Showing papers on "Thermal shock published in 2000"

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Abstract: Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.

Cold Shock and Its Effect on Ribosomes and Thermal Tolerance in Listeria monocytogenes

Abstract: Differential scanning calorimetry (DSC) and fatty acid analysis were used to determine how cold shocking reduces the thermal stability of Listeria monocytogenes. Additionally, antibiotics that can elicit production of cold or heat shock proteins were used to determine the effect of translation blockage on ribosome thermal stability. Fatty acid profiles showed no significant variations as a result of cold shock, indicating that changes in membrane fatty acids were not responsible for the cold shock-induced reduction in thermal tolerance. Following a 3-h cold shock from 37 to 0°C, the maximum denaturation temperature of the 50S ribosomal subunit and 70S ribosomal particle peak was reduced from 73.4 ± 0.1°C (mean ± standard deviation) to 72.1 ± 0.5°C (P ≤ 0.05), indicating that cold shock induced instability in the associated ribosome structure. The maximum denaturation temperature of the 30S ribosomal subunit peak did not show a significant shift in temperature (from 67.5 ± 0.4°C to 66.8 ± 0.5°C) as a result of cold shock, suggesting that either 50S subunit or 70S particle sensitivity was responsible for the intact ribosome fragility. Antibiotics that elicited changes in maximum denaturation temperature in ribosomal components also elicited reductions in thermotolerance. Together, these data suggest that ribosomal changes resulting from cold shock may be responsible for the decrease in D value observed when L. monocytogenes is cold shocked.

Thermal and mechanical properties of aluminum titanate–mullite composites

Abstract: The thermal and mechanical properties of aluminum titanate–mullite composites prepared by a gel-coated powder processing method, namely a mullite precursor gel coating aluminum titanate particles (containing 2.5 wt% MgO), were investigated. A microstructure with both elongated and equiaxed mullite grains was observed in the composite samples. Both the mechanical strength and the thermal expansion coefficient increased with the mullite amount. The mechanical strength showed a strong dependence on the grain size of aluminum titanate with an exponent of −3. Inhibition of aluminum titanate grain growth due to the presence of the mullite phase enhanced the mechanical strength of the composites. The thermal shock resistance tended to decrease as the mullite content increased. The thermal stability of aluminum titanate was improved in the present composite system.

Residual shear strength of Sn-Ag and Sn-Bi lead-free SMT joints after thermal shock

Abstract: In this study, two lead-free solder alloys, namely 50 tin-50 bismuth (Sn-Bi) and 96.5 tin-3.5 silver (Sn-Ag), were studied for their use in surface mount solder joints. They have been considered as potential replacements for 63 tin-37 lead (Sn-Pb) solder. All joints were subjected to various cycles of thermal shock with temperature ranging from -25 to 125/spl deg/C. Shear tests were conducted on joints with and without thermal shock treatment. Another thermal shock cycle (-25 to 85/spl deg/C) was carried out on Sn-Bi solder joints for comparison. Their performance against thermal shock was compared with eutectic Sn-Pb solder by evaluating their residual shear strength and studying their microstructural change. For the Sn-Ag solder, a fine rod-like Ag/sub 3/Sn intermetallic was formed in the solder matrix after the thermal shock. On the other hand, Bi-rich and Sn-rich phases appeared in the Sn-Bi solder after the -25 to 125/spl deg/C thermal shock. Moreover, fine cracks were observed along the Bi-rich grain-like phase boundary. These were not observed in the Sn-Bi solder with the -25 to 85/spl deg/C thermal shock treatment. Voids and cracks were also observed in the joint of Sn-Bi solder alloy after 1000 thermal shock cycles. In addition, the thickness of intermetallic compound (IMC) of three solder alloys gradually grew with the number of thermal shock cycles. These defects reduced the strength of solder joint and led to thermal fatigue failure. In general, the shear strength is found to decrease with increasing number of thermal shock cycles. The Sn-Ag solder was better than the Sn-Bi solder in terms of residual thermal shock shear strength. Sn-Bi solder showed good properties when it was treated with the -25 to 85/spl deg/C thermal shock. It has a strong potential to replace Sn-Pb solder in low temperature applications such as consumer electronics. The Sn-Ag solder is suitable for high temperature applications.

