Technical field • _. The present invention concerns a process for the prepa- ration of an extrahard material based on etal carbides, the properties of which are essentially similar to that of tungsten carbide but that is substantially lighter than the latter.

This material can be used for the manufacturing of tools for the machining of metals and which have excellent proper¬ ties of resistance to ear, shock and heat.

This material is represented by the formula Mo W. C, wherein x is between 0.01 and 1, and presents, as was estab- lished by X ray diffraction analysis, a hexagonal crystal structure identical with that of tungsten carbide WC.

Hence, it consists of a solid solution containing tung¬ sten and molybdenum onocarbides WC and MoC. Background of the Art

It is known that tungsten carbide WC is much used as an extrahard material, na ely for the manufacturing of machining tools such as cemented carbide tools for cutting metals and drilling rocks and minerals. It has been established that some of the desirable properties of WC, such as for instance its resistance to rupture and to developing cracks under moderate stress are related, at least partly, to its hexago¬ nal crystal structure. Further this carbide is very hard and resistant under hot conditions and its wettability by ce en- ting bonding metals, such as Co, Ni ^' and Fe, is excellent.

However tungsten is heavy and expensive and, for econo- ical reasons, it is desirable to replace it by a lighter and more abundent metal having similar properties. Molybdenum is one of such metals: indeed, molybdenum onocarbide MoC has a hexagonal crystal structure identical with that of WC and is also very hard. Unfortunately, it is not stable above about 1200 C (whereas WC well resists up to about 2700°C)

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which prevents it to be used in all applications suited by WC.

It was however shown that solid Solutions of tungsten and molybdenum monocarbides, that is materials of the formu- la Mo W.. C defined hereinabove, possessed excellent physi- cal properties which practically resemble those of WC, even with proportions of the molybdenum carbide as low as 1% approximately. Such materials are therefore much advanta- geous, as co pared to pure WC, with regard to lightness and price since the density of MoC is only 9 (as compared with 15.7 for WC) and molybdenum is a relatively abundent and cheap metal. Further, their heat resistance and decomposition temperature, in between that of MoC and WC, is in direct relationship with the agnitude of the W/Mo ratio. However, in order to exploit these advantages, the method of manu- facturing such solid solution should be also economical. There exists already some ethods to fabricate such Solutions or, more specifically, such Solutions with, in addi tion, significant proportions of iron-group metals such as Co, Ni and Fe. It was indeed found that such metals promote the formation of materials consisting of carbides of hexago¬ nal structure and of formula MoxW1..-xC wherein x is the same as in the above given definition.

Thus, there was obtained a hexagonal carbide of formula Mo ₄₄ ^W _{n c ς} C in admixture with cobalt by heating together WC, Mo-C, and C with 4.8% of Co at 2000°C (Z. Anorg. Chem. 262 (1950), 212-217). In this respect, the Applicant has found that when mixtures of compressed powders containing WC, Mo and C or W, Mo and C were reacted in the presence of 2.5- 10% Co by heating 4 hrs to 1200-2000°C (the temperature being dependent on the W/Mo ratio) under an inert atmosphere, there were also obtained materials containing mainly, as the car¬ bide p^hase, MoxW1_.-xC in which 0 x 0.8.

According to another method, mixtures of sintered pow¬ ders containing, in atom-percents, about 41% Mo, 57% C and 2% Co were fused together with variable proportions of WC and the product was annealed 300 hrs at 1200 C under lθ

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Torr. There was thus obtained a series of materials con¬ taining ainly the carbide Mo W.._ C where x can reach the value of 0.88 (Monatsh. Chem. 107 (1976), 1167-1176.

According to another process (Ger an Patent Application DOS 2.623.990) concerning the preparation of solid Solutions of WC in MoC containing iron-group metals, a mixture of graphite, metallic W and Mo in desired proportions, or a corresponding mixture of Mo ₂ C, WC and graphite, together with 0.5-1% of Ni or Co is heated to a temperature high enough to promote the formation of high-temperature stable phases. In this process, the temperatures are chosen as follows: Above 1975 C for inducing the formation of a solid solution of cubic carbides WC. and MoC. (y being defined

1-y 1-y ^J ' in the recited reference and being called x in this refe- rence) ; above 1680 C for inducing the formation of a solid solution of pseudo-cubic carbides W-C« and Mo_,C . Then, the temperature is lowered to the ränge where the solid solution of the desired monocarbides is stable and maintained in this ränge for the time required to form the desired solution of hexagonal crystal structure.

However, these methods of the prior-art all have at least one of the following drawbacks: need of a high tempe¬ rature premelting of the ingredients, need of a prefor ation of the carbide phases at high temperature. Moreover, having an iron-group metal present together with the desired extra-hard material is not always desirable in regard to further uses of the product and it may be wan- ted to maintain the level of such added metal as low as possible. It will also be noted that, according to the prior-art, (Planseeber. für Pulver Metallurgie _4_ (1956) , 2-6) , it is recognized that solid Solutions of hexagonal carbide Mo W. C do not form easily when there are used, as starting substan- ces, mixtures of C, W and Mo in the form of separate distinct phases. Thus, if graphite mixed with Mo metal (or carbide

Mo ₂ C) and W metal (or carbide WC) is heated to 1700°C; a

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solid solution of hexagonal type monocarbides does not form but, instead, there are formed two separate phases: a mono- carbide WC and a carbide of formula (Mo,W)_C. Disclosure of the Invention ^' - The process of the invention remedies, at least partly, these disadvantages. It comprises heating between 1000 C and a temperature T lower than the maximum temperature limit of the do ain of stability of the Mo W-_ C phase a mixture, intimate to the atomic or molecular scale, of tungsten and molybdenum, i.e. at least one compound or at least one solid solution of these two elements, the total Ni, Co and Fe con¬ tent of which does not exceed 0.1%, in the presence of carbon and/or of a carbon containing compound. The upper limit T of the heating temperature embodied in the process is defined as follows: For the mixtures leading to a phase of the above- mentioned formula where 0.01 x 0.8, Tx = 2700 - 1375°x°C. F the cases where 0.8 x 1, T = 3400 - 2250x°C. Preferably, t heating temperature is chosen between 1100 or more and a • value somewhat 2-10% below the limit calculated as above but this is in no way compulsory and can be adapted to the needs in each case.

The proportions of the ingredients comprising the star- ting mixture will preferably conform to the molecular frac- tions corresponding to the desired carbide phase. Thus, for ^' instance, for a material of formula Mo _n yd _C, the corres¬ ponding starting Mo, W and C compounds will be in the mole ratio 1 : 1 : 2. The proportions are, moreover, not immutable and can be varied within certain limits, e.g. + 10% depen- ding on the operating conditions as will be seen in the experimental part of this specification.

As carbon containing compounds, one can use gases such as CO or gaseous hydrocarbons such as CH or acetylene; carbon powders such as soot, charcoal, carbon-black or, together, a carbon powder and a gas as defined above. The starting solid solution containing tungsten and molybdenum can consist of a mixture of the metals in the

elementary or combined State. A mixture of the metalic ele- ments comes to be an alloy; in the combined State, such solid solution can be, for instance, molybdo-tungstic oxide or molybdo-tungstic acid or a lower carbide of formula (Mo W. )_C where x is defined as hereintofore. Indeed, it has been found that the simultaneous presence of Mo and W in the crystals of the starting solid solution which consists of a mixture of these elements at the molecular level, is one of the key factors leading to the formation of a practi- cally sole hexagonal carbide Mo ΛW_.L ^— ΛC under relatively mild reaction conditions and at a rate sufficient for industrial application. Thus, the formation of the solid solution of hexagonal carbides is surprisingly rapid, even at relatively moderate temperatures, when the starting association W-Mo confor s to the above criteria and the reaction has a very high yield even in the absence of cobalt, the usual promoter.

It will also be remarked that the partial replacement of some of the W atoms of tungsten carbide by molybdenum, which is achieved in principle by the application of the present process, leads to a very significant lowering of the weight of said carbide (about 30% reduction when repla- cing 2 atoms over 3) without involving significant deficien- cies. On the contrary, such mixed carbides have a better strain resistance at high temperatures and a better stability regarding granulär growth than ordinary carbides (see DOS 2.623.990) .

Best mode of carrying out the Invention For implementing the present process, the classical metallurgical techniques will preferably be applied, for instance, with respect to the starting alloy, sintering and melting. Moreover, for economical reasons, common techniques for manufacturing pure WC are applied as uch as possible. Thus, preferably, the following steps will be carried out: 1) Preparation of a mixture of tungsten and molybdenum compounds, preferably intimate at the molecular scale.

2) Reduction by hydrogen of the above mixture into a Mo-W alloy, this alloy consisting of a solution at the atomic scale of Mo and W.

3) Carbidization of the powder of the alloy thus ob- taJ-ned.

In accord with the above steps, it is possible to Start fro a solution of ammonium molybdo-tungstate which is acidi- fied, which results in the precipitation of molybdo-tungstic acid (intimate mixture of molybdic and tungstic acids) or to directly use the ammonium salts after evaporation of the solution. Then, the solid acids or salts obtained are roasted in air which converts the into oxides which are then re- duced to Mo-W alloys by heating in the presence of hydrogen. It is also possible to effect the direct reduction of the acids and salts without going through the oxide step. Roastin is preferably effected by heating in air at 400-500 C for several hours. For the reduction, one can heat the oxide between 900 and 1300 C under an atmosphere of hydrogen. For this, one preferably raises the temperature gradually, for instance 150 /hour, until 900 is reached, which temperature is then kept constant for several hours. After cooling, the alloy is obtained under the form of a metallic powder which can be further milled in finer form if desired. By X-ray diffraction analysis, the extent of reduction can be checked as well as the purity of the alloy. Then the metal powder is mixed with a measured quantity of carbon powder, this quantity being calculated for obtaining a product in accord with the formula Mo W. C defined above. It is important that this mixing should be as intimate as possible and it can be obtained, for instance, by milling for several hours in a ball-mill. Then, the mixture is heated for several hours under a non-oxidizing atmosphere (for instance of hydrogen) at a temperature defined as a function of the alloy compo- sition as indicated hereinabove. This temperature will preferably be at least 1100 C but always lower than the temperature corresponding to the stability limit of the

carbide considered; it will be determined by the formulae given heretofore.

By way of exa ple, the maximum heating temperatures for several compositions of mixed carbides are: Mo _W _C 2287°; ^;Mό 0.5 0.5 ^{C 2012} ° ^C ' ^MO 0.6 ^W 0.4 ^{C 1875} ° ^C ' ¹⁶ °°° ^C - ^If the heating temperatues exceed the above- entioned values, a material is obtained which contains significant proportions of lower carbide (Mo,W) ₂ C and free carbon.

The mixed carbide obtained by the present process can be thereafter converted according to usual metallurgical techniques into other form. For instance, it can be powdered, mixed with binders (cements) , e.g. Co and Ni, shaped by compression and sintered as usual. It can also be mixed with other powdered extra-hard materials, such as cubic type car- bides, e.g. TiC, and binders before sintering. Preferably, the carbides of formulae between the limiting formulae

^M °0.8 0.2 ^{C Hnd M} °0.2 ^W 0.8 ^{C are USed} '

It is also useful to mention at this stage that it is

^' possible to use (Mo,W)C carbides as catalysts for chemical reactions, e.g. in the cases where the simple carbide WC is active and has a catalytic effect similar to that of platinum. An example of such catalytic effect is the catalytic oxi- dation of hydrogen in fuel-cells using an acid electrolyte (see Annual Report of the Institute for Material Research, National Bureau of Standards, Washington DC; issued May 1977) . It will also be re arked that it is possible to obtain a mixture of the solid solution of the desired monocarbide of crude formula (Mo,W)C with the lower carbide of formula (Mo,W) ₂ C and only traces of free carbon by heating the starting Mo-W alloy with an amount of carbon lower than the stoechiometric quantity corresponding to (Mo,W)C but higher than the stoechiometric quantity corresponding to (Mo,W) ₂ C at temperatures not exceeding the above mentioned maxima. The material obtained then has properties different from the pure monocarbide of the invention and is also interesting industrially, because the presence of a small amount -of

(Mo,W)„C in the hexagonal monocarbide (Mo,W)C of the inven¬ tion imparts to the hard material a very fine grain struc¬ ture.

Industrial Applicability The following Examples illustrate the invention in a more detailed manner. Example 1

In 1 liter of 13% aqueous ammonia, there were dissolved 0.67 mole of H ₂ WO. and 0.67 mole of MoO-.. The solution was acidified at pH 1 with HN0 ₃ which co-precipitated the molyb- dic and tungstic acids formed and the precipitate was collected by filtration. After drying in air, the solid was placed in the tube of an oven and roasted 2 hrs at 400 , whereby it was converted to the oxide. Then, the tube was heated to 900 C and a current of H ₂ was passed for 2 hrs on the oxide which was thus reduced to a 1 : 1 molar-ratio alloy of Mo and W (solid solution) . X-ray diffraction ana- lysis showed that the reduction of the oxides was complete because the oxide "bands were no more present and because the spectrum contained the bands characterizing the alloy. Thus, the formation of a (110) band at d = 2.3 Ä was observed which was situated between the values for pure Mo (2.225 Ä) and pure W (2.238 Ä) and was characteristic of the alloy. One hundred gram of the alloy powder was mixed in a ball-mill with 9.44 g of carbon powder to give a mixture (109.44 g) for which the molar ratio of the components was W/Mo/C = 1 : 1 : 2.2, the excess of C being for compensating possible operating losses and to avoid the formation of lower carbides. Pellets were formed by subjecting 10 g portion of

2 the mix to a pressure of 6.4 ton/cm in a hydraulic press.

The pellets were placed in a graphite Container and were heated for 4 hrs at 1600 under H_ in an electric oven. After cooling, there was obtained a very pure metal carbide of approximate formula Mo _W _C. The purity of the product was evidenced by the following analytical results:

X-ray diffraction analysis: The spectrum showed only

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the bands of the desired monocarbide.

Elemental analysis:

Calculated for Mo _{Λ C} W _{Λ C} C W 60.5% Mo 31.6% C 7.91%

O.o U.o found W 60.5% Mo 31.3% C 7.95% The discrepancy originated in the following impurities: free carbon 0.12%, N 0.01%, O 0.03%. The total of Fe, Ni and

Co was below 0.09%.

In order to show the differences between the prior-art and this embodiment of the present invention, there was pre- pared an intimate mixture of very finely ground powders of molybdenum, tungsten and carbon in amounts corresponding to the overall formula Mou _Λ .o _C Wu,_..o _C C + 10% excess of C. The mixture was pelletized and carbidized as described above. After this treatment, it was noticed by X-ray diffraction analysis that the obtained product showed two carbide phases in the mole ratio 0.4 : 1; the first phase had the lower- carbide structure (Mo,W)_C; the second phase had the hexago¬ nal structure of the WC type. Example 2 There was proceeded exactly as described in Example 1 but using a starting solution wherein the molar ratio W : Mo was 1 : 2 (0.5 moles of W and 1 mole of Mo as ammonium tungstate and molybdate respectively in 1000 ml of 13% NH.0H) . This was acidified at pH 2.5 and the resulting solid was dried, roasted and reduced. Thereafter, a mixture of alloy powder and C was prepared using amounts of the components calculated for having a molar composition Mo _Λ --W _Λ ■_ _. -,-- , plus 10% carbon. Pellets were formed under 1.6 ton/cm and carbi- dization was carried out for 4 hrs at 1400 C under H ₂ con- taining 5% (by volume) of acetylene.

According to X-ray diffraction analysis, the obtained product consisted of very pure molybdenum and tungsten car¬ bide of approximate composition Mo ₆₇ ^w _{0 33} C containing only traces of the lower carbide (Mo,W) ₂ C, that is less than 2%. Example 3

There was proceeded exactly as described in Example 1

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by means of the following products and under the following conditions:

Molar ratio W : Mo in the starting solution 3 : 1. Acidification at pH 1 with HC1. Roasting temperature 500°C. •Reduction 2 hrs at 900 under H ₂ . Respective quantities of the components of the powder mixture of the pellets corres¬ ponding to Mo_ ₂₅ ^W ₇₅ ^C * Carbidization 2 hrs at 2000°C under a mixture 3 : 1 (vol.) of H_ and N- . The analytical results were as follows (by weight) : W 79.3%, Mo 13.6%, C (total) 6.95%, C (free) 0.08%, N 0.011%, 0 0.02%. X-ray diffraction analysis showed that the obtained solid solution of carbide w very pure.

