Characterization of the Structure and Wear Resistance of Ni65-Based Coatings via HVOF Flame Spraying and Its Application to Potato Digging Shovels

Abstract

To improve the tribological properties of a potato digging shovel (PDS), Ni65-based coatings with rare earth oxides additions were fabricated on 65Mn# steel via High-Velocity Oxygen-Fuel (HVOF) flame spraying, the effect of macroscopic surface shape of PDSs on their wear resistance of PDSs was examined, and finally a kind of PDS with a specific macroscopic surface shape and satisfied wear resistance was obtained. The addition of CeO₂ and Y₂O₃ decreased the defects in coatings, refined the microstructure, made hard phases distributed more uniformly and ultimately improved coating properties. According to the XRD analysis results, the Ni65-based coatings were composed of the matrix phase γ-Ni and hard phases formed by Cr, Fe and Ni with B, C and Si. More Cr₇C₃ phases were detected in coating B than in coating A, but the phases related to Y and Ce were also not detected because of the low content in both EDS and XRD analyses. Heat treatment and HVOF flame sprayed coatings both increased the hardness of specimens, and coating A (621HV_1.0) provided a hardness nearly equivalent to that of the heat treatment specimens (617HV_1.0), while coating B provided the highest hardness (664HV_1.0). For all specimens, laser surface texturing (LST) structures weakened their corrosion resistance. However, the Ni65-based self-fluxing alloy coatings significantly improved the corrosion resistance of specimens, and coating B provided the best corrosion resistance. SEM images show that the main wear mechanism for worn specimens was abrasive wear, and less wear signs were observed on the surface of coating B. Abrasive wear examinations indicate that specimen BJ had the best wear resistance and, compared with specimen W, the mass loss of specimen BJ decreased by 28.56% and 20.83% at relative sliding speeds of 2.35 m/s and 3.02 m/s, respectively. However, considering the negative effect of LST structures on the corrosion resistance of specimens, the processing techniques of specimen A and specimen B are more applicable to PDSs. The macroscopic surface shapes affected the wear resistance of PDSs and ZF had the lowest mass loss but the highest draught force; comparatively, YS had a better balance on the draught force reduction and wear resistance. Finally, YS with coating B, which decreased the mass loss by more than 27.17%, is recommended in this paper. On the whole, the conclusions in this paper provide a reference for the design of potato digging shovels with lower draught force and better tribological properties.

1. Introduction

Wear failure accounts for more than 80% of the failure and material consumption of agriculture tools [1,2]. Soil-engaging components, including potato digging shovels (PDSs), inevitably interact with soil particles and suffer from low stress abrasive wear [3,4], which results in more than 50% of the wear loss of soil-engaging components [1,2]. Meanwhile, the pesticides and chemical fertilizer, the oxygen in the air [2], as well as the secretions and body decomposition products of soil creatures [5], always remain on the surface of PDSs and result in corrosive wear, which will accelerate the abrasive wear and ultimately reduce the life and working performance of PDSs. In addition, the replacement of worn PDSs will lead to the increase of time cost [4]. Thus, studies and designs on the wear resistance improvement of PDSs are beneficial to the improvement of their life and working performance, and ultimately decrease the material consumption and the cost of potato production.

The wear behavior of soil-engaging components is affected by their structures, surface morphology, material properties, working conditions and so on. The macroscopic surface shapes and surface morphology of soil-engaging components have an important influence on their interaction properties with soil particles, which helps to decrease the soil adhesion and stress to soil-engaging components and finally reduce the wear loss. In addition, the surface morphology (e.g., pits and grooves) is able to store up or excrete the abrasive particles and wear debris [6], which is helpful for the reduction of surface damage and the wear loss. For example, Zhang [3] fabricated a kind of ridge-shaped subsoiler tine inspired by the surface morphology of pangolin and shellfish, and his results showed that the wear resistance of subsoiler tines were improved by 7.1%–44.0%. However, the effects of macroscopic surface shapes and surface morphology on the wear resistance of PDSs are scarcely reported in the literature.

