Abstract
The hardness and wear resistance of nano-sized chrome oxide (Cr2O3) coating are superior to those of micro-sized Cr2O3 coating. Aimed to the preparation nano-sized Cr2O3 coating, the study on a complete process has been developed on the basis of the summarized research progress of plasma spray micro-sized Cr2O3 coating and nano-sized Cr2O3 coating. In the complete process, nano-sized spherical Cr2O3 powder was firstly prepared and the agglomerated powder of nano-sized Cr2O3 was then achieved by spray prilling and sintering and nano-sized Cr2O3 coating is fabricated by air plasma spray (APS) eventually. The performance of the coating was further studied.
The main subjects in this dissertation are as follows:
(1) Low-temperature hydrogen-reduction sintering method and milling technique were combined to prepare the nano-sized spherical Cr2O3 powder. Orthogonal experiments were conducted to explore the effect of the weight ratio of ball to powder, the milling speed and the milling time on the size and morphology of Cr2O3 powder. The milling technique was also optimized. The particle size of the spherical Cr2O3 powder products of good crystallization is between 60 nm and 120 nm and the apparent density is determined as about 1.21 g/cm3.
(2) The spray dring technique and activated sintering method were used to prepare the agglomerated powder of nano-sized Cr2O3, because the nano-sized Cr2O3 powder is unsuitable for direct APS. The spray dring technique was further optimized on the basis of the study of solid content of slurry, binder, water reducer and the working parameters of spary dryer, such as air temperature, blower rate and feed rate on the morphology of the agglomerated nano-sized Cr2O3 powder.
The size of the agglomerated powder is about 50 µm. The apparent density and the vibration density are determined as 1.67 g/cm3 and 2.37 g/cm3 respectively and the liquidity is also achieved as 48.5 s/50 g. The agglomerated nano-sized Cr2O3 powder product is proved suitable for APS.
(3) Orthogonal experiments were conducted to study the effect of air plasma spraying parameters, such as plasma current, Ar gas flow rate, electric voltage and spray distance on the Vickers microhardness and the tribological behaviours. The wear mechanisms of nano-sized Cr2O3 coating were further studied.
Key words: plasma spray; nano-sized Cr2O3 coating; spray dring; sintering;wear-resisting property
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