Ablation mechanism of C/C–SiC and C/C–SiC–ZrC composites in hypersonic oxygen-enriched environment

In this study, C/C–SiC and C/C–SiC–ZrC composites were prepared via chemical vapor infiltration and polymer infiltration pyrolysis, and the ablation mechanism under hypersonic oxygen-rich environmental conditions was investigated. The C/C–SiC composites demonstrate an excellent ablation resistance in a hypersonic oxygen-rich environment with a relatively low temperature and speed of approximately 1800 K and 1100 m/s, respectively. It is only in the ablation center area with higher temperatures that a certain degree of thermochemical ablation was observed. The mass and linear ablation rates of C/C–SiC composites (0.027 g/s and 0.117 mm/s, respectively) showed a significant increase in a hypersonic oxygen-rich environment with a temperature and velocity of approximately 2050 K and 2000 m/s, respectively. The high-temperature ablation resistance of ZrC-modified C/C–SiC–ZrC composites improved significantly. However, the ZrC ceramic component had a considerable impact on the ablation resistance of the material. The structural integrity of C/C–20SiC–30ZrC composites was relatively high in hypersonic oxygen-rich environments with a jet temperature and velocity of 2050 K and 2000 m/s, respectively, and mass and linear ablation rates were 0.012 g/s and 0.015 mm/s, respectively. When the ZrC content increased by 40%, the ablation resistance of the composite reduced significantly, whereas the mass and linear ablation rates increased to 0.043 g/s and 0.130 mm/s, respectively.     

Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite

The present study investigates the effect of Ti addition on the microstructure development and phase evolution during spark plasma sintering of ZrB₂–SiC ceramic composite. A ZrB₂–20?vol% SiC sample with 15?wt% Ti was prepared by high-energy milling and spark plasma sintering at 2000?°C for 7?min under 50?MPa. The X-ray diffraction test, microstructural studies and thermodynamic assessments indicated the in-situ formation of several compounds due to the chemical reactions of Ti with ZrB₂ and SiC. The Ti additive was completely consumed during the sintering process and converted to the ceramic compounds of TiC, TiB and TiSi₂. In addition, another refractory phase of ZrC was also formed as a result of sidelong reaction of ZrB₂ and SiC with the Ti additive.     

Decomposition reactions in the SiC–Al–Y–O system during gas pressure sintering

The substantial densification, that occurred in the SiC–Al–Y–O system was explained in the present work by analysing possible chemical reactions and their dependence on initial particle associations, i.e. homogeneity of mixing, the physical and chemical state of additives, pressurised sintering environment over the reactants and temperature of sintering. Hydroxyhydrogel powder precursors were found to be better than mechanically mixed SiC–YAG powder and pre-forming of YAG by holding the specimens at the temperature of 1400°C for 2 h were found to be the best. Decomposition reactions within the system could be controlled by using finer SiC and applying gas pressure over the reactants.     

In situ synthesis and interfacial bonding mechanism of SiC in MgO–SiC–C refractories

Adding SiC directly to MgO–C refractories possesses the disadvantages of low dispersion and interfacial bonding strength. Herein, the in situ synthesized SiC was introduced into the MgO–SiC–C refractories to maintain the original excellent performance of MgO–C refractories and reduce the carbon dissolution in molten steel. With the increase of Si and C content in raw materials, the morphology of SiC changed from whisker to network, whose growth mechanism was vapor–solid and vapor–liquid–solid. The network structure and uniform distribution of SiC improved the thermal shock resistance of MgO–SiC–C refractories. According to the analysis of molecular dynamics simulation by Materials Studio software, SiC strengthened the relationship between periclase and graphite to enhance the structure of the compound.     

