Transient FGM joining of silicon carbide ceramics: a feasibility study

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Composites Part B 28B (1997) 85-91 © 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359-8368/97/$17.00

Transient FGM joining of silicon carbide ceramics: a feasibility study

K. Kakegawa Department of Applied Chemistry, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan

and A. M. Glaeser Department of Materials Science and Mineral Engineering, University of California, USA and Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA (Received 22 April 1996; revised 20 May, 1996) The feasibility of joining SiC and SiC/SiC composites using Ge-based transient liquid phases was examined by measuring the contact angles of Ge and Ge-rich Ge-Si alloys on both SiC and reaction bonded SiC (RBSIC). The contact angle decreased with increasing anneal time and with increasing anneal temperature. The contact angle of molten Ge on SiC at 1150°C after 180 min was ~80°; contact angles on RB-SiC under the same conditions were greater than those on SiC. The addition of small amounts of Si to the Ge resulted in a significant decrease in the contact angle. This beneficial change was observed when experiments were conducted using SiC, RB-SiC, and even C. A powder of Ge or of a mechanical mixture of Ge and Si powders was applied to the interface between polished SiC blocks and the assembly was heated to 1150°C to form a bond via a Ge-based interlayer. While the blocks were joined and exhibited some strength, the resulting strengths were low, and further research is required to produce the level and reproducibility of strength required for practical applications. © 1997 Elsevier Science Limited. All rights reserved (Keywords: SiC; SiC/SiC composites; joining; transient liquid; S i - G e alloys; wetting)

INTRODUCTION The limitations of forming technologies make the fabrication of large, complex ceramic assemblies difficult. Thus, the joining of smaller components is often a necessary step towards production of useful structures and devices. This is true irrespective of whether the components are oxides, carbides, nitrides, borides or ceramic matrix composites that may also contain metals. In addition, there is often also a need to join ceramic components to metals. A combination of particular interest is the joining of silicon-based structural ceramics to high performance structural metals, notably the nickel-base superalloys. A variety of metallic alloys and glasses suitable for producing ceramic-ceramic joints for low temperature applications have been developed 1 5 . If metals are used in conventional brazing processes, the bonding temperature needs to exceed the melting temperature of the metal. If glasses are used, the bonding temperature must

be sufficiently high to reduce the viscosity of the glass to low levels. In addition to these thermal requirements, it is also necessary for the liquid to have suitable wetting behavior. The wetting behavior is often determined in a sessile drop experiment, in which the contact angle 0, Figure 1, is measured. The value of the contact angle is determined by a force balance at the line of intersection of the three interfaces involved, and is generally expressed in terms of the Young equation: ")/sv = 7sl ~- "-)'IvCOS0

( 1)

where %v, %1, and 7~v are the interfacial energies between the solid (substrate) and vapor, the substrate and liquid, and the liquid and vapor, respectively. This equation considers only a balance of force in the horizontal plane, and thus assumes that the substrate is not dissolved by the liquid. If the chemistry of the droplet is changed, the values of ~sl, and 7Iv can change, and thus, changes in the contact angle can occur. Since a change in the droplet

85

Transient FGM joining of SiC: a feasibility study: K. Kakegawa and A. M. Glaeser

vapor

~v Substrate (SIC, RB-SiC)

(a)

Substrate (SIC, RB-SiC)

(b)

Figure 1 Schematic of a sessile drop experiment in which a liquid droplet on a substrate forms a contact angle 0. The value of the contact angle satisfies equation (1). When %1 is greater than 7sv, an obtuse contact angle is formed, as shown in (a). If%1 can be reduced to a value less than 7sv, then an acute contact angle is formed, as shown in (b)

chemistry will also change the chemistry of the equilibrium vapor, %v can also be affected through interaction between the solid and the vapor, however, this effect is often ignored. A particularly useful rearrangement of this equation is: cos 0 - %v - 7sl ")'Iv

(2)

