V. Heyer, M. Willert-Porada, Universität Bayreuth, LS für Werkstoffverarbeitung, Bayreuth
A. Schmidt-Rodenkirchen, InVerTec, Bayreuth
Attempts to further improve the high temperature mechanical performance of sintered and in-situ beta-Si3N4 fiber reinforced Silicon Nitride ceramic by addition of short carbon fibers as strengthening element independent of in-situ formation of the elongated ?-Si3N4-inclusions were not very successful so far. The main obstacles are the mismatch of thermal expansion coefficient (CTE) between C-fiber and Si3N4 – tensile stress builds up at the fiber-matrix interface due to higher CTE of the Si3N4 ceramic - and reactions between Carbon, Oxide additives and Silicon Nitride at sintering temperature. Two processing strategies were mainly followed in the past: infiltration of a carbon template with Si3N4-ceramic precursor followed by pyrolysis of the precursors, and low temperature hot pressing of powder derived C-Si3N4-CMC’s. As a new approach we synthesize compositionally graded C-SiC-Si3N4-coatings on short carbon fibers and consolidate the fibers with the ceramic powder by hot pressing. The matrix powder consists of commercial Si3N4 powder (HC Starck) and in-house synthesized Yb2SiO5 as sintering additive [1,2].
The fibers are coated by Silicon via CVD deposition of Silicon in a fluidized bed reactor. As precursor Silane, SiH4 or Trichlorosilane, HSiCl3 (TCS) are utilized. With TCS as precursor Plasma Enhanced atmospheric pressure CVD is applied while with Silane thermal CVD conditions are used. Transformation of Silicon into SiC and Si3N4 proceeds by subsequent heating of the Si-coated fibers in Nitrogen atmosphere, up to 1500°C at 1 atm N2. The relative thickness of SiC:Si3N4 layers depends upon the morphology and thickness of the original Silicon coating, which is itself strongly depending upon the choice of Si-precursor and CVD reaction conditions as well as fluidized bed process parameters.
In the paper the different Si-coating morphologies obtained from different variants of CVD-coating are discussed with respect to specific surface and reactivity for formation of SiC as well as Si3N4. The phase composition is analyzed by XRD and Raman spectroscopy while the compositional gradients are studied on cross sections of the coated fibers using SEM-EDX and Monte Carlo simulations. The achievable compositional gradients are compared with results of thermodynamic calculations.
 M. Knoll, M. Willert-Porada, Synthesis of porous silicon nitride ceramic with a compositional Gradient, Materials Science Forum Vols. 631-632 (2010) pp 477-482. doi:10.4028/www.scientific.net/MSF.631-632.477
 P. Pontiller, M. Willert-Porada, High Temperature Behavior of Si3N4 and Yb2SiO5 Coated Carbon Fibers for Silicon Nitride CMC, Advanced Engineering Materials 16(5), May 2014