CVD COATING OF SAPPHIRE FIBERS WITH hBN TO IMPROVE MECHANICAL PROPERTIES OF REINFORCED NiAl COMPOSITES

摘要

Thermal atmospheric pressure CVD of hexagonal boron nitride from trimethylborazine (TMB) in H_2 + 3 % NH_3 at 925℃ has been used to deposit 1 to 3 μm single-phase hBN on sapphire fibers with the aim of forming a weak bonding in an Al_2O_3 fiber/NiAl matrix composite to overcome low-temperature brittleness of NiAl. The hexagonal structure of BN was clearly identified by means of reflexion electron energy loss spectrometry (REELS). During high-temperature applications, the fiber-matrix bond has to be strengthened to overcome poor NiAl creep resistance. We have shown that hBN reacts with NiAl at elevated temperatures to form A1N (and boron dissolved in NiAl), thus loosing its lubricating property. Furthermore, yttrium was added to the composite with the aim of increasing the fiber-matrix bond strength. At 1100 to 1300℃, Y gradually reacted with Al_2O_3 to form the Y-Al garnet Y_3A_5O_(12) (YAG), thus roughening the sapphire fibers which is expected to strengthen the interface bonding.