Symposium CF
High and Ultra High Temperature Ceramics for Extreme Environments


Session CF-1 - Synthesis and Processing

CF-1:IL01  Advances in Ultra-High Temperature Ceramic Research at University of Arizona: Investigating Oxidation Resistant UHTCs Using Relevant Testing Facilities and Direct Current Assisted Processing, Joining, and Physical Properties of Large Scale Complex Shape UHTC Parts
E. CORRAL, Materials Science and Engineering Department, Arizona Materials Laboratory, University of Arizona, Tucson, AZ USA

Ultra-high temperature ceramics (UHTCs) are ideal candidates for non-ablative thermal protection system materials and high temperature application engine components due to their high refractory nature and high thermal conductivity. This presentation will focus on a range of research investigations focused on understanding the fundamental relationships between processing science and physical properties of UHTCS in extreme environments. Three areas of research will be highlighted that range from precursor chemical synthesis of UHTC particles for application as coatings to carbon-carbon composites, to large scale near net shape processing of diborides using direct current sintering and rapid joining. Specifically, self protective high temperature oxidation resistant UHTC filled carbon-carbon composites show high temperature oxidation resistance using dynamic non-equilibrium thermal gravimetric methods; densification using direct current sintering shows densification behavior as a function of current density and high strength mechanical properties of high purity diboride composites; bulk UHTCs tested for oxidation resistance using multiple high temperature testing methods such as DNE-TGA and oxyacetylene torch testing show excellent ablation resistance under oxygen rich environments; rapid direct current joining of borides, carbides, and nitrides show seamless joint microstructures with retained mechanical strength at the joint; and large scale near net shape processing of UHTCs using direct current sintering can be uniformly processed.

CF-1:IL02  Amorphization, Field Activated Sintering and Superplastic Forming of UHTCs
H. KIMURA, Department of Mechanical Engineering, School of Systems Engineering, National Defense Academy, Yokosuka, Kanagawa, Japan

The solid state amorphous and nanocrystalline synthesis is now becoming one of the most fruitful ways of material, processing and function innovation in the technological field of ultra-high temperature ceramics (UHTCs)(1-3). Mechanically driven amorphization occurs in a variety of covalent typed ceramics such as SiC, B4C, HfC, ZrB2 and binary B4C・SiC, when using reaction ball milling that provides judicious selection of process variables and parameters on the basis of thermodynamics and kinetics. Pulsed electric current sintering and millimeter wave pressure sintering of amorphous UHTCs make it possible to obtain rapid densification during non-Newtonian flow and viscous flow enhanced under electric field at ultralow temperatures. Then, fully dense amorphous B4C・SiC shows high-speed superplastic forging with compressibility of 0.75 around 1400 K. This presentation describes a comprehensive approach to novel phenomenon involved in amorphous UHTCs with the special attention of quantitative description, including some recent topics such as high-pressure compaction at an ambient temperature.
(1)H. Kimura: "Integrated Materials System for Bulk Nanocrystalline Ceramics" Proc. of PRICM4, 163-166(2001).
(2)H. Kimura: "Mechanically Driven Amorphization and Bulk Nanocrystalline Synthesis of UHTCs", Ceramic Transactions, 203, 119-130(2009).
(3)H. Kimura: "Amorphous and Nanocrystalline UHTCs -Materials Design and Solid State Synthesis-", AeroMat 2011 (ASM Int.), Conf. recording (2011).

CF-1:IL03  Bringing Modelling to UHTCs
A.I. DUFF, T. DAVIES, B. LEE, M. FINNIS, Imperial College London, UK; A. GLENSK, B. GRABOWSKI, Max Planck Institut fuer Eisenforschung, Germany

An understanding of UHTCs behaviour at very high temperatures is being pursued by many groups using experimental (e.g. arc jet) testing which is empirically difficult and hard to reproduce and control. As a result, modelling approaches are also being pursued. Recently, fully anharmonic DFT calculations (prohibitively expensive up until now) have become computationally amenable. Methods such as the UP-TILD approach, for example, which efficiently sample the atomic configuration space and up-sample only for a select number of configurations, have enabled a great increase in computational efficiency. This presentation will describe use of this approach to calculate the thermodynamic properties of ZrC and ZrB2 at high temperatures. Defect formation free energies for ZrC will also be presented, along with a description of the methodological development necessary to enable such calculations. Equilibrium vacancy concentrations at high temperatures will also be computed by means of highly accurate interatomic potentials fitted to vacancy-vacancy interactions within ZrC.

CF-1:IL04  Nonoxide High-melting Point Compounds as Materials for Extreme Conditions
S. ORDANIAN, Saint-Petersburg State Technology Institute, Technical University, Saint-Petersburg, Russia

Nonoxide high-melting compounds such as transition metal carbides, borides, nitrides, and silicides, and covalent-bonded B4C, SiC, Si3N4, AlN - are the base of many materials for high temperature applications including wear, aggressive, impact and radiation conditions. Author with coworkers have studied the interaction between compounds from various classes, such as MedC-Med', MedC-MedB2, MedN-Med', MedN-MedB2, MedN-Med'B2, MedC-SiC, MedB2-SiC, MedSi2-MedB2, MedB2-LnB6, MedB2-B4C, MedSi2-SiC, LnB6-B4C, LnB6-SiC, LnB6-W2B5, MedC-W2B5, WSi2(MoSi2)-MedB2, WSi2(MoSi2)-MedC etc. (over 160 phase diagrams), ternary B4C-SiC-MedB2, SiC-TiB2-TiC and other eutectics, which is important for optimizing the sintering temperature, material design and tailoring of properties.
A vast identified group of eutectics with component number n ≥ 3 to 6 has reduced Тeut. (> 600 oC). Larger number of component phases suppresses grain growth, which is particularly important for developing nanostructured ceramics via pressureless sintering and for controlling the ceramics' performance.
Multiphase ceramics (TiC-TiB2-SiC, B4C-SiC-MedB2, B4C-W2B5-MedB2, B4C-LnB6-MedB2, etc.) feature improved mechanical parameters and high wear and impact resistance; these materials were developed at VIRIAL Ltd. (Russia).

