SAPS Combination to Protect

May 31, 2016 - Silicide Coating Fabricated by HAPC/SAPS Combination to Protect. Niobium Alloy from Oxidation. Jia Sun, Qian-Gang Fu,* Li-Ping Guo, and...
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Silicide Coating Fabricated by HAPC/SAPS Combination to Protect Niobium Alloy from Oxidation Jia Sun, Qian-Gang Fu,* Li-Ping Guo, and Lu Wang State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern Polytechnical University, Xi’an 710072, China ABSTRACT: A combined silicide coating, including inner NbSi2 layer and outer MoSi2 layer, was fabricated through a two-step method. The NbSi2 was deposited on niobium alloy by halide activated pack cementation (HAPC) in the first step. Then, supersonic atmospheric plasma spray (SAPS) was applied to obtain the outer MoSi2 layer, forming a combined silicide coating. Results show that the combined coating possessed a compact structure. The phase constitution of the combined coating prepared by HAPC and SAPS was NbSi2 and MoSi2, respectively. The adhesion strength of the combined coating increased nearly two times than that for single sprayed coating, attributing to the rougher surface of the HAPC-bond layer whose roughness increased about three times than that of the grit-blast substrate. After exposure at 1200 °C in air, the mass increasing rate for single HAPC-silicide coating was 3.5 mg/cm2 because of the pest oxidation of niobium alloy, whereas the combined coating displayed better oxidation resistance with a mass gain of only 1.2 mg/cm2. Even more, the combined coating could significantly improve the antioxidation ability of niobium based alloy at 1500 °C. The good oxidation resistance of the combined silicide coating was attributed to the integrity of the combined coating and the continuous SiO2 protective scale provided by the oxidation of MoSi2. KEYWORDS: niobium-based alloy, HAPC, plasma spray, oxidation, structure Metal silicide,4,28 such as WSi2, TiSi2, MoSi2, and their related alloy, presents excellent physical and chemical properties. For example, a properly alloyed MoSi2 would show a high bending strength at room temperature29 as well as an anomalous strengthening at high temperature.30 Moreover, because of the formation of a sealed glassy film at high temperature, MoSi2based coating could block the further inward diffusion of oxygen.31−34 It is worthwhile to note that MoSi2 has a close thermal expansion coefficient (CTE, αMoSi2 = 8.0 × 10−6/°C31) to niobium-based alloy (7.8−8.2 × 10−6/°C35). Recently, Mo− Si−Al36,37 has been sprayed immediately on niobium based alloy and it could efficiently protect niobium-based alloy from oxidation at 1250 °C. However, the bonding strength between sprayed coatings and substrates is not strong enough because of their mechanical adhesion.38 The combination, including HAPC plus spraying (or slurrying), has been considered as an innovative way to protect niobium-based alloy from oxidation. This technique usually involves two steps: spraying a Mo layer on the niobium-based alloy surface at first and depositing a silicide layer by HAPC subsequently.35,39 The coating structure commonly consists of a residually unsiliconized Mo layer and a MoSi2 based layer. A

1. INTRODUCTION Poor oxidation resistance of niobium-based high temperature alloys has received a great deal of attention in recent years.1−8 Halide activated pack cementation (HAPC), an economic and efficient way to prepare oxidation resistant coating, is widely employed to protect niobium based alloy from oxidation.9−13 Nevertheless, the coating prepared by HAPC exhibits many cracks or defects due to its rather high preparation temperature (1100 °C−1300 °C).9,14−16 In addition, the Nb-oxide products are not volatile and grow gradually with the oxidation prolonging.4 The accumulated growth of the nonvolatilized Nb2O5 often leads to a stress-concentrated zone. It is easy for cracking in this zone. Previous works mainly concentrated on selective oxidation protection for niobium based alloy by adding reactive elements such as Al,16−18 Cr,19,20 Zr,21 Ge,15 B,1 Hf, 16 and Y 17,22 to decrease the niobium depletion. Unfortunately, the service life of components is limited because of the low additive and the gradually increased Nb-oxide products. Plasma spraying is widely used to prepare thermal barrier coatings (TBCs) on high-temperature alloys.23−25 The high heat energy provided by plasma flame could melt spraying materials quickly and the solidification of the melted materials would form a lamellar structure coating.24,26 The coating prepared by plasma spraying often exhibits good thermal shock resistance27 and outstanding oxidation protective ability.24 © XXXX American Chemical Society

