1. Introduction
To improve the efficiency of aviation turbine engines, the hot components must endure an extreme environment, including high temperature and pressure. Therefore, an advanced thermal barrier coating technology is required. The thermal barrier coating consists of two parts: a ceramic thermal barrier coating and a bond coat. MCrAlY (M = Ni, Co, NiCo) coatings have been widely used as a bond coat for thermal barrier coatings or as a high-temperature protective coating alone [
1,
2,
3,
4,
5,
6,
7,
8]. The advantages of MCrAlY coatings have been researched by Saeidi et al. [
6].
Methods for the deposition of MCrAlY coatings include electron beam physical vapor deposition, electroplating, and thermal spraying [
3]. In particular, high-velocity oxygen fuel (HVOF) spraying is the most widely used technique [
3,
6]. During the HVOF process, powder particles have a high velocity, short exposure time and low temperature in the jet; these properties can substantially reduce the oxygen content and porosity of MCrAlY coatings to enhance their high-temperature oxidation resistance [
9]. HVOF spraying is also considerably cheap and easy to operate [
2,
3].
MCrAlY coatings normally include NiCoCrAlY, CoCrAlY, and NiCrAlY. The properties of NiCoCrAlY and CoCrAlY with certain cobalt show much more hot corrosion resistance than that of NiCrAlY coatings under the environment of salt contaminants [
10]. The composition of NiCrAlY coatings, which often have high nickel, chromium, and aluminum contents, is designed for nickel-based superalloys to endure high-temperature oxidation. At a temperature range of 900 to 1300 °C, NiCrAlY coatings react with oxygen and form an oxide scale—thermally grown oxide (TGO)—on the surface, which will prevent the substrate from being oxidized [
9]. The main TGO component is α-Al
2O
3, which is slow growing and thermodynamically stable and is formed via diffusion of Al
3+ inside the coating on the surface and its reaction with oxygen [
11]. However, the presence of other oxides, such as spinel oxides (Ni(Cr,Al)
2O
4, NiO, Cr
2O
3) and metastable Al
2O
3 (θ-Al
2O
3) will accelerate the TGO growth rate and degradation of NiCrAlY coatings [
9,
12].
The oxidation resistance of NiCrAlY coatings is considerably affected by the thermal spray powder composition, the parameters of thermal spraying, the microstructure of the coatings, and the parameters of the processing technology [
8,
13]. In the early stage of oxidation, a pure, continuous, and dense α-Al
2O
3 scale on the surface of deposited coatings can remarkably improve the oxidation resistance of NiCrAlY coatings [
10]; thus, much research has been attempted, including the addition of reactive elements, such as Pt Cr Zr, Hf, and Ta [
14,
15], the use of cryomilled powder [
16,
17], and the application of laser treatment on the surface of deposited coatings [
18]. Recent studies have shown that vacuum heat treatment (VHT) can affect the oxidation resistance of MCrAlY coatings. Doolabi et al. [
9] reported that VHT decreased the oxidation resistance of CoNiCrAlY because VHT transformed the coating microstructure from a single γ-NiAl phase to a two-phase γ + β. The two-phase microstructure, together with coarse grain size and low dislocation density, of VHT coatings reduced the aluminum diffusion rate and decreased the possibility of having a single layer of alumina. On the contrary, Han [
10] showed a different conclusion. VHT could improve the high-temperature oxidation resistance of CoCrAlY coating, due to the tensile stress in the TGO of the VHT coating decreasing, which might reduce the TGO growth rate and the spinel oxide generated on the surface of the VHT coating. Currently, few people have researched the effect of the change in temperature of VHT on the oxidation resistance of MCrAlY.
This study aimed to investigate the changes in coating microstructure under different VHT temperatures and explore the effects of these changes on the high temperature oxidation in the early stage of HVOF NiCrAlY coatings. The microstructural evolution of the coatings under different VHT conditions was analyzed, and the process parameters of VHT were comparatively optimized.
2. Materials and Methods
2.1. Powder Material
The powder used in this study was a NiCrAlY alloy powder produced by H.C. Starck (AMPERT 413, Munich, Germany). The powder is a type of spherical powder with a size range of 15 to 45 µm, and the average powder particle size is 22 µm. The morphology of the powder is shown in
Figure 1. The nominal chemical composition of the powder is listed in
Table 1.