CF-1:IL05  Processing and Properties of UHTC Composites
J. BINNER1, A. PAUL1, S. VENUGOPAL1, PENXIANG ZHENG2, B. VAIDHYANATHAN2, P. BROWN3, 1University of Birmingham, UK; 2Loughborough University, UK; 3 Defence Science and Technology Laboratory (DSTL), UK

Ultra-high temperature ceramic (UHTC) materials are required for future hypersonic vehicle applications. It is widely accepted in the UHTC community that the major disadvantages with monolithic UHTCs are their poor defect tolerance and thermal shock resistance. In order to overcome this and to meet the oxidation resistance at extreme temperatures, UHTC composites are being prepared using carbon fibre (Cf) preforms and a variety of UHTC powder combinations via a slurry impregnation route. This presentation will present the latest results in terms of the composition of the UHTC powders used and their subsequent performance when tested using an ultra-high temperature oxyacetylene flame at temperatures of around 2700°C.

CF-1:IL06  Processing and Sintering of Fiber-containing UHTCs

Ultra high temperature ceramics (UHTC) include borides and carbides of early transition metals and are presently considered a class of promising materials for several applications, the most appealing ones being in the aerospace and energy sectors. Crucial issues for application in severe environment is the improvement of fracture toughness and thermal shock resistance.
Incorporation of SiC or C fibers in UHTC matrices is currently adopted to overcome these problems and effects on densification, microstructure and thermo-mechanical properties are studied.
In this talk, processing and sintering are first shown for short SiC of C fibers incorporation, considering different kinds of commercial products and different starting compositions. On the basis of fundamental studies on boride/fiber interaction, the best systems are selected for subsequent processing of continuous SiC or C fibers - ZrB2 composites. Reactivity at the fiber/matrix interface is one of the most critical issues for this kind of composites, either in case of short or long fiber use due to high temperatures involved in the sintering stage. Preliminary characterization and oxidation studies are shown.

CF-1:L07  A novel Field Assisted Sintering Technique for Ultra-high Temperature Ceramics
E. ZAPATA-SOLVAS, Materials Science Institute of Seville, CSIC-University of Seville, Seville, Spain; D. GÓMEZ-GARCÍA, A. DOMÍNGUEZ-RODRÍGUEZ, Dpt. Condensed Matter Physics, University of Seville, Seville, Spain; R.I. TODD, Dpt. Materials, University of Oxford, Oxford, UK

Sintering is the process whereby a pre-shaped ceramic green body is compacted at high temperature. This process could be significantly enhanced by external agents, such as an electromagnetic field or mechanical load. Field Assisted Sintering Techniques (FAST) have been intensively investigated during the last 10-15 years for sintering a wide variety of ceramic systems. Rapid heating and cooling rates, lower sintering temperatures and shorter dwelling times in addition to better microstructure control compared to other conventional techniques, such as pressureless sintering or hot press, are responsible for the growth in popularity of FAST among the scientific community. The most well-known FAST for sintering ceramics are Spark Plasma Sintering (SPS) and microwave sintering. In this work, a novel FAST has been developed and used for sintering ultra-high temperature ceramics (UHTCs) such as ZrB2. The main advantages of this novel FAST are shorter processing times and better energy efficiency compared to SPS. Results for ZrB2, MoSi2 and ZrB2/20 vol. % MoSi2 sintering using this novel FAST will be discussed and compared to SPS data in terms of sintering cycle, densification mechanisms, microstructure after sintering and energy efficiency, among others.

CF-1:L08  Dual Composite Architectures for Toughening of ZrB2-MoSi2 UHTC Composites Produced by Polymer Co-extrusion
R.J. GROHSMEYER, G.E. HILMAS, W.G. FAHRENHOLTZ, Missouri University of Science and Technology, Rolla, MO, USA; A. D'ANGIO, F. MONTEVERDE, CNR-ISTEC, Faenza, Ravenna, Italy

Dual composite (DC) architectures have been shown to increase fracture toughness while maintaining hardness in cemented carbides designed for room temperature applications. The goal of this research is to study fundamental microstructure-processing-property relationships in DC ceramics composed of zirconium diboride (ZrB2) sintered with molybdenum disilicide (MoSi2) additions, specifically to increase toughness at high temperatures. DC ceramic architectures were produced with mm-sized granules of ZrB2 with an engineered quantity of MoSi2, uniformly dispersed in a ZrB2 matrix containing a different volume fraction of MoSi2 and a different ZrB2 particle size, with MoSi2 quantity and ZrB2 particle size chosen to optimize the mechanical property relationship of the individual ZrB2-MoSi2 composites in the matrix and granules. The research has focused on the development of DC architectures in two main steps: production of ZrB2-MoSi2 granules and matrix by co-extrusion of high-solids-loaded thermoplastic polymer preforms, and the consolidation and densification of these simple composites into DC bulk ceramics. Attainable granule size ranges and morphologies will be discussed with respect to particle size and composition, as well as mechanical properties of the composites in bulk form.

CF-1:L09  Design of Grain Growth Resistant Nanograined YSZ
R.H.R. CASTRO, D.V. QUACH, CHI-HSIU CHANG, S. DEY, University of California, Davis, CA, USA

This work presents the results on the design of nanograined structures resistant to grain growth. From a comparative thermodynamic analysis on the enthalpy of segregation and enthalpy of mixing, lanthanum was selected as a potential efficient dopant to suppress grain growth in YSZ. As expected, La was observed to segregate to the boundary of nanograined YSZ consolidated by Spark Plasma Sintering, and showed significant resistance to grain growth as compared to undoped YSZ. The interface energy of pure and doped YSZ was measured by using differential scanning calorimetry and revealed a drop in the energy caused by La doping, decreasing driving force for grain growth. Pinning effects are also believed to play a major role in the phenomenon.

CF-1:IL10  Densification of UHTCs Assisted by Electromagnetic Fields
M.J. REECE, S. GRASSO, Materials Research Institute, Queen Mary University of London, London, UK

We have extended the envelope of processing conditions available during rapid sintering by Spark Plasma Sintering (SPS). We have demonstrated that High Pressure Spark Plasma Sintering (HPSPS) allows full densification of Ultra-High Temperature Ceramics (e.g. ZrB2, HfB2) even in the absence of any sintering aid in a temperature range which is 400-500 ᵒC lower than conventional SPS. Due to the lower sintering temperature, HPSPS is an ideal technique to achieve nanostructured and fully dense ceramics.
It is well know that due to both localized heating and the reduced sintering time in SPS processing can produce a significant energy saving compared to Hot Pressing. In order to further improve the energy saving we have developed a very rapid sintering technique called SPS flash with heating rates of the order of 5000 ᵒC/minute. SPS flash allows densification of ZrB2 up to 95% under a discharge time as short as 35 seconds, which results in an energy saving greater than 95% compared to conventional SPS.

