Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating porous product
Reexamination Certificate
2001-10-01
2004-11-30
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating porous product
C264S430000, C264S432000, C264S628000, C264S654000, C264S683000
Reexamination Certificate
active
06824727
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to SiAlON-based ceramics useful as cutting tools for the machining of metallic materials.
BACKGROUND OF THE INVENTION
Cutting tools with high wear resistance and reliability are critical to improving industrial productivity. It has been found that ceramic cutting tools allow considerable increase in the rate of machining or improvements in the dimensional tolerances achieved through reduction in wear of the tool.
Such ceramic cutting tools are made from alumina, alumina-titanium carbide composites, silicon nitride or SiAlON. Of these, the alumina and alumina titanium carbide composites exhibit very good wear performance due to their high hardness but suffer from very poor reliability due to their tendency to chip. The SiAlON and silicon nitride grades are considered more reliable because they show less tendency to chip.
However, existing ceramic cutting tools are inadequate due to their poor combination of hardness and toughness and processability. There remains a need for a ceramic material with a combination of high wear resistance and reliability provided by a combination of high hardness and resistance to chipping.
a) Desired Properties of Ceramic Cutting Tools
It is taught, (see for example K. Jack, “Ceramic Cutting Tool Materials”, Materials & Design Vol. 7, September/October 1986, pg. 267-270, see especially pg. 270. and C. Chatfield, T. Ekstrom & M. Mikus, J. Mat. Sci. 21, pg. 2297-2307 (1986)) that the properties of interest in metal cutting inserts are resistance to mechanical and thermal shock, resistance to plastic deformation at high temperatures, on the order of 1000° C., resistance to abrasive wear, and resistance to chemical or dissolution wear. Resistance to mechanical shock is improved by high toughness, while resistance to abrasive wear is improved by both high toughness and high hardness. Lack of toughness leads to inserts being unreliable because they are susceptible to chipping, while too low a hot hardness can result in failure due to excessive plastic deformation. Low hardness results in poor resistance to abrasive wear as discussed below. Resistance of oxide based cutting tools such as alumina or alumina titanium carbide composites to chemical wear while machining steel is much better than that of silicon nitride or SiAlON.
The wear of a ceramic is taught by S. T. Buljan & V. K. Sarin (“Design and Wear Resistance of Silicon Nitride-Based Composites”, Inst. Phys. Conf. Ser. 75, Chap. 9, pg. 873 (1986)) to be related to the hardness and toughness of the material according to a factor K
ic
3/4
H
1/2
where H is hardness and K
ic
is fracture toughness, with improved wear resulting fro higher hardness and higher toughness.
Ceramic materials made from SiAlONs typically have mechanical and physical properties similar to those of beta silicon nitride, including a low thermal expansion, but chemical properties similar to aluminum oxide (see for example, K. Jack, “Sialons and Related Nitrogen Ceramics”, J. Mat. Sci. 11 (1976) 1135-1158, pg. 1146).
b) SiAlON: Silicon Nitride with Alumina
“SiAlON”s are phases in the silicon-aluminum-oxygen-nitrogen and related systems. SiAlON ceramic materials differ from silicon nitride because aluminum and oxygen are contained in the crystal structure (see K. Jack, “Sialons and Related Nitrogen Ceramics” J. Mat. Sci. 11 (1976) 1135-1158, and F. Riley, “Silicon Nitride and Related Materials”, J. Amer. Ceram. Soc. 83 [2] 245-65, February 2000).
Ceramic items made from SiAlON typically have high elevated temperature strength and hardness and are desirable for industrial applications. In particular SiAlON retains hardness at high temperatures better than alumina (see for example Jack, Materials & Design, Vol 7, No 5, October 86, pg. 271, FIG. 10).
In addition to the aluminum and oxygen incorporated into the structure, compounds such as yttria are commonly added to assist sintering. During sintering these compounds react with silica on the surface of the silicon nitride, silica deliberately added or otherwise present as an impurity. Similarly such compounds react with any alumina present, either deliberately added or present on the surface of aluminum nitride, or otherwise added as an impurity.
These additional elements greatly increase the complexity of the phase relations affecting SiAlON materials and thus increase the difficulty in processing SiAlON materials to achieve the desired properties. It is known, for example, that the phase chemistry of the intergranular phases in SiAlON is more complex than that of the corresponding silicon nitride ceramic systems (see for example F. Riley J. Amer. Ceram. Soc. 83 [2] pg. 259, February 2000). On the other hand, the complexity of the phase relations for these materials enables articles made from SiAlONs to be fabricated with much more economical processes. For example, in the case of silicon nitride, dense bodies can generally only be made by hot pressing or the use of high gas pressure sintering techniques to prevent the decomposition of the silicon nitride phase during densification. The SiAlON material typically may be processed to a high density without the application of high pressures. This process is typically known as pressureless sintering and consists of cold pressing followed by sintering at normal atmospheric pressures of an inert gas. The use of this process enables considerable reduction in the cost of fabricated articles.
The complex phase relations of the SiAlON materials makes it very difficult to accurately or definitively define the nature of the crystal structure in a finished ceramic. Thus it is useful and common to define such ceramic compositions in terms of the raw materials from which they are fabricated (i.e., formulations) in addition to attempting to fully characterize the finished materials.
c) Alpha′ & Beta′ Phases of SiAlON
The two best known crystal phases in the SiAlON family are the alpha′ and beta′ phases, based on corresponding alpha and beta silicon nitride crystal structures. In these SiAlON phases a portion of the silicon and nitrogen atoms are replaced by aluminum and oxygen atoms.
The beta′ SiAlON phase is generally considered to be represented by the formula Si
6−z
Al
z
O
z
N
8−z
, wherein 0<z<4.2. This structure does not incorporate additional metal ions in the crystal lattice.
Microstructurally, beta′ SiAlON mostly appears as elongated high aspect ratio fiber like grains which contribute to high strength and toughness in the ceramic material.
Ceramic articles made from beta′ SiAlON can show high values of toughness but show low hardness, that is their hardness is, for example, on the order of 92 Rockwell (A scale) (see U.S. Pat. No. 4,547,470 to Tanase et al.). As a result of the low hardness such ceramic cutting tools do not show satisfactory wear resistance.
The alpha′ SiAlON phase is generally considered to be represented by the formula M
x
(Si,Al)
12
(O,N)
16
wherein 0<x<2 and M is an element such as Mg, Y, Ce, Sc or other rare earth elements. More precisely, the crystal stoichiometry is represented by M
m/v
S
12−m−n
Al
m+n
OnN
16−n
(see G. Z. Cao and R. Metselaar, “Alpha′-Sialon Ceramics: A Review”, Chem. Mat. Vol 3 No 2, 242-252 (1991)), wherein v is the valence of M. The two formulas are used interchangeably in this specification. This structure accommodates additional M ions that are not accommodated within the beta′ SiAlON structure.
Typically alpha′ SiAlON appears mostly as equiaxed grains in the microstructure of the ceramic and is associated with higher hardness in the material. This equiaxed microstructure does not provide the high toughness associated with the fiber-like beta′ SiAlON microstructure.
Thus, in attempts to provide ceramic SiAlON compositions which are usable in high temperature applications such as cutting tools, various authors and patentees have taught the combination of alpha′ SiAlON with beta′ SiAl
Allan David
Roy Robert Donald
Fiorilla Christopher A.
Greenlee Winner and Sullivan P.C.
Indexable Cutting Tools of Canada Limited
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