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LiF-film-assisted diffusion bonding of MgF2 ceramics
Materials Science and Engineering, A 154 (1992) 215-221
215
LiF-film-assisted diffusion bonding of MgF 2 ceramics T. F. Yen and Y. H. Chang Department of Materials Engineering, National Cheng Kung University, Tainan (Taiwan) D. S. Tsai, S. L. Duh and S. J. Yang Materials Research and Development Centre, Chung-San Institute of Science and Technology, Lung-Tan, Taoyuan (Taiwan) (Received September 13, 1991 )
Abstract The joining of MgF by hot pressing was studied. A layer of LiF with 10-12 ~m thickness was used as agent to decrease the bonding temperature and to avoid breaching. The main factors that influenced the light scattering loss were the abnormal grain growth and voids in the interlayer, near-layer and matrix regions. Parameters of 550 °C for 5 h and 575 °C for 3 h under a loading pressure of 300 kgf cm-2 were the best conditions for obtaining high transmittance. High transmittance was related to the preferred orientation of the LiF film (0 ] 1) and slower grain growth. According to the grain growth, the value of the light scattering efficiency factor was calculated as 1.9308 which is independent of the wavelength of the incident light and of the layer region of the samples. Increasing the loading pressure to 430 kgf cm- -~ could improve the relative transmittance by about 10% and 40% for bonding at 650 °C for 3 h and 700 °C for 3 h respectively, and for the other samples the transmittance was lowered by refining the grain size via the recrystallization process.
1. Introduction Diffusion bonding has led to the development of a range of special fabrication techniques for a long time. However, the difficulty of fabrication of complexshaped and large components requires a joining technology to be developed for ceramics to ceramics [1-3] and ceramics to metals or glasses [4-6]. For example, the bonding of SiC and Si3N 4 is an example of the anticipated use of structural ceramics for turbine blades and vanes. The bonding of fluoride-based optical ceramics could be of technological importance for use in laser window and other military applications [7, 8] because of the multispectral capability, low dispersion in the infrared, low refractive index, low nonlinear coefficient, low birefringence and low thermal distortion of such materials. Some key advantages of MgF 2 over crystals for infrared applications include flexibility in size and shape, and the easiness with which the properties can be tailored by doping and processing. Two main difficulties must be taken into consideration in manufacturing the diffusion bonding components. One is the easiness of crack formation under pressure due to the softening properties of fluorides at elevated temperatures. The second is that the interfacial structure should not influence the infrared trans-
mittance, even if a thin layer of dissimilar compound exists at the interface. Therefore in this paper the effects of a thin LiF film used as an assistant for bonding MgF2, the influence of the film on the relative transmittance in the infrared range, and the changes in the interfacial microstructures that governed predominantly the relative transmittance at various temperatures, soaking times and pressures were studied. The LiF-MgF 2 binary system was previously investigated by Tacchini [9], who indicated that over a large range of compositions the material is a solid solution, and a eutectic point exists near 50 tool.% of MgF2. This is the advantage of choosing LiF as an interlayer in the diffusion bonding process, and the other is that the lattice mismatch problem could be eliminated.
2. Experimental details The MgF 2 ceramics used for examination were provided by Chung-San Institute of Science and Technology. They were manufactured by pressing and ground with abrasive initially. All the test samples were cut from a large hot-pressed MgF 2 plate and sliced into discs 10 mm × 10 mm × 4 mm before they underwent different diffusion bonding conditions. Elsevier Sequoia
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Diffusion bonding of MgF2 ceramics
Both faces of the discs were ground with emery paper labelled from 200 to 1200 mesh, and then were polished with 1 and 0.1 /am A1203 powders (Linde A polish abrasive). It was necessary to control the thickness of the discs at 4 mm in order to compare the transmittance between the bonded and unbonded samples. For degrease, the discs were washed with trichloroethylene (CICH: CC12), acetone and distilled water for 20 min each step in an ultrasonic bath, and then ovenbaked at 80 °C for 12 h. After the processing, the contamination on the ceramic surfaces by organic materials was identified by infrared spectroscopy. The transmittance of the unbonded specimens was measured by infrared spectroscopy (Hitachi, 270-30) with wavelengths ranging from 2.5 to 10/am. The LiF film (the purity of the LiF was 99.9%) was produced on the carefully cleaned MgF2 discs by evaporation in a vacuum chamber at 1 x 10 -5 Torr. The distance from substrate to source was 5 cm which gives a deposition rate of 2.5-2.67/am min- ~. The substrate was kept at 200 °C. The thickness of the films was determined by weighing. The structure of the LiF films was checked with X-rays. After the deposition of LiF, the MgF 2 discs maintained their relative transmittance at 9 8 - 1 0 0 % of that of the bare samples. Figure 1 shows the relative transmittance of the LiF-coated MgF2 ceramics; the slight waviness at a wavenumber of 3440 cm- ~(2.9/am) is the absorption band of HzO. For examination of the diffusion bonding, samples were assembled and lightly clamped together before they were introduced into the hot-pressing facility. The hot-pressing chamber was evacuated to a few millitorr and then the samples were heated to the bonding temperature. The temperatures selected ranged from 550 to 750 °C with a heating period of 2-5 h. When the bonding temperature was reached, the samples were subjected to a compressive stress of 300 kgf cm-2 (P0)
or 430 kgf cm -2 (P2) (load divided by the bonding surface area; the load pressure was indicated by the load cell) and held at the temperature for 2-5 h. Finally, the power supply of the chamber heater was switched off and the pressure was released. The samples were allowed to furnace cool. The bonded layer was examined by scanning electron microscopy (JOEL-JSM-35-SEM) using the plain specimens, cut perpendicular to the bonded layer. They were polished with 0.1 /am AI203 powders, etched with acid solution (HzSO4:B203=10:l) at 120-140 °C for 3-5 min and coated with gold film. Electron microprobe analysis was performed across the bonded layer to examine the oxygen content so as to find the degree of oxidation of MgF 2 during the processing. The crystal structures of the bonded specimens were examined by X-ray diffractometry (Rigaku XRD) using nickel-filtered Cu K a radiation, operated at 30 kV and 10 mA.
