Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-05-23
2002-08-13
Marcheschi, Michael (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S096400, C501S098400, C501S097100, C501S097200, C501S097300, C501S098500, C264S299000, C264S319000
Reexamination Certificate
active
06432855
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a new and unique ceramic superabrasive material.
BACKGROUND OF THE INVENTION
Hardness is a fundamental material parameter that measures the resistance of a material to an applied compressive load. It is a function of both the strength of the interatomic bonding and of the rigidity (or compliance) of the lattice framework. Diamond is the hardest known bulk material (approximately 70 GPa “giga pascals”), due to strong covalent sp
3
bonding in a tetrahedral lattice configuration, and its hardness would make it an excellent material for grinding ferrous-based materials such as engine blocks and other automotive components. However, it is expensive and reacts with iron (graphitization) leading to rapid wear when applied against ferrous and high nickel-content alloys. Cubic boron nitride, &bgr;-BN, is a material with the diamond crystal structure in which the carbon atoms of diamond are replaced by boron and nitrogen. The resulting material exhibits much lower chemical reactivity with iron, but also possesses a significantly lower hardness than diamond, approximately 45 GPa. The cubic polymorph of BN can be prepared only by a combination of high temperatures (1800 to 2000° C.) and extremely high pressures (65 kbar=50,000 atmospheres). As a result, the cost of &bgr;-BN can exceed $7,000 per pound ($15,000 per kg). Consequently, &bgr;-BN is prohibitively expensive for all but the most high value, specialized applications. For example, grinding and finishing operations for automotive hardened steel stamping dies are constrained to the use of &bgr;-BN for milling applications due to lack of a suitable cost-effective alternative. The next hardest abrasive bulk materials are silicon carbide and vanadium carbide, each with a hardness within the 24 to 28 GPa range. The lower hardness of these materials renders them inappropriate for the demands of high volume, industrial grinding and finishing operations. Therefore, development of a cost-effective abrasive material with a hardness comparable to that of &bgr;-BN would be a significant benefit to industry for which there is a real need.
A recent study of complex ternary borides revealed a new class of lightweight, ultra-hard ceramics. These alloys, aluminum magnesium boride alloyed with a few atomic percent group IV or group V element (AlMgB
14
:X where X=Si, P, C), were prepared by mechanical alloying, a high energy solid state technique, and consolidated by vacuum hot pressing. The AlMgB
14
intermetallic compound is based on four B
12
icosahedral units positioned within an orthorhombic unit cell containing 64 atoms. The icosahedra are positioned at (0,0,0), (0,1/2,1/2), (1/2,0,0), and (1/2,1/2,1/2) while the Al atoms occupy a four-fold position at (1/4,3/4,1/4) and the Mg atoms occupy a four-fold position at (0.25,0.359,0). The unique electronic, optical, and mechanical properties of this material are due to a complex interaction within each icosahedron (intrahedral bonding) combined with interaction between the icosahedra (intericosahedral bonding). The hexagonal icosahedra are arranged in distorted, close-packed layers. Table I provides a comparison with several hard materials along with their corresponding density, bulk, and shear moduli.
