Abstract:
A method for forming a ballistic-impact armor structure Improved composite armor designs use optimally shaped ceramic cross-pellets and a web system for patterning the cross-pellets, improving manufacturability, and providing additional structural reinforcement. The result is lightweight, composite hybrid structures A dense, hard body having good fracture toughness, hardness and a high capacity to absorb impacts for ballistic protection particularly suited to tactical ground vehicles.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to ballistic armor and other structures made of ceramic materials, in more particularly to methods of making impact-resistant bodies from materials which have high fracture toughness, to a lightweight composite ballistic armor for military and tactical vehicles. 
       BACKGROUND OF THE INVENTION 
       [0002]    During the last few decades, efforts have been made to produce ceramic-based armors which will be lower in mass than metals and thus be potentially more suitable for applications where weight is of significant importance, for example aircraft armor and armor for the human body. Some of these efforts have looked towards silicon carbide as a potential candidate for such applications whereas others have used fiber-reinforced ceramic materials. 
         [0003]    The present invention relates generally to ballistic resistant devices and systems and to methods of manufacture of such ballistic resistant devices and systems, and particularly, to ballistic resistant devices and systems for use in body armor and to methods of manufacture of such ballistic resistant devices and systems. 
         [0004]    Ballistic resistant armor is used in many applications including, for example, protection of vehicles and persons from ballistic threats. Body armor to be worn on a person for protection from, for example, ballistic threats, has been available for several decades. In general, body armor protects vital parts of the human torso against penetration and severe blunt trauma from ballistic projectiles. In the development of body armor, there is a continuing effort to develop lighter, stronger, thinner, and more durable armor. 
         [0005]    This invention pertains to ballistic-impact armor structure specifically precision formed container armor structure and to methodology for making such structure. Ballistic resistant armor is used in many applications including, for example, protection of vehicles and persons from ballistic threats. Body armor to be worn on a person for protection from, for example, ballistic threats, has been available for several decades. In general, body armor protects vital parts of the human torso against penetration and severe blunt trauma from ballistic projectiles. In the development of body armor, there is a continuing effort to develop lighter, stronger, thinner, and more durable armor. 
         [0006]    The present invention relates in general to protective armor, and, in particular, to ceramic-based integral armor. None of these ceramic armors produced to date have been considered to be entirely satisfactory, and thus the search has gone on for processes for producing more effective ceramic armors. 
         [0007]    Ceramic materials have long been considered for use in the fabrication of armor components because ceramic materials have a high hardness, are potentially capable of withstanding armor-piercing projectiles, and are relatively lightweight. However, the use of ceramic materials in armor applications has been limited by the low impact resistance of these materials, which results from ceramic&#39;s brittleness and lack of toughness. Indeed, one of the significant drawbacks to the use of ceramic materials in armor applications is that they lack repeat hit capability. In other words, ceramic materials tend to disintegrate upon the first hit and cease to be useful when subjected to multiple projectiles. For a more effective utilization of ceramic materials in armor applications, it is necessary to improve the impact resistance of this class of materials. 
         [0008]    Desired armor protection levels can usually be obtained if weight is not a consideration. However, in many armor applications, there is a premium put on weight. Some areas of application where lightweight armor are important include ground combat and tactical vehicles, portable hardened shelters, helicopters, and various other aircraft used by the Army and the other Services. Another example of an armor application in need of reduced weight is personnel body armor worn by soldiers and law enforcement personnel. 
         [0009]    State-of-the-art integral armor designs typically work by assembling arrays of ballistic grade ceramic tiles within an encasement of polymer composite plating. Such an armor system will erode and shatter projectiles, including armor-piercing projectiles, thus creating effective protection at reduced weight. Various designs are in current use over a range of applications. Substantial development efforts are ongoing with this type of armor, as it is known that its full capabilities are not being utilized. For example, there is a large body of information which shows that confining the ceramics results in an increase in penetration resistance. 
