Method of manufacturing sintered ceramic body

A method of manufacturing a sintered ceramic body which has a conductive layer and which is suitable for bonding with a metal member, has the steps of coating at least part of a surface of an unsintered ceramic powder compact with a coating material which has a melting point or decomposition point higher than a sintering temperature of the compact and which is selected from the group consisting of a metal, a metal salt, a conductive inorganic material and a nonconductive inorganic material which becomes conductive after sintering, and sintering a resultant structure, thus forming the sintered ceramic body having a conductive layer on at least part of the surface thereof.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of manufacturing a sintered 
ceramic body having a conductive layer thereon which is suitable for 
bonding with a metal member, especially a steel member. 
A typical conventional method of bonding a sintered ceramic body to a metal 
member comprises applying an Mo-Mn paste or the like to a surface of the 
sintered ceramic body which is to be bonded, sintering the resultant 
structure in a reducing atmosphere to form a conductive layer which can be 
electroplated on this surface, electroplating nickel on the conductive 
layer and bonding an electroplated nickel layer to the metal member by 
brazing. 
This method is very effective when the ceramic body comprises an oxide 
series sintered body such as alumina, but is not suitable when an 
non-oxide series sintered body such as silicon nitride is used. In 
particular, when a metal member comprises steel, direct bonding through 
diffusion cannot be substantially achieved at low temperature. When the 
steel member is bonded at high temperature, properties of the steel member 
is greatly degraded, resulting in inconvenience. 
Since a non-oxide series sintered ceramic body such as a silicon nitride 
sintered body has good wearing and high-temperature properties, this 
sintered body shows promise for application in automobile parts and gas 
turbine parts. A strong demand has arisen for establishing a technique for 
bonding such a non-oxide series ceramic body with a metal member, 
especially a steel member. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of 
manufacturing a sintered ceramic body suitable for bonding with a metal 
member, especially a steel member. 
In order to achieve the above object of the present invention, there is 
provided a method of manufacturing sintered ceramic body, comprising the 
steps of coating at least part of a surface of an unsintered ceramic 
powder compact with a coating material which has a melting point or 
decomposition point higher than a sintering temperature of said compact, 
and which is selected from the group consisting of a metal, a metal salt, 
a conductive inorganic material and a nonconductive inorganic material 
which becomes conductive after sintering, and sintering a resultant 
structure, thus forming the sintered ceramic body having a conductive 
layer on said at least part of said surface thereof. 
A ceramic material used in the present invention comprises an oxide series 
ceramic material such as alumina, silica, titania or zirconia, or a 
non-oxide series ceramic material such as silicon nitride, silicon carbide 
or SIALON. 
The use of, particularly, non-oxide series ceramic materials is effective 
in that it is possible to utilize the wearing and high temperature 
properties of the ceramic materials. It should also be noted that, when it 
comes to, particularly, silicon ceramics such as silicon nitride and 
SIALON, a conductive layer of a silicide such as Mo.sub.5 Si.sub.3 or 
W.sub.5 Si.sub.3 can be easily formed on the surface of the sintered body. 
Further, in the case of nitride ceramics such as silicon nitride, a 
conductive layer of a nitride such as TiN or ZrN can be easily formed on 
the surface of the sintered body. 
According to the method of the present invention, the material coated on 
the surface of the ceramic powder compact comprises a metal such as Ti, 
Zr, Hf, V, Nb, Ta, Cr, Mo or W which is selected from the metal elements 
belonging to Groups IV, V, VI, VII and VIII of the Periodic Table and 
which has a melting point, decomposition point or sublimation point higher 
than the sintering temperature. The coating material may comprise a metal 
salt such as a nitrate, a nitrite, a sulfate, a sulfite, a borate, a 
carbonate, a silicate, a phosphate, a phosphite, a chloride, a fluoride, a 
chlorate, an ammonium salt, an oxalate, a hydroxide, a hydride, an iodide, 
a bromide, or an alkoxide of a metal selected from the above-mentioned 
metals. These metals or metal salts can be used singly or as a mixture 
containing at least two metals or metal salts. The conductive inorganic 
material is selected from a silicide, a carbide, a boride, a nitride or an 
oxide of the above-mentioned metals and can be exemplified by molybdenum 
silicide (MoSi.sub.2, Mo.sub.5 Si.sub.3), tungsten silicide (WSi.sub.2, 
W.sub.5 Si.sub.3), tungsten carbide (WC), molybdenum carbide (Mo.sub.2 C), 
titanium nitride (TiN), or zirconium nitride (ZrN). The conductive 
inorganic material may comprise carbon. The nonconductive inorganic 
material which becomes conductive after sintering comprises TiO.sub.2, 
ZrO.sub.2 or the like. TiO.sub.2 and ZiO.sub.2 form conductive materials 
TiN and ZrN, respectively, after being sintered in a nitrogen atmosphere. 
