Effect of Si3N4 Addition on Compressive Creep Behavior of Hot‐Pressed ZrB2–SiC Composites

Compressive creep studies have been carried out on hot‐pressed ZrB₂–SiC (ZS) and ZrB₂–SiC–Si₃N₄ (ZSS) composites in air under stress and temperature ranges of 93–140 MPa and 1300°C–1425°C, respectively for time durations of ≈20–40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350°C, and as ≈470 ± 20 kJ/mol in the range of 1350°C–1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB₂ grain boundaries and ZrB₂–SiC interfaces. These results along with calculated values of activation volumes suggest grain‐boundary sliding as the major damage mechanism, which is controlled by O^2? diffusion through SiO₂ at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si₂N₂O near ZrB₂–SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values <1 suggests that grain‐boundary sliding involving solution‐precipitation‐type mechanism is operative at these temperatures.     

Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing

A refractory fused silica based integrally cored ceramic mold, the ceramic core with a ceramic mold shell in a single patternless construction, is fabricated by ceramic stereolithography of additive manufacturing. Refractory ceramic molds should satisfy the following restrictions such as similar strength to that of cast metal during solidification, thermal stability for dimensional accuracy, and easy removal of core after casting. Here, we report mechanical and transformation kinetic studies on the refractory fused silica of integrally cored ceramic mold. The flexural strength of sintered silica continually increases with higher density of better densification up to 11.4 MPa at 1300 °C, while it decreases from 11.3 MPa at 1350 °C to 4.6 MPa at 1500 °C. The degradation of the flexural strength is related to the larger amount of the cristobalite and microcracks generated by the abrupt contraction induced during the transformation of beta to alpha cristobalite. Given the quantitative x-ray diffraction study on transformation kinetics, an apparent activation energy Q is 674 ± 53 kJ/mol and the average time exponent 1.85, suggesting that the transformation kinetic is controlled by 1-dimensional interfacial growth.     

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2-SiC composites

A study has been carried out to examine the effect of LaB₆ addition on the compressive creep behavior of ZrB₂-SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB₂-20 vol% SiC composites containing LaB₆ (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B₄C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300°C than the ZSBCL-10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB₂ grain boundaries and ZrB₂-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL-14 composite at 1300–1400°C suggest that ZrB₂ grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep-based deformation and damage at the inner core of samples.     

Tensile creep behavior of a ytterbium silicon oxynitride–silicon nitride ceramic

Tensile creep behavior of hot pressed silicon nitride on the Si₃N₄–Yb₄Si₂O₇N₂ tie line was investigated at temperatures of 1300 and 1400 °C under an applied stress of 125 to 200 MPa. During the tests, the creep strain increased with time and the creep rate monotonically decreased both with time and strain. On the basis of minimum strain rates, the stress exponents for 1300 and 1400 °C were determined to be 3.1 and 1.7, respectively. All the specimens tested at 1400 °C lead to failure while exhibiting a large scatter in the time-to-failure data. The activation energy was determined to be 879 kJ/mol from a comparison between creep rates at different temperatures. The creep mechanism is discussed on the basis of the creep parameters and creep damage observation.     

Ultra-high creep resistant SiC ceramics prepared by rapid hot pressing

The high-temperature compression creep of additive-free β/α silicon carbide ceramics fabricated by rapid hot pressing (RHP) was investigated. The creep tests were accomplished in vacuum at temperature range 1500 °C–1750 °C and compressive loads of 200 MPa to 400 MPa. Under investigated condition the RHP ceramics possessed the lowest creep rate reported in the literature. The observed strain rates changed from 2.5 × 10^?9 s^?1 at 1500 °C and a lowest load of 275 MPa to 1.05 × 10^?7 s^?1 at 1750 °C and a highest load of 400 MPa. The average creep activation energy and the stress exponent remain essentially constant along the whole range of investigated parameters and were 315 ± 20 kJ?mol^?1, and 2.22 ± 0.17, respectively. The suggested creep mechanism involves GB sliding accommodated by GB diffusion and β?α SiC phase transformation.     