         [0010]    The current war in Iraq, have heightened the need for ballistic armor. Military vehicles, in particular, are vulnerable to higher-potency weapons such as rocket-launched grenades and other projectiles. Military personnel want lightweight, fast and maneuverable vehicles, but they also want vehicle occupants to be fully protected. Ballistic steel armor plates, while relatively inexpensive, add thousands of pounds to a vehicle, many of which were not designed to carry such loads. This has resulted in numerous engine and transmission failures as well as problems with vehicle suspensions and brakes. The additional weight reduces fuel efficiency and makes it impossible to carry additional personnel in the vehicle in case of emergency. For these reasons, designers are beginning to adopt more lightweight composite armor across the board for military and tactical vehicles. 
         [0011]    Multiple hits are a serious problem with ceramic-based armors. Armor-grade ceramics are extremely hard, brittle materials, and after one impact of sufficient energy, the previously monolithic ceramic will fracture extensively, leaving many smaller pieces and a reduced ability to protect against subsequent hits in the same vicinity. Further, when the impact is at sufficient energy and velocity, collateral damage typically occurs to the neighboring ceramic tiles. Schade, et al. (U.S. Pat. No. 5,705,764) used a combination of polymers and polymer composites to encase the ceramic tiles in a soft surround to isolate the tiles from one another, reducing collateral damage. In response to ever-increasing anti-armor threats, improvements are warranted in the field of blast and fragment protection from explosive devices as well as ballistic mitigation. 
         [0012]    The invention provides a composite armor plate for absorbing and dissipating kinetic energy from high-velocity projectiles, the plate comprising a single internal layer of cross pellets which are bound and retained in plate form the cross pellets. Composite armor plate comprising: a single internal layer of cross pellets made of ceramic material disposed in a plurality of spaced-apart rows and columns, which are bound and retained in plate form by an elastic material; a majority of said cross pellets having at least one convexly curved end face; an outer impact receiving major surface defined by said convexly curved end faces of said cross pellets for absorbing and dissipating kinetic energy from high-velocity projectiles; said convexly curved end faces of said cross pellets receiving impact from high-velocity projectiles and absorbing and dissipating kinetic energy therefrom; 
         [0013]    A composite armor plate opening occupies a volume of up to 25% of said cross pellet and said valley space is approximately 2.2 mm. A composite armor plate cross pellets have at least one axis of at least 8 mm. A composite armor plate cross pellets have at least one axis of at least 16 mm. A composite armor plate wherein each of said cross pellets is formed of a ceramic material selected from the group consisting of sintered oxide, nitrides, carbides and borides of alumina, magnesium, zirconium, tungsten, molybdenum, titanium and silica. 
         [0014]    A composite armor plate wherein each of said cross pellets is formed of a material selected from the group consisting of alumina, boron carbide, boron nitride, titanium diboride, silicon carbide, silicon oxide, silicon nitride, magnesium oxide, silicon aluminum oxynitride and mixtures thereof. 
         [0015]    The present invention relates to a ceramic body for deployment in a composite armor panel, for absorbing and dissipating kinetic energy from projectiles and for ballistic armor panels incorporating the same. More particularly, the invention relates to improved ceramic bodies for use in armored plates for providing ballistic protection for body armor, light and heavy mobile equipment and for vehicles against high-velocity, armor-piercing projectiles or fragments. 
         [0016]    The present invention is a modification of the inventions described in U.S. Pat. Nos. 5,763,813; 5,972,819; 6,289,781; 6,112,635; 6,203,908; and 6,408,734 and in WO-A-9815796 the relevant teachings of which are incorporated herein by reference. 
         [0017]    In U.S. Pat. No. 5,763,813 there is described and claimed a composite armor material for absorbing and dissipating kinetic energy from high velocity, armor-piercing projectiles, comprising a panel consisting essentially of a single internal layer of high density ceramic pellets said pellets having an Al.sub.2O.sub.3 content of at least 93% and a specific gravity of at least 2.5 and retained in panel form by a solidified material which is elastic at a temperature below 250.degree. C.; the majority of said pellets each having a part of a major axis of a length of in the range of about 3-12 mm, and being bound by said solidified material in plurality of superposed rows, wherein a majority of each of said pellets is in contact with at least 3 adjacent pellets, the weight of said panel does not exceed 45 kg/m.sup.2. 
