Electrically-conductive sintered compact of silicon nitride machinable by electrical discharge machining and process of producing the same

An electrically-conductive sintered compact of silicon nitride which is machinable by electrical discharge machining and a process to produce the same. TiN and/or TiC powder is added to a powder of silicon nitride in an amount 15-40% by volume to act as a conductivity-supplying agent while 0.01-3.0% by volume MgO and/or Al.sub.2 O.sub.3 powder is added as a sintering assistant. The mixed powders are then preformed in a desired shape and sintered in a nonoxidizing environment at 1,600.degree. C.-2,000.degree. C. to obtain a compact of silicon-nitride machinable by electrical discharge machining due to its electrical conductivity being at least 1 S.multidot.cm.sup.-1.

FIELD OF THE INVENTION 
The present invention relates to a sintered compact of silicon nitride 
which has a high electrical conductivity and can be machined by electric 
discharge machining, and to a process for producing the same. 
BACKGROUND AND SUMMARY OF THE INVENTION 
A sintered compact of silicon nitride (hereinafter called a "sintered 
compact of Si.sub.3 N.sub.4 " for ease of reference) is known not only as 
a material having excellent oxidation resistance but also as a material 
having a low coefficient of thermal expansion and intensive strength 
properties at high temperatures. Moreover, research and development 
activities are being conducted to enable sintered compacts of Si.sub.3 
N.sub.4 to be utilized as a high-temperature structural material for 
turbine engine blades and nozzles and for heat exchange members, to name a 
few. 
However, because powder metallurgy is normally employed to produce sintered 
compacts of Si.sub.3 N.sub.4, it is difficult to obtain sintered compacts 
having complicated shape configurations, accurate dimensions and planes. 
Finished shaped products of sintered compacts of Si.sub.3 N.sub.4 are 
therefore typically produced by machining, or grinding after sintering. 
As is commonly known, a sintered compact of Si.sub.3 N.sub.4 is a very hard 
material and thus very difficult to machine. Consequently, development in 
the field of sintered compact applications has been hampered by the 
technical restrictions imposed due to the difficulties of machining 
sintered compacts of Si.sub.3 N.sub.4. Such technical restrictions 
include, for exmple, a large amount of time and labor required even if 
such machining is feasible; only relatively simple shape configurations 
are available with such machining; and particularly, thin parts such as 
turbine blades typically cannot be produced. 
Electric discharge machining is generally known as one of the means for 
machining into finished parts having complicated shape configurations. 
However, sintered compacts of Si.sub.3 N.sub.4 conventionally produced are 
electrically insulative and thus have not been conventionally thought of 
as being suitable for electric discharge machining which requires the 
compact to be electrically conductive. 
According to the present invention, however, there has been obtained a 
conductive sintered compact of Si.sub.3 N.sub.4 which is machinable by 
electric discharge machining. The present invention is realized by the 
addition of powders including a conductivity-supplying agent and a 
sintering assistant to the Si.sub.3 N.sub.4 powder. The resulting Si.sub.3 
N.sub.4 powder is then sintered so that the excellent properties 
associated with conventional sintered compacts of Si.sub.3 N.sub.4 are 
maintained while yet producing a sintered compact of Si.sub.3 N.sub.4 
capable of being machined by electrical discharge machining techniques. 
More specifically, TiN and/or TiC powders are employed as 
conductivity-supplying agents and MgO and/or Al.sub.2 O.sub.3 powders are 
employed as sintering assistants. The TiN and/or TiC in addition to the 
MgO powders are crushed to particles measuring 2 .mu.m or smaller in 
average size before being uniformly dispersed in the Si.sub.3 N.sub.4 
powder and shaped into a compact preform. The resulting preform is then 
subjected to hot isostatic pressing at an elevated temperature within the 
range 1,600.degree. to 2,000.degree. C. in a nonoxidizing atmosphere to 
obtain a sintered compact of Si.sub.3 N.sub.4 which has a conductivity of 
1 S.multidot.cm.sup.-1 or greater and is machinable by electrical 
discharge machining. 
TiN and/or TiC powders are utilized as a conductivity-supplying agent due 
to their high electrical conductivity (i.e., electrical conductivity of 
TiN is 4.times.10.sup.4 S.multidot.cm.sup.-1, while that of TiC is 
3.times.10.sup.4 S.multidot.cm.sup.-1) which is substantially equivalent 
to that of metal, their greater hardness and their stability at high 
temperature. The use of MgO and/or Al.sub.2 O.sub.3 on the other hand are 
used as sintering assistants since addition of a small amount of either is 
not only effective for the sintering of a Si.sub.3 N.sub.4 matrix but also 
contributive to the sintering of TiN and/or TiC. 