Stress resistance parameters of brittle solids under laser/plasma pulse heating

Abstract: The present study analyses the stress and fracture response of brittle materials under the effect of high-power pulse laser/plasma sources. It establishes the relationship between the material properties and the degree of fracture damage and threshold loading parameters which initiate fracture. The crack stability is analyzed using the generalized fracture mechanics approach for a non-uniform stress distribution. The thermal stress field is determined using a quasi-static thermomechanical diagram appropriate for a pulsed heating under a uniaxial temperature variation. The stress intensity factor is calculated for a crack propagating in the stressed layer considered to be much smaller compared to the body dimensions. The calculations show that stress intensity versus crack length curves display maxima which allow one to determine the critical conditions under which the existing cracks start to propagate and the advancing cracks are arrested. The results are used to evaluate the thermal stress resistance and the degree of fracture damage as a function of thermal and mechanical properties of materials and the intensity and duration of thermal shock loading. The critical temperature variation within which the crack stability is independent of the crack size is established. The relation of this parameter to the thermomechanical properties of materials is discussed.

Mechanical and thermal shock properties of size graded MgO–PSZ refractory

Abstract: The effects of the mixture of coarse powder with fine PSZ powder on the thermal-mechanical properties of 10 Mg–PSZ samples were studied. The size graded specimens were injection-molded using 3.5 m% MgO–ZrO 2 powders. The physical properties of the ZrO 2 samples and five thermal shock parameters were measured and calculated. These properties included density ( ρ ), porosity ( p ), the ratio of m/(t+c+m) phase, fracture toughness ( K IC ), strength ( σ f ), Young's modulus ( E ), shear modulus ( G ), Poisson's ratio ( ν ), and the thermal expansion ( α ) between ambient temperature to 1100°C. The toughness and thermal shock resistance of the PSZ are controlled by the states of porous microstructure which can be represented by a parameter (nominal largest tolerable length of defects) a t . The PSZ samples show two types of thermal shock behavior differentiated by comparing the value of a t to the characteristic length L f of the defects in the sintered PSZ. The states of the defects, i.e. porosity, are the microstructural evidence to explain the relationship between the thermal shock properties.

Crack channelling and spalling in a plate due to thermal shock loading

Abstract: The propagation of a pre-existing edge crack across a finite plate subjected to cold shock has been studied. The plate, initially at uniform temperature, is exposed to a cold shock on one surface whilst three different types of heat transfer boundary condition are separately considered for the opposing face: cold shock, thermal insulation and fixed temperature. For all three boundary conditions, the plate experiences tensile stress near the cold-shocked surface and compressive stressing near the mid-plane. Consequently, a Mode I edge crack extending into the compressive region may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified, with a focus on crack channelling and spalling. The dislocation method is employed to calculate the energy release rates for plane strain cracking and steady-state channelling. For steady-state spalling, the energy release rate is obtained by an energy analysis of elastic beams far ahead and far behind the crack tip. Analytical solutions are also obtained in the short crack limit in which the problem is reduced to an edge crack extending in a half space; and the parameter range over which the short crack solution is valid for a finite plate is determined. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock.

4.13 – Carbon Matrix Composites

Abstract: Carbon matrix composites (CMCs) are generally used in aerospace industry for missile nose tips, re-entry heat shields, rocket motor nozzles, and brakes for aircrafts because of its attractive properties like light weight, low density, high strength, stiffness, high thermal conductivity, low coefficient of thermal expansion, high fracture toughness, good fatigue, creep resistance, and excellent thermal shock resistance. The unique property of this material is that its strength increases with increase in temperature. But there are some problems associated with this material like oxidation at moderately elevated temperatures and high manufacturing cost. Therefore, this chapter presents a critical review of carbon matrix composites. The attention is focused on manufacturing of carbon fiber, different techniques of making carbon–carbon matrix materials, microstructural examination of CMCs, mechanical properties of CMCs, oxidation protection and applications of CMCs.

