High Temperature Flexural Strength Research Articles

High Temperature Flexural Strength, Microstructure, and Phase Evolution of Quartz Fiber/Boron Phenolic Resin Ceramizable Composite Modified with W and B4C

In order to investigate the effect of refractory metal on the high temperature properties of phenolic resin matrix composites, modified quartz fiber reinforced ceramizable composites were prepared by a molding process with a refractory component, tungsten, as the functional component and boron carbide as the ceramic forming agent. The effects of the tungsten and the boron carbide on the heat resistance of the composite were investigated. The results showed that the introduced refractory metal tungsten and the boron carbide can react to form tungsten borides and tungsten carbides at high temperature, and form a ceramic layer on the surface of the composite, which can fill the defects caused by pyrolysis of matrix and improve the high temperature performance of the composite. When the content of boron carbide was 10 wt% and the content of tungsten powder was 30 wt%, the flexural strength of the composite before and after heat treatment at 1200 °C were increased by 54.6% and 30.2% respectively compared with that without filler.

Infiltration of silica slurry into silica-based ceramic cores prepared by selective laser sintering based on pre-sintering

The silica-based ceramic core is an important mold for manufacturing hollow blades. The preparation of silica-based ceramic cores by combining selective laser sintering (SLS) and vacuum infiltration (VI) technologies is a promising approach. To enhance infiltration effect, pre-sintering was introduced to obtain the bodies with high porosity and hydrophilicity. The infiltrated silica content increased from 29.58 wt% for samples infiltrated once to 68.91 wt% for samples infiltrated three times. The increase of silica content in the matrix reduced the porosity of final ceramics, thus improving the room-temperature flexural strength. The infiltrated silica gradually transformed into cristobalite during the high-temperature test, which improved the high-temperature flexural strength and creep resistance. When the final-sintering temperature was 1225 °C, room-temperature and high-temperature flexural strength reached 21.17 MPa and 21.21 MPa, respectively, due to the enhanced sintering necks and low porosity. These results indicate that our work provides a promising route to fabricate silica-based ceramic cores by SLS technology.

High-Temperature Flexural Strength of Aluminosilicate Ceramic Shells for the Investment Casting of Nickel-Based Superalloy

High-Temperature Flexural Strength of Aluminosilicate Ceramic Shells for the Investment Casting of Nickel-Based Superalloy

High temperature flexural strength, microstructure, phase evolution and anti-oxidation mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C

Carbon fiber/phenolic resin composites (CF/Ph) have attracted great interests in the field of thermal protection materials for their characteristics of high specific strength and easy manufacturing. However, CF/Ph are inherently susceptible to oxidation failure at elevated temperatures. In this study, a novel Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C was fabricated by an impregnating and compression molding route. Thermal stability, flexural strength, microstructure and phase evolution of the resulting ceramizable composite were studied. The residue yield at 1400 °C and flexural strength after treated at 1400 °C for 15min was 90.4% and 53.1 MPa, respectively, which was increased by 15.9% and 532.1% than that without ceramizable fillers. Surface defects generated by matrix pyrolysis were well healed, and PyC and carbon fibers were covered with dense ceramic layers while the fracture surface was covered with relatively continuous ceramic layers without visible pores. Multiphase ceramics composed of TiB2, TiO2, TiC and PyC were identified. Furthermore, oxidation failure and anti-oxidation mechanism was revealed based on the aforementioned characterizations and thermodynamic calculation results. Oxidation resistance got enhanced markedly for synergistic effects of oxygen consuming, carbon fixation, oxygen barrier and endothermic effect, which were derived from ceramization reactions between TiB2, B4C, O2, Al and PyC.

Microstructure and properties of silica-based ceramic cores by laser powder bed fusion combined with vacuum infiltration

The silica-based ceramic core has attracted much attention in the preparation of hollow blades due to its great leachability. In this paper, the silica-based ceramic cores reinforced with ZrSiO4 were prepared by laser powder bed fusion (LPBF) combined with vacuum infiltration (VI). To enhance the infiltration effect, the pre-sintered bodies with high porosity and hydrophilicity were obtained by pre-sintering at 1100 °C. Results showed that a large number of silica particles infiltrated into the pre-sintered bodies. The infiltrated silica promoted the generation of liquid phase in sintering, thereby promoting the removal of pores and the connection of grains. Nevertheless, the dispersed ZrSiO4 grains prevented the viscous flow of the liquid phase, thereby increasing the porosity. ZrSiO4 grains could hinder the propagation of cracks due to their high strength. When the addition of ZrSiO4 was 10 wt.%, room-temperature flexural strength of silica-based ceramic cores infiltrated with slurry S1 (the mass ratio of silica sol to silica powder was 10:1) reached 17.21 MPa due to the reinforcement of sintering necks. Moreover, high-temperature flexural strength reached 13.90 MPa. Therefore, the pre-sintering process could greatly improve the mechanical properties of silica-based ceramic cores prepared by LPBF-VI technology.