CF-1:L11  Electro Discharge Machinable Alumina-based Nanocomposites
L.A. DÍAZ1, S. RIVERA2, A.A. OKUNKOV3, YU.G. VLADIMIROV3, F.J. GOTOR4, R. TORRECILLAS1, 3, 1Centro de Investigación en Nanomateriales y Nanotecnología (CINN) Consejo Superior de Investigaciones Científicas (CSIC) - Universidad de Oviedo (UO) - Principado de Asturias (PA), Llanera, Asturias, Spain; 2Nanoker Research, S.L., Polígono de Olloniego, Oviedo, Asturias, Spain; 3Moscow State University of Technology "STANKIN", Moscow, Moscow Oblast, Russian Federation; 4Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Sevilla, Spain

This work presents the results of an electro-discharge machined ceramic composites consisting of a base non-conductive ceramic component such Al2O3, to which is added sufficient amounts of an electro-conductive ceramic nanoparticles such as TiC, TiNC, NbNC, TaNC, and SiC (whiskers) to achieve an electrical resistance of less than about 100 The compositions studied were prepared with the same proportion of raw materials: alumina 42 (vol %) + conductive material (TiC, TiNC, NbNC, and TaNC) 22 (vol %) + SiCw 36 (vol %). SiC whiskers were used to optimize mechanical properties. Fracture toughness is improved up to three times that the corresponding to the individual ceramic components. Strength and hardness is also improved.
Cutting tools with 2, 54 x 2, 54 cm2 dimensions and other complex ceramic pieces were machined by using EDM machine, from Pulse Electrical (PECS) sintered compacts. These cutting tools were tested to machine high Cr-base superalloy industrial laminating cylinders without any further dressing, showing different performance that are discussed in this paper.

CF-1:IL13  Recent Developments of High Pressure Sintering of Advanced High Temperature Nanoceramics
V.S. URBANOVICH, Scientific-Practical Materials Research Centre NAS of Belarus, Minsk, Belarus

Currently in the advanced ceramics development the nanostructured approach becomes more and more widespread. The possibility to improve physical, chemical and mechanical properties of SiC, TiB2, B4C, Si3N4 and other promising high-melting point compounds is very attractive. However, the pore-free bulk processing of these compounds with the grain size lower than 20-30 nm is not so easy. High-pressure sintering at 4-8 GPa tends to decrease the densification temperature and retains the initial structure of nanopowders. This review generalizes world experience in high-pressure sintering of advanced nanoceramics with analysis of their main properties.

CF-1:L14  Reaction Bonded Si3N4 (RBSN) / BN Composites for Industrial Applications
L. CAVALLI, Petroceramics spa, Stezzano (BG), Italy

Hexagonal Boron Nitride (h-BN) shows remarkable physical properties, including high thermal stability, low density, low metals wettability, high corrosion resistance and microwave transparency. These features make it extremely interesting for several industrial applications such as furnaces manufacturing and metallurgy industry.
Usually, h-BN parts are sintered by expensive high-temperature/high-pressure processes, strongly limiting their size.
Here we present a new, cost-effective technique to obtain materials with high h-BN content and large dimensions, suitable for wide-scale industrial applications.
Using h-BN and silicon powders as raw materials, reaction-bonded Si3N4/BN composites were obtained by complete nitriding of silicon.
Two shaping techniques were exploited: slip casting and uniaxial compression molding using a thermosetting resin (in this case also Silicon Carbide was obtained), leading to materials with different properties.
Both large (plates with diameter up to 400mm) and/or complexly-shaped objects (i.e. crucibles) were produced.
Such materials were prepared using a Gas Pressure Sintering oven with different process parameters. The as-prepared samples were characterized and tested in a real application, as parts of liquid-silicon infiltration crucibles.

CF-1:L15  Development and Processing of SiAlON Nano-ceramics by Spark Plasma Sintering
A.S. HAKEEM1, T. LAOUI2, F. PATEL2, A.I. BAKARE1, S. ALI2, 1Center of Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia; 2Mechanical Engineering Department, King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia

The development of SiAlON-based ceramics has been great impact in the field of cutting/drilling tool industry and other engineering applications. It is highly desirable to cut-down the cost of the cutting tools by increasing the lifetime of the tools while is directly associated with the cost of energy sector. Therefore, an improved tool life impacts both time and energy by preparing these ceramics adopting alternate synthesis route and use of various precursors. This work presents results of synthesis SiAlON-based nano-ceramic via a non-conventional synthesis technique namely; spark plasma sintering (SPS). The properties of SiAlON will be tailored by optimizing the processing parameters during synthesis. Generally, metal nitrides and oxides precursors are used for processing self-reinforced SiAlON ceramics. In this work the use of nano-size (such as; Si3N4, SiO2, AlN and Al2O3) and metallic precursors would be used, which could be novel, to synthesize SiAlONs at low temperature(s) having improved properties in these oxynitride nano-ceramics. The synthesis samples are characterized by X-ray diffraction, field emission scanning electron microscopy, density, hardness and fracture toughness measurements.

Session CF-2 - Oxidation, Corrosion, and Testing

CF-2:IL01  Oxidation Mechanisms of ZrB2 - 30 vol% SiC
K.N. SHUGART, E.J. OPILA, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, USA

It is well known that SiC additions to ZrB2 result in increased oxidation resistance, however, the oxidation mechanisms remain elusive. Four aspects of the oxidation mechanisms of ZrB2 - 30 vol% SiC are addressed here. First, variability in the oxidation kinetics at temperatures between 1300 and 1550°C was quantified. The variability was attributed to gaseous oxidation products forming bubbles in the glassy surface layer. Next, the compositions of the borosilicate layer formed at temperatures between 1300 and 1500°C were determined. XPS of the oxide surface and ICP-OES of the bulk oxide were used to characterize the B2O3 gradient in the glass layer. The surface was depleted in B2O3 while the average composition of the glass layer contained 20 to 45 mol% B2O3. The resulting B2O3 gradient should affect oxidation kinetics. Third, the time and temperature conditions (>1600°C) were established under which SiC depletion in the base material occurred. Kinetics for depletion layer formation were evaluated in terms of reaction and diffusion limited mechanisms. Last, 18O2 tracer diffusion experiments in oxide layers previously formed on the ZrB2-SiC were attempted. The goal of this work is to provide detailed understanding of rate limiting mechanisms to enable life prediction.

