a function of the temperature drop. The present paper concerns the physical fundamentals of heat conduction in current ceramic materials and refractories based on MgO, A12O3, and Si3N4. The classical stationary and nonstationary methods for determining thermal conductivity are considered. Special attention is devoted high-temperature processes and the difficulties arising in this case. It is recommended to solve high-temperature problems by using methods based on solving inverse problems of heat conduction

Increasing the Si3N4/1.25Cr-0.5Mo steel joint strength by using the method of drilling holes by laser in the surface layer of brazed Si3N4

Silicon nitride (Si3N4) ceramic is a kind of structural ceramic material with a combination of many superior properties, and has been used widely in industry. In most applications Si3N4 is used together with structural metals [1], so it is very important to obtain reliable Si3N4/metal joints. However, Si3N4 is one of the most difficult ceramics to join to any metal because its coefficient of thermal expansion is much lower than that of the important structural metals such as steels and nickel-base superalloys. This difference gives rise to the residual thermal stresses within the Si3N4/metal joints, and causes a very low joint strength. Recently a wide variety of methods using interlayers have been developed for ceramic/metal joints to reduce the residual stresses, e.g. composite interlayers [2], soft metal interlayers [3–5], laminated interlayers [3, 6, 7] and microcracked interlayers [8]. For the composite interlayers such as silicon nitride–metal mixtures, the strength and reliability of the interlayer itself is still a problem [9]. The soft metal interlayers such as aluminum, copper and nickel can relax the residual stresses by their elastic, plastic and creep deformation, but only a single layer cannot remove the residual stresses efficiently. Good effect of reducing the residual stresses can be obtained by using laminated interlayers (including multi-interlayer) with suitable structure and suitable thickness. But using the laminated interlayers itself signifies that not only a relatively complicated interlayer-machining procedure is required, but also the possibility of occurring joining defects in this chain system is increased when producing joints with a large area. For the method using microcracked interlayers, its application is very limited. Furthermore, the research on other methods without using interlayers to reduce the residual stresses within the ceramic/metal joints has rarely been reported. A new method to reduce the residual stresses and concurrently to strengthen the ceramic/metal interface, the method of drilling holes by laser in the surface layer of ceramics, is proposed in this letter. As a preliminary study, the factual effect of this method is examined, and the mechanism of increasing the ceramic/metal joint strength by this method is also discussed. The active brazing method was chosen for joining of ceramic to ceramic and metal. The ceramic and metal to be joined are hot-pressed sintered Si3N4

Elastic Properties of Porous Structural Ceramics Produced by Plasma-Spraying

ABSTRACTThis study focuses on the relationship between porosity and elastic stiffness of structural ceramic materials produced by plasma-spraying. Mg-AI-spinel was chosen as a model material and studied in as-sprayed condition as well as upon annealing to increasingly higher temperatures up to 1650°C. The detailed analysis of the stiffness and mass density evolution offers a deep insight into the initial microstructural state of the material and into the morphological changes that occur upon heat treatment.

Microstructural Design in Alumina-Alumina/Zirconia Layered Composites

Very recently several authors have pointed out the extremely important role of microstructural design in developing structural ceramic materials for long term high temperature applications. In this sense Raj has identified several boundary conditions: (1) Resistance to oxidation, (ii) Resistance to grain boundary sliding and cavitation, (iii) Good strength and toughness at room temperature. The aspiration is to eliminate grain boundaries which can act as cavitation sites, without using single crystals which typically exhibit low toughness. In this regard ceramics with single crystal-like morphologies, e.g., large elongated grains, with good fracture toughness and high bending strength have been proposed. One route to find these apparently contradictory characteristic is by building up layered microarchitectures where layers with high toughness and high bending strength coexist with layers with high creep resistance. These conditions can be met in the case of Al{sub 2}O{sub 3}/Al{sub 2}O{sub 3}-ZrO{sub 2} laminates. The present work was directed to the study of the microstructural features and properties of Al{sub 2}O{sub 3}/Al{sub 2}O{sub 3} + unstabilized ZrO{sub 2} and Al{sub 2}O{sub 3}/Al{sub 2}O{sub 3} + t-ZrO{sub 2} (3 mol% Y{sub 2}O{sub 3}) layered composites.

