Cutting tool composed of silicon nitride

A cutting tool composed of silicon nitride comprising chiefly a .beta.-silicon nitride crystal phase and containing a grain boundary phase of rare earth elements, silicon, aluminum, oxygen and nitrogen, wherein the cutting surface is constituted by at least a fired surface or a surface formed by removing and polishing the fired surface by an amount of not larger than 10 .mu.m, and has in the mirror surface thereof a micro-Vicker's hardness which is higher than that of the interior thereof and is not smaller than 16 GPa. The cutting tool has a cutting surface that can be easily worked enabling the cost of working to be decreased, and has a highly hard layer that is suited for cutting cast iron and the like materials featuring increased durability and extended cutting life.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an inexpensive cutting tool composed of a 
sintered product of silicon nitride which is suited for cutting cast iron. 
2. Description of the Prior Art 
Owing to their excellent heat resistance, thermal shock resistance and 
oxidation resistance, sintered products of silicon nitride have until now 
been used as engineering ceramics and, particularly, as components for 
thermal engines such as turbo rotors. Owing to their high toughness and 
abrasion resistance, furthermore, the sintered products of silicon nitride 
have also been used as cutting tools. 
In recent years, it has been attempted to utilize a sintered product having 
a high density and a high mechanical strength as a cutting tool as 
disclosed in Japanese Laid-Open Patent Publications Nos. 32785/1980 and 
73670/1981. 
Such a sintered product of silicon nitride is usually obtained by adding 
thereto, as a sintering assistant, a rare earth oxide such as Y.sub.2 
O.sub.3 or an aluminum compound such as Al.sub.2 O.sub.3 or AlN followed 
by firing in a nonoxidizing atmosphere containing nitrogen. 
When fired at a high temperature, however, the sintered product of silicon 
nitride brings about such a problem that the surface of the sintered 
product is coarsened since silicon nitride is subject to be decomposed. To 
suppress the decomposition of the silicon nitride, the silicon nitride has 
heretofore been fired in a pressurized atmosphere of a nitrogen gas. 
Even though the silicon nitride can be fired in a nitrogen atmosphere, 
however, there still remains a problem in that it is not possible to 
completely suppress the surface from being coarsened. When such a sintered 
product were to be used as a cutting tool, therefore, the fired surface 
has to be worked like a mirror surface by removing and polishing the 
coarsened surface by an amount of not smaller than 100 .mu.m. Besides, the 
cutting tool having a very complex shape requires a very extended period 
of time for polishing the surface resulting in an increase in the working 
cost and an increase in the cost of production. 
The sintered product of silicon nitride generally has a Vicker's hardness 
of about 15 MPa in the mirror surface thereof, and it is desired to 
further increase the hardness and strength so that it can be used as a 
cutting tool. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide a sintered 
product of silicon nitride which exhibits excellent property for cutting 
cast iron and the like materials, making it possible to simplify the 
working at the time of producing a cutting tool and to greatly decrease 
the cost of production. 
The present inventors have first forwarded the study based on such a 
standpoint that, to simplify the working, it is important to improve 
mechanical properties of the fired surface and to minimize the coarsening 
on the fired surface. The inventors have discovered the fact that when a 
rare earth oxide, an aluminum compound and a silicon oxide are added as 
sintering assistants and are sintered in an atmosphere containing a 
nitrogen gas and an SiO gas, the sintering property is improved and, at 
the same time, the surface coarseness decreases. The inventors have 
further discovered the fact that a layer having a hardness higher than 
that of the interior of the sintered product can be formed on the surface 
of the sintered product when the amounts of the rare earth oxide and the 
silicon oxide are controlled to lie within a particular composition range, 
and have thus arrived at the present invention. 
That is, a cutting tool composed of silicon nitride of the present 
invention comprises chiefly a .beta.-silicon nitride crystal phase and 
contains a grain boundary phase of rare earth elements, silicon, aluminum, 
oxygen and nitrogen, wherein the cutting surface is constituted by at 
least a fired surface or a surface formed by removing and polishing the 
fired surface by an amount of not larger than 10 .mu.m, and has in the 
mirror surface thereof a micro-Vicker's hardness which is higher than that 
of the interior thereof and is not smaller than 16 GPa. 