Characterization of thermophysical and mechanical properties of hafnium carbonitride fabricated by hot pressing sintering

Dense HfCxN1−x carbonitride ceramics are very promising as potential ultra-high temperature ceramics (UHTCs) for application under extremely harsh environments. However, the thermophysical and mechanical properties of the HfCxN1−x carbonitrides have not been investigated clearly. The present work prepared HfCxN1−x (x = 0.3, 0.4, 0.5, 0.6, 0.7) ceramics at 1950 °C under 30 MPa in flowing Ar atmosphere by using hot pressing sintering method. The relative densities of the samples obtained reached above 96%. Thermal conductivity of the as-prepared HfCxN1−x carbonitrides ranged from 19 to 24 W m−1 K−1 at room temperature. The increased role of electrons in thermal conduction caused by both increasing nitrogen content and increasing temperature, resulted in improved thermal conductivity, varying from 32 to 39 W m−1 K−1. With increasing nitrogen content, the electrical conductivity also increased, ranging from 149 to 213 × 104 Ω−1 m−1. With the increase of nitrogen content, Hf-C covalent bonds are gradually replaced by Hf-N covalent bonds with lower bond strength, resulting in HfC0.7N0.3 exhibiting the highest room-temperature flexural strength and hardness, HfC0.3N0.7 exhibiting the highest fracture toughness. Their mechanical properties are greatly improved over the binary HfC and HfN. The high-temperature flexural strength of the HfC0.7N0.3 decreased from 324 MPa at 1000 °C, to 139 MPa at 1600 °C and 100 MPa at 2000 °C. Meanwhile, it was revealed that the high-temperature flexural strength decreased with increasing nitrogen content for the as-prepared HfCxN1−x carbonitrides, similar to the changing trend of room-temperature flexural strength. The HfC0.3N0.7 possessed high-temperature plasticity at 2000 °C, attributed to the ability of the coarser grain to produce numerous layer dislocations.

Effects of ZrSiO4 content on properties of SiO2-based ceramics prepared by digital light processing

SiO2-based ceramic cores are widely used in the preparation of gas turbine engine hollow blades due to their excellent chemical stability and easy removal after casting. In this paper, ZrSiO4 reinforced SiO2-based ceramics were fabricated using digital light processing (DLP) technology. The results showed that the addition of ZrSiO4 reduced the cure depth due to its high UV light absorptivity and refractive index. When the content of ZrSiO4 increased to 15 wt%, the cristobalite content reached the maximum, and radial shrinkage reached the minimum of 1.4%. ZrSiO4 grains could hinder the propagation of cracks, enhancing the room-temperature flexural strength. At 1550 °C, fracturing across SiO2 grains in SiO2-based ceramics led to the great improvement of high-temperature flexural strength. When the content of ZrSiO4 reached 15 wt%, the flexural strength at room temperature and high temperature was 11.5 MPa and 36.7 MPa, respectively. Therefore, the SiO2-based ceramics prepared using DLP technology have good room temperature and high temperature properties, and are expected to be used for hollow blade casting.

Preparation of high-performance silica-based ceramic cores with B4C addition using selective laser sintering and SiO2–Al2O3 sol infiltration

Silica-based ceramic cores are commonly employed to fabricate hollow aero-engine blades during investment casting. However, achieving excellent performance and high precision remains an extremely challenging problem. In this study, silica-based ceramic cores with boron carbide (B4C) addition were prepared by selective laser sintering (SLS) and SiO2–Al2O3 sol infiltration. The effects of B4C content and mass ratio of alumina to silica in the SiO2–Al2O3 sol on the mechanical properties of the obtained silica-based ceramic cores were explored. The results suggested that the room temperature flexural strength was gradually improved from 3.39 MPa to 27.35 MPa at B4C content from 0 to 2 wt% due to the oxidation of B4C, and the linear shrinkage gradually increased to 4.61%. Furthermore, the high temperature flexural strength was enhanced from 4.86 MPa (non-infiltrated) to 11.84 MPa at mass ratio from 0 to 1 because of the generated mullite phase and the linear shrinkage decreased to 1.38%, which can meet the application requirements for the preparation of ceramic cores. Consequently, it shows the capability to prepare high-performance silica-based ceramic cores with B4C addition by combining SLS technology and SiO2–Al2O3 sol infiltration process.

