Comparative Study of Microstructure, Phase Composition, and Oxidation Resistance of CoNiCrAlY Coating Deposited by HVOF and LPPS processes
Subject Areas :
pejman zamani moghaddam
1
(surface and coating engineering, faculty of materials, Malek Ashtar university of technology, Tehran, Iran)
Zia Valefi
2
(surface and coating engineering, faculty of materials engineering, Malek Ashtar University of Technology, Tehran, Iran)
Keywords: CoNiCrAlY Coating, HVOF, LPPS, Thermal spray, High-temperature oxidation.,
Abstract :
In this research, CoNiCrAlY powder was deposited by high-velocity oxy-fuel (HVOF) and low-pressure plasma spraying (LPPS) processes on IN738 nickel-based superalloy substrates. The high-temperature oxidation test was performed on the coatings at a temperature of 1050 ̊C and a time of 200 hours in a muffle furnace. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The porosity (volume percentage) and surface roughness (micrometer) were measured for HVOF coating as 0.6 and 4.4, and for LPPS coating as 2 and 6.62, respectively. The HVOF coating consisted of γ-CoNiCr and β-(Co,Ni)Al, while the LPPS coating included a single phase γ-CoNiCr. The disappearance of the β phase in the LPPS coating after spraying was due to dissolution in the plasma jet and its non-recovery in the conditions of rapid quenching and non-equilibrium solidification. This phase was recovered after heat treatment. The microstructure of the LPPS coating had much less oxide than the HVOF coating due to depositing at low oxygen pressure in the vacuum chamber. After 200 hours of oxidation test, the amount of β phase (as an oxidation resistance criterion) was completely consumed in the LPPS coating, while the HVOF coating contained the retained β deposits. The average thickness of TGO layer for HVOF and LPPS coatings was 5.2 and 7.1 μm, respectively. The dispersed oxides in the microstructure, lower roughness and denser structure of HVOF coating were reasons for the higher oxidation resistance of HVOF coating than LPPS.