GB2233322A

GB2233322A - Strengthening refractory oxide ceramics

Info

Publication number: GB2233322A
Application number: GB8914449A
Authority: GB
Inventors: David James Godfrey
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1989-06-23
Filing date: 1989-06-23
Publication date: 1991-01-09
Also published as: GB8914449D0

Abstract

High strength refractory oxide ceramics of silicon nitride and silicon carbide become oxidised at temperatures in excess of 1400 DEG C and their protective film of SiO2 forms glasses which flow to reduce the protection. Refractory oxide ceramics of yttria or ceria possess high melting points but are mechanically weak. These may be strengthened by producing the ceramic with upto 50 mole percent of a strengthening material selected from zirconia, silicon carbide, alumina, magnesia, tentalum carbide, zirconium carbide, aluminium nitride or silicon nitride. Zirconia is the preferred strengthening material and is preferably present in its tetragonal crystal form. Uses of the strengthened yttria and ceria ceramics are in relation to heat engine technology.

Description

Strenatheninq Refractory Oxide Ceramics This invention relates to the strengthening of refractory oxide ceramics specifically Yttria and Ceria.

Ceramics made of silicon nitride and silicon carbide offer higher strength and improved oxidation resistance over metals at temperatures above 1000or and have been seen as a means of extending heat engine technology to higher operating temperatures. However at higher temperatures in excess of 1400or oxidation becomes a serious problem and their protective oxide film of SiCz forms glasses which exhibit flow and are therefore less protective. Oxides do not require protection against oxidation, and provided they do not exhibit extreme volatility, may be serious contenders for components operating at temperatures above the practical limits for silicon nitride and silicon carbide ceramics in air or hydrocarbon combustion environments.

Alumina has a melting point of 2054or, but is generally reported to have rather poor mechanical properties at veryhigh temperature, although there is little specific information in the scientific literature on this.

Yttria Y2O3 and Ceria CeO2 have respective melting points of 2439or and 2600oC and present in this respect an improvement over silicon nitride and silicon carbide ceramics. However Yttria and Ceria do not possess sufficient mechanical strength to be viable alternatives to silicon nitride and silicon carbide ceramics.

Whilst Yttria has been incorporated up to about 30 mole percent into zirconia Zero2 to prevent undesirable phase changes taking place and to stabilise zirconia in the cubic phase no attention has been paid to the effects on Yttria of incorporating zirconia.

Consequently it is an object of the present invention to provide strengthened refractory oxide ceramics.

Accordingly there is provided a strengthened refractory oxide ceramic wherein the oxide ceramic contains up to 50 mole per cent of a strengthening material selected fran zirconia, silicon carbide, alumina, magnesia, tantalum carbide, zirconium carbide, aluminium nitride or silicon nitride.

Preferably the refractory oxide ceramic is Yttria or Ceria. The invention should not be limited in this respect and other suitable ceramics will be apparent to those skilled in the art.

Preferably zirconia is added to the ceramic.

It has been found that by producing ceramics containing up to 50 mole per cent of zirconia that the strength of Yttria and Ceria can under optimal conditions be approximately doubled with a general trend of greater mechanical strength with increased zirconia content.

Where zirconia is used to strengthen the ceramic the zirconia is preferably in the tetragonal crystal form as this may yield a tougher material. The strengthening material is preferably present in the ceramic in a finely particulate form. Preferably the strengthening material is substantially evenly dispersed throughout the ceramic material.

According to a further aspect of the present invention there is provided a method of making a refractory oxide ceramic which contains up to 50 mole per cent of a strengthening material selected from zirconia, silicon carbide, alumina, magnesia, tantalum carbide, zirconium carbide, aluminium nitride or silicon nitride which comprises the steps of: (a) providing the strengthening material in a particulate form; (b) mixing the particulate strengthening material with a divided form of the ceramic; and (c) sintering the resulting mixture to produce a modified ceramic with improved mechanical strength.

Preferably the ceramics of the method are Yttria and Ceria.

Preferably the strengthening material referred to is zirconia.

Preferably both the ceramic and strengthening material are divided to a fine particulate form and mixed together by milling. To aid this process an alcohol is preferably added and this is preferably a lower alcohol containing 1 to 6 carbon atans such as propan-2-ol.

After being milled for an appropriate duration the mixture is dried, ground and sieved. Typical of the gauge of sieve used is of the order of 50 um. Where necessary binding agents may be used with the divided ceramics and strengthening material. Typical binding agents are organic polymers and two examples are polyvinylpyrrolidine (WP) and polyethylene glycol (PEG).

The temperature and duration over which the sintering takes place will be dependant upon the individual constituents and required final properties but a typical preocedure is heating up at 60oC per hour to 1700 C holding the temperature constant and then cooling at 1200 per hour.

The invention will now be described by way of example only and with reference to the accompanying Drawings of which: Figure 1 shows a graph of strength against % probability of failure for Yttria and Ceria; Figure 2 shows a graph of strength against % probability of failure for Yttria with zirconia ceramics; Figure 3 shows a graph similar to that of Figure 2 but with different proportions of Yttria to zirconia; Figure 4 shows a graph of the mean strength of Yttria/zirconia ceramic discs with different mole percentages of zirconia; Figure 5 shows a graph of strength against % probability of failure for Yttria/zirconia and Ceria/zirconia ceramics; Example 1 - Ceramic Strength of Yttria Procedure 60g of Y203 powder (Berkshire ores lot No.7465) was mixed with 22ml of 20% polyethylene glycol 8000 binder.The powder binder mixture was over dried and sieved to 53 um, discs of 18.88mm were pressed at 11 tons/4" diameter pressure. The discs were then fired following a sequence of temperature climb rate of hour till l700oC, then held at 1700 C for 4 hours and subsequently cooled with a rate of descent of 120 C/hour. After firing the discs were broken by the ringload method, as described in GB Patent Appln 8705195, using a 6 x 2mm disc on a 9 x 4mm jig.

Example 1A - Ceramic Strength = Ceria Procedure 30g of CeO2 (Analar) powder was mixed with llml of 10% polyvinylpyrrole/ polyethylene glycol (1:1 mixture) binder. The powder was oven dried and the resulting cake ground in a silica mortar and sieved through a 53 um sieve.

18.88mm discs were pressed at 11 tons/4" diameter pressure as above. The discs were then fired and broken using the procedure described in Example 1.

The dimensions, weights and strenghts of 8 discs of Yttria and 8 discs of Ceria were measured. Figure 1 shows the room temperature strength of these discs of Yttria and Ceria to be very similar.

Example 2 - Ceramic Strength st Yttrip/Zirconia Procedure Powders of the following compositions were trade using Yttria (Berkshire ores) and Zirconate (Toyosoda Tz-0 lot Z006265P): Mass Y203 Mass ZrO2 Mole% ZrO2 Run No.

38.818 21.182 50 1 41.48 18.52 45 2 43.995 16.005 40 3 46.37 13.63 35 4 48.629 11.371 30 5 50.766 9.234 25 6 52.797 7.203 20 7 54.73 5.27 15 8 56.57 3.43 10 9 58.325 1.675 5 10 The powders were milled in plastic pots in propan-2-ol slurry with zirconia cylpebs (cylindrical pebbles) for 20 hours, dried, ground and then sieved through a 425um sieve. 20ml of 20% PEG binder was added to each powder, and the mixtures then oven dried and sieved to 53 um. 18.88 nun discs were pressed at 11 tons/4" diameter pressure and fired and broken as described in Example 1 above.The weights, dimensions and strengths of 8 samples of each run were taken and Figures 2 and 3 show the strength against % probability of failure for each sample and demonstratetheincreased strength of zirconia reinforced Yttria over Yttria alone.

The values of Mol% Zr 2 against mean strength according to the table below are shown in Figure 4 and show the tendency for strength to increase with increasing Mo1% of ZrO2.

Molt ZrO2 Mean Strength Run No 50 167.7 1 45 160.4 2 40 162.6 3 35 151.7 4 30 99.0 5 25 116.5 6 20 92.3 7 15 110.9 8 10 94.9 9 5 82.0 10 Example iL - Ceramic Strength z Ceria/Zirconia Procedure The experimental procedure was as in Example 2 except that Ceria was used instead of Yttria, and 20 mol% of Zr 2 was used.

A graph illustrating the strength of Yttria, Ceria, Yttria/Zirconia and Ceria/Zirconia is shown at Figure 5 and demonstrates the improvement in ceramic strength produced by zirconia at a level of 20 mol%.

Claims

1. A strengthened refractory oxide ceramic wherein the oxide ceramic

contains upto 50 mole per cent of a strengthening material selected from zirconia, silicon carbide, alumina, magnesia, tantalum carbide, aluminium nitride or silicon nitride.

2. A strengthened refractory oxide ceramic as claimed in claim 1 wherein the refractory oxide ceramic is Yttria or Ceria.

3. A strengthened refractory oxide ceramic as claimed in claim 1 or 2 wherein zirconia is added to the ceramic.

4. A strengthened refractory oxide ceramic as claimed in claim 3 wherein the zirconia is in its tetragonal crystal form.

5. A strengthened refractory oxide ceramic as claimed in any of the preceding claims wherein the strengthening material is present in the ceramic in a finely particulate form.

6. A strengthened refractory oxide ceramic as claimed in claim 5 wherein the strengthening material is substantially evenly dispersed throughout the ceramic material.

7. A strengthened refractory oxide ceramic substantially as herein described and with reference to the accompanying Drawings.

8. A method of making a refractory oxide ceramic which contains up to 50 mole per cent of a strengthening material selected from zirconia, silicon carbide, alumina, magnesia, tantalum carbide, zirconium carbide, aluminium nitride or silicon nitride which comprises the steps of: (a) providing the strengthening material in a particulate form; (b) mixing the particulate strengthening material with a divided form of the ceramic; and (c) sintering the resulting mixture to produce a modified ceramic with improved mechanical strength.

9. A method of making a refractory oxide ceramic as claimed in claim 8 wherein the ceramics of the method are Yttria and Ceria.

10. A method of making a refractory oxide ceramic as claimed in claim 8 or 9 wherein the strengthening material is zirconia.

11. A method of making a refractory oxide ceramic as claimed in claim 8 9 or 10 wherein both the ceramic and strengthening material are divided to a fine particulate form and mixed together by milling.

12. A method of making a refractory oxide ceramic as claimed in claim 11 wherein to aid the process of division and milling an alcohol is added.

13. A method of making a refractory oxide ceramic as claimed in claim 12 wherein the alcohol is a lower alcohol containing 1 to 6 carbon atoms.

14. A method of making a refractory oxide ceramic as claimed in claim 11, 12 or 13 wherein after being milled for an appropriate duration the mixture is dried, ground and sieved.

15. A method of making a refractory oxide ceramic as claimed in any one of claims 8 to 14 wherein binding agents are used with the divided ceramics and strengthening material.

16. A method of making a refractory oxide ceramic substantially as herein described and with reference to the accompanying Diagrams.