key: cord-0066650-rj9pxz4r
authors: Korobov, Yu.; Alwan, H.; Soboleva, N.; Makarov, A.; Lezhnin, N.; Shumyakov, V.; Antonov, M.; Deviatiarov, M.
title: Cavitation Resistance of WC-10Co4Cr and WC-20CrC-7Ni HVAF Coatings
date: 2021-08-13
journal: J Therm Spray Tech
DOI: 10.1007/s11666-021-01242-7
sha: 01cd08f75c8bc0efa5c164a64b63e1c944a044e8
doc_id: 66650
cord_uid: rj9pxz4r

Machines operating in aqueous environments may be subjected to cavitation damage during operation. This study aims to evaluate the cavitation resistance of WC-10Co4Cr and WC-20CrC-7Ni coatings under cavitation erosion conditions with additional electrochemical effects. The coatings were deposited on AISI 1040 steel substrates using a high velocity air fuel thermal spray process. The microstructure of the coatings was observed by a scanning electron microscope, while their phase composition was analyzed using an energy-dispersive microanalysis system. In addition, the microhardness of the coatings and substrate was measured, and the surface topography of the eroded surface layers was observed using a 3D optical profilometer. The results revealed that the cavitation resistance of the WC-20CrC-7Ni coatings was better than that of the WC-10Co4Cr coatings. The observation of the structure and surface topography made it possible to identity the reasons for the differences between the cavitation resistance of both coatings: The WC-20CrC-7Ni coatings had a finer grain structure, lower pore density, and lower as-sprayed surface roughness. These differences, along with the presence of a high Cr and Ni content in the feedstock powder, that increased the coating corrosion resistance, contributed to improving the cavitation resistance and reducing the material loss of the WC-20CrC-7Ni coatings.

Hydro turbine blades, valves, pump impellers, and marine ship propellers are critical components of machines operating in aqueous environments. They are often exposed to cavitation damage during operation (Ref 1). This article is an invited paper selected from presentations at the 2021 International Thermal Spray Conference, ITSC2021, that was held virtually May 25-28, 2021 due to travel restrictions related to the coronavirus (COVID- 19) pandemic. It has been expanded from the original presentation.

A series of small holes or pits are formed in the component's surface due to the frequent formation and collapse of cavitation bubbles (cavities) near the surface, causing material damage ( . High pressure shock waves and liquid micro-jets can be produced during the collapse of cavitation bubbles, resulting in plastic deformation, fatigue, and, consequently, material loss ( . The shock wave pressure emitted during the collapse process can reach as high as 1500 MPa ( Ref 2, 11) . The velocities of liquid micro-jets exceed 120 m/s ( .

One of the most effective ways to combat cavitation damage is covering the surfaces of components with coatings (Ref 1). Various coatings and surface engineering techniques are used to prevent cavitation erosion or reduce it as much as possible. Ceramic-metallic (cermet) coatings are extensively used in thermal spraying to prepare denseprotective coatings with a high level of hardness to serve in a wide range of industrial applications requiring resistance against wear, corrosion (Ref [15] [16] [17] [18] [19] [20] [21] [22] , and cavitation (Ref [23] [24] [25] [26] [27] [28] . Cermet coatings are generally constituted of a combination of hard materials, e.g., WC and Cr 3 C 2 , with one or more matrix metal, e.g., Co and Ni ( Ref 15, 29) .

A matrix with a tungsten carbide, e.g., WC-Co with 12 or 17% Co, has previously been used to improve wear resistance (Ref 21) and increase cavitation erosion resistance ( Ref 30, 31) . Moreover, adding Cr to the matrix, e.g., WC-CoCr, WC-CrCNi, and Cr 3 C 2 -NiCr coatings, can improve the corrosion resistance and reduce the cavitation erosion of these coatings ( Ref 15, 23, 25, [32] [33] [34] . At the same time, the effect of corrosion can reach 50-60% of the total loss of WC-Co-Cr HVOF coating under corrosionerosion conditions ( Ref 35) .

Some studies have been published on the comparative wear behavior of WC-12Co-4Cr and WC-20Cr 3 C 2 -7Ni HVOF coatings. In particular, these coatings were compared by a dry sliding wear test ( Ref 36) . Feedstock powders used in this study were manufactured by agglomeration and sintering. Comparative dry sliding wear tests of WC-12Co-4Cr and WC-20Cr 3 C 2 -7Ni coatings were performed at room temperature, 400°C, 600°C, and 750°C. Of course, the nature of loading during dry sliding wear tests and during cavitation is different. However, the value of the specific load in both cases is of the same level, namely 1050 (Ref 36) and 1500 MPa (see above), respectively. This allows comparing the test results.

