Patent Application
20080146433
Not Applicable
Not Applicable
1. Field of Invention
The invention relates to ceramics. More particularly, this invention pertains to a ceramic material product suitable for use in ballistic armor, mirrors, optics, structural ceramics, and the like, and a method for manufacturing the same.
2. Description of the Related Art
Ceramic materials are used in applications such as mirrors, optics, structural ceramics, ballistics, and the like. In many mounting and structural applications, ceramic materials are desirable for use due to the high achievable purity and complex structural shapes possible with a ceramic material, as well as the relatively high stiffness and low creep of ceramic materials. In other ceramic applications, a lightweight ceramic material is desirable. For example, in recent decades, certain hard ceramic materials have been developed for certain armor applications. These ballistic ceramic materials, such as alumina, boron carbide, silicon carbide, and titanium diboride ceramics provide the advantage of being lighter in mass than steel and provide ballistic stopping power comparable to steel. Ballistic ceramics are extraordinarily hard, strong in compression, and relatively light weight, making them efficient at eroding and shattering armor-piercing threats. Thus, in applications in which having armor with the lowest possible mass is important, such as human body armor and aircraft armor, ballistic ceramic materials are useful.
A common method of manufacture of structures made of ceramic materials is to sinter components of ceramic materials to form the structure using hot isostatic pressing (“HIP”). In an HIP process, particles of ceramic material are subjected to elevated temperature and isostatic gas pressure in an autoclave. An inert gas is used to discourage chemical reaction of the ceramic material. The increased pressure and temperature causes the ceramic material to undergo a process called sintering, whereby the particles adhere to each other.
In using an HIP process to manufacture ballistic ceramic materials, pressures exceeding 2,000 psi and temperatures in the range of 2,225 to 2,240 degrees Centigrade are necessary to achieve sufficient adhesion of the ballistic ceramic particles. The necessity for achievement and maintenance of such high temperatures and pressures make HIP manufacture of ballistic ceramics a costly endeavor, thereby resulting in increased cost to the consumer of the ballistic armor.
Reaction bonding has been used as an alternative in manufacturing ballistic ceramic structures. In reaction bonding, a matrix or composite of ballistic ceramic particles are bonded with in situ formed silicon carbide. In this process, ballistic ceramic material particles are mixed with carbon and silicon. The mixture is then heated to a point in which a portion of the carbon and silicon react to form an amount of silicon carbide distributed throughout the matrix. This reaction results in a semi-continuous phase of silicon distributed throughout the matrix, with discontinuous silicon carbide phases bonding discontinuous phases of filler material.
The reaction bonding process poses several attractive advantages over HIP sintering. Less pressure and temperature are necessary to carry on the reaction bonding process as opposed to HIP sintering, thereby making reaction bonding more economical. Also, reaction bonding is accomplished using the relatively inexpensive raw materials of the ballistic ceramic materials, such as silicon and carbon.
However, despite the advantages of reaction bonding, the performance and quality of reaction sintered ballistic ceramic matrix has traditionally been deemed inferior to HIP manufactured ballistic armor. Ceramics inherently contain flaws such as micro cracks, porosity, voids, impurities, and residual stresses from processing that can serve as sites for initiation of failure by mechanisms such as intergranular flow, sliding and micro cracking within the initial or reflected stress wave states. U.S. Pat. Nos. 7,104,177; 6,995,103; and 6,862,970 each disclose the use of silicon as an agent to react with carbon and form silicon carbide as a phase that bonds a filler ceramic, either boron carbide or silicon carbide, together with approximately 10-20 percent of unreacted silicon remaining in the composite. This amount of excess silicon is deleterious to the ballistic performance of the finished ballistic material, since silicon undergoes a pressure induce phase transformation in the range of 9-16 GPa that results in densification up to 20 percent. As is set out in V. Domnich and Y. Gogotsi, Phase Transformation in Silicon Under Contact Loading, Rev. Adv. Mater. Sci. 3, 1-36 (2002), this phase transformation ultimately leads to void formation and failure sites upon impact by a ballistic projectile.
Moreover, in traditional reaction bonding, organic materials are added to the suspended ballistic ceramic particles without assuring that the organic materials would cover the surface of all suspended ballistic ceramic particle grains. As a result, the reaction bonded silicon carbide occurs in a discontinuous phase throughout the matrix, with uneven distribution of silicon carbide and relatively low surface area contact between the silicon carbide and filler particles. This lack of uniformity of silicon carbide distribution leads to imperfections within the ballistic ceramic composite, which in turn leads to decreased ballistic-stopping performance of the ballistic ceramic composite. Thus, quality control of the resulting ballistic ceramic matrix is difficult to maintain using traditional reaction bonding techniques.
For the composite matrix to be a more effective ballistic armor, more uniform bonding between the suspended ballistic ceramic particles and the in situ formed silicon carbide is important. Ceramic bodies tend to exhibit stronger and more reliable properties when they are uniformly fine grained, fully dense, and non-porous. Microstructure is known to influence dynamic properties thought to relate to penetration resistance. For example, N. K. Bourne, et al. “The Effect of Microstructural Variations upon the Dynamic Compressive and Tensile Strengths of Aluminas” Proceedings: Mathematical and Physical Sciences, Vol. 446, No. 1927 (Aug. 8, 1994), pp. 309-318, discloses that smaller grain size corresponds to a higher Hugonit Elastic Limit (HEL) in ceramic materials. In sintered ceramics, a higher glassy grain boundary phase is associated with lower spall strength, thus indicating that a ceramic with smaller grain size and reduced amount of second “bonding” phase exhibits improved dynamic properties and resistance to failure in dynamic impact conditions. Thus, suspending small particles of a relatively hard ballistic ceramic material, such as boron carbide, within a uniform and substantially continuous phase of softer yet tougher ballistic ceramic material, such as silicon carbide, would allow for a composite ballistic ceramic material capable of exhibiting significant ballistic stopping power.
In manufacturing a reaction bonded ceramic composite having a fine grained filler distribution, a problem arises in appropriating a filler material of sufficiently small grain size. Specifically, finely divided ceramic material suitable for use as filler is significantly more expensive than large grained ceramic material of equal purity. Moreover, a second problem arises in disbursing organic materials evenly throughout the fine grained filler material. In traditional processes of dividing a large grained ceramic material into a more fine grained material, the opportunity for contaminants to adhere to the surface of the filler granules is great. Similarly, in traditional processes of dividing carbon and other organic materials into material of sufficient grain size to allow for disbursement within the fine grained filler material, the opportunity for contaminants to oxidize portions of the organic material is great. Such contamination of the fine grained filler material and the organic material leads to reduced bonding between the filler material and the organic material, thereby reducing the amount of surface to surface contact between the continuous-phase in-situ formed ceramic material and the filler material. Such reduced surface to surface contact ultimately results in greater instances of weak points in the product ballistic armor.
A method for producing a ceramic material product is disclosed. A large grained filler material is provided, which is generally a ballistic ceramic material. The filler material is divided into a number of filler granules, with the filler granules collectively having a median diameter less than approximately 10 microns. An amount of carbon is provided. In a coating step, the carbon is divided into carbon dust and the carbon dust is allowed to coat the filler granules. In one embodiment, the grinding step and coating step are accomplished simultaneously by combining the filler material with the amount of carbon in one or more grinding apparatus.
Following the grinding and coating steps, the mixture is formed into a selected shape. In one embodiment, this forming is preceded by an optional addition of an organic binder to the mixture to aid in forming the mixture to a desired shape, and to aid in retaining the mixture within the desired shape. The formed mixture is then placed in a substantial vacuum, and the mixture is then exposed to a amount of fluid silicon. The formed mixture and fluid silicon are heated to a temperature at or exceeding 1,450 degrees Centigrade. Upon reaching a temperature at or around 1,450 degrees Centigrade, the silicon within the mixture reacts with the carbon dust substantially coating the filler particles. The silicon and carbon form a matrix of silicon carbide throughout the mixture, thereby suspending the filler granules within the silicon carbide matrix.
In one embodiment of the present invention, a amount of silicon particles is provided. After agitation of the filler granules and carbon dust sufficient to substantially cover the filler granules in carbon dust, the resultant coated filler granules are mixed with the amount of silicon particles. To accomplish proper disbursement of the silicon particles within the coated filler mixture, the silicon particles are selected to have an average diameter between 10 microns and 200 nanometers. In this embodiment, the disbursed silicon particles react with the carbon dust substantially coating the filler particles during the heating step to produce the silicon carbide matrix.
The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which:
Accordingly, a method for producing a ceramic material is disclosed herein. The method of the present invention provides for the manufacture of a reaction bonded ballistic ceramic composite product having a relatively uniformly fine grained distribution, including a relatively uniformly distributed and finely divided ballistic filler suspended throughout a matrix of silicon carbide. The ballistic ceramic composite includes finely divided filler bonded by in situ silicon carbide covering substantially all of the surface area of the filler. In one embodiment, free metallic silicon is distributed within the matrix to fill the voids remaining within the ballistic ceramic composite following the in situ reaction of silicon to carbon.
Referring now to the Figures, a process for the manufacture of ballistic ceramic material is schematically represented generally at 100 in
The filler material is divided at 104 into a number of filler granules, with the filler granules collectively having a median diameter less than approximately 10 microns. In several more discreet embodiments, the filler material is divided 104 such as to exhibit a plurality of modes. For example, in one embodiment, the filler granules exhibit a first mode having a median diameter of approximately 10 microns, a second mode having a median diameter of approximately 5 microns, a third mode having a median diameter of approximately 3 microns, and a fourth mode having a median diameter of approximately 1 micron. Those skilled in the art will recognize that numerous combinations of grain size exist which are suitable for the filler granules. To this extent both a bimodal grain-size distribution and a trimodal grain-size distribution are contemplated.
