Structure and properties of polycarbosilane synthesized from polydimethylsilane under high pressure

The polycarbosilane (PCS), which is the precursor of SiC fiber, was synthesized under high pressure by thermal decomposition of polydimethylsilane. The composition, structure, and properties of the PCS were characterized by the measurements of softening point, elemental analysis, IR, GPC, NMR, TG‐DTG‐DTA, XRD, and oxidative reaction activity, respectively. Structure model of the PCS was therefore inferred. The results showed that the PCS was the polymer with a Si? C backbone with M_n about 1587. IR and NMR showed the presence of SiC₄ and SiC₃H structure units containing Si? CH₃, Si? CH₂? Si, and Si? H groups. The ratio between H in C? H bond and H in Si? H bond was about 8.84 with SiC₃H/SiC₄ and about 0.51 from ¹H NMR and ²⁹Si NMR, respectively. Elemental analysis gave an empirical formula of SiC_1.87H_7.13O_0.03. TG analysis showed that the ceramic yield of the PCS at 1200°C in a N₂ flow was about 78.9%. β‐SiC microcrystal could be obtained when PCS was pyrolyzed at 1250°C with the crystal size about 37.5 Å. Compared with the PCS with similar softening point synthesized under normal pressure, the PCS synthesized under high pressure had approximate elemental composition, higher Si? H bond content and reaction activity, higher molecular weight, and higher ceramic yield, but lower ratio of SiC₃H and SiC₄. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1188–1194, 2006     

Green synthesis,formation mechanism and oxidation of Ti3SiC2 powder from bamboo charcoal,Ti and Si

Using low-cost and highly reactive bamboo charcoal, Ti and Si elemental powders as starting materials, Ti₃SiC₂ powder was synthesized via a simple and cost-efficient pressureless sintering technique in argon atmosphere. The influences of synthesis temperature, holding time and Si content on the Ti₃SiC₂ content of the synthesized products were investigated, and the analysis indicated that the relative content of Ti₃SiC₂ reached 98.9 wt% with a molar ratio of 3Ti/1.2Si/2.2C at 1400 °C for 1.5 h. The Ti₃SiC₂ with good crystallinity and homogeneous nanolayered structure was synthesized at lower temperatures due to the high reactivity and high specific surface area of bamboo charcoal. The non-isothermal oxidation behavior showed that Ti₃SiC₂ powder was stable in air below 540 °C. With the temperature increasing up to 1300 °C, continuous and dense TiO₂ and SiO₂ oxidation layers were formed on the surface of Ti₃SiC₂ particles, which conferred good oxidation resistance to Ti₃SiC₂ powder.     

In Situ Neutron Powder Diffraction Study of Ti3SiC2 Synthesis

The synthesis of Ti₃SiC₂ by pressureless reactive sintering of Ti/SiC/C mixtures under an Ar atmosphere has been studied using in situ neutron diffraction. The intermediate phases TiC_x and Ti₅Si₃C_x (x≤ 1) form first at ∼800–1400°C. These phases are consumed in the formation of Ti₃SiC₂, at ∼1500°C. After sintering, Ti₅Si₃C_x disappears but an amount of TiC_x remains in the sample primarily as a surface layer. The studies appear to support a suggestion that the intermediate phases react to form Ti₃SiC₂ through a diffusion-controlled process. Prolonged stepwise heating under argon in some experiments resulted in decomposition of Ti₃SiC₂ above ∼1400°C and significant disproportionation of the sample.     

Phase Evolution of Ti3SiC2 Annealing in Vacuum at Elevated Temperatures

The thermal decomposition of Ti₃SiC₂ in vacuum furnace up to 1500°C has been investigated. The results show that the mild decomposition of Ti₃SiC₂ commences at 1300°C and the higher the holding temperature, the larger the volatilization of Si atoms. The Ti₃SiC₂ decomposition occurs simultaneously on the surface and in the bulk. Four phases coexist at 1400°C and 1450°C and the Ti₅Si₃C_x phase appears in the bulk and/or surface. Diffusion distance, rate, and volatilization of Si contribute to the porous structure and the presence of Ti₅Si₃C_x. The evolution of furnace pressure reflects the decomposition kinetics of Ti₃SiC₂.     

