Rapidly solidified thick cobalt-based alloy deposit with carbide particles produced by plasma spraying

Abstract

Thermal spraying can produce rapidly solidified thick layers, because the droplets that accumulate on the substrate experience cooling rates of 10⁵–10⁸ K s^–1. It is thus an attractive method to produce composite deposits containing fine particles formed in situ. In the present work, Co–1·5C, Co–1·5C–14Cr and Co–1·5C–29Cr (wt-%) alloy powders produced by argon atomisation were low-pressure plasma sprayed onto water cooled or uncooled substrates. The as sprayed Co–1·5C deposit formed on a water-cooled substrate consists of a metastable β-Co phase, supersaturated with carbon due to the high cooling rate. Heat treatment at >673 K led to the formation of α-Co and graphite. The deposit heat treated at 1073 K contained graphite particles ∼0·8 μm in size. At higher treatment temperatures, coarsening of graphite and the elimination of carbon supersaturation result in lower hardness. As-sprayed Co–1·5C deposit on uncooled substrates consisted of β-Co, α-Co and graphite. As-sprayed deposit of Co–1·5C–14Cr and Co–1·5C–29Cr powders on uncooled substrates consisted of β-Co, α-Co, Cr₃C₂, Cr₇C₃ and graphite. The fine (0·5 μm) precipitates in the as-sprayed Co–1·5C–14Cr deposit on uncooled substrates were identified as chromium carbide. As-sprayed Co–1·5C–29Cr deposits on uncooled substrates contained many precipitates ∼0·6 μm in size and showed the highest microhardness of ∼600 HV (2·94 N).

Keywords:

• CARBIDES
• PLASMA SPRAYING
• RAPID SOLIDIFICATION
• COBALT-BASED ALLOY DEPOSIT
• PRECIPITATION