Influence of LaH2 on oxidation characteristics and irradiation melting characteristics of a tungsten coating fabricated by atmospheric plasma spraying

Surface & Coatings Technology 299 (2016) 143–152

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Influence of LaH2 on oxidation characteristics and irradiation melting characteristics of a tungsten coating fabricated by atmospheric plasma spraying Qing Yu Hou a,b,⁎, Lai Ma Luo a, Zhen Yi Huang b, Ping Wang b, Ting Ting Ding b, Yu Cheng Wu a,⁎⁎ a b

National-local Joint Engineering Research Center of Nonferrous Metals and Processing Technology, Hefei University of Technology, Hefei, Anhui 230009, China Anhui Key Laboratory of Metal Materials and Processing, Maanshan, Anhui 243002, China

a r t i c l e

i n f o

Article history: Received 5 January 2016 Revised 1 May 2016 Accepted in revised form 10 May 2016 Available online 12 May 2016 Keywords: Atmospheric plasma spraying (APS) Tungsten coating LaH2 Oxidation characteristics Irradiation melting characteristics

a b s t r a c t In this work, as a kind of rare earth hydrides, 1.5 wt.% LaH2 powder was introduced into a tungsten powder and then processed by atmospheric plasma spraying (APS) to form a coating. The oxidation characteristics and laser irradiation melting characteristics of the tungsten coatings with or without LaH2 addition were investigated and compared. The results showed that the coatings exhibited typical splat/lamellar microstructure. WO3, which was distributed between lamellar layers or in splats, formed in the APS-W coating by the oxidation reaction between W and O2. The introduction of LaH2 in the tungsten-based coating led to forming WO2 by a deoxidization of WO3. La2O3 with a morphology of long strip, which was distributed mainly between lamellar layers and had a filling effect on the pores, formed in the APS-W/LaH2 coating by the oxidation reaction between La/LaH2 and WO3/ O2. The oxide/oxygen content and the porosity of the APS-W/LaH2 coating were apparently lower than those of the APS-W coating; and the relative density and thermal conductivity of the former were higher than those of the latter. The trends of the crack to form in the APS-W/LaH2 coating were lower than that in the APS-W coating when the two types of coatings were irradiated by a laser. The coarse elongated tungsten grains formed in laser irradiation melting sample for the APS-W coating. But the fine equiaxial tungsten grains formed in laser irradiation melting sample for the APS-W/LaH2 coating, which was mainly the result of the pinning effect of the reprecipitated spherical La2O3 particles on the re-crystalline tungsten grain boundaries. The ability of the APS-W/ LaH2 coating to resist high heat loading was greater than that of the APS-W coating. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tungsten coatings are thought as promising candidate armor materials for plasma-facing components in the future International Thermonuclear Experimental Reactor (ITER) [1–4]. Among the potential coating techniques to be used to fabricate tungsten coatings, atmospheric plasma spraying (APS) looks particularly attractive owing to its ability to cover large areas with thick coatings and its relative simplicity and cost-effectiveness [5–7]. However, APS coatings commonly have some drawbacks such as higher oxygen/oxide content and porosity, which would reduce the mechanical and thermal properties of the tungsten coatings [8–11]. In order to prevent the negative effects of APS coatings, vacuum plasma spraying (VPS), low-pressure plasma spraying (LPPS), and inert gas atmosphere plasma spraying (IPS) techniques have been applied to

⁎ Correspondence to: Q.Y. Hou, National-local Joint Engineering Research Center of Nonferrous Metals and Processing Technology, Hefei University of Technology, Hefei, Anhui 230009, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (Q.Y. Hou), [email protected] (Y.C. Wu).

http://dx.doi.org/10.1016/j.surfcoat.2016.05.029 0257-8972/© 2016 Elsevier B.V. All rights reserved.

prepare tungsten coatings [1,7,12–15]. However, it is not possible to spray a large area using these spraying techniques to fabricate tungsten coatings in a cost-effective manner [11,16]. Huang et al. [16] proposed that high-speed atmospheric plasma spraying (HAPS) technique could be used to fabricate high-quality tungsten coatings with low cost and simplicity as compared with VPS, LPPS, and IPS. Nevertheless, the cost of HAPS technique is still relatively high as compared to APS technique. Alternatively, post-processing on APS tungsten coatings using vacuum annealing with or without pressure is practical and feasible way to achieve high-quality tungsten coatings, attributing to a reduction of tungsten oxides [10,11]. However, extra time consuming vacuum annealing and the complexity of applying pressure in vacuum annealing should not be ignored. Therefore, other effective way to overcome the negative effects of tungsten coatings fabricated by APS technique should be explored. It might be effective to reduce the content of tungsten oxide in an APS-W coating by the introduction of rare earth (RE) because it would react with tungsten oxide to form tungsten and RE oxide. However, it was reported that the activity of RE element in air was very high [17]. It might be easily oxidized in air before it was introduced in to a tungsten material, limiting its positive effect in a tungsten material.

