Microstructure and friction-wear performances of laser-cladded nano-CeO 2 –reinforced NiCoCrAlY coatings at high temperature

Abstract

Nano-CeO₂–reinforced NiCoCrAlY coatings were fabricated on Ti6Al4V alloy using laser cladding. The morphologies, chemical compositions, and phases of obtained coatings were analyzed using a scanning electron microscope, energy-dispersive spectrometer, and X-ray diffraction, respectively, and the effects of nano-CeO₂ mass fraction on the coefficient of friction and the wear rate of NiCoCrAlY coating at 600 °C were investigated using a ball-on-disk wear test. The results show that the nano-CeO₂–reinforced NiCoCrAlY coating is mainly composed of NiTi₂, Ti₃O, CoO, and β-Ti phases, while the new phases of AlTi₃ and (Ni, Co)₂Ti₄O are formed after the wear test. The average coefficients of friction of NiCoCrAlY coatings with the nano-CeO₂ mass fractions of 0%, 2%, 4%, and 6% are 0.699, 0.655, 0.636, and 0.615, respectively, and the corresponding wear rates are 4.04 × 10⁻⁷, 3.95 × 10⁻⁷, 3.13 × 10⁻⁷, and 2.35 × 10⁻⁷ mm³ N⁻¹ m⁻¹, respectively, which decrease with the increase of nano-CeO₂ mass fraction. The wear mechanism is primary adhesive wear and oxidation wear, accompanied by slight abrasive wear, and the addition of nano-CeO₂ is the main factor that is enhancing wear resistance.

Keywords

Laser cladding NiCoCrAlY coating coefficient of friction wear rate wear mechanism

Introduction

As a (α+β)-type structure,¹ Ti6Al4V alloy accounts for 50% of the total amount of titanium alloy, and its wear resistance is poor because of low hardness,^2,3 which further limits its use under friction conditions.⁴ As an excellent thermal barrier, MCrAlY (M = Ni, Co, or combination of these) coatings have been widely used at high temperatures to elevate the friction wear performance of Ti6Al4V alloys.⁵ The addition of CeO₂ improved the oxidation resistance of high-velocity oxygen fuel (HVOF)-sprayed MCrAlY coating;⁶ especially, CeO₂ in the nano form has the advantages of large specific surface area and high chemical activity and still retains the unique characteristics of rare-earth elements,⁷ which is used as a reinforced phase to add into the NiCoCrAlY coating.⁸ It has the advantage over traditional coatings at high temperatures because of their hardness and wear resistance.^9–11

At present, thermal spraying is usually used to fabricate NiCoCrAlY coating,¹² and the irregular oxide layer grows on the coating surface,¹³ which decreases its wear performance. As a new kind of coating technology, laser cladding (LC) prevents the formation of the oxide layer under the protection of Ar gas.^14–16 Chun et al.¹⁷ prepared the NiCoCrAlY/ZrB₂ coating on the pure Ti substrate using an LC and found that the ZrB₂ improved the wear resistance of the substrate at high temperatures; Hao et al.¹⁸ investigated the wear resistance of NiCoCrAlYTa coating on the Inconel 718 substrate, which was improved by the post-oxidized to form the oxidative layer; Meng et al.¹⁹ analyzed the laser-cladded NiCoCrAlY coating by adding Ni-based powder, which reduced its wear rate; Pereira et al.²⁰ prepared NiCoCrAlY and CoNiCrAlY coatings on AISI 304 steel by LC and found that the coefficients of friction (COFs) decreased with the increase of temperature; Mahesh et al.²¹ investigated the oxidation behavior of NiCrAlY–CeO₂ coating on the Ni-based substrate and revealed that the incorporation of CeO₂ elevates its oxidation resistance; and Yanan et al.²² investigated the effects of CeO₂ on the microstructure and properties of TiC/Ti₂Ni-reinforced Ti-based coating by LC and found that the grains were refined on the coating with the CeO₂ mass fraction of 2%, while excess CeO₂ hindered grain refinement, resulting in the decrease of wear resistance. The above-mentioned researches showed that the wear resistance of NiCoCrAlY coating was mainly used for the protection of Ni- and Fe-based substrates; however, the wear resistance of NiCoCrAlY coating by the addition of nano-CeO₂ on the Ti6Al4V alloy has rarely been reported, which affects its friction wear performance and service life.

