CMAS-resistant abradable coatings

A system comprising:

• a ceramic or ceramic matrix composite (CMC) substrate;
• an abradable coating on the CMC substrate, the abradable coating comprising a plurality of first rare earth (RE) silicate layers in an alternating layer arrangement with a plurality of second RE silicate layers such that the abradable coating includes a first layer, a second layer, a third layer, a fourth layer, and a fifth layer, in that order, on the ceramic or the CMC substrate with the fifth layer being located between the fourth layer and the ceramic or the CMC substrate,
• wherein the plurality of first RE silicate layers includes the first layer, the third layer, and the fifth layer,
• wherein the plurality of second RE silicate layers includes the second layer and the fourth layer,
• wherein the first RE silicate layers include a rare earth monosilicate and the second RE silicate layers include a rare earth disilicate, and wherein the first RE silicate layers include a greater concentration of the rare earth monosilicate than the second RE silicate layers;
• and
• an opposing abrasive element configured to abrade the abradable coating to a maximum penetration depth such that at least the first layer and the third layer of the plurality of first RE silicate layers and at least the second layer and the fourth layer of the plurality of second RE silicate layers are abraded by the opposing abrasive element, and the fifth layer of the plurality of first RE silicate layers is between the substrate and the maximum penetration depth such that the fifth layer of the plurality of first RE silicate layers defines a continuous and undisturbed CMAS resistant barrier layer over the CMC substrate after the opposing abrasive element abrades to the maximum penetration depth,
• wherein the first layer, the third layer, and the fifth layer of the plurality of first RE silicate layers each include at least about 20 wt. % rare earth monosilicate, and wherein the second layer and the fourth layer of the plurality of second RE silicate layers each include at least about 50 wt. % of the rare earth disilicate,
• wherein a thickness of each of the first layer, the third layer, and the fifth layer is less than a thickness of each of the second layer and the fourth layer,
• wherein a porosity of each of the second layer and the fourth layer is greater than a porosity of each of the first layer, the third layer, and the fifth layer,
• wherein the plurality of first rare earth (RE) silicate layers in the alternating arrangement with the plurality of second RE silicate layers reduces thermal stress on the abradable coating compared to a similar abradable coating having a similar amount of material but having only a one or two layer structure, and
• wherein the plurality of first rare earth (RE) silicate layers in the alternating arrangement with the plurality of second RE silicate layers reduces crack propagation through the abradable coating compared to a similar abradable coating having a similar amount of material but having only a one layer structure.

A method comprising:

• forming an abradable coating on a ceramic or CMC substrate, the abradable coating comprising a plurality of first rare earth (RE) silicate layers in an alternating layer arrangement with a plurality of second RE silicate layers such that the abradable coating includes a first layer, a second layer, a third layer, a fourth layer, and a fifth layer, in that order, on the ceramic or the CMC substrate with the fifth layer being located between the fourth layer and the ceramic or the CMC substrate,
• wherein the plurality of first RE silicate layers includes the first layer, the third layer, and the fifth layer,
• wherein the plurality of second RE silicate layers includes the second layer and the fourth layer,
• wherein the first RE silicate layers include a rare earth monosilicate and the second RE silicate layers include a rare earth disilicate, and wherein the first RE silicate layers include a greater concentration of the rare earth monosilicate than the second RE silicate layers;
• and
• abrading, via an opposing abrasive element, the abradable coating to a maximum penetration depth such that least the first layer and the third layer of the plurality of first RE silicate layers and at least the second layer and the fourth layer of the plurality of second RE silicate layers are abraded by the opposing abrasive element, and the fifth layer of the plurality of first RE silicate layers is between the substrate and the maximum penetration depth such that the fifth layer of the plurality of first RE silicate layers defines a continuous and undisturbed CMAS resistant barrier layer over the CMC substrate after the opposing abrasive element abrades to the maximum penetration depth,
• wherein the first layer, the third layer, and the fifth layer of the plurality of first RE silicate layers each include at least about 20 wt. % rare earth monosilicate, and wherein the second layer and the fourth layer of the plurality of second RE silicate layers each include at least about 50 wt. % of the rare earth disilicate,
• wherein a thickness of each of the first layer, the third layer, and the fifth layer is less than a thickness of each of the second layer and the fourth layer, and
• wherein a porosity of each of the second layer and the fourth layer is greater than a porosity of each of the first layer, the third layer, and the fifth layer.