AU645721B2 - Process for manufacturing ceramic-metal composites - Google Patents

Description

64572 1 S F Ref: 151055 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class 0 Complete Specification Lodged: Accepted: Published: I* Priority: Related Art: Name and Address of Applicant: Her Majesty The Queen in Right of New Zealand DSIR Chemistry Gracefield Road Lower Hutt NEW ZEALAND 0* 0 Address for Service: Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia to Complete Specification for the invention entitled: Process for Manufacturing Ceramic-Metal Composites The following statement is a full description of this invention, including the best method of performing it known to me/us 5845/3 The invention relates to a method of manufacturing ceramic-metal composites or "cermets" in powdered or solid form.

Background A cermet is a composite of a metal such as aluminium, tungsten, chromium and titanium, and a ceramic component such as metallic carbides, oxides, borides and nitrides. Cermets display physical properties superior to those of either component alone.

The metal provides toughness and the ceramic component provides hardness. They are used for refractory purposes, in aerospace equipment, in wear resistant high-speed tools, and in nuclear engineering devices.

Cermet composites containing iron and titanium carbide have been disclosed in US patent specification No 2,828,202. Those composites were fabricated by pre-shaping and pre-sintering titanium carbide and infiltrating molten steel into the pore structure of the titanium carbide under vacuum. These early Fe-TiC cermets however had large titanium carbide grains, partly due to limitations in the raw material particle size and partly due to grain growth resulting from the high temperature processing conditions which were necessary for the molten steel to infiltrate the titanium carbide pore structure.

A composite of iron and titanium is disclosed in British patent specification No 1,531,151. That composite was made by reacting a pre-manufactured alloy of iron and titaniuin in a nitrogen atmosphere to form a powder, cold iso-statically pressing that powder and then hot extruding the pressed powder to produce a fine grained material. The properties of this material are superior to those of the earlier Fe-TiC composites and are considered to be intermediate between those of cemented carbides (or "hard metals") such as Co-WC and those of high speed steels.

ri The hardness is superior to that of high speed steels and the toughness is superior to that of cemented carbides. The Fe-TiN material disclosed in British patent specification No 1,531,151 also has a desirable fine grained micro structure, however, a major disadvantage of this method is the expense of the pre-made Fe-Ti alloy.

The synthesis of cermets has also been described by H Lehuy, G Cliche and S Dallaire in Minerals Science and Engineering, A125 (1990) Lll-L14, however, expensive ferro-titanium alloys are again required.

Obiect of the Invention It is an object of this invention to provide a method for manufacturing ceramic-metal composite powders or solid ceramic-metal composite products which is an improvement on or at least an alternative to those methods described above.

Statement of the Invention 0 "The invention in broad terms may be said to be a method for S.o. manufacturing a ceramic-metal composite comprising blending an l iron titanium oxide with an excess of a source of carbon and heating the blend in a non-oxidizing atmosphere to a temperature sufficiently elevated to form a ceramic-metal composite in a powdered form.

Preferably the iron titanium oxide is ilmenite (FeTiO3), which occurs naturally but may also be produced synthetically, and the source of carbon is carbon, such as carbon lampblack, coal, charcoal and graphite or alternatively a material that will decompose to form carbon at an elevated temperature such as a sugar or other organic compounds.

-3- Preferably the blend is heated to a temperature of at least substantially 1200°C and most preferably between substantially 1250°C and substantially 1450 C. The blend may be heated under nitrogen alternatively under NH 3 or a mixture of hydrogen and nitrogen to form iron metal and titanium nitride, or under argon, vacuum, natural gas or methane to produce iron metal and titanium carbide.

o Preferably the cermet powder is densified, shaped and fired by art-known methods to form a solid ceramic-metal composite S product.

0 0 Preferably any of TiO 2 cobalt, nickel, molybdenum, manganese, chromium, silicon or tungsten, or their oxides, nitrides or carbides is/are added to the blend or incorporated following powder synthesis.

0 •e •..Detailed Desc-:iption In accordance with the invention iron titanium oxides, such as ilmenite (FeTiO3) are reacted with carbon in a non-oxidizing atmosphere at an elevated temperature to form a powder mixture 0" of iron metal and titanium nitride (TiN) and/or titanium carbide *0 (TiC). The iron and titanium are mixed at the molecular level in the iron titanium oxide starting material thus enabling the synthesis of very fine grain cermet powders directly from the oxides. This powder can be shaped into solid bodies to form a ceramic-metal composite by the conventional ceramic or powder metallurgical forming processes, for example pressureless sintering, hot pressing, hot isostatic pressing or hot extrusion. The iron metal matrix is capable of being modified to a steel composition by adding suitable alloying materials, such as cobalt, nickel, molybdenum, manganese, chromium, silicon or tungsten, to either the original oxide powder or the carbothermally reduced iron-titanium nitride or iron-titanium carbide powder. The cermet can be heat treated like conventional steel to increase its hardness.

