CH685051A5 - Silicon nitride sintered abrasive grain and process for its production - Google Patents

Abstract

Sintering of silicon nitride powder with oxidic sinter additives, in particular aluminium oxide/yttrium oxide gives abrasive grain having high toughness.

Description

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description

The present invention relates to an abrasive grain based on silicon nitride, which can be produced by sintering silicon nitride powder with oxidic sintering additives, and a method for the production thereof.

Sintered silicon nitride has so far been used to a limited extent for technical ceramics, for example for turbine blades or roller bearings. Crucial for these uses is the high strength at room temperature as well as at high temperatures. Silicon nitride is also sometimes used as cutting ceramics, but so far not as an abrasive grain.

This is due to both the high price of silicon nitride and the lower hardness compared to other materials used as abrasives, such as corundum or silicon carbide.

However, it has been shown that in some applications the hardness of an abrasive is of less importance than previously thought and that the toughness, which is usually expressed by the Kic value, plays a very important role.

The object of the present invention was to provide an abrasive grain with great toughness and a less expensive manufacturing process for this abrasive grain.

According to the invention, this object is achieved by the abrasive grain according to claim 1 and the manufacturing method according to claim 5.

It has been found that silicon nitride powder with oxidic sintering additives can be sintered at 1650 to 1800 ° C under normal pressure, ie “without pressure”, or under increased gas pressure to form particularly tough abrasive grain.

Other hard materials can optionally be added to the silicon nitride powder. These include carbides such as silicon carbide, tungsten carbide, boron carbide or titanium carbide, other nitrides such as aluminum nitride, titanium nitride, tantalum nitride, boron nitride, borides such as titanium di-boride or high-melting metals such as tungsten.

Oxides, double oxides or mixed oxides of aluminum, magnesium or rare earths or mixtures of the compounds mentioned are expediently used as sintering additives. Double oxides are understood here to mean stoichiometrically or approximately stoichiometrically composed oxidic compounds of two or more metals with their own crystal structure, for example spinels, garnets or perovskites. Mixed oxides are to be understood as oxidic compounds of two or more metals which are derived from a pure metal oxide by replacing part of the metal atoms with atoms of one or more other metals without the structure changing, i.e. mixed crystals or solid solutions. Rare earths are to be understood here as the metals scandium, yttrium, lanthanum and the lanthanides and their mixtures.

Preferred sintering additives are mixtures of aluminum oxide with magnesium oxide or at least one rare earth oxide and the corresponding double or mixed oxides such as spinels or garnets.

A mixture of aluminum oxide and yttrium oxide or a corresponding compound such as yttrium aluminum garnet (YAG; Y3AI5O12) is particularly preferred as the sintering additive. Particularly good results were achieved with a mixture of Al2O3 and Y2O3 in a weight ratio of 3: 5 (corresponding to a molar ratio of approx. 1.3: 1).

The abrasive grain according to the invention has a Vickers mi hardness of preferably at least 16 GPa and a fracture toughness Kic of at least 6 MPa • m 1/2. Typical values are approx. 18 GPa hardness and approx. 7 MPa • m1 / 2 fracture toughness.

The abrasive grain according to the invention can be produced by mixing silicon nitride powder, optionally with the above-mentioned carbides, other nitrides, borides or metals, with the sintering additives, solidifying them and sintering them at 1650 to 1800 ° C. under a protective gas. Instead of the above-mentioned oxides, double oxides or mixed oxides, corresponding precursor compounds such as hydroxides or salts with volatile inorganic or organic acids, for example nitrates, carbonates or acetates, can also be used. These precursor compounds decompose before reaching the sintering temperature to the corresponding oxides, which then act as actual sintering additives.

Mixing can be done dry or preferably wet, ie in suspension. Water is preferably used as the suspending agent, if appropriate with the addition of customary auxiliaries such as plasticizers, defoamers or pH regulators. It has been found that particularly good results are achieved with a weakly alkaline aqueous suspension with a pH of about 8-11. For example, ammonia solution or an amine can be added to adjust the pH. It is also possible to use organic liquids such as alcohols as suspending agents. Other auxiliaries such as lubricants (for better compression if this is done by pressing) or temporary binders (for sufficient strength before sintering) are also advantageously added in this process step.

The mixing process is preferably carried out in a colloid mill, an attritor, an agitator ball mill, an annular gap mill or a vibration mill. Attractors and agitator ball mills are particularly preferred, especially those with grinding media made of silicon nitride.

The mixing is particularly advantageously carried out in such a way that agglomerates present in the silicon nitride powder or in the other solid starting materials are destroyed and any coarse particles present are comminuted.

After mixing, the mixture or the suspension is suitably solidified in order to obtain sinterable “green bodies”. For this purpose, the suspension can be filtered, for example, and the filter cake obtained can be dried. The dried Fil5

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The cake can then be comminuted to the desired abrasive grain size, if necessary after additional compaction by dry pressing. The crushing can take place, for example, with a roller crusher and is neither associated with high energy expenditure nor with heavy wear. If the suspension already has a high solids content, it can also be dried directly without a separate filtration step.