To further improve the wear resistance of soil-engaging components, surface engineering technologies (e.g., heat treatment, surface modification and surface coatings) accompanied by structure optimizations are recommended. High-Velocity Oxygen-Fuel (HVOF) flame spraying is a kind of widely used surface coating technology that produces a high-velocity (over 1500 m/s) flame [7,8] using high-pressure fuel and a combustion-supporting agent in a combustion chamber or injector. The coating powders are then processed into molten or semi-molten state and accelerated to a velocity of over 300–800 m/s [7,9], and finally high-quality wear-resistant coatings are fabricated on the surface of substrates. Benefitting from the low processing temperature and high spraying velocity, HVOF flame sprayed coatings are characterized by high bonding strength, low porosity, dense and even microstructure as well as low oxidation [9,10,11].

Self-fluxing alloy powders, ceramic powders and composite powders are all applicable to HVOF flame spraying, of which Ni65 powder is one kind of Ni-based self-fluxing alloy powder suitable in abrasive and corrosive situations. Zhong [12] fabricated a kind of Ni65-based coating with TiB₂ additions on Q235 steel via laser cladding, whose hardness was five times more than that of the substrates, and compared with ASTM304 steel, the mass loss of the coating in abrasive conditions decreased by 2/3–4/5. Pang et al. [13] fabricated three kinds of Ni-based coatings and found that the erosion resistance of the Ni65 coating was significantly higher than that of either the Ni67 coating or the Ni60 + 35% WC composite coating. Guo et al. [14] fabricated Ni65-based coatings adding YL10.2 hard particles, and the results showed that the YL10.2 additions with proper size and contents were conducive to improving the hardness of Ni65-based coatings.

Currently, considering the limitation of a single surface engineering technology, composite surface engineering technologies are catching increasing attention. A laser process can replace many traditional process methods because of its high energy density, stable directivity, high efficiency, non-contact and convenient control [8,15], such as laser quenching [16], laser shock hardening [17], laser cladding [18], laser alloying [19], laser ablation [20,21] and so on. For example, after the laser remelting for NiCrCoFeCBSi/WC coating, the hardness increased by 20%–30%, the friction coefficient decreased by over 30% and the wear loss decreased concomitantly [22].

In contrast to soil-engaging components such as subsoilers, the small thickness of PDSs makes them easier to be out of shape resulting from the high processing temperature of some other surface strengthening techniques (e.g., surface cladding and surface welding), which is harmful to the working performance of PDS. Comparatively, HVOF provides high-quality coatings and is not likely to result in the serious deformation of PDSs. Nevertheless, the defects (e.g., the oxidation, pores and inhomogeneous accumulations of coating particles) are inevitable, which are detrimental to the performance of coatings. Thus, the objectives of the study are to (1) fabricate a kind of Ni65-based self-fluxing alloy coating with Y₂O₃ and CeO₂ additions via HVOF flame spraying, (2) prepare biomimetic laser surface texturing (LST) structures on the surface, (3) characterize the surface morphology and material characteristics via a scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis, (4) examine the corrosion and abrasion resistance via neutral salt spray tests and abrasive wear tests, (5) design and fabricate potato digging shovels (PDSs) with different macroscopic surface shapes and coating contents and examine their wear resistance via abrasive wear tests, and finally provide a reference for the design and improvement of potato digging shovels or other soil-engaging components with lower draught force and better tribological properties.

2. Materials and Methods

2.1. Substrates and Preparation of Ni65-Based Self-Fluxing Alloy Coatings

The substrates were 65Mn# steel [23], which is a kind of widely used material for tillage tools. Two kinds of coatings, i.e., coating A(100 wt.% Ni65) and coating B(98.5 wt.% Ni65 + 1 wt.% CeO₂ + 0.5 wt.% Y₂O₃), were prepared on the substrates via HVOF flame spraying. The Ni65 powders with a particle size of 38–106 μm were obtained from Shanghai Xinzuan Alloy Materials Co., Ltd. (Shanghai, China), while the CeO₂ and Y₂O₃ powders with a particle size of not more than 1 μm were obtained from Heibei Chuancheng Metal Materials Co., Ltd. (Xingtai, China). The nominal chemical composition of Ni65 alloy powder is listed in

Table 1. Nominal chemical composition of Ni65 alloy powder (wt.%).

C	Cr	Mo	W	B	Si	Fe	Ni
≥1.0	≥25			3.0~4.5	3.5~5.5	≤14	Bal.

