A method of forming an impurity barrier layer on a CMC substrate may include introducing, to a heated plume of a thermal spray gun, a composite feedstock that includes a first coating material including a plurality of first particles; and a second coating material that may be different from the first coating material, where the second coating material at least partially encapsulates at least a portion of respective surfaces of the plurality of first particles; and directing, using the heated plume, at least the first coating material to a surface of a CMC substrate to deposit an impurity barrier layer including at least the first coating material.