Thermal Shock Resistance and Fracture Behavior of ZrB2–Based Fibrous Monolith Ceramics

The thermal shock resistance and fracture behavior of zirconium diboride (ZrB₂)-based fibrous monoliths (FM) were studied. FMs containing cells of ZrB₂–30 vol% SiC with cell boundaries composed of graphite–15 vol% ZrB₂ were hot pressed at 1900°C. The average flexure strength of the FMs was 375 MPa, less than half of the strength of hot-pressed ZrB₂–30 vol% SiC. Flexure specimens failed noncatastrophically and retained 50%–85% of their original strength after the first fracture event. A critical thermal shock temperature (Δ T _c) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ T _c values for ZrB₂ and ZrB₂–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ T _c values of 1350°C and below. As Δ T _c increased, the stiffness of the flexure specimen decreased linearly. The lower stiffness and improvement in thermal shock resistance is attributed to crack propagation in the cell boundary and crack deflection around the load-bearing cells. The critical thermal shock was attributed to the fracture of the ZrB₂–30% SiC cell material.     

Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

Fracture Toughness of TiB2 and B4C Using the Single-Edge Precracked Beam, Indentation Strength, Chevron Notched Beam, and Indentation Strength Methods

The mode I fracture toughness ( K _Ic) of boron carbide (B₄C) and titanium diboride (TiB₂) was determined using four competing techniques. The indentation strength (IS), chevron notched beam (CNB), and indentation fracture (IF) methods are common techniques that were compared to the recently standardized single-edge precrack beam (SEPB) method. The SEPB method was more difficult to apply, but it represents the most rigorous method for K _Ic determination, because it uses few assumptions and requires a direct measurement of crack length. The IS method was an expeditious and economical alternative when low indentation loads were used. CNB K _Ic values were virtually rate-independent when displacement rates less than or equal to 0.5 mm/min were used. The IF method was the least satisfactory technique, because of high variability in K _c values and because of the low differentiation between the two materials studied.     

Fracture Toughness and Thermal Shock Behavior of Silicon Nitride–Boron Nitride Ceramics

Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless-sintered Si₃N₄-BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear-elastic and exhibit no R -curve behavior, their flaw resistance is different from that of other linear-elastic materials. Whereas the critical thermal shock temperature difference (Δ T _c) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe thermal shocks.     

Mechanical Properties of Partially Dense Alumina Produced from Powder Compacts

The elastic modulus ( E ), the critical strain energy release rate ( G _c), and the flexural strength (σ) have been determined for two partially dense alumina bodies produced from the same powder but with different initial densities. The mechanical properties were measured for specimens fabricated at four different relative densities. The measured elastic modulus, critical strain energy release rate, and a calculated critical stress intensity factor ( K _c) were observed to be linearly related to (ρ–ρ₀)/(1 –ρ₀), where ρ is the current relative density and ρ₀ is the initial relative density of the powder compact. With the observed linear relations for E, G _c (or K _c), and the assumption that the crack length responsible for failure was present in the initial powder compact and shrunk in proportion to the relative density change, a Griffith equation was constructed to estimate the strength at any relative density. This relation was in good agreement with measurements.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.     

Aging Behavior of xPb(Fe2/3W1/3)O3· (1 –x)Pb(Fe1/2Nb1/2)O3 Ceramic

The aging behavior of a series of lead perovskite dielectrics with the compositions x Pb(Fe_2/3W_1/3)O₃·(1 – x )Pb(Fe_1/2-Nb_1/2)O₃, where 0 ≤ x ≤ 1, and the effect of dopants were studied. Below the Curie temperature ( T _c), the capacitance and the dissipation factor (tan δ) decrease approximately linearly with logarithmic time. The aging rate depends on the temperature difference, Δ T , between the aging temperature and T _c, and on the dopant concentration, but is independent of the measurement frequency between 1 and 1000 kHz. The maximum aging rate is about 3% per decade of time for capacitance and 5% per decade for tan δ at 1 mol% dopant concentration, and increases to 6.3% for capacitance and 8.5% for tan δ at 0.7 mol% dopant concentration. These results are consistent with an aging mechanism caused by changing ferroelectric domain structure with time, as proposed for BaTiO₃.     

