Carbonaceous bodies

A unitary carbonaceous body consists of turbostratic carbon formed with a superficial graphitized portion in situ, preferably by passing a high-amperage electric current through this portion.

FIELD OF THE INVENTION 
The present invention relates to carbonaceous bodies and, more 
particularly, to a mechanical carbon material suitable to form bearings, 
dies, molds and electric conductors or brushes and the like parts where 
resistance to mechanical and thermal stresses is important. 
BACKGROUND OF THE INVENTION 
These parts have been made heretofore commonly from carbon which proves to 
have satisfactory mechanical strength in use. Carbon bodies which have 
been employed are, however, found to be defective in lubrication 
properties and hence are desired to be improved. 
OBJECT OF THE INVENTION 
Accordingly, it is the object of the present invention to provide an 
improved carbonaceous body which is not only mechanically strong but is 
excellent also in lubrication and other properties including wear 
resistance and separability from mating surfaces in use. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided a carbonaceous body has 
at least one selected superficial portion which is more graphitized in 
situ than the remainder that is basically nongraphitized carbon, said 
superficial portion serving to form a frictional, bearing and/or die or 
molding surface. Preferably, there are provided a plurality of such 
superficial portions serving to form such surfaces of a base body which is 
substantially of non-graphitized or turbostratic carbon. The more 
graphitized portion or portions should, in accordance with a more specific 
aspect of the invention, have a p value in the range less than 0.6 and 
practically between 0.3 and 0.6 determined with X-ray diffrection analysis 
and defined by the Franklin's expression: d=3.440-0.086(1-p.sup.2) where d 
is the means stratic spacing of turbostratic structure obtained from an 
X-ray diffraction diagram. The terms "p value" and "Franklin's expression" 
are well known in the carbon art and described, for example, in Takei and 
Kawashima: "Atarashii Kogyo Zairyo no Kagaku" A-8 "Tanso to Kokuen Seihin 
(Carbon & Graphite Articles)", Kinbara Shuppan (1967), and in R. E. 
Franklin: Proc. Roy, Soc., 209A, 196 (1951). 
SUMMARY OF THE INVENTION 
The invention will now be described with reference to the accompanying 
drawing in which: 
FIG. 1 is a sectional view in elevation illustrating a thrust bearing 
embodying the invention; 
FIG. 2 is a cross-sectional view as taken along the line II--II and viewed 
in the direction of the arrow in FIG. 1; and 
FIG. 3 is a diagrammatic view illustrating an apparatus for preparing a 
carbonaceous composite body according to the invention.

