Ceramic joining is recognized as one of the enabling technologies for the application of silicon carbide-based materials in a number of high temperature applications. An affordable, robust technique for the joining of silicon carbide-based ceramics has been developed. This technique is capable of producing joints with tailorable thickness and composition. Microstructure and mechanical properties of reaction formed joints in a reaction bonded silicon carbide have been reported. These joints maintain their mechanical strengths at high temperatures (up to 1350 C) in air. This technique is capable of joining large and complex shaped ceramic components

Singh, Mrityunjay

English

NASA Technical Reports Server

A NEW APPROACH TO JOINING OF SILICON CARBIDE-BASED MATERIALS FOR
HIGH TEMPERATURE APPLICATIONS j_j "_._
Mrityunjay SINGH
f6
FDC-NYMA, Inc., MS 106-5, NASA Lewis Research Center Group, Cleveland, OH
44135, USA
Ceramic joining is recognized as one of the enabling technologies for the application of
silicon carbide-based materials in a number of high temperature applications. An
affordable, robust technique for the joining of silicon carbide-based ceramics has been
developed. This technique is capable of producing joints with tailorable thickness and
composition. Microstructure and mechanical properties of reaction formed joints in a
reaction bonded silicon carbide have been reported. These joints maintain their
mechanical strengths at high temperatures (up to 1350°C) in air. This technique is
capable of joining large and complex shaped ceramic components.
1. INTRODUCTION
Silicon carbide-based ceramic materials are either being used or are under active
consideration for use in a number of high temperature applications in aerospace and
ground-based systems [1-2]. The interest in these materials is due to their good
mechanical properties, high thermal conductivity, and good oxidation and thermal shock
resistance. The engineering design often requires fabrication of complex shaped silicon
carbide-based ceramic components which are quite expensive. In many instances, it is
much more economical to build up complex shapes by joining together geometrically
simple shapes. However, the joints must have good mechanical strength and
environmental stability comparable to the bulk materials, and the joints must retain their
structural integrity at high temperatures. In addition, the joining technique should be robust,
practical, and reliable.
Overviews of various joining techniques for monolithic ceramics, i.e., mechanical
• fastening, adhesive bonding, welding, brazing, and soldering have been provided in recent
publications [1-8]. The majority of the techniques used today are based on the joining of
monolithic ceramics with metals either by diffusion bonding, metal brazing, brazing with
oxides and oxynitrides, or diffusion welding. These techniques require either high
https://ntrs.nasa.gov/search.jsp?R=19980231075 2020-06-18T00:38:18+00:00Z
temperatures for processing or hot pressing.The joints i:,roducedby these techniques have
different thermal expansion coefficients than the parent materials, which contributes to
stress concentration in the joint area. Normally, the use temperatures for these joints are
around 700 °C.
Ceramic joint interlayers have been developed as a means of obtaining high
temperature joints in silicon carbide-based ceramic materials. These joint interlayers have
been produced via pre-ceramic polymers, in-situ displacement reactions, and tape
casting/reaction bonding techniques [8]. Joints prodL_ced by the pre-ceramic polymer
approach exhibit large amounts of porosity and poor mechanical properties. On the other
hand, hot pressing or high temperature fixtures are needed for in-situ displacement
reactions and reaction bonding techniques. Due to the equipment required, these
techniques are not well suited for joining large or complex shaped components.
The reaction forming technique reported here is unique in terms of producing joints
with tailorable microstructures. The formation of joints by this approach is attractive since
the thermomechanical properties of the joint interlayer can be tailored to be very close to
those of the silicon carbide base materials. In addition, high temperature fixturing is not
needed to hold the parts at the infiltration temperature. A variety of silicon carbide-based
ceramics and fiber reinforced composites have been joined using this approach [9-15].
In this paper, the microstructure and mechanical properties of reaction formed
joints in a reaction bonded silicon carbide material (Cerastar RB-SiC) have been
discussed. The room and high temperature mechanic_.l behavior of as received Cerastar
RB-SiC material has been presented, too, for comparison.
2. EXPERIMENTAL PROCEDURES
The Cerastar reaction bonded silicon carbide (Cerastar RB-SiC) materials used in
this study were obtained from Carborundum Co., Gardener, MA. Appropriate size pieces
were cut from the large plates and as machined surfaces were used for joining. Before
joining, these pieces were cleaned in acetone and dried
Details of the reaction forming method for tile joining of silion carbide-based
ceramics and fiber reinforced composites have been discussed in earlier publications [9-
15]. The joining steps include the application of a carbonaceous mixture in the joint area
and curing at 110-120°C for 10 to 20 minutes in a fixture. Silicon or a silicon-alloy in tape,
paste, or slurry form is applied around the joint region and heated up to 1250-1425°C
(depending on the composition of the infiltrant) for 5 to 10 minutes. The molten silicon or
silicon-alloy reacts with carbon to form silicon carbide with controllable amounts of silicon
and other phases as determined by the alloy composition. Joint thickness can be readily
controlled in this process by controllingthe fixturing force during the curing step.
Flexure bars were machined from the joined pieces, with joints in the middle of the
flexure bars. Four-point flexural strength testing was carried out using MIL-STD-1942
(MR) configuration B specimens with 20 mm inner and 40 mm outer spans. Flexure tests
were conducted at room temperature, 800, 1200, and 1350°C in air. At least six to nine
specimens were tested at room temperature, and four specimens were tested at each
high temperature. After testing, fracture surfaces were examined by optical and scanning
electron microscopy to identify the failure origins.
3. RESULTS AND DISCUSSION
An optical micrograph of joined Cerastar RB-SiC material with a ~ 25 l.u'n thick
reaction formed joint is shown in Fig. 1. This micrograph shows a non-uniform distribution
of coarse and fine silicon carbide grains (grey) and a silicon phase (white) in the base
material. The joint contains silicon carbide and silicon phase. Reaction formed joints with
different thicknesses have been fabricated using this process. The joint thickness and
composition have a strong influence on both the room and high temperature properties of
• the joined materials.
Fig. 1 : Microstructure of reaction formed joints in Cerastar RB-SiC material (joint thickness
-25 pm).
The room and high temperature flexural strengths of the as-received and joined
Cerastar RB-SiC materials are shown in Fig. 2. The average room temperature strengths
• of as-received and joined specimens were 157+11 MPa and 212+10 MPa, respectively.
The flexural strength of as-received bars increases at high temperatures. Healing of
machining flaws is one possible explanation. The flexural strengths of joined bars are
comparable to those of as-received materials at high temperatures. In the joined bars,
fracture always occurs away from the joint regions. The fracture origins appear to be
inhomogeneities inside the parent material. SEM examination of fracture surfaces
indicates that the inhomogeneous silicon distribution and porosity are the most common
fracture origins.
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Test Temperature (°C)
Fig. 2: Flexural strengths of as-received and joined Cerastar RB-SiC at room and high
temperatures.
This joining approach has been used to join a wde variety of silicon carbide-based
materials in different sizes and shapes as shown in Fig. 3. There is a potential to extend
this joining approach to the repair of silicon carbide components in service.
Fig. 3: Photograph showing joined monolithic silicon carbide pieces.
4. CONCLUSIONS
It has been demonstrated that the reaction forming approach can be used to
produce strong joints in commercially available reaction bonded silicon carbide-based
materials. These joints maintain their strength at temperatures up to 1350°C in air. The
joining technology is affordable and robust and it can be used for the joining of large and
complex shaped components.
ACKNOWLEDGEMENT
The author would like to thank Mr. Steve Cogoli of Carborundum Co. for providing
the monolithic silicon carbide materials. Technical assistance of Mr. R.F. Dacek in this work
is greatly appreciated.
REFERENCES
1) M.M. SCHWARTZ, "Ceramic Joining", ASM Intemational, Materials Park, OH (1990).
2) M.G. NICHOLS, "Joining of Ceramics", Chapman and Hall, London (1990).
3) R.W. MESSLER, Jr., "Joining of Advanced Materials", Butterworth-Heinemann, Boston,
MA (1993).
4) M.M. SCHWARTZ, "Joining of Composite Matrix Materials", ASM Intemational,
Materials Park, OH (1994).
5)
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C.R. BATES, M.R. FOLEY, G.A. ROSSI, G.J. SANDBERG, and F.J. WU, Ceramic
Bulletin, 69, 3 (1990) 350-356.
T. ISEKI, 'Joining of SiC Ceramics' in "Silicon Carbide Ceramics-l: Fundamental and
Solid Reaction", Edited by S. Somiya and Y. Inomata, Elseveier Applied Science, New
York, NY (1991) 239-261.
N. IWAMOTO, 'Joining of SiC' in "Silicon Carbide Ceramics-2: Gas Phase Reactions,
Fibers and Whisker, Joining", Edited by S. Somiya and Y. Inomata, Elseveier Applied
Science, New York, NY (1991)279-295.
J.M. FRAGOMENI AND S.K. EL-RAHAIBY, "Review of Ceramic Joining Technology",
Rept. No. 9, Ceramic Information Analysis Center, Purdue University, Indiana (1995).
M. SINGH, J.D. KISER, and S.C. FARMER, Ceram. Engg. and Sci. Proceedings, 18, 3
(1997) 161-166.
10) M. SlNGH and J.D. KISER, 'Physics & Process Modeling and Propulsion R&T
Conference', NASA CP-10193 (1997) 5:1-10.
11) M. SINGH, Scripta Materialia, 34, 8 (1997) 1151-1154.
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15) M. SINGH, "Microstructure and Mechanical Properties of Reaction Formed Joints in
Reaction Bonded Silicon Carbide Ceramics", J. Mater. Sci., (1998) in press.


A New Approach to Joining of Silicon Carbide-Based Materials for High Temperature Applications

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