Example 4

With reference to Example 1, there was proceeded under the following conditions: Molar ratio W : Mo in the starting solution 1 : 4. Acidification at pH 0.8 (HC1) . Roasting 500°C Reduction 2 hrs at 850 C (H_) . Pellets, ratio of components in accord with O _Q WU_,. Δ-C. Carbidization 2 hrs at 1450 C,

■ mixture H„ : N_ 3 : 1. Analytical results: ^" hexagonal, WC type structure con- firmed by X-ray diffraction patterns. Elemental analysis (by weight): W 29.5%, Mo 60.8%, C (total) 9.64%, C (free) 0.15%, N 0.032%, 0 0.05%. Example 5 An aqueous solution containing 1 mole of ammonium tungstate and 5 oles of ammonium molybdate was evaporated near to dryness (98.2%). The solid formed (yield 97%) was dried and, with reference to the previous Examples, was subjected to the following treatments: Roasting 500 C, 2 hrs; reduction 900 , H_; pellets, ratio of components according to Mo_ ₀₀ W _{Λ 10} C; carbidization 2 hrs, 1500 C, H. : N„ 3 : 1 ( H^ cracking gas) .

Analytical results: monocarbide containing only traces of lower carbide (Mo,W)_C (X-ray diffraction). Elemental analysis (% by weight): W 25.1, Mo 64.7, C (total) 9.91, C (free) 0.15, N 0.038, 0 0.08.

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Example 6

Ten g of the lower carbide (W,. _π Mo _{Λ r} ) C were mixed u. o O. o Δ with 0.45 g of carbon powder. After good grinding and

2 milling, pellets were formed under 1.6 ton/cm and heated

5 •4 hrs at 1600 C under H ₂ . Analysis of the obtained product (elemental and X-ray diffraction) showed it to be a hexago¬ nal monocarbide of formula W _c ^Mo _{n c} ^c practically pure. Example 7

As in the method described in Example 1, there were 0 prepared four samples of mixtures of molybdenum and tungsten oxides, respectively S_. , S ₂ , S- and S.. They were obtained by first precipitating (pH 1, HCl) ammonia Solutions of ammonium molybdate and tungstate the molar ratio (Mo/W) of which were: S1 _n 0.49, S2 0.96,' S3_ 1.63 and S4. 3.35. Then the 5 mixed acids were roasted 2 hrs at 500 which provided the desired oxides. Thereafter, the oxides were reduced by hydrogen in a tubular electric oven. The temperature was progressively raised to 900 C (200 /hr) and it was main- tained for 4 hrs at 900°C. The hydrogen used was 99.9997% 0 purity to minimize as much as possible nitrogen contamination. After the reaction was terminated, the alloy products were checked for homogenity and purity. The "d" values of the X-ray diffraction patterns (321) are given below and also for pure W and Mo.

25 Sample Alloy.composition d ^α (321)8 ^A

Mo Mo 0.8411

^S 2 ^M °0.41 ^W 0.51 . 0.8435

30 444

^S l Mo W 0.8 0.33 0.67 w W 0.8459

It is noted that the "d" values are practically line- arly dependent on the atomic ratios of the alloying consti- tuents. 35 The alloy powders were thereafter mixed for 125 hrs

in a ball-mill with the stoechiometric quantity (+ 10%) of carbon powder required for forming the carbides (Mo,W)C.

2 Then, pellets were molded under 1.6 ton/cm which were heated at various temperatures, for 4 hrs under very pure H ₂ . The figures reported below show the degree of purity of the obtained monocarbide in term of the mole % of lower-carbide

(Mo,W) ₂ C present in the samples as a function of the heating temperature. Said figures have been estimated from the X-ray diffraction pattern intensity. Sample Molar % of (Mo,W) .<___C after heating at (°C)

1000 1100 1200 1400 1600 1800 2000 2200

^S 1 9.7 0.0 • • • • 0.0 2.0

^S 2 18 . 1.8 0.0 • 0.0 16 38

^S 3 53 0.0 • • 0.0 4.8 • ^— •

^S 4 36 2.9 1.2 0.0 24 83 • •

These figures clearly illustrate the fact that heating temperatures ust be lowered in proportion to the amount of molybdenum present in the alloy.