It is desirable to select a material having high strength and toughness 
from among these materials. Composite material containing these material 
may be effectively used. 
To be "conductive" in the present invention is to have a specific 
resistance of 1 k.OMEGA.cm or less, preferably 1 .OMEGA.cm or less. 
A reinforcing material (e.g., glass fiber) or a reaction accelerator (e.g., 
calcium oxide or calcium phosphate) may be added in the amount of less 
than 50% by weight to the material coated on the surface of the ceramic 
powder compact. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A method of the present invention will be described in detail hereinafter. 
A material which serves as a conductive layer after sintering is coated on 
the surface of an unsintered ceramic powder compact. The unsintered 
ceramic powder compact signifies a formed body prepared by adding a 
sintering accelerator and an organic binder to ceramic powder and forming 
the mixture, a body obtained by degreasing this formed body, or a body 
obtained by presintering this degreased body. A material suspension is 
coated on the surface of the unsintered ceramic powder compact. 
Alternatively, the powder compact is dipped in this suspension, or a paste 
material is coated on the surface of the powder compact, or a powdered 
material is applied by a step press to the surface of the powder compact. 
In addition, the coating may be deposited by chemical vapor deposition or 
physical vapor deposition on the surface of the powder compact. When a 
metal salt soluble in a given solvent is used as the coating material, a 
solution prepared by dissolving this metal salt in the given solvent is 
applied to the surface of the powder compact, or the powder compact is 
dipped in this solution. The conductive layer preferably contains at least 
50% by volume of conductive material. 
The ceramic powder compact coated with a predetermined material is heated 
in a non-oxidizing atmosphere, thereby obtained a ceramic body having a 
conductive layer thereon. A sintering temperature can be that of a typical 
ceramic material, and falls within the range of 1500.degree. to 
2000.degree. C., more preferably 1700.degree. to 2000.degree. C. when the 
ceramic powder compact contains Si.sub.3 N.sub.4 as a main component.

The present invention will now be described by way of examples. 
EXAMPLE 1 
An organic binder (paraffin) was added to a powder mixture comprising 5% by 
weight of yttrium oxide and 4% by weight of aluminum oxide, 3% by weight 
of aluminum nitride and 1.5% by weight of titanium oxide as sintering 
accelerators. A resultant mixture was formed in molds to obtain a formed 
body. 
This formed body was degreased and presintered at a temperature of about 
350.degree. C., and a paste containing molybdenum as a major constituent 
was coated on the resultant body and was presintered in a nitrogen 
atmosphere at a temperature of 800.degree. C. for two hours. Thereafter, 
the presintered body was sintered in the nitrogen atmosphere at a 
temperature of 1,700.degree. to 1,800.degree. C. to form a conductive 
layer on a silicon nitride series sintered ceramic body. This conductive 
layer comprises mainly Mo and Mo.sub.5 Si.sub.3. 
Nickel was electroplated on the conductive layer of the sintered ceramic 
body and on an iron member (SS41). The sintered ceramic body and the iron 
member were baked in a forming gas at a temperature of about 700.degree. 
C. A silver brazing agent was inserted between the sintered ceramic body 
and the iron member and was brazed at a temperature causing the brazing 
agent to melt, thereby bonding the sintered ceramic body and the iron 
member together. The bonding strength was measured to be not less than 560 
kg/cm.sup.2 by shear. 
