Effect of Nano-Fly Ash Additive on the Mechanical and Microstructural Properties of Plasma-Sprayed Mullite Coatings

The effect of MCrAlY and nano-fly ash additive powders on the mechanical and microstructural properties of mullite coatings was examined in this work. Three distinct mullite-based coatings, namely M (100% mullite), MM (95% mullite − 5% MCrAlY), and MMF (90% mullite − 5% MCrAlY − 5% nano-fly ash), were deposited onto a martensitic stainless steel (AISI 410) substrate through air-plasma spraying. MMF coatings achieved the best coating integrity during the experimental trials, with a porosity of 7.65%, and an optimum fracture toughness of 1.40 MPa m^0.5. The results revealed that incorporating MCrAlY particles into mullite coatings resulted in an optimal hardness of 638 HV₁. The addition of nano-fly ash significantly increased the adherence of MMF coatings to the AISI 410 substrate, which is critical to their durability and efficacy. Furthermore, the MMF coatings demonstrated a remarkable 60% reduction in crystallite size, yielding a finer size of 47 nm. Furthermore, dislocation density increased by 125%, reaching 44.8 × 10⁻⁵ nm⁻², compared to MM coatings. It was also revealed that the presence of MCrAlY and fly ash nanoparticles increased shear resistance by restricting the mobility of the shear plane, obtaining the highest adhesion strength of 76 MPa. These findings show that combining MCrAlY particles with nano-fly ash in mullite coatings provides various benefits, including enhanced hardness, crystal characteristics, adhesion, and shear resistance.