CF-2:IL02  Improvement of Thermal Stability and Oxidation Resistance of UHTC above 2000 °C
F. REBILLAT1, A.-S. ANDRÉANI1, A. POULON-QUINTIN2, 3, 1Univ. Bordeaux, LCTS, Pessac, France; 2CNRS, ICMCB, UPR 9048, Pessac, France; 3Univ. Bordeaux, ICMCB, UPR 9048, Pessac, France

Solar furnace is an original method for testing ultra-high-temperature ceramics (UHTC) at very high temperature in air. This method enables a large temperature-time composition parameter space to be covered by rapidly producing a large set of oxidized samples. This heating system is based on concentrated light radiance supplied by the sun: around 15.5 MW/m2 on 13 mm2. Samples tested are massive cylindrical specimens of UHTC material prepared by spark plasma sintering (SPS).
The well known ZrB2+SiC material shows a limited temperature of use in an oxidizing environment due to the too low stability above 2000°C of silica formed by oxidation. Tests on samples with different SiC/ZrB2 ratio showed that the decrease of SiC content enhances oxidation resistance. Thus, few new systems without silicon are proposed, starting from the Hf or Zr, C, B and rare earth elements. In our work, the choice of rare earths is motivated by the formation of oxides with melting points higher than 2000°C. The complex oxide scales formed during oxidation are accurately described, in term of presence of porosity and gradients of composition. Similarities with the mechanism of oxidation described for ZrB2+SiC materials are shown, with an improvement of the protection capacity of the oxide scale.

CF-2:L03  Synthesis, Oxidation Resistance, Emittance, and Thermal Conductivity of ZrB2-SiC Multiphase Ceramics
R.F. SPEYER, School of Materials Science and Engineering, Georgia Inst. of Technology, Atlanta, GA, USA; FEI PENG, Clemson University, USA

ZrB2-SiC multiphase ceramics were synthesized by pressurless sintering and post-HIPing to theoretical density. Oxidation resistance was characterized using thermogravimetry up to 1900°C. The spectral emittance was measured up to 1300°C and out to 6 micrometers. Thermal conductivity measurements were performed using flash diffusivity and correlated with heat transfer models using finite difference calculations.

CF-2:L04  High-temperature Passive Oxidation Mechanism of CVD Silicon Carbide
T. GOTO, H. KATSUI, Institute for Materials Research, Tohoku University, Sendai, Japan

Silicon carbide (SiC) has been used in extremely harsh environment such as a heat-shield of space shuttle and a gas turbine blade due to its high oxidation resistance, thermal shock resistance and mechanical properties. Since the oxidation kinetics is strongly affected by impurity and defects of SiC, high-purity and dense SiC (single crystalline or chemical vapor deposited SiC) should be studied to understand the intrinsic nature. Although many studies have been devoted, the oxidation kinetics are still controversial particularly at high temperature.
The excellent oxidation resistance of SiC is caused of stable SiO2 scale on the surface (i.e., passive oxidation). We have investigated the oxidation behavior of CVD SiC at more than 1800K, and found a carbon layer at the SiC/ SiO2 interface by Raman spectroscopy and TEM suggesting that the passive oxidation could be limited by CO outward diffusion. At further high temperature, SiO and CO vapors accumulated at the SiC/SiO2 interface causing bubbles. The bubble formation condition (temperature and pressure) can be calculated from SiC-SiO2-C or SiC-SiO2-Si or SiC-SiO2 equilibria. We have identified the bubble formation at 1850K. This coincides to the SiC-SiO2-C equilibrium suggesting the CO outward diffusion limited process.

CF-2:L05  Corrosion Properties of Hafnia Based Silicon Carbonitride Ceramics
S. JOTHI, R. SUJITH, R. KUMAR, Materials Processing Section, Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai, India

Advanced ceramic and polymer derived ceramics (PDC) coatings are considered as potential candidate materials for high-performance gas turbine engines, where they can be subjected to multiple hot-corrosion attack by sodium, chlorine and sulphur environments. Nevertheless, there is lack of study on the corrosion and reliability performance of these ceramics under extreme environment conditions. In this aspect, hot corrosion studies were carried out in NaCl (mp 801 ˚C) and Na2SO4 (mp 884 ˚C) as sources of corrosive species for sodium, chlorine and sulphur at a constant temperature of 1000 ˚C for 24 h. Similarly, a critical need exists in fluoro chemical industries to withstand corrosive halogenated inorganic acids. In this aspect, HF (azeotropic; 38.26% HF) was used as source of corrosive species for fluoride at a fixed temperature of 90 ˚C for 2 weeks. From the scanning electron micrographs, we inferior that the surface degradation was proportional to the degree and uniformity of corrosion pitting attack as controlled by the chemistry of the molten salt. No significant mass loss was observed in crystalline ceramic, while the amorphous ceramic was completely disintegrated in fluoric acid. The full experimental details were explained in line with corrosion mechanism.

CF-2:IL07  UHTC Oxidation using Concentrated Solar Energy
M. BALAT-PICHELIN, PROMES-CNRS Laboratory, Font-Romeu Odeillo, France

UHTC could be used in the future for several high temperature applications like in the fields of space and energy. During this lecture, different UHTC materials and their physico-chemical behavior and thermal radiative properties at high temperature will be presented.
For space applications, ZrB2-SiC based ceramics that could be used as thermal protection systems have been studied in air plasma conditions simulating the atmospheric re-entry phase. The influence of different additives such as Si3N4, ZrSi2. will be shown. Oxidation, emissivity and catalycity are mandatory to be known in order to be able to develop heat shields that can withstand very high heat flux. Several results will be given.
In concentrated solar power plants, solar receivers have to absorb the solar energy to transfer it to the working fluid, so they need to have a high solar absorptivity and a low emissivity. Such absorbers require materials able to support thermal and mechanical stresses with the slowest possible oxidation kinetics at very high temperatures. ZrC or HfC based UHTC can be envisaged as materials for solar receiver to replace the currently used metals in order to increase the temperature of the working fluid and so the efficiency of the CSP plant. Some examples of results will be given.

CF-2:L08  Influence of Oxidation Processes on Mechanical Properties of Silicon Nitride
H. KLEMM, W. KUNZ, Fraunhofer IKTS Dresden, Germany

Various materials based on silicon nitride with different sintering additives (SiO2, Y2O3, Sc2O3, Y2O3/Al2O3) have been fabricated. The materials were characterized regarding their mechanical properties at ambient and elevated temperatures. Oxidation treatment has been performed prior and during the mechanical tests. As the consequence of their oxidation behaviour the materials exhibited different failure behaviour in short and long-term tests at ambient and elevated temperatures.
The composition of the intergranular phase between the silicon nitride grains (grain boundary phase) formed consequently from silica in the silicon nitride raw powder and the sintering additive system used was found to be the main influencing factor during oxidation and the mechanical behaviour finally obtained. Dependent on the properties of the surface layer formed during oxidation (oxygen diffusion) different oxidation damages were observed in the microstructure of the materials. These oxidation enhanced damages were found to have significant influence on the mechanical behaviour of the materials at ambient and elevated temperatures.

