NO136971B - CERAMIC MATERIAL AND PROCEDURES FOR THE PREPARATION OF THIS. - Google Patents

Description

The present invention relates to a ceramic material

and a method for producing this, which is a particularly uniform, fine-grained aluminum oxide material.

Aluminum oxide (A^O^) and aluminum oxide compositions have for many years been used as a material where high temperature resistance and high strength are desired. For example, for refractory materials and for metalworking tools that are exposed to high speeds and high wear and tear, such materials have found wide industrial application.

There are indications that the strength of such material is in a specific relationship to the density and grain size, as denser and smaller grain sizes produce more durable tools. Great emphasis is therefore placed on coming up with ceramic cutting tools with such material properties. When aluminum oxide is used as a material in a cutting egg, fractures occasionally occur, and such fractures are believed to be due to the presence of relatively large aluminum oxide crystals or "grains" in an otherwise predominantly fine-grained structure. Much of the effort in terms of research in this area has therefore been aimed at developing methods for producing a material with a high density and uniform fine-grained structure.

The crystal growth that occurs when starting materials in the form of powder are heated so much that they sinter, is often delayed by the addition of magnesium oxide (MgO) in an amount of 0.5% or less. This heating can take place in a vacuum oven where the material's temperature is raised to between 1400 and 1550°C. It is believed that methods of this kind produce a material that has a grain size of 2-3 microns. To achieve such a result, however, heating times of more than 4 hours are required during sintering.

In terms of efficiency and production economy

it is clear that a reduction of the heating time is desirable, especially if the reduced heating time is followed by a more uniform fine-grained structure. Due to the risk of breakage in aluminum oxide tools, there is also a need for a technique that leads to an even smaller grain size to achieve greater strength.

The invention, both the ceramic material and the method are characterized by the features reproduced in the claims and the invention must be described in more detail in the form of an example. Example

α-alumina (alumina) powder with a particle size of less than 1 micron is processed or ball milled in a dry mill from 4-8 hours. Preferably, alumina from the W.R. Grace Company under the name "Grace-KA 210" is used as raw material for practicing the invention. This alumina powder has a surface area of the order of 9 m 2 /g. It also has a high degree of purity and contains an additive of 0.1% of MgO. Other aluminas may also be used, although experimental data seem to indicate that the best results are obtained with the "Grace-KA 210" material.

To maintain the purity of the alumina powder, the ball mill should also be made of pure alumina.

At the end of the painting, the powder is baked between 4 and

8 hours at a temperature of between 50 and 10Q°C. Baking the powder at 72°C seems to be an optimal temperature for this step in the process. These ball grinding and drying operations have the effect of removing excess surface gases and thereby producing a finer-grained product. The relationship between surface gas and grain size in the fully finished material is not known with certainty. However, it is possible that the surface gas. behaves as an impurity phase that causes selective grain growth at high temperatures.

After degassing, the powder is sieved through a 200 mesh U.S. standard sieve to break up any .agglomerates that may have formed. The sieved powder is placed in a high-temperature, high-pressure mold...A graphite mold in a slow vacuum or reducing atmosphere may typically be suitable for the purpose. A compression pressure of from 210-570 kgp/cm 2 is exerted in the mould.. In

in most cases it has been found that an initial packing

or "prepressing" under a pressure of 400 kgp/cm2 they provide

.best results. This prepack is then reduced to a pressure in the range of between 14 and 70 kgp/cm 2. Usually a reduction of the pressure to 70 kgp/cm 2 will give good results.

The powder and mold are placed in a hot press or other high temperature and high pressure sintering device. A protective atmosphere is established in the system to protect the mold. A vacuum/a helium atmosphere or a mixed atmosphere of helium and 8% by weight has been found suitable for the purpose.

Starting with reduced pressure on the compressed powder, the temperature of the powder and the mold is raised using induction heat to between 400 and 1000°C within 1 min. In most cases, raising the temperature up to 800°C, measured with an optical pyrometer, will give a good result. According to a feature of the invention, when the temperature is raised to e.g. 800°C, the pressure against the powder is increased to 254 kgp/cm 2 at the end of the heating period, which is from 1-3 min. Shrinkage usually begins when the temperature of 800°C is reached. This shrinkage can be observed using a linear displacement transducer which is attached to the piston which applies the pressure against the powder. When this shrinkage begins, the pressure against the powder is kept constant. Although 250 kgp/cm <2> is a preferred pressure in this connection, good results can also be achieved with pressures from 140 up to 420 kgp/cm 2 .

As the pressure continues to increase, the temperature is also increased, but at a lower rate than during the initial heating period to 800°C. During between 6 and 10 min. the maximum process temperature is reached, which is in the range from 1200-1800°C.

