Dip Coating of Boron Nitride Thin Films on Nicalon Fibers
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DIP COATING OF BORON NITRIDE THIN FILMS ON NICALON FIBERS 3 Shen1 , Beng Jit Tan1 .3 , and Steven L. Suibl,2, * Lu 1 Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060 2 Department of Chemical Engineering, Univ. of Connecticut, Storrs, CT 06269 3 lnstitute of Material Science, University of Connecticut, Storrs, CT 06269 Francis S. Galasso, United Technologies Research Center, East Hartford, CT 06108
* To whom correspondence should be addressed.
ABSTRACT
A process involving dip coating of ceramic fibers in H3 BO3 solution followed by reaction with NH3 has resulted in the formation of a BN coating on Nicalon and a carbon coated Nicalon fiber. BN coated C-Nicalon fiber maintained its strength during the coating process, while the BN coated Nicalon did not. INTRODUCTION Fiber-reinforced ceramic matrix composites (CMC) are new materials developed for use at high temperature in corrosive or oxidizing environments [1-5]. The high toughness of these composites has been characteristically associated with weak bonding between the fibers and the matrix in order to obtain substantial fiber pull-out as a major source of toughening. Strong fibermatrix bonding may result in brittle failure and flat fracture surfaces with little fiber pull-out. Thus, a proper thin film coating on the fiber can serve to limit the interfacial bonding and also act as a diffusion barrier for fiber-matrix combinations that otherwise may be incompatible due to excessive reactions leading to fiber degradation [6]. Some studies [7-12] have shown that BN can be a good interface coating for fiber-reinforced ceramic matrix composites (CMC). Typically chemical vapor deposition methods [3-17] have been used to prepare BN coatings. EXPERIMENTAL SECTION 1. Synthesis
Nicalon fibers, which are basically SiC with certain amounts of 0 and N, are now widely used for CMC reinforcements. C-Nicalon are carbon coated Nicalon fibers. They are both available commercially.* Nicalon fibers were dipped in fresh acetone twice, 3 mins each to remove the0 sizing (modified epoxy) while C-Nicalon fibers were dipped in hot water (-80 C) to remove its polyvinyl acetate (PVA) sizing. The Nicalon with sizing removed or C-Nicalon *Nippon Carbon, Tokyo, Japan
Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
228
Temperature Controller r Mixing chamber
Furnace
Trap
NH 3
2H 3 BO 3 B20
3
+ 2NH 3
-
*
0
B20 2BN
3
+ 3H 20
+ 3H 20
Fig. 1. Experimental set-up for the conversion of B20 3 to BN fibers were dipped in boric acid solution (0.6 M ). After being dried in air, the coated fibers were placed in a quartz tube reactor and heated in He (flow rate: -20 mL/min) to 1000 0C in about 40 mins. NH3 gas (flow rate: -25 mL/min) was then introduced into the reactor and reacted with the coating (which had been converted to B20 3 ) for 30 mins at 10000 C. After the reaction, the coated fibers were cooled in the furnace to room temperature in flowing He (flow rate: -20 mL/min). The experimental set-up and the chemical reactions occurring i
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