Received: April 18, 2016 Accepted: May 31, 2016

A

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Morphology of the atomized powder and (b) the corresponding particle size distribution. The inset image in a indicates that the main phase of sprayed powder is MoSi2 with high purity. The proper particle size, 24.41 μm (D50), is suitable for thermal spray technique.

Figure 2. Diagrammatic sketch of HAPC employed in this work. HAPC is an in situ chemical deposition process. Detailed information can be found in our previous work.34 compositional homogeneity. The substrate used in this work with a nominal composition (wt %) of W 5−6, Zr 1.5−1.7, Mo 2.1−2.5 (Bal. Nb) in the dimension of 10 mm × 10 mm × 3 mm, was obtained by wire-cutting then subsequently grit-blasted with SiC grit (600 μm on average) under 0.80 MPa pressure to get a relatively rough and clean surface. The specimens were then cleaned in an ultrasonic acetone bath and dried at 80 °C for 1−2 h. The pack powders for HAPC were the mixture of silicon powder, NH4F and the balance Al2O3 powder. Silicon powder in analytically pure (AR) level was the donor source and NH4F was used as the activator with a purity of 99.9%. Al2O3 (AR grade) acted as the inert filler in the pack providing the connective porosity for gaseous phase to deposit under high temperature.34 Spraydry atomized MoSi2 powders were used for spraying. The morphologies, phase, and particle size distribution of the powders are shown in Figure 1. The MoSi2 powder exhibits spherical geometry. The average diameter of the MoSi2 powders is 24.41 μm. 2.2. Coating Deposition. The niobium-based alloy substrate was first coated by HAPC and then deposited by SAPS. The diagram for HAPC is shown in Figure 2. Before deposition, the pack powders were weighed accordingly and mixed up using ball-milling equipment in the dry condition. The rotation speed of milling was kept at 90 r/min for 2 h to get uniformly mixed pack. Further details of mixing of the powders can be seen in ref 34. Subsequently, the mixed pack was filled in a cylinder Al2O3 crucible followed by embedding the cleaned substrate into the mixed pack. The crucible was sealed with an Al2O3 lid using high-temperature alumina-base cement binder before transferring into the reaction furnace. After the pack was loaded, the furnace was vacuumed to 60 Pa and then filled with high-purity Ar (99.999 wt %) gas to reach a standard atmospheric pressure. The reaction furnace was heated at a ratio of 12 °C/min up to the deposition temperature. The deposition process was conducted at 1200 °C for a certain time. The deposited layer was controlled within thickness of 110 ± 5 μm. After the deposition, the furnace was

Mo−Si−B coating system was fabricated successfully through the combined method.40,41 Employing vacuum annealing could further densify the sprayed Mo layer so as to improve the quality of as-deposited coating.42 However, the unsiliconizing retained zone, Mo layer with defects, restricts the service life because of the potentially generated volatile MoO3. Therefore, eliminating Mo layer could be proposed. Inspired by this idea, we designed a two-layer coating by conversely combining these methods without using Mo layer. Namely, the niobium-based alloy substrate was first treated through HAPC, and then deposited with MoSi2 by plasma spraying immediately. It would be expected that the following sprayed layer could both increase coating adhesion (supported by the HAPC-retained rough interface) and enhance oxidation resistance of the HAPC-coating. The present work combined HAPC and thermal spraying to fabricate a double-layer silicide protective coating on niobium based alloy. The HAPC layer was considered as a strong adhesive layer which was beneficial to bond for spraying. The microstructure change of the combined coating before and after oxidation was investigated by comparing with the single HAPCcoating and the single-sprayed MoSi2 coating. The microstructures and compositions of the oxide layers after oxidation were analyzed. The oxidation mechanisms of the coatings were discussed as well.

