CH654564A5 - Process for making a shaped object from nitride of elements selected from groups III, IV, V or VI of the Periodic Table, in particular from silicon nitride - Google Patents

Abstract

The porous shaped object is placed into a hot-isostatic press. In the second sintering phase, the shaped object is exposed for at least 30 minutes to a nitrogen atmosphere pressure of at least 100 bar and a temperature of at least 1200 DEG C. The second sintering phase is carried out without addition of sinter aids and without an extraneous gas-tight envelope. This treatment gives a solid shaped object whose density is at least 2.0 g/cm<3> and whose HVO.05 hardness is in the range from 13,000 to 15,000 N/mm<2>. Nitriding throughout the entire shaped object is achieved.

Description

** WARNING ** Beginning of DESC field could overlap end of CLMS **.

PATENT CLAIMS 1. Process for the production of a shaped body from nitride of elements selected from the III., IV., V or VI group of the periodic table, in which process the powder of the pure element or a powder mixture of the pure element and its nitride is used as the starting material , which starting material is shaped into a porous, further machinable shaped body in a first sintering phase, characterized in that the porous shaped body is placed in a hot isostatic press, where it is subjected to a nitrogen atomic pressure for at least 30 minutes in a second sintering phase without sintering additives and without an external gas-tight envelope of at least 100 bar and a temperature of at least 1200 C, after which the pressure and temperature drop evenly.

2. The method according to claim 1, characterized in that the following combinations of pressure and temperature can be selected depending on the time: - applying first a form, later a temperature, the pressure and the temperature rise together until the holding phase, after which the pressure and the temperature drop evenly, - simultaneous application of a pressure and a temperature, the temperature first rising rapidly and the pressure being delayed to the heating-up time, after which the pressure and the temperature drop uniformly, simultaneous application of a pressure and a temperature, holding the pressure - and temperature rise according to a stair step effect, application of the pressure and the temperature to the holding phase, after which the pressure and the temperature drop evenly, - simultaneous application of a pressure and a temperature, the pressure rising rapidly first and the temperature being delayed at the time of printing, after which de r Pressure and temperature drop evenly.

3. The method according to claims 1 and 2, characterized in that the pressure drops to 100 bar.

4. The method according to claim 1, characterized in that the nitrogen atmosphere contains pure nitrogen or a gas mixture with N2.

The invention is based on a method for producing a shaped body from nitride from III., IV., V. or VI. Group of the periodic system selected elements, in particular made of silicon nitride, in which process the powder of the pure element or a powder mixture of the pure element and its nitride is used as the starting material, which starting material is shaped in a first sintering phase into a porous, further machinable shaped body.

The use of shaped bodies made of nitride from III., IV., V. or VI. Group of the periodic system selected elements, in particular a silicon nitride shaped body, enables applications in which metallic materials usually fail or which applications can only be carried out by adding alloying elements.

The mechanical and thermal properties such as low coefficient of friction, low density, high temperature resistance and, in particular, high hardness and wear resistance speak in favor of silicon nitride or nitrides of the above-mentioned elements. These properties also result in the many possible applications such as structural parts for gas turbines and high-temperature machines. Apparatus construction, chemical engineering, wear and tear seals and bearing parts of machines, metallurgy and furnace construction.

Such nitrides can replace metallic structural parts such as nickel, molybdenum and cobalt or their combinations. A shortage of these substances and thus an increase in price is to be expected in the future. In contrast, z. B.

Quartz sand, from which the elementary silicon is extracted, is abundant on earth.

It is known to cast elemental silicon with admixture of sintering additives as a slip or to cold isostatically pressed. It is also possible to inject the plastic compound into a steel mold. After such shaping, sintering takes place under nitrogen atmospheric pressure, with the reaction

expires.

Moldings produced in this way, however, have very strong porosities, since the equilibrium for the formation of elements in the equation is shifted to the left.

In order to solve this problem, it is proposed in DE-OS 23 51162 to impregnate the molded bodies thus obtained with metallic-organic liquids.

Instead of elementary silicon, it is also known to also shape silicon nitride powder with the addition of sintering additives to give shaped bodies and then to sinter them in a nitrogen atmosphere. However, decomposition of the silicon nitride Si3 N4 can also occur. In DE OS 28 55 859 a method is described in which use is made of protective powders, whereby such decomposition is to be prevented.

It is also known that elemental silicon or a powder mixture of silicon and silicon nitride can be hot-pressed and then sintered in a nitrogen atmosphere. In order to improve the density, the moldings are pressed in heated press dies with the addition of oxidic pressing aids (hot-pressing aids that form glass phases).

Such a method is described in DE-PS 24 12 637.

In all of these known processes, aggregates such as sintering additives, pressing aids, metallic-organic liquids, oxidic hot-pressing aids, etc., are required in order to obtain sintered molded bodies.

In the journal Power Metallurgy International, Volume 7, No. 2 from 1975, a process for reaction-sintered molded bodies is described. The reaction-sintered molded bodies are produced in a normal sintering furnace in a nitrogen atmosphere at a temperature of up to 1500 ° C over a period of two to five days. However, the shaped bodies produced in this way are disadvantageous in the sense that they generally have a porosity of about 20%, nitrogen penetrating only into an upper layer of the shaped body which is about 10 mm thick during sintering.

The invention specified in claim 1 is based on the object of proposing a method in which the reaction time during the sintering of the shaped body is significantly reduced and the nitriding of the shaped body is achieved not only on the surface but through the entire shaped body. No supplements should be used in the process. The density of the finished shaped body should be at least 2.0 g / cm3 and its hardness HV005 should be in the range from 13,000 to 15,000 Nm2.

According to the invention, this object is achieved in the method mentioned in the preamble of claim 1

the process steps listed in the characterizing part of claim 1 solved.

The following combinations of pressure and temperature can advantageously be selected with regard to the time: - applying first a pre-pressure, later a temperature, whereby the pressure and the temperature rise together until the hold phase, after which the pressure and the temperature drop evenly, - simultaneous Applying a pressure and a temperature, whereby the temperature rises rapidly first and the pressure is delayed to the heating time, after which the pressure and the temperature drop evenly, - simultaneous application of a pressure and a temperature, stopping the pressure and temperature rise according to a stair-step effect, Applying the pressure and the temperature to the holding phase, after which the pressure and the temperature drop uniformly, - simultaneous application of a pressure and a temperature, the pressure rising rapidly first and the temperature being delayed at the time of printing, after which the pressure and the temperature are uniform waste n.

These combinations are shown graphically in FIGS. 1-4.

A shaping of a shaped body can be done directly by z. B. cold isostatic pressing. As a starting material z. B. elemental silicon powder or a powder mixture of elemental silicon with silicon nitride is used. Others from III., IV., V. or VI.

Group of the periodic table selected elements such as boron, aluminum, titanium, zircon, vanadium, tantalum, chromium, molybdenum can be used. The silicon powder should contain a maximum of 2% of the impurities such as Fe, Ca, Al.

In the case of slip or injection molding, the shaping additives must be removed by an additional heat treatment of the molding. The shaped body is then sintered in a nitrogen atmosphere in a first sintering phase. The result is a posöser molded body that continues, e.g. B. by milling, can be processed and which has a low strength.

The porous shaped body is then placed in a hot isostatic press. After closing the press, it is evacuated several times and flushed with nitrogen. Then nitrogen or a gas mixture with N2 is pumped into the press and the gas is heated. In the second sintering phase, the shaped body is exposed to a nitrogen atmosphere pressure of at least 100 bar and a temperature of at least 1200 ° C. for at least 30 minutes. After the end of the holding phase, the pressure and temperature drop evenly.

The second sintering phase is carried out without sintering additives and without an external gas-tight envelope.

