Pressureless sintering-based method for making a two-phase ceramic composite body

Abstract

Inventive manufacture of CrB.sub.2—Al.sub.2O.sub.3 composites is based on pressureless sintering. According to typical inventive practice, CrB.sub.2 powder and Al.sub.2O.sub.3 powder are mixed together in selected volumetric proportions so that the volume of the CrB.sub.2 does not exceed 50% of the overall volume of the CrB.sub.2—Al.sub.2O.sub.3 mixture. The CrB.sub.2—Al.sub.2O.sub.3 mixture is shaped into a green body. The green body is pressureless sintered in a non-oxidizing atmosphere at a firing temperature in the approximate range between 1600° C. and 2050° C. The present invention succeeds in preparing, via pressureless sintering, a proportionality-associated range of compositions in the CrB.sub.2—Al.sub.2O.sub.3 system, which is a potentially “advanced” ceramic system. A typical inventively fabricated CrB.sub.2—Al.sub.2O.sub.3 composite is inventively configured in a complex shape, and has “advanced” material (e.g., mechanical) properties that are favorable for a contemplated application. Inventive manufacture of ceramic-ceramic composites is thus dually attributed, and uncommonly so, with complex shape-ability and advanced capability.

Claims

1. A method for making a chromium diborid-aluminum oxide two-phase ceramic composite body, the method comprising: mixing chromium diboride powder and aluminum oxide powder to produce a mixture, said mixture consisting of said chromium diboride and said aluminum oxide, wherein said chromium diboride constitutes a percentage by volume of said mixture in the range between 21% and 30%, and said aluminum oxide constitutes a percentage by volume of said mixture in the range between 70% and 79%; shaping said mixture to form a green body, said green body including said chromium diboride and said aluminum oxide; pressureless sintering said green body in a non-oxidizing atmosphere at a firing temperature in the range between 1600° C. and 1699° C., the pressureless sintered said green body including said chromium diboride and said aluminum oxide; wherein said pressureless sintering of said green body is performed to obtain a chromium diboride-aluminum oxide two-phase ceramic composite that is fully dense.

2. The method of claim 1 wherein the method further comprises milling said mixture prior to said shaping.

3. The method of claim 1 wherein said non-oxidizing atmosphere is one of a vacuum, an inert gas, and a reducing gas.

4. The method of claim 1 wherein said shaping includes using at least one shaping technique selected from the group consisting of die pressing, cold isostatic pressing, extrusion, slip casting, and injection molding.

5. The method of claim 1 wherein the method further comprises cooling the pressureless sintered said green body.

6. The method of claim 5 wherein the method further comprises at least one of: machining said green body prior to said pressureless sintering; and machining the cooled pressureless sintered said green body.

7. The method of claim 1 wherein said chromium diboride constitutes a percentage by volume of said mixture in the range between 22% and 30%, and said aluminum oxide constitutes a percentage by volume of said mixture in the range between 70% and 78%.

8. The method of claim 1 wherein said chromium diboride constitutes a percentage by volume of said mixture in the range between 23% and 30%, and said aluminum oxide constitutes a percentage by volume of said mixture in the range between 70% and 77%.

9. A method for making a CrB.sub.2—Al.sub.2O.sub.3 two-phase ceramic composite article, the method comprising: preparing a blend of chromium diboride powder and aluminum oxide powder, said blend consisting of said chromium diboride and said aluminum oxide, wherein the prepared said blend of chromium diboride powder and aluminum oxide powder is in volumetric proportions in the range between 21% CrB.sub.2-79% Al.sub.2O.sub.3 and 30% CrB.sub.2-70% Al.sub.2O.sub.3; shaping said blend into a green body, said green body including said chromium diboride and said aluminum oxide; densifying said green body in the absence of applied pressure in a non-oxidizing atmosphere, said densifying of said green body including heating said green body in a furnace to a temperature in the range between 1600° C. and 1699° C., the densified said green body including said chromium diboride and said aluminum oxide; wherein said densifying of said green body is performed to obtain a chromium diboride-aluminum oxide two-phase ceramic composite that is fully dense.

10. The method of claim 9 wherein said shaping of said blend includes cold isostatic pressing.

11. The method of claim 9 wherein said non-oxidizing atmosphere is either a vacuous atmosphere or a gaseous atmosphere.

12. The method of claim 11 wherein said gaseous atmosphere is either an inert gaseous atmosphere or a reducing gaseous atmosphere.

13. The method of claim 9 wherein the method further comprises at least one of: machining said green body prior to said densifying of said green body; and machining the densified said green body.

14. The method of claim 9 wherein the prepared said blend of chromium diboride powder and aluminum oxide powder is in volumetric proportions in the range between 22% CrB.sub.2-78% Al.sub.2O.sub.3 and 30% CrB.sub.2-70% Al.sub.2O.sub.3.

15. The method of claim 9 wherein the prepared said blend of chromium diboride powder and aluminum oxide powder is in volumetric proportions in the range between 23% CrB.sub.2-77% Al.sub.2O.sub.3 and 30% CrB.sub.2-70% Al.sub.2O.sub.3.