CF-2:L09  Oxidation Behavior of Cf/SiC Composites Protected by SiC-ZrC-LaB6 Multi-component Coatings
LE GAO, XIANGYU ZHANG, SHAOMING DONG, YANMEI KAN, CHUNJING LIAO, YUSHENG DING, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China

Carbon fiber reinforced silicon carbide (Cf/SiC) composites are usually considered as one of the most promising engineering materials in aerospace area because of their excellent performances, especially at high temperatures. Antioxidant coating is often needed to protect Cf/SiC composite from oxidation. To extend the protecting temperature range, a new coating system consisting of silicon carbide, zirconium carbide and lanthanum hexaboride were developed in present study. The multi-component coatings were prepared on Cf/SiC composites by chemical vapor deposition (CVD) and slurry dip coating method. The oxidation resistances at temperatures below 1800 °C are tested in stagnant air and the oxidation resistances at higher temperature are evaluated using arc jet tests. The results indicated that the SiC-ZrC-LaB6 coated Cf/SiC composites showed better oxidation resistance at a wide temperature range. The variation in the structure and phase assemblages of the coating after oxidation experiment was analyzed by scanning electron microscopy(SEM) and X-ray diffraction(XRD), and the oxidation mechanism was discussed.

Session CF-3 - Mechanical and Thermal Properties

CF-3:IL01  Nanoceramics for Extreme Environments
R.A. ANDRIEVSKI, Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, Russia

During the last years the great interest has been attracted by nanomaterials for their high physical, chemical, mechanical, and biological properties which are connected with specific nanostructure. However, because of the presence of large interfaces, segregations, and other defects, nanomaterials are nonequilibrium by nature and special attention must be devoted to their stability in extreme conditions such as high temperatures, radiation/deformation fields, and corrosion environments. Nanomaterials-based high-melting point compounds (carbides, borides, nitrides and oxides) such as TiN, SiC, ZrO2, TiB2, and others have some possibilities to use in these conditions. The role of size effect in mechanical and thermal properties including those in radiation conditions is discussed in details. Some little-explored and unexplored problems are pointed.

CF-3:IL02  High-temperature Mechanical Behaviour of Super-hard Carbides: The Special Case of Boron Carbide
D. GOMEZ-GARCIA1, B.M. MOSHTAGHIOUN1, M. CASTILLO-RODRIGUEZ2, A. DOMINGUEZ-RODRIGUEZ, 1Department of Condensed Matter Physics, University of Seville, Spain; 2 Institute for Materials Science, CSIC-USE, Spain

High-temperature refractory ceramics are at the forefront of the research in the ceramic field due to their potential application for aircraft industry together with the energy saving. Among them, carbides have received particular attention thanks to their singular properties. This presentation will focus on the high-temeprature plasticity of super-hard carbides, with a special attention to their unexpected behaviour of boron carbide, which makes this system a paradigm by itself in the theory of ceramic plasticity. A thorough analysis will be outlined.

CF-3:L03  Cationic Diffusion Coefficient in Ceria-Zirconia from Plasticity Studies
S. DE BERNARDI-MARTIN, B.M. MOSHTAGHION, D. GOMEZ-GARCIA, A. DOMINGUEZ-RODRIGUEZ, Department of Condensed Matter Physics, University of Seville, Seville, Spain

Ceria-zirconia ceramics are receiving considerable attention due to their potentialities as a solid-electrolyte. Contrary to the case of the case of yttria-zirconia, the transport properties in this system are far from being fully understood, particularly those for cation motion in the lattice. In this presentation, the cation diffusion coefficient is obtained from analysis of the strain rate-strain creep curves. The analytical technique is discussed in detail together with the physical implications of the numerical outputs achieved for this crucial quantity.

CF-3:L04  Spark Plasma Sintering of Fine-grained Alumina Polycrystals and their High-temperature Plasticity
Y. TAMURA1, E. ZAPATA-SOLVAS2, D. GOMEZ-GARCIA1, A. DOMINGUEZ-RODRIGUEZ1, 1Department of Condensed Matter Physics, University of Seville, Spain; 2Institute of Materials Science, CSIC-USE, Seville, Spain

Spark plasma sintering technique has been used to sinter pure alumina powders. The optimization parameters for full-dense sintering are reported as well as the grain growth kinetics. These specimens are revealed to have a very fine microstructure with average grain size well below in the submicrometric scale. The high.temperature creep mechanisms are identified and discussed carefully in this presentation.

CF-3:L05  Anisotropic Mechanical Properties and Plasma Sputtering Resistance Performances of Textured h-BN Composite Ceramics
XIAOMING DUAN, DECHANG JIA, NAN JING, ZHIHUA YANG, ZHUO TIAN, SHENGJIN WANG, YU ZHOU, DAREN YU, YONGJIE DING, Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin, China; School of Energy Science and Technology, Harbin Institute of Technology, Harbin, China

Hexagonal boron nitride (h-BN) grain shows the typical layer plate structures, so the textured materials can be achieved by aligning the h-BN grains along the same direction. In our work, textured h-BN composite ceramics were manufactured by hot-press sintering using mullite as the sintering additives. The sintering pressure is an important factor that affects the density and the textured characteristics of composite ceramics, and then also influences mechanical properties and plasma sputtering resistance performances. Based on the textured microstructure features, the composite ceramics show obviously anisotropic mechanical properties, and fracture mechanisms also varied. After plasma sputtering, the phases kept stable, while the element compositions and the morphology of surfaces changed. For h-BN grains, sputtered damage mechanisms by Xe ions are B-N bond broken and BN layers delaminated. Deleterious mechanisms of composite ceramics are h-BN grains damage, mullite damage and h-BN grains detached from surfaces.

