MANUFACTURE OF TRANSITION METAL CARBIDE AND CARBONITRIDE WHISKERS WITH LOW RESIDUAL AMOUNTS OF OXYGEN AND INTERMEDIATE OXIDE PHASES

Abstract: The present invention relates to a method of producing whiskers, with low residual amount of oxygen, in large volumes and at low cost to be used as reinforcing material. The whiskers consist of solid solutions between two or more transition metal carbides, nitrides and carbonitrides, (Me1-X-YMe'XMe''Y..etc.)C1-ZNZ, having preferably submicron diameters. The whiskers are suitable for use as a reinforcement material in a wide range of materials, including metals, intermetallics, plastics, ceramics and metallic bonded hard material. The transition metal oxides, hydroxides or alkali compounds thereof are mixed with carbon powder. The carbon powder is added in a hyperstoichiometric amount with respect to the stoichiometric requirements. A halogenide salt is used as a volatilisation agent for the transition metals and a catalyst such as Ni or Co that is able to dissolve transition metals plus C and/or N. The reactant powders are blended so as to intimately mix them. Finally, the mixed starting material is subjected to nitriding, carbonising or carbo-nitriding heat treatments in order to produce the desired whiskers.

Description: Manufacture of transition metal carbide and carboni ri.de whiskers with low residual amounts of oxygen and intermediate ox e phases This invention relates to improvements in the car- bothermal whisker synthesis process of transition metal carbides, nitrides and carbonitrides containing two or more transition metals with respect to the yield of whiskers and also with respect to the high temperature properties, especially thermal stability. The latter property is extremely crucial during high temperature sintering of whisker reinforced composite materials. US 4,888,084 discloses a method for the preparation of titanium nitride whiskers by carbothermal reac- tion of mixtures of TiC>2 , carbonized organic (e.g. cotton) fibres and a catalyst such as Ni or Co. During the process a halogen, preferably a chlorine containing gas is passed over the mixture. In US 5,256,243 a process of making TiC whiskers from a titanium source comprising TiC>2 and/or alkali metal titanate, transition metal chlorides and a carbon powder is described. US 5,795,384, US 5,853,477, US 5,851,285 and EP-A- 754782 relate to a method of producing, in large volumes and at low cost, transition metal carbide, nitride or carbonitride whiskers. The basic idea is the use of a carbon powder with a volatile part which volatilises at temperatures up to 1000°C. Transition metal oxide or alkali compounds thereof in amounts to satisfy the stoi- chiometric requirements of the desired carbide or nitride is mixed with the carbon powder along with an alkali metal chloride powder as a volatilization agent for the metal and a catalyst for the whisker growth such as Ni or Co. The reactant powders are blended and subjected to nitriding, carbonizing or carbonitriding heat treat- ments in order to produce the desired whiskers. US 5,835,477 particularly relates to Ta, Nb, Zr and Hf carbide, nitride or carbonitride whiskers, EP-A-754782 to titanium carbide, nitride and carbonitride whiskers and finally US 5,851,285 to whiskers which are solid solutions between two or more transition metal carbides, nitrides and carbonitrides . It is the object of the present invention to provide a method of producing, in large volumes and at low cost, high strength whiskers of the transition metal carbides, nitrides, or carbonitrides having a low residual oxygen content and which do not contain residual intermediate oxide phases formed during the synthesis process . Fig 1 is a SEM-micrograph of a whisker composite with whiskers according to the invention. Fig 2 is a SEM-micrograph of a whisker composite with whiskers according to prior art . It has now been discovered that the overall oxygen content in the prepared whisker material is very important. The exact nature of this finding is not clear since oxygen can be present as a solid solution in the lattice of the ceramic whisker, or concentrated on the surface as an oxycarbonitride, or present as small amounts of remaining residual intermediate formed oxide phases. The oxygen content will directly influence the morphology of the whisker when annealed at temperatures