Deposition and thermal diffusion of borides and carbides of refractory metals

Abstract

The invention provides a substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate of a workpiece. A layer of the refractory metal is deposited on the substrate. At least one of the elements boron and carbon is deposited from a source other than the workpiece on the workpiece having the refractory metal layer. The workpiece is heated at a temperature and for a time period sufficient to diffuse at least a portion of the deposited refractory metal into the substrate and at least a portion of the deposited boron or carbon into the refractory metal layer and the substrate to form a substantially uniform layer of the boride or carbide of the refractory metal metallurgically bonded on the substrate.

I claim:  
     
         1 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate of a workpiece, the layer produced by the steps of: 
 (a) providing a first molten salt bath of an anhydrous fused salt electrolyte in an inert container, the molten salt bath comprising a substantially eutectic mixture of at least one halide from the group consisting of alkali metal halides and alkaline earth metal halides and a reducing agent for the refractory metal;    (b) immersing an anode comprising the refractory metal in the first molten salt bath;    (c) immersing a cathode comprising the workpiece in the first molten salt bath, the workpiece being electrically conductive;    (d) electrodepositing a layer of the refractory metal on the workpiece;    (e) heating the workpiece with the electrodeposited refractory metal thereon to a first temperature in a range of about 700° C. to about 900° C. for a first time period sufficient to diffuse at least a portion of the electrodeposited refractory metal into the substrate such that a refractory metal layer is metallurgically bonded to the substrate;    (f) providing a second molten salt bath in an inert crucible, the second molten salt bath comprising an anhydrous fused salt electrolyte comprising at least one halide from the group consisting of alkali metal halides and alkaline earth metal halides and a compound containing at least one second element from the group consisting of B and C;    (g) immersing the cathode comprising the workpiece having at least a portion of the electrodeposited refractory metal diffused therein in the second molten salt bath;    (h) electrodepositing a layer of the second element from the second molten bath on said workpiece having the refractory metal layer; and    (i) heating the workpiece having the second element electrodeposited on the layer of the refractory metal to a second temperature in the range of about 700° C. to about 900° C. for a second time period sufficient to diffuse at least a portion of the B or C into the refractory metal layer and the substrate to form a boride or carbide of the refractory metal and to provide a substantially uniform and metallurgically bonded layer of the boride or carbide of the refractory metal on the substrate.    
     
     
         2 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 1  in which the inert container contains a protective atmosphere of argon, for preventing contaminants from entering the container.  
     
     
         3 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 2  in which the reducing agent is selected from the group consisting of a fluoride of the refractory metal and a chloride of the refractory metal.  
     
     
         4 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 3  in which electrodepositing of the refractory metal is effected by passing direct current at a current density in the range of between about 5 mA per square centimetre to about 100 mA per square centimetre through the first molten salt bath between the anode and the cathode.  
     
     
         5 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 4  in which electrodepositing of the second element is effected by passing direct current at a current density in the range of between about 200 mA per square centimetre to about 300 mA per square centimetre through the second molten salt bath.  
     
     
         6 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 5  in which the refractory metal is selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.  
     
     
         7 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 5  in which the refractory metal is Nb.  
     
     
         8 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 7  in which the second element is B and the second molten salt bath additionally comprises a second reducing agent for B.  
     
     
         9 . A substantially uniform layer of niobium diboride metallurgically bonded on a substrate according to  claim 8  in which the first time period is between about five minutes and about three hours.  
     
     
         10 . A substantially uniform layer of niobium diboride metallurgically bonded on a substrate according to  claim 9  in which the second temperature is about 800° C. and the second time period is between about one hour and about nine hours.  
     
     
         11 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 7  in which the second element is C.  
     
     
         12 . A substantially uniform layer of niobium carbide metallurgically bonded on a substrate according to  claim 11  in which the first time period is between about five minutes and about three hours.  
     
     
         13 . A substantially uniform layer of niobium carbide metallurgically bonded on a substrate according to  claim 12  in which the electrodepositing of C is effected by passing direct current through the second molten salt bath between the anode and the cathode at a current density of about 100 mA per square centimetre.  
     
     
         14 . A substantially uniform layer of niobium carbide metallurgically bonded on a substrate according to  claim 13  in which the second temperature is about 750° C. and the second time period is about four hours.  
     
     
         15 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 5  in which the refractory metal is Ta.  
     
     
         16 . A substantially uniform layer of a boride or a carbide of a refractory metal metallurgically bonded on a substrate according to  claim 15  in which the second element is B and the second molten salt bath additionally comprises potassium tetrafluoroborate.  
     
     
         17 . A substantially uniform layer of tantalum diboride metallurgically bonded on a substrate according to  claim 16  in which the first time period is between about five minutes and about three hours.  
     
     
         18 . A substantially uniform layer of tantalum diboride metallurgically bonded on a substrate according to  claim 17  in which the second temperature is about 900° C. and the second time period is about seven and one-half hours.  
     
     
         19 . A substantially uniform layer of a compound comprising a refractory metal and at least one of the elements B and C metallurgically bonded on an electrically conductive substrate formed by electrodepositing and thermally diffusing the refractory metal on the substrate to form a refractory metal layer, and electrodepositing from a source other than the workpiece and thermally diffusing at least one of the elements B and C on the refractory metal layer and into the refractory metal layer and the substrate to form the compound.  
     
