Ceramic composite of silicon carbide and aluminum nitride

A ceramic composite is disclosed which may be used as lightweight armor or for other impact or wear resisting purposes. The ceramic composite may comprise distinct phases of AlN and SiC; may be a solid solution of SiC; or may contain AlN or SiC, or both AlN and SiC as residual phase(s) in a solid solution matrix of SiC and AlN.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the use of silicon carbide/aluminum 
nitride as a solid solution ceramic composite for use in armor, cutting 
tools, wear parts, nozzles, parts subject to high temperature, and similar 
articles subjected to severe impacts or abrasive action. 
2. Description of the Prior Art 
Cutler et al U.S. Pat. No. 4,141,740 discloses a refractory product formed 
as a solid solution from aluminum nitride, silicon carbide, and aluminum 
oxycarbide and the process for producing the refractory product. This 
patent is cited as background pertaining to the subject invention. 
It is well known that silicon carbide ceramic faced armor systems are light 
weight and offer substantially better ballistics performance than that of 
monolithic metallic plates. The silicon carbide ceramic faced armor 
systems are better because the ceramics which are used have greater 
compressive mechanical properties, especially dynamic compressive yield 
strengths, than do the metals. These ceramics then cause the projectile to 
deform more than do the metals, either through plastic deformation and 
erosion, or through fracture. The penetration capabilities of the 
projectile is reduced by this increased deformation in two ways. 
Projectile plastic deformation and consequent erosion reduces the kinetic 
energy of a projectile through plastic flow and reduction in projectile 
mass. Alternately, a fractured projectile is essentially defocused; its 
impact footprint is increased which allows a large volume of the target 
material to work against the projectile. 
While silicon carbide ceramic faced armor systems have performances which 
are known to be superior to most other systems, it is not presently used 
on light armored combat vehicles for two major reasons. The first is cost; 
the high performance silicon carbide ceramic materials now typically cost 
in the range of $50.00 to $100.00 per pound. Our Government is presently 
unwilling to pay the resulting high cost of an armor system utilizing 
these high cost ceramics for application to a light weight armored 
vehicle. The other reason is the typical poor multi-hit performance of 
silicon carbide ceramic systems caused by the highly brittle nature of the 
ceramic material. Silicon carbide ceramic faced armored systems are 
typically built with ceramic plates or "tiles". A ballistic impact on one 
tile is sufficient to fracture that tile such that it will not prevent a 
penetration if hit a second time. In addition, the impact into the first 
tile can often induce fracture of adjacent tiles, reducing their 
performance such that they will no longer prevent projectile penetration. 
Hence, the multi-hit performance, or number of projectiles that can be 
stopped at any given area is limited. 
SUMMARY OF THE INVENTION 
The ceramic composite materials of the present invention are primarily 
intended for use as a lightweight armor for deflecting armor piercing 
projectiles through erosion and shattering of the projectile. The 
composite material may be used for other uses where they would be 
subjected to severe wear, high temperatures, abrasive action or other 
severe impacts. 
When used in lightweight armor, the ceramic composites are specifically 
formulated and processed for the best trade-off between both economy of 
manufacture and performance. More particularly, the composites are 
comprised of microstructures containing two or three phases which include: 
silicon carbide, (SiC), aluminum nitride (AlN), or a solid solution of AlN 
and SiC. 
Through proper selection of powders and firing conditions, one of the 
following different microstructural types can be produced: 
TYPE 1 comprises distinct phases of SiC and AlN; 
TYPE 2 comprises a solid solution of SiC and AlN; and 
TYPE 3 contains an AlN, or SiC, or both AlN and SiC as residual phase(s) in 
a solid solution matrix of SiC and AlN. 
The firing may be either sintering or hot-pressing, and firing conditions 
of temperature and time are the controlling factors in obtaining the 
desired microstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Prior to describing the ceramic composite material of the present 
invention, it is believed that a brief description of the preferred use of 
the material as armor for preventing the penetration of high velocity 
steel, tungsten and similar piercing projectiles, would be helpful. 
As indicated above, it is well known that excessively heavy armor added to 
military combat vehicles is undesirable since it lowers the performance of 
the vehicles. When using a ceramic composite of AlN and SiC it has been 
determined that the composite is not only much lighter in weight but is 
also lower in cost than steel silicon carbide when defeating the same 
ballistic threats. Also a ceramic composite of AlN and SiC when used as 
armor exhibits considerable toughness during ballistic impact which 
provides large granular fractures and chunks after impact. Toughness is 
also exhibited by the ceramic composite of the present invention since its 
ballistic performance is constant at all angles of obliquity, i.e., the 
same resistance to penetration occurs when the armor is impacted by a 
weapon when contacted at 90.degree. or at an angle less than 90.degree. to 
the surface of the armor. It is well known that other ceramic armor, such 
as boron carbide fractures differently with changes in obliquity. 
It is also believed that chemical definitions of "solution", "phase" and 
"residual" may be helpful in understanding the invention. 
"Solution" is defined as a "single homogeneous, liquid, solid or gas phase 
that is a mixture in which the components "liquids, gas, solid, or 
combinations thereof" are uniformly distributed throughout the mixture. 
"Phase" is defined as "portions of a physical system (liquid, gas, solid) 
that is homogeneous throughout, has definable boundaries, and can be 
separated physically from other phases. 
"Residual" relates to a mineral deposit formed by a chemical concentration 
of residue left in place. 