A pre-selected amount of carbon sufficient to substantially coat the filler granules is provided at 106. In an embodiment utilizing boron carbide as the filler material, an amount of carbon selected to weigh from 5-10% of the weight of the filler material is typical for filler material of particle size as described above. In a coating step 108, the carbon is divided into carbon dust and the carbon dust is allowed to coat the filler granules.
In one embodiment, an attrition mill, such as those attrition mills manufactured by UNION PROCESS®, is selected to provide the grinding apparatus, due to the ability of the attrition mill to provide rapid division of the filler material with relatively high energy efficiency as compared to other grinders. However, such rapid division of the filler material is not necessary to accomplish the process of the present invention. Therefore, those skilled in the art will recognize other devices suitable for use to accomplish the grinding apparatus.
The grinding apparatus agitates the filler material, carbon, and grinding media at 208 such that the filler material is divided 104 into a number of filler granules, with the filler granules collectively having a median diameter of approximately 10 microns. At the same time, the carbon is divided into carbon dust having a sufficiently small average diameter to facilitate covering each of the filler granules with carbon dust 108. The agitation 208 of the filler material, carbon, and grinding media further serves to distribute 108 the carbon dust within the number of filler granules such that the carbon dust is allowed to substantially cover each filler granule.
By accomplishing grinding of the filler material 104 and grinding of the filler with carbon simultaneously, as illustrated in
Referring again to
Referring now to
After agitation 208 of the filler granules and carbon dust sufficient to substantially cover the filler granules in carbon dust, the resultant coated filler granules are optionally mixed at 114 with the amount of silicon particles. To accomplish proper disbursement of the silicon particles within the coated filler mixture, the silicon particles are selected to have an average diameter between 10 microns and 200 nanometers. As shown in
Referring again to
In one embodiment, the forming step 118 occurs through the process of slip casting, which is depicted in the flow diagram of
In one embodiment of the process invention 100 in which the inert cover is provided 210 by soaking the filler material and carbon in liquid ethanol during the grinding process, such liquid ethanol is used as the slip liquid 502 during the slip casting process. Thus, in one embodiment, the provision of the inert cover 210 is combined with the provision of the slip liquid 502. Furthermore, those skilled in the art will recognize other methods, such as freeze casting, dry pressing, iso-pressing, extrusion, and other such methods suitable for use to form the mixture 118 into a desired shape. Such methods may be used to accomplish forming of the mixture 118 without departing from the spirit and scope of the present invention.
Referring again to
The mixture is then exposed to the amount of fluid silicon 122. The formed mixture and fluid silicon are heated 124 to a temperature at or exceeding the melting point of the silicon, whereby the fluid silicon substantially permeates among the particles of the mixture. Upon reaching a temperature at or around the melting point of the silicon, the silicon within the mixture, including any silicon particles optionally added at 114, reacts with the carbon dust substantially coating the filler particles. The silicon and carbon form a substantially continuous phase of silicon carbide throughout the mixture. Thus, heating of the silicon vapor and the mixture results in creation of a matrix of silicon carbide with the filler particles suspended within a substantially continuous phase of silicon carbide.
In certain embodiments, illustrated in
During the heating process 124, the silicon vapor 604 permeates the grains of the formed mixture 600 to react with the carbon dust 304 to form silicon carbide. In one embodiment of the process invention 100, the provision of silicon 112 and the provision of carbon 106 occur in a substantially stoichiometrically balanced fashion, such that the silicon 112 is substantially consumed during the formation of the silicon carbide matrix. In another embodiment, sufficient silicon is provided 112 to allow silicon vapor 604 to continue to permeate and accumulate within the formed mixture 600, thereby filling at least a portion of any voids within the silicon carbide matrix with silicon.
From the foregoing description, it will be clear to one of skill in the art that a silicon carbide matrix suspending particulates of filler material is formed to produce a ceramic material. The ceramic material produced by the process invention 100 contains suspended particulates of filler material having sufficient small grain size, high surface area, and small facets, such that the ceramic material exhibits desirable strength, high weibull modulus, high thermal conductivity, high K1C and attractive thermal and wear properties for use in ballistic applications.
One skilled in the art will recognize that the ceramic material product disclosed herein is suitable for use in a number of applications, including ballistic armor, mirrors, optics, structural ceramics, and the like. For example, mirror mounts, optical elements, and other such structures composed at least in part from the ceramic material of the process invention 100 would benefit from the relatively high stiffness, low creep, and low coefficient of thermal expansion of the ceramic material. Semiconductor fixtures and furniture composed at least in part from the ceramic material of the process invention 100 would benefit from the high achievable purity, low coefficient of thermal expansion, low creep, and the complex structural shapes possible with the ceramic material of the process invention 100.
While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants general inventive concept.