Improved creep resistance of high entropy transition metal carbides

The creep behaviour of (Ta-Hf-Zr-Nb)C high entropy ceramic (HEC) was investigated at temperatures between 1400 and 1600 °C in vacuum under compressive stresses from 150 to 300 MPa. The measured steady-state creep rates ranged from 2 × 10⁻⁹/s to 8 × 10^-8/s, which are approximately 10 times lower than the published creep rates of the corresponding monocarbides. The stress exponent n is in the range of 2.34 ∼ 2.89 and the average activation energy is 212 kJ/mol. The creep mechanisms involve dislocation glide/climb and the formation of voids and cracks. The voids formed at the grain boundaries parallel to the loading direction, which often connected to form cracks at the highest load/temperature The active dislocation slip system during creep was &lt,110&gt,{111}. The reason why (Ta-Hf-Zr-Nb)C has enhanced creep resistance compared to the monocarbides can be explained by lattice distortion and the higher thermodynamic stability of HEC ceramic at high temperatures.     

Effect of LaB6 additions on densification,microstructure, and creep with oxide scale formation in ZrB2-SiC composites sintered by spark plasma sintering

A comparative study has been carried out on densification, microstructure, and creep with oxide-scale formation in ZrB₂-20 vol.% SiC-(7, 10 or 14 vol.%) LaB₆ composite containing B₄C and C as additives, and prepared by spark plasma sintering at 1800 °C under 70 MPa ram pressure. Addition of LaB₆ has promoted densification of composites by scavenging oxygen impurity, thereby increasing their hardness. Constant load compressive creep tests at 1300 °C under 47 and 78 MPa stresses have shown the lowest creep rate in the 10 vol.% LaB₆ composite. The stress exponents obtained for composites having 10 vol.% LaB₆ (～1.3 ± 0.1) and 14 vol.% LaB₆ (～2.6 ± 0.2) suggest respectively, grain boundary diffusion with intergranular glassy phase formation and dislocation glide as operating mechanisms. Intergranular cracking caused by grain boundary sliding appears as the damage mechanism. Oxide scales formed during creep exhibit greater thickness and defect concentration than those by isothermal exposure at 1300 °C within similar duration.     

Oxidation resistance of AlN–(TiB2–TiSi2) ceramics in air up to 1450 °C

With the aid of DTA, TG, XRD, SEM and EPMA methods the kinetics and mechanism of oxidation of both composite powders and monolithic ceramics of AlN–(TiB₂–TiSi₂) system with different content of components were studied in the air up to 1450 °C under isothermal and non-isothermal conditions. It was established that the oxidation isotherms of monolithic ceramics follow the parabolic and paralinear rate law. According to the kinetic data and results of investigation of composition, morphology and structure of oxide films that are formed at different temperatures, the AlN–based ceramics containing up to 30% (TiB₂–TiSi₂) solid solutions are the corrosion-resistant and have the high adhesion of oxide layer in relation to substrate material. The activation energies of oxidation calculated for ceramics with 10% (TiB₂–TiSi₂) are: E₁ =180 kJ/mol for the temperature range up to 1300 °C and E₂=630 kJ/mol at 1350–1450 °C. The change of activation energy value is associated with the change of oxide layer composition: at the oxidation till 1300 °C the oxides of individual elements are, mainly, formed on the samples while above 1350 °C the formation of very dense surface film containing β-tialite Al₂TiO₅ takes place.     

Interfacial microstructure and mechanical properties of ZTA/ZTA joints brazed with Ni-Ti filler metal

The reliable brazing of the ZTA ceramic joints was successfully obtained using Ni-Ti filler metal. The microstructure and mechanical properties of the joints brazed at different temperatures were investigated. During the process of brazing, both Al₂O₃ and ZrO₂ in the ZTA reacted with the Ni-Ti filler, resulting in the formation of the AlNi₂Ti + Ni₂Ti₄O reaction layer adjacent to the ZTA substrate when brazed at 1350 °C for 30 min. NiTi and Ni₃Ti compounds precipitated at the center of brazing seam. When the brazing temperature increased from 1320 °C to 1380 °C, the thickness of AlNi₂Ti + Ni₂Ti₄O layer increased gradually. As the brazing temperature varied from 1400 °C to 1450 °C, TiO was formed adjacent to the ZTA substrate, along with the reduction of Ni₂Ti₄O. AlNi₂Ti distributed at the interface and center of brazing seam. The maximum shear strength of 152 MPa was obtained when brazed at 1420 °C for 30 min.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Creep behaviour of dense and porous SrTi0.75Fe0.25O3-δ for oxygen transport membranes and substrates

