Thermal Shock Resistance Parameters Research Articles

Toughness and thermal shock resistance of YBa 2Cu 3O 7− x composite superconductors containing Y 2BaCuO 5 or Ag particles

Macroscopic toughness measurements and the Vickers indentation technique have been used to characterise sintered or melt-textured superconducting YBa 2Cu 3O 7− x ceramics. A newly developed thermal shock resistance parameter, based on the Vickers indentation technique, has been used. The crack propagation mechanisms observed in sintered ceramics have been analysed with respect to residual stress fields due to the anisotropy in thermal expansion coefficients, or to the difference with that of the Y 2BaCuO 5 (‘211’) inclusions. The addition of the ‘211’ brittle phase does not increase the toughness and thermal shock resistance of sintered ceramics, but toughens the textured materials. An enhancement of the thermal shock resistance, through an increased R-curve behaviour, has been achieved by introducing a metallic silver ductile phase into the microstructure, both in sintered and textured ceramics. The evaluation of the latter needed the extension of the thermal shock resistance parameter to the case of Palmqvist indentation cracks. Parallelly, a modelling of the thermal shock behaviour of textured ceramics is proposed through the description of the microstructure as a stacking of YBa 2Cu 3O 7− x layers cracked along the (001) planes, joined by Y 2BaCuO 5 particles, corresponding to a low-stiffness material.

Thermal shock behaviour of industrial carbon anodes

Thermal shock resistance of industrial carbon anodes was studied from both the direct thermal shock test and indirect measurements of thermal shock resistance parameters derived from thermal and mechanical properties. The direct thermal shock test was conducted using the so-called arc discharge technique. In conjunction with finite element and fracture mechanics analysis, the thermal shock fracture initiation and crack propagation behaviour of carbon anodes were characterised in the thermal shock tests. The thermal shock test results were correlated with the thermal shock resistance parameters derived from thermal and mechanical properties. It was found that increased extent of non-linear stress–strain behaviour, which can be described by a lower brittleness function, χ, and a higher γWOF/γi ratio, leads to significantly higher thermal shock crack propagation resistance.

Evaluation of the thermal shock behaviour of tape cast ceramic substrates by means of lamp irradiation

Thermal shock behaviour of tape-cast Y-PSZ is characterised by lamp irradiation, i.e. the generation of strong lateral temperature gradients in thin circular discs by rapid heating with a high power halogen lamp. The accuracy of the method could be strongly improved by coating the samples with a circular graphite spot in the disc centre. This remarkably reduces the elliptical distortion of the temperature profile due to the plane wound filament of the lamp. The time resolved in situ measurement of the temperature profile with a high speed thermal imaging system enables the calculation of the arising thermal stresses. The application of the maximum temperature in the disc centre at the moment of failure, T max, as an easily measurable thermal shock quality control parameter for industrial applications is demonstrated and the relation between T max and the thermal shock resistance parameter under plane stress conditions is discussed. The results are compared with ring-on-ring strength tests which show the same trend.

The thermal shock behaviour of ductile particle toughened alumina composites

Over recent years, it has been established that the incorporation of metallic particles into a ceramic matrix can lead to enhanced fracture properties. Relatively few attempts, however, have been made to establish whether or not the improved fracture toughness typically observed in such composite systems can offer improved performance in demanding environments. The current study is concerned with the thermal shock behaviour of a ceramic matrix composite consisting of an alumina matrix containing 20vol% of discrete iron particles. The composite material has been produced by both hot pressing and conventional sintering techniques. The hot pressed composite shows a greater resistance to thermal shock than the monolithic matrix, both in terms of the critical temperature differential and retained strength, whereas the sintered material has been found to behave as a typical low strength refractory ceramic. The calculation of thermal shock resistance parameters for the composites and the monolith has indicated possible explanations for the differences in thermal shock behaviour.

Study and modelling of the thermo-mechanical behaviour of melt-textured YBaCuO-composites containing Ag and/or 211 particles

YBa 2Cu 3O x high- T c ceramic superconductors have been elaborated by the melt-textured-growth method with additions of Ag particles and/or Y 2BaCuO 5 (211 green phase) in order to study the fracture properties and the thermal shock resistance. The Vickers indentation technique, initially devoted to the sole determination of the toughness has been used in conjunction with a new thermal shock resistance parameter. It makes use of the initial stage of stable propagation of Vickers indentation cracks when submitted to a thermal stress, and results in a generalisation of the fourth parameter of Hasselman. Whereas the toughness increases with increasing 211 content, does the thermal shock resistance decrease. This is explained with respect of the strongly micro-cracked (along the 001 planes) microstructure which is seen as a stacking of dense laminates bound by the 211 particles and subjected to a linear temperature gradient. The higher the number of bounding particles, the stiffer the quenched material, and the higher the induced thermal stress. This micro-mechanical model is implemented into a finite element software. The first results of the calculations confirm the above approach.

