High Temperature Bending Strength Research Articles

Brittleness reduction of Al2O3–C refractories: Microstructure evolution and role of carbon fibers

Carbon fibers (Cf) were incorporated into Al2O3–C refractories to enhance their toughness in this work. The results indicated the 0.3 wt% Cf-adding improved the high-temperature bending strength to 1.93 MPa from 1.07 MPa, and led to a 55 % enhancement in thermal shock resistance. The above enhancement could attributed to the in-situ formed SiC coating which was observed on the carbon fiber's surface and strengthened the interface bonding between the carbon fiber and refractory matrix. A solid-liquid diffusion reaction was employed to describe the formation process of in-situ SiC layer on the carbon fibers. Furthermore, the wedge-splitting test with digital images was carried out to investigate the crack-growth trajectory during the splitting process. It was confirmed that Cf-adding could enhance the strength and reduce brittleness with the crack-bridging, pullout, and interlock behaviors of carbon fibers in the refractory matrix.

Enhancement effects of FeCoNiCrMn high-entropy alloy doping on the Ti(C, N)-based cermets: Microstructure and mechanical properties

The enhancement effects of doping Ti(C, N)-based cermets with different contents of FeCoNiCrMn (HEA, high entropy alloy) on the mechanical properties, microstructure, cutting performance and high temperature bending strength of the Ti(C, N)-based cermets were investigated. Ti(C, N) particles had good wettability to the metal liquid phase of HEA, and with the increase of HEA content, the strength of the binder gradually improved. When the content of HEA was 12%, the cermets had the best comprehensive mechanical properties, with a relative density of 99.8%, Vickers hardness of 19.5 ± 0.2 GPa, fracture toughness of 8.2 ± 0.1 MPa·m1/2, and bending strength of 1229 ± 82 MPa. Compared with Co binders, HEA binders had better softening resistance at high temperatures. When the content of HEA exceeded 6%, the bending strength at 700 °C of the Ti(C, N)-based cermet tools were enhanced by 78%–96%, and the cutting life was increased by 35%–65%.

Effect of Si/Al ratio on in-situ synthesis of Al2O3–β-Sialon composite ceramics for solar thermal storage by aluminothermic and silicothermic nitridation

In-situ Al2O3–β-Sialon composite thermal storage ceramics were one-step synthesized by aluminothermic and silicothermic reduction nitridation method, using solid waste coal-series kaolin, Al powder and Si powder as raw materials and firing at 1600 °C in N2 atmosphere. The effects of Si/Al ratio and firing temperature on the phase composition, structure and properties of fired samples were researched by XRD, SEM, DSC-TG, etc. The results show that increasing Si/Al ratio and firing temperature can improve the properties by increasing the content of β-Sialon. However, with the increasing Si/Al ratio, the deviations between actual and designed contents of β-Sialon increase due to Si volatilization, and the properties cannot be further improved when Si content is larger than 16.3 wt%. The fired samples with Si metal and Al metal mass ratio = 64.9% obtained optimal properties: water absorption 8.4%, bulk density 2.58 g·cm−3, room temperature bending strength 82.7 MPa, high-temperature bending strength 81.1 MPa, thermal expansion coefficient (room temperature-1000 °C) 5.06 × 10−6·°C−1, thermal conductivity (at room temperature) 7.77 W·(m·K)−1, specific heat capacity (at room temperature) 0.7 J·(g·K)−1, which mean that it is a good candidate for the thermal storage material.

Enhanced 3D printed Al2O3 core via in-situ mullite

The ceramic core plays an important role in the preparation of aero-engine hollow blades, as it can directly determine the precision and pass rate of the cooling passage in the blade cavity. Sufficient high-temperature bending strength, high open porosity, and low sintering shrinkage are the main requirements for qualified ceramic cores. In order to prepare qualified ceramic cores with complex structures, in-situ mullite-reinforced Al 2 O 3 -based ceramic cores were successfully prepared using vat photopolymerization 3D printing technology. The in-situ synthesis of mullite was designed through thermodynamic analysis. The influence mechanisms of the sintering temperature and doping amount of fused silica on the open porosity, sintering shrinkage, and bending strength of the core were systematically investigated. With an increase in the sintering temperature and doping amount of fused silica, the porosity of the in situ mullite-reinforced ceramic core decreased. Sintering shrinkage increased with an increase in the sintering temperature and doping amount of fused silica. The cores containing 20 wt% fused silica showed the smallest sintering shrinkage. The bending strength increased with an increase in the sintering temperature and the doping amount of fused silica. A core doped with 20 wt% fused silica had the highest bending strength at 1773.15 K. Furthermore, 20 wt% fused silica doping, sintered at 1673.15 K, can be used to prepare in-situ mullite enhanced Al 2 O 3 -based cores with the best comprehensive properties. This core has a higher open porosity (40%), suitable bending strength of 25 MPa (at 1773.15 K), and lower sintering shrinkage in the Z direction. The significance of this study lies in the regulation of in-situ mullite generation and viscous flow in solid-liquid sintering by adjusting the doping amount of fused silica and the sintering process, and this opens up new pathways by the coordinated regulation of strength, open porosity, and sintering shrinkage of 3D printed ceramic cores. • An in situ mullite-enhanced ceramic core was successfully prepared using 3D printing technology. • In-situ mullite was synthesized through thermodynamic analysis in 3D printing ceramic cores. • Strength, porosity and sintering shrinkage of 3D printing ceramic cores were controlled. • A novel method for the coordinated regulation of strength, porosity and sintering shrinkage of 3D printing ceramic cores.

