Sintered silicon carbide-titanium diboride mixtures and articles thereof

Sintered articles made from binary compositions of silicon carbide and titanium diboride are described. The articles may be prepared by initially mixing finely-divided silicon carbide, carbon or a carbon source material, a densification or sintering aid, and finely-divided titanium diboride, forming the mixture into a desired shape and subsequently heating at temperatures sufficiently high to form a sintered ceramic article of silicon carbide and titanium diboride. When the present sintered ceramic articles contain high amounts of titanium diboride, generally between about 65 and about 95 percent, and more preferably, between about 80 and about 95 percent, by weight, they are quite electrically conductive, generally having less than 0.2 ohm-cm electrical resistivity, and are useful as electrical ignitors. Such articles are also extremely resistant to corrosion by molten aluminum and aluminum alloys; thus, they are aptly suited to use as electrodes in aluminum refining processes. When the present sintered ceramic articles contain high amounts of silicon carbide, generally between about 50 and about 95 percent by weight, they are characterized by high densities and high strengths, typically having MOR above 50,000 psi, and an extraordinary resistance to thermal shock. Such articles are particularly useful in the fabrication of diesel engine precombustion chambers and as honeycomb structures, such as those utilized in automobile emission control units.

TECHNICAL FIELD 
This invention relates to sintered silicon carbide-titanium diboride 
mixtures and articles thereof, which can particularly be used for 
electrodes in aluminum refining or precombustion chambers and honeycomb 
structures, such as those utilized in automobile emission control units. 
BACKGROUND ART 
Silicon carbide, a crystalline compound of silicon and carbon, has long 
been known for its hardness, its strength and its excellent resistance to 
oxidation and corrosion. Silicon carbide has a low coefficient of 
expansion, good heat transfer properties and maintains high strength at 
elevated temperatures. In recent years, the art of producing high density 
silicon carbide bodies from silicon carbide powders has been developed. 
Methods include reaction bonding, chemical vapor deposition, hot pressing 
and pressureless sintering (initially forming the article and subsequently 
sintering under substantially pressureless conditions). Examples of these 
methods are described in U.S. Pat. Nos. 3,852,099; 3,853,566; 3,954,483; 
3,960,577; 4,080,415; 4,124,667; and 4,179,299. The high density, sintered 
silicon carbide bodies produced are excellent engineering materials and 
find utility in the fabrication of components for turbines, heat exchange 
units, pumps and other equipment or tools that are exposed to severe wear 
and/or operation under high temperature conditions. The present invention 
relates methods of producing compositions of sintered silicon carbide and 
titanium diboride and to articles made of such binary ceramic material. 
DISCLOSURE OF INVENTION 
The present sintered articles are made from binary compositions of silicon 
carbide and titanium diboride and are prepared by initially mixing 
finely-divided silicon carbide, carbon or a carbon source material, a 
densification or sintering aid, and finely-divided titanium diboride, 
forming the mixture into a desired shape and subsequently heating at 
temperatures sufficiently high to form a sintered ceramic article of 
silicon carbide and titanium diboride. 
The silicon carbide component may suitably be selected from alpha or beta 
phase silicon carbide. Mixtures of alpha and beta phase material may be 
utilized. The silicon carbide starting material of the present invention 
does not require separation or purification of phases to obtain a 
sinterable material. Minor amounts of amorphous silicon carbide may be 
included without deleterious effect. The silicon carbide component is 
utilized in finely-divided form. A suitable finely-divided material may be 
produced by grinding, ball milling or jet milling larger particles of 
silicon carbide and subsequently classifying or separating a component 
suited to use in the present invention. Preferably, the silicon carbide 
starting material has a maximum particle size of about 5 microns and an 
average particle size of about 0.10 to about 2.50 microns. It is difficult 
to obtain accurate particle size distribution for silicon carbide powders 
having a size less than about 1 micron in size, and, therefore, surface 
area may be considered relevant in determining suitable material. 
Accordingly, the preferred silicon carbide particles for use in the 
present powders have a surface area of from about 1 to about 100 m.sup.2 
/g. Within this range, it is more preferred that the surface area of 
particles range between about 2 and about 50 m.sup.2 /g, and, within that 
range, a range from about 2 to about 20 m.sup.2 /g has been found 
eminently useful. 