In this study, NiCoCrAlY coatings with the different nano-CeO₂ mass fractions were fabricated on Ti6Al4V alloy using a LC.²³ The aim was to investigate the effects of nano-CeO₂ mass fraction on the friction wear performance of obtained coatings, which provided an experimental basis for the wear resistance of nano-CeO₂–reinforced NiCoCrAlY coating.

Experimental procedures

Sample preparations

The substrate was Ti6Al4V alloy with the following chemical composition (wt, %): Fe ≤ 0.30, C ≤ 0.10, N ≤ 0.05, H ≤ 0.015, O ≤ 0.20, Al 5.5–6.8, V 3.5–4.5, and Ti was balanced. The chemical composition of nano-CeO₂ (wt, %) was as follows: CeO₂ 99.99, Ln₂O₃≤ 0.01, Nd₂O₃+ Pr₆O₁₁≤ 0.03, Fe₂O₃≤ 0.01, SiO₂≤ 0.02, CaO ≤ 0.01, and Al₂O₃≤ 0.02; and that of NiCoCrAlY powder (wt, %) was as follows: Cr 20–22, Al 5, Fe ≤ 1, Y 0.5, Co 6, and C 0.4, and Ni was balanced. The laser-cladded NiCoCrAlY powders with the nano-CeO₂ mass fractions of 2%, 4%, and 6% were milled using a planetary ball miller for 2 h. The morphologies and phases of nano-CeO₂–reinforced NiCoCrAlY powders were analyzed using a JSM–6360LA type scanning electron microscope (SEM) and D/Max 2500/PC type X-ray diffraction (XRD), respectively.

The LC test was conducted on a ZKSX–2008 type fiber-coupled laser spraying system with the operation mode of continuous wave (CW). Technical parameters were as follows: laser wavelength of 1064 nm, beam distribution of circular Gaussian, spot diameter of 4 mm, laser power of 1400 W, scanning speed of 4 mm s⁻¹, overlap rate of 50%, Ar gas speed of 8 L min⁻¹, and powder feeding speed of 8 g min⁻¹.

Characterization methods

The morphologies and phases of obtained coatings were analyzed using a JSM–6360LA type SEM and D/Max 2500/PC type XRD, respectively. The hardness was measured using a micro-Vickers hardness tester; test parameters were as follows: load of 5 N and pressure holding time of 10 s, and the hardness was characterized with the average value of three times.

The wear test was conducted on a HT–1000 type high temperature wear tester, and the COF and wear rate were the average value of three tests. Test parameters were as follows: friction mode of ball–disk contact, friction couple of a Si₃N₄ ceramic ball, temperature of 600 °C, wear load of 3 N, speed of 500 r min⁻¹, and time of 30 min. The morphologies and chemical elements of worn tracks were analyzed using a JSM–6360LA type SEM and its configured energy-dispersive spectrometer, respectively; and the profiles of worn tracks were analyzed using a VHX–700F type ultra–depth microscope (UDM).

According to the contoured curve of the worn track, the wear area S is calculated by an Origin 8.5 software, and the wear volume was given as

V = lS

(1)

where l is the length of worn track (mm) and S is the area of worn track (mm²). Wear rate was given as

Δ = V / Lp

(2)

where V is the wear volume/mm³; L is the wear distance/m; and p is the wear load/N. According to equations (1) and (2), the wear rate (Δ) was positively correlated with wear volume (V) and the wear area (S). The small wear area meant the low wear rate, that is, the wear resistance of coating was high.

Analysis and discussion

Morphologies and XRD analysis of powders

Figure 1(a) shows the morphology of NiCoCrAlY powder. The powders were spherical and cylindrical, the irregular particles adsorbed on the spherical surface, and the particle sizes were more uniformly distributed. Figure 1(b) shows the morphology of nano-CeO₂ powder. The size value of nano-CeO₂ powder was less than 200 nm, and the particles were loosely distributed with no agglomerations. The morphology of NiCoCrAlY powder with the nano-CeO₂ mass fractions of 2% is shown in Figure 1(c). The NiCoCrAlY particle had an obvious bulge, and the nano-CeO₂ was adsorbed on its surface. The morphologies of NiCoCrAlY powder with the nano-CeO₂ mass fractions of 4% and 6% are shown in Figure 1(d) and (e), respectively. The bulges on the NiCoCrAlY particles decreased, and more nano-CeO₂ particles were adhered on the NiCoCrAlY particles.