CF-3:IL06  Ultra High Temperature Mechanical Testing of ZrB2 Based Ceramicss
G.E. HILMAS, W.G. FAHRENHOLTZ, E.W. NEUMAN, Missouri University of Science and Technology, Department of Materials Science and Engineering, Rolla, Missouri, USA

Mechanical properties of zirconium diboride (ZrB2), zirconium diboride-silicon carbide (ZrB2-SiC), and zirconium diboride-zirconium carbide (ZrB2-ZrC) composites were tested at room temperature and at temperature exceeding 2000°C. ZrB2 with 0.5 wt% C as a sintering aid, and ZrB2-SiC and ZrB2-ZrC composites containing 30 vol% SiC and 10 vol% ZrC, as a dispersed particulate phase, were hot pressed to full density at temperatures from 1900°C to 2150°C. Hot pressed billets were machined to ASTM standard flexure bars and four-point bend tests were performed from room temperature to 1600°C in air and from 1000°C to 2300°C in flowing Ar. Fracture toughness was measured using the chevron notch method and was also performed from room temperature to 1600°C in air and from 1000°C to 2300°C in flowing Ar. Elastic modulus was estimated from the stress-strain curves generated during strength testing. Room temperature strength of ZrB2 was measured to be 380 MPa, with a strength of 170 MPa at 2300°C. Strength of ZrB2-30SiC at room temperature was 680 MPa, and a strength of 275 MPa at 2200°C. ZrB2-10ZrC exhibited a strength of 660 MPa at room temperature, and 290 MPa at 2300°C. The influence of microstructure and testing environment on the mechanical properties will be discussed.

CF-3:IL07  Mechanical Behaviour under Fatigue at High Temperature of Ceramic-matrix Composites
P. REYNAUD, N. GODIN, M. R'MILI, G. FANTOZZI, INSA Lyon, MATEIS (UMR CNRS 5510), Villeurbanne, France

Ceramic-matrix composites are interesting materials for long-term uses due to their non-brittle behaviour even at high temperature under air. The mechanical behaviour is studied under cyclic and static fatigue. Under these loading modes, CMC are subjected to damage mechanisms like multiple cracking of matrix, fibre breaks, crack initiation and propagation in transverse yarns, fibre/matrix interfaces evolutions... These experimental mechanical behaviours are analysed by a micromechanical approach based on matrix-crack density and on local evolutions of fibre/matrix interactions along a continuous fibre bridging a matrix-crack. As fatigue mechanism a progressive decrease of interfacial shear stress by a wear mechanism of fibre and matrix surfaces during cyclic fatigue or a recession of interphases during static fatigue at high temperature is proposed. This mechanism introduced in a 1D equivalent composite describes theoretically the experimental evolutions of composite strain and mean elastic modulus observed during long term tests. For 2D composites cracks in transverse yarns are also observed. This specific damage mechanism is analysed in order to improve 1D model.

CF-3:L08  Modelling Damage and Creep Crack Growth in Ultra-High Temperature Ceramics
M. PETTINA', K. NIKBIN, Mechanical Engineering Department, Imperial College London, London, UK; A. HEATON, P. BROWN, Defence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire, UK; W.E. LEE, Materials Department, Imperial College London, London, UK

Damage and failure assessment of components operating at ultra-high temperatures is an area that needs substantial development. A continuum damage model based on multiaxial ductility exhaustion is proposed to predict creep crack growth (CCG) in structural ceramics at ultra-high temperatures. The work focuses on monolithic ZrB2 ultra-high temperature ceramic (UHTC), for which a reasonable set of material creep data is available. The predominant deformation mechanism shown by ZrB2 at temperatures greater than 1800 K and at stresses above 200 MPa is power law creep. Using the creep constitutive properties that have been found for this material, the proposed methodology is applied to a representative three point bend geometry, which is planned to be tested. Relevant Fracture Mechanics parameters such as stress intensity factor, K, or steady state creep parameter, C*, are evaluated and compared with available models. In this way the essential properties required to develop predictive damage simulations are investigated, underlining the importance of having accurate material test data.

CF-3:L09  Superhard Boron Carbide Ceramics with Ultrafine-grained and Dense Microstructures Sintered by Spark Plasma Sintering(SPS)
A.L. ORTIZ1, B.M. MOSHTAGHIOUN2, D. GOMEZ-GARCIA2, A. DOMINGUEZ-RODRIGUEZ2, 1Department of Mechanical, Energy and Materials Engineering, University of Extremadura, Badajoz, Spain; 2Department of Condensed Matter Physics, University of Sevilla, Spain

We studied the combination of high-energy ball-milling, annealing, and spark-plasma sintering to process superhard B4C ceramics with ultrafine-grained, dense microstructures from commercially-available powders, without sintering additives. It was found that the ultrafine powder prepared by high-energy ball-milling is hardly at all sinterable, but that B2O3 removal by gentle annealing in Ar provides the desired sinterability. A parametric study was also conducted to elucidate the role of the temperature, time and heating ramp in the densification and grain growth, and thus to identify optimal spark-plasma sintering to densify completely the B4C ceramics with ultrafine grains (~370 nm). Super-high hardness of ~38 GPa without relevant loss of toughness (~3 MPa∙m1/2) was thus achieved. The role played by the different sintering parameters are carefully discussed.

CF-3:L11  Thermochemistry of Metal Borosilicate Glasses
P. KROLL, Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX, USA

Metal borosilicate glasses appear as oxidation products of ultra-high temperature ceramics such as HfB2/SiC composites, but are also developed for immobilization of nuclear waste and as luminescent materials for optical applications. An important property of the multicomponent system is its thermochemistry, which characterizes metal solubility as well as the thermal stability of the glass system.
In our studies we target mixtures of metal oxides, MOx (M=Hf, Nb, Ta, W, Mo), with boria and silica. Using a melt-quench approach and ab-initio molecular dynamic simulations we generate mixtures with low amounts of metal oxide. After final optimization we analyze structure and energy of the models and compute the enthalpy of mixing, which relates to the solubility of the metal oxide. A comparison with experimental thermochemical data, which is only available for some specific systems, provides confidence for major differences we observe.
Our results show various trends for different metal oxide glass systems. For instance, the enthalpy of mixing HfO2 in boria is lower than in silica, which indicates that hafnia solubility in boria is higher than in silica. The opposite trend is found for MoO3, which mixes better in silica than in boria. Ta2O5 and Nb2O5 show significantly lower enthalpies of mixing than WO3. However, the enthalpy of mixing of WO3 in a mixed borosilicate glass shows a minimum along the B2O3-SiO2 line. We will discuss our results in the light of recent oxidation experiments of some high-temperature materials.