Sample selection for models to represent ceramic cores fabricated by stereolithography three-dimensional printing

In this study, ceramic cores were fabricated by stereolithography three-dimensional printing (SLA-3DP) with excellent synthetic mechanical properties. To this end, green body samples were first three-dimensionally printed in three models, after which the samples were sintered at different temperatures. High-temperature flexural strengths of the ceramic core samples printed in model 2 reached a maximum value of 24.2 MPa at a sintering temperature of 1250 °C during casting heat test at 1550°C. Meanwhile, the high- and room-temperature flexural strengths of ceramic core samples printed in model 2 were much stronger than those printed in models 1 and 3. Moreover, shrinkage, porosities, bulk densities and phase compositions of ceramic core samples printed in three models at different sintering temperatures did not show obvious differences. The ceramic core samples showed distinct mechanical properties as they had different fracture modes. We call this the “surface effect”. The anisotropy flexural strength was induced by weak binding between layers. The recommended choice of sample for different models to represent ceramic cores fabricated by stereolithography three-dimensional printing is model 2. Herein, based on the results of systematic characterization of high-throughput samples, we report the basic research, evaluation and prediction system of composition design, process optimization, microstructure and property defect analysis of ceramic cores made of SLA-3DP.

Influence of equiatomic Zr/(Ti,Nb) substitution on microstructure and ultra-high strength of (Ti,Zr,Nb)C medium-entropy ceramics at 1900 °C

High-temperature mechanical properties of medium-entropy carbide ceramics have attracted significant attention. Tailoring the microstructure is an effective way to improve these high-temperature mechanical properties, which can be affected by the evolution of the enthalpy and entropy, as well as by lattice distortion and sluggish diffusion. In this study, the effects of equiatomic Zr/(Ti,Nb) substitution (Zr content of 10–40 at%) on the microstructure and high-temperature strength of (Ti,Zr,Nb)C medium-entropy ceramics were investigated. The grain size of the (Ti,Zr,Nb)C medium-entropy ceramics was refined from 9.4±3.7 to 1.1±0.4 μm with an increase in the Zr content from 10.0 to 33.3 at%. A further increase in the Zr content to 40 at% resulted in a slight increase in the grain size. At 1900 °C, the (Ti,Zr,Nb)C medium-entropy ceramics with the Zr contents of 33.3 and 40 at% exhibited ultra-high flexural strengths of 875±43 and 843±71 MPa, respectively, which were higher than those of the transition metal carbides previously reported under similar conditions. Furthermore, relatively smooth grain boundaries, which were detected at a test temperature of 1000 °C, transformed into curved and serrated boundaries as the temperature increased to 1900 °C, which may be considered the primary reason for the improved high-temperature flexural strength. The associated mechanism was analyzed and discussed in detail.

Improved mechanical properties of SiC fiber reinforced silica-based ceramic cores fabricated by stereolithography

• The SiC fiber reinforced silica-based ceramics were prepared via stereolithography. • Excellent high temperature mechanical properties were obtained by the reinforcement of SiC fibers. • SiC fiber can reduce the linear shrinkage of silica-based ceramics greatly through the oxidation reaction of SiC. • The manufacturing precision of ceramic parts is improved a lot. Silica-based ceramic cores have been widely used to fabricate aero-engine hollow blades due to their moderate high temperature mechanical properties and excellent leachability. In this study, silica-based ceramics with SiC fiber addition were prepared via stereolithography, and the influence of SiC fiber content on mechanical properties of the obtained silica-based ceramics was investigated. With the increase of SiC fiber content, linear shrinkage gradually decreased, while room temperature flexural strength and high temperature flexural strength first increased and then decreased. As SiC fiber content increased to 4.0 wt%, linear shrinkage was reduced to 0.62% resulting from the oxidation of SiC. Furthermore, room temperature flexural strength was improved from 11.79 MPa to 23.83 MPa and high temperature flexural strength was enhanced from 15.64 MPa to 34.62 MPa with 4.0 wt% SiC fiber addition due to the reinforcement of fibers and the enhanced β-cristobalite content, which meets the need of ceramic cores. Therefore, it demonstrates the capability of fabricating high-performance and high-precision silica-based ceramic cores reinforced by SiC fibers via stereolithography for rapid manufacturing of hollow blades.

