CA1336102C - Silicon nitride based article with improved fracture toughness and strength - Google Patents
Silicon nitride based article with improved fracture toughness and strengthInfo
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- CA1336102C CA1336102C CA 591219 CA591219A CA1336102C CA 1336102 C CA1336102 C CA 1336102C CA 591219 CA591219 CA 591219 CA 591219 A CA591219 A CA 591219A CA 1336102 C CA1336102 C CA 1336102C
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Abstract
A densified silicon nitride-based ceramic body of improved fracture toughness and improved strength. The ceramic body comprises a first phase of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, and an intergranular, silica-based second phase of silica and one or more suitable oxide densification aids. In the first phase, if the aspect ratio of the beta-silicon nitride grains is less than about 1.5, the equivalent diameter of the grains is at least about 0.4 microns.
Similarly, if the equivalent diameter of the grains is less than about 0.4 microns, the aspect ratio is at least about 1.5. The starting formulation used to form the ceramic body comprises silicon nitride and about 0.5-12%
by weight of the densification aids. The ceramic body has a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa.
A composite body is described in which 10-50% by volume of the silicon nitride and densification aids is substituted by refractory whiskers or fibers having an aspect ratio of 3-150 and an equivalent diameter greater than that of the beta-silicon nitride grains. The whiskers or fibers are uniformly distributed in the ceramic body.
Processes for producing and using the ceramic body are also described.
Similarly, if the equivalent diameter of the grains is less than about 0.4 microns, the aspect ratio is at least about 1.5. The starting formulation used to form the ceramic body comprises silicon nitride and about 0.5-12%
by weight of the densification aids. The ceramic body has a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa.
A composite body is described in which 10-50% by volume of the silicon nitride and densification aids is substituted by refractory whiskers or fibers having an aspect ratio of 3-150 and an equivalent diameter greater than that of the beta-silicon nitride grains. The whiskers or fibers are uniformly distributed in the ceramic body.
Processes for producing and using the ceramic body are also described.
Description
87~3-343 CN 1 336 1 02 SILICON NITRIDE BASED ARTICLE WITH IMPROVED FRACTURE
TOUGHNESS AND STRENGTH
This application contains subject matter related to matter disclosed and claimed in commonly assigned co-pending Canadian Patent Application No. 591,218-1, filed concurrently herewith.
This invention relates to fracture and abrasion resistant materials and to articles of manufacture made therefrom. More particularly, it is concerned with silicon nitride-based ceramic bodies exhibiting both improved fracture toughness and improved strength.
The need for improved materials for cutting tool applications, with enhanced toughness and strength at room and elevated temperatures, and with chemical inert-ness, has generated a widespread interest in ceramic materials as candidates to fulfill these requirements.
Conventional ceramic cutting tool materials have failed to find wide application, primarily due to their low fracture toughness. Therefore, many materials have been evaluated to improve ceramic performance, such as silicon nitride-based monolithic and composite materials for cutting tool applications.
Silicon nitride-based ceramics are also under active development as structural components for advanced turbine engines. A critical factor which limits the widespread application of these materials in such heat engines is their tendency to catastrophic failure. Accordingly, improvement in strength and fracture toughness of these materials would improve their performance in the demand-ing heat engine environment, contributing to a more rapid development of these high performance turbine engines.
Many improvements have been made in the toughness, abrasion resistance, high temperature strength and A~
chemical inertness of silicon nitride materials, but the stringent conditions encountered by heat engine compo-nents and cutting tools demand even further improvement in material characteristics. In many applications, for example in gray cast iron and high nickel alloy machin-ing, silicon nitride tool wear has been found to be dominated by abrasion. Even at cutting speeds as high as 5000 sfm, chemical reactions between tool and workpiece are negligible in comparison. It has been found that abrasion resistance for silicon nitride ceramic cutting tool materials is directly proportional to KIC / H / , where KIC is the fracture toughness and H is the hard-ness. It may be seen, therefore, that further improve-ment in the fracture toughness of silicon nitride ceramic materials could bring about significant increases in both reliability and abrasive wear resistance, providing materials for cutting tools with new and improved charac-teristics.
Attempt has been made to increase the fracture toughness of silicon nitride materials through the development of a composite, in which dispersed partic-ulate, fiber, or whisker materials are included in a silicon nitride-based matrix. The added complexity of composites, however, can result in improvements in fracture toughness at the expense of strength. The present invention provides new and improved monolithic and composite silicon nitride-based ceramic materials exhibiting both improved fracture toughness and improved strength.
The wear-resistant silicon nitride-based bodies of the invention are also useful in other wear part and structural applications, for example in dies, nozzles, etc.
According to one aspect of the invention, there is provided a densified silicon nitride-based ceramic body 87-3-343 CN -3- 1 33~ 1 ~2 of improved fracture toughness and improved strength comprising: a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase con-sisting essentially of silica and one or more suitableoxide densification aids; wherein the ceramic body is formed from a starting formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
and the ceramic body has a fracture toughness of at least 5.0 MPa m~ and a modulus of rupture of at least 700 MPa.
The ceramic body according to the invention option-ally further includes refractory whiskers or fibers having an aspect ratio of about 3-150, uniformly distrib-uted in the ceramic body. The equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains. The starting formulation com-prises the silicon nitride, the one or more densification aids, and about 10-50% by volume refractory whiskers or fibers, based on the total volume of the ceramic body.
