Outer airseal abradable rub strip

1. A blade outer airseal having:

a body comprising:

an inner diameter (ID) surface;

an outer diameter (OD) surface;

a leading end;

a trailing end;

a metallic substrate; and

a coating system atop the substrate along at least a portion of the inner diameter surface,

wherein:

at least over a first area of the inner diameter surface, the coating system comprises an abradable layer system comprising a plurality of layers including a first layer atop a second layer;

the first layer is relatively erosion-resistant relative to the second layer;

the second layer is relatively abradable relative to the first layer;

the first layer has at most 10% porosity;

the second layer has at least 40% porosity;

the second layer comprises a boron nitride; and

the first layer comprises a lower, if any, weight content of boron nitride than does the second layer.

2. The blade outer airseal of claim 1 wherein the plurality of layers have a metallic matrix.

3. The blade outer airseal of claim 2 wherein the metallic matrix comprises an MCrAlY in the second layer and an MCrAlY or a Ni-based alloy in the first layer.

4. The blade outer airseal of claim 2 wherein the metallic matrix comprises, by weight:

≥50% combined cobalt and nickel.

5. The blade outer airseal of claim 1 wherein the plurality of layers further comprises:

a third layer below and more erosion-resistant than the second layer; and

a fourth layer below and less erosion resistant than the third layer.

6. The blade outer airseal of claim 5 wherein:

the first and third layers are essentially the same; and

the second and fourth layers are essentially the same.

7. The blade outer airseal of claim 5 wherein:

the first and third layers are each between 0.020 mm and 0.15 mm thick;

the second layer is between 0.040 mm and 2.0 mm thick; and

the fourth layer is at least 2.0 mm thick.

8. The blade outer airseal of claim 7 wherein:

the first and third layers are thinner than the second layer; and

the second layer is thinner than the fourth layer.

9. The blade outer airseal of claim 1 wherein:

the first layer has a bentonite filler.

10. The blade outer airseal of claim 1 wherein the first layer and the second layer comprise metallic matrix compositions differing by no more than 1.0 weight percent of any component.

11. The blade outer airseal of claim 1 wherein one or more of:

the coating system has a bondcoat between the abradable layer and the substrate; and

the substrate is a nickel-based superalloy.

12. A method for manufacturing the blade outer airseal of claim 1 , the method comprising:

thermal spray of the first layer and the second layer.

13. The method of claim 12 wherein:

the thermal spray comprises spraying matrix for the first layer and the second layer from the same source while varying one or more non-matrix components.

14. The method of claim 13 wherein:

the varying the one or more non-matrix components comprises using less of the one or more non-matrix components when spraying the first layer than when spraying the second layer.

15. A method for using the blade outer airseal of claim 1 , the method comprising:

installing the blade outer airseal on a turbine engine; and

running the turbine engine so that blade tips rub the abradable coating.

16. The method of claim 15 wherein the rub causes the blade tips to locally fully penetrate the first layer.

17. The method of claim 16 wherein:

the plurality of layers further comprises:

a third layer below and more erosion-resistant than the second layer; and

a fourth layer below and less erosion resistant than the third layer;

the rub does not cause the blade tips to penetrate the third layer.

18. The method of claim 17 further comprising:

a damage event causing imbalance so as to produce further rub which causes the blade tips to penetrate the third layer but not reach the substrate.

19. A blade outer airseal having:

a body comprising:

an inner diameter (ID) surface;

an outer diameter (OD) surface;

a leading end;

a trailing end;

a metallic substrate; and

a coating system atop the substrate along at least a portion of the inner diameter surface,

wherein:

at least over a first area of the inner diameter surface, the coating system comprises an abradable layer system comprising a plurality of layers including a first layer atop a second layer;

the first layer is relatively erosion-resistant relative to the second layer;

the second layer is relatively abradable relative to the first layer;

the first layer has at most 10% porosity;

the second layer has at least 40% porosity;

the first layer has a bentonite filler; and

the second layer has a boron nitride filler.

20. The blade outer airseal of claim 19 wherein the plurality of layers have a metallic matrix.