above 1200°C as when sintering a composite material. .In order to reduce the oxygen content it has been found that hyperstoichiometric amounts of carbon must be used during the synthesis process. A high carbon content is in itself not detrimental to the synthesis process since carbon acts as a filler and helps to keep up the growth space for the whisker. The present invention thus relates to a method of producing whiskers consisting of solid solutions between two or more transition metal carbides, nitrides and carbonitrides having preferably submicron diameters . Low residual amount of oxygen and high yield is obtained by using a hyperstoichiometric amount of carbon powder with a primary grain size (that is, a majority of the powder) of 10-50 nm. The carbon powder contains a volatile part in amounts between 5 wt% and 30 wt%, which partly vola- tilizes at temperatures exceeding 500°C, whereby the porosity of the starting mixture is kept at a high level through the whole reaction. This facilitates the growth of whiskers within the volume of the starting mixture and provides the whisker with an accurate mixture of gas species that are generated from the starting mixture. The high void content also makes it possible for nitrogen gas to penetrate the reaction mixture in case of synthesising a nitride or carbonitride whisker. The carbon powder is a type of carbon black that has oxygen- containing functional groups such as carboxyl and chinon on the surface of the powder which is the volatile portion and which volatilises at temperatures above 500°C. It may also contain some hydrocarbons. Such a carbon powder is easily produced in large amounts and is com- mercially available on the market, e.g. Degussa Color Black F 200 or F 2 , Degussa Special Black 4, 5 or 6. The porosity of the reaction mixture can easily be controlled by varying the type and amount of volatile component to a greater extent compared to when chopped carbon fi- bers are used as described in the prior art. The reaction mixture contains two or more separate transition metal, Me, Me' and Me" etc, oxides, hydroxides or alkali oxide compounds thereof with grain sizes typically between 1 and 100 μm, preferably with a fluffy appearance. Me, Me' and Me" are equal to metals in group 4,5 or 6 such as Ti , Ta, Nb, Zr, and Hf respectively. The above-mentioned carbon powder is added in amounts to exceed the stoichiometric requirements of the desired compound to be produced. The amount of carbon shall preferably be 1.1 times or more, most preferably 1.2-1.5 of what is required for balancing the chemical equation. An alkali, alkali earth halogenide containing salt like NaF, KF, LiF, LiCl, NaCl , KC1 , MgCl2 or CaCl2 , alone or in combination, in a halogenide : transition metal molar ratio of (0.1-2), preferably (0.3-0.7), is added as a volatilisation agent for the transition metals. A chlorine-containing gas (e.g. HC1 or CI2) may also be added to the synthesis atmosphere as a volatilising agent, alone or in combination with a halogenide- salt. A metal catalyst for the whisker growth that is able to dissolve the transition metals plus C and/or N, e.g. Ni and/or Co powder of conventional grain size, or a Ni- or Co-compound, preferably NiCl2, that will decompose to the metal and a volatile component at the reac- tion temperature, is added in a (Ni+Co) : transition metal molar ratio of (0.01-0.5), preferably (0.02-0.1). The reactant powders are intimately mixed, wet or dry, in some conventional way. The bulk density of the reaction mixture should be low 0.4-0.6 g/cm^ so that the surfaces of the powders are accessible to reaction gases and so that the reaction products can be removed. The desired bulk density for a particular mixture has to be optimised for each system and composition and can easily be made by the skilled artisan. The reactant mixture is heated in a furnace at 1000-1800°C, preferably 1100-1600°C, in a an inert or non oxidising atmosphere such as Ar, N2 , H2 , NH3 or He or mixtures thereof, with a low gas flow (~40 ml/min) . The reaction time from 0.5 to 15 hours, and the pressure in the reaction zone from 10 mbar to 50 bar, preferably 0.5 to 5 bar. The reaction takes place in a reaction chamber made of some inert material, preferably graphite. Different designs can be used but all allowing a controlled gas exchange between the reactor chamber and the