3. Results and discussion 3.1. X - r a y structure analysis
From the transmittance measurements, it was found that a higher transmittance of 92-95% of the value for the bare sample occurred at the bonding conditions of 550 °C for 5 h and 575 °C for 3 h for samples of LiP0 (LiF film as interlayer and the bonding pressure was 300 kgf cm- 2) as shown in Fig. 2. The X-ray diffraction
100 \~
-= ~. ~ 0----13
90 ~ "~.
5hr 4hr 3hr
8O 70
2.~5 100
3
4
Wavelength (um) 5 6
~
~
,9 101,1!2
--60 50
80 4C 30 20
4C .~aC N
10
1( 4( )0
0 550
\ 3,500
3 6 0 0 2,500 2000 1BOO I~X) ~ Wavenumber ( cm-1 )
1;~00 1(~00 800
6~)
Fig. 1. The transmittance of MgF2ceramic coated with LiF film.
600
650 Temperature
700
750
( °C ]
Fig. 2. The variation of transmittance with bonding temperature and soaking time for LiP0 samples: e, 5 h; zx, 4 h; tz, 3 h; o, 2 h.
T. F. Yen et aL /
Diffusion bonding of MgF2 ceramics
217
(2) In Rayleigh-Gans scattering [15], when m is close to unity, m - 1 < 1, and when the phase shift p is small, p = 2x(m - 1 ) < 1. Then
Q=(m-1 )215_1_2x2 [2
g 20
30
40
50
60
70
80
2e
Fig. 3. The X-ray diffraction pattern of the bonded specimen around the bonding face. Arrows indicate the LiF peaks. The matrix is MgF2. showed that the higher-transmittance specimens exhibited peaks for (100) and (311) of LiF as in Fig. 3. Counts et al. [10] pointed out that the solid solution between LiF and MgF 2 is essentially complete at temperatures above 670 °C, but that conversion from solution occurs on cooling below this temperature. So we concluded that the interlayer of samples bonded at 550 °C (5 h) and at 575 °C (3 h) was LiF and that of other samples was LiF solid solution (LiF(ss)). Lilley and Newkirk studied MgF2-doped LiF crystals [11], and discovered a metastable phase of MgF 2 precipitates under rapid cooling or even slow cooling of lightly doped crystals (e.g. 0.045 mol.% of MgF2).The precipitates of MgF 2 in LiF crystals could diffuse the light, making the transmittance of the LiF crystals decrease. So we proposed that the higher transmittance of samples bonded at lower temperatures was due to a cleaner LiF layer (low MgF 2 precipitate content), while the lower transmittance for the samples bonded at higher temperatures was due to the LiF(ss) interlayer (high MgF2 precipitate content). 3.2. E v a l u a t i o n o f scattering loss Hattori et al. [13] have evaluated the light scattering
loss characteristics of current fluorides and concluded that the scattering loss was due to extrinsic defects. However, it is known that the approximate analytical equations for calculating the scattering efficiency factor Q are given for restricted conditions. Approximate expressions for estimating scattering characteristics are as follows. (1) In Rayleigh scattering [14], when the particle size is smaller than the wavelength ~., then x - 2ream2~ ~. < 1 (a is the particle radius). In this case the efficiency factor is given by 8 [m2--lt2x4
0 = 1m 75+ 2 ]
(11
where m =ml/m2, m t is the refractive index of the particle and m 2 that of the surroundings.
1
sin(4x)
7[1-cos(4x)]
4x
16x 2
] x 2
- 2[7 + ln(4x)- Ci(4x)]~ J
(2)
where 7 = 0.577 is Euler's constant and Ci(x) = - f [cos(u)/u] d u x
is the cosine integral. (3) In geometrical optics and diffraction [16], when the relative refractive index is close to unity (m - 1 < 1 ) and when the particle size is larger than the wavelength (x > 1 ), then Q=2-4
s i n P + 4 (1-cosP)2 P P
(3)
where p = 2x(m - 1 ). As described above, the relative refractive index and particle size are important parameters for calculating the Q value. We examined the effects of the changes in grain size in the three regions (interlayer, near layer and matrix) on the light scattering loss. Various parameters were checked: (a) the grain sizes in the three regions are all larger than the incident wavelength; (b) x = 2aram2/ 2 > 1, m - 1 < 1, i.e. the relative refractive index m = m l / m 2 is close to unity [17], regardless of the incident wavelength. From the grain size, which changed under different bonding conditions (temperature, 550-750 °C; time, 2-6 h) for LiP0 samples (as shown in Figs. 4, 5 and 6 for the three regions respectively) and eqn. (3), the light scattering efficiency factor Q was calculated as 1.9308. This value is independent of wavelength. The absorption efficiency factor Qabs was neglected owing to the results of Hardy and Agrawal [18] and Bendow [19]. By calculating the number of grains per unit volume of samples from the equation for the Bouguer law I =I0 e x p ( - a d ) (where I is the photon intensity that passed through the thickness d of sample; I 0 is the incident light photon intensity; a is the scattering extinction coefficient and a = nAQ; n is the number of grains per unit volume and A is the cross-section area n a 2 that faces the light going-through), it is implied that the scattering loss is proportional to n n a 2. The size of grains varied in the interlayer, near-layer and matrix regions of bonded samples according to the
