Effect of Nano-La₂O₃ and Mo on Wear Resistance of Ni60a/SiC Coatings by Laser Cladding

Abstract

To improve the wear resistance of the TMR blade and investigate the effect of nano-La₂O₃ and Mo on the wear resistance of laser cladding coating. 65Mn blade as the substrate, La₂O₃, Mo and Mo-La₂O₃ composite powders were added into Ni60a/SiC composite powder. Using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and the CFT-I surface synthesizer, the phase composition, element distribution and friction and wear properties of the coating were analyzed to obtain the best composition of the composite coating. The results showed that the wear resistance of Mo-La₂O₃-Ni60a/SiC composite coating was the best. The coating was analyzed by X-ray diffraction and X-ray photoelectron spectroscopy. The coating contained hard phases such as CrB, CrC and Cr₇C₃, and the element distribution was uniform. It can be seen from the scanning electron microscope that the addition of nano-Mo and La₂O₃ improves the toughness and compactness of Ni60a/SiC composite coating, and the microstructure is refined. The friction coefficient of Mo-La₂O₃-Ni60a/SiC composite coating is 0.5, and the wear depth is 12.35 μm, 23% and 89% lower than that of 65Mn substrate, respectively. The surface roughness of the Mo-La₂O₃-Ni60a/SiC coating after wear is 2.06 μm, and the wear amount is 0.001 g. The wear mechanism of the coating is mainly adhesive wear, abrasive wear, and oxidation wear. The wear surface of the Mo-La₂O₃-Ni60a/SiC composite coating is mainly composed of micro furrows, accompanied by the generation of new wear-resistant layers.

1. Introduction

Wear is one of the main failure modes of agricultural machinery parts [1]. For example, a key component of a total mixing ration (TMR) forage mixer for kneading and cutting agricultural materials is the 65Mn blade. The blade surface is damaged via wear after long use, which seriously affects the working efficiency and service life of a TMR forage mixer [2,3,4].

Laser-cladding technology uses a high-energy-density laser beam to melt and solidify the cladding material on the substrate surface [5]. Laser absorption and other indicators have a large influence on the quality of the cladding [6,7]. Laser-cladding coated substrates significantly improve material savings, processing accuracy, surface quality and mechanical properties compared to conventional manufacturing [8]. A matrix with a laser-cladding coating provides improved mechanical properties and wears resistance. Thus, laser-cladding technology has been applied in industry and agriculture [9,10,11]. Following the emergence of composite materials in recent years, laser-cladding materials have gradually changed from single alloys and ceramics to composite materials [12]. In the composites, both rare earth oxides (La₂O₃, CeO₂ powder) and Mo powder can improve the microstructure and morphology of the laser cladding coating, refine the grains, and improve the microstructure and toughness [13,14]. In the two composite coatings, Yuan et al. [15] used 45CrNi steel as the substrate and added Mo and CeO₂ to the Ni-based alloy by laser cladding technology, respectively, to study the microstructure and abrasive wear properties of the coating. The results showed that the coating’s microstructure toughness and wear resistance were improved by adding Mo to the Ni-based alloy, and the strengthening effect of Mo on the abrasive wear properties of the coating was better than that of CeO₂. Wenhu Li et al. [16] prepared the composite powder of Mo-Si-B and La₂O₃ on the metal surface by liquid-liquid (L-L) doping, mechanical alloying (MA), and hot-pressing (HP) sintering technology to improve the wear resistance of 1173 K metal. The relationship between wear resistance and the load was analyzed. The results showed that the formation of MoO₃ film on the worn surface changes the contact state of the friction pair. The friction coefficient of Mo-Si-B alloy decreases and the wear rate of the alloy increases with the test load.

