GB2299579A

GB2299579A - Water resistant ceramic articles

Info

Publication number: GB2299579A
Application number: GB9605413A
Authority: GB
Inventors: Graham Paul Dransfield; Timothy Andrew Jennett
Original assignee: Tioxide Specialties Ltd
Current assignee: Tioxide Specialties Ltd
Priority date: 1995-04-01
Filing date: 1996-03-14
Publication date: 1996-10-09
Also published as: GB9605413D0

Abstract

A zirconia ceramic article is exposed to water at at least 60{C. At least the surface of the article which is exposed to water is prepared by forming an aqueous suspension containing particulate zirconia and particulate yttria, agitating the suspension in a bead mill for at least 2 minutes using a mixing medium having an average particle size of from 0.2 mm to 3 mm, separating and drying the mixed particulate material so formed, shaping said mixed particulate material into a green body and sintering said green body at a temperature in the range 1200{C to 1450{C. The amount of yttria used is at least 4 molar per cent with respect to zirconia and less than 1 per cent alumina by weight with respect to zirconia is present in the green body when said body is sintered. The articles mentioned include components of internal combustion engines e.g. valve guides, valves and valve seats.

Description

WATER RESISTANT CERAMIC ARTICLES This invention relates to water-resistant ceramic articles prepared from stabilised zirconia and in particular to the use of such ceramic articles in applications involving contact with hot water.

Ceramics formed from a composition containing zirconia and an agent such as yttria which has the effect of stabilising the zirconia in the cubic and/or tetragonal phase at room temperature are well known. Such ceramics have desirable properties and, in particular, are tough.

However, generally, ceramics which have been prepared by stabilisation of zirconia with yttria are not considered to be useful in situations involving prolonged contact with hot water since contact with the water causes degradation. Degradation is rapid at a temperature of about 200"C but is also of significance at temperatures above about 60"C.

A number of patent applications have been published recently which describe the preparation of ceramic articles from yttria-stabilised zirconia, said articles being resistant to attack by hot water.

It is an object ofthis invention to provide an altemative and surprisingly effective means of preparing ceramic articles which are resistant to attack by hot water.

According to the invention, a method of using a ceramic article comprises exposing said ceramic article to water at a temperature of at least 60"C wherein at least the surface of said ceramic article which is exposed to the water is formed from a ceramic material which is prepared by forming an aqueous suspension containing particulate zirconia and particulate yttria, said yttria being present in an amount equal to at least 4 molar per cent with respect to moles of zirconia, agitating said suspension in a bead mill in the presence of a particulate mixing medium for at least 2 minutes, said mixing medium having an average particle size of from 0.2 mm to 3 mm, separating and drying the mixed particulate material so formed, shaping said mixed particulate material into a ceramic green body and sintering said green body at a temperature in the range 1 2000C to 1 4500C and wherein less than 1 per cent alumina by weight with respect to zirconia is present in the green body when said green body is sintered.

It is preferred that the particulate zirconia and the particulate yttria have a small particle size. A convenient indicator of average particle size of a powder is a measurement of the specific surface area of the powder by a BET nitrogen desorption method. Preferably the zirconia used in the method of the invention has a specific surface area as determined by this technique greater than lm2/g. More preferably, the specific surface area is from 5 to 25m2/g.

The specific surface area of zirconia depends to some extent upon the surface characteristics and the shape of the particles but usually an average particle size of less than 1.0 micrometre is preferred and more preferably the average particle size is from 0.1 to 0.5 micrometre, the particle sizes being determined using a Malvern MS20 Mastersizer.

A particularly useful form of particulate zirconia is produced by the vapour phase oxidation of an oxidisable vaporised zirconium compound.

Typical zirconium compounds which can be oxidised in the vapour state are the zirconium halides, particularly zirconium tetrachloride. The oxidation in the vapour state usually is carried out by mixing the zirconium tetrachloride with an excess of heated oxygen under such conditions that oxidation of the zirconium tetrachloride takes place and the desired sized zirconia is obtained directly on cooling and separating from the gas stream. A preferred method of heating the oxygen to react with the zirconium tetrachloride is to pass the oxygen through an electric arc between two electrodes supplied with electrical power at an appropriate voltage and amperage in order to generate a so-called electrical plasma flame.This form of manufacture of the particulate zirconia has an advantage in that the product is obtained in the oxide form directly and that the oxidation process can be controlled so as to control the particle size of the product. A product which is very useful for carrying out the process of the invention is formed by the oxidation of zirconium tetrachloride in a plasma flame and has a specific surface area of from 10 to 20m2/g.

The particulate yttria preferably has a specific surface area as determined by BET nitrogen desorption greater than 1 mug and preferably greater than 5m2/g. It has been shown that particulate yttria having a specific surface area about 50m2/g can be used in the method of the invention to produce a useful mixture of zirconia and yttria. Preferably, however, the specific surface area of the yttria is less than 25m2/g.