surrounding atmosphere. Selecting the synthesis temperature and the particle size of the transition metal oxides, hydroxides or alkali oxides in the starting mixture can influence the homogeneity in the composition. The carbon: transition metal molar ratio to be used depends among other things also on the selected oxide sources and the desired composition within the transition metal solid solution and if a nitride, carbide or carbonitride is desired. Also, depending on the type of transition metal sources and the type of whiskers to be produced, different synthesis parameters such as temperature and gas phase composition must be chosen. N2 or NH3-gas is utilized as the nitrogen source for the production of nitrogen containing whiskers according to the present invention. By choosing mixtures of e.g. two transition metal oxides e.g. TiC>2 and Ta2C>5 a whisker of the composition (Ta 1 . x Ti x )C 1 . y y with 0<x<l and 0<y<l can be produced. The obtained x- and y-values depend on the composition of the starting mixture, reaction temperature and atmos- phere used. The balanced carbothermal nitridation reaction can be written: (x)Ti0 2 (s) + ( (l-x)/2)Ta 2 0 5 (s) + (7/2-x/2-y) C (s) + (y/2)N 2 (g) => Ta 1 . x Ti x C 1 . y N y (s) + (5/2-x/2) CO (g) The temperature is preferably held between 1100 and 1600°C in a nitrogen atmosphere for three hours. It is essential for this reaction to proceed to the right, that the CO (g) -partial -pressure is held sufficiently low and that nitrogen gas is provided to the interior of the reaction mixture. This means that the nitrogen gas must penetrate the mixture whose porosity is held at a high level using the carbon powder according to the present invention. The mole fraction carbon/transition metal (s) depends on the transition metal oxide (s) and the desired product. There should, however, according to the invention, be a surplus of carbon. Also depending on the type of transition metal and the type of whiskers to be produced different synthesis parameters such as temperature and gas phase composition must be chosen. During the reaction intermediate oxygen containing phases are formed such as sodium tantalates, sodium niobates or sodium titanates. The result of the synthesis according to the present invention, is a mixture of whiskers containing two or more transition metals in an amount of 70 to 90% by volume and excess carbon. The whiskers have a diameter less than 5 μm, preferably less than 1 μ and are straight with a smooth surface. The excess carbon is removed in a special refine- ment step. This step includes first deagglomeration of the whisker product in a mixer and then dispersing it in water. Petroleumether is added and the mixture is shaken in a separation funnel for a couple of minutes. The funnel is then at rest for a while and the two liquid phases are separating. The whiskers are dispersed in the water phase and the carbon is dispersed in the petroleumether. The whisker containing water phase is then separated and taken care of . The catalyst metal can be removed by leaching the product for a couple of minutes in 1% HCl and then wash with water 4-5 times. In addition, submicron particles of the same compound are formed. This high whisker yield makes an extra separation of whiskers and particles unnecessary. However, if an even higher yield is desired, a subsequent refinement step can be applied. The preparation conditions given above, both with regard to the starting formulation and to the synthesis parameters, are related to the equipment used, design of the reaction chamber, the choice of raw materials etc. It is within the purview of the skilled artisan using other equipment and other raw materials to determine the optimal conditions by experiments. The reason for the improved whisker quality is probably that a surplus of carbon is necessary in order to control the local carbon activity during the whole whisker synthesis cycle. A local deficit of carbon will lead to higher oxygen content of the whisker and possibly also to residual oxygen containing intermediate phases . Rxampl.fi 1 (invention) Tiθ2 rutile powder (Aldrich) and Ta2θ5 (Starck ceramic grade) powder was mixed with carbon powder (Degussa FW200) , NaCl (Akzo) and Ni powder (Cerac 325 mesh) with mole fractions: Ta2U5 : C : Ni : NaCl = 1 : 9: 0.05 :0.5; Ti0 2 : C : Ni : NaCl =1: 4 : 0.05 : 0.5; T 