However, the processing technology is too complex. Houan Zhang et al. [17] combined self-propagating high-temperature synthesis (SHS) and hot pressing (HP) to prepare La₂O₃-WSi-MoSi₂ composite cladding layer on the surface of alloy steel. The wear resistance of MoSi2 against steel is significantly improved by adding both WSi₂ and La₂O₃, and it is attributed to the increase in hardness and toughness of the composite. Li et al. [18] used Mo-25Si-8.5B alloy as the basic formula to explore the effect of La₂O₃ content on its hardness and friction, and wear properties. The results showed that the La₂O₃ content affected the hardness of Mo-25Si-8.5B alloy coating. When the La₂O₃ content was 0.9%, the hardness and friction resistance of the composite coating was the best. Jing et al. [19] melted Ni-Mo-Si composite powder on AISI 1045 steel surface by laser cladding but only explored the oxidation resistance of the surface.

The research on the improvement of the wear resistance of the composite coating by rare earth La₂O₃ and Mo is extensive, but the treatment process is complex. Through the laser cladding technology, there are few studies on the influence of nano-La₂O₃ and Mo powder on the wear resistance of Ni60a/SiC composite powder coating. In this paper, the effects of the optimal addition of 2 wt.% nano-rare-earth La₂O₃ and 3.5 wt.% nano-Mo powder on the microstructure and wear resistance of Ni60a/SiC composite powder cladding layer were analyzed by laser cladding technology.

2. Materials and Methods

2.1. Materials

A steel 65Mn was cut into 60 mm × 34 mm × 5 mm specimens using an EDC wire cutting machine (ZTE CNC machine tool plant, Shenzhen, China) as a matrix for laser-cladding. The element composition of the 65Mn blade was analyzed using a direct reading spark spectrometer (FOUNDRY-MASTER PRO, Oxford, UK), as listed in

Table 1. Elemental composition of 65Mn blade.

Material	Element Composition (wt.%)
	C	Mn	Cr	Si	Ni	Cu	P	S	Fe
65Mn	0.189	1.034	0.224	0.272	0.014	0.1782	0.011	0.0112	Bal

. Before laser-cladding, the cladding surface was polished using #80, #120, #240, #360 and #800 sandpaper. We select Ni60A (45~106 µm; purity > 99.9%) as the basic powder of the cladding layer, SiC (70 μm, purity > 99.9%), La₂O₃ rare earth powder (40 nm, purity > 99.9%) and Mo powder (50 nm, purity > 99.9%) as the reinforcing phase, and the matching parameters are shown in

Table 2. Proportioning scheme of laser cladding powder.

Sample	Name
65Mn	Y1
80 wt.% Ni60a + 20 wt.% SiC	Y2
78 wt.% Ni60a + 20 wt.% SiC + 2 wt.% La2O3	Y3
76.5 wt.% Ni60a + 20 wt.% SiC + 3.5 wt.% Mo	Y4
74.5 wt.% Ni60a + 20 wt.% SiC + 2 wt.% La2O3 + 3.5 wt.% Mo	Y5

. The ND-PM-1L ball mill (Nanjing Nanda Instrument Factory, Nanjing, China) was used to mix the powder evenly. The speed of the ball mill was 400 r/min, and the mixing time was 4 h. For ease of expression, each mixed powder and corresponding coating is labeled in Table 2.

2.2. Laser-Cladding Parameters

Laser-cladding experiments were carried out under maximum power of 5 kW of a CO₂ laser-cladding system (Jiangsu Zhufeng Optoelectric Technology Co., Ltd., Nantong, China). Figure 1 shows the schematic diagram of the laser cladding. The process parameters of the laser cladding are shown listed in

Table 3. Parameters for laser cladding.

Process Parameters	Operation Range
Laser power (W)	2000
Scanning speed (mm/min)	400
Spot diameter (mm)	3
Overlap rate (%)	45
Defocus distance (mm)	10
Powder feeding rate (g/min)	8
Protecting gas (Ar, L/min)	8

. The cladding dimensions of the 65Mn matrix length × width = 40 mm × 20 mm, the micro-hardness of the coating was measured by a micro-hardness tester. The test load was 0.98 N, and the retention time was 10 s. The average value of five points was measured in the same area.

