1 Introduction

WC-Co cemented carbide coatings sprayed by HVOF were widely used in many fields, such as aviation, aerospace, metallurgy industry, as well as energy and electric power generation owing to their excellent wear resistance (Ref 1,2,3,4). The microstructure and mechanical properties of WC-Co coatings are of great importance for their application. The microstructure and mechanical properties were always associated with the WC particle size, distribution of Co binder phase, porosities, cracks, impurities and interconnection between carbides and metal (Ref 5). The wear resistance enhancement of HVOF sprayed WC-Co coatings strongly depended on the improvement of microstructure and mechanical properties.

In order to achieving high-qualified WC-Co coatings, researchers have been showing great interest and done a lot of work to improve the performance of deposited coatings, including the improvement of the morphological properties of feedstock powder, the design of coating composition, the optimization of the spray parameters, and so on. Wang et al. proposed a possible method for the preparation of completely densified and spherical WC-Co particles by using the new alumina-assisted treatment technology. It was shown that the wear resistance of the lately proposed WC-Co coating by HVOF spraying was four times higher compared with that of the conventional coating because of the intensifying impacts of nanocrystalline Co as well as WC/Co interfacial bonding (Ref 6). Wang et al. optimized spray parameters of the WC-12Co powder with liquid fuel JP-8000 HVOF spray system which had the mass flow meter regulating the flux of the medium and found that the coating deposited based on the optimal spray parameter exhibited higher hardness and better wear resistance owing to the decrease in the mean free path of the matrix (Ref 7). Mi et al. prepared WC-Co coatings by high-velocity oxygen-fuel, and then, the coatings were heat treated for 4 h at 350, 450 and 550 °C in air atmosphere. They found that the hardness initially increased and then, decreased with the increasing heat-treated temperature, and the coating heat-treated at 450 °C exhibited better wear resistance in comparison with the as-sprayed coating (Ref 8). Mohanty et al. prepared 1wt% carbon nanotube-reinforced WC-Co coatings sprayed by HVOF and found that the addition of carbon nanotube in the coating produced an enhancement in performance such as hardness, elastic modulus, fracture toughness as well as wear resistance (Ref 9).

Design of material components is one way of improving the mechanical properties of materials. Tungsten borides is an important superhard material which hardness could be compared to cubic boron nitride and diamond (Ref 10, 11). Based on this, adding a certain amount of WB to the WC-Co is expected to produce a new coating material with greater mechanical properties.

In the study, we found that WB would react with Co to form CoWB. In order to obtain the coating materials with improved mechanical properties, we need to minimize the content of the metal phase in the coating. In view of this, WC-30WB-10Co composites were fabricated and the microstructure, phase composition, interfacial bonding strength, porosities, microhardness, as well as fracture toughness of the as-sprayed coatings were studied in detail. For the purpose of comparison, the similar experiments were conducted on conventional WC-12Co coatings.

2 Experimental

2.1 Coating Preparation

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.

Table 1 Chemical composition for WC-30WB-10Co powders and WC-12Co powders
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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.

Table 2 Technological parameters of HVOF spraying for WC-30WB-10Co coatings and WC-12Co coatings
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2.2 Characterization

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.

3 Results and Discussion

3.1 Microstructure of Raw Material Powders

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.

Fig. 1
figure 1

SEM images of raw materials: (a) WC; (b) WB; (c) Co

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3.2 Phase Constitution and Microstructure of Composite Powders

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:

$$ {\text{WB }} + {\text{ Co }} \to {\text{ CoWB}} $$
(1)
$$ 2{\text{WB }} + {\text{ Co }} \to {\text{ CoW}}_{2} {\text{B}}_{2} $$
(2)
Fig. 2
figure 2

X-ray diffractograms for feedstock powders: (a) WC-30WB-10Co; (b) WC-12Co

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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.

Fig. 3
figure 3

SEM images of feedstock powders at different multiples: (a) and (b) WC-30WB-10Co; (c) and (d) WC-12Co; (a) and (c) × 300; (b) and (d) × 3000

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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).

Fig. 4
figure 4

Cross-sectional metallographic images of feedstock powders: (a) WC-30WB-10Co; (b) WC-12Co

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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.

3.3 Composition and Microstructure of Deposited Coatings

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:

$$ 4{\text{WC }} + {\text{ O}}_{2} \to 2{\text{W}}_{2} {\text{C }} + \, 2{\text{CO}} $$
(3)
$$ 12{\text{Co }} + \, 12{\text{WC }} + \, 5{\text{O}}_{2} \to 2{\text{Co}}_{6} {\text{W}}_{6} {\text{C}} $$
(4)
$$ 3{\text{Co }} + \, 3{\text{WC }} + {\text{ O}}_{2} \to {\text{Co}}_{3} {\text{W}}_{3} {\text{C}} $$
(5)
Fig. 5
figure 5

X-ray diffractograms for the coatings: (a) WC-30WB-10Co; (b) WC-12Co

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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 14,15,16).

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).

Fig. 6
figure 6

Cross-sectional SEM images for the coatings at different multiples: (a) and (b) WC-30WB-10Co; (c) and (d) WC-12Co; (a) and (c) × 100; (b) and (d) × 2000

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3.4 Mechanical Properties of Coatings

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 17,18,19). 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.

Fig. 7
figure 7

Cross-sectional microhardness and fracture toughness for WC-30WB-10Co coatings and WC-12Co coatings

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Fig. 8
figure 8

X-ray diffractograms for CoWB-12Co cemented carbides

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Fig. 9
figure 9

SEM image of CoWB-12Co cemented carbides

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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·m0.5, which is much lower than that of the conventional WC-12Co coatings (4.17 MPa·m0.5). 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.

4 Conclusions

In this work, the WC-30WB-10Co and WC-12Co thermal spray powders were prepared and their corresponding coatings were deposited by high velocity oxy-fuel spraying. The microstructure, phase composition, interfacial bonding strength, porosities, microhardness, as well as fracture toughness of both coatings were investigated. The following conclusions can be drawn.

  1. (1)

    The CoWB phase and a small amount of CoW2B2 phase were present in the WC-30WB-10Co powders, and no new phases were detected in the WC-12Co powders. The apparent density of WC-30WB-10Co powders was much higher than that of WC-12Co powders. The flowability of WC-30WB-10Co powders was significantly better than that of WC-12Co powders.

  2. (2)

    The low intensity diffraction peaks of W2C, Co3W3C and Co6W6C were present in the two coatings because of the oxidation and decarburization occurred during HVOF spraying of the powders.

  3. (3)

    The microhardness of WC-30WB-10Co coatings reached 1639HV0.3, which far exceeded the microhardness of the conventional WC-12Co coatings (1222HV0.3). The fracture toughness of WC-30WB-10Co coatings was only 1.34 MPa·m0.5, which was much lower than that of the conventional WC-12Co coatings (4.17 MPa·m0.5). This was attributed to the formation of the ternary compounds and the consumption of the ductile Co binder in the WC-30WB-10Co coatings.