Study on functionally gradient coatings of Ti–Al–N

Abstract: Functionally gradient coatings of Ti–Al–N were deposited on the 5CrNiMo steel substrate by multi arc ion deposition (MAID) technique by gradually increasing the nitrogen pressure during the deposition process Scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to investigate the composition, microstructure and phases of coatings Thermal shock resistance tests were carried out in a resistance furnace The adhesion strengths of Ti–Al–N functionally gradient coatings were tested by an automatic scratch tester It was shown that there are three phases, (Ti,Al)N, (Ti 2 ,Al)N and Al, in the coatings Along the thickness direction of Ti–Al–N functionally gradient coatings, the content of ceramic phase increases and the content of metal phase decreases gradually The hardness distributed along the thickness direction increases with increasing the N 2 pressure, namely increasing the content of ceramic It is found that the adhesion strength and the thermal shock resistance property of Ti–Al–N functionally gradient coatings are better than that of homogeneous (Ti,Al)N coatings and increase with decreasing the composition gradient

Crack propagation in a functionally graded plate under thermal shock

Abstract: Thermal cracking in a ceramic/metal functionally graded plate is discussed. When a functionally graded plate is cooled from high temperature, curved or straight crack paths are experimentally observed on the ceramic surface. One of the reasons that make the crack paths to differ are the thermal or mechanical conditions. In order to clarify the influence of these conditions on the crack path, the crack propagation is simulated using finite element method.

Evaluation of the heat transfer coefficient in thermal shock of alumina disks

Abstract: Disks of a high-purity commercial alumina powder were fabricated by slip casting, precalcined, sintered and machined with SiC paper (120 and 320 grit). The specimens were tested in thermal shock conditions from several temperatures ( T i ) between 870 and 980°C using a high-velocity air jet at room temperature ( T 0 ). The temperature differential between the disk and the air jet was incremented in 10°C until crack propagation was detected. During the air impinging, the temperature was recorded on the lower specimen surface at the central point and at a peripheral one. The coefficient governing the convective heat transfer on the specimen surface, h , was estimated by fitting the calculated temperature profiles with those measured during the test. Three alternative models were proposed for the temperature calculations using a finite element analysis.

Glass substrate for flat panel display device

Abstract: PROBLEM TO BE SOLVED: To provide a glass substrate for flat panel display device which is excellent in thermal shock resistance, crack resistance, and molding property, generates no thermal deformation and thermal contraction under heat treatment at a temperature of 570-600°C and has characteristic of ≥10.5 Ω.cm volume electric resistivity (log ρ). SOLUTION: The glass substrate for flat panel display device is characterized by having a composition (expressed by mass %) of 61 to 74% SiO2, 0 to <10% Al2O3, 1 to 10% MgO, 0 to 8% CaO, 1 to 15% SrO, 0 to 7% BaO, MgO+CaO+ SrO+BaO=12 to 23%, 0 to 8% Na2O, 2 to 12% K2O, Na2O+K2O=6 to 14%, and 0 to 7% ZrO2 and 65 to <75×10-7/°C coefficient of thermal expansion. COPYRIGHT: (C)2002,JPO

New [NZP] materials for protection coatings. Tailoring of thermal expansion

Abstract: [NZP] (the NaZr2P3O12-Family) materials can be selected for synthesizing new thermal shock resistant ceramic coatings with a thermal expansion that can be tailored to match that of the substrate, and to possess a low thermal expansion anisotropy. The tailoring technique will involve the selection of a suitable pair of compositions which, upon being mixed to form a crystalline solution, will possess the desired thermal expansion coefficient and will have negligible thermal expansion anisotropy. This can be done when the axial thermal expansion for one end member is larger in the a-direction than in the c-direction and vice versa for the other end member.