2U5 : Tiθ2 = 1 : 0.667. The carbon powder contained 21 wt% volatile components, which were compensated for. The powders were mixed in a high speed blender and then subjected to a carbothermal heat treatment with the following process parameters: T = 1250°C, t = 3h, atmosphere: Ar(g), P (Ar) = 1 atm The whisker yield was about 80% by volume. The whiskers produced consisted of (Tag 75Tig 25)^. The whiskers had smooth surface morphology, low oxygen content and no remaining residuals of sodium tantalate. Washing the whiskers in a mixture of two liquids namely water and petroleumether removed the excess carbon. The whiskers disperse in the water phase whereas the carbon is concentrated in the petroleumether and can be removed . Fxampl e 2 (prior art) Tiθ2 rutile powder (Aldrich) and Ta2U5 (Starck ceramic grade) powder was mixed with carbon powder (Degussa FW200) , NaCl (Akzo) and Ni powder (Cerac 325 mesh) with mole fractions: Ta 05 : C : Ni : NaCl = 1 : 7 : 0.05 :0.5; Ti0 2 : C : Ni : NaCl = 1: 3 : 0.05 : 0.5; Ta2θ 5 : Ti0 2 = 1 : 0.0526. The powders were mixed in a high speed blender and then subjected to carbothermal heat treatment with the following process parameters T = 1250°C for 1.5 h followed by 1300 °C for 1.0 h The whiskers were (Tag SO^O 2θ) c - Tne whiskers had smooth surface morphology, medium oxygen content (0.34 wt%) and probably small residuals of sodium tantalate. Fxampl 3 The whiskers from Example 1 and Example 2 were mixed with alumina powder at a concentration of 20 % by volume of whiskers and then hot pressed at 1720 °C for 90 min at 28 MPa to study the effect of oxygen content on the high temperature stability of the whiskers. SEM-micrographs of the corresponding composites show that the whiskers with low oxygen content manufactured according to example 1 show no morphological changes during sintering (Figure 1) whereas the whiskers produced according to example 2 have degraded during sintering (Figure 2) . The reason for the different behaviour of the whiskers during sintering with alumina is probably the presence of intermediate oxidation products such as sodium tantalate in the whiskers according to example 2 forming low melting compounds during sintering with alu- mina.

Claims: Claims 1. A method of producing whiskers in large volumes having an overall composition of (Meι_χ_ γMe'χMe ' Υ.. etc . ) Ci-z N z with Me, Me' and Me" equal to transition metals in group 4, 5 or 6 such as Ti , Ta, Nb, Zr and Hf respectively by intimately mixing a carbon powder containing a volatile part, a mixture of two or more Me, Me' and Me" oxides, hydroxides or alkali oxide compounds, at least one of the alkali, alkali earth halogenide containing salts LiCl , NaCl, KC1 , CaCl2, MgCl2, LiF, NaF or KF in a halogenide : transition metal molar ratio of (0.1-2), preferably (0.3-0.7), and Ni and/or Co or a compound thereof in (Ni+Co) transition metal molar ratio of 0.01-0.5 and heating the mixture to 1000-1800°C for 0.5-15 hours, at 10 mbar-50 bar pressure in an inert or non oxidising atmosphere depending on the desired whiskers c h a r a c t e r i s e d in using a hyperstoichiometric amount of said carbon powder with respect to the stoichiometric requirements of the de- sired whisker. 2. The method of claim l c h a r a c t e r i s e d in that said hyperstoichiometric amount of carbon is 1.2 times or more. 3. The method of any of the preceding claims c h a r a c t e r i s e d in that the atmosphere is Ar, N2 , H2 , NH3 or He or mixtures thereof . 4. The method of any of the preceding claims c h a r a c t e r i s e d in that the carbon powder has a primary grain size of 10-50 nm. 5. The method of any of the preceding claims c h a r a c t e r i s e d in a volatile part of the carbon powder of 5-30 weight- %. 6. The method of any of the preceding claims c h a r a c t e r i s e d in that the mixture is heated to a temperature of 1100-1600°C. 7. The method of any of the preceding claims c h a r a c t e r i s e d in that the atmosphere contains a chlorine-containing gas either in addition to or instead of the halogenide-containing salt.

Inventors: AHLEN NIKLAS, BRANDT GUNNAR, EKELUND MAGNUS, JOHNSSON MATS, PETTERSSON PERNILLA, AHLEN, NIKLAS, BRANDT, GUNNAR, EKELUND, MAGNUS, JOHNSSON, MATS, PETTERSSON, PERNILLA
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 2000
Main classification: C30B25
IPC codes: C30B25