PREFERRED EMBODIMENTS OF THE INVENTION 
According to the present invention, the cutting tool is composed of a 
sintered product of silicon nitride which comprises chiefly a 
.beta.-silicon nitride crystal phase and contains a grain boundary phase 
of rare earth elements, silicon, aluminum, oxygen and nitrogen. According 
to the present invention, the cutting surface of the sintered product is 
constituted by at least a fired surface or a surface formed by removing 
and polishing the fired surface by an amount of not larger than 10 .mu.m. 
That is, the sintered product has a favorable surface without being 
coarsened even on the fired surface. Even when the surface has to be 
polished, the polishing needs be effected by an amount of 10 .mu.m at the 
greatest to obtain a favorable surface. 
According to the present invention, furthermore, the cutting tool has a 
surface layer having a hardness higher than that of the interior thereof. 
The cutting surface formed by the highly hard surface layer has a hardness 
of not smaller than 16 GPa as measured by a micro-Vicker's hardness 
measuring method. The surface layer having a high hardness exhibits high 
performance for cutting cast iron and the like materials. When the 
hardness is lower than 16 GPa, excellent cutting performance is no longer 
exhibited. It is desired that the hardness in the interior of the sintered 
product is not smaller than 15 GPa. 
The sintered product of silicon nitride constituting the cutting tool 
contains, in addition to silicon nitride, rare earth elements, aluminum 
and impurity oxygen in a total amount of from 5 to 25 mol % and, 
particularly, from 8 to 20 mol % reckoned as oxides of rare earth 
elements, as an oxide of aluminum and as SiC.sub.2 of impurity oxygen. 
More concretely speaking, rare earth elements are contained in amounts of 
from 1 to 10 mol % reckoned as oxides thereof, aluminum is contained in an 
amount of from 1 to 10 mol % reckoned as Al.sub.2 O.sub.3, and impurity 
oxygen is contained in an amount of from 3 to 20 mol % reckoned as 
SiO.sub.2. The impurity oxygen stands for the amount of remaining oxygen 
of when the amounts of oxygen bonded as oxides of rare earth elements and 
as oxide of aluminum are subtracted from the whole amount of oxygen in the 
sintered product. According to the present invention, the molar ratio 
SiO.sub.2 /RE.sub.2 O.sub.3 of rare earth elements reckoned as RE.sub.2 
O.sub.3 to remaining impurity oxygen reckoned as SiO.sub.2 is from 2 to 5 
and, particularly, from 2.3 to 4. 
Examples of rare earth elements used in the present invention include Y, 
Er, Yb and Lu. Among them, Y is desired from the standpoint of low cost. 
Described below is a method of producing a cutting tool composed of silicon 
nitride. First, a silicon nitride powder is provided as a starting powder. 
The silicon nitride powder is an .alpha.-Si.sub.3 N.sub.4 powder or, 
preferably, the one containing not less than 80% of .alpha.-Si.sub.3 
N.sub.4. It is desired that the powder has a particle diameter of from 0.4 
to 1.2 .mu.m. There are further provided powders of oxides of rare earth 
elements, a powder of silicon oxide and a powder of an aluminum compound 
such as Al.sub.2 O.sub.3 or AlN, as additives for the silicon nitride 
powder. These components are contained in a total amount of from 5 to 25 
mol % and, particularly, from 8 to 20 mol %. In this case, the silicon 
oxide also includes oxygen that is inevitably contained in the starting 
silicon nitride powder and that is reckoned as SiO.sub.2 in addition to 
the one added as the silicon oxide powder. 
Concretely speaking, these additives are added in amounts as follows: 1 to 
10 mol % of oxides of rare earth elements, 1 to 10 mol % of an aluminum 
compound reckoned as Al.sub.2 O.sub.3, and 3 to 20 mol % of silicon oxide. 
According to the present invention, furthermore, the composition is so 
adjusted that the molar ratio SiO.sub.2 /RE.sub.2 O.sub.3 of oxides of 
rare earth elements (RE.sub.2 O.sub.3) to the silicon oxide (SiO.sub.2) is 
from 2 to 5 and, particularly, from 2.3 to 4. This ratio is limited 
because of the reason that when the ratio is not smaller than 2, the 
sintering proceeds and resistance against oxidation increases, too. When 
the ratio is not smaller than 5, however, the surface layer undergoes 
decomposition, whereby voids are formed and the hardness decreases. 
