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The Influence of Tungsten Carbide Spray Particle Size on Coating Microstructure and Properties
Release time:
2020-08-30 22:09
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The Influence of Tungsten Carbide Spray Particle Size on Coating Microstructure and Properties
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| The particle size of WC in the original powder determines the size of tungsten carbide particles in the spray coating, and the particle size of tungsten carbide in the coating directly affects the performance of the sprayed coating. Nanocrystalline tungsten carbide coatings prepared using nanoscale powders exhibit higher hardness and toughness as well as superior wear resistance compared to micron-scale tungsten carbide coatings made from conventional micrometer-scale powders. Recent studies have shown that tungsten carbide coatings prepared using micro-nano composite powders outperform those made from pure nanoscale powders. However, there are currently relatively few reports on this topic. Therefore, this paper systematically investigates the microstructure and properties of supersonic flame-sprayed nanoscale, micro-nano composite, and micron-scale tungsten carbide coatings, and explores the influence of the original powder WC particle size on the microstructure and performance of the coatings. |
As shown by the XRD diffraction comparison (Figure 2) and the analysis results (Table 1), the WC particle size in FN-WC powder is slightly smaller, followed by that in PN-WC powder, while the WC particle size in M-WC powder is slightly larger.
Table 1: Full Width at Half Maximum of the WC Main Peak
Powder FWHM/(°)
FN-WC 0.194
PN-WC 0.157
M-WC 0.148
The substrate material is 0Cr13Ni5Mo martensitic stainless steel. Before spraying, the substrate surface was cleaned and then sandblasted. The spraying equipment used is an HVOF system manufactured by SulerMecto of the United States. During spraying, propane serves as the fuel gas, high-pressure oxygen acts as the oxidizing gas, and nitrogen is used as the powder-feeding gas. The spraying process parameters are shown in Table 2.
Table 2: Spraying Parameters
Parameters Values
Pressure (O2)/MPa 1.0
Flow rate (O2) / (L·min⁻¹) 240
Pressure (C3H8) / MPa 0.6
Flow rate (C3H8) / (L·min⁻¹) 68
Spray distance/mm 200
1.2 Coating Microstructure and Mechanical Property Testing
The microstructural analysis of the coating was performed using a FEI Quanta 400HV scanning electron microscope, and the porosity of the coating was determined according to JB/T75059-1994. The bonding strength of the coating was tested using the paired specimen tensile test method specified in GB/T 8642-2002. The bonding strength tests were conducted on a CSS-44300 electronic testing machine at an loading rate not exceeding (1,000 ± 100) N/s. The microhardness of the coating was measured using an HDX-1000TMC/LCD microhardness tester, with a load of 300 g and a dwell time of 15 s. Nine measurement points were taken on each specimen, and the average value of the microhardness was calculated as the final result for the coating.
1.3 Coating Erosion and Wear Testing
The test was conducted using a particle erosion wear testing machine with a structure similar to that of the Japanese ACT-JP testing machine. The erosion test parameters were as follows: erosion distance, 100 mm; nozzle inner diameter, 3.6–4.0 mm; nozzle length, 22 mm; abrasive material, brown corundum with a particle size of 149 μm (100 mesh); compressed air pressure, 0.3 MPa; and erosion angles, 15° and 90°.
2 Results and Discussion
2.1 Microstructure of the Coating
Figure 3 shows the SEM micrographs of the cross-sections of the sprayed coatings. The porosity of the three coatings was statistically analyzed at a magnification of 5,000×. The results indicate that the porosities of the FN-WC, PN-WC, and M-WC coatings are 2.1%, 2.0%, and 2.3%, respectively, showing comparable porosity levels among the coatings. In the magnified microstructures, most of the WC particles in the FN-WC coating have diameters smaller than 100 nm, as shown in Figure 3(a2). Furthermore, at the boundaries of some flattened particles, regions devoid of nano-sized WC particles can be observed, as indicated by the brightly white metallic areas in Figure 3(a2). These regions likely result from partial melting of the nanoparticles in the supersonic flame jet, causing the fine nano-sized WC particles to decompose under high temperatures. In the PN-WC coating, most of the WC particles have diameters greater than 500 nm, with some even reaching around 2 μm; however, a few nanoparticles are also present within the coating, as shown in Figure 3(b2). In the M-WC coating, the WC particles are on the micrometer scale. During the spraying process, larger WC particles undergo fragmentation, and this fragmented morphology is retained in the coating. After fragmentation, the metallic bonding phase between the smaller tungsten carbide particles is insufficient, as illustrated in Figure 3(c2).
2.2 Coating Phase Structure
Figure 4 shows the XRD analysis results for the FN-WC coating, PN-WC coating, and M-WC coating. After treatment, the FN-WC coating exhibited a slightly higher relative peak intensity for the W2C phase, indicating that the FN-WC coating underwent a greater degree of decomposition during the spraying process. This result is consistent with the findings from the microstructural analysis of the coatings. Nanoscale tungsten carbide particles tend to decompose easily in supersonic flame jets; the more of the WC in the powder particles exists as nanoparticles, the more severe the decomposition will be in the flame jet. In contrast, the WC particles in the PN-WC and M-WC coatings mostly exist in micrometer- or submicrometer-sized forms, making them less prone to decomposition. Consequently, only a small amount of the W2C phase was observed in the XRD patterns of these coatings.
2.3 Microhardness of the Coating
Coating hardness refers to a material's ability to resist deformation or fracture within a small volume on its surface; its magnitude significantly influences the coating's wear resistance and erosion resistance. Figure 5 shows the microhardness test results for three tungsten carbide coatings. The Vickers hardness values of the PN-WC and FN-WC coatings are 1,241 HV0.3 and 1,254 HV0.3, respectively, slightly higher than that of the M-WC coating, which is 1,229 HV0.3. Notably, the hardness of the PN-WC coating exhibits greater uniformity. Combined with the microstructural photographs of the three coatings shown in Figure 3, it can be observed that the FN-WC coating contains regions of pure metal resulting from the decomposition of WC particles, whereas the M-WC coating features numerous fractured WC particles. The non-uniform microstructures in these two coatings lead to a scattered distribution of hardness values. In contrast, the PN-WC coating has a uniform distribution of micron- and nanoscale WC particles, thus resulting in a more concentrated distribution of hardness values.
2.4 Coating Adhesion Strength
Table 3 shows the coating adhesion strengths. As can be seen, the average adhesion strength of the PN-WC coating exceeds 73.5 MPa, and during tensile testing, fracture predominantly occurs within the bonding adhesive. The average adhesion strength of the M-WC coating is comparable to that of the PN-WC coating, and both of these coatings exhibit slightly higher adhesion strengths than the FN-WC coating. This difference may be attributed to phenomena such as oxidation and decomposition of WC particles in the supersonic flame flow experienced by the all-nanometer FN-WC coating. Among the samples from the FN-WC all-nanometer coating, specimens Nos. 3 through 5 all fractured within the coating itself.
Table 3: Coating Adhesion Strength
Coating Bonding Strength/MPa Mean Value/MPa Fracture Position
1 2 3 4 5
FN-WC 72.3 74.8 62.6 70.4 63.2 68.7 Coating + glue
PN-WC 70.7 75.8 72.0 76.9 72.3 73.5 Glue
M-WC 73.2 69.4 77.2 76.3 68.6 72.9 Glue
2.5 Coating Erosion and Wear Performance
The erosion test was conducted by using sand particles of a specified mesh size corresponding to 15 seconds and 100 grams. After each erosion cycle, the sample was weighed. Once the erosion process was completed, the cumulative mass loss due to erosion was used as the basis for calculating the volumetric loss caused by coating wear. This volumetric loss from wear was then adopted as the evaluation criterion for erosion resistance performance. A linear regression was performed on the cumulative volumetric loss, and the reciprocal of the slope of the fitted straight line was defined as the erosion wear coefficient Re. Under identical experimental conditions, a higher Re value indicates better erosion resistance performance of the coated material being tested.
The erosion wear results of the FN-WC coating, PN-WC coating, and M-WC coating at a small angle (15°) are shown in Figure 6. As can be seen, despite the use of all-nano WC particles, the FN-WC coating did not exhibit a significant improvement in erosion wear resistance. On the contrary, the PN-WC coating, prepared using a mixture of nano- and micro-sized WC particles, demonstrated a markedly enhanced resistance to erosion wear. Compared to the M-WC coating material, the PN-WC coating prepared with a mixed nano-micro WC particle size showed a substantial improvement in erosion resistance, with an erosion resistance approximately 1.5 times that of conventional coatings and 2.3 times that of the 0Cr13Ni5Mo substrate. In the coatings, the hard WC phase is bonded by the CoCr alloy matrix phase; however, under actual turbine operating conditions—specifically, under small-angle erosion conditions—the metal matrix phase exhibits poorer wear resistance compared to the hard WC phase.
Figure 6: Erosion Wear Volume Loss of the Coating at a 15° Angle of Attack
The PN-WC coating contains both large and small WC particles as hard phases, which are uniformly distributed within the CoCr alloy matrix, as shown in Figures 3(b1) and 3(b2). When sand particles erode the coating at a small angle, the metallic phase in the coating is easily abraded, exposing the larger hard-phase particles. As the erosion process continues, the CoCr alloy matrix surrounding the large hard-phase particles will be eroded away, eventually causing these large particles to detach. However, thanks to the small WC particles dispersed in the metallic binder phase around the large hard-phase particles in the PN-WC coating, the coating can better resist sand particle erosion and mitigate the erosion of the metallic binder phase, thereby delaying the detachment of the large hard-phase particles. Consequently, the PN-WC coating exhibits superior erosion resistance. In the FN-WC coating, the degradation of WC particles is the primary factor leading to its reduced erosion performance. In the M-WC coating, the insufficient metallic binder phase between the fragmented small tungsten carbide particles after fracture adversely affects the coating's erosion resistance.
Figure 7 shows the erosion wear volume loss of the FN-WC coating, PN-WC coating, and M-WC coating at a 90° impact angle. As can be seen, the trends in erosion volume loss for the three coatings are quite similar. The erosion resistance Re values of the PN-WC and FN-WC coatings are 4.70×10⁴ g/cm³ and 4.68×10⁴ g/cm³, respectively, slightly higher than that of the M-WC coating. Moreover, the erosion volume loss of all three coatings at a 90° impact angle is significantly higher than that of the substrate. The study indicates that, at a 90° impact angle, metallic materials exhibit better erosion resistance than WC coatings.
Figure 8 shows a comparison of the erosion wear morphologies of FN-WC, PN-WC, and M-WC coatings. Under an impingement angle of 90°, the erosion wear marks on all three coatings are more pronounced than those observed at an impingement angle of 15°. The erosion wear marks, which appear as furrow-like grooves, are significantly coarser under the 90° impingement angle, indicating that the volumetric material loss during erosion is more severe at 90° than at 15°. At the 15° impingement angle, the erosion furrow marks on the surface of the PN-WC coating are less distinct compared to those on the surfaces of the FN-WC and M-WC coatings; only a few minor furrows can be observed.
Combining the erosion resistance of the three coatings at an angle of attack of 15°, PN-WC exhibits superior erosion resistance, resulting in shallower and narrower grooves on its surface. In contrast, FN-WC and M-WC coatings have lower erosion resistance, leading to more pronounced grooves.
Conclusion
(1) Tungsten carbide coatings with three different WC particle sizes were prepared using the HVOF process. The FN-WC, PN-WC, and M-WC coatings exhibited similar porosity levels, but their phase structures differed. In the FN-WC coating, tungsten carbide showed severe decomposition, whereas in the PN-WC and M-WC coatings, only a small amount of tungsten carbide particles underwent decomposition.
(2) The PN-WC coating exhibits a more uniform microstructure, with slight decomposition of the WC phase. Its hardness distribution is more concentrated, and its bonding strength is slightly higher, resulting in excellent overall performance. At an attack angle of 15°, it demonstrates superior erosion resistance, showing promising application potential in the field of wear and corrosion protection for hydraulic turbine flow components.
(3) The particle size of tungsten carbide is an important factor influencing coating performance, and coatings with excellent performance can be prepared by selecting powders with appropriately sized tungsten carbide particles.
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