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Therefore, other way to introduce RE into a tungsten material should be explored. It was reported that the stability of RE hydrides in air, such as LaH2, was higher than of their parent metal [17]. It is reasonable to propose that it might be more effective to introduce RE in the form of its hydride into an APS-W coating than that in the form of RE element. In addition, because it has been reported that a tungsten-based material with RE oxide dispersion commonly had excellent properties [1,2,18– 22], the properties of a tungsten coating fabricated by APS technique might be also improved by the introduction of RE hydride. However, no works have been focused on fabricating a RE hydride doped tungsten-based coating using APS technique. It would be interesting and important to investigate the influence of RE hydride addition on the microstructure and properties of a tungsten-based coating fabricated by APS technique. On the other hand, melting and evaporation of tungsten-based material under plasma impact is anticipated in ITER because the possible loss of plasma control cannot be excluded in the actual operations [23]. Although some attempts have been made to study the characteristics of the melted La2O3-doped tungsten-based material by melting experiments [18,21,24], phenomena on the melted tungsten-based coatings with La2O3 formation were rarely concerned. Therefore, in this work, as a try, one of the RE hydrides, 1.5 wt.% LaH2 powder was introduced into a tungsten powder then processed by APS technique to form a coating. The obtained tungsten coating was purposefully over oxidized in order to highlight the effect of LaH2 addition on the oxidation characteristics of the tungsten coating; and the parameters were also to be used to fabricate LaH2-doped tungsten-based coating. The oxidation characteristics of the tungsten coatings without or with LaH2 addition were investigated and compared. The influence of LaH2 on the oxygen/oxide content of the tungsten-based coatings was tested and discussed. In addition, some properties, such as porosity, relative density, and thermal conductivity of the two kinds of coatings were tested and compared. Finally, the behavior of the tungsten-based coatings under high heat loading was evaluated by irradiation melting the coatings using a CO2 continuous laser. 2. Experimental procedures

the Rietveld refinement method program RIETAN-FP [25]. The goodness-of-fit of the refinement results was evaluated by minimizing weighted-profile R-factors (Rwp) and Rietveld standard deviations of the structural parameter χ2 (S) quantities [26,27]. The characteristics of tungsten oxide and the existence of La2O3 phase in the LaH2-doped tungsten-based coating was verified using X-ray photoelectron spectrometer (XPS, Escalab250Xi, Thermo Scientific, USA). The Al Kα was used for X-ray source. Both chambers were evacuated to b10−7 Pa in order to remove the gaseous impurities, such as H2O, O2, and H2. 2.2. Property characterization The oxygen content of the coatings was detected by a Nitrogen/Oxygen analyzer (TC600, Leco, USA). The densities of the coatings were measured according to the Archimedes' principle [28]. The relative densities were calculated from the volume fraction and theoretical densities of W and LaH2 adopted as 19.25 g/cm3 and 5.36 g/cm3, respectively. Three measurements were conducted to determine the averaged values of the oxygen content and the density of each coating. The porosities of the coatings were estimated from SEM micrographs taken from several locations of a specimen by image analyzing software of ImageJ; at least 15 SEM images (which cover most of the typical areas of the cross-section of the coatings) were used to achieve a statistic on the porosities. The specific heat capacity (Cp) was measured on the free-standing coatings of Φ 5 mm × 200 μm using a synchronous TG-DSC thermal analyzer (STA449C, Germany) at room temperature. The thermal diffusivity (α) was conducted on the free-standing coatings of Φ 12.7 mm × 200 μm using a laser flash diffusivity system (LFA457, Germany) at room temperature. The thermal conductivity (λ) was calculated from the measured Cp, α and density (ρ) of the coatings based on the formula of λ = α·Cp·ρ. The behavior of the fabricated tungsten-based coatings under high heat loading was evaluated by irradiation melting the coatings using a CO2 continuous laser (DJ-HL-T5000B, China) with a spot diameter of 2 mm and power of 2500 W. The scanning rate was set at 3000 mm·min− 1. Then the treated specimens were observed using the above-mentioned SEM and XPS.

