3,199,993 United States Patent 0 ice Patented Aug. 10, 1965

1 2 As non-metals may be mentioned C, N, and O, forming 3,199,993 SINTERED BODIES BEING RESISTANT T0 carbides, nitrides, and oxides, respectively with the HEAT, QXIDATION AND WEAR boron and/or silicon of the binder. Nils Gustav Scln'ewelius, Hallstaharnmar, Sweden, A preferred combination of silicon boride and binder is SiB3 and MoSi2, to which may ‘be added colloidal silica, assiguor to Aktiebolaget Kanthal, Hallstahammar, 5 Sweden . The above-mentioned transition metals may be used in No Drawing. Filed Mar. 12, 1959, Ser. No. 798,856 combination with or be replaced by said non-metals, Claims priority, application Sweden, Mar. 13, 1958, preferably oxygen, carbon or nitrogen. The liquids 2,459/58 temperature exceeds preferably 1400° .C. and the total 4 Claims. (Cl. 106-55) quantity of Si-l-B constitutes 10 to 80% of the weight 10 The present invention relates to .a sintered body of the binder. According to an embodiment of the in being resistant to heat, oxidation and wear and manu vention a ceramic binder is used having the composition: factured by compacting and sintering powder mixtures. 20 to 100% SiOZ by weight, 0 to v5% B203, 0 to 80% ‘The invention is particularly concerned with such A1203, and O to 10% Na2O or K20. sintered bodies which contain silicon boride. In this Particularly advantageous binding means have been connection, silicon boride is understood to include found to be bron-silicides the compound being chemical compounds or solid solutions, or mixtures of M(Si,B)2 or M5(Si,B)3 in which M is a transition metal. chemical compounds and/ or of solid solutions, where any A further :binder composition is 15 to 50% Si by weight, such composition contains exclusively silicon and boron. 0 to 20% Al, and 50 to 85% M0 by weight, preferably in the ‘form of MoSiz. Alternatively, the binder may con~ The possibility of producing such materials containing 20 silicon and boron has been realized for a long time, how sist exclusively of silicon carbide or so called boron car ever up until now they have found no practical applica bide. With boron ca-rib-ide is meant a solid solution with tion. The compositions of such silicon borides have the approximate composition B40. Also boron nitride been stated to be, inter alia, SiB3, SiB6 and SiBm. They (BN) and silicon nitride (Si3N4) may form or be in are characterized by great hardness as well as by a rela 25 cluded in the binder. tively good resistance to chemicals and to oxidation. It has not been possible to explain with certainty why The most widely known compound is SiB3. ‘binders of the types above speci?ed are so useful in sin Insofar as the density of SiB3, is 2.6 g./cm.3 and its tered bodies of silicon ‘boride. An explanation may be that hardness very high, one would expect that such a silicon ‘both silicon boride and the binders according to the in boride could ?nd use as an abrasive or as a heat re 30 vention have a very thin coating of SiOz and/0r B203 sistant construction material, for example. It has been which may result in that the two components “Wet” one found, however, that the great brittleness-of the ma another during sintering whereby this process is fa terial puts obstacles in the way of its practical application. cilitated. The function of the binder in the sintered body This is true also for other silicon borides having compo is before all to prevent the grain growth at high tempera sitions differing from SiB3. The silicon borides con ture, it being understood that the grain size is of consid cerned in connection with the present invention, are of erable importance both in the silicon boride .and in the the general composition SiBx in which x ranges from 2.5 binder as will be explained below. to 14. , Sintered bodies according to the invention which are In connection with the present invention it has been to be used at high temperature in an oxidizing atmosphere found, quite surprisingly, that it is possible to produce 40 should preferably contain silicon triboride, SiB3. This in a powder-metallurgical way a sintered body being silicon boride, as a matter of fact, has a particularly high resistant to heat, oxidation and wear and containing resistance to oxidation which is related to its high silicon silicon-boride, provided that there is used as a binder, content. Owing to the fact that the heat of formation or cementing medium, a pulverulent material evenly - per oxygen atom of S102 exceeds that per oxygen atom distributed in the sintered body and containing silicon 45 of B203, solid bodies of silicon triboride, on oxidation at and/or boron and, provided the quantity and composi high temperature, will be surrounded by a thin coating tion of the binder are determined according to certain of mostly SiO2, whereas, on the other hand, the formation rules, it has ‘been found that products are obtainable hav~ of B203 will be suppressed, which results in a high degree ing sufficient mechanical strength to render the product of resistance to oxidation. If the silicon content of the practical for use for a great variety of purposes. silicon boride is a lower one, such as in the case of a The sintered body according to the invention is char compound having the composition SiB12, then the silicon acterized thereby that it consists of 20 to 98% of silicon content of the product will be insufficient to ensure the boride by weight, and 2 ‘to 80% of a binder by weight, formation of a protective coating of SiO2. It is possible, said silicon boride having the composition SiBx, where however, to attain improved resistance to oxidation by x ranges between 2.5 and 14, and the binder contains 55 using as binders heat resistant silicides of a high silicon silicon and/or boron. The total weight of boron and content, such as MoSi2. Combinations of silicon triboride silicon in the binder constitutes preferably 10 to 80% and Mosiz, for example, show sui?ciently good charac by