The mixture having the above-mentioned composition is molded into the shape 
of a predetermined cutting tool and is fired. The molding method may be 
press molding, cast molding, extrusion molding, injection molding or cold 
hydrostatic pressure molding. Here, the molding and the firing may be 
simultaneously carried out like a hot press method, as a matter of course. 
According to the present invention, the firing is executed in an atmosphere 
which contains a nitrogen gas and an SiO gas at a temperature of from 
1700.degree. to 2000.degree. C. and, particularly, from 1750.degree. to 
1850.degree. C. When no SiO gas is contained in the atmosphere, the 
decomposition of the surface layer is not completely suppressed. Besides, 
in an atmosphere of nitrogen gas only, the decomposition of the silicon 
nitride is not completely suppressed. When the silicon nitride is to be 
used as a cutting tool, therefore, the fired surface must be polished by 
an amount of not smaller than 10 .mu.m. When the atmosphere contains the 
SiO gas only without nitrogen, the silicon nitride undergoes the 
decomposition at the above-mentioned firing temperature and the surface is 
coarsened. 
Into a firing furnace are introduced the nitrogen gas, the molded article, 
as well as the SiO.sub.2 powder or a mixture powder of the silicon powder 
and the SiO.sub.2 powder in amounts depending upon the volume of the 
molded article. 
The firing temperature is limited to lie within the above-mentioned range. 
This is because when the firing temperature exceeds 2000.degree. C., the 
decomposition of the surface layer is no longer suppressed. When the 
firing temperature is not higher than 1700.degree. C., on the other hand, 
the firing does not proceed to a sufficient degree, and a densely sintered 
product is not obtained. 
Examples of the firing method include hot press method, normal pressure 
firing method, nitrogen gas pressure firing method, as well as hot 
hydrostatic pressure firing method (HIP) after the firing and the HIP 
firing method while sealing the molded article with a glass. 
In addition to the above-mentioned components, the sintered product of 
silicon nitride of the present invention may further contain dispersed 
therein metals of the Groups 4a, 5a and 6a of periodic table such as Ti, 
Ta, V, W and Mo, as well as carbides, nitrides and silicates thereof or 
SiC as particles or whiskers, in order to improve properties of the 
sintered product. 
In the thus obtained cutting tool, the face and the flank that serve as a 
cutting surface can be used in the form of the fired surface, or may be 
polished by an amount of not larger than 10 .mu.m. The cutting surface can 
be polished by sand blasting or the like method inexpensively. 
According to the cutting tool of the present invention as described above, 
additives and, particularly, the amounts of their addition to the silicon 
nitride are so controlled that the molar ratio SiO.sub.2 /RE.sub.2 O.sub.3 
of the oxides of rare earth elements (RE.sub.2 O.sub.3) to the silicon 
oxide (SiO.sub.2) is from 2 to 5, and the composition is fired in an 
atmosphere containing a nitrogen gas and an SiO gas at a relatively low 
temperature of from 1700.degree. to 2000.degree. C. for a short period of 
time. At the same time, components of the molded article are suppressed 
from being decomposed, and voids are little formed in the fired surface. 
Besides, the sintered product exhibits improved oxidation resistance and 
enhanced cutting property. 
Moreover, since the SiO gas is contained in the atmosphere at the time of 
firing the sintered product, a highly hard layer having a micro-Vicker's 
hardness of not smaller than 16 GPa can be formed on the surface. 
As a result, there is obtained a favorably fired surface, i.e., a highly 
dense surface without voids by polishing the cutting surface by an amount 
which is not larger than 10 .mu.m. It is therefore allowed to decrease the 
cost that was not possible with the working according to the prior art. 
According to the cutting tool of the present invention, furthermore, the 
surface layer has a hardness of not smaller than 16 GPa which is higher 
than that of the interior thereof. Therefore, the cutting tool exhibits 
excellent abrasion resistance, excellent cutting property particularly 
when a east iron is cut, and extended cutting life.