2.1. Coatings preparation and microstructure characterization 3. Results and discussion Pure tungsten powder (purity N 99.9%) and tungsten-based composite powder with 1.5 wt.% LaH2 (purity N 99.5%) addition (W/LaH2) were used as feedstocks to fabricate coatings using an atmospheric plasma spraying (APS, APS-2000A, China) system with power of 4.2 kW (70 V and 600 A). Argon gas was used as plasma forming gas (primary gas) and carrier gas, respectively. Reduced activation steel substrate was grit-blasted and ultrasonically cleaned prior to the plasma spraying process. The roughness Ra of the substrate was about 10.0 μm. During APS process, the backside of the substrate was kept at a low temperature by air jet cooling to avoid the detachment of the coating from the substrate. Detailed spraying parameters were shown in Table 1. The microstructures and element compositions of the powder before and after APS treatments were investigated by scanning electron microscope (SEM, SU-8020, JSM-6490, SU1510, Japan) with energy dispersive X-ray spectrometer (EDX, Oxford, Japan). The size of the powder was estimated by image analyzing software of ImageJ. The phase constitutes of the powder and the coating were analyzed by X-ray diffractometer (XRD, D/MAX-2500V, Japan) operating with Cu Kα (λ = 1.5406 Å) radiation. XRD profile of the W/LaH2 powder was then analyzed using Table 1 Spraying parameters to fabricate tungsten-based coatings using APS machine. Parameters Primary gas

Carrier gas

Spray distance

Traverse speed

Powder feed rate

Units Value

L/min 5

mm 80

mm/s 400

g/min 10

L/min 60

3.1. Characteristics of the blended powders SEM microscopy of W, LaH2 and W/LaH2 powders is shown in Fig. 1a–c. It can be seen from Fig. 1a and b that the morphologies of W and LaH2 powders are all irregular and their sizes are about 4–20 μm and 0.2–11 μm, respectively. When the two types of powders were mixed in a planetary ball mill, the W/LaH2 composite powders consisted of W particles with a large size and LaH2 particles with a small size, indicated by the EDX analyses which were not provided, as shown in Fig. 1c. The morphology and size of the W powder hardly changed, but that of the LaH2 powder changed much when compared with those before mixing. LaH2 powder with a large and sharp edge was milled into a small and round blunt one, attributing to its high brittleness [29]. Fig. 1d shows the XRD Rietveld refinement result of the W/LaH2 powders. α-W, LaH2 and LaH2(H2O) phases can be indexed. The formation of LaH2(H2O) might be ascribed to the introduction of H2O during the mixing process. Table 2 illustrates the refinement result from Fig. 1c. It can be seen that the relative contents of α-W, LaH2 and LaH2(H2O) phases in the W/LaH2 powder are respectively 98.1 wt.%, 1.2 wt.% and 0.7 wt.%, which seem to be different from that the above designed compositions. It would not be questioned that a completely homogenizing W/LaH2 powder was not easily obtained by mechanical mixing in a planetary ball mill. Therefore, the relative content of tungsten phase in the specimen to conduct XRD measurement might be different to that of the nominal composition. In addition, the relative content of tungsten phase in the W/LaH2 powder would also be

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Fig. 1. SEM micrograph of the feedstock: (a) W powder; (b) LaH2 powder; (c) W/LaH2 powder prepared by ball mill; (d) XRD Rietveld refinement result of the W/LaH2 powder.

decreased because of the introduction of H2O in LaH2 to form LaH2(H2O) phase during the mixing process. Finally, calculation error in the XRD Rietveld refinement would also be considered. Considering the aforementioned factors, therefore, it can be proposed that the refined result agreed well with the given nominal compositions of the W/LaH2 powder. 3.2. Phase characterization of the coatings Fig. 2a shows the phase constituents of the APS-W coating. Phases of α-W and WO3 can be indexed from its XRD patterns. Some researches had reported that WnO3n − 2 and/or WnO3n − 1 might be formed in tungsten materials [30–33]; and the atom ratio O/W is between 2 and 3, depending on temperature [30–32]. The various oxidation states of tungsten oxide, such as WO2, WO3, WO2.9, and so forth, might be obtained at different temperature [33]. The typical reduction sequence might be expressed as follows [10,11,34]: WO3 →WO2:9 at 893 K

ð1Þ

WO3 & WO2:9 →WO2 at 1066 K

ð2Þ

WO2 →W at N 1173 K:

