WO1988000578A1

WO1988000578A1 - Method of forming a ceramic product

Info

Publication number: WO1988000578A1
Application number: PCT/AU1987/000205
Authority: WO
Inventors: Michael Vincent Swain; Mark Brian Trigg
Original assignee: Commonwealth Scientific And Industrial Research Or
Priority date: 1986-07-10
Filing date: 1987-07-07
Publication date: 1988-01-28

Abstract

Ceramic materials which comprise a transition metal carbide, nitride or carbonitride and from 5 to 90 volume percent zirconia. In general, these materials have a bulk density greater than 75 percent of theoretical and a modulus of rupture greater than 200 MPa. The materials may be formed by mixing together a finely divided particulate transition metal carbide, nitride or carbonitride and zirconia, fabricating the mixture into a ceramic product of the desired shape, and heating the fabricated mixture in a low oxygen containing atmosphere to a temperature in the range from 1300oC to 2000oC to produce the ceramic product.

Description

METHOD OF FORMING A CERAMIC PRODUCT

The present invention relates to ceramic products and is particularly concerned with tough and dense ceramic products and with methods of forming tough and dense ceramic products.

It is widely accepted that transition metal carbides and nitrides in general possess excellent intrinsic properties such as high strength, hardness, chemical stability, thermal and electrical conductivity. However, traditional ceramic fabrication techniques such as pressureless sintering used with these materials do not lead to the production of tough and dense bodies. In order to achieve these goals in production of materials, cermet techniques are usually employed. This leads to impairment of the properties as the result of the incorporation of a metal binder phase. In addition, these metals are usually classed as being strategic materials.

An alternative technology to produce dense specimens from transition metal carbides and nitrides is by hot pressing of powders. The limitations of this technique are well known, with specimens being limited to relatively simple shapes and high costs associated with their manufacture. In addition, components fabricated in this manner tend to be brittle at room to moderate temperatures (approximately 1000°C), with corresponding low values for fracture toughness. This has limited the widespread utilization of these materials in engineering applications. It is appreciated that the brittle nature of these materials is common to ceramic materials in general.

The discovery by Garvie et al, reported in Nature, "Ceramic Steel", 258, 703-704, 1975, that zirconia materials could be toughened as a result of the crystallographic transformation of tetragonal to monoclinic phases, has created a new class of tough ceramic materials. The transformation of tetragonal to monoclinic zirconia is associated with a volume expansion. By suitable control of the starting composition and the firing cycles employed it is possible to retain metastable tetragonal grains in the final microstructure. Up to moderate temperatures and under certain applied stress fields, such as an advancing crack tip, the metastable phase is able to transform to the thermodynamically stable monoclinic phase. The martensitic type transformation results in the dissipation of the energy about the advancing crack tip and leads to the observed increases in the strength and toughness. These materials are considered low to moderate temperature (approximately 800°C) materials, as high temperature exposure leads to coarsing of the tetragonal phase and subsequent loss of toughness. Also above the equilibrium phase transformation temperature, no toughening can be derived by this mechanism.

European patent publication No.0157487 (Sumitomo Electric Industries Limited) describes a ceramic product, comprising 55 to 96% by volume of zirconia and 4 to 45% by volume of a carbonitride, carboxide, oxynitride or carboxynitride of an element in Group IVa, Va or Via.

The zirconia is essentially in the tetragonal and/or cubic forms and contains a stabilizing oxide in solid solution, for example yttrium oxide in an amount of from 2.5 to 7.5 mole percent. The products are produced by pressure-assisted sintering (hot pressing). The strength of the thus prepared materials is very modest, ranging from 60 to 170 MPa. The reference also indicates that when the level of zirconia addition is below 55% it was not possible to maintain the desired properties of the product.

It is an object of this invention to produce a ceramic product which can readily be densified and yet maintains the excellent intrinsic properties of transition metal carbide, nitride or carbonitride together with values of fracture toughness traditionally not associated with this class of material.

