Mechanical and thermal properties and microstructural evolution of Ta-doped Nb4AlC3

(Nb_1-xTa_x)₄AlC₃ (x = 0–0.5) ceramics were prepared by the hot press sintering method. The XRD results show that the second phase (Nb_1-xTa_x)C is formed when the Ta content increases to 25 mol%. The SEM micrographs show that (Nb_1-xTa_x)C has a core/rim structure, whose formation mechanism was also investigated. Substituting some Ta for Nb can significantly improve the mechanical properties of Nb₄AlC₃. (Nb_0.75Ta_0.25)₄AlC₃ exhibits an excellent fracture toughness of 8.3 ± 0.3 MPa m^1/2 at room temperature (RT). The highest Young's modulus (349 ± 16 GPa) and Vickers hardness (4.5 ± 0.3 GPa) at RT are exhibited by the (Nb_0.5Ta_0.5)₄AlC₃ sample, which correlate to increases of 18% and 80%, respectively, compared with those of Nb₄AlC₃. The flexural strengths of (Nb_0.5Ta_0.5)₄AlC₃ are 439 ± 18 MPa at RT and 344 ± 22 MPa at 1100 °C, which correlate to increases of 27% and 45%, respectively, compared with those of Nb₄AlC₃. The solid solution of Ta and the formation of (Nb_1-xTa_x)C are beneficial to the strengthening of Nb₄AlC₃. The coefficient of thermal expansion (CTE) increases slightly from 7.08 × 10⁻⁶ K⁻¹ for Nb₄AlC₃ to 7.24 × 10⁻⁶ K⁻¹ for (Nb_0.75Ta_0.25)₄AlC₃ at 25–1400 °C. The thermal conductivity of (Nb_0.75Ta_0.25)₄AlC₃ (28.4–29.8 W/m·K) is higher than that of Nb₄AlC₃ (18.1–21.2 W/m·K) over the whole test range (25–1000 °C). Owing to their excellent mechanical and thermal properties, Ta-doped Nb₄AlC₃ ceramics have good potential as structural materials.     

High-temperature microstructural evolution of SiCN fibers derived from polycarbosilane with different C/N ratios

Research into the high-temperature microstructural evolution of SiCN ceramic fibers is important for the aerospace application of advanced ceramic matrix composites in harsh environments. In this work, we studied the microstructural evolution of SiCN fibers with different C/N ratios that derived from polycarbosilane fibers at the annealing temperature range of 1400∼1600 °C. These results showed that the phase separation of SiC_xN_y phase and the two-dimension grain growth process of free carbon nanoclusters could be processed at the researched temperature range. As the annealing temperature increased to 1600 °C, the crystallization of amorphous SiC and Si₃N₄ could be detected. SEM and Raman analysis showed that the decomposition and carbothermal reduction of the Si₃N₄ phase at high temperatures played primary roles in contributing to the fiber strength degradation. Thus, a higher C/N ratio, which is beneficial for inhibiting the decomposition of amorphous Si₃N₄, helps SiCN fibers retain high tensile strength at high temperatures.     

Ti3AlC2材料的制备及其高温抗氧化性能研究

以TiC粉、Ti粉和Al为原料,按TiC:Ti:Al=2:1:1(原子比)混合,采用真空热压烧结法制备Ti3AlC2材料.XRD和SEM分析表明,合成产物中几乎全为Ti3AlC2相,TiC含量极少,层片状的Ti3AlC2发育良好,晶粒细小,分布均匀.该材料在900 ℃宅气中断续氧化30 h后的氧化动力学遵循抛物线规律,氧化层主要成分为TiO2和Al2O3,与基体粘结紧密,起到良好的保护作用.     

Changes in the microstructure and mechanical properties of Al2O3/YSZ directionally solidified eutectic ceramics during long-time heat treatment

Al₂O₃/YSZ directionally solidified eutectic ceramics (DSECs) were prepared using the induction heating zone melting (IHZM) method. The microstructures of the DSECs formed with various crucible pulling rates (2.5–300 mm/h) changed from the discontinuous lamellar into the colony structure, finally into the cell structure. The intracolony had better microstructural stability than the intercolony during heat treatment due to two factors: the size effect and the t-m transformation effect. The hardness changes were mainly related to the release and generation of residual stress caused by repeated heating and cooling. The quantity of cracks in DSECs increased with the heat treatment time, leading to a gradual decrease in the fracture toughness, 300 mm/h DSEC had the lowest reduction in fracture toughness after 500 h heat treatment due to fine microstructure, the fracture toughness was 7.15 MPa·m^1/2, which was 76 % of that before the heat treatment (9.39 MPa·m^1/2).     

