Methods for making aluminum nitride armor bodies

Abstract

A method of making aluminum nitride armor bodies is provided. The method starts with low cost bulk raw material, in the form of aluminum or aluminum alloy, cryogenically mills the raw material into a precursor powder, which is essentially free of oxides and other undesirable impurities. The precursor powder is formed into a pre-form using low cost, short residence time molding processes. Finally, the pre-form is exposed to a nitriding process to convert the pre-form into the aluminum nitride armor body. In this manner, the method avoids the use of high cost aluminum nitride as a starting material and avoids the need for the high cost, single axis densification processes of the prior art.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Application Ser. No. 61 / 406,801, filed Oct. 26, 2010, the contents of which are incorporated by reference herein in their entirety.BACKGROUND OF THE DISCLOSURE[0002]1. Field of the Disclosure[0003]The present disclosure relates to armor bodies such as, but not limited to, plates, bar shapes, and / or complex structures. More particularly, the present disclosure is related to methods for producing a low cost, high efficiency, high yield aluminum nitride material and forming armor bodies therefrom.[0004]2. Description of Related Art[0005]Armor bodies are often assembled into a protective panel or system, which have been used to protect buildings, vehicles (air, land, or water), and people from projectiles. When used to protect buildings, people, and vehicles, the armor plates have been made from variety of materials such as metal, ceramic, fiber composite, glass, and others materials.[00...

AI Technical Summary

Benefits of technology

[0011]The present disclosure provides a method to eliminate several processing steps and take a novel equiaxed aluminum powder, form it into the shapes desired for armor plating, and then, after formation into the desired shape, directly reacts the aluminum powder with nitrogen in a controlled exothermic reaction in a nitrogen atmosphere. The aluminum powder can, in some embodiments, be an aluminum alloy that contains desired sintering aids such as, but not limited to, rare earth metals, alkaline metals, and any combinations thereof. These alloys can also be selected to aid in sintering and / or aid in milling performance at cryogenic temperatures.

[0013]In the case of aluminum nitride the ability to make an aluminum nitride with no oxygen in the matrix through direct nitridation of a suitable equiaxed precursor has been determined by the present disclosure as one way to achieve low cost and high density.

[0014]The methods of the present disclosure provide an aluminum powder precursor which was more equiaxed, of a size between 400 nanometers and a few dozen microns, and free of oxygen. As such the aluminum powder precursor of the present disclosure is an important step to making the desired aluminum nitride armor bodies with respect to cost and to thermal conductivity.

[0021]The process in some embodiments of the present disclosure includes placing the aluminum into a grinding mill equipped with a center line powered shaft having a multitude of orthogonal bars, preferably made of or coated with or shielded with ceramics. Typically, the mill will be filled up to about 50% or more of its volume with each media and aluminum, to a level covering the upper bar. Then, liquid nitrogen will be poured in and the mill will be covered. Some of the nitrogen will sublime, further lowering the temperature of the liquid. Upon addition of the liquid nitrogen, the materials will to some degree thermal shock and upon rotation of the shaft, the thermal shock and transition from ductile to brittle materials will provide very quick attrition of the aluminum. Typically, the shaft will rotate between 50 and 400 RPM.

[0022]Advantageously, during the milling according to the present disclosure, the energy in the mill, or the attrition zone, is very low at the circumference and at the center of the mill near the shaft. Most of the attrition and energy is found in the middle of the orthogonal bars. In this case since the shaft, in a preferred embodiment the orthogonal shafts will be metal fitted with zirconia or silicon nitride covers, never comes into anything with strength, hardness or fracture toughness exceeding that of the brittle aluminum, there is little or no wear on the bars, and thus no contamination. The energy of the rotating bars crushes the aluminum autogenously, creating no contamination and quickly producing the particles size, distribution and shape and purity desired. It is to be expected that some intrinsic nitriding may occur during the milling process. However, such nitriding is merely an artifact of the milling process and does accounts for a small percentage of the aluminum precursor.

[0023]Upon separation from the nitrogen and with protection from oxygen or other contamination, a precursor as required for forming the armor body is provided. The precursor can be processed into a pre-form of a desired shape, which is then directly processed into aluminum nitride with low cost and net shape and high theoretical density by the reaction with nitrogen from net shape aluminum powder pre-forms made of this precursor in furnace operations that will control nitrogen to control the exothermic reaction typically with cross sections of 25 mm or less; or aluminum nitride in any desired ballistic protecting shape such as, but not limited to, plate, tile, sphere, or cylinder structures that can be used to construct armor systems; or aluminum nitride in complex or even honeycomb structures where the wall thicknesses are 25 mm or less.

Problems solved by technology

Thus, the process of the present disclosure results in an aluminum nitride body, which starts with pure aluminum or aluminum alloy powder and lacks the expensive high temperature, high pressure densification process of the prior art.

Because the bonds created by these contaminations are most all covalent bonds, these contaminants are nearly impossible to remove once they have joined.

In many cases, the contaminant itself is difficult to measure and the negative effects are understood and recognized, but poorly quantified.

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