The carbon or carbon source material is one that will supply from about 0.5 
to about 6.0 percent by weight of the silicon carbide component in excess 
or combinable carbon to the mixture to be sintered. The carbon component 
facilitates the subsequent sintering operation and aids in reducing the 
amounts of oxides that might otherwise remain in the finished sintered 
product. In a more preferred range, the combinable carbon is present in 
amounts between about 2.0 and about 5.0 percent by weight of silicon 
carbide. Liquid organic materials or solutions or suspensions of organic 
materials may be used as the carbon source. Particularly useful are 
furfuryl alcohol and resin materials that also perform as a temporary 
binder during the initial cold pressing operation which leave a carbon 
residue in the formed body upon heating. A resin material particularly 
adopted to use in the present invention is a liquid thermosetting 
phenolformaldehyde resin typified by that sold by Varcum Chemical Division 
of Reichhold Chemicals, Inc. under the designation of Varcum B-178. 
Generally, such carbonizable organic materials provide from about 30 to 
about 50 percent of their original weight in combinable carbon. If 
desired, both a carbon source, such as petroleum coke, finely divided 
graphite or carbon black, and a carbonizable binder may be included in the 
mixture. Most preferred and useful in the present invention are 
thermosetting resin materials which function as both binding materials and 
as a carbon source. 
The densification, or sintering aids useful in the present invention are 
those found in the prior art, for example, those described in U.S. Pat. 
Nos. 4,080,415; 4,124,667; and 4,179,299. Boron or boron-containing 
compounds are preferred densification aids. Examples of useful 
boron-containing aids are boron carbide, boron nitride, boron oxide, 
aluminum diboride, metallic boron and silicon hexaboride. Densification 
aids are generally effective in the range of from about 0.2 to about 3.0 
percent by weight, for example, the weight of boron as compared to the 
weight of the silicon carbide component. A particularly useful 
densification aid in the present composition is B.sub.4 C. The sintering 
aid may be added, in part or in whole, by carrying out the sintering 
process in an atmosphere of a sintering aid, for example, boron. 
The suitable titanium diboride starting component of submicron size and 
satisfactory purity may be obtained by treating commercially available 
titanium diboride or may be produced by reacting high purity titanium 
dioxide, B.sub.2 O.sub.3 and carbon at elevated temperatures and milling 
the titanium diboride product to obtain a finely-divided product. The 
titanium diboride component is preferably utilized in a particle size 
range similar to that size range described above for the silicon carbide 
component. 
The silicon carbide and the titanium diboride components may contain minor 
amounts of impurities, such as iron, calcium, magnesium and aluminum, 
without deleterious effect on the product. 
The components are thoroughly mixed to obtain an intimate mixture and 
formed, suitably by being cold molded or pressed, at pressures between 
about 6,000 and 20,000 psi, and more preferably, between about 12,000 and 
18,000 psi, to obtain a green body. The green body is subsequently 
furnaced at temperatures between about 1900.degree. C. and 2200.degree. 
C., under substantially pressureless conditions, to sinter the silicon 
carbide component and obtain a sintered composite ceramic article 
comprised of silicon carbide and titanium diboride. The present composite 
sintered ceramic articles typically have densities ranging between about 
85 and about 98 percent of the theoretical density of silicon 
carbide/titanium diboride composites (based on 3.21 g/cc and 4.50 g/cc for 
SiC and TiB.sub.2, respectively). 
In addition to being hard and dense, the composite ceramic articles of the 
present invention possess many other desirable characteristics, being 
tough, wear-resistant, abrasion-resistant and resistant to most acids and 
alkalis. The thermal shock resistance of the articles increases with 
increasing titanium diboride content, articles of high titanium content 
having particularly outstanding thermal shock resistance. 