It is also an object of this invention to provide ceramic products which have strengths superior to the prior art materials, even when they contain less than 55% zirconia.

There is accordingly provided a method of forming a ceramic product which comprises mixing together a finely divided particulate transition metal carbide, nitride or carbonitride and zirconia, fabricating the mixture into a ceramic product of the desired shape and heating the fabricated mixture in a low oxygen containing atmosphere to a temperature in the range from 1300°C to 2000°C to produce a dense ceramic product comprised of the transition metal carbide, nitride or carbonitride and zirconia.

Further according to the present invention there is provided a ceramic material comprised of transitional metal carbide, nitride or carbonitride and from 5 to 90 volume percent zirconia.

The transition metal carbide, nitride or carbonitride is preferably represented by the general formula M(C_x,N_1+y-x) where "M" is one of the elements scandium, titanium, vanadium, chromium, zirconium, niobium, hafnium or tantalum and "x" is greater than or equal to zero but less than or equal to about 1.2 and "y" is greater than or equal to zero but less than 0.2. It is appreciated that these phases may not be stoichiometric and may also contain some oxygen picked up from the oxide layer on the non oxide powder, from the high temperature reaction with the zirconia, the furnace atmosphere or a combination of these factors.

Preferably, the transition metal carbide carbonitride or nitride has a particle size less than 10 microns and advantageously a particle size less than 2 microns. It is also preferable that the zirconia has a crystallite size less than 1 micron and, advantageously, less than 0.1 microns. The preferred range for the zirconia content is between 30 to 80 volume percent. The zirconia may be also be pre-alloyed to stabilize or partially stabilize the tetragonal modification at room and moderate temperatures by the addition of magnesium, calcium, scandium, yttrium, cerium or members of the rare earths group. It will be appreciated that it is also possible for the transition metal in the carbide, carbonitride or nitride to enter the zirconia lattice and effect stabilization of the tetragonal modification of zirconia. Conveniently, yttrium can be employed as the stabilizing agent at a rate of 1 to 6 mole percent based on the zirconia; preferably this range is 2 to 4 mole percent, in order to obtain high levels of strength and toughness.

Less than five percent zirconia may be added but the improvement in properties will generally be negligible. Further more than 90% zirconia may be added, but the advantageous properties of the transitional metal carbide,- nitride or carbonitride may then be minimised.

in addition, alumina in finely divided form, preferably with a particle size of less than 5 microns, may be added to the mixture in an amount sufficient to give up to 40 weight percent in the final product.

The transition metal carbide, nitride or carbonitride may also be alloyed with one or more other transition metals (e.g. those listed above) to form solid solutions with enhanced properties. For example, to titanium carbide, scandium may be added to improve the hardness, or chromium or niobium and tantalum may be added to improve the oxidation resistance.

Temperatures required to sinter the materials range from 1300°C to 2000°C. The preferred range is between 1500° to 1700°C, as at lower temperatures, it is difficult to achieve fully dense materials and at higher temperatures, grain growth of the zirconia crystals can occur leading to the destabilization of the tetragonal phase at room and moderate temperatures and as a consequence a reduction in the mechanical properties are observed. The firing temperature required to achieve low levels of porosity is also related to the composition of the transition metal compound employed and the amount of zirconia present. Higher temperatures are required in order to densify materials with increasing carbon to nitrogen ratio and increasing amounts of transition metal compound(s) present.

It is an advantage of the present invention that the mixed materials may be sintered to a high density at or- below atmospheric pressure in an atmosphere with a low oxygen content in the temperature range of 1300°C to 2000°C. In some instances sintering at or below atmospheric pressure may result in less than optimum density, for example, only 90%, in which case it may be followed up by the techniques known as hot pressing or hot isostatic pressing. In other instances hot pressing and hot isostatic pressing may be used alone.