2.2. Sample Preparation
The NiCrAlY coatings were deposited on FeCrAl alloy using an HVOF process (Sulzer Metco DJ2700, Winterhur, Switzerland).
Table 2 lists the spraying parameters used in this study. To obtain a free-standing coating, the sandblasting was skipped before spraying to reduce the bonding strength of the coating. Due to the low bonding strength, the coating was separated from the substrate by bending it around a mandrel of a thickness of approximately 0.8 mm when it was being sprayed. The free-standing coating was then cut into 15 mm × 15 mm × 0.5 mm pieces by using a surface grinder and a cutter. The surfaces of the pieces were polished to metallic luster by using sandpaper (360 mesh).
2.3. VHT
The VHT furnace (Weike, VHA-150/200, Shengyang, China) (<5 × 10−2 Pa, Air) was heated to 1000, 1100, and 1200 °C for 2 h, and the heat rate was set to 10 °C/min from ambient and cooled with the furnace. The samples were marked as shown—VHT1000, VHT1100 and VHT1200. Six samples were treated in each batch of VHTs. Three samples were used for the oxidation test and other samples were used for characterization research. The maximum heating temperature of the furnace is 1500 °C, and the size of the effective heating zone is 150 mm × 120 mm × 200 mm.
2.4. High-Temperature Oxidation Test
All of the specimens were subjected to a high-temperature oxidation test in a muffle furnace (Xinyu, XY-1700, Nanyang, China). The high-temperature oxidation test was conducted at 1100 °C for 21 h in air. The weight gain of the coatings was measured using an analytical balance of 0.1 mg precision at 1, 6, 11, 16, and 21 h, and a weight gain curve was drawn.
2.5. Sample Characterization
The phase identification of the specimens was performed via X-ray diffractometer (XRD) (Rigaku, D/Max-Ultima+, Tokyo, Japan, Cu Kα). The scanning speed was 8°/min, and the 2θ values ranged from 20° to 95°. The phase compositions were analyzed using Jade (Intelligent XRD Analysis Software, Material Data TM, Version: 6). The cross-sectional and surface microstructures of the samples were evaluated by SEM (Supra 55, Carl Zeiss, Oberkochen, Germany). The microporosity of the coating was evaluated by image analysis using OLYCIA m3 software (Version: m3), and the porosity was calculated from a larger area. The chemical composition of the samples was studied by an energy spectrometer (EDS) (Oxford X-Max Extreme, Abingdon, UK).
4. Discussion
The comparison of
Figure 3 and
Figure 4a showed no evident change in the microstructure of powders and unmelted particles in the as-sprayed coating. This finding was mainly due to the fast powder feeding speed and low spraying temperature during the HVOF spraying [
9] and the powder contact with the substrate surface without complete melting. Therefore, the as-sprayed coating was formed by incompletely melted particles, and the powder microstructure was preserved [
1].
VHT coatings presented different microstructures (
Figure 4b–d). The appearance of dispersed irregular block β-NiAl could be attributed to the diffusion and the grain growth during VHT [
9,
19]. At the cooling stage of VHT (<950 °C), the phase transformation of γ + β → γ’ + α was conducted [
20,
21]. Therefore, γ’-Ni
3Al was generated in VHT coatings (
Figure 4 and
Figure 9). The great number of grain/phase boundaries provides numerous fast diffusion paths of aluminum and therefore benefits the establishment of an exclusive Al
2O
3 scale [
22]. The large amount of lamellar and granular γ’-Ni
3Al in VHT coatings provides plenty of phase boundaries and promotes the establishment of an Al
2O
3 scale. VHT resulted in a reduction in porosity levels in the as-sprayed coating. This reduction was due to diffusion, which decreased the porosity via a sintering effect [
6,
23]. The high defect concentration within the coating acted as “easy passages” for oxygen diffusion into the coating, and the internal oxides were rapidly formed in the initial stage of oxidation [
24]. Thus, low porosity is beneficial to the oxidation resistance of NiCrAlY coatings. This point could be proven by
Figure 6. The compact microstructure of VHT1200 coatings obviously contain less internal oxide (
Figure 6d), which means better oxidation resistance.