Fracture Resistance Characteristics of a Metal-Toughened Ceramic

The fracture characteristics of an Al₂O₃/Al composite are examined. Measurements of resistance curves and work of rupture are compared with predictions of a micromechanical model, incorporating the effects of crack bridging by the Al reinforcements. The bridging traction law is assumed to follow linear softening behavior, characterized by a peak stress, σ_c, and a critical stretch-to-failure, u _c. The values of σ_c and u _c inferred from such comparisons are found to be broadly consistent with independent measurements of stretch-to-failure, along with the measured flow characteristics of the Al reinforcement. The importance of large-scale bridging on the fracture resistance behavior of this class of composite is also demonstrated through both the experiments and the simulations.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

Influence of a Dispersion of Aluminum Titanate Particles of Controlled Size on the Thermal Shock Resistance of Alumina

The possibility of developing fine-grained (∼0.5–3 μm) and dense (≥0.98ρ_th) alumina (90 vol%)–aluminum titanate (10 vol%) composites with improved thermal shock resistance and maintained strength is investigated. One alumina material and one composite with similar microstructures (porosity and grain-size distribution) were fabricated to investigate the effect of Al₂TiO₅ on thermal shock behavior. The size of the Al₂TiO₅ particles was kept under 2.2 μm to avoid spontaneous microcracking. The mechanical and thermal properties of the materials involved in their response to thermal shock and the results for the evolution of indentation cracks of equal initial crack length with increasing Δ T in samples quenched in glycerine are described. The combination of thermal and mechanical properties—thermal conductivity, thermal expansion coefficient, Young's modulus, and toughness—improve the thermal shock resistance of the alumina–aluminum titanate composite in terms of critical temperature increment (>30%). The suitable structural properties of alumina—hardness and strength—are maintained.     

Fracture Toughness and Spalling Behavior of High-Al2O3 Refractories

The fracture energies and spalling resistance of high-Al₂O₃ refractories were studied. The fracture energies, γ _WOF and γ _NBT , were measured by the work-of-fracture and the notched-beam-test methods, respectively. Spalling resistance, as measured by the relative strength retained in a water quench, correlated well with the thermal-stress resistance parameter applicable to stable crack propagation under conditions of thermal shock, (γ _WOF /α² E ₀). Many of the refractories exhibited high ratios of γ_WOF to γ_NBT; such high ratios were shown analytically to maximize the parameter ( R ¹¹¹¹= E _0γWOF/S₁²) which describes the resistance to catastrophic spalling. The increase of crack length with increasing quenching temperature difference (Δ T ) was somewhat less than that predicted theoretically; the discrepancy was attributed to an increase of crack density with Δ T . In general, the results show that fracture energy is important in establishing the spalling resistance of high-Al₂O₃ refractories.     

Superconducting Characteristics of Polycrystalline Magnesium Diboride Ceramics Fabricated by a Spark Plasma Sintering Technique

Highly densified MgB₂ superconductors were successfully fabricated using a spark plasma sintering (SPS) technique, and their superconductivity with respect to microstructural evolution was evaluated. Full densification with final density close to the theoretical density was achieved at a temperature of 1000°C within a total SPS processing time of 40 min. Both an MgB₂ specimen sintered at 1000°C for 30 min and one sintered at 1050°C for 10 min exhibited a high critical transition temperature ( T _c) similar to that of an MgB₂ single crystal (39 K), and a very sharp superconducting transition width (Δ T ) less than 0.5 K. In addition, high critical current densities ( J _c) of 7.7 × 10⁵ A/cm² in a field of 0.6 T at 5 K and of 8.3 × 10⁴ A/cm² in a field of 0.09 T at 35 K were obtained. These excellent superconducting characteristics of the SPS-processed MgB₂ are attributed to uniformly distributed secondary MgO phase nanoparticles and well-developed dislocations in the microstructure that may act effectively as extrinsic flux pinning sites, resulting in the strong pinning force showing a high J _c of 8.7 × 10⁴ A/cm² even in the condition of a field of 4 T at 5 K.     