Description

DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

(1) According to the present invention's typical methodology for making a CrB.sub.2—Al.sub.2O.sub.3 two-phase ceramic composite article, aluminum oxide and chromium diboride powders are intimately mixed in proportions to yield the desired volume percent ratio, based on the densities of the two species. The volumetric proportions will usually fall within the range between about 0% CrB.sub.2-100% Al.sub.2O.sub.3 and about 50% CrB.sub.2-50% Al.sub.2O.sub.3. Frequent inventive practice provides for volumetric proportions falling within the range between about 20% CrB.sub.2-80% Al.sub.2O.sub.3 and about 30% CrB.sub.2-70% Al.sub.2O.sub.3.

(2) Optionally, the mixture (blend) of powders can then be milled to achieve smaller particle sizes. Additionally or alternatively, either or both of the CrB.sub.2 powder and the Al.sub.2O.sub.3 powder can be milled prior to mixing, for similar purpose. In terms of “sinterability” of particulate, finer is generally better.

(3) A “green ceramic” specimen is then prepared from the mixture using a known shaping technique. Any of several known shaping techniques may be applied to the CrB.sub.2—Al.sub.2O.sub.3 mixture so as to render the CrB.sub.2—Al.sub.2O.sub.3 mixture a CrB.sub.2—Al.sub.2O.sub.3 green body. Conventional shaping techniques suitable for inventive practice include die pressing, cold isostatic pressing (OP), extrusion, slip casting, and injection molding. Cold isostatic pressing may be a preferred shaping technique for many inventive embodiments, because cold isostatic pressing tends to yield a higher “green density” as compared with other known shaping techniques. Generally speaking, the higher is the density of the green body, the easier is the sintering.

(4) The green specimen is then fired in a non-oxidizing atmosphere to a temperature ranging from about 1600° C. to about 2050° C. The non-oxidizing atmosphere can be a vacuum, or an inert gas (e.g., argon), or a reducing gas. The 2050° C. temperature, which represents the upper limit of the firing temperature range, is near the melting point of aluminum oxide, which is 2054° C. In inventive practice, the specific firing temperature will vary with the particular CrB.sub.2 and Al.sub.2O.sub.3 powders used, their volumetric proportions, and the firing atmosphere. Typically, the pressureless sintered ceramic body will be permitted to remain and cool in the furnace for a period of time after heating has concluded. This may be accomplished, for instance, by simply turning the furnace off, or alternatively, by programming the specific cooling rate.

(5) Machining of ceramic entities is commonplace in the ceramics industry. The skilled artisan reading the instant disclosure will appreciate how machining may be effected for shape refinement of inventive ceramic entities. If necessary or appropriate, “green machining” of the present invention's green ceramic body, and/or “final machining” of the present invention's pressureless sintered ceramic body, may be implemented.

(6) In testing their invention, the present inventors prepared a mixture of 20 volume-% CrB.sub.2 and 80 volume-% Al.sub.2O.sub.3. The mixture was cold isostatic pressed to form a specifically shaped green body. The green body was fired in helium (He) at 1800° C., yielding a fully dense ceramic with flexural strength of 220 MPa.

(7) According to usual inventive practice, the Al.sub.2O.sub.3 constituent represents at least about fifty percent of the overall volume of the inventive CrB.sub.2—Al.sub.2O.sub.3 two-phase composite body, and hence the CrB.sub.2 constituent represents no more than about fifty percent of the overall volume of the inventive CrB.sub.2—Al.sub.2O.sub.3 two-phase composite body. Inventive green body embodiments having higher chromium diboride contents generally require higher firing temperatures, but should result in densified ceramic material that is stronger, tougher and harder, e.g., with better ballistic properties. Chromium diboride content from about twenty percent to about thirty percent is believed by the present inventors to be the optimum range for inventive practice, in order to achieve the best combinations of processing and properties.

(8) Ballistic armor is a notable realm of inventive application. The present invention provides a methodology for cost-effectively producing complexly shaped bodies composed of an advanced ceramic material system that has been demonstrated in testing to be formidable when subjected to great force/energy such as associated with projectile impact. For instance, complexly shaped articles useful for ballistic personal/personnel armor protection, such as helmets and curved body-armor plates, can be propitiously manufactured using the inventive methodology.

(9) Inventive practice is a viable option for diverse applications, including but not limited to ballistic armor applications. The present inventors' novel discovery that CrB.sub.2—Al.sub.2O.sub.3 materials can be densified without being pressured expands the possibilities of armor applications and other applications. The present invention's pressureless sintered CrB.sub.2—Al.sub.2O.sub.3 two-phase ceramic composite materials promise increased availability (e.g., larger-scale production) because of reduced costs and increased article-shaping capability/versatility. The potential of the inventive CrB.sub.2—Al.sub.2O.sub.3 composites exceeds that of many ceramic materials manufactured via pressure-assisted sintering.

(10) The present invention, which is disclosed herein, is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the instant disclosure or from practice of the present invention. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.