Microstructure, Wear and Corrosion Properties of Inconel 718-CeO₂ Composite Coatings

Abstract

Based on laser cladding technology, six composite coatings with different amounts of Inconel 718 and 0~5% CeO₂ were successfully prepared on the 316L stainless steel substrate. The effect of different amounts of CeO₂ particles was investigated and discussed, such as microstructure, phases, elemental distribution, microhardness, wear resistance and corrosion resistance. The results show that the phases are composed of γ~(Fe, Ni), Ni₃Nb, (Nb_0.03Ti_0.97)Ni₃, and MC_X(M = Cr, Nb and Mo). When the amount of CeO₂ particles is higher than 1%, some Ce₂O₃ compounds can be detected in coatings. The average microhardness values of N0~N5 are 604.6, 754.5, 771.6, 741.4, 694.5 and 677.3 HV_0.2, respectively. There is a trend that the microhardness increases firstly and then decreases, because an appropriate amount of CeO₂ can improve the solid solution strength. The average wear rate values of N0~N5 are 2.97 × 10⁻⁵, 1.22 × 10⁻⁵, 0.94 × 10⁻⁵, 1.53 × 10⁻⁵, 1.81 × 10⁻⁵ and 2.26 × 10⁻⁵ mm³∙N⁻¹∙min⁻¹, respectively. The N2 coating has the smallest corrosion current density of 2.05 × 10⁻⁴ A·cm⁻², which is about 56% of the N0 coating. When the amount of CeO₂ particles is 2%, the coating has the best wear resistance and corrosion resistance due to fine grains and Cr, Nb and Mo compounds.

1. Introduction

The 316L stainless steel, a low-carbon austenitic stainless steel, exhibits exceptional resistance to oxidation, corrosion and elevated temperatures. It is commonly used in transportation, medical devices, petrochemical machinery and nuclear power plants [1,2,3,4,5]. In the field of nuclear power plants, it is used as light water reactors, piping, pressure vessels and heat exchangers due to its excellent formability, high-temperature strength and corrosion resistance. However, 316L stainless steel is single-phase austenitic, which has low surface hardness and poor wear resistance. This could cause failures or accidents during service [6,7,8,9]. For instance, components in petrochemical machinery frequently fail due to wear, which not only increases maintenance costs but also negatively impacts production quality. Therefore, a good surface property is essential for reducing operational failures and ensuring personnel safety.

Some surface modification techniques have been used to enhance the mechanical and chemical properties of steel, such as thermal spraying [10,11], electroplate [11,12] and laser cladding [13,14]. Laser cladding technology is widely adopted due to its ability to produce coatings with low dilution rates, superior performance, and metallurgical bonding to substrates. Ertugrul et al. [15] studied the effect of different powders and grinding times. It showed that a large number of TiC particles were attached to 316L particles after grinding for two hours, which was helpful to refine grains during solidification and generate some carbides. Abouda et al. [16] prepared some high chromium steel coatings on the 316L substrate, which found that the Cr₂₃C₇ and Cr₂₃C₆ particles were uniformly distributed. Pejakovic et al. [17] developed the TiCN particle to reinforce the 316L coating, which found that the uniform distribution of TiCN particles increased the wear resistance extremely. At the same time, some researches show that the rare earth elements were used to obtain good properties [18,19,20,21]. Shu et al. [21] used CoFeCrNiSiB and CeO₂ to make coatings. When the amount of CeO₂ particles increased from 2% to 4%, the wear resistance and corrosion resistance of the cladding coatings decreased. Chen et al. [22] also used Fe-based alloy powder and CeO₂ to prepare coatings, which found that the addition of CeO₂ could increase the nucleation of particles and further refine the microstructure. With the increase in CeO₂, the microhardness and wear resistance of the coating first increased and then decreased. Therefore, the proper quantity of rare earth elements will enhance the qualities of the cladding coating. As a result, it is worth studying the effect of varying CeO₂ in Inconel 718 composite coatings.

In this paper, all six composite coatings with different amounts of Inconel 718 and 0~5% CeO₂ were fabricated on the 316L stainless steel substrate with laser cladding technology. The microstructure, phase and element distribution were observed. And the wear resistance and corrosion resistance were also tested and analyzed.

2. Experimental Materials and Methods

2.1. Materials

The chemical composition of 316L stainless steel is included in

Table 1. Chemical composition of 316L stainless steel (wt.%).