EXAMPLE 2 
The silicon nitride series formed body prepared in Example 1 was 
presintered in a nitrogen atmosphere at a temperature of 1,350.degree. C. 
for two hours. The resultant body was dipped in a suspension containing 
molybdenum powder for about 30 seconds and was then removed from the 
suspension. Thereafter, the resultant body was placed in the nitrogen 
atmosphere at a temperature of 1,700.degree. to 1,800.degree. C. to form a 
conductive layer on the surface of the sintered ceramic body. This 
conductive layer comprises mainly Mo and Mo.sub.5 Si.sub.3. 
The sintered ceramic body having the conductive layer was bonded to the 
iron member in the same manner as in Example 1. The bonding strength was 
measured to be 520 kg/cm.sup.2. 
EXAMPLE 3 
The silicon nitride series presintered body obtained in Example 2 was 
dipped in an aqueous solution of phosphomolybdic acid for about 15 seconds 
and was sintered in a nitrogen atmosphere at a temperature of 
1,700.degree. to 1,800.degree. C. to form a conductive layer on the 
sintered ceramic body. 
The resultant sintered ceramic body having the conductive layer thereon was 
bonded to the iron member in the same manner as in Example 1. The bonding 
strength was measured to be 400 kg/cm.sup.2. 
EXAMPLES 4 AND 5 
Paraffin was mixed as an organic binder with silicon nitride series powder 
(containing as sintering accelerators 5% by weight of yttrium oxide and 2% 
by weight of aluminum oxide), and a trichlene solvent was evaporated. The 
resultant powder was sieved by a sieve. The sieved powder was filled into 
molds such that a top surface thereof became flat. 
In one case, tungsten powder was poured on top of the silicon nitride 
series powder. The resultant powder layers was pressed and hot-pressed at 
a temperature of 1,740.degree. C. for one hour to prepare a ceramic body 
having a conductive layer thereon. This conductive layer comprises mainly 
W and W.sub.5 Si.sub.3. 
In an additional case, molybdenum powder was used in place of tungsten 
powder to prepare another ceramic body having a different conductive layer 
(Mo, Mo.sub.5 Si.sub.3) thereon. 
The resultant sintered ceramic bodies having different conductive layers 
were respectively bonded to iron members, in the same manner as in Example 
1. The bonding strengths of these bonded bodies were measured to be not 
less than 620 kg/cm.sup.2 (in the case when tungsten powder was used) and 
not less than 660 kg/cm.sup.2 (in the case when molybdenum powder was 
used). 
EXAMPLES 6 TO 11 
Various sintered ceramic bodies having different conductive layer were 
prepared and bonded to iron members, in the same manner as in Examples 1 
to 5. Table 1 shows conditions for preparating the ceramic bodies. The 
bonding strengths of these bonded bodies were similar to that in Examples 
1 to 5. 
TABLE 1 
______________________________________ 
Ceramic Main component 
Example 
powder Coating Coating 
of conductive 
No. compact material method layer 
______________________________________ 
6 formed powder of pressing 
WC, W.sub.5 Si.sub.3 
body of WC 
Si.sub.3 N.sub.4 
7 formed powder of pressing 
Mo.sub.2 C, Mo.sub.5 Si.sub.3 
body of Mo.sub.2 C 
Si.sub.3 N.sub.4 
8 pre- suspension 
dipping 
TiN 
sintered of TiO.sub.2 
body of 
Si.sub.3 N.sub.4 
9 pre- suspension 
dipping 
ZrN 
sintered of ZrO.sub.2 
body of 
Si.sub.3 N.sub.4 
10 pre- aqueous dipping 
Mo.sub.5 Si.sub.3 
sintered solution 
body of of 
SIALON phospho- 
molybdic 
acid 
11 pre- suspension 
dipping 
TiN 
sintered of TiO.sub.2 
body of 
SIALON 
______________________________________ 
According to the method of the present invention, a conductive film was 
formed on a unsintered, porous ceramic powder compact, and the resultant 
structure was sintered. Therefore, a rigid conductive layer was formed on 
the surface of the resultant sintered ceramic body. Unlike in the 
conventional method, a sintered ceramic body of this type can be bonded 
with high bonding strength to a metal member, especially a steel member.