Advances in Functionalized Materials Research 2016View this Special Issue
Synthesis and Characterization of Nanostructured WC-Co/Al Powder Prepared by Mechanical Alloying
Nanostructured WC-Co/Al powder was synthesized from WC-12Co powder and pure Al powder by mechanical alloying (MA). The morphology and microstructural evolution of WC-Co/Al powder were investigated by a series of characterization methods. The results showed that the β-Co phase in the initial WC-12Co powder was replaced by the AlxCo phases (such as Al9Co2 and Al13Co4). As the ball milling time increased, the average grain size of WC in the WC-Co/Al powder decreased firstly and then remained at a constant value of around 40 nm. The deposition behavior of powders sprayed by high velocity oxygen fuel (HVOF) spraying was investigated. During spraying, the WC-Co/Al powder had a better flattening than the WC-12Co powder without ball milling, which is beneficial to fabricate compact coatings with lower porosity.
WC based cemented carbide has been widely used in geoengineering and machining fields because of its high hardness and good red hardness [1–3]. Metal Co is the most often used material as a binder of these WC based cermets. Hard WC skeleton and flexible Co binder phase ensure the WC-Co high hardness and toughness, thus ensuring the good abrasion resistance [4, 5]. Fabrication of a WC-Co coating with thickness of hundreds of microns on the surface of traditional parts is an effective and economic method to improve the wear resistance and to increase the service life of parts [6, 7]. In recent years, with the rise of nanotechnology, people look forward to preparing more superior WC-Co coatings using nanostructured powder rather than conventional powder [8–12]. However, some authors reported that the decomposition and decarbonization of nanostructured WC grains were serious during the process of spraying, resulting in a decline in wear resistance [9–12]. Therefore, optimizing the feedstock powder structure and chemical composition has become an important way to obtain excellent performance of nanostructured WC-Co based coating [13–16]. Metal Al, which has a low melting point and an unfilled 3p1 orbital, also can be used as a binder of WC based cermets . Recently, Wu et al.  reported an Al/WC composite coating prepared by high energy milling and found that Al can react with WC, which was beneficial to bond Al and WC. Basak et al.  also reported that Al alloying can improve the wear resistance of the WC-Co coatings. However, there is still no evidence in literature regarding the deposition behavior of nanostructured WC-Co/Al powder and the details of the microstructural evolution during preparation of these powders.
In this work, a nanostructured WC based composite powder was synthesized using mechanical alloying (MA) method through adding the metal Al powder to the WC-12Co powder. Afterward, the laboratory-made WC-Co/Al powders were sprayed onto mild steel by high velocity oxygen fuel (HVOF) spraying. Morphology, microstructure, phase constitution, and deposition behavior of the powders were investigated using a series of characterization methods.
2. Materials and Experimental Methods
Commercially available WC-12Co powder, obtained from Zhangyuan Tungsten Industry New Materials Co., Ltd., China, and pure Al powder, provided by Beijing General Research Institute of Mining & Metallurgy, China, were used to prepare the WC-Co/Al composite powder. The morphology and microstructure of these powders were examined by a Hitachi S-4800 scanning electron microscope (SEM), and the typical SEM photos are shown in Figure 1. The WC-12Co powders, which showed near-spherical shapes with rough and porous surfaces (Figure 1(a)), were fabricated by the agglomerating and sintering method. These WC-12Co particles had a size range from 15 to 45 μm (Figure 1(a)). Figure 1(b) is the cross-section SEM image of WC-12Co powder. It can be seen that the WC grain size ranges from 0.2 to 1 μm. Figure 1(c) shows that the Al powders have a globular shape and smooth surface, and the size distribution is −5 + 1 μm.
2.2. Mechanical Alloying
The WC-12Co powder and Al powder were mixed evenly, with a mass proportion of 97 : 3, in a QM3-SP4J planetary ball mill machine. The ball milling time was 10 h, 30 h, and 50 h, respectively. Before ball milling, the stainless steel ball jar was vacuumized and then filled with Ar gas as shielding gas. Stearic acid (C18H36O2) was added into the jar as a process control agent (PCA), in order to prevent the cold welding between the grinding balls, powders, and jar wall. The revolving speed of the jar was set at 500 RPM. The mass ratio of ball to powders was 10 : 1. After the ball milling, we crushed the powder manually by a mortar and pestle. Then a fine screen was used to screen the WC-Co/Al powder to a size range from 15 to 40 μm.
The morphology and microstructure of powders were investigated by the Hitachi S-4800 SEM. A Bruker D8 FOCUS X-ray diffractometer (XRD, Cu-Kα radiation, voltage 40 kV, electric current 40 mA, and scanning speed 2°/min) was used to analyze the phase structure of powders. In this work, average grain size of WC was determined by measuring the line broadening at half the maximum diffraction intensity of the WC phase (2θ = 35.6°, nm on the face (100)) and then was calculated using Scherrer’s equation : where is the grain size, is the X-ray wavelength ( nm for Cu-Kα radiation in this work), is the line broadening at half the maximum intensity (in radians), and θ is the Bragg angle.
A TJ-9000 HVOF spraying system (China, Tianjin University) was used to spray powders onto the mild steel plates for investigating the deposition behavior of the powders. The spraying parameters were the oxygen pressure of 0.7 MPa, the oxygen flow rate of 245 L/min, the fuel gas pressure of 0.65 MPa, the fuel gas flow rate of 30 L/min, and the spraying distance of 270 mm, respectively. The splats were collected and the coatings were also prepared. The morphology of splats and the microstructure of coatings were investigated by the Hitachi S-4800 SEM. Porosity of coatings was measured using the metallographic method . 10 cross-section images of each coating obtained by OLYMPUS GX51 optical microscope (OM), with a magnification of 200 times, were used for the measurement of porosity.
3. Results and Discussion
3.1. XRD Analysis of Powders
In this paper, a total of three sets of WC-Co/Al powder were prepared. These powders were denoted by 1#, 2#, and 3#, which corresponded to the ball milling time of 10 h, 30 h, and 50 h, respectively. For convenience, the initial WC-12Co powder was denoted by 0# powder. Figure 2 is the XRD patterns of 1#, 2#, and 3# WC-Co/Al powders and the WC-12Co powder. We can see that the WC phase (PDF-ICDD: 73-0471) and the β-Co phase (PDF-ICDD: 15-0806) constitute the initial WC-12Co powder. The height of the WC phase diffraction peaks becomes lower and wider with the increase of milling time. This indicated that WC grains were repeatedly crushed and broken by balls, becoming smaller and smaller during the process of ball milling. The β-Co phase diffraction peak, which was detected in the initial WC-12Co powder (2θ = 44°, nm), disappeared in the XRD patterns of WC-Co/Al powders after the ball milling. This is because of the solution of Al in the Co matrix. One dump was detected at the 2θ = 45°~47° in each WC-Co/Al powder instead of the sharp diffraction peak, which showed that the binding phase became amorphous to some extent during the ball milling . The reason for this phenomenon can be attributed to the fact that the grinding balls repeatedly squeezed the mixed powders making the Al element dissolve into the Co phase and facilitating the formation of intermetallic compounds (IMCs) of Al and Co, such as AlCo (PDF-ICDD: 65-0672), Al5Co2 (PDF-ICDD: 65-3638), Al9Co2 (PDF-ICDD: 65-2369), or Al13Co4 (PDF-ICDD: 65-1165) . Because the diffraction peaks of these IMC phases are very closed in the XRD patterns, we noted them as AlxCo in this paper.
Figure 3 is the average WC grain size as a function of the milling time. Grain size of WC phase is a crucial factor of the microstructure of WC based powders and coatings . The calculated average WC grain size of 0#, 1#, 2#, and 3# powders from (1) was 248.5 nm, 93.1 nm, 39.0 nm, and 44.8 nm, respectively. As we know, Scherrer’s formula is only applicable to nanoscale materials . When the value of D was larger than 100 nm, the calculated error is great, so the result of the initial WC-12Co powder was inaccurate and presented in Figure 4 only for comparison. It can be seen from the powders’ cross-section SEM images (Figure 1(b)) that the average grain size of WC-12Co powder is about 0.5 microns. The calculated values of 1#, 2#, and 3# WC-Co/Al powders were all lower than 100 nm. At the early stage of the ball milling process, the WC grain size decreased sharply with the milling time due to the drastically mechanical crushing. However, when the grain refinement reached a critical level, high specific surface area increased the materials’ surface energy significantly . Consequently, at the subsequent milling, the average grain size of WC in the WC-Co/Al powder did not decrease further but nearly remained at a constant value of around 40 nm. Therefore, a too long milling time is worthless.
3.2. Morphology and Microstructure of the Powders
Figure 4 shows the typical morphology and cross-section microstructure of the 2# WC-Co/Al powder. As can be seen from Figure 4(a), the powders had a very rough surface and presented a polygonal morphology. This is the typical powder morphology after ball milling process as a result of the collision and crushing between the ball and powders [2, 8]. From Figure 4(b), we can see that the interior structure of powders is very dense with fine WC grains dispersed in the binding phase uniformly. There are almost no pores inside the powder. In addition, it can be seen that the WC grains size is very small, most of which were under 100 nm, indicating that the calculated results from Scherrer’s equation are credible.