Element	Fe	C	Mn	S	P	Ni	Si	Mo	Cr
Amount	Bal	≤0.08	≤2.00	≤0.04	≤0.03	10.00~14.00	≤1.00	2.00~3.00	16.00~18.50

, which is used as the substrate. The size of the substrate is 50 × 30 × 10 mm. The amounts of Cr, Ni and Mo elements are 16.00~18.50%, 10.00~14.00% and 2.00~3.00%, respectively. Before the experiment, the substrate was mechanically ground with diamond sandpaper of grit sizes (No. 400, 600, 800 and 1200), followed by ultrasonic cleaning with absolute ethanol for 5 min, and final drying with a heat gun.

The laser coating material is Inconel 718 alloy powder (10~50 μm). The detailed chemical composition of this alloy powder is included in

Table 2. Chemical composition of Inconel 718 alloy powder (wt.%).

Element	Ni	Si	Mo	Cr	Co	Ti	Nb	Fe
Amount	Bal	0.35	2.80~3.30	17.00~21.00	1.00	0.65~1.15	4.75~5.50	14.15

. The alloy powder was dried before the experiment. At the same time, the particles of rare earth CeO₂ (50~60 μm) were mixed with Inconel 718 alloy powder. Before the experiment, the alloy powder was ground in a ball mill machine for 2 h. Then it was put on the surface of 316L substrate with 1 mm thickness.

2.2. Experimental Scheme

The experimental scheme is that the amount of CeO₂ is 0~5%. Figure 1 shows the laser cladding system. This system is mainly composed of the CNC system, CO₂ laser, laser nozzle, input gas and water-cooling system. In this system, a high-energy laser beam is generated in a mixed gas box. In order to ensure equipment security, a water-cooling system is employed to cool the equipment. The laser nozzle, controlled by a CNC system, follows multi-trajectory paths to achieve different laser cladding directions. The laser processing parameters employed in this study have been previously optimized and validated [23,24]. The laser power is 1600 W. The scanning speed is 2 mm∙s⁻¹. The spot diameter is 3 mm. The overlap rate is 30%.

2.3. Experimental Methods

Before the experiment, the substrates were mechanically ground using diamond sandpaper of grit sizes (400, 600, 800 and 1200 meshes) and polished for 30 min. Then they were cleaned by ultrasonic cleaning with absolute ethanol for 5 min. A DL-HL-T2000 CO₂ laser system was employed for the laser cladding process. All six composite coatings were fabricated. Then the samples were sectioned into cubic specimens measuring 10 × 10 × 10 mm³ in order to repeat tests more than 3 times. Subsequently, the sample was mechanically polished to obtain a similar test surface followed by chemical etching using aqua regia (HCl:HNO₃ = 3:1). Microstructural characterization was performed using a scanning electron microscope (TESCAN MIRA, TESCAN, Brno, Czech Republic). The elemental distribution was analyzed by energy dispersive spectrometer. The phases were detected by TD-3500 X-ray diffractometer (Dandong Tongda Science and Technology Co., Ltd., Dandong, China). The microhardness of the cladding layer was tested repeatedly 3 times to obtain the average value. The microhardness tester was HXD-1000TMC/LCD (Shanghai Optical Instrument Co., Ltd., Shanghai, China). The load was 200 gf for 15 s. The MGW-02 high-speed reciprocating fatigue machine (Jinan Yihua Tribology Testing Technology Co., Ltd., Jinan, China) was used. The frequency and loading load were 2 Hz and 10 N. The electrochemical measurements were performed with a CHI600E electrochemical workstation. The potential range was −1.6~0.8 V. The scan rate was 0.01 V∙s⁻¹.

3. Results

3.1. Phase Analysis

Figure 2 shows the macroscopic morphologies of Inconel 718 cladding coatings with different CeO₂ contents. The N0 coating (without rare earth oxide) exhibits a smooth and clean surface. In contrast, the coatings N1–N5 (with CeO₂ addition) display a whitish suspended layer on their surfaces. Due to the high melting point (about 2600 °C) and low density (7.65g/cm³) of CeO₂, part of the unmelted CeO₂ particles floated on the coating surface during rapid solidification [25]. As shown in Figure 2a–d, coatings N0–N3 demonstrate relatively flat macroscopic morphologies with distinct laser tracks, indicating high cladding quality. However, Figure 2e,f reveal that coatings N4 and N5 develop numerous protrusions.

