Environmental barrier coating for silicon based substrates
An article comprises a silicon based substrate, a bond layer and a protective top layer. The top layer is selected from the group consisting of rare earth disilicates, yttrium disilicates, rare earth monosilicates, yttrium monosilicates, silica and mixtures thereof. The protective layer described above is used in combination with a bond layer provided between the protective layer and the silicon based substrate which functions as oxygen getter and includes an oxygen gettering agent. By oxygen gettering agent is meant a refractory metal oxide former which forms an oxide at operational condition of (high temperature and aqueous environment) having a melting point of greater than 1500° C. wherein the negative free energy of formation of the refractory metal oxide from the refractory metal is more than 100 Kcal/mole. Suitable oxygen gettering agents include silicon and other refractory metals. An oxygen gettering agent may also be added to the protective layer.
1. An article comprising a silicon based substrate, an oxygen gettering bond layer on the substrate and a protective top layer, the protective top layer comprises an oxygen gettering agent and a constituent selected from the group consisting of rare earth disilicates, yttrium disilicates, rare earth monosilicates, and mixtures thereof.
2. An article according to claim 1 , wherein the protective layer is selected from the group consisting of yttrium disilicate, lutetium disilicate, erbium disilicate, lanthanum disilicate, and mixtures thereof.
3. An article according to claim 1 , wherein the protective layer is selected from the group consisting of the disilicates in combination with the monosilicates.
4. An article according to claim 1 , wherein the protective layer is selected from the group consisting of the disilicates in combination with silicon.
5. An article according to claim 1 , wherein the monosilicates are selected from the group consisting of silicates of lanthanum, ytterbium, lutetium, erbium and mixtures thereof.
6. An article according to claims 1 , 2 , 3 , 4 or 5 , wherein the bond layer is selected from the group consisting of:
(a) pure silicon;
(b) a bond layer comprising two phases wherein the first phase is selected from the group consisting of refractory metal oxides, refractory metal silicates and mixtures thereof and the second phase is selected from the group consisting of refractory metal oxide forming metals, silicon carbide, silicon nitride, and mixtures thereof;
(c) a bond layer which comprises an alloy comprising a refractory metal disilicide/silicon eutectic;
(d) a bond layer which comprises a silicon layer having a dispersion of particulate material therein, wherein the particulate material is selected from the group consisting of (1) alumina, (2) yttria, (3) aluminum silicate, (4) compounds of rare earth elements, alkaline earth elements, aluminum, oxygen, yttrium, nitrogen, carbon, and mixtures thereof;
(e) a bond layer which comprises a silicon metal containing layer having a dispersion of fibers wherein at least some of the fibers extend between the bond layer and the further layer, wherein the fibers are formed of a material selected from the group consisting of (1) alumina, (2) yttria, (3) aluminum silicate, (4) silicon carbide, (5) silicon nitride, (6) compounds of rare earth elements, alkaline earth elements, aluminum, silicon, niobium, tantalum, hafnium, oxygen, yttrium, nitrogen, carbon and mixtures of (1) thru (6); and
(f) a bond layer which comprises a metal refractory oxide having a thickness of between 0.1 to 15 microns, wherein the refractory metal oxide is selected from the group consisting of oxides of chromium, tantalum, niobium, silicon and mixtures thereof.
7. An article according to claim 1 , wherein at least one intermediate layer is provided between the bond layer and the protective layer.
8. An article according to claim 7 , wherein the intermediate layer is selected from the group consisting of silicon oxide, mullite, cordierite, sillimanite, and mixtures thereof.
9. An article according to claim 1 , wherein the silicon substrate is a silicon nitride substrate.
10. An article according to claim 1 , wherein the top layer further includes yttrium monosilicates.