The percentages of the conductivity-supplying agent and the sintering 
assistant to be added should preferably be 15 to 40% by volume and 0.01 to 
3% by volume, respectively, and the percentage should be determined in 
consideration of the following points. First, while TiN and TiC as 
conductivity-supplying agents both exhibit excellent stability at high 
temperatures, they are both less stable than Si.sub.3 N.sub.4. Thus, the 
amount of the TiN and/or TiC additive should be minimized to the extent 
that satisfactory properties for electrical discharge machining are 
obtainable. Secondly, the amount of the sintering assistant to be added 
should also be minimized to the extent that a high sintering density can 
be obtained. 
As described later, the addition of a large amount of the sintering 
assistant will be followed by excessive growth of Si.sub.3 N.sub.4 
particles, whereby the conductivity-supplying agent and the electrical 
conductivity (that is, the properties for electrical discharge machining 
and oxidation resistance at high temperature) will be deleteriously 
affected. 
Further aspects and advantages of the present invention will become more 
clear after consideration is given to the detailed description in 
conjunction with the examples.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS 
Referring to the drawings, a detailed description of the exemplary 
embodiments of the present invention will be given below. 
It has been discovered (as shown in FIG. 1) that the electrical 
conductivity of a sintered compact of Si.sub.3 N.sub.4 changes when 
prepared by adding TiN and MgO powders to Si.sub.3 N.sub.4 powder, 
crushing and blending the powders and then sintering them using hot press 
techniques, provided that the percentage of MgO powder added is constant 
(0.5% by volume). 
The properties for electrical discharge machining are closely related to 
the electrical conductivity properties of the resulting sintered compact. 
Electrical conductivity of a sintered compact to permit electrical 
discharge machining thereof should be in the order of 10.sup.-2 
S.multidot.cm.sup.-1. However, such conductivity must be 10.sup.0 
S.multidot.cm.sup.-1 or greater to allow electrical discharge machines 
generally in conventional usage to machine the sintered compact and thus 
even in greater conductivity regions, the properties for electrical 
discharge machining are improved commensurate with conductivity. The 
removal rate of the sintered compact during electric discharge machining 
is seen to increase up to 10.sup.2 S.multidot.cm.sup.-1, whereas the 
surface roughness is also seen to improve even in the conductivity region 
of greater than 10.sup.2 S.multidot.cm.sup.-1. When electrical discharge 
machinability are solely taken into consideration, the larger the amount 
of TiN added, the better. 
On the other hand, the bending strength of the sintered compact and its 
weight gain by oxidation after it is processed at 1,200.degree. C. in 
atmosphere for 100 hours change as shown in FIG. 2. The bending strength 
minimally decreases until the addition of TiN reaches 25% by volume and 
gradually decreases when the addition thereof exceeds 25% by volume, 
whereas the increase in weight gain quickly rises as the amount of the TiN 
additive is increased. Thus, the excellent properties typically associated 
with Si.sub.3 N.sub.4 will be lost if large amounts (i.e., greater than 
25% by volume) of TiN are added. Accordingly, the amount of the 
conductivity-supplying agent to be added should be limited to the extent 
that satisfactory properties for electrical discharge machining are 
available. 
On the other hand, if the amount of MgO as an additive is changed while the 
amount of the additive TiN is kept constant (i.e., 25% by volume), the 
bending strength of the sintered compact will be that as shown in FIG. 3. 
The bending strength and the relative density of the sintered compact 
saturate respectively within the range of 0.2.about.0.5% by volume of MgO 
and at 0.2% by volume thereof--that is, in a region where the amount of 
MgO as additive is extremely small. On the contrary, the electrical 
conductivity and the weight gain by oxidation (at 1,200.degree. C. in 
atmosphere for 100 hours) change as is shown in FIG. 4. The electrical 
conductivity is seen as sharply decreasing at about the point when the 
amount of MgO exceeds 1% by volume and renders electrical discharge 
machining impossible at 5% by volume thereof. Weight gain by oxidation is, 
however, seen to start with a lower range of the percentage of the MgO and 
also rises drastically. 
The reason for the limitation of the amount of the sintering assistant as 
an additive to the extent that higher density and greater bending strength 
are obtainable is believed attributed to a reduction in the properties for 
the resulting sintered compact. The increase in the amount of MgO as an 
additive allow for excessive growth of TiN particles and excessive 
formation of a solid TiN-MgO solution. Based on the observation of a 
lapped face of the sintered compact, it has been found that as the average 
size of the TiN particles increased (as shown in FIG. 5) the particles 
became more spherical in shape with corresponding increase in the amount 
of MgO additive.* 
FNT *4.pi..times.area/(circumference).sup.2 is used as a parameter indicative 
of the similarity between the shape of a particle and a sphere. That is, 
the greater the similarity, the larger the value becomes such that for a 
sphere, the value is 1. 