Figure 3 shows the XRD pattern of Inconel 718 cladding coatings with different amounts of CeO₂ particles. It can be seen that N0~N5 coatings have the similar peak angle and are all mainly composed of the γ~(Fe, Ni), Ni₃Nb, (Nb_0.03Ti_0.97)Ni₃, and MC_X(M = Cr, Nb and Mo) phases. Therefore, the different amounts of CeO₂ particles do not influence the main phases. In the N0~N5 coatings, the diffraction peak intensity of γ~(Fe, Ni), (Nb_0.03Ti_0.97)Ni₃ and MC_X(M = Cr, Nb, Mo) first decreases and then increases at about 43°. And the Ce₂O₃ phase appears in the N2~N5 coatings, because when the amount is enough, CeO₂ will decompose and generate some new oxides (Ce₂O₃) due to the high energy from laser beam. The following chemical reactions will occur in the molten pool.

$$Ce{O}_2\to [Ce]+2O$$

(1)

$$3Ce{O}_2+[Ce]\to 2C{e}_2{O}_3$$

(2)

3.2. Microstructure

Figure 4 shows the top microstructure of Inconel 718 cladding coatings with different amounts of CeO₂ particles. In Figure 4a, the microstructure of the N0 coating is mainly composed of dendrites and equiaxed grains. The thermal behavior during laser cladding influences the grain structure of the coating. The equiaxed grains are more likely to form when the G/R ratio is low [26]. In Figure 4b, when the CeO₂ particles are added to the coating, the number of dendrites reduces significantly. Some coarse compounds appear in the N1 coating. In Figure 4c, the N2 coating shows no dendrites but some small irregular gray clumps, which demonstrates a uniform phase distribution. In Figure 4d, the gray clumps in the N3 coating increase significantly and distribute in the dark gray background area. In Figure 4e, the number of gray lumps in N4 coating decreases, which has a large size and distributes unevenly. In Figure 4f, the size of irregular gray clumps in the N5 coating increases further. It can be seen that the addition of CeO₂ significantly alters the coating’s microstructure, promoting the formation of coarse compounds and a gray lump. Furthermore, the gray lump and compounds in the N1–N5 coatings can effectively impede dislocation motion, thereby enhancing coating performance [27]. When the amount of CeO₂ is 3%, it has the best positive effect on the refinement of the coating microstructure and uniform distribution. When the amount of CeO₂ is more than 4%, the crystal size will grow nonuniformly. When the amounts of CeO₂ are 4% and 5%, more CeO₂ particles will decrease the fluidity of liquid metal, because the melting point of CeO₂ is about 2600 °C.

Figure 5 shows the middle microstructure of Inconel 718 cladding coatings with different amounts of CeO₂ particles. In Figure 5a, the microstructure of the N0 coating without CeO₂ is mainly composed of dendrites, but the dendrite arms are relatively thick. In Figure 5b,c, the microstructure of the N1 and N2 coatings is more refined and distributes uniformly. When CeO₂ particles are added to the coating, the number of dendrites is reduced significantly due to the nuclei with a high melting point. In Figure 5d–f, when the amount of CeO₂ particles is up to 3%, the grains will grow again and form some large compounds in the N3~N5 coatings. Meanwhile, there are an increasing number of white substances precipitated in the N3~N5 coatings, which is continuous mesh or dispersed granular.

In order to investigate the white substance, the point scanning method was used to detect the elemental distribution in the N5 coating. The selected P1~P4 points are shown in Figure 6.

The elemental distributions of P1~P4 are included in

Table 3. Amount of elements at P1~P4 points (%).

Elements	P1	P2	P3	P4
Fe	17.97	14.03	11.31	32.52
Ni	34.16	32.67	37.25	41.96
Cr	13.25	14.68	15.36	19.94
Mo	6.98	7.56	6.74	2.36
Nb	25.48	28.56	26.74	1.65
Si	1.12	1.52	1.48	0.75
Mn	1.04	0.98	1.12	0.82

. In the N5 coating, the average amount of the Nb element is 26.93% in the white substance, which is higher than that in the gray mass (1.65%). The average amount of Mo element (7.09%) is also higher than that of the gray mass (2.36%). The average amount of Fe element (14.43%) is significantly lower than that of the gray mass (32.52%). The white substance enriched with Nb and Mo elements is usually Laves phase, which has strong electron reflection ability and shows bright white under electron microscope. The precipitation of Laves phase is due to the enrichment and segregation of alloying elements, such as Nb and Mo. The Laves phase is a brittle and hard phase, which needs more Nb and Mo elements. Therefore, it will result in the decrease in hard phases and weaken the mechanical properties of the coating.