Facile synthesis of Ti3SiC2 powder by high energy ball-milling and vacuum pressureless heat-treating process from Ti–TiC–SiC–Al powder mixtures

In this article, Ti/TiC/SiC/Al powder mixtures with molar ratios of 4:1:2:0.2 were high energy ball-milled, compacted, and heated in vacuum with various schedules, in order to reveal the effects of temperature, soaking time, thickness of the compacts, and carbon content on the purity of the sintered compacts. X-ray diffraction and scanning electron microscopy were employed to investigate the phase purity, particle size and morphology of the synthesized samples. It was found that the Ti₃SiC₂ content nearly reached 100 wt.% on the surface layer of the sintered compacts prepared in the temperature range from 1350 °C to 1400 °C for 1 h. Powder containing 91 wt.% Ti₃SiC₂ was successfully synthesized by heating 6 mm green compacts of 4Ti/1TiC/2SiC/0.2Al at 1380 °C for 1 h in vacuum. The excessive carbon content failed to improve the purity of Ti₃SiC₂ powder. TiC phase was the main impurity in the formation process of Ti₃SiC₂.     

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.     

Microstructure and ablation mechanism of C/C–SiC composites

C/C–SiC composites were prepared by molten infiltration of silicon powders, using porous C/C composites as frameworks. The porosities of the C/C–SiC composites were about 0.89–2.8 vol%, which is denser than traditional C/C composites. The ablation properties were tested using an oxyacetylene torch. Three annular regions were present on the ablation surface. With increasing pyrocarbon fraction, a white ceramic oxide layer formed from the boundary to the center of the surface. The ablation experimental results also showed that the linear and mass ablation rates of the composites decreased with increasing carbon fraction. Linear SiO₂ whiskers of diameter 800 nm and length approximately 3 μm were formed near the boundaries of the ablation surfaces of the C/C–SiC composites produced with low-porosity C/C frameworks. The ablation mechanism of the C/C–SiC composites is discussed, based on a heterogeneous ablation reaction model and a supersaturation assumption.     

Modeling of thermal explosion in constrained dies for B4C–Ti and BN–Ti powder blends

The process of reactive in-situ synthesis of dense particulate reinforced TiB₂/TiC and TiB₂/TiN ceramic matrix composites from B₄C–Ti and BN–Ti powder blends with and without the addition of Ni has been modeled. The objective of modeling was the determination of optimal thermal conditions preferable for production of fully dense ceramic matrix composites. Towards this goal heat transfer and combustion in dense and porous ceramic blends were investigated during heating at a constant rate. This process was modeled using a heat transfer–combustion model with kinetic parameters determined from the differential thermal analysis of the experimental data. The kinetic burning parameters and the model developed were further used to describe the process of combustion synthesis in a constrained die under pressure. It has been shown that heat removal from the reaction zone affects the ignition temperature of thermal explosion.     

Sol-Gel synthesis and characterization of SiC–B4C nano powder

In the present research, SiC–B₄C nano powders were synthesized through sol–gel process in water–solvent–catalyst–dispersant system. In order to evaluate the formation mechanism of the product during sol-gel process, TEM, SEM, DTA/TG, BET, XRD, FTIR and DLS analysis methods were employed. The nanometric size of precursor was controlled by dispersing agents and controlling pH inside the sol. DLS analysis revealed that the particles of the precursor inside the sol were below 10 nm. FTIR results indicated that the (Si–O–B) bonds were formed in the dried gel powder, due to hydrolysis and condensation reactions. DTA analysis confirmed that the synthesis temperature was lower than 1400 °C. XRD results implied the presence of cubic β-SiC and the rhombohedral B₄C phases, which were formed simultaneously in the SiC–B₄C nanopowder. BET analysis indicated a high surface area for the particles of about 171.42 m²/g, and that the surfaces of these particles were meso porous. SEM analysis exhibited that SiC– B₄C particle size was in the range of 20–40 nm with homogenous morphology. Ultimately, the TEM/EDS microstructural analysis showed that B₄C and SiC particles were formed simultaneously and uniformly in the final product.     

Study on microstructure evolution and reaction mechanism of in-flight Ti–Si–C agglomerates during reactive plasma spraying using in situ water quenching

An in situ water quenching method was used to explore the microstructure evolution and reaction mechanism of Ti–Si–C agglomerates during reactive thermal spraying. The quenched powders exhibit a melting process from outside to inside and small particles to large ones, forming complete droplets with increasing spray distance. The formation of the liquid phase is the basis of the reaction; once the liquid phase is formed, reactions begin to form new phases (TiC and Ti₅Si₃). The melting point of the new phase and the temperature of the droplet determine the formation mechanism and morphology of the new phase in the coating. As the melting point is higher than the droplet temperature, the new phase, TiC, grows to a large submicron size in flight. When the melting point is lower than the droplet temperature, like Ti₅Si₃, it dissolves into the liquid phase and re-precipitates in nanometer size at impact. Moreover, the droplet surface absorbed and dissolved O element from the atmosphere, and thus Ti₃O coexisted with Ti₅Si₃ in lamellae of coatings.     

