The Influence of Tungsten Carbide Spray Particle Size on Coating Microstructure and Properties

The Influence of Tungsten Carbide Spray Particle Size on Coating Microstructure and Performance The size of WC particles in the original powder directly affects the microstructure and performance of tungsten carbide coatings sprayed onto a substrate. Nanocrystalline tungsten carbide coatings prepared using nanoscale powders exhibit higher hardness and toughness, as well as superior wear resistance, compared to micron-sized 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 purely nanoscale powders. However, there are currently relatively few reports on this topic. Therefore, this paper systematically investigates the microstructure and performance of supersonic flame-sprayed nanoscale, micro-nano composite, and micron-sized tungsten carbide coatings, exploring the impact of the original powder’s WC particle size on coating microstructure and performance. 1 Experimental Setup 1.1 Preparation of Tungsten Carbide Coatings Three types of WC-CoCr powders—FN-WC, PN-WC, and M-WC—with nano-, nano-micron mixed, and micron-sized WC particles were used as spray powders. In the FN-WC powder, aggregated nanoscale WC particles form micron- or submicron-sized agglomerates, which are then bonded together by a binder, as shown in Figures 1(a) and 1(b). The average particle size of these agglomerates ranges from 18.85 to 37.56 μm, with a loose bulk density of 5.48 g/cm³. The morphology of the PN-WC powder is illustrated in Figures 1(c) and 1(d); its cross-sectional microstructure reveals some free nanoscale particles along with predominantly submicron- or micron-sized particles. The average particle size of the PN-WC powder is 23.33–44.79 μm, with a loose bulk density of 5.6 g/cm³. The morphology of the M-WC powder is shown in Figures 1(e) and 1(f); the WC particles in the powder end-face microstructure range from 0.5 to 3 μm in diameter, with an average particle size of 21–50 μm and a loose bulk density of 5.0 g/cm³. XRD diffraction comparisons (Figure 2) and analysis results (Table 1) indicate that the WC particle size in the FN-WC powder is slightly smaller, followed by the PN-WC powder, while the M-WC powder has slightly larger WC particles. Table 1: Half-Width at Half-Maximum of Main WC Peaks Powder FWHM/(°) FN-WC 0.194 PN-WC 0.157 M-WC 0.148 The substrate used was 0Cr13Ni5Mo martensitic stainless steel. Before spraying, the substrate surface was cleaned and sandblasted. The spraying equipment was an HVOF system manufactured by SulerMecto in the United States. Propane was used as the fuel gas, high-pressure oxygen as the oxidizer, and nitrogen as the powder feed gas during spraying. The spraying process parameters are listed 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 Microstructural and Mechanical Property Testing The microstructure of the coatings was analyzed using a FeiQuata400HV scanning electron microscope. Coating porosity was determined according to JB/T75059-1994. The bond strength of the coatings was tested using the paired specimen tensile test method specified in GB/T8642-2002. The bond strength tests were conducted on a CSS-44300 electronic testing machine at a loading rate not exceeding (1000 ± 100) N/s. The microhardness of the coatings was measured using an HDX-1000TMC/LCD microhardness tester under a load of 300 g for 15 seconds; nine points were tested per sample, and the average microhardness value of each coating was calculated. 1.3 Abrasive Wear Testing The abrasive wear tests were performed using a particle erosion wear testing machine similar in structure to the ACT-JP testing machine from Japan. 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 of 15° and 90°. 2 Results and Discussion 2.1 Microstructure of the Coatings Figure 3 shows the SEM images of the cross-sections of the sprayed coatings. The porosity of the three coatings was statistically analyzed at 5000x magnification. The results show that the porosities of the FN-WC, PN-WC, and M-WC coatings were 2.1%, 2.0%, and 2.3%, respectively, indicating comparable porosity levels among the coatings. At higher magnifications, most of the WC particles in the FN-WC coating had diameters less than 100 nm, as shown in Figure 3(a2). Additionally, some flat particle boundaries showed no presence of nanoscale WC particles, as seen in the bright white metallic regions in Figure 3(a2). These regions likely resulted from partial melting of the nanoparticles in the supersonic flame jet, causing the small nanoscale WC particles to decompose at high temperatures. In the PN-WC coating, most of the WC particles had diameters greater than 500 nm, even reaching around 2 μm; however, some nanoscale particles were also present in the coating, as shown in Figure 3(b2). In the M-WC coating, the WC particles were micron-sized, and large WC particles broke during the spraying process, retaining their fractured morphology within the coating. The metal bonding phase between the fragmented tungsten carbide particles was insufficient, as shown in Figure 3(c2). 2.2 Phase Structure of the Coatings Figure 4 shows the XRD analysis results for the FN-WC, PN-WC, and M-WC coatings. After processing, the FN-WC coating exhibited a slightly higher relative peak intensity for the W₂C phase, indicating a higher degree of decomposition during spraying. This result is consistent with the microstructural analysis. Nanoscale tungsten carbide particles tend to decompose easily in the supersonic flame jet; when most of the WC particles in the powder exist as nanoparticles, their decomposition becomes more severe. In the PN-WC and M-WC coatings, most of the WC particles were micron- or submicron-sized and did not easily decompose, resulting in only minor traces of the W₂C phase observed in the XRD patterns. 2.3 Microhardness of the Coatings Coating hardness refers to the material's ability to resist deformation or fracture within a small volume on the surface, and its magnitude significantly influences the coating's wear resistance and erosion resistance. Figure 5 shows the microhardness test results for the three tungsten carbide coatings. The Vickers hardness values of the PN-WC and FN-WC coatings were 1241 HV0.3 and 1254 HV0.3, respectively, slightly higher than the M-WC coating’s hardness of 1229 HV0.3. The hardness of the PN-WC coating was more concentrated. Combined with the microstructural photographs of the three coatings shown in Figure 3, it can be seen that the FN-WC coating contains pure metal areas resulting from the decomposition of WC particles, whereas the M-WC coating has many fragmented WC particles. The uneven microstructures of these two coatings led to dispersed hardness values, while the PN-WC coating had uniformly distributed micron-nano WC particles, resulting in a more concentrated distribution of hardness values. 2.4 Bond Strength of the Coatings Table 3 shows the bond strength of the coatings. The average bond strength of the PN-WC coating was over 73.5 MPa, and the primary fracture during the tensile test occurred within the adhesive layer. The average bond strength of the M-WC coating was comparable to that of the PN-WC coating, and both of these coatings had slightly higher bond strengths than the FN-WC coating. This may be related to oxidation and WC particle decomposition phenomena occurring in the FN-WC all-nanoscale coating during the supersonic flame jet. In the FN-WC all-nanoscale coating, samples 3–5 all fractured within the coating. Table 3: Bond Strength of the Coatings 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 Erosion Resistance Performance The erosion tests were conducted by subjecting the coatings to abrasive particles corresponding to 100 g every 15 seconds, weighing the coatings after each test cycle. After the erosion tests, the cumulative mass loss due to erosion was calculated and converted into volumetric loss caused by coating erosion, using this volumetric loss as a criterion for evaluating erosion resistance. A linear fit was applied to the cumulative volumetric loss data, and the reciprocal of the slope of the fitted line was defined as the erosion resistance coefficient Re. Under the same test conditions, a higher Re value indicates better erosion resistance of the coated material. The erosion resistance results for the FN-WC, PN-WC, and M-WC coatings at a small angle (15°) are shown in Figure 6. Although the FN-WC coating used all-nanoscale WC particles, its erosion resistance did not improve significantly. On the contrary, the PN-WC coating, prepared using a mixture of nano- and micron-sized WC particles, showed a significant increase in erosion resistance. Compared to the M-WC coating, the PN-WC coating prepared with a nano-micron mixed WC particle composition demonstrated markedly improved erosion resistance, with an erosion resistance approximately 1.5 times that of traditional coatings and 2.3 times that of the 0Cr13Ni5Mo substrate. The hard WC phase in the coating is bonded by the CoCr alloy matrix; under real turbine operating conditions—specifically, at small-angle erosion—the metal matrix exhibits poorer wear resistance compared to the hard phase. Figure 6: Volumetric Loss of Coating Erosion at 15° Angle The PN-WC coating contains both large and small WC particles, evenly distributed within the CoCr alloy matrix, as shown in Figures 3(b1) and 3(b2). When abrasive particles erode the coating at a small angle, the metal matrix is easily worn away, exposing the large hard particle phase. As the erosion continues, the CoCr alloy matrix surrounding the large particles is gradually removed, leading to the detachment of these large particles. However, since the PN-WC coating contains small WC particles distributed within the metal bonding phase surrounding the large particles, these small particles can resist the erosion of the abrasive particles, mitigating the erosion of the metal bonding phase and delaying the detachment of the large WC particles. Thus, the PN-WC coating demonstrates superior erosion resistance. The decomposition of WC particles in the FN-WC coating is the main reason for its reduced erosion resistance. In the M-WC coating, the insufficient metal bonding phase between the fragmented small tungsten carbide particles negatively impacts the coating’s erosion resistance. Figure 7 shows the volumetric loss of erosion for the FN-WC, PN-WC, and M-WC coatings at a 90° angle.As can be seen, the erosion volume loss trends of the three coatings are quite similar. The erosion resistance Re 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. Under a 90° impingement angle, the erosion volume loss of all three coatings is significantly greater than that of the substrate. The study indicates that metallic materials exhibit better erosion resistance than WC coatings at a 90° impingement angle. Figure 8 shows a comparison of the erosion wear morphologies of the FN-WC, PN-WC, and M-WC coatings. The erosion wear marks on all three coatings under a 90° impingement angle are more pronounced than those observed at a 15° impingement angle. The furrow-like erosion wear marks appear coarser, confirming that the volume loss during erosion at a 90° impingement angle is more severe than at a 15° angle. At a 15° impingement angle, the erosion furrow marks on the surface of the PN-WC coating are less distinct compared to those on the FN-WC and M-WC coatings; only a few shallow furrows can be observed. Combined with the erosion resistance of the three coatings at a 15° impingement angle, the PN-WC coating, owing to its superior erosion resistance, exhibits shallower and narrower furrows on its surface. In contrast, the FN-WC and M-WC coatings have lower erosion resistance, resulting in more prominent furrow marks. 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 have comparable porosity but differ in their phase structures. The FN-WC coating shows severe decomposition of tungsten carbide, whereas the PN-WC and M-WC coatings exhibit only slight decomposition of tungsten carbide particles. (2) The PN-WC coating has a more uniform microstructure, with minimal decomposition of the WC phase, a more concentrated hardness distribution, and slightly higher bonding strength, giving it excellent overall performance. It demonstrates superior erosion resistance at a 15° impingement angle and holds great potential for application in the field of anti-abrasive damage control for hydraulic turbine flow components. (3) The size of tungsten carbide particles is an important factor influencing coating performance; by selecting powders with appropriately sized tungsten carbide particles, high-performance coatings can be produced. We have accumulated extensive experience in material, equipment, process, and solution development for coating applications, and we are currently replicating these successful cases. We will guide you through the entire coating manufacturing transformation process, ensuring: rapid production start-up; a reliable supply solution covering materials, equipment, and processes; coating trials conducted either on-site at your facility or at our technical center; and consistently high coating quality and efficiency. Start collaborating with us today and reap success tomorrow!

Undertaking on-site construction of lithium-ion battery industry equipment, plasma cladding coatings, and nano-ceramic coatings.

Guangzhou Sanxin Company utilizes American Miller plasma cladding equipment and technology to perform on-site cladding of nano-ceramic coatings, which are suitable for hopper, screw, and ribbon mixing and conveying systems in the lithium-ion new energy industry.

[Coating Preparation] Wear-resistant Coating

Thermal Barrier Coating Preparation Methods

Plasma Spraying—Plasma spraying involves feeding metal or ceramic powders into a high-temperature plasma flame, where the plasma jet heats the spray material to a molten or highly plastic state. Guided by the high-speed plasma jet, these particles rapidly impact the surface of the workpiece. During the spraying process, the spray material first undergoes heating, reaching either a fully molten or semi-molten state; then enters a flight phase propelled forward by the gas stream; and finally strikes the substrate surface with sufficient kinetic energy, resulting in intense collisions that flatten the particles into a thin, planar layer and cause them to solidify instantly. The resulting coating consists of countless deformed particles interlaced and stacked in a wavy, layered structure. There are inevitably some voids or pores between the particles, with porosity typically ranging from 4% to 20%. The coating also contains oxides and inclusions. By employing a high-temperature plasma arc heat source, supersonic speeds, and low-pressure or protective atmospheres during spraying, these defects such as voids and pores can be significantly reduced. Since the coating has a layered structure, its performance exhibits certain directional characteristics. The bonding between the coating and the substrate surface is generally considered to occur in two ways: mechanical bonding, where the flattened particles formed by collision interlock with the irregularities on the substrate surface, creating a mechanical “anchoring” effect; and metallurgical bonding, which occurs when diffusion and alloying take place at the interface between the coating and the substrate, forming intermetallic compounds or solid solutions at the bonding zone. Among these bonding mechanisms, plasma-sprayed coatings primarily rely on mechanical bonding. The performance of the coating is closely related to both the quality of the spray powder and the spraying process itself. Therefore, the treatment of the spray powder is extremely important. The spraying process itself also greatly influences the coating’s performance. If the power is too high during spraying, the coating becomes dense, and improper control of the substrate temperature can lead to residual stresses, causing the coating to peel off and fail. Electron Beam Physical Vapor Deposition (EB-PVD)—In recent years, EB-PVD thermal barrier coatings have been developed. These coatings are produced by heating and vaporizing ceramic sources using a high-energy electron beam, with the ceramic vapor deposited onto the substrate atom by atom. When preparing gradient thermal barrier coatings, this method enables a continuous transition in both structure and composition between the metallic bond coat and the ceramic layer. After subsequent high-temperature treatment, diffusion occurs between the bond coat and the ceramic layer, effectively eliminating the internal interface. The resulting coating microstructure features columnar crystals oriented perpendicular to the substrate surface. The bonding between the columns and the substrate is metallurgical, providing excellent stability. Moreover, at high temperatures, the columnar microstructure demonstrates outstanding strain tolerance, greatly enhancing the coating’s resistance to thermal fatigue. In thermal cycling tests, coating failure is typically caused by cracking within the Al2O3 layer. Additionally, the coating surface is smooth and requires no further machining, the process parameters are easy to control, and the coating is repairable—all of which represent significant advantages compared to thermal barrier coatings prepared by plasma spraying. However, drawbacks such as uncontrollable coating thickness, complex surface cleaning procedures, expensive and sophisticated equipment, relatively low deposition rates, and cumbersome processing procedures still require further research and improvement. Schematic diagram of the EB-PVD principle—EB-PVD columnar crystal structure. Liquid Injection Plasma Spraying—Liquid injection plasma spraying is a promising coating preparation method that has emerged in recent years. Although there are almost no domestic reports on this technique, some exploratory studies have been conducted abroad. The principle behind liquid-injection plasma spraying for thermal barrier coatings is that a zirconium salt solution is drawn out by a conveying motor and, under the action of a carrier gas, passes through an atomizing nozzle before entering the plasma. Within the hot plasma, physicochemical reactions occur, and the material is deposited onto the metal substrate. Conventional thermal barrier coatings prepared by powder injection can withstand approximately 400 thermal cycles, while EB-PVD-prepared coatings can endure around 780 cycles. The new liquid-injection-based thermal barrier coatings can withstand an average of 1018 thermal cycles, significantly improving their thermal cycling performance. The phase structure of the coating mainly consists of a non-transformative tetragonal phase, and no phase transformation occurs even at 1121°C during thermal cycling. The width of cracks increases with the number of thermal cycles. The hardness of the coating initially rises during the early stages of thermal cycling. The columnar crystal structure of the coating remains intact throughout the thermal cycling process. The depth of penetration of the liquid into the plasma nozzle greatly affects the deposition efficiency of the coating. Coating failure primarily occurs within the ceramic top layer, near the interface between the ceramic layer and the bond coat. Overall, thermal barrier coatings prepared by the liquid-injection plasma spraying process exhibit the following characteristics: (1) A unique microstructure: The grain size of the coating is 10–30 nm; it features uniform nanopores and micropores; contains longitudinal microcracks; and lacks layered particle structures or lamellar grain boundaries; (2) The growth of nanocrystals is inhibited; (3) The coating displays excellent thermal shock resistance. Sol-Gel Composite Slurry Hot-Press Filtration Method for Preparing Ceramic Coatings—By using the sol-gel composite slurry hot-press filtration method, we can prepare Al2O3-ZrO2-Y2O3 composite coatings featuring YPSZ particles embedded within an Al2O3-Y2O3 spatial network membrane structure. This approach combines the advantages of both Al2O3-Y2O3 and ZrO2-Y2O3 coatings, achieving superior overall performance. The PYSZ coatings prepared by the hot-press filtration method possess a nano/micro/microporous composite structure, effectively reducing phonon thermal conduction and convective heat transfer, thus delivering enhanced thermal barrier performance. The thermal barrier effect of the coating increases with the increase in sol content within the slurry. In the Al2O3-ZrO2-Y2O3 composite coating, the Al2O3-Y2O3 network membrane can block oxygen ion transport, while the embedded YPSZ helps adjust the thermal expansion match between the coating and the substrate. Meanwhile, the nano/micro/microporous composite structure of the coating facilitates stress relaxation. As a result, the Al2O3-ZrO2-Y2O3 composite coating demonstrates exceptional resistance to high-temperature oxidation and spalling of oxide layers. Materials, Equipment, Processes, and Solutions—We have accumulated extensive experience in coating applications and are currently replicating these successful cases. We will guide you through the entire coating manufacturing transformation process, ensuring: rapid production start-up; a reliable supply solution covering materials, equipment, and processes; coating trials conducted either on-site at your facility or at our technical center; and consistently high coating quality and efficiency. Start collaborating with us today and reap success tomorrow!

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