3.3. Deposition Behavior of the Powders
Among these three ball-milled powders, 2# powder had the finest WC grain size, and the ball milling time of 2# powder was moderate. Therefore, in this work, we take 2# powder as a preferred WC-Co/Al powder to prepare the splats and coatings using HVOF spraying for investigating the deposition behavior of the WC-Co/Al powders. For comparison, the WC-12Co splats and coatings were also prepared by the HVOF spraying. Figure 5 shows the HVOF deposited splats’ morphology of the 2# WC-Co/Al powder and the WC-12Co powder. It can be seen that the flattened degree of the ball-milled WC-Co/Al powder (Figures 5(a) and 5(b)) is better than that of WC-12Co powder (Figures 5(c) and 5(d)). This suggested that, under the same spraying parameters, ball-milled WC-Co/Al powder was easier to spread around than the initial WC-12Co powder. Several reasons are responsible for this phenomenon. One is the finer WC grains of the WC-Co/Al powder compared with the WC-12Co powder, which can be seen clearly in Figures 5(b) and 5(d). The WC grain size of WC-12Co powder without ball milling ranged from 0.2 to 1 μm, while the average size of WC grains after the ball milling was only 39 nm. Li and Yang  reported that the refinement of carbide grains was favorable for the carbide based powders to flatten during the HVOF spraying. In addition, He and Schoenung  reported that, under the same condition of HVOF spraying, nanostructured WC-Co powders would experience higher temperature than the micron-structured WC-Co powders, which is also advantageous to the particle flattening. The other reason is that the aluminum has a low melting point (660°C) and good thermal conductivity, making the WC-Co/Al powder easier to melt. Furthermore, comparing Figures 1(a) and 4(a) we can find that the size of WC-Co/Al powder is smaller than that of WC-12Co powder, which also makes the WC-Co/Al powder easier to melt and flatten.
The cross-section SEM images of the WC-Co/Al coating and the WC-12Co coating are shown in Figure 6. It can be seen that both the two kinds of coating have a lamellar structure, which is determined by the nature of thermally spraying process [13, 21]. HVOF spraying gun provided particles with high speed impacting to the substrate, and then the particle flattened and solidified rapidly. The subsequent particles will be gradually superimposed onto the deposited layers, and finally the coatings were prepared. However, it is worth noting that the thickness of each layer in the WC-Co/Al coating (Figure 6(a)) is thinner than that of the WC-12Co coating (Figure 6(c)). This is associated with the flattening degree of two powders, as shown in Figure 5. The WC-Co/Al powder was easier to flatten during deposition, so the thickness of splats was thinner and thus the coating got the finer microstructure. From Figure 6(c) we can see clearly that the cross-section profiles of the splats in the WC-12Co coating are thick relatively, which also demonstrates the results observed in Figure 5(c). In the WC-Co/Al coating (Figure 6(b)), the WC grains were very fine and most of them retained nanostructure. It can be seen from Figure 6(d) that the WC grains basically remain of similar size and shape compared with that in the WC-12Co powders (Figure 1(b)). This suggested that the decomposition of WC phase is slight during the process of HVOF spraying. In addition, comparing Figure 6(a) with Figure 6(c) we can find that the WC-Co/Al coating exhibits more compact microstructure than WC-12Co coating. The porosity of WC-12Co coating calculated using the metallographic method was 1.62%, while the porosity of nanostructured WC-Co/Al coating was only 0.57%. Tillmann et al.  reported that the porosity of coatings decreased with the improvement of the single particles flattening. The phenomena observed in Figures 5 and 6 are in agreement with the conclusion from Tillmann et al.
The nanostructural WC-Co/Al powder was prepared by mechanical alloying from the pure Al powder and submicron-structured WC-12Co powder. During the process of ball milling, the Al powder reacted with the Co phase in WC-12Co powder and intermetallic compound of Al and Co, noted by AlxCo, generated. Ball milling time was an important factor for determining the average WC grain size. After milling time of 10 h, 30 h, and 50 h, the average WC grain size of WC-Co/Al powder was 93.1 nm and 39.0 nm and 44.8 nm. During HOVF spraying, the WC-Co/Al powder had a better flattening degree than the WC-12Co powder without ball milling, which is beneficial to form a uniform and compact microstructure in the WC-Co/Al coating. The porosity of the WC-Co/Al coating, which was only 0.57%, decreased significantly than that of the WC-12Co coating (1.62%).
The authors declare that there are no competing interests regarding the publication of this paper.
The authors are grateful for the financial support by the National Natural Science Foundation of China (no. 51375332).
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