Size Sensitivity to Thermal Shock of Plasma-Sprayed Ceramics and Factors Affecting the Size Effect

The influence of specimen size on thermal shock resistance is investigated for relatively large plasma-sprayed alumina tubes of varying diameter, length, and wall thickness. The observations suggest that an increasing wall thickness has a significant effect on the critical temperature difference for the onset of fracture, Δ T _C, compared to the relatively weak effect of tube diameter and length. A plot of Δ T _C versus the ratio of surface area and volume of the quenched specimens enables one to predict the respective value for larger tubes. The various parameters influencing the thermal shock size effect are addressed. According to the well-known thermal stress equations, all measures to increase Δ T _c, such as higher strength, lower Young's modulus, higher thermal conductivity, lower thermal expansion, and even less severe heat transfer conditions, simultaneously lead to an enhanced sensitivity of a ceramic component with respect to its size. For water quench tests, this effect may be intensified by a steep decrease of the surface heat conductance with temperature for Δ T > 400°C, resulting in a pronounced size sensitivity to thermal shock, particularly for ceramics of excellent thermal shock resistance.     

A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa2Cu3O7−x Films

Shortly after the discovery of high-temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density ( J _c) across grain boundaries in the perovskite-like superconductor YBa₂Cu₃O_{7− x} (YBCO) falls off exponentially below the intragrain J _c beyond a critical misorientation angle θ_c of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of J _c well beyond the conventional J _c(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical J _c(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved J _c values over a wide range of applied magnetic fields. Consequently, very high J _c values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.     

NiZnCu Ferrite Thick Film with Nano Scale Crystallites Formed by the Aerosol Deposition Method

This paper describes the magnetic properties of NiZnCu ferrite film deposited at room temperature by an aerosol deposition method (ADM). The thickness of the film was 6 μm and the deposition rate was estimated as 2 μm/min. The microstructure of as-deposited at room temperature films consists of randomly oriented nanocrystallites with a size of 20 nm. As-deposited and annealed films exhibited the following magnetic properties: intensity of magnetization M _s= 0.147 T (117 emu/cm³), coercivity H _c= 40.58 kA/m (510 Oe); and M _s= 0.3 T (250 emu/cm³), H _c= 14.95 kA/m (188 Oe), respectively.     

High-Tc Hydroxide- and Fluoride-Containing Ceramic Superconductors

Experimental data for maximum T _c , up to 125 K with temperature cycling, are reported with regard to Y-Ba-Cu-O.OH systems. This increase in T _c is presumably associated with structural transition or phase equilibrium for [CuO₆] local octahedra, occupied by hydroxyl or fluorine ions in the oxygen vacancy sites. Possible improvement in T _c using hydroxide ceramic compounds is described briefly.     

Base-Metal-Electroded BaTiO3 Capacitor Materials with Duplex Microstructures

The effect of dopants and processing conditions on the dielectric properties of base-metal-electroded materials was investigated. BaTiO₃ materials simultaneously doped with MgO and Y₂O₃ additives can achieve small capacitance variation (Δ C / C ), which meets the X7R specification, when the proportion of additives is abundant enough and the materials are not over-fired. Presumably, small Δ C / C values of thus obtained materials are the result of the formation of core–shell structure, which requires stringent control of material processing conditions. In contrast, X7R-type materials can be obtained in a much wider processing window, when prepared by mixing two BaTiO₃ materials of suitable dielectric constant–temperature ( K – T ) characteristics. Duplexed materials prepared from these two end-point BaTiO₃ materials with ratios ranging from 3:1 to 1:2 exhibit K – T behavior within the X7R specification, provided that one of the components possesses flat K – T behavior. Moreover, the dielectric properties of these materials were simulated using a simplified microstructural model. Simulation results indicate that the effective dielectric constant of core–shell materials, ( K _e)_CS, varies significantly not only with the dielectric properties of cores and shells, but also with the shell-to-core thickness ratio, whereas the effective dielectric constant of duplexed materials, ( K _e)_D, can be maintained at a very small Δ C / C value for a wide range of end-point constituent ratios, which agrees very well with the measured K – T properties for the materials.     