The composition of TGO is related to oxidation resistance. An exclusive α-Al
2O
3 scale ensures a good oxidation resistance. θ-Al
2O
3 is a type of metastable alumina. It will convert into α-Al
2O
3 during the oxidation process. θ-Al
2O
3 is minimally protective against oxidation for two reasons. One is that the high growth rate of θ-Al
2O
3 accelerates the consumption of aluminum [
25]. The other is the volume expansion caused by the transformation of θ-α may lead to a crack in TGO [
18]. The reason why XRD did not detect the θ-Al
2O
3 could because the transformation of θ-α had been completed after the oxidation test [
26,
27]. However, the morphology of θ-Al
2O
3 is similar to whiskers, and the whisker-like oxide will be preserved on the surface of α-Al
2O
3 after the transformation of θ-α [
25,
28,
29,
30]. Hence, we inferred that the surface of the coatings did not generate θ-Al
2O
3 after VHT but directly generated α-Al
2O
3. No θ-Al
2O
3 grew on the surface of VHT coatings (
Figure 8), probably because of the microstructure and the concentration of yttrium during VHT. The reason could be concluded as follows. During the oxidation test, the following reactions can occur at the high temperature stage (>1000 °C) in VHT coatings [
15,
17,
19]:
Hence, the γ’-Ni
3Al in VHT coatings will completely transform to γ-NiCr in the early stage of the oxidation test. This result suggests that VHT coatings contain more γ-NiCr. Relevant studies [
9,
12] have shown that γ-NiCr tends to form a uniform oxide layer of α-Al
2O
3 and β-NiAl tends to form θ-Al
2O
3. Therefore, the aluminum and γ-NiCr produced by this reaction promote the generation of an exclusive α-Al
2O
3 scale. Meanwhile, dispersed β-NiAl prevented continuous θ-Al
2O
3 scale being generated on the VHT coatings’ surface. The formation of θ-Al
2O
3 can also be reduced by yttrium [
14]. High temperature enhances the concentration of yttrium from the coating surface (
Table 6), which prevented the generation of θ-Al
2O
3.
Figure 6 presents the order of mass gain as follows: as-sprayed > VHT1000 > VHT1100 > VHT1200. Combined with the above discussion, the following results can be concluded. The mass gain of the as-sprayed coating was maximum, which means the worst oxidation. The low content of yttrium and high concentration of β-NiAl caused the generation of continuous θ-Al
2O
3 scale on the surface of the as-sprayed coating (
Figure 8a,b). The rapid growth rate of θ-Al
2O
3 caused the thickest TGO of the as-sprayed coating (
Figure 7a). The unprotected θ-Al
2O
3 layer and high porosity made oxide diffusion in the as-sprayed coating easy and caused serious internal oxidation (
Figure 6a). VHT1200 presented minor mass gains because of the following reasons. (1) The γ’-Ni
3Al generated in the coating during VHT promotes the establishment of α-Al
2O
3 scale. (2) The dispersion distribution of β-NiAl and the concentration of yttrium on the coatings’ surface restrained the θ-Al
2O
3 generated on the coatings’ surface. (3) The extremely low porosity made it difficult for oxygen to enter the coatings and decreased the internal oxidation.
5. Conclusions
In this study, NiCrAlY coatings were prepared via HVOF spraying and VHT was carried out at different temperatures. Then, high temperature oxidation resistance was compared. The conclusions were as follows:
Compared with the as-sprayed, VHT1000 and VHT1100 coatings, the VHT1200 coating has the best high temperature oxidation resistance, which is related to the microstructure transformation under different VHT temperature conditions. After VHT, the γ’-Ni3Al phase appears in the coating, and with the increase in VHT temperature, more γ’-Ni3Al phase boundaries promote the formation of α-Al2O3. On the other hand, the concentration of yttrium and the phase transformation of γ’-Ni3Al → γ-NiCr on the surface of VHT1200 prevents the generation of θ-Al2O3. In addition, VHT is beneficial for reducing the porosity of the coating, and the VHT1200 coating has the lowest internal oxidation.