Microstructure and Oxidation Behavior of Boron-Added WSi_{2}

Materials Transactions, Vol. 49, No. 9 (2008) pp. 2047 to 2053 #2008 The Japan Institute of Metals

Microstructure and Oxidation Behavior of Boron-Added WSi2 Compact*1 Akira Yamauchi1 , Tatsuya Sasaki2; *2 , Akira Kobayashi3 and Kazuya Kurokawa1 1

Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Sapporo 060-8628, Japan Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan 3 Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japan 2

In order to improve the oxidation resistance of WSi2 at 873–1473 K, B added WSi2 was fabricated by a spark plasma sintering method and oxidation tests were carried out in air. The fabricated B added WSi2 consists of WSi2 , Si and W2 B5 . The addition of B into WSi2 leads to the formation of a protective borosilicate scale, resulting in improvement of the oxidation resistance. Requisite concentration of B for the formation of a protective borosilicate scale decreases as the temperature is raised. Consequently, the addition of 2 or 3 mass% B is the most effective for improvement of the oxidation resistance of WSi2 in the temperature range of 873–1473 K. Such effect of B on high-temperature oxidation of WSi2 is also discussed. [doi:10.2320/matertrans.MRA2008002] (Received December 29, 2007; Accepted June 5, 2008; Published July 16, 2008) Keywords: high temperature oxidation, tungsten disilicide, boron addition, borosilicate scale

1.

Introduction

From the viewpoint of energy resource saving and reduction in CO2 emissions, we have to attain high efficiency for energy conversion systems such as gas turbine and jet engine systems. However, at present this depends much on the high-temperature performance of Ni-base superalloys. Future high-temperature structural applications will require materials with increasing performances. This is the reason why intermetallics,1–4) ceramics,5) carbon composites,6–8) and refractory metals9,10) are candidates for replacing the current Ni-base superalloys. Silicide becomes one of the above candidates, because it has high strength and can form a protective SiO2 scale at very high temperature. When metal disilicides form a protective SiO2 scale, they show excellent oxidation resistance. The structures of oxide scales formed on metal disilicides are classified into three groups: (1) silica scale, (2) mixed oxide (silica + metal oxide) scale, and (3) double layered (silica(outer side)/metal oxide) scale.13) On WSi2 , it has been reported that its oxidation behavior is complicated, and the structures of their scales show the 3 types,13–16) depending on temperature ranges. At temperatures below 1073 K, the scales consist of mixed oxides of WO3 and SiO2 , indicating a low oxidation rate. In an intermediate temperature range (1073–1473 K), scales having double-layer structure are formed. The outer and inner layers are consisting of SiO2 and mixed oxides of WO3 and SiO2 , respectively. The outer porous SiO2 layer must be formed by evaporation of WO3 . Consequently, at these temperatures, WSi2 forms the thick and porous oxide scales and thus shows an extremely poor oxidation resistance. On the other hands, at temperatures above 1573 K, WSi2 shows excellent oxidation resistance because of the formation of a thin and dense SiO2 scale on it. Therefore, understanding the classification of oxidation behavior in silicides is essential for investigating the oxidation resistance of next generation’s ultra high temperature material.

A few papers demonstrated the improvement of oxidation resistance by the addition of B to metal silicide. Meyer et al. reported that Mo5 Si3 formed a protective borosilicate scale by the addition of B and thus showed a good oxidation resistance.17) According to Kurokawa et al., the oxidation resistance of NbSi2 is dramatically improved by the addition of B with concentrations of 2 mass% or higher at temperatures ranging from 1073 to 1673 K in air.4) Consequently, their results suggested that the formation of a borosilicate scale having high plasticity suppressed the high porosity of the scale, making the scale much more protective.4,17,18) The purpose of this work is to clarify the effect of B addition on the improvement of oxidation resistance for WSi2 compacts, especially at intermediate temperatures (1073–1473 K). 2.

