Microstructure,Mechanical, and Thermal Properties of (ZrB2 + ZrC)/Zr3[Al(Si)]4C6 Composite

The microstructure, mechanical, and thermal properties of in situ hot‐pressed 30 vol% (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composite have been investigated and compared with monolithic Zr₃[Al(Si)]₄C₆ ceramic. The composite is composed of ZrB₂ and ZrC grains embedded in a Zr₃[Al(Si)]₄C₆ matrix. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 415 GPa), strength (bending strength of 621 MPa), and toughness (fracture toughness of 7.37 MPa·m^1/2) compared with monolithic Zr₃[Al(Si)]₄C₆. The composite retains high modulus of 357 GPa at 1430°C (86% of that at ambient temperature) due to clean grain boundaries with no glassy phase. In addition, the composite exhibits higher specific heat capacity and thermal conductivity but slightly lower coefficient of thermal expansion compared with monolithic Zr₃[Al(Si)]₄C₆. The calculation of the thermal stress fracture resistance parameter (R) predicts a much improved thermal shock resistance of the composite. Based on these results, (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composites show promising potential for high‐temperature and ultra high‐temperature applications.     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.     

Multicomponent synergistically affected mechanical properties,microstructure, and oxidation resistance of Zr–Al(Si)–C based composites

Herein, in-situ Zr₃[Al(Si)]₄C₆-based composites with 10–40 vol% ZrB₂–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB₂–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB₂–SiC, while the hardness increases near linearly. Zr₃[Al(Si)]₄C₆–ZrB₂–SiC shows high strength (623 MPa), toughness (7.59 MPa m^1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB₂–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10⁻⁵ kg² m⁻⁴ s⁻¹ when oxidized at 1400 °C.     

A dense and fine-grained SiC/Ti3Si(Al)C2 composite and its high-temperature oxidation behavior

A dense SiC/Ti₃Si(Al)C₂ composite was synthesized by in situ hot pressing powders of Si, TiC and Al as a sintering additive at 1500 °C for 2 h under 30 MPa in Ar atmosphere. This composite has a fine-grained and homogeneous microstructure with grain sizes of 5 μm for Ti₃Si(Al)C₂ and of 1 μm for SiC. The SiC/Ti₃Si(Al)C₂ composite possesses an improved oxidation resistance, with parabolic rate constants of 4.57 × 10^?8 kg²/m⁴/s at 1200 °C and 1.31 × 10^?7 kg²/m⁴/s at 1300 °C. This study provides an experimental evidence to confirm the formation of amorphous phases in the oxide scale of the SiC/Ti₃Si(Al)C₂ composite. Microstructure and phase composition of the SiC/Ti₃Si(Al)C₂ composite and oxide scales were identified by X-ray diffractometry and scanning electron microscopy. The mechanism for the enhanced oxidation resistance has been discussed.     

Si/B/C/N/Al precursor-derived ceramics: Synthesis,high temperature behaviour and oxidation resistance

The Si/B/C/N/H polymer T2(1), [B(C₂H₄Si(CH₃)NH)₃]_n, was reacted with different amounts of H₃Al·NMe₃ to produce three organometallic precursors for Si/B/C/N/Al ceramics. These precursors were transformed into ceramic materials by thermolysis at 1400 °C. The ceramic yield varied from 63% for the Al-poor polymer (3.6 wt.% Al) to 71% for the Al-rich precursor (9.2 wt.% Al). The as-thermolysed ceramics contained nano-sized SiC crystals. Heat treatment at 1800 °C led to the formation of a microstructure composed of crystalline SiC, Si₃N₄, AlN(+SiC) and a BNC_x phase. At 2000 °C, nitrogen-containing phases (partly) decomposed in a nitrogen or argon atmosphere. The high temperature stability was not clearly related to the aluminium concentration within the samples. The oxidation behaviour was analysed at 1100, 1300, and 1500 °C. The addition of aluminium significantly improved the oxide scale quality with respect to adhesion, cracking and bubble formation compared to Al-free Si(/B)/C/N ceramics. Scale growth rates on Si/B/C/N/Al ceramics at 1500 °C were comparable with CVD–SiC and CVD–Si₃N₄, which makes these materials promising candidates for high-temperature applications in oxidizing environments.     