ð3Þ

In the present work, when tungsten was oxidized to form WO3 in the APS-W coating, reactions (1)–(3) might not occur. This is due to the fact that the velocities of plasma spraying are high (200–600 m/s [35]); and the solidification of liquid/semi-liquid droplets is rapid (105–7 K/s [35]). Therefore, there is not enough resident time for the transformation from Table 2 Phase constitution, structural parameters, and phase abundance of W/LaH2 powders as determined by XRD. Materials

Phases

Space group

Lattice parameters (nm) a

Powders

α(W)

Im3m

0.31671 (4)

Rwp = 8.45%

LaH2

0.56303 (8)

S = 6.52

LaH2(H2O)

Fm3m P4/nmm

0.65378 (9)

b

c

Abundance (wt.%) 98.1 1.2

0.38668 (8)

0.7

WO3 to other tungsten oxides. Additionally, when droplets impinged on the substrate, their solidification rates were also high because of the large temperature gradient [35], limiting the transformation shown in Eqs. (1)–(3). Therefore, WO3 would be formed and preserved in the APS-W coating, as indicated by an enlarged local graph of its XRD patterns shown on the top right of Fig. 2a. Although it has been reported that the crystal structure WO3 is different because it has been reported that there are as high as eleven variants of WO3 [33], the formed one in the present work was just one of them. In comparison, only α-W can be indexed in the XRD patterns of the APS-W/LaH2 coating, as shown in Fig. 2b. It was no doubt that tungsten oxide would form in the APS-W/LaH2 coating, though it might be reduced by the introduction of LaH2. If the formed tungsten oxide was reduced by LaH2, La2O3 should form in the APS-W/LaH2 coating. However, no XRD peaks for tungsten oxide and La2O3 can be indexed in the XRD patterns of the APS-W/LaH2 coating. As pointed out by our previous work that tungsten oxide and an introduced particle located mainly between lamellar layers of an APS tungsten-based coating [36]. Because the distribution of the lamellae in the APS coating was perpendicular to the coating surface, most of the tungsten oxide and La2O3 in the APS-W/LaH2 coating would be covered by the lamellae, decreasing their opportunity to be detected by XRD tested on the coating surface. The other possible reason for that might be attributed to the relative contents of tungsten oxide and La2O3 in the APS-W/LaH2 coating might be less, decreasing their chance to be detected by XRD. In order to show whether tungsten oxide and La2O3 formed in the APS-W/LaH2 coating or not, XPS technique was used, as shown in Fig. 2c–f. It can be obtained from Fig. 2c and d that the binding energies of the W4f7/2 and O1s are 31.35 eV and 531.25 eV, respectively. The binding energies can be assigned to those values in WO2 configurations [37]. Similarly, the binding energies of the La3d5/2 and O1s being about of 834.9 eV and 530.59 eV can be assigned to those values in La2O3 configurations [37], as shown in Fig. 2e and f. In other words, it can be concluded from Fig. 2c–f that WO2 and La2O3 formed in the APS-W/LaH2 coating. The result is similar to a vacuum sintering Ti-Fe-Mo alloy that La2O3 would be formed in the alloy because of the introduction of LaH2 in the alloy [17]. Therefore, it can be proposed that the above-mentioned reasons to explain why tungsten oxide and La2O3 could not be indexed by XRD patterns for the APS-W/LaH2 coating was logical.

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Fig. 2. Phase characteristics of the coatings: (a) XRD pattern of the APS-W coating; (b) XRD pattern of the APS-W/LaH2 coating; (c) (d) W4f7/2 and O1s XPS spectra of WO2 in the APS-W/ LaH2 coating; (e) (f) La3d5/2 and O1s XPS spectra of La2O3 in the APS-W/LaH2 coating.

Nevertheless, it should be clarified why WO3 can be indexed in the XRD patterns for the APS-W coating shown in Fig. 2a. This might be due to the fact that the relative content of tungsten oxide in the APS-W coating might be higher than that in the APS-W/LaH2 coating, increasing its chance to be detected by XRD. In addition, the distribution of tungsten oxide in the APS-W coating might be different from that in the APSW/LaH2 coating. Far detailed information about these considerations would be provided by following microstructure observations. Compared to the APS-W coating, it can be seen that WO3 located in the APS-W coating changed into WO2 in the APS-W/LaH2 coating. This might be the reduction of WO3 by the introduced LaH2. 3.3. Microstructure and oxidation characteristics of the coatings Fig. 3a shows a typical cross-sectional morphology of the APS-W coating. Generally, splat/lamellar structure and splat boundary of thermal spraying coating are observed; and the other phase can also be seen mainly around lamellar gaps. The well-melted tungsten particles forming the lamellar layers are called splats; and the dark lines between splats are called splat boundaries [38]. Another typical microstructure of