weight of the binder. According to a preferred em teristics to enable them to be used in an oxidizing atmos bodiment the liquids temperature of the binder exceeds phere at temperatures as high as 1600° C. 1400° C. The silicon boride has preferably the compo 60 Sintered bodies according to the invention are pre sition SiB3. pared by compacting and sintering pulverulent mixtures The binder according to the invention may be any of in a suitable atmosphere or under pressure, such as in a a variety of different compositions. Thus, its silicon graphite mold. Such sintering conditions as pressure, and/or boron may be chemically bound to one or time, temperature and nature of the atmosphere will 65 more metals or non-metals. depend to a large extent on the composition of the powder As examples of metals may be mentioned the transi mixture and, therefore, cannot be stated generally. tion metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, and The particle sizes of the raw materials concerned should the compounds of said metals may be silicides and preferably be very small, such as less than 40 microns and preferably less than 10 microns It is often preferred borides respectively, preferably disilicides, such as MoSi2, 70 and silicides with the composition M5Si3 in which M is to use silicon boride and binder with still less grain size, one of said metals. such as less than 6 microns. In order cases the grain 3,199,993 3 4 sizesv of the starting material may be selected according bodies thus obtained consist of 23% SiB3, 68% MoSiz to two or more speci?ed grain fractions with a view that andv 9% of a balance containing SiOz and molybdenum grains of different sizes are combined to produce an boride. The bodies are oxidation resistant at temper volume weight as high as possible. This is often prefer atures up to 1600° C. and are suitable for use as electric able in the case of silicon carbide. If a ceramic binder resistance elements at high temperatures, particularly as is used, itmay, to advantage, be admixed in a colloidal supply conductors .to incadescent zones“ condition, such as in the form of hydrated silicon dioxide or clay. Example 2 Heat resistant Silicides or borides will be admixed 75 parts by weight of silicon boride of a grain size of either in the form of pre-reacted powders, or in the nature 10 10 to 40 microns androf the approximate composition of the various components, such as molybdenum powder SiB12 are mixed with 25 parts by weight of bentonite hav and silicon powder. It is possible also to admix the ing‘a particle size less than 1 micron. binder in the form of powdered metal, provided that the The mixture is compacted into blocks weighing about sintering is carried out in a manner to cause the metal 1 kg, and these are then sintered for 10 minutes in hydro powder to react with a part of the silicon boride while 15 gen at 1600“ C. The product thus obtained consists of forming metal silicides and/or metal borides. Instead 75% silicon boride and 25 % of an oxide component con of adding a powder of a single metal it is possible to add taining 35% Si. It is crushed and screened, whereupon a pulverulent mixture of several metals or an alloy powder. the particle-size fraction ranging'from 3 to 0.5 mm. is During sintering this metallic addition will take up silicon removed. These particles may, to advantage, be used and/ or boron. Also oneor more oxides, such a alumini as an abrasive’ in the production of ceramically bound um oxide, may be added. The sintering is then carried ' grinding wheels which can be calcined in an oxidizing out so that the silicon boride is oxidized, the silica then _ atmosphere at 1500° C. without destruction of the abrasive reacting with oxide addition and forming, for instance, grains by oxidation.v aluminium silicate, at least in a boundary layer between ’ Example 3 the oxide grains and the silicon boride grains. 25 A sintered body resistant to grinding is manufactured In another embodiment the silicon boride powder may by pressure sinteringr for 1 minute in a graphite mold at be sintered in an oxidizing atmosphere to form silicon 1700° C. under a pressure. of 180 kg./cm.2 a mixture dioxide and/or boron oxide in situ. The latter may sisting of 70% by weight of silicon boride having grain constitute the sole hinder or may react withadded binder sizes of 1 to 5 microns and the approximate composition materials. ' 30 SiBm and, further, 25% of B4C by weight, having grain Silicon boride reacts at a very high temperature with sizes-of 3 to 8 micronsand, still further, 5% of carbonyl most substances which must be noted in the manufacture nickel powder. of sintered bodies according to the invention. To cause Example 4 that only a controlled fraction of the silicon boride is then dissolved in the binder quite short sintering times A refractory sintered body is‘preparedby sintering at might be used, for instance, by pressure sintering. The 2050° C. in'a CO atmosphere of a mixture consisting of sintering is preferably carried out in two steps, i.e., ?rst equal parts by weight of SiC with particle sizes of 10 to a presintering in a protective atmosphere and then a 200 microns and of silicon boride of the approximate com secondary sintering in an oxidizing atmosphere, for in position SiB6 and grain sizes of 3 to' 8 microns. After stance, as described in the British Patent 791,324. sintering the composition is 45% silicon boride and the The silicon borides are preferably prepared separately balance binder containing SiC and 134C. by the employment of melt-metallurgical or powder-metal Example 5 lurgical processing techniques. According to the powder matellurgical process mixtures of silicon powder and. A heat resistant sinteredybody suitable for different boron powder are sintered, if desired, under presure at structural parts is prepared by pressure sintering at 1600° high tempeature, for instance in a graphite mold at C. during 2 minutes at a pressure of 200‘ l