Strong, damage tolerant oxide-fiber/oxide matrix composites.

摘要

Electrophoretic deposition (EPD) is an easy and cost effective method to fabricate fiber-reinforced green composites. Non-conductive Nextel(TM) 720 fibers were successfully coated with a transient, conductive polypyrrole submicron surface layer for use directly as an electrode in EPD processing. However, electric-field shielding limits particle infiltration into the conductive fiber bundles and they mostly deposit on the outer surface of the fiber bundle. When the bundle is large, central cavities exist after deposition.; The EPD cell was modified for electrophoretic infiltration deposition (EPID). Non conductive fibers were laid on an electrode and charged particles in an ethanol suspension are driven there through by an electric field, infiltrate and deposit on the electrode to then build up into the fiber preform and fill the voids therein. Dense, uniform, green fiber composites were successfully fabricated via constant current EPID. The EPID process is modeled as capillary electrophoretic infiltration. The process consists of two steps: particle electrophoresis outside the capillaries and electrophoretic infiltration inside the capillaries. Due to the zero net flow of the ethanol across the capillary cross-section, there is no electro-osmotic flow contribution to the deposition rate. Hamaker's law was extended to the EPID process, i.e., the deposition yield is proportional to the electric field inside the capillaries. The total deposition yield is controlled by the slow step of the process, i.e., the rate of electrophoresis in the open suspension outside the capillaries.; AlPO4 was proposed as a weak layer between oxide fibers and oxide matrix in fiber-reinforced ceramic matrix composites (CMC's). AlPO 4 nano particles were synthesized by chemical co-precipitation of Al 3+ and HPO42- with urea at 95°C. The solution pH basic region and amorphous AlPO4 precipitated of narrow size distribution with a mean particle size 50nm. Nextel 720 fibers were pretreated with cationic polyelectrolytes to have a positive surface charge and then dipped into diluted, negatively-charged AlPO4 colloidal suspension (0.05M) at pH 7.5. Amorphous AlPO4 (crystallizes to tridymite- and cristobalite-forms at 1080°C) nano particles were coated on fibers layer-by-layer using an electrostatic attraction protocol. A uniform and smooth coating was formed which allowed fiber pullout from the matrix of a Nextel 720/alumina mini-composite hot-pressed at 1250°C/20MPa.; Reaction-bonded mullite (RBM), with low formation temperature and sintering shrinkage was synthesized by incorporation of mixed-rare-earth-oxide (MREO) and mullite seeds. Pure mullite formed with 7.5wt% MREO at 1300°C. Introduction of 5wt% mullite seeds gave RBM with less than 3% shrinkage and 20% porosity. AlPO4-coated Nextel 720/RBM composites were successful fabricated by EPID and pressureless sintering at 1300°C. Significant fiber pullout occurred and the 4-point bend strength was around 170MPa (with 25-30vol% fibers) at room temperature and 1100°C and a Work-of-Fracture 7KJ/m2. At 1200°C, the composite failed in shear due to the MREO-based glassy phase in the matrix. AlPO4-coated Nextel 720 fiber/aluminosilicate (no MREO) showed damage tolerance at 1200°C with a bend strength 170MPa.