Microstructure, oxidation & mechanical properties of as-sprayed and annealed HVOF & VPS CoNiCrAIY coatings

• Main Author: Saeidi, Saman
• Published: University of Nottingham 2011
• Subjects: 669; TS Manufactures
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.539168

Three HVOF and a VPS CoNiCrAlY coating were sprayed using Praxair CO-210-24 and Sulzer Metco Amdry 9951 powders. All coatings were subject to vacuum annealing treatments at different temperatures for different exposure times in order to achieve equilibrium microstructures in all test samples. Both HVOF and VPS, as sprayed and heat treated coatings were oxidised at 850°C and 1100°C for periods of up to 250 hours. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and laser diffraction powder size analyses were used in order to analyse the powder particle size distribution and characterise the microstructure of the as-received powders, as-sprayed coatings and annealed and oxidised coatings. Thermogravimetric analysis (TGA) was carried out on as-sprayed and heat treated HVOF and VPS coatings at 1100°C for 96 hours in order to compare the oxidation rate of HVOF and VPS coatings and investigate the effect of microstructure and heat treatment on the oxidation rate of both types of coatings. Furthermore, micro hardness, nano hardness and dynamic mechanical analysis (DMA) with a 3-point bend clamp were used in order to study the hardness and Young’s modulus of as-sprayed and annealed HVOF and VPS coatings. The Eshelby inclusion model and image analyses were used in order to further investigate the effect of microstructure on the mechanical properties of these coatings. It was observed that heat treatment prior to oxidation will change the dual oxide layer observed for the as-sprayed oxidised coatings to a single alumina layer for the heat treated and oxidised coatings. Furthermore, it was observed that microstructural features such as porosity and oxide stringers have an effect on the oxidation rate of coatings. A model was presented indicating the role of oxide stringers and porosity on the resulting heat treated microstructure and oxidation rate. It was also understood that the beginning stage of oxidation can play an important role on the oxidation behaviour (oxidation rate and different oxides present) of both HVOF and VPS coatings. It was observed that annealing will change the microstructure of the coatings and affect their mechanical properties. It was seen that annealing reduced the hardness of the thermally sprayed coatings while it had increased the Young’s modulus of the coatings. It was also observed that features such as porosity levels and β volume fraction have an effect on the mechanical properties of the coatings.