As is the case with zirconia, the relationship between particle size and specific surface area depends to some extent on surface characteristics and particle shape. Usually, however, yttria useful in this invention has an average particle size less than 10 micrometre and preferably less than 1.0 micrometre.

Particles having an average size as small as 0.05 micrometre have been shown to be useful but, generally, an average size above 0.1 micrometre is preferred, the particle sizes being determined using a Malvern MS20 Mastersizer.

It is, however, possible to utilise zirconia and/or stabilising agents having a particle size somewhat larger than these preferred ranges since the mill employed in the method of the invention is capable of reducing the particle size of powders. When it is desired to reduce the particle size of the zirconia and/or yttria the period during which the suspension is agitated is adjusted to ensure the necessary size reduction. it is preferred that the period during which the suspension is agitated is such that at least 90 per cent by weight of the yttria has a particle size less than 2 micrometre at the end of the period.

Alternatively, the zirconia and/or the yttria can be separately milled to reduce the particle size before introducing them to the bead mill.

The proportions of zirconia and yttria used in the invention are such that the amount of yttria is more than 4 molar per cent with respect to moles of zirconia. Preferably at least 5 molar per cent is used and, generally, the preferred amount is up to 10 molar per cent with respect to moles of zirconia.

The invention is also characterised by a relatively small proportion of alumina in the mixture which is prepared for sintering. The proportion of alumina present is less than 1 per cent by weight with respect to zirconia and preferably the proportion is less than 0.5 per cent by weight with respect to zirconia. Most preferably, the sintering takes place in the substantial absence of alumina although trace quantities of alumina do not have any significant detrimental effect on the properties of the ceramic article finally prepared.

If alumina is deliberately added it may be incorporated into the aqueous suspension during mixing of zirconia and yttria or may be subsequently mixed with the mixed particulate material.

An aqueous suspension of the particulate zirconia and the particulate yttria is used and this may be formed by stirring the particulate materials with water. Preferably, however, a dispersing agent is added to aid the dispersion of the particulate material in the water. Generally, an organic dispersing agent is preferred and suitable dispersing agents include alkanolamines such as monoisopropanolamine and polymeric derivatives of acrylic acids and salts thereof such as ammonium polyacrylate.

When a dispersing agent is used it can be mixed with the particulate materials to form a filly dispersed system before agitation in the bead mill or, alternatively, the particulate materials may be partially dispersed initially and the agitation produced by the bead mill is relied upon to complete the dispersion process.

The equipment used to agitate the suspension of zirconia and yttria is known as a bead mill and is characterised in that it uses a particulate mixing medium to effect agitation and mixing. This particulate mixing medium has an average particle size of from 0.2 mm to 3 mm. Preferably, the average particle size is from 0.5 mm to 1.5 mm and most preferably from 0.6 mm to 1.2 mm.

The particulate mixing medium is suitably sand, glass beads or ceramic beads. Preferably, ceramic beads formed from yttria stabilised zirconia are used as the mixing medium.

In the mixing step of the method of the invention, the aqueous suspension is agitated in the bead mill for at least 2 minutes. The optimum period for which the aqueous suspension is agitated depends upon a number of factors including the design and efficiency of the mill used, and the concentration of the suspension. Generally, it is preferable to agitate for at least 5 minutes and frequently agitation continues for at least 8 minutes.

Usually, agitation for greater than 45 minutes will produce no additional benefit. The mixing step of the invention can be operated as a batch process or a continuous process. In a batch process the aforementioned minimum time of 2 minutes refers to the time for which a batch is agitated in the bead mill.

When the bead mill is operated continuously the minimum time of2 minutes refers to the average residence time of the aqueous suspension in the mill. It is sometimes convenient to pass the aqueous suspension through the bead mill more than once or to pass it through more than one bead mill. In such circumstances the time of agitation refers to the total time or total average residence time for which the aqueous suspension is agitated.

Suitable bead mills include those equipped with one or more agitators and particularly useful are those mills which operate at high speed. For example, mills which operate at a speed above 500 revolutions per minute have been found to be useful but usually a speed of 1000 to 6000 revolutions per minute is preferred and typically a speed of 2500 revolutions per minute is used. Agitator mills in which the tip speed of the agitator is 10 metres per second or greater are of use. If desired the bead mill can be cooled.

If required, other additives which are conventionally used in ceramic compositions such as sintering aids, binders or colours can be added to the aqueous suspension before, during or after the suspension is agitated in the bead mill.

After agitation in the bead mill the particulate mixing medium is separated from the aqueous suspension and the mixture of zirconia and yttria is dried to form a powder which is subsequently formed into a ceramic article by shaping and firing. The mixture may be separated from the water by, for example, filtration and subsequently dried by heating but, preferably, the suspension is passed to a spray dryer for conversion to a dried powder.

The method employed to shape the mixture into a ceramic green body depends on the shape of the article which is to be made but generally the mixture will be shaped by die pressing or moulding. Suitable die pressing techniques include uniaxial pressing and isostatic pressing. If desired the mixture can be mixed with a binding agent prior to shaping.

After shaping the green body is fired at a temperature in the range of 1 2000C to 1450"C to form a sintered article. Usually the sintering temperature is not greater than 1400"C and preferably the green body is fired at a temperature between 1300"C and 1400"C.

The resistance to attack by hot water of the articles produced for use in this invention can be assessed by immersion of the sintered articles in distilled water in an autoclave under pressure, if necessary, following the method of Nakajima et al., Advances in Ceramics, Volume 12, pages 399-403 (1984). Yttria-stabilised zirconia ceramic articles prepared by conventional methods have been shown to degrade on exposure to water at about 60"C and are very rapidly destroyed by water at a temperature of 1 800C to 2000C.

Ceramic material which has been formed using the method described hereinbefore has been shown to be extremely resistant to degradation by water.

Consequently, the use of such material to form at least the surface of a ceramic article enables the article to be used in applications where exposure to water above 600C occurs. The ceramic articles are particularly useful in situations where exposure to water above 100 C occurs and exposure of the surface water at a temperature of 1 800C for several hundred hours has been shown to result in little or no loss in strength using the aforementioned test.

According to the invention ceramic articles are employed where a surface is contactable in use with hot water. The water may be present as a liquid or vapour or may be gaseous. Typical articles include various components for internal combustion engines such as valves, valve guides, valve seats, cylinder linings, exhaust ports and piston crowns, components for mills such as impact surfaces in high pressure jet mills, rotors, impellers, shafts, casings and bearing surfaces for pumps.

In addition to their resistance to attack by hot water, the ceramics articles prepared as described hereinbefore have been shown to be resistant to attack by acids and alkalies.

This invention is illustrated by the following examples.

EXAMPLE 1 Particulate zirconium oxide, prepared by vapour phase oxidation of zirconium tetrachloride and having a mean size of 0.4 micrometre as determined by a Malvern MS20 Mastersizer was dispersed in water together with 9 weight per cent (5 molar per cent) yttria and 0.3 weight per cent hafnia based on weight of zirconia.

The overall solids content of the dispersion was 400 grams per litre.

Four portions of this dispersion were taken and alumina was added to three of the portions at concentrations of 0.15%, 0.25% and 0.5% by weight with respect to weight of zirconia. All four portions were separately subjected to attritor milling using a Netzch PE 075 mill with zirconia balls of 0.6 mm diameter at a speed of 500 rpm for 45 minutes. The pH of the milled dispersions was adjusted to a value of 9, the zirconia balls were separated and the mixed powder filtered and dried overnight at 1 1 0 C.

The resulting powders were isostatically pressed into discs of approximately 30 mm diameter and sintered at 1400" C in air, holding the samples at 1 4000C for 2 hours. The densities of the discs were measured by water immersion and the strengths tested by biaxial disc flexure. Discs were immersed in an autoclave at 180"C and a pressure of 1 MPa, following the method of Nakajima et al, Advances in Ceramics, Vol. 12, pages 399-403 (1984) and examined at intervals. The resulting densities, strengths and times to failure are presented in Table 1, the time to failure being defined as the time taken for the sample to lose sufficient strength so as to break into more than one piece.

TABLE 1

Sample % Al2O3 Sintered Density Modulus of Ageing time Ref. /Mg m-3 Rupture/MPa to failure/hrs 1/1 0 5.99 697 > 1000 1/2 0.15 5.98 737 700 1/3 0.25 5.99 739 550 1/4 0.5 5.96 879 400 EXAMPLE 2 (Comparative) Example 1 was repeated except that a sintering temperature of 1 5000C was employed. The results are presented in Table 2.

