Oxide-forming protective coatings for niobium-based materials
Coatings suitable for use as protective oxide-forming coatings on Nb-based substrates exposed to high temperatures and oxidative environments. The coatings contain chromium and/or molybdenum, preferably contains silicon, and optionally contains niobium, titanium, hafnium, iron, rhenium, tantalum, and/or tungsten, which in combination form multiple intermetallic phases, which in combination form one or more intermetallic phases that promote the formation of a slow-growing oxide scale. Depending on the particular coating composition, the intermetallic phases may be: a silicon-modified Cr 2 Nb Laves phase and optionally a chromium solid solution phase, a CrNbSi intermetallic phase, and/or an M 3 Si intermetallic phase where M is niobium, titanium, and/or chromium; or M 5 Si 3 , MSi 2 and/or M 3 Si 2 where M is molybdenum, niobium, titanium, chromium, hafnium, iron, rhenium, tantalum, and/or tungsten.
1. A coating capable of promoting the oxidation resistance of a surface of a niobium-based substrate, wherein the coating is formed from: a composition that contains, in atomic percent, about 31 to about 99% chromium, about 1 to about 37% silicon, and about 0 to about 40% niobium+titanium+hafnium+iron where titanium, hafnium and iron are optional, and wherein the coating comprises at least about 72 volume percent of silicon-modified Cr 2 Nb Laves phase, up to about 28 volume percent of chromium solid solution, up to about 28 volume percent of CrNbSi intermetallic phase, and up to about 25 volume percent of M 3 Si intermetallic phase, wherein M is selected from at least one of niobium, titanium, and chromium.
2. The coating according to claim 1 , wherein the coating has an oxide scale on a surface thereof that is a complex oxide containing chromia, titania, niobia, and silica.
3. The coating according to claim 1 , wherein the coating contains hafnium.
4. The coating according to claim 1 , wherein the coating is on the surface of the niobium-based substrate, and the niobium-based substrate has a composition consisting of, by weight, up to 5% aluminum, up to 1% carbon, up to 5% cobalt, up to 20% chromium, up to 5% iron, up to 10% hafnium, up to 3% manganese, up to 15% molybdenum, up to 0.2% silicon, up to 30% tantalum, up to 20% titanium, up to 8% vanadium, up to 15% tungsten, and up to 10% zirconium, up to 5% of a rare earth element, up to 25% of a platinum group metal, the balance niobium and incidental impurities.
5. The coating according to claim 4 , wherein the substrate is a component of a gas turbine engine.
6. The coating according to claim 1 , wherein the coating is on the surface of the niobium-based substrate, and the niobium-based substrate is formed of a material containing a silicide reinforcement material dispersed in a niobium-based matrix.
7. The coating according to claim 6 , wherein the substrate is a component of a gas turbine engine.
8. A gas turbine engine component comprising: a substrate formed of a niobium-based alloy or a niobium-silicide composite;
a coating on the substrate and capable of promoting the oxidation resistance of a surface of the substrate; and
an oxide scale on a surface of the coating;
wherein the coating is formed from a composition that contains, in atomic percent, about 31 to about 99% chromium, about 1 to about 37% silicon, and about 0 to about 40% niobium+titanium+hafnium+iron where titanium, hafnium and iron are optional, and wherein the coating comprises at least about 72 volume percent of silicon modified Cr 2 Nb Laves phase, up to about 28 volume percent of chromium solid solution, up to about 28 volume percent of CrNbSi intermetallic phase, and up to about 25 volume percent of M 3 Si intermetallic phase, wherein M is selected from at least one of niobium, titanium, and chromium.
9. The gas turbine engine component according to claim 8 , wherein the coating contains hafnium.