IN-SITU FAILURE ANALYSIS OF A NI COATING DEPOSITED BY ELECTROLYTE BRUSHING

The worn shaft of a turbine was restored by electrolytic brushing with Ni. Grinding performed after deposition produced partial flaking of t he coating. A field analysis was required to identify the process stag e where the failure was induced, i.e. substrate cleaning, coating depo sition or grinding. Since a traditional failure analysis by destructiv e sampling could not be performed, metallography by replica was adapte d to observe the morphology inside the pits produced by flaking. In ad dition. fragments of the flaked coating were analyzed. Comparison of t he morphological features of these samples led to the conclusion that the cause of failure was the lack of adhesion between contiguous layer s of the coating, probably produced by a defective deposition. When lo cated below the surface, these defects can be detected by field hardne ss measurements.