Abstract
Two commercial 7–8wt.% Yttria Stabilised Zirconia (YSZ) suspensions were sprayed by Suspension High Velocity Oxy Fuel (SHVOF) thermal spraying for advanced high temperature coatings. Heat treatments of the free-standing coatings were conducted at 800°C and 1000°C for 72h. The SHVOF coatings using two liquid carriers: water and ethanol, behaved differently in terms of micro-structure and phase stability. The ethanol coatings retained a fully tetragonal composition after heat treatments; while the aqueous coatings, however, underwent the undesirable tetragonal to monoclinic phase transformation at 1000°C, which is lower than previously reported temperatures (>1200°C) in thermal sprayed YSZ coatings. The heat treatments not only resulted in densification of both coatings, but also caused excessive crystallite growth in aqueous coatings promoting the undesirable phase transformation. On the contrary, the ethanol suspension improved the phase stability by favouring the homogenization of yttrium during spraying.
Highlights
High-velocity oxy-fuel (HVOF) is a well-established thermal spraying technique that has been widely used for various engineering applications at high temperatures [1,2]
Yttria Stabilised Zirconia (YSZ) particles from suspensions form agglomerates due to drying and the typical particle diameters were in the order of 400–500 nm, which agrees with the measurement of the particle size distribution (D50 = 0.47 μm) as provided by the supplier
It indicates that each YSZ particle is consisted of several smaller nano-crystallite, which could even be identified from Fig. 3 (B) under a higher magnification
Summary
High-velocity oxy-fuel (HVOF) is a well-established thermal spraying technique that has been widely used for various engineering applications at high temperatures [1,2]. Suspensions of oxide ceramics such as alumina [6,7,8], zirconia [9,10,11] and titania [12,13,14] have been widely sprayed by both plasma and HVOF techniques designated for wear-resistant coatings, solid oxide fuel cell (SOFC), thermal barrier coatings (TBC), and photocatalytic applications, etc. In case of high melting point oxides like zirconia (2715 °C), a higher flame temperature is indispensable, the use of organic solvent is more favourable owing to the extra heat generated by the combustion of organic solvent as fuel (the mass enthalpy generated by ethanol is 29.75 MJ/kg [3])
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