plasma spraying coating, which was made up of un-melted or semimelted tungsten particles and splashed debris from the impacting tungsten splats can also be seen. EDX results (Fig. 3b) obtained from Fig. 3a give evidence that the pale gray phase is tungsten and the dark gray phase is tungsten oxide. More details about the composition distribution in the APS-W coating, which might be used to indicate the phase composition of the microstructure, could be obtained by EDX mapping, as shown in Fig. 3c–e. It can be proposed from Fig. 3c–e that the dark gray phase shown in Fig. 3a is O-rich, indicating that tungsten oxide formed in the APS-W coating. It can be proposed from the XRD analysis shown in Fig. 2a that the formed tungsten oxide in the APS-W coating was WO3. It can also be seen from Fig. 3a that WO3 not only distribute between the lamellar layers, but in the splats. The WO3 distributed in the splats increased its chance to be detected by XRD conducted on the coating surface. In comparison, the lamellar structure can also be seen in the APS-W/ LaH2 coating, as shown in Fig. 4a. However, the relative content of the dark gray phases in the coating is far lower than that in the APS-W coating shown in Fig. 3a. The details of few splats and inter-splat regions in the APS-W/LaH2 coating show clearly that the dark gray phases in the

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Fig. 3. SEM cross-sectional micrograph and composition distribution of the APS-W coating: (a) SEM micrograph; (b) EDX results of the different phases; (c) SEM micrograph to be used as EDX mapping; (d) W distribution in micrograph of (c); (e) O distribution in micrograph of (c).

APS-W/LaH2 coating distribute mainly between lamellar layers and they might be composed of different phases, as shown in Fig. 4b. These phases with various contrasts being marked by different labels in Fig. 4b were distinguished by EDX analysis, as shown in Fig. 4c–h. Combined with the XRD and XPS results shown in Fig. 2c–f, it can be proposed that the matrix phase (labeled by A in Fig. 4b), the dark phase (labeled by B1 and B2 in Fig. 4b), and the dark gray phase (labeled by B3, C1 and C2) are tungsten, WO2 and La2O3, respectively. In order to further verify the existence of La2O3 in the APS-W/LaH2 coating, EDX mapping analysis was also carried out; Fig. 5 is a representative example. It can be seen from Fig. 5b–d that the dark gray microstructure located between lamellar layers (Fig. 5a) was La-rich and Orich, indicating that lanthanum oxide was formed in the APS-W/LaH2 coating. The other position being O-rich but W-scarce and La-scarce should be WO2, as circled in Fig. 5a and d. In the present work, the temperature of the plasma jet (typically 10,000 to 20,000 K hot [4]) is so high that most of the injected W/ LaH2 powders were melted/semi-melted. Droplets that mainly consisted of [W], [La] and [H] were formed during the flight of the melted/semi-melted powders. It has been reported that the W-La and W-H systems exhibited nearly complete immiscibility in both the liquid

and the solid states; and no compounds could be formed in the two systems [33]. In comparison, the La-H system has miscibility in both the liquid and the solid states [33]. During APS process, tungsten thin shells would be formed first on the droplet surfaces because the melting point of tungsten is the biggest among the elements in the system; and the cooling rate of the droplet surfaces were higher than that inside of the droplets. The formed tungsten thin shells would separate the formed La-H liquid from exposing to the air. Therefore, the following reaction would occur at temperature range of 1747–1803 K [33] because of the intrusion of the cold air on the thin shells. x W þ O2 →WOx 2

ð4Þ

where x = 2–3 depending on temperature [30–32] and T is the temperature. In the present work, x should be equal to 3 as discussed above. Therefore, the formed La-H liquid would react with WO3 due to the permeation of liquid: 2½La þ 3WO3 →3WO2 þLa2 O3 and 2½H þ WO3 →WO2 þ H2 O ðgÞ:

ð5Þ

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Fig. 4. SEM cross-sectional micrograph of the APS-W/LaH2 coating: (a) SEM micrograph; (b) detail of few splats and inter-splat regions in the coating; (c)–(h) EDX results of the different phases.