As shown in FIG. 1, the electrical conductivity of the sintered compact of 
Si.sub.3 N.sub.4 --TiN--MgO system approaches the value based on the 
assumption that all the TiN particles are coupled together as the 
percentage of the additive TiN increases. Accordingly, the TiN particles 
in contact with each other are considered present in a region where the 
amount of the additive TiN is large. The reason for the reduction in the 
electrical conductivity despite the amount of the additive TiN being 
constant is considered attributable to the reduced probability of contact 
of the TiN particles with each other because TiN particles have been 
rendered coarse and more spherical in shape. 
On the other hand, the reason for the growth of the weight gain by 
oxidation as the amount of the additive MgO increases is considered due to 
the fact that the rate of the movement of a substance within a TiN 
particle or the intergranular movement thereof is accelerated because 
oxidation is inwardly facilitated as the size of the TiN increases and 
because the formation of the solid TiN--MgO solution or a solid Ti--N--O 
solution is progressed. 
As set forth above, the amounts of the powders of the 
conductivity-supplying agent and the sintering assistant should preferably 
be limited to the extent that each of them is capable of maintaining 
electrical discharge machining properties and the mechanical properties 
associated with a Si.sub.3 N.sub.4 sintered compact. For that purpose, it 
is therefore necessary to increase the effect of the 
conductivity-supplying agent and the sintering assistant as additives to 
Si.sub.3 N.sub.4 powder by respectively furthering the dispersion of the 
powders and increase the driving force at the time of sintering by 
selecting the average size of each type of powder prior to sintering at 2 
.mu.m or smaller and preferably 0.5 .mu.m or smaller. 
The powder prepared in consideration of the above-described points is 
sintered in a nonoxidizing atmosphere, that is, a gaseous atmosphere 
preferably containing more than one inert gas of N.sub.2, CO, N.sub.3, Ar, 
Ne, and H.sub.2 or a vacuum atmosphere at 
1,600.degree..about.2,000.degree. C. or by the use of hot isostatic 
pressure (HIP) under similar conditions. The sintering temperature is 
preferably 1,600.degree. C. or above because high density and increased 
bending strength are unavailable unless a large amount of sintering 
assistant is added at temperatures lower than 1,600.degree. C. The 
sintering temperature is preferably kept below 2,000.degree. C. since 
resolution and vaporization becomes uncontrollable due to the rise in 
vapor pressures of the other additives (in addition to that of Si.sub.3 
N.sub.4) at temperatures greater than 2,000.degree. C. even though the 
pressure of, e.g., N.sub.2 is also raised. 
Further understanding of this invention will be obtained by reference to 
the following nonlimiting examples. 
The following reference symbols are used in the columns of the properties 
for electrical discharge machining in Tables 1-3 illustrated later. 
______________________________________ 
Symbols 
Electrical conductivity: 
Symbol: 
(S .multidot. CM.sup.-1) 
EDM conditions: 
______________________________________ 
.circleincircle. 
10.sup.2 .ltoreq. R 
EDM is possible under 
normal conditions. 
Stability and removal rate 
are high. 
.circle. 
10.sup.0 .ltoreq. R &lt; 10.sup.2 
EDM is possible under 
normal conditions. 
Stability and removal rate 
are low. 
.DELTA. 
10.sup.-2 .ltoreq. R &lt; 10.sup.0 
EDM is possible but 
requires high voltage. Not 
for practical use because 
of instability. 
X R .ltoreq. 10.sup.-2 
EDM is impossible. 
______________________________________ 
EXAMPLE 1 
Sintered compacts of Si.sub.3 N.sub.4 were prepared by adding the 
conductivity-supplying agents and the sintering assistants shown in Table 
1 below to Si.sub.3 N.sub.4 powder, crushing, mixing, and sintering the 
products of 1,750.degree. C. for one hour in a gaseous atmosphere of 
N.sub.2 at a pressure of 200 kg.multidot.cm using a hot press so as to 
examine the density, bending strength, electrical conductivity, electrical 
discharge machining properties and weight gain by oxidation (at 
1,200.degree. C. under the atmospheric pressure for 100 hours). Table 1 
below shows the results obtained. 