218
T. F. Yen et al.
/
Diffusion bonding of MgF2 ceramics
20
1c
18
9
16
8
14
7
~1o N ~a
~6 ~5 ~4 E
~a 2q
4
1 0
I
I
I
600 650 700 Temperature (*C)
I
750
550
I
I
600
650
I
700
I
750
Temperature (°C)
Fig. 4. The dependence of grain size around the interlayer of LiP0 samples on bonding temperature and soaking time: o, 2 h; zx, 3 h; m,4 h; e, 5 h.
Fig. 6. The variation of the grain size in the matrix of LiP0 samples with bonding temperature for different soaking times (symbolsas in Fig. 4).
18
TABLE 1. Control mechanisms as evidenced by grain size in particular regions
16
Temperature
20
Region(s)
14
~12 600 650 700 750
2a
3
4
5
NL NL L NL+L
NL NL+ L NL+ L NL+L
L L L L
M + L + NL NL NL M+NL+L
('94
M, matrix;NL, near layer; L, layer. aprocessingtime (h).
2q 0
550
I
I
600
.650
I
700
I
750
Temperature ('C)
Fig. 5. The relationship of the grain size around the near layer of LiP0 samples to bonding temperature for different soakingtimes (symbolsas in Fig. 4). creep localized during the bonding process, and also depended on the different bonding conditions. Thus the grain size affects the transmittance. For example, the relative transmittance of the sample bonded at 600 °C for 4 h was about 62%, far below that of the unbonded sample; the main reason was assumed to be the interlayer because its grain size was about 16/am larger than the 2-3/am of the near layer and the matrix. Comparing the grain sizes of the three regions (Figs. 4-6) in a sample, the regions that affect predominantly the scattering loss were deduced and are listed in Table 1 for LiP0 samples (for the samples in this study, which were cut from o n e M g F 2 plate, the voids and anisotropy are assumed to be of the same level, and the assessment in the table is a relative representation). It was found that after bonding for 4 h the grain growth of the interlayer (LiF) became an important mechanism to reduce the light transmittance. For shorter
bonding times, the near-layer grains played the most important role in influencing the scattering loss. These results are similar to those of Ciarlo and Nicholson's studies [20] for the initial sintering of MgF2-LiF solid solution. They proposed the possible defect equations for the system as: LiFMgF2 ~
Li'MgJr .t' F~ + V F"
Esu b
(4)
L i i" + F i'
Eint
(5)
and LiFMgF~ ~
Also, Ciarlo and Nicholson gave the ratio of the volume diffusivities D,, of DvooJD ...... =1.97_+0.2 (where 0.05 and 0.035 denote LiF content (mol.%)). More LiF additive could enhance the volume diffusivity and grain growth. The promotion of grain growth was also expected in the model proposed by James and Catlow [21] who pointed out that the existence of both octahedral and tetrahedral interstitials is possible in the rutile structure. They also reported values of Esub=l.57 eV, Eint=4.30 eV, and selfcompensating energy Esc-0.39 eV. These results described the transport properties and suggested that
7. F. Yen et al.
/
Diffusion bonding of MgF2 ceramics
the interstitial Li + may have a very high mobility parallel to the c axis. Abnormal grain growth was observed in the interlayer, near-layer and matrix areas. The rate of grain growth in the matrix of MgF 2 was measured as 0.49 # m h- t at 600 °C and 1.92/~m h- 1 at 750 °C. 3.3. Effects of joining pressure Figure 7 illustrates the variation in relative transmittance with bonding temperature under a pressure of 430 kgf cm -2 (P2) for different soaking times. Under this P2 load, the samples were broken as the temperature was below 600 °C. Whether the load of P0 or P2 was applied, some complicated phenomena between the relative transmittance and the interfacial microstructure were observed. The relative transmittance was observed to decrease with increase in the bonding temperature for the samples that were subjected to soaking for a time of 2 h. Increasing the pressure load was found not to increase the transmittance except at 650 °C for which the transmittance was raised by about 6%. Examining the microstructure of the interface of the sample bonded at 650 °C under different pressures for 2 h (Fig. 8), there obviously existed some interfacial voids in the sample which was subjected to a low pressure load to reduce the transmittance. Compared with the results for the P0 samples, the transmittance of the P2 sample increased by about 10% and 40% after bonding at 650°C for 3 h and 700 °C for 3 h respectively (Fig. 7), and for the other
100
219
samples the transmittance decreased. The main reason was the deformation around the interface under higher loads. The deformed structure around the interface of P2 samples would induce a strain field as a convex hill around the interface and is shown in Fig. 9. The strain field would decrease the relative transmittance [9] and the activation energy of recrystallization. The fine grain structure would be caused through the process of recrystallization. In contrast, on 190 samples abnormal grain growth was observed near the interlayer. After bonding at 650 °C for 4 h, in the sample with higher pressure P2, plate-like grains appeared at the interlayer (Fig. 10(a)) and the 20% transmittance was diminished compared with that of the 190 sample (Fig. 10(b)). However, after bonding at 700°C for 4 h, abnormal grain growth of size about 10 ~m at the interface was observed (shown in Fig. 11 ), resulting in a reduction in the transmittance by about 20%. Extending the soaking time to 5 h, large grains were formed at the near layer and fine grains at the interlayer for samples bonded at 650 and 700 °C with t90 pressure (Fig. 12). The transmittance of these samples fell by about 10%. Figure 13 shows micrographs of the interlayer of samples bonded at 750 °C for 5 h under 190 and P2
u
.=
90
= 2hr 3hr z ~ . ~ 4 hr o-----o 5hr
80 7O
~_60 50 b- 40
~ 30 20 10
550
' 600
' ' 650 700 Temperature (°C)
~0
7
Fig. 7. The dependence of the transmittance of LiP2 samples on bonding temperature for different soaking times: o, 2 h; rn, 3 h; zx,4 h;o, 5 h.
