Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-02-24
2002-07-09
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S087000, C501S089000, C501S092000, C501S096300, C451S102000
Reexamination Certificate
active
06417126
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
None.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
None.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is 97% or more of the theoretical density. The composites are prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and provides a boride-containing equilibrium atmosphere during the process. The present invention further relates to an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention relates to a protective coating for an article comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising a silicon carbide particles wherein the protective coating is able to withstand repeated exposure to high temperature.
(2) Description of Related Art
Aluminum oxide (alumina) based ceramics which contain at least 50% alumina and carbides produced by hot pressing or hot isostatic pressing have high strength and excellent resistance to corrosion, oxidation, and wear. However, these alumina-based ceramics have poor strength and toughness when compared to silicon nitride-based ceramic materials. Of particular importance is that alumina-based ceramics generally have relatively poor strength and toughness and are sensitive to thermal crack formation because the aluminum oxide has relatively poor thermal conductivity. In the case of metal cutting tools, this leads to very short tool lives in machining steel, particularly under conditions with short operating times and varying cutting depths. Various attempts have been made to improve the strength, toughness, and thermal conductivity of alumina-based ceramics. To some extent, the thermal properties of alumina-based ceramics has been improved by addition of titanium carbide or titanium nitride to improve the thermal conductivity of the ceramic. The carbide
itride also had the effect of increasing the hardness of the ceramic. However, the toughness of the ceramic material was insufficient for fabricating tools to use for cutting steel. When zirconium oxide was added to the aluminum oxide a composite was produced which had increased strength and toughness but with thermal properties not much better than those of pure aluminum oxide. The addition of silicon carbide to aluminum oxide has resulted in an alumina-based ceramic containing silicon carbide whiskers which has increased strength and toughness when compared to pure alumina-based ceramics.
Alumina-based ceramics have been described in the prior art. For example, U.S. Pat. No. 4,732,878 discloses an oxidation resistant alumina-based ceramic comprising alumina-silica or alumina-boria-silica as a first phase and an in situ generated discontinuous carbon second phase. U.S. Pat. No. 5,418,197 discloses fabrication of an alumina-based ceramic containing homogeneously dispersed silicon carbide whiskers. U.S. Pat. No. 5,538,926 discloses fabrication of alumina-based ceramic containing silicon carbide whiskers and one or more oxides of Mg, Si, Ca, Ti, Zr, C, Ni, Y, and rare earth elements. U.S. Re. 32,843 and Re. 34,446 disclose fabrication of alumina-based ceramics comprising silicon carbide whiskers using a hot-press method.
However, despite the research into developing high density alumina-based ceramics, there still remains a need for a method that enables the efficient production of alumina-based ceramics with a density greater than 97% of the theoretical density for the ceramic and which has improved thermal conductivity, strength, and toughness, and increased resistance to wear, corrosion, and oxidation.
SUMMARY OF THE INVENTION
The present invention provides a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is 97% or more of the theoretical density. The composites are preferably prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and provides boride. During firing, the protective coating provides boride in an equilibrium atmosphere in the container wherein the boride is at a concentration that inhibits leeching of the boron or boron carbide from the green preform. The equilibrium atmosphere also prevents carbon from outside the container from entering the container and impregnating the green preform. Thus, the equilibrium atmosphere enables alumina-based ceramics to be fabricated to higher densities. The present invention further provides an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention provides a protective coating for an article comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising silicon carbide particles wherein the protective coating is able to withstand repeated exposure to high temperature.
Thus, the present invention provides a process for preparation of a dense alumina-based ceramic composition which comprises: (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried; (b) introducing into the container a dried green preform made from a mixture of an alumina and a metal carbide powder and a boride powder, wherein the mixture has been milled together; (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.
In particular, the present invention provides a process for preparation of a dense alumina-based ceramic composition which comprises: (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried; (b) introducing into the container a dried green preform made from a mixture of an alumina and a metal carbide powder and a boride powder, wherein the mixture has been milled together; (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.
In a preferred embodiment, the present invention provides a process wherein the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures the
C-Max Technology, Inc.
Group Karl
McLeod Ian C.
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