         [0018]    In U.S. Pat. No. 6,112,635 there is described and claimed a composite armor plate for absorbing and dissipating kinetic energy from high velocity, armor-piercing projectiles, said plate consisting essentially of a single internal layer of high density ceramic pellets which are directly bound and retained in plate form by a solidified material such that the pellets are bound in a plurality of adjacent rows, wherein the pellets have an Al.sub.2O.sub.3 content of at least 93% and a specific gravity of at least 2.5, the majority of the pellets each have at least one axis of at least 12 mm length said one axis of substantially all of said pellets being in substantial parallel orientation with each other and substantially perpendicular to an adjacent surface of said plate and wherein a majority of each of said pellets is in direct contact with 6 adjacent pellets, and said solidified material and said plate are elastic. 
         [0019]    In WO-A-9815796 there is described and claimed a ceramic body for deployment in a composite armor panel, said body being substantially cylindrical in shape, with at least one convexly curved end face, wherein the ratio D/R between the diameter D of said cylindrical body and the radius R of curvature of said at least one convexly curved end face is at least 0.64:1. 
         [0020]    In U.S. Pat. No. 6,289,781 there is described and claimed a composite armor plate for absorbing and dissipating kinetic energy from high velocity projectiles, said plate comprising a single internal layer of pellets which are directly bound and retained in plate form by a solidified material such that the pellets are bound in a plurality of adjacent rows, characterized in that the pellets have a specific gravity of at least 2 and are made of a material selected from the group consisting of glass, sintered refractory material, ceramic material which does not contain aluminum oxide and ceramic material having an aluminum oxide content of not more than 80%, the majority of the pellets each have at least one axis of at least 3 mm length and are bound by said solidified material in said single internal layer of adjacent rows such that each of a majority of said pellets is in direct contact with at least six adjacent pellets in the same layer to provide mutual lateral confinement there between, said pellets each have a substantially regular geometric form and said solidified material and said plate are elastic. 
         [0021]    In U.S. Pat. No. 6,408,734 there is described and claimed a composite armor plate for absorbing and dissipating kinetic energy from high velocity, armor-piercing projectiles, as well as from soft-nosed projectiles, said plate comprising a single internal layer of high density ceramic pellets, characterized in that said pellets are arranged in a single layer of adjacent rows and columns, wherein a majority of each of said pellets is in direct contact with at least 3 adjacent pellets and each of said pellets are substantially cylindrical in shape with at least one convexly-curved end face, further characterized in that spaces formed between said adjacent cylindrical pellets are filled with a material for preventing the flow of soft metal from impacting projectiles through said spaces, said material being in the form of a triangular insert having concave sides complimentary to the convex curvature of the sides of three adjacent cylindrical pellets, or being integrally formed as part of a special interstices-filling pellet, said pellet being in the form of a six sided star with concave sides complimentary to the convex curvature of the sides of six adjacent cylindrical pellets, said pellets and material being bound and retained in plate form by a solidified material, wherein said solidified material and said plate material are elastic. 
         [0022]    The teachings of all of these specifications are incorporated herein by reference. 
         [0023]    As described and explained therein, an incoming projectile may contact the pellet array in one of three ways: 1. Center contact. The impact allows the full volume of the pellet to participate in stopping the projectile, which cannot penetrate without pulverizing the whole pellet, an energy-intensive task. 2. Flank contact. The impact causes projectile yaw, thus making projectile arrest easier, as a larger frontal area is contacted, and not only the sharp nose of the projectile. The projectile is deflected sideways and needs to form for itself a large aperture to penetrate, thus allowing the armor to absorb the projectile energy. 3. Valley contact. The projectile is jammed, usually between the flanks of three pellets, all of which participate in projectile arrest. 
         [0024]    The high side forces applied to the pellets is resisted by the pellets adjacent thereto as held by the substrate or plate, and penetration is prevented. 
         [0025]    There are four main considerations concerning protective armor panels. The first consideration is weight. Protective armor for heavy but mobile military equipment, such as tanks and large ships, is known. Such armor usually comprises a thick layer of alloy steel, which is intended to provide protection against heavy and explosive projectiles. However, reduction of weight of armor, even in heavy equipment, is an advantage since it reduces the strain on all the components of the vehicle. Furthermore, such armor is quite unsuitable for light vehicles such as automobiles, jeeps, light boats, or aircraft, whose performance is compromised by steel panels having a thickness of more than a few millimeters, since each millimeter of steel adds a weight factor of 7.8 kg/m.sup.2. 