Whether the contact angle is obtuse [Figure l(a)] or acute [Figure l(b)] hinges purely on the relative value of 7sv and 7sl. Thus, if it is assumed that 7sv is unaffected by changes in liquid chemistry, the transition from a nonwetting (obtuse contact angle) to a wetting (acute contact angle) configuration requires that the value of the solid-liquid interfacial energy, %1, be reduced by the change in chemistry. The contact angle formed by the glass or molten metal on the ceramic components will play a critical role in determining the success of the joining process. Glasses and metal alloys that form an acute contact angle on the ceramic when molten will flow and fill the gap between two ceramic objects due to capillarity, and produce a good vacuum-tight seal when the assembly is cooled and the interlayer solidifies. In addition, when the contact angle is low, a more benign interfacial flaw geometry is favored6. Since, in general, molten metals form an obtuse contact angle on ceramics, chemical modification of metal-based interlayers that improves the wetting characteristics is often required. In some cases, an addition can reduce the value of 7sl, and bring about the desired

86

change. Alternatively, 'reactive' metals, metals that react with the ceramic to form a more easily wetted reaction product on the ceramic, can be added as dilute components to the alloy. Common reactive metal additives are Ti, Zr and, for some materials, Cr. In forming stable and strong joints, the thermal expansion mismatch between the components to be joined, the thermal expansion mismatch between the materials to be joined and the interlayer, and the ductility of the interlayer are all important issues 6-1°. In general, if there is a substantial thermal expansion mismatch, some ductility in the system is desirable to allow relaxation of stresses by plastic flow. If the joint is symmetric, i.e. ceramic/metal/ceramic, then the thermal expansion mismatch between the interlayer and the ceramic becomes the key concern. Elastic modulus mismatch also plays a role in determining the failure behavior. When attempting to extend these joining methods to high temperature applications, several issues assume increased importance. Since for conventional joining routes the joining temperature typically exceeds the intended use temperature, joining for high temperature applications is synonymous with high temperature joining. As a result, chemical compatibility and stability are likely to become important if not limiting issues. At low temperatures, chemical reactions between the interlayer and ceramic may be thermodynamically prohibited or kinetically inhibited. At high temperatures, reactions become more common, and the kinetics of reaction are less likely to be limiting. The temperature range over which thermal expansion mismatch can accumulate increases, potentially leading to larger mismatch induced stresses. To further complicate matters, only a limited number of metals are suitable for applications that involve high temperature service, and relatively little attention has been given to the development of refractory glasses for such applications 11. The joining of Si-based ceramics poses an extremely challenging set of problems4'5'12-14. When thin metal interlayers are used, a thermal expansion mismatch and a modulus mismatch will arise. The thermal expansion coefficients of both Si3N4 and SiC are lower than those of all metals, and thus, substantial thermal expansion mismatch stresses arise when these ceramics are joined to each other with metallic interlayers, or to bulk structural metal components. Ductile metal interlayer approaches have been used, and commercial brazes like Cusil ABA work quite well for low temperature applications. However, no commercial brazes are suitable for use at T _> 7 0 0 ° C 13'14. Oxynitride glasses have been explored for joining Si3N4, however, they are difficult to synthesize and stabilize at high temperatures 15-17. The silicon ceramics react with virtually all of the refractory metals at temperatures below their melting temperatures 18'19. One of the more successful methods of joining SiC ceramics relies on the reaction between molten Si (Tm= 1410°C) and C at ~1500°C to form SiC. Rabin 2° has shown that this reaction, when initiated in the joint region between two SiC bodies produces a