Experimental Procedures

The starting materials were WSi2 powder (purity: 99.5 mass%, average grain size: 2.5 mm, Japan New Metals Co., LTD) and B powder (purity: 99 mass%, average grain size: 40 mm, Kojundo Chemical Laboratory Co., LTD). Percent in this paper means mass percent, unless otherwise stated. B and WSi2 mixed powders consisting of WSi2 -0.5, 1.0, 2.0 and 3.0%B were ball-milled for 1.8 ks at a rate of 150 rpm. These powder mixtures were sintered by using a spark plasma sintering (SPS) method. The sintering conditions are shown in Table 1. The specimens for oxidation tests were cut into 10
3:5
1 in mm pieces from the compacts. Their surfaces were polished with SiC paper of up to #1500, and then polished with a 1 mm diamond abrasive for mirror finishing. Prior to oxidation tests, the specimens were ultrasonically

Table 1 Sintering conditions of B added WSi2 powders. 1. RT-873 K : 2 K/s Sintering process

3. 873–1473 K : 0.167 K/s

*1This

Paper was Originally Published in Japanese in J. Japan Inst. Metals 71 (2006) 9–15 *2Undergraduate Student, Hokkaido University, Present address: DAIHATSU MOTOR CO. LTD.

2. hold for 3 min at 873 K

Compressive stress

40 MPa

Atmosphere

high vacuum (510 Pa)

2048

A. Yamauchi, T. Sasaki, A. Kobayashi and K. Kurokawa

(a)

(b)

50µm

50µm (c)

(d)

50µm

50µm Fig. 1

Microstructures of sintered B added WSi2 . (a) 0.5 mass%B, (b) 1.0 mass%B, (c) 2.0 mass%B, and (d) 3.0 mass%B.

mol% (a)

W2B5

Intensity (a.u.)

WSi2

Si

3.0mass% B

W

0.16

Si

99.84

mol% W

35.34

B

64.66

(b)

mol%

2.0mass% B

W 32.46 1.0mass% B

10µm

0.5mass% B

20

40

60

Si

66.29

B

1.25

80

Diffraction angle, 2 θ Fig. 2

(a) X-ray diffraction patterns of B added WSi2 , and (b) SEM micrograph and EPMA analysis of 3.0 mass%B added WSi2 .

cleaned in ethanol, and the mass and the surface area were measured. Isothermal oxidation tests were performed at temperatures ranging from 873 to 1473 K for up to 360 ks in the laboratory air. The oxidation kinetics was obtained by measuring the difference in masses before and after the oxidation. The oxidized specimens were characterized by using X-ray diffractometer (XRD), field-emission scanning electron

microscope (FE-SEM) and electron probe microanalyzer (EPMA). 3.

Results and Discussion

3.1 Microstructure of B added WSi2 compact Figure 1 shows the microstructures of B added WSi2 compacts (B-WSi2 ) fabricated by SPS. The analysis by XRD

Microstructure and Oxidation Behavior of Boron-Added WSi2 Compact

0.8

0.8

0.4

0 WSi2 0.5B 1B 2B 3B

36 ks

Mass Change, ∆ M / kgm-2

Mass Change, ∆ M / kgm-2

(a) 180 ks oxidation

-0.4

(b) 360 ks oxidation 0.4

0

1000

1100

1200

1300

1400

WSi2 0.5B 1B 2B 3B

-0.4

-0.8

-0.8 900

2049

900

1000

1500

36 ks

1100

1200

1300

1400

1500

Temprature, T / K

Temprature, T / K

Fig. 3 Mass change of WSi2 and B added WSi2 as a function of temperature for oxidation in air for (a) 180 ks and (b) 360 ks.

