Coated silicon nitride based cutting tools

Details Coated silicon nitride based cutting tools Coated silicon nitride based cutting tools

2000-03-28

2001-12-04

Hail, III, Joseph J. (Department: 3723)

Abrading

Rigid tool

C451S541000, C451S548000, C451S556000

Reexamination Certificate

active

06325710

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to coated silicon nitride based cutting tools and a method of making the same. By a special heat treatment prior to the deposition of the coating, an improved bonding strength of the coating is obtained.
Silicon nitride is a highly covalent compound with a number of interesting engineering properties. An adverse effect of the strong bonding is a low self diffusivity which is why the material cannot be consolidated by solid state sintering. Sintering additives such as Al
2
O
3
, Y
2
O
3
and MgO are used to form a liquid with the SiO
2
which is always present on the surface of the Si
3
N
4
grains. The resulting material has a two-phase microstructure of silicon nitride grains embedded in an intergranular bonding phase, which is normally a glass. During sintering, the silicon nitride grains obtain an elongated shape which has a positive effect on the fracture toughness of the material.
Uncoated silicon nitride cutting tools are used in cutting or milling applications of the cast irons. Attempts have been made to improve the tool life of the silicon nitride insert by application of wear resistant coatings. Coated silicon nitride cutting tool inserts are known, e.g., by the Sarin et al. U.S. Pat. Nos. 4,440,547, 4,431,431, etc.
However, the coating adhesion is not always satisfactory.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a method of improving the adhesion of a coating on a silicon nitride based insert.
In one aspect of the invention there is provided a silicon nitride based cutting tool comprising a silicon nitride containing substrate including a glass phase, a single or multilayer outer coating and a continuous or semicontinuous transition zone of 2-10 &mgr;m thickness between the substrate and the inner layer of the outer coating in which transition zone the glass phase is replaced by the material of the inner layer.
In another aspect of the invention there is provided a method of making a coated silicon nitride based cutting tool comprising a substrate and a single or multilayer outer coating comprising treating the substrate in a carburizing atmosphere at 850-1375° C. and thereafter depositing the single or multilayer coating.


REFERENCES:
patent: 4406670 (1983-09-01), Sarin et al.
patent: 4431431 (1984-02-01), Sarin et al.
patent: 4440547 (1984-04-01), Sarin et al.
patent: 4670024 (1987-06-01), Bhat et al.
patent: 5021372 (1991-06-01), Pyzik et al.
patent: 5915162 (1999-06-01), Uchino et al.
patent: 61160494 (1986-05-01), None
patent: 63040785 (1988-02-01), None

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