CZ9903572A3 - Sintered silicon nitride, process for its production, structural elements made of it, especially valves, process for their production and its use - Google Patents

Abstract

Sintered silicon nitride (Si3N4) having a chlorine content in the range of 100 to 500 ppm, a characteristic n value of subcritical crack growth of 150, an average flexural strength at room temperature of 1850 MPa and a Weibull modulus of 118. Sintered silicon nitride is produced by dispersing silicon nitride powder, which either contains chlorine in an amount of 500 to 1500 ppm or is alternatively used together with a metal chloride, together with a sintering additive in water, mixing it with an organic processing aid, grinding the aqueous dispersion to a fineness of 90% < m < 1 mm and then drying it so that the silicon nitride granulate has a moisture content in the range of 1.0 to 4% by weight and an average granulate size of 40 to 80 mm, which is then compressed and, after heating the organic processing aid is sintered under a nitrogen pressure of 0.1 L p L 1.0 MPa.

Description

The invention relates to sintered silicon nitride (Si^N ₄ ), a method of its production, structural elements made from it, in particular valves, a method of their production and its use.

State of the art

Silicon nitride materials can be used in many applications. However, in addition to their unavailability, the structural elements have disadvantages such as insufficient reliability in continuous operation, which prevent the widespread use of these silicon nitride structural elements in the above-mentioned applications, which would be economically and environmentally advantageous. Thus, DE-A 4 312 251 claims a high-strength silicon nitride material with a defined failure probability, but no instructions are given as to how this failure probability can be achieved. This is derived only from classical bending strength measurements and their statistical evaluation.

The reliability of ceramic materials is determined from the short-term strength with its dispersion width and from the long-term behavior under load.

The short-term strength results from the Griffith equation:

• φ · · * φ φ · · φ · • •ΦΦ 4 · · ♦ Φ · · ·

ΦΦΦΦ Φ · · ····

Φ ΦΦ · Φ ΦΦ ΦΦ Φ· Φ Φ Φ · Φ · Φ Φ Φ Φ · · «

ΦΦΦ · Φ « · Φ · · · · σ = - (1) a|c Υ where σ denotes the strength in MPa ,

Kj _c denotes fracture toughness in MPa.

c denotes the critical crack length in pm and

Y denotes the shape factor of the critical crack.

According to this equation, strength is directly dependent on the length of the crack, or defect, in the material.

The dispersion of short-term strength, important for reliability, is described by the Weibull statistic and is referred to as the Weibull modulus according to DIN 51 110.

In contrast, for stress under tension leading to catastrophic fracture, the vk-concept applies v = A . K ¹ ^ (2) where v denotes the crack growth rate in m/s ,

Kj denotes the stress intensity factor in MPa . m^/2 stress 1 = tensile stress and at strain • · *· ♦ ·· ·· 99 • 9 9 · ··· · 9 « · · • · β · t « · · · · · • · 9 9 9 9 9 9 9 999 999

9 9 9 9 9 9 · · • · ·· · · · · · · 9 9

A, n parameter for subcritical crack growth (lifetime).

This concept is appropriate for materials and structural elements that are loaded with alternating stresses below the described maximum stress, leading immediately to failure according to the Griffith equation, and is therefore relevant for ceramic materials and structural elements for a large number of technical applications, for example for piston engine valves.

The crack growth value is determined according to the description of the preliminary standard ENV 843-3 by determining the bending strengths at different strain rates.

The determination of bending strength is described in DIN 51 110.

The applied stress rate, which should lead to fracture within 5 to 10 s, is usually about 100 MPa/s. The bending strength determined in this case is referred to as the short-term or inertial strength σθ.

To determine the crack growth parameter, this measurement is carried out at a reduced strain rate. In this case, the cracks present have the opportunity to grow, which results in fracture at a lower strain rate, i.e., so-called subcritical crack growth occurs. If the stress at the tensile strength is plotted over the strain rate in a double logarithmic scale and the mean values of multiple measurements for a defined strain rate are connected by a regression line, the crack growth value at A is obtained from the slope of the line and the intercept of the axis of this line. Typical ceramic materials have n values of 30 to 40 (see Kingery, Introduction to Ceramics, John Viley & Sons, New York, 1976, p. 804) and are thus demonstrably • · · · · · · frfr ♦ * « · ··« ·

9 9 9 9 9 9 9 · ·· · · ·99 99 9 9 99 qualified as subcritical crack growth, so their service life is limited in practical use.

