Study on Preparation and Properties of WC/Co-based Thermal Spray Powder and Coating
XIANG Jintao
CNKI
Abstract
Thermal spraying is an emerging and important area of current scientific research technology to prepare tungsten carbide/cobalt based metal ceramic coating.The conventionnal spray technologies include:arc spraying, plasma spraying,detonation gun spraying and high velocity oxygen fuel (HVOF) spray. Nowdays HVOF is considered best and most widely used technology for thermal spray tungsten carbide/cobalt based metal ceramic with combination of excellent adhesion and wear resistance.
In this paper the WC-10Co4Cr compound powder was prepared by means of sintered-crushed method with micron WC,Co and Cr3C2 powder as raw materials.The addition of Cr3C2, sintering time for the preparation of WC-10Co4Cr composite powder were investigated, structuer and performance of the sintered powder were characterized. Two kinds of WC-12Co and WC-10Co4Cr wear-resistant coating were sprayed using HVOF technology.The powder composition,structure,the use of liquid fuel for coating structure, anti-corrosion and sliding wear resistance of the WC-10Co4Cr coating were studied.The conclusions are listed as following:The WC-10Co4Cr composite powder is prepared with 1.5μmWC, 1.5μm Co and Cr3C2 powder by mechanical milling and sintered-crushered method. After sintered for 1.5h-5h in 1250 ℃ hydrogen atmosphere,the powder morphology and phase composition change significantly.After sintered for 1.5h, the powder consists of the main phase WC and Cr3C2 phase, together with small amount of Co3W3C and Cr7C3 phase;after 2.5h of sintering the Cr3C2 phase disappears, indicating that complete diffusion of Cr3C2, the WC grain size does not grow significantly; after 5h of sintering, massive decarburized η phase (Co3W3C, and Co6W6C), or even metallic W phase emerge,together with some oxidation phase of WO3, WO2and Cr2O3 phase.
Also, the WC grain grows.Two kinds of dense coating were sprayed with JP-5000 gun. The WC-10Co4Cr coating has porosity of 0.8% and average microhardness of HV0.31322 while the WC-12Co coating has porosity of 1.1% and average microhardness of HV0.31238.5.The main phase of WC-10Co4Cr and WC-12Co prepared with the same spraying parameters is WC phase with low decarburization degree. For the WC-10Co4Cr coating there are a small amount of new phase W2C without η phase (Co3W3C,Co6W6C); while for the WC-12Co coating there are a small amount of new phases W2C and trace phase of Co6W6C.
The volume wear ratio of the WC-10Co4Cr coating is 1/146 that of the stainless steel(1Cr18Ni9Ti) meterial. The friction coefficient after 1000s stabilizes at about 0.8 under the conditions of load 10N, speed 600r/min and wear time 120min. The sliding wear mechanism is fragmentation of WC and three-body abrasive wear. The neutral salt spray corrosion test and electrochemical polarization were conducted in 3.5%NaCl solution at 25±0.5°C, with potentiodynamic scanning speed 0.001V/s, scanning potential range for Eocp in -300~+200mV. The WC-10Co4Cr coating shows excellent,the corrosion resistance decreases with the order corrosion-resistant performance WC-10Co4Cr coating>the hard chrome plating>stainless steel (1Cr18Ni9Ti).
Key Words: High Velocity Oxygen Fuel spray (HVOF); WC/Co-based coating;Sintered-crushed; Sliding wear; Corrosion resistance
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