Synthesis of SiBNC-Al ceramics with different aluminum contents via polymer-derived method

This study reports the synthesis of three types of aluminium (Al)-modified polyborosilazane ceramic precursors (PBSAZ) from C₈H₁₉Al/HSiCl₃/HMDZ/BCl₃ and their thermal conversion to SiBNC-Al ceramics at 1000°C. FT-IR, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) are used to characterize the structures and properties of the PBSAZ. PBSAZ is found to be built up of the Si─N─B framework, with Al successfully introduced into the ceramic network structures. The ceramic yield is 63.5 wt.% for the Al-poor polymers (PBSAZ-5) and 65.1 wt.% for the Al-rich polymers (PBSAZ-1). The high-temperature cracking behavior and the crystal phase structures of the ceramics were characterized by XRD and Raman, which revealed that SiBNC-Al ceramics contained Si₄N₃, SiC, and AlN (+SiC) crystals after heat treatment at 1600°C. The oxidation behavior of SiBNC and SiBNC-Al ceramics at 1500°C shows that the introduction of Al improves the quality of the oxide layer, effectively suppresses the oxide layer cracking phenomenon, and reduces gas bubble generation.     

Elastic,mechanical, electronic,and defective properties of Zr–Al–C nanolaminates from first principles

By means of first principles calculations, Zr–Al–C nanolaminates have been studied in the aspects of chemical bonding, elastic properties, mechanical properties, electronic structures, and vacancy stabilities. Although the investigated Zr–Al–C nanolaminates show crystallographic similarities, their predicated properties are very different. For (ZrC)_nAl₃C₂ (n = 2, 3, 4), the Zr–C bond adjacent to the Al–C slab with the C atom intercalated in the Zr layers is the strongest, but the one with the C atom intercalated between the Zr layer and Al layer is the weakest. In contrast, the situation for (ZrC)_nAl₄C₃ (n = 2, 3) is just the opposite. For Zr–Al–C nanolaminates, the calculated bulk, shear and Young's modulus increase in the sequence of Zr₂AlC < Zr₃AlC₂ < Zr₂Al₄C₅ < Zr₃Al₄C₆ < Zr₂Al₃C₄ < Zr₃Al₃C₅ < Zr₄Al₃C₆. The (ZrC)_nAl₃C₂ (n = 2, 3, 4) series exhibit the most outstanding elastic properties. In the presence of the external pressure, the bulk and shear moduli exhibit a linear response to the pressure, except for Zr₂AlC and Zr₃AlC₂, both of which belong to the so‐called MAX phases. The two materials also exhibit very distinct properties in the strain‐stress relationship, electronic structures and vacancy stabilities. As the intercalated Al layers increase, the formation energy of V_Zr and V_Al increases, while the formation energy of V_C decreases.     

Ultra-high temperature ceramics based on ZrB2 obtained by pressureless sintering with addition of Cr3C2, Mo2C,and WC

ZrB₂-MeC and ZrB₂-19 vol% SiC-Me_xC_y where Me=Cr, Mo, W were obtained by pressureless sintering. The capability to promote densification of ZrB₂ and ZrB₂-SiC matrices is the highest for WC and lowest for Cr₃C₂. The interaction between the components results in the formation of new phases, such as MeB (MoB, CrB, WB), a solid solution based on ZrC, and a solid solution based on ZrB₂. The addition of Cr₃C₂ decreases the mechanical properties. On the other hand, the addition of Mo₂C or WC to ZrB₂-19 vol% SiC composite ceramics leads increased mechanical properties. Long-term oxidation of ceramics at 1500 °C for 50 h showed that, in binary ZrB₂-Me_xC_y, a protective oxide scale does not form on the surface thus leading to the destruction of the composite. On the contrary, triple composites showed high oxidation resistance, due to the formation of dense oxide scale on the surface, with ZrB₂-SiC-Mo₂C displaying the best performance.     

Ti3Si(Al)C2-based ceramics fabricated by reactive melt infiltration with Al70Si30 alloy

Dense Ti₃Si(Al)C₂-based ceramics were synthesized using reactive melt infiltration (RMI) of Al₇₀Si₃₀ alloy into the porous TiC preforms. The effects of the infiltration temperature on the microstructure and mechanical properties of the synthesized composites were investigated. All the composites infiltrated at different temperatures were composed of Ti₃Si(Al)C₂, TiC, SiC, Ti(Al, Si)₃ and Al. With the increase of infiltration temperature from 1050 °C to 1500 °C, the Ti₃Si(Al)C₂ content increased to 52 vol.% and the TiC content decreased to 15 vol.%, and the Vickers hardness, flexural strength and fracture toughness of Ti₃Si(Al)C₂-based composite reached to 9.95 GPa, 328 MPa and 4.8 MPa m^1/2, respectively.     

Spontaneous pulverisation of Al4C3 containing MAX bulk ceramics at room temperature

Abstract

The deleterious effect of Al₄C₃ on the performance of MAX ceramics has not attracted much attention. In the present study, Al₄C₃ containing Cr₂Al(Si)C and SiC/Cr₂AlC ceramics have been prepared. It was found that spontaneous pulverisation of the as synthesised samples took place at room temperature after exposure in air and water for various durations. Volume expansion induced by the interaction of Al₄C₃ with moisture is held responsible for the disintegration of Al₄C₃ containing MAX ceramics. Preventive measures to avoid the formation of Al₄C₃ upon preparing MAX ceramics were herein proposed.     

Inhibition of liquid Si infiltration into carbon-carbon composites by the addition of Al to the Si slurry pre-coating: mechanism analysis

The mechanism of the inhibition of liquid Si infiltration (LSI) into a two-dimensional carbon-carbon composite (2D-C/C) by the addition of Al to the Si slurry pre-coating was investigated. It was shown by means of a vapor treatment experiment designed intentionally that the surface composition of the inner pores beneath the Si slurry pre-coating before the occurrence of LSI was pure carbon and SiC, while before the occurrence of the LSI with the Si-6 wt.%Al slurry pre-coating, the surface composition of the inner pores was Al₄C₃, SiC and a small amount of pure carbon. The formation of the SiC and the Al₄C₃ was the result of the evaporation of almost all the Al additive and a little Si during the heating. For reactive infiltrations, reactions at the vapor-liquid-solid triple line are believed to affect the final infiltration depth. Faster reactions at the triple line lead to faster infiltration velocity and hence deeper reactive infiltration. The reaction at the triple line for the LSI with the Si-6 wt.%Al slurry pre-coating was mainly between liquid Si and the surface Al₄C₃, which was probably slower than the reaction of liquid Si with the pure carbon at the triple line corresponding to the LSI with the Si slurry pre-coating. Therefore, the extent of the penetration of liquid Si during the LSI with Si-6 wt.%Al slurry pre-coating was lower than that with the Si slurry pre-coating.     

Synthesis and structure–property relationships of a new family of layered carbides in Zr-Al(Si)-C and Hf-Al(Si)-C systems

The layered ternary and quaternary carbides in Zr-Al(Si)-C and Hf-Al(Si)-C systems with general formulae of (TC)_nAl₃C₂, (TC)_nAl₄C₃ and (TC)_n[Al(Si)]₄C₃ (where T = Zr or Hf, n = 1, 2, 3…) have attracted increasing attentions due to their fascinating properties such as high specific stiffness, high strength and fracture toughness, refractory, machinability by electrical discharge method, thermal shock resistance, as well as high-temperature and ultrahigh-temperature oxidation resistance. The combination of these properties makes them promising as structural components or coatings for high- and ultrahigh-temperature applications. In this review, the progresses on processing, and structure–property relationships of the novel layered carbides are comprehensively outlined. The crystal structure characteristics are introduced first. Then, methods for processing powders and bulk samples are summarized. The third section focuses on the multi-scale structure–property relationships. Finally, the potential applications and further trends in tailoring the properties and developing low cost processing methods are highlighted.     

On the coordination of Al in ill-crystallized C-S-H phases formed by hydration of tricalcium silicate and by precipitation reactions at ambient temperature

High-resolution solid-state ²⁷Al MAS NMR measurements suggest that Al incorporated into C-S-H phases, prepared by precipitation reactions from sodium silicate and calcium chloride solutions and by hydration of tricalciumsilicate (C₃S), may occur tetrahedrally (Al^[4]) as well as octahedrally coordinated (Al^[6]). The amount of which depends on the composition of the C-S-H phase. With increasing CaO/SiO₂-ratio the portion of Al^[6] increases and that of Al^[4] decreases. The products of paste hydration of C₃S contain about 80 % of the Al as Al^[6] and 20 % as Al^[4].     

