Alumina grit blasting parameters for surface preparation in the plasma spraying operation

The paper examines how the grit blasting process influences the surface roughness of different substrates, the grit residue, and the grit erosion. The influence of grit blasting conditions on induced substrate residual stresses is also discussed. Aluminum alloy, cast iron, and hard steel were blasted with white alumina grits of 0.5, 1, and 1.4 mm mean diameters. Grit blasting was performed using either a suction-type or a pressure-type machine equipped with straight nozzles made of B₄C. The influence of the following parameters was studied: grit blasting distance (56 to 200 mm), blasting time (3 to 30 s), angle between nozzle and blasted surface (30°, 60°, 90°), and blasting pressure (0.2 to 0.7 MPa). The roughness of the substrate was characterized either by using a perthometer or by image analysis. The grit residue remaining at the blasted surface was evaluated after cleaning by image analysis. The residual stresses induced by grit blasting were determined by using the incremental hole drilling method and by measuring the deflection of grit-blasted beams. Grit size was determined to be the most important influence on roughness. The average values of R_a and R_t and the percentage of grit residue increased with grit size as well as the depth of the plastic zone under the substrate. An increase of the pressure slightly increased the values of R_a and R_t but also promoted grit breakdown and grit residue. A blasting time of 3 to 6 s was sufficient to obtain the highest roughness and limit the grit breakdown. The residual stresses generated under the blasted surface were compressive, and the depth of the affected zone depended on the grit diameter, the blasting pressure, and the Young's modulus of the substrate. Moreover, the maximum residual stress was reached at the limit of the plastic zone (i.e., several tenths of a millimeter below the substrate surface).