-4- The process enables fine grained Fe-TiN and Fe-TiC powders to be made directly from iron-titanium oxides such as the mineral ilmenite. The iron titanium oxides can be synthetic or naturally ocurring. Naturally occurring ilmenite grains can typically range from 100-200% in diameter. It is preferable to grind the ilmenite to yield grains not substantially greater than 10, in diameter.

Iron titanium oxides, such as ilmenite, are thoroughly blended with carbon, such as carbon lampblack, coal, charcoal and graphite, or a material that decomposes to form carbon at an elevated temperature, such as a sugar or other organic compounds S. and heated in a non-oxidizing atmosphere to a temperature of at least substantially 1200 C and most preferably 1250-1450 C. The method of the present invention is most conveniently carried out at atmospheric pressure. Typically but not exclusively, the blend is heated for 2 to 4 hours.

A variety of cermet powders of iron and titanium nitride or iron and titanium carbide can be obtained depending upon the reaction atmosphere employed and the carbon content.

The overall reactions for the two processes can be summarized as follows: Formation of Fe TiN under nitrogen FeTiO 3 3C N Fe TiN 3CO J 2 (ii) Formation of Fe-TiC under argon or vacuum FeTiO 3 4C-- Fe TiC 3CO Both reaction paths follow a complex route through a number of intermediates. An excess of carbon is used to ensure that no unreacted oxide phases remain in the cermet powder. The excess (free) carbon has a useful function in that it can react with the iron to form a low melting point Fe-C phase which facilitates subsequent densification to form a solid body.

Excess carbon produced during the synthesis of Fe-TiN can be simultaneously used to synthesise Fe-TiC powder thus enabling Fe-Ti(C,N) cermets to be produced. This is possible since both TiN and TiC have substantially similar simple cubic structures.

These Fe-Ti(C,N) compositions enable the production of cermets having a wide range of properties.

The composition of the cermets produced depends on the S. atmosphere employed. Fe-TiN powder can alternatively be produced under H 2 /N (for example, containing 2-10% hydrogen) or *2

NH

3 and Fe-TiC can alternatively be produed using natural gas or S methane. In the former case the hydrogen or ammonia acts as a reducing agent to either replace or assist the added carbon. In the latter case the natural gas or methane acts as a reducing agent to either replace or assist the added carbon.

The ilmenite employed in the process of the invention may be natural or synthetic. Alternative iron titanium oxides having alternative stoichiometries may be used to produce cermet powders having alternative Fe/Ti ratios.

Additives can be blended with the ilmenite prior to reaction with the carbon to modify either the metal or ceramic phase.

For example, TiO 2 may be adder to achieve a higher Ti(C,N):Fe ratio. Cobalt, nickel, molybdenum, manganese, chromium, silicon or tungsten, or their oxides, nitrides or carbides can also serve as additives in order to modify the characterisitics of the metal phase by improving its performance (e.g by modifying the iron phase towards a high speed steel or similar composition) or changing the wetting behaviour at the ceramic-metal interface.

Alternatively the additives can desirably be incorporated following the production of the Fe-Ti(C,N) powder.

-6- Solid bodies having zero porosity have been made by hot-pressing uniaxially or isostatically pressed pre-shaped powders by conventional methods. For example, a force of 1000kg can be applied to a 25mm disc-shaped sample at 1330 C, under oxygen-free nitrogen or argon, or in vacuum.

The following examples further illustrate the method of the invention: *0 SExample 1.

Finely ground ilmenite was blended with sufficient carbon lampblack to give a stoichiometric carbon excess for the formation of Fe and TiN, thus ensuring complete carbothermal reduction of the ilmenite. The ilmenite-carbon powder mixture was gradually heated at a rate of 5 C/min in an oxygen-free nitrogen atmosphere to 1390°C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium nitride.

SExample 2.

Finely ground ilmenite was blended with sufficient carbon lampblack to give a stoichiometric carbon excess for the formation of Fe and TiC, thus ensuring complete carbothermal reduction of the ilmenite. The ilmenite-carbon powder mixture was gradually heated at a rate of 50C/min in an oxygen-free argon atmosphere to 1390 C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium carbide.

Example 3 Finely ground ilmenite was blended with sufficient carbon lampblack to give a stoichiometric carbon excess for the formation of Fe and TiC, thus ensuring complete carbothern l reduction of the ilmenite. The ilmenite-carbon powder mixture was gradually heated at a rate of 50C/min in a vacuum to 13900C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium carbide.

-7- Example 4.

Finely ground ilmenite was gradually heated at a rate of 5 C/min in an oxygen-free methane atmosphere to 1390 C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium carbide.

Example Finely ground ilmenite was gradually heated at a rate of 5 C/min under an oxygen-free methane or natural gas atmosphere to 13900C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium carbide.

e* Example 6.

