GB2214178A

GB2214178A - Reinforced ceramics

Info

Publication number: GB2214178A
Application number: GB8901737A
Authority: GB
Inventors: R David Malcolm Dawson; R Melissa Jane Crimp
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-01-26
Filing date: 1989-01-26
Publication date: 1989-08-31
Anticipated expiration: 2009-01-26
Also published as: GB2214178B; GB8801634D0; GB8901737D0

Abstract

A reinforced ceramic material in which a reinforcing material such as whiskers of SiC or Si3N4 is dispersed in a matrix of a ceramic material such a sialon is prepared by forming a mixed suspension of a reinforcing material and of a powder of a ceramic matrix material in an aqueous medium, the pH of the suspension being controlled so that the zeta potential at the reinforcing material is of opposite polarity to that at the powder thereby to cause the reinforcing material and the powder to agglomerate to form a coalesced, homogeneous mixture, for example in a flocculated suspension. A reinforced ceramic material is then formed by drying, for example by freeze or spray drying, followed by fabricating using methods such as hot pressing or pressing and then sintering.

Description

Reinforced Ceramics This invention relates to a method of making a reinforced ceramic material.

It is known to reinforce a ceramic material to enhance its strength and toughness for engineering applications.

For example, US Patent No 4 543 345 describes a ceramic composite defined by a matrix of a ceramic material having silicon carbide whiskers homogeneously dispersed therein, and a method of making such a composite. As observed in the above-mentioned patent, it is, however, difficult to provide a homogeneous dispersion of the whiskers in a matrix powder due to their tendency to agglomerate or cling during mixing. The latter is undesirable because it gives rise to flaws in the reinforced ceramic material.

Conventional mixing methods such as ball-milling and fluidised bed mixing are ways of dispersing whiskers to meet this problem but have the disadvantage of breaking the whiskers into shorter lengths which reduces their ability to reinforce the material.

The invention enables whiskers to be dispersed in a ceramic matrix in a way that does not cause damage thereto, and is also applicable to dispersing other reinforcing materials in a ceramic matrix. Whiskers are thin strong filaments or fibres typically of size range 0.1 to 3 micrometres diameter and 5 to 200 micrometres length.

Thus, in the invention, a method of making a reinforced ceramic material comprises the steps of (i) preparing an admixture comprising a mixed suspension of a reinforcing material and of a powder of a ceramic matrix material in an aqueous medium of a controlled pH such that the zeta potential at the reinforcing material is of opposite polarity to that at the powder thereby to cause the reinforcing material and the powder to agglomerate to form a coalesced, homogeneous mixture; (ii) drying the admixture; and (iii) treating the dried admixture to form a coherent reinforced ceramic material.

The reinforcing material in the resulting reinforced ceramic material is not damaged by the method of the invention which is therefore suitable for making ceramics reinforced with high aspect ratio whiskers. For example, in experimental demonstrations of the invention, whiskers have been found not to degrade or clump significantly.

In step (i), the admixture may be prepared by preparing a first suspension comprising the reinforcing material in an aqueous medium and a second suspension comprising the powder in an aqueous medium, the pH of each of the suspensions being such that the reinforcing material and the powder are each dispersed and do not agglomerate in the suspensions, and admixing the first and second suspensions. The pH of the first and second suspensions may be controlled by use of an acid such as nitric acid or an alkali such as aqueous potassium hydroxide. Preferably, pH is controlled using a reagent that is volatile and can therefore readily be removed in a subsequent process step.

In some cases, it may be necessary to use additional means such as ultrasonics to maintain a reinforcing material or powder in suspension.

After admixture in step (i), the pH may or may not have to be adjusted. For example, the pH's of the first and second suspensions may be such as to cause the reinforcing material and the powder to agglomerate at that pH. In step (i), electrostatic attraction causes the powder and the reinforcing material to coalesce, for example so that the powder coats the reinforcing material to form a floculated suspension (or sludge).

Determination of the magnitude and polarity of the zeta potentials of powders and materials in aqueous suspension at various pH's in order to assess their suitability for use in the present invention and establish operating conditions for the invention may be peformed by methods known in the art, for example as described in the examples herein. It should be noted that the reinforcing material or the powder or both may comprise more than one constituent having different zeta potentials. However, the invention may still work satisfactorily even if one such constituent, say of the powder, has a zeta potential of opposite polarity to that of other constituents of say the powder, provided that the nett polarity of say the powder is opposite to that of the reinforcing material.

Step (ii) may be carried out by methods known in the art such as air drying, spray drying or freeze drying.

Where a sludge is formed in step (i), step (ii) conveniently comprises filtering and drying the sludge and removing any volatile medium present by heating. Where a dispersion is formed in step (i), step (ii) conveniently comprises spray-drying or freeze-drying.

Step (iii) may be carried out by fabrication methods known in the art such as hot pressing and pressing followed by sintering.

The volume fraction of reinforcing material in the ceramic material and the particle sizes of the reinforcing material and the powder may be varied subject to product and process limitations and requirements.For example, the volume fraction in the ceramic material may be in the range 20% to 30%.

The reinforcing material may, for example be a refractory oxide such as alumina, titania or zirconia, or a refractory carbide such as silicon carbide, or a refractory nitride, such as silicon nitride. It may, for example be in particulate or whisker form where, as indicated above, the latter is preferred.

The ceramic matrix material may, for example be silicon nitride, sialon, silicon carbide, zirconia or alumina.

The invention will now be illustrated in the following examples, where silicon carbide or silicon nitride whiskers are the reinforcing materials and sialon is the ceramic matrix material.

MEASUREMENT OF ZETA POTENTIAL An electrolyte solution (0.001N) was prepared by dissolving KNO3 in deionised water using ultrasonic agitation, and the test powder or whiskers suspensed in a sample thereof at a concentration of about 0.01% by weight.

The zeta potential was then measured by means of a Malvern Zetasizer II c automated electrokinetics analyser, which determines electrophoretic parameters using the laser-doppler effect.

Some values (in mV) at different pH's and for different materials are summarised in the table below: Material pH 2.3 pH 3.0 pH 4.5 Si3N4 powder 34 35 28 A1203 powder 41 41 -9 Y203 powder 48 29 25 SiC whiskers -8 -25 Si3N4 whiskers - - -28 The polarity of the potential was positive except where otherwise indicated.

The above table was used, in conjunction with other results, to predict operating conditions for the present inventions and as used in Examples 1 and 2 below. It dshould be noted that the Deryguin-Landou-Verwey-Overheek theory predicts that the zeta potential should be greater than + 25mV for a stable suspension to form.

EXAMPLE 1 The appropriate amounts of sialon mixture (in powder form) and SiC whiskers to give a composite containing 20 volume % whiskers were weighed and placed in separate containers. The sialon powder was then slowly added to 100 ml Analar water, and dispersed ultrasonically for 5 minutes. The pH of the suspension was measured, and then, while the suspension was stirred continuously, adjusted to pH 3 to give the required zeta potential to maintain the dispersion. The SiC whiskers were similarly dispersed, the pH of the suspension was measured, and its pH adjusted to the same value. Whilst the whisker suspension was stirred, the powder suspension was slowly added, to give a homogeneous whisker/powder suspension. This suspension was frozen in an ethanol/water bath maintained at -50 C, then dried under 0.1 m bar pressure. The product was shaken to break up soft agglomerates and sifted through a 1 mm sieve.

It was then die-pressed and isostatically pressed to form 'green' composites, and sintered by methods known in the art to produce densified composite materials. SEM examination of the material showed the whiskers to be well-dispersed and to have survived the sintering process.

EXAMPLE 2 The appropriate amour.ts of sialon mixture and Si3N4 whiskers to give a composite containing 20 volume % whiskers were weighed and placed in separate containers.

The sialon powder was then slowly added to 100 ml Analar water, and dispersed ultrasonically for 5 minutes. The pH of the suspension was measured, and then, while the suspension was stirred continuously, adjusted to pH 4.5 to give the required zeta pptential to maintain the dispersion. The Si3M4 whiskers were similarly dispersed, the pH of the suspension was measured, and its pH adjusted to the same value. Whilst the whisker suspension was stirred, the powder suspension was slowly added, to give a homogeneous whisker/powder suspension.

This suspension was frozen in an ethanol/water bath maintained at -500C, then dried under 0.1 m bar pressure.

The product was shaken to break up soft agglomerates, and sifted through a 1 mm sieve. It was then die-pressed and isostatically pressed to form 'green' composites, and sintered by methods known in the art to produce densified composite materials. SEM examination of the material showed the whiskers to be well-dispersed in the 'green' state.

Claims

1. A method of making a reinforced ceramic material comprises the steps of (i) preparing an admixture comprising a mixed suspension of a reinforcing material and of a powder of a ceramic matrix material in an aqueous medium of a controlled pH such that the zeta potential at the reinforcing material is of opposite polarity to that at the powder thereby to cause the reinforcing material and the powder to agglomerate to form a coalesced, homogeneous mixture; (ii) drying the admixture; and (iii) treating the dried admixture to form a coherent reinforced ceramic material.

2. A method according to claim 1 wherein, in step (i), the admixture is prepared by preparing a first suspension comprising the reinforcing material in an aqueous medium and a second suspension comprising the powder in an aqueous medium, the pH of each of the suspensions being such that the reinforcing material and the powder are each dispersed and do not agglomerate in the suspensions, and admixing the first and second suspensions.

3. A method according to claim 2 wherein the pH of the admixture is adjusted after admixing.

4. A method according to claim 3 wherein the pH is adjusted by means of a volatile medium.

5. A method according to any of the preceding claims wherein step (ii) is carried out by air drying, spray drying or freeze drying.

6. A method acording to any of the preceding claims wherein step (iii) is carried out by hot pressing or by pressing followed by sintering.

7. A method according to any of the preceding claims wherein the volume fraction of the reinforcing material in the reinforced ceramic material is in the range from 20% to 30%.

8. A method according to any of the preceding claims wherein the reinforcing material is a refractory oxide, refractory carbide or a refractory nitride.

9. A method according to claim 8 wherein the reinforcing material is silicon carbide or silicon nitride.

10. A method according to claim 8 or claim 9 wherein the reinforcing material is in whisker form.

11. A method according to any of the preceding claims wherein the matrix material is silicon nitride, sialon, silicon carbide, zirconia, or alumina.

12. A method of making a reinforced ceramic material substantially as described herein with reference to either of the examples.

13. A reinforced ceramic material made by a method according to any of the preceding claims.