DE975182C - Process for making silicon carbide abrasive tools - Google Patents

Description

Process for the manufacture of silicon carbide abrasive articles Manufacture of vitrified bonded silicon carbide grinding wheels was a thing of the past tried so far to protect the silicon carbide grain against oxidation during firing, those used to inflate the abrasive, d. H. led to a loosening of its structure. This has been done in different ways.

One has z. B. the firing is carried out under reducing conditions or binders are used whose melting points are below the temperature, in which an oxidation of the Si C grain can occur in an oxidizing atmosphere.

Another possibility was that as a binder or as an addition to him substances related, which are preferred with the oxygen to the oxidizing React atmosphere and thereby keep it from the Si C grain. But it will a blistered layer on the surface of the abrasive as a result of the reaction formed, which must be removed by dressing before using the grinding wheel.

In making refractories, you already have bonds used, through their effect on the Si C of the body on its surface a Protective layer of Si 02 is created when using the refractory body prevents further oxidation. Such a function is not with grinding tools necessary, for which it is only important to prevent inflation in the event of a fire and to bind the grain as tightly as possible.

In contrast to the known processes mentioned above, silicon carbide grinding tools their abrasive grain completely or predominantly (at least 75 percent by volume) There is silicon carbide and its bond is a glass with 7o to 9o percent by weight Silica is conscious at a temperature (above 400 ° C), at which makes the bond thin, fired under oxidizing conditions, so that SiC grain oxidized over the surface. The Si O2 produced in the process goes in its formation state with the S'02 of the bond a particularly strong connection.

The firing takes place with such a content of the binder in alkali oxide, that the burnt abrasive contains 2 to 10 percent by volume of it, of which is at least 1% / 0 K2 O. This causes the product to swell due to the formation of bubbles avoided.

It has been suggested to use Si C abrasive grain before mixing to subject the binder to an oxidation process in order to create a layer on it of silicon oxide to form. That way should better connect with the S 'O2-containing binders are secured. Even if that was the case, it will anyway this purpose can be achieved more effectively if the additional Si 02 is oxidized generated during firing, since it is easier to deal with the Si 02 in the state of origin Bonding connects, with the addition of the complication of prior special treatment of the Si C grain is avoided.

The new process gives the possibility of manufacturing firmly joined Abrasive media with a very low proportion of binder, because when firing with such a high amount Temperatures can be worked so that the binder is almost completely liquefies and the abrasive grain is perfectly wetted. With perfectly liquefied Binders can also move the abrasive grains closer together, without that due to the denser grain structure, the degree of porosity and thus the freedom from cuts is reduced because as a result of the better adhesion of the grain to the binder The need for this can be reduced to an extent beyond the extent of the reduction of the grain gap goes out.

The high abrasive grain content gives the opportunity to use the raw abrasive to burn without drying beforehand, because they are densely stored after pressing Abrasive grains form a stable framework.

The grinding tools also correspond to the high firing temperature fireproof.

A binder composition that corresponds to the concept of the invention is, for example, the following: Panel I. Weight percent Silicon dioxide, SiO2 .......... 6o Alumina, Al2 03. . . . . . . . . . . . . . . . 25th Potassium oxide, K20 .............. I4 The content of the bond increases by the amount of S'02 formed by the oxidation of silicon carbide during firing with a corresponding decrease in the percentage of the other constituents.

A convenient way of producing the molding compound is that silicon carbide grains are first moistened with a potassium silicate solution and the Wet grains are coated with finely powdered clay, preferably kaolin. To this In this way, the desired K2 O content is readily obtained and at the same time as a result the high plasticity of the compound of an alkali silicate solution with kaolin is easy malleable mixture which is shaped and pressed in the usual way.

Molded bodies produced in this way, such as. B. grinding wheels, have such high strength when wet that they cannot be used before the fire needs to dry.

The abrasive grit can be made in an ordinary ceramic furnace, e.g. B. a tunnel furnace, in an oxidizing atmosphere with a heat treatment approximately accordingly Seger cone I6, which corresponds to a peak temperature of around 1425 to 1450ºC, to be burned.

The invention has proven to be particularly valuable for grinding tools which consist of coarse silicon carbide grains, in particular of such products, in which at least two-thirds of the abrasive from crown size 30 (pass through sieve with I44 meshes per cm2) or coarser. However, of course you can Using the same process, grinding wheels with a finer grain can also be advantageously manufactured will.

Mixtures of silicon carbide grains with other grains can also be used. A product which contains at least 75 percent by volume of the abrasive content in the form of silicon carbide, for example, would still have to be regarded as a silicon carbide abrasive in the present sense. Example I 36.25 kg of silicon carbide abrasive grain I4 (passage through sieve with 16 meshes per cm2) were first mixed with 2.265 kg of a potassium silicate solution with a content of 26.80 / uSiO2 and 12.64 / o K, O and then with 1.675 kg of a mixture of 9 parts Florida kaolin and 1 part dry sodium silicate powder (Na2 O - 3.22 Si 02) mixed. During this mixing process, the grains were coated with the sticky potassium silicate solution and, in this state, easily adhered an even layer of the dry powdery ingredients. In the resulting mixture, the individual grains were relatively dry on the outside, but stuck together when pressed together due to the moisture under the powder coating. The composition of the binder at this stage on a dry basis was as follows: A portion of the mixture was then pressed in a mold into a disc 61 cm in diameter, 5.1 cm in thickness with a 6.35 cm hole. The density of the freshly formed disk was 2.33 g / cm3. This now had the following composition and structure on a dry basis. Plate III Weight volume percent I percent Silicon carbide ......... 93.9 65.8 Binder ............ 6.1 5.7 Pores ................. 28.5 The disk was then fired at Kegel 16 in a large tunnel kiln with a cycle time of 8 days. The fired disc was completely black, with no superficial irregularities, and had the following properties: Plate IV Volume weight ............... 2.25 g / cm3 Modulus of elasticity ............ 92 - 1o10 dynes / cm2 Weight gain ............ 2.60 / o Dimensional changes ............. 2, o percent by volume enlargement Weight percent silicon carbide after analysis .............. 86.51 / o Weight percent binder after analysis .............. r3.50 / 0 The chemical analysis of the binder in the burned disc showed: From this data the final structure of the disk was calculated as follows: Table VI 6o, 8 percent by volume silicon carbide, 12.6 percent by volume binder, 26.6 percent by volume pores.

Microscopic examination showed that the binder was almost complete Glass with traces of mullite crystals was.

A series of such disks that pass a normal grinding test were found to be free cutting and gave a higher by 380 / o Grinding quality number than similar grinding wheels with conventional porcelain-like Binding. The quotient from the square of the abrasion of the test object is used as the quality number in the hour and the wear of the grinding wheel in cm3 per hour, which experience has shown A useful measure of the economy of a grinding wheel gives when grinding wheels of a certain degree of hardness can be compared, as is the case here was.

Example II In this example, part of the binder is a inert material that remains solid and undissolved in the fire and in the final Product present in the form of crystalline particles dispersed in the glass bond is. The inert material was purposely added to make up the volume of the binder without reducing the volume of liquid present at the peak temperature Glass is enlarged.

Mixing, shaping and firing was carried out essentially as in Example I, but the mixture had the following composition: Table VII 7.525 kg silicon carbide abrasive, grain 20, 0.726 kg 40% potassium silicate solution, 0.48 kg Florida kaolin, o.113 kg of ground zircon.

This mixture was pressed to a density of 2.34 g / cm3. To the product had similar good properties as the pane according to the fire Example I.

Material other than zirconium can be used, such as zirconia. B. silicon carbide, vitrified silica, pottery pebbles, etc.

Example III It is not essential that the clay used be Florida kaolin or kaolin at all, although clay is considered a desirable ingredient and kaolin is high in clay. However, low alumina clay can be used with success as in the following example in which mixing, molding and firing were carried out essentially as in Example I. Plate VIII 4 = 54 kg silicon carbide, grain i mm, 180 pounds Mississippi Shaped Clay, 0.317 kg of 400% potassium silicate solution. Articles made with this had very similar physical properties to the articles made according to Example I.