Transient FGM joining of SiC: a feasibility study: K. Kakegawa and A. M. Glaeser strong joint with failure characteristics like that of reaction bonded SiC (RB-SiC). Although the residual Si in RB-SiC limits applications to T < 1410°C, this may be adequate for numerous applications. Currently, continuous fiber ceramic matrix composites (CFCCs) based on SiC/SiC are under development for use in a variety of high temperature applications. These materials have outstanding strength and fracture toughness. However, these composites are even more difficult to join than their single phase SiC counterparts. The NICALON ~ fibers in these materials begin to degrade when the temperature exceeds ~1200°C. Thus, it is desirable to have a joining process that proceeds at T < 1200°C, but provides a use capability to temperatures approaching 1200°C. Transient liquid phase (TLP) bonding approaches seem ideally suited to satisfying this unusual combination of requirements 21. In the early 1980s, Iseki and coworkers 21'22 investigated the use of Ge as an interlayer for bonding RBSiC. Ge melts at 937°C, and alloys with the residual Si to form a more refractory Si-Ge alloy. Iseki's results showed quite respectable joint strengths could be achieved, and that strength actually peaked at high temperature; the strength increase was attributed to a reduction of the thermal expansion mismatch induced residual stress as the test temperature was increased. In principle, by extending this method, it could be applied to the joining of SiC, and also to the joining of SiC/SiC composites. Ge would provide a liquid for joining at low temperature; the degradation of NICALON fibers that occurs at more elevated temperature could be avoided or reduced. By developing interlayers that include a component that either reacts with or absorbs Ge by interdiffusion, a more refractory interlayer structure could be developed. This interlayer structure then involves a functional chemical gradient that disappears with time, an approach explored in oxide systems, and described in ref. 24. As discussed previously, the wetting behavior of an interlayer material and its chemical compatibility with the material to be joined play an important role in determining the viability of a liquid phase based joining approach. As a result, a series of sessile drop experiments were conducted, using Ge, and Ge-Si alloys in conjunction with a variety of substrate materials that might be useful in designing and fabricating a variety of interlayer chemistries for the fabrication of joints between SiC/SiC composites. The work was designed to address several critical issues relevant to joining: • Are the contact angles of Ge on SiC and RB-SiC, sufficiently low to allow joining? u What effects do the time, temperature and droplet composition have on the wetting behavior of the liquid? • Is it possible to produce assemblies that have some integrity at room temperature and at higher temperature?

EXPERIMENTAL Wetting experiments were conducted in the temperature range extending from 950°C to 1150°C in a Ta element furnace. The test samples consisted of cubes or powders of high purity Si, Ge, or Si-Ge mixtures which were placed on polished and cleaned substrates. The samples were heated, typically, at a rate of 10°C/min to the desired test temperature while a vacuum of 5-7 x 10-6 torr was maintained. For the Si-Ge mixtures, alloying occurred in situ. The wetting behavior of molten Ge and Si-Ge alloys on SiC, RB-SiC, Si, and C substrates was recorded as a function of time in this temperature range by measuring the contact angle 0 formed during a sessile drop experiment. The contact angles on both sides of each sessile drop were measured periodically using a telegoniometer with a precision of 4-2°. The difference between the two measurements was generally less than 2 °, and thus, the average value of 0 is reported. The wetting experiments were of at least 4 h duration. Efforts were also made to join SiC, RB-SiC and Si3N4 using pure Ge and Si-Ge alloys. Prior to bonding, the surfaces of the SiC, RB-SiC and Si3N4 substrates were polished to a l#m finish with diamond paste. The substrates were coated with a powder of Ge or Ge-Si alloy to form the interlayer, and two coated substrates were stacked to form a sandwich. A small weight, approximately 100 g, was put on the sample to provide a small contact pressure. The furnace chamber was evacuated to a vacuum below 3 x 10-6 torr. The samples were heated at a rate of 2°C/min to 1150°C and held at temperature for 6h to allow interdiffusion and disappearance of the liquid. They were subsequently cooled at a rate of 4°C/min to room temperature. Joined pieces were subjected to manual loading at room temperature as a quick qualitative test of joint integrity. Some joined SiC assemblies were reheated to the joining temperature with the joined face aligned vertically, and one piece supported as indicated in Figure 2. The application of a modest load to the interface was expected to result in some sliding of the samples if a significant amount of liquid reformed.

