The effect of silicon and zirconium additions on the microstructure of vacuum-plasma-sprayed CoNiCrAlY coatings

Abstract

Abstract To improve the oxide scale adherence to high temperature alloys, active elements such as yttrium are added in amounts of 0.5–1 wt.%. Recently, silicon and zirconium have proved to be successful as alternative active elements in CoNiCrAlY and NiCrAlY alloys. To understand the mechanisms behind this improvement and to compare various alloy systems, it is useful to investigate the effect of active elements on the microstructure. In this study, the effect of silicon and zirconium additions on the microstructure of the CoNiCrAlY system was investigated. The CoNiCrAlSiZrY coatings were prepared by vacuum plasma spraying in a full-scale production unit and subsequent vacuum heat treatment (at 1120 °C for 2 h and at 845 °C for 24 h). The microstructural features were revealed by scanning and transmission electron microscopy. The chemical composition of the secondary phases was determined by energy-dispersive X-ray analysis. In the as-deposited condition, the matrix structure of the CoNiCrAlSiZrY coatings is single-phase γ (f.c.c.). As a consequence of the silicon and zirconium additions the ternary silicide G phase is frequently present as a dispersion in the coating microstructure. It is proposed that the presence of silicides explains the observed improvements of oxide scale adherence. Other secondary phases are Cr23C6, Cr7C3 and Y2O3. Furthermore, both the secondary and the matrix phase increase in grain size with increasing substrate temperature and with heat treatment. During heat treatment the matrix structure transforms into a three-phase γ (f.c.c.) + γ′-Ni3Al (ordered f.c.c.) + α-Cr (b.c.c.) structure.