Scratch resistance and damage mechanism of laser remelted thermally sprayed ceramic coating

Scratch resistance of plasma sprayed and laser remelted alumina and chromia coatings were assessed under constant and variable load. Failure load and cohesive strength of the coatings increased up to 65% and 94%, respectively upon laser remelting. The scratch wear resistance increased up to 88%. This is attributed to an improvement in hardness, elastic modulus and indentation fracture toughness of the remelted coatings. Upon laser remelting, lamellar microstructure of thermally sprayed coating was transformed into a columnar microstructure. A phase change from y-alumina to a-alumina also occurred. Principal mechanisms of failure of the coatings were plastic deformation, tensile cracking, and spallation. In both types of coatings, a subsurface crack was found to grow in a direction parallel to the scratch. Propagation of this crack to the surface produced spallation of the coating. In the ball on disc test, the coatings were found to fail by splat delamination and radial cracking. The wear coefficients of the laser remelted chromia and alumina coating, respectively were 80% and 76% lower than those of the as-sprayed coatings.