Boron Nitride Interphase in Ceramic‐Matrix Composites

Abstract

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3-NH3, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with Sic deposited from CH3SiCI3-H2 at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF3-NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The Sic infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a Si02/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/Si02 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface. [

Advanced Ceramic Materials for High Temperature Applications

“…The interphase coats the fibre with a film of 0.1-1 lm thickness and it must be strongly bonded to the fibre surface. Pyrolytic carbon (PyC) or hexagonal boron nitride (hex-BN) have been commonly used as interphase, [14] but they are being replaced by more complex structures based on multilayered systems containing "n" nanometric layers of PyC-SiC or BN-SiC. These multilayered structures have been designed, firstly, to improve their mechanical response due to an increase on toughnening mechanisms, mainly crack deflection and debonding, and secondly, to enhance the oxidation resistance of the interphase incorporating SiC layers which are able to act as self-healing interphases if they are oxidized.…”

Platinum-enhanced graphitisation in sandwich structures of silicon carbide and borosilicate glass

“…Within the hexagonal basal planes, the planar, trigonal sp 2 -hybridisation of the C-atoms yields strong -bonds, whereas perpendicular to these basal planes the electrons of the 2p z -orbitals are forming only weak -bonds. The resulting anisotropy of properties of graphitic layers allows to control debonding and pull-out [4,5].…”

Silicon and Boron Containing Components by CVD and CVI for High Temperature Ceramic Composites

“…Particular attention has been heeded to the interfacial zones in terms of chemistry and microstructure [7,75]. These studies revealed that when Sic fibers, as NicalonTM or ZTyrannoTM, are coated with BN, many interfacial C and/or SiO2-rich sublayers can be present between the fiber and the boron nitride film [76].…”

“…This bonding must be weak enough to allow crack deflection along the interface and strong enough to retain load transfer from matrix to fiber [3][4][5]. An additional coating, the interphase, is often intentionally inserted between the fiber and the matrix to reach this goal [6,7], However, one important problem is to preserve the fibers and matrix integrity and to maintain the properties of cracks deviation and load transfer at the fiber/matrix interface, especially in oxidative environments, so as to retain the mechanical properties of the composite.…”