Machinable coating with environmental barrier properties

1 . An article, comprising:

a ceramic based substrate, a bond coat on the substrate;

a barrier layer disposed on the bond coat, the barrier layer including diffusive particles and gettering particles disposed in a matrix, and there being a dispersion of the diffusive particles and the gettering particles within the same layer of matrix;

a machinable coating disposed on the barrier layer;

wherein a surface roughness of the machinable coating is between about 20 and about 100 microinches Ra; and

wherein the article includes a slot, and wherein the barrier layer and machinable coating are disposed on at least one surface of the slot; and

wherein the machinable coating is mullite.

2 . The article of claim 1 , wherein the matrix is silica.

3 . The article of claim 1 , further comprising a topcoat disposed between the barrier layer and the machinable coating.

4 . The article of claim 1 , wherein the surface roughness of the machinable coating is between about 25 and about 65 microinches Ra.

5 . The article of claim 1 , wherein the diffusive particles are barium-magnesium alumino-silicate.

6 . The article of claim 1 , wherein the gettering particles are SiOC or SiC.

7 . A method of applying a coating to an article, comprising:

applying a barrier layer to a ceramic based article, the barrier layer including diffusive particles and gettering particles disposed in a matrix, and there being a dispersion of the diffusive particles and the gettering particles within the same layer of matrix;

applying a machinable coating of mullite on the barrier layer;

machining the machinable coating;

wherein a surface roughness of the machinable coating is between about 20 and about 100 microinches Ra after the machining; and

wherein the article includes a slot, and wherein the barrier layer and machinable coating are applied to at least one surface of the slot.

8 . The method of claim 7 , wherein the machining is by at least one of grinding, ultrasonic machining, water guided laser, milling and reaming.

9 . The method of claim 7 , wherein the matrix is silica.

10 . The method of claim 7 , wherein the diffusive particles are barium-magnesium alumino-silicate and wherein the gettering particles are SiOC or SiC.

11 . The method of claim 7 , wherein a surface roughness of the machinable coating is about 350 microinches Ra before the machining.

12 . The method of claim 7 , wherein a surface roughness of the machinable coating is between about 25 and about 65 microinches Ra after the machining.

13 . The method of claim 7 , further comprising applying a topcoat to the barrier layer before applying the machinable coating.