Under this condition, if some of the liquid La had no opportunity to react with WO3, they might react with oxygen during solidification. Therefore, La2O3 might be formed as follow: 4La þ 3O2 →2La2 O3 :

ð6Þ

Additionally, some of W powder even LaH2 powder was not melted due to the rapid heating characteristic of the APS process [38]. As discussed above, WO3 would be formed at about 1747–1803 K. When the temperature cooled to about 1473 K, which is the temperature to form LaH2, the dehydrogenation of LaH2 was inevitable, leading to forming La element [17]. The formed La element would react with WO3, resulting in forming La2O3. Therefore, combining with the oxidation characteristics of tungsten shown in Eq. 4, it can be concluded that the formation of La2O3 in the APS-W/LaH2 coating might be attributed to the following routes: LaH2 →La þ H2 ðgÞ

ð7Þ

2La þ 3WO3 →3WO2 þLa2 O3 or 4La þ 3O2 →2La2 O3

ð8Þ

2H2 ðgÞ þ O2 ðgÞ→2H2 O ðgÞ:

ð9Þ

It is logical to show the formation of WO2 and La2O3 in the APS-W/ LaH2 coating by the above routes. However, it should not be ignored that La2O3 might be formed by a reaction between LaH2 and O2 during spraying process because some of the LaH2 might not be melted or dehydrogenated, as shown in Eq. (10). 4LaH2 þ5O2 ðgÞ¼ 2La2 O3 þ4H2 O ðgÞ

ð10Þ

In addition, it should be pointed out that WO3 might also exist in tungsten powder though its purity was as high as 99.9%. As proposed above, WO3 would be reduced by the introduced LaH2 to form WO2 and La2O3 in the APS-W/LaH2 coating. From the above discussions, it can be proposed that the oxidation characteristics of the APS-W/LaH2 coating might be attributed to the following four pathways. (1) In the case of the injected W and LaH2 powders were melted/ semi-melted during plasma spraying process, liquid phase including [W], [La] and [H] would be obtained. During the flight

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Fig. 5. Composition distribution of the APS-W/LaH2 coating obtained by EDX mapping: (a) SEM micrograph to be used as EDX mapping; (b) W distribution in micrograph of (a); (c) La distribution in micrograph of (a); (d) O distribution in micrograph of (a).

of the formed liquid in the cold air, tungsten thin shells would be formed first; and oxidation would occur on the surface of the primary-precipitated tungsten thin shells, leading to form WO3. Then the non-solidified liquid [La] would react with WO3 to form WO2 and La2O3. The liquid [La] that had no opportunity to react with WO3 would react with oxygen to form La2O3. (2) In the case of the injected W and LaH2 powders were not melted during the spraying process, WO3 would be formed first. Then elemental La and H2 would be obtained because of the dehydrogenation of LaH2. The obtained La and H2 would react with WO3 or O2 to form WO2, La2O3 and H2O (g). (3) In the case of the injected LaH2 powder was not melted during the spraying process, some of the LaH2 powder, which was not dehydrogenated, would react with O2 to form La2O3. (4) In the case of the existence of WO3 in tungsten powder, La2O3 would be formed by the reduction of WO3 by the introduced LaH2.

Thus, the problem now is to clarify which of the four types does the oxidation of the APS-W/LaH2 coating follows. It is no question that the oxidation characteristics of the APS-W/LaH2 coating were mainly attributed to pathway (1) because the temperature of plasma spraying is so high that most the injected powder was melted. However, such oxidation characteristics cannot be used to explain the notable difference in oxygen/oxide content between the APS-W and APS-W/LaH2 coatings, as shown in Table 3, where porosity, relative density and thermal conductivity were also summarized. It is commonly recognized that the oxidation characteristics produced by pathway (2) was also inevitable because some of the injected powder would not be melted, though the temperature of plasma spraying is high enough. The oxidation produced by pathway (2) would decrease the oxygen/oxide content of the APS-W/LaH2 coating because the formed H2O in a gaseous state would be released from the coating. In comparison, the priority of the oxidation characteristics produced by pathways (3) and (4) was very low

