Refractory hard metals in powder form for use in the manufacture of electrodes

Abstract

The invention relates to a refractory hard metal in powder form comprising particles having an average particle size of 0.1 to 30 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a refractory hard metal of the formula (I): A x B y X z wherein A is a transition metal, B is a metal selected from the group consisting of zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, manganese, tungsten and cobalt, X is boron or carbon, x ranges from 0.1 to 3, y ranges from 0 to 3 and z ranges from 1 to 6. The refractory hard metal in powder form according to the invention is suitable for use in the manufacture of electrodes by thermal deposition or powder metallurgy.

1 . A refractory hard metal in powder form comprising particles having an average particle size of 0.1 to 30 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a refractory hard metal of the formula:  
       A x B y X z    (I)  
       wherein A is a transition metal, B is a metal selected from the group consisting of zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, manganese, tungsten and cobalt, X is boron or carbon, x ranges from 0.1 to 3, y ranges from 0 to 3 and z ranges from 1 to 6.  
     
     
         2 . A refractory hard metal in powder form according to  claim 1 , wherein A is a transition metal selected from the group consisting of titanium, chromium, zirconium and vanadium.  
     
     
         3 . A refractory hard metal in powder form according to  claim 2 , wherein A is titanium, X is boron and y is 0.  
     
     
         4 . A refractory hard metal in powder form according to  claim 3 , wherein x is 1 and z is 1.8.  
     
     
         5 . A refractory hard metal in powder form according to  claim 3 , wherein x is 1 and z is 2.  
     
     
         6 . A refractory hard metal in powder form according to  claim 3 , wherein x is 1 and z is 2.2.  
     
     
         7 . A refractory hard metal in powder form according to  claim 2 , wherein A is titanium, X is carbon and y is 0.  
     
     
         8 . A refractory hard metal in powder form according to  claim 7 , wherein x is 1 and z is 1.  
     
     
         9 . A refractory hard metal in powder form according to  claim 2 , wherein A is titanium, B is zirconium or hafnium, X is boron and y is other than 0.  
     
     
         10 . A refractory hard metal in powder form according to  claim 9 , wherein B is zirconium, x is 0.5, y is 0.5 and z is 2.  
     
     
         11 . A refractory hard metal in powder form according to  claim 9 , wherein B is zirconium, x is 0.9, y is 0.1 and z is 2.  
     
     
         12 . A refractory hard metal in powder form according to  claim 2 , wherein B is hafnium, x is 0.5, y is 0.5 and z is 2.  
     
     
         13 . A refractory hard metal in powder form according to  claim 2 , wherein A is zirconium, B is vanadium, X is boron and y is other than 0.  
     
     
         14 . A refractory hard metal in powder form according to  claim 13 , wherein x is 0.8, y is 0.2 and z is 2.  
     
     
         15 . A refractory hard metal in powder form according to  claim 1 , wherein said average particle size ranges from 1 to 5 μm.  
     
     
         16 . A process for producing a refractory hard metal in powder form as defined in  claim 1 , comprising the steps of: 
 a) providing a first reagent selected from the group consisting of transition metals and transition metal-containing compounds;    b) providing a second reagent selected from the group consisting of boron, boron-containing compounds, carbon and carbon-containing compounds;    c) providing an optional third reagent selected from the group consisting of zirconium, zirconium-containing compounds, hafnium, hafnium-containing compounds, vanadium, vanadium-containing compounds, niobium, niobium-containing compounds, tantalum, tantalum-containing compounds, chromium, chromium-containing compounds, molybdenum, molybdenum-containing compounds, manganese, manganese-containing compounds, tungsten, tungsten-containing compounds, cobalt and cobalt-containing compounds; and    d) subjecting said first, second and third reagents to high-energy ball milling to cause solid state reaction therebetween and formation of particles having an average particle size of 0.1 to 30 μm, each particle being formed of an agglomerate of grains with each grain comprising a nanocrystal of a refractory hard metal of the formula (I) as defined in  claim 1 .    
     