TABLE I
Density, Hardness, Bulk and Shear Moduli of
Selected Hard Materials
Bulk
Shear
Density
Hardness
Modulus
Modulus
(g/cm
3
)
(GPa)
(GPa)
(GPa)
C(diamond)
3.52
70-90
443
535
BN(cubic)
3.48
50-60
400
409
C
3
N
4
(cubic)
†
†
496
332
SiC
3.22
24-28
226
196
Al
2
O
3
3.98
21-22
246
162
TiB
2
4.50
30-33
244
263
WC
15.72
23-30
421
—
AlN
3.26
12
203
128
TiC
4.93
28-29
241
188
AlB
12
2.58
26
Si
3
N
4
3.19
17-21
249
123
AlMgB
14
2.66
35-40
*
*
* unknown, or not well characterized
† presently available in quantities too small to permit measurement of density and hardness
Prior work on these complex orthorhombic borides has mainly involved determination of crystal structure. I. A. Bairamashvili, L. I. Kekelidze, O. A. Golikova, and V. M. Orlov
J. Less Comm. Met
. 67 (1979) 461 initially examined the thermoelectric properties of this and related borides prepared by hot pressing powders produced from crystallization of aluminum melt solutions. They observed that these compounds exhibited high melting points and were relatively brittle. W. Higashi and T. Ito
J. Less Comm. Met
. 92(1983)239 conducted an extensive crystallographic study on the 1:1:14 compound and established the lattice parameters and atom positions but performed no property measurements. More recently, H. Werheit, U. Kuhlmann, G. Krach, I. Higashi, T. Lundstrom, and Y. Yu
J. Alloys and Compounds
202(1993)269 examined the optical and electronic properties of the orthorhombic AlMgB
4
prepared by growing single crystals in alumina crucibles from Al-B solutions containing Li or Mg. In addition to their unique mechanical properties, evidence suggests that these systems exhibit novel electronic properties such as hopping conduction. Crystallographic studies indicate that the metal sites are not fully occupied in the lattice so that the true chemical formula may be closer to Al
0.75
Mg
0.78
B
14
, which is contemplated by the formula here used as AlMgB
14
.
The primary objective of this invention is to provide a new, lightweight, extremely hard ceramic by intentionally modifying the composition of the baseline alloy.
SUMMARY OF THE INVENTION
A ceramic material which is an orthorhombic boride of the general formula: AlMgB
14
:X, with X being a doping agent. The ceramic is a superabrasive, and in most instances provides a hardness of 40 GPa or greater.
DETAILED DESCRIPTION OF THE INVENTION
The ceramic, as hereinbefore explained, is an orthorhombic boride of the general formula: AlMgB
14
:X. In the formula X represents from about 5 atomic or weight percent to about 30 atomic or weight percent of a doping agent which dramatically increases the hardness of the baseline boride ceramic. If a single element addition, it is atomic percent; if a compound, it is weight percent. In particular, it can, and often does increase the hardness by as much as 10-20%, depending upon the particular doping agent used. Preferred doping agents are elements from group III, IV and V of the periodic chart, and borides and nitrides derived from those such as TiB
2
, AlN, and BN. The most dramatic results have been observed when TiB
2
is the doping agent. Preferred amounts are 10% to 20%, based on atomic/weight percents. It is not known why a dramatic increase in hardness occurs, and while applicant does not wish to be bound by any theory, one possibility might be that the Si atoms substitute for Al in the orthorhombic unit cell. If the interstitial sites corresponding to the Al atoms were smaller than the Mg sites, it is conceivable that the slightly smaller Si atoms might give rise to a higher occupancy than the 74.8% reported by Higashi and Ito. Consequently, the overall intericosahedral bonding might be stronger, leading to a more rigid unit cell. Moreover, the additional electron provided by the Si would be available for forming an additional bond with the boron icosahedra.
The invention also relates to a process of preparing this new superabrasive ceramic material. The process can generally be described as mechanical alloying, coupled with hot pressing. In a typical operation, the baseline alloy and consolidation of its powder into the dense compact are prepared in the following manner. Mechanical alloying (MA) was selected as the initial route to alloy formation because of its ability to generate fine, sinterable powders. As the particle size is reduced, diffusion distances become smaller, thereby enhancing sintering. An additional benefit of mechanical alloying is the high surface-to-volume ratio which promotes chemical reactivity and alloy formation. The large number of independent variables complicate identification of an optimum processing route. These variables include milling type (vibratory vs. planetary vs. attritor), vial geometry (convex curvature vs. flat-ended), ball-to-charge weight ratio,
Cook Bruce A.
Harringa Joel L.
Russell Alan M.
Iowa State University Reseach Foundation, Inc,.
Marcheschi Michael
McKee Voorhees & Sease, P.L.C.
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