Several product types from a silica glass-based structural ceramic material

Some aspects in the manufacture of several types of products from a TsNIIM-developed silica glass-based structural material and its properties are discussed. The material is suitable for the fabrication of microwavetransparent inserts, electrical insulating plates and elements of intricate shape, refractories for induction metal heating and melting furnaces, toughening-furnace hearth segments, indicator plates, and extrusion-die crucibles for the fabrication of amorphous metal and alloy ribbons.

Properties and applications of some structural refractory ceramic materials based on nonmetal nitrides

New composite ceramic materials developed at the Central Materials Research Institute, including nitride-oxide composites based on hexagonal boron nitride and nitride composites based on silicon nitride are described. Some important technological aspects of the production of articles and parts from these materials are considered. Data on the properties and main applications of the composites are reported.

Microwave-assisted Acid Dissolution of Sintered Advanced Ceramics for Inductively Coupled Plasma Atomic Emission Spectrometry

The microwave-assisted acid dissolution of sintered bodies of 28 structural and electronic advanced ceramic materials was systematically evaluated. These materials included zirconia-based ceramics, such as m-ZrO2 (a non-stabilized monoclinic zirconia), Ca-PSZ and Mg-PSZ (two partially stabilized zirconias), Y-FSZ (a fully stabilized zirconia) and Ce-TZP, Yb-TZP and Y-TZP/Ce (three tetragonal polycrystalline zirconias); alumina-based ceramics, such as Al2O3, mullite and spinel; ceria-based ceramics, such as CeO2–Gd2O3 (cubic ceria gadolinia); titania-based ceramics, such as TiO2; titanate-based ceramics, such as Al2TiO5, BaTiO3 and BIT (bismuth titanate); lead titanate-based ceramics, such as Ca-PT, La-PT, Nd-PT, Sm-PT and Gd-PT; lead zirconate titanate-based ceramics, such as PZT and PLZT; niobate-based ceramics, such as PMN (lead magnesium niobate); non-oxide-based ceramics, such as AlN, BN, Si3N4 and SiC; and oxide and non-oxide-based ceramics, such as β′-sialon (silicon aluminium oxynitride). Fifteen acids or mixtures of acids were tried, including HCl, HNO3, H2SO4, aqua regia, H2SO4–(NH4)2SO4 and mixtures of these acids with HF and with H2O2. A commercially available laboratory medium pressure microwave oven was used. Eleven optimized microwave methods were developed. These methods are simple (three stages maximum), fast (15–35 min digestion time) and mild (20–60% of the microwave oven power). By applying these microwave methods, it was possible to dissolve completely all the sintered advanced ceramics, except SiC and β′-sialon. These two non-oxide ceramics were the only samples that could not be dissolved by any of the acids or mixtures of acids tested. The microwave-assisted acid dissolution was compared for ICP-AES with conventional dissolution procedures, i.e., alkali fusion in a platinum crucible and in a graphite crucible and acid decomposition by conductive heating at elevated pressure (in a PTFE bomb). It was demonstrated that microwave-assisted dissolution presents many advantages over the other procedures. When compared with acid decomposition by conductive heating in a PTFE bomb, one of the most important advantages is the drastic shortening of the digestion time from hours to minutes. When compared with alkali fusions, one of the most important advantages is the use of smaller amounts of high-purity acids, which contain less impurities than the fluxes; because of this, matrix effects and contamination from the attack reagents are lower, and consequently there is an improvement in the analytical figures of merit of ICP-AES.

Mechanical properties of structural ceramic materials based on silicon nitride

The creation of structural ceramic materials based on silicon nitride is considered. The results of a study of the properties of these materials in a wide temperature range are presented, including data on the short-term strength, elastic characteristics, and parameters of fracture mechanics and results of long-term static and dynamic tests of some silicon nitride materials.