When shredding, the shrinkage during sintering is appropriately taken into account. Unwanted fines can be screened off and returned to the process.

The suspension can also be spray dried, so that a flowable granulate is obtained which can be compressed, for example, by dry pressing. When using appropriate pressing tools, abrasive grains of a defined shape can also be produced.

The sintering is conveniently carried out under nitrogen at 1650 to 1800 ° C, preferably at about 1750 ° C.

The sintering time depends on the temperature, type and amount of the additives and the sintering atmosphere, for example at 1750 ° C with AI2O3 / Y2O3 in a nitrogen atmosphere for approx. 3 h.

After sintering, any agglomerates formed are broken up and the abrasive grain is sieved to give the desired grain size distribution.

The following examples illustrate the production of the abrasive grain according to the invention, without any restriction to the described embodiments.

example 1

In a stirred ball mill (251 usable volume) with a circulation container, 46 kg of silicon nitride powder with a specific surface area of 10 m2 / g and an oxygen content of 1.5% by weight (type SN-E 10, ÜBE, Japan) and 1.5 kg of aluminum oxide powder (Type CS 400, Martinswerk GmbH, Bergheim, Germany) and 2.5 kg of yttrium oxide powder (HC Starck, Goslar, Germany) and 2 kg of auxiliaries (liquefier, defoamer, amino alcohol as base) mixed in 50 l of demineralized water for 4 hours and deagglomerated. Balls of silicon nitride were used as grinding media. The suspension was then dried in a layer thickness of initially 30 mm at 100 ° C. in 24 hours. The dried mass was crushed with a roller crusher and the fraction with a grain size of 0.1 to 1.4 mm was sieved out. The grains were sintered under nitrogen at normal pressure for 3 hours at 1750 ° C. After sintering, a P-36 grain (according to the FEPA standard) was sieved out.

The sintered density was determined pycnometrically to be 3.2 g / ml. A micro hardness determination according to Vickers showed a value of 18.3 GPa, the fracture toughness was Kic = 7 MPa • m1 / 2.

Example 2

The procedure was as described in Example 1, 1.5 kg of polyvinyl alcohol (PVA) as a temporary binder and 1.5 kg of polyethylene glycol (PEG) as a lubricant being added to the suspension.

The suspension was spray-dried and the granules obtained in this way were pressed at a pressure of 100 MPa into plates measuring 90 × 90 × 20 mm 3. The plates were crushed with a roller crusher. The comminuted material was further processed as described in Example 1.

The finally obtained abrasive grain had the same properties as the product from Example 1 within the scope of the measurement accuracy.

Claims (1)

Claims 1. Sintered abrasive grain made of silicon nitride, optionally with the addition of carbides, further nitrides, borides or refractory metals, and at least one sintering additive from the group of oxides, double oxides and mixed oxides of aluminum, magnesium and rare earths. 2. Sintered abrasive grain according to claim 1, characterized in that it contains as a sintering additive a mixture of aluminum oxide and magnesium oxide or at least one rare earth oxide and / or a corresponding double or mixed oxide. 3. Sintered abrasive grain according to claim 2, characterized in that it contains a mixture of aluminum oxide and yttrium oxide and / or a corresponding double oxide as a sintering additive. 4. Sintered abrasive grain according to one of claims 1 to 3, characterized by a Mi-hardness according to Vickers of at least 16 GPa and a fracture toughness Kic of at least 6 MPa • m1 / 2. 5. A method for producing sintered abrasive grain according to claim 1, characterized in that silicon nitride powder, optionally with the addition of carbides, further nitrides, borides or refractory metals, with at least one sintering additive from the group of oxides, double oxides and mixed oxides of aluminum, magnesium and the rare earths and / or corresponding precursor compounds, which pass into the oxides, double oxides or mixed oxides mentioned when heated, mixed, solidified and sintered at 1650 to 1800 ° C. under normal pressure or elevated gas pressure. 6. The method according to claim 5, characterized in that a mixture of aluminum oxide and magnesium oxide or at least one rare earth oxide and / or a corresponding double or mixed oxide is used as the sintering additive. 7. The method according to claim 6, characterized in that a mixture of aluminum oxide and yttrium oxide and / or a corresponding double oxide is used as the sintering additive. 8. The method according to any one of claims 5 to 7, characterized in that the mixing is carried out in a suspension in a device from the group of colloid mills, attritors, agitator ball mills, annular gap mills and vibration mills and the suspension is then dried. 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 3rd 5 CH 685 051 A5 9. The method according to claim 8, characterized in that the mixing is carried out simultaneously with the deagglomeration of the silicon nitride. 10. The method according to one or more of claims 5 to 9, characterized in that the solidification of the mixture of silicon nitride powder and sintering additives is carried out by dry pressing and the compacted mass is then comminuted to abrasive grain size. 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 4th