Promising WC-30WB-10Co Cemented Carbide Coating with Improved Density and Hardness Deposited by High Velocity Oxy-Fuel Spraying: Microstructure and Mechanical Properties

WC-30wt.%, WB-10wt.%Co powders and WC-12wt.%Co powders were prepared by the same procedure, including the steps of wet grinding, spray granulation, vacuum degreasing sintering, mechanical crushing and screening in this work. Both powders were prepared under exactly the same conditions, including ball milling time, granulation process, sintering temperature, holding time, and so on. The raw materials were produced by Luoyang Golden Egret Geotools Co., Ltd, including WC powders, WB powders and Co powders. The particle size distribution of both powders prepared was 15-45 μm. The chemical compositions of the powders were confirmed quantitatively by inductive coupled plasma (ICP) with an emission spectrometer, as shown in Table 1.

The substrate material used in this study was low-carbon steel samples with a dimension of 100 mm × 100 mm × 5 mm. Prior to the coating spraying, the substrates were firstly grit-blasted with 60 meshes Al₂O₃, and then, the surface of steel was cleaned in an ultrasonic ethanol bath for 10 min in order to remove oil stains and contaminants of corundum. A Praxair JP-8000 HVOF thermal spray equipment was employed to prepare the WC-30WB-10%Co coating and WC-12Co coating. The temperature of the samples sprayed was controlled below 200 °C during the spraying process. Both coatings were prepared with the same spraying parameters, and the spraying parameters are shown in Table 2.

The chemical compositions of the powders were examined using inductive coupled plasma with an emission spectrometer (ICAP-7000, Thermo Electron, USA). The phases of the powders and coatings were analyzed by x-ray diffraction (Rigaku Dmax/2550VB + , Tokyo, Japan) at 40 kV and 250 mA using Cu-Kα radiation. The microstructural analysis of the powders and coatings was investigated using scanning electron microscopy (Evo18, Carl Zeiss, Germany). The apparent density and flow rate of these powders were measured using the Hall Flowmeter Funnel (LC-XF-02, Lichen, China) in accordance with the ASTM B212 and ASTM B213 standard test method. The cross-sectional morphology of the powder and the porosity of the coating on the cross section were analyzed using a optical microscope (Imager.A2m, Carl Zeiss, Germany) with an image analysis system. The hardness and the fracture toughness were tested on the cross section of the coating by a Vickers hardness tester (Vickers 402 MVD, Wilson, USA) with a load of 300 g. The fracture toughness of the coating was obtained according to Evans and Wilshaw equation (Ref 12). The results of the porosity, hardness and fracture toughness of the coatings were obtained by the average of ten readings. The interfacial bonding strength between coating and substrate was measured by tensile test according to the ASTM C633-2001 standard test method, and the result was obtained by the average of five readings.

Figure 1 shows the scanning electron micrographs of the raw material powders of WC, WB and Co. As can be seen from the figure, all three powders are irregular in shape and show some level of agglomeration. The average grain size of WC powders is about 2.9 μm, that of WB powders is about 3.2 μm, and that of Co powders is about 0.8 μm.

Figure 2 shows the x-ray diffraction patterns of WC-30WB-10Co powders and WC-12Co powders. It was clearly observed from Fig. 2(b) that the WC-12Co powders were comprised of WC and Co, and there were no impurity phases. As shown in Fig. 2(a), the WC phase, CoWB phase and a small amount of CoW₂B₂ phase were present in the WC-30WB-10Co powders; however, WB and Co phases were not detected. The CoWB and CoW₂B₂ phases were formed by the reaction of WB and Co during the sintering process of the spray-dried powders. The reaction equations were as follows:

Figure 3 shows the SEM images of WC-30WB-10Co powders and WC-12Co powders at different multiples. Both powders have a spherical or nearly spherical shape as shown in Fig. 3(a) and (c). It was observed from Fig. 3(b) and (d) that the WC-30WB-10Co particles show a smoother and denser surface than that of the WC-12Co particles.

The cross-sectional microstructures of the two powders are shown in Fig. 4. No pores are visible on the cross section of WC-30WB-10Co particles, while some pores are found in WC-12Co particles. The apparent density of WC-30WB-10Co powders reached up to 6.61 g/cm³, which was much higher than that of WC-12Co powders (5.06 g/cm³). Both WC-30WB-10Co and WC-12Co particles were prepared at the sintering temperature of 1230 °C, which was much lower than the eutectic temperature of WC-Co system (1340 °C). No liquid phase appeared during sintering of both powders. The mechanism of sintering densification of WC-30WB-10Co particles was the exothermic heat generated by both chemical reactions (1) and (2) during sintering, which promoted the diffusion of grain boundaries and atoms, and resulted in improved density. On the contrary, the sintering of WC-12Co particles mainly depended on the diffusion of atoms and grain boundaries in the solid phase at low temperature, and the particles sintered were not fully densified, as demonstrated by the cross-sectional metallographic image in Fig. 4(b).

