DE686017C - Process for the production of porous abrasive bodies - Google Patents

Description

D 73574

The invention solves the problem in the production of porous abrasive bodies, in particular Grinding wheels, made of vitrified bonded abrasive grain that can be oxidized at the firing temperature, such as silicon carbide, boron carbide or diamond, the feared oxidation of the abrasive grain during firing, which is on the one hand in a Blunt cutting edges whose sharpness is a prerequisite for a perfect grinding effect is, on the other hand, would result in a puffing up of the abrasive, so that the body is not the desired Structure and have the required strength and hardness, possibly also jump.

The solution is that the raw

Mixture of abrasive grit and ceramic binder a lighter at firing temperature as the abrasive grain oxidizable substance is added in such amounts that the entire existing in the pores of the body and when burning under oxidizing Conditions in them entering oxygen is chemically bound to it. As an additive for example aluminum, silicon, magnesium, boron, boron carbide, boron suboxide or a mixture of several of them into consideration.

It has already been proposed, for example one made of aluminum oxide or silicon carbide and a refractory ceramic composition composed of an organic binder add oxidizing metal, its oxidation product in a thin outer layer of the body, in which the binder burns at the same time, forms a glaze in order to prevent that when the body is used as intended, oxygen penetrates the interior of the body at a high temperature and the binder can also oxidize there. Such a measure is neither necessary nor necessary in the case of a grinding wheel permissible because, on the one hand, the abrasive is never oxidizing again after firing Conditions and on the other hand with regard to its determination by and must have the same quality. Incidentally, this is the case Suggestions, even if the oxidizable silicon carbide is used as a filler, not about the protection of this material against dulling, which is important only for abrasive grain Edges due to oxidation.

The use of a metal alloy which oxidizes at the firing temperature is also known as a binder for a refractory

A mass whose filler consists of silicon carbide or chamotte. This is also some of the metal is oxidized by burning. Here, too, it is not a question to protect the filler, rather it should be formed from a part of the binding metal Oxide of the mass has an increased mechanical Give strength and fire resistance.

Furthermore, it is known in the production of refractory products, in particular crucibles, an addition of aluminum or Magnesium powder to the optionally also containing silicon carbide as filler Provide raw mass in order to be carried out with the implementation of the metal powder the heat developed from the mixing water to metal oxide to accelerate the drying process. If the metal powder addition is appropriately dimensioned, there is no remaining Metal powder left over for the firing process. In addition, any remainder would be added to the Intended use of the crucible or other refractory object in be gradually consumed in an oxidizing atmosphere. According to the invention on the other hand, the special purpose according to the addition of metal so dimensioned that on In any case, there is enough metal present in the pores at the start of firing the body's existing oxygen or oxygen entering it during the burning process to oxidize and thus to prevent attack on the oxidizable abrasive grain.

Among the various substances that can be used for the protective effect is preferably aluminum or silicon or a mixture or an alloy of both used. The choice of the protective substance is also largely dependent on the type of Depend on the abrasive. The selection must always be made in such a way that the protective substance is lighter oxidizes as the abrasive. Therefore, for example, one does not, of course, use boron carbide at the same time use as an abrasive and protective material, although it cannot be overlooked that an and the same substance depending on its physical state, especially its degree of fineness, reacts differently.

Aluminum is particularly valuable as a protective material for objects made of silicon carbide, if the refractory grains in moderately fine Grain size are used. Silicon has been found to be more beneficial in silicon carbide articles proven with coarse grain. Magnesium is of more powerful chemical Effect, and its oxide is also more basic in the chemical reaction.