     
         20 . A substantially uniform layer of a carbide of a refractory metal on a substrate of a workpiece, the layer produced by the steps of: 
 (a) providing a first molten salt bath of an anhydrous fused salt electrolyte in an inert container, the molten salt bath comprising a substantially eutectic mixture of at least one halide from the group consisting of alkali metal halides and alkaline earth metal halides and a reducing agent for the refractory metal;    (b) immersing an anode comprising the refractory metal in the first molten salt bath;    (c) immersing a cathode comprising the workpiece in the first molten salt bath, the workpiece being electrically conductive;    (d) electrodepositing a layer of the refractory metal on the workpiece;    (e) heating the workpiece with the electrodeposited refractory metal thereon to a first temperature in a range of about 700° C. to about 900° C. for a first time period sufficient to diffuse at least a portion of the deposited refractory metal into the substrate such that a refractory metal layer is metallurgically bonded to the substrate;    (f) providing a second molten salt bath in an inert crucible, the second molten salt bath comprising a substantially eutectic mixture of at least one of the fluorides of Li, Na, or K, including about two percent to about ten percent by weight the reducing agent of the refractory metal and about two percent to about ten percent by weight crystalline powder graphite;    (g) immersing the cathode comprising the workpiece having at least a portion of the electrodeposited refractory metal diffused therein in the second molten bath;    (h) electrodepositing a layer of carbon from the second molten salt bath on said workpiece having the refractory metal layer;    (i) heating the workpiece having the carbon electrodeposited on the layer of the refractory metal to a second temperature in the range of about 850° C. to about 900° C. for a second time period sufficient to diffuse at least a portion of the carbon into the refractory metal layer and the substrate to form a carbide of the refractory metal and to provide a substantially uniform and metallurgically bonded layer of the carbide of the refractory metal on the substrate.    
     
     
         21 . A substantially uniform layer of a carbide of a refractory metal on a substrate according to  claim 20  in which the refractory metal is Ta.  
     
     
         22 . A substantially uniform layer of tantalum carbide on a substrate according to  claim 21  in which the second molten salt bath comprises about five percent by weight potassium heptafluorotantalate and about five percent by weight crystalline powder graphite.  
     
     
         23 . A substantially uniform layer of a carbide of a refractory metal on a substrate of a workpiece, the layer produced by the steps of: 
 (a) providing a first molten salt bath of an anhydrous fused salt electrolyte in an inert container, the molten salt bath comprising a substantially eutectic mixture of at least one halide from the group consisting of alkali metal halides and alkaline earth metal halides and a reducing agent for the refractory metal;    (b) immersing an anode comprising the refractory metal in the first molten salt bath;    (c) immersing a cathode comprising the workpiece in the first molten salt bath, the workpiece being electrically conductive;    (d) electrodepositing a layer of the refractory metal on the workpiece;    (e) heating the workpiece with the electrodeposited refractory metal thereon to a first temperature in a range of about 700° C. to about 900° C. for a first time period sufficient to diffuse at least a portion of the electrodeposited refractory metal into the substrate such that a refractory metal layer is metallurgically bonded to the substrate;    (f) providing a crystalline graphite powder in an inert crucible;    (g) burying the workpiece having at least a portion of the electrodeposited refractory metal diffused therein in the crystalline graphite powder;    (h) compressing the crystalline graphite powder with pressure in the range of up to about 5,000 grams per square centimetre;    (i) evacuating air from the inert crucible;    (j) depositing a layer of carbon from the crystalline graphite powder on said workpiece having the refractory metal layer; and    (k) heating the workpiece to a second temperature in the range of about 1,000° C. to about 1,200° C. for a second time period sufficient to diffuse at least a portion of the carbon in the refractory metal layer and the substrate to form a carbide of the refractory metal and to provide a substantially uniform and metallurgically bonded layer of the carbide of the refractory metal on the substrate.    
     
     
         24 . A substantially uniform layer of a carbide of a refractory metal on a substrate of a workpiece, the layer produced by the steps of: 
 (a) providing a first molten salt bath of an anhydrous fused salt electrolyte in an inert container, the molten salt bath comprising a substantially eutectic mixture of at least one halide from the group consisting of alkali metal halides and alkaline earth metal halides and a reducing agent for the refractory metal;    (b) immersing an anode comprising the refractory metal in the first molten salt bath;    (c) immersing a cathode comprising the workpiece in the first molten salt bath, the workpiece being electrically conductive;    (d) electrodepositing a layer of the refractory metal on the workpiece;    (e) heating the workpiece with the electrodeposited refractory metal thereon to a first temperature in a range of about 700° C. to about 900° C. for a first time period sufficient to diffuse at least a portion of the electrodeposited refractory metal into the substrate such that a refractory metal layer is metallurgically bonded to the substrate;    (f) depositing a layer of carbon on said workpiece having the refractory metal layer by gas carburizing; and    (g) heating the workpiece to a second temperature in the range of about 1,000° C. to about 1,400° C. for a second time period sufficient to diffuse at least a portion of the carbon in the refractory metal layer and the substrate to form a carbide of the refractory metal and to provide a substantially uniform and metallurgically bonded layer of the carbide of the refractory metal on the substrate.