(b)

(a) Cu plated

resin

Cu plated

scale scale

substrate

50µm

(c)

substrate

50µm

(d)

scale

scale

substrate

1µm

substrate

1µm

Fig. 4 Cross-sectional micrographs of oxide scales formed on B added WSi2 , after oxidation at 1073 K for 180 ks in air. (a) 0.5 mass%B (polished), (b) 1.0 mass%B (polished), (c) 2.0 mass%B (fractured), and (d) 3.0 mass%B (fractured).

and EPMA (Fig. 2) showed that the matrix of the compact is WSi2 , the dark-colored second phase is Si, and the third phase around Si is W2 B5 . The amount of the second phase increases with an increase in the B concentration. The reaction between B and WSi2 at their interface can be expressed as 5B þ 2WSi2 ! 4Si þ W2 B5 :

3.2 Oxidation behavior Figure 3 shows the temperature dependence of mass changes of WSi2 and B-WSi2 oxidized for 180 and 360 ks in the temperature range. In the oxidation of WSi2 , W and Si are simultaneously oxidized to form WO3 and SiO2 in the temperature range of 873 K to 1473 K.13,14) As the temperature increases, the mass gain increases and then reaches a maximum value at about 1300 K. Above 1300 K, a remarkable decrease in mass change occurs. At 1473 K a pro-

2050

A. Yamauchi, T. Sasaki, A. Kobayashi and K. Kurokawa

(b)

(a)

scale

scale substrate

substrate

1µm

(c)

1µm

(d)

scale scale

substrate

1µm

1µm

substrate

nounced mass loss of WSi2 is confirmed for the oxidation time of 36 ks. This mass loss is attributable to the evaporation of WO3 .13,14) On the other hand, the addition of B leads to outstanding suppression of the severe mass loss of WSi2 . At 1073 K, 0.5 and 1.0% of B-WSi2 show the almost same mass change for the 180 ks oxidation as WSi2 . Their mass changes for the 360 ks oxidation are larger than those of WSi2 for the same oxidation time. In comparison, the mass changes of 2.0 and 3.0% of B-WSi2 are negligible even for the 360 ks oxidation, showing an excellent oxidation resistance. As the temperature is increased, the effect of B concentration on the mass change of the compacts diminishes. In other words, the B concentration necessary to improve the oxidation resistance decreases as the temperature increases. 3.3 Structure and composition of oxide scale Figure 4 shows the cross-sectional SEM micrographs of WSi2 containing various B concentrations which were oxidized at 1073 K for 180 ks. As shown in Figs. 4(a) and (b), many pores are formed in the scales for 0.5 and 1.0% of B-WSi2 . The thickness of their scales consisting of yellow oxide is 100 mm or larger. On the contrary, the scales formed on 2.0 and 3.0% of B-WSi2 are about 2 mm in thickness and dense. The scales formed at higher temperatures were similarly observed. The cross-sectional SEM micrographs of the various B-WSi2 oxidized at 1473 K for 180 ks are shown in Fig. 5. For all the specimens the scales are about 1–2 mm in

Thickness of oxide scale, δ / µ m

Fig. 5 Fractured oxide scales formed on B added WSi2 , after oxidation for 180 ks at 1473 K in air. (a) 0.5 mass%B, (b) 1.0 mass%B, (c) 2.0 mass%B, and (d) 3.0 mass%B.

200

8

0.5B 1B 2B 3B

150

6 4 2 0 1100

100

1200

1300

1400

1500

50

0 900

1000

1100

1200

1300

1400

1500

Temperature, T / K Fig. 6 Change of the thickness of scales formed on B added WSi2 with temperature for 180 ks oxidation in air.

thickness and dense. Thus, the influence of B concentration on the growth rate of scales decreases as the oxidation temperature increases. This is attributable to the fact that the viscosity of the SiO2 scale with B (probably borosilicate) decreases with increasing the B concentration and temperature,19,20) and the SiO2 scale is easily formed due to the evaporation of WO3 in the initial oxidation stage.13) Since the evaporation of WO3 affects the oxidation kinetics of WSi2 ,13,14) there is a possibility that the temperature dependence of mass change by the oxidation (Fig. 3)

Microstructure and Oxidation Behavior of Boron-Added WSi2 Compact

A crack

2051

80

B

crack

Scale

Substrate

A

B

Concentration (mol%)