2.3. Wear Test

Friction and wear experiments were performed using a CFT-I surface comprehensive tester (China Kehua Science and Technology Development Co., Ltd., Fujian, China). Before the wear test, surface polishing of the 65Mn base cladding layer was sequentially carried out by an MP-2B metallographic sample grinder (Yantai Huayin Test Instrument Co., Ltd., Yantai, China) using #80, #120, #240, #400, #800, #1000 and #2000 round gold coherence abrasive paper. A JPS-20B ultrasonic cleaner was used to oscillate for 6 min at 40 °C. The solution was anhydrous ethanol (concentration ≧ 99.7%). In the wear test, the loading force was 30 N, the rotating speed was 300 r/min, and the loading time was 30 min. The rotating friction mode was adopted, and the rotating radius was 3 mm. The wear amount was measured using a JM-B5003 electronic balance (Grade Ming Weighing and Checking Equipment Co., Ltd., Shenzhen, China) with an accuracy of 0.001 g. Three experiments were performed on each sample to reduce errors.

2.4. Microscopic Morphology Characterization

Metallographic samples were intercepted along the direction perpendicular to the processing of the cladding samples. The samples were ground, polished and corroded (HF:HNO₃:H₂O = 1:3:46). Field emission scanning electron microscopy (SEM) and three-dimensional surface topography (Nanovea ST400, NANOVEA, Irvine, CA, USA) were used to investigate the wear scar and polished sections of the coating, It enables the testing of material surfaces from nanometres to millimetres in terms of roughness, depth and other indicators, with high measurement accuracy. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical and valence compositions of the coating. The phase composition of the coating was determined by X-ray diffraction (XRD). The scanning speed was 3°/min, and the scanning angle was 10° to 90°.

3. Results and Discussion

3.1. Micro-Hardness and Phase Analysis of the Coatings

Figure 2a shows the micro-hardness of the sample, and the hardness of 65Mn substrate is 608 HV. the hardness of the cladding layer on 65Mn substrate is improved by laser cladding technology. The surface hardness of Y2, Y3, Y4 and Y5 are 870 HV, 991 HV, 914 HV and 1285 HV, respectively. The hardness of the cladding layer is higher than that of the Ni60a/SiC cladding layer after adding nano-La₂O₃ and Mo, which indicates that nano-La₂O₃ and Mo refine the grains of the Ni60a/SiC composite cladding layer. The refinement of the grains is beneficial to the improvement of the hardness of the cladding layer, and the ability of nano-La₂O₃ to inhibit grain growth is better than that of Mo powder. The hardness of the Ni60a/SiC composite coating reached 1285 HV when the nano-La₂O₃ and Mo powder were mixed, which indicates that the growth of the crystal is further inhibited, and the hardness is significantly improved.

Figure 2b shows that the formation of Y2, Y3, Y4 and Y5 coatings is accompanied by a variety of new phases, indicating that the composite powder is decomposed and ablated at high temperatures of laser cladding. CeO₂, CrB, CrC and FeNi3 are formed in the coating as primary phases, and CeO₂ and CrC have no obvious diffraction peaks. Y3 and Y5 are added with nano-La₂O₃, and the rare earth lanthanum is stable in the oxide coating. Mo powder, Si, S, and other elements formed MoSi2 and MoS compounds in the coating added with nano-Mo powder. The addition of nano-La₂O₃ and Mo powder improved the microstructure of the coating and are beneficial to the refinement of other grains. At high temperatures, the coating recrystallization occurred, and the elements were combined with various intermetallic phases [20]. Cr₇C₃ and Cr3Ni2SiC are the hard phases of secondary precipitation in the coating. Under the same laser cladding parameters, the diffraction peak intensity of Cr₇C₃ in the Y5 coating is the largest, related to the precipitation of Cr₇C₃ in the coating. Combined with the CrC phase in the primary phase, the formation of the hard phase of the coating is conducive to improving the hardness of the composite [21]. The affinity between C and Cr is strong, and CrC and Cr₇C₃ phases result from an in-situ reaction between Cr and graphite in the molten pool [22]. Studies have shown that carbides precipitated at grain boundaries can improve the wear resistance of materials [23].

3.2. Wear Test Analysis

Figure 3 is the data diagram of the friction coefficient and wear amount of coating samples, and Figure 3c is the wear schematic diagram. From Figure 3a,b, it can be seen that the friction coefficient of Y1 jumps greatly within 30 min and does not tend to a stable value. The friction coefficient changes between 0.5 and 0.8, and the average friction coefficient is about 0.65. After 15 min running-in stage, the friction coefficient gradually tends to be stable at around 0.5. The running-in stage shows that the wear layer has been destroyed and tends to be stable after 15 min. The friction coefficient of Y2 is 23% lower than that of Y1.

It can be seen from Figure 3b that the friction coefficients of Y3 and Y5 are stable around 0.46 and 0.63 within 30 min, respectively, and there is no long wear running-in stage, indicating that the wear failure of the wear layer is relatively light. The friction coefficients of Y3 and Y5 are reduced by 29% and 3% compared with Y1, respectively. The friction coefficient of Y5 has a noticeable jump within 30 min, which indicates that the wear layer of Y5 was varied during wear failure, and the average friction coefficient is about 0.5. The friction coefficient of Y5 is reduced by 23% compared with Y1. Figure 3d shows the wear analysis of the sample. The average wear amount of Y1 is 0.021 g, and the wear amounts of Y2, Y3, Y4 and Y5 are 0.003 g, 0.002 g, 0.002 g and 0.001 g, respectively. It can be seen that the wear amount after laser cladding is significantly reduced. Combined with Figure 2, it can be seen that the reduction of wear amount is related to the generation of the hard phase of the coating, and the wear amount of Y5 is the least.

3.3. Microscopic Wear Morphology Analysis

Figure 4a,e shows the three-dimensional morphology and SEM images of the wear scar of the coating sample. Combined with the data in Figure 4f, it can be seen that the surface roughness of the Y1 coating sample after wear is 23.43 μm, and the average wear depth is 110.06 μm. It can be seen from the observation of the wear scar (Figure 4a) that there are serious cracks and deep grooves on the worn surface, a serious drawing phenomenon and no new wear-resistant layer. The analysis of laser cladding coating shows that the wear scar depth and surface roughness under the coating is significantly reduced. The surface roughness of Y2, Y3, Y4 and Y5 are 3.93 μm, 2.20 μm, 3.41 μm and 2.06 μm, respectively. The surface roughness of Y3 and Y5 is the lowest. The average wear depths of Y2, Y3, Y4 and Y5 are 19.21 μm, 15.34 μm, 15.79 μm and 12.35 μm, respectively. The average wear depth of Y5 is the smallest, and the wear depth of Y5 is reduced by 89% compared with Y1.

Figure 4b,e shows that the wear surface of the coating is significantly improved compared with that of 65Mn substrate. The wear surface of Y2 appears delamination, and with the emergence of a new wear-resistant layer, the wear surface of Y3 appears to have abrasive grains and grooves. Combined with Figure 3b, the generation of abrasive particles reduced wear to a certain extent, so the friction coefficient decreased. The wear surface of Y4 appeared to be coating cracking, and the coating peeled obviously. The instability of the wear-resistant layer caused an increase in the friction coefficient. The wear surface of Y5 appeared to have abrasive particles and furrows, but the wear of the coating was lighter and accompanied by the emergence of new wear layers. Combined with Figure 3b,d, the low friction coefficient of Y5 is related to the surface roughness of the worn surface and the formation of a new wear-resistant layer, which leads to a decrease in wear depth.

Figure 4 shows that the wear width of Y1 is the largest and Y5 is the smallest. Serious fatigue and mechanical wear occur on the wear surface of Y1, and the wear resistance is poor. In contrast, the wear surface of the sample containing the coating is significantly improved. Combined with Figure 2, the formation of the coating hard phase is one of the reasons for the improvement of wear resistance. The Y2 wear surface is mainly adhesive, and Y3 has significant abrasive wear. There are abrasive particles on the wear surface, generally generated under cyclic stress, and most of the abrasive particles are oxides. In the friction heat environment, CeO₂ abrasive particles can be retained on the interface as a lubricant, forming a wear-reducing layer under cyclic stress, reducing the further wear of the surface and reducing the friction coefficient [24]. The delamination phenomenon caused by fatigue wear occurs in Y4, and the wear surface of Y5 is dominated by micro-furrows. Based on the above analysis, La₂O₃ improves the wear resistance of Ni60a/SiC coating better than Mo powder, and Y5 shows the best comprehensive performance and wear resistance.