The present composite ceramic articles containing high amounts of titanium 
diboride, generally between about 65 and about 95 percent, and, more 
preferably, between about 80 and about 95 percent, by weight, are quite 
electrically conductive, generally having less than 0.2 ohm-cm 
resistivity, and are useful as electrical ignitors. Such articles are also 
extremely resistant to corrosion by molten aluminum and aluminum alloys; 
thus, they are aptly suited to use as current conducting elements used in 
contact with molten aluminum and alloys thereof, as electrodes in aluminum 
refining processes. In addition, such articles are also useful as parts of 
pumps used for pumping molten aluminum or alloys thereof, for example, 
pistons, cylinders and impellers. 
The present sintered ceramic articles containing high amounts of silicon 
carbide, generally between about 50 and about 95 percent, and more 
preferably, between about 80 and about 95 percent, by weight, are 
characterized by high densities and high strengths, typically having a 
modulus of rupture (MOR) of about 50,000 psi. Such articles, surprisingly, 
have extraordinary resistance to thermal shock, and are particularly 
useful in the fabrication of diesel engine precombustion chambers, or 
honeycomb structures, such as those utilized in automobile emission 
control units, which require the combination of high strength and high 
resistance to thermal shock. Such honeycomb structures typically have 
various cell configurations with cell widths varying between about 0.075 
and about 5.0 cm, wall thicknesses between about 0.0025 and about 0.25 cm, 
and lengths between about 2.5 and about 60 cm. Generally, such structures 
are formed by extrusion. The present compositions are aptly suited to such 
forming processes and yield a honeycomb product having high mechanical 
strength and excellent thermal shock resistance.

BEST MODE FOR CARRYING OUT INVENTION 
The invention will now be described in greater detail partly with reference 
to the following examples, which are intended to illustrate, and not to 
limit the scope of the invention. In the following examples, all parts are 
parts by weight and all temperatures are in degrees Centigrade. 
EXAMPLE I 
95 parts of submicron silicon carbide, having an average particle size of 
about 0.45 microns, was mixed with 5 parts of finely-divided titanium 
diboride, 0.5 parts of boron carbide, having a size less than 35 microns, 
and 4.0 parts of Varcum B-178 liquid thermosetting phenol-formaldehyde 
resin. The mixture was ball-milled with acetone using tungsten carbide 
balls for two hours in a plastic jar. The mixture was then allowed to dry 
at room temperature in air and was subsequently screened through an 80 
micron silk screen. 
The mixture was then cold pressed into a round disc, 3.8 cm in diameter and 
0.6 cm high, using a metal mold at a pressure of 15,000 psi. The disc was 
removed from the mold and sintered under substantially pressureless 
conditions in an argon atmosphere at a temperature of 2150.degree. C. for 
a period of one hour. 
The product, sintered silicon carbide and titanium diboride, was found to 
have a bulk density of 3.157 g/cc and a relative density of 97.0 percent. 
This relative density is computed by the formulas: 
##EQU1## 
where W.sub.1 =weight fraction of SiC; 
d.sub.1 =theoretical density of SiC (3.21 g/cc); 
W.sub.2 =weight fraction of TiB.sub.2 (=1-W.sub.1); and 
d.sub.2 =theoretical density of TiB.sub.2 (4.50 g/cc); 
##EQU2## 
For this Example I, 
##EQU3## 
3.157.div.3.256.times.100%=97.0%. The product, as determined by 
microscopy, was found to have a porosity of 0.3 percent. The electrical 
resistivity, as determined using a four probe method at room temperature, 
was found to be 294.1 ohm-cm. The average grain size of the titanium 
diboride component was found to be 6.2 microns and of the silicon carbide 
9.0 microns. The modulus of rupture (MOR), using a four point method at 
room temperature, was found to be 46,000 pounds per square inch (expressed 
for convenience as 46.0 kpsi). 
Examples II through XI were carried out in a similar manner, varying the 
proportions of the silicon carbide and titanium diboride components. The 
results are set out in Table A below, in which calculations were made in 
the same manner as those described above for Example I. 
EXAMPLE XII 
A mixture containing 80 parts silicon carbide and 20 parts titanium 
diboride was compounded as in Example I. The mixture was injection molded 
into the form of a precombustion chamber for a diesel engine and sintered 
at 2150.degree. for one hour. The sintered product was then heated 
uniformly using a gas burner to a temperature of about 900.degree. and 
quenched in cold water. After quenching, visual inspection revealed no 
cracking or chipping. Similar tests were conducted using chambers 
fabricated solely of sintered silicon carbide. The sintered silicon 
carbide chambers developed large cracks and a plurality of chips. 