At dry sliding wear tests, the hardness of the coatings was roughly the same, 11.2 and 10.7 GPa, respectively, but the rate of wear varies greatly ( Ref 36) . At room temperature, the volumetric wear loss of the WC-20Cr 3 C 2 -7Ni coating was two times higher, which is due to the abrasion of a softer Ni-based matrix by a counterbody. A similar ratio in dry sliding wear conditions at room temperature was shown by Song et al. (Ref 37) . Here, the wear of the WC-20Cr 3 C 2 -7Ni coating was 1.8-3 times higher, depending on the applied load.

At the same time, increasing of the test temperature up to 750°C leads to reverse wear ratio. The WC-20Cr3C2-7Ni coating showed 10 times greater wear resistance compared to the WC-CoCr coating due to the accelerated high-temperature oxidation of the Co matrix ( Ref 36) .

Similar to the case of dry sliding wear at the room temperature, the cavitation resistance of the WC-12Co-4Cr coating in deionized water (neutral media) exceeds this parameter for the WC-20Cr3C2-7Ni coating by a factor of 2.6 (Ref 38). However, in actual conditions (for example, river water, and tap water), the pH of media differs from neutral by 1-1.5 units.

High velocity air fuel (HVAF) spraying has been increasingly used in industry and research over the last decade ( Ref 39) . Past studies have reported higher hardness, density, elastic modulus, fracture toughness, and compressive stresses for HVAF-sprayed coatings in comparison with high velocity oxygen-fuel (HVOF) ones (Ref [40] [41] [42] [43] . Cavitation erosion tests have demonstrated that 86WC-10Co4Cr and Cr 3 C 2 -25NiCr HVAF coatings perform better than HVOF ones ( Ref 21) .

The purpose of this study is to compare the cavitation erosion resistance of WC-20CrC-7Ni and WC-10Co4Cr HVAF coatings in a weakly alkaline environment, which is typical for operating conditions of the critical components of machines operating in aqueous environments.

Commercial agglomerated and sintered thermal spray powders were used as feedstock materials for coating manufacturing. They include TAS WC-340 (further WC-CoCr) and TAS WC-390 (further WC-CrC-Ni) powders (C&M Technologies GmbH, Germany) ( Table 1) . Both powders were deposited on AISI 1040 steel (wt.%: Fe bal.; 0.40 C; 0.28 Si; 0.60 Mn according to our chemical analysis) via HVAF (SB9500-gun, Uniquecoat Technologies, VA, USA). Prior to applying the coatings, the AISI 1040 steel specimens were grit-blasted by steel particles with an average size of 0.3 mm using an air pressure of 0.5 MPa. The HVAF spraying parameters are listed in Table 2 . After spraying, all specimens were ground under the same conditions using diamond grinding wheel to maintain a uniform surface for each. The coating thickness, as sprayed and as ground, was checked using digital coating thickness gauge. Also, cross sections of specimens were made to measure the thickness of coatings according to the standard ISO 1463 ( Ref 44) .

The ultrasonic vibration method (ultrasonic generator with 500 W power and a frequency of 20 kHz) was used as the main tool for evaluating the resistance of the WC-CoCr and WC-CrC-Ni coatings to cavitation erosion. The test specimens were prepared according to the requirements of ASTM G32-10 (Ref 45), as shown in Fig 1(a) . However, the test scheme used in the current work was different from that described in the standard. Usually, the specimen is immersed in a liquid during the test, while in this case, the cavitation effect was achieved due to the contact of the specimen end face with the jet of a test media ( Ref 46) . The liquid supply system included main reservoir (30 litters) located at a height of 1 m relative to the feeding nozzle exit. This provides sufficient head for liquid to flow upward from a 6-mm-diameter nozzle to a height of several millimeters. The liquid drains into a collection tank (not shown) with a water pump maintaining a certain liquid level within the main reservoir and providing continuous circulation of the liquid. The sample, fixed to the end of the vibratory horn, was placed at a distance of 2 ± 0.1 mm above the nozzle exit. Tap water was used as a liquid. Its pH value 7.5 corresponds to a weakly alkaline solution, which brings testing conditions closer to a real operating situation. A voltage of 8.5 V was applied between the test specimen and the nozzle acting as a counter electrode to accelerate the cavitation process (Fig. 1b) . The current resulted from applying this voltage was measured using an ammeter in accordance with the electrical circuit shown in Fig. 1(c) . According to Ohm's law, the resistance was calculated from the measured current and voltage data.