Study on the microstructural evolution of high temperature adhesives for graphite bonding

Development of high-temperature adhesives (HTAs) can provide a way to produce carbon materials with a large size or a complex shape. Some HTAs were developed for joining graphite materials. These HTAs were prepared using phenol formaldehyde (PF) resin as matrix and B₄C powders as fillers. The results showed that these HTAs had satisfactory adhesive properties for graphite bonding even though the bonded graphite components were heat-treated at temperatures up to 1500 °C. In order to clarify the effects of the microstructure of HTAs on adhesive strength, the bonded graphite parts were heat-treated at high temperatures ranging from 400 to 1500 °C. The microstructural evolution of HTAs was investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Analyses of the HTAs at the interface revealed that during microstructural evolution, carbonization of PF resin and complex interactions between PE and B₄C powders occurred. A high chemical bonding force was introduced at the interface through interactions between the glassy carbon from the PF resin and B₄C fillers, and the B₄C fillers restrained the volume shrinkage of PF resin during carbonization, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

Spark plasma sintering of TiB2-based ceramics with Ti3AlC2

A TiB₂–Ti₃AlC₂ ceramic was manufactured by spark plasma sintering at 1900 °C temperature for 7 min soaking time under 30 MPa biaxial pressure. The role of Ti₃AlC₂ additive on the microstructure development, densification behavior, phase evolution, and hardness of the ceramic composite were studied. The phase characterization and microstructural investigations unveiled that the Ti₃AlC₂ MAX phase decomposes at the initial stages of the sintering. The in-situ formed phases, induced by the decomposition of Ti₃AlC₂ additive, were identified and scrutinized by XRD and FESEM/EDS techniques as well as thermodynamics principles. The sintered TiB₂–Ti₃AlC₂ ceramic approached a near full density of ~99% and a hardness of ~28 GPa. The densification mechanism and sintering phenomena were discussed and graphically illustrated.     

Microstructural evolution and mechanical properties of TiB2-TiC-SiC ceramics joint brazed using Ti-Ni composite foils

A novel TiB₂-based ultra-high-temperature ceramic containing 60 vol.% TiB₂, 20 vol.% TiC, and 20 vol.% SiC was fabricated by hot pressing and subsequently joined using the brazing technique. Ti-based filler was used as the brazing alloy by taking advantage of the reaction between Ti and TiB₂-TiC-SiC. The effects of the brazing temperature on the microstructure and mechanical properties of the brazed joint were investigated. The results showed that Ti in the filler reacted with the TiB₂-TiC-SiC ceramics and formed a reaction layer I that comprised TiB and TiC. The brazing seam was composed of TiB, TiC, Ti₅Si₃, Ti₂Ni, and TiNi. When the brazing temperature was increased, the reaction between TiB₂-TiC-SiC ceramics and the filler was observed to become vigorous; this led to an increase in the growth of the reaction layer I. Meanwhile, the continuous Ti₂Ni layer in the brazing seam gradually disappeared; it was replaced by TiB and Ti₅Si₃. The room temperature shear strength reached a maximum value of 168 MPa when the joint was brazed at 1040 °C for 30 min; while it was 104 and 81 MPa at test temperature of 600 °C and 800 °C, respectively. In addition, the effects of TiB whiskers on the coefficient of thermal expansion of the brazing seam and fracture of the brazed joint were discussed.     

Oxidation and creep behavior of textured Ti2AlC and Ti3AlC2

The oxidation and creep behaviors of textured Ti₂AlC and Ti₃AlC₂ ceramics were characterized. The oxidation behavior of the two materials, which was studied in air at temperatures ranging from1000 to 1300 °C, was observed to be anisotropic and the materials exhibited a better oxidation resistance along a direction transverse to the c-axis. The correlation between the overall parabolic rate constant and oxidation temperature of both textured materials was characterized, providing new insights into the oxidation kinetics. The results indicate that the texturing has a negligible influence on the creep behavior in the assessed temperature range of 1000?1200 °C in air, for the applied stresses ranging from 40 to 80 MPa. In this stress regime, the creep behavior of textured Ti₂AlC and Ti₃AlC₂ appears to be controlled by grain boundary sliding. This behavior can be rationalized based on a model for superplastic deformation, indicating pure-shear motion under stationary conditions accommodated by lattice or grain-boundary diffusion.     