         [0026]    Armor for light vehicles is expected to prevent penetration of bullets of any type, even when impacting at a speed in the range of 700 to 1000 meters per second. However, due to weight constraints it is difficult to protect light vehicles from high caliber armor-piercing projectiles, e.g. of 12.7 and 14.5 mm, since the weight of standard armor to withstand such projectile is such as to impede the mobility and performance of such vehicles. 
         [0027]    A second consideration is cost. Overly complex armor arrangements, particularly those depending entirely on composite materials, can be responsible for a notable proportion of the total vehicle cost, and can make its manufacture non-profitable. 
         [0028]    A third consideration in armor design is compactness. A thick armor panel, including air spaces between its various layers, increases the target profile of the vehicle. In the case of civilian retrofitted armored automobiles which are outfitted with internal armor, there is simply no room for a thick panel in most of the areas requiring protection. 
         [0029]    A fourth consideration relates to ceramic plates used for personal and light vehicle armor, which plates have been found to be vulnerable to damage from mechanical impacts caused by rocks, falls, etc. 
         [0030]    Fairly recent examples of armor systems are described in U.S. Pat. No. 4,836,084, disclosing an armor plate composite includes a supporting plate consisting of an open honeycomb structure of aluminum; and U.S. Pat. No. 4,868,040, disclosing an antiballistic composite armor including a shock-absorbing layer. Also of interest is U.S. Pat. No. 4,529,640, disclosing spaced armor including a hexagonal honeycomb core member. 
         [0031]    Other armor plate panels are disclosed in British Patents 1,081,464; 1,352,418; 2,272,272, and in U.S. Pat. No. 4,061,815 wherein the use of sintered refractory material, as well as the use of ceramic materials, are described. 
         [0032]    In the majority of the patents by the present inventor, the preferred embodiments are pellets having a cylindrical body and at least one convexly curved end face while as indicated above U.S. Pat. No. 6,408,734 teaches the use of special triangular inserts or pellets in the form of a six sided star with concave sides for filling the interstices between cylindrical pellets. 
         [0033]    It has now been found that when using pellets of increased diameter especially for light and heavy armored vehicles for dealing with large projectiles, the valley space between four adjacent cylindrical pellets increases as the diameter of the pellets increase. 
         [0034]    While a pellet of regular polygonal cross-section, such as a hexagon, reduces and almost eliminates said valley space, it has been found that the maintenance of a valley space between 4 adjacent cross-pellets has several major advantages including assuring the elasticity and flexibility of the plate, reducing the overall weight of the plate and serving to attenuate the propagation of shock waves between adjacent plates. 
         [0035]    With this state of the art and these considerations in mind, there is now provided according to the present invention a composite armor plate for absorbing and dissipating kinetic energy from high-velocity projectiles, said plate comprising a single internal layer of cross pellets which are bound and retained in plate form said cross pellets. 
         [0036]    It has now been found that armor formed with cross pellets according to the present invention have major advantages in that it enables the use of cross pellets of large diameter with only a small valley space there between. 
         [0037]    Thus while the large size pellets described e.g. in U.S. Pat. No. 6,112,635 are effective for stopping larger size projectiles, there is always a danger that a small caliber projectile or a projectile fragment could find its way into the valley gap between said large diameter pellets. As will be realized. The cross pellets of the present invention result in a much smaller valley gap than that obtained with pellets having cylindrical cross-sections of comparable diameter. 
         [0038]    Furthermore, as will be realized, the cross pellets of the present invention are formed by effectively cutting away arcuate segments of a cross pellet having formed from cylindrical body and which preferably has at least one convexly curved end face and then cutting away the corners of the polygon formed thereby to form a cross pellet. As a result, segments of the composite cross pellet which are less in weight than the weight of the regular pellets. 
         [0039]    Thus, using cross-pellets according to the present invention to form composite armor plates, one no longer has to worry that an increase in pellet size results in an accompanying increase in valley gap since the size of the valley gap can be controlled by creating straight cut around edge of the cross pellets or by decreasing the arch edge, in conversely to regular pellet which creating space between three pellet the cross-pellets valley space is between four cross-pellets therefore the valley space is much smaller. The cross pellets shape can be of repeating straight sides and still with convex shape in top and under the cross pellets shape. 