Submicron SiO2 Powder: Characterization and Effects on Properties of Cement-Free Iron Ditch Castables

Submicron materials are those with particle size diameters between 0.1 and 1 μm. Submicron SiO2 generally refers to SiO2 powder with a D90 < 1 μm (D90 refers to the particle size distribution exhibited by the sample and corresponds, in this case, to 90% of the particles not exceeding a diameter of 1 μm). In this study, a new type of cement-free iron ditch castable was prepared using dense corundum and silicon carbide as the primary raw materials with submicron SiO2 powder as the binder. The effects of submicron SiO2 powder content on the bulk density, apparent porosity, linear rate of change, compressive strength, and bending strength were investigated. The mechanism of action of the submicron SiO2 powder was also investigated by analyzing its microstructure and particle size distribution. The results revealed that (1) the submicron SiO2 powder can be used as the sole bonding agent in the preparation of cement-free iron ditch castables; (2) in comparison to traditional castables, the cement-free castable developed in this study demonstrated strong service performance and high-temperature bending strength.

Effects of various sintering additives on the properties of β-SiAlON–SiC ceramics obtained by liquid phase sintering

β-SiAlON–SiC ceramics were obtained by the liquid phase sintering method under N2 atmosphere. The effects of various sintering additives (including Y2O3, ZrO2, ZrSiO4, Suzhou kaolin, petalite, and cordierite) on the structure, thermal expansion coefficients (room temperature–1000 °C), and high-temperature (1400 °C) mechanical properties of β-SiAlON–SiC ceramics were studied. The results indicate that 10 wt% Y2O3 promoted the formation of columnar β-SiAlON crystals resulting in samples with the highest room-temperature bending strength of 103.7 MPa, but an increased thermal expansion coefficient of 4.7 × 10−6 °C−1. A 10 wt% addition of ZrO2 (or ZrSiO4) fractured the samples greatly due to the transformation of cubic ZrO2 to monoclinic ZrO2 during the cooling stage, and the use of Y2O3 as a stabilizer was indispensable for the ZrO2-containing sintering additives. The combined use of Y2O3, ZrO2, and ZrSiO4 exhibited a better effect on the densification of the ceramics than the other sintering additives, and the lowest water absorption of 12.1% was obtained. Y2O3-containing additives caused severe oxidation of the β-SiAlON–SiC ceramics at 1400 °C, decreasing the high-temperature bending strength to 10.3–25.3 MPa. Suzhou kaolin, petalite, and cordierite were also helpful for increasing the nitridation ratio by carbothermic or aluminothermic nitridation of SiO2 and endowed the samples with low thermal expansion coefficients of 3.7–4 × 10−6 °C−1. The addition of Suzhou kaolin resulted in the highest high-temperature bending strength of 50.5 MPa by improving the oxidation resistance of β-SiAlON–SiC ceramics.

Исследование длительной термообработки при 1350 °С высокотемпературного композиционного материала с Nb матрицей, армированной волокнами -Al2O3

Solving the problems when developing high-temperature composite materials (HTCM) requires non-standard approaches, as for example, using the long-term high-heat treatment (HHT), which has a significant effect on mechanical properties of the HTCM at high temperatures. To obtain a niobium-based HTCM reinforced with α-Al2O3 single crystal fibers (“Nb – SCF α-Al2O3”) hot pressing technique was used in the research. The HHT effect at 1350 °C of the HTCM on its high-temperature (1300 °C) bending strength, hardness and density at 22 °C after 100 hours HHT with a step in 25 hours was investigated. The samples structure and elements distribution at the interfacial boundaries of HTCMs were studied. It was established that the elements interdiffusion width at the interphase boundary of the continuous composition “Nb – SCF α-Al2O3” doesn’t exceed 2 μm for the whole HHT term; in outside the interphase boundary, only niobium oxides and carbides were detected. It was found that the bending strength after 25 hours HHT slightly exceeded the strength of the initial sample (before HHT); with further high-temperature HHT, the strength increased by 1.7 – 2 times in comparison with the initial sample. The hardness (HV 0.5) after 25 hours HHT remained actually unchanged (70), and subsequently sharply increased and the average hardness in three aging stages (50, 75 and 100 hours) was 330. The density of HTCMs increased with HHT, and after 100 hours HHT increased in comparison with the original sample by 1.3 times.

Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

ZrB2/x·vol%B4C (x = 30–90) composites were fabricated from ZrB2 and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 102 s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B4C phase was formed at 1600 °C, the relative density (Dr) was evaluated; the composites sintered at 1900 °C attained the highest Dr. Their Dr values increased gradually from 99.35% to 99.99% with increasing B4C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (Hv), fracture toughness (KIC) and three-point bending strength (σb) were measured. Hv exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, KIC and σb revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m1/2 for KIC and 870 MPa for σb, in the 60 vol%B4C sample, and then the KIC decreased gradually to 9.73 MPa m1/2, and σb dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σb in the 90 vol% B4C sample. Next, the high-temperature σb values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σb (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B4C composites at 1600 °C, and then the σb of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.

High-temperature strength of boron carbide with Pt grain-boundary framework in situ synthesized during spark plasma sintering

Grain boundaries, twins, and defects are considered to influence the thermomechanical behavior of any covalent ceramic, as a result, monolithic B4C samples show different curve shapes of bending strength vs temperature and the present theoretical models fail to fit them over the entire temperature range. To overcome these issues, we fabricated a novel high-density boron carbide and evaluated its high-temperature bending strength. The as-obtained ceramic is composed of boron carbide grains and a fine grain-boundary metal Pt framework. The material shows a decreased strength, which is due to a non-linear increase in the volume expansion coefficient of the B4C. Recovery in strength above 1000 °C is due to the presence of twins, their growth and rearrangements. We consider twins rearrangements are the pieces of evidence for a novel ‘micro’ mechanism of high-temperature stress accommodation for the boron carbide bulks.

High temperature mechanical properties of zirconia metastable t'-Phase degraded yttria stabilized zirconia

Abstract Air plasma sprayed (APS) 8 wt%-yttria stabilized zirconia (8YSZ) with metastable tetragonal prime phase (t′) has been widely applied as thermal barrier coatings (TBCs) for gas turbine blades because of its outstanding mechanical properties at high temperatures. In the present research, a carefully designed process was used to prepare 8YSZ samples with different phase composition (t′, t and c) simulating the phase degradation of the material during operation conditions. High temperature (1000–1200 °C) bending strength, elastic modulus, and thermal expansion coefficient were measured, which exhibit strong dependence on the phase degradation during heat treatment. Effect of the phase composition on high temperature thermo-mechanical properties and the enhancement of the bending strength have been discussed, providing a new perspective for further improvements.

Oxidation Resistance and Bending Strength at High Temperatures of Ni/(ZrO<sub>2</sub>+Al<sub>2</sub>O<sub>3</sub>) Hybrid Materials

Oxidation resistance and bending strength at high temperatures of 5 vol% Ni/(10 vol% ZrO2+Al2O3) were investigated in this paper. Oxidation tests were conducted at temperature ranging from 1100 to 1300oC for 1 to 24 h in air. The oxidation resistance of the composites was studied via observation of oxidized-zone development from a cross-section view after oxidation. Three-point bending tests were conducted at temperatures ranging from room temperature to 1200oC in order to estimate its performance at high temperatures. Bending strength of the composites achieved 1200 MPa at room temperature and 460 MPa at 1200oC. Dispersion of ZrO2in Ni/Al2O3composites enhanced both their room and high temperature bending strength. Nevertheless, ZrO2slightly degraded the oxidation resistance of the composites. The oxidation rate of Ni/(ZrO2+Al2O3) was one order of magnitude higher than that of Ni/Al2O3.