2. EXPERIMENTAL PROCEDURES 2.1. Materials. The niobium based alloy was prepared by vacuum nonconsumable arc melting under an argon atmosphere, and the obtained ingot was remelted at least six times in order to ensure B

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. XRD patterns of (a) the HAPC-coating and (b) the plasma sprayed coating. naturally cooled down to the ambient temperature. As shown in Figure 2, the deposition proceeds in a sealed crucible located at the hot-zone of the high-temperature reaction furnace. With the increased temperature, silicon resource catalyzed by NH4F transforms into gaseous silicon fluoride which occupied the pores among pack powders (2NH4F + Si → SiF2 + 2NH3 + H2). Then, pyrolysis of silicon fluoride leads to the activated Si atom. When the active Si atom reactivated with Nb-based substrate, a deposited layer presents. Summarily, during the HAPC deposition, it involves mainly three chemical reactions:43 initially gaseous halide generation, gradually forming reactive silicon atom (2SiF2 → SiF4+Si) and inward reaction diffusion of silicon source (2Si+Nb → NbSi2). Detailed description about the deposition mechanism of HAPC can be found in our previous work.34 The deposited resultants were cleaned ultrasonically in alcohol several times and dried at 80 °C for 2 h. For the second step, a high efficiency plasma spray system was used to deposit the MoSi2 coating. The HAPC-treated substrate was only ultrasonically cleaned without sand blasting before spraying. The spray system is composed of plasma torch, power feeder, gas supply, water cooling circulator and PC-control unit. The spraying power conducted in this work was 40 kW. To accelerate the particles melting, an interlet feedstock nozzle with an interdiameter of 5.5 mm was used. The feedstock rate was controlled at 4.1−4.5 r/min. The distance between the spray nozzle and substrate was kept at 100 mm. The sprayed coating was deposited in a thickness of 100 ± 10 μm. Two other kinds of silicide coatings, single HAPC coating and single SAPS coating were also deposited on niobium-based alloy for comparison. 2.3. Microstructure Characterization. The phase of coated samples was characterized by X-ray diffraction (XRD) with a Cu Kα (λ = 0.1542 nm) radiation, 0.03° step size and 0.28°/s scan speed using X̀ Pert Pro MD apparatus (PANalytical Netherlands). The morphologies of the coatings were determined by scanning electron microscopy (SEM, VEGAT-2, Germany). The chemical compositions were examined by energy dispersive spectroscopy (EDS). The roughness values of substrate before and after the HAPC process were analyzed using a 3D confocal laser microscope (Optelics C130, Lasertec, Japan). 2.4. Pull-off Test. The bonding strength of the coatings was tested according to ASTM C633, which was especially designed for plasmasprayed coatings. Cylindrical aluminum rods with dimensions of Φ25 mm × 25 mm were used as matching parts. Specimens were bonded with the end surface of matching parts by a modified acrylate adhesive. Then they were positioned perpendicularly for 5−10 min and solidified for 6−8 h at 80 °C. The pull-off test was carried out in a universal test machine (CMT5304−30 kN), and the largest force before failure, Fmax, of each specimen was recorded. For each kind of coating, five specimens were measured. The bonding strength was calculated by means of dividing Fmax by the real contact area. 2.5. Oxidation Test. The thermogravimetric-dynamic thermal analysis (TG-DTA) of the uncoated specimens was conducted using a thermogravimetric equipment (TGA/SDTA851, METTLER TOLEDO, Switzerland). The substrate specimens were charged inside the alumina crucibles of 5 mm inner diameter and 4 mm long and

