SiAION material and cutting tools made thereof

Abstract

A SiAlON-based ceramic particularly suited for use as a cutting tool in the high speed chip forming machining of metals composed of a SiAlON matrix including a) a phase of alpha′ SiAlON represented by the general formula of Mx(Si,Al)12(O,N)16, wherein 0<x<2 and M is at least two cationic elements, a first cationic element being 0.2 to 4 weight percent Mg and optionally between 0.5 and 15 weight percent of one or more of Ca, Sr, and Ba calculated as oxide, based on the SiAlON matrix, and a second cationic element being one or more of Y, Sc, La and the rare earth (RE) elements; b) a phase consisting of beta′ SiAlON represented by the general formula Si6-zAlzOzN8-z wherein 0<z<4.2; and c) a component containing glass, and at least one additional intergranular crystal phase that is detectable using X-ray diffraction techniques is provided.

Description

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 a...

AI Technical Summary

Benefits of technology

A significant and surprising advantage of the present invention is the unexpected results of using Mg to form one or more intergranular crystal phases that may be detected by XRD. Such elements were previously considered by the prior art to be harmful to the properties of the SiAlON body by the formation of low melting glasses, as described above.

is the unexpected results of using Mg to form one or more intergranular crystal phases that may be detected by XRD. Such elements were previously considered by the prior art to be harmful to the properties of the SiAlON body by the formation of low melting glasses, as described above.

It should be understood that no assertion is being made that any metal, oxide or nitride exists as separate phases within the ceramic unless explicitly described as a separate or dispersed phase. Thus, a reference to an amount of a component expressed as a metal, oxide or a nitride is made for the purposes of calculation only, without implying that the component is present in that form in either a precursor or final formulation.

Depending on the application for the SiAlON ceramic material of the invention, the ceramic material of this invention may also contain a substantially inert filler such as a known oxide, nitride, silicide, carbide, carbo-oxy-nitride, oxy-carbide, carbo-nitride, or boride of one or more of the elements Ti, Zr, Hf, Nb, Ta, V, Cr, Mo, W, B, Si. By “known” is meant only those compounds which are known to exist, thus excluding impossible or improbable combinations such as borides of boron. Preferably the inert filler is one or more of TiN, Ti(C,N) (with the atomic ratio of C:N between 0 and 1) Mo2C, TiC and SiC, with TiN, Mo2C and Ti(C,N) being most preferred. The inert filler is included in amounts from 1 to 50 volume percent, based on the final ceramic material. Preferably such additional particles may constitute between 1.5 and 40 volume percent. Most preferably the range is between 2 and 25 volume percent. The inclusion of the inert filler may result in somewhat softer SiAlON ceramic materials for use as a composite which contains the SiAlON matrix phases with the filler in a dispersed phase.

It has been found that the new SiAlON ceramic material can provide wear performance better than that of previously known SiAlON and / or silicon nitride cutting tools. A significant and unexpected advantage of this new material is that it combines high wear resistance and fracture resistance with low cost since it may be easily fabricated by the inexpensive cold pressing and sintering method.

Ceramic materials of the present invention having the best wear and fracture resistance properties are formed by microwave sintering, which avoids the necessity of pressure sintering.

Problems solved by technology

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.

However, existing ceramic cutting tools are inadequate due to their poor combination of hardness and toughness and processability.

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.

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.

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.

As a result of the low hardness such ceramic cutting tools do not show satisfactory wear resistance.

This equiaxed microstructure does not provide the high toughness associated with the fiber-like beta′ SiAlON microstructure.

This patent states that a density of >95% can be obtained with good retention of hot strength by an undefined pressureless sintering method; however, a method to achieve a useful product simultaneously having high density, high toughness and high hardness is not disclosed.

Many papers and patents note that a common problem is that the intergranular phases degrade the properties of ceramics.

460, all teach that these intergranular phases are undesirable because they generally cause high temperature degradation and reduction in strength.

As a result, the high temperature properties are expected to deteriorate.

. . However the material will lack good high-temperature properties.” (see col.

However these methods produce ceramic bodies that are difficult if not impossible to fully densify.

It is also taught that eliminating the additives changes the microstructure and impairs the mechanical properties (see col.

However the process is very difficult to apply in manufacturing, and the hardness is insufficient for practical application for cutting tools.

Amer. Ceram. Soc. 259 (1992))—These methods suffer from the problem that complete crystallization may be inhibited by kinetic factors and do not reduce the glass content sufficiently to be effective.

Such methods are further complicated in SiAlON materials because of their complex phase relations which in turn can produce numerous undesirable phases with even slight changes in starting compositions (see U.S. Pat. No. 5,413,972 to Hwang et al., col.

This approach restricts the growth of the fiber-like beta′ grains and thus will reduce the toughness of the ceramic.

The third phase is RE2M2-UO7-2U, where M is at least one of Hf, Zr and U. This approach also has the limitations that it is difficult to achieve due to the complexity of the phase system and difficult if not impossible to process into a useful article.

The above approaches typically produce ceramic bodies that have inferior properties or are difficult if not impossible to fully densify and fabricate into useful products.