Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping by extrusion
Reexamination Certificate
2001-12-04
2004-05-25
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping by extrusion
C264S640000, C264S656000
Reexamination Certificate
active
06740286
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for consolidation and densification of multiple-phase composite materials, including fibrous monolith composites.
BACKGROUND OF INVENTION
The process of fabricating high strength materials from powders such as ceramic and metal powders generally involves preparing “green” materials that include the powder and a thermoplastic binder of variable composition. As part of the fabrication process, the binder typically is removed from the material in a binder burnout step and the powder consolidated and densified in order to obtain a final structure having the desired properties, including strength and hardness. Methods of consolidation and densification include sintering processes such as, uniaxial hot pressing, hot isostatic pressing, overpressure sintering and atmospheric (pressureless) sintering.
Sintering processes, are critical in the fabrication of materials from ceramic and metal powders. Equipment used in pressure sintering processes including hot isostatic pressing (HIP) and uniaxial hot pressing must be designed to accommodate the high temperatures and high pressures associated with these sintering methods. Purchase, operation and maintenance costs for the HIP and uniaxial hot press equipment may be high as a result of the need to incorporate vessels capable of withstanding high pressures or hydraulic controlled rams into their respective designs. There are also additional costs in addressing safety requirements and designs for the safe and reliable operation of high pressure equipment. Additionally, the capacity of HIP and uniaxial hot press equipment is limited by these requirements. Thus, production volume capabilities are reduced, which further increases production costs. Furthermore, pressing is generally limited and cannot be used effectively with three-dimensional structures having more complex geometries.
Pressureless sintering furnaces generally are less expensive to purchase, operate and maintain as compared to equipment for pressure sintering. They also provide larger production volume capabilities and lower overall production costs. However, an important disadvantage associated with pressureless sintering is the potential inability to achieve effective sintering of a material in the absence of pressure.
Fibrous monoliths (FMs) are a unique class of structural ceramics. FMs are monolithic ceramics that are manufactured by powder processing techniques using inexpensive raw materials. Methods of preparing FM filaments are known. U.S. Pat. No. 5,645,781 describes methods of preparing FM composites by extrusion of filaments having controlled texture. As a result of the combination of relatively low costs of manufacture and benefits of enhanced materials performance, FMs have been used in a wider range of applications than heretofore typical for ceramics. Fibrous monoliths typically have been formed to various fibrous textures. For example, FM filaments have been woven into thin, planar structures. Alternatively, the filaments have been formed into three-dimensional structures having complex geometries.
Generally, the macroarchitecture of an FM composite includes a plurality of filaments each including a primary phase in the form of elongated polycrystalline cells surrounded by at least a thin secondary phase in the form of a cell boundary. The material selected for the cell phase differs from the material selected for the cell boundary phase in type and/or composition. Thus, the various materials comprising a FM composite each have different material properties.
This “multi-phase” nature of FM composites, along with the possibility that the composites are formed into complex structures, can increase the difficulties encountered when attempting to sinter such composites. Significantly, when two or more materials are used and are to be maintained essentially separate from each other in a composite component, the ability to effectively sinter the FM composite component can be severely limited or even prevented. Because the material properties of the two phases differ, the range of physical and chemical conditions that lead to effective sintering of the composite can be restricted. The difficulty in identifying an effective sintering regime increases further as additional materials are included in the composite. Moreover, the potential for unfavorable interactions between materials that can limit sinterability increases as additional materials are added to the composite.
There remains a need for more efficient, cost-effective sintering processes that can be utilized during fabrication of fibrous monolith composite structures, particularly those having complex geometries.
SUMMARY OF THE INVENTION
The present invention overcomes the problems encountered in conventional methods by providing efficient, cost-effective processes for consolidation and densification of composites formed of more than one composition. More specifically, the present invention provides methods of pressureless sintering that are effective for sintering fibrous monolith composite structures, including those having complex geometries. Pressureless sintering of FM composites provides for the consolidation and densification of two- and three-dimensional components in less time and at a lower cost as compared to other sintering processes. Additionally, FM composites with geometries too complicated to be processed by uniaxial hot press techniques can be sintered in accordance with the method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to methods of consolidating and densifying ceramic composite components by pressureless sintering. Components that can be consolidated and densified in accordance with the invention include those formed of composites that have two or more materials present in essentially separate phases. Such composites include fibrous monolith (FM) composites, which are made up of a plurality of filaments having a core phase that is surrounded by a shell phase.
In a pressureless sintering process, composites are heated to high temperatures without high pressure in a large volume, high temperature furnace. In comparison to various pressure sintering processes, pressureless sintering significantly lowers the overall production cost of FM composites, in part due to lower equipment purchase, operation and maintenance costs. Pressureless sintering also provides large production volume capabilities, so that mass production of FM components is possible. The processes of the present invention thus provide increased effectiveness and efficiencies in the overall fabrication of FM composite components.
As used herein. “fibrous monolithic composite” and “fibrous monolith” are intended to mean a ceramic composite material that includes a plurality of monolithic fibers, or filaments, each having at least a cell phase surrounded by a boundary phase but may include more than one core and/or shell phase. Fibrous monoliths exhibit the characteristic of non-brittle fracture, such that they provide for non-catastrophic failure.
As used herein, “cell phase” is intended to mean a centrally located primary material of the monolithic fiber that is dense, relatively hard and/or strong. The cell phase extends axially through the length of the fiber, and, when the fiber is viewed in cross-section, the cell phase forms the core of the fiber. The “cell phase” also may be referred to as a “cell” or “core”.
As used herein, “boundary phase” is intended to mean a more ductile and/or weaker material that surrounds the cell phase of a monolithic fiber in a relatively thin layer. The boundary phase is disposed between the various individual cell phases, forming a separate layer between the cell phase and surrounding cell phases when a plurality of fibers are formed in a fibrous monolithic composite. The “boundary phase” also may be referred to as a “shell,” “cell boundary,” or “boundary”.
Fibrous monoliths (“FMs”) are a unique class of structural ceramics that have mechanical properties similar to continuous fi
Artz Gregory J.
Cipriani Ronald A.
Mulligan Anthony C.
Rigali Mark J.
Sutaria Manish P.
Advanced Ceramics Research, Inc.
Fiorilla Christopher A.
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