Plastic and nonmetallic article shaping or treating: processes – Disparate treatment of article subsequent to working,... – Effecting temperature change
Reexamination Certificate
2000-02-29
2003-04-08
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Disparate treatment of article subsequent to working,...
Effecting temperature change
C264S237000
Reexamination Certificate
active
06544458
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to ceramic materials and specifically to such materials and their preparation to achieve increased density and thermal shock resistance.
DESCRIPTION OF THE PRIOR ART
Ceramic materials are increasingly becoming an integral part of modern industry. Applications of novel advanced ceramic materials can provide significant economies, increase productivity and expand product markets. Industry requires materials which have both high strength at room temperature and high strength at elevated temperatures that resist deformation, are damage tolerant, and that resist cracking, corrosion and erosion where thermal stress and mechanical stress is a factor. Usually the thermal shock resistance of high density ceramic materials is relatively low. New materials are being developed worldwide using a number of principles relating to different-compositions and several different structural characteristics, including atomic, electronic, grain boundary, micro-structure, and macro-structure.
Certain ceramic products are desirably fabricated from high purity materials, e.g., materials that are substantially entirely comprised of a particular ceramic component or material. Such materials are polycrystalline in structure and are utilized for such products as the fabrication of cutting tools and other applications where wear resistance is an important property and capability. Representative techniques for the preparation of such high density materials include those set forth in U.S. Pat. No. 5,342,564 to Wei et al. Wei et al. propose to prepare alumina-titanium carbide composites of high density by a specific rapid sintering regime which heats the green body in question to a temperature up to which thermal shock is experienced. Generally, however, improvements in properties such as thermal shock resistance and fracture toughness can only be achieved, if at all, by substantial doping with other ceramic or refractory materials such as titanium carbide. Moreover, the process so disclosed is careful to control heating so as to avoid thermal shock of the green body, and clearly, is neither expected to achieve nor actually achieves any improvements in these properties.
One of the drawbacks in the processing of high density ceramic materials of the type described is the development of extensive crack propagation and corresponding susceptibility to fracture in use. For example, the fabrication from ceramic materials of metal casting sleeves of the type regularly utilized in the fabrication of automotive parts, involves a highly critical and correspondingly expensive fabrication owing to the tendency of the dense body to develop unwanted cracks and fractures in fabrication. Generally, such sleeves are first fabricated, then cooled to shrink-fit them into the casings into which they are disposed for use. In use, the bodies experience extreme temperature fluctuations and because of their high density and corresponding susceptibility to thermal shock damage, must be frequently replaced, at great cost to the manufacturer in terms of equipment and downtime. At the same time, however, the density of materials utilized as shot sleeves must necessarily be high, and, in fact, as close to theoretical as possible, so that these materials will withstand the abrasive forces of a continuous casting process.
U.S. Pat. No. 3,887,524 to Kirchner et al. seeks to prepare an alumina body with a combination of improved strength and thermal shock properties, and employs a quenching procedure where the body is fired up to temperatures on the order of 1750° C. and is then quenched in a liquid quenching medium held at a temperature below about 250° C. This specific quenching medium is an emulsion of oil with water, and several oils are proposed and illustrated. The materials prepared in accordance with Kirchner et al., however, did not purport to exhibit the high density characteristics of products of interest herein and, in fact, by the flexural strength properties revealed, would not be expected to so perform. This same problem is reflected in U.S. Pat. No. 5,139,979 to Anderson et al. where aluminum titanate composites are prepared, which are purported to offer the properties of improved thermal shock resistance, high mechanical strength and low coefficient of thermal expansion. As reflected in Table 4 of Anderson et al., materials prepared with the described properties set forth by the patentees exhibited densities that were substantially lower than theoretical. This further evidences the understanding in the art that the properties of high density and improved thermal shock resistance are generally incapable of conjoint achievement in a single ceramic body.
As both properties are desirably achieved, and particularly inasmuch as the combination of these properties would yield a material having a broad range of commercial and other industrial applications, a need is therefore perceived to exist which is believed to be fulfilled by the present invention as set forth below.
SUMMARY OF THE INVENTION
In accordance with the present invention, a ceramic material is disclosed which exhibits a combination of properties not found in any known ceramic materials. Specifically, the ceramic materials of the present invention exhibit a combination of extremely high density approaching theoretical levels, together with an increased modulus of elasticity and, notably, increased thermal shock resistance as measured by the ability of the ceramic material to withstand a change in its temperature of 300° C. or greater, as is caused by the rapid upquenching of said ceramic material, without the development by the ceramic material or a structure formed from it, of extensive fracture or like structural degradation. Exemplary ceramic materials include alumina, zirconia, titania, thoria, silica, magnesia, calcia, nitrides thereof, carbides thereof, aluminum nitride, silicon nitride, boron nitride, boron carbide, and mixtures thereof, with alumina, aluminum nitride and silicon nitride being particularly exemplary. In a particular embodiment, the ceramic material comprises alumina, and more particularly, alumina of 85%, 96% and 98.8% purity, respectively. As illustrated herein, densities on the order of 3.8 g/cm
3
and thermal shock resistance of 650° C. or greater are attained by the within invention. The invention also extends to composites prepared with materials such as alumina, zirconia, magnesia, silica and calcia. Also, mixtures of materials are contemplated, so that, for example, composites prepared to contain 75% to 90% alumina, remainder zirconia, are included.
A ceramic structure or product in accordance with the invention is prepared by a process which comprises, in a first embodiment:
(A) forming the ceramic structure;
(B) applying a pressure or stress to the ceramic structure of Step (A) on the order of 50,000 psi as determined in relation to the size and shape of said ceramic structure, and a particular K factor relating to modulus of elasticity;
(C) concurrently with step (B), adjusting the temperature of said ceramic structure to a level on the order of 650° C. or other source of energy of activation up to and exceeding the elastic limit of said material, as determined in relation to alumina and to a particular K factor relating to the heat capacity, thermal conductivity and thermal expansion of the ceramic structure; and
(D) performing the treatment in accordance with the conditions of Steps (B) and (C) at a frequency of at least once and for a period sufficient to reach temperature equilibrium within said ceramic structure, and to thereby achieve the pressure parameter of at least the magnitude of Step (B).
A similar process that may be practiced in accordance with the method of preparation of the present invention comprises:
(A) providing a ceramic structure and subjecting it to at least one first heat treatment to promote the formation of controlled macro- and micro-fracture domains therein; and
(B) subjecting the ceramic structure treated in accordance with Step (A) to a
A. H. Casting Services Limited
Fiorilla Christopher A.
Klauber & Jackson
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