Abstract
An extraordinary property by refining the grain size to nanocrystalline level has become of great concern in the field of materials science. An explosion of the activity in this subject results from the success in both the fabrication of nanostructured powder and the process development for the powder. In this study, microcrystalline Ni coating was fabricated by HVOF spraying of mechanically alloyed Ni powder. Both the mechanical property and the thermal stability of the coating obtained were evaluated. The powder characteristic with milling period and mass of methanol addition in the mechanical alloying was fundamentally investigated. To gain the extremely finer grain size in the powder, it was revealed that the attritor milling was more effective than the ordinary rotational milling. Moreover, finer grain size in the powder was given under the conditions of smaller amount of methanol addition and longer milling period. The micro hardness of the coating made from the MA powder was approximately 32% higher than that of the coating made from the conventional powder. As the trace of the microcrystalline property of the MA powder was recognized in the coating, it was revealed that the grain growth in the powder was effectively suppressed during spraying in the HVOF process. From the measurement results of the micro hardness for the heated coating under the various heating conditions, it was confirmed that the coating microstructure has fairly high thermal stability.
