Advanced design of EBC based on mass-transfer mechanisms in oxides under oxygen potential gradients at high temperatures
November 5-9, 2017
An environmental barrier coating (EBC) must exhibit excellent oxygen/water vapor shielding and thermomechanical durability in severe combustion environments. Thus, a multilayered structure, in which the individual layers have a particular function, is generally adopted when designing EBCs to enhance their overall performance. As an example, some EBCs incorporate a bonding layer on a SiC/SiC substrate, followed by an oxygen shielding layer and a water vapor shielding layer. The top coating of such systems is fabricated with a segmented structure to reduce thermal stresses during temperature cycling. Naturally, the oxide layers that provide the excellent gas shielding required for EBCs, which should be achieved by using fully dense coatings, are exposed to a large oxygen chemical potential gradient (dµO) at high temperatures. This results in an inward diffusion of oxygen ions and an outward diffusion of cations, as described by the Gibbs-Duhem equation. It should be noted that cation transport induces decomposition of the oxides and collapse of the layered EBC structure. Therefore, to develop robust EBCs with excellent gas shielding, it is very important to elucidate and control mass transfer within them. However, quantitative information on the oxygen shielding and mass transfer mechanisms of EBC candidate materials remains insufficient, and in particular, no diffusion data under a dµO has been reported.
In the present study, as a prelude to improving the environmental shielding and structural stability of EBCs, we evaluated the oxygen permeability of polycrystalline Yb2Si2O7 (Y2S2) and Al4+2xSi2-2xO10-x (mullite) wafers, which served as models for the EBC layers, using an oxygen tracer gas (18O2) [1,2]. Wafers with thicknesses of several hundred micrometers were exposed to a dµO at high temperatures, with each surface of the wafer deliberately subjected to a different oxygen partial pressure. Oxygen permeation occurred along grain boundaries (GBs) in both oxides. For Y2S2, the oxygen permeability constant normalized by the GB density, which was independent of grain size, was about ten times larger than that for mullite. However, the water vapor volatilization resistance of Y2S2 is significantly better than that of mullite. Hence, Y2S2 is suitable as the water vapor shielding layer, and mullite is suitable as the underlying oxygen shielding layer.
Oxygen permeation occurred by GB diffusion of oxygen ions from the high-oxygen-partial-pressure (high-PO2) surface to the low-PO2 surface, with simultaneous GB diffusion in the opposite direction of Yb ions for Y2S2 and Al ions for mullite. Oxygen permeation related to the GB diffusion of Si ions in both oxides was negligibly small compared to that due to the GB diffusion of other cations, resulting in structural instability of the oxides near both PO2 surfaces. The oxygen and cation fluxes at the outflow side in the oxides were significantly larger than those at the inflow side, in accordance with dominant cation transport at the high-PO2 side and dominant oxygen transport at the low-PO2 side. The structural stability of a mullite layer under a dµO can be improved by utilizing an underlying bonding layer that can function as an Al reservoir. Thus, even if outward diffusion of Al ions occurs in the mullite layer, the Al-containing bonding layer can supply Al ions to the Al-deficient zone in the mullite layer. Another approach is to utilize an upper Y2S2 water vapor shielding layer with limited oxygen shielding. In other words, if the PO2 at the interface between the mullite and Y2S2 layers, which is in equilibrium with the corresponding µO, falls below 10-5 Pa at 1673 K, the Al flux in the mullite layer will be significantly reduced. Structural stabilization of the Y2S2 layer can also be achieved by decreasing the driving force for outward diffusion of Yb ions in the Y2S2 layer to set an upper layer of Yb2SiO5.
 S. Kitaoka et al., J.Am.Ceram.Soc., DOI: 10.1111/jace.14834
 M. Wada et al, Acta Materialia, 135, 372 (2017)
Satoshi Kitaoka, Tsuneaki Matsudaira, Masashi Wada, Taishi Yokoi, and Masasuke Takata, "Advanced design of EBC based on mass-transfer mechanisms in oxides under oxygen potential gradients at high temperatures" in "Advanced Ceramic Matrix Composites: Science and Technology of Materials, Design, Applications, Performance and Integration", Yutaka Kagawa, Tokyo University of Technology, Japan Dongming Zhu, NASA Glenn Research Center, USA Ram Darolia, GE Aviation (retired), USA Rishi Raj, University of Colorado, Boulder, USA Eds, ECI Symposium Series, (2017). http://dc.engconfintl.org/acmc/19