Model-aided fabrication of fiber-reinforced ceramic composite tubes using forced-flow chemical vapor infiltration

Fiber-reinforced ceramic composites possess high thermal conductivity, high fracture toughness, and corrosion resistance, having potential for use in fossil-energy steam plants, where corrosive environments at high temperature and pressure exist. The utilization of fiber-reinforced ceramic composite tubes may enable plant operation at higher temperatures, and may extend the lifetime of specific plant operations, improving overall efficiencies and reducing down-time.; Dense, fiber-reinforced ceramic composite tubes were fabricated using forced-flow, chemical vapor infiltration. This process involved gaseous ceramic precursor infiltration throughout a fibrous preform, where a temperature gradient was applied and a ceramic precursor was forced through its surface at lower temperature. The application of a suitable temperature gradient and total flow enabled the ceramic matrix deposition to preferentially translate from the preform hot-surface to the cold-surface, resulting in a dense, ceramic composite in a reasonable total process time.; Fibrous tube preforms were fabricated with Nextel(TM) 312 fiber. Silicon carbide was the reinforced ceramic matrix, which was deposited throughout the tube preform using methyltrichlorosilane. A standard set of process conditions was attempted to evaluate the feasibility in achieving dense composites. Tube preform infiltrations with variation in temperature and total flow were performed to determine effects on final density and total process time.; Density characterization was performed on tube preforms infiltrated with the same process conditions for various time lengths to study the transient tube densification. Tube density profiles were characterized using X-ray computed tomography and digital image analysis, and the results from both were compared for their effectiveness in the prediction of the transient tube densification.; A comprehensive process model simulated the transient tube infiltration using multiple, steady-state intervals. Transport properties were input into the model and were determined either by experiment or by calculation using a descriptive model. The model prediction was validated to experimental tube density results by the adjustment of model parameters. The model was then used to determine minimum process times that yielded acceptable tube density, while these times were compared to those seen in experiment. The model density profiles were compared to those found with the density characterization methods to evaluate the process simulation.