Studies on the scale-up of the microwave-assisted nitridation and sintering of reaction-bonded silicon nitride Page: 1 of 6
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Studies On The Scale-Up Of The Microwave-Assisted Nitridation And Sintering
Of Reaction-Bonded Silicon Nitride
J. O. Kiggans*, T.N. Tiegs*, H. D. Kimrey*, and Jon-Paul Maria**
*Oak Ridge National Laboratory, Oak Ridge, Tenn.
**Pennsylvania State University, State College, Pa. Vz u
N1AV 0 6 19
Abstract o wl
Studies using laboratory test samples have shown that microwave heating produces sintered
reaction-bonded silicon nitride materials with improved properties [1,2]. The final challenge for
processing this material by microwave heating is the development of a technology for processing
larger batch-size quantities of these materials. Initial microwave scale-up experiments were
performed using powder compacts of a bucket tappet geometry. In experiments using
microwave-transparent boron nitride sample crucibles, temperature gradients within some
crucibles led to larger variations in the sample densities than were obtained with the
conventionally processed samples. The use of a microwave-suscepter type crucible made of
silicon carbide and boron nitride resulted in an improved temperature uniformity and in density
variations comparable to those obtained for the control groups.
Silicon nitride-based materials are the leading candidates for use as high-temperature
structural ceramics due to their excellent overall combination of mechanical and physical
properties [3,4]. Significant progress has been made in recent years in producing materials with
superior strength, fracture toughness, wear, and creep resistance. However, these materials
tended to be prohibitively expensive due to the high cost of the silicon nitride powders used to
produce them. Reduction of cost has been recognized as a major factor for the introduction of
silicon nitride ceramics into the marketplace .
Sintered reaction-bonded silicon nitride (SRBSN) is an attractive alternative to the use of
expensive high purity powders for a number of reasons. Most importantly, silicon is economical
compared to high purity silicon nitride powders. Another advantage of SRBSN is that during
sintering there is less shrinkage than for compacts starting with silicon nitride powders. During
the nitridation reaction, growth of silicon nitride occurs in the pore space between the initial
silicon particles, consequently, densities for SRBSN materials prior to sintering are relatively
high; typically about 65 to 75% theoretical density [6,7].
Traditionally, the production of SRBSN has been accomplished using resistance-heated
furnaces in a two-step process. In recent years, microwave heating has been introduced as an
alternative and an advantageous process for producing SRBSN materials[1,2,8]. The present
study is meant to build on previous studies in which small lab-scale SRBSN shapes were
processed by microwave heating to include the ability to uniformly process multiple industrial-
Injection-molded powder compactsa of a bucket tappet geometry were purchased for the
microwave scale-up studies. The green bucket tappet samples consisted of a cup-like structure
with the dimensions of 4.08 cm outside diameter, 3.38 cm height, and with a 0.381 cm wall
thickness. The buckets were bisque fired in nitrogen by the supplier to provide green strength for
shipment. The samples had a nominal composition of silicon containing 30 wt.% Si3N4 and
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Kiggans, J.O.: Tiegs, T.N. & Kimrey, H.D. Studies on the scale-up of the microwave-assisted nitridation and sintering of reaction-bonded silicon nitride, article, May 1, 1996; Tennessee. (digital.library.unt.edu/ark:/67531/metadc670341/m1/1/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.