Multicomponent (Hf0.25Zr0.25Ti0.25Cr0.25)B2 ceramic modified SiC–Si composite coatings: In-situ synthesis and high-temperature oxidation behavior

High-entropy ceramics, a novel type of multicomponent materials with broad application prospects, have stirred up world-wide interests for over a decade. In the current work, in-situ high-entropy (Hf_0.25Zr_0.25Ti_0.25Cr_0.25)B₂ ceramic modified SiC–Si (HETMB₂-SiC-Si) coating was deposited on carbon/carbon (C/C) composites via gaseous reactive infiltration of Si assisted slurry painting (GRSI-SP) method, to improve the oxidation protective ability of C/C composites at 1973 K. The formation and oxidation mechanisms of the coating was explored by first-principles simulation, experiment and thermodynamic analyses. The coating prepared at 2373 K shows dense mosaic structure filled with HETMB₂-rich Si-based multiphase. This coating adheres well with the C/C substrate, which is ascribed to the formed zigzagged SiC–Si transition layer. This coating protected C/Cs from oxidation for more than 205 h at 1973 K. The enhanced oxidation protective ability is mostly ascribed to the subsequently generated compact and stable Hf-Zr-Ti-Cr-Si-O composite oxidation scale. This research will start up novel research ares of developing high-entropy materials modified coatings with improved protective ability under extreme environments.     

Improved oxidation resistance properties of Si3N4/O′–SiAlON composite ceramics by a repeated sintering method

Si₃N₄/O′–SiAlON composite ceramics with superior oxidation resistance properties were fabricated by a repeated sintering method. The effects of sintering time on the phase evolution, microstructure, and oxidation resistance properties of the Si₃N₄/O′–SiAlON composite ceramics were investigated. The results indicated that the content of the O′–SiAlON phase and the densification of Si₃N₄/O′–SiAlON composite ceramics increased after two-time sintering. Furthermore, the thickness of the oxide layer of the Si₃N₄/O′–SiAlON composite ceramics after oxidation at 1100–1500°C for 30 h was not significant. Compared to the oxidation weight gain after the one-time sintering process, the oxidation weight gain of Si₃N₄/O′–SiAlON composite ceramics was 0.432 mg/cm² after two-time sintering when oxidized at 1500 C for 30 h, which was reduced by 43.3%. The mechanism of the improved oxidation resistance properties was ascribed to the formation of more O′–SiAlON and the enhancement of the densification.     

The C-S-H gel of Portland cement mortars: Part I. The interpretation of energy-dispersive X-ray microanalyses from scanning electron microscopy, with some observations on C-S-H, AFm and AFt phase compositions

Scanning electron microscopy (SEM) microanalyses of the calcium-silicate-hydrate (C-S-H) gel in Portland cement pastes rarely represent single phases. Essential experimental requirements are summarised and new procedures for interpreting the data are described. These include, notably, plots of Si/Ca against other atom ratios, 3D plots to allow three such ratios to be correlated and solution of linear simultaneous equations to test and quantify hypotheses regarding the phases contributing to individual microanalyses. Application of these methods to the C-S-H gel of a 1-day-old mortar identified a phase with Al/Ca=0.67 and S/Ca=0.33, which we consider to be a highly substituted ettringite of probable composition C₆A₂S?₂H₃₄ or {Ca₆[Al(OH)₆]₂·24H₂O}(SO₄)₂[Al(OH)₄]₂. If this is true for Portland cements in general, it might explain observed discrepancies between observed and calculated aluminate concentrations in the pore solution. The C-S-H gel of a similar mortar aged 600 days contained unsubstituted ettringite and an AFm phase with S/Ca=0.125.     