Thermal shock resistance of the Al 2O 3-metal composites made by reactive infiltration of Al into oxide fiber board

Al 2O 3-metal composites were prepared by oxidation and reaction of molten Al alloy in two types of commercial Al 2O 3-SiO 2 fibrous insulation board heat treated at 1000°C and 1200°C. The 4-point bending test specimens were heated up to test temperature and then quenched in a water bath of 20°C. Fracture toughness, elastic modulus, Poisson ratio, and thermal expansion coefficient were determined, to obtain the specific fracture energies and the thermal shock resistance parameters. Commercially available, dense, sintered Al 2O 3 was tested for comparison. The fracture surface of specimens was investigated by scanning electron microscopy. Composites that are more resistant to thermal shock and are shrinkage free could be produced with inexpensive fibers through the reactive oxidation and infiltration process.

Thermal shock resistance of SiC whisker reinforced Si 3N 4 ceramic composites

Thermal shock resistance of hot-pressed SiCw/Si 3N 4 (w = whisker) composites was investigated by thermal shock resistance parameters and water quenching test. The results showed that the thermal shock resistance to fracture initiation and crack-stable repropagation deteriorated with increasing SiCw content in spite of the improvement of fracture toughness, K IC. This could be attributed to the great decrease of strength, σ, and the considerable increase of Young's modulus, E, and thermal expansion coefficient, α, resulting from the addition of SiC whiskers. The curves of the residual strength, σ r, after water quenching versus thermal shock temperature difference, ΔT, e.g. σ r − ΔT, showed good relationships with the calculated thermal stress fracture resistance parameter, R, and thermal stress crack stability parameter, R st. The crack propagation of SiC whisker reinforced Si 3N 4 occurred primarily in a quasi-static manner, and it could be mainly attributable to SiC whiskers' pulling-out and bridging toughening effects.

Effect of the temperature dependence of thermal properties on the thermal shock tests of ceramics

Thermal stress generated during a thermal shock is closely related to the fracture of ceramics. An attempt has been made to obtain thermal stress in a specimen by numerical calculation. The temperature dependence of thermal conductivity and diffusivity were introduced to realize the practical thermal conditions. The maximum thermal stress, σmax*, was recognized at the Fourier number, but differed from the temperature dependence. Correlative equations of σmax* and ηmax* with the Biot number, βi, under cooling or heating tests, have been proposed. These equations resulted in the exact σmax* and ηmax* compared with the previous equations, in which temperature dependence was ignored. The thermal shock resistance parameter was expressed by the correlative equations of σmax* in order to suggest adequate experimental conditions and specimen size. A comparison of the measured and calculated time to failure of the specimen led to confirmation of the fracture criterion. The measured time disagreed with the calculated one, if the fracture by thermal shocking was not predominant. The correlative equations were also useful to select the kind of ceramics subjected to thermal shocking.

Thermal Shock and Size Effects in Castable Refractories

To study accurately the effects of thermal shock on castable refractories, it is necessary to model the fracture process zone. The fictitious crack model, which is often used for the analysis of cementitious materials, is a suitable way to include the fracture process zone in thermal shock analysis. The analysis reinforces the importance of the thermal shock resistance parameter, R″″, which has the units of length and is very similar to the characteristic length, lch, used as a size scale in studies of the fracture of cementitious materials. Whether a cementitious bar behaves in a brittle or ductile fashion depends on the size of the bar relative to its characteristic length. Therefore, the behavior of refractories after they have been quenched is dependent on their size relative to R″″. It is also shown that the critical temperature to cause strength degradation by thermal shock is dependent on the relative size of the bar.

Thermal Shock Fracture of Ceramic Disk under Rapid Heating

Thermal shock fracture resistance on several kinds of ceramics was evaluated by the rapid heating thermal shock test using disk specimens. The thermal shock resistance parameter R c = F /E α ( F : thermal fracture strength, E: Young's modulus, α: thermal expansion coefficient) was obtained by measuring the temperature distributions in the disk at the fracture. The measured thermal shock resistance parameter R c was compared with other thermal shock test data including water quenching test. Compatible data were obtained regardless of the difference between heating test or quenching test.