Figure 1.

Morphologies of NiCoCrAlY powders with different nano-CeO₂ mass fractions: (a) NiCoCrAlY powder, (b) Nano-CeO₂ powder, (c) with nano-CeO₂ mass fraction of 2%, (d) with nano-CeO₂ mass fraction of 4%, and (e) with nano-CeO₂ mass fraction of 6%.

Figure 2 shows the XRD analysis of NiCoCrAlY powders with the different nano-CeO₂ mass fractions. The powders were mainly composed of Ni, Co, Cr, and Al phases, while Fe was in the impurity phase, Y was not detected because of its low content, indicating that the addition of nano-CeO₂ powder had no effect on the phase compositions of NiCoCrAlY powders.

Figure 2.

XRD analysis of NiCoCrAlY powders with different nano-CeO₂ mass fractions.

Morphologies and XRD analysis of coating surfaces

Figure 3(a) shows the morphology of a NiCoCrAlY coating surface. The surface grains were mainly short dendrite NiTi₂ as the reinforced phase, which was evenly distributed on the coating surface.

Figure 3.

Morphologies of NiCoCrAlY coating surfaces with different nano-CeO₂ mass fractions: (a) with a nano-CeO₂ mass fraction of 0%, (b) with a nano-CeO₂ mass fraction of 2%,(c) with a nano-CeO₂ mass fraction of 4%, and (d) with anano-CeO₂ mass fraction of 6%.

Figure 3(b) shows the morphology of NiCoCrAlY coating surface with the nano-CeO₂ mass fraction of 2%. The contained CeO₂ grains were obviously found on the coating. The porosity decreased markedly. The dendrites increased on the coating surface, and a small amount of cellular crystals was dispersed around the dendrite, indicating that the nano-CeO₂ promoted the grain nucleation.

Figure 3(c) shows the morphology of NiCoCrAlY coating surfaces with the nano-CeO₂ mass fraction of 4%. The coating surface was covered with dendrites, and the cellular crystals almost disappeared, indicating that the addition of nano-CeO₂ promoted the formation of nuclei²⁴ and increased the nucleation rate. However, a small amount of CeO₂ did not inhibit the growth of grains, and the large nuclei were connected to each other and grew up, resulting in the coarsening of grains.

Figure 3(d) shows the morphology of NiCoCrAlY coating surfaces with the nano-CeO₂ mass fraction of 6%. The coarse dendrites were broken, and the cellular crystals were increased. The short dendrites and cellular crystals were dominant on the coating surface, indicating that a small amount of nano-CeO₂ promoted the formation and growth of grains.²⁵ The nano-CeO₂ destroyed the stable state of grain growth with the increase of nano-CeO₂ mass fraction, and the grains grew into the small dendrites and cellular crystals, which played a role in fine grain strengthening.²⁶ After crystallization, nano-CeO₂ became enriched at the grain boundary and reduced the surface tension and interfacial energy, which decreased the critical nucleation. The number of nucleation particles was increased to avoid grain coarsening,²⁷ and the NiTi₂ phase experienced the process of grain increase, grain growth, and grain refinement.

Figure 4 shows XRD analysis of NiCoCrAlY coatings with the different nano-CeO₂ mass fractions. The coatings were mainly composed of NiTi₂, Ti₃O, CoO, and β-Ti phases, and the addition of nano-CeO₂ had little effect on the phase compositions,^22,26 with no new phases. Ti on the coating was positively correlated with nano-CeO₂, which increased the intensity of the Ti₃O peak.²² The molten pool contained nano-CeO₂ and had better fluidity, which caused more Ti atoms in the substrate float up to form the reinforced phase. Ti and Co on the coating absorbed O in the air, which was preferentially adsorbed by α-Ti to form Ti₃O.²⁸ This was because the oxygen solubilities in the α-Ti and β-Ti phases were 33.9% and 3.8%, respectively.

Figure 4.

XRD analysis of NiCoCrAlY coatings with different nano-CeO₂ mass fractions.

Morphologies of coating cross-sections

Figure 5(a) shows the morphology of a NiCoCrAlY coating cross-section. The bonding interface of coating substrates was in a zigzag shape with irregular protrusions. The deep cladding layer was formed on the region with fast thermal conductivity.

Figure 5.

