Surface hardening of Ti alloys by gas-phase nitridation: kinetic control of the nitrogen activity

Abstract

The present invention relates to surface hardening of a metal surface of a work piece formed of titanium and titanium alloys. The method comprises the steps of heating the work piece; exposing the metal surface to a nitrogen gas having a partial pressure lower than about 10 −2 Pa; and maintaining the work piece in the nitrogen gas so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.

1 . A method for surface hardening of a metal surface of a work piece formed of a metal from the group consisting essentially of titanium and titanium alloys, said method comprising the steps of: 
 heating the work piece to a desired temperature of less than about 1000° C.;    exposing the metal surface to a nitrogen gas having a partial pressure lower than about 10 −2  Pa; and    maintaining the work piece in the nitrogen gas for a period of time so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.    
   
   
       2 . The method of  claim 1  including the step of exposing the metal surface to a nitrogen gas having a partial pressure of between about 10 −7  Pa and 10 −2  Pa and a temperature of less than about 1000° C.  
   
   
       3 . The method of  claim 2  including the step of exposing the metal surface to a nitrogen gas having a partial pressure of between about 10 −5  Pa and 10 −3  Pa and at a temperature of between about 700° C. and 1000° C.  
   
   
       4 . The method of  claim 3  including the step of exposing the metal surface to a nitrogen gas having a partial pressure of between about 10 −5  Pa and 10 −3  Pa and at a temperature of about between about 800° C. and 900° C.  
   
   
       5 . The method of  claim 1  including the step of exposing the metal surface so that nitrogen is diffused into the surface of the workpiece for a desired distance of about 2 and 250 μm.  
   
   
       6 . The method of  claim 1  including the step of maintaining the work piece in the nitrogen gas for a period of time of from about 1 to 250 hours.  
   
   
       7 . A method for surface hardening of a metal surface of a work piece formed of a metal from the group consisting essentially of titanium and titanium alloys, said method comprising the steps of: 
 heating the work piece to a first temperature of less than about 1000° C.;    exposing a surface of the work piece to a nitrogen gas having a first pressure that is determined by a second temperature to which a first powder of metal nitride and a second powder of the corresponding metal forming the metal nitride are exposed; and    maintaining the work piece in the nitrogen gas for a period of time so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.    
   
   
       8 . The method of  claim 7  including the step of exposing the surface of the work piece to a nitrogen gas having a first pressure of between about 10 −7  Pa and 10 −2  Pa.  
   
   
       9 . The method of  claim 8  including the step of exposing the surface to a nitrogen gas having a first pressure of between about 10 −7  Pa and 10 −2  Pa created by the decomposition of the nitride powder and simultaneously reacting the metal powder with the nitrogen to form metal nitride.  
   
   
       10 . The method of  claim 8  including the step of exposing the metal surface to a nitrogen gas having a first pressure of between about 10 −5  Pa and 10 −3  Pa and at a second temperature of between about 700° C. and 1000° C.  
   
   
       11 . The method of  claim 10  including the step of exposing the metal surface to a nitrogen gas having a first pressure of between about 10 −5  Pa and 10 −3  Pa and at a second temperature of about between about 800° C. and 900° C.  
   
   
       12 . The method of  claim 7  including the step of exposing the metal surface so that nitrogen is diffused into the surface of the work piece for a desired distance of about 2 and 250 μm.  
   
   
       13 . The method of  claim 12  including the step of encapsulating the work piece together with powders of a metal nitride and the corresponding metal in an evacuated inert container.  
   
   
       14 . The method of  claim 13  wherein the inert container is an ampoule of fused silica.  
   
   
       15 . The method of  claim 7  including the steps of: 
 exposing the surface of the work piece to a nitrogen gas whose pressure is at a second pressure different than the first pressure.    
   
   
       16 . The method of  claim 15  including the step of: 
 exposing the surface of the work piece to a nitrogen gas whose pressure is at a third pressure different than the second pressure.