Microstructure characterization of oxide coatings deposited by pulsed excimer laser ablation
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H. Chung and J. Mazumderb) Center for Laser-aided Materials Processing, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 16 January 2003; accepted 11 April 2003)
This work addresses the issues of coating of oxide fibers or laminates with debondable oxide interphases. It fabricates a model system for an alumina matrix reinforced with alumina fibers, wherein an enstatite interphase is transformation weakened, resulting in interphase debonding. A suitable multilayer coating sequence was chosen to act as a chemical bridge between the alumina fiber and matrix. The pulsed excimer laser ablation method (KrF excimer laser of ⳱ 248 nm) was used to deposit several oxide materials individually onto silicon wafers. Titania (TiO2 or T), aluminum titanate (Al2O3 · TiO2, Al2TiO5 or AT), and enstatite (MgO · SiO2, MgSiO3 or EN) layers were deposited from sintered target materials. X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy investigations indicated that as-deposited coatings were amorphous or partially crystallized into nanosize grains, and their thicknesses were uniformly distributed over the Si-substrate, growing in columnar texture (although not as pronounced for enstatite). Transmission electron microscopy/energy dispersive spectroscopy analysis confirmed that the chemical composition of the coating materials was the same as that of the target materials and that the coatings were completely crystallized into nano- or submicrometer grain size after annealing at 1200 °C for 1 h. With these data, sapphire monofilaments were sequentially coated with five layers of Al2TiO5, TiO2, MgSiO3, TiO2, and Al2TiO5. This construction provided a chemical bridge between the alumina monofilament and the enstatite debondable interphase.
I. INTRODUCTION
The fabrication of stable, high-temperature, oxide ceramics in the form of continuously reinforced, ceramicmatrix composites remains a lofty and elusive goal.1–3 Toughening mechanisms are based on fiber pullout4 and crack bridging by fibers.5 These necessitate debonding at a weak fiber–matrix interface6,7 or within an interphase between a matrix and fiber.8–11 Thus, interfacial phenomena such as chemical composition, microstructure, and residual stress state play key roles in determining the properties of a fiber-reinforced ceramic matrix composite.
a)
Present address: Institute für Neue Materialien Gmbh, D-66123 Saarbrücken, Germany. b) Presently Robert H. Lurie Professor of Engineering, University of Michigan, Ann Arbor, Michigan 48109.
Debonding mechanisms include crack deflection along porous interphases12,13 or “transformation weakening” where the coating undergoes a displacive phase transformation accompanied by a large negative volume change or a unit cell shape change.13–19 This mechanism has been demonstrated in -␣ cristobalite transformationweakened interphases between mixed mullite/cordierite laminates.13–17 It is also seen in fibrous monolit
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