The following combinations of pressure and temperature are possible depending on the time.

Fig. 1 Application of first a form, later a temperature, the pressure and the temperature rise together until the holding phase, after which the pressure and the temperature drop evenly, 2 simultaneous application of a pressure and a temperature, the temperature rising rapidly first and the pressure being delayed for the heating-up time, after which the pressure and the temperature drop evenly, 3 simultaneous application of a pressure and a temperature, stopping the pressure and temperature rise according to a stair-step effect, application of the pressure and the temperature to the holding phase, after which the pressure and the temperature drop uniformly, and 4 simultaneous application of a pressure and a temperature, the pressure rising rapidly first and the temperature being delayed at the time of printing, after which the pressure and the temperature drop uniformly.

During these processes, the reaction takes place as follows:

Since the processes take place under a very high nitrogen atmosphere, the reaction can only proceed from left to right. Since the nitrogen atmospheric pressure does not fall below 100 bar even during the cooling of the shaped body, there cannot be any decomposition of the silicon nitride.

The cold isostatically pressed silicon nitride molding has a density of approx. 50% of the theoretical density.

In the first temperature phase, up to approx. 1000 "C, the silicon grains bake together with little nitrogen absorption. This creates a porous silicon nitride molded body of low strength. The reaction mentioned begins at approx. 1000 ° C

The porous molded body absorbs nitrogen and binds it directly. The pores grow closed when nitrogen is absorbed. If this reaction is carried out too quickly, the increase in volume can cause the molded body to tear.

The invention is to be explained in more detail by the following examples: example 1 Silicon powder of quality 14 STA 325/2 was pressed with the aid of the cold isostatic press at 600 bar to form a cylinder of 22 x 100 mm. The subsequent sintering under nitrogen pressure at 1060 "C resulted in a porous Si body with a density of 2.1 g / cm3. This body did not yet have any strength and could be broken by hand. Only the subsequent sintering at 1760" C during a Hour at 1300 bar nitrogen pressure resulted in a solid body with a density of 2.6 g / cm3. The hardness HVo.05 was 18000-31000 N / mm2.

Example 2 Powder of quality 14 STA 352/2 was cold isostatically pressed to a cylinder 20 x 100 mm. Pressure sintering at 1350 C for 3 hours at 1200 bar nitrogen resulted in a body with a density of 2.0 g / cm3. The uneven structure consisted of a + ss silicon nitride with a hardness of HV0.05 = 1300-1500 kp / mm2.

Example 3 Bodies treated in the same way as listed in example 2 were additionally exposed to a nitrogen pressure of 1300 bar at 1800 C for one hour. The density increased to 3.1 g / cm3 and the hardness HVo 05 to 17000-22000 N / mm2. The structure consists of evenly distributed a + ss nitride.

Claims (4)