EXAMPLE IV Although it is convenient and advantageous, because of the great raw strength which it imparts to the shaped bodies, to add the potassium silicate in solution, it can also be added as a dry powder. The following mixture has been successfully molded and fired. Plate IX 3.63 kg silicon carbide, grain i mm, 0.14 kg potassium silicate powder, o, 196 kg Florida kaolin, 0.17 kg of water, 0.036 kg of dextrin. The silicon carbide was moistened in a mixer with the water containing the dextrin dissolved; Then a mixture of dry potassium silicate and Florida kaolin powder was sieved in with a further processing mixer until the individual silicon carbide grains were covered with them. This mixture was then molded in the usual manner. The shaped panes were dried and then glazed at cone I6.

Example V Discs of lower hardness and with a finer abrasive material can be made by this binding process. Such slices become common used for grinding hardened carbide tools and sometimes with raw silicon carbide manufactured. As an example of this, grinding wheels were made from the following mixture 8 "diameter and 1.6" thick with a 3.17 "diameter hole that were fired in a tunnel kiln at Kegel I6. Panel X 2.265 kg Silicon carbide, grain 0.25 mm, 0.166 kg 40% potassium silicate solution, 0.097 kg Florida kaolin. After firing, these disks had a structure of 45 volume percent S'C, 9.3 Volume percent binding and 45.7 volume percent pores. They behaved favorably in an attempt at grinding hardened carbide.

Other disks of similar structure have produced satisfactory results made using silicon carbide, grain 0.09 mm. Mixing, shaping and baking were carried out essentially as in Example I.

One way of realizing the purposes and advantages of the invention is to provide a vitrified binder whose composition with respect to silicon oxide, aluminum oxide, potassium oxide and other alkaline oxides is within the following limits: Plate XI Weight percent Silicic acid, Si02 ......... ....... 7o to 9o Alumina, Al2 03 ............. . . . . . 4 to 2o K20. . . . .............. at least I. Total content of alkali oxides, K2 O, Na2 O, Li2 O, Rb2 O, CS2 O 2 to IO The fact that five alkali oxides are mentioned here does not mean that they should all be present in the binder. The total content of 2 to 10% is particularly important, but what is important is the presence of at least 1% / o K2 O. This alone can make up the entire amount of alkali oxide, but Na2O will usually form a considerable part of it, while of course Lit O , Rb2 O and Cs2 O will be present in smaller amounts, if at all.

Other glass bonds can be made from components such as K20, Si02, AI203, E "alkali oxide, boric acid, manganese oxide, etc. are produced. Examples are in the Range from 50 to 70'0 / o S'02, 10 to 25'0 / o A1203, 3 to 150 / o alkali inclusive more than 1% / 9 K20, together with at least i% / a of another flux under Inclusion of up to 50% alkaline earth oxide, up to 5% boric acid and up to 5% Manganese oxide, all by weight.

Claims (5)

PATENT CLAIMS: I. Process for the production of silicon carbide grinding wheels, whose abrasive grain is completely or predominantly (at least 75 percent by volume) silicon carbide and the bond of which is a 70 to 90 weight percent silica glass, thereby characterized that the firing with such a content of alkali oxide in the binder, that the burnt abrasive from him in the binder 2 to 10 percent by volume, of that at least I'0 / 0 K20, under oxidizing conditions at one temperature is carried out above 400 ° C, at which the bond becomes thin. 2. Procedure according to claim I, characterized in that the abrasive grain with an aqueous Solution of a silicate, mainly of potassium silicate, is coated on the the admixed powdered ceramic dry material adheres, and that from the thus produced mixture molded compacts is fired without prior drying. Considered publications: German Patent Specifications No. 4,693, 37,668, 323 875, 4I2 I57, 648 232, 649 3I 2, 738 490, 858 378; German patent applications D 73864 IVc / 8ob (announced on 9. 3. 1939), N 37370 IVc / 8ob (published on I. 4. 1937); French Patent No. 5I4 103; British Patent Nos. 488 644 526 130, 563 652, 572 770, 575 237, 575 318; U.S. Patents Nos. 20,868, 978,679, I I72,659, 1444 I62, I 546 II5, 1918 242, 1987 86I, 2,158,034; U 11 man: "Encyclopedia of Technical Chemistry", 2nd edition, I93o, vol. 5, p. 779; Rauh: "The practical basics of grinding wheel manufacture", 1934, P.26; Zeitschrift "Talksaal", 195I, p. 297; Report FIAT F. R. No. 370, p. II ; BIOS report FR No. 1407, p. 5.