The change in material surface behavior due to test acceleration was not taken into account in this study. However, the damage caused by this combination is referred to as cavitation erosion-corrosion, which contributes to increasing the mass loss significantly (Ref 47, 48) .

The cavitation resistance of the coatings was evaluated as the average result of three tested specimens. The testing duration was 330 min, divided into several periods. The first two periods were 15 min long, while the others were divided into five equal periods (60 min each). Cleaning the test specimens with acetone and drying them in warm air for 30-40 s were repeated before and after each period in order to weigh the specimens via a laboratory mechanical balance (VLR-200, Gosmetr, Russia) with maximum weight of 200 grams and an accuracy of 0.5 mg.

The mass loss of the tested specimens was initially determined, since the cavitation tests were interrupted at period intervals to weigh the specimens. The difference between the initial weight and the measured weight represents the mass loss at each test interval. Due to the two coatings having somewhat different densities, the volume loss criterion was adopted in comparing their cavitation wear resistance. Thus, the mass losses of cavitated specimens were converted into volumetric one according to the following equation:

where V-specimen volume loss; m-specimen mass loss; q-density of material.

The density of steel AISI 1040 was taken as 7850 kg/m 3 (Ref 49), while the density of the coatings was calculated via measuring the mass and volume of the coatings. The volume was measured by immersing the separated coating in the water.

The microstructure of the initial material was observed via a scanning electron microscope (SEM) Tescan VEGA II XMU (Tescan, Czech Republic) with an energy-dispersive microanalysis system (EDS). The WC particle's size of the 

Initial Microstructural SEM Analysis, Microhardness, and Roughness Parameters Figure 2 visualizes the SEM micrographs of the feedstock materials, and Table 1 provides chemical composition and powder particle's size distribution. Both the WC-CrC-Ni and the WC-CoCr powders exhibit the typical spherical morphology of agglomerated and sintered feedstock powders with some surface porosity. The initial coating thickness (as sprayed) was of 380 ± 20 lm, while after grinding, it was of 280 ± 20 lm. Figure 3 shows the cross-sectional images of the initial microstructure of the WC-CoCr and WC-CrC-Ni coatings. Both coatings have dense structures, and no delamination was revealed on the coating-substrate border. However, it is observed that the WC-CrC-Ni coating showed lower (Fig. 3a, b) . The microstructure of the WC-CrC-Ni coating is characterized by a fine WC particle size (no more than 3.4 lm, Fig. 3d) , while the microstructure of the WC-CoCr coating is coarser (WC particles size up to 4.7 lm, Fig. 3c ). The presence of pores, 0.8 ± 0.1% (A3 in Fig. 3c) , can also be seen in the structure of the WC-CoCr coating. Such pores are almost completely absent in the structure of the WC-CrC-Ni coating, less than 0.1% (B4 in Fig. 3d) . Obviously, the difference in particles size and porosity is due to the smaller particle size of the WC-CrC-Ni feedstock powder.

Additionally, because of the increase in the share of chromium carbide, the melting point of WC-CrC-Ni is lower than that of WC-CoCr, namely 1250 versus 1495 C°( Ref 52, 53) . This increases the duration of the WC-CrC-Ni droplet being in the liquid state comparing to WC-CoCr, which creates favorable conditions for extracting gases from the melt and, accordingly, reducing porosity.

In the WC-CoCr coating, blocky tungsten carbide particles with rounded edges (A1 in Fig. 3c) were detected in the CoCr matrix (A2 in Fig. 3c ). The tungsten carbide particles are distributed relatively evenly, and no fractured carbide particles were observed. Luo et al. have shown that the dissolution of WC in the matrix during HVOF/HVAF spraying is strongly reduced when compared to plasma spraying (Ref 54). X-ray analysis (Fig. 4a) points to the dissolution of WC-CoCr coating components into each other and formation of complex carbides (Co, Cr) 2 W 4 C (g-phase). As confirmed by other researchers, g-phases are often contained in coatings with a Co/CoCr matrix (Ref [55] [56] [57] , and g-phase-containing coating withstands corrosive attack better than pure WC-Co ( Ref 58) . The EDS analysis reveals that some tungsten carbides demonstrate a stoichiometric composition closer to W 2 C than WC (A1 in Fig. 3c, Table 3 ). Such decarburization transformation in the structure of WC-CoCr HVOF/HVAF coatings usually occurs because of carbon burning from the WC during the deposition process (Ref 54, 55, 59-61). However, W 2 C was not revealed by x-ray analysis. This is probably because its volume concentration is insufficient for detection by the x-ray method. W 2 C is harder than WC (30 GPa and 24 GPa, respectively), but has a much lower elastic modulus of 420 GPa (compared to 680 GPa in case of WC) (Ref 62). Thus, W 2 C is more prone to degradation by cavitation due to its higher brittleness.

In contrast to the WC-CoCr coating, the WC-CrC-Ni coating does not contain complex carbides of the M 6 C type (g-phase), despite many researchers observe different NixWyC phases, especially Ni 2 W 4 C in the structure of WC-Ni or WC-NiCrBSi composite coatings (Ref [63] [64] [65] [66] . According to the EDS analysis, the structure also contains chromium carbides and decarburized tungsten carbides (B1, B3 in Fig. 3d, Table 3 ), whose stoichiometric composition is similar to W 2 C and Cr 23 C 6 , respectively. As discussed previously, a small amount of decarburized tungsten carbides could form during spraying process, but the phase volume is below the detection limit of x-ray diffractometer. Their existence in the structure of WC-Ni hardfacing coating is also reported in (Ref 67). Despite the fact that the powder contains chromium carbides and the EDS analysis of the WC-CrC-Ni coating also showed a high chromium content in the matrix, it was not possible to obtain a diffraction pattern that would clearly correspond to any of the chromium carbides by x-ray diffraction analysis (Fig. 4b) . This might be probably associated with the complete melting of carbide particles during HVAF, mutual dissolution of powder elements, and extremely nonequilibrium crystallization conditions, as well as residual compressive stresses in the coating. All could significantly affect the lattice parameters of the forming The microhardness and the roughness parameters (Ra and Rt) of the two coatings and AISI 1040 steel are listed in Table 4 . The values shown in the table were measured before the cavitation tests. The microhardness values of both coatings (WC-CoCr and WC-CrC-Ni) were measured on the cross section after grinding for two different loads (HV0.05 and HV0.5). Differences in microhardness at each load are within the measurement error. Higher level of mean hardness at lower loads can be more affected by the location of the indentation.

The electrical current measured during the cavitation test was of 6.5 ± 0.1 lA, and the electrical resistance of the The mass loss increments obtained at the end of each weighing interval during the cavitation tests and the calculated values of volume losses for all tested materials are shown in Table 5 . It can be seen that the coatings WC-CoCr and WC-CrC-Ni are characterized by the same steady-state erosion rate values during the last period of the test, specifically 1.2 mm 3 /h. The overall volume loss for the AISI 1040 steel is about 3.3-3.8 times higher compared to the above coatings. The maximum thickness loss rates (converted by dividing over the nominal area of each eroded specimen), as required by ASTM G32 standard, were the following: 32 lm/h at 210 min, 8 Moreover, the fine structure of the WC-CrC-Ni coating (Fig. 3b) leads to an increase in the specific surface area of carbide particles, and thus, the cracks propagation energy required. This, in turn, increases their resistance against chipping and, therefore, against cavitation in comparison with the coarser WC-CoCr coating. In the same context, Pugsley and Allen (Ref 71) showed that the grain size of coating affects the cavitation erosion resistance, since the erosion resistance increases with decrease in grain size. In terms of solid steel, Bregliozzi et al. have reported that finer grain size in the structure corresponds to greater cavitation resistance due to an increase in the surface density of the grain boundary. This provides a dominant supporting action against cavitation (Ref 72).

In addition, the presence of porosity in cermet coatings significantly lowers cavitation erosion resistance. Comparing the two coatings, the density of porosity (number of pores per unit of area) in the structure of the WC-CrC-Ni (Fig. 3b) is very low, and very small pores (less than 0.5 lm) can be observed. Sugiyama et al. (Ref 73) have reported that porosity affects the cavitation erosion resistance of thermally sprayed cermet coatings, namely the cavitation erosion resistance is inversely proportional to the density of pores. Thus, the WC-CoCr coating is more In addition to the previously discussed factors of increased cavitation resistance of WC-CrC-Ni coatings, the higher ductility of the Ni-based matrix may also have an effect. As known, nickel elongation is 38-42%, while the elongation of cobalt is an order of magnitude lower, approximately 3% (Ref 77). Figure 6 and 7 shows the SEM micrographs and 3D-topography profiles of the eroded surfaces for both coatings and the AISI 1040 steel after 330 min of cavitation exposure. The WC-CoCr coating is characterized by many large and relatively deep cavities, which are located at a distance of 50 lm from each other (Fig. 6a) . One also can see microcrack propagation between the cavities as marked by the arrows in Fig. 6(a) . With regard to the WC-CrC-Ni coating, surface mostly consists of relatively flat areas and separate smaller pores. Microcracking of the WC-CrC-Ni coating tends to be near the voids (marked by arrow-1, Fig. 6b ) and along the borders of flat areas with dimensions exceeding 50 lm (marked by arrow-2, Fig. 6b ). AISI 1040 steel experienced significant extent of failure mainly concentrated at the interfaces in the form of deep cracks network, as shown in Fig. 6 (c).

3D images of the cavities (Fig. 7) confirm the SEM observations. Indeed, examining the microrelief of WC-CoCr coating, one can notice relatively deep pits at the bottom of cavitation craters (Fig. 7a) , while the craters on the WC-CrC-Ni surface have flat bottom (Fig. 7b) . This fact could explain the almost 1.5 times higher values of the average surface roughness (Ra) and maximum peak-tovalley roughness (Rt) of WC-CoCr ( Table 6 ). Detachment of the surface layer of WC-CoCr coating leads to revealing pre-existing porosity under surface, which explains the considerable distinctions in the surface appearance of coatings (Fig. 6a, b) , despite a similar volume loss and wear rate.

AISI 1040 steel exhibits the non-uniform fracture pattern (Fig. 7c ) due to lower cavitation resistance of ferritic phase compared to pearlitic one. Typically, fracture occurs at the interface and is further enhanced by corrosive conditions, resulting in the highest values of Ra and Rt (Table 6 ) in comparison with the two coatings.

The cross-sectional SEM micrographs of the WC-CoCr and WC-CrC-Ni eroded coatings (after cavitation erosion tests) with magnified details are shown in Fig. 8 and 9 . Generally, failure begins to form beneath the surface of the two coatings. With respect to the WC-CoCr coating layer, some visible evidence indicates a greater tendency toward damage. This evidence includes the large damage area, depth, and size of the pits formed beneath the coating surface. Cracks were observed to develop along the boundaries of particles in the near-surface layers of the (Fig. 8a) . When cracks joined and emerged to the surface, the material was detached and a crater was formed at the place of separation (Fig. 8b) .

Regarding the WC-CrC-Ni coating layer surface, the damaged area is 2-3 times smaller, and the pits are smaller by an order of magnitude. It can be observed that the J Therm Spray Tech development of cracks occurs along ''matrix-WC particles'' boundary. This is confirmed by the angular shape of the trajectory of the fractured surface (marked by arrows (Fig. 9a) ). The defects in the coatings act as powerful sites of crack propagation (marked by arrow-1, (Fig. 9b) ). Furthermore, when the developed microcracks connect together (marked by arrow-2, (Fig. 9b) ), the detachment of the coating surface particles occurs.

According to studies of solid superalloys (Ref 69, 70) , it can also be assumed that the increased corrosion resistance of the Ni-Cr matrix also lessened the cavitation damage of the coating, but this should be the subject of further research.

The cavitation tests in weakly alkaline environment revealed that volume loss of the WC-CoCr coating was greater than that of the WC-CrC-Ni coating. The study of the structure, phase composition, and surface topography was combined with the analysis of the results of third-party studies. They showed that the reasons for the lower susceptibility of WC-CrC-Ni coating to erosion cavitation damage comparing to WC-CoCr coating are as follows:

• A finer structure and lower porosity, which leads to the reduction in material loss of this coating and, consequently, reduced cavitation damage; • The lower average values of roughness parameters of the WC-CrC-Ni coating's surface; • In case of the WC-CrC-Ni coating, the hardening of the matrix due to the dissolution of Cr is combined with an increased plasticity of the matrix base. It helps to prevent chipping of the carbides from the matrix. • The combination of Cr and Ni in the WC-CrCNi coating results in improved corrosion resistance. As in the previous paragraph, it helps to prevent chipping of the carbides from the matrix. 

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X-ray Optics and Microanalysis

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