100 m O 60

40

W Si 20

B 0

100

0

200

300

400

-6

Distance, d / 10 m Fig. 7

Cross-sectional microstructure and concentration profiles of 1.0 mass%B added WSi2 , after oxidation at 1073 K for 360 ks in air.

will be inconsistent with that of the scale thickness. Therefore, the thicknesses of scales formed at various temperatures were measured on the SEM photographs. Figure 6 indicates the temperature dependence of the scale thickness formed by the 180 ks oxidation. The temperature dependence in Fig. 6 is in good agreement with that in Fig. 3. Furthermore, it was found that the negligible mass change in the high temperature range is due to the formation of SiO2 scale, not owing to the evaporation of WO3 but owing to the effect of the B addition. Next, the B concentration in the scale was examined. Figure 7 shows the cross-sectional SEM micrograph and concentration profiles of W, Si, B and O in the scale formed on 1.0% of B-WSi2 oxidized at 1073 K for 360 ks. WO3 and SiO2 are uniformly distributed throughout the scale. In addition, the atomic ratio of Si to W in the scale is close to 2. Regarding B, the whole scale has the uniform B concentration of about 1.1%. These results mean that the effect of the B addition on the oxidation of WSi2 containing low B concentration at 1073 K is very little. It is concluded that W and Si must be simultaneously oxidized to form WO3 and SiO2 , and a thick scale is formed similarly with WSi2 . At this temperature, WO3 evaporated hardly. Moreover, the changes of composition of the scales formed on B-WSi2 with high B concentration and oxidation temperature were investigated. Table 2 summarizes the results of EPMA for the scales formed on 1.0 and 3.0% of B-WSi2 oxidized for 360 ks in the temperature range of 1073–1473 K. Though these concentrations are mean values analyzed by EPMA, the distributions of each element are almost uniform throughout the scale regardless of its thickness. These results demonstrate that the B concentration in the scale tends to decrease as the oxidation temperature increases. In addition, these results indicate that the content of WO3 in the scale formed at the intermediate temperatures decreases outstandingly with an increase in the B concentration. Consequently

Table 2 Compositions of scales formed on B added WSi2 by oxidation for 360 ks at 1073, 1273 and 1473 K. (mass%) 3 mass%B-WSi2

1 mass%B-WSi2 Si

W

B

O

Si

W

B

O

1073 K

20.73

52.43

1.11

25.72

39.59

7.43

1.75

51.23

1273 K

35.35

19.79

0.6

44.26

39.92

3.33

1.38

55.38

1473 K

41.19

9.44

0.83

48.54

40.07

5.73

0.83

53.38

it can be concluded that increasing B concentration leads to the formation of a protective oxide scale having little content of WO3 . 3.4 Formation of SiO2 scale containing B2 O3 B-WSi2 showed excellent oxidation resistance except for the compacts having 0.5 and 1.0% B oxidized at 1073 K. It must be emphasized that the scales formed on B-WSi2 showing excellent oxidation resistance consist of SiO2 with B and their thicknesses are about 1–2 mm. In order to clarify the growth mechanism of the SiO2 scale with B, the morphologies of protective scales formed at high temperatures during the short time were examined. Figure 8 shows the surface features of the scales formed on WSi2 containing the various B concentrations in the oxidation at 1473 K for 180 ks. The observation revealed that the ratio of surface area covered with the circular black oxide increases with the B concentration for the same oxidation time. The EPMA analysis indicated that this black-colored oxide consists of SiO2 with B. Figure 9 shows the cross section of the SiO2 scale with B formed on B-WSi2 . The relatively thick SiO2 scale with B is locally formed on the region where Si and W2 B5 phases coexist. It was observed that this oxide widens with oxidation time because of its low viscosity.19,20) In the case of high B

2052

A. Yamauchi, T. Sasaki, A. Kobayashi and K. Kurokawa

(a)

(b)

(c)

(d)

Fig. 8 Surface features of B added WSi2 oxidized for 90 ks at 1473 K in air. (a) 0.5 mass%B, (b) 1.0 mass%B, (c) 2.0 mass%B, and (d) 3.0 mass%B.

