Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-11-23
2001-03-20
Krynski, William (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S367000, C428S375000, C501S095100, C501S096300
Reexamination Certificate
active
06203904
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns an inexpensive method for producing high-strength silicon carbide (SiC) fibers with uniform boron nitride (BN) coatings and improved creep resistance. The process defined herein will produce BN coatings on boron-doped SiC bodies or fibers originally formed by various known processes, such as by sintering, chemical vapor deposition, etc.
Silicon carbide is a material with excellent mechanical properties at high temperatures. SiC fibers are of great interest for the fabrication of composite materials, especially composite materials for use in high temperature structural applications. Composite materials generally consist of three components: a reinforcement phase, a matrix phase, and an interface phase (i.e., a phase between the reinforcement and the matrix phases). Each phase is of critical importance in determining the properties of the composite. High-strength fibers are a common reinforcing phase. The fibers are usually coated with the interface phase prior to being consolidated with the matrix to form the overall composite. However, processes for the coating of fibers are usually expensive. In addition, many processes do not provide uniform coatings of the interphase phase over the entire surface of the fibers. Non-uniform coatings adversely affect the thermomechanical properties of the composite.
In ceramic-matrix and glass-matrix composites prepared with SiC fibers, very common interface materials include carbon (C) and boron nitride (BN). Fibers are usually coated with these interface materials by chemical vapor deposition (CVD) methods. However, CVD methods for depositing BN coatings on SiC fibers are usually very expensive. In addition, the CVD methods often do not give coatings with good uniformity, particularly for fine-diameter fibers that comprise multifilament fiber tows.
It is also possible to deposit coatings on fibers by pulling the fibers through liquid solutions or sols which contain precursors to the desired coating phase either in a dissolved form or a fine particulate form. These precursors usually need to be heat treated in an appropriate temperature range in order to be converted to the desired coating phase. However, liquid solution or sol methods may be expensive, the uniformity of the coatings formed is often poor, especially for multifilament fiber tows, and it can be difficult to form dense, non-porous, crack-free coatings, especially with coatings prepared from fine particulate sols.
In this invention, it was unexpectedly discovered that boron-doped SiC fibers with high relative density can be heat treated in a nitrogen-containing atmosphere, at a temperature which is the same or higher than the temperature used to originally sinter the fibers, to develop a BN layer at the SiC fiber surface. Surprisingly, BN-coated SiC fibers prepared in this manner still retain high tensile strengths, i.e., strengths that are approximately the same as the original uncoated SiC fibers. Furthermore the BN coated SiC fibers prepared in this manner have improved creep resistance compared to the original uncoated SiC fibers. This invention is a low-cost method for forming BN coatings on SiC fibers. The cost is low because the BN coatings are formed on the fiber in-situ by a simple heat treatment in which the surrounding inexpensive nitrogen atmosphere reacts with a constituent which is already within the fiber. In contrast, other methods are more complicated. CVD methods, for example, require careful control over processing conditions involving two or more gaseous species in order to form stoichiometric BN and to deposit it such that the coating is reasonably smooth and uniformly distributed. Liquid solution or sol methods require more complicated steps of synthesis, deposition, and subsequent decomposition of suitable precursors. Another very important cost benefit of the current invention is that it allows the fiber production and fiber coating steps to be combined into a single fabrication process.
The method of this invention can also be applied to the development of low-cost boron nitride coatings on other SiC carbide ceramics, i.e., SiC substrates, surface coatings, bulk shapes, etc. In addition, the process is expected to be applicable to forming BN coatings on other refractory carbides fibers or refractory carbide bodies which are doped with boron. This would include, but not be limited to, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), molybdenum carbide (MoC), zirconium carbide (ZrC), etc. which are doped with boron.
There are many methods of forming SiC ceramics. SiC ceramics are often prepared by forming and consolidating fine SiC particles into a desired shape and subsequently heat treating (i.e., sintering) the “green” shape in order to eliminate the interparticle pores or void space and to obtain a high-strength body with high relative density having little or no residual porosity. SiC ceramics are also prepared by other methods, especially by chemical vapor deposition (CVD) and by heat treatment of organosilicon polymers. For example, organosilicon polymers have been used to prepare fine SiC particles, fibers, bulk samples, coatings, etc. Some samples prepared using organosilicon polymers develop fine SiC crystallites and fine pores during processing and a sintering step is required to produce a dense SiC sample. The last step is carried out in a similar manner as in conventional powder processing methods, except that lower sintering temperatures are usually needed because the size of the SiC particles or crystallites and of the pores are usually considerably smaller.
It is well known that SiC ceramics with high relative density and fine grain sizes are desirable in order to attain excellent mechanical properties. However, it is very difficult to prepare pure SiC with high relative density and fine grain sizes by sintering methods, especially by pressureless sintering methods. In samples comprised of fine particles or fine crystallites, pure SiC generally undergoes growth of particles and pores during high temperature heat treatment because of the dominance of surface diffusion and/or vapor phase diffusion processes. Thus, very little densification occurs in pure SiC during high temperature heat treatment. As a result of this problem, sintering aids are almost always used to enhance the densification (and prevent coarsening) during sintering and, thereby, allow the fabrication of SiC with high relative density and fine grain sizes. Several additives have been found effective as sintering aids for SiC, but boron-containing compounds are the most commonly used additives. Varying amounts of boron compounds have been reported as effective for sintering (e.g., 0.2-5 wt %), but boron concentrations on the order of approximately 0.5-1 wt % are most common. The typical sintering temperatures for preparing dense SiC are in the range of approximately 1700-2300° C. The required temperature is highly dependent upon the size of the SiC particles or crystallites which comprise the porous body that is being sintered. For example, SiC bodies fabricated from the more conventional powder processing routes generally require higher sintering temperatures (typically from 1900-2300° C.) and this results in sintered bodies comprised of coarser grain sizes (greater than 1 micron). In contrast, organosilicon polymer-derived SiC bodies such as fibers can be sintered at lower temperatures (e.g., approximately 1700-1900° C.) and, consequently, the resulting grain sizes are usually smaller (less than 1 micron).
In addition to controlling the sintering temperature and using the proper amount and type of sintering additive, it is also important to control the gas atmosphere during sintering of SiC in order to achieve high relative density and fine grain size. It is well known that oxidizing atmospheres are undesirable, while atmospheres which are usually referred to as inert or chemically inert are desirable. Argon is the most common atmosphere used in sintering of SiC. Helium, nitrogen, and a vacuum have al
Gray J. M.
Krynski William
Saitta Thomas C.
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