SPECIFIC DESCRIPTION 
The thrust bearing 1 shown in FIGS. 1 and 2 comprises a carbonaceous body 
prepared in accordance with the invention, the body having a 
non-graphitized base carbon body 1a and a plurality of superficial 
portions 1b which are graphitized advantageously by an electrical heating 
process as will be described in connection with FIG. 3. 
With reference to FIG. 3, a homogeneous carbon body 2 comprises a sintered 
mass of carbon from which the composite body 1 of the invention is 
prepared with an arrangement shown. Thus, the sintered ungraphitized or 
carbon body 2 is held on a base 3 and has a pair of cylindrical graphite 
electrodes 4a, 4b urged thereon by an insulating head 5 under pressure W. 
A power supply for energizing the electrodes 4 is shown comprising, in a 
preferred embodiment, a direct-current branch 6 consisting of a DC source 
7 and an AC-blocking reactor or choke 8 and an alternating-current branch 
9 consisting of an AC source 10 and a DC-blocking capacitor 11, the two 
electrical branches being connected in parallel with each other across the 
electrodes 4a and 4b via a switch 12. 
Initially, the carbonaceous body consists entirely of a non-graphitized 
carbon material 1a which may be made in accordance with usual practice by 
a known powder metallurgy or sintering process from a usual carbon 
precursor derived from coal, petroleum or pitch-coke system containing in 
part natural or man-made graphite powder or soot system with coal tar, 
pitch and/or heatsettable resin added therein as a binder. Such precursor 
mixture may be compacted by means of molding processes such as compression 
or extrusion molding at a room temperature or an elevated temperature from 
50.degree. to 100.degree. C. under pressure of around 1500 Kg/cm.sup.2 
with the compaction being followed by baking at a temperature of about 
100.degree. C. or calcination to form the carbonized body 2 of a desired 
shape and size as shown in FIG. 3. 
Carbon of the turbostratic structure is hard and strong but inherently poor 
in lubricity whereas graphitized carbon excells in lubrication properties 
and thermal and electrical conductivities but is poor in mechanical 
strengths due to its laminated structure. In addition, in order to achieve 
graphitization, a high-temperature heat-treatment or secondary baking 
after the calcination or primary baking of a carbonaceous substance is 
required involving an elevated temperature as high as 2000.degree. to 
3000.degree. C. and a long period of treatment time, this entailing an 
extremely large amount of energy. This, coupled with shortened life of 
dies or molds used, tends to render a product undesirably expensive and 
make a volume productive thereof impractical or uneconomical. 
In accordance with the present invention, problems of conventional 
carbon-made mechanical articles are overcome by initially preparing a 
carbonaceous body consisting substantially entirely of carbon by means of 
low-temperature molding and calcination and subsequently graphitizing 
preferentially preselected superficial portions of the carbon body by 
localized heat-treatment, advantageously by a process involving an 
arrangement as shown in FIG. 3. Thus the unitary or one-piece carbon body 
has the graphite superficial portions formed in situ on the body. 
In the arrangement of FIG. 3, a sintered carbon mass 2 is positioned on the 
base 3 of a press not shown, being sandwiched between the upper surface of 
the base 3 and the lower ends of a a pair of electrodes 4a, 4b attached to 
the electrode holder or head 5 under pressure W exerted by such press. The 
lower ends of the compressing electrodes 4a, 4b are positioned in contact 
with the mass 2 so as to be diametrically symmetrical with respect to the 
center axis thereof. 
In operation, upon closure of the switch 12, a high-amperage electric 
current primarily from the main DC source 7 and also containing a 
high-frequency component, of a frequency from 1 to 10 kHz or more but not 
greater than 10 MHZ, from the AC source 10 is passed through the mass 2 
between the electrodes 4a and 4b so that electrical heating is effected 
among particles constituting the mass 2. Superficial layers of the mass 2 
in the vicinity of the interfaces between the mass 2 and the electrodes 
4a, and 4b are thus selectively graphitized or at least more graphitized 
than the remainder so that the portions 1b have a p value (referred to 
previously) of less than 0.6 or practically between 0.3 and 0.6 while the 
remainder portions 1a remain to have a p value in excess of 0.6 or 0.7. By 
repeating this cycle while altering the contact position of the electrodes 
4a, 4b with the mass 2 successively, a plurality of locally graphitized 
zones 1b are formed in succession on the body 1 as illustrated in FIG. 2. 
Since the carbonaceous body 1 while consisting basically of turbostratic 
carbon is provided with one or more effective superficial zones 
selectively graphitized or enhanced of graphitization, the body 1 is given 
excellent frictional properties possessed by graphite while retaining 
mechanical strength of original carbon and hence permits the satisfactory 
use as bearings, dies, molds, conductors and the like mechanical or 
contact articles. 
Particular locations of graphitization as well as area, depth and the 
degree of graphitization thereof on the base carbon body may be variably 
achieved by the corresponding configuration of the electrodes 4a, 4b, the 
current magnitude (current density) of the output of the power supply, the 
relative proportion of the AC component and DC component of the power 
supply and other operating parameters. 
EXAMPLE 
A carbonaceous precursor contains as its principal component 7 parts 
thermal petroleum black (soot) plus 3 parts pitch coke and as the binder 
10 % by weight high-carbon pitch. A mass of the precurser is compacted 
calcinated by electrical sintering with an electric power of 8 w.hr/g, a 
pressure of 450 Kg/cm.sup.2 and a sintering period of 90 seconds to form a 
sample B having a turbostratic carbon structure. A compacted body 
constituting the sample B is then locally graphitized by an arrangement 
basically shown in FIG. 3 with an electric power of 15 w.hr/g (containing 
7 parts DC component and 3 parts 3 KHz AC component), a pressure 350 
Kg/cm.sup.2 and a treatment period of 100 seconds to form a sample C as 
shown in FIG. 2 of which graphitized zones (1b) have a p value of 0.33. 
Physical properties of sample C are examined and compared with those of 
sample B examined as well as those of sample A which represents a typical 
graphite body, as shown in Table I. The first, calcination step of the 
samples is conveniently conducted with the same arrangement as in the 
local graphitization step. 
TABLE I 
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Sample Sample Sample C 
A B (graphitized zones) 
______________________________________ 
Density (g/cm.sup.3) 
1.84 1.80 1.80 
Bending strength 
450 670 670 
(Kg/cm.sup.2) 
Compression strength 
1000 2100 2100 
(Kg/cm.sup.2) 
Hardness (Hs) 
60 79 79 (50) 
P-value 0.35 0.75 (0.33) 
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Abrasion tests (dry-type) are carried out of sample A, B and C with a 
mating surface of FC25 material (cast iron) moving at a relative rate of 
displacement of 8 meters/seconds and with a contact pressure of 5 
Kg/cm.sup.2. Test results are shown in Table II. 
TABLE II 
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Testing Sample A Sample B Sample C 
temp. (.degree.C.) 
.mu. .DELTA.m 
.mu. .DELTA.m 
.mu. .DELTA.m 
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50 0.17 1.4 0.24 5 0.18 1.0 
150 0.29 130 0.35 8 0.30 15.0 
250 0.13 44 0.51 11 0.17 12.6 
300 0.14 22 0.46 10 0.15 13.5 
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.mu.: friction coefficient 
.DELTA.m: wear (.times. 10.sup.-7 cm.sup.3 /Kg .sup.. m) 
It is seen that a body when purely of graphite exhibits a low friction 
coefficient but suffers extreme wear in a temperature range of 150.degree. 
to 250.degree. C. On the other hand, a non-graphitized turbostratic carbon 
body suffers a relatively low wear but is found, due to a large friction 
coefficient it exhibits, to require a great amount of torque. In contrast 
to these, however, bodies according to the invention are shown to be 
ideally low both in friction coefficient and wear, thus quite satisfactory 
as a mechanical bearing material. 
There is thus provided, in accordance with the invention, an improved 
carbonaceous body which is excellent in mechanical strength, wear 
resistance and lubricity, and hence highly suitable as mechanical surface 
bearing components and which permits a volume production at a reduced 
cost.