Nanomechanical Characterization of High-Velocity Oxygen-Fuel NiCoCrAlYCe Coating

Abstract

MCrAlY (M = Ni or/and Co) coatings have played an indispensable role in the high-temperature protection system for key components of aero-engines due to their excellent high-temperature oxidation and hot corrosion resistance. Nanoindentation is a useful and highly efficient method for characterizing the nanomechanical properties of materials. The rich information reflecting materials can be gained by load-displacement curves. In addition to common parameters such as elastic modulus and nanohardness, the indentation work and creep property at room temperature can also be extracted. Herein, nanomechanical properties of NiCoCrAlYCe coatings using high-velocity oxygen-fuel (HVOF) spraying were investigated systematically by nanoindentation. The microstructure of as-sprayed NiCoCrAlYCe coatings present mono-modal distribution. Results of nanoindentation reveal that the elastic modulus and nanohardness of NiCoCrAlYCe coatings are 121.08 ± 10.04 GPa and 6.09 ± 0.86 Gpa, respectively. Furthermore, the indentation work of coatings was also characterized. The elastic indentation work is 10.322 ± 0.721 nJ, and the plastic indentation work is 22.665 ± 1.702 nJ. The ratio of the plastic work to the total work of deformation during indentation is 0.687 ± 0.024, which can predict excellent wear resistance for NiCoCrAlYCe coatings. Meanwhile, the strain rate sensitivity determined by nanoindentation is 0.007 ± 0.001 at room temperature. These results can provide prediction of erosion resistance for MCrAlY coatings.

1. Introduction

MCrAlY coatings, as a high-temperature protective coating of superalloys (bond layer of thermal barrier coatings and overlay coatings), have been widely used in aerospace, shipping, energy, and other industries due to the excellent resistance to high-temperature oxidation and thermal corrosion [1,2]. The evolution of bond coats has proceeded from diffusion coatings, including aluminum and platinum aluminum, MCrAlY (M = Ni or/and Co) coatings to thermal barrier coatings. As for MCrAlY coatings, the rich-Ni and rich-Co provide resistance to oxidation and hot corrosion, respectively. Al and Cr are used to promote the formation of oxide film, and Y is used to improve the adhesion of oxide film [3]. Furthermore, MCrAlY coatings can also relieve thermal mismatches between the substrate and top layer [4]. However, with the increasing work temperature of hot aero-engine components, MCrAlY coatings are subjected to complex interactions, such as thermal stress and mechanical stress [5,6]. Therefore, to improve the efficiency of engines and prolong the service life of hot components of engines, it is urgent to improve the comprehensive properties of MCrAlY coatings.

Currently, there are many methods by which to prepare MCrAlY coatings, such as arc spraying [7], electro-plating [8], plasma spraying [9], flame spraying [10], and so on. Among them, high-velocity oxygen-fuel (HVOF) spraying is one of the most commonly used industrial thermal spraying technologies due to its high flame velocity and relatively low temperature, which is suitable for spraying MCrAlY powders [11,12,13,14]. Karaoglanli et al. [15] compared the oxidation behavior of CoNiCrAlY coatings fabricated by atmospheric plasma spraying (APS), supersonic atmospheric plasma spraying (SAPS), HVOF, and detonation gun methods. The results indicated that the HVOF-CoNiCrAlY coatings had a better oxidation resistance. Srivastava et al. [16] fabricated an Al_1.4Co_2.1Cr_0.7Ni_2.45Si_0.2Ti_0.14 high-entropy alloy (HEA) as a bond coat for the TBC system on a Ni-based superalloy by HVOF spraying, and showed that coatings containing HEA had more outstanding high-temperature properties compared with MCrAlY. Praveen et al. [17] investigated the erosion resistance of the NiCrSiB-Al₂O₃ coating on AISI304 stainless steel by HVOF thermal spraying, and indicated that the NiCrSiB-Al₂O₃ coating had a ductile erosion behavior. Rajendran et al. [18] prepared the WC-10Ni-5Cr coatings on 35 Mo Cr steel by HVOF process, and implied that the oxygen flow rate had a larger effect on the porosity and microhardness of coatings. Sacriste et al. [19] prepared MCrAlY coatings by twin wire arc spray, and found that the NiCrAlY coatings arc sprayed by using nitrogen had a lower roughness due to the lower oxide content compared to air atomization. In addition, the composition and structure of MCrAlY coatings determine chemical and mechanical properties of coatings to a certain degree [20]. To ensure that the hot components of engine can work stably at high temperature for a long time, some researchers optimize the composition of MCrAlY alloy. Zakeri et al. [21] investigated the high-temperature oxidation behavior of the NiCoCrAlY-CeO₂ coatings fabricated by HVOF spraying and found that the oxidation rate of the nano-CeO₂ modified coatings decreased by 87% compared to the NiCoCrAlY coatings. Yu et al. [22] studied the oxidation behaviors of the NiCrAlY alloy with different silicon content, and indicated that doping silicon was beneficial to improve the oxidation resistance of coatings. Yang et al. [23,24] demonstrated that the addition of Pt in NiCoCrAlY coatings promoted the diffusion of Al, which was conducive to selective oxidation and the formation of highly protective Al₂O₃ film. In short, doping alloying elements or nanoparticles is an effective way to improve the oxidation resistance of MCrAlY coatings. As for MCrAlY coatings, except for high-temperature oxidation and corrosion, it is well known that the mechanical properties of coatings are also one of the important parameters by which to evaluate the service life. Hence, studying the mechanical properties of coatings, especially the nanomechanical properties, has vital practical significance for the improvement of MCrAlY coatings system.