Method of power density determination in microwave discharge,sustained in hydrogen–methane gas mixture

We present the results of studying a continuous microwave discharge maintained at a frequency of 2.45 GHz in a CVD reactor based on a cylindrical resonator excited at the TM₀₁₃ mode. The discharge was ignited in hydrogen and a gaseous mixture of hydrogen and methane and was studied by the method of optical emission spectroscopy. Density of atomic hydrogen and gas temperature were measured, as well as the spatial distribution of both optical emission intensity of the plasma and intensity of the Hα line of atomic hydrogen. The main parameters of the discharge were calculated numerically using the two-dimensional self-consistent model of the discharge. Basing on the obtained results, we proposed a method for high-precision experimental determination of the plasma volume and calculation of the specific energy contribution to the plasma, i.e., microwave power density in plasma (MWPD), with minimal errors. According to the calculations, in the experiment performed, the microwave power density in the plasma varied from 50 to 550 W/cm³ as the gas pressure increased from 80 to 350 Torr. The method allows one to perform unified MWPD calculations in different CVD reactors and to compare diamond film deposition regimes.     

Effect of nitrogen impurity on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid mechanism on 6H–SiC substrates

This work reports on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid (VLS) on on-axis and 2° off-axis 6H–SiC(0001) substrates using Si–Ge as liquid phase. It was found that, depending on growth conditions (mainly temperature or nitrogen amount in the reactor), the deposit could be either a complete 3C or 6H–SiC layer or even a mixture of both polytypes. The proportion of 3C inside the deposit increases when 1) nitrogen amount in the reactor increases or 2) temperature is decreased. Though the effect of temperature could be explained in terms of 3C–SiC initial island dissolution, the influence of nitrogen is less obvious but it is shown to be effective at the early stage of growth. Several hypotheses are proposed such as SiC lattice modification by N incorporation or surface effects during the early stage of growth.     

The Effect of Si Substitution for SiC on SHS in the Ti–Si–C System

Investigated was the effect of Si substitution for SiC on SHS in the Ti–Si–C system. Starting powders were intermixed to obtain 3Ti–SiC–C and 3Ti–Si–2C green mixtures and then green compacts by uniaxial pressing. The influence of heating rate, reactor temperature, and replacement of SiC by Si was studied by XRD, SEM, and TEM. In combustion products obtained in optimized conditions, Ti₃SiC₂ was found to be predominant. In comparison with conventional methods, our products obtained in a one-step low-temperature process contained minimal amounts of undesired impurities and required no finishing processes such as chemical purification.     

Structural evolution of Fe–Y2O3–Ti powder during ball-milling and thermal treatment

Y₂O₃ dispersion is widely used in ceramics and steels strengthening. The improved performance results from very fine oxide particles being dispersed within the matrix by ball milling; however, during ball-milling and subsequent heat-treatment, the mechanism underpinning the evolution of Y₂O₃ and additive Ti remains uncertain. In this study ball-milling was performed on Fe+10% Y₂O₃+5%Ti powders for different times without adding a process control agent. Heat-treatment was then applied at 900 to 1200 °C. Then the powder after ball-milling and heat treatment was characterised, which showed that, with increased milling time, the average particle size increased while Fe and Y₂O₃ underwent an amorphous transition. After ball-milling for 30 h, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) both verified the generation of Y₂Ti₂O₇. In the subsequent heat-treatment, differential scanning calorimetry (DSC) showed that the amorphous Fe and Y₂O₃ had been transformed back to crystalline at 736.3 °C and 991.3 °C, respectively. With increased heating temperature, the Y₂O₃ content increased, while that of Y₂Ti₂O₇ remained stable throughout.     