         [0040]    In preferred embodiments of the present invention said cross-pellets have a special shape. Which If theoretically the projectiles can be designed to change the penetration angle of the bullet, the armor will be much more effective. Therefore, the bigger the angle change is, the better the armor performances will be. Arch and curved shapes cause oblique impacts often reorienting &amp; fracturing impacting projectiles. 
         [0041]    In the embodiments of the present invention a majority of said cross pellets preferably have at least one convexly-curved end face oriented to substantially face in the direction of an outer impact receiving major surface of said plate as view of the top slop cross pellets in  FIG. 5 . 
         [0042]    It has thus now been found that utilizing the cross pellets of the present invention according to this preferred embodiment allows a reduction in weight and height of the cross pellets equal to the difference in height between the cut and the uncut segments thereof since projectiles react to the entire height of a cross-pellet at their point of impact including the height of the convex end face. 
         [0043]    The solidified material can be any suitable material, such as aluminum, a thermoplastic polymer such as polycarbonate, or a thermoset plastic such as epoxy or polyurethane and in preferred embodiments of the present invention said solidified material and said plate are elastic. 
         [0044]    The term “elasticity” as used herein relates to the fact that the plates according to the present invention are bent when a load is applied thereto however upon release of said load the plate returns to its original shape without damage. 
       SUMMARY OF THE INVENTION 
       [0045]    The present invention improves upon existing composite armor designs through the use of optimally shaped ceramic cross-pellets and a web system for patterning the cross-pellets, improving manufacturability, and providing additional structural reinforcement. The result is lightweight, composite hybrid structures for ballistic protection particularly suited to tactical ground vehicles and body armor. 
         [0046]    The composite armor system exhibits the following features: Ultra-light-weight compared with existing armor structures. Flexibility to fit various vehicle bodies and contours. Superior impact energy absorption capability due to the unique design of the armor unit. Superior strength for structural integrity due to the tendon-reinforcements. Capability to resist heat and flame due to the flexibility to select desirable parent materials for the composite. Ease of manufacture, maintenance and repair and low life-cycle cost due to the fact that armor units can be installed and removed individually. Applicability to other military applications and to commercial vehicle systems. 
         [0047]    Examples of problems with existing composite materials and products made from the materials can include high weight, high cost of the materials, high manufacturing costs, and long manufacturing times. Additional examples of problems have included insufficient heat transfer resistance, poor acoustic properties, poor chemical resistance, poor moisture or water resistance, and inferior electrical properties. Existing composite materials have also been proven marginally cost effective for use as structural members or high strength materials. Desired material properties which have been insufficiently addressed by existing composite materials, include, for example, high strength to weight ratios, hot and cold insulation, high impact and compressive resistance, high flex modulus/stiffness, low specific gravity, chemical stability, sandability, formability, machineability, acoustics, reduced dielectric constant, non-combustible, water resistance, reduced warpage and shrinkage, and the ability to adhere or attach to other materials via conventional hardware or glues. Furthermore, existing composite materials insufficiently combine various desired material properties together into a single material. 
         [0048]    There are ongoing needs for improved materials and products made from the materials, such as lighter weight, lower cost, lower manufacturing costs, structural strength, and other properties. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    In the drawings: 
           [0050]      FIG. 1  and  FIG. 1A  is an elevational view of a first preferred cross-pellet according to the present invention. 
           [0051]      FIG. 2  and  FIG. 2A  (3D) cross-pellet  15  is provided with a channel  5 ,  6 , 9 , 11  as described in  FIG. 1 . In addition said cross-pellet  15  is provided with a channel  5 ,  6 , 9 , 11  which will be assembled as showing in  FIG. 3 . and  FIG. 12 , Increasing strength, toughness, hardness and a high capacity to absorb impacts for ballistic protection of the armor plate. 
           [0052]      FIG. 12  is an elevational view of a preferred -cross-pellet embodiments according to the present invention. 
       
    
    
       [0053]    Referring to  FIG. 1  there is seen an elevational view of a preferred cross-cross-pellet  8  according to the present invention having a substantially cylindrical body  7  and two convexly curved end faces  10  and  14 . As indicated in the drawing, end face  10  which is designed to serve as the outer impact receiving end face of the cross-pellet  8  has a radius as indicated by the letter R. In contradistinction the end face  14  designed to be disposed substantially opposite to the outer impact receiving end face  10  has a spherical surface. In addition said cross-cross-pellet  8  is provided with a channel  5 ,  6 , 9 , 11  creating triangles shapes to increase strength to the final armor plate. 
         [0054]    Referring to  FIG. 2  there is seen an elevational view of a further preferred cross-cross-pellet  15  according to the present invention having a substantially channel  5 ,  6 , 9 , 11  creating triangles shapes to increase strength to the final armor plate. 
         [0055]    Referring to  FIG. 12  there is seen an elevational view of plate embodiments with cross-pellet  17 ,  18 , and  21  according to the present invention all plate embodiment with the same pattern. As with the embodiments discussed with reference to  FIGS. 1 and 2 . 
       DESCRIPTION OF THE INVENTION 
       [0056]    Basic armor configuration according to the invention is illustrated in  FIG. 12 ,  5 ,  3  shows construction of armor plate. The ceramic layer used in the front plate is preferably composed of a single- or multi-layered fabric network filled by thermoplastic polymer material and ceramic cross-pellets, which are arranged in a periodic pattern designed for improving the ballistic resistance, especially for multiple hits. The ceramic cross-pellets will have an optimally designed shape, which enhances the transferring of impact load onto surrounding cross-pellets. This feature results in desired compress stress among the cross-pellets, which reduces the crack propagation and improves the out-of-plane impact resistance performance. 
         [0057]    The ceramic cross-pellet in the plate is molded with the selected thermoplastic polymer material, which functions as impact absorber and position keeper of the cross-pellet. The fabric web in the ceramic cross-pellet has two major functions: one is to keep the cross-pellet in a desired arrangement and the other is to reinforce the ceramic cross-pellets during the ballistic impact. 
         [0058]    The back plate features ultra-light weight and outstanding out-of-plane stiffness, strength. It is designed to have improved bending stiffness and strength for optimizing the armor performance. The fabric web is designed to hold the armor in place and form an integrated armor kit that can fill into various vehicle contours. The optimally designed supporting structure also provides the advanced features for low cost and easiness to install, replace, and repair. 
       Effectiveness 
       [0059]    Ceramic materials are hard and brittle. The high hardness contributes to flatten the nose part of the incoming projectiles, which increases the forces to stop the projectiles. The brittle properties of ceramics are not good for sustained defeating of projectiles, however, the damage zone forms due to this helps to distribute the impact force over a larger area. Another effect of brittleness of ceramic material is the long cracks usually expand from the point of hit due to bending. The long cracks and resulting small pieces of ceramic material are harmful for the defeat of projectiles, because not much constraint exist in-plane to keep the material in the damage zone and to contribute resistance forces. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0060]    Although carbides and borides have been well known as candidates for producing armor materials because of their general high strength, hardness, elastic modulus, sonic velocity and lightweight, the brittleness of these materials as flat plate has posed a problem. Frequently such materials have fractured or fragmented upon ballistic impact, and as a result, it has heretofore been generally necessary to confine such embodiments of this aspect of the present invention there is provided a cross-pellet for use in a composite armor plate for absorbing and dissipating kinetic energy from high-velocity projectiles, wherein the height of the end face disposed substantially opposite to an outer impact receiving end face of said cross-pellet is less than the height of the impact receiving end face. 
         [0061]    As stated, the present invention also provides cross-pellet for use in a composite armor plate for absorbing and dissipating kinetic energy from high-velocity projectiles, said cross-pellet being made of a ceramic material and said cross-pellet being characterized by a substantially regular geometric cross-sectional area, and first and second end faces, each of said end faces projecting from said body and having an outwardly decreasing cross-sectional area wherein the height of the end face disposed substantially opposite to an outer impact receiving end face of said cross-pellet is less than 18% of the length of the diameter of the cross-pellet body from which it projects. 
         [0062]    In the event that it is desirable to insure a situation in which the ceramic cross-pellets or ceramic bodies are distanced one from another and still retain their full ballistic resistance capabilities one may add an “ear” or a pin-like protrusion to the ceramic body which acts to sufficiently slow and erode the penetrating projectile or fragment. By adding this pin-like protrusion or ear to the ceramic cross-pellet or ceramic body. 
         [0063]    It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.