Some aspects of the structure and properties of spray-fused coatings

The investigation has been concerned with a number of aspects of the structure and properties of Co-based Stellite SF6 coatings on Type 316 stainless steel and Nimonic Alloy PE16 substrates.

A wide range of literature relevant to the work has been reviewed. This includes the metallurgy and mechanical, wear and corrosion properties of Co-based alloys, wear processes, wear testing, surface treatments for wear resistance, spray-fused coatings and the available methods for the evaluation of grit blasted surfaces.

Grit blasted surfaces have been characterised using scanning electron microscopy and surface profilometry techniques. It has been found that the total average roughness (R_tm) and the centreline average (R_a) measurements are sensitive to variations in surface hardness in the range 200 - 240 VPN. The greatest surface roughnesses were obtained using G.30 cast iron grit and angles of 45 and 90° were preferred with blasting times not less than 30s.

Tensile adhesion test results have indicated that as-sprayed coatings have cohesive strength values of 6 - 15 MN m^-2. For spray-fused coatings the bond strength was in excess of 80 MN m^-2.

It has been shown that variations in the temperature and time cycle in refusion have a considerable effect upon coating chemical composition, microstructure and general surface hardness. The results of morphological studies using stain etching, X-ray diffraction and electron probe microanalysis techniques have indicated that M₇C₃, M₇BC₄, M₂₃C₆ and M₆C type carbides are the most prevalent in the coatings.

The mechanical properties of the coatings have been determined using two test configurations. Coatings with fine, coarse and dendritic structures were examined. The modulus of elasticity (E) of the coatings is in the range 258 - 286 GN m^-2. The fracture strength, σ_F, is in the range 346 -529 MN m^-2 (ring test configuration) and 855 - 991 MN m^-2 (beam test configuration). Both E and σ_F showed little dependence upon coating structure or heat treatment history. The strain-to-fracture of the coatings is in the range 0.31 - 0.62 % and was found to be sensitive to coating structure and thermal history.

The adhesive wear resistances of the coatings have been estimated using a cross-cylinder arrangement. The wear resistance was largely determined by coating microstructure and is in the range 0.63 – 13.43 x 10¹⁰g cm^-2. Coatings with a fine structure had the greatest wear resistance and those with dendritic structure least. No obvious relationship was found between wear resistance and either surface hardness or the area of the coating outer surface occupied by carbides.

The corrosion of the coatings in N₂-saturated dilute HC1 solutions has been found to proceed by pitting and intergranular attack and penetration of the carbide - solid solution matrix areas. The rate of attack is affected by the coating structure. The greatest intergranular penetration occurred in coatings with coarse structures whilst dendritic structures showed the least. The solid solution is 'leached out' whilst the carbides remain apparently uncorroded.