Considering the challenging conditions imposed by application of membranes in an asymmetric design, in particular creep resistance of the substrate material is an important parameter for the stability in long-term operation. As promising material, in terms of chemical stability, the perovskite SrTi_0.75Fe_0.25O_3-δ has been identified in previous works. Porous supports with different microstructures have been produced using different manufacturing methods and compared to the material in its fully dense state regarding creep behaviour. The creep deformation of pressed, porous tape-cast and freeze-dried SrTi_0.75Fe_0.25O_3-δ specimens has been investigated in the application relevant temperature range of 850–1000?°C under compressive stresses of 15, 30 and 45?MPa. A global fitting method considering all experimental data was used to derive stress exponent and activation energy of SrTi_0.75Fe_0.25O_3-δ, which are 2.9?±?0.4 and 402?±?25?kJ/mol, respectively. Thus, it is suggested that the mechanism controlling creep is mainly related to dislocation climb/glide.     

A novel microwave dielectric ceramic La5Sn4O15 with medium-permittivity and low loss

A novel low-loss La₅Sn₄O₁₅ ceramic is fabricated via traditional solid-state route. The investigation mainly focused on phase composition, dielectric characteristics and crystalline from 1300 to 1400 °C. The XRD analyses confirmed the La₅Sn₄O₁₅ ceramic is consisted of cubic and hexagonal structure. The effects of relative density on the Q × f and permittivity are also investigated. The satisfactory properties (ε_r of 17.4, Q × f of 84,760 GHz and τ_f of ?19.08 ppm/°C) are obtained at optimum sintering condition (sintered at 1350 °C for 4 h), indicating La₅Sn₄O₁₅ ceramic is a promising ceramic for using in wireless applications region.     

Compressive creep of SiC whisker/Ti3SiC2 composites at high temperature in air

The compressive creep of a SiC whisker (SiC_w) reinforced Ti₃SiC₂ MAX phase-based ceramic matrix composites (CMCs) was studied in the temperature range 1100-1300°C in air for a stress range 20-120 MPa. Ti₃SiC₂ containing 0, 10, and 20 vol% of SiC_w was sintered by spark plasma sintering (SPS) for subsequent creep tests. The creep rate of Ti₃SiC₂ decreased by around two orders of magnitude with every additional 10 vol% of SiC_w. The main creep mechanisms of monolithic Ti₃SiC₂ and the 10% CMCs appeared to be the same, whereas for the 20% material, a different mechanism is indicated by changes in stress exponents. The creep rates of 20% composites tend to converge to that of 10% at higher stress. Viscoplastic and viscoelastic creep is believed to be the deformation mechanism for the CMCs, whereas monolithic Ti₃SiC₂ might have undergone only dislocation-based deformation. The rate controlling creep is believed to be dislocation based for all the materials which is also supported by similar activation energies in the range 650-700 kJ/mol.     

Preparation and characterization of mullite microspheres based porous ceramics with enhancement of in-situ mullite whiskers

High-strength insulating ceramic materials were prepared using lightweight mullite microspheres with dense surfaces and high internal porosity as the main raw material and silica sol as a binder. The effects of AlF₃·3H₂O content on the in situ formation and growth of mullite whiskers were analyzed by X-ray diffraction and scanning electron microscopy. The obtained results showed that mullite whiskers were formed in large quantities at 1200 °C using AlF₃·3H₂O and V₂O₅ as additives; their optimal growth was observed at 4 wt% AlF₃·3H₂O and 1 wt% V₂O₅. The apparent porosity of the produced specimens was 39%; the MOR and CCS of the specimens were 31 and 152 MPa, respectively; the HMOR at 1300 °C was 11.32 MPa; and the thermal conductivity at 900 °C was 0.783 W m⁻¹ K⁻¹. The staggered whisker network structure formed between mullite microspheres not only improved the mechanical properties of the material, but also refined its pore size, reduced the thermal conductivity, and enhanced the thermal insulation properties.     

