Abstract
The effect of preheating on rupture properties of substrate coated by low pressure plasma spraying was investigated in this study. Creep-rupture specimens of Ni-base superalloy, Mar-M247 were coated after preheating at several temperature ranges. Especially, a relationship between preheating temperature and microstructures of sprayed substrates was discussed. The results are summarized as follows.
(1) Creep properties of Mar-M247, as a substrate, coated with CoCrA1Y were kept at the level of as cast material when preheated at the range of 1073-1173 K in low pressure plasma spraying.
(2) The substrates of specimen coated after preheated at the range of 1473-1573 K were severely damaged in rupture life, strain and reduction of area. It is considered that liquation induced by high preheating in low pressure plasma spraying should occur and grow on the solidification grain boundaries of substrate. Consequently, creep properties were deteriorated, because of opening the grain boundaries during cooling period.
(3) The mechanism of liquation and opening on the grain boundaries was deduced as the following. At first, selective decomposition of tungsten carbide in complex carbides is brought by heat. Simultaneously, carbon is dissolved around the grain boundaries which is one of elements decreasing melting point. After spraying, cooling shrinkage makes opening of solidification grain boundaries.
