Aqueous slurry for the production of thermal and environmental barrier coatings and processes for making and applying the same
An improved slurry formulation for the production of a thermal and environmental barrier coatings are provided which can withstand high temperature applications. The slurry includes a combination of a coarse ceramic powder fraction having close porosity particles and a fine ceramic powder fraction. The combination of the two powders produces a bimodal particle size distribution having a controlled amount of closed porosity that imparts desirable properties to the coating produced. The finer solid particles are interdispersed within an aqueous binder to produce a ceramic matrix with sufficient mechanical strength. The closed porosity containing coarse particles are embedded within the resultant ceramic matrix and do not disintegrate under high temperature conditions to impart a temperature resistant, non-collapsing closed porosity to the coating which can also act as an environmental barrier.
1. An aqueous slurry composition for production of a porous thermal or environmental barrier coating on a metallic or a ceramic substrate, comprising:
a first powder comprising an oxide material with a thermal conductivity lower than about 5 W/m K, coarse particles having a first median size ranging from about 5 microns to about 60 microns, with at least a portion of the coarse particles having a closed porosity that is temperature resistant and substantially impermeable to gas and liquid;
a second powder comprising an oxide material with a thermal conductivity lower than about 5 W/m K, the second powder characterized as fine particles having a second median size ranging from about 0.1 to about 5 microns, wherein the second median size is at least about 5 times smaller than the first median size of the first powder, whereby the coarse particles of the first powder and the fine particles of the second powder form a bimodal particle size distribution in the slurry;
a plurality of elemental Boron particles provided in an effective amount, wherein said elemental Boron is contained in an amount from 0.2 to 2.0 weight %; and
an inorganic binder suspending at least a portion of the plurality of elemental Boron particles, the coarse particles and the fine particles in an aqueous solution;
wherein the closed porosity of the coarse particles is temperature resistant and imparts a non-collapsing closed porous structure to the coating.
2. The aqueous slurry composition of claim 1 , the first median particle size ranging from about 20 microns to about 50 microns, and the second median particle size ranging from about 1.0 micron to about 4.0 microns, wherein the closed porosity of the coarse particles is no less than 14 percent.
3. The aqueous slurry composition of claim 1 , the first median particle size ranging from about 20 microns to about 50 microns, and the second median particle size ranging from about 1.0 micron to about 4.0 microns, wherein the coarse and the fine particles are composed of yttria-stabilized zirconia.
4. The aqueous slurry composition of claim 1 , wherein the coarse particles and the fine particles are composed of yttria stabilized zirconia, calcia stabilized zirconia, magnesia stabilized zirconia or mixtures thereof.
5. The aqueous slurry composition of claim 1 , wherein the first powder comprises from about 30 wt. % to about 60 wt. % of the aqueous slurry composition, and the first and the second powders in combination comprise a total of at least 55 wt. % up to about 85 wt. % of the aqueous slurry composition.
6. The aqueous slurry composition of claim 1 , wherein the binder is selected from the group consisting of a metal silicate binder or a metal phosphate binder.
7. The aqueous slurry composition of claim 6 , wherein the binder is a metal silicate binder with a SiO2/M2O ratio higher than about 2.5, wherein M is a metal selected from Na, K and Li or a combination thereof.
8. The aqueous slurry composition of claim 6 , wherein the binder is a metal phosphate binder with a P2O5/M ratio of not less than 0.1, wherein M is a metal selected from Group I, II, III or IV of Periodic Chart of Elements or a combination thereof.
9. The aqueous slurry composition of claim 1 , further comprising an anticorrosive pigment, a viscosity modifier or a combination thereof.
10. The aqueous slurry composition of claim 1 , wherein the fine oxide particles comprise a mixture of oxide compound with a median particle size distribution ranging from about 0.1 microns to about 5 microns, the mixture of the oxide compounds having a pyrochlore-type crystal structure represented by the formula Ln 2 M 2 O 7 , wherein
M is Zr, Ce, and/or Hf; and
Ln is La, Gd, Sm, Nd, Eu, Yb or any combination thereof.
11. The aqueous slurry composition of claim 1 , wherein the fine oxide particles comprise a mixture of oxide compounds having a perovskite-type crystal structure represented by the formula AMO3, and wherein
M is Zr and/or Ti;
A is an alkali earth element, rare earth element or any combination thereof.
12. The aqueous slurry composition of claim 1 , wherein the fine oxide particles are a mixture of oxide compounds comprising aluminates of rare earth metals.
13. The aqueous slurry composition of claim 3 , the first powder and the second powder comprise yttria stabilized zirconia, the binder selected from the group consisting of an alkali metal silicate binder and metal phosphate binder, wherein said first powder comprises from 35% to 55% by weight, ratio of the second powder to the first powder is in the range from about 1:1 up to about 1:2.5 by weight, and the first and the second powders combined comprise between about 60% to about 80% by weight of the aqueous slurry composition.