Thermal shock resistance of alumina-sialon composites

The thermal shock behaviour of pure alumina and an alumina-sialon composite was examined. The critical temperature difference of the thermal shock, ΔTc, had a good relationship with the calculated thermal shock resistance parameter,R. The thermal shock resistance of the composite was remarkably improved by simple modification, such as oxidation at 1400 °C for 100 h. The oxide layer, with porous microstructure, consisted mainly of mullite and a small amount of alumina. The ΔTc of pure alumina, the composite and the oxidized composite was 210, 225 and 360 °C, respectively. This improvement was considered to be due to the characteristics of the oxide layer, which had high strength as well as a low elastic constant and a low thermal expansion coefficient.

Dependence of Mechanical Properties of MgO-C Bricks on Graphite Content

MgO-C bricks containing 0 to 30 vol% flake graphite and expanded graphite were prepared and heat-treated at 800 and 1400°C. The dependence of mechanical properties of the MgO-C bricks on the graphite content was studied. The maximum values of bending strength, elastic modulus and fracture toughness were obtained at 2 to 3 vol% expanded graphite and at around 5 vol% flake graphite. The effective fracture energy increased with an increase in graphite content up to 30 vol% for expanded graphite. However, no slight increase in effective fracture energy was observed above 20 vol% flake graphite. Thermal shock resistance parameters R″″ and Rst were calculated based on the data obtained. R″″ was not correlated with graphite content. On the other hand, Rst remarkably increased above 7 vol% graphite and larger Rst was obtained with expanded graphite. Consequently, improvement of the spalling resistance of MgO-C brick was expected with expanded graphite.

Non-Steady State Temperature Distribution and Thermal Stress History in Ceramic Disks Heated under Constant Heat Flux Conditions

Temperature distribution of ceramic disks subjected to suddenly heating from outside under constant heat flux conditions was measured to estimate the thermal stress history and evaluate the first kind thermal shock resistance parameter of ceramic materials. Also, numerical calculations were conducted to compare with experimental data for each heating condition. Temperature dependence of thermal conductivity and thermal diffusivity was introduced into the calculation to realize the practical heat conductive condition. Two types of ceramics were used for the experiment; alumina ceramics having high and varying thermal conductivies and mica ceramics having low and constant thermal conductivities. As a result, it was found that the thermal stress history of heated disks is easily estimated from measurement of temperature distribution of the specimen. Temperature dependence of thermal properties was substantially affected on temperature distribution and thermal stress history, and it is shown that varying thermal properties in alumina ceramics can lead to a significant increase in the maximum tensile thermal stress generated in the center of the disks. As demonstrated by the numerical calculation for many kinds of other ceramics, maximum thermal stress of heated disks under constant heat flux condition shows non-steady-state value.

Thermal shock criteria for ceramics

Based on the theory of heat conductivity and thermo-elasticity a detailed fracture mechanical thermal shock description is given, which emphasizes the transient nature of this phenomenon. Resulting from these considerations, various thermal shock resistance parameters are introduced that depend upon crack initiation and crack arrest criteria. The role of thermal shock experiments is discussed. Si 3N 4 and SiC as two important high temperature structural ceramics are studied with regard to thermal shock applications. Experimental results from a developed high temperature thermal shock apparatus are shown and compared with water quench experiments. Measurements of final crack lengths strongly support the energy balance criterion for crack arrest.

Thermal shock-induced fracture of ion-implanted LiF crystals

Monocrystals of LiF, ion implanted with Ar+, were exposed to thermal shock in a plasma of different intensities. Ion implantation substantially alters the fracture pattern and characteristics of the material, particularly in reducing the thermal shock resistance parameter, S′, and in increasing the damage resistance parameter, S″. The former parameter indicates that ion implantation allows fracture to be initiated at lower thermal shock temperature differences and the latter parameter is associated with higher crack densities and lower crack penetration depths. The increase in the parameter S″ indicates that ion implantation can result in a higher mechanical stability and greater durability of the crystals damaged by thermal shock. Surface melting at very high heat fluxes eliminates any effect of ion implantation.