The effect of grain boundary phase characteristics on the crack deflection behavior in a silicon nitride material

Silicon nitride is a promising structural ceramic material due to its inherent properties such as strength, thermal shock resistance and high temperature creep and oxidation resistance. Recent attempts to densify monolithic and composite materials without the use of additives have been very successful. Tanaka et al. densified different commercial silicon nitride materials by hot isostatic pressing without the use of additives, where the intrinsic SiO{sub 2} acted as the liquid phase sintering aid. Symons et al. have developed a laser-synthesized high purity amorphous Si{sub 2}N{sub 4} powder with a very fine particle size, and a controllable oxygen content. This powder was also densified without the use of additives. More recently, monolithic (not containing whiskers or fibers) silicon nitride materials have been developed with fracture toughness values as high as 10--12 Mpa.m{sup 0.5}. These are the so-called self- (or in-situ) reinforced materials reported by Pyzik et al. and Li and Yamanis. The reason for the extremely high fracture toughness values has been attributed to the presence of large, elongated {beta}-Si{sub 3}N{sub 4} grains, which activate various toughening mechanisms, such as crack deflection, crack bridging and grain pull-out. However, it was correctly pointed out that little is known about the effects ofmore » the grain boundary phase and its physical properties on the interfacial debonding/intergranular fracture behavior and fracture toughness in silicon nitride materials. This report is a brief description of a hypothesis developed to explain the large differences in fracture toughness between high purity (additive-free) Si{sub 3}N{sub 4} and conventionally sintered Si{sub 3}N{sub 4} materials.« less

Use of superhard nonmetallic materials for making machine parts and tools

Superhard nonmetallic structural materials, such as oxide, carbide, and nitride fine ceramics and zirconium dioxide and aluminum oxide (leucosapphire) single crystals, are currently actively replacing existing materials used in making machine parts and tools in machine building and other branches of industry. According to their set of unique physical and mechanical properties, the stability of dimensional characteristics, and disinclination to setting, superhard nonmetallic materials occupy an intermediate position between hard alloys and diamonds (while their price is relatively low). These materials are needed in making tools used in rolling extremely thin strip and foil, drawing fine wire of especially precise dimensions, and some other cases. Structural ceramics. The specific operating conditions of cutting, deforming, and guiding tools for machines and units led to the appearance of new structural ceramic materials with unique physical and mechanical properties that prevent adhesion of metal on the tool, permit bringing the surface of parts and tools to a mirrorlike state, and provide high wear resistance, the ability to operate without cooling under severe thermocyclic loads in oxidizing and aggressive environments, and stability of the dimensions of articles at various temperatures. Foreign experience testifies to the high efficiency of using ceramic materials to make cutting tools, rolls, drawing dies, and other tools. Thus, according to data of Hitachi Co. (Japan) rolls made of sialon are highly effective in cold rolling of corrosion-resistant steel: they provide the same surface quality as in buffing strip; in comparison with steel rolls they make it possible to roll thinner strip; the durability of sialon rolls is 20 times higher than that of steel ones. Application of such roils in warm rolling of thin tungsten strip eliminates adhesion of metal on the rolls, which makes their service life nearly 250 times longer than that of hard-alloy rolls.

Synthesis of partially stabilized zirconia by a sol-gel process

Synthesis of chemically homogeneous and agglomerate free powder of a potential structural ceramic material, partially stabilized zirconia (PSZ) was developed in this investigation. Although many routes for synthesizing PSZ are currently available the route adopted in the present study uses inexpensive starting materials as compared to other sol-gel routes. This study clearly showed that uniformly sized and chemically homogeneous powders can be produced by this route.

Evaluation of strength, toughness and subcritical crack extension of a commercial HP-Si3N4 exposed to a sulphidizing environment

Structural ceramic materials, in particular silicon nitride, have attracted interest for coal gasification technology at temperatures in excess of 1200 °C. In this study, a commercially available hot pressed silicon nitride is exposed for 200 h in a 0.4% H2S + 0.75% H2O + H2 (balance) atmosphere, at 1200 and 1300 °C, simulating coal gasification environments. The crack growth behaviour of the as-received material is compared to that of the exposed condition. Measurements are carried out on specimens with small natural flaws (dynamic strength tests) and on specimens containing long cracks (chevron notch), both tested in four point bending. Results of strength tests (Modulus of rupture (MOR) and Weibull modulus) as well as toughness in both conditions are also presented. In view of the significant changes in the high temperature mechanical properties caused by crystallization of the intergranular glassy phase at triple points, further investigations of the effect of controlled devitrification seem surely worthwhile.

NEUTRON RADIOGRAPHY FOR Si3N4FINE CERAMICS

The nondestructive testings for structural fine ceramic materials have been evaluated by film method. Thickness effects on the detectability of neutron radiography have been studied theoretically and experimentally. Both methods with and without contrast enhancer are studied by using Kyoto University Reactor facility. The experimental detectabilities obtained have been in good agreement with the theoretical analyses. By using contrast enhancer, the small defects on the surface of silicon nitride ceramics being near 15 microns in width and depth have been clearly detected. For the case of non-enhancer, 0.6% of the detectability has been obtained over the thickness of 30 to 40 mm for the silicon nitride ceramics

Designing ceramic components for structural applications

Despite significant advances in the properties of structural ceramic materials in recent years, ceramics are not being utilized to their fullest potential due to their inherent brittle nature and a general lack of understanding of how to design reliable ceramic components. The characteristics of ceramics are much different from those of metals, and the design process must account for those differences. Weibull statis-tical analysis, in conjunction with finite element analysis, provides a basis for designing reliable ceramic components.

Silicon carbonitrides - A novel class of materials

Silicon carbonitride monoliths derived from polymer powder compacts via polymer pyrolysis represent a new class of structural ceramic materials due to the complete coalescence of the powder particles during polymer decomposition and the formation of a uniform ceramic matrix free from any grain boundaries or secondary phases. The submicron pore channel system penetrating the material can be minimized in volume by infiltration of liquid polysilazane solution or by post-HIPing so that relative densities of 96% can be reached. The monoliths are oxidation resistant up to 1600°C in air due to the formation of a highly pure silica oxidation layer and they exhibit a creep rate of 1.10 -6 s -1 at 1650°C and 90 MPa load. Initially x-ray amorphous silicon carbonitride can be crystallized in nitrogen or in air to form Si 3 N 4 /SiC or Si 3 N 4 /SiC/C nanocomposites

Coordination compounds of aluminum as precursors to aluminum nitride

Two different systems have been examined as potential sources of aluminum nitride, an important electronic and structural ceramic material. Cyclic organoaluminum amides obtained as intermediates in the thermolysis of trialkylaluminum : ammonia Lewis acid-base complexes have been used to obtain AlN powder and as precursors for the chemical vapor deposition of AlN films. The structures of two of these intermediates were determined by single-crystal XRD and the kinetics and thermodynamics of their formation and thermal decomposition reactions were also studied. The second system employs ethylenediamine as the Lewis base in combination with the R 3Al (R = Me, Et) compounds. A 2:1 ratio of Et 3Al with en yields a hydrocarbon-soluble, polymeric amide on thermolysis, which can be used to prepare AlN films by solution coating followed by pyrolysis in NH 3. Lower proportions of R 3Al to en, on thermolysis, lead to the formation of R mAl(en-2H) n cluster species that contain 5- and 6-coordinated Al atoms chelated and bridged by en-2H ligands. On further heating, these cluster species apparently go on to form cross-linked, insoluble, polymeric networks through condensation reactions involving the multiple N-H and R-Al groups on the periphery of the cluster molecules. The structures of two of these novel Al-en cluster compounds were determined by single-crystal XRD.