SiC ~

Joined Interface

1

Figure 2 Schematic indicating geometry used to test whether a significant amount of liquid would reform at the joining temperature

87

Transient FGM joining of SiC: a feasibility study." K. Kakegawa and A. M. Glaeser

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RESULTS A N D DISCUSSION

Contact angles The contact angles formed by pure liquid Ge on SiC at 950°C, 1050°C and 1150°C as a function of time are shown in Figure 3. In general, the contact angles decreased with time at temperature, and as the temperature was raised, the values of the contact angle at fixed time decreased. At 950°C, the contact angle decreased slowly and steadily, but remained above 90 ° even after 6 h at temperature. At 1050°C, the decrease is more rapid and more pronounced, and one would anticipate that the contact angle would fall below 90 ° after roughly 6 h. At

88

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1150°C, after an initial oscillation in the contact angle, 0 became acute after ~200min and reached ~70 ° after ~5 h. The results suggest that there is a temperature window within which a wetting condition (0 < 90 °) is achieved and fibers would not be degraded. However, while it would be possible to use Ge for joining SiC at temperatures _>1050°C, the wetting behavior is clearly not optimal. As a practical matter, lower values of the contact angle are desired, and it would be desirable to achieve this lower value of the contact angle at shorter times and at lower temperatures. Si is known to wet SiC ceramics well, and thus, in an effort to improve the wetting of Ge-based liquids on SiC, the effect of small additions of Si to Ge on the contact angle was examined. Figure 4 shows the contact angle data for pure Ge and for a G e - S i alloy on SiC at 1150°C. Clearly, Si additions had a strong beneficial effect on the contact angle. Since Si and Ge form ideal solutions, and the surface tension of pure Si exceeds that of pure Ge, small additions of Si to Ge should have a minimal effect on the liquid-vapor surface tension, 7iv; the surface should be enriched in Ge relative to the average composition, and only a slight increase in %v would be expected as Si is added. If this were the sole effect of adding Si, one would expect a slight decrease in the contact angle. However, from Equation (2), it is evident that the transition from a nonwetting to a wetting condition implies that Si additions substantially reduce the value of %1. The observation that the contact angle reduction occurs even at very short times is particularly significant in transient liquid phase processing since solidification can compete with liquid redistribution. Since one would like to minimize the joining time, it is desirable to reach low values of 0 early enough in the process to allow filling of interfacial gaps by the liquid.

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If Ge is used as the TLP former in combination with a (large) source of pure Si as the absorber of the TLP component, then one can expect that the composition of the liquid formed will reflect the liquidus temperature of the phase diagram, and thus, will vary with the joining temperature. Specifically, as the joining temperature is decreased, the amount of Si dissolved in the Ge will decrease. Thus, it was of interest to examine the dependence of the contact angle on liquid composition. The contact angles achieved on SiC substrates after 10 rain at 1150°C are plotted against the concentration of Si in the liquid in Figure 5. Even small additions of Si improved the wetting behavior substantially, suggesting

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Figure 8 Contact angles of Ge on pure Si for times up to 10min at 950°C

that Ge-Si alloys would exhibit sufficiently low contact angles even at lower joining temperatures where the Ge-rich Ge-Si liquid can accommodate only a small amount of Si in solution. Additions of Si to Ge were also effective in reducing contact angles on RB SiC. The results of contact angle measurements for pure Ge and Ge-Si alloys on RB-SiC are shown in Figure 6. At 1150°C, the addition of Si into Ge again decreased the wetting angle significantly. For the G e - l . 8 w t % Si and Ge-4.6wt% Si alloys, the molten alloy infiltrated the residual porosity in the substrate. This indicates that with these highly wetting compositions, mechanical interlocking can occur between the interlayer and the ceramic. We expected the contact angle of pure Ge would also be low, since dissolution of the residual Si in the RB-SiC should occur at temperature. The relatively high contact angle indicates little Si is being dissolved. This suggests that either the material contained little or no free Si, or that some barrier to dissolution was present. The barrier could take the form of a SiC reaction layer that 'protects' the Si core, or of SiO2. In principle, it should only be the unreacted Si core of grains that are exposed at the surface that will be accessible to the Ge as an alloying element. Figure 7 provides information paralleling that of Figure 5. It shows the Si concentration dependence of the contact angle of Ge-Si alloys on RB-SiC. As was the case for SiC substrates, small additions of Si to Ge resulted in a substantial decrease in the contact angle. We also note that the figure does not show the lowest or 'steady-state' contact angles. Several experiments were undertaken to better understand the possible combinations of materials that could be used in an interlayer based on a multiphase mechanical mixture. These experiments were also designed to help determine why in wetting experiments on RB-SiC,