Fig. 8. Scanning electron micrographs at the bonded layer of samples of(a) P0 and (b) P2.
220
T.F. Yen et aL /
Diffusion bonding of MgF2 ceramics
Fig. 9. Scanning electron micrographs at the bonded layer of samples treated at 700 °C for 3 h under a load of (a) P0 and (b) /°2.
Fig. 10. Scanning electron micrographs at the bonded layer of samples bonded at 650 °C for 4 h under a pressure of (a) P0 and (b) P2.
pressure respectively for different soaking times. Because at this temperature a liquid phase and MgF2(ss) phase coexisted in the LiF-MgF2 system, the grain of MgF 2 matrix near the interface grew as the result of liquid-phase sintering and LiF particles also grew to larger sizes. The liquid-phase sintering process may improve the cohesion of the diffusion bonding at the cost of poor transmittance.
4. Conclusion To promote the diffusion bonding of MgF 2 ceramics by using a layer of LiF film, a good joint for infrared transmission (with about 95% of the relative transmittance of an unbonded sample) was obtained at lower bonding temperatures (550 °C for 5 h and 575 °C for 3 h). For bonding at higher temperatures, the formation of LiF(ss) film made the infrared transmittance fade away. The lower transmittances (less than 90%) were observed as a result of light scattering by the grain growth at the interlayer, near layer and matrix. The light scattering efficiency factor was calculated as
Fig. 11. Scanning electron micrograph at the bonded layer of an LiP0 sample treated at 700 °C for 4 h.
1.9308 using the geometrical optics and diffraction method, which was found to be independent of incident wavelength. Use of a higher bonding pressure (420 kgf cm-:) could produce a higher relative transmission (95% of the value for an unbonded sample), and a fine grain structure was observed, caused by the strain field around the interface. However, for lower
T. F. Yenetal.
/
Diffusion bondingofMgF_,ceramics
221
Fig. 12. Scanning electron micrographs at the bonded layer of LIP() samples treated at (a) 650 °C and (b) 700 °C for 5 h.
Fig. 13. Scanning electron micrographs of samples treated at 750 °C for 5 h and under different pressures: (a) LIP0; (b) LIP2.
b o n d i n g pressures (300 kgf c m 2), m a n y voids existed o n the interface, leading to p o o r relative transmittance.
7 D. A. Ducker, H. C. Hafner and N. J. Kreidl, J. Am. Ceram. Soc., 41 (1962) 435. 8 T. E. Nappier and H. S. Halbedel, Emycl. Chem. Technol., 10(1980) 700. 9 T. E Yen, Y. H. Chang, D. L. Yu, E S. Yen, D. S. Tsai and I.-N. Lin, Mater Sci. Eng. A, 147(1991) 121. 10 W. E. Counts, R. Roy and E. F. Osborn, J. Am. Ceram. Soc., 36(1953) 15. 11 E. Lilley and J. B. Newkirk, J. Mater. Sci., 2 (1967) 567. 12 S. Miyake and K. Suzuki, J. Phys. Soc. Jpn., 9 (1954) 702. 13 H. Hattori, S. Sakaguchi, T. Kanamori and Y. Terunuma, Appl. Opt., 26(1987) 2683. 14 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 6. 15 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 7. 16 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 11. 17 J.W. Robinson (ed.), CRCHandbook of Spectroscopy, Vol. 2, 1974. 18 J. R. Hardy and B. S. Agrawal, Appl. Phys. Lea., 22 (1973) 236. 19 B. Bendow, Appl. Phys. Lett., 23(1973) 133. 20 J. Ciarlo and P. S. Nicholson, J. Am. Ceram. Soc., 64 (1981) 460. 21 R. James and C. R. A. Catlow, J. Phys. C, 10 (1977) L237.
Acknowledgment T h e authors express their thanks to the National Science C o u n c i l of the R e p u b l i c of C h i n a for s u p p o r t ing this project ( N S C 7 9 - 0 2 1 0 - D 0 0 6 - 2 7 ) .
References 1 T. Kaba, M. Shimada and M. Koizumi, J. Am. Ceram. Soc., 66(1983) C135. 2 H. Tabata, S. Kanzaki and M. Nakamura, Proc. Conf. on Ceramic Components for Engines, Hakone, 1983, p. 387. 3 R.S. Wilks, Br. Patent 1,417,169. 4 S.S. Cole, Jr. and G. Sommer, J. Am. Ceram. Soc., 44(1961) 265. 5 J. T. Klomp and Th. P. J. Botden, Am. Ceram. Soc. Bull., 49 (1970) 204. 6 C. R. Faulkner and L. S. Williams, Planseeber. Pulvermetall., 12(1964) 111.