PATENT CLAIMS 1. Process for the production of a shaped body from nitride of elements selected from the III., IV., V or VI group of the periodic table, in which process the powder of the pure element or a powder mixture of the pure element and its nitride is used as the starting material , which starting material is shaped into a porous, further machinable shaped body in a first sintering phase, characterized in that the porous shaped body is placed in a hot isostatic press, where it is subjected to a nitrogen atomic pressure for at least 30 minutes in a second sintering phase without sintering additives and without an external gas-tight envelope of at least 100 bar and a temperature of at least 1200 C, after which the pressure and temperature drop evenly. 2. The method according to claim 1, characterized in that the following combinations of pressure and temperature can be selected depending on the time: - applying first a form, later a temperature, the pressure and the temperature rise together until the holding phase, after which the pressure and the temperature drop evenly, - simultaneous application of a pressure and a temperature, the temperature first rising rapidly and the pressure being delayed to the heating-up time, after which the pressure and the temperature drop uniformly, simultaneous application of a pressure and a temperature, holding the pressure - and temperature rise according to a stair step effect, application of the pressure and the temperature to the holding phase, after which the pressure and the temperature drop evenly, - simultaneous application of a pressure and a temperature, the pressure rising rapidly first and the temperature being delayed at the time of printing, after which de r Pressure and temperature drop evenly. 3. The method according to claims 1 and 2, characterized in that the pressure drops to 100 bar. 4. The method according to claim 1, characterized in that the nitrogen atmosphere contains pure nitrogen or a gas mixture with N2. The invention is based on a method for producing a shaped body from nitride from III., IV., V. or VI. Group of the periodic system selected elements, in particular made of silicon nitride, in which process the powder of the pure element or a powder mixture of the pure element and its nitride is used as the starting material, which starting material is shaped in a first sintering phase into a porous, further machinable shaped body. The use of shaped bodies made of nitride from III., IV., V. or VI. Group of the periodic system selected elements, in particular a silicon nitride shaped body, enables applications in which metallic materials usually fail or which applications can only be carried out by adding alloying elements. The mechanical and thermal properties such as low coefficient of friction, low density, high temperature resistance and, in particular, high hardness and wear resistance speak in favor of silicon nitride or nitrides of the above-mentioned elements. These properties also result in the many possible applications such as structural parts for gas turbines and high-temperature machines. Apparatus construction, chemical engineering, wear and tear seals and bearing parts of machines, metallurgy and furnace construction. Such nitrides can replace metallic structural parts such as nickel, molybdenum and cobalt or their combinations. A shortage of these substances and thus an increase in price is to be expected in the future. In contrast, z. B. Quartz sand, from which the elementary silicon is extracted, is abundant on earth. It is known to cast elemental silicon with admixture of sintering additives as a slip or to cold isostatically pressed. It is also possible to inject the plastic compound into a steel mold. After such shaping, sintering takes place under nitrogen atmospheric pressure, with the reaction expires. Moldings produced in this way, however, have very strong porosities, since the equilibrium for the formation of elements in the equation is shifted to the left. In order to solve this problem, it is proposed in DE-OS 23 51162 to impregnate the molded bodies thus obtained with metallic-organic liquids. Instead of elementary silicon, it is also known to also shape silicon nitride powder with the addition of sintering additives to give shaped bodies and then to sinter them in a nitrogen atmosphere. However, decomposition of the silicon nitride Si3 N4 can also occur. In DE OS 28 55 859 a method is described in which use is made of protective powders, whereby such decomposition is to be prevented. It is also known that elemental silicon or a powder mixture of silicon and silicon nitride can be hot-pressed and then sintered in a nitrogen atmosphere. In order to improve the density, the moldings are pressed in heated press dies with the addition of oxidic pressing aids (hot-pressing aids that form glass phases). Such a method is described in DE-PS 24 12 637. In all of these known processes, aggregates such as sintering additives, pressing aids, metallic-organic liquids, oxidic hot-pressing aids, etc., are required in order to obtain sintered molded bodies. In the journal Power Metallurgy International, Volume 7, No. 2 from 1975, a process for reaction-sintered molded bodies is described. The reaction-sintered molded bodies are produced in a normal sintering furnace in a nitrogen atmosphere at a temperature of up to 1500 ° C over a period of two to five days. However, the shaped bodies produced in this way are disadvantageous in the sense that they generally have a porosity of about 20%, nitrogen penetrating only into an upper layer of the shaped body which is about 10 mm thick during sintering. The invention specified in claim 1 is based on the object of proposing a method in which the reaction time during the sintering of the shaped body is significantly reduced and the nitriding of the shaped body is achieved not only on the surface but through the entire shaped body. No supplements should be used in the process. The density of the finished shaped body should be at least 2.0 g / cm3 and its hardness HV005 should be in the range from 13,000 to 15,000 Nm2. According to the invention, this object is achieved in the method mentioned in the preamble of claim 1 ** WARNING ** End of CLMS field could overlap beginning of DESC **.