89

Transient FGM joining of SiC: a feasibility study." K. Kakegawa and A. M. Glaeser 140

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time. These facts suggest that the dissolution of an oxide film does not promote a decrease in contact angle. Thus, the behavior of RB-SiC suggests that the free Si is either shielded by SiC or thick SiO2 skins, or that there is little or no free Si present in the material. In developing interlayers suitable for the joining of CFCCs, it would or could be advantageous to allow reaction between Si and C, so that SiC is formed in situ during the bonding process. For this to be viable, the TLP must wet both Si and C. To assess the potential for the latter, the effect of Si additions to Ge on the contact angle formed on C substrates was examined. The results, shown in Figure 10, indicate that Si additions were also effective in decreasing the contact angle of Ge on carbon. In this particular situation, a reaction product layer may be forming at the interface between the droplet and the substrate. A more detailed investigation of the interface is planned.

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dissolution of the Si-containing substrate was ineffective in promoting the improved wetting. Contact angles formed by initially pure Ge on a Si wafer at 950°C are shown in Figure 8. The contact angles were very low. The results suggest that the Ge can dissolve a relatively thin native oxide film, and gain access to the underlying Si. Similar behavior would be expected for thin native oxides on the excess Si in RB-SiC. To assess the effect of a thicker oxide layer on wetting, the wetting behaviour of Ge and Ge-Si alloys on SiO2 substrates was examined. Figure 9 summarizes the contact angles formed by pure Ge sessile drops and Ge-Si alloy sessile drops on SiO2 at 1050°C. The contact angle of pure Ge is high and does not change substantially with

90

Fracture strength For the purposes of joining, either a Ge powder or a Ge-Si alloy powder was inserted between blocks of SiC, RB-SiC and Si3N4. Bonded assemblies were obtained for all the combinations examined. This suggests that the joining of Si-based ceramics with Ge and Ge-Si alloys is possible. However, all samples were weak, and failed under manually applied loading. These results indicate a need to investigate the fracture surfaces and to assess the cause of the low failure loads. In prior and parallel work using metallic interlayers25 28 and described in part in a companion paper 29, similar examination led to the identification of interfacial flaws that resulted from inadequate wetting. When processing changes that improved wetting were implemented, improvements in the joint strength and strength reproducibility were also obtained. In the present case, the lack of ductility of the interlayer may also play an important contributory role. The inclusion of SiC particles may be required to reduce the thermal expansion mismatch and yield more robust joints. A number of bonded samples were reheated to 1150°C with a shear imposed on the interface, as shown in Figure 2. All the samples remained intact, and no evidence for sliding was found suggesting that little if any liquid formed. This would appear to be consistent with the desired alloying occurring during the bonding cycle. Chemical analysis and more systematic studies of the interlayer microstructure and microchemistry are needed.

SUMMARY AND CONCLUDING REMARKS The results of this feasibility study indicate that it is possible to produce a Ge-rich Ge-Si liquid that wets a variety of Si-based ceramics and C at relatively low joining temperatures. This is attractive for the joining of SiC-fiber/SiC-matrix composites, because the reduced

Transient FGM joining of SIC." a feasibility study. K. Kakegawa and A. M. Glaeser processing temperature should make it possible to avoid the fiber degradation that can be expected at the higher joining temperatures used by more conventional techniques. It is, however, important to realize that there are differences among SiC/SiC composites, and that the coatings placed on the SiC fibers may interfere with the wetting and adhesion required for the formation of a strong bond. Feasibility studies showed that some strength could be achieved, however, strong joints were not obtained. Further work will be required to identify the causes of failure, and to suggest modifications in the joining chemistry and process. As in prior work, the combination of strength measurements, fractography, microchemical and microstructural analysis, and wetting studies is expected to lead to the development of improved joint properties.

l0 11 12

13

14 15

16

ACKNOWLEDGEM ENTS

17

The research was performed at Berkeley, and materials, supplies, and instruments were provided by the Lawrence Berkeley National Laboratory. We are grateful for support by the Director, the Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. De-AC03-76SF00098.

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