Oxidation behavior of silicon carbide based biomorphic ceramics prepared by chemical vapor infiltration and reaction technique
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Chemical vapor infiltration
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Silicon carbide(SiC) ceramic foams with a continuously connected open-cell structure were prepared and characterized for their mechanical performance.The apparent densities of the SiC ceramic foams are controlled between 0.4 g/cm~3 and 1.3 g/cm~3,with corresponding compressive strengths ranging from 8 MPa to 50 MPa and flexural strengths from 3 MPa to 30 MPa.Stress-strain curves with only one linearelastic region,which is different from those previous reports on ceramic foams, are obtained by compressive test.This can be attributed to the existence of filaments with fine,dense and high strength microstructures in SiC.The SiC and the filaments can respond homogeneously to applied loading.
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Silicon carbide matrix composites have been fabricated from either ceramic‐grade Nicalon TM or Hi‐Nicalon TM fibers coated with an interface material consisting of six alternating carbon and silicon carbide layers. Initial efforts involved the use of chemical vapor infiltration to produce minicomposites (single tows of fibers). In subsequent work, forced‐flow thermal‐gradient chemical vapor infiltration was used to produce a single composite plate with a multilayer interface from ceramic‐grade Nicalon fabric and two plates from Hi‐Nicalon fabric, one with a single carbon layer and one with a multilayer interface. Tensile testing of the minicomposites and of specimens cut from the plates revealed typical composite behavior and strengths for the as‐processed samples. Exposure of tensile specimens to 950°C air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coating. The results demonstrate that forced‐flow thermal‐gradient chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are susceptible to oxidation that causes the loss of composite properties.
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Two dimensional carbon fiber-reinforced silicon carbide-carbon binary matrix composites(2D Cf/(SiC-C)) were fabricated by means of isothermal and isobaric chemical vapor infiltration(ICVI).The matrix structures of the 2D Cf/(SiC-C) composites were characterized by the backscattered electron imaging(BSE) of scanning electron microscope(SEM).Furthermore,their room temperature mechanical properties and fracture surfaces were compared with two dimensional carbon fiber-reinforced silicon carbide matrix composite(2D Cf/SiC).The results indicate that the matrices in the 2D Cf/(SiC-C) composites are multilayered structures composed of SiC and PyC layers.The PyC matrix layers are homogeneous and continuous,which are bonding well with SiC matrix layers.The 2D Cf/(SiC-C) composite with a thicker PyC matrix layer in fiber bundles exhibits better mechanical properties.Meanwhile,its tensile strength,failure strain,fracture toughness and fracture work are 3%,142%,22% and 58% higher than those of the 2D Cf/SiC composite,respectively.The multilayered matrices composed of SiC and PyC layers,cause the fibers in the 2D Cf/(C-SiC) composites to pull out twice in a concentrated mode.Moreover,the first pull-out fibers play a leading role in enhancing the strength and toughness.
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Piezoelectric lead zirconate titanate (PZT) ceramic-cement composites using a high volume of ceramic content (80%) in 0–3 connectivity were fabricated. Piezoelectric Force Microscope (PFM) characterization was carried out and ferroelectric hysteresis behavior of the composites were investigated. Domain configurations of PZT ceramic can be seen at the interfacial zone of PZT-cement composites. Ceramic particles were seen to bind well with the cement matrix. The ferroelectric hysteresis loop at 50 Hz and 10–25 kV/cm showed a slim loop with low loss behavior for this type of composite due to increase in the PZT ceramic in 0-3 PZT-cement composites.
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Silicon carbide fiber reinforced silicon carbide (SiCf/SiC) composites were fabricated with a pyrocarbon interface layer between porous 2.5-dimensional SiC fiber preforms and SiC matrix deposited by the low pressure chemical vapor infiltration method. The effects of the interface layer thickness and matrix fabrication conditions on the mechanical properties of composites were investigated. The results show that a pyrocarbon inter face layer 0.1 μm thick makes the flexural strength of composites increase by 104.2% from 144 to 294 MPa, and the fracture of the composite is in a non-catastrophic mode. But when the interface layer thickness is increased further to about 0.16 μm the reinforcement effectiveness of the fiber decreases, and this causes the composite to have bad fracture performance. When the deposition temperature of the matrix is decreased from 1 050 ℃ to 950 ℃, the flexural strength of the composite increases by ～45% from 188 MPa to 274 MPa. The porosity of composites increases and the SiC texture changes from rhomboid to spherical when the deposition pressure for SiC matrix fabrication is increased from 8 kPa to 16 kPa. Due to the fine SiC matrix, the strength of the composite increases slightly though the porosity is a little higher.
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A two-dimensional model was developed according to infiltration-induced structural changes in C/SiC composites and physicochemical phenomena involved in isothermal chemical vapour infiltration (ICVI) process. The mathematical model was implemented to simulate the densification behaviour of the C/SiC component of a small-scale thruster liner for rocket engine. The calculated results show that infiltration efficiency is high at first and then decreases dramatically, which is in agreement with the corresponding experimental results. The correspondence between calculated results and experimental data implied that this mathematical model is reasonable and feasible for characterizing the densification behaviour of C/SiC composites in the ICVI process. The dependence of densification behaviour of C/SiC composites on infiltration temperature has also been investigated. Calculation results show that the densification rate increases while density uniformity of overall composites decreases evidently with elevated temperature.
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