High-temperature compressive creep of novel fine-grained orthorhombic ZrO2 ceramics stabilized with 12 mol% Ta doping

A novel fine-grained orthorhombic ZrO₂ ceramic stabilized with 12?mol% Ta doping was fabricated by spark-plasma sintering from home-made powders, and its high-temperature mechanical properties evaluated for the first time by compressive creep tests in both Ar and air. It was found that the high-temperature plasticity of the ceramic deformed in Ar, under which the Ta-doped orthorhombic ZrO₂ is a black suboxide with abundant oxygen vacancies in its crystal structure, is controlled by grain boundary sliding (stress exponent ～2, and activation energy ～780–800?kJ/mol). However, the high-temperature plasticity of the ceramic deformed in air, under which the Ta-doped orthorhombic ZrO₂ is a white oxide due to the elimination in situ of oxygen vacancies, is controlled by recovery creep (stress exponent ～3, and activation energy ～750?kJ/mol). It was also observed that black Ta-doped orthorhombic ZrO₂ is more creep resistant than its white counterpart with the same grain size, and that the former deforms as the more conventional Y₂O₃-stabilized ZrO₂ does.     

Flexural creep of zirconium diboride–silicon carbide up to 2200 °C in minutes with non-contact electromagnetic testing

Flexural creep of ZrB₂–30 vol% SiC ultra high temperature ceramic composite was studied at 1700–2200 °C and 20–50 MPa using the novel method of electromagnetic Lorentz force loading of electrically heated specimens. Experiments were conducted in air and in non-oxidizing atmospheres. The apparent activation energy for creep was 344 ± 35 kJ/mol for non-oxidizing conditions. The stress exponent was 1.4 ± 0.4. The creep rate was slightly higher in air due to a decrease in the size of the load bearing substrate because of oxidation. There was no evidence of electric field effects. Creep experiments could be performed up to 2200 °C very quickly, with experiments conducted in a few minutes.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Mechanical loss,creep, diffusion and ionic conductivity of ZrO2-8 mol%Y2O3 polycrystals

Polycrystalline ZrO₂-8 mol%Y₂O₃ was investigated by combining several experimental techniques on identical materials sintered out of the same high purity powder. The mechanical loss spectrum (damping and elastic modulus) was measured in a large frequency and temperature range (10⁻²Hz–1.5kHz; −150 to 1400°C). Damping due to point defect relaxation at low temperature and to viscoelastic relaxation at high temperature was revealed. The creep resistance was investigated with four-point bending tests (stress and temperature ranges: 20–75 MPa, 1100–1290°C), indicating Nabarro-Herring creep as the main rate-controlling mechanism. Both viscoelastic deformation and creep seem to be controlled by cation diffusion. Measurements of the ⁹⁶Zr tracer diffusivity by secondary ion mass spectrometry at 1125–1460°C yielded an activation enthalpy of 460 kJ/mol. Close values were obtained for creep (440 kJ/mol) and viscoelastic relaxation (530 kJ/mol). Finally, the ionic DC-conductivity of these electrolytes was measured with high accuracy in the range 300–1250°C.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Multiple high-temperature resistant phases modified phosphate-based adhesive for engineering ceramic connection in extreme environment

The lower-strength defect of inorganic phosphate adhesive had been definitely improved by self-generating multiple high-temperature resistant phases. Compared to our previous product, the best bonding performance of this novel adhesive for mullite was increased by 270%, which was close to some popular preceramic polymer-based adhesives. The apparent shear strength at room temperature was up to 33.1?MPa after calcination at 900?°C, while the high-temperature strength researched 23.3?MPa at 900?°C and maintained above 17?MPa from 700° to 1200?°C. The reinforced effect of adhesive owed to the introduction of various Cu-based intermetallics, the premature generation of Al₄B₂O₁₈ at 900?°C, and the structure optimizing through the oxidization of Si and B₄C. Besides, the novel adhesive displayed good resistance to thermal-shock, especially for air-cooling test. After 15 thermal cycles in air, the residual strength of 1300?°C-calcined joints was still above 13?MPa (~40%).