A method for evaluating the adhesion of metallic thermally sprayed coatings by measuring the electrical resistance of the coating sprayed on a metal substrate was carried out. The thermal sprayed coatings were made of zinc alloy on carbon steel substrates. The electrical resistance levels between the substrates and coatings were evaluated. The electrical resistance increases with increasing measurement time. The larger the rate of increase of the electrical resistance, the lower the adhesive strength. There is a possibility to evaluate qualitatively the adhesion properties.

Adhesion properties on the substrate blasted by multi-angle blasting with white alumina grit were examined. The low carbon steel substrates were blasted at two blasting angles (high and low angles) in a multi-angle blasting process. The sprayed coating of zinc alloy was evaluated by an adhesion test. The surface roughness levels after the multi-angle blasting were lower than those in the case of normal blasting. The multi-angle blasting was an effective blasting process to obtain improved adhesive properties. The low blasting angle may have contributed for removing contamination from the substrate surface. The high blasting angle may have contributed for an effective surface roughness to enhance the adhesive properties.

The effect of the hardness in the substrate surface blasted by a grit blasting process on the adhesive strength of Zn-Al sprayed coatings is investigated to find the adhesive strength is improved by work hardening of the substrate surface. The adhesive strength between a substrate of a carbon steel and sprayed coatings of Zn-Al alloy sprayed by a wire flame spraying process is measured. The substrate is roughened by the grit blasting process with white alumina girt in various blasting angles and blasting time. The hardness is measured in around 20 micro-meter depth from the substrate surface. The adhesive strength increases with increasing the hardness even if the surface roughness is almost same. There is the definite correlation between the adhesive strength and the hardness rather than the surface roughness.

The substrate surface roughening and cleaning are very important process for pre-treatment of a substrate for thermal spraying. There are some methods for the pre-treatment. Especially, the grit blasting process is the most general process. However, some grits embed into the blasted substrate, and the embedded grits remain on it. The girt embedment is affected by various conditions of the blasting process. It is important to make known the effect of the blasting time on the residual grit amount. A carbon steel substrate was blasted by white alumina grit. The blasting time was changed. The surface roughness of the blasted substrate increased monotonically with increasing blasting time. During very short time at the starting of blasting, the residual grit amount increased with increasing blasting time. Then the amount of the residual grit decreased with increasing blasting time. The penetration depth of the residual grit on the blasted substrate denoted the same tendency of the residual grit amount. There may be a critical time that the residual girt amount becomes the local minimum value.

The residual grit weight and the penetration depth of the embedded grit were investigated by dissolution of the blasted substrate with a mixed acid solution. A carbon steel substrate was blasted by white alumina grit (WA #30). The blasting angle varied from 30° to 90°. The residual grit amount and the penetration depth increased with increasing blasting angle. The residual grit amount was 2-12 g/m 2 , and the penetration depth was 26-34 µm. When the blasting angle increased from 30° to 90°, the increase ratio of the residual grit amount was 6.0, although the increase ratio of perpendicular component of impact energy by blasting was 2.0. To reduce the residual grit amount, a decrease of blasting angle is more effective than the decrease of grit mass. For the reduction of the penetration depth of the embedded grit, the blasting angle effect is small.

A carbon steel was blasted by white alumina, and removability of residual grit was investigated. Blasting parameters such as grit particle size were varied. Residual grit was removed from substrate surface by dissolving the substrate in mixed acid solution, and the amount of residual grit was measured. Removal of residual grit was carried out by ultrasonic cleaning. The amount of residual grit was 7-17 g/m 2 , and penetration depth of embedded grit of most residual grit was 25-40 µm. The amount of residual grit increased with decreasing the grit size. However, the minimum amount of the substrate dissolution required for the grit removal increased with increasing the grit size.

Generally, the surface of a substrate is roughened by blasting in pretreatment thermal spraying. Since some of the grit remains on the substrate surface, sprayed coating and the adhesion property characteristics are degraded. However, it seems that there is almost no research on this problem. In this study, residual grit is quantitatively evaluated, and a technique for reducing grit is proposed. For residual grit reduction, a solid lubricant was used, and applied to the surface of a substrate or grit; reduction of residual grit was confirmed. After blasting with solid lubricant, it remained on the substrate surface, which could be removed by heating the substrate.