When it is said that one system (A) offers better performance than another 
system (B); it means either that the minimum weight of a system (A) is 
less than that of (B) when both are designed to prevent penetration of a 
projectile under identical impact conditions, or that when both systems 
are of the same weight, system (A) can prevent a given projectile from 
penetrating under more severe impact conditions than can system (B). 
As mentioned previously, silicon carbide faced armor systems when used 
alone are too costly and lack multi-hit. 
The ceramic composite of the present invention is formed from a mixture of 
powders of AlN and 1 to 99% SiC. It has been determined that just 1% of 
AlN is enough to modify the microstructure since the grain size of the 
composite is refined at 1% and above. It has also been determined that 75 
to 95% of SiC yields the higher fracture toughness. The powders are 
blended followed by molding and sintering or by hot-pressing. The firing 
temperature is selected dependent upon the microstructure desired. For a 
TYPE 1 microstructure, a mixture of AlN and SiC, firing is conducted in 
the range of 1600 to 1800 degrees centigrade. For TYPE 2 or 3 
microstructure, with a matrix of a solid solution of AlN and SiC, the 
firing temperature ranges from about 1800 to 2300 degrees centigrade. A 
significant fraction of the powders react at this higher temperature, to 
form a solid solution matrix, which densifies around the unreacted 
particles. 
The above ceramic composites of the present invention is intended to 
address the problem of prior art silicon carbide ceramic armored systems 
discussed previously. AlN has been ballistically tested and found to have 
about two-thirds of the ballistic performance of SiC. However, the cost of 
AlN is projected to drop to near half of the price of SiC in the near 
term, due to a large market potential in electronic applications. The 
addition of AlN to SiC decreases final costs into two ways. First, the 
inclusion of the lower cost powder decreases the cost of the final 
product. Secondly, the inclusion of AlN allows for a significant reduction 
in the processing temperature compared to SiC without the AlN. TYPE 1 
microstructure AlN/SiC composite parts, with 38 volume percent SiC, have 
been hot-pressed at 1650.degree. C. compared to 1950.degree. C. typically 
required without the AlN. This reduction in temperature significantly 
increases the life of the hot-pressing graphite tooling, resulting in 
reduced final part costs. 
Regarding the multi-hit performance, AlN in ballistic tests has 
demonstrated higher toughness compared to plain SiC. This was demonstrated 
both by the very course rubble (compared to that of SiC) of AlN after 
ballistic impact, and by the constant performance of the AlN over a range 
of angles of attack of the projectiles. The ballistic performance of other 
high performance ceramics such as boron carbide (B4C) and plain SiC drops 
when the ceramics are impacted by projectiles at angles lower than 
90.degree. to the surface of ceramic. This performance drop is caused by 
the extensive fracture and consequential loss in shear properties of the 
more brittle ceramics. The higher toughness of the AlN eliminates this 
performance drop. The solid solution matrix in TYPES 2 and 3 of the 
proposed ceramic composite exhibit a toughness between that of AlN and 
SiC. The unreacted particles which remain in the matrix impart additional 
toughness in the composite by the crack deflection mechanisms common in 
other particulate ceramic composite materials. The same crack deflection 
mechanisms give the TYPE 1 microstructure greater toughness than that of 
plain SiC as well. The proposed ceramic composite will have: sufficiently 
high ballistic performance to perform as lightweight armor systems; 
toughness such that the problems with limited multi-hit performance are 
substantially reduced or eliminated; and costs low enough to justify use 
in armor applications. 
The ballistic performance, multi-hit performance, and cost requirements of 
an armor system guides the selection of the appropriate ceramic composite 
formulation and microstructural type. For instance, for the lowest 
possible costs a high percentage of AlN composite would be processed to 
form a TYPE 1 microstructure. Higher ballistic performance requirements 
would require the formulation to have higher SiC contents with a TYPE 2 or 
3 microstructure. TYPE 3 formulation would be used for greater toughness 
and multi-hit performance. When used as armor, the ceramic composite of 
the present invention is preferably formed from aluminum nitride with 1 to 
99 percent silicon carbide. The aluminum nitride and silicon carbide are 
received as 0.5 to 5 micron powders which are placed in molds and are 
either hot pressed together or sinter fired together at about 2000 psi for 
4 to 6 hours at about 1700.degree. C. The aluminum nitride and silicon 
carbide form a solid solution which when cooled are in the shape of hard 
tiles which are connected to the vehicle surface to be protected. 
The aluminum nitride has about two-thirds the efficiency of silicon carbide 
to stop the armor piercing projectiles as compared to silicon carbide 
alone. However, the aluminum nitride, at the present time, costs about 
one-third that of silicon carbide. The silicon carbide when hot pressed 
with aluminum nitride improves the efficiency of the armor by increasing 
the projectile erosion properties of the composite. 
Although the primary intended use of the ceramic composite of the present 
invention is as armor for military vehicles which will defeat armor 
piercing munitions, it will be understood that the composite may be used 
for cutting tools, wear parts, nozzles, electronic components, high 
temperature components and many other applications subjected to high 
impact forces and/or wear. 
From the foregoing description it is apparent that the ceramic composite of 
the present invention is formed from a selected percentage of the silicon 
carbide and aluminum nitride which react when processed to form the 
ceramic composite. The solid solution microstructure is formed by 
processing at a higher temperature compared to the mixed individual AlN 
and SiC phase microstructure. 
Although the best mode contemplated for carrying out the present invention 
has been herein shown and described, it will be apparent that modification 
and variation may be made without departing from what is regarded to be 
the subject matter of the invention.