According to our previous studies, we designed the NiCoCrAlYCe coatings by HVOF spraying, and indicated that the NiCoCrAlYCe coatings exhibited a good bonding strength and thermal shock resistance [25,26]. The structural properties of multicomponent coatings have been investigated in previous studies, such as the composition, uniformity of the distribution of elements, and phase composition [26]. Nevertheless, the nanomechanical properties of NiCoCrAlYCe coatings are not analyzed systematically. In this work, the NiCoCrAlYCe coatings were fabricated by HVOF spraying, and the nanomechanical properties of NiCoCrAlYCe coatings were discussed in detail.

2. Experimental

2.1. Material Preparation

The substrates are K417G nickel-based superalloy (Φ25.4 × 6 mm²) and feedstocks are customized Ni-20Co-19.75Cr-11.4Al-0.68Y-0.79Ce (wt.%) powders (NiCoCrAlYCe, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China) according to composition requirements. Before spraying, the pretreatment of the substrates’ surface is necessary, including ultrasonic cleaning in the acetone solvent for 10 min and sand blasting with parameters of 46 mesh brown corundum and 0.4 MPa pressure. The average surface roughness determined by the 3D profiler of sand-blasted substrates is 4.01 μm. Subsequently, NiCoCrAlYCe coatings with the thickness of 200 ± 30 μm were deposited onto substrates by high-velocity oxygen fuel spraying (HVOF, JP-5000, Praxair, Inc., Danbury, CT, USA). The main parameters are set in

Table 1. Main parameters for HVOF-sprayed NiCoCrAlYCe coatings.

Flow rate of kerosene (L/h)	18.9
Flow rate of N2 (L/min)	10.4
Flow rate of O2 (L/min)	873.2
Feeding rate (g/min)	70
Spray distance (mm)	360

.

2.2. Characterization Technology

The microstructure of NiCoCrAlYCe feedstocks and as-sprayed coatings was characterized by scanning electron microscope (SEM, Nova NanoSEM 430, FEI Company, Eindhoven, The Netherlands). The nanoindentation test (Anton-Paar, Graz, Austria) was employed on the polished cross-section of NiCoCrAlYCe coatings to analyse the elastic–plastic properties. In order to reduce the error, 20 indentation points are used. The maximum loading force of nanoindentation is 98 mN with 10 s dwelling time and the loading rate is 3.27 mN/s. To ensure the reproducibility of results, twenty samples of each configuration were analysed.

3. Results and Discussion

Figure 1a shows the surface morphology of NiCoCrAlYCe feedstocks. It is clear that feedstocks are spherical and smooth. The particle size ranges from 10 μm to 45 μm and this meet the demands of HVOF spraying. In the case of thermal-sprayed coatings, the flowability of feedstocks is necessary. The spherical and smooth NiCoCrAlYCe feedstocks can ensure the flowability. In addition, whether the microstructure inside the powder is dense or not has a great influence on the wear resistance of as-sprayed coatings. Figure 1b shows the cross-section morphology of feedstocks. It is significant that there is a dense microstructure inside the feedstocks. The feedstocks with dense microstructure can obtain dense coatings after suitable spraying parameters and the wear resistance of coatings will be improved in turn.

Figure 2a shows surface morphology of as-sprayed NiCoCrAlYCe coatings. It can be observed that the melting state of feedstocks is good and only a small amount of unmelted particles exists in the coatings. This indicate that NiCoCrAlYCe coatings can be regarded as a fully melted zones, i.e., mono-modal distribution. The cross-section image further demonstrates the coatings are compact, as displayed in Figure 2b.

As for the NiCoCrAlYCe coatings, nanomechanical properties are vital. Figure 3a shows load-displacement curves measured by nanoindentation of NiCoCrAlYCe coatings. It can be seen that these curves are relatively close and have good repeatability. Based on characteristics of load-displacement curves, it can be inferred that coatings are mainly composed of molten regions. The elastic modulus (E) and nanohardness (H) are 121.08 ± 10.04 GPa and 6.09 ± 0.86 GPa, respectively. Further Weibull analysis of E and H for NiCoCrAlYCe coatings is shown in Figure 3b. There is only a single line for the Weibull plots of elastic modulus or nanohardness, and this demonstrates the mono-modal microstructure in NiCoCrAlYCe coatings.