To meet ever-increasing demands, especially in the automotive industry, there is a need for silicon nitride materials and components with improved reliability.

The object of the present invention is therefore to produce sintered silicon nitride and reliable structural elements, in particular valves, based on silicon nitride (SiN), which would have this property profile and would also be simple and therefore inexpensive to produce.

The essence of the invention

It has now surprisingly been found that sintered silicon nitride with a certain chlorine content has improved subcritical crack growth behavior while simultaneously having higher flexural strength and a high Weibull modulus.

The subject of the present invention is therefore sintered silicon nitride having a chlorine content in the range of 100 to 500 ppm, a characteristic value n of subcritical crack growth of s 50, preferably s 60, a mean flexural strength at room temperature of 850 MPa and a Weibull modulus of s: 18.

The chlorine content of sintered silicon nitride is determined by pressure decomposition using hydrofluoric acid at a temperature in the range of 100 to 120 °C and subsequent potentiometric titration of the chloride using silver nitrate.

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The sintered silicon nitride according to the present invention preferably contains alkaline earths, Sc ₂ O ₂ , Y ₂ 0 ₂ > ^θχ ΐύγ rare earths, TiO ₂ , Zr0 ₂ , HfO ₂ , and/or A1 ₂ 0^ as sintering additives, which in the sintered material form a secondary phase concentration of 7.5 to 20% by volume in addition to crystalline silicon nitride and/or mixed silicon nitride crystals.

This secondary phase concentration is determined by determining the total oxygen content of the sintered silicon nitride by hot extraction. From this result, the known oxygen concentration introduced by the added sintering aids is subtracted. The difference represents the oxygen content of the silicon nitride after processing, which is present in the form of silicon dioxide. This silicon dioxide concentration is added to the concentration of the sintering aids, which represents the total proportion of oxide components in addition to silicon nitride.

For the proportion of silicon nitride in the material, it is used for o

calculation of the volume fraction its net density is 3.18 g/cm , for the secondary phase, which is formed by the reaction of sintering additives with silicon dioxide of silicon nitride powder during sintering, the net density is calculated using the equation i=n

Φ _κ = G-cel/Σ (Gi/ΦβΡ in g/cm ³ (3) i=l where

G-cel indicates the total mass of oxide components in g

ΦΦ ΦΦ φ φ φ φ • ΦΦΦ • · φ · • φ φ φ

ΦΦ φ φ φ φ φ · φ φ

ΦΦΦ φ

denotes the mass of individual oxide components in g

Φβ£ denotes the net densities of individual oxide components

Q in g/cm .

Thus, the volume fractions of silicon nitride and the secondary phase are determined, the latter being in the range of 7.5 to 20% by volume for the material according to the present invention.

The silicon nitride according to the present invention is characterized by a high degree of densification (slight porosity), so that neither the density nor the elasticity of the material changes during subsequent sintering at a temperature of up to 50°C higher than the sintering temperature.

The present invention further relates to a method for producing sintered silicon nitride according to the present invention, in which silicon nitride powder, which either contains chlorine in an amount of 500 to 1500 ppm or is alternatively used with a metal chloride, is dispersed together with a sintering additive in water, mixed with a processing aid, the aqueous dispersion is ground to a fineness of 90% < 1 pm and then dried, preferably by spray drying or fluidized bed drying, so that the silicon nitride granulate has a moisture content in the range of 1.0 to 4% by weight, preferably 1 to 3% by weight and the average granulate size is 40 to 80 pm, this is then compressed and, after heating the organic processing aid under a nitrogen pressure of 0.1 p =£ 1.0 MPa, it is sintered.

The pressing is preferably carried out axially and/or isostatically.

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In a preferred embodiment of the method according to the present invention, pressing is carried out at a pressure < 250.0 MPa, heating of the organic processing agent and moisture in air, protective gas or under vacuum at a temperature of 650 °C and sintering at a nitrogen pressure of 0.1 p 1.0 MPa at a temperature of 2000 °C.

Preferably, the silicon nitride powder used has a chlorine content in the range of 500 to 1500 ppm, which results in a chlorine content of 100 to 500 ppm in the sintered silicon nitride.

As silicon nitride powder, for example, Baysinid ST from Bayer AG, Germany, can be used. If the chlorine content of the silicon nitride powder is too low, metal chlorides, such as magnesium chloride or barium chloride, can be added additionally.