The good spherical shape of the particles ensures a high flowability during spraying. The flowability was obtained by measuring the time of 50 g of the powders naturally flowing through a Hall flowmeter in accordance with the ASTM B213 standard test method. The flow rate of WC-30WB-10Co powders and WC-12Co powders was 8.9 and 12.6 s, respectively. The flowability of WC-30WB-10Co powders was significantly better than that of WC-12Co powders. This may be attributed to the smooth surface and the high apparent density for WC-30WB-10Co powders.

Figure 5 shows the x-ray diffraction patterns of WC-30WB-10Co coatings and WC-12Co coatings. It could be seen that the low intensity diffraction peaks of W₂C, Co₃W₃C and Co₆W₆C were presented in the two coatings, indicating that slight oxidation and decarburization occurred during HVOF spraying of the powders (Ref 7, 13). The reaction equations can be expressed as follows:

In addition, it was observed from Fig. 5(b) that the Co diffraction peaks disappeared compared with that of Fig. 2(b). This was attributed to the rapid solidification and high cooling rates of the metal Co phase (∼10⁶-10⁷ K/s) during thermal spray deposition, which led to the formation of amorphization Co phase (Ref 14-16).

Figure 6 shows the cross-sectional SEM images of WC-30WB-10Co coatings and WC-12Co coatings at different multiples. SEM images at low magnification show that both coatings exhibit dense microstructure and good adhesion to the substrates. The thickness of both coatings was approximately 350 μm. The macroscopic flaws such as cracks or delamination were not observed. SEM images at high magnification clearly show that both coarse and fine WC particles are observed, and most of them are blocky and angular. A small amount of micro-pores are present in both coatings. According to the measurements of metallographic method, the porosities of WC-30WB-10Co coatings were 0.56 vol.%, that of WC-12Co coatings were 0.73 vol.%, and the porosities of both coatings were less than 1 vol.%. As shown in Fig. 6(b), the CoWB particles were distributed around WC particles. The WC particles were dissolved into the Co binder phase homogeneously in Fig. 6(d).

As shown in Fig. 6(a) and (c), both coatings exhibit good adhesion to the substrates. Based on the tensile test according to the ASTM C633-2001 standard, the average interfacial bonding strength of WC-30WB-10Co coatings and WC-12Co coatings were 76.7 and 72.4 MPa, respectively. Judging from the fracture surface of the tensile specimens, all the fracture occurred in the bonding glue, which indicated that the actual interfacial bonding strength of both coatings was greater than their measured values. The strong bond between the coating and the substrate benefited from the sandblasting of the substrate surface before spraying and the high velocity impact of semi-molten particles during thermal spray deposition.

Figure 7 shows the cross-sectional microhardness and fracture toughness of WC-30WB-10Co coatings and WC-12Co coatings. It can be seen that the microhardness of WC-30WB-10Co coatings reached 1639HV0.3, which far exceeded the microhardness of the conventional WC-12Co coatings (1222HV0.3). The high hardness of the ternary CoWB compounds has been reported by many researchers (Ref 17-19). However, to the best of our knowledge, few previous literatures reported the actual measured hardness values of the ternary CoWB compounds. Therefore, it is necessary to test the hardness of CoWB in order to clarify the reasons for the high hardness of WC-30WB-10Co coatings. Since the single-phase CoWB ceramic was difficult to achieve sintering densification, and the excessive pores would cause large errors in the hardness measurements. Based on this, CoWB-12%Co cemented carbides were prepared by the traditional powder metallurgy process. Figure 8 and 9 shows the x-ray diffractograms and SEM image of CoWB-12Co cemented carbides, respectively. The surface microhardness was tested, and the result was obtained by the average of ten readings. The final result shows that the microhardness of CoWB-12Co cemented carbides is up to 1931HV0.3. From the above results, it was concluded that the high microhardness was attributed to the formation of the ternary compounds in the WC-30WB-10Co coatings. Moreover, the porosities have an important effect on the microhardness (Ref 20). The porosities of WC-30WB-10Co coatings were lower compared to those of WC-12Co coatings. The low porosities usually mean high hardness. However, it is noteworthy that the porosities of the coatings were not the primary factor for the higher hardness. The difference in the hardness of both coatings was mainly caused by the formation of the ternary CoWB compounds in the WC-30WB-10Co system.

As shown in Fig. 7, the fracture toughness of both coatings changed in a way inverse to their microhardness. The fracture toughness of the 30WB-10Co coatings is only 1.34 MPa·m^0.5, which is much lower than that of the conventional WC-12Co coatings (4.17 MPa·m^0.5). The low fracture toughness of 30%WB-10%Co coatings may be attributed to the consumption of the ductile Co binder during the sintering process of spherical powders due to the formation of the ternary compounds, which leads to a significant loss in crack resistance and breakage strength. In order to improve the fracture toughness of WC-WB-Co coatings, the following methods can be adopted: (1) ultra-fine WC and WB powders should be used as raw materials; (2) the amount of WB need to be reduced; (3) the spraying parameters need to be optimized. Overall, WC-30WB-10Co hard alloy coatings are expected to be well applied in aggressive wear and corrosion environments due to the high hardness and low metal phase content.