subsequently placed inside the TG apparatus. Specimens were heated up to 1400 °C with a heating rate of 10 °C/min and held that temperature for 5 min in air atmosphere. The static isothermal oxidation behavior of the substrate and the coated samples was evaluated at 1200 and 1500 °C in static air. Prior to the oxidation test, the crucible was heated at 1350 °C until no mass change was observed. The specimens were placed in an alumina crucible for a certain period in an electric furnace. Subsequently, the mass of the oxidized sample was obtained by weighing the specimen in an electric balance with a sensitive of ±0.1 mg at ambient environment after taking out the specimen from the electric furnace. Then, the specimens were put into the furnace again for the next cycle. The mass change ratio (ΔW, mg/cm2) of sample was calculated using equation ΔW = (m1 − m0) /S, where m0 and m1 represent the initial and final mass (mg) of samples for each cycle during isothermal oxidation process and S represents the total surface area (cm2) of the initial specimen. The mass change rates of the specimens were obtained by calculating the average mass change rates of three specimens. The oxidation curves of tested specimens were reported as a function of oxidation exposure.

3. RESULTS AND DISCUSSION 3.1. Microstructure and Adhesion of the As-Deposited Coating. XRD patterns of the as-deposited coatings are shown in Figure 3. The phase constitutions of the coatings after HAPC and SAPS are NbSi2 (JCPDS No. 00−008−0450) and MoSi2, respectively. Figure 4 shows the microstructure and composition of the HAPC-coating. Seen from Figure 4a, the surface of the HAPC-coating is relatively coarse. Figure 4b indicates that a uniform layer is formed with some vertical microcracks in the coating. By EDS analysis (Figure 4d), NbSi2 is further confirmed for the HAPC-deposited coating. No intertransition layer, such as Nb5Si3 reported in other literatures,10,12 is detected due to the rather short deposition duration.33 Due to the inward diffusion of silicon atom perpendicularly to the surface, it reveals a typical column crystallization for NbSi2 coating as shown in Figure 4c. Vishwanadh et al.44 detected the crystallization growth of HAPC-coating with vertical column crystal grains in the deposited coating through transmission electron microscope. Column structure, which is vertical to the substrate surface, benefits adhesion between the coating and the substrate.23 Roughness is a critical parameter associated with adhesion for the sprayed coatings.45−47 A rougher adhesive interface would provide a stronger bonding of the deposited coatings.47,48 3D morphologies of the grit-blast and the HAPC-treated substrate are shown in Figure 5. The characteristic value of the substrate corresponding to the uneven level, Sa, increases from 4.71 C

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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The cross-sectional back scatter electron (BSE) images of the single deposited coatings and the combined coating are shown in Figure 6. It can be seen that the NbSi2 phase (Figure 6a) is

Figure 6. Cross-sectional BSE morphologies of (a) the single-sprayed, (b) the single HAPC and (c) the combined coatings. Some defects exist on each single coating, especially for the single-sprayed coating as magnified in b. It can be seen that the combined coating exhibits a denser structure comparing with other two single coatings.

Figure 4. (a) Surface, (b) polished cross-sectional, and (c) fracture cross-sectional morphologies of the HAPC coating and (d) the corresponding EDS result of spot A in b. Panel a indicates a rough surface presents on niobium based alloy after HAPC treatment. The HAPC procedure brings a column-growth coating (NbSi2, as shown in d) perpendicular to the substrate (c), whereas some defects can also be found (b).

