혼성분사 공정을 이용한 금속기지 복합소재의 제조 및 코팅 형성 기구에 대한 연구

Abstract

Recently, spraying techniques have been considered as one of the novel processes for manufacturing the diamond tool wheel based on its superior economic feasibility and relatively less thermal damage. However, during atmospheric plasma spraying (APS), it is obvious that the diamond particles may undergo severe oxidation and amorphization in plasma flame as its temperature is much higher than those for oxidation and amorphization of diamond. In contrast to APS, kinetic spraying technique is based on high-strain-rate impact with extremely low heat input during coating. Therefore, it is a desirable method for depositing oxygen-sensitive materials such as diamond. However, one of the most notable disadvantages of kinetic spraying is the difficulty in retaining the original diamond particle size distribution in the coating, as high velocity impacts during the process can cause fracturing of the diamonds. Hence, application of a protective nickel film onto the diamond particles for uniform flight behavior and reduced impact stress was attempted. However, the broken ratio of nickel-coated diamond particles increased sharply with increasing particle size due to high kinetic energy.

To this end, an innovative hybrid spraying technique was used in this study in order to obtain novel bronze/diamond composite coatings with relatively large diamond particles (mean size of 80 μm excluding that of nickel film). During hybrid spray process, bronze powders were deposited using atmospheric plasma spraying (APS) (3MB-II, Sultzer Metco, Switzerland) in order to obtain a thermally softened matrix. Simultaneously, diamond powders were deposited using kinetic spraying (KINETIC 3000, CGT, Germany) in order to embed thermally less-exposed diamond particles into the thermally softened bronze matrix. Hence, size protection of the relatively large diamond particles in bronze/diamond composite coatings was successfully achieved by hybrid spraying by the reduced impact stress of diamond particles due to the effect of a thermally softened bronze matrix. (broken ratio was estimated to be less than 10%).

In addition, during hybrid spraying, the nickel layer was closely attached to the surface of diamond particle excluding the bronze due to the relatively low temperature process and appropriate CTE value of nickel. Hybrid sprayed diamond particles likewise have short exposure time to plasma flame compared with APS. For these reasons, the diamond surface was not severely oxidized, and the crystal-structure of diamond could also be protected during the hybrid spraying process. Only 10 nm thickness of amorphous layer was observed on the diamond surface.