. Powders of coating B were mechanically mixed using a planetary ball mill (QXQM-2, Changsha Tencan Powder Technology Co., Ltd., Changsha, China) for 6 h at a ball-to-powder weight ratio of 10:1 and rotation speed of 500 r/min; and then, these two kinds of powders were dried at a temperature of 150 °C for 2 h using a vacuum drying oven (ZK-35S, Tianjin Sanshui Scientific Instrument Co., Ltd., Tianjin, China). Before the spraying, specimens were ultrasonic cleaned for 10 min using an acetone solution and absolute alcohol successively. This was followed by a sandblasting process, and finally coatings with a thickness of about 400 μm were obtained. The process parameters employed for these two kinds of coatings is listed in

Table 2. Process parameters employed for HVOF spraying process.

Parameters	Coating Type
	A	B
Fuel type	Kerosene
Fuel flow rate (L/min)	0.38	0.35
Fuel pressure (Bar)	8
Oxygen flow rate (L/min)	873
Oxygen pressure (Bar)	11.8
Carrier gas type	N2
Carrier gas flow rate (L/min)	12
Combustor pressure (Bar)	7.5
Spray distance (m)	0.32
Spray gun speed (m/min)	25
Powder feed rate (g/min)	5.5	5

.

Inspired by the wave-like surface structure of pangolin, shellfish and other animals influenced by abrasive wear, after the HVOF spraying process, a laser surface texturing (LST) process utilizing a laser marking machine (FB20-SBGZ, Changchun New Industries Optoelectronics Tech. Co., Ltd., Changchun, China) was conducted using the following parameters: a laser wavelength of 1064 nm, a pulse duration of 100 ns, an average power of 20 W, a pulse repetition rate of 20 kHz, a laser line width of 10 μm and a scanning speed of 500 mm/s. Finally, all of the specimens fabricated in this work and their treatments are listed in

Table 3. Specimens fabricated in this work and their treatment.

Specimen Type	Surface Treatment
W	/
WJ	LST
R	HT
RJ	HT + LST
A	Coating A
AJ	Coating A + LST
B	Coating B
BJ	Coating B + LST

Note: The heat treatment (HT) with low-temperature tempering process after quenching was carried out to obtain a surface hardness of 48–56 HRC [24], which is also a traditional surface strengthening treatment for soil-engaging components made of 65Mn# steel.

.

2.2. Design and Fabrication of Potato Digging Shovels

Our previous research found that macroscopic surface modification was conducive to reduce the draught force of potato digging shovels (PDSs) [25], but the effect of the macroscopic surface shape on the wear resistance of PDSs was still not clear. Therefore, PDSs with three kinds of different macroscopic surface shapes as well as an original plane potato digging shovel (DZ) were designed and fabricated in this study. To save experimental costs, these four kinds of PDSs were made of 45# steel [23]. When it comes to the wear resistance examinations for PDSs with both macroscopic surface shapes and the Ni65-based coatings, 65Mn# steel was used as the substrates. Meanwhile, PDSs with or without heat treatment were also fabricated and employed for comparative study.

Table 4. Nominal chemical composition of the steel of 65Mn# and 45# (wt.%).

Chemical Element	65Mn#	45#
C	0.62–0.70	0.42–0.50
Si	0.17–0.37	0.17–0.37
Mn	0.90–1.20	0.50–0.80
P	≤0.035
S	≤0.035
Cr	≤0.25
Ni	≤0.30
Cu	≤0.25
Fe	Bal.

shows the nominal chemical composition of the steel of 65Mn# and 45#.

As shown in Figure 1a, limited by the size of abrasive wear tester, the overall dimension of DZ in this paper was 150 mm × 80 mm × 5 mm (length × width × thickness), which is half the size of a potato digging shovel (PDS) in practice. A PDS can be divided into three parts (P-a, P-b and P-c) based on its functions: the P-a penetrates and cuts soil, the P-b conveys the soil backward and the P-c guarantees the firm attachment with the potato harvester’s frame, while the dash lines represent the boundary of the three parts mentioned above, as shown in Figure 1. The difference between DZ and the other three kinds of shovels (YS, SS and ZF, see Figure 1b–d) is that the planar structure of DZ’s P-b is replaced by different spatial surface while P-a and P-c remain the same, and the equations of their collimation lines are shown in

Table 5. Equation of the collimation line of PDSs with a spatial surface.

Shovel Type	Equation of the Collimation Line
YS	$\mathrm{y}=-\frac{0.0008}{9}·{\mathrm{x}}^3+0.0102·{\mathrm{x}}^2+0.5645·\mathrm{x}, \mathrm{where} 40 \le \mathrm{x} \le 85$
SS	$\mathrm{y}=-4.2586 \times {10}^{-3}·{\mathrm{x}}^4+1.2736 \times {10}^{-4}·{\mathrm{x}}^3+2.6944 \times {10}^{-2}·{\mathrm{x}}^2-3.0260·\mathrm{x}, \mathrm{where} 5 \le \mathrm{x} \le 30$
ZF	$\mathrm{y}=-1.9893 \times {10}^{-4}·{\mathrm{x}}^4-1.2251 \times {10}^{-2}·{\mathrm{x}}^3+0.2151·{\mathrm{x}}^2+0.8847·\mathrm{x}, \mathrm{where} 0 \le \mathrm{x} \le 25$

.

2.3. Characterization of the Surface

The microstructure and chemical composition of the substrate and coatings were examined using a scanning electron microscope (SEM, JSM-IT500, JEOL Ltd., Tokyo, Japan) equipped with energy-dispersive spectroscopy (EDS), and the SEM images were taken at an acceleration voltage of 15 kV.

The phase compositions of these two kinds of coatings were examined by X-ray diffraction (XRD, DX2700BH, Haoyuan Instrument Co., Ltd., Dandong, China) with Cu Kα radiation at 40 kV, 30 mA, 2θ range of 20–100° and step size of 0.02°.

Microhardness evaluations in the cross-sectional area of the coatings were done using a Vickers microhardness tester (HV-1000, Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd., Shanghai, China) with a 1 kg load for 10 s. The average value of five measurements was taken as the microhardness indicator for each specimen.

2.4. Neutral Salt Spray Tests

The neutral salt spray (NSS) tests were conducted to evaluate the corrosion resistance of the specimens using a salt spray test cabinet (KE-90D, Dongguan Kaier Xinke Instrument Technology Co., Ltd., Dongguan, China) according to ISO 9227:2017 [26]. The tests lasted for seven days and the surfaces of specimens were observed and recorded at 24-h intervals. The size of the specimens was 20 mm × 20 mm × 10 mm (length × width × thickness).

2.5. Abrasive Wear Tests

Abrasive wear tests were conducted on a JMM abrasive wear tester (designed by Jilin University and Chinese Academy of Agricultural Mechanization Sciences) using specimens with a size of 60 mm × 20 mm × 10 mm (length × width × thickness). As shown in Figure 2a,b, the key components in JMM abrasive wear tester include a specimen holder with four specimen mounting positions at 90-degree intervals, a subsoiler, three press rollers, a scraper and a rotary bin. The rev of the rotary bin is controlled by a frequency converter. The distance between the position of the specimens being tested and the spindle of the rotary bin r is 400 mm (see Figure 2a). The depth of the testing specimen in abrasive materials D is 80 mm (see Figure 2c), and the impact angle of the abrasive material against the specimen surface is 35° (see Figure 2d). As shown in Figure 2e, the abrasive materials used in this paper were a mixture of 96.5 wt.% quartz sand (particle size of 0.214–0.420 mm) and 3.5 wt.% bentonite (particle size of not more than 76 μm) [27,28], and the water content of the abrasive materials was 3–5 wt.%. In addition, the abrasive materials were updated for the test of each set of four specimens. The positions of a set of four specimens were changed successively to abrade them in turn at every 803.4 m of sliding distance, and the total wear distance for every set of four specimens was 25708.8 m. The relative sliding speed of abrasive materials against the specimen was considered to be the same as that of the rotary bin, and two different speeds (i.e., 2.35 m/s and 3.02 m/s) were adopted in this paper. The tests were conducted indoors at an air temperature of 16 °C– 18 °C and a humidity of 40%–45%. The wear resistance of the specimens was indicated by their weight loss. On the other hand, considering the difference of the wear surface of different PDSs, the wear resistance of PDSs was indicated by their weight loss per unit wear surface area. Finally, the morphologies of the abraded surfaces were examined by SEM.