continuous and uniform in the single HAPC coating. The single sprayed coating displays an obvious lamellar structure as seen in Figure 6b. SiO2 located among lamellar splats may derive from the somewhat oxidation of the in-flight melting powders during spray process. Many defects, such as cracks (Figure 6a) and holes (inset image of Figure 6b), can be found in the single deposited coatings. From Figure 6c, the combined coating exhibits a double-layer structure, i.e., the compact bond layer, NbSi2, and the relatively loose top layer, MoSi2. It can be found that some defects at MoSi2/NbSi2 interface in the HAPCcoating are sealed to some extent by the sprayed materials. Besides, some holes and unmelted particles still remain in the outer-sprayed coating, which are the characteristic of thermal sprayed coatings.23,49 The adhesion strength between coating and substrate was tested through pull-out test and the results are shown in Table 1. The average adhesion strength of the combined coating is Table 1. Adhesion Strength of the Single-Sprayed MoSi2 Coating and the Combined Coating after Pull-out Test coating method adhesion strength (MPa)

spray only

combined technique

14.6 ± 2.1

20.9 ± 5.4

20.9 ± 5.4 MPa and that of the single-sprayed coating is only 14.6 ± 2.1 MPa. It is well-known that the interface between sprayed coating and substrate is mechanical jointed, exhibiting a relatively weak bonding under service.23 The interdiffusion layer (HAPC-coating, exhibiting chemical adhesion) is in situ synthesized and has strong adhesion to substrate.50 The rougher surface of HAPC-layer could further provide a better joint to the sprayed coating.48,51 Therefore, the combined coating exhibits better adhesion strength than the one with the single spray. 3.2. Microstructure of the Coatings after Oxidation. Figure 7 shows the microstructure and phase constitution of the single HAPC-coating after oxidation. As can be seen from the cross sectional BSE image (Figure 7a), a lot of vertical cracks present. They grew continuously toward the coatingsubstrate interface and stopped their propagation at the front of Nb5Si3, which was a new interdiffusion zone with relatively compact structure (Figure 7b). The formation of Nb5Si3 could

Figure 5. Roughness evolution of the substrate (a) before (gritblasting) and (b) after HAPC process.

(Figure 5a) to 15.53 μm (Figure 5b) after HAPC treatment, indicating a rougher surface has been obtained by HAPC. D

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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formed due to the thermal mismatch between coating and substrate. Figure 8b indicates that the surface oxide scale has a thickness of about 10−20 μm. The XRD patterns in Figure 8c reveal that the protective SiO2 is the main constitution of the surface scale. Delamination fracture of the coating can be found in some location (Figure 8b). This might be caused by the lower adhesion strength (Table 1) of the single sprayed MoSi2 coating. The combined coating exhibits continuous structure, as shown in Figure 9. Relatively flat and smooth surface can be obtained after oxidation (Figure 9a). XRD indicates that a continuously protective SiO2 was formed on the surface (Figure 9g). From the cross-sectional view in Figure 9b, the uniform and compact phase can be clearly found in each layer on the surface. A five-layer structure appeared on the surface after oxidation (Figure 9c). To accurately identify each layer’s composition, we employed map-scanning (Figure 9d), linescanning (Figure 9e), and spot-scanning (Figure 9f) EDS characterizations. From Figure 9e, it can be concluded that the O mainly located in the outer layer (E zone), the Mo in the near-top layer (D zone), and the Nb in the inner layer (A, B, and C zones). It is interesting to find that the amount of the Si element in the C zone is small while the one of the O in this zone seems to be high. Figure 9d verifies this speculation. It just likes a Si channel between outer D zone and inner B zone. From Figure 9d, e, it can be identified all zones’ compositions except C zone, namely, Nb3Si5 for A zone, NbSi2 for B, MoSi2 for D, and SiO2 for E. SiO2 is even found to disperse in MoSi2 layer and at the MoSi2/NbSi2 interface. As shown in Figure 9f, we can see that C zone is a mixture of the loose Nb2O5 and the dense Nb5Si3. In addition, a small amount of SiO2 located at Nb5Si3 + Nb2O5 layer was also detected in Figure 9f. It can be inferred that Nb5Si3 and Nb2O5 are the oxidation products of the NbSi2 coating. The SiO2 layer is related to the oxidation of MoSi2 coating, which possesses a successive and uniform distribution on the surface. Microstructure evolution of the combined coating during exposure will be discussed subsequently. 3.3. Oxidation Behavior. 3.3.1. Oxidation Behavior of Bare Substrate. Figure 10a displays the thermal gravity (TG) curve of the alloy substrate. The substrate shows three distinct behaviors during continuous heating process in air. Since heated between 350 and 800 °C (stage I), the alloy starts to be oxidized. There is a slow weight gain due to the formation of nonstoichiometric oxides of niobium, NbOx (1 ≤ x ≤ 2)3,53 and orthorhombic-Nb2O5. XRD patterns in Figure 10b can also verify this argument. With further heating from 800 to 1200 °C, the weight of the specimen increases rapidly and then becomes steady due to the formation and growth of monoclinic-Nb2O5 (stage II). It was reported that orhtorhombic-Nb2O5 was an unstable phase.54 Its transformation from orthorhombic to monoclinic crystalline was quickly fulfilled in temperature range of 800−930 °C. One exothermic event at approximately 800 °C in the inset DTA curve of Figure 10a might be assigned to this phase transformation of Nb2O5. TG curve presents a continuous weight gain up to 1200 °C due to the nonisothermal heating process. Beyond 1200 °C (stage III), a sharp increase gain in weight is observed because more oxidation products would be formed after the surface scale peeling out. It can be seen from the inset image of Figure 10c that a new surface presents after spallation of 0.5 h oxidized surface scale. Meanwhile, the newly bare substrate gains its mass quickly thereafter (Figure 10a).

