Alumina-Titanium Carbide-Silicon Carbide Composition

The present invention relates to ceramic materials containing alumina, titanium carbide and silicon carbide and especially those compositions possessing a combination of high toughness and wear resistance.

Materials for cutting tool inserts fall into several well-known categories. These include high speed steels, cast alloys of cobalt and chromium, sintered carbides and ceramic materials such as alumina with the corundum crystal structure, and even diamonds. Each material has an advantage depending upon the particular application. Some are much more expensive than others. High speed steel has the greatest resistance to shock of all the materials. For this and other reasons, it is the preferred cutting material for many applications. Because of their resistance to wear, cast alloys and sintered carbides often cost less per piece machined than the steels.

Ceramic materials are used in especially difficult applications. They have high hardness, chemical inertness and wear resistance even at elevated temperatures. This makes them useful, for example, for cutting cast iron and hardened steel at high cutting speeds. The inertness prevents welding of the tool insert to the metal being machined at the temperatures created by machining. Generally, however, ceramic tool inserts cannot be used where there are heavy interrupted cuts. Also, at slower machining speeds, tool loads are markedly higher and ceramic tools are likely to chip or fracture because of their lower tensile strength and toughness.

Tougher ceramic tools have been developed. These may comprise the addition of a second ceramic phase. Each phase is comprised of equiaxed grains as a result of combining equiaxed powders prior to hot pressing to form the tool insert. The addition of a second equiaxed phase increases toughness to some extent and provides a wear resistant tool insert.

Ceramic cutting tools made of alumina-titanium carbide composites have been successful in machining ferrous and non-ferrous alloys. See, for example, U.S. Pat. No. 3,580,708. These ceramic composites possess excellent high temperature mechanical strength and chemical wear resistance which are needed for superior performance in metalcutting. The utility of the material may be limited by its low fracture toughness in applications where tools tend to fail by fracture, say, in milling or high speed roughing.

Toughness of equiaxed ceramic composites is known to increase with increasing volume of the second phase up to a maximum that depends upon the particular phases and generally reaching maximum between 30 and 40 volume percent of the second phase. Fracture toughness of ceramic composites may be further increased by altering the morphology or shape of the second phase. It has been shown by Faber and Evans, in "Crack Deflection Processes - I. Theory," Acta Metall., Volume 31, No. 4, Pages 565-576 (1983) that the fracture toughness of certain ceramic composites can be increased by as much as four times by using rod-shaped second phases. The shape of the second phase is characterized by its aspect ratio (length to diameter ratio).

A composition disclosed in Wei U.S. Pat. No. 4,543,345 comprises the addition of silicon carbide whiskers to an alumina matrix to increase fracture toughness. It is explained in the Wei patent that the improved fracture toughness and resistance to slow crack growth is the result of energy spent in pulling whiskers out of the matrix. It is also pointed out in the Wei patent that not all matrix compositions are toughened by the addition of silicon carbide whiskers. Selected compositions disclosed in the Wei patent are finding use as materials for tool inserts. The tool inserts made with silicon carbide whiskers have limited use. They are useful for machining Inconel and other nickel base superalloys but have poor service life with soft steel or cast iron due to their poor wear resistance in these applications caused by their reactivity with iron at the high temperatures encountered.

It has been surprisingly found that the fracture toughness of fired ceramic compositions containing silicon carbide whiskers dispersed in an alumina based matrix phase can be substantially increased through the addition of dispersed titanium carbide phase to the alumina based matrix without a significantly adverse effect on the hardness of the composition. In addition, it has also been surprisingly found that, in the machining of soft steels, the wear resistance of silicon carbide whisker reinforced ceramic cutting tools can be increased by the addition of titanium carbide phase. In this manner, a ceramic cutting tool is provided which has the fracture toughness of silicon carbide whisker reinforced composites but with significantly improved wear resistance in soft steel machining applications.

In accordance with the present invention, a fired ceramic composition is provided having an alumina based matrix phase containing a dispersion of silicon carbide whiskers and titanium carbide phase. The ceramic composition contains about 1.0 to less than 30 v/o (volume percent), preferably 2.5 to 25 v/o, and more preferably 2.5 to 20 v/o silicon carbide whiskers, about 5 to about 40 v/o titanium carbide phase, and preferably up to about 3 v/o sintering aid residue, with the remainder essentially an alumina based matrix. The sum of the silicon carbide whisker and titanium carbide is preferably less than about 60 v/o, and more preferably less than about 50 v/o. The titanium carbide phase may be either substantially equiaxed titanium carbide particles, titanium carbide whiskers or a mixture thereof.

