SE409202B - PROCEDURE FOR MAKING A CERAMIC PRODUCT - Google Patents

Description

731-3794-5 Preferably, a part of the alumina present at said temperature in the mixture is obtained by adding to the starting material an aluminum compound, which at said temperature decomposes to alumina.

Preferably, at least half of the silicic acid present in the mixture at said temperature is produced by adding a silicon compound to the starting material, which is decomposed into silicic acid at said temperature.

In order to obtain the desired ingredients of alumina and silicic acid in the mixture at the temperature concerned, an aluminum silicon compound is introduced into the starting material used to prepare the mixture. At least a part of the composition present in the mixture at said temperature which forms molten glass is obtained by adding to the starting material a compound which on heating to said temperature decomposes into a glass-forming substance.

The temperature is preferably between 15 ° C and 180 ° C and the mixture is suitably subjected to pressure at this temperature.

In a first example, aluminum nitride powder of the type marketed by Koch-Light under the name 8006H and with an average particle size of 50fl 'was mixed with magnesium oxide powder marketed by Hopkin and Williams Limited under the name "light" and silicon nitride powder containing 89% an average particle size of 8fl. It is known, however, that silicon nitride powder regularly contains silicon as a coating on the individual particles and furthermore that aluminum nitride powder usually contains alumina as an impurity. It is obvious that both of these impurities affect the following reaction, which is intended to produce silicon aluminum aluminitride ceramic material, since it will add silicon, aluminum and oxygen to the reaction. Before the starting materials were mixed, the amount of impurities in the silicon nitride resp. in the aluminum nitride first by means of so-called rapid neutron activation analysis.

Using the starting material indicated above, it was found that the silicon content of the silicon nitride powder was 6% by weight and the alumina content of the aluminum nitride was 5% by weight. These powders were ground in ball mills fl for 2ü hours in isopropyl alcohol with alumina beads to give a mixture consisting of 9ü, 5% by weight of silicon nitride, 5% by weight of aluminum nitride and 0.5% by weight of magnesium oxide.

The mixture was then dried and introduced into a graphite sink, which rested on a gyafite plug, which closed one end of the sink cavity. A graphite stamp was then passed down to the powder. All graffiti surfaces that came into contact with the powder had been pre-sprayed with boron nitride to a thickness of 0.25 mm. This unit was then introduced into a press, after which the sample was hot-pressed by increasing the temperature and pressure simultaneously to 1680 ° C resp. 210 kp / cmg. The sample was kept at this temperature and this pressure until full density was reached, which is indicated by the fact that no further movement of the piston can be felt, which usually occurs after 20 minutes. The temperature was then increased to 1.60 ° C over a period of Ä minutes, maintaining the pressure at the stated value of 210 kp / cm 2. The sample was retained under these conditions for an additional approximately # 0 minutes. During the hot pressing, the mixture produced the desired single phase silicon aluminum oxynitride. In addition to what was included in the reactive composition, some silicon and alumina were added from the mill, which are believed to have reacted with the magnesium oxide during the hot pressing operation to obtain a magnesium-aluminosilicate glass, which increased the density of the product.

Namely, this was found to have a high density value of 3.09 g / cm 2, 7313794-5 with an average breaking strength of 5200 kp / cm 2 at 110,000, 14500 kp / cm 2 at 120 ° C and 3300 kp / cm 2 at 137 ° C.

The first example was repeated, reducing the quantity of glass by reducing the amount of magnesium oxide.

As expected, this was found to result in a decrease in the density and strength of the hot pressed product. Thus, when the procedure of the first example was repeated with 0.25% by weight of magnesium, the resulting density became 3.08 kp / omg and the average breaking point at room temperature was 4000 kp / cm

It is obvious that although it is easy to introduce the glass-forming agent into the starting mixture in connection with the treatment in the ball mill, if this is provided with balls of suitable material, this procedure reduces the possibilities for controlling the amount of glass produced. Preferably, a grinding technique should therefore be used, where the best control is possible, e.g. colloid grinding, and to introduce all the glass-forming components in measured quantities.

Thus, a second example was based on a mixture containing 2.5% by weight of aluminum nitride, 1% by weight of silicic acid, 1% by weight of magnesium dioxide and 95.5% by weight of silicon nitride. The mixture was introduced into a colloid mill and treated in isopropyl alcohol until the average particle size was 5%. / (, after which the mixture was dried and sieved so that any remaining lumps of material were removed.

To undergo hot pressing, the mixture was introduced as previously described into a graphite sink and then heated to 171,000 for 30 minutes, during which time the mixture was subjected to a gradually increasing pressure of uëp to 235 kp / omg.

The mixture was kept at this temperature and pressure for 1 hour, after which the temperature was raised to 177,500 and the mixture was kept at this second temperature for a further 35 minutes. At these elevated temperatures, the mixture contained the desired silicon aluminum oxynitride, but in addition to the reactive composition, a magnesium glass was formed during the hot pressing as a result of the reaction between the added silicic acid and the magnesium oxide. This glass increased the density of the material during hot pressing, so that the final product had a density of 3.2 g / cm 2 and a maximum breaking stress at room temperature of 10000 kp / cm 2 with a minimum value of 5200 kp / cm 2, and with an average value of 8000 kp / cm2. Creep tests were also performed on the product and at 120,000 it was found that it had 0.5% creep, when exposed to a load of 780 kp / cmg for 30 hours.

The second example was repeated with decreasing amount of alumina, and as expected, the density of the obtained product was reduced.

In a third example, the procedure was repeated in the previous example but with 1% by weight of silicon nitride replaced by aluminum nitride powder. It was found that this change increased the creep strength of the resulting product, so that at 120000 it showed only 0.3% creep, when subjected to a load of 780 kp / cm 2 for 100 hours. However, the strength of the material was slightly reduced compared to what was obtained in the second example. The mean breaking strength at room temperature was 7000 kp / cmz. It is believed that these results were caused by some of the silicic acid from the glass-forming silicic acid additive reacting with excess aluminum nitride, thereby reducing the overall glass content.

To further confirm the observations in the third example, the process was repeated with a slightly increased aluminum nitride content, at the expense of the silicon nitride content. In this way, it was found that it was possible to achieve further improvements in creep strength. At 4.5% addition of aluminum nitride, the product had a creep of 0.15% for 100 hours under the same conditions as indicated above. However, as the aluminum nitride content decreased, the hot pressing became more difficult, which was reflected in a decreasing density and strength of the product. With the addition of ü, 5% aluminum nitride, the hot-pressed product had a density of 3.15 g / cm2 and an average breaking strength at room temperature of 5500 kp / cm2. In the same way, there were variations in the aluminum nitride content in the starting material that could be used to modify the creep properties of silicon-aluminum oxynitride.

In a fourth example, two starting mixtures were prepared, both of which were intended to result in a silicon-aluminum oxynitride with the same composition but with different aluminum nitride contents.

One of these mixtures consisted of 83% by weight of silicon nitride, 10% by weight of aluminum nitride, 6% by weight of silicic acid and 1% by weight of alumina, while the other mixture contained 85% by weight of silicon nitride, 7.62% by weight of aluminum nitride, 3.33% by weight of silicic acid , 2.56% by weight alumina and 1% by weight magnesium oxide.

The first batch thus contained a higher amount of aluminum nitride and resulted in a product which, when subjected to a load of 780 kp / cm 2 for 100 hours at 120,000, showed only a creep of 0.057%. As expected, this was an improvement over the value of 0.108% obtained for the product prepared from the second mixture, subjected to the same treatment. Thus, even in this case, it was thus found that increasing the creep strength of the product according to the first example coincided with a decrease in the density and the mean value of the breaking strength at room temperature (3.16 g / cm 2 and 5600 kp / cm 2) compared with the product prepared from the second mixture (3.18 g / cmg and 6000 kp / cmz).

In all the examples given, the reaction took place at elevated temperature at the same time as pressure was applied to the reacting 7z1zv94fs mass. This has been done so that only as few parameters as possible, which affect the product, are changed, whereby a reliable analysis of the results obtained can be made.

Similar experiments have been performed, however, in the absence of pressure, whereby test pieces were pressed in the cold state at 140 kp / cm 2 in steel molds to obtain a self-supporting block, which was removed from the mold and coated in boron nitride in a graphite crucible. This was then placed in an oven and the heating procedure according to the examples given followed. In each case, a product was obtained consisting exclusively of the previously obtained silicon-aluminum oxynitride.

Furthermore, in the examples the reactions have taken place between 17000 and 1780 ° C. These temperature ranges were chosen because it turned out to be an area where the products could be produced in a relatively short time. However, completely satisfactory products can be obtained with temperatures as low as 120,000 and as high as 200 ° C, but it is advisable to choose 140 ° C or higher in order to obtain an economical reaction at high rates. The upper limits of temperature will obviously be determined by the economics of the process and the desire to prevent the product from being destroyed by being exposed for too long at such high temperatures. 180000, however, is a practically applicable limit.

It is obvious that where alumina has been added to the starting material, it can be replaced by any aluminum alloy which will decompose to alumina during heating to the elevated temperature necessary to produce the desired ceramic material. Where both silicic acid and alumina are added to the starting material in the above examples, the alternative may be to add to the starting material an alloy of these substances which will give off the desired amount of silicic acid and alumina at the elevated temperature and subsequent treatment. In the same way, the magnesium oxide used in the starting material can be replaced by a magnesium alloy which decomposes to oxide during the further treatment. However, it has been found that the preferred amount of alumina or aluminum alloy added to the starting material is less than 5%, preferably not more than 1% by weight of the starting material.

Claims (7)

731379l | '-5 P A T E N T K R A V 1. A process for the preparation of a ceramic product, characterized in that at a temperature of between 1200-200000 a mixture of 0 to 97.5% of silicon nitride, aluminum nitride in an amount not exceeding 50%, of silicic acid in a amount not exceeding 51.5% and alumina in an amount not exceeding 75% when silicon nitride is present in the mixture and not more than 35% when silicon nitride is absent in the mixture, all% indications indicating weight percent, and a further composition forming molten glass at said temperature, and the heating is continued until the ceramic phase of the product consists essentially only of a silicon aluminum oxynitride ceramic material with a uniform phase structure, with the molten glass as promoter of the product densification. 2. A process according to claim 1, characterized in that a part of the alumina present at said temperature in the mixture is produced by adding to the starting material an aluminum compound, which at said temperature decomposes to alumina. 3. A process according to any one of claims 1 or 2, characterized in that at least half of the silicic acid present in the mixture at said temperature is produced by adding a silicon compound to the starting material, which decomposes to silicic acid at said temperature. Ü. 4. A process according to any one of the preceding claims, characterized in that in order to supply the mixture with alumina and silicic acid at said temperature, an aluminum silicon compound is introduced into the starting material used to prepare the mixture. 10 5. A process according to any one of the preceding claims, characterized in that at least a part of the composition present in the mixture at said temperature which forms molten glass is obtained by adding to the starting material a compound which on heating to said temperature decomposes into a glass-forming substance. 6. A method according to any one of the preceding claims, characterized in that the mixture is subjected to a pressure at said temperature. A method according to any one of the preceding claims, characterized in that the temperature is kept between 15 ° C and 180 ° C. i Elm-g PRESENTED PUBLICATIONS: Germany 2,262,785