Preparation,microstructure, and mechanical properties of ZrB2-based ceramics with layered Zr2Al4C5 inclusions

ZrB₂/Zr₂Al₄C₅ composite ceramics with different volume contents of Zr₂Al₄C₅ formed in situ were fabricated by the spark plasma sintering technique at 1800 ^°C. The content of Zr₂Al₄C₅ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂/Zr₂Al₄C₅ ceramics. The results indicated that sinterability of the composites was remarkably improved by the addition of Zr₂Al₄C₅ compared to the single-phase ZrB₂ ceramic. The microstructure of the resulting composites was fine and homogeneous, the average grain size of the ZrB₂ decreased, and the average aspect ratio of the Zr₂Al₄C₅ increased with the increase in the amount of Zr₂Al₄C₅. As the content of Zr₂Al₄C₅ increased, both the Vickers hardness and Young's modulus of the composites first increased and then decreased. The fracture toughness of the ZrB₂–40 vol% Zr₂Al₄C₅ composite was 4.25 MPa m^1/2, which increased by approximately 70% compared to the monolithic ZrB₂ ceramic. The improvement was mainly attributed to the toughening mechanisms such as the layered structure toughening, crack deflection and crack bridging, caused by the in situ formed layered Zr₂Al₄C₅ inclusions.     

Formation of (Al2OC)1−x(AlN)x solid solution starting from Al–Si–Al2O3 powder matrix at 1300°C in flowing nitrogen

(Al₂OC)_1−x(AlN)_x solid solution-reinforced Si–Al₂O₃ composite was successfully synthesized by designed heating of the Al–Si–Al₂O₃ composite to 580°C and held for 8 hours, followed by heating to 1300°C at a rate of 12°C/h in flowing nitrogen. The reaction mechanism is as follows: after the Al–Si–Al₂O₃ composite is heated to 580°C and held for 8 hours, an AlN cladding is formed on the surface of the Al powder, thus the composite is preconverted into (Al–AlN cladding structure)–Si–Al₂O₃ system. With increasing temperature, the AlN cladding ruptures and the reactive Al(l) flows out. The Al(l) preferentially undergoes active oxidation to form metastable Al₂O(g), which lowers P_O2 inside the composite and inhibits the active oxidation of Si. Moreover, ultrafine carbon is produced by the pyrolysis of the phenolic resin binder. Both metastable Al₂O(g) and ultrafine carbon are highly reactive. Therefore, under the induction of AlN and N₂, (Al₂OC)_1−x(AlN)_x solid solution is formed by the reaction which easily occurs at a relatively low temperature. In the presence of a large amount of Al₂O(g), the P_O2 in the composite does not satisfy the condition required for both Si nitridation and active oxidation, so the free Si remains stable in the composite, forming a metal-non-oxide-oxide composite. The cold crushing strength of the composites is up to 305 MPa, and the composites do not show hydration after 20 months of storage in the environment.     

High‐Strength ZrC Ceramics Doped with Aluminum

High‐strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC_1?x, Zr₃Al₃C₅ and Zr–Al compound such as AlZr₃ and Al–C–Zr. The intermediate product AlZr₃ below 1600°C and remained Al–C–Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C–1900°C) could be attributed to the formation of nonstoichiometric ZrC_1?x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC_0.85–Zr₃Al₃C₅ and ZrC_0.80–Zr₃Al₃C₅ based ceramics had 3‐point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2–18.3 GPa and 3.3–3.5 MPa m^1/2, respectively.     

Phase evolution mechanism of non‐oxide bonded Al–Al2O3–MgO–ZrO2 composites at 1873 K in flowing nitrogen

In flowing nitrogen, non‐oxides such as Al₄O₄C, Al₂OC, Zr₂Al₃C₄, and MgAlON bonded Al₂O₃‐based composites were successfully prepared by a gaseous phase mass transfer pathway using aluminum, zirconia, alumina, and magnesia as raw materials at 1873 K, after an Al–AlN core‐shell structure was formed at 853 K. Resin bonded Al–Al₂O₃–MgO–ZrO₂ composites after sintering were characterized and analyzed by X‐ray diffraction (XRD), scanning electron microscope (SEM) and, energy dispersive spectrometer (EDS), and the influence of the MgO content on the sintered composites was studied. The results show that after sintering, the phase composition of the Al–Al₂O₃–ZrO₂ composite is Al₂O₃, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄, while the phase composition of the Al–Al₂O₃–ZrO₂ composite with the addition of MgO 6 wt% and MgO 12 wt% is Al₂O₃, MgAlON, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄ as well as Al₂O₃, MgAlON, Al₂OC, and Zr₂Al₃C₄, respectively. The addition of MgO changed the phase composition and distribution for the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites after sintering. When the added MgO content is equal to or more than 12 wt%, the Al₄O₄C in the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites is unable to exist in a stable phase.