CA1058841A

CA1058841A - Method of obtaining cast refractory inorganic materials

Info

Publication number: CA1058841A
Application number: CA255,715A
Authority: CA
Inventors: Alexandr G. Merzhanov; Vladimir I. Jukhvid; Inna P. Borovinskaya; Fedor I. Dubovitsky
Original assignee: INSTITUT KHIMICHESKOI FIZIKI AKADEMII NAUK SSSR
Current assignee: INSTITUT KHIMICHESKOI FIZIKI AKADEMII NAUK SSSR
Priority date: 1975-06-26
Filing date: 1976-06-25
Publication date: 1979-07-24
Also published as: CH623019A5; ATA427876A; SE7607145L; DE2628578B2; AT374160B; IT1063627B; DE2628578C3; SU617485A1; DE2628578A1; FR2317253A1; GB1497025A; FR2317253B1

Abstract

METHOD OF OBTAINING CAST REFRACTORY INORGANIC
MATERIALS

Abstract of the Disclosure A method of obtaining cast refractor inorganic materials comprises mixing at least one oxide of a metal selected from the IV, V and VI groups of the periodic system with a metallic reducer and a non-metal or an oxide thereof. Then a small por-tion of the mixture surface is ignited to produce a combustion zone. Combustion is conducted in a gaseous medium under a pres-sure of 1 to 5,000 atm.
The proposed method permits obtaining cast carbides, bo-rides, silicides and nitrides of metals of the IV, V and VI
groups of the periodic system, as well as cast hard alloys based thereon. The resulting compounds exhibit high hardness, strength and wear resistance.

Description

lOS8841 ~ he present invention relates to p~oducing rsfractory in-organic materials, a~d more speci~ically to a method of obtain-ing cast refractory inorganic materials, including æuch compo-unds as carbides, borides, silicides, and nitrides of metals selected ~rom the IV, V and VI groups of the periodic system, as well as hard alloys and hard alloys with alloying additi-ves. ~hese materials ~eature high hardness, strength, wear resistance and ~ind extensive application in the manufacture of cutting tools, chemical~, refractories, abrasives and ma-chine tools.
~ here is known a method o~ obtaining cast refractorg inorganic material~ by way o~ hèating the powder of inor-ganic compounds in electric orens or electric arc under at-mospheric pressure to a melting point as high as 2,000 to 4,000E.
~ his method is disadvantageous in that it is impossible to obtain cast rsfractory inorganic materials, such as borides, silicides, and carbides o~ tungsten, chromium, molybdenum pos-~essing properties meeting industrial requirements because o~
their partial decomposition at the melting point. ~his leads to ~ormation of a ~ree metal and a non-metal, which results in lower melting point and hardness o~ the material~ as well as results in ~ormation o~ pores and cavities.
Other disadvantages of the prior art method are the pos-sibility of producing onlg small castings, the necessity to

- 2 -obtain, first~ a powder o~ the refractory inorganic compound, the use of sophisticated equipment, low capacity of this equip-ment, and high electric power co~sumption.
Industrial hard alloys are also produced by sintering pow-ders o~ a refractory inorganic compound and a binder metal at elevated temperatures and under high pressures.
Disadvantages inherent i~ this method are the poQsibility of obtaining only small si~tered specimen~, their porosity and presence of free carbon therein, which affects their strength, the use o~ sop-histicated equipment, low capacity of this e~uip-ment and high consumption o~ electric power.
~ here is known still another method o~ obtaining starting powders of refractory inorganic compounds for producing cast materials, by igniting a small portion of the sur~ace layer o~
the metal-non-mctal mixture, i.e. heating it to a temper~ture suf~icient for initiating the process of combustion in a thin layer of the starting components, in which case the propaga-tion of the combustio~ zone ~rom one layer to another is due to the heat reIeased as a result of the reaction between the starting co~ponents and heat transfer. ~hus, the reaction pro-ceeds in a thin layer of the mixture, which is known as the combu~tion zone, where temperature is as higrh as 2,000 to 4.000E. The combustion zone propàgates in the mixture at a rate of 1 to 15 cm/sec. The process is conducted in an air-tight vessel, in an inert gas medium.

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It is an object of the present invention to pro~ide a method of obtaining cast re~ractory inorganic materials with chemical composition, hardness, mechanical properties and po-rosity such as to meet industrial re~uirement3.
Another object of the invention is to provide a method of obtaining cast refractory inorganic materials, such as will permit producing castings of any size and shape.
Still another object of the invention is to provide a me-thod o~ obtaining cast refractory inorganic materials, such as will permit producing castings in the process of synthetizing refractory materials, which eliminates the step of preparing, in advance, powder~ of refracto~y inorganic compounds.
With these and other objects in view, the present inve~-tion is aimed at providing a method of obtaining cast refrac-tory inorganic materials, wherein, according to the invention, at least one oxide of a metal selected from the IV, V and VI
groups o~ the periodic system is mixed with a metallic reducer and a ~on-metal or an oxide thereo~, whereafter a small portion of the mi~ture surface is ignited in a gaseous medium, which results in propagation of the mixture combustion zone with the mixture burning under a pressure of 1 to 5.000 atm, to obtain the end product.
~ o obtain cast hard alloys based on refractory inorganic compounds, introduced into the mixture should pre~erably be nickel, cobàlt, molybdenum or oxides thereof in an amount o~
~rom 5 to 20 wt %.

~o5884~

To improve the mechanical properties of hard alloys, it is expedient to introduce into the mixture such alloying additives as manganese or magnesium in an amount o~ from 1 to 5 wt %.
To prevent the volatile components of the mixture from evaporating ~rom the mixture and dissociation of the end pro-duct, synthesis should pre~erably be conducted under a pressure o~ 00 to 5,000 atm.
~ o prevent porositg and cavities in the end product, synthesis should preferably be conducted in steady rotation conditions at centrifugal accelerations of 100 to 1,500 g, wherein g is the ~ree fall accele~ation~ and in a gaseous me-dium under a pressure of 1 to 100 atm.
~ o obtain cast re~ractory inorganic materials the synthe-~is temperature whereof i~ below their melting point, the mix-ture should preferably be heated prior to ignition to a tempe-rature equal to the difference between the melting point .
of the end product and the normal synthesis temperature.
The herein proposed method of obtaining cast refractory inorganic materials is realized as follows.
Introduced into a reaction mixture including oxides of me-tals selected ~rom the IV, V and VI groups of the periodic system, a metallic reducer, such as aluminum or magnesium, is a non-metal or an oxide thereo~, for example~ carbon~ boron, boron oxide, silicon, silicon oxide, nitrogen; then, a small ~o58~4~

portion of the mixture sur~ace is ignited in a gaseous medium under a pressure o~ 1 to 5,000 atm. In obtaining nitride~, u~ed as the ~aseous medium is ~itrogen, while in the other ca-ses use is made of an inert medium, such as argon.
Pressure is required in the process of synthesis to pre-clude violent escape and explosion o$ the reaction mixture.
I~ addition, to prevent evaporation of the volatile components a~d dissociation of the refracto~y inorganic components, synt-hesis is conducted under a pres~ure of 1,000 to 5,000 atm. The max~ mum pressure is limited by the strength o~ the reactor in which synthesis takes place.
~ ynthesis is based on utilizing the heat of the chemical reaction of interactio~ of the reaction mixture components and takes the $orm of combustion. The mixture is ignited by a heated tungsten coil or any other conventional means locally in the æurface layer. Combustion proceeds as $ollowss the sur-face layer of the mixture is ignited and reacts evolving a great amount of heat which is partially trans$erred to the adjacent cold layer; the latter is heated, in turn, ignited, enters into reaction, part of itæ heat being transferred to the next adjacent layer by way of heat conduction.
Formed in the course of synthesi~ are the end product and slag, i.e. oxide o$ the metallic reducer, which are liquid at the syntheæis temperature and separated because of difference in their specific gravities~ At the end o$ the process, they æolidi~y $orming distinct layers and can easily be separated $rom each other.

.

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~ he sizes and shapes of the resulting cast refractory inorganic materials are determined by those of the reactor, which may be various.
In order to obtain hard alloys, introduced into the reac-tion mixture in addition to the above-mentioned components is a binder metal, for example, nickel, cobalt, molybdenum or their oxides in an amount of from 5 to 20 wt %~ Introduction of a lesser amount of the binder metal does not ensure the required plasticity of hard alloys, while an excess of the binder metal substantially reduces their hardness.
~ o prevent porosity and cavitie as well as to accelerate separation of the end product from slag, synthesis is conduct-ed with a centrifugal acceleration of 100 to 1,500 g (g being the free ~all acceleration) in steady rotation conditions.
Centrifugal acceleration is created by a centrifuge. The centrifuge is a thick-walled reactor in which synthesis is conducted and which rotates about its axis.
~ he centrifuge provides for a gaseous medium pressure from 1 to 100 atm and centrifugal acceleration from 100 to 1,500 g-~ o impart to hard alloys the required mechanical proper-ties, ~or example, impact resilience or bending strength, in-troduced into the reaction mixture is an alloying additive, such aæ manganese or magnesium, in an amount of from 1 to 5 wt %.

Introducing more than 5 wt % o~ the additive is not recommended for this may result in lower hardness and melting point of the allogs.
When it is necessary to obtain ca~t refractory inorganic materials the synthesis temperature whereo~ is less than their melting point, for example, carbides of tungsten, zirconium, hafnium, or tantalum, the starting mixture i~ heated, prior to ignition.
~ he temperature of heating the starting mixture must be su~ficient for the inorganic refracto~y material to melt in the process o~ synthesis noDmally the heatin~ temperature ~or different mixtures is selected e~perimentally. In the case of mixtures, whose synthesis temperature is known, the starting mi~ture heating temperature is equal to the difference between the melting point of the refractory compound and temperature of its synthe is under normal conditions. In the case o~ tungs-ten carbide, for example, the mixture is heated to a tempera-ture o~ 300 to 550C-The proposed method permits obtaining cast refractory in-organic materials of any size and shape with properties ~atis-fying industrial requirements. Obtaining a cast material by this method enables dispensing with sophisticated equipment including presses and ovenæ a~ well as some of the steps nor-mally indispensable when such materials are produced b~ sinter-ing u~der industrial conditions, namely:

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1) the step of producing metal from its oxide and non--metal from its o~ide~
¦ 2) the step o~ synthetizing the starting refractory com-pound.
The method of this invention will be better understood from the following example~ illustrating the manner in which it can be realized.
Exam~le 1 Obtaining ¢ast Mo2C.
288 g of MoO3~ 12 g of carbon and 108 g o~ Al were mixed in a mixer for one hour. The re~ulting mixture was charged into a reiractory cylindrical container and pac~ed therein~
bhen, the container with the mixture was placed in a reactor.
The mixture was ignited by mean~ of a heated tungsten coil, the pressure of the gas in the reactor being equal to 1 atm.
Combustion was accompanied by violent escape of the mixture.
After combustion, the sample was allowed to stay in the reactor for one hour during which period the end product (Mo2C) wa~ completely separated from slag (Al203), both compo-nents be mg liquid at the s~nthesis temperature; then, both components solidified in two distinct layers and were cooled down to room temperature.
.

End Product _haracteristics Obtained as a result of synthesis was a cast cylindrical piece divided into two layers. The top layer was grey and brittl ~ _ g _ le (A1203), while the bottom layer was ~ilvery and hard (Mo2C). ~he yield was 20 wt % o~ the expected value because o~ the mi~ture escaping violently from the container, resulting in tin~ pores and cavities in the end product.

X-Ra~ Phase Ana~is_Data The end product wa~ a mixture of Mo2C and Mo.

Qhemical_~nalysis Data_ Content of ~ixed carbon~ in wt ~: 5.85 - calculated for Mo2C~ 4.2 -Iound.
., Microhardness _nd De_s_t~ Ana~ysis_Data Microhardness, in kg/mm2s 1,500 to 1,800 - according to literature~ 1,790 - found.
De~ity, in g/cm3: 9.2 - according to literature; 9.1 found.
Exam~le 2 Obtaining ca~t Mo2C.
288 g of MoO3, 12 g o~ carbon and 108 g of Al were mi~ed in a mixer for one hour. ~he resulting mixture was charged into a refractory cylindrical container ~nd packed therein.
~he container with the mixture was then placed in a reactor.
~he mixture was ignited by a heated tungsten coil, the pres-sure of argo~ in the reactor bei~g 100 atm.

- 10 _ 105884'1 During combustion, no Yiolent escape of the mixture was observed. After combustion, the sample was removed from the container.
End Product Ch3racteristics The synthesis product was a cast cylindrical piece di-vided into two distinct layers. The top laye~ was grey and brittle (A1203), while the bottom layer was silve~y and hard (Mo2C). ~he yield of Mo2C was 100 wt % of the expected value.
The end product is ~ree o~ pores and cavities.

Chemical Ana~y~is Data Cont~nt of fixed carbo~, in wt %: 5.85 - calculated for Mo2C; 5.6 - ~ou~d.
X-Ra~ Phase Anal~s~s_Data ~ he end product was Mo2C.

1 , Microhardness and Den~it~ Ana~is Data Microhardness, in ~g~mm2: 1,500 to 1,800 - according to literature~ 1,790 - found.
Density, in ~/cm3: 9.2 - ac¢ording to literature; 8.8 -- found.
ExamPle 3 Obtaining cast Mo2C.
288 g o~ ~oO3, 12 g o~ carbon and 108 g of Al were mixed in a mixer for one hour. The re~ulti~g mixture wa~ charged into lOS8841 a re~ractory cylindrical container and packed therein. The container with the mixture was then placed in a reactor. ~he mixture was ignited by a heated tungsten coil, the pressure of argon in the reactor being equal to 2,000 atm. Combustion proceeded without violent escape of the mixture. After combus-tion, the sample was removed from the container.

End Product Characteristics The synthesis product was a cast cylindrical piece divided into two distinct layers, the top layer being grey and brittle (A1203) and the bottom layer being silvery and hard (Mo2C). ~he yield of Mo2C was 100 wt % o~ the expected value. ~o pores and cavities were found.

Ch_mic_l_Ana~ysis Data Content of fixed carbon, in wt %: 5.85 - calculated ~or Mo2C~ 5.8 - found.
g-Ra~ Phase A~al~sis_Data The end product was Mo2C.

Microhardness 3nd Densit~ Ana~y is Data Microhardness, in kg/mm2: 1,500 to 1,800 - according to literature; 1,790 - found.
Density, in g/cm3s 9.2 - according to literature; 8.8 -~ound.

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~xam~le 4 Obtaining cast Mo2C
288 g o~ ~oO3, 12 g of carbon and 108 g o~ Al were mixed in a mixer for one hour. The resulting mixture was charged in-to a refractory cylindrical container and packed therein. The container with the mixturs was then placed on a centrifuge which was rotated. The centrifugal acceleration was brought up to 1,000 g, wherea~ter the mixture was i~nited with a heated tungsten coil, the pressure in the reactor being 1 atm. Comb~ -tion proceeded without violent escape of the mixture. After combustio~, the sample was withdrawn from the container.

~nd Product Characteristics _ _ _ _ _ _ _ _ _ _ _ _ _ _ The synthe~is product was a cast cylindrical piece divid-ed into two distinct layers. The top layer wa~ gre~ and brittle (A1203), while the bottom layer was silvery and hard (~o2C).
The yield of ~o2C was 100 wt % o~ the expected value. The end product was free of pores and cavities.

X-Ra~ Phase Anal~s_s_Data ~ he end product was a mixture o~ Mo2C and Mo.

Ch_mical_Ana~ysis Data_ Content of fixed carbon, in wt %: 5.85 - calculated for Mo2Ct 4.4 - found.

Microhardness Anal sis_Data Microhardness, in kg/mm2: 1,500 to 1,800 - accordin~ to literature~ 1,790 - found.
Examples 5-8 Obtai~ing cast Mo2~.
The process was conducted as in ~xample 4. The pressure of argon was 100 atm and centri~ugal accelerations were 100, 300, 1,00 and 1,500 g, respectively. For end product charac-teristics ~ee Table.
Example 9 Obtaining cast VC.
The process was conducted as in Example 1. For end pro-duct characteristics see Table.
Exam~le, 10 Obtaining cast ~C.
The proces3 was condu¢ted as in ~xample 2. ~or end pro-duct characteristics see Table.
ExamPle 11 Obtaining cast VC.
The process was conducted as in Example 3. For end pro-duct characteristics see Table.
Example ?2 Obtaining cast VC.
The process was conducted a~ in Example 4. ~or end pro-duct characteristics see Table.
Examples 13, and 14 Obtaining cast VC.
The process was conducted as in Example 4, the preqsure being equal to 100 atm a~d centrifugal accelerations being 100 and 1,500 g, respectivel~. For end product characteristic~
see ~able.
- ~4 -Exam~le 15 Obtaining cast Cr3C2.
The process was conducted as in Example 1. For end pro-duct characteristics see '~able.
ExamPle 16 Obtaining cast Cr3C2.
~ he process was conducted as in Exa~ple 2. For end pro-du¢t characteristics see ~able.
i E~am~le 17 Obtaining cast Cr C .

3 2 The process was conducted as in Example 3. Eor end pro-duct characteristics see ~ablé.
ExamPle 18 Obtaining cast Cr3C2.
The process was conducted as in Example 4. ~or end pro-duat characteristics see ~able.
~ xamples 19 and 20 Obtaining cast Cr3C2.
~ he process was conducted as in ~xample 4 under a presqure o~ 100 atm and at a centrifugal acceleration of 100 and 1,500g, respectively. For end product characteristics see Table.
ExamPle 21 Obtai~ing cast ~C.
232 g o~ W03, 12 g o~ carbon and 54 g o~ Al were mixed in a mixer for one hour. The reæulting mixture was charged into a refractory container, conical in shape, and packed therein.
Then, the container with the mixture was placed in a reactor and heated to a temperature o~ 300C. The mix*ure was ignited b~ a heated tungsten coil, the pressure in the reactor being equal to 5 atm. The temperature in the combustion zone was 2~600 to 2,700C under normal conditions, the melting point o~ WC being equal to 2,870C. For e~d product characteristics see ~able.

Examples 22 and 23 Obtaining cast WC.
The process was conducted a~ in Example 21, the starting mixture being heated to 550C under an argon pressure of 100 and 2,000 atm, respectively. ~or end product characteristics see ~able.
ExamPles 24-32 Obtaining cast Mo~.
For process parameters and end product characteristics see ~able.
xample 32 Obtaining cast VN.
552 g of V205 and 270 g of Al were mixed in a mixer for one hour. The resulting ~ixture was charged into a re~ractory cylindrical container and pac~ed therein. The mixture was ig-nited by means o~ a heated tungsten coil, the nitrogen pressu-re i~ the reactor being 100 atm. Combustion proceeded without violent escape of the mixture. After combustion, the sample was extracted from the container.

, .
End Product _haracteristics he synthesis product was a cast piece divided into two distinct layers: the top layer was grey and brittle (Al203), while the bottom layer was grey brown and hard (V~). The yield o~ VN was 100 wt % of the expected ~alue. The end pro-duct had pores and cavities.

X-Ra~ Phase Anal~sis Data ~he end product was a mixture o~ VN, V3~, and V.

.
Chemical Ana~sis Data ~.~",, Content o~ ~ixed nitrogen, in wt %: 21.6 - calculated for VN~ 11.5 - found.
ExamPle 33 Obtaining cast VN.
~ he process wa~ conducted as in Example 32. ~or end pro-duct characteri~tics see Table.
Ex~mPle 34 Obtaining a hard alloy containing 30 wt ~ of TiC, 60 wt % of Mo2C, 10 wt % oP Ni.
87 g of MoO3, 40 g of ~iO2, 10.5 g of NiO, 11 g of carbon~ and 36 g of Al were mixed ~or one hour, the resulting mixture being char~ed into a re~ractory cylindrical container and packed therein. ~he container with the ~ixture was then placed in a reactor. The mixture was ignited by a heated tungsten coil, the reactor pressure being 1 atm. Combustion was accompanied by violent escape of the mixture. A~ter com-bustion, the sample was removed from the container.

- ~nd Product Characteristics -- _ _ _ _ _ The product was a cast cylindrical piece dirided into two layers. ~he top layer was greay and brittle (Al203), while the bottom la~er was silvery and hard (hard alloy). ~he yield o~
the end product was 30 wt % of the expected value.

: 1058~41 ,, .
~-Ra~ Phase _nal~si~Data ' ~he end product was a mixture of ~iC, Mo2C, C~ ~i, and Mo.
,~ Ch_mical_Analysis Data s Content of ~ixed carbon, in wt %: 7.5 - calculated for ~ l 30 wt % of TiC, 60 wt ~ of Mo2C, 10 wt % of Ni; 7.0 - found.
,. . .

Dens_t~ Anal~sis_Data Density, in g/cm3: 8.2 - obtained.
Examples 35-39 Obtaining a cast hard alloy containing 95 wt % of TiC, 60 wt % o~ Mo2C, 10 wt %
of ~i.
For end product characteristics see Table.
Examples 40-42 Obtaining a cast hard alloy containin~
95 wt % of WC, 5 wt % of Co.
~or end product characteristi~s see ~able .
ExamPles 43-45 Obtaining a cast hard alloy containing 90 wt % of WC, 10 wt % of Co.
~or end product characteristics see ~able.
Exam~les 46-48 Obtaining a cast hard alloy containing 80 wt % of WC, 20 wt % of Co.
~or end product charactexistics see Table.
~am~les 49-51 Obtaining a cast hard alloy containing 50 wt ~0 o~ ~iC, 40 wt % of Cr3C2, 10 wt % of Ni.
~or e~d product charac~eristics see ~able.

, _xamDles 52-54 Obtaining a cast hard alloy containing 90 wt % of VC, 10 wt % of Co.
~or end product characteristics see ~able.
Exam-ples 55-57 Obtaining a cast hard alloy containing : .
90 wt % o~ MoB, 10 wt % o~ Ni.
For end product characteristics see ~able.
Examples 58-60 Obtaining a cast hard alloy with alloy-ing additi~es, containing 85 wt % of VC, 10 wt % of Ni, 5 wt % of Mn.
~or end product characteristics see ~able.
~xamles 61-63 Obtaining a cast hard alloy with allo~ing additives, containing 85 wt % o~ VC, 14 wt % of Ni, 1 wt % of Mn.
For end product characteristics see Table.
Examples 64-66 Obtalnin~ a cast hard alloy with alloyin~
. .
additives, containing 45 wt % of ~iC, 40 wt % o~ Cr3C2, 10 wt ~ o~ ~i, 5 wt %
o~ Mn.
~or end product characteristics see ~able.
~xamPle 67 Obtaining a cast hard alloy with alloying additives, containing 45 wt % o~ ~iC, 44 wt%
of Cr3C2, 10 wt % of Ni, 1 wt % o~ Mg.
For end product characteristics see ~able.
Examles 68-71 Obtaining cast W2~.
For end product characteristics see ~able.

xamples 72-?4 Obtaining a cast hard alloy containing 90 wt ~ of WC, 10 wt % o~ Mn. or end product characteristics see ~able. xamples 75-77 Obtaining a cast hard alloy containing 85 wt % of Cr3C2, 15 wt % of Mn.
~or end product characteristics see l'able.
xamPle~ 78-BO Obtainin~ cast NbSi2.
~or end product characteristic~ see Ilable.
Examples 81-83 Obtainin~ cast V3Si.
~or end product characteristics see Table.
xamples 84-86 Obtainin~ cast WSi2.
For end product characteristics ~ee ~able.
ExamPle 87 Obtaining cast MoSi2.
For end product characteristics see ~able.
xamples 88 and 89 Obtaining a cast complex carbide containning 30 wt % of TiC, 70 wt %
o~ Mo2C.
~or end product characteristics see Table. xamPles 90 and 91 Obtaining a cast complex silicide containing 50 wt % of MoSi2, 50 wt %
of ~iSi2. or end product characteristics see Table. amPles 92 and 93 Obtaining a cast complex boride containing 50 wt % o~ MoB, 50 wt %
of ~iB2. or end product characteristics see ~able.

xamPle 94 Obtaining a cast hard alloy containing 90 wt % of WC, 10 wt % of Mo.
For end product characteristics see ~able.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of producing cast refractory carbides borides, nitrides or silicides, comprising mixing an oxide of at least one metal from Groups 4b, 5b and 6b of the Periodic Table with aluminium or magnesium and with carbon, boron, silicon or nitrogen, or an oxide thereof, and igniting and melting a small portion of the surface of the mixture by the application of heat in a gaseous atmosphere comprising either the aforesaid nitrogen or an inert gas, at a pressure of 1-5,000 atm., whereby the molten combustion zone is propagated throughout the entire reaction mixture, cooling the mixture to form a layer of the said carbide, boride, silicide or nitride and a layer of alumin-ium or magnesium oxide, and separating the two layers to obtain the said cast refractory material.

2. A method as claimed in claim 1, wherein at least one of nickel, cobalt or molybdenum or an oxide thereof is intro-duced into the mixture in an amount of from 5 to 20 wt %.

3. A method as claimed in claim 2, wherein magnesium and/
or manganese is introduced into the mixture in an amount of from 1 to 5 wt % as an alloying additive.

4. A method as claimed in claims 1, 2 or 3, wherein the inert gaseous atmosphere is provided by argon.

5, A method as claimed in claims 1, 2 or 3, wherein the method is carried out in the gaseous atmosphere under a pressure of from 1,000 to 5,000 atm.