Torque rheometry and rheological analysis of powder–polymer mixture for aluminum powder injection molding

Selection of desired powder–polymer mixture (feedstock) formulation is a key factor in manufacturing perfect parts via powder injection molding. In the present study, feedstock characteristics of an aluminum-based powder were investigated by torque rheometry and rheological analyses. Several binders containing various amounts of polypropylene (PP), paraffin wax (PW), and stearic acid (SA) were selected for torque mixing and viscosity evaluation. Then, feedstocks consisting of 54, 58, 62, and 66 vol. % solid contents were prepared with modified binder. Feedstock flow behaviors were investigated regarding the rheological parameters such as mixing torque, viscosity, flow behavior index, flow activation energy and moldability index. It was found that increasing solid loading from 54 to 62 vol. % led to improved rheological behavior. This improvement was not observed in high solid contents, i.e., 66 vol. %. Based on experimental results, the optimized binder composition (60PW,35PP,5SA vol. %) and the optimum powder loading (62 vol. %) were selected as the best formulations for injection of aluminum powder. These values are supported by critical powder volume concentration measurements deduced from the oil absorption method. The resulting aluminum molded green parts with no defects exhibited the straightforward injection molding process of selected feedstock.     

Mullite–molybdenum composites fabricated by pulse electric current sintering technique

Pulse electric current sintering of monolithic mullite and mullite/0–100 vol.% Mo composites was performed in vacuum of 4.5×10⁻⁵ Torr at temperatures and pressures of 1500 °C and 20 MPa, respectively. No traces of additional phases were observed by SEM and XRD for these composites. Microstructural observations reveal that Mo (molybdenum) particles dispersed uniformly at lower Mo contents and exhibited flaky and elongated structure at higher content. Simultaneous increase of fracture strength and toughness occurred with increase in Mo content. It attained a maximum of 1.1 GPa and 9.2 MPa m^1/2, respectively for 90 vol.% Mo composites. The increase in flexural strength is due to smaller initial flaws in mullite/Mo composites for lower Mo contents and due to plastic deformation of Mo phase for higher Mo contents. Similarly, frontal process zone toughening and crack bridging are expected to be the responsible mechanisms for enhanced toughness in these composites. Partial debonding in the mullite–Mo interface, giving rise to plastic deformation of Mo phase also contributes in the increase of toughness values.     

TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Direct synthesis of NiAl intermetallic matrix composite with TiC and Al2O3 reinforcements by mechanical alloying of NiO–Al–Ti–C powder mixture

NiAl intermetallic matrix nanocomposite with TiC and Al₂O₃ was directly fabricated by mechanical alloying of NiO, Al, graphite, and Ti as the starting powder mixture. The phase and morphological evaluations during mechanical alloying were characterized by X-ray diffraction and scanning electron microscopy equipped by energy-dispersive X-ray spectroscopy, respectively. The thermal behavior of the 40 h milled powder was obtained via differential thermal analysis. Followed by 10 h milling, no new phases were formed; but after 20 h milling, Al reduced a fraction of NiO, NiAl and Al₂O₃ were formed, and the released energy promoted TiC formation. When milling time reached 40 h, the remained raw materials were completely consumed and NiAl/TiC–Al₂O₃ nanocomposite was formed. Therefore, the main stage to synthesize NiAl/TiC–Al₂O₃ nanocomposite was the reduction of NiO by Al. The microstructural evaluation revealed formation of a homogeneously distributed composite and thermal analysis showed that the synthesized product was stable.     

Mechanochemical synthesis of MoSi2–SiC nanocomposite powder

MoSi₂–25 wt.%SiC nanocomposite powder was successfully synthesized by ball milling Mo, Si and graphite powders. The effect of milling time and annealing temperature were investigated. Changes in the crystal structure and powder morphology were monitored by XRD and SEM, respectively. The microstructure of powders was further studied by peak profile analysis and TEM. MoSi₂ and SiC were synthesized after 10 h of milling. Both high and low temperature polymorphs (LTP and HTP) of MoSi₂ were observed at the short milling times. Further milling led to the transformation of LTP to HTP. On the other hands, an inverse HTP to LTP transformation took place during annealing of 20 h milled powder at 900 °C. Results of peak profile analysis showed that the mean grain size and strain of the 20 h milled powder are 31.8 nm and 1.19% that is in consistent with TEM image.