Alumina disks with different surface finish: thermal shock behavior

Abstract: Disks of commercial alumina powder were fabricated by slip casting and sintering (1600°C, 2 h). Two different surface treatments were performed: a coarse wear, using a 70 grit diamond wheel (CAW) and a fine one with SiC paper of 120 and 320 grit (FAW). The thermal shock resistance of the worn specimens was evaluated testing the disks by sudden cooling with a high-velocity air jet. The critical temperature differential ( ΔT C ) was determined increasing the initial temperature of the sample in 10°C until the crack propagation was detected. The values of the mean ΔT C were 940°C for FAW specimens and 765°C for CAW ones. The statistical distributions of the results were analyzed in function of the fracture strength of the specimens measured in biaxial flexion and the characteristics of the different surface finish determined by scanning electron microscopy, profilometry, and residual stresses measured by X-ray diffraction.

Behavior of plasma-sprayed thermal barrier coatings during thermal cycling and the effect of a preoxidized NiCrAlY bond coat

Abstract: The failure mechanisms of thermal barrier coatings (TBCs) subjected to a thermal load are still not entirely understood. Thermal stresses and/or oxidation cause the coating to fail and hence must be minimized. During the present investigation, TBCs up to 1.0 mm were sprayed and withstood high thermal stresses during thermal testing. Owing to the substantial thickness, the temperature at the top coat/bond coat interface was relatively low, resulting in a low oxidation rate. Furthermore, bond coats were preoxidized before applying a top coat. The bond strength and the behavior during three different thermal loads of the preoxidized TBCs were compared with a standard duplex TBC. Finite-element model (FEM) calculations that took account of bond coat preoxidation and interface roughness were made to calculate the stresses occurring during thermal shock. It is concluded that the thick TBCs applied during this research exhibit excellent thermal shock resistance and that a preoxidizing treatment of the bond coat increases the lifetime during thermal loading, where oxidation is the main cause of failure. The FEM analysis gives a first impression of the stress conditions on the interface undulations during thermal loading, but further development is required.

Synthesis and Characterization of Ti3AlC2.

Abstract: Polycrystalline bulk samples of Ti 3 Al 1.1 C 1.8 have been fabricated by reactively hot isostatically pressing a mixture of titanium, graphite, and Al 4 C 3 powders at a pressure of 70 MPa and temperature of 1400°C for 16 h. The hot isostatically pressed samples are predominantly single phase (containing ∼4 vol% Al 2 O 3 ), fully dense, and have a grain size of ∼25 μm. This carbide is similar to Ti 3 SiC 2 , with which it is isostructural, and has an unusual combination of properties. It is relatively soft (Vickers hardness of ∼3.5 GPa) and elastically stiff (Young's modulus of 297 GPa and shear modulus of 124 GPa); yet, it is lightweight (density of 4.2 g/cm 3 ) and easily machinable. The room-temperature electrical resistivity is 0.35 ± 0.03 μΩ.m and decreases linearly as the temperature decreases. The temperature coefficient of resistivity is 0.0031 K -1 . The coefficient of thermal expansion, in the temperature range of 25°-1200°C, is 9.0 (± 0.2) x 10 -6 K -1 . The room-temperature compressive and flexural strengths are 560 ± 20 and 375 ± 15 MPa, respectively. In contrast to flexure, where the failure is brittle, the failure in compression is noncatastrophic and is accompanied by some plasticity. The origin of that plasticity is believed to be the formation of a shear band that is oriented at an angle of ∼45° to the applied load. Ti 3 Al 1.1 C 1.8 also is a highly damage-tolerant material; a 10-kg-load Vickers indentation made in a bar 1.5 mm thick reduces the postindentation flexural strength by ∼7%. This material also is quite resistant to thermal shock. At temperatures of >1000°C, the deformation in compression is accompanied by significant plasticity and very respectable ultimate compressive stresses (200 MPa at 1200°C).