In addition to the nanohardness and elastic modulus, the indentation work is also a crucial parameter by which to evaluate the elastic–plastic property of coatings. The indentation work can be obtained from load-displacement curves in nanoindentation. The total indentation work (W_t) consists of elastic work (W_e) and plastic work (W_p). The equation can be written as follows [27]:

$$W_t=W_e+W_p$$

(1)

where W_t is the total indentation work, W_e is the elastic work, and W_p is the plastic work.

The total indentation work can be calculated by the following equation [27]:

$$W_t={\displaystyle \underset{0}{\overset{h_{\max }}{\int }}F(h})\mathrm{d}h$$

(2)

where h_max is the maximum indentation depth during loading, and F(h) is the function of load with indentation depth during the loading curve. We have

$$W_e={\displaystyle \underset{h_{\mathrm{f}}}{\overset{h_{\max }}{\int }}P(h})\mathrm{d}h$$

(3)

where h_max is the maximum indentation depth during loading, P(h) is the function of load with indentation depth during the unloading curve, and h_f is the remaining indentation depth [27]. According to Equations (1)–(3), the indentation work of NiCoCrAlYCe coatings is displayed in Figure 4a. The elastic indentation work ranges from 8.551 nJ to 11.969 nJ, and the plastic indentation work ranges from 19.960 nJ to 26.169 nJ. Figure 4a also shows that the plastic work of the coating is sensitive to the microstructure, whereas the elastic work is the opposite. The wear resistance of coatings can be evaluated by a new parameter, the microhardness dissipation parameter (MDP). The MDP is defined as the ratio of plastic indentation work to total indentation work, as shown in the following equation [27],

$$\mathrm{M}\mathrm{D}\mathrm{P}=W_p/W_t$$

(4)

where MDP is the microhardness dissipation parameter, W_p is the plastic work, and W_t is the total indentation work.

The higher the MDP value of the coating, the better the wear resistance. As shown in Figure 4b, the calculated MDP of NiCoCrAlYCe coatings is 0.687 ± 0.024. This result suggests that NiCoCrAlYCe coatings have the ability to dissipate most of the deformation energy (about 69%), predicting excellent wear resistance under the erosion or abrasive wear condition. Furthermore, there are no cracks near the indentation impression displayed in Figure 4c.

Furthermore, it is obvious that there is a platform, namely creep process, in the load displacement curve (Figure 3a). The stable creep stage can be described by the following equation [28,29],

$$\frac{1}{h}\frac{\mathrm{d}h}{\mathrm{d}t}=K{\sigma }^{\frac{1}{m}}$$

(5)

$$\sigma =C_1\frac{P_{\max }}{h^2},$$

(6)

where h is the indentation depth, t is time, σ is the flow stress, P_max is the maximum load (10 gf), K and C₁ are constant, and m is the strain rate sensitivity. Based on Equations (5) and (6), the integral result is written as follows [30]:

$$h=A{(t-t_c)}^{\frac{m}{2}},$$

(7)

where h is the indentation depth, t is time, t_c is the constant, and m is the strain rate sensitivity. In the process of creep behavior, the indentation depth (h) versus holding time can be fitted by using Equation (7). The fitting curves are shown in Figure 5a (different colored lines represent randomly selected indentation points), and the fitting results are very good (R² > 99%). The strain rate sensitivity of NiCoCrAlYCe coatings at room temperature is 0.007 ± 0.001 (Figure 5b). As for high-temperature protection coatings, creep property is very important. This indentation method can provide a new way to calculate the strain rate sensitivity.

Based on the above results, NiCoCrAlYCe coatings exhibit outstanding nanomechanical properties due to the mono-modal microstructure of NiCoCrAlYCe coatings. Furthermore, compared to nanostructured La₂(Zr_0.75Ce_0.25)₂O₇ thermal barrier coatings, the elastic modulus and nanohardness of NiCoCrAlYCe coatings is dramatically improved [28]. In addition, the ratio of the plastic work to the total work of deformation during indentation is larger than that of La₂(Zr_0.75Ce_0.25)₂O₇ coatings, which can predict excellent wear resistance for NiCoCrAlYCe coatings.

4. Conclusions

In this work, NiCoCrAlYCe coatings were fabricated by HVOF spraying by using spherical and dense feedstocks. The microstructure of NiCoCrAlYCe coatings was characterized by SEM. Moreover, the nanomechanical properties of coatings were investigated by the nanoindentation method. Some meaningful conclusions can be drawn as follows:

(1) The nanohardness and elastic modulus of NiCoCrAlYCe coatings are 6.09 ± 0.86 GPa and 121.08 ± 10.04 GPa, respectively.

(2) The Weibull distribution reveals the mono-modal microstructure in NiCoCrAlYCe coatings.

(3) The elastic indentation work is 10.322 ± 0.721 nJ, and the plastic indentation work is 22.665 ± 1.702 nJ. The microstructure of NiCoCrAlYCe coatings has little effect on the elastic work, whereas it has a great influence on the plastic work.

(4) It can be predicted that NiCoCrAlYCe coatings have a good wear resistance based on the value of MDP (0.687 ± 0.024).

(5) The strain rate sensitivity of NiCoCrAlYCe coatings is 0.007 ± 0.001 under room temperature.