According to another aspect of the invention, there is provided a process for producing a densified silicon nitride-based ceramic body of improved fracture toughness and improved strength comprising the step of: densifying a blended powder mixture comprising silicon nitride and about 0.5-12% by weight of one or more suitable oxide densification aids, based on the combined weight of the one or more densification aids and the silicon nitride, in a nitrogen or inert atmosphere at about 1650-1850C
and about 3-30,000 psi, for a time sufficient to produce a ceramic body comprising: a first phase consisting essentially of beta-silieon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 mierons, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase eonsisting essentially of silica and the one or more densification aids; and having-a fracture toughness of at least 5.0 MPa-m~ and a strength of at least 700 MPa.
In the process according to the invention, the blended powder mixture densified in the densifying step optionally further includes about 10-50% by volume of refractory whiskers or fibers having an aspect ratio of about 3-150, based on the total volume of the ceramic body. The equivalent diameters of the whiskers or fibers and the silicon nitride in the blended powder mixture, and the densification time are selected to produce the densified ceramic body in which the equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains.
In accordance with other aspects of the invention, there are provided methods according to the invention for continuous or interrupted machining of grey cast iron stock or nickel-based superalloy stock involve milling, turning, or boring the stock with a cutting tool com-prising a densified silicon nitride-based ceramic body having a fracture toughness of at least 5.0 MPa-m~ and a modulus of rupture of at least 700 MPa. The ceramic body includes a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, 87-3-343 CN 1 336 t 02 and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids. In the first phase, if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns, and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5. The ceramic body is formed from a starting formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride.
The machining speed for the grey cast iron stock is about 800-6000 sfm, and the feed rate is about 0.01-0.04 in/rev. The machining speed for the nickel-based stock is about 200-1500 sfm and the feed rate is about 0.005-0.04 in/rev.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which:
The Figure is a graphical representation of the variation of strength with temperature for a known material and for materials according to the invention.
The densified silicon nitride-based ceramic bodies of the present invention comprise beta-silicon nitride grains bonded together by an intergranular phase of a silica-based material. The silica in the intergranular phase is normally present in the silicon nitride compo-nent of the starting formulation. The oxide densifica-tion aids are also present in the intergranular phase.
The preferred densification aid is yttria, included in the starting formulation in an amount of about 2-8% by weight based on the combined weight of the silicon nitride and the densification aids. The yttria may be used alone or in combination with other suitable 87-3-343 CN -6- l 336 1 02 densification aids, for example alumina, present in the starting formulation in an amount of about 0.5-12% by weight. Other suitable oxide densification aids may be included in the starting formulation with or without the yttria and/or alumina. The densification aid, or combination of densification aids is selected to optimize properties desired in the ceramic body, for example high temperature strength, chemical resistance, or oxidation resistance. Such other suitable oxide densification aids include, but are not limited to magnesia, ceria, zirconia, and hafnia. The total amount of densification aids included in the starting formulation preferably should not exceed 12% by weight.
Impurities may be present in the starting materials used for the manufacture of the ceramic body. The impurities tend to become concentrated in the intergran-ular phase during preparation of the ceramic body.
Therefore high purity starting materials are desired, preferably those having less than about 0.1% by weight cation impurities. A typical undesirable impurity is calcium, which tends to deleteriously affect the second phase and the high temperature properties.
The monolithic ceramic bodies described above have a microstructure of beta-silicon nitride grains bonded together by a continuous, bonding, intergranular second phase formed from the densifying additive. Because the intergranular second phase is continuous, its character-istics profoundly affect the high temperature properties of the monolithic ceramic material. The monolithic ceramic bodies of the present invention possess high fracture toughness and high strength at temperatures in excess of 1200C, preferably in excess of 1500C.
In another aspect of the present invention whiskers or fibers of hard refractory silicon carbide or a transi-tion metal carbide, nitride, or carbonitride, or mixtures or solid solutions thereof are dispersed in a two-phase matrix. By the term "transition metal carbide, nitride, or carbonitride", as used throughout this specification and claims, is meant any carbide, nitride, or carboni-tride of titanium, hafnium, tantalum, niobium, or tungsten.
The hard refractory whiskers incorporated into materials in accordance with this invention each comprise a single crystal, while the fibers are polycrystalline.
The whiskers or fibers preferably have an average diame-ter of about 1-5 microns and an average length of about 10-250 microns, with a preferred aspect ratio of length to diameter of about 3-150.
These dispersoids may be coated if desired with a different hard refractory material deposited as one or more polycrystalline layers on the fiber or whisker.
Suitable coatings for the silicon carbide whiskers or fibers include refractory oxides and nitrides. Those for the metal carbide, nitride, or carbonitride dispersoids include refractory oxides, nitrides, or carbides. Such coated dispersoids may be selected to optimize bulk (e.g.
mechanical) properties and surface (e.g. chemical) properties of the dispersoid materials in the matrix.
The useful life and performance of composite bodies in accordance with this aspect of the invention depend, in large part, on the volume taken up by the dispersed phase in the article. The whiskers or fibers should comprise about 10-50% by volume of the densified composite.
In accordance with the principles of the present invention, the hard refractory dispersoids are uniformly distributed in a two-phase matrix. The first phase of the matrix consists essentially of grains of beta-silicon nitride, as described above for the monolithic ceramic body not including the whiskers or fibers. The inter-granular phase or second phase of the matrix is formed from one or more densification aids, as also described above. The degree of purity of the materials used in the starting formulation for the composite ceramic bodies of the invention is as described above for the monolithic bodies.
The composite ceramic bodies described herein have a composite microstructure of refractory whiskers or fibers uniformly dispersed in a matrix containing a first phase of beta-Si3N4 grains and a continuous, bonding, intergranular second phase formed from the densifying additive. Because the intergranular phase is continuous, its characteristics profoundly affect the high temperature properties of the composite material. The composite ceramic bodies of the present invention possess high fracture toughness and high strength at temperatures in excess of 1200C, preferably in excess of 1500C.