Morphologies of NiCoCrAlY coating cross-sections with different nano-CeO₂ mass fractions: (a) with a nano-CeO₂ mass fraction of 0%, (b) with a nano-CeO₂ mass fraction of 2%, (c) with a nano-CeO₂ mass fraction of 4%, and (d) with a nano-CeO₂ mass fraction of 6%.

Figure 5(b)–(d) shows the morphologies of NiCoCrAlY coating cross-sections with the nano-CeO₂ mass fractions of 2%, 4%, and 6%, respectively. The large serrated teeth at the interface between the coating and the substrate disappeared, suggesting that the nano-CeO₂ improved the fluidity and heat transfer of the molten pool. The NiCoCrAlY coating with the nano-CeO₂ mass fraction of 6% exhibited the micro-serration bonding at the interface, and the micro-serration enhanced its bonding strength and reduced crack sensitivity and shedding risk on the bonding sites.

Figure 6 shows the hardness distributions of NiCoCrAlY coating cross-sections with the different nano-CeO₂ mass fractions. The average hardness values of NiCoCrAlY coatings with the nano-CeO₂ mass fractions of 0%, 2%, 4%, and 6% were 867, 942, 957, and 1003 HV, respectively, which increased with the increase in the nano-CeO₂ mass fraction; this was because the NiTi₂-reinforced phase enriched on the skeleton structure improved their hardness.²⁹

Figure 6.

Hardness distribution of NiCoCrAlY coating cross-sections with different nano-CeO₂ mass fractions.

COFs and wear rates

Figure 7(a) shows the COFs of nano-CeO₂–reinforced NiCoCrAlY coatings versus wear time. The large fluctuations of COFs in the running-in period (0–2 min) were due to the removal and redistribution of particles on the contact surfaces. The COFs remained at 0.69 in the range of 2–20 min, while those in the range of 20–30 min presented an upward trend and reached 0.771 at 30 min. The COF fluctuation was related to the oxides. The COFs of nano-CeO₂–reinforced NiCoCrAlY coatings were lower than 0.7; this was because the nano-CeO₂ increased the adhesion of oxides on the coating. The COFs of nano-CeO₂–reinforced NiCoCrAlY coatings presented a rapid decrease, rise, and stable states; this was because the adhered oxides on the coating were broken, which caused the COFs to fluctuate.³⁰ The average COFs of NiCoCrAlY coatings with the nano-CeO₂ mass fractions of 2%, 4%, and 6% were 0.655, 0.636, and 0.615, respectively, suggesting that the nano-CeO₂–reinforced coatings had lower COFs.³¹ The standard deviation of NiCoCrAlY coatings with the nano-CeO₂ mass fractions of 2%, 4%, and 6% in the time range of 5–30 min were 0.030, 0.019, and 0.027, respectively, while that of NiCoCrAlY coating was 0.035, showing that the nano-CeO₂–reinforced coatings also had more stable friction processes.

Figure 7.

Coefficients of friction versus wear time, and profiles of worn tracks and wear rates on NiCoCrAlY with differentnano-CeO₂ mass fractions: (a) coefficients of friction versus wear time, (b) profiles of worn tracks, and (c) wear rates.

Figure 7(b) shows the profiles of worn tracks on the nano-CeO₂–reinforced NiCoCrAlY coatings. The depths of worn tracks with the nano-CeO₂ mass fractions of 0%, 2%, 4%, and 6% were 8.157, 6.48, 6.81, and 4.886 µm, respectively, and the corresponding average wear depths were 4.359, 3.685, 2.976, and 2.717 µm, respectively, which decreased with the increase of nano-CeO₂ mass frictions.

Figure 7(c) shows the wear rates of nano-CeO₂–reinforced NiCoCrAlY coatings. The wear rates of NiCoCrAlY coating with the nano-CeO₂ mass fractions of 0%, 2%, 4%, and 6% were 4.04 × 10⁻⁷, 3.95 ×10⁻⁷, 3.13 ×10⁻⁷, and 2.35 × 10⁻⁷ mm³ N⁻¹ m⁻¹, respectively, which decreased with the increase of nano-CeO₂ mass fraction. The fined grains were distributed on the coating with the nano-CeO₂ mass fraction of 6%, which effectively deduced the wear rate, and the effect of nano-CeO₂ on the wear resistance was consistent with the hardness.^19,32

Morphologies and XRD analysis of worn tracks

Figure 8(a) shows the morphology of worn tracks on the NiCoCrAlY coating. The plastic deformation was obviously found on the worn track, which was a typical adhesive wear feature. The felled oxides scratched the coating, leading to abrasive wear.

Figure 8.

Morphologies of worn tracks with different nano-CeO₂ mass fractions: (a) with a nano-CeO₂ mass fraction of 0%, (b) with a nano-CeO₂ mass fraction of 2%, (c) with a nano-CeO₂ mass fraction of 4%, and (d) with a nano-CeO₂ mass fraction of 6%.

Figure 8(b) shows the morphology of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 2%. There was also obvious plastic deformation on the worn track, and the wear debris was produced on the wear couples, which caused micro-cutting and grooving.^33,34 A large number of fallen oxides were observed on the grooves of worn tracks, indicating that the wear mechanism was adhesive wear and oxidation wear, accompanied by slight abrasive wear.

Figure 8(c) shows the morphology of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 4%. The plastic deformation was more obviously observed on the worn tracks; this was because the coarse grains reduced the plasticity, and the felled oxides reduced the degree of oxidation wear.

Figure 8(d) shows the morphology of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 6%. More nano-CeO₂ refined the coating structure and improved the plastic deformation of worn tracks, and the high hardness increased deformation resistance with the addition of nano-CeO₂.³⁵ The shedding oxides on the worn track reduced the oxidation wear.¹⁹ It was concluded that the degrees of plastic deformation and oxide on the worn track with the nano-CeO₂ mass fraction of 6% were the lowest among the four kinds of coatings.

Figure 9(a) shows the wear mechanism of NiCoCrAlY coating. The oxide layer was formed on the worn track at a high temperature because of the oxidation of alloy elements,^36,37 and the tribo pair rubbed violently with the coating and produced a large amount of debris on the marked green zone. At the same time, at high temperatures, the welding junctions occurred on purple zone between the tribo pair and the coating, which lead to adhesive wear.

Figure 9.

Schematic diagram of the wear mechanism on NiCoCrAlY coatings with different nano-CeO₂ mass fractions:³⁶ (a) NiCoCrAlY coating and (b) nano-CeO₂–reinforced NiCoCrAlY coating.

Figure 9(b) shows the wear mechanism of nano-CeO₂–reinforced NiCoCrAlY coating. Under the action of cyclic shearing stress, the hard phases were worn out slowly,³⁶ and the number of hard phases were obviously refined and increased.^22,26 The wear degree of hard phases was slowed down, and the hard wear-resistant layer was formed on the light blue zone, which was aggregated by the hard phases.

Figure 10 shows XRD analysis of worn tracks with the different nano-CeO₂ mass fractions. The new phases of AlTi₃ and (Ni, Co)₂Ti₄O were formed on the worn tracks; among them, Ni₂Ti₄O was a solid solution formed by the dissolution of O in NiTi₂, which was oxidized to reduce the Ti oxide;³⁸ Co₂Ti₄O was a solid solution formed by the Co atoms replacing the Ni atoms in Ni₂Ti₄O; and the AlTi₃ phase was a new enhanced phase formed at high temperatures. The Ti₃O phase vanished on the surface of worn tracks. The peaks of Ti₂O₃ were observed at 40.22°, 42.627°, and 48.756°, indicating that Ti₃O was oxidized to Ti₂O₃, which was related to the low contents of NiTi₂ and Ni₂Ti₄O.

Figure 10.

XRD analysis of worn tracks on NiCoCrAlY coatings with different nano-CeO₂ mass fractions.

Line scan analysis of worn tracks

Figure 11(a) shows the line-scanned position of worn tracks on the NiCoCrAlY coating. The mass fractions of chemical elements (mass, %) were as follows: C 1.7, O 26.05, Al 3.02, Si 0.66, Ti 42.4, V 1.84, Cr 4.47, Co 0.74, Ni 18.57, and Y 0.57, as shown in Figure 11(b). The Ni, Co, Cr, Al, and Y contents on the worn track decreased slightly, as shown in Figure 11(c)–(f). The Si content increased obviously on the worn track, which came from the tribo pair, and the wear mechanism was adhesive wear, as shown in Figure 11(g). The O content increased on worn tracks; there was oxidation wear, as shown in Figure 11(h).

Figure 11.

Line scan analysis of worn tracks on NiCoCrAlY coating: (a) line scan position, (b) result of line scan analysis, (c) Ni content, (d) Cr content, (e) Al content, (f) Y content, (g) Si content, and (h) O content.

Figure 12(a) shows the line-scanned position of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 2%. The mass fractions of chemical elements (mass, %) were as follows: O 30.18, Al 3.59, Si 0.81, Ti 51.46, V 2.47, Cr 2.61, and Ni 8.89, as shown in Figure 12(b). The Ni, Cr, and Al contents on the worn track decreased slightly, as shown in Figure 12(c)–(e). The contents of Cr and Ni decreased, while those of Ti and V increased, indicating that nano-CeO₂ caused more Ti to enter the molten pool.²⁸ The additional reinforced phases were formed, and the coating elements were further diluted. The contents of Si and O increased on the worn track, and the wear mechanism was adhesion wear and oxidation wear, as shown in Figure 12(f) and (g).

Figure 12.

Line scan analysis of worn tracks on NiCoCrAlY coating with a nano-CeO₂ mass fraction of 2%: (a) line-scanned position, (b) result of line scan analysis, (c) Ni content, (d) Cr content, (e) Al content, (f) Si content, and (g) O content.

Figure 13(a) shows the line-scanned position of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 4%. The mass fractions of chemical elements (mass, %) were as follows: O 34.3, Al 3.12, Si 1.49, Ti 48.58, V 1.92, Cr 2.06, Ni 7.48, and Fe 1.04, as shown in Figure 13(b). The Ni, Cr, Al, and Y contents on the worn track decreased slightly, as shown in Figure 13(c)–(e). The Si content was higher than that with the nano-CeO₂ mass fraction of 2%, showing that more Si was stuck on the worn track. The wear mechanism was adhesive wear, as shown in Figure 13(f). The content of O increased at the worn track. This meant that there was oxidative wear on the worn track, as shown in Figure 13(g).

Figure 13.

Line scan analysis of worn tracks on NiCoCrAlY coating with nano-CeO₂ mass fraction of 4%: (a) line-scanned position, (b) Result of line scan analysis, (c) Ni content, (d) Cr content, (e) Al content, (f) Si content, and (g) O content.

Figure 14(a) shows the line-scanned position of worn tracks on the NiCoCrAlY coating with the nano-CeO₂ mass fraction of 6%. The mass fractions of chemical elements (mass, %) were as follows: O 31.67, Al 4.1, Si 0.91, Ti 49.89, V 2.34, Cr 2.22, and Ni 8.87 as shown in Figure 14(b). Co was detected on the worn track, which was related to the reduction of CoO peak. The Ni, Cr, and Al contents on the worn track decreased slightly, as shown in Figure 14(c)–(e). The Si content increased on the worn track, indicating that there was adhesive wear, as shown in Figure 14(f). The O content did not change, indicating that the degree of oxidation wear was reduced, as shown in Figure 14(g). Ce was not detected, which further proved that the addition of nano-CeO₂ had no obvious effect on the phase composition and phase composition.^26,28

Figure 14.

Line scan analysis of worn tracks on NiCoCrAlY coating with a nano-CeO₂ mass fraction of 6%: (a) line-scanned position, (b) result of line scan analysis, (c) Ni content, (d) Cr content, (e) Al content, (f) Si content, and (g) O content.

Conclusion

Based on the experimental results presented above, the conclusions can be drawn as follows:

The nano-CeO₂–reinforced NiCoCrAlY coatings are mainly composed of NiTi₂, Ti₃O, CoO, and β-Ti phases. Nano-CeO₂ promotes grain refinement, while the Ni₂Ti₄O and AlTi₃ phases are formed after the wear test, which enhance coating hardness.

The average COFs of NiCoCrAlY coatings with the nano-CeO₂ mass fractions of 0%, 2%, 4%, and 6% are 0.699, 0.655, 0.636, and 0.615, respectively, and the corresponding wear rates are 4.04 × 10⁻⁷, 3.95 × 10⁻⁷, 3.13×10⁻⁷, and 2.35×10⁻⁷ mm³ N⁻¹ m⁻¹, respectively, which decrease with the increase of nano-CeO₂ mass fraction.

The wear mechanism of nano-CeO₂–reinforced NiCoCrAlY coating is adhesive wear and oxidation wear, accompanied by slight abrasive wear, while the degree of oxidation wear decreases with the increase of nano-CeO₂ mass fraction.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Key Research and Development Project of Jiangsu Province (BE2016052).

ORCID iD

Kong Dejun

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