     
         17 . A process according to  claim 16 , wherein said first reagent comprises a transition metal selected from the group consisting of titanium, chromium, zirconium and vanadium.  
     
     
         18 . A process according to  claim 17 , wherein said transition metal is titanium.  
     
     
         19 . A process according to  claim 16 , wherein said first reagent comprises a titanium-containing compound selected from the group TiH 2 , TiAl 3 , TiB and TiCl 2 .  
     
     
         20 . A process according to  claim 16 , wherein said second reagent comprises boron.  
     
     
         21 . A process according to  claim 16 , wherein said second reagent comprises a boron-containing compound selected from the group consisting of AlB 2 , AlB 12 , BH 3 , BN, VB 2 , H 2 BO 3  and Na 2 B 4 O 7 .  
     
     
         22 . A process according to  claim 16 , wherein said second reagent comprises carbon.  
     
     
         23 . A process according to  claim 16 , wherein said second reagent comprises tetraboron carbide.  
     
     
         24 . A process according to  claim 16 , wherein said third reagent is a compound selected from the group consisting of HfB 2 , VB 2 , NbB 2 , TaB 2 , CrB 2 , MoB 2 , MnB 2 , Mo 2 B 5 , W 2 B 5 , CoB, ZrC, TaC, WC and HfC.  
     
     
         25 . A process according to  claim 16 , wherein step (d) is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz.  
     
     
         26 . A process according to  claim 25 , wherein said vibratory ball mill is operated at a frequency of about 17 Hz.  
     
     
         27 . A process according to  claim 16 , wherein step (d) is carried out in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.  
     
     
         28 . A process according to  claim 27 , wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.  
     
     
         29 . A process according to  claim 16 , wherein step (d) is carried out under an inert gas atmosphere.  
     
     
         30 . A process according to  claim 29 , wherein said inert gas atmosphere comprises argon or helium.  
     
     
         31 . A process according to  claim 16 , wherein step (d) is carried out under a reactive gas atmosphere.  
     
     
         32 . A process according to  claim 31 , wherein said reactive gas atmosphere comprises hydrogen, ammonia or a hydrocarbon.  
     
     
         33 . A process according to  claim 16 , wherein step (d) is carried out for a period of time of about 5 hours.  
     
     
         34 . A process according to  claim 16 , wherein a sintering aid is added during step (d).  
     
     
         35 . A process for producing a refractory hard metal in powder form as defined in  claim 5  or  8 , comprising subjecting TiB 2  or TiC to high-energy ball milling to cause formation of particles having an average particle size of 0.1 to 30 μm, each particle being formed of an agglomerate of grains with each grain comprising a nanocrystal of TiB 2  or TiC.  
     
     
         36 . A process according to  claim 35 , wherein said high-energy ball milling is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz.  
     
     
         37 . A process according to  claim 36 , wherein said vibratory ball mill is operated at a frequency of about 17 Hz.  
     
     
         38 . A process according to  claim 35 , wherein said high-energy ball milling is carried out in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.  
     
     
         39 . A process according to  claim 38 , wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.  
     
     
         40 . A process according to  claim 35 , wherein said high-energy ball milling is carried out under an inert gas atmosphere.  
     
     
         41 . A process according to  claim 40 , wherein said inert gas atmosphere comprises argon or helium.  
     
     
         42 . A process according to  claim 35 , wherein said high-energy ball milling is carried out under a reactive gas atmosphere.  
     
     
         43 . A process according to  claim 42 , wherein said reactive gas atmosphere comprises hydrogen, ammonia or a hydrocarbon.  
     
     
         44 . A process according to  claim 35 , wherein said high-energy ball milling is carried out for a period of time of about 20 hours.  
     
     
         45 . A process according to  claim 35 , wherein a sintering aid is added during said high-energy ball milling.