TABLE 2

Sample % Altos Sintered Density Modulus of Ageing Time Ref /Mg m3 Rupture/MPa to Failurelhrs 2/1 0 5.99 582 100 2/2 0.15 5.98 604 < 100 2/3 I 0.25 5.98 641 < 100 2/4 0.5 5.97 705 < 100

Claims

CLAIMS 1. A method of using a ceramic article comprising exposing said ceramic article to water at a temperature of at least 60"C wherein at least the surface of said ceramic article which is exposed to the water is formed from a ceramic material which is prepared by forming an aqueous suspension containing particulate zirconia and particulate yttria, said yttria being present in an amount equal to at least 4 molar per cent with respect to moles of zirconia, agitating said suspension in a bead mill in the presence of a particulate mixing medium for at least 2 minutes, said mixing medium having an average particle size of from 0.2 mm to 3 mm, separating and drying the mixed particulate material so formed, shaping said mixed particulate material into a ceramic green body and sintering said green body at a temperature in the range 1 2000C to 1 4500C and wherein less than 1 per cent alumina by weight with respect to zirconia is present in the green body when said green body is sintered.
2. A method according to claim 1 in which the particulate zirconia has a specific surface area as determined by BET nitrogen desorption greater than 1 m2g-k
3. A method according to claim 2 in which the specific surface area of the zirconia is from 5 to 25 m2g~l
4. A method according to claim 1 in which the particulate zirconia has an average particle size less than 1.0 micrometre as determined by Malvern MS20 Mastersizer.
5. A method according to claim 4 in which the average particle size of the zirconia is from 0.1 to 0.5 micrometre.
6. A method according to any one of the preceding claims in which the zirconia is produced by oxidation of zirconium tetrachloride in a plasma flame and has a specific surface area of from 10 to 20 m2g'
7. A method according to any one of the preceding claims in which the particulate yttria has a specific surface area as determined by BET nitrogen desorption greater than 1 m2g-'
8. A method according to claim 7 in which the specific surface area of the yttria is greater than 5 m2g-'
9. A method according to any one of the preceding claims in which the particulate yttria has a specific surface area as determined by BET nitrogen desorption less than 50 m2g-'
10.A method according to claim 9 in which the specific surface area of the yttria is less than 25 m2g'
11 A A method according to any one of claims 1 to 6 in which the particulate yttria has an average particle size less than 10 micrometres as determined by Malvern MS20 Mastersizer.
12. A method according to claim 11 in which the average particle size of the yttria is less than 1.0 micrometre.
13. A method according to any one of claims 1 to 6, 11 and 12 in which the particulate yttria has an average particle size greater than 0.05 micrometre as determined by Malvern MS20 Mastersizer.
14. A method according to claim 13 in which the average particle size of the yttria is greater than 0.1 micrometre.
15. A method according to any one of the preceding claims in which the particulate yttria is present in an amount equal to at least 5 molar per cent with respect to moles of zirconia.
16. A method according to any one of the preceding claims in which the particulate yttria is present in an amount up to 10 molar per cent with respect to moles of zirconia.
17. A method according to any one of the preceding claims in which less than 0. 5 per cent alumina by weight with respect to zirconia is present in the green body.
18. A method according to any one of the preceding claims in which sintering takes place in the substantial absence of alumina.
19. A method according to any one of the preceding claims in which the mixing medium has an average particle size from 0.5 mm to 1.5 mm.
20. A method according to any one of the preceding claims in which the mixing medium has an average particle size from 0.6 mm to 1.2 mm.
21. A method according to any one of the preceding claims in which the mixing medium is sand, glass beads or ceramic beads.
22. A method according to claim 21 in which the ceramic beads are formed from yttria stabilised zirconia.
23. A method according to any one of the preceding claims in which the aqueous suspension is agitated for at least 5 minutes.
24. A method according to any one of the preceding claims in which the aqueous suspension is agitated for at least 8 minutes.
25. A method according to any one of the preceding claims in which the aqueous suspension is agitated until at least 90 per cent by weight of the yttria has a particle size less than 2 micrometre.
26. A method according to any one of the preceding claims in which the bead mill is operated at a speed above 500 revolutions per minute.
27. A method according to any one of the preceding claims in which the bead mill is operated at a speed of 1000 to 6000 revolutions per minute.
28. A method according to any one of the preceding claims in which the bead mill is an agitator mill in which the tip speed of the agitator is greater than or equal to 10 metres per second.
29. A method according to any one of the preceding claims in which the suspension of mixed particulate material is converted to a dried powder by passing through a spray dryer.
30. A method according to any one of the preceding claims in which the green body is sintered at a temperature not greater than 1400"C.
31. A method according to any one of the preceding claims in which the green body is sintered at a temperature between 1300"C and 1400"C.
32. A method according to any one of the preceding claims in which the ceramic article is exposed to water at a temperature above 1 00 C.
33. A method according to any one of the preceding claims in which a dispersing agent is added to the aqueous suspension of zirconia and yttria.
34. A method according to claim 33 in which the dispersing agent is an alkanolamine or a polymeric derivative of an acrylic acid or a salt thereof
35. A method according to any one of the preceding claims in which the ceramic article is a valve, a valve guide, a valve seat, a cylinder lining, an exhaust port or a piston crown for an internal combustion engine, an impact surface in a high pressure jet mill or a rotor, an impeller, a shaft, a casing or a bearing surface for a pump.