3.4. Elemental Analysis of Coating

Figure 5a shows that the content of C and Mn in the wear scar of the 65Mn matrix is 58.43% and 22.38%, and hard phase elements Si, Cr and Ni are 2.08%, 0.04% and 13.52%, respectively. In the coating sample, the content of hard phase elements is significantly increased. The contents of Si in the wear scars of Y2, Y3, Y4 and Y5 are 7.59%, 7.23%, 9.70% and 5.39%, respectively. The contents of Cr are 15.92%, 19.65%, 14.44% and 15.83%, respectively. Combined with Figure 2, Figure 3 and Figure 4, hard phase elements could reduce the friction coefficient and wear amount of the wear surface, improve the wear state of the wear surface, and improve the wear resistance of the coating. Compared with the element content in the wear scar and unweared parts of the coating (Figure 5d,f,h), the Si content in the unweared parts of Y3, Y4 and Y5 is 7.28%, 7.38% and 7.20%, respectively, and the Cr content is 21.12%, 29.77% and 28.54%, respectively. The element content was higher than the wear area, indicating that the coating element was slightly shifted when the wear occurred. Compared with the change in the coating’s oxygen (O) content, the content of unweared Y3, Y4 and Y5 is 6.46%, 1.69% and 2.66%, respectively. And oxygen content is 11.69%, 22.50% and 23.11%, respectively. The oxygen content increased obviously, indicating the oxidation reaction occurred in the wear layer, in which CeO₂ as oxide abrasive particles had the effect of lubricant. Combined with Figure 3 and Figure 4, the friction coefficient of the Y5 surface is small, and the change of wear surface was small.

To analyze the influence of nano-rare earth La₂O₃ and Mo on the Ni60a/SiC composite powder cladding layer, the element distribution in the cross-section of Y2, Y3, Y4 and Y5 coatings is observed (Figure 5i). It can be seen from the figure that the coating is rich in Cr, Ni and Si, which is the main element of the cladding powder. Compared with the Y2 composite powder coating, the composite coating with nano-rare-earth La₂O₃ and Mo is dense without obvious cracks and holes, indicating that nano-rare-earth La₂O₃ and Mo can promote the uniform refinement of microstructure and promote the uniform distribution of Cr, Ni and Si elements in the coating, indicating that nano-rare-earth La₂O₃ and Mo can improve the fluidity of Cr, Ni and Si elements in the coating. The ionic radius of La³⁺ is large, which can effectively hinder the transfer of substances [13]. The distribution of Si and Ni in Y2 coating with nano-Mo is uniform, and the Cr element is partially vacant. Compared with Y2 coating, the distribution of elements in Y3 and Y5 coating with rare earth La₂O₃ is more uniform, and the mutual infiltration of Cr and Si elements with the substrate promotes the better combination of the coating and the substrate, which is beneficial to improve the comprehensive performance of the coating.

To further understand the effect of nano-La₂O₃ and Mo powder on the Ni60a/SiC composite cladding layer and analyze the composition and chemical state of the coating, XPS analysis was carried out on Y3, Y4 and Y5. From Figure 6, it can be seen that the coating contains basic hard phase elements such as Cr, Ni and Si. On the Cr_2p orbital, the binding energies of Cr elements are in the range of 570~590 eV. The valence states of Cr elements in the three coatings are composed of multiple binding energies, and the difference is small. Y3, Y4 and Y5 contain Cr₂O₃, and the binding energy is about 575.6 eV. Cr₂O₃ is easy to form, which is related to the high affinity between O and Cr and the low Gibbs free energy of Cr₂O₃ [25,26]. Compared with the change of the binding energy of nano-La₂O₃ and Mo compounds in the coating, the 3d orbitals of La in Y3 and Y5 have multiple linear spectral splittings (Figure 6a,c). This is because La mostly exists in the form of compounds. Combined with the binding energy diagram of O1s, the binding energy of La and O1s is low, about 528.6 eV, and the La3d_5/2 orbit does not show obvious diffraction peaks. The binding energy of the La3d_5/2 orbit in Y3 is 837.9 eV, and that of the La3d_5/2 orbit in Y5 is 836.7 eV. There is a significant deviation in the binding energy.