1mass%B, 1273 K, 50hr

borosilicate

W2B5

Si 10µm

Fig. 9 Cross-sectional micrograph of scale formed on 0.5 mass%B added WSi2 by oxidation at 1473 K for 180 ks in air.

compacts, dense SiO2 scales with B were readily formed throughout the surface during the initial stage of oxidation, because of the homogeneous distribution of W2 B5 over the whole specimen surface. 3.5

Effect of B on improvement of oxidation resistance of WSi2 The effect of B on the high temperature oxidation resistance of WSi2 has been considered on the basis of the results, in which high temperature oxidation behavior of

WSi2 was classified according to the temperature range as follows. (1) T 5 1273 K: Formation of the scales consisting of mixed oxides of WO3 and SiO2 results in a large mass gain. (2) 1473 K 5 T < 1673 K: In the initial stage of oxidation, scales consisting of WO3 and SiO2 are formed, and then SiO2 only is left in the scales owing to the evaporation of WO3 . However, these scales are porous. (3) T = 1673 K: Protective scales consisting of only SiO2 are formed at 1673 K and above, from the initial stage because of their low viscosity. At temperatures below 1573 K, WSi2 shows a poor oxidation resistance because of the formation of either porous scale or mixed oxides scale consisting of WO3 and SiO2 . Therefore, it is needed to form a protective oxide scale below this temperature for the suppression of inward oxygen diffusion by densifying the oxide scale by B addition. The effect of B addition on the improvement of oxidation resistance of metal silicides such as Mo5 Si3 ,17) NbSi2 4) and ReSi1:75 21) has been confirmed. Such an effect results from densifying the scale due to the formation of borosilicate phase having low viscosity. Though the equilibrium dissociation oxygen pressure of borosilicate is not identified, that of WO3 is eight orders of magnitude higher than that of SiO2 over the whole temperature range.22) That of borosilicate was estimated to be the almost same as that of SiO2 . Therefore, when the surface of B-WSi2 was entirely covered with a

Microstructure and Oxidation Behavior of Boron-Added WSi2 Compact

dense SiO2 (or borosilicate) scale, oxidation of W at the scale/substrate interface could not take place. In other words, selective oxidation of Si proceeds. The composition of borosilicate has been shown as (SiO2 )1x (B2 O3 )x , and increasing x leads to decreasing its viscosity.19,20) In this study, the formation of borosilicate phase was not verified by XRD, probably because this phase was amorphous. However, the EPMA analysis confirmed that the presence of B in the scale (see Table 2). The formation of a borosilicate scale having low viscosity would lead to sealing pores in the scale and consequently improves the oxidation resistance of WSi2 . The high concentration of B is required for showing this effect at low temperatures. In other words, at higher temperatures the addition of smaller amounts of B is sufficient to give WSi2 excellent oxidation resistance. Thus, this result implies that the increasing temperature could cause SiO2 with B to soften easily. Moreover, it is well-known that B2 O3 evaporates heavily above 1273 K.23) Consequently this causes a decrease in the concentration of B in the oxide scale formed at higher temperatures. 4.

Conclusions

In the present study, the oxidation behavior of B-WSi2 (WSi2 -0.5, 1.0, 2.0, 3.0%B) was investigated in a temperature range of 873 to 1473 K. The following results were obtained. (1) The oxidation resistances of WSi2 containing B concentrations above 2.0% are remarkably improved owing to the formation of the dense SiO2 scale with B in a temperature range of 1073 to 1473 K. (2) The B concentration of WSi2 to form the dense SiO2 scale with B decreases with an increase in the oxidation temperature. The structural features of the scales formed on B-WSi2 at 1273 K and above are almost the same irrespective of B concentration. (3) The formation of a SiO2 scale with B progresses around the Si and W2 B5 phases. The covering rate of the surface with the oxide scale increases with temperature and B concentration.

2053

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