GB2245557A - Metal-ceramic composites - Google Patents

Abstract

A compacted and sintered metal-ceramic composite comprises sintered grains of a ceramic matrix and sub-micron particles of a metal, selected from Pt, Pd, Mo, Ru and W, preferably wholly included within the ceramic grains. The composites are used in metal cutting tools and as catalysts. The grains and/or metal particles may be produced by sol-gel techniques.

Description

IHPROVEHENTS IN HATERIALS This invention concerns improvements in materials, and more especially it concerns metal modified ceramic matrix composites.

It is known to incorporate toughening materials such as metal, carbide, nitride or oxide fibres or particulates into ceramic materials for engineering applications, because so-called "engineering ceramics" are inherently brittle. It is also known, from GB 1,283,608, to incorporate iridium metal into uranium dioxide ceramic fuel rods for nuclear reactors, to reduce disastrous disintegration caused by fission gas evolution. The iridium metal is distributed in the uranium dioxide ceramic as an intergranular grain boundary precipitate.

It has recently be reported in "Chemistry of Materials" Vol. 1, 6, 1989, pp 576-578, that it is possible to disperse palladium in silica by forming palladium oxide particles in a silica xerogel, and subsequently reducing to palladium metal. We do not consider that a ceramic material based exclusively on silica would have adequate properties for engineering applications such as cutting tools. It is noted that no use of such composites is postulated in the paper.

It has also been reported in Mat. Res. Bull. 19, 1984, pp 169-177, that ceramic-metal composites may be obtained by sol-gel technology, and the incorporation of Cu, Pt and Ni into alumina, zirconia, silica and titania was achieved. We are not aware, however, that there has been any practical application of this either in the form of study of the engineering properties of the composites or in the form of any commercial product.

Our studies using conventional powder processing technology show that when a dense composite is formed, as would be required for engineering applications, under conditions of heat and pressure, there is agglomeration and sintering of the metal particles. The agglomeration is undesirable because it reduces the numerical distribution of metal particles throughout the composite, which reduces the probability of a metal particle interacting with a crack propagating through a composite. Additionally, such agglomerated metal particles may be large enough to cause crack initiation-in the ceramic matrix.

There remains a need for a ceramic material which is capable of acting as an engineering ceramic, having desirable toughness and resistance to cracking or disastrous failure.

The present invention provides a compacted and sintered metal-ceramic composite material comprising a ceramic matrix of sintered grains, preferably selected from zirconia, alumina, silicon nitride, silicon carbide, titanium carbide, titanium nitride, titania and mixtures thereof, and dispersed sub-micron size particles of a metal selected from platinum, palladium, molybdenum, ruthenium, tungsten and mixtures thereof. Preferably, said particles are substantially wholly included within said grains.

The invention further provides a method of producing the material of the invention which comprises compacting and sintering under conditions of heat and pressure, a composite material comprising grains of a ceramic, preferably selected from grains of zirconia, alumina, silica, silicon nitride, silicon carbide, titanium carbide, titanium nitride, titania and mixtures thereof having dispersed sub-micron size particles of a metal or precursor of a metal selected from platinum, palladium, rhodium, molybdenum, ruthenium, tungsten and mixtures thereof.

The ceramic grains for compaction may conveniently be prepared by sol-gel techniques which are modification of methods known per se, and which are more particularly described in the Examples hereinafter. Other preparations may be used in place of or in combination with the sol-gel technique, and in particular the deposition of sub-micron sized particles of Mo onto grains of composite containing Pd, Pt etc. is to be considered.

The ceramic grains may suitably be in the range 0.05 to 1 micron, and the metal or precursor particle sizes are suitably 2 to lOOOnm.

The preferred quantity of metal particles relative to the composite generally varies with their particle size distribution, and in general, it is appropriate to use larger amounts with smaller particle sizes. Tests show that the fracture toughness of composites peaks at an interparticle distance approximately identical to the particle diameter. It is possible to calculate that a metal loading of 32% offers ideal interparticle distances for the range of particle sizes of prime interest, although other properties and cost will of course influence the actual loading chosen.

Preferred composite materials are those in which the metal is Pt, Pd, Mo or a mixture of Mo with Pt or Pd.

The compaction and sintering are done under conditions to achieve the desired result, and these are suitably compaction of the composite material into a mould of desired size and the application of a pressure of at least about lOMPa and a temperature of at least about 13500C for 30 mins. Desirably, for most engineering applications, densification to a nominal 100% of theoretical maximum is achieved; for a palladium-alumina composite for example, suitable conditions are a temperature of 13500C to 15100C under a pressure of 30MPa for 30 minutes under a controlled inert and/or reducing atmosphere. Hot Pressing or Hot Isotatic Pressing ("HIP") processes may conveniently be used.Those skilled in the art are aware that additional components may be added to the composite material for such purposes as to prevent grain size growth during subsequent processing or in use, and small amounts of refractory oxides may be used providing there is no undesirable effect on performance parameters which are critical for the intended use.

It has been found that the new composite material of the invention, especially when compacted and sintered to 100% nominal density, offers interesting properties as a cutting tool for steel.