The binding energy of La3d_3/2 orbits of Y3 and Y5 is around 852.0 eV. After multiple line spectrum splitting, the difference in the binding energy of La3d_3/2 orbits in Y3 is 2.7 eV, and that of La3d_3/2 orbits in Y5 is 3.2 eV. The diffraction peak of Mo in Figure 6b is observed, and the photoelectron peak of Mo is very weak. On the other hand, the diffraction peak of the Mo3d3/2 orbit is relatively high, and the binding energy is 231.1 eV.

In the Ni2p orbit, Y3, Y4 and Y5 have obvious spin-orbital splitting and have satellite peaks. The binding energy of Ni2p_3/2 orbit in Y3 is 851.8 eV and 854.6 eV. The binding energy of Ni2p_3/2 orbit in Y4 is 852.4 eV and 855.4 eV. The binding energy of Ni2p_3/2 orbit in Y3 is 852.1 eV and 854.6 eV. The binding energy of Ni2p_1/2 orbit in Y3, Y4 and Y5 are 870.5 eV, 869.7 eV and 868.3 eV. The binding energy difference between the Ni2p_3/2 orbit and Ni2p_1/2 orbit is 18.7 eV, 17.3 eV and 16.2 eV. It can be seen from the binding energy that Ni exists in the oxide film in the chemical state of Ni²⁺. On the 2p orbit of Si, the binding energies of Y3 at 103.2 eV, 101.9 eV and 99.6 eV correspond to the orbits of CeO₂, Si3N4 and Si2p3/2, respectively. The binding energies of Y4 at 102.5 eV, 101.0 eV and 99.4 eV correspond to the orbits of CeO₂, SiC and Si2p_3/2, respectively. The binding energies of Y5 at 101.9 eV and 99.7 eV correspond to the orbits of CeO₂ and Si2p_3/2, respectively. Compared with Y3, Y4 and Y5, Si3N4 and SiC were not found in Y5, indicating that Si₃N₄ and SiC gradually replaced CeO₂, which promoted the anti-wear performance of Y5 coating. Combined with Figure 2, Y3, Y4 and Y5 coatings had hard phase and anti-wear elements. Compared with Y3 and Y4, the anti-wear CeO₂ content of Y5 coating increased, and the improvement of wear resistance of Y5 coating was closely related to the decrease of friction coefficient and CeO₂ content.

4. Conclusions

(1) Ni60a/SiC, Mo-Ni60a/SiC, La₂O₃-Ni60a/SiC and Mo-La₂O₃-Ni60a/SiC coatings produced hard phases such as CrC and Cr₇C₃ and lubrication CeO₂ phases under the same laser cladding process. Compared with Ni60a/SiC coating, the coating with nano-La₂O₃ and Mo powder is accompanied by the generation of new phases such as FeNi3, MoSi2 and Cr3Ni2SiC, which significantly improved the structure of the coating and improved the hardness and wear resistance of the coating.

(2) Compared with the 65Mn substrate, the coating samples show better wear resistance, and the Mo-La₂O₃-Ni60a/SiC coating had the best wear resistance. In the same test conditions, the friction coefficient is 0.5, and the wear depth is 12.35 μm, 23% and 89% lower than the substrate, respectively. The surface roughness of the Mo-La₂O₃-Ni60a/SiC coating after wear is 2.06 μm, and the wear amount is 0.001 g, a slight wear failure occurred on the coating surface.

(3) The wear mechanism of 65Mn substrate is fatigue wear and mechanical wear. The wear surface with the coating is mainly adhesive wear, abrasive wear, and oxidation wear. Adding nano-Mo powder and rare earth La₂O₃ improves the compactness of the coating. Mo powder enhances the toughness of the coating microstructure. Rare earth La₂O₃ inhibits the growth of grains and refines the microstructure. Mo-La₂O₃-Ni60a/SiC coating shows the best wear resistance, and the wear surface is dominated by micro furrows.