Session CF-4 - Characterization and Analysis

CF-4:IL02  Characterization of UHTCs Containing Various Kinds of Fibers

Different kinds of fibers, such as carbon or silicon carbide, were added to UHTC matrices to improve the fracture toughness.
Several sintering additives were chosen for the densification (ZrSi2, Si3N4 and TaSi2) using various sintering cycles to identify the best temperature allowing densification of the matrix and minimal fiber degradation.
The microstructure of the composites was examined through scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyse the microstructure at nanoscale level, the evolution of the fiber morphology and the interfaces with the matrix.
Subsequently, a thermo-mechanical characterization was carried out. The fracture toughness increased of 20-50% as compared to the unreinforced matrix, whilst the mechanical strength nearly halved owing to the introduction of 100-200 μm long reinforcement. The room-temperature strength was preserved up to 1200°C, but at 1500°C in air it generally dropped to 80-300 MPa, depending on the sintering additive and fiber type.
This study allowed the understanding of the chemical stability of each type of fiber towards different UHTC matrices and suggested possible mechanisms leading to the final microstructure and properties.

CF-4:IL03  First Principles Calculations of Interfaces in Ultra High Temperature Ceramics
V. TOMAR, School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN, USA

This work focuses on understanding correlations between thermal conduction and mechanical strength in a model high temperature material interfaces. Analyses examine single crystal ZrB2, single crystal SiC, and a <0001>-<111> ZrB2-SiC interface using a framework based on Car Parrinello molecular dynamics (CPMD) ab-initio simulation method from 500 K to 2500 K. Analyses indicate that the strength reduction with increase in temperature is strongly correlated to phonon and electron thermal diffusivity change. With increase in temperature, phonon thermal diffusivity increases in the case of ZrB2 and reduces in the cases of SiC as well as the interface. Electron contribution to thermal diffusivity increases with temperature increase in the case of interface. Examination of change in thermal properties at different mechanical strain levels reveals that the mechanisms of strength and thermal property change with increase in temperature may be similar to the mechanisms responsible for property change with change in applied strain. The analyses are accompanied by thermomechanical experiments at high temperatures that point out important correlations between thermal and mechanical properties.

CF-4:L05  Micro-structural Approach of the Mechanisms of the Carbothermal Reduction of Hafnia by TEM and XRD
F. REJASSE, O. RAPAUD, A. MAITRE, G. TROLLIARD, Laboratoire SPCTS, Limoges Cedex, France

Group IVb transition metal carbides (TiC, ZrC, HfC) are ultra-refractory materials which can be used under severe conditions of pressure and temperature thanks to their thermo-mechanical properties. The carbothermal reduction of the corresponding oxides is the most common route which allows to produce under a controlled atmosphere a large amount of powder. Many researchers have undertaken thermodynamic and kinetic studies on the carbothermal reduction of zirconia and titanium oxides. Conversely, the reduction of hafnia has not been deeply studied. They have suggested the formation of an intermediate M-O-X oxycarbide where M is the transition metal. The present work is focused on the better understanding of the mechanisms during the carbothermal reduction of hafnia. Hence a structural approach by TEM and a detailed analysis of the XRD patterns by refinement of the oxycarbide phase lattice were undertaken. This approach allowed us to compare these mechanisms with those determined on the ZrO2-C system [1]. The second part of this study is devoted to the determination of the oxygen limit of solubility in the oxycarbide to improve the available thermodynamical data.
[1] J. DAVID, M. GENDRE, A. MAÎTRE, G.TROLLIARD, J. Eur. Ceram. Soc., 2013, 33, 165-179.

CF-4:L06  Si3N4-SiC Nanocomposites Sintered with Various Rare-earth Oxide Additives for High Temperature Applications
P. TATARKO1, M. KASIAROVÁ1, J. DUSZA1, P. SAJGALÍK2, 1Institute of Materials Research, SAS, Kosice, Slovak Republic; 2Institute of Inorganic Chemistry, SAS, Bratislava, Slovak Republic

Silicon nitride (Si3N4) based ceramics are prime candidates for such diverse high-temperature applications items as rotors and stator vanes for advanced gas turbines, valves and cam roller followers for petrol and diesel engines. The main aim of the work is to contribute to the further improvement of mechanical properties and reliability of silicon nitride based ceramics performing at high temperature and hazardous environments.
High temperature mechanical properties of Si3N4-SiC nanocomposites have been investigated in order to study the influence of various rare-earth oxide sintering additives (La2O3, Sm2O3, Nd2O3, Y2O3, Yb2O3 and Lu2O3) and SiC addition on their wear, thermal shock, oxidation and creep resistance, respectively. Simultaneously, set of six reference monolithic Si3N4 materials sintered with the same sintering rare-earth oxide additives were prepared in order to compare their mechanical properties with that of the composites. The experimental materials were prepared by hot-pressing and the SiC nanoparticles were produced by the carbothermal reduction of SiO2 during the sintering process. It has been found out that the liquid phase with lower viscosity (larger ion of RE3+) tends to form intragranular SiC particles (30 - 40 nm) whereas the higher viscosity (smaller ion of RE3+) promotes the formation of intergranular particles of SiC (150 - 170 nm). This finding is considered as the most important result because it has been proved that the nanoparticles at the grain boundaries are favourable for mechanical properties, especially at high temperature.
Wear resistance of the materials was investigated by the tribology measurements in the temperature range from 22°C to 900°C. The improved wear resistance was observed in the case of the materials sintered with the additive with smaller RE3+ and the nanocomposites sintered with Lu2O3, Yb2O3 and Y2O3 exhibited almost constant specific wear rate up to 700°C. The oxidation resistance measurements were carried out at a temperature range from 1300°C to 1400°C for 204 hours. The nanocomposites sintered with the rare-earth with smaller ionic radius exhibited the higher oxidation resistance because of the slower cation diffusion of the additives to the oxide layer. Reduction in diffusion rate was caused by the presence of SiC nanoparticles at the grain boundaries, which serve as the obstacles for diffusion, as well as by the higher refractory phase at the grain boundaries. Therefore, the nanocomposite sintered with Lu2O3 additive (the smallest RE3+) exhibited the best oxidation resistance. Similarly, beneficial effect of the SiC nanoparticles located at the grain boundaries and the higher viscosity of the secondary phase were observed in the case of Lu-doped nanocomposites. On the other hand, no improvement in creep resistance of the nanocomposites sintered with La2O3 (the largest RE3+) was observed. This material contains the SiC nanoparticles located inside the Si3N4 grains rather than at their boundaries, and those are ineffective to suppress the creep rate. However, on the contrary, the critical quench temperature increased with increasing radius of RE ions indicating the lower thermal shock resistance of the Lu-doped nanocomposites compared to the materials sintered with the larger RE3+.
It can be concluded that the Si3N4-SiC nanocomposite sintered with Lu2O3 additive exhibited superior high temperature mechanical properties, with the only exception in the case of thermal shock resistance. Therefore it provides the best combination of the room and high temperature mechanical properties indicating its potential for high temperature applications.