The thermal shock of ß-alumina

The thermal shock of sodiumβ-alumina with relative densities from 60 to 98% theoretical has been investigated over the temperature range 150 to 700° C by quenching into water. The samples were ring segments cut from electrolyte tubes and were subsequently tested in both compression and tension. For relative densities of 75% and below the thermal shock damage was typical of stable crack growth and a steady decline in strength with sintering temperature was observed. For relative densities of 95% and above, thermal shock causes unstable crack growth and a critical value of ΔT was observed in the range 170 to 250° C depending on initial strength. From the linear relationship between observed ΔTc and the thermal shock resistance parameter,R, it was concluded that the rapid heat transfer during quenching was nucleate water boiling and that cooling from ∼110° C to 0° C was not responsible for damage. The fracture stress after thermal shock above ΔTc was consistent and showed little dependence on initial strength for relative densities ⩾95%. However, the fractional reduction in strength was related to the damage resistance parameterR‴. An estimate of the energy expended in fracture has been made, based on microscopic observation and compared with estimates of the stored strain energy due to thermal stresses.

Oxidation effects on CTE and thermal shock fracture initiation in polycrystalline graphites

The coefficient of thermal expansion (CTE) was measured as a function of oxidation for three commercial fine-grained graphites derived from petroleum cokes and coal tar pitches and fabricated by extrusion, undirectional molding, and isostatic molding. The CTE was observed to vary with the crystallite size and the preferred orientation and to decrease as much as 20% with increasing oxidation. This CTE decrease was attributed to an increase of the accommodation by Mrozowski cracks enlarged by the oxidation process. Effects on thermal shock fracture initiation were examined by estimating changes in the thermal shock resistance parameter, R. It is concluded that in spite of the continuous decrease in CTE, changes in R with oxidation are not continuous for these graphites. The complexity is a consequence of the different extents to which graphite oxidation affects CTE, strength and the Young's elastic modulus.

Some techniques for the characterization of cokes and graphites

Methods used at the Coal Research Establishment of the UK NCB for characterizing the structure and physical properties of cokes and graphites are briefly reviewed. These include the use of X-ray diffraction and optical microscopy, and the measurement of a range of mechanical, thermal and electrical properties. Some relations between the structural parameters of graphites and their physical properties are discussed. Examples illustrating the application and practical importance of the results obtained are grouped loosely under three headings: 1. (1) Prediction of graphite properties from tests on cokes (e.g. prediction of coefficient of thermal expansion from coke mosaic texture). 2. (2) Exploration and monitoring of changes in process conditions (e.g. use of graphite crystallite size to monitor the effects of changes in graphitization temperature, use of coke mosaic texture to indicate effects of changes in temperature and time during coal-digestion stage of the solvent-extraction process). 3. (3)Prediction of the performance of graphite electrodes in an arc furnace (e.g. concept of thermal-shock resistance, estimation of thermal-shock resistance parameters from physical properties, and correlation between these and thermal-shock performance in laboratory tests).

New Determination Method of Thermal Shock Resistance Parameter of Ceramics

A new heat flux method is proposed for determining a thermal shock resistance parameter of ceramics. A ring shaped disc specimen is heated by the electro-conductive thin film glazed on its inner surface, and the time to fracture initiation is measured. At the same time, nondimensional thermal stresses occurring in the specimen under various heat flux conditions are theoretically calculated as a function of nondimensional time to give a set of reference curves. By combining the curves with the measured time, voltage and resistivity of electro-conductive film etc., the thermal shock resistance parameter, S/Eα, is easily determined. The experimental date obtained by this method are useful for designing ceramic materials used in firing kilns.

WC-20%Co系超硬合金の耐酸化，耐熱衝撃性に及ぼすWC粒度の影響

The relations among oxidation resistance, thermal shock resistance and metallurgical structure parameters of WC-20% Co sintered alloys with different WC-grain sizes (3.8–14 micron-m) were investigated by means of oxidation tests in air at temperatures of 750 to 850°C, and using a newly developed thermal shock test apparatus. To evaluate the thermal shock resistance of these alloys, tests were carried out using an electro-spark discharge apparatus and measuring the total length of thermal crack produced 0.05 mm below the sparking surface. Structure parameters of WC-20% Co alloys were measured with the photomicrographs quantitatively.The results obtained are as follows:(1) Oxidation process of WC-20% Co alloy is controlled by the OXIDE/METAL interfacial reaction. It obeys a linear law which has an apparent activation energy of 42.1 kcal/mol.(2) Oxidation resistance of these alloys depends mainly upon the cobalt content, which exists on the oxidation front surface, and upon the WC-grain size in the sintered structure.(3) Thermal shock resistance increases with increase in WC-grain size.