Ceramic bodies formed from the densified monolithic or composite materials according to the present invention may be coated with one or more adherent layers of hard refractory materials, for example by known chemical vapor deposition or physical vapor deposition techniques. The hard refractory materials suitable for coating monolithic or composite ceramic bodies according to the present invention include the refractory carbides, nitrides, and carbonitrides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, and mixtures and solid solutions thereof, and alumina, zirconia, and hafnia, and mixtures and solid solutions thereof. Each layer may be the same or different from adjacent or other layers. Such coatings are especially advantageous when applied to cutting tools formed from the densified composites of the present invention.
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In accordance with yet another aspect of the inven-tion, a process is provided for preparing the monolithic or composite bodies described above, densifying the materials to densities approaching theoretical density, i.e. greater than 98~ of theoretical, while achieving optimum levels of mechanical strength and toughness at both room temperature and elevated temperature, making the bodies particularly useful as cutting tools in metal removing applications, or as structural components for turbine engines.
The silicon nitride, the densification aid, and optionally the hard refractory whiskers or fibers are blended to form a starting formulation or powder mixture.
The powder mixture is then densified or compacted to a high density, for example by sintering, hot pressing, or hot isostatic pressing techniques. A star~ing composi-tion for the production of the strong, tough, abrasion resistant materials according to the present invention may be made by employing Si3N4 powder, normally predomi-nantly alpha-Si3N4, and preferably of average particle size below about 3 microns.
Densification of the silicon nitride-based mono-lithic material or the silicon nitride/whisker composite material is aided by the incorporation of one or more of the above-described densification aids into the initial composition. In the starting formulations employed in the fabrication, hard refractory whiskers or fibers optionally comprise about 10-50% of the total volume of the densified article, as described above, while the densification aid comprises about 0.5-12~ by weight, based on the combined weight of the densification aid and the silicon nitride in the starting composition. In the starting formulation, the balance of the mixture normally comprises the silicon nitride powder.
The starting materials may be processed to a powder compact of adequate green strength by thoroughly mixing the matrix starting materials by processes such as dry milling or ball milling in a nonreactive liquid medium, such as toluene or methanol; admixing the whiskers or fibers, if included, by blending, preferably in a non-reactive liquid medium; and forming the mixture, for example by pressing, injection molding, extruding, or slip casting. Processing may also optionally include a presintering or prereacting step in which either the uncompacted materials or the compact is heated at moder-ate temperatures.
Since the strength of monolithic or composite articles in accordance with this invention decreases with increasing porosity in the total compact, it is important that the compact be densified to a density as nearly approaching 100~ of theoretical density as possible, preferably greater than 98% of theoretical density. The measure of percent of theoretical density is obtained by a weighted average of the densities of the components of the compact.
The microstructural tailoring of the ceramic mate-rials described herein is critical to providing mono-lithic and composite bodies exhibiting both improved fracture toughness and improved strength. This micro-structural tailoring involves careful control of the silicon nitride grain size and aspect ratio. In the composite materials it also involves careful control of the dispersoid content and its size relative to the matrix grains. These sizes are expressed as the equiva-lent diameters of the silicon nitride grains and the whiskers or fibers. By the term "equivalent diameter", as used throughout this specification and claims, is meant the average diameter of an equiaxed particle of the same volume as the particle, grain, whisker, or fiber.
The terms "equivalent diameter", "grain size", "aspect ratio", and the like, as used throughout this ~7-3-343 CN -11- 1 336 1 02 specification and claims, refer to the average values of these measurements within the ceramic body.
An increase in both fracture toughness and strength of silicon nitride monolithic and composite materials with an increase in grain size is unexpected, since ceramics in general are expected to exhibit lower strength with increased grain size (Kingery, Introduction to Ceramics, John Wiley & Sons Inc., NY, London, 624 (1960); Evans, J. Am. Cer. Soc., 65, 127-137 (1982)). It has been further observed that silicon nitride ceramics containing alumina and yttria sintering aids exhibit this behavior (G. Watting et al., Sci. of Ceramics, Proc. Non Oxide Tech. and Eng. Ceramics, Limerick, Ireland (1986)).
It has been found, however, that an increase in silicon nitride grain size and control of the silicon nitride aspect ratio, through the densification process according to the invention, can achieve an increase in fracture toughness with an unexpected concomitant in-crease in the strength of the silicon nitride body.
Further, when reinforcing whiskers or fibers are included in the material, the whiskers or fibers being of a prescribed aspect ratio and relative size, a further increase in resistance to fracture may be achieved, again with a concomitant increase in strength. This too is unexpected in light of the teachings of U.S. Patent No.
4,543,345, which states that additions of silicon carbide whiskers to silicon nitride bodies do not produce in-creased toughness.
To achieve a monolithic or composite ceramic body according to the invention, the powder mixture described above is compacted and densified in nitrogen or an inert atmosphere, e.g., argon, at a pressure of about 3-30,000 psi and a temperature of about 1650-1850C, and held at the maximum temperature for a prolonged time, normally about 2-12 hours. The time at maximum temperature is sufficient to achieve grain growth in the beta-silicon 87-3-343 CN -12- l 3 3 6 1 G 2 nitride component of the ceramic body and the microstruc-ture described above. The improved properties of the resultant body are unexpected, in light of statements found in the prior art that extended times at high temperatures result in a decrease of fracture strength as well as fracture toughness (Ziegler, Heinrich, and ~otting, J. Mater. Sci, 22, 3041-3086 (1987)).
The following Examples are presented to enable those skilled in the art to more clearly understand and prac-tice the present invention. These Examples should not beconsidered as a limitation upon the scope of the present invention, but merely as being illustrative and represen-tative thereof.
A powder mixture of silicon nitride, 1.5% by weight alumina, and 6% by weight yttria was dry ball milled for 24 hours using silicon nitride milling media. The powder was processed in a graphite die coated with boron nitride, and hot pressed at 3500 psi and 1725C in nitrogen for 90 minutes for Example la, or 400 minutes for Example lb. The properties and grain sizes of the resulting densified ceramic bodies at room temperature and elevated temperatures are shown in Table 1 below and in Figure 1. These results show an increase in both fracture toughness and modulus of rupture (i.e. strength) at room and elevated temperature with an increase in the equivalent grain diameter of the beta-silicon nitride grains resulting from the increase in hot pressing time.
E~AMPLE 2 A powder mixture of the composition described above for Example 1, but with 30% by volume of the powder mixture substituted by silicon carbide whiskers having an equivalent diameter of 1.95 microns and an aspect ratio of 33, was wet blended in methanol in a high shear -blender to disperse the whiskers throughout the mixture.
The blended mixture was then hot pressed as described above for Example 1 for 400 minutes. The properties of the resulting composite ceramic body are shown in Table 1, while the elevated temperature properties of the composite are compared in Figure 1 to those for the bodies of Example 1. These composite bodies exhibit increases in both fracture toughness and strength over both the conventional bodies and the improved monolithic bodies of Example 1.
H.P.Time, Aspect Equiv. KIC, MOR @ 25c Ex # min. % T.D. Ratio Dia, microns MPa m~ MPa la 90 99.3 1.8(G) 0.37 (G) 4.7 773 lb 400 99.1 1.8(G) 0.59 (G) 5.4 886 2 400 99.1 1.8(G) 0.84 (G) 6.4 975 12(W) 1.95 (W) 20 (G) = Si3N4 grains; (W) = SiC whiskers A cutting tool (3b) of the composite material according to the invention was compared with a standard silicon nitride cutting tool (3a) in a machining applica-tion. A turning operation was performed on an Inconel workpiece (Inconel 718 ( Rc45)) at a cutting speed of 800 sfm and a feed rate of 0.006 in/rev. The depth of cut was 0.040 in. The averagé notch wear of the two types of cutting tool after 1 minute is shown below in Table 2.
The cutting tool according to the invention exhibited significantly reduced notch wear during machining, as compared to the standard silicon nitride cutting tool.
H.P. Time, Ave.Notch Wear, Ex # Mat'l. min in 3a Si3N4 90 1.5% A12O3 6% Y2O3 10 3b 3 4 0.017 1.5% Al2O3 6% Y2O3 30% vol SiC W
The densified monolithic and composite ceramic bodies according to the invention are hard, non-porous, and exhibit room and elevated temperature strength and fracture toughness higher than that of conventional silicon nitride materials. These bodies are useful for ceramic articles including, but not limited to cutting tools, extrusion dies, nozzles, dies, bearings, and wear resistant structural parts. These bodies are especially useful as ceramic components for heat engines and as shaped cutting tools for continuous or interrupted milling, turning, or boring of grey cast iron stock or high nickel (at least 50% nickel) superalloy stock, e.g.
Inconel.
While there has been shown and described what are at present considered the preferred embodiments o the invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the claims.
TOUGHNESS AND STRENGTH
This application contains subject matter related to matter disclosed and claimed in commonly assigned co-pending Canadian Patent Application No. 591,218-1, filed concurrently herewith.
This invention relates to fracture and abrasion resistant materials and to articles of manufacture made therefrom. More particularly, it is concerned with silicon nitride-based ceramic bodies exhibiting both improved fracture toughness and improved strength.
The need for improved materials for cutting tool applications, with enhanced toughness and strength at room and elevated temperatures, and with chemical inert-ness, has generated a widespread interest in ceramic materials as candidates to fulfill these requirements.
Conventional ceramic cutting tool materials have failed to find wide application, primarily due to their low fracture toughness. Therefore, many materials have been evaluated to improve ceramic performance, such as silicon nitride-based monolithic and composite materials for cutting tool applications.
Silicon nitride-based ceramics are also under active development as structural components for advanced turbine engines. A critical factor which limits the widespread application of these materials in such heat engines is their tendency to catastrophic failure. Accordingly, improvement in strength and fracture toughness of these materials would improve their performance in the demand-ing heat engine environment, contributing to a more rapid development of these high performance turbine engines.
Many improvements have been made in the toughness, abrasion resistance, high temperature strength and A~
chemical inertness of silicon nitride materials, but the stringent conditions encountered by heat engine compo-nents and cutting tools demand even further improvement in material characteristics. In many applications, for example in gray cast iron and high nickel alloy machin-ing, silicon nitride tool wear has been found to be dominated by abrasion. Even at cutting speeds as high as 5000 sfm, chemical reactions between tool and workpiece are negligible in comparison. It has been found that abrasion resistance for silicon nitride ceramic cutting tool materials is directly proportional to KIC / H / , where KIC is the fracture toughness and H is the hard-ness. It may be seen, therefore, that further improve-ment in the fracture toughness of silicon nitride ceramic materials could bring about significant increases in both reliability and abrasive wear resistance, providing materials for cutting tools with new and improved charac-teristics.
Attempt has been made to increase the fracture toughness of silicon nitride materials through the development of a composite, in which dispersed partic-ulate, fiber, or whisker materials are included in a silicon nitride-based matrix. The added complexity of composites, however, can result in improvements in fracture toughness at the expense of strength. The present invention provides new and improved monolithic and composite silicon nitride-based ceramic materials exhibiting both improved fracture toughness and improved strength.
The wear-resistant silicon nitride-based bodies of the invention are also useful in other wear part and structural applications, for example in dies, nozzles, etc.
According to one aspect of the invention, there is provided a densified silicon nitride-based ceramic body 87-3-343 CN -3- 1 33~ 1 ~2 of improved fracture toughness and improved strength comprising: a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase con-sisting essentially of silica and one or more suitableoxide densification aids; wherein the ceramic body is formed from a starting formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
and the ceramic body has a fracture toughness of at least 5.0 MPa m~ and a modulus of rupture of at least 700 MPa.
The ceramic body according to the invention option-ally further includes refractory whiskers or fibers having an aspect ratio of about 3-150, uniformly distrib-uted in the ceramic body. The equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains. The starting formulation com-prises the silicon nitride, the one or more densification aids, and about 10-50% by volume refractory whiskers or fibers, based on the total volume of the ceramic body.
According to another aspect of the invention, there is provided a process for producing a densified silicon nitride-based ceramic body of improved fracture toughness and improved strength comprising the step of: densifying a blended powder mixture comprising silicon nitride and about 0.5-12% by weight of one or more suitable oxide densification aids, based on the combined weight of the one or more densification aids and the silicon nitride, in a nitrogen or inert atmosphere at about 1650-1850C
and about 3-30,000 psi, for a time sufficient to produce a ceramic body comprising: a first phase consisting essentially of beta-silieon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 mierons, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase eonsisting essentially of silica and the one or more densification aids; and having-a fracture toughness of at least 5.0 MPa-m~ and a strength of at least 700 MPa.
In the process according to the invention, the blended powder mixture densified in the densifying step optionally further includes about 10-50% by volume of refractory whiskers or fibers having an aspect ratio of about 3-150, based on the total volume of the ceramic body. The equivalent diameters of the whiskers or fibers and the silicon nitride in the blended powder mixture, and the densification time are selected to produce the densified ceramic body in which the equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains.
In accordance with other aspects of the invention, there are provided methods according to the invention for continuous or interrupted machining of grey cast iron stock or nickel-based superalloy stock involve milling, turning, or boring the stock with a cutting tool com-prising a densified silicon nitride-based ceramic body having a fracture toughness of at least 5.0 MPa-m~ and a modulus of rupture of at least 700 MPa. The ceramic body includes a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, 87-3-343 CN 1 336 t 02 and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids. In the first phase, if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns, and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5. The ceramic body is formed from a starting formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride.
The machining speed for the grey cast iron stock is about 800-6000 sfm, and the feed rate is about 0.01-0.04 in/rev. The machining speed for the nickel-based stock is about 200-1500 sfm and the feed rate is about 0.005-0.04 in/rev.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which:
The Figure is a graphical representation of the variation of strength with temperature for a known material and for materials according to the invention.
The densified silicon nitride-based ceramic bodies of the present invention comprise beta-silicon nitride grains bonded together by an intergranular phase of a silica-based material. The silica in the intergranular phase is normally present in the silicon nitride compo-nent of the starting formulation. The oxide densifica-tion aids are also present in the intergranular phase.
The preferred densification aid is yttria, included in the starting formulation in an amount of about 2-8% by weight based on the combined weight of the silicon nitride and the densification aids. The yttria may be used alone or in combination with other suitable 87-3-343 CN -6- l 336 1 02 densification aids, for example alumina, present in the starting formulation in an amount of about 0.5-12% by weight. Other suitable oxide densification aids may be included in the starting formulation with or without the yttria and/or alumina. The densification aid, or combination of densification aids is selected to optimize properties desired in the ceramic body, for example high temperature strength, chemical resistance, or oxidation resistance. Such other suitable oxide densification aids include, but are not limited to magnesia, ceria, zirconia, and hafnia. The total amount of densification aids included in the starting formulation preferably should not exceed 12% by weight.
Impurities may be present in the starting materials used for the manufacture of the ceramic body. The impurities tend to become concentrated in the intergran-ular phase during preparation of the ceramic body.
Therefore high purity starting materials are desired, preferably those having less than about 0.1% by weight cation impurities. A typical undesirable impurity is calcium, which tends to deleteriously affect the second phase and the high temperature properties.
The monolithic ceramic bodies described above have a microstructure of beta-silicon nitride grains bonded together by a continuous, bonding, intergranular second phase formed from the densifying additive. Because the intergranular second phase is continuous, its character-istics profoundly affect the high temperature properties of the monolithic ceramic material. The monolithic ceramic bodies of the present invention possess high fracture toughness and high strength at temperatures in excess of 1200C, preferably in excess of 1500C.
In another aspect of the present invention whiskers or fibers of hard refractory silicon carbide or a transi-tion metal carbide, nitride, or carbonitride, or mixtures or solid solutions thereof are dispersed in a two-phase matrix. By the term "transition metal carbide, nitride, or carbonitride", as used throughout this specification and claims, is meant any carbide, nitride, or carboni-tride of titanium, hafnium, tantalum, niobium, or tungsten.
The hard refractory whiskers incorporated into materials in accordance with this invention each comprise a single crystal, while the fibers are polycrystalline.
The whiskers or fibers preferably have an average diame-ter of about 1-5 microns and an average length of about 10-250 microns, with a preferred aspect ratio of length to diameter of about 3-150.
These dispersoids may be coated if desired with a different hard refractory material deposited as one or more polycrystalline layers on the fiber or whisker.
Suitable coatings for the silicon carbide whiskers or fibers include refractory oxides and nitrides. Those for the metal carbide, nitride, or carbonitride dispersoids include refractory oxides, nitrides, or carbides. Such coated dispersoids may be selected to optimize bulk (e.g.
mechanical) properties and surface (e.g. chemical) properties of the dispersoid materials in the matrix.
The useful life and performance of composite bodies in accordance with this aspect of the invention depend, in large part, on the volume taken up by the dispersed phase in the article. The whiskers or fibers should comprise about 10-50% by volume of the densified composite.
In accordance with the principles of the present invention, the hard refractory dispersoids are uniformly distributed in a two-phase matrix. The first phase of the matrix consists essentially of grains of beta-silicon nitride, as described above for the monolithic ceramic body not including the whiskers or fibers. The inter-granular phase or second phase of the matrix is formed from one or more densification aids, as also described above. The degree of purity of the materials used in the starting formulation for the composite ceramic bodies of the invention is as described above for the monolithic bodies.
The composite ceramic bodies described herein have a composite microstructure of refractory whiskers or fibers uniformly dispersed in a matrix containing a first phase of beta-Si3N4 grains and a continuous, bonding, intergranular second phase formed from the densifying additive. Because the intergranular phase is continuous, its characteristics profoundly affect the high temperature properties of the composite material. The composite ceramic bodies of the present invention possess high fracture toughness and high strength at temperatures in excess of 1200C, preferably in excess of 1500C.
Ceramic bodies formed from the densified monolithic or composite materials according to the present invention may be coated with one or more adherent layers of hard refractory materials, for example by known chemical vapor deposition or physical vapor deposition techniques. The hard refractory materials suitable for coating monolithic or composite ceramic bodies according to the present invention include the refractory carbides, nitrides, and carbonitrides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, and mixtures and solid solutions thereof, and alumina, zirconia, and hafnia, and mixtures and solid solutions thereof. Each layer may be the same or different from adjacent or other layers. Such coatings are especially advantageous when applied to cutting tools formed from the densified composites of the present invention.
.~ ,,~
In accordance with yet another aspect of the inven-tion, a process is provided for preparing the monolithic or composite bodies described above, densifying the materials to densities approaching theoretical density, i.e. greater than 98~ of theoretical, while achieving optimum levels of mechanical strength and toughness at both room temperature and elevated temperature, making the bodies particularly useful as cutting tools in metal removing applications, or as structural components for turbine engines.
The silicon nitride, the densification aid, and optionally the hard refractory whiskers or fibers are blended to form a starting formulation or powder mixture.
The powder mixture is then densified or compacted to a high density, for example by sintering, hot pressing, or hot isostatic pressing techniques. A star~ing composi-tion for the production of the strong, tough, abrasion resistant materials according to the present invention may be made by employing Si3N4 powder, normally predomi-nantly alpha-Si3N4, and preferably of average particle size below about 3 microns.
Densification of the silicon nitride-based mono-lithic material or the silicon nitride/whisker composite material is aided by the incorporation of one or more of the above-described densification aids into the initial composition. In the starting formulations employed in the fabrication, hard refractory whiskers or fibers optionally comprise about 10-50% of the total volume of the densified article, as described above, while the densification aid comprises about 0.5-12~ by weight, based on the combined weight of the densification aid and the silicon nitride in the starting composition. In the starting formulation, the balance of the mixture normally comprises the silicon nitride powder.
The starting materials may be processed to a powder compact of adequate green strength by thoroughly mixing the matrix starting materials by processes such as dry milling or ball milling in a nonreactive liquid medium, such as toluene or methanol; admixing the whiskers or fibers, if included, by blending, preferably in a non-reactive liquid medium; and forming the mixture, for example by pressing, injection molding, extruding, or slip casting. Processing may also optionally include a presintering or prereacting step in which either the uncompacted materials or the compact is heated at moder-ate temperatures.
Since the strength of monolithic or composite articles in accordance with this invention decreases with increasing porosity in the total compact, it is important that the compact be densified to a density as nearly approaching 100~ of theoretical density as possible, preferably greater than 98% of theoretical density. The measure of percent of theoretical density is obtained by a weighted average of the densities of the components of the compact.
The microstructural tailoring of the ceramic mate-rials described herein is critical to providing mono-lithic and composite bodies exhibiting both improved fracture toughness and improved strength. This micro-structural tailoring involves careful control of the silicon nitride grain size and aspect ratio. In the composite materials it also involves careful control of the dispersoid content and its size relative to the matrix grains. These sizes are expressed as the equiva-lent diameters of the silicon nitride grains and the whiskers or fibers. By the term "equivalent diameter", as used throughout this specification and claims, is meant the average diameter of an equiaxed particle of the same volume as the particle, grain, whisker, or fiber.
The terms "equivalent diameter", "grain size", "aspect ratio", and the like, as used throughout this ~7-3-343 CN -11- 1 336 1 02 specification and claims, refer to the average values of these measurements within the ceramic body.
An increase in both fracture toughness and strength of silicon nitride monolithic and composite materials with an increase in grain size is unexpected, since ceramics in general are expected to exhibit lower strength with increased grain size (Kingery, Introduction to Ceramics, John Wiley & Sons Inc., NY, London, 624 (1960); Evans, J. Am. Cer. Soc., 65, 127-137 (1982)). It has been further observed that silicon nitride ceramics containing alumina and yttria sintering aids exhibit this behavior (G. Watting et al., Sci. of Ceramics, Proc. Non Oxide Tech. and Eng. Ceramics, Limerick, Ireland (1986)).
It has been found, however, that an increase in silicon nitride grain size and control of the silicon nitride aspect ratio, through the densification process according to the invention, can achieve an increase in fracture toughness with an unexpected concomitant in-crease in the strength of the silicon nitride body.
Further, when reinforcing whiskers or fibers are included in the material, the whiskers or fibers being of a prescribed aspect ratio and relative size, a further increase in resistance to fracture may be achieved, again with a concomitant increase in strength. This too is unexpected in light of the teachings of U.S. Patent No.
4,543,345, which states that additions of silicon carbide whiskers to silicon nitride bodies do not produce in-creased toughness.
To achieve a monolithic or composite ceramic body according to the invention, the powder mixture described above is compacted and densified in nitrogen or an inert atmosphere, e.g., argon, at a pressure of about 3-30,000 psi and a temperature of about 1650-1850C, and held at the maximum temperature for a prolonged time, normally about 2-12 hours. The time at maximum temperature is sufficient to achieve grain growth in the beta-silicon 87-3-343 CN -12- l 3 3 6 1 G 2 nitride component of the ceramic body and the microstruc-ture described above. The improved properties of the resultant body are unexpected, in light of statements found in the prior art that extended times at high temperatures result in a decrease of fracture strength as well as fracture toughness (Ziegler, Heinrich, and ~otting, J. Mater. Sci, 22, 3041-3086 (1987)).
The following Examples are presented to enable those skilled in the art to more clearly understand and prac-tice the present invention. These Examples should not beconsidered as a limitation upon the scope of the present invention, but merely as being illustrative and represen-tative thereof.
A powder mixture of silicon nitride, 1.5% by weight alumina, and 6% by weight yttria was dry ball milled for 24 hours using silicon nitride milling media. The powder was processed in a graphite die coated with boron nitride, and hot pressed at 3500 psi and 1725C in nitrogen for 90 minutes for Example la, or 400 minutes for Example lb. The properties and grain sizes of the resulting densified ceramic bodies at room temperature and elevated temperatures are shown in Table 1 below and in Figure 1. These results show an increase in both fracture toughness and modulus of rupture (i.e. strength) at room and elevated temperature with an increase in the equivalent grain diameter of the beta-silicon nitride grains resulting from the increase in hot pressing time.
E~AMPLE 2 A powder mixture of the composition described above for Example 1, but with 30% by volume of the powder mixture substituted by silicon carbide whiskers having an equivalent diameter of 1.95 microns and an aspect ratio of 33, was wet blended in methanol in a high shear -blender to disperse the whiskers throughout the mixture.
The blended mixture was then hot pressed as described above for Example 1 for 400 minutes. The properties of the resulting composite ceramic body are shown in Table 1, while the elevated temperature properties of the composite are compared in Figure 1 to those for the bodies of Example 1. These composite bodies exhibit increases in both fracture toughness and strength over both the conventional bodies and the improved monolithic bodies of Example 1.
H.P.Time, Aspect Equiv. KIC, MOR @ 25c Ex # min. % T.D. Ratio Dia, microns MPa m~ MPa la 90 99.3 1.8(G) 0.37 (G) 4.7 773 lb 400 99.1 1.8(G) 0.59 (G) 5.4 886 2 400 99.1 1.8(G) 0.84 (G) 6.4 975 12(W) 1.95 (W) 20 (G) = Si3N4 grains; (W) = SiC whiskers A cutting tool (3b) of the composite material according to the invention was compared with a standard silicon nitride cutting tool (3a) in a machining applica-tion. A turning operation was performed on an Inconel workpiece (Inconel 718 ( Rc45)) at a cutting speed of 800 sfm and a feed rate of 0.006 in/rev. The depth of cut was 0.040 in. The averagé notch wear of the two types of cutting tool after 1 minute is shown below in Table 2.
The cutting tool according to the invention exhibited significantly reduced notch wear during machining, as compared to the standard silicon nitride cutting tool.
H.P. Time, Ave.Notch Wear, Ex # Mat'l. min in 3a Si3N4 90 1.5% A12O3 6% Y2O3 10 3b 3 4 0.017 1.5% Al2O3 6% Y2O3 30% vol SiC W
The densified monolithic and composite ceramic bodies according to the invention are hard, non-porous, and exhibit room and elevated temperature strength and fracture toughness higher than that of conventional silicon nitride materials. These bodies are useful for ceramic articles including, but not limited to cutting tools, extrusion dies, nozzles, dies, bearings, and wear resistant structural parts. These bodies are especially useful as ceramic components for heat engines and as shaped cutting tools for continuous or interrupted milling, turning, or boring of grey cast iron stock or high nickel (at least 50% nickel) superalloy stock, e.g.
Inconel.
While there has been shown and described what are at present considered the preferred embodiments o the invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the claims.
Claims (21)
1. A densified silicon nitride-based ceramic body of improved fracture toughness and improved strength comprising:
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride; and the ceramic body has a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa.
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride; and the ceramic body has a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa.
2. A ceramic body in accordance with claim 1 wherein the one or more other densification aids consist essentially of yttria, present in the starting formula-tion in an amount of about 2-8% by weight.
3. A ceramic body in accordance with claim 1 wherein the one or more other densification aids consist essentially of yttria, present in the starting formula-tion in an amount of about 2-8% by weight, and alumina, present in the starting formulation in amount of about 0.5-4% by weight.
4. A ceramic body in accordance with claim 1 wherein the beta-silicon nitride grains have an equiva-lent diameter of about 0.25-3.5 microns, and an aspect ratio of about 1.5-10.
5. A coated silicon nitride-based ceramic body of improved fracture toughness and improved strength com-prising a ceramic body in accordance with claim 1 having deposited thereon an adherent, wear-resistant, refractory coating comprising one or more refractory layers.
6. A ceramic body in accordance with claim 1 further comprising refractory whiskers or fibers having an aspect ratio of about 3-150, uniformly distributed in the ceramic body;
wherein the equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains; and the starting formulation comprises the silicon nitride, the one or more densification aids, and about 10-50% by volume refractory whiskers or fibers, based on the total volume of the ceramic body.
wherein the equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains; and the starting formulation comprises the silicon nitride, the one or more densification aids, and about 10-50% by volume refractory whiskers or fibers, based on the total volume of the ceramic body.
7. A ceramic body in accordance with claim 6 wherein the beta-silicon nitride grains have an equiva-lent diameter of about 0.25-3.5 microns, and an aspect ratio of about 1.5-10.
8. A ceramic body in accordance with claim 6 wherein the refractory whiskers or fibers are of mate-rials selected from the group consisting of silicon carbide, silicon carbide coated with a refractory mate-rial, refractory metal carbides, refractory metal car-bides coated with a refractory material, refractory metal nitrides, and refractory metal nitrides coated with a refractory material.
9. A coated silicon nitride-based ceramic body of improved fracture toughness and improved strength com-prising a ceramic body in accordance with claim 6 having deposited thereon an adherent, wear-resistant, refractory coating comprising one or more refractory layers.
10. A process for producing a densified silicon nitride-based ceramic body of improved fracture toughness and improved strength comprising the step of:
densifying a blended powder mixture comprising silicon nitride and about 0.5-12% by weight of one or more suitable oxide densification aids, based on the combined weight of the one or more densification aids and the silicon nitride, in a nitrogen or inert atmosphere at about 1650-1850°C and about 3-30,000 psi, for a time sufficient to produce a ceramic body comprising:
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and the one or more densification aids;
and having a fracture toughness of at least 5.0 MPa?m and a strength of at least 700 MPa.
densifying a blended powder mixture comprising silicon nitride and about 0.5-12% by weight of one or more suitable oxide densification aids, based on the combined weight of the one or more densification aids and the silicon nitride, in a nitrogen or inert atmosphere at about 1650-1850°C and about 3-30,000 psi, for a time sufficient to produce a ceramic body comprising:
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and the one or more densification aids;
and having a fracture toughness of at least 5.0 MPa?m and a strength of at least 700 MPa.
11. A process in accordance with claim 10 wherein the one or more densification aids consist essentially of yttria, the yttria being present in the powder mixture in an amount of about 2-8% by weight.
12. A process in accordance with claim 10 wherein the one or more densification aids consist essentially of yttria, the yttria being present in the powder mixture in an amount of about 2-8% by weight, and alumina, the alumina being present in the powder mixture in an amount of about 0.5-4% by weight.
13. A process in accordance with claim 10 further comprising the step of depositing on the densified ceramic body an adherent, wear-resistant, refractory coating comprising one or more refractory layers.
14. A process in accordance with claim 10 wherein the beta-silicon nitride grains in the densified ceramic body have an equivalent diameter of about 0.25-3.5 microns, and an aspect ratio of about 1.5-10.
15. A process in accordance with claim 10 wherein the blended powder mixture densified in the densifying step further comprises about 10-50% by volume of refrac-tory whiskers or fibers having an aspect ratio of about 3-150, based on the total volume of the ceramic body; and the equivalent diameters of the whiskers or fibers and the silicon nitride in the powder mixture, and the densification time are selected to produce the densified ceramic body in which the equivalent diameter of the whiskers or fibers is greater than that of the beta-silicon nitride grains.
16. A process in accordance with claim 15 wherein the refractory whiskers or fibers are of materials selected from the group consisting of silicon carbide, silicon carbide coated with a refractory material, refractory metal carbides, refractory metal carbides coated with a refractory material, refractory metal nitrides, and refractory metal nitrides coated with a refractory material.
17. A process in accordance with claim 15 wherein the equivalent diameter of the whiskers or fibers in the densified ceramic body is about 0.3-10.0 microns, and the equivalent diameter of the beta-silicon nitride grains is about 0.25-3.5 microns.
18. A process in accordance with claim 15 further comprising the step of depositing on the densified ceramic body an adherent, wear-resistant, refractory coating comprising one or more refractory layers.
19. A method for continuous or interrupted machining of grey cast iron stock comprising the step of milling, turning, or boring the stock with a cutting tool com-prising a densified silicon nitride-based ceramic body having a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa, the ceramic body comprising:
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
wherein the machining speed is about 800-6000 sfm and the feed rate is about 0.01-0.04 in/rev.
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
wherein the machining speed is about 800-6000 sfm and the feed rate is about 0.01-0.04 in/rev.
20. A method for continuous or interrupted machining of nickel-based superalloy stock comprising the step of milling, turning, or boring the stock with a cutting tool comprising a densified silicon nitride-based ceramic body having a fracture toughness of at least 5.0 MPa?m and a modulus of rupture of at least 700 MPa, the ceramic body comprising:
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
wherein the machining speed is about 200-1500 sfm and the feed rate is about 0.005-0.04 in/rev.
a first phase consisting essentially of beta-silicon nitride grains having an aspect ratio of about 1-10 and an equivalent diameter of about 0.25-10 microns, wherein if the aspect ratio is less than about 1.5 the equivalent diameter is at least about 0.4 microns and if the equivalent diameter is less than about 0.4 microns the aspect ratio is at least about 1.5; and an intergranular, bonding, silica-based second phase consisting essentially of silica and one or more suitable oxide densification aids;
wherein the ceramic body is formed from a start-ing formulation comprising silicon nitride and about 0.5-12% by weight of the one or more densification aids, based on the combined weight of the one or more densification aids and the silicon nitride;
wherein the machining speed is about 200-1500 sfm and the feed rate is about 0.005-0.04 in/rev.
21
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US15678788A | 1988-02-17 | 1988-02-17 | |
| US156,787 | 1988-02-17 |
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| CA1336102C true CA1336102C (en) | 1995-06-27 |
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