215
LiF-film-assisted diffusion bonding of MgF 2 ceramics T. F. Yen and Y. H. Chang Department of Materials Engineering, National Cheng Kung University, Tainan (Taiwan) D. S. Tsai, S. L. Duh and S. J. Yang Materials Research and Development Centre, Chung-San Institute of Science and Technology, Lung-Tan, Taoyuan (Taiwan) (Received September 13, 1991 )
Abstract The joining of MgF by hot pressing was studied. A layer of LiF with 10-12 ~m thickness was used as agent to decrease the bonding temperature and to avoid breaching. The main factors that influenced the light scattering loss were the abnormal grain growth and voids in the interlayer, near-layer and matrix regions. Parameters of 550 °C for 5 h and 575 °C for 3 h under a loading pressure of 300 kgf cm-2 were the best conditions for obtaining high transmittance. High transmittance was related to the preferred orientation of the LiF film (0 ] 1) and slower grain growth. According to the grain growth, the value of the light scattering efficiency factor was calculated as 1.9308 which is independent of the wavelength of the incident light and of the layer region of the samples. Increasing the loading pressure to 430 kgf cm- -~ could improve the relative transmittance by about 10% and 40% for bonding at 650 °C for 3 h and 700 °C for 3 h respectively, and for the other samples the transmittance was lowered by refining the grain size via the recrystallization process.
1. Introduction Diffusion bonding has led to the development of a range of special fabrication techniques for a long time. However, the difficulty of fabrication of complexshaped and large components requires a joining technology to be developed for ceramics to ceramics [1-3] and ceramics to metals or glasses [4-6]. For example, the bonding of SiC and Si3N 4 is an example of the anticipated use of structural ceramics for turbine blades and vanes. The bonding of fluoride-based optical ceramics could be of technological importance for use in laser window and other military applications [7, 8] because of the multispectral capability, low dispersion in the infrared, low refractive index, low nonlinear coefficient, low birefringence and low thermal distortion of such materials. Some key advantages of MgF 2 over crystals for infrared applications include flexibility in size and shape, and the easiness with which the properties can be tailored by doping and processing. Two main difficulties must be taken into consideration in manufacturing the diffusion bonding components. One is the easiness of crack formation under pressure due to the softening properties of fluorides at elevated temperatures. The second is that the interfacial structure should not influence the infrared trans-
mittance, even if a thin layer of dissimilar compound exists at the interface. Therefore in this paper the effects of a thin LiF film used as an assistant for bonding MgF2, the influence of the film on the relative transmittance in the infrared range, and the changes in the interfacial microstructures that governed predominantly the relative transmittance at various temperatures, soaking times and pressures were studied. The LiF-MgF 2 binary system was previously investigated by Tacchini [9], who indicated that over a large range of compositions the material is a solid solution, and a eutectic point exists near 50 tool.% of MgF2. This is the advantage of choosing LiF as an interlayer in the diffusion bonding process, and the other is that the lattice mismatch problem could be eliminated.
2. Experimental details The MgF 2 ceramics used for examination were provided by Chung-San Institute of Science and Technology. They were manufactured by pressing and ground with abrasive initially. All the test samples were cut from a large hot-pressed MgF 2 plate and sliced into discs 10 mm × 10 mm × 4 mm before they underwent different diffusion bonding conditions. Elsevier Sequoia
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Diffusion bonding of MgF2 ceramics
Both faces of the discs were ground with emery paper labelled from 200 to 1200 mesh, and then were polished with 1 and 0.1 /am A1203 powders (Linde A polish abrasive). It was necessary to control the thickness of the discs at 4 mm in order to compare the transmittance between the bonded and unbonded samples. For degrease, the discs were washed with trichloroethylene (CICH: CC12), acetone and distilled water for 20 min each step in an ultrasonic bath, and then ovenbaked at 80 °C for 12 h. After the processing, the contamination on the ceramic surfaces by organic materials was identified by infrared spectroscopy. The transmittance of the unbonded specimens was measured by infrared spectroscopy (Hitachi, 270-30) with wavelengths ranging from 2.5 to 10/am. The LiF film (the purity of the LiF was 99.9%) was produced on the carefully cleaned MgF2 discs by evaporation in a vacuum chamber at 1 x 10 -5 Torr. The distance from substrate to source was 5 cm which gives a deposition rate of 2.5-2.67/am min- ~. The substrate was kept at 200 °C. The thickness of the films was determined by weighing. The structure of the LiF films was checked with X-rays. After the deposition of LiF, the MgF 2 discs maintained their relative transmittance at 9 8 - 1 0 0 % of that of the bare samples. Figure 1 shows the relative transmittance of the LiF-coated MgF2 ceramics; the slight waviness at a wavenumber of 3440 cm- ~(2.9/am) is the absorption band of HzO. For examination of the diffusion bonding, samples were assembled and lightly clamped together before they were introduced into the hot-pressing facility. The hot-pressing chamber was evacuated to a few millitorr and then the samples were heated to the bonding temperature. The temperatures selected ranged from 550 to 750 °C with a heating period of 2-5 h. When the bonding temperature was reached, the samples were subjected to a compressive stress of 300 kgf cm-2 (P0)
or 430 kgf cm -2 (P2) (load divided by the bonding surface area; the load pressure was indicated by the load cell) and held at the temperature for 2-5 h. Finally, the power supply of the chamber heater was switched off and the pressure was released. The samples were allowed to furnace cool. The bonded layer was examined by scanning electron microscopy (JOEL-JSM-35-SEM) using the plain specimens, cut perpendicular to the bonded layer. They were polished with 0.1 /am AI203 powders, etched with acid solution (HzSO4:B203=10:l) at 120-140 °C for 3-5 min and coated with gold film. Electron microprobe analysis was performed across the bonded layer to examine the oxygen content so as to find the degree of oxidation of MgF 2 during the processing. The crystal structures of the bonded specimens were examined by X-ray diffractometry (Rigaku XRD) using nickel-filtered Cu K a radiation, operated at 30 kV and 10 mA.