Effect of alloying time on fabrication of Ti3Si(Al)C2 by spark plasma sintering

Abstract

3Ti–Si–2C–0·2Al mixture powders were used to fabricate high purity Ti₃SiC₂ ceramic through mechanical alloying (MA) and spark plasma sintering (SPS). The effect of ball milling time on the fabrication of Ti₃SiC₂ by SPS was also investigated. The results showed that the mixed powders were obviously refined after the MA of 5 h. After milling of 10 h, the mixed powders containing TiC, Ti₃SiC₂ were synthesised by a mechanically induced self-propagating reaction. After further milling to 20 h, the yield powders were refined. Ball milling time had a remarkable effect on SPS fabrication of Ti₃SiC₂. A shorter milling time of 5 h only helped to increase the Ti₃SiC₂ content in the sintered bulk. The samples subjected to the MA treatment of 20 h had a fine and dense organisation. Ball milling of 10 h was most beneficial for fabricating dense and high purity Ti₃SiC₂. Ti₃SiC₂ bulk with a purity of 96 wt-% was obtained by MA for 10 h and subsequent SPS at 850°C. When sintered at 1100°C, Ti₃SiC₂ bulk with a purity of 99·3 wt-% and a relative density of 98·9% was obtained.     

Ti3SiC2 friction material prepared by novel method of infiltration sintering

Titanium silicon carbide (Ti₃SiC₂) is a remarkable friction material for its combination of the best properties of metals and ceramics. The high purity Ti₃SiC₂ ceramic has been prepared by infiltration sintering (IS), and the effect of a small amount of Si on Ti₃SiC₂ ceramic formation was investigated. The results show that the purity of Ti₃SiC₂ ceramic could be increased significantly and the sintering time for Ti₃SiC₂ could be decreased remarkably when proper amount of Si was added in the starting mixture. The Ti₃SiC₂ sintered compact with a purity of 99.2?wt-% and a relative density of 97% was obtained by the IS from a starting mixture composed of n(Ti):n(Si):n(TiC)?=?1:0.3:2 at 1500°C with holding time of 2/3?h.     

Effect of liquid reaction on the synthesis of Ti3SiC2 powder

Synthesis of Ti₃SiC₂ powder was carried out by heat treating powder mixtures of Si, TiC and coarse Ti (−150 μm) in a temperature range of 1000–1400 °C. The phase content of Ti₃SiC₂ in the synthesized powder was improved to 99% when heat treated at 1400 °C for 4 h. Ti–Si liquid reaction was found to occur above the binary eutectic temperature, and this liquid reaction is believed to have assisted the synthesis reaction of Ti₃SiC₂.     

High-temperature hydrogenation behaviour of bulk titanium silicon carbide

Thermal stability of Ti₃SiC₂ was investigated at 1200–1400°C in hydrogen atmosphere for 3 hours. The hydrogenation mechanism was clarified by a combination of X-ray diffraction, scanning electron microscope, Raman spectroscopy and first principles calculation. At 1200°C, a dense and uniform TiSi₂ layer formed on the sample surface, which originated from both the preferable lose of silicon from the Ti₃SiC₂ substrate and the dissociation of Ti₃SiC₂. As temperature increased to 1300°C, TiSi₂ layer began to scale off and presented laminated Ti₃SiC₂ grains beneath this layer, which indicated preferential hydrogenation occurred along the basal planes. This phenomenon was ascribed to the fact that the introduction of H interstitial atom weakened the combination between titanium and silicon interface layer, which was confirmed by first principles calculations. In addition, the formation of TiSi₂ owing to the dissociation of Ti₃SiC₂ caused the volume expansion after hydrogenation, resulting in that majority of TiSi₂ layer spelled off at 1400°C.     

Joining of SiCf/SiC using a layered Ti3SiC2-SiCw and TiC gradient filler

A layered filler consisting of Ti₃SiC₂-SiC whiskers and TiC transition layer was used to join SiC_f/SiC. The effects of SiC_w reinforcement in Ti₃SiC₂ filler were examined after joining at 1400 or 1500 °C in terms of the microstructural evolution, joining strength, and oxidation/chemical resistances. The TiC transition layer formed by an in-situ reaction of Ti coating resulted in a decrease in thermal expansion mismatch between SiC_f/SiC and Ti₃SiC₂, revealing a sound joint without cracks formation. However, SiC_f/SiC joint without TiC layer showed formation of cracks and low joining strength. The incorporation of SiC_w in Ti₃SiC₂ filler showed an increase in joining strength, oxidation, and chemical etching resistance due to the strengthening effect. The Ti₃SiC₂ filler containing 10 wt.% SiC_w along with the formation of TiC was the optimal condition for joining of SiC_f/SiC at 1400 °C, showing the highest joining strength of 198 MPa as well as improved oxidation and chemical resistance.     

Microstructure and microindentation of Ti3SiC2 – Titanium filler brazed joints by tungsten inert gas (TIG) process

Herein we study the joining of Ti₃SiC₂ - a MAX phase - with a Ti filler (Ti₃SiC₂/Ti-filler) using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₃SiC₂ comes into contact with the molten Ti - filler during the TIG-brazing operation, it starts decomposing into TiC_x and a Si-rich liquid. Simultaneously, the molten Ti infiltrates into the Ti₃SiC₂ resulting in a 200 µm thick duplex region, comprised of TiC_x and a Ti-rich phase with some dissolved Si. Both Si and C are found in the solidified Ti; the Si source is from the Si-rich liquid, while the presence of C indicates that some of the C diffused into the Ti. Upon cooling, C- containing Ti- rich lamellae form the solidified Ti. Microindentation results of the decomposed Ti₃SiC₂ layer show an increase in hardness and a decrease in elastic modulus relative to T₃SiC₂. Notably, no cracks were observed.     

Interfacial microstructure and mechanical properties of TC4/Ti3SiC2 contact-reactive brazed joints using a Cu interlayer

Reliable contact-reactive brazed joints of TC4 alloy and Ti₃SiC₂ ceramic were obtained using a Cu interlayer. The interfacial microstructure of a TC4/Ti₃SiC₂ joint brazed at 920?°C for 10?min was TC4/Ti₂Cu +?α-Ti +?β-Ti/Ti₂Cu +?AlCu₂Ti +?Ti₅Si₃/Ti₅Si₃ +?Ti₅Si₄/Ti₃SiC₂. The interfacial microstructure and mechanical properties of TC4/Ti₃SiC₂ joints brazed at different temperatures were investigated. With increasing temperature, the shear strength of the brazed joints first increased and then decreased. The maximum shear strength was 132?±?8?MPa, and the corresponding fracture occurred along the Ti–Si reaction layer and the Ti₃SiC₂ substrate adjacent to the Ti–Si reaction layer. The microhardness test also demonstrated that the Ti–Si reaction layer possessed the highest microhardness, 812?±?22 HV. The Ti-Si reaction layer was the weakest part of the brazed joints. To eliminate the Ti-Si reaction layer and improve the mechanical properties of TC4/Ti₃SiC₂ brazed joints, a 40-μm Ni layer was plated on the surface of the Ti₃SiC₂ ceramic before brazing. The results showed that the Ti–Si reaction layer that formed adjacent to the Ti₃SiC₂ ceramic was thin and intermittent. Moreover, the interface between the Ti₃SiC₂ ceramic and the TC4 alloy became jagged. The shear strength of the TC4/nickel-plated Ti₃SiC₂ brazed joints improved to 148?±?8?MPa; the corresponding fracture occurred mainly in the Ti₃SiC₂ ceramic and only a small portion of the fracture occurred in the brazing seam.     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

Rapid synthesis of dense Ti3SiC2 by spark plasma sintering

Ti₃SiC₂ was rapidly synthesized and simultaneously consolidated from the starting mixture of Ti/Si/2TiC by spark plasma sintering (SPS). An intensive reaction leading to the formation of Ti₃SiC₂ occurred at the measured temperature of around 1200 °C, which is several hundreds degrees lower than that of conventional reactive hot pressing. The phase composition of the product could be tailored by adjusting the process parameters. An axisymmetric preferred orientation of the Ti₃SiC₂ grains with well-developed (008) planes was formed, resulting in an anisotropic hardness in respect to the textured product.     

Accommodation,Accumulation, and Migration of Defects in Ti3SiC2 and Ti3AlC2 MAX Phases

We have determined the energetics of defect formation and migration in M_n+1AX_n phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti₃SiC₂ structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti₃AlC₂ structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti₃AlC₂ is different and the energy is lower compared with the Ti₃SiC₂ structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti₃SiC₂, will have important implications for radiation damage response in these materials.     

Ablation properties of C/C-UHTCs and their preparation by reactive infiltration of K2MeF6 (Me = Zr,Ti) molten salt

Ultra-high temperature ceramic-modified C/C composites (C/C-UHTCs) were prepared by the reactive infiltration of K₂MeF₆ (Me = Zr, Ti) mixed with Si and Zr-Si powders. Molten salt infiltration can be divided into two stages: salt ion melt and Me-Si alloy melt. In the temperature range below 1400 °C, Zr and Si dissolve in the molten salt, are carried by the ion melt, and precipitate at the PyC interface to form carbides. Above 1400 °C, a large amount of molten salt volatilises and thermally decomposes. The Me-Si alloy forms a melt and infiltrates the C/C matrix, and finally forms C/C-ZrC-SiC, C/C-Ti₃SiC₂-SiC, and C/C-ZrC-TiC-SiC composites. The C/C-ZrC-SiC composite with the highest ZrC content exhibited the lowest mass rate (2.6 ± 0.02 mg/s) and linear ablation rate (0.82 ± 0.04 μm/s), which were reduced by 43.5 and 50.8 %, respectively, compared to the unmodified C/C-ZrC-SiC composite.     

Improving hardness and toughness of plasma sprayed Ti–Si–C nano-composite coatings by post Ar-annealing

Ti–Si–C (TSC) composite coatings were fabricated by plasma spraying using Ti/Si/graphite agglomerates as feedstock. Ar-annealing was carried out to reduce the intrinsic defects and increase the performance of the as-sprayed TSC coating. The effects of the annealing temperature (500–900 °C) on the microstructures and mechanical performances of the TSC coatings were investigated. All TSC coatings consisted of TiC, Ti₅Si₃ and MAX phase Ti₃SiC₂. With the increase in temperature (>700 °C), TiC became predominant, while the Ti₃SiC₂ phase content increased, which was accompanied by a decrease in Ti₅Si₃ content. The high -temperature annealing (>700 °C) led to a homogenous microstructure with a relatively low porosity and increased number of micro-cracks. Notably, the hardness and fracture toughness of the TSC coating were simultaneously increased after the annealing, from 1164 HV to 1.96 MPa m^1/2 to 1560 HV and 3.45 MPa m^1/2, respectively. The formation of nanoscale TiC and Ti₅Si₃ with a network distribution, uniform and dense microstructure, and toughening effects of Ti₃SiC₂ and micro-cracks provided the high mechanical performances of the TSC composite coatings.     

Influence of small amounts of Fe and V on the synthesis and stability of Ti3SiC2

Polycrystalline bulk samples of (Ti_1-yMe_y)₃SiC₂, where Me=Fe or V and y=0.01 to 0.1, were fabricated by reactive hot isostatic pressing of a mixture of Ti, C (graphite), SiC and Fe or V at 1450°C for 4 h under a pressure of 60 MPa. X-ray diffraction and scanning electron microscopy of the fully dense samples have shown that small amounts of Fe and V interfere with the reaction between Ti, C and SiC leading to the presence of SiC, TiC_x, as well as different Fe and V-containing phases in the final microstructures. The presence of these impurity phases also reduces the temperature at which Ti₃SiC₂ decomposes. The decomposition is manifested by the formation of a network of pores when the samples are annealed at 1600°C, a temperature at which pure Ti₃SiC₂ is thermally stable. The concentration threshold for this decomposition is as low as 1 at%.     

Surface strengthening of Ti3SiC2 through magnetron sputtering of Mo and Zr and subsequent annealing

Magnetron sputtering deposition of Mo and Zr and subsequent annealing were conducted with the motivation to modify the surface hardness of Ti₃SiC₂. For Mo-coated Ti₃SiC₂, Si diffused outward into the Mo layer and reacted with Mo to form molybdenum silicides in the temperature range of 1000–1100 °C. The MoSi₂ layer, however, cracked and easily spalled off. For Zr-coated Ti₃SiC₂, Si also diffused outward to form Zr–Si intermetallic compounds at 900–1100 °C. The Zr–Si compounds layer had good adhesion with Ti₃SiC₂ substrate, which resulted in the increased surface hardness.     

Effects of Mo addition on tribological performance of plasma-sprayed Ti–Si–C coatings

Ti–Si–C–Mo composite coatings were fabricated by plasma spraying using Ti, Si, graphite and Mo powders. The effect of Mo on microstructure and tribological performance of the Ti–Si–C coatings were investigated. The results showed that the Ti–Si–C coating consisted of TiC, Ti₃SiC₂, Ti₅Si₃, and residual graphite. The Ti–Si–C–Mo coatings consisted of TiC, Ti₃SiC₂, Ti₅Si₃, residual graphite, Mo and Mo₅Si₃ phases. With increasing Mo contents, the fractions of Mo and Mo₅Si₃ phases increased, and the fractions of Ti₃SiC₂ and Ti₅Si₃ phases decreased. All the coatings existed a typical lamellar structure. The addition of Mo enhanced the hardness and fracture toughness of Ti–Si–C coating by 16% and 52%, respectively. The coating porosity decreased by 57.6%. The wear resistance of the Ti–Si–C coating was also improved and the mass loss decreased by 83%. The wear mechanism of the Ti–Si–C–Mo coatings was the combination of abrasive wear, adhesive wear, and tribo-oxidation wear.     

Dependence of the microstructure and properties of TiC/Ti3SiC2 composites on extra C addition

TiC/Ti₃SiC₂ composites were synthesized with Ti/Si/C and Al (in which extra C addition ranges from 0 to 25 wt.%) as starting powders by hot-pressed sintering method at 1400 °C under 30 MPa. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the phase composition and the fracture surface. The results reveal that with the increase of extra C addition, the content of Ti₃SiC₂ phase decreases while the content of TiC phase increases. Graphite phase is detected in the samples with extra C addition of 20 wt.% and 25 wt.%. The bending strength decreases from 554.81 MPa to 57.44 MPa due to the decrease of the densification and Ti₃SiC₂ phase content. The electrical conductivity falls from 42,474.52 s/cm to 1524.95 s/cm, resulting from lower Ti₃SiC₂ phase content and higher contact resistance.