Figure 7 shows the microstructure of the binding zone of Inconel 718 cladding coating with different amounts of CeO₂ particles. The substrate and coating have an obvious boundary and a good metallurgical bond. When CeO₂ is not added, the structure of the binding zone of the N0 coating is composed of coarse columnar crystal and cellular crystal. In Figure 7b,c, the grains are small and distribute uniformly. In Figure 7d, the coarse columnar grains and Laves phase appear in the N3 coating. In Figure 7e,f, the structure is mainly composed of irregular chunks dendrite in the binding zone of the N4 and N5 coatings.

Combined with the XRD result, when the amounts of CeO₂ particles are 0%, 1% and 2%, the grain size is small in the N0~N2 coatings. The corresponding diffraction peak intensity (43°) is lower. When the amounts of CeO₂ particles are 3%, 4% and 5%, the crystal size is large in the N3~N5 coatings. The corresponding diffraction peak intensity (43°) is higher.

3.3. Elemental Distribution

In order investigate the elemental distribution, the EDS line scanning was used. The irregular clumps were selected to obtain the main elements in the compounds in the N1 and N5 cladding coatings as shown in Figure 8.

Figure 9 is the EDS line scanning result of irregular lumps in the N1 and N5 coatings; the elemental distribution is similar. The amount of Fe and Ni elements is very low, which is lower than that in the background region. The Cr, Mo and Nb elements are enriched in the irregular clumps. Combined with the results of XRD analysis, the irregular clumps are hard phases of MC_X(M = Cr, Mo and Nb). Because Inconel 718 powder is an alloy with a high amount of Cr element, more Cr compounds will form in the coating due to the high temperature. During this process, Nb and Mo elements will also be attracted by the C atom and form the MC_X (M = Cr, Nb and Mo) compounds. When the CeO₂ was added, some Cr, Mo and Nb carbides appeared increasingly in the coating.

3.4. Microhardness

Figure 10 shows the cross-section microhardness of Inconel 718 coatings with different amounts of CeO₂ particles. The microhardness curve can be divided into three regions, the coating, binding zone and substrate. There is a decreasing trend between the three regions. The microhardness values of the N0~N5 coatings are 604.6, 754.5, 771.6, 741.4, 694.5 and 677.3 HV_0.2, respectively. Among them, the maximum microhardness of the N2 cladding coating is 797.9 HV_0.2, which is 4.2 times higher than that of the substrate. The change of the N0 coating microhardness is small due to the lower amount of CeO₂, which produced fewer strengthening compounds. The values of the N1~N5 coatings microhardness are higher than that of the N0 coating without rare earth CeO₂ particles. It can be seen that the addition of CeO₂ can improve the microhardness of the Inconel 718 composite coating. With the increase in CeO₂, the microhardness of the coating increases first and then decreases.

When the amounts of CeO₂ particles are 1~3%, the grains are fine in the N1~N3 coating, which has the high average microhardness. When the amounts of CeO₂ particles are 4 and 5%, the grains are large in the N4 and N5 coatings, which make the microhardness decrease. The reason is that there is a negative correlation between the microhardness and the grain size. According to the Hal-Petch Formula (3), if the grain is fine, the strength and microhardness will be high. Therefore, the average hardness of the coating shows a trend of first increasing and then decreasing.

$${\sigma }_{\mathrm{y}}={\sigma }_o+k{d}^{-{\displaystyle \frac{1}{2}}}$$

(3)

where ${\sigma }_y$ is the yield limit of the material; ${\sigma }_0$ is the lattice friction resistance; $k$ is a constant and $d$ is the average grain size.

3.5. Friction and Wear

Figure 11a shows the friction coefficient of Inconel 718 coatings with different amounts of CeO₂. The friction coefficients of the N0~N5 coatings are 0.302, 0.154, 0.126, 0.169, 0.224 and 0.245, respectively. Along with the addition of CeO₂, the friction coefficients also have a trend of increasing first and then decreasing. The N2 coating has the lowest friction coefficient (0.126). Figure 11b shows the wear rate of Inconel 718 cladding coatings with different amounts of CeO₂ particles. The wear rates of N1~N5 coatings are lower than that of the N0 coating (2.97 × 10⁻⁵ mm³∙N⁻¹∙min⁻¹). With the increase in CeO₂, the wear rate of the coating decreases first and then increases, which is consistent with the change of the friction coefficient. Among them, the wear rate of the N2 coating is the lowest, which is 0.94 × 10⁻⁵ mm³∙N⁻¹∙min⁻¹, which is only about 1/3 of the N1 coating. After the addition of CeO₂, the grains become fine, which plays a key role in resisting the friction and wear. In addition, the precipitation of the Laves phase also has a decreasing trend. More Cr, Mo and Nb carbides appear and increase the anti-friction wear strength, but the Laves phase intensifies when the addition of CeO₂ is more than 3%, which increases the wear rate.