The atmosphere used in the sintering operation affects the properties of the fired materials. Generally, the oxygen partial pressure of the atmosphere should not exceed about 10 ^-5 atmosphere. Nitrogen and/or argon (or other inert gas) atmospheres are the most suitable for titanium nitride based materials. When carbonitride alloys are used with intermediate carbon to nitrogen ratios, the use of argon (or other inert gas) and carbon monoxide containing atmosphere is advantageous. For carbides or other high carbon content alloys, the use of carbon monoxide containing atmospheres is most suitable.

Using the above-described procedures, in accordance with the present invention, it is possible to obtain ceramic materials with bulk densities in excess of 75 percent of theoretical and modulus of rupture (MOR) values greater than 200 MPa with a zirconia content in the range of 5 to 90 volume percent. The ceramic materials of the present invention are thus greatly superior to those of the prior art, e.g. as described in EP-A-0157487.

Ceramic materials can also be produced which have excellent electrical conductivity. This allows the materials to be spark-machined. This is an important consideration as the materials produced are extremely hard so that the alternative to spark-machining is the use of diamond cutting tools. The ceramic products of the invention may be used as cutting tools in place of tungsten carbide cermets or as components subjected to highly erosive or corrosive conditions.

The invention is further described and illustrated by the following non-limiting examples

EXAMPLE 1

Titanium nitride (Japan New Metals), having an average particle size of 11 microns and zirconia doped with 2 mole percent yttria (Toyo Soda Manufacturing Company, Japan), having a crystallite size of 230 Angstroms were milled together in iso-propanol for 20 hours. The composition of the batches is shown in Table 1. After milling, the titanium nitride had an average particle size of 1.5 microns. It is believed that the milling operation is beneficial in both reducing the particle size of the transition metal carbonitride, and breaking down agglomerates of zirconia and thus obtaining an intimate mixture of the raw materials. The slurry was then dried to powder and uniaxially pressed followed by cold isostatic pressing at 210 MPa to obtain bars. The samples were then sintered in a furnace at a pressure of 0.1 MPa. with a hold time of 60 minutes at the maximum temperature. The fired bulk density, modulus of rupture, hardness and fracture toughness of the samples is shown in Table 1.

* by volume

Note (a): This sample was hot pressed at 1500°C at 35 MPa. EXAMPLE 2

Samples of titanium carbonitrides of varying carbon to nitrogen ratios were prepared as described in Example 1 and heated in nitrogen, argon or carbon monoxide containing atmospheres. The carbon monoxide was generated in a powder bed which surrounded the specimens by the reaction of titanium dioxide with the carbon in the bed.

* Titanium carbonitride to zirconia ratio of 1:1 by volume. EXAMPLE 3

Samples were prepared as described in Example 1. The sintering operation was evaluated in four different atmospheres; nitrogen, argon, carbon monoxide containing and vacuum. The results of the sintering operation are given in Table 3. From these observations, high densities and strength are seen to be affected by the sintering atmosphere. The preferred atmosphere depends on the carbon to nitrogen ratio of the carbonitride, carbon monoxide for high ratios, carbon monoxide or argon atmosphere for intermediate ratios, and argon or nitrogen for low ratios.

11

EXAMPLE 4

Specimens were prepared as described in example 1, from titanium carbonitride with a carbon to nitrogen ratio of 3 : 7 and zirconia with varying amounts of yttria stabilizing content .

* Titanium carbonitride to zirconia ratio of 1 : 1 by volume .

EXAMPLE 5

Samples were prepared as described in example 1 and were processed either by pressureless sintering in nitrogen (as outlined above) or by hot pressing in a graphite die assembly at 1500°C at a pressure of 35 MPa. Selected properties are given in Table 5. Hot pressing resulted in higher bulk density of the fired product. This is also reflected in the improved mechanical properties such as MOR and hardness. This demonstrates that when the use of pressureless sintering techniques do not yield a high density body, it is still possible to obtain high density materials by other fabrication methods,

