Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1999-11-23
2001-02-13
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S088000
Reexamination Certificate
active
06187705
ABSTRACT:
BACKGROUND OF THE INVENTION
Silicon carbide (SiC) 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. This invention concerns a method for producing silicon carbide (SiC)-based fibers with improved creep resistance and high tensile strength, and the fibers so produced, by exposing the sintered fibers to nitrogen gas at temperatures above the sintering temperature and exposing the fibers to carbon monoxide gas at lower temperature, with the exposure steps occurring in either order.
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 (i.e., void space) and to obtain a high-strength body with high relative density (i.e., with 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. Continuous SiC fibers with fine diameters (e.g., ~5-25 &mgr;m) are generally prepared from organosilicon polymers. Some SiC fibers (and other types of samples) prepared using organosilicon polymers develop fine SiC crystallites and fine pores at some stage during processing and, hence, a sintering step is required to ultimately produce a dense (low porosity), high-strength SiC sample.
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 (~1900-2300° C.) and this results in sintered bodies comprised of coarser grain sizes (>1 &mgr;m). In contrast, organosilicon polymer-derived SiC bodies (e.g., fibers) can be sintered at lower temperatures (e.g., ~1700-1900° C.) and, consequently, the resulting grain sizes are usually smaller (<1 &mgr;m).
For high temperature applications, it would be desirable to have SiC fibers with high strength and high resistance to creep. As discussed by DiCarlo (in Composites Science and Technology, 51 213-222, 1994), it is difficult to achieve both of these properties simultaneously in SiC fibers. The strength of SiC fibers is controlled by “flaws” (e.g., voids, particulate impurities, kinks, grains, etc.). When other flaws are avoided through careful processing, the strength is controlled by the size of the SiC grains comprising the fibers. Hence, it is generally observed that carefully processed organosilicon polymer-derived SiC fibers have much better strength compared to powder-derived SiC fibers because the grain sizes are much smaller. The tensile strengths for organosilicon polymer-derived SiC fibers are typically in the range of ~2.0-3.5 GPa. (Fiber tensile strengths ≧2 GPa are desirable for the development of high-strength, fiber-reinforced composites.) In contrast, the tensile strengths for powder-derived SiC fibers are usually much lower, e.g., ~0.5-1.5 GPa.
The creep resistance of SiC fibers is usually highly dependent upon grain size also. However, the effect of grain size on creep behavior is the opposite of its effect on strength. The creep resistance increases as the grain sizes increases. Hence, powder-derived SiC fibers usually show better creep resistance than polymer-derived SiC fibers. Thus, it has not been possible in the past to make SiC fibers with both high tensile strength and high creep resistance. The difficulty in simultaneously achieving high tensile strength and high creep resistance in SiC fibers was demonstrated by Takeda et al. (in Ceram. Eng. Sci. Proc., 17[4] 35-42, 1996) using the organosilicon polymer-derived fibers known as HI-NICALON TYPE S™ which are produced by Nippon Carbon Co. As prepared HI-NICALON TYPE S™ fibers have an overall composition close to that of stoichiometric SiC, low-oxygen-content (≦0.2 wt %), high tensile strength (~3 GPa), and fine diameter (~12 &mgr;m). Takeda et al. heat treated these fibers at temperatures up to 1900° C. for 10 hours in an argon atmosphere. The fiber tensile strengths decreased with increasing heat treatment temperature, reaching values below 1 GPa for the longest heat treatment carried out at the highest temperature. X-ray diffraction line broadening measurements showed that the SiC crystallite (grain) sizes increased as the re-heat treatment temperature (and/or time) increased. Hence, the decrease in tensile strength with heat treatment was attributed to the increase in grain sizes. The creep behavior of the SiC fibers was assessed using a bend stress relaxation (BSR) test. In contrast to the strength behavior, the creep resistance of the fibers increased as the grain size increased. Fibers given the longest heat treatment carried out at the highest temperature showed the best creep resistance, as assessed by the BSR test. Heat treatments of the organosilicon polymer-derived HI-NICALON TYPE S™ fibers at the highest temperature and longest time resulted in fibers that were similar to powder-derived fibers in terms of good creep resistance and low strength values. This combination was observed because the heat treatments resulted in fibers with relatively large grain sizes.
It is well known that SiC ceramics with high relative density (i.e., low residual porosity) and fine grain sizes are desirable in order to attain high strength. 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 coarsening (growth) of particles (crystallites) and pores during high temperature heat treatment because of the dominance of surface diffusion and/or vapor phase diffusion processes. Thus, very little densification (i.e., pore removal) occurs in pure SiC during high temperature heat treatment. As a result of this problem, additives (i.e., “sintering aids”) are 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 ~0.5-1 wt % are most common. When sintering aids are used, the typical siltering temperatures for preparing dense SiC are in the range of approximately 1700-2300° C. As noted earlier, the required temperature is highly dependent upon the size of the SiC particles (or crystallites) which comprise the porous body that is being sintered.
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. There have been numerous studies in which SiC was sintered in various atmospheres. It is well known that oxidizing atmospheres are undesirable, while atmospheres which are usually referred to as “inert” or “chemically inert” are considered desirable. Argon is the most common atmosphere used in sintering of SiC. Helium, nitrogen, and vacuum have also been widely reported as useful atmospheres for sintering of SiC.
As described in more detail below, there have been many prior studies in which SiC was fabricated using nitrogen as the atmosphere during the sintering (dens
Group Karl
Saitta Thomas C.
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