3. Results and discussion 3.1. X - r a y structure analysis
From the transmittance measurements, it was found that a higher transmittance of 92-95% of the value for the bare sample occurred at the bonding conditions of 550 °C for 5 h and 575 °C for 3 h for samples of LiP0 (LiF film as interlayer and the bonding pressure was 300 kgf cm- 2) as shown in Fig. 2. The X-ray diffraction
100 \~
-= ~. ~ 0----13
90 ~ "~.
5hr 4hr 3hr
8O 70
2.~5 100
3
4
Wavelength (um) 5 6
~
~
,9 101,1!2
--60 50
80 4C 30 20
4C .~aC N
10
1( 4( )0
0 550
\ 3,500
3 6 0 0 2,500 2000 1BOO I~X) ~ Wavenumber ( cm-1 )
1;~00 1(~00 800
6~)
Fig. 1. The transmittance of MgF2ceramic coated with LiF film.
600
650 Temperature
700
750
( °C ]
Fig. 2. The variation of transmittance with bonding temperature and soaking time for LiP0 samples: e, 5 h; zx, 4 h; tz, 3 h; o, 2 h.
T. F. Yen et aL /
Diffusion bonding of MgF2 ceramics
217
(2) In Rayleigh-Gans scattering [15], when m is close to unity, m - 1 < 1, and when the phase shift p is small, p = 2x(m - 1 ) < 1. Then
Q=(m-1 )215_1_2x2 [2
g 20
30
40
50
60
70
80
2e
Fig. 3. The X-ray diffraction pattern of the bonded specimen around the bonding face. Arrows indicate the LiF peaks. The matrix is MgF2. showed that the higher-transmittance specimens exhibited peaks for (100) and (311) of LiF as in Fig. 3. Counts et al. [10] pointed out that the solid solution between LiF and MgF 2 is essentially complete at temperatures above 670 °C, but that conversion from solution occurs on cooling below this temperature. So we concluded that the interlayer of samples bonded at 550 °C (5 h) and at 575 °C (3 h) was LiF and that of other samples was LiF solid solution (LiF(ss)). Lilley and Newkirk studied MgF2-doped LiF crystals [11], and discovered a metastable phase of MgF 2 precipitates under rapid cooling or even slow cooling of lightly doped crystals (e.g. 0.045 mol.% of MgF2).The precipitates of MgF 2 in LiF crystals could diffuse the light, making the transmittance of the LiF crystals decrease. So we proposed that the higher transmittance of samples bonded at lower temperatures was due to a cleaner LiF layer (low MgF 2 precipitate content), while the lower transmittance for the samples bonded at higher temperatures was due to the LiF(ss) interlayer (high MgF2 precipitate content). 3.2. E v a l u a t i o n o f scattering loss Hattori et al. [13] have evaluated the light scattering
loss characteristics of current fluorides and concluded that the scattering loss was due to extrinsic defects. However, it is known that the approximate analytical equations for calculating the scattering efficiency factor Q are given for restricted conditions. Approximate expressions for estimating scattering characteristics are as follows. (1) In Rayleigh scattering [14], when the particle size is smaller than the wavelength ~., then x - 2ream2~ ~. < 1 (a is the particle radius). In this case the efficiency factor is given by 8 [m2--lt2x4
0 = 1m 75+ 2 ]
(11
where m =ml/m2, m t is the refractive index of the particle and m 2 that of the surroundings.
1
sin(4x)
7[1-cos(4x)]
4x
16x 2
] x 2
- 2[7 + ln(4x)- Ci(4x)]~ J
(2)
where 7 = 0.577 is Euler's constant and Ci(x) = - f [cos(u)/u] d u x
is the cosine integral. (3) In geometrical optics and diffraction [16], when the relative refractive index is close to unity (m - 1 < 1 ) and when the particle size is larger than the wavelength (x > 1 ), then Q=2-4
s i n P + 4 (1-cosP)2 P P
(3)
where p = 2x(m - 1 ). As described above, the relative refractive index and particle size are important parameters for calculating the Q value. We examined the effects of the changes in grain size in the three regions (interlayer, near layer and matrix) on the light scattering loss. Various parameters were checked: (a) the grain sizes in the three regions are all larger than the incident wavelength; (b) x = 2aram2/ 2 > 1, m - 1 < 1, i.e. the relative refractive index m = m l / m 2 is close to unity [17], regardless of the incident wavelength. From the grain size, which changed under different bonding conditions (temperature, 550-750 °C; time, 2-6 h) for LiP0 samples (as shown in Figs. 4, 5 and 6 for the three regions respectively) and eqn. (3), the light scattering efficiency factor Q was calculated as 1.9308. This value is independent of wavelength. The absorption efficiency factor Qabs was neglected owing to the results of Hardy and Agrawal [18] and Bendow [19]. By calculating the number of grains per unit volume of samples from the equation for the Bouguer law I =I0 e x p ( - a d ) (where I is the photon intensity that passed through the thickness d of sample; I 0 is the incident light photon intensity; a is the scattering extinction coefficient and a = nAQ; n is the number of grains per unit volume and A is the cross-section area n a 2 that faces the light going-through), it is implied that the scattering loss is proportional to n n a 2. The size of grains varied in the interlayer, near-layer and matrix regions of bonded samples according to the
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/
Diffusion bonding of MgF2 ceramics
20
1c
18
9
16
8
14
7
~1o N ~a
~6 ~5 ~4 E
~a 2q
4
1 0
I
I
I
600 650 700 Temperature (*C)
I
750
550
I
I
600
650
I
700
I
750
Temperature (°C)
Fig. 4. The dependence of grain size around the interlayer of LiP0 samples on bonding temperature and soaking time: o, 2 h; zx, 3 h; m,4 h; e, 5 h.