TABLE 1 
__________________________________________________________________________ 
Conductivity 
Sintering 
Relative 
Bending 
Electrical 
Properties 
weight gain by 
supplying agent 
assistant density 
density 
strength 
conductivity 
for oxidation 
(% by vol) 
(% by vo.) 
(%) (kgm-m.sup.2) 
(S .multidot. cm.sup.-1) 
EDM (mgcm.sup.-2) 
__________________________________________________________________________ 
*TiN 25 MgO 0.5 98 97 3 .times. 10.sup.2 
.circleincircle. 
1.8 
*TiN 25 Al.sub.2 O.sub.3 1.0 
98 89 2 .times. 10.sup.2 
.circleincircle. 
1.5 
*TiC 25 MgO 0.5 96 88 3 .times. 10.sup.2 
.circleincircle. 
1.4 
*TiC 25 Al.sub.2 O.sub.3 1.0 
98 88 3 .times. 10.sup.2 
.circleincircle. 
1.1 
**TaN 25 
MgO 0.5 98 95 1 .times. 10.sup.2 
.circleincircle. 
5.8 
**TaC 25 
MgO 0.5 98 94 2 .times. 10.sup.2 
.circleincircle. 
5.4 
__________________________________________________________________________ 
*Present Invention 
**Comparative Example 
As is evident from Table 1, a series of sintered compacts according to the 
present invention using conductivity-supplying agents of TiN or TiC and 
sintering assistants of MgO or Al.sub.2 O.sub.3 as additives were 
machinable while excellent properties of Si.sub.3 N.sub.4 were maintained, 
whereas those using TaN or TaC as additives showed a disadvantageous 
increase in weight gain and a reduction in oxidation resistance. 
EXAMPLE 2 
Sintered composites of Si.sub.3 N.sub.4 were prepared by adding TiN and MgO 
powder in Si.sub.3 N.sub.4 powder at the percentages shown in Table 2, 
crushing, mixing, and sintering the products at 1,750.degree. C. for one 
hour in a gaseous atmosphere of N.sub.2 at a pressure of 200 
kg.multidot.cm.sup.-2 using a hot press so as to examine the density, 
bending strength, electrical conductivity, electrical discharge machining 
properties and weight gain by oxidation (at 1,200.degree. C. for 100 
hours). The results obtained are shown in Table 2. 
EXAMPLE 3 
Sintered composites of Si.sub.3 N.sub.4 were prepared by adding TiC and MgO 
powder to Si.sub.3 N.sub.4 powder at the percentages shown in Table 4, 
crushing, mixing, and sintering the products of 1,800.degree. C. for one 
hour in a gaseous atmosphere of N.sub.2 at a pressure of 200 
kg.multidot.cm.sup.-2 using a hot press to examine the density, bending 
strength, electric conductivity, electrical discharge machining properties 
and weight gain by oxidation (at 1,200.degree. C. for 10 hours). The 
results obtained are shown in Table 3. 
TABLE 2 
__________________________________________________________________________ 
Relative 
Bending 
Electrical 
TiN MgO density 
strength 
conductivity 
Properties 
weight gain 
(% by vol) 
(% by vol) 
(%) (kgmm.sup.-2) 
(S .multidot. cm.sup.-1) 
for EDM 
by oxidation 
__________________________________________________________________________ 
0 0.5 99 101 &lt;10.sup.-12 
X -- 
5 100 98 &lt;10.sup.-12 
X 0.6 
10 0.5 99 97 .sup. 3 .times. 10.sup.-4 
X 0.5 
15 0.01 91 48 5 .times. 10.sup.0 
.circle. 
-- 
0.1 97 75 1 .times. 10.sup.0 
.circle. 
-- 
0.5 98 95 .sup. 1 .times. 10.sup.-2 
.DELTA. 
0.8 
3 99 98 .sup. 2 .times. 10.sup.-4 
X -- 
20 0.1 98 81 2 .times. 10.sup.1 
.circle. 
-- 
0.5 98 97 .sup. 9 .times. 10.sup.-1 
.DELTA. 
1.4 
3 99 100 .sup. 4 .times. 10.sup.-2 
.DELTA. 
-- 
25 0 80 20 2 .times. 10.sup.2 
.circleincircle. 
1.3 
0.01 91 45 2 .times. 10.sup.2 
.circleincircle. 
0.9 
0.03 95 62 3 .times. 10.sup.2 
.circleincircle. 
0.5 
0.1 97 79 3 .times. 10.sup.2 
.circleincircle. 
0.6 
0.2 98 85 2 .times. 10.sup.2 
.circleincircle. 
1.1 
0.5 98 97 3 .times. 10.sup.2 
.circleincircle. 
