Characterization of the W 2 C phase formed during the high velocity oxygen fuel spraying of a WC + 12 pct Co powder
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I. INTRODUCTION
MANY thermal spraying techniques have been developed to produce WC 1 Co coatings. One of the first systems used to obtain this kind of coating to meet industrial requirements was the plasma process, but the emphasis has now changed to favor the high velocity oxygen fuel (HVOF) process. With the HVOF process, one of the main problems is the formation of the brittle W2C phase. When the WC 1 M powder particles travel with the HVOF spraying flame, the WC can be degraded and new phases are formed because of the loss of carbon by oxidation and reaction of the W with the metallic phase present, e.g., Co. The new formed phases have been identified as W2C, CoWO4, CoxWyCz , and metallic W.[1,2] These new phases introduced unsatisfactory performance of the coatings in terms of wear resistance. With the identification of the problems leading to poor coating characteristics, it has become possible to modify the coating methods used to produce WC-M coatings.[3,4,5] The following spraying methods are still used for producing WC-M coatings. A. Atmospheric Plasma Spray A temperature of 12,000 K is attained, which favors oxidation as a result of reaction with the atmospheric oxygen.[6] B. Low Pressure Plasma Spray With this process, the oxidation reactions are decreased by the reduction of the oxygen partial pressure, and at the
J.M. GUILEMANY, Professor, J.M. de PACO, Project Coordinator, and J.R. MIGUEL, Director of the Thermal Spray Centre (CPT), are with the Department of Chemical Engineering and Metallurgy, University of Barcelona, 08028 Barcelona, Spain. J. NUTTING, formerly Emeritus Professor, Department of Chemical Engineering and Metallurgy, University of Barcelona, is deceased. Manuscript submitted June 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
same time, because of the lower total pressure, the particle velocity is increased and the temperature is similar to the atmospheric plasma spray process; therefore, there is less time for oxidation to take place.[7] C. High Velocity Oxygen Fuel Spray The particle velocity is high (500 to 800 m/s), thus reducing the time for oxidation to occur. The flame temperature is relatively low, 3000 K, and this, together with the low transit time of the particles, reduces the extent of oxidation.[8] Vinayo et al.[9] have described the reactions that take place during the thermal spraying of WC-Co as follows: 2WC 1 O2 ⇔ W2C 1 CO2
[1]
W2C 1 –21 O2 ⇔ W2 (CO)
[2]
W2 (CO) ⇔ 2W 1 CO
[3]
[10]
Tu et al. confirmed that the WC phase undergoes a degradation process to produce W2C, which then transforms to metallic W when high temperatures are attained in an oxygen-rich atmosphere. Further modifications to the reaction sequence were proposed by Fincke et al.,[11] who believed that during thermal spraying, carbon diffuses from the WC into the metallic matrix and that Co diffuses from the matrix into the carbide as follows: 4Co 1 4WC 1 O2 ⇔ 2Co2W4C 1 2CO
[4]
3Co 1 3WC 1 O2 ⇔ Co3W3C 1 2CO
[5]
12Co 1 12WC 1 502 ⇔ 2Co6W6C 1 10CO
[6]
Finke et al. also maintained that R
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