Fracture toughness, strength and thermal shock behaviour of bulk plasma sprayed alumina— effects of heat treatment

Abstract: Bulk plasma spraying Al2O3 produces a material with a defect rich splat-internal microstructure and a non-equilibrium phase composition which are both subject to extensive change by heat treatment. In the present work heat treatment induced changes in the strength, fracture and thermoshock characteristics of this material have been investigated and related to microstructural development. It is found that the as-sprayed intrasplat defect structure provides low energy paths for crack propagation. This results in low toughness and strength but excellent thermoshock resistance. Heat treatment increases overall porosity, but heals the defect structure and increases splat integrity, leading to a large increase in strength, accentuated R-curve behaviour and a disproportionate increase in toughness, but also a reduction in thermoshock resistance.

High power IGBT modules: thermal fatigue resistance evaluation of the solder joints

Abstract: Extensive accelerated aging tests have been carried out with representative test structures in order to evaluate the thermal fatigue resistance of five solder alloys intended for high power IGBT modules. With regard to the chip-to-substrate samples, Pb-free preforms have given excellent results since the thermal impedance of these hybrid assemblies has hardly changed during thermal cycling, even after 2000 shocks between +125/spl deg/C and -55/spl deg/C (less than 5%). The best results have been clearly obtained with the finest solder microstructures. In addition, the analysis of the results has led to an evaluation of the acceleration factor of the thermal fatigue tests. Finally, SEM observations of cross-sectioned samples have shown crack propagation during thermal cycling which is responsible for the increased thermal impedance. However, with regard to substrate-to-baseplate samples, the best results have been obtained with a Pb-bearing solder alloy strongly subject to coarsening during thermal cycling. Moreover, it has been demonstrated that fast cooling can cut crack growth rate in the solder joints by half. In addition, EDX analyses have shown copper diffusion all over the solder joint when the DBC (direct bonded copper) substrate is not nickel-plated, which seems to slightly improve its fatigue resistance. Finally, these experiments have shown that fine solder microstructures do not necessarily lead to good fatigue performances at high levels of stress or strain.

Limitations of the Weibull Theory in Stress Fields With Pronounced Stress Gradients

Abstract: The Weibull theory for brittle fracture of ceramic materials has to be modified for stress fields with pronounced spatial inhomogeneities, e.g. for severe thermal shock or for material joints. This is due to the fact that the stress intensity factor determining the failure behavior of the natural flaws is affected by the stress gradients. This leads to a rather involved relationship between critical flaw size and applied stress and affects the strength.Copyright © 2000 by ASME

Numerical method of thermal shock resistance estimation by quenching of samples in water

Abstract: A temperature dependence of a transient heat transfer for cylindrical and ball samples (of different surface roughness) of 3–60 mm diameters heated up to the temperature range from 150 to 1200° C and quenched in a water bath of large volume was established. The measurement errors of the transient heat transfer defined by different methods with regard to hysteresis and statistical nature of boiling phenomena were evaluated. The study revealed, that the transition point from bubble to film boiling and vice versa differs essentially. The transient heat transfer in the field of bubble boiling did not depend on the size and the shape of the samples, their surface roughness and thermo-physical properties. But the magnitude of hysteresis in changing between the boiling regimes were substantially governed by the geometrical and thermo-physical characteristics of the samples. The examples of thermal stresses estimation which caused quenching damage to ZrC samples, heated up to a wide range of temperature from 150 to 1200 C, are given. The obtained data on the transient heat transfer and proposed recommendation on the temperature regimes of quenching for convenient sample sizes can form a basis of a standard for the numerical evaluation of the thermal shock resistance.