* by volume

EXAMPLE 6

One specimen of a titanium carbonitride-zirconia composite of composition and preparation conditions identical with sample 23 (TiC_{0 .3}N_{0 .7}-50% 2Y ZrO₂) in Table 3 of Example 3 was fabricated into a wire drawing die. This die (2mm diameter) was used for drawing nickel-silver wire, a material particularly severe on traditional hardmetal dies. It was found that a reduction of 15% of the original diameter was possible with the said titanium carbonitride material whereas the maximum sustainable with hardmetal materials was only 5%. in the drawing of two coils of wire (30-35kg each) only minimal wear was experienced and only towards the end of the second drawing was the surface finish deemed unsatisfactory. EXAMPLE 7

Specimens of titanium nitride-zirconia material with composition 60 vol% TiN the remainder 2Y-ZrO₂ were fabricated by hot pressing at 1500°C for 30 min. These were then shaped in the form of cutting tools suitable for metal machining. This material was compared with a range of current cutting tools (see Table 6) for machining of a low alloy steel (En 26) with a Vickers hardness of 317 (30kg). Details of the steel and testing procedures to enable comparison have been published by N. Gane and L.W. Stephens, Wear 88 (1983) 67-83. The extent of the flank wear, crater depth and cutting speeds required for a tool life of 10 minutes, corresponding to a flank wear of 300μm and crater depth of 140μm are listed in Table 7. The TiN-ZrO₂ material had comparable if not better performance than the alumina tool and surpassed all the other materials for continuous cutting applications.

Claims

1. A method of forming a ceramic product, characterized in that a finely divided particulate transition metal carbide, nitride or carbonitride and zirconia are mixed together, the mixture is fabricated into a ceramic product of the desired shape, and the fabricated mixture is heated in a low oxygen containing atmosphere to a temperature in the range from 1300°C to 2000°C to produce a dense ceramic product comprised of the transition metal carbide, nitride or carbonitride and zirconia.

2. A method as claimed in Claim 1, characterized in that the transition metal carbide, nitride or carbonitride has the general formula M(C_x,N_1+y-x) where "M" is one of the elements scandium, titanium, vanadium, chromium, zirconium, niobium, hafnium or tantalum and "x" is greater than or equal to zero but less than or equal to about 1.2 and "y" is greater than or equal to zero but less than or equal to 0.2.

3. A method as claimed in Claim 1 or Claim 2, characterized in that the transition metal carbide, carbonitride or nitride has a particle size less than 10 microns.

4. A method as claimed in Claim 3, characterized in that the particle size is less than 2 microns.

5. A method as claimed in any one of Claims 1 to 4, characterized in that the zirconia has a crystallite size less than 1 microns and preferably less than 0.1 microns.

A method as claimed in any one of Claims 1 to 5, characterized in that the zirconia content of the mixture is between 30 to 80 volume percent

7. A method as claimed in any one of Claims 1 to 6, characterized in that the zirconia is pre-alloyed to stabilize or partially stabilize the tetragonal modification at room and moderate temperatures by the addition. of magnesium, calcium, scandium, yttrium, cerium or another member of the rare earth group.

8. A method as claimed in any one of Claims 1 to 7, characterized in that alumina in finely divided form, preferably with a particle size of less than 5 microns, is added to the mixture so that product contains up to 40 weight percent of alumina.

9. A method as claimed in any one of Claims 1 to 8, characterized in that the transition metal carbide, nitride or carbonitride is alloyed with another metal of the transition series to form a solid solution.

10. A method as claimed in any one of Claims 1 to 9, characterized in that the heating temperature is between 1500°C and 1700°C.

11. A method as claimed in any one of Claims 1 to 10, characterized in that the atmosphere is selected from nitrogen, argon or another inert gas, carbon monoxide or mixtures of two or more of said gases.

12. A ceramic material characterized in that it comprises a transition metal carbide, nitride or carbonitride and from 5 to 90 volume percent zirconia.

13. A ceramic material as claimed in Claim 12, characterized in that it has a bulk density greater than 75 percent of theoretical and a modulus of rupture greater than 200 MPa.

14. A ceramic material as claimed in Claim 12 or Claim 13, characterized in that the zirconia content is less than 55 volume percent.