1.8 
1 99 95 6 .times. 10.sup.1 
.circle. 
2.5 
3 99 94 5 .times. 10.sup.0 
.circle. 
3.5 
5 99 96 .sup. 4 .times. 10.sup.-2 
.DELTA. 
5.2 
30 0.1 96 72 8 .times. 10.sup.2 
.circleincircle. 
-- 
0.5 98 90 5 .times. 10.sup.2 
.circleincircle. 
3.3 
3 99 92 1 .times. 10.sup.2 
.circleincircle. 
-- 
35 0.1 96 70 3 .times. 10.sup.3 
.circleincircle. 
-- 
0.5 98 84 2 .times. 10.sup.3 
.circleincircle. 
5.0 
3 99 85 6 .times. 10.sup.2 
.circleincircle. 
-- 
40 0.5 97 79 4 .times. 10.sup.3 
.circleincircle. 
7.3 
50 0.5 97 72 6 .times. 10.sup.3 
.circleincircle. 
-- 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
Relative 
Bending 
Electrical Increase 
TiN MgO density 
strength 
conductivity 
Properties 
in Oxidation 
(% by vol) 
(% by vol) 
(%) (kgmm.sup.-2) 
(S .multidot. cm.sup.-1) 
for EDM 
amount (mgcm.sup.-2) 
__________________________________________________________________________ 
0 0.5 99 95 &lt;10.sup.-12 
X -- 
5 100 96 &lt;10.sup.-12 
X 0.7 
10 0.5 97 98 .sup. 2 .times. 10.sup.-3 
X -- 
15 0.01 88 41 8 .times. 10.sup.0 
.circle. 
-- 
0.1 96 75 4 .times. 10.sup.0 
.circle. 
-- 
0.5 97 92 2 .times. 10.sup.0 
.circle. 
1.0 
3 99 95 .sup. 2 .times. 10.sup.-3 
X -- 
20 0.1 96 71 5 .times. 10.sup.1 
.circle. 
-- 
0.5 97 91 5 .times. 10.sup.1 
.circle. 
1.0 
3 99 101 .sup. 8 .times. 10.sup.-2 
.DELTA. 
-- 
25 0 75 15 1 .times. 10.sup.2 
.circleincircle. 
-- 
0.01 85 37 1 .times. 10.sup.2 
.circleincircle. 
-- 
0.03 92 51 3 .times. 10.sup.2 
.circleincircle. 
-- 
0.1 95 65 4 .times. 10.sup.2 
.circleincircle. 
-- 
0.2 96 77 3 .times. 10.sup.2 
.circleincircle. 
0.9 
0.5 96 88 3 .times. 10.sup.2 
.circleincircle. 
1.4 
1 98 97 2 .times. 10.sup.2 
.circleincircle. 
-- 
3 99 92 6 .times. 10.sup.1 
.circle. 
3.0 
5 99 94 2 .times. 10.sup.2 
.circle. 
4.8 
30 0.1 95 61 3 .times. 10.sup.3 
.circleincircle. 
-- 
0.5 96 85 1 .times. 10.sup.3 
.circleincircle. 
2.1 
3 98 91 5 .times. 10.sup.2 
.circleincircle. 
-- 
35 0.1 93 55 6 .times. 10.sup.3 
.circleincircle. 
-- 
0.5 95 81 4 .times. 10.sup.3 
.circleincircle. 
4.0 
3 97 88 1 .times. 10.sup.3 
.circleincircle. 
-- 
40 0.5 96 81 6 .times. 10.sup.3 
.circleincircle. 
-- 
50 0.5 93 70 8 .times. 10.sup.3 
.circleincircle. 
-- 
__________________________________________________________________________ 
As set forth above, TiN and/or TiC powder as a conductivity-supplying agent 
and MgO and/or Al.sub.2 O.sub.3 powder as a sintering assistant were added 
to Si.sub.3 N.sub.4 powder with the powders being crushed into particles 
measuring 1 .mu.m in average size before being blended together. The 
resulting mixture was sintered in a nonoxidizing atmosphere under pressure 
to obtain a sintered compact of Si.sub.3 N.sub.4 having sufficient 
electrical conductivity and thus machinability by electrical discharge 
machining while yet maintaining excellent properties inherent in a 
conventional electrically-insulative sintered composite of Si.sub.3 
N.sub.4. 
Accordingly, while the present invention has been herein claimed in what is 
presently conceived to be the most preferred and exemplary embodiments 
thereof, those in this art may recognize that many modifications may be 
made which shall be accorded the broadest scope of the appended claims so 
as to encompass all equivalents thereof.