Gradient Armor System

Abstract

An armor system consisting of a ceramic matrix having a gradient of fine allic particles dispersed therein in an amount of from 0.0 percent commencing at the front or impact surface of the armor system to about 0.5 to 50 percent by volume along the interface of the system.

Description

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

The present invention relates to an armor system comprising an aluminum oxide matrix having a gradient of fine molybdenum dispersed therein.

Early lightweight armor systems comprised, for example, monolithic ceramic armor materials such A1.sub.2 0.sub.3 and B.sub.4 C. During development of the lightweight armor, it was realized that the brittle ceramic material required an energy absorbing material, e.g., "woven-roving" or impregnated fiberglass, in order to contain the spallation caused by impact and thus prevent damage by secondary missiles. It was also found that the backup material resulted in a more efficient armor system in that the combination stopped projectiles at slightly higher velocities than did the ceramic materials when utilized without a backup material.

The present armor system represents an improvement over the prior art in that it combines a hard, bullet shattering medium and an energy absorbing medium into one integral plate. The system consists of an A1.sub.2 0.sub.3 matrix in which fine molybdenum particles, i.e., approximately 1 micron, are dispersed.

Work in the field has shown that the most important property of a ceramic for defeating a projectile is its dynamic tensile strength. This property influences the time after impact at which the fractures begin to move toward each other from front and rear surfaces of the ceramic plate. The dynamic tensile strength also influences the volume fraction of a plate which will bear the loading of the projectile. It is considered that providing a cermet having increasing volume fraction toward the rear of the plate will increase the time necessary to form the crack at the rear of the plate and also increase the volume of the plate which bears the impact loading of the projectile.

The total time during which a projectile, e.g., a 30 caliber bullet, interacts with a ceramic plate is of the order of 200 to 300 microseconds. During this time many events take place during which the bullet is either defeated or sufficient fragments of the bullet and/or armor perforate the backup material causing defeat of the armor. The present armor has the tendency to increase the "incubation" period before formation of one or both of the initial cracks in an armor plate, a period extending from the instant of contact to about 5 microseconds after contact. Increasing the time necessary to form the crack at the rear of the plate while increasing the volume of the plate which bears the impact loading of the projectile is considered to result in a significant increase in ballistic limit for the gradient material.

In addition to increasing the "incubation" period before formation of one or both of the initial cracks in a gradient armor plate, the utilization of said gradient material is also considered to result in the lengthening of the total time of interaction between the projectile and the armor. It is considered that the presence of a phase in which the shock velocity is quite different from that of the matrix will influence the wave front in two important ways. First, when the wave front moves through the region having different materials, it becomes broken up or dispersed since portions of it will have different velocities in the different particles. Secondly, each time a portion of the wave moves from one material to the other, there should be an absorption of energy from the wave at the transition region between the two materials. Thus, one would expect these transition regions to be extremely important in determining the properties of the gradient armor system. A result of this importance is that nothing less than a chemical bond should be acceptable for these interface areas, i.e., there should be an interdiffusion layer between the particles or a reaction layer as a result of chemical reaction at the interface. Concurrently, there should be negligible or no porosity.

It is an object of this invention to provide and disclose an improved lightweight ceramic material.

It is a further object of this invention to provide and disclose an integral material comprising a hard exterior impact surface in combination with an energy absorbing interior medium.

It is a further object of this invention to provide and disclose an armor material comprising a ceramic matrix having a gradient of fine metal particles dispersed therein.

It is a further object of this invention to provide and disclose an armor material comprising a ceramic matrix having a gradient of fine metal particles dispersed therein, and also having an interdiffusion layer between the particles or a reaction layer as a result of the chemical reaction at the interface.

It is a further object of this invention to provide and disclose an armor material comprising an aluminum oxide matrix having a gradient of fine molybdenum oxide particles dispersed therein.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing in which:

FIG.. 1 is a schematic illustration of the gradient armor system.

FIG. 2 shows a schematic of a specific example of an alternative of the invention.

FIG. 3 shows a graphic illustration of the variation of the hardness of the material as a function of the distance within the armor material from the impact to the rear surfaces.

FIG. 4 is a graphic illustration of the variation of the dynamic tensile strength of the material as a function of the distance within the armor material from the impact to rear surfaces.

FIG. 5 is a graphic illustration of the variation of the energy absorbing capacity of the material as a function of the distance within the armor material from the impact to the rear surfaces.

Referring now to FIG. 1 of the drawing, the present armor material comprises impact surface 11 and rear surface 13. The armor material is divided into two segments. Segment 15 comprises a hard layer having high compressive properties. This is the hard, bullet shattering medium. Segment 17 comprises a layer having gradually increasing dynamic tensile strength and energy absorbing capacity. These properties are obtained by the incorporation of a metallic or intermetallic phase 19 into the matrix of the material in a progressively increasing volume fraction.

An illustration of the distribution of the fine metallic particles in the ceramic matrix obtainable in practice is shown in FIG. 2. In addition, backup plate 21 consisting of "woven-roving" fiberglass may be bonded to the rear surface of the armor. The thickness of the backup material is dependent upon the threat to be defeated and the volume fraction of metallic phase which has been incorporated into the ceramic matrix.

Line AB of FIG. 3 represents the distance from impact surface 11 to rear surface 13 of FIG. 1. Line AC represents the hardness of the material. Thus, it is seen that the hardness of the material progressively decreases commencing from the impact surface 11 to the rear surface 13 of the material.

Line AB of FIG. 4 represents the distance from impact surface 11 to rear surface 13 of FIG. 1. Line AC represents the dynamic strength of the material. Thus, it is seen that the dynamic strength of the material progressively increases commencing from the impact surface 11 to the rear surfaces 13 of the material.

Line AB of FIG. 5 represents the distance from impact surface 11 to rear surface 13 of FIG. 1. Line AC represents the energy absorbing capacity of the material. Thus, it is seen that the energy absorbing capacity of the material progressively increases commencing from impact surface 11 to rear surface 13 of the material.

The A1.sub.2 0.sub.3 powder, utilized to illustrate a specific example of this invention was developed under U.S. Pat. No. 3,305,349. The material consisted of A1.sub.2 0.sub.3 containing a fine dispersoid of 5 percent by volume of Mo. Utilizing reagent A1.sub.2 0.sub.3 powder, eight different powders containing 0.0; 0.5; 1.0; 1.5; 2.0; 3.0; 4.0 and 5 percent volume Mo were prepared. The gradient material is constructed by simply placing successive layers of each powder composition on top of the previous layer. Layer thickness is not critical. The thickness of each layer of the prototype sample was one-eighth of the total thickness. The total thickness chosen depends on the threat to be defeated. A typical total thickness is about one-third to 1.0 inch.

A hot-pressing type, commercially available apparatus was utilized to prepare the gradient armor. The final product is prepared by positioning the layers of the material, prepared as described above, in a graphite die and onto a graphite bottom plunger. A top plunger is placed in the die in contact with the powder layers, and the entire ensemble placed within an induction coil. Powder is applied to the coil to raise the temperature of the die and sample to 800.degree. C. as quickly as possible. The temperature of the sample is held constant at 800.degree. C. for 30 minutes to permit outgassing of gasses present within the sample. After 30 minutes the temperature is raised to 1,600.degree. C. as quickly as possible, and pressure is applied to the plunger and sample system so as to reach 8,000 p.s.i. when the sample reaches 1,600.degree. C. The actual heating and rate of application of pressure will depend on the specific pressure and power application equipment used of which a variety are commercially available. The pressure and temperature are held at 8,000 p.s.i. and 1,600.degree. C., respectively, for a period of 5 minutes. The sample is then allowed to cool in the die to room temperature and recovered. While I have specifically disclosed a metallic gradient of 0.5 to 5 percent by volume at the interface, a metallic gradient of 0.5 to 50 percent by volume is considered operable.

It is considered that spallation could be significantly reduced utilizing the present gradient armor thus reducing the thickness of a backup material to contain it. In certain applications, e.g., a purely fragment protective personnel armor, the backup material may not even be necessary. This would result in a significant weight reduction since the "woven-roving" backup material typically weighs 2.5 pounds per square foot. In addition, it is considered that the fragments of gradient armor which the backup material would have to contain would not be as sharp and knifelike as fragments from a monolithic ceramic plate such as A1.sub.2 0.sub.3 or B.sub.4 C.

Although I have described my invention with a certain degree of particularity, I wish to be understood that I do not desire to be limited to the exact details of formulation shown and described, for obvious modifications will occur to a person skilled in the art.

Claims

Having described my invention, I claim:

1. An armor system having front projectile impact and rear surfaces, comprising an aluminum oxide matrix having a gradient of fine molybdenum metal particles dispersed therein in an amount of 0.0 commencing at the projectile impact surface of the armor system to about 0.5 to 50 percent by volume along the interface of the system.

2. An armor system in accordance with claim 1 wherein the molybdenum metal particles are dispersed in the aluminum oxide matrix in an amount of about 0.5 to 5.0 percent by volume along the interface of the system.

3. An armor system in accordance with claim 1 wherein the molybdenum metal has a particle size of approximately 1 micron.

4. An armor system having a front projectile impact and rear surfaces, comprising an aluminum oxide matrix having a gradient of fine molybdenum metal particles dispersed therein in an amount of 0.0 commencing at the projectile impact surface of the armor system to about 0.5 to 50 percent by volume along the interface of the system, and a backup plate attached to the rear surface of the system.

5. A armor system in accordance with claim 4 wherein the molybdenum metal particles are dispersed in the aluminum oxide matrix in an amount of about 0.5 to 5.0 percent by volume along the interface of the system.

6. An armor system in accordance with claim 4 wherein the molybdenum metal has a particle size of approximately 1 micron.