Refractory metal silicides reinforced by in-situ formed Nb2O5 fibers and mullite nanoclusters

There is keen interest in the use of refractory metal silicides as structural materials or thermal barrier coatings for a high temperature environment. However, a long-standing problem for these materials is their poor thermal shock property. To address this challenge, Nb-Al-SiC elements were introduced into the MoSi2 matrix and consolidated by in-situ hot pressing. We find that this treatment leads to improved performance of MoSi2 composites in high temperature thermal shock resistance and bending strength. After in-situ HPing, the Nb, Al2O3 particles, and SiC nanoclusters were uniformly dispersed in the MoSi2 matrix and inhibited the movement of dislocation, resulting in a strengthening effect. During the thermal shock process, the fragmentized oxide layer present in the surface of the pure MoSi2 alloy disappeared completely, and a dense multi-component oxide layer was formed in-situ on the surface of the MoSi2 composites. The dense multi-component oxide layer was composed of SiO2 glass, fiber-structured Nb2O5, and nano-sized mullite phases. The fiber structured and nano-sized oxide phases play an important role in strengthening the oxide layer.

Enhanced Sintering of Boron Carbide-Silicon Composites by Silicon

Boron carbide (B4C)-silicon (Si) composites have been prepared by aqueous tape casting, laminating, and spark plasma sintering (SPS). The influences of silicon (Si) content on the phases, microstructure, sintering properties, and mechanical properties of the obtained B4C-Si composites are studied. The results indicate that the addition of Si powder can act as a sintering aid and contribute to the sintering densification. The addition of Si powder can also act as a second phase and contribute to the toughening for composites. The relative density of B4C-Si composites samples with adding 10 wt.% Si powder prepared by SPS at 1600 °C and 50 MPa for 8 min is up to 98.3%. The bending strength, fracture toughness, and Vickers hardness of the sintered samples are 518.5 MPa, 5.87 MPa m1/2, and 38.9 GPa, respectively. The testing temperature-dependent high-temperature bending strength and fracture toughness can reach a maximum value at 1350 °C. The B4C-Si composites prepared at 1600, 1650, and 1700 °C have good high-temperature mechanical properties. This paper provides a facile low-temperature sintering route for B4C ceramics with improved properties.

Interfacial microstructure and high-temperature strength in silicon nitride/nickel-based superalloy bonding

Reliable Si3N4/nickel-based superalloy joints were obtained by partial transient liquid phase bonding with a Ni/Cu/Ti multi-interlayer. The flexural strengths of the joints were investigated at elevated temperatures. The maximum high-temperature bending strength of 115 MPa was achieved when the joint was tested at 1073 K. Microstructures and compositions of the joints after high-temperature testing were analyzed by scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy (TEM). A reaction layer with a thickness of about 3 μm was developed at the interface. The TEM results revealed a fine grain microstructure in the reaction layer of the joint after undergoing bending test at 1073 K. Cu-rich precipitates are observed in the fine-grained reaction layer by TEM. In addition, the high-temperature fracture mechanism of the joints was discussed. The fractures of the joints at room temperature exhibited a brittle fracture characteristic, while plastic deformation of the interlayers in the joint occurred at elevated temperatures.

Facile and Economical Preparation of SiAlON-Based Composites Using Coal Gangue: From Fundamental to Industrial Application

The present study aims to synthesize SiAlON-based composites utilizing coal gangue. Different types of SiAlON-based composites were synthesized using coal gangue by carbothermal reduction nitridation method through control of different reaction atmospheres. The experimental results indicate that the oxygen partial pressure was an essential factor in the manufacture of SiAlON-based composites and under proper control of the atmospheres, SiAlON-based composites with different crystal structures could be synthesized. The optimum conditions of synthesis of different SiAlON-based composites were respectively determined. Based on the laboratory results, a prototype plant was proposed and constructed, and β-SiAlON composite was successfully produced using coal gangue. The synthesized β-SiAlON composite was applied in preparation of iron ladle brick instead of SiC, which showed that the compression strength, refractoriness under load and high temperature bending strength were increased from 44.5 ± 6.7 MPa, 1618 ± 21 °C and 5.4 ± 1.2 MPa to 64.1 ± 2.5 MPa, 1700 ± 28 °C and 7.1 ± 1.6 MPa, respectively. Compared with the traditional synthesis method, the present technique is expected to save energy both in raw materials and technical process.

Dopant effects on high temperature mechanical properties of zirconium carbide ceramics

Combining theoretical and experimental methods, the effects of 5 vol.-% WC dopant on the microstructure evolution, sintering behaviour and mechanical properties of ZrC ceramics were investigated. WC dopant was found to improve the high temperature elastic modulus and bending strength of ZrC ceramics. Both calculations and experimental results showed that the formation of (Zr, W)C solid solution promote dissolution of the impurity oxygen from the starting powder into the lattice sites, resulting in less oxygen defects in grain boundaries. Internal friction curve can also conform that the ZrC ceramics doped with WC have cleaner grain boundaries, which improved higher elastic modulus and bending strength in WC doped ZrC ceramics at elevated temperature.

Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies

TiAl intermetallic and SiC ceramic are two kinds of important high-temperature structural material, but it is difficult to fabricate complex high-performance composites of them by traditional methods. In the paper, it is a pioneer study to prepare their composites so as to fabricate turbine blade by the hybrid technology of Stereolithography and gelcasting. Impregnation and pyrolysis behaviors of polycarbosilane were explored for SiC ceramic preparation, and the microstructure evolution and mechanical properties of TiAl-based composites were mainly analyzed. The results showed that adding trace amount of nickel and aluminum could remarkably promote the sintering of gelcasting TiAl intermetallic samples, the micropores inside TiAl samples were connected after debinding, and the metallurgical structures were all intermetallic. After the porous TiAl samples impregnated with polycarbosilane were then pyrolyzed three times, the final metallurgical structures were TiAl-based composites of TiAl, NiAl, SiC, TiC and Ti3SiC2. Ti3SiC2 was the interface layer formed between TiAl intermetallic and SiC ceramic, and their high-temperature bending strength was between 82MPa and 90MPa at 1100°C. Therefore, it was a promising method for the fabrication of complex high-performance TiAl-based parts such as turbine blade.

Mechanical and thermal physical properties, and thermal shock behavior of (ZrB2 + SiC) reinforced Zr3[Al(Si)]4C6 composite prepared by in situ hot-pressing

The mechanical and thermal physical properties, as well as thermal shock behavior of in situ hot-pressed 30vol.% (ZrB2+SiC)/Zr3[Al(Si)]4C6 composite have been investigated and compared with monolithic Zr3[Al(Si)]4C6 ceramic. The composite shows superior hardness (Vickers hardness of 17.5GPa), stiffness (Young’s modulus of 418GPa), strength (room-temperature bending strength of 648MPa, and high-temperature bending strength of 439MPa at 1300°C in air), and toughness (fracture toughness of 7.69MPam1/2) compared with Zr3[Al(Si)]4C6. The composite can retain a high modulus of 362GPa at 1350°C (87% of that at ambient temperature). In addition, the composite exhibits higher specific heat capacity and thermal conductivity but slightly lower coefficient of thermal expansion compared with Zr3[Al(Si)]4C6. The critical thermal shock temperature difference (ΔTc) increases from 225°C for Zr3[Al(Si)]4C6 to 345°C for the composite, and the calculation results of thermal shock resistance parameters also indicate a much improved thermal shock resistance of the composite.

J0470103 長繊維強化自己治癒セラミックスの高温機械的特性([J047-01]自己治癒材料・システム(1),機械材料・材料加工部門)

Mechanical properties, especially high-temperature bending strength and bending creep strength, were investigated on self-healing fiber reinforced ceramics, shFRC, at elevated temperatures. shFRC consists of alumina continuous fiber bundle, Al_2O_3 matrix and SiC interlayer oriented along fiber bundle as self-healing agent. Bending strength of smoothed samples were obtained 82-120 MPa ranging at 1000-12000℃, higer than 85 MPa of ones at room temperature. Every specimens were deformed plasticaly. Bending creep was obtained over 100 h durability at 1000℃, weareas specimen was fractured for 2 h at 120℃. Both of specimen conducted high-temperature tests were broken from compressed surface at 1000℃, weareas and tensio surface at 1200℃. Different fracture mechanism would be resulted from tmeperature dependence of plasticity of Al_2O_3 matrix and leads higher bending strength than room temperature. Although bending test accomplished to evaluated the values of mechanical properties at high-temeprature, precise evaluation is expected using tensile test.

A dense and tough (B4C–TiB2)–B4C ‘composite within a composite’ produced by spark plasma sintering

A (B 4 C–TiB 2 )–B 4 C “composite within a composite” was spark plasma sintered using a mixture of B 4 C and B 4 C–TiB 2 eutectic powders. The effect of the eutectic powders’ morphology on the mechanical characteristics of composites is presented. Composites of eutectic powder with naked TiB 2 fibers consolidated in nitrogen and argon atmospheres resulted in dense ceramics with improved mechanical properties . The room temperature and high temperature (1600 °C) bending strength and fracture toughness of the (B 4 C–TiB 2 )–B 4 C composite consolidated in a nitrogen atmosphere reached 387.2 MPa, 399.8 MPa and 7.76 MPa m 1/2 , respectively.