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si₃N₄) fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si₃N_4f/BN/Si₃N₄ composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m^1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m^1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si₃N_4f/BN/Si₃N₄ composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.     

Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti₃AlC₂ powder is prepared in a very mild condition via Ti₃Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti₃AlC₂ can be obtained at 900°C with the 1:1 salt-to-material ratio. The formation mechanism of Ti₃AlC₂ through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti₃Al and C → TiAl and TiC → Ti₂AlC and TiC → ψ and TiC → Ti₅Al₂C₃ and TiC → Ti₃AlC₂, where the key ψ, a modulated Ti₂AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti₂AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti₅Al₂C₃ through topotactic transition, followed by Ti₅Al₂C₃ converting into Ti₃AlC₂ by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti₂AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti₅Al₂C₃ (0 0 0 3), Ti₅Al₂C₃ (0 0 0 3)//Ti₃AlC₂ (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.     

热压烧结工艺制备Ti2AlC/Ti3AlC2陶瓷材料

以Ti，Al，C为原料，采用热压工艺制备出相组成为Ti2AlC/Ti3AlC2块体材料，合成材料的X—射线衍射和扫描电镜(SEM)分析的结果表明：当烧结温度为1400℃时，材料中的主晶相为Ti2AlC,大小为10μm的板状多晶体；而在1500℃的温度下烧结所得材料的主晶相为Ti3AlC2，其板状多晶体的晶粒尺寸平均约为20μm。     

The role of Al in the reactive synthesis of porous Mo2Ti2AlC3 ceramics

The mixed powders of TiH₂, molybdenum, aluminum and graphite with molar ratios of 2/2/n/2.85 (n ranges from 1.0 to 1.4 mol with an interval of 0.1 mol) were used as raw material powders for this work, and a novel porous Mo₂Ti₂AlC₃ was synthesized via reactive synthesis. Through systematic research on the pore structure parameters of porous Mo₂Ti₂AlC₃ prepared with different aluminum content, the results show that there is a clear correlation between the aluminum content and the pore structure parameters. With the aluminum content rising from 1.0 to 1.2 mol, the viscous permeability coefficient and pore size decreased, while the porosity increased; When the aluminum content increased from 1.2 to 1.4 mol, the pore structure parameters of porous Mo₂Ti₂AlC₃ displayed an opposite trend. The reasons for the evolution laws of these pore structure parameters were also discussed in depth. In addition, the pore structure forming mechanism of porous Mo₂Ti₂AlC₃ ceramics during the activation reaction sintering process has been explored. This work can provide an important reference for the subsequent preparation of quaternary porous MAX phase ceramics.     

Pyrolysis mechanism of magnesium citrate nonahydrate and microstructural evolution during the process

The key characteristics of the porous carbon materials and ceramic composites derived from citrates are directly affected by the pyrolysis mechanism of parent citrates and the microstructural revolution during the process. The pyrolysis mechanism of magnesium citrate nonahydrate (MCN) and the microstructural evolution during its pyrolysis were investigated by analysing the C/MgO nanocomposite powders from MCN pyrolyzed in carbon embedded condition and flowing argon atmosphere. The pyrolysis process of MCN was composed of the following stages: (1) MCN dehydrated to magnesium citrate at about 150 °C; (2) magnesium citrate decomposed into itaconic acid magnesium and MgO at about 300 °C; (3) itaconic acid magnesium decomposed into carbon, MgO and CH₄ at around 500 °C; (4) CH₄ was pyrolyzed and graphene was deposited on MgO. The carbon produced in stage (3) was turbostratic while that derived from the pyrolysis-deposition of CH₄ was few-layered-graphene. The MgO nano grains produced in stage (2) precipitated and agglomerated while those derived from itaconic acid magnesium were much smaller in size. In carbon embedded condition, the few-layered-graphene not only deposited on the MgO aggregates surface but also inserted into the MgO nano grain boundaries, which suppressed the growth and sintering of MgO nano grains.     

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti₂AlC and Ti₃AlC₂ ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti₂AlC and Ti₃AlC₂ were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al₂O₃ scale on it. Furthermore, Ti₂AlC had larger difference regarding oxidation behavior between the top and side surface than Ti₃AlC₂, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al₂O₃ layer already at the initial oxidation stage.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

Role of boron addition on phase composition,microstructural evolution and mechanical properties of nanocrystalline SiBCN monoliths

Nanocrystalline SiBCN monoliths with the same Si/C/N mole ratio and various boron additions ranged from 0 to 3.0 mol were prepared by mechanical alloying plus reactive hot pressing methods. Correction of boron content and microstructural/morphological evolution was investigated in detail by XRD, SEM, TEM and STEM-EDX structure characterization. Except for SiC and BN(C), boron addition contributes to B_xC formation. Besides, boron addition promotes the crystallization of SiC, leading to the formation of poor crystallinity of spherical structures in inner SiC. Furthermore, boron addition significantly promotes the grain growth of SiC and B_xC and therefore increases the relative volume ratios of B_xC/BN(C) and B_xC/SiC. Amorphous-like BN(C) changes to belt-like structures as boron addition increases. The new formed B_xC effectively contributes to the improvement of Vicker’s hardness while pull-out of BN(C) belt-like structures benefits the fracture toughness.     

Comparison of corrosion behavior of Ti3SiC2 and Ti3AlC2 in NaCl solutions with Ti

Corrosion behavior of self-sintered, ternary-layered titanium silicon carbide (Ti₃SiC₂) and titanium aluminum carbide (Ti₃AlC₂) fabricated by an in-situ solid-liquid reaction/hot pressing process was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy in a 3.5% NaCl solution. Commercially pure titanium (Ti) was selected for comparison through XRD, XPS, SEM and EDS examinations for elucidating both the passivation behavior and corrosion mechanism of the alloys. Both Ti₃SiC₂ and Ti₃AlC₂ exhibited significantly superior passivation characteristics compared to Ti; Ti₃SiC₂ also showed better corrosion resistance. The silicon/aluminum site is prone to attack, and the difference in the diffusion rate between the A-site atoms and titanium decreases the passivation ability of the MAX phase. CP titanium exhibited a lower passivation current density and did not undergo breakdown in the test potential region while two MAX phases are destroyed. Nevertheless, the corrosion resistances of Ti₃SiC₂ and Ti₃AlC₂ are comparable to that of CP titanium.     

Influence of Ti3AlC2 content and load on the tribological behaviors of Ti3AlC2p/Al composites

In this study, Ti₃AlC₂ particles doped aluminum matrix composites were prepared by ultrasonic agitation casting method. Microstructure, mechanical properties, and tribological properties of pure aluminum and Ti₃AlC_2p/Al composites were characterized. Influence of different loads (10, 20, 30, and 40 N) and Ti₃AlC₂ contents (1.0, 2.0, 3.0, and 4.0 wt%) on the tribological behaviors of the composites were studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersion spectroscopy (EDS), and 3D laser confocal were used to assist the analysis. The results indicated that fine and uniformly microstructure and the optimum comprehensive mechanical properties were exhibited on 2.0 wt%-Ti₃AlC_2p/Al composites. The abrasive grooves were widened and deepened with an increase in the load. The abrasion performance of composites improved distinctly with the addition of the Ti₃AlC₂ particles, which changed the wear mechanism from adhesive wear to abrasive wear. The 30 N load and the composites of 2.0 wt% Ti₃AlC₂ revealed the optimum tribological properties. The improvement of the tribological behavior of composites was attributed to the refinement of microstructure, the improvement mechanical properties and the three dimensional layered Ti₃AlC₂ phases with self-lubricating properties.     

Improved mechanical properties and toughening mechanism of mullite ceramics reinforced by introducing Ti3AlC2 particles

Ti₃AlC₂, as a toughening phase, was introduced into mullite ceramics for the first time by the pressureless sintering process aiming at improving the mechanical properties. Significant enhancement in density and mechanical performance of mullite ceramics was achieved through the introduction of Ti₃AlC₂ particles. The density of as-prepared mullite–Ti₃AlC₂ composites was increased by 23% (from 2.86 g/cm³ to 3.51 g/cm³) with Ti₃AlC₂ increasing from 0 wt% to 20 wt%. The formation of the liquid phase and decomposed particles from Ti₃AlC₂ are supposed to be responsible for the densification of mullite–Ti₃AlC₂ composites. The optimal mechanical properties were obtained in the mullite–Ti₃AlC₂ composites with 15 wt% Ti₃AlC₂. The bending strength, fracture toughness as well as Vickers hardness were reached 214.36 MPa, 4.84 MPa·m^1/2, and 9.21 GPa, which are 40%, 74%, and 113% higher than pure mullite ceramics, respectively. The improved mechanical performance was mainly attributed to the synergetic action of crack deflection, crack branching and bridging, and strengthened grain boundary.