Finely ground ilmenite was gradually heated at a rate of 5 C/min in an ammonia atmosphere to 1390 0 C for 2 hours. The resultant powder was an intimate mixture of iron metal and titanium nitride.

Example 7 Finely ground ilmenite was gradually heated at a rate of 5 C/min S in an H /N atmosphere to 1390 C for 2 hours. The resultant 2 2 powder was an intimate mixture of iron metal and titanium nitride.

Example 8 An intimate powder mixture of iron metal and titanium nitride, carbonitride or carbide, derived from iron titanium oxides as in examples 1-4 (above), was shaped by uniaxial or isostatic pressing at room temperature. 3% by weight of oleic acid dispersed in ethanol to ensure uniform distribution throughout the powder mixture, was employed as a pressing aid to assist compaction of the unfired body. After drying at 100 0 C, the pressing aid and residual solvent were removed by heating the shaped, compacted body in flowing oxygen-free nitrogen at C/minute to 450 C and then held at this temperature for 2 hours to completely eliminate all organic species from the compacted body. The body was then fully densified by hot-pressing at 13300C for 1 hour at 20MPa in an atmosphere of flowing oxygen-free nitrogen. Annealed iron-titanium carbonitride cermet bodies were produced which displayed porosities of less than 0.01% and Vickers Hardness values of over 1100 (for a 30kg load).

The foregoing describes the invention including preferz:ed "methods of carrying it out. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope of the invention as defined by the following claims.

S*.

A

AS

Claims (23)

1. A method of manufacturing a ceramic-metal composite powder comprising blending an iron titanium oxide powder with an excess of a source of carbon and heating the blend in a non-oxidizing atmosphere to a temperature sufficiently elevated to form a ceramic-metal composite in powdered form.

2. A method according to claim 1, wherein said iron titanium oxide is ilmenite.

3. A method according to claim 1 or 2, wherein the source of S, carbon is carbon or a material that will decompose to form carbon at an elevated temperature.

4. A method according to any one of the preceding claims, wherein the source of carbon is carbon lampblack, coal, charcoal or graphite or a material that will decompose to form carbon at an elevated temperature.

5. A method according to any one of the preceding claims, wherein the blend is heated to a temperature of at least substantially 1200°C. 04

6. A method according to any one of the preceding claims, wherein the blend is heated to a temperature of between substantially 13000C and substantially 1400 0 C.

7. A method according to any one of claims 1 to 6, wherein the blend is heated in an atmosphere of nitrogen to produce iron metal and titanium nitride.

8. A method according to any one of claims 1 to 6 wherein the blend is heated in an atmosphere of argon or in a vacuum to produce iron metal and titanium carbide.

9. A method according to claim 7, wherein excess carbon is introduced to the blend which is heated in an atmosphere of nitrogen to produce iron metal and titanium carbonitride.

A method according to any one of claims 1 to 6 wherein the blend is heated in the presence of a reducing agent.

11. A method according to claim 10,wherein the reducing agent is hydrogen/nitrogen or ammonia and the ceramic-metal composite produced is iron-titanium nitride. as

12. A method according to claim 10, wherein the reducing agent is natural gas or methane and the ceramic-metal composite produced is iron-titanium carbide.

13. A method according to any one of the preceding claims, wherein a number of additives are included in the blend for "a modifying the metal and/or ceramic phase. S"

14. A method according to claim 13, wherein TiO 2 is employed as an additive. a a

15. A method according to claim 13 or 14, wherein any of cobalt, nickel, molybdenum, manganese, chromium, silicon or tungsten, or their oxides, nitrides or carbides is employed as an additive.

16. A method of manufacturing a solid ceramic-metal composite product comprising densifying, shaping and firing a powdered ceramic-metal composite produced by the method of any one of the preceding claims to form a solid ceramic-metal composite. -11-

17. A method according to claim 16, wherein a number of additives are incorporated following powder synthesis for modifying the metal and/or ceramic phase.

18. A method according to claim 17, wherein TiO 2 is'employed as an additive.

19. A method according to claim 17 or 18, wherein any of S cobalt, nickel, molybdenum, manganese, chromium, silicon or tungsten, or their oxides, nitrides or carbides is employed as ia an additive. S

20. A method of manufacturing a ceramic-metal composite powder substantially as herein described with reference to Examples 1 to 4.

21. A ceramic-metal composite powder when prepared by the method of a.v one of claims 1 to

22. A method of manufacturing a solid ceramic-metal composite produce substantially as herein described with reference to Example

23. A solid ceramic-metal composite product when prepared by Sthe method of any one of claims 16 to 19. DATEL this TWENTY SEVENTH day of DECEMBER 1990 Her Majesty The Queen in Right of New Zealand Patent Attorneys for the Applicant SPRUSON FERGUSON