Figure 7. (a, b) Cross-sectional, (c) surface morphologies, and (d) XRD pattern of the single HAPC-coating after oxidation at 1200 °C for 24 h. The existence of Nb2O5 in the surface scale leads to a loose structure (b) failing to provide an efficient protection from oxidation.

result from the depletion of Si atoms in the coating due to the outward diffusion of Si leading to the formation of silica on the NbSi2 coating surface.44 According to the XRD patterns (Figure 7d), Nb2O5 and SiO2 are the predominant phases of the surface oxide scale. In Figure 7c, it can be found that the shape of Nb2O5 is rod with a length of 5−10 μm and width of 1−3 μm. It is reported that Nb2O5 with special feature often coexists with SiO2 during the oxidation of NbSi2.52 The discontinuous distribution of Nb2O5 produced some pores in surface scale (Figure 7b) and oxygen could penetrate through the loose Nb2O5 layer further, leading to internal oxidation of the HAPC coating.4 The single-sprayed coating displays different morphologies of the surface scale, as shown in Figure 8. Many cracks and pores spread all over the surface (Figure 8a). The cracks might be

Figure 8. (a) Surface, (b) cross-sectional morphologies, and (c) XRD pattern of the samples with single-sprayed coating after oxidation at 1200 °C for 4 h. E

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 9. Surface and cross-sectional analysis of the combined coating after oxidation. (a) Continuous and uniform silica film formed on the surface; (b) compact layered structure, all of which improved the antioxidation capacity; (c) magnified layered structure includes five layers: Nb5Si3, NbSi2, Nb5Si3+Nb2O5, MoSi2, and SiO2 from inner to outer; (d, e) main element distribution through the cross-section; and (f) further verification of the mixture layer of Nb5Si3+Nb2O5; (g) XRD pattern of the surface scale after oxidation.

Pilling−Bedworth theory, the Nb2O5 PBR (Pilling-Bedworth rate) is so large (2.69) that volume expansion of Nb2O5 would lead to the large interface stress, which would even tear the adhesive surface oxide scale, i.e., spallation of the surface scale.55,56 Therefore, the Nb based alloy losses its weight due to the continuous spallation of oxides (see the inset image in Figure 10c).