3. Results and Discussion

3.1. Characterization of the Surfaces

Figure 3 shows the photo of specimens before the abrasion while the SEM images of specimen WJ, specimen RJ, specimen AJ and specimen BJ are shown in Figure 4. The grooves fabricated via the laser surface texturing (LST) process are clearly visible on the surface of these four kinds of specimens. Owing to the relatively higher and uneven cooling rate, there were some unavoidable pores and cracks on the coating surface of specimen AJ and specimen BJ, and then the melted and deformed materials influenced by laser flowed and filled into these pores and cracks; as a result, compared with those of specimen WJ and specimen RJ, there were less upward and outward accumulated materials on the sides of the grooves. Compared with specimen AJ (see Figure 4c), there are less unmelted particles, cracks and pores on the surface of specimen BJ; hence coating B has a higher quality than coating A. Referring to the reports from other researchers, rare earth oxides, such as CeO₂ and Y₂O₃, can provide surface-active elements that are able to improve the fluidity of melted coating materials, promote the emission of harmful gas, and form stable compounds with harmful elements (e.g., S, P) and emit out of the coating so as to reduce the harmful inclusions [29,30]; consequently, cracks and pores decrease.

As shown in Figure 4 and Figure 5a,b, the grooves with a triangular cross-section were fabricated on the surfaces of specimens (i.e., WJ, RJ, AJ and BJ) via LST process, and the dimensions of the grooves measured in the cross-section are illustrated in Figure 5c. It is clear that the intervals between neighbouring grooves are about 800 μm and there is no significant difference among these four kind of samples. However, things differ when it comes to the widths and depths. The grooves of specimen WJ (about 104 μm wide) and specimen RJ (about 90 μm wide) are much wider than those of specimen AJ (about 49 μm wide) and specimen BJ (about 47 μm wide), but the depths of specimen AJ (about 290 μm deep) and specimen BJ (about 310 μm deep) are slightly higher than those of specimen WJ (about 272 μm deep) and specimen RJ (about 257 μm deep).

The EDS analyses of the surface of specimen W, specimen R, specimen A and specimen B were conducted as shown in Figure 6. It is clear that (1) the Ni-based coatings were successfully prepared on the surface of Fe-based substrates; (2) compared with specimen W, more O was detected on the surface of specimen R because of the oxidation in heat treatment process, and the Si content also increased; (3) compared with specimen W and specimen R, more convergently distributed elements (e.g., C, O, Cr, Si and B) were detected on the surface of specimen A and specimen B at some specific locations, and these elements mainly came from the Ni65 powders, while it is also inferred that hard phases may exist at these locations because these detected elements are the typical elements of hard phases; (4) compared with specimen A, the proportions of C, O, Si and B in coating B increased; (5) finally, Ce and Y were not detected in coating B because of the low additions of CeO₂ and Y₂O₃.

The XRD analysis results for coating A and coating B are shown in Figure 7. It clearly demonstrates that the main phases in the two kinds of coatings both included γ-Ni, Ni₃Fe, (Fe,Cr)₂₃C₆, Cr₂₃C₆, Cr₇C₃, Cr₂B, Fe₂₃ (C,B)₆, Ni₃B and Ni₃₁Si₁₂ that have also been reported by other researchers [31,32]; however, Chen et al. [31] also detected CrB phases in the coatings that may result from the high B content and fabrication technique of the coatings. Accordingly, γ-Ni is the matrix phase while the left phases are typical strengthening phases of Ni-based self-fluxing alloy, and these strengthening phases can provide good hardness and wear resistance for coatings. However, the phases related to Y and Ce were also not detected in the XRD analysis because of the low content and tiny particle size of these rare earth additions, which is similar to the reports from Ghadami et al. [33]. Compared with coating A, the addition of CeO₂ and Y₂O₃ did not significantly affect the phase composition of the coating, but some broad diffused diffraction peaks with relatively lower intensity (e.g., the diffraction peak at a 2θ of about 44.5°) were observed in coating B, which means the formation of more amorphous phases and lattice distortion [11,34], but Ai et al. [32] reported that the addition of Y₂O₃ increased the peak strength of some reinforcement phases. In addition, more Cr₇C₃ phases were detected in coating B, which may result from the increase of C content in coatings, as has been mentioned in the EDS analysis of specimen surface.

Figure 8 shows the cross-sectional images of specimen A and specimen B, and

Table 6. The chemical elements of point Ⅰ, point Ⅱ and point Ⅲ in Figure 8a (wt.%).

Element	Point Ⅰ		Point Ⅱ		Point Ⅲ
	Coating A	Coating B	Coating A	Coating B	Coating A	Coating B
B	5.26	5.51	7.83	3.32
C	5.11	4.71	7.39	4.50	19.31	6.41
O	0.59		1.01		38.02	39.20
Si	4.07	3.78	4.16	4.52
Cr	16.49	16.18	15.58	12.08
Mn	0.17
Fe	3.75	4.19	6.20	3.76
Ni	64.56	65.63	57.83	71.82
Al	100				42.67	40.32
Total	5.26	100	100	100	100	100

lists the chemical elements of point Ⅰ, point Ⅱ and point Ⅲ in Figure 8a via EDS analyses. An obvious interface can be seen between coatings and substrates, and there are also some cracks and pores at the interface or in the coatings that are harmful to the properties and service life of coatings. According to the reports from Liu et al. [35], the formation of these cracks and pores is usually the result of the overheating of the molten pool, gas entrapment, surface impurities, etc. In coating A, vertical cracks perpendicular to the interface were also observed. By contrast, the pores and cracks were significantly less in coating B, which have also been observed on the surface of coatings. In addition, the figure also illustrates that there are many black phases (see point Ⅰ in Figure 8a) and white phases (see point Ⅱ in Figure 8a) scattering in both coating A and coating B, which were also reported by other studies on Ni-based coatings [9,33]. According to the EDS analysis results in Figure 8c that correspond to the XRD analysis results in Figure 7, these phases are mainly hard phases formed by Cr, Fe and Ni with B, C and Si, and a small amount of W-based phases were also detected where white phases are. Compared with coating A, more refined hard phases were observed in coating B. For the phases at point Ⅲ (see Figure 8a), a large proportion of Al and O were detected, but the coatings and substrates do not contain any Al, so these phases may be the sand particles inlaid on the surface of substrates in the sandblasting process.

Studies have shown that surface-active elements provided by CeO₂ and Y₂O₃ are able to form stable compounds that have a high melting point with the elements in the coatings [32,36], and these stable compounds will become the core of heterogeneous phase nucleation so as to form more hard phases. In addition, these surface-active elements will also gather at the grain boundary to inhibit the growth of grains, and finally the microstructure is refined and hard phases are distributed more uniformly, which has been reported by Ai et al. [32]. Therefore, the decrease of pores and cracks as well as the increase of hard phases in coating B can be contributed to the positive effect of the addition of rare earth oxides, as has been mentioned above.

Figure 9 shows the microhardness of substrates and coatings as well as that of specimen W and specimen R, which was measured in the cross-section. As can be observed, the microhardness of specimen W as well as the substrates of specimen A and specimen B varies in a range of 200–240 HV_1.0, and both the heat treatment (HT) and the HVOF spraying process significantly contribute to the improvement of the microhardness. Meanwhile, the average microhardness of coating A is 621HV1.0, which is similar to that of specimen R (617HV_1.0), while that of coating B is 664HV1.0; thus, the addition of CeO₂ and Y₂O₃ is conducive to further improving the microhardness of the Ni65 coating. The decrease of pores and cracks as well as the increase of refined and more uniformly distributed hard phases resulting from the addition of rare earth oxides serve as the key contributors for the increase of the hardness of coating B; in addition, as has been reported by Fan et al. [37], the increase of the content of harder Cr₇C₃ phases also plays a catalytic role.

3.2. Corrosion Behavior

Figure 10 shows the surface change of specimens during the seven-day tests. Corrosion signs were observed on the surface of all specimens except for specimen A and specimen B on the first day. The original spot-shaped corrosion signs appeared randomly on the surface of flat specimens, then spread and finally covered the whole surface; differently, the corrosion signs on the surface of specimens after the laser surface texturing (LST) process tended to be strip-shaped and always originated at edge positions. The LST treated specimens were much more seriously corroded than smooth specimens; according to the reports from Sun et al. [38], the reason for this observation is that LST treatment generated lattice point defects in the coating, which are likely to be the corrosion originations. In addition, the anti-corrosion property of specimens with the Ni65-based coatings is better than that of uncoated specimens (i.e., W, WJ, R and RJ), which is the result of the elements (e.g., Ni, Cr, Mo and Si) in the coatings; meanwhile, benefitting from the refining of the coating phases and the decrease of coating defects due to the addition of rare earth oxides, which led to the decrease of corrosion originations, specimens with coating B had a better anti-corrosion property than specimens with coating A. On the whole, specimen WJ was corroded the most seriously, while specimen B had the best corrosion resistance.

3.3. Abrasive Wear Behavior

Figure 11 shows the specimens after the abrasion. Meanwhile, the SEM images of worn surfaces and mass loss of specimens at different speeds after the abrasive wear examinations are shown in Figure 12. It is very clear that the major surface damage types on the surface of the specimens represented as micro grooves produced by the micro cutting or micro ploughing of abrasive particles and craters produced by the high speed impact of abrasive particles. These phenomena are the typical characteristics of abrasive wear as reported by Li and Kong [39] and Singh et al. [40]. The amount and size of micro grooves and craters depend on the surface hardness, and so larger amounts of longer, wider and deeper micro grooves or craters were observed on the surface of specimen W because of its low hardness. In addition, there existed some flake debris or deformed lips with different sizes and amounts along the grooves and craters, which is the severe plastic deformation of coating materials resulting from the repeated interactions with abrasive particles. These observations were similar to the reports by Nath and Kumar [41] in their studies on the erosion behavior of HVOF sprayed WC–10Co–4Cr cermet in slurry wear tests. For the HVOF flame sprayed coatings, owing to their high hardness, fatigue fracture occurred and bigger spalling pits were observed, especially along the LST structures, which resulted from the spallation or brittle fracture of the hard phases in coatings as reported by Singh et al. [40] as well as Nath and Kumar [41]. Moreover, some unmelted particles became visible after the material ruptures beside the LST structures. Therefore, the major wear mechanism of tested specimens was abrasive wear.

As shown in Figure 12i, compared with specimen W/R/A/B, the mass loss of specimen WJ/RJ/AJ/BJ decreased, which indicates that the LST structures were conducive for improving the abrasion resistance of specimens. On the one hand, similar to the studies on the wear behavior of LST treated cemented carbide reported by Tong et al. [42], nonsmooth structures, such as grooves and pits, are able to store up or excrete the abrasive particles and wear debris; on the other hand, these nonsmooth structures can change the motion state of abrasive particles and even form a vortex layer, which stimulates more sliding abrasive particles to roll on the surface; thus, the contact probability and intensity between abrasive particles and surfaces decrease, and finally the surface damages and abrasions decrease. Figure 12i also illustrates that the increase of the relative sliding speed of abrasive materials against specimens resulted in the significant increase of the mass loss of specimens, according to the studies by Halila et al. [43] and Mattetti et al. [44]. The main reason for this phenomenon is that the contact stress between surfaces and abrasive particles increases as the relative sliding speed increases [44,45]. At these two different relative sliding speeds, the mass loss of coating B was lower, which indicates that the abrasion-resistant coatings were fabricated on substrates and the addition of rare earth oxides helped to further improve the abrasion resistance of the Ni65 coating, and these are corresponding to the SEM observations. As has been mentioned above, compared with coating A, the addition of rare earth oxides helped to form more hard phases, refine the microstructure, reduce the defects in coatings, make hard phases distributed more uniformly and finally improve the dislocation slip resistance, which decreased the severe deformation of materials [32,45]. Thus less abrasion signs were observed on coating B. The specimen with the best abrasion resistance was specimen BJ, and compared with specimen W, the mass loss of specimen BJ at relative sliding speeds of 2.35 m/s and 3.02 m/s decreased by 28.56% and 20.83%, respectively. However, considering the negative effect of LST structures on the corrosion resistance of specimens, the process techniques for specimen BJ were not recommended in this study. Comparatively, the mass loss of specimen B at relative sliding speeds of 2.35 m/s and 3.02 m/s decreased by 19.96% and 15.66%, respectively, while the mass loss of specimen A at relative sliding speeds of 2.35 m/s and 3.02 m/s decreased by 13.32% and 11.34%, respectively, which also exhibited great abrasion resistance. However, the wear loss reduction rates in this paper are lower than those in the studies on the abrasion behavior of Fe-based overlaid ploughshares in field tests by Singh et al. [40], which may result from the discrepancy in coating contents and fabrication techniques. Considering the different application situations, however, the process techniques for specimen B and specimen A without LST structures were applied to PDSs afterwards.

Figure 13 shows the comparison of PDSs before and after the abrasion. It can be seen from Figure 13a that the macroscopic surface shapes have an influence on the abrasion resistance of PDSs. The change of the macroscopic surface shapes of PDSs made soil mass easier to be broken and prevented some positions of PDSs from being seriously abraded, which reduced the abrasion intensity and thus reduced the wear loss of PDSs. Meanwhile, after the abrasion, the surface of DZ-W (DZ without heat treatment) became darker while the surfaces of DZ-R (DZ with heat treatment), YS-A (YS with coating A) and YS-B (YS with coating B) became brighter after the removing of surface materials (see Figure 13b). In addition, some positions of PDSs in Figure 13a as well as DZ-W and DZ-R in Figure 13b were slightly corroded because of their weak corrosion resistance.