Fig. 6. The variation of the grain size in the matrix of LiP0 samples with bonding temperature for different soaking times (symbolsas in Fig. 4).
18
TABLE 1. Control mechanisms as evidenced by grain size in particular regions
16
Temperature
20
Region(s)
14
~12 600 650 700 750
2a
3
4
5
NL NL L NL+L
NL NL+ L NL+ L NL+L
L L L L
M + L + NL NL NL M+NL+L
('94
M, matrix;NL, near layer; L, layer. aprocessingtime (h).
2q 0
550
I
I
600
.650
I
700
I
750
Temperature ('C)
Fig. 5. The relationship of the grain size around the near layer of LiP0 samples to bonding temperature for different soakingtimes (symbolsas in Fig. 4). creep localized during the bonding process, and also depended on the different bonding conditions. Thus the grain size affects the transmittance. For example, the relative transmittance of the sample bonded at 600 °C for 4 h was about 62%, far below that of the unbonded sample; the main reason was assumed to be the interlayer because its grain size was about 16/am larger than the 2-3/am of the near layer and the matrix. Comparing the grain sizes of the three regions (Figs. 4-6) in a sample, the regions that affect predominantly the scattering loss were deduced and are listed in Table 1 for LiP0 samples (for the samples in this study, which were cut from o n e M g F 2 plate, the voids and anisotropy are assumed to be of the same level, and the assessment in the table is a relative representation). It was found that after bonding for 4 h the grain growth of the interlayer (LiF) became an important mechanism to reduce the light transmittance. For shorter
bonding times, the near-layer grains played the most important role in influencing the scattering loss. These results are similar to those of Ciarlo and Nicholson's studies [20] for the initial sintering of MgF2-LiF solid solution. They proposed the possible defect equations for the system as: LiFMgF2 ~
Li'MgJr .t' F~ + V F"
Esu b
(4)
L i i" + F i'
Eint
(5)
and LiFMgF~ ~
Also, Ciarlo and Nicholson gave the ratio of the volume diffusivities D,, of DvooJD ...... =1.97_+0.2 (where 0.05 and 0.035 denote LiF content (mol.%)). More LiF additive could enhance the volume diffusivity and grain growth. The promotion of grain growth was also expected in the model proposed by James and Catlow [21] who pointed out that the existence of both octahedral and tetrahedral interstitials is possible in the rutile structure. They also reported values of Esub=l.57 eV, Eint=4.30 eV, and selfcompensating energy Esc-0.39 eV. These results described the transport properties and suggested that
7. F. Yen et al.
/
Diffusion bonding of MgF2 ceramics
the interstitial Li + may have a very high mobility parallel to the c axis. Abnormal grain growth was observed in the interlayer, near-layer and matrix areas. The rate of grain growth in the matrix of MgF 2 was measured as 0.49 # m h- t at 600 °C and 1.92/~m h- 1 at 750 °C. 3.3. Effects of joining pressure Figure 7 illustrates the variation in relative transmittance with bonding temperature under a pressure of 430 kgf cm -2 (P2) for different soaking times. Under this P2 load, the samples were broken as the temperature was below 600 °C. Whether the load of P0 or P2 was applied, some complicated phenomena between the relative transmittance and the interfacial microstructure were observed. The relative transmittance was observed to decrease with increase in the bonding temperature for the samples that were subjected to soaking for a time of 2 h. Increasing the pressure load was found not to increase the transmittance except at 650 °C for which the transmittance was raised by about 6%. Examining the microstructure of the interface of the sample bonded at 650 °C under different pressures for 2 h (Fig. 8), there obviously existed some interfacial voids in the sample which was subjected to a low pressure load to reduce the transmittance. Compared with the results for the P0 samples, the transmittance of the P2 sample increased by about 10% and 40% after bonding at 650°C for 3 h and 700 °C for 3 h respectively (Fig. 7), and for the other
100
219
samples the transmittance decreased. The main reason was the deformation around the interface under higher loads. The deformed structure around the interface of P2 samples would induce a strain field as a convex hill around the interface and is shown in Fig. 9. The strain field would decrease the relative transmittance [9] and the activation energy of recrystallization. The fine grain structure would be caused through the process of recrystallization. In contrast, on 190 samples abnormal grain growth was observed near the interlayer. After bonding at 650 °C for 4 h, in the sample with higher pressure P2, plate-like grains appeared at the interlayer (Fig. 10(a)) and the 20% transmittance was diminished compared with that of the 190 sample (Fig. 10(b)). However, after bonding at 700°C for 4 h, abnormal grain growth of size about 10 ~m at the interface was observed (shown in Fig. 11 ), resulting in a reduction in the transmittance by about 20%. Extending the soaking time to 5 h, large grains were formed at the near layer and fine grains at the interlayer for samples bonded at 650 and 700 °C with t90 pressure (Fig. 12). The transmittance of these samples fell by about 10%. Figure 13 shows micrographs of the interlayer of samples bonded at 750 °C for 5 h under 190 and P2
u
.=
90
= 2hr 3hr z ~ . ~ 4 hr o-----o 5hr
80 7O
~_60 50 b- 40
~ 30 20 10
550
' 600
' ' 650 700 Temperature (°C)
~0
7
Fig. 7. The dependence of the transmittance of LiP2 samples on bonding temperature for different soaking times: o, 2 h; rn, 3 h; zx,4 h;o, 5 h.