Figure 12 shows the friction and wear micromorphology of 718 cladding coatings with different amounts of CeO₂. The N0~N5 coatings have the same type of adhesive wear and different degrees of spalling on the surface. Along with the increase in CeO₂, the spalling area decreases first and then increases. Among them, the N0 coating has more spalling and deeper furrows due to the low microhardness as shown in Figure 12a. In Figure 12b, the spalling area of the N1 coating decreases and has shadow furrows. After the addition of CeO₂, the coating grains are refined. The hard phase distribution is more uniform, which can resist the ploughing effect better. In Figure 12c, the peeling area of the N2 coating continues to decrease. This coating has the maximum microhardness, and the scratches left by the friction pair are shallow. It also has the best wear resistance. In Figure 12d,f, the peeling area of the N3~N5 coating gradually increases, because excessive CeO₂ will make the grains become larger again in the coatings. It not only reduces the microhardness but also results in the lower wear resistance. Therefore, it can be seen that the N2 coating has the lowest friction coefficient, the lowest wear rate and the best wear resistance. The appropriate CeO₂ amount can improve the wear resistance of cladding coating.

3.6. Electrochemical Corrosion

Figure 13 shows the polarization curve of Inconel 718 cladding coatings with different amounts of CeO₂ particles in the 3.5% NaCl solution. The N0~N5 coatings have two curves and obvious passivation regions.

The electrochemical corrosion potential (Ecorr) and corrosion current density (Icorr) are analyzed by the Tafel extrapolation method. The values are shown in Figure 14. The corrosion potentials of the N0~N5 coatings are −0.512, −0.537, −0.495, −0.553, −0.573 and −0.738 V, respectively. The corrosion current densities of the N0~N5 coatings are 3.67 × 10⁻⁴, 3.57 × 10⁻⁴, 2.05 × 10⁻⁴, 5.76 × 10⁻⁴, 8.94 × 10⁻⁴ and 6.42 × 10⁻⁴ A∙cm⁻², respectively. The N2 coating has the highest corrosion potential and the lowest corrosion current density, which shows the better performance of the corrosion resistance. The addition of CeO₂ particles can reduce the precipitation of Laves phase and enrich the Cr, Mo and Nb elements in the cladding coatings [28]. The Cr, Mo and Nb compounds can improve the corrosion resistance of the laser cladding coating further.

4. Conclusions

• All six Inconel 718/CeO₂ coatings are prepared successfully and have good metallurgical bonding with the 316L substrate. The phases of the N0~N5 coatings are similar, including the γ~(Fe, Ni) solid solution, Ni₃Nb, (Nb_0.03Ti_0.97)Ni₃, MC_X(M = Cr, Nb, Mo) and others. When the amount of CeO₂ particles is greater than 1%, the Ce₂O₃ phase appears in the N1~N5 coatings. The background region of the N0~N5 coatings is γ~(Fe, Ni) solid solution. The MC_X carbides distribute among the γ~(Fe, Ni) solid solution.
• The values of the average microhardness of the N0~N5 coatings are 604.6, 754.5, 771.6, 741.4, 694.5 and 677.3 HV_0.2, respectively. Along with the increase in CeO₂, the values of the average microhardness increase first and then decrease. When the amounts of CeO₂ are 1% and 2% in the N1 and N2 coatings, the Laves phase decreases. When the amount of CeO₂ is more than 3%, some big compounds and Laves appear more than the N2 coating in the coatings. And the formation of Laves phase will consume more Cr, Mo and Nb elements in the coating, which will reduce the number of carbides.
• The wear rates of the N0~N5 coatings are 2.97 × 10⁻⁵, 1.22 × 10⁻⁵, 0.94 × 10⁻⁵, 1.53 × 10⁻⁵, 1.81 × 10⁻⁵ and 2.26 × 10⁻⁵ mm³∙N⁻¹∙min⁻¹, respectively. With the increase in CeO₂, the wear rates of the N0~N5 coatings increase first and then decrease. The N2 coating has the lowest wear rate due to the uniform phase distribution. The corrosion potential of the N2 coating is −0.495 V, and the corrosion current density is 2.05 × 10⁻⁴ A∙cm⁻². It has the highest corrosion potential and the lowest corrosion current density due to the lower precipitation of the Laves phase and more Cr, Mo and Nb compounds, which shows the better performance of the corrosion resistance. In future studies, it will be beneficial to study a detailed amount of CeO₂, such as 1.5%.