The best results seem to be achieved at a temperature of approx. 1600°C which is reached approx. 8 min. after the temperature of 800°C was reached. These high temperatures are observed with an optical pyrometer. This maximum temperature is maintained over a period of

from 2-6 min. and preferably for 3 min. if a maximum temperature of 1600°C is reached.

After this period at maximum temperature, the induction heat source or other heat source is switched off and the pressure on the material in the mold is reduced to zero. A cooling period of from 1-5 min. will usually be sufficient for the mold and the sintered alumina powder to cool to room temperature and can be removed from the press and from the mold.

Samples of sintered alumina, prepared as described above, have shown in carefully performed laboratory tests the following average properties:

In this connection, it is noted that the term "standard deviation" used here is equal to the square root of the arithmetic mean of the squares of the deviations of the physical experimental data from their arithmetic mean. The superior properties which on average can be obtained from sintered alumina if "Grace-KA 210" powder is used as basic raw material in the process according to the invention are obvious. It is noted that "Grace-KA 100" powder does not have 0.1% MgO crystal growth inhibitor added. During the work to establish the above-mentioned experimental data, it was found that the mixing of the samples had a great influence on the result. Chemical polishing of the samples gives e.g. more realistic data for bending strength. Mechanical polishing, on the other hand, appears to have an adverse influence on the strength of the sample being tested.

Fracture surface studies with a scanning electron microscope at 10,000 times magnification and on a representative sample of ceramic alumina material that was produced as described above show that the material has a grain size distribution as follows:

Alumina ceramic materials produced according to the principles of the invention thus have a grain structure that is different from the grain size distributions that characterize previously known materials. Crystals of much larger average size, ie 2 or 3 microns, usually occurred in these previously known structures. Consequently, a new alumina ceramic material with a fine-grained structure and better grain size distribution than has previously been achievable is proposed.

The invention is also not limited to use in connection with alumina, but can be used in connection with other metal oxides. E.g. it is possible to improve the production of uranium oxide (UO2) pellets by using the method according to the invention. A pellet density that is within k% of the theoretical maximum achievable can be achieved with the help of this pressure and. temperature rate controlled sintering process.

In order to achieve 95% of the theoretical maximum density, the powder is exposed to a maximum process temperature of the order of 800-

900°C over an 8-9 min. , heat cycle.. During this time cycle, physical pressure is exerted against the powder to be sintered. There is of course an initial or preliminary heating period of approx.

1 min. characterized by incipient shrinkage during which

period the temperature in the powder is raised rapidly and the powder is exposed to increasing physical or mechanical pressure. The resulting uranium dioxide pellets do not require grinding or other surface treatment.

end operations because they are produced in molds with the correct diameter. Eliminating a machining operation during the production of uranium dioxide pellets for reactor fuel is particularly desirable because it reduces processing costs and eliminates

are a main source of waste substances of reactive material.

The various new features that characterize inven-

nelsen is pointed out in the requirements that form part of the present specification. For a better understanding of the invention, its operational advantages and specific objectives it aims to achieve,

reference is made to the preceding description where preferred embodiments of the invention are described.

Claims (1)

]- Ceramic material consisting of grains of sintered aluminum oxide, characterized in that the grain size distribution, observed on a fracture surface with a scanning electron microscope with ten thousand times magnification, is this: 2. Ceramic material as specified in claim ], characterized in that it contains 0.1% by weight or less of magnesium oxide. 3. Ceramic material as stated in claim 1, characterized in that it has an average breaking strength of 38,200 kgp/cm2' with a standard deviation of 8620 kgp/cm 2. 4. Ceramic material as stated in claim 3, characterized in that it has an average bending strength of 5820 kgp/cm2', with a standard deviation of 1635 kgp/cm ?. 5. Ceramic material as specified in claim 4, characterized in that it has an average Knopp hardness value of approx. 2334. 6. Process for the production of the ceramic material specified in the preceding claims, characterized by the fact that the material is heated to more than 400°C during 1 min., that an increasing mechanical pressure is exerted on the material while this is heated to the aforementioned 400 C C for a o oppnco. et. maximum hot pressure, that the temperature is increased from the aforementioned 40°C to more than 1200°C in a period of approx. 7 min., that the mechanical pressure is maintained at maximum value throughout the heating process. and that said pressure and said temperature of over 1200°C are maintained for more than 2 min. 7. Method as stated in claim 6, characterized in that the step which includes heating the powder to 400°C also includes the application of mechanical pressure of approx. 70 kgp/cm<2> before the heating to 400°C begins. 8. Method as stated in claim 7, characterized in that maximum process pressure is applied. in the range 14 0-4 20 kgp/cm'.