The choice of protective material, apart from the abrasive, also depends according to the nature of the other constituents of the mixture, the firing conditions to be used and the properties desired for the product. Its quantity and its degree of delicacy are chosen so that the intended purpose is achieved with certainty. The protective substance is therefore preferably used in a certain excess the calculated or assumed demand. If the protective material itself is an abrasive such as B. boron carbide, a residue remaining in the grinding wheel acts as a Abrasives with.

example

for aluminum as a protective material

The binder can have the following composition:

Feldspar. 55.5 ° / ₀ wt.,

Plastic tone .. 18.5 ° / ₀ -,

Flint 18.5% -,

Chalk ......... 3.5 ° / ₀ -,

Aluminum powder,. 5.0 ° / ₀ -

Silicon carbide grain or some other refractory grain which z. B. the sieve number 5 (25 meshes per cm ² ) may correspond, is mixed with this ceramic binder in a ratio of 455 g of grains to 155 g of binder and so much water or another liquid that a plastic and malleable mass is formed can be shaped as desired. After drying, the shaped body is fired in a conventional type of kiln at a temperature corresponding to Seger cone 13.

This binder, if it does not contain aluminum, creates a swollen or puffed object which - comes out of the mold completely; but with the addition of 5 per cent. aluminum it does not swell to any significant degree and it certainly satisfies the requirements. The addition of aluminum can be varied within wide limits, but amounts of up to 10 or 15% of the total amount of the binder are usually sufficient. Under certain conditions, give even less than 5 ° / ₀ a correct operation.

Of course, the binder composition may vary depending on the degree of hardness or the degree of porosity and the type of refractory material used and the Structure and the desired product can be modified within wide limits. Everyone A person skilled in the art can easily create a ceramic binder from the usual ingredients, viz plastic clay, sintered clay, kaolin, feldspar and flint Addition of various fluxes, such as lime, magnesia, etc., which in the presence of a suitable protective agent with advantage in large

Larger amounts can be used than would result in the absence of the protective agent the accelerated oxidation of the grains would be possible.

example

for silicon as a protective substance
Silicon carbide grains are to be bound by a vitrifiable binder to produce a ίο grinding tool. A frit is used as a binding agent, which is obtained from the following mass:

Borax (Na., B ₄ O ₇ -10 H, O) 33.8% by weight,

Boric acid (H ₃ BO ₃ ) 26.7 ° / ₀ -,

Ground flint (SiO ₂ ). 39.5% -

This mass is melted and poured into water or otherwise granulated and then ground to a desired degree of fineness. This frit will then mixed with plastic clay and silicon in the following ratio:

Powdered frit .... 75 parts by weight,

Plastic clay 25 -,

Silicon powder 10

The silicon carbide grains can again correspond to the sieve number s and with this Binder in the ratio of 45Sg of grains and 15sg of binder and the necessary Mixing water can be mixed.

After molding and drying the body, the firing may be performed at the relatively low temperature of 925 ⁰ C.

The silicon metal is preferably, as indicated in the example, used in an amount of from about 10 ° / ₀ of the binder, however, its amount may vary depending on the type of object within wide limits.

example
for boron carbide as a protective substance

It can be a binder mixture similar to the last mentioned in connection with Silicon carbide with a grain size corresponding to sieve number 5 can be used replacing the silicon metal with 2 to 10 parts of finely powdered boron carbide. In this particular case, only 98 g of binder are needed for 455 g of silicon carbide to be used to create a medium hard object.

Boron suboxide acts similarly to boron carbide and can be used in similar proportions will.

It can be seen that the protective material for silicon carbide bodies is such that it itself or its oxidation product is incapable of contacting silicon carbide directly

or its oxidation product (silicon dioxide) near the firing temperature enter into a reaction which produces harmful substances and that he at Use in a sufficient amount is capable of using the silicon carbide under the firing conditions and protect in the presence of the chosen ceramic binder.

The same principles apply to the protection 7 "of diamonds, boron carbide, etc. For example, a perfectly satisfactory grinding wheel can be made from vitrified bonded diamond abrasive. A certain weight of diamond grains of the grain size corresponding to sieve number 40 is taken and this is moistened with a 4 ° / ₀ igen aqueous solution of cellulose ether, then sets 21 g binder per 100 grams of diamond at and provides an intimate mixture forth. Various binders may be used, preferably those which are at relatively low temperature, for example 1 100 ⁰ C or below to burn. Such a low-burning binder »5 can be made from the following mixture:

Powdered frit .... 75 parts by weight,

Plastic Kentucky clay, 25

Aluminum or silicon powder 10

The powdered frit can be made by melting a mass of the the following composition to a liquid glass:

Borax (Na ₂ B ₄ O ₇ · 10 H ₂ O) 3.2.2 _{% / 0} wt.,

Boric acid (H ₃ BO ₃ ) ...... 25.4 ° / ₀ -,

Ground flint (SiO ₂ ). 37.6 "/" -, aluminum hydroxide

(Al ₂ O ₃ -3 H ₂ O) 1.0 o / ₀ -,

Chalk (CaCQ ₃ ). 3.8 ° / ₀ -,

100.0 ° / ₀ wt. _Lo5

After melting, the mass is granulated by pouring it into water and dried and finally ground to a fine powder in a ball mill.

The diamond grains and the raw binder are mixed with the addition of water in such a ratio that the desired density and porosity is achieved, whereupon the firing takes place at a temperature of 925 ⁰ C in an atmosphere, which mainly consists of nitrogen with only a small amount Amount of oxygen there is. This atmosphere, is preferred the pure air which contains about 2i ° / ₀ oxygen to the required performance of the protective substance be reduced. The result is a porous and free-cutting grinding

disc of extreme hardness of the securely bound grain.

In the production of abrasives made of boron carbide grain in a ceramic bond aluminum and silicon are suitable as protective substances for the boron carbide grain. Man can use them with a ceramic binder burning at low temperature · the above use specified type for silicon carbide and diamond grains. The molded article is useful in a Burned atmosphere with a low oxygen content in order to reduce the tendency to oxidize and to be able to keep the amount of protective material required very low. For the practice of the invention It is often desirable to choose binders and preservatives that are consistent with the chemical nature and reactivity of the oxidizable substances to be bound refractory grains stand. Silicon carbide is essentially acidic in character and reacts much more easily with binders or oxides with harmful puffiness of a basic nature than those of an acidic nature. It is therefore used for silicon carbide preferably selected binders and protective substances that are not strongly basic to reduce the chemical reactivity to a minimum. Diamonds that are a form of carbon are essentially neutral, so that the acidic or basic character of binders and protective substances is less important for them. It goes without saying that a protective substance must not be used in this way under the conditions of use act chemically so that it attacks or decomposes the grains to be bound.

The physical form of the protective substance can vary according to the particular request judge. It can be finely chopped or in flake form or in other suitable form To be in a state of affairs. It is usually preferred to use it as a fine powder, when a quick response is required. Its reaction effect not only depends depends on its exposure to oxygen, but also on its size and shape of the grains, and these are chosen so that they are suitable for the particular type of protective substance and the size as well as the type of oxidizable abrasive grains are desired Give protective effect. So there is some kind of regulation of the degree of Oxidation of the oxidizable grains in the change in the size of the protective substance particles in relation to the size of the oxidizable grains, so that the oxygen during the the whole burning time is prevented from acting on the grains. By request Mixtures of different protective substances can be used.

Used powdered glass or powdered frit as part of the raw binder is used, it may be desirable under certain conditions to use the finely divided Introduce protective substance into the glass (the frit) during the manufacture of the glass. This can be done by adding the protective substance to the molten one Glass or the molten frit under suitable conditions before cooling and Mixing in powders of the glass or the frit.

The invention is also applicable to the manufacture of anti-slip tiles from oxidizable ones refractory grains and a ceramic binder.

Claims (1)

Claim: Process for the production of porous grinding bodies, in particular grinding wheels, made of vitrified bonded abrasive grain that can be oxidized at the firing temperature, such as silicon carbide, boron carbide or Diamond, characterized in that the raw mixture of abrasive grain and ceramic binder is lighter than the abrasive grain at the firing temperature oxidizable substance such as aluminum, silicon, magnesium, boron, boron carbide, boron suboxide or a mixture of several of them, is added in such amounts that the whole is in the pores of the Oxygen that is present in the body and when it burns under oxidizing conditions is chemically bound to it will.