Reactive consolidation and high-temperature strength of HfB2–SiB6

During the spark plasma sintering at 1900 °C, SiB6 decomposes into cubic silicon carbide and boron carbide, owing to the reducing environment of the furnace. For the HfB2-SiB6 ceramic improvement in hardness (24.5 ± 0.7 GPa) was attributed to the formation of the B12(C,Si,B)3. Fracture toughness by indentation (6.8 ± 2.4 MPa·m1/2), single-edge notched bend specimens (4.6 ± 0.4 MPa·m1/2) and room-temperature strength (513 ± 21 MPa) of the HfB2–SiB6 composite produced by spark plasma sintering was higher or on the same level as the HfB2–SiC ceramics. The high-temperature flexural strength tests suggested that the strength would decrease monotonically with an increase in temperature. At or below 1600 °C, only a linear stress-strain response was observed, and resulted into a mean strength of ~320 MPa. During the tests at 1800 °C, we observed a nonlinear deformation indicating ongoing plastic deformation which led to a strength decrease down to 230 ± 30 MPa.

Improved mechanical properties of SiB6 reinforced silica-based ceramic cores fabricated by 3D stereolithography printing

Silica-based ceramic core is an extremely critical component in the manufacture of hollow blades during investment casting. However, the traditional preparation methods rely more on the molds, and the manufacturing costs are relatively high. In this study, silica-based ceramics with silicon hexaboride (SiB6) addition were prepared via 3D stereolithography printing. And the effects of the SiB6 content on mechanical properties of the obtained ceramic samples were explored. As the SiB6 content increased to 2.0 wt%, the linear shrinkage gradually decreased, while the room temperature and high temperature flexural strength were enhanced at the SiB6 content from 0 to 1.0 wt% and reduced as the SiB6 content further rose. As the SiB6 content increased to 1.0 wt%, the linear shrinkage was reduced to 1.86% resulting from the oxidation reaction of SiB6. Furthermore, with 1.0 wt% SiB6 addition, the flexural strength of the samples at room temperature was enhanced from 6.75 MPa to 14.63 MPa due to the sintering promotion of oxidation product B2O3, and the flexural strength at 1550 °C was improved from 7.68 MPa to 13.08 MPa because of the enhanced β-cristobalite content, which is suitable for high temperature casting of ceramic cores. Therefore, it demonstrates the capability of fabricating SiB6 reinforced silica-based ceramic cores with high performance via stereolithography.

Fracture peculiarities and high-temperature strength of bulk polycrystalline boron

We report the mechanical behavior of a bulk boron ceramic prepared by spark plasma sintering of commercially-available β-boron powder. In order to fabricate polycrystalline boron ceramic, we used a protective tantalum foil reacted with carbon from the graphite die or graphite foil forming a thin layer of TaB2 and TaC covering the boron specimen. This is the first study to show the high-temperature flexural strength, toughness, and Young's moduli of boron up to 1400 °C. At 1600 °C and above, boron will react with testing environment forming an outer shell.The flexural strength and fracture toughness at room temperature reached an average of 340 MPa and 4.1 MPa m1/2, respectively. Despite showing clear signs of plastic deformation on the strain-stress curves, the yield strength of the monolithic boron ceramic exceed 1 GPa at 1200 °C. It was determined that fracture at elevated temperatures follows a quasi-transgranular mechanism, where the sub-grains of the boron fracture as plate-like structures. An interpretation for the observed fracture behavior was proposed.

Experimental study on high temperature performances of silica-based ceramic core for single crystal turbine blades

Silica-based ceramic cores are widely utilized for shaping the internal cooling canals of single crystal superalloy turbine blades. The thermal expansion behavior, creep resistance, and high temperature flexural strength are critical for the quality of turbine blades. In this study, the influence of zircon, particle size distribution, and sintering temperature on the high-temperature performance of silica-based ceramic cores were investigated. The results show that zircon is beneficial for narrowing the contraction temperature range and reducing the shrinkage, improving the creep resistance and high-temperature flexural strength significantly. Mixing coarse, medium and fine fused silica powders in a ratio of 5:3:2, not only reduced high temperature contraction, but effectively improved the creep resistance. Properly increasing the sintering temperature can slightly reduce the thermal deformation and improve the high-temperature flexural strength of the silica-based core, but excessively high sintering temperature negatively impacts the creep resistance and high-temperature flexural strength.

