Characterization of supersonic plasma sprayed WC-12Co coatings
Wang Haijun¹  Han Zhihai^{1, 2}  Zhang Ping¹  Zhou Shikui¹ 
1.State Key Laboratory of Remanufacture Technology, Armored Force Engineering Institute, Beijing 100072,China
2. State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China

 

The WC-12Co particle in-flight speed in plasma jet from the recently developed supersonic plasma gun were measured using the Spray Watch-2i CCD on-line measure system. The histograms showed the mean value of velocity is about 400~500m/s that is faster than obviously it (about 200m/s) by common plasma spraying. That result in the characterization of coating sprayed by supersonic plasma spraying (S-PS) on the bonding strength, micro-hardness, porosities and XRD phases analysis, SEM morphologies, as well as the decarburization and oxidation of tungsten carbide are all better than those of WC-12Co coating by common plasma spraying.

1  Intruduction

Thermal spraying WC-Co hard coatings are widely used in wear situations because they combine several advantages such as resistance to abrasion, erosion, high temperature and corrosive atmospheres ^[1]. But the WC-Co powders have to be exposed to high-temperature plasma jet for heating and acceleration during conventional plasma-sprayed (APS), lead to WC-Co may decompose to W₂C, even to metallic tungsten and metallic cobalt it can further formed an amorphous phase or complex carbides such as Co₃W₃C, Co₂W₄C, Co₆W₆C and other oxides^[2,3]. All that result in higher porosity and lower hardness of coatings sprayed by APS is a major disadvantage compared with the high velocity thermal spraying processes such as high velocity oxy-fuel (HVOF) spraying coatings^[4,5].

This paper investigate the influence of particle in-flight speed on the microstructure, hardness and bonding strength of supersonic plasma sprayed (S-PS) WC-12Co coatings.
2  Experimental conditions
The WC-12Co powder with grain size –45+22 μm used for this experiment. The coating were deposited on 45 steel substrates using common plasma sprayed (APS,by Metco 9M system) and supersonic plasma sprayed (S-PS, by HEPJet developed by China) with the spraying conditions described in table.1. The thickness of the coatings was fixed to 300μm approximately. The block specimens with 25×16×6i‡ojfor microstructure analyzing and micro hardness testing and the φ25.4×60i‡ojcylinder samples for the bonding strength testing.
Table 1. Spray conditions for WC-12Co

Process parameter	HEPJet	9M system
Spray distance / mm	110	110
Ar flow rate / m3h-1	3.6	2.8
H2 flow rate / m3h-1	0.32	0.15
Voltage / V	420	68
Arc current / A	148	650
Feed rate / g min-1	60	80

The bonding strength of coating samples measured according to GB9796-88 China standard , the E-7 glues were will glues to cylinder samples for tensile bonding strength test by WE-10 A all-purpose material test machine and the mean values of bonding strength from measuring data of three samples.

Microhardness were measured by Micro Vickers indenter with a load of 200g applied 15s. the value given is average of 10 measurements.

The porosities of coatings using S-PS (by HEPJet) and APS (by Metco 9M) were measured through the  SEM image analysis of coating section according to metallographic standard. The percentages value of porosities was the average of five measure areas.

The recorded histograms of particles in-flight speed in plasma jet from the recently developed supersonic plasma gun (by HEPJet ) were measured using the Spray Watch-2i CCD on-line measure system as shown in the Fig.1. The histograms present the mean value of velocity is about 400~500m/s that is much faster than it (about 200m/s) by common plasma spraying (APS).

Fig.1. Histograms for in-flight speed, temperature and their distribution by S-PS using Spray Watch-2i system

The microstructure and phase analysis of coatings by two different plasma spraying were carried out by means of SEM and XRD.

3         Results and discussion

The WC-12Co particles flying velocity and temperature by the supersonic plasma spraying (S-PS, by HEPJet ) on-line measured using the Spray Watch-2i CCD system are shown in fig.1. It showed the particle average velocity of WC-12Co powder is about 400~500m/s and is much higher than the velocity of particle (about 200m/s) using  a common plasma sprayed (by 9M system) and the av erage temperature of particles is about 2100Ž which is very suit to melt the WC-Co.

Table 2 summatizes the properties of coatings include bonding strength, micro hardness and porosity obtained from two kinds of plasma spraying processes by HEPJet and 9M system.It is obvious that the bonding strength and micro hardness of the WC-12Co coatings by S-PS were much high that by APS.

Table 2. Properties of the WC-12Co coatings by S-PS and APS

Coating properties	HEPJet (S-PS)	Metco.9M (APS)
Bonding strength / MPa	>65	48
Hardness / HV0.2	1303	949
Porosity/ %	0.82	8.35

Fig.2 Show the cases of decarburization and decomposition of tungsten carbide by APS is obviously severe than that of WC-12Co coating by S–PS due to XDR spectrum exhibits more peaks of mixed carbides (Co₂W₄C, Co₆W₆C) and W₂C (as seen in fig.2.b)

Fig.2 XRD patterns of WC-12Co coatings (a)by HEPJet (S-PS) and (b)by 9M system (APS)
The SEM morphologies of cross-sections of the WC-12Co coatings are shown in fig.3.
 
(a)    specimen by S-PS
 
(b)    specimen by APS
Fig.3. SEM morphologies of the WC-12Co coatings deposited by S-PS (HEPJet) and APS (9M system)
The coating obtained with HEPJet is more dense than the deposited by 9M system because it has little porosity. On the micrographs, the lighter grey corresponds to the WC particles which are homogeneously distributed in the coatings, the darker grey corresponds to the cobalt-rich matrix and the darkest spots are pores.

Table3 is a EDS compositions of WC-12Co powder and its coatings by HEPJet and 9M system.

Table 3. EDS composition of powder andWC-12Co coatings by HEPJet (S-PS) and 9M system (APS)

	powder	HEPJet	Metco.9M
C wt %	11.97	7.80	5.97
Co wt %	11.76	11.55	10.49
W wt %	76.27	77.90	78.95
O wt %	-	3.75	6.59

 

It shows the wt% of elements (include C,Co,W and O) of WC-12Co powder produced change during plasma spraying. It also indicate that decrease of decarburization, decomposition and oxidation of  tungsten carbide by S-PS compared that by APS as 7.80 C% to HEPJet sprayed and 5.97 C% to Metco 9M as well as oxygen content of coating sprayed by S-PS is only half of it by APS.

4   Conclusion

The in-flight behavior of WC-12Co particles during the supersonic plasma sprayed (S-PS, by HEPJet ) were measured in this work.. The result show that its average velocity is about 400~500m/s in the supersonic plasma jet and is much higher than the speed (only lower 200m/s)of powder particle  by common plasma sprayed (APS,by 9M system).

Compared with bonding strength, micro-hardness and analysis of the microstructure, phases and porosity of two kinds of WC-12Co coatings, The results showed the bonding strength, micro-hardnees and porosity of WC-12Co coatings sprayed by S-PS , as well as the decarburization and oxidation of tungsten carbide, are obviously better than those of common plasma sprayed ( by 9M system). It is One of important factors that the flying velocity of particles is much faster than the it of common plasma spraying.

5          Aknowledgements

This work was support by the Science and Technology Fund of national key laboratory No. 51489020405JS9101.

References
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