Calorimetric Study of Nickel Molybdate: Heat Capacity, Enthalpy, and Gibbs Energy of Formation

The thermodynamic properties of the α and β polymorphs of NiMoO₄ were directly investigated by calorimetry. The standard entropies of formation, Δ_f S ° _T , of α and β were determined from measuring the molar heat capacity, C _p,m, from near absolute zero (2 K) to high temperature (1380 K) by a relaxation method and differential scanning calorimetry. The standard enthalpies of formation, Δ_f H ° _T , of α and β were determined by combining C _p,m with the standard enthalpy of formation, Δ_f H °₂₉₈, at 298 K obtained from drop solution calorimetry in molten sodium molybdate at 973 K. The standard Gibbs energies of formation, Δ_f G ° _T , of α and β were determined from their Δ _f S ° _T and Δ _f H ° _T values. The Δ _f G ° _T values indicate that the polymorphic transformation from α to β occurs at 1000 K, consistent with the observed phase transformation at 1000 K.     

Processing and Electrical Properties in Lead-Based (Pb(Mg1/3Nb2/3)O3, Pb(Yb1/2Nb1/2)O3, PbTiO3) Systems

Lead-based ferroelectric (FE) ceramics exhibit superior electromechanical properties; therefore, there has been an increased focus on developing new lead-based FE materials with high Curie temperature ( T _c) and enhanced properties. The aim of this study was to investigate new compositions in the Pb(Mg_1/3Nb_2/3)O₃–Pb(Yb_1/2Nb_1/2)O₃–PbTiO₃ ( PMN–PYbN–PT) system to enhance the electromechanical properties while increasing the T _c and lowering the sintering temperature. The 0.575[0.5PMN–0.5PYbN]–0.425PT composition at PMN/PYbN (50/50) mole ratio were prepared by reactive sintering PMNT and PYbNT powder mixtures at 950°–1200°C for 4 h. PMNT and PYbNT powders were calcined via the columbite method. Samples were prepared by cold isostatic pressing at 80 MPa. Dense and fully perovskite 0.575[0.5PMN–0.5PYbN]–0.425PT ceramics were fabricated at 975°C for 4 h, and these samples displayed a remnant polarization ( P _r) of 32 μ C/cm², coercive field ( E _c) of 17 kV/cm, and a piezoelectric charge coefficient ( d ₃₃) of 475 pC/N. It is proposed that this ternary system can be tailored for various applications.     

Effect of Slow Crack Growth on the Thermal-Stress Resistance of an Na2O-CaO-SiO2 Glass

The effect of slow crack growth on the thermal-shock resistance of an Na₂O-CaO-SiO₂ glass was studied. An analytic and numerical technique was developed to calculate the critical quenching temperature difference, Δ T_c , of circular rods quenched in water from known crack velocity data. For rods of a specific radius and crack depth, Δ T_c , was calculated to be 147°C, in favorable correspondence with experimentally observed values of 155° to 160°C. In the absence of crack growth, Δ T_c was estimated to be 238°C, well in excess of the observed value and indicative of the significant effect of slow crack growth on thermal-stress resistance. It is also shown that crack growth significantly extends the time-to-failure to a value much greater than the time of maximum thermal stress. A form of subcritical crack instability is predicted at stress intensity factors well below the critical stress intensity factor, at which catastrophic failure becomes inevitable over the duration of the transient thermal stress. It is suggested that, when all other factors are equal, the effect of slow crack growth on thermal-stress resistance can be minimized by maximizing thermal diffusivity. It is also argued that surface-compression strengthening will be more effective than reduction in flaw size in increasing thermal-stress resistance. Recommendations are made for the design and selection of brittle materials subjected to thermal stress in stress-corrosive environments.