Where equiaxed titanium carbide particles are utilized, the particles have an average size of between 0.1 to 10 micrometers, preferably, 1 to 10 microns, and more preferably, 3 to 7 microns. Where titanium carbide whiskers are used, the whiskers have a diameter between 0.25 to 10 microns, and preferably about 1 to 10 microns. Preferably, the composition is comprised of about 10 to about 35 v/o titanium carbide phase, and more preferably, about 15 to 35 v/o titanium carbide phase. Preferably, the titanium carbide content is at least equal to the silicon carbide content and, more preferably, the titanium carbide content is greater than the silicon carbide content.

The alumina based (i.e., greater than 50 volume percent of the alumina based matrix is Al.sub.2 O.sub.3) matrix is preferably entirely alumina except for impurities and sintering aid residue.

The sintering aid utilized in the present invention may preferably be zirconia, magnesia, a rare earth oxide such as yttria, or a combination of the foregoing not exceeding about 3 v/o. The sintering aid residue observed in the hot pressed composition is preferably between about 0.05 to 3.0 v/o and, more preferably, between 0.25 to 1.5 v/o. The sintering aid is preferably magnesia since this is believed to provide improved toughness.

Further features and other objects and advantages will become apparent from the following detailed description of the invention made with reference to the drawings in which:

FIG. 1 is a scanning electron photomicrograph of large diameter, angular cross section titanium carbide whiskers (2000 .times.).

FIG. 2 is a scanning electron photomicrograph of small diameter, round cross section titanium carbide whiskers (2000 .times.).

FIG. 3 is a scanning electron photomicrograph of equiaxed titanium carbide particles (5000 .times.).

FIG. 4 is a scanning electron photomicrograph of silicon carbide whiskers (2000 .times.).

FIG. 5 is an optical photomicrograph of a ceramic composition containing silicon carbide whiskers, equiaxed titanium carbide phase and alumina (as polished, 625 .times.).

FIG. 6 is an optical photomicrograph of another ceramic composition containing silicon carbide whiskers, titanium carbide whiskers and alumina (as polished, 625 .times.).

FIG. 7 shows an embodiment of an indexable cutting tool in accordance with the present invention.

FIG. 8 shows plots of Rockwell A hardness and fracture toughness, K.sub.IC, in MPam.sup.1/2 (18.5 kg load), as functions of titanium carbide content and silicon carbide whisker (SiC.sub.w) content, where .=small diameter titanium carbide whiskers; o=large diameter titanium carbide whiskers; and .DELTA.=substantially equiaxed titanium carbide particles.

FIG. 9 shows a plot of fracture toughness, K.sub.IC, versus flank wear resistance of compositions in the high speed rough turning of AISI 1045 steel.

The present invention will be further clarified by consideration of the following examples which are intended to be purely exemplary of the present invention.

Mixes A through I as shown in Table I were made utilizing the following starting materials. Large diameter titanium carbide whisker starting material was composed of particles having an angular cross section with diameters between about 1 to about 6 microns and lengths up to about 100 microns (see FIG. 1). Small diameter titanium carbide whisker starting material was composed of particles having a round cross section and a diameter in the range of about 0.25 to about 3.0 microns and lengths up to about 150 microns (see FIG. 2). The substantially equiaxed titanium carbide particles had a diameter in the range of about 1 to about 10 microns with an average size of about 5 microns (see FIG. 3). These titanium carbide starting materials were at least 98 percent pure.

The titanium carbide whiskers were prepared in a chemical vapor deposition reactor using the technique described in Bauer et al U.S. patent application Ser. No. 354,641, filed on May 19, 1989, (Kennametal Inc. Case No. K-0963), but without the cleaning step utilized therein.

The silicon carbide whisker starting material had a diameter of about 0.3 to 0.7 microns, and a length of about 20 to 50 microns (see FIG. 4). The silicon carbide whiskers were purchased from Tokai Carbon Co. (>98 percent pure, grade No. 2; however, higher purity grade No. 1 may also be used).

The alumina (>99 percent pure, ALCOA A16-SG) had a median particle size after milling of about 0.5 to 0.6 microns.

The magnesia starting material had an average particle size of about 1.0 micron.

The foregoing particulate materials were measured out in the proportions required to produce about 60 grams of each of the nominal compositions (A through I) shown in Table I. Each mix was prepared by first ultrasonically dispersing the titanium carbide whiskers, silicon carbide whiskers and/or substantially equiaxed titanium carbide particles in propanol for about 20 minutes. Weighed amounts of alumina, dispersed titanium carbide whiskers and silicon carbide whiskers, and magnesia sintering aid were thoroughly, but gently, blended in a ball mill for about 30 minutes using propanol as the solvent and alumina cycloids as the media. The mixed slurry was pan dried, and passed through a 100 mesh screen. Each mix was then hot pressed in a one inch diameter graphite die using a pressure of about 4,000 psi under argon of one atmosphere at the approximate temperature shown in Table I to produce fired billets having a density of at least 98 percent of theoretical density.

Typical cross sections through resulting compositions are shown in FIGS. 5 and 6. In FIG. 5, the large white phase is substantially equiaxed titanium carbide particles, the acicular light gray phase is silicon carbide whiskers and the dark gray matrix in which the foregoing phases are substantially homogeneous dispersed is alumina containing the residue (not visible at this magnification) of the magnesia sintering aid. FIG. 6 is similar to FIG. 5 except that this composition contains large diameter titanium carbide whiskers (white phase) instead of equiaxed titanium carbide particles.

The foregoing billets were then sectioned and ground into SNGN-433T style (American National Standard Designation in accordance with ANSI B212.4--1986 (cutting edge preparation: 0.008 inch.times.20.degree. chamfer)) indexable cutting inserts. An example of such a cutting insert 10 is shown in FIG. 7. The cutting insert 10 has a rake face 30, a flank face 50 and a cutting edge 70 at the junction of the rake and flank faces. The cutting edge 70 is preferably in a chamfered condition as mentioned above and shown in FIG. 7.

These materials were then subjected to hardness testing (Rockwell A), fracture toughness testing by the Palmqvist method (Evans and Charles, Fracture Toughness Determination by Indentation," J. American Ceramic Society, Vol. 59, No. 7-8, pages 371, 372, using an 18.5 kg load) and cutting tests the results of which are described in Tables I and II and plotted in FIGS. 8 and 9.

Review of these test results lead us to believe that, to obtain optimum combinations of flank wear resistance in cutting soft steel and fracture toughness, the titanium carbide phase content of the material preferably should, at least, be equal to the silicon carbide content, and most preferably, greater than the silicon carbide content. In addition, it is our belief that, for optimum toughness and wear resistance, the average titanium carbide whisker diameter should preferably be between about 1 to 10 .mu. and, more preferably, should be greater than the average silicon carbide whisker diameter.

It can clearly be seen that the present invention provides a range of compositions containing a combination of K.sub.IC fracture toughness exceeding 5.5 MPam.sup.1/2, and more preferably, exceeding about 6 MPam.sup.1/2, in conjunction with high flank wear resistance in the high speed roughing of soft steels such as AISI 1045 steel. Preferably, these cutting tools in accordance with the present invention are characterized by a flank wear resistance of greater than 400, more preferably greater than 500, and most preferably greater than 650 minutes/inch when turning AISI 1045 steel having a hardness of 180-195 BHN at a speed of 1000 surface feet/minute (sfm), a feed rate of 0.024 inch/revolution (ipr) and a depth of cut (doc) of 0.100 inch.

These compositions in accordance with the present invention also provide cutting tools having improved thermal shock resistance compared with similar compositions containing titanium carbide without silicon carbide. Silicon carbide increases the hardness and the thermal conductivity, and decreases the thermal expansivity, of these materials. The combination of the improvements in thermal conductivity and expansivity in conjunction with high fracture toughness, provides the aforementioned improvement in thermal shock resistance.

The combination of properties possessed by the present invention--improved flank wear resistance in the high speed roughing of soft steels, fracture toughness and thermal shock resistance--cannot be found in the prior art alumina compositions containing only silicon carbide or titanium carbide as a reinforcing agent.

It is contemplated that the ceramic compositions in accordance with the present invention will be useful as cutting inserts with or without a coating. If coated, they may be coated with one or more refractory coatings such as alumina with or without titanium nitride as described in our U.S. Pat. No. 4,801,510.

It is further believed that the manufacturing cost of the present invention may be reduced if the hot pressing step utilized herein to at least substantially fully densify the substrate is replaced by the densification method described in P. K. Mehrotra et al copending U.S. Pat. No. 4,820,663, the whisker containing ceramic substrate is sintered to substantially full density by a method including the steps of: (a) forming a compact of a sinterable ceramic composition; (b) applying a coating to the compact by vapor deposition of a ceramic composition that does not become vitreous before or during the subsequent isostatic pressing step; and (c) heating and isostatically pressing the coated compacts in an atmosphere that reacts with the coating and/or the compact at pressing temperatures and pressures to cause the compact to approach theoretical density. U.S. Pat. No. 4,820,663 and all other patents, patent applications and publications referred to herein are hereby incorporated by reference.

Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein.

For example, it should be recognized that one skilled in the art can develop optimized compositions within the scope of the invention for machining carbon steel or other materials, perhaps using machining conditions differing from those used herein.

It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims: