US 2844492 A
Description (OCR text may contain errors)
E. FlTZER MATERIALS AND FORMED BODIES Filed Feb. 23, 1954 HALIDES'OF Si Dr B ACTIVATIONIN REACTION WITH vAPOR AND CARRIER :HEATING zONE BODIES To BE GAS I INICONTACT TREATED WITH METALLIC Si or B INITIAL METAL POWDER CONVERTED METAL MOLDING POwDER A ACTIvATI'ID ATMOSPHERE CONvERTED METAL POWDER AND Mo MOLDING sINTERINO ACTIVATION OF HALIDES OF B or Si MOLDING T SINTERING FIG. 4. ACTIVATED E ATMOSPHERE METALLIC SKELETON f 8 METALLIC POWDER SINTERING ACTIVATED F/ 6. 5. ATMOSPHERE INVENTOR.
WERI D FITZER BY 2 ATT'Y.
2,844,492 METHOD OF PRODUCING HEAT RESISTING METALLIC Werke Aktiengesellschaft 'fuer rKohlfahrikate, Mei- -t ingen, near 'Augsburg Germany, a-corporationof Ger- 7 "application February 2s,' 1 as4,=serial No. 4113 49 aims pi'iority applicafion Austria February 26 ,1-1953 I 4 Claims. Cl. 117-66) This invention relates to the production of metallic.
materials :and is particularly concerned with a method 1 of producing heat resisting metallic materials, that is, *metal- StatesPatent chlorides are not subjected to any -cooling-prior'tofthe vaporization reaction, at any rate, no cooling be lowjthe' temperature of the vaporization reaction. The activation presumably causes formation of low gradehalogen compounds which are unstable at a lower temperature; The catalytic reduction of the semimetal chlorides-by thehydrogen carrier. gas, during the vaporization reduc tion or rather the heterogeneous metallic exchange reaction of the semimetal chlorides with the solid rnetal that is toform the hard :material would thusseem'to be facilitated and in some cases' actually eliectedyby the formation of unstable halogen compounds. These heterolic'nnaterials which remain substantially :constantin the presence of high temperatures-and whichiare adapted-for making e'lectrica'l heating resistorsor structural apparatus and machine elements exposed to temperatures on the order of about 1750" C., for example, protective-tubing, nozzles, burner heads, parts-of gas turbines an'd thelik'e.
Such materials are made of chemical compositions of" the so calledtransition-metals of the'fourth to sixth groups ofi'thmperiodic system of elements (Ti, Zr,"-Hf, V', Nb, la,
Cr, Mo, W) containing semimetals such as Si,- B; Di-' silicid'es "and "diborides of molybdenum and chromium are particularly suit-able for the above mentioned :pur-
"These :compositions are also referred "to as hardmaterials because of the great :hardn'ess exhibited there-u by. Formed= ormolded bodies are usually produced from these' hard materials by powdered metal, taking cor-re spending initial materials or starting with form'e'd i'metallic bodies whose metal is converted into th desired semimetal composition.
' Of theknown methods of producing ane'talliodisilicideis I I and metallic diboride's, that method which'is daemon- "tedlinolog'i'c'al interest is :the one in which the bonversion I of tl1e metals or their alloys which are to form the hard materials is accomplished with vaporized halogen compounds of boron orsilicon. This vaporizingmethod which is applicable for siliconiz ing or 'boronizing of metallic powders as Well as "of metallic 'b'odi'es h'as, however; the drawback of -requiring long reaction tim'es "and exhibiting a spatially differentiated rea'ction, causing formation-of-dis'turbing coating on h :t a metallic bodies.
e mvention accelerates the reaction rand improves its -efliciency, making it not only possible t'o convert hardmater-ial into disilicides or diborides but -als'o Tto form on the surfaces ofrelatively'thick metallic "parts :a "well pounds oifer inaddition purification possibilities and therefore the making of purest hard materials free of foreign metallic admixtures; I
, In accordance with the invention, these advantages are obtained lay-activating vaporized halides of silicon 'or bOIGll, prior to the reaction with the metal that is zto form -thechardymaterial, by conducting it over largesur faces of metallic silicon or boron, at a temperature which metallic powder or thin bodies of metals forming the adhering-protective coating of metallicidisilicide or metallic di-boride, respectively. The volatile halogen conrcauses the formation. of unstable halogen compounds.
, .Th'e fs'iliconor boron-halides are thereby suitably .in't'erm'ixed with- :a neutral or .a reducing carrier gas: and are r in .a' heating zone activated by the Silicon or boron'or alloys thereof. vIt was found that the best actlvation takes place at the .highest temperatures provided that the activated formed at the ligher temperatures.
geneous reactions may also be superimposed'upon contact with the metallic surface which is at lower tempera-.
ture by a thermal decomposition of the subchlorides The activationmethod according to the inventionoffers the further advantage"oftpermitting incident to circulation of the vaporization atmosphere, ina closed cycle, during the activation, a regeneration-of the-halogen or hydrogen halidesproduced in the vaporization reaction.
The method according to the invention may be practiced-in'various ways, for*example, to produce from cor-' responding metal powders hard materialpowder for use as an initial substance for making formed. or molde bodies by molding'or pressing and sinteringflq V The metalsand alloys which are to form thehardma- 'terial are for this purpose exposed to thetreatmentwhile in powdered form at desired particle size[ It is merely necessary :to provide the greatest possible exposed surface area and :tO renew continuously the vaporization atmos phere while maintaining the temperature. A slight uniform. agitation of the metallic powder is of advantage. The co-called flowingmaterial bed-may be advantageously;
used for forming such'hard materials from corresponding metallic powders. :It may be mentioned as an example thatit is possibleto convert molybdenum ,power with an average particlesize-of In, at l 200 in a-few minutes completely into MoSi -by the use of silicon chlorides free of HCl and activated at 1300 C. over silicon powder. The resulting disilicide powder can be directly. pressed and worked without any prior comminutionor screening.
It is possible to produce in this manner "heating conductors, structural furnace parts, protective tubing, nozzles,
burner heads, parts for turbines and internal combustion engines, in short, all constructional parts and elements which are exposed to temperatures 'on the order of about 1750" C. Particularly favorable is the constancy of or resistance against V 0 corrosion, of such MoSi bodies made in accordance with the invention, making them usable in'interna'l combustion engines operated with crude oil without having to fear by clinkering.
.However, these hard metallic disilicides the temperatures that may be reached in a protective .gas. Molybdenum powder is added to improve'the' molding properties of MoSi powder.
atmosphere containing activated boronor silicon-halide. The sinter atmosphere is thereby continuously conducted through the sinter chamber and is subjected to activation prior to entering the sinter chamber byconducting itgover metallic boron or silicon. The treatment of the molded body with boron or silicon halide is suitably continued during the sintering until an excess of a semimetal, that is, silicon or boron is precipitated upon the surface of the molded body.
2,844,492 I Patented July 22, 519 58 accelerated oxidation 7 and dib'orides, for example, MoSi can be molded only by the use of. very high pressures and their sinterability is wanting at In order to convertthe metallic powder contained in the powdered mixture, upon.
' sintering,- into the corresponding disilicide or dib'oride, the invention proposes to sinter the molded body in an- It was found, for example, with disilicides that a particularly favorable oxidation resistance of the treated molded body is present when such body exhibits a relatively lowsilicon excess as compared with the stoichiometric composition of the disilicide. Such an excess of semimetal which settles in the pores of the hard material body may also be obtained by thermal decomposition of vapor-formed unstable halogen compounds of the semi-metal during the sintering procedure. purpose, advantage may be taken, for example, of compounds which are stable at lower temperatures, for example, silicon iodide, which decomposes thermally on the highly heated molded body in known manner. A halogen compound which is stable at higher temperature and which is formed directly prior to decomposition will also decompose incident to cooling upon contacting the molded body, and will thereby precipitate the desired semimetal thereon. Such a thermal decomposition may also take place in the present method when the activation temperature of the vaporlike halogen compound of the semirnetal is above the reaction temperature of the metal which is to form the hard material. The catalytic reaction during the vaporizing procedure, that is, the metal exchange reaction between metal and semimetal, for example, molybdenum and silicon, is facilitated by the formation of unstable compounds and is, silip ported by a thermal decomposition of the halogen compositions.
Alloys may also be used for the powdered mixture of the hard materials which is to be sintered, that is, disilicides or diborides of the metals which are to form the hard material. The thermal decomposition ofthe vaporlike halogen compounds not only causes a conversion of the metallic particles of the sinter body to form the corresponding metallic disilicides or diborides, but there also occurs an enrichment with metallic silicon or boron especially along the surface layer. A sinter body is thus obtained which is characterized by an especially dense surface and which is oxidation proofat high temperatures.
In the treatment of pressed or molded bodies, made of metal powders which are to form hard materials, with activated reaction gas, during the sintering, it is sufiicient if this reaction treatment is carried on only during part of the sintering time.
The new method of producing hard materials by conversion of activated halogen compounds of the semimetals with the metals that are to form the hard materials is not limited to the treatment of powders of such materials. It was found that the method is excellently adapted for treating formed or molded bodies made of the co'rresponding metals. According to the thickness or wall thickness,
respectively, of the molded bodies, annealed in a reaction gas, a hard material coating or layer will thereby form upon such bodies or they will undergo conversion int-o the corresponding hard material. The formation of the hard material layer or coating is accomplished very quickly with the present method, and the layer will accordingly be of fine grain and will not tend to separate. Prior methods provided coarse grained layers or coatings and caused recrystallization of the base metal.
The present method of boronizing or siliconizing transition metals of the fourth to sixth groups of the periodic system may also be advantageously used in cases where highly temperature proof bodies of relatively large dimensions are to produced which cannot be made solely from powdered metal in molds or press forms. In such cases, the hard material powder is pressed or molded into parts forming a metallic skeleton. The resulting molded body consisting of a powder mixture and a solid metal body is thereafter subjected to sintering in an atmosphere containing activated vaporized halogen For this compounds of silicon or boron so that the metallic skeleton part is converted into the corresponding disilicide or diboride at least in its surface layer. The disilicides or diborides which are present penetrate thereby into the metal so that a solid body will result which exhibits great mechanical strength even at high temperatures.
It is possible to produce in this manner, for example, satisfactory heat conductors of MoSi by using molybdenum parts, for example, in the form of concentric tubes and molding MoSiz powder into the annular space therebetween. The resulting body is thereafter sintered in an atmosphere containing an activated silicon halogen compound so that the metallic molybdenum tubes are covered with a protective layer of molybdenum disilicide.
It is understood of course that different metallic skeleton bodies may be employed, for example, wires to be used as heaters, and that hard material powder may be molded thereabout to increase the mechanical strength thereof.
It is possible to produce in this manner long thin heater wires with desired resistance, which could not be made before because only relatively short and thick molded bodies could be produced of pressed and sintered pulver mixtures alone which had suflicient mechanical strength.
The various phases of the method described herein are illustrated in identical sequence in the accompanying diagrammatic drawings Fig. 1-5. Known apparatus may be used to carry out the required operations.
What is believed new and desired to have protected by Letters Patent is defined in the appended claims.
1. Method of producing high-melting material from a base metal taken from transition metals of the fourth to sixth groups of the periodic system of elements by reaction at elevated temperature of one of said metals with a vaporized halide compound of a semi-metal taken from the group consisting of silicon and boron, comprising forming a vaporized halide of said semi-metal, activating said halide prior to reaction with the base metal by large area contact at elevated temperature with a corresponding metallic semi-metal to form an un-.
'of la, reaction of said molybdenum powder with said activated halide compound being efiected at 1200 C., said reaction causing conversion of said molybdenum powder to form molybdenum disilicide.
4. A method according to claim 3, comprising molding said molybdenum disilicide to form a desired body, sintering said body, and exposing said body during the sintering thereof to the action of a gas mixture comprising said activated halide and a carrier gas.
References Cited in the file of this patent UNITED STATES PATENTS 2,206,395 Gertler July 2, 1940 2,307,005 Ruben Dec. 29, 1942 2,657,128 Stern et a1. Oct. 27, 1953 FOREIGN PATENTS 435,754 Great Gritain Sept. 23, 1935 598,181 Great Britain Feb. 12, 1948