Properties of alumina-based ceramic cores

Fused corundum is a rather promising raw material for preparing an alumina-based ceramic core due to its excellent high temperature resistance and chemical inertness. In this study, alumina-based ceramic cores were prepared using fused corundum as the matrix material, and the effect of varying silica powder contents on the properties of the alumina-based ceramic cores, including the sintering shrinkage, the flexural strength, and the high temperature deformation was investigated. The mineralization mechanisms of the silica on the alumina-based ceramic core were also analyzed. The optimum addition amount of silica in this experiment is 8% in weight. At that moment, the aluminum-based core has both a low sintering shrinkage coefficient of 0.66% and better properties: the room temperature flexural strength is 22.19 MPa, the high temperature flexural strength is 21.54 MPa, the high temperature deformation is 0.93 mm, and the residual flexural strength is 47.41 MPa.

Influence of Al2O3 content on mechanical properties of silica-based ceramic cores prepared by stereolithography

Silica ceramic cores have played an important part in the manufacture of hollow blades due to their excellent chemical stability and moderate high-temperature mechanical properties. In this study, silica-based ceramics were prepared with Al2O3 addition by stereolithography, and the influence of Al2O3 content on mechanical properties of the silica-based ceramics was investigated. The Al2O3 in silica-based ceramics can improve the mechanical properties by playing a role as a seed for the crystallization of fused silica into cristobalite. As a result, with the increase of Al2O3 content, the linear shrinkage of the silica-based ceramics first decreased and then increased, while the room-temperature flexural strength and the high-temperature flexural strength first increased and then decreased. As the Al2O3 content increased to 1.0 vol%, the linear shrinkage was reduced to 1.64% because of the blocked viscous flow caused by Al2O3. Meanwhile, the room-temperature flexural strength and the high-temperature flexural strength were improved to 20.38 and 21.43 MPa with 1.0 vol% Al2O3, respectively, due to the increased α-cristobalite and β-cristobalite content. Therefore, using the optimal content of Al2O3 in silica-based ceramics can provide excellent mechanical properties, which are suitable for the application of ceramic cores in the manufacturing of hollow blades.

Enhanced 3D printed alumina ceramic cores via impregnation

Abstract3D‐printed alumina ceramic cores often face the problem of low flexural strength at its application temperature of 1500℃, and the requirements on porosity and strength limit its fabrication process. In this research, three types of solutions were employed to impregnate the ceramics to improve the strength of the 3D‐printed alumina ceramic cores. The results showed that silica sol (SS), tetraethyl orthosilicate (TO), and 3‐(Trimethoxysilyl)propyl methacrylate (PM) all could improve the (room temperature) flexural strength of the 3D printed ceramics effectively compared to the control sample (CS) without impregnation, and the SS solution could improve the high temperature flexural strength at 1500℃ greatly compared to other solutions. The improvement in strength was due to the introduction of Si, which formed a stable mullite phase during the sintering process. Computed tomography (CT) analysis showed there is no cracks formed in the ceramic cores impregnated with the SS solution, and they exhibited the overall best performance (21.2 ± 0.8 vol% open porosity, shrinkage of 3.2 ± 0.3% in the X direction, 3.4 ± 0.2% in the Y direction, 2.1 ± 0.4% in the Z direction, room temperature flexural strength of 85.7 ± 6.9 MPa, and 1500℃ flexural strength of 19.2 ± 1.1 MPa).

Effect of CaO and SiO2 on the properties of Y2O3-based ceramic core materials

ABSTRACT Nb-Si alloy is an important candidate material to produce hollow turbine blades in the aerospace industry. However, the ultra-high melting temperature of Nb-Si alloy (≥1750°C) makes the traditional ceramic core made of Al2O3 (poor leachibility) or SiO2 (melting temperature) invalid in the production of Nb-Si hollow turbine blades. In this study, Y2O3 were used as the ceramic core matrix due to its high melting points, high strength, and low interfacial reactivity. CaO and SiO2 were added in Y2O3 matrix to improve the leachability and high temperature flexural strength (1500°C) of ceramic cores. The other properties such as porosity, shrinkage and bulk density were investigated as well. The results showed that the introduction of CaO in Y2O3-based ceramic cores improved the leachability. The addition of SiO2 in the Y2O3-CaO matrix could greatly improve the high temperature flexural strength, and keep a good leachability at the same time. The best comprehensive properties of Y2O3-based ceramic cores were obtained when it contained 12 wt% CaO and 4 wt% SiO2. The new developed Y2O3-based ceramic cores with the addition of CaO and SiO2 has the potential to meet the extreme requirements in the casting of Nb-Si alloy hollow turbine blades.

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si3N4) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si3N4 and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La2O3, Sm2O3, Y2O3, and Lu2O3) on the phase compositions, development of Si3N4 grains and flexural strength (especially high-temperature flexural strength) were researched. Si3N4 ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si3N4 ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.