because of the lower stability of LaH2 and the lower content of oxygen/oxide in the powder. Similarly, for the APS-W coating, its oxidation characteristics were mainly attributed to the above-mentioned pathways (1) and (2) neglecting the effect of LaH2. In summary, the oxidation characteristic of the APS-W was the oxidizing of tungsten during plasma spraying. In comparison, the oxidation characteristic of the APS-W/LaH2 coating was different to that of the APS-W coating, as proposed above. Therefore, it can be proposed that the introduced LaH2 would react with WO3 or O2 to form WO2, La2O3 and H2O in the gaseous state in the APS-W/LaH2 coating, decreasing the oxygen content of the coating when compared with the APS-W coating. A lower oxygen content was believed to help reduce the porosity and improve the relative density of one given sprayed coating [4,35,38,39]; and the present study confirms this, as indicated in Table 3. Although reducing oxygen content and porosity was beneficial to improve the thermal conductivity of one tungsten material [4,35,38, 39], it was not suitable to only use this rule to explain the difference of the thermal conductivity between the present two types of coatings. This is due to the fact that the thermal conductivity of a material is mainly determined by its thermal diffusivity (α), specific heat capacity (Cp), and density (ρ) [35,39], as formulated in Section 2.2. From the measured α, Cp, and ρ (about 3.9 × 10−5 m2/s, 0.13 J/g·K, 14.03 g/cm3 for APS-W coating and about 14.03 g/cm3, 0.095 J/g·K, 16.97 g/cm3 for APS-W/LaH2 coating), thermal conductivity (λ) of the two types of coatings could be calculated, as illustrated in Table 3. It can be seen that the

Table 3 Some basic properties of APS coatings. Coatings

W coating

W/LaH2 coating

Oxygen content (wt.%) Porosity (%) Relative density (%) Thermal conductivity (W/m K)

1.44 ± 0.06 27.14 ± 0.67 ~73 71.5 ± 0.15

0.38 ± 0.02 8.40 ± 0.58 ~92 90.1 ± 0.18

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calculated thermal conductivity of the APS-W/LaH2 coating is higher than that of the APS-W coating. Although porosity and oxygen/oxide impurity in the two types of coating could be used to explain the difference in the thermal conductivity, the effects of texture structure and the interfacial contact between lamellae should be concerned in the future work. In addition, it should be clarified why the relative content of oxygen in the APS-W/LaH2 coating was far lower than that in the APS-W coating, as indicated in Table 3. The releasing of the formed H2O in a gaseous state might not be used to fully explain the result. It can be seen from Figs. 4 and 5 that La2O3 distributed mainly between lamellar gaps of the APS-W/LaH2 coating, having a filling effect on the pores, which was similar to that of copper in tungsten-based coatings obtained by APS technique [39,40]. The filling effect of La2O3 in the APS-W/LaH2 coating reduced the tendency of tungsten to be oxidized as compared with the APS-W coating. Combining with the releasing of H2O in a gaseous state in the APS-W/LaH2 coating, therefore, the oxygen content of the APS-W/LaH2 coating was far lower than that of the APS-W coating. The higher oxygen content in the APS-W coating lead to forming more tungsten oxide in this coating as compared with the APS-W/LaH2 coating, as shown in Fig. 3a and 4a, respectively. It should be noticed from Table 3 that the oxygen content and porosity of the present fabricated APS-W coating were almost worse than those of the APS-W coatings fabricated by other works where the oxygen content and porosity of the plasma sprayed tungsten coating were about 0.21–0.49% and 6–16%, respectively [6,8–10,41]. This is attributed to the spraying parameters used in the present work were not an optimized one for the purpose of strengthening the influence of LaH2 addition on the oxidation characteristics of the tungsten coating. In other words, a remarkable difference in the oxidation characteristics between the APS-W coatings with or without LaH2 addition could be easily found if the obtained APS-W coating without LaH2 addition was over oxidized. It is reasonable to predict that the oxygen content and some of the properties of an APS-W coating would be improved effectively because of the introduction of LaH2 if the spraying technology was optimized. Similar work using supersonic atmospheric plasma spraying technique to prepare LaH2 doped tungsten-based coating had been developed; and the positive results had been obtained, which has been submitted for consideration. It has been found that the formed La2O3, which distributed mainly between lamellar layers had a filling effect on the pores, which was similar to that of copper in tungsten-based coatings obtained by APS technique [39,40]. Nevertheless, due to the melting point of La2O3 being far higher than that of copper, the stability of La2O3 in a tungsten-based material should be greater than that of copper. Therefore, the properties of a tungsten-based material to resist high heat loading would be improved due to the formation of La2O3 in a LaH2 doped tungsten-based material. 3.4. Laser irradiation melting characteristics of the coatings The surface morphologies of the laser irradiation melting APS-W and APS-W/LaH2 coatings as revealed by SEM microscopy are shown in Fig. 6a and b. For the two types of coatings, the melted laser scanning tracks were continuous and some W-drops were formed by balling phenomena. It should be pointed out that the overlap between two tracks was incomplete in order to study the laser irradiation melting characteristics of the two types of coatings to be exposed to a transient high heat loading. It can also be seen from Fig. 6a and b that cracks were formed on the laser irradiation melting surface. Comparing the characteristics of the cracks in the two types of coatings, it can be obtained that the cracks were more easily formed in the laser irradiation melting sample for the APS-W coating than that for the APS-W/LaH2 coating. In addition, for the APS-W coating, the laser irradiation melting specimen not only has a crack along the scanning track, but has a crack at 45° along the scanning track. In comparison, the laser irradiation melting specimen for the APS-W/LaH2 coating only has a crack at 45° along the scanning

track. The above results indicated that the ability of the APS-W/LaH2 coating to resist high heat loading was greater than that of the APS-W coating. It was proposed in reference [42] that the mechanical properties would be the key factors to understand the tungsten behavior under high heat loading. In the present work, the different ability of the two laser irradiation melting coatings to resist high heat loading might be also attributed to their different mechanical properties. Therefore, further information about the mechanical properties of the two laser irradiation melting coatings should be concerned in the future. More details about the microstructure of the two types of laser irradiation melting specimens are shown in Fig. 6c and d, respectively. The coarse elongated tungsten grains were formed in the laser irradiation melting specimen for the APS-W coating, as shown in Fig. 6c. In comparison, the fine equiaxial grain was formed in the laser irradiation melting specimen for the APS-W/LaH2 coating (Fig. 6d). From a detailed observation on the microstructure shown in Fig. 6d, it can be seen that some spherical particles with about 2.5–6.8 μm existed mainly on the tungsten grain boundaries of the laser irradiation melting sample, helping to hinder the growth of the re-crystalline tungsten grains. The EDX mapping results shown in Fig. 6e–g give evidence that these spherical particles existing on the tungsten grain boundaries are La-rich and O-rich, indicating that these particles were La2O3. As obtained in Fig. 4 and Fig. 5, La2O3, which were distributed mainly between lamellar gaps by morphology of long strip, formed in the APS-W/LaH2 coating. The formed long strip of La2O3 in the APS-W/LaH2 coating would be remelted and re-precipitated in the form of spherical particles under the effect of laser irradiation. The re-precipitated La2O3 particles were located on the tungsten grain boundaries, pinning the movement of the tungsten grains. Therefore, the grain size of the tungsten grains in the APS-W/LaH2 coating was smaller than those in the APS-W coating. 4. Conclusions One kind of rare earth hydrides, LaH2 powder was introduced into a pure tungsten powder in 1.5 wt.% (W/LaH2) and sprayed by an atmospheric plasma spraying (APS) machine to form a coating. The influence of LaH2 addition on oxidation characteristics and laser irradiation melting characteristics were studied. The following results were obtained: (1) WO3 was formed in the APS-W coating by the oxidation reaction between W and the invasive O2 from the cold air. The introduction of LaH2 led to forming WO2 and La2O3 in the APS-W/LaH2 coating because of the oxidation of La and/or LaH2 by WO3 and/ or O2 during spraying. (2) La2O3 with a morphology of long strip distributed mainly between lamellar layers of the APS-W/LaH2 coating had a filling effect on the pores, decreasing the oxygen content and the porosity of the APS-W/LaH2 coating when compared to the APS-W coating. The relative density and the thermal conductivity of the APS-W/LaH2 coating were higher than those of the APS-W coating. (3) Crack was more easily formed in the APS-W coating than in the APS-W/LaH2 coating under the effect of laser irradiation, indicating that the ability of the former coating to resist laser irradiation melting was worse than that of the latter coating. The smaller grain size in the laser irradiation melting APS-W/LaH2 coating was mainly because of the pinning effect of the re-precipitated spherical La2O3 particles on the re-crystalline tungsten grain boundaries.

Acknowledgements This work was supported by Natural Science Foundation of Anhui Province of China (No. 1508085ME95), National Magnetic Confinement Fusion Program of China (No. 2014GB121001), China Postdoctoral Science Foundation (No. 2014M551796) and the Fundamental Research Funds for the Central Universities (No. 20136h2x0063).

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Fig. 6. SEM surface morphology, micrograph and composition distribution of the laser irradiation melting coatings: (a) surface morphology of laser irradiation melting APS-W coating; (b) surface morphology of laser irradiation melting APS-W/LaH2 coating; (c) SEM micrograph of laser irradiation melting APS-W coating; (d) SEM micrograph of laser irradiation melting APSW/LaH2 coating; (e) W distribution in micrograph of (d); (f) La distribution in micrograph of (d); (g) O distribution in micrograph of (d).

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