Other properties continue to be studied, which indicate potential uses as tools or tool parts and as an engineering grade ceramic material with diverse applications including engine parts, especially for internal combustion engines. The materials demonstrate good corrosion and tribological properties. The materials have also been shown to demonstrate a surprising catalytic activity, for materials that had been sintered at approaching lSOO0C, and having a low surface area. Conventional catalyst studies indicate that there is little or no exposed metal surface, and hence those skilled in the art would expect a negligible catalytic activity. The materials may find uses in engine parts such as cylinder liners or heads, valves, turbocharger parts, etc., or, for applications where high mechanical performance is not required (in comparison to machine tools or engine parts) such as burner heads, a less dense sintered composite is indicated, which will have a higher surface area. For specific catalytic use, the composite material may include other components to promote and/or stabilise desired catalytic effects.

The invention will now be described by way of example only.

EXAMPLE 1 A palladium sol was prepared by dissolving 1.5g of palladium chloride in lOOml of distilled water containing lml of concentrated HC1. This was then made up to 250ml with distilled water. A solution containing 4g of poly(vinylpyrollidone) in SOml of water was added to this. The pH of this solution was adjusted to 7 by addition of NaOH. 25ml of ethanol was added and the solution refluxed for 4 hours. A clear black solution formed after approximately 1 hour.

An alumina sol-gel was prepared by adding 25g of aluminium sec-butoxide to 25ml of boiling water with vigorous stirring. This solution was refluxed for 1 hour resulting in a turbid solution. To this was then added 20ml of water containing 1.5ml of concentrated HC1 and the solution refluxed for a further 3 hours.

The palladium sol was then added to the gel with stirring and then regelled at room temperature in air. The solid was dried at 900C for 48 hours. The dried solid was ground in an agate mortar and pestle and then reduced at 5000C for 30 minutes under an atmosphere of nitrogen containing 10% hydrogen, resulting in a black composite powder. TEM analysis of this powder showed that the metal particles present in the ceramic host were only marginally bigger than the initial metal particles of the starting sol, and were encapsulated within the alumina particles. The metal particles were approximately 15-20nm in diameter.

The dried black composite powder was then compacted and densified by HIP or hot pressing. The resulting 1% by wt Pd in Al203 composite was considered as composite material 1, and tested for mechanical properties as described below.

EXAMPLES 2-5 1% Pt and 1% Mo black in a conventional alumina ceramic (composites 2 and 3 respectively) were prepared and tested.

The composite materials were tested for flexural strength and fracture toughness in comparison with a ceramic consisting of solely alumina but treated in the same way, and the results are shown in Table 1 below.

TABLE 1 Flexural Strength Alumina Composite 2 Composite 3 (N/mm2) 550 630 599 Fracture Toughness (MPa m2) 4.4 4.8 5.6 The composites 1, 3 and 5 were tested for performance as cutting tools, in comparison with a commercial ceramic composite consisting of zirconia and alumina (comparison). Test 1 was a medium speed cutting test on steel, with time to breakage given in minutes; Test 2 was identical except that the workpiece was rotated at high speed, and Test 3 was a shock test involving cutting a steel piece with an interrupted profile. The results are shown in Table 2 below.

TABLE 2 Test 1(mien) Test 2(min) Test 3 (arbitrary units) Comparison 10 5.4 32 Composite 1 6.2 2.0 5 Composite 2 8 6.7 6 Composite 3 7.6 5.7 40 It can be seen that the composites of the invention, while not showing an improvement over existing ceramic technology at medium speed, do show better performance at high speeds, and the lUMo composite shows a marked improvement in the shock test.

Other composites were prepared with 0.1 wt % Pd and 0.5 wt % Mo black in similar manner.

Claims (15)

1. A compacted and sintered composite material comprising a ceramic matrix of sintered ceramic grains and dispersed sub-micron size particles of a metal selected from platinum, palladium, molybdenum, ruthenium, tungsten, and mixtures thereof.

2. A material according to claim 1, wherein the ceramic grains are selected from zirconia, alumina, silica, silicon nitride, silicon carbide, titanium carbide, titanium nitride, titania and mixtures thereof.

3. A material according to claim 2, wherein the ceramic grains are alumina.

4. A material according to any one of the preceding claims, wherein the metal is platinum, palladium, molybdenum, or a mixture of molybden um with platinum or palladium.

5. A material according to any one of the preceding claims, wherein the ceramic grains have a particle size of 0.5 to 1 micron.

6. A material according to any one of the preceding claims, wherein the metal particle sizes are from 2 to lOOOnm.

7. A material according to claim 1, substantially as hereinbefore described.

8. A machine tool incorporating a metal-cutting part of a material according to any one of the preceding claims.

9. The use of a material according to any one of claims 1 to 7, as a catalyst.

10. A method of producing a composite material according to any one of claims 1 to 7, which method comprises compacting and sintering under conditions of heat and pressure, a composite material comprising grains of a ceramic having dispersed sub-micron size particles of a metal or a precursor of a metal selected from platinum, palladium, rhodium, molybdenum, ruthenium, tungsten and mixtures thereof.

11. A method according to claim 10, wherein the metal particles are substantially wholly included within said grains.

12. A method according to claim 10 or 11, wherein the grains and/or the metal particles are produced by sol-gel techniques.

13. A method according to claim 10, 11 or 12, wherein the compaction and sintering is carried out at a pressure of at least 10 MPa and a temperature of at least about 13500C.

14. A method according to any one of claims 10 to 13, wherein compaction and sintering effects densification to 100% of theoretical maximum.

15. A method according to claim 10, substaintially as hereinbefore described.