It can be seen from Figure 14a that the mass loss of PDSs follows the order: DZ > SS > YS > ZF, and thus ZF has a macroscopic surface shape that enables it to possess the best abrasion resistance. However, a good PDS should also be characterized by low draught force, and so the working process of these four kinds of PDSs was simulated using the analysis model reported in our previous study [25]. The simulation results showed that the draught force of DZ, YS, SS and ZF was 7.99 kN, 6.40 kN, 6.65 kN and 8.58 kN, respectively. Thus the abrasion resistance of YS with coating A (YS-A) and coating B (YS-B) as well as DZ with (DZ-R) and without (DZ-W) heat treatment were compared. As shown in Figure 14b, YS with coating B (YS-B) exhibited the best abrasion resistance, and compared with DZ, the mass loss of YS-B decreased by more than 27.17%. Nath and Kumar [41] fabricated HVOF sprayed WC–10Co–4Cr cermet on the plane martensitic stainless steel, but only reduced the wear loss by not more than 25%, which is slightly smaller than the data in this paper. Hence macroscopic surface modification will further improve the wear resistance of tools. Consequently, the process techniques for YS-B were recommended for adoption in practice.

4. Conclusions

In this paper, Ni65-based self-fluxing alloy coatings with rare earth oxides additions via HVOF flame spraying and LST structures were fabricated on 65Mn# steel. Meanwhile, the effect of macroscopic surface shape on the wear resistance of PDSs was also examined. Finally, a kind of PDS with a macroscopic surface shape and satisfied wear resistance was obtained. The following conclusions can be drawn:

(1) The addition of CeO₂ and Y₂O₃ decreased the defects in the coatings, refined the microstructure, made hard phases distributed more uniformly and consequently improved coating properties.

(2) In the EDS analyses, more C, O, B and Si were detected in coating B than coating A. According to the XRD analysis results, the Ni65-based coatings were composed of the matrix phase γ-Ni and some strengthening phases such as Ni₃Fe, (Fe,Cr)₂₃C₆, Cr₂₃C₆, Cr₇C₃, Cr₂B, Fe₂₃(C,B)₆, Ni₃B and Ni₃₁Si₁₂; and compared with coating A, more Cr₇C₃ phases were detected in coating B; in addition, the phases related to Y and Ce were also not detected in both EDS and XRD analyses because of the low content.

(3) Heat treatment and HVOF flame sprayed coatings both increased the hardness of the specimens, and coating A (621HV_1.0) provided a hardness nearly equivalent to that of heat treatment specimens (617HV_1.0), while coating B provided the highest hardness (664HV_1.0).

(4) For all specimens, LST structures weakened their corrosion resistance, but the Ni65-based coatings significantly improved the corrosion resistance of specimens and coating B provided the best corrosion resistance.

(5) Based on the SEM images, the main wear mechanism for worn specimens were abrasive wear, and less wear signs were observed on the surface of coating B because of the improvement of its coating quality. Abrasive wear examinations indicate that specimen BJ exhibited the best wear resistance, and compared with specimen W, the mass loss of specimen BJ decreased by 28.56% and 20.83% at relative sliding speeds of 2.35 m/s and 3.02 m/s, respectively. But considering the negative effect of laser surface texturing structures on the corrosion resistance of specimens, the processing techniques of specimen A and specimen B are more applicable to PDSs.

(6) The macroscopic surface shapes affected the wear resistance of PDSs and ZF exhibited the lowest mass loss but the highest draught force. Comparatively, YS had a better balance on the draught force reduction and wear resistance. Finally, YS with coating B, which decreased the mass loss by more than 27.17%, was recommended in this paper.

On the whole, conclusions in this paper provide a reference for the design of potato digging shovels or other soil-engaging components with lower draught force and better tribological properties. Finally, due to the limitations of this study and the effects of COVID-19, further work will be necessary in order to explore the corrosion behavior of Ni65-based coatings with rare earth oxides additions via an electrochemical test and to characterize their corroded surfaces via SEM analysis; in addition, chemical treated LST structures as well as the field application of YS with coating B will also be further examined.