Isothermal oxidation curve of the alloy is shown in Figure 10c. The oxidation process continues with a rapidly increased weight gain indicating catastrophic disintegration. The uncoated sample experiences nearly linear oxidation with a final weight gain of 517.8 mg/cm2. It is easy to find that the oxidation products, mainly Nb2O53,53,55 (Figure 10b), existed on the alloy surface after exposure to 1200 °C. According to the F

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 10. Thermal dynamic analysis for bare substrate in static air: (a) thermogravimetry plots and the corresponding DTA curve in local range of 600−1000 °C; (b) XRD patterns of the alloy after oxidation at 800 and 1200 °C, respectively; (c) thermal kinetic curve at 1200 °C as a function of exposure time (the inset image indicates the spallation of surface oxide scale after 0.5 h exposure).

Figure 11. (a) Oxidation kinetic curves of three different kinds of coating at 1200 °C in static air and (b) the fitted curves related to the two-step coated sample and the single HAPC sample.

3.3.2. Oxidation Behavior of the Coated Samples. Figure 11a displays the isothermal oxidation kinetics of the coated samples. It can be found that the single plasma-sprayed coating possesses the worst oxidation protective capacity with a big mass loss of 14.38 mg/cm2. The HAPC-coating exhibits a mass gain of 3.5 mg/cm2 after the whole oxidation process while only 1.2 mg/cm2 is found for the combined coating system. The kinetic curves of the samples with HAPC-coating and the twostep coating seem to follow a parabolic law. After fitting process (Figure 11b), the parabolic rate constant kp (mg2 cm−4 h−1) is 0.53 for the HAPC-coating and 0.057 for the two-step coating, indicating the combined coating possesses better oxidation resistance. It can be concluded that the combined coating, with inner HAPC layer and outer plasma sprayed layer, could effectively protect niobium-based alloy against oxidation. In Figure 11, it is worth noting that the kinetic curve does not contain weight loss even though the coating is a

combination of the single coating in the two forms, i.e., the HAPC-coating and the plasma sprayed coating. This phenomenon can be attributed to the effect of interface adhesion. In the single-sprayed coating, the adhesion strength is not large enough to resist interface delamination fracture during oxidation (see Figure 8b) because of the small roughness (see Figure 5a). The single-sprayed coating fails to protect the alloys after 4 h oxidation. HAPC treatment can provide the alloy substrate a rougher surface (see Figure 5b), which benefits the interface adhesion. The improved adhesion (Table 1) of the combined coating makes its oxidation kinetics different. No interface delamination fracture is found from the section morphologies of the two-step coated samples (Figure 9b) after oxidation. It is clear that the uniform five-layer structure is formed on the surface. The weight gain in the kinetic curve of the two-step coating might be related to the presence of SiO2, G

DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Oxidation experiment at higher temperature (1500 °C) was also performed and the corresponding thermal kinetic curves of the bare alloy substrate and the two-step coated samples are shown in Figure 13. The bare alloy experienced seriously pest

Mo5Si3, Nb2O5 and Nb5Si3, which offset and exceed the weight loss due to the evaporation of MoO3 on the surface. To better understand the oxidation process of the combined coating, the main mechanism at 1200 °C is sketched in Figure 12. As can be seen from Figure 12a, a duplex layer structure was

Figure 12. Schematic illustration of the formation process of the scale upon oxidation at 1200 °C for different stages: (a) as-sprayed, (b) during oxidation, (c) after oxidation.

deposited after the two-step preparation. The main chemical reactions taking place on the coating exposed to 1200 °C in dry air are listed in eqs 1−6. Si in the outer MoSi2 layer outward diffused to react with oxygen to form SiO2 film and Mo reacted with O to form volatile MoO334,39,57,58 upon exposure (Figure 12b). In addition, partial oxidation byproduct SiO2 (eq 1) was dispersed in the outer MoSi2 layer (see Figure 12c) due to the internal oxidation through defects of the sprayed coating. When the inward-diffusion O reached the MoSi2/NbSi2 interface, the under near-layer would convert to Nb 5 Si 3 + Nb 2 O 5 intermediate layer with the coexistence of little SiO2 (see Figure 12c and eqs 5 and 6) attributed to the oxidation of NbSi2.13,52 In addition, since the diffusion coefficient of Si in NbSi2 is higher than that of Nb,59 most Si diffused inward to substrate promoting the interdiffusion zone, Nb3Si5, while the remainder Nb in NbSi2 contributed to the formation of Nb2O5 (Figure 12b).12,60 After oxidation, there appeared five layers on the alloy surface as shown in Figure 12c. 2MoSi 2(s) + 7O2 (g) → 2MoO3(s) + 4SiO2 (l)

