Polycarbosilane Infiltration Research Articles

Processing and characterization of Cf/ZrB2-SiC-ZrC composites produced at moderate pressure and temperature

Cf/ZrB2-SiC/ZrC composites were prepared by hot pressing of carbon fibre (Cf) cloth, ZrB2, polycarbosilane (PCS) and Zr metal at 4MPa and 1200°C. The bulk density of Cf/ZrB2-SiC/ZrC composite was improved from 2.29g/cc to 2.69g/cc as a function of PCS infiltration cycles. Microstructural investigation revealed that the SiC encapsulates the Cf and also present at grain boundaries. This resulted in enhanced scratch critical load from 1500 to 3200 mN and flexural strength from 30 to 100MPa. Further, oxyacetylene flame at 2100°C for a shorter duration of 20–60s showed a decrease in the mass ablation rate from 0.0062 to 0.0028g/s. This was due to the formation of protective Zr-Si-O layer and confirmed by elemental analysis.

Fabrication and Properties of 3D-C<sub>f</sub>/Mullite Composites with SiC Interfacial Coatings

Three-dimensional carbon fiber reinforced mullite composites (3D-Cf/mullite) were fabricated by sol-gel method with diphasic mullite sol as raw materials. SiC interfacial coatings were deposited by polycarbosilane infiltration and pyrolysis before sol-gel process. The as-fabricated composites showed 285.7MPa in flexural strength, relatively close to 277.6MPa of those without coatings. After treated at 1300°C and 1500°C in Ar atmosphere for 1h, the retention rates of flexural strength were 80.0% and 69.9%, respectively, which showed a great improvement compared to the composites without coatings. This was attributed to the prevention of carbothermal reduction between carbon fiber and matrix by SiC coatings.

Ablation behavior of three-dimensional braided C/SiC composites by oxyacetylene torch under different environments

SiC ceramic matrix composites reinforced by three-dimensional braided carbon fibers were prepared via polycarbosilane infiltration pyrolysis (PIP). The ablation behavior of the composites was characterized by an oxyacetylene torch under different environments. The morphology and microstructure of the as-ablated composites were examined by scanning electron microscopy and the composition of the new phase was confirmed by energy dispersive spectroscopy. Two conditions showed different ablation mechanisms. The erosion mechanism of the high speed oxyacetylene torch was the main ablation behavior under oxygen free environment. Thermo-chemicals ablation was the main ablation behavior under abundant oxygen environment.

Morphology and Microstructure of Three‐Dimensional Orthogonal C/SiC Composites Ablated by an Oxyacetylene Flame at 2900°C

C/SiC composites were prepared by polycarbosilane infiltration pyrolysis and ablated by oxy‐acetylene flame at 2900°C for 180 s. The morphology and microstructure of C/SiC were observed by scanning electron microscopy. The phase transition and the composition were confirmed by energy‐dispersive spectroscopy and X‐ray diffraction. The ablation rates of the center region and the outer region were 2.5 and 1.1 μm/s, respectively. The ablated C/SiC was covered by a turbostratic carbon coating resulting from the pyrolysis of acetylene. White dross attached on the surface was composed of SiO2 resulting from the sublimation and decomposition of SiC during ablation and oxidization of Si and SiC during cooling. The results indicated that the ablation was due to a combination of carbon coating deposition, decomposition of SiC, oxidation, and mechanical erosion.

Effects of ablation at different regions in three-dimensional orthogonal C/SiC composites ablated by oxyacetylene torch at 1800 °C

C/SiC composites were prepared by polycarbosilane infiltration pyrolysis and ablated by oxyacetylene flame at 1800 °C for 180 s. Morphology and microstructure of C/SiC have been studied by scanning electron microscopy/energy dispersive spectroscopy, and X-ray diffraction analysis. Two concentric ring regions appeared on the surface of the ablated C/SiC. In the centre region of the ablated C/SiC was composed of irregular SiC particles with a lot of pores. The pores were resulted from (i) gas expansion in closed pores during ablation, and (ii) from the release of SiO gas produced by the oxidation of SiC with sufficiently low oxygen partial pressure. The results indicated the ablation was due to a combination of oxidation, mechanical erosion and recrystallization of the surface SiC.