EXAMPLE XIII 
A mixture containing 80 parts of silicon carbide and 20 parts titanium 
diboride is compounded as in Example I and extruded to form a green body 
in the form of a honeycomb. The honeycomb body has square cells about 0.5 
cm in width; the cell walls are about 0.025 cm thick, and the cell is 
about 15 cm in length. Such structures are eminently useful in the 
fabrication of automobile emission control units. The honeycomb green body 
is initially freeze dried at a temperature of less than 10.degree. and 
subsequently vacuum dried under a vacuum (absolute pressure 10.sup.-1 to 
10.sup.-3 mmHg) for about six hours to prevent cracking or distortion of 
the body during the drying step. The green body is then sintered under 
substantially pressureless conditions at 2100.degree. for one hour in an 
argon atmosphere. The sintered product will be found to have a density of 
about 97% of theoretical, a modulus of rupture of more than 50,000 psi, 
and, when subjected to the quench test as described in Example XII, shows 
excellent thermal shock resistance. 
As will be appreciated from the foregoing, the present new ceramic 
articles, comprising small particles of titanium diboride in a matrix of 
sintered silicon carbide, have a variety of desirable characteristics, 
depending on the amount of silicon carbide in the initial composition. 
TABLE A 
__________________________________________________________________________ 
Composition 
Bulk Theoretical 
Relative Electrical 
Grain Size 
Example 
(wt%) Density 
Density 
Density 
Observed 
Resistivity 
(Microns) MOR 
No. SiC/TiB.sub.2 
(g/cc) 
(g/cc) 
(%) Porosity 
(ohm-cm) 
TiB.sub.2 
SiC (Kpsi) 
__________________________________________________________________________ 
I 95/5 3.157 
3.256 97.0 0.3 294.1 6.2 9.0 46.0 
II 90/10 3.185 
3.304 96.4 2.1 23.9 5.6 9.0 48.7 
III 80/20 3.270 
3.405 96.0 1.0 2.0 8.6 9.0 52.7 
IV 70/30 3.345 
3.512 95.2 1.3 0.14 9.0 9.6 42.1 
V 60/40 3.421 
3.625 94.4 1.2 0.30 9.2 9.6 38.3 
VI 50/50 3.473 
3.747 92.7 4.7 0.20 12.0 12.0 35.2 
VII 40/60 3.541 
3.876 91.4 7.6 0.04 9.9 12.0 26.8 
VIII 30/70 3.622 
4.015 90.2 18.1 0.01 10.2 16.2 23.8 
IX 20/80 3.693 
4.165 88.7 19.9 0.172 15.2 21.2 17.1 
X 10/90 3.825 
4.326 88.4 43.7 0.003 30.0 37.0 14.8 
XI 5/95 3.877 
4.411 87.9 40.9 0.001 32.3 42.5 15.2 
__________________________________________________________________________ 
Articles made from compositions containing lower amounts, less than about 
30 percent, and, more particularly, less than about 20 percent, by weight, 
silicon carbide exhibit excellent thermal shock resistance. Such materials 
are good electrical conductors, making them useful as home heater-range 
ignitors. They are also highly resistant to molten aluminum, aluminum 
alloys and molten silicates, making them useful as industrial electrodes 
in smelting processes. These compositions also exhibit desirable 
properties as ceramic armor materials. In contrast, the present ceramic 
articles may be produced using compositions containing higher amounts, 
greater than about 60 percent, and, more particularly, greater than about 
80 percent, by weight, silicon carbide. Such ceramic materials are hard, 
dense materials having an extraordinary resistance to thermal shock and 
are useful as abrasives, in fabrication of tools and other wear-resistant 
articles, and particularly in processes or operations in which the article 
undergoes rapid and extreme temperature changes. 
While the invention has been described herein with reference to certain 
examples and preferred embodiments, it is to be understood that various 
changes and modifications may be made by those skilled in the art without 
departing from the concept of the invention, the scope of which is to be 
determined by reference to the following claims.