The sintering additives which are useful in the process according to the present invention are preferably alkaline earths, Sc ₂ 0 ₂ , Y 2 O 3 » rare earth oxides, TiO ₂ , ZrO ₂ , HfO ₂ , B ₂ 0 ₂ and/or Al ₂ O 3 .

These are preferably added in such amounts that during sintering, by reaction with oxygen, which is always present in silicon nitride powders and assumed as silicon dioxide, a liquid phase is formed which occurs in the sintered material as a predominantly glassy secondary phase in a concentration of 7.5 to 20% by volume.

The organic processing aids which can be used in the process according to the present invention are preferably dispersing aids and/or molding aids, the latter having the function of binders and plasticizers.

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Dispersion aids are preferably citric acid and polyacrylic acid derivatives as well as amino alcohols in a concentration of 0.1 to 2.5% by weight, based on the solids content of the dispersion.

A large number of substances can be used as molding aids, such as polyurethane dispersions, cellulose derivatives, starches, polysaccharides, polyamide solutions, polyvinyl alcohol, polyvinyl acetate, polyethylene glycols and/or stearates. These agents are preferably used in amounts of 0.2 to 5% by weight.

Pressing at a pressure < 250.0 MPa can be carried out by axial and/or dry isostatic pressing, for example in a mold corresponding to the structural element.

The materials obtained in this way have no granulate relics in the material structure. Therefore, reproducible strengths of over 85.0 MPa at a Weibull modulus of 18 are achieved.

Another subject of the invention are structural elements, in particular valves, made of silicon nitride according to the present invention.

Another object of the present invention is to provide valves with a failure probability of less than 10% made of sintered silicon nitride.

The subject of the present invention is further a method of manufacturing valves according to the present invention, in which the valves • · 9 * 9 · · 9« 99

99 9 9 9 9 9 9 99 9

99 999 9999

99 99 99 99 999 999

9999 999 9 9

99 999 99 99 99 are selected using oscillation analysis by dividing the frequency of the resonant frequency peak by 0.0125%. The other steps of the process are analogous to the method for producing silicon nitride according to the present invention, with the pressing being carried out in a form corresponding to the valve being produced.

It has been found that using the latter method, it is possible to quickly and unambiguously detect faultless structural elements by dividing the frequency of the resonant frequency peak, regardless of the geometric shape of the valve.

However, this type of selection is also possible for structural elements with a different geometric shape.

In the method according to the present invention, the valve under test is placed on the side of the plate on three electrodynamic transducers, one transmitter and two receivers, such that the valve is supported at a distance of about 1 to 2 mm from the edge of the plate with an angular separation of the sensors of 120° each. By varying the frequency in the range of 0.1 to 2 MHz of the transmitter, the oscillation of the stem bending and the oscillation of the plate bending are excited in the ceramic valve and the oscillations are sensed by both receivers.

Macroscopic defects such as cracks, material inclusions of foreign components or inhomogeneities with respect to density and/or modulus of elasticity are manifested in a uniquely assigned splitting of the resonant frequency into two-part natural oscillations, the frequency splitting increasing with increasing error size.

Another object of the invention is the use of silicon nitride according to the present invention and structural elements made therefrom in engine construction, in particular as valves in piston ·· ·· • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · * ·

·» ·· • · · · · · · · · ·· ·· ··· • · • · ·· motors, in general mechanical engineering, in storage techniques and in instrument construction.

Engine parts made from it are characterized above all by their long durability and high reliability. For example, engine valves made of silicon nitride have shown excellent durability without failure in test sites operated under conditions far exceeding those of normal piston engines.

Examples of embodiments of the invention

Different material compositions are produced and characterized identically using different starting silicon nitride powders. The silicon nitride powder used is the completely chlorine-free LC12S (H.C. Starek, Germany) and the chlorine-containing Baysinid^ST (Bayer AG, Germany). These products have the following typical properties:

LC12S Baysinid^ST 2 Specific surface area, m/g 20 > 10 content 0 , % wt. 2.0 1.2-1.4 table of contents C , % wt. 0.2 < 0.05 Fe content , ppm 60 < 50 contents Al , ppm 300 < 20 Ca content , ppm 30 < 20 content Cl , ppm < 100 700-1500 share a-! s ₃ n ₄ , % about 95 about 96 related on α+β

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ΦΦ ΦΦ ·· ·· • φ φ φ φ φφφ φ · · · φ φ φ φ φ

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The sintering additives used are yttrium oxide (Υ2θ3) (available under the trade name Grade C from H.C. Starek, Germany), aluminum oxide (Al2O3) (available under the trade name CT 3000 SG from Alcoa, Germany) and magnesium oxide (MgO) (available as MgO p.a. from Merck, Germany). The raw materials are introduced into the water p.a. provided, to which a dispersing aid (KV 5088 from Zschimmer & Schwarz, Germany) is added and dispersed using a high-speed mixer. The viscosity is adjusted to 20 mPas, which corresponds to a solids content of about 64%.