CF-4.1:L07  Modelling and Experimental Thermodynamic Approach of High Temperature IVB-metal Carbides and Oxycarbides

In this work, some critical experimental information concerning the M-C-O systems where M is a transition metal belonging to the IVB group have been determined. These systems are of interest at very high temperatures in nuclear applications as fuel cladding. From the thermodynamic point of view, the underlying data used for the optimization through the CalPhaD method have to be improved. Consequently, this study is devoted to the determination of the reaction and specific heats of the ZrCxOy, TiCxOy, and HfCxOy oxycarbides and of their stability domains. These data will then be used to assess more accurately the corresponding ternary systems.
Carbides and oxides were mainly shaped and sintered by Spark Plasma Sintering and were synthetized by using the usual carbothermal route. The mastering of this ceramic process allows us to obtain very dense materials (close to the theoretical density) and carbides or oxycarbides powders for thermal analysis, respectively. The sintered pellets were used as carbide / oxide couples and heated at different high temperatures for diffusion experiments. Chemical compositions at the equilibrium of the oxycarbide phase were then analysed in order to determine the range composition and the structural properties of the solid solutions so formed.

Poster Presentations

CF:P01  Two-step Pressureless Sintering of Silicon Carbide-based Materials below 2000 °C
G. MAGNANI, G. SICO, ENEA-UTTMATF, Faenza, Italy; A. Brentari, Certimac S.C.a.r.l., Faenza, Italy

Pressureless sintering of silicon carbide powder requires addition of sintering aids and high sintering temperature in order to achieve high sintered density. The high sintering temperature normally causes an exaggerated grain growth which can compromise the mechanical properties. Two-step sintering can be used to overcome this problem. This process consists in heating the sample to a peak temperature T1 followed by a rapid cooling down to a lower dwell temperature T2. By this method, high sintered density is obtained avoiding the grain growth associated to the last step of the sintering. So, two-step sintering was applied to different commercial silicon carbide powders with different sintering mechanism: solid state and liquid phase sintering. In both cases the sintering temperature was set 100-150 °C below the temperature conventionally required. The microstructure was consisted of fine and equiaxed grains. The beneficial effects of the two-step sintering process were more evident in the solid state sintering. In this case sintered density higher than 96% was achieved with T<2000 °C and boron and carbon as sintering additives.

CF:P03  Dispersion of CNTs in Alumina using a Novel Mixing Technique and Spark Plasma Sintering of the Nanocomposites with Improved Fracture Toughness
N. BAKHSH1,  F. AHMAD KHALID1, A.S. HAKEEM2, 1Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, Swabi, KPK, Pakistan; 2Centre of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, Kingdom of Saudi Arabia

The present study emphasizes the fabrication of CNT- reinforced alumina nanocomposites used for structural applications. A new technique for mixing and dispersing CNTs in alumina powder was employed. Spark plasma sintering (SPS) technique was used for the fabrication of nanocomposites with varying amounts of as-received CNTs (1, 2 and 3 weight %) in the alumina matrix. Densification behavior, hardness and fracture toughness of the nanocomposites were studied. A comparison of mechanical properties of the desired nanocomposites is presented. An improvement in fracture toughness of approximately 14% at 1 wt% CNT-alumina nanocomposite over monolithic alumina compacts was observed due to better dispersion of CNTs in the alumina matrix that ultimately helped in grain growth suppression to provide finer grain in the nanocomposites. The fractured surfaces also revealed the presence of CNTs bridging and pull out that aided the improvement of mechanical properties. The synthesized samples were characterized using field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, densification, Vicker's hardness testing and fracture toughness measurements.

CF:P05  Fracture Mechanics of Y2O3 Ceramics at High Temperatures
M. BONIECKI, Z. LIBRANT, W. WESOLOWSKI, Institute of Electronic Materials Technology, Warsaw, Poland; M. GIZOWSKA, M. OSUCHOWSKI, K. PERKOWSKI, I. WITOSLAWSKA, A. WITEK, Institute of Ceramic and Building Materials, Warsaw, Poland

Fracture toughness KIc and four-point bending strength σc at high temperature (up to 1500 °C) of Y2O3 ceramics of various grain size were measured. The ceramics were prepared by pressureless air sintering and next hot isostatic pressing of high purity (99.99%) Y2O3 powder. Relative density of about 99.0 % was achieved. Photos of microstructures revealed small pores distributed mainly inside grains. As a results, σc values at room temperature are smaller than given in the literature, however KIc values obtained by the SENB methods are in good agreement with those. For smaller grain size (about 6 µm) ceramics KIc and σc are almost constant from 20 ° to 1200 °C and next they decrease. For bigger grain size (about 18 µm and bigger) they increase up to 800 °C and next they keep constant up to 1200 °C. The micrographs analyses of fracture surfaces indicated that transgranular mode of fracture at room temperature changes to almost intergranular at 1500 °C.

CF:P07  Development of Cordierite Ceramics from Natural Raw Materials
M. RUNDANS, G. SEDMALE, I. SPERBERGA, Riga Technical University, Riga, Latvia; I. PUNDIENE, Vilnius Gediminas Technical University, Vilnius, Lithuania

Cordierite ceramics are known for their low CTE and high compressive strength values which affords them place in fields where demanding thermal and mechanical properties are required. Development of such ceramics is greatly dependent on materials used. If raw materials are used formation of additional phases and pore/glass formation is expected.
The purpose of this research is to examine the process of cordierite development from mixed compositions formed from precursors of the natural raw materials as illite clay, dolomite and quartz sand and synthetic additives - MgO, γ-Al2O3 and their influence on thermal and mechanical properties. It is verified that the addition of up to 8.4 wt.% of illite clay and 20-21 wt.% dolomite in starting compositions at the sintering temperature of 1200 °C results in the development of dense ceramic material with perfect-shaped crystalline cordierite phase and secondary spinel phase. If the additive of mineral raw materials in starting cordierite compositions exceeds more than above mentioned, development of the secondary phases - forsterite and anorthite takes place. Sintered cordierite ceramics have been tested for their compressive strength, coefficient of thermal expansion and modulus of elasticity after 20 cycles of thermal shock treatment.

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