(1)

MoO3(s) + H 2O(g) → MoO2 (OH)2 (g)

(2)

2CO2 (g) → 2CO(g) + O2 (g)

(3)

MoO3(s) + 2CO(g) → MoO(g) + 2CO2 (g)

(4)

4NbSi 2(s) + 13O2 (g) → 2Nb2O5(s) + 8SiO2 (l)

(5)

5NbSi 2(s) + 7O2 (g) → Nb5Si3(s) + 7SiO2 (l)

(6)

Figure 13. Thermal kinetic curves of (a) the alloy substrate and (b) the two-step coating sample employed at 1500 °C. The inset image displays the actual surface scale at certain exposure.

degradation as soon as being put into hot condition. After 10 min exposure, its surface oxide scale peeled quickly. Again, the degraded surface was oxidized further (Figure 13a). The alloy finally lost its mass of 422.8 mg/cm2 in less than an hour. Nevertheless, the two-step fabricated coating could provide an efficient protection for the alloys. Although the two-step coating initially exhibited somewhat mass loss due to the evaporation of MoO3 (eqs 2 and 4) and gradually rebound to gain its mass due to the internal oxidation (eqs 5 and 6), its integrity remained the same as the original (Figure 13b). Importantly, a glassy silica film, detected on the sample surface after 8 h exposure, indicates a long-term protection would be expected thereafter.

4. CONCLUSIONS Plasma spraying combined with HAPC was used to deposit a two-layer silicide coating on the niobium-based alloy. The combined coating consisted of a NbSi2 transition layer and a sprayed MoSi2 outer layer. NbSi2 layer prepared with HAPC exhibited a relatively uneven surface which was beneficial to the spray-coating adhesion. The adhesion strength of the combined coating increased by 20% compared to that of the single sprayed coating. The mass change rate for the single HAPCsilicide coating and the single sprayed MoSi2 coating was 3.5 and 14.38 mg/cm2, respectively, whereas the combined coating possessed better oxidation resistance with only 1.2 mg/cm2 after the same duration at 1200 °C. Furthermore, the combined coating provided even better antioxidation performance at 1500 °C. The outer protective film formed under high temperature and the thermal match among each layer attributed to the improving oxidation resistance of the combined coating.

Although water vapor and carbon dioxide during oxidation exposure were very little, it might influence the chemical reaction proceeding more or less. K. Hansson et al.61 reported that water vapor could promote the volatility of MoO3 at initially lower temperature (600 and 700 °C) due to the formation of gaseous MoO2(OH)2 (eq 2). Besides, reduction reaction (eq 4) of MoO3 might be fulfilled soon due to the existence of unstable nonstoichiometric oxides (MoOx, x < 3).62 Hence, it can be seen that the gaseous product is the unique substance leading to the mass loss of the combined coating systems. On the other hand, the mass gain for the formation of Mo5Si3, SiO2, Nb2O5, and Nb5Si3 quickly offsets the loss during the oxidation. Thereby, the specimens exhibit mass gain without mass loss in the whole oxidation process. In addition, improved by the outer sprayed layer, the mass of the two-step prepared specimens increases slowly in comparison with that of the single HAPC case.

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DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by National Natural Science Foundation of China (51521061) and the “111” Project (Grant B08040).

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REFERENCES

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DOI: 10.1021/acsami.6b04599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX