WO1998008778A1

WO1998008778A1 - Ceramic materials

Info

Publication number: WO1998008778A1
Application number: PCT/AU1997/000552
Authority: WO
Inventors: Charles S. Montross
Original assignee: The University Of Queensland
Priority date: 1996-08-29
Filing date: 1997-08-29
Publication date: 1998-03-05
Also published as: AUPO195896A0

Abstract

An engineering ceramic which is useful for producing long life components for internal combustion engines. It comprises a partially stabilised zirconia ceramic alloy having a transformable tetragonal phase, wherein the stabiliser is selected from magnesium oxide, calcium oxide and indium sesquioxide, and mixtures thereof. The ceramic alloy is prepared by sintering the mixture of oxides in the cubic solid solution region followed by controlled cooling and heat treatment.

Description

CERAMIC MATERIALS

BACKGROUND OF THE INVENTION THIS INVENTION relates to engineering ceramics and is particularly concerned with zirconia based ceramics having high thermal stability and high toughness.

Engineering ceramics are becoming increasingly important for the manufacture of components subject to wear, corrosion, stress and high temperature environments, such as the componentry of jet engines and internal combustion engines. Typical ceramics used for this purpose comprise zirconia and magnesia alloyed with strontia or yttria. Such ceramics, while presenting major advances and good utility, nevertheless have limitations which are undesirable. Such limitations include lack of thermal stability above 800°C and/or brittleness due to lack of significant amounts of transformable tetragonal phase .

OBJECT OF THE INVENTION

It is therefore an object of the invention to provide a ceramic which does not have these limitations and which has useful longevity and retained toughness, making the ceramic particularly suitable for use in internal combustion engines and the like.

SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided a partially stabilised zirconia alloy having a transformable tetragonal phase, wherein the stabiliser is selected from the group consisting of magnesium oxide, calcium oxide, indium sesquioxide, and mixtures thereof.

According to another aspect of the present invention there is provided a process for preparing a partially stabilised zironia alloy having a transformable tetragonal phase which comprises sintering a mixture of zironia with a stabiliser selected from the group consisting of magnesium oxide, calcium oxide, indium sesquioxide, and mixtures thereof, in the cubic solid solution region followed by controlled cooling and heat treatment to precipitate the transformable tetragonal phase .

DESCRIPTION OF THE INVENTION

The compositional range of the alloy is preferably 0-9.0 mole % indium sesquioxide, 0-9.0 mole % calcium oxide, and O-10.0 mole % magnesium oxide, wherein the total stabiliser content does not exceed about 10 mole

%, with the remainder being zirconia.

Most preferably magnesium oxide and indium sesquioxide are simultaneously present in the alloy composition. Suitably, magnesium oxide and indium sesquioxide are present in approximately equimolar amounts, most suitably in about 4-5 mole % amounts for each oxide . The controlled cooling heat treatment suitably consists of sintering in the cubic solid solution region, cooling to below 900°C, then reheating to between 1150° and 1300°C, and then holding for a sufficient time to produce transformable tetragonal precipitates. A typical ceramic alloy consists of 4.75 mole % ln₂0₃, 4.75 mole % MgO, and the remainder Zr0₂. A typical controlled cooling heat treatment consists of sintering in the cubic solid solution field at 1700°C for about 2 hours, controlled cooling at 200°C/hr to 800°C, holding for about 10 hours, heating at 200°C/hr to 1200°C, then heat treating at that temperature for about 50 hours, followed by cooling at 200°C/hr to room temperature. The toughness of this ceramic alloy after the 20 hours heat treatment at 1200°C was approximately 7MPaVm. The indentation technique was used for all fracture toughness values reported here.

The maximum toughness measured with this heat treatment was lOMPaVm after holding FOR 50 hours at 1200°C. The same composition when rapidly cooled from 1700°C to 1200°C and isothermally held for 100 hours would develop a maximum toughness of only 7MPaVm with 7 volume % transformable tetragonal phase .

The ceramic alloy according to the present invention has a desirable microstructure consisting of a transformable tetragonal phase in a cubic plus monoclinic zirconia matrix. This microstructure gives rise to a desirable property which results in toughness while retaining thermal stability at 1000°C for several thousand hours. The composition consisting of one or more of indium sesquioxide, magnesium oxide, and calcium oxide together with zirconia can be used for making partially stabilised zirconia components where resistance to thermal and mechanical shock for long times between room temperature and 1000°C is required. A prime application is for producing long life components for internal combustion engines such as the adiabatic diesel engine. A property of retained toughness after thousands of hours at operating temperatures is highly desired and would permit the construction of fuel efficient internal combustion engines. These engines currently cannot be made due to the lack of both thermal stability and toughness in other zirconia ceramics.

DESCRIPTION OF PREFERRED EMBODIMENT A preferred embodiment of the invention will now be described in the following example.

EXAMPLE A mixture of oxides was prepared having the following ingredients: ln₂0₃ : 4.75 mole %

MgO : 4.75 mole %

Zr0₂ : 90.5 mole % This mixture can be suitably prepared by either the ceramic method with a combination of the appropriate oxide powders or through powders produced by chemical coprecipitation and/or other methods.

To demonstrate the feasibility to precipitate the transformable tetragonal phase, a series of heat treatments were carried out at temperatures of 1400°C, 1300°, and 1200°C for times logarithmically varying from 5 to 500 hours. These treatments were conducted to form a temperature-time-transformation diagram by rapid cooling from the cubic solid solution temperature to the isothermal hold temperature. After the appropriate aging time, the sample was further rapidly cooled to room temperature .

Transformable tetragonal phase was precipitated at all temperatures from 1400°C to 1200°C. Maximums of 7 volume % transformable tetragonal phase were achieved at the temperatures of 1300°C and 1200°C for aging times of

100 to 200 hours. Polished monoclinic contents did not increase significantly till approximately 500 hours at

1300°C and 1200°C wherein they attained 20 to 30 volume %.

The maximum toughness recorded was 7MPaVm for 100 hours at 1200°C and UMPaVm for 500 hours at 1300°C.

Long term aging studies were conducted to investigate the thermal stability and the overageing and decomposition behaviour at 1000°C. Specimens that had the specified heat treatment were aged at 1000°C for 1000 hours. The mechanical properties such as strength were measured by four point bedding both before and after aging. The average strength before aging was 828Mpa and after aging at 1000°C was measured at 766Mpa, a change of - 7.5%. The Vickers hardness showed only a - 3.7% change due to aging. The fracture toughness was measured at 13.6MPaVm before aging and decreased to 13.2MPaVm, a change of - 2.7%. This same fracture toughness testing method gave a toughness of 11-13 MPaVm on Nilcra Mg-PSZ prepared by the commercial process. These values are comparable when tough specimens are measured with the indentation method. Specimens that did not have the specified heat treatment were aged at 1000°C for times to 5000 hours with specimens removed at 1000 hour intervals. Specimens aged to 5000 hours have been investigated for polished monoclinic content and transformable tetragonal phase content with the toughness and hardness also measured. For the example composition, starting from 0 volume % polished monoclinic content, the content increases to 40 volume % by 4000 to 5000 hours. The transformable tetragonal phase content increases to 5 volume % by 1000 hours then remains constant to 5000 hours. The fracture toughness reached approximately 9MPaVm by 1000 hours then slowly decreased to a minimum of βMPaVm by 4000 hours. The Vickers Hardness remained between 900 and 1000 for all heating times measured.

The specified controlled heat treatment and cooling was conducted on the alloy to demonstrate the ability to precipitate larger quantities of the transformable tetragonal phase. That composition had 8.2 volume % transformable tetragonal phase and only 0.8 volume % polished monoclinic content with an aging time of 20 hours at 1200°C. The tetragonal content then increased to 15 volume % at 50 hours aging, followed by loss of transformable tetragonal phase content at 100 hours at 1200°C. Heat treatment of this composition at 1250° and 1300°C yielded only moderate amounts of transformable tetragonal content. The hardness decreased after the maximum in toughness and with subsequent overageing of the microstructure. The composition with a maximum transformable tetragonal phase content of 15 volume %, had a corresponding toughness of approximately lOMPaVm as measured by the indentation method. This same method gave a toughness of 11-13 MPaVm on Nilcra Mg-PSZ prepared by the commercial process. These values are comparable when tough specimens are measured with the indentation method.

Comparative Example 1 A commercially produced ceramic consisting of magnesia, zirconia, and a small quantity (0.25 weight %) of strontia is sold as a tough partially stabilised zirconia. This composition was selected as a comparison to the proposed composition discussed above. This ceramic is produced with optimised properties by ICI Advanced Ceramics Pty Ltd (formerly Nilcra Ceramics Pty. Ltd.) using the ceramic method and heat treated according to patented and proprietary methods. Since this commercially available ceramic has demonstrated the feasibility to precipitate the transformable tetragonal phase, the isothermal temperature-time-transformation analysis was not conducted.

Long term aging was conducted to investigate the thermal stability of the tough partially stabilised zirconia. Five samples of MS grade Nilcra MgPSZ were aged at 1000°C for times ranging from 100, 200, 500 and 1000 hours. Polished monoclinic content, transformable tetragonal phase content, grain boundary phase content, toughness and hardness were measured. The polished monoclinic content increased rapidly for the first couple of hundred hours then asymptotes to approximately 60 volume % by 1000 hours. The transformable tetragonal content increased from 17 to 25 volume % in the first 100 hours then rapidly overaged. The transformable tetragonal phase content then decreased to approximately 5 volume % by 200 hours and 0 volume % at longer times of heating. The grain boundary phase content increases for the first 200 hours as does the polished monoclinic content. The grain boundary phase content then asymptotes to approximately 30 volume %, which is about half the polished monoclinic content asymptote. Mechanical properties, including fracture toughness and Vickers Hardness, are measured. The toughness decreases from the initial value of 13MPaVm to about 7MPaVm by 500 hours of heating at 1000°C. The toughness then remains at approximately 7MPaVm through to 1000 hours. The Vickers Hardness undergoes a sharp drop from approximately 1500 to approximately 850 by 100 hours. The hardness then remains about the same value for the duration of the aging experiments . The major changes in phase content occur within the first 200 hours, then the changes become asymptotic by 500 hours. The transformable tetragonal phase content reached a peak by 100 hours, then decreases in value by 200 hours. The polished monoclinic content increases to 40 volume % by 200 hours then asymptotes to 60 volume % by 500 hours.

Physical properties such as fracture toughness and hardness change sharply within the first 200 hours. Vickers Hardness drops by almost 50% within the first 100 hours. The fracture toughness drops from 13MPaVm to about 8MPaVm by 200 hours and levels off to 7MPaVm by 500 hours.

Summary: the undoped commercially available magnesia partially stabilised zirconia ceramic exhibits little thermal stability at 1000°C. This composition undergoes rapid degradation of properties by 200 to 500 hours at 1000°C.

Comparative Example 2 One zirconia-based ceramic is commercially sold as a tough and thermally stable ceramic. This ceramic is an alloy of magnesia, yttria and the remainder zirconia. A composition, consisting of 1.6 mole % Y₂0₃ 8.75 mole % MgO and 89.65 mole % Zr0₂, was prepared by the ceramic method as a comparison to the proposed compositions discussed above . To demonstrate whether the ceramic can precipitate the transformable tetragonal phase, a series of heat treatments were conducted at temperatures of 1400°, 1300°, and 1200°C for times logarithmically varying from 5 to 500 hours. These treatments were conducted to form a temperature-time-transformation diagram by rapid cooling from the cubic solid solution temperature to the isothermal hold temperature. After the appropriate aging time, the sample was further rapidly cooled to room temperature. The yttria-magnesia-zirconia composition was found to have little precipitation of the transformable tetragonal phase occurring above 1200°C. A maximum of approximately 6 volume % transformable tetragonal phase was measured for a sample aged 500 hours at 1200°C with a resulting toughness of 6MPaVm and a Vickers Hardness of 1050.

A long term aging study was conducted to investigate the thermal stability and the overawing and decomposition at 1000°C. Specimens that did not have the specified heat treatment were aged at 1000°C for times ranging to 5000 hours with specimens removed at 1000 hour intervals . The volume fraction of transformable tetragonal phase slowly increased from 0 to 8.5 volume % by 4000 hours followed by rapid overawing. The toughness peaked at approximately 8 MPaVm at 4000 hours while the Vickers Hardness slowly decreased to 950 by 5000 hours. The polished monoclinic content, which is an indication of degradation of the ceramic content, exhibited little growth until 5000 hours where it reached 12 volume %.

The specified controlled cooling that was applied to the india-magnesia-zirconia alloy was also applied to the yttria-magnesia-zirconia alloy. The yttria-magnesia-zirconia composition after the controlled cooling had poor overall volume % transformable tetragonal phase and very poor ratios of tetragonal phase to polished monoclinic content. With increasing transformable tetragonal phase content the polished monoclinic content also increased for all temperatures investigated. A maximum of 3 volume % transformable tetragonal phase was achieved with a resulting 4 MPavm and Vickers Hardness of 1100.

Summary: the yttria-magnesia-zirconia composition exhibits thermal stability at all temperatures studied. However, this composition shows limited ability to precipitate transformable tetragonal phase necessary for the development of transformation toughening. Yttria- magnesia-zirconia is, therefore, a brittle ceramic due to the lack of significant amounts of transformable tetragonal phase.

Conclusion Comparing the ceramic alloy of the example with the two comparative ceramic alloys, the following was noted. In Comparative Example 1, the ceramic rapidly overages and reached 60 volume % polished monoclinic content by 1000 hours. The example ceramic reached only 40 volume % by 4000 to 5000 hours under the same conditions. The example ceramic can be controlled heat treated to reach equivalent volume % transformable tetragonal phase and fracture toughness and can retain these properties for longer times than ceramic of Comparative Example 1. The example ceramic does not significantly decrease and change in strength when aged at 1000 °C for 1000 hours while the ceramic of comparative Example 1 rapidly degrades within 200 hours.

The ceramic of Comparative Example 2 is difficult to overage since it is a very stable alloy system. It takes 5000 hours at 1000°C to reach 12 volume % polished monoclinic content. It also took 4000 hours at 1000°C to reach a maximum of 8.5 volume % transformable tetragonal phase at 1000°C and was followed by rapid overawing. The specified heat treatment that was used on the ceramic of Comparative Example 2 was only able to produce a maximum of 3 volume % transformable tetragonal phase with a resulting toughness of only 4MPaVm and Vickers Hardness of 1100. The stability of the Comparative Example 2 ceramic alloy system makes it difficult to produce any transformation toughening which is essential to making tough, wear resistant ceramic engine components. Table Summarising the Properties

Heat Example Comparative Comparative

Treatment Composition Example 1 Example 2 ln₂0₃, MgO, Zrθ₂ Nilcra Mg-PSZ Y₂0₃, MgO, ZrO, MgO, SrO, Zr0₂

Isothermal Heat Transformable Not applicable Little

Treatment tetragonal phase transform- ble precipitated tetragonal phase precipitated

Long Term Aging Thermally stable Not thermally Thermally stable stable

Specified Transformable Not Applicable Little

Controlled tetragonal phase transfor -able

Cooling precipitated tetragonal phase precipitated

Summary of Tough, Thermally Tough, Not Not tough,

Properties stable thermally stable Thermally stable

Claims

1. Partially stabilised zirconia alloy having a transformable tetragonal phase, wherein the stabiliser is selected from the group consisting of magnesium oxide, calcium oxide and indium sesquioxide, and mixtures thereo .

2. A partially stabilised zirconia alloy as claimed in claim 1, in which the compositional range of the alloy is 0-9.0 mole % indium sesquioxide, 0-9.0 mole % calcium oxide, and 0-10.0 mole % magnesium oxide, wherein the total stabiliser content does not exceed about 10 mole %, with the remainder being zirconia.

3. A partially stabilised zirconia alloy as claimed in claim 1, in which the stabiliser comprises equimolar amounts of magnesium oxide and indium sesquioxide.

4. A partially stabilised zirconia alloy as claimed in claim 3, in which each stabiliser is present in an amount of about 4-5 mole %.

5. A partially stabilised zirconia alloy as claimed in claim 1 which comprises 4.75 mole % indium sesquioxide,

4.75 mole % magnesium oxide and 90.5 mole % zirconia.

6. An internal combustion engine manufactured from a partially stablised zirconia alloy having a transformable tetrahedral phase as claimed in claim 1.

7. A process for preparing a partially stabilised zironia alloy having a transformable tetragonal phase which comprises sintering a mixture of zironia with a stabiliser selected from the group consisting of magnesium oxide, calcium oxide, indium sesquioxide, and mixtures thereof, in the cubic solid solution region followed by controlled cooling and heat treatment to precipitate the transformable tetragonal phase.

8. A process as claimed in claim 7, in which the mixture is sintered at about 1700°C.

9. A process as claimed in claim 7 or claim 8, in which the controlled cooling and heat treatment comprises cooling to below 900°C, reheating to between 1150°C and 1300°C, and holding for a sufficient time to produce transformable tetragonal precipitates.

10. A process for preparing a partially stabilised zirconia alloy having a transformable tetragonal phase, which comprises sintering a mixture of about 4.75 mole % indium sesquioxide, about 4.75 mole % magnesium oxide and about 90.5 mole % zirconia in the cubic solid solution field at 1700°C for about 2 hours, followed by controlled cooling at 200°C, heat treating at 1200°C for about 50 hours, and cooling at 200°C/hr to room temperature.

11. Partially stabilised zirconia alloy having a transformable tetragonal phase substantially as herein described with reference to the non-comparative example.

12. A process for preparing a partially stabilised zirconia alloy having a transformable tetragonal phase substantially as herein described with reference to the non-comparative example.

AMENDED CLAIMS

[received by the International Bureau on 29 December 1997 (29.12.97); original claims 1 and 7 amended; remaining claims unchanged (1 page)]

1. Partially stabilised zirconia alloy having a transformable tetragonal phase, wherein the stabiliser is indium sesquioxide optionally in admixture with magnesium oxide and/or calcium oxide .

3. A partially stablised zirconia alloy as claimed in claim 1, in which the stabiliser comprises equimolar amounts of magnesium oxide and indium sesquioxide.

5. A partially stablised zirconia alloy as claimed in claim 1 which comprises 4.75 mole % indium sesquioxide, 4.75 mole % magnesium oxide and 90.5 mole % zirconia.

7. A process for preparing a partially stabilised zirconia alloy having a transformable tetragonal phase which comprises sintering a mixture of zirconia with indium sesquioxide optionally in admixture with magnesium oxide and/or calcium oxide, in the cubic solid solution region followed by controlled cooling and heat treatment to precipitate the