The batch is ground using an agitator ball mill until a simultaneously determined particle size distribution of 90% < 1 gm is achieved.

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Then 1% of Bayceram 4305 as a binding and plasticizing agent, available from Bayer AG, Germany, is added and the mixture is ground for a further 5 minutes.

The dispersion thus obtained is granulated by spray drying, the conditions always being adjusted to the batch so that the proportion of granulate < 150 gm is as small as possible. Granulate > 150 gm is separated from the processed batch by sieving.

Shaped bodies are pressed from this granulate at a pressure of > 250 MPa, heated to a maximum temperature of 650 °C and sintered as described in DE-A 4 233 602 without inclusion in a protective powder in graphite vessels at a temperature of 1800 °C ± 50 °C in a nitrogen atmosphere.

The material properties are listed in the following table. The flexural strength and Weibull modulus are determined according to »· ·· · ·· ·· ·· ··· · ··· · · · · · · · · · · · · · · · · · · · · · · ··

DIN 51 110 and the subcritical crack growth parameter according to the European pre-standard ENV 843-3.

The following abbreviations have been used in the following table:

The TGV corresponds to the proportion of dust particles in the suspension after grinding that are smaller than 1 pm (% < 1 gm);

GGV refers to the granulate size distribution, which is characterized by the mean value of the D50 distribution in gm.

Sintered density is the density of sintered bodies, determined according to the buoyancy process in water.

The O content and Cl content are the oxygen and chlorine contents analyzed in the sintered bodies. From the oxygen content, after subtracting the known oxygen contents introduced by the sintering additives, the proportion of oxygen introduced via the silicon nitride powder after processing is calculated. Assuming that this occurs as silicon dioxide, the total concentration in weight percent of the oxide component is calculated by adding this calculated silicon dioxide content in the sintered material and the concentration of the added sintering additives. This is converted into the volume fractions of the secondary phase and crystalline silicon nitride in accordance with the above explanation. The sec.-phase column in the table shows the important concentration of the secondary phase in volume percent thus determined.

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Claims (8)

Sintered silicon nitride, characterized in that it has a chlorine content in the range of 100 to 500 ppm, a characteristic value of subcritical crack growth růstu: 50, a mean flexural strength at room temperature of 850 MPa and a Veibull modulus of: 18. Sintered silicon nitride according to claim 1, characterized in that it contains alkaline earths, SC 2 O 3, Y 2 O 3. rare earth oxides, T1O2, ZrO2, HfO2, ^and Β2θ3 / ^No b 2θ3 ° ^Α ^ J ^ ° ^and sintering additives, wherein the sintered material is in the form a secondary phase concentration of 7.5 to 20% by volume in addition to crystalline silicon nitride and / or mixed silicon nitride crystals. A process for the production of sintered silicon nitride according to claim 1 or 2, characterized in that the silicon nitride powder, which either contains chlorine in an amount of 500 to 1500 ppm or, alternatively, is used together with the metal chloride, is dispersed together with the sintering additive. water, mixed with an organic processing aid, the aqueous dispersion is ground to fineness 90% < 1 µm and then dried so that the silicon nitride granulate has a moisture content of 1.0 to 4% by weight and a mean granulate size of 40 to 80 µm, which is then compressed and after heating the organic processing aid under a nitrogen pressure of 0.1 p, 1.0 MPa sintered. Process according to claim 3, characterized in that the pressing is carried out at a pressure of <250.0 MPa, heating of the organic process medium and moisture in air, a shielding gas or under vacuum at 650 ° C and sintering at a temperature of £ 2000 ° C. . 5. The sintered silicon nitride components of claim 1 or 2. Valves with a probability of failure less than 10 ^-8 from sintered silicon nitride according to claim 1 or 2. Valve production method according to claim 6, characterized in that the valves are selected by oscillation analysis by dividing the frequency of the resonance frequency peak by 0.0125%. Use of sintered silicon nitride or structural elements thereof according to claims 1, 2 or 5, for engine construction, machine construction, storage technology and instrument construction.