Influence of glass bead blasting of 9%Cr steel substrate on development of aluminide diffusion coatings

Abstract

The influence of two surface pretreatments, glass-bead blasting and grinding, on the development of aluminide diffusion coatings on 9%Cr steel substrates has been studied by optical, scanning electron and transmission electron microscopies. The results showed that pretreatment of the surface by glass-bead blasting yields an aluminised layer with a more uniform structure and less thickness variation than pretreatment by grinding. These beneficial consequences of glass-bead blasting are introduced by an increase in surface roughness which, by increasing the specific surface area that is accessible to gaseous metal halides to react with the substrate, to release aluminium, facilitates the surface reactions step in the coating development process, and by an increase in dislocation density. Furthermore, changes in dislocation arrangement take place as a result of glass-bead blasting, as the dislocations form into an arrangement of cells and tangles. As such defects contribute to coating nucleation and solid-state diffusion, it is apparent that glass-bead blasting of the substrate surface affects the surface reaction and the solid-state diffusion stages of the aluminising process. However, the dislocation arrangement developed during the glass-bead blasting pretreatment changes and the density of dislocation decreases during annealing at 650°C, the pack aluminising temperature. Coating development is, therefore, influenced by the treatment only in the early stages of the aluminising process.