Fig. 8. Scanning electron micrographs at the bonded layer of samples of(a) P0 and (b) P2.
220
T.F. Yen et aL /
Diffusion bonding of MgF2 ceramics
Fig. 9. Scanning electron micrographs at the bonded layer of samples treated at 700 °C for 3 h under a load of (a) P0 and (b) /°2.
Fig. 10. Scanning electron micrographs at the bonded layer of samples bonded at 650 °C for 4 h under a pressure of (a) P0 and (b) P2.
pressure respectively for different soaking times. Because at this temperature a liquid phase and MgF2(ss) phase coexisted in the LiF-MgF2 system, the grain of MgF 2 matrix near the interface grew as the result of liquid-phase sintering and LiF particles also grew to larger sizes. The liquid-phase sintering process may improve the cohesion of the diffusion bonding at the cost of poor transmittance.
4. Conclusion To promote the diffusion bonding of MgF 2 ceramics by using a layer of LiF film, a good joint for infrared transmission (with about 95% of the relative transmittance of an unbonded sample) was obtained at lower bonding temperatures (550 °C for 5 h and 575 °C for 3 h). For bonding at higher temperatures, the formation of LiF(ss) film made the infrared transmittance fade away. The lower transmittances (less than 90%) were observed as a result of light scattering by the grain growth at the interlayer, near layer and matrix. The light scattering efficiency factor was calculated as
Fig. 11. Scanning electron micrograph at the bonded layer of an LiP0 sample treated at 700 °C for 4 h.
1.9308 using the geometrical optics and diffraction method, which was found to be independent of incident wavelength. Use of a higher bonding pressure (420 kgf cm-:) could produce a higher relative transmission (95% of the value for an unbonded sample), and a fine grain structure was observed, caused by the strain field around the interface. However, for lower
T. F. Yenetal.
/
Diffusion bondingofMgF_,ceramics
221
Fig. 12. Scanning electron micrographs at the bonded layer of LIP() samples treated at (a) 650 °C and (b) 700 °C for 5 h.
Fig. 13. Scanning electron micrographs of samples treated at 750 °C for 5 h and under different pressures: (a) LIP0; (b) LIP2.
b o n d i n g pressures (300 kgf c m 2), m a n y voids existed o n the interface, leading to p o o r relative transmittance.
7 D. A. Ducker, H. C. Hafner and N. J. Kreidl, J. Am. Ceram. Soc., 41 (1962) 435. 8 T. E. Nappier and H. S. Halbedel, Emycl. Chem. Technol., 10(1980) 700. 9 T. E Yen, Y. H. Chang, D. L. Yu, E S. Yen, D. S. Tsai and I.-N. Lin, Mater Sci. Eng. A, 147(1991) 121. 10 W. E. Counts, R. Roy and E. F. Osborn, J. Am. Ceram. Soc., 36(1953) 15. 11 E. Lilley and J. B. Newkirk, J. Mater. Sci., 2 (1967) 567. 12 S. Miyake and K. Suzuki, J. Phys. Soc. Jpn., 9 (1954) 702. 13 H. Hattori, S. Sakaguchi, T. Kanamori and Y. Terunuma, Appl. Opt., 26(1987) 2683. 14 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 6. 15 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 7. 16 H. C. Van de Hulst, Light Scattering by Small Particles, Dover, New York, NY, 1981, Chapter 11. 17 J.W. Robinson (ed.), CRCHandbook of Spectroscopy, Vol. 2, 1974. 18 J. R. Hardy and B. S. Agrawal, Appl. Phys. Lea., 22 (1973) 236. 19 B. Bendow, Appl. Phys. Lett., 23(1973) 133. 20 J. Ciarlo and P. S. Nicholson, J. Am. Ceram. Soc., 64 (1981) 460. 21 R. James and C. R. A. Catlow, J. Phys. C, 10 (1977) L237.
Acknowledgment T h e authors express their thanks to the National Science C o u n c i l of the R e p u b l i c of C h i n a for s u p p o r t ing this project ( N S C 7 9 - 0 2 1 0 - D 0 0 6 - 2 7 ) .
References 1 T. Kaba, M. Shimada and M. Koizumi, J. Am. Ceram. Soc., 66(1983) C135. 2 H. Tabata, S. Kanzaki and M. Nakamura, Proc. Conf. on Ceramic Components for Engines, Hakone, 1983, p. 387. 3 R.S. Wilks, Br. Patent 1,417,169. 4 S.S. Cole, Jr. and G. Sommer, J. Am. Ceram. Soc., 44(1961) 265. 5 J. T. Klomp and Th. P. J. Botden, Am. Ceram. Soc. Bull., 49 (1970) 204. 6 C. R. Faulkner and L. S. Williams, Planseeber. Pulvermetall., 12(1964) 111.