Preparation and Mechanical Properties of C/C-SiC Composites

Porous C/C composite with certain porosity prepared by Chemical vapor infiltration (CVI) was chosen as the preforms to develop the C/C-SiC composites through precursor infiltration and pyrolysis(PIP), using PCS (polycarbosilane) as the precursor and divinylbenzene as solvent and cross-linking reagent for PCS. The effect of the infiltration solution with different PCS/DVB ratio on the final density, microstructure, and mechanical properties of composites was investigated and the proper PCS/DVB ratio to prepare the C/C-SiC composites was suggested. The experimental results showed that the final densities and the mechanical properties of the composites were close related to the PCS/DVB ratio. Higher PCS/DVB ratio resulted in higher final density and better mechanical properties, but not the highest PCS/DVB ratio could get the best mechanical properties. The main reason is that too high PCS/DVB ratio will make the infiltration process become difficult and lead to the formation of lots of pores in the final composite, at last lowers the mechanical properties. It is believed that the 50% PCS content is proper to prepare the C/C-SiC composites. The composite from 50% PCS infiltration solution could get the final density of1.696g/cm3, the flexural strength of 171Mpa, and shearing strength of 21.6Mpa, which are the best mechanical results among the obtained materials.

Fabrication and Characteristics of Carbon Fiber Reinforced C–SiC Dual Matrix Composites

A series of carbon fiber reinforced C-SiC dual matrix composites (C/C-SiC composites) were developed through precursor infiltration of polycarbosilane (PCS) and pyrolysis (PIP), using porous C/C composites with different density from chemical vapor infiltration (CVI) as the preform. The density, mechanical properties, and microstructure of the composites were investigated and the effects of the preform density and the PCS concentration of the infiltration solution on the final density and the mechanical properties of the composites were discussed in detail. The results show that the final density of the C/C-SiC composites prepared at the infiltration concentration of 50% is the highest, indicating that 50% is the proper PCS concentration of the PCS/ Xylene solution to prepare the C/C-SiC composites. The final densities of C/C-SiC composites were closely related to the preform density and the highest final density corresponds to the highest original preform density. For the composites prepared using infiltration solution of 50% PCS, the C/C-SiC composite whose preform density is 1.23 g/cm3 possesses the best mechanical properties while that whose preform density is 1.49 g/cm3 the worst mechanical properties.

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-C f/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C f/SiC composites fabricated with vacuum infiltration were 1.794 g cm −3 and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm −3 and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.

Enhanced Sintering of TiB<sub>2</sub> with SiC Addition Prepared by Polycarbosilane Infiltration

The sintering of TiB2 was limited due to its high degree of covalent bonding and low self diffusion coefficient. To solve this problem, hot pressing has been used for fabricating TiB2 ceramics, but this process was expensive and limited to the production of simple shapes. Pressureless sintering has also been studied, but its sintering temperatures is, as high as more than 2000°C, resulting in exaggerated grain growth and decrease of mechanical properties. The addition of SiC was found to be effective in improving the densification. However, a supplementary HIP process was needed to enhance the mechanical properties. In this study, the densification behavior and microstructural change of TiB2 ceramics infiltrated by the polycarbosilane of SiC precusor were investigated. The infiltrated polycarbosilane was converted to SiC during heating and densification was accelerated.

Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane

Abstract Three-dimensional braided carbon fiber reinforced silicon carbide composites (3D-B Cf/SiC) were prepared through eight cycles of vacuum infiltration of polycarbosilane (PCS) and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of Cf/SiC were discussed. It was found that the heating rate had great effect on the mechanical properties of Cf/SiC composites. With the increase of heating rate, the density of Cf/SiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of Cf/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 °C/min. The results showed that the flexural strength of the Cf/SiC composites fabricated at a heating rate of 15 °C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600 °C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300 °C in vacuum, the Cf/SiC showed higher flexural strength (680 MPa) than that (557 MPa) at room temperature.

Fabrication of short-fiber-reinforced SiC composites by polycarbosilane infiltration

Abstract Green sheets of the composites using polycarbosilane (PCS)-infiltrated (coated) Tyranno SA short fibers, submicron SiC powder and Al 2 O 3 –Y 2 O 3 –CaO mixtures as sintering additives were prepared by tape-casting. These green sheets were stacked and hot-pressed at 1750 °C. Most of all SiC short fibers were oriented along the tape casting direction. The bulk density of the composites was about 2.66 g/cm 3 . The bending strength was measured at room and high temperatures and the highest value was about 260 MPa at room temperature and 1000 °C. The fracture behavior of the composites showed a completely brittle fracture at room and high temperatures up to 1400 °C. Results of scanning electron microscope investigation showed that there were little pulled-out fibers in the tensile stress zone of the fracture surface independent of test temperature. A mechanically weak and chemically tight interface layer was not successfully formed by the present coat method.

A new process for Al 2O 3/SiC nanocomposites by polycarbosilane infiltration

Conventional powder processing is commonly used to fabricate Al 2O 3/SiC nanocomposites. Recently a new processing route using SiC polymeric precursor such as polysilazane, polysilastyrene or polycarbosilane (PCS) has also been attempted, where a polymeric precursor solution is mixed with alumina powder. The present work investigated a new process to make the nanocomposites by modifying such a polymer precursor route. Unlike the existing process, the solution of a SiC polymer precursor was infiltrated into partially sintered (i.e. porous) alumina bodies, sintered at normal pressure. With PCS precursor, it was studied how the precursor solution concentration and sintering condition affected the SiC content, densification, phase and microstructure.

Carbon fibre-reinforced CMCs by PCS infiltration

C/SiC-based composites were produced by infiltration of a woven carbon fabric preform with a polycarbosilane (PCS) solution. Progress of the infiltration process up to eight infiltrations was followed by density measurements, microscopy and mechanical testing. After six infiltration cycles, the density increase tended to a plateau. Dense deposition of the matrix on the outer surface sealed off the core of the composite, which hindered successive infiltrations. With further re-infiltration cycles, the development of the matrix and the degree of bonding between the fibre and matrix increased. There was also a transition in the failure mode of the composite, when tested in 3-point bending, from shear to plain brittle tensile failure, as the number of infiltrations increased.

Fabrication and Mechanical Properties of 3-Dimensional Tyranno Fiber Reinforced SiC Composites by Repeated Infiltration of Polycarbosilane

Three-dimensional Tyranno fiber reinforced SiC composites were fabricated by repetitive infiltration of molten polycarbosilane into 3-dimensional textile preforms (volume fraction 35-39%) with different fiber orientation ratio, followed by a pyrolysis process up to 1200°C in flowing argon. The composites displayed both high fracture toughness and work of fracture. In addition, a semistable fracture behavior was maintained even at 1200°C in air. Bending strength, fracture toughness, and work of fracture of the composite were strongly affected by the fiber orientation ratio and fiber diameter of the preform. This study suggested that the volume fraction of fiber along the tensile direction was the critical parameter for increasing the mechanical properties of the composite, and that the use of thin fibers involved higher values of work-of-fracture.

Fabrication and Evaluation of 3-Dimensional Tyranno Fiber Reinforced SiC Composites by Repeated Infiltration of Polycarbosilane

3-Dimensional Tyranno fiber reinforced SiC composites were fabricated by infiltration of molten polycarbosilane into a 3-dimensional textile preform (volume fraction: 39%) in an autoclave followed by pyrolysis in an argon atmosphere up to 1200°C. An attempt was made to improve the density of the composite by repeating the infiltration and pyrolysis process. Ten times repetition increased the density of the composite to 2.4×103kg·m-3 (relative density: 83%, open porosity: 5%). The bending strength and fracture toughness of the composite were 420 MPa and 11.5 MPa·m1/2, respectively. The composite showed semistable fracture behavior because of bridging of fibers parallel to the tensile direction.

Fabrication of SiC-SiC Whisker Composite by Repeated Infiltration of Polycarbosilane

SiC-SiC whisker composite was fabricated by infiltration of polycarbosilane (PCS) as a precursor for matrix into the whisker preform followed by the pyrolysis process. The density was improved by repetitive infiltration into the composite with low density. The bending strength at room temperature and 1200°C was also improved by increased density. The strength at high temperature was the same as that room temperature. Two liquids were employed for infiltration; n-hexane solution of polycarbosilane and molten polycarbosilane were infiltrated. The density was improved to ca. 2g⋅cm-3 by the infiltration of these liquids. The composite modified by solution infiltration tended to have higher bending strength than that by molten PCS infiltration for the same density.

Preparation of Unidirectionally Reinforced Carbon-SiC Composite by Repeated Infiltration of Polycarbosilane

SiC composites (C/SiC) reinforced with unidirectionally oriented carbon fibers were fabricated by the pressureless sintering method completed with infiltration of polycarbosilane (PC). This method is effective for increasing density and strength of the composite.The open porosity of C/SiC of 10% was achieved after 10 times infiltration of PC and the strength reached 380MPa. Fracture behavior of C/SiC changed from delamination parallel to the fibers to fiber failure after the infiltration.