Nitriding Method for Improving Surface Characteristics of Cobalt-Chromium Based Alloys

Abstract

A method for improving the surface characteristics of Cobalt chromium alloys by plasma nitriding. By creating a glow discharge around the parts with selective gases at partial pressures a compound layer is formed at the surface beneficial for wear applications. A further benefit is an increase in the load bearing capacity of the surface as compared to standard material.

1 . A method for enhancing the surface hardness and smoothness of cobalt-chromium alloys through exposure in a reaction vessel to a mixture of reaction gases at a partial pressure (less than atmospheric) within a temperature range of 250° C. to 1000° C. for process time sufficient to create a substantial compound layer.  
   
   
       2 . The method of  claim 1  wherein the reaction vessel utilizes a pulse plasma glow discharge as a media for the reaction.  
   
   
       3 . The method of  claim 1  wherein the total pressure of reaction gases is 0.5 to 100 mbars.  
   
   
       4 . The method of  claim 1  where the temperature is 400° C. to 600° C.  
   
   
       5 . The method of  claim 1  wherein the reaction gases are Ar, N 2 , H 2 .  
   
   
       6 . The method of  claim 3  wherein the reaction gases are Ar, N 2 , H 2 .  
   
   
       7 . The method of  claim 1  wherein the reaction gases are Ar, N 2 , H 2 , CH 4 .  
   
   
       8 . The method of  claim 3  wherein the reaction gases are Ar, N 2 , H 2 , CH 4 .  
   
   
       9 . The method of  claim 1  wherein the reaction time is 6-42 hours  
   
   
       10 . The method of  claim 1  wherein the reaction the reaction time is approximately 24 hours.  
   
   
       11 . The method of  claim 1  where the pulse pause ratio is 1:0 to 1:50.  
   
   
       12 . The method of  claim 3  where the pulse pause ration is 5:1 to 1:20.  
   
   
       13 . The method in  claim 1  in which the identified hardened surface contains a compound layer of Cr—N of 1 to 20 microns thickness.  
   
   
       14 . The method in  claim 1  in which the identified hardened surface contains a compound layer of Cr—N of 3 to 15 microns thickness.  
   
   
       15 . A method for enhancing the surface hardness and smoothness of a cobalt chromium material and with surface hardness and smoothness improved through exposure in a reaction vessel to a mixture of reaction gases (Ar, N 2 , H 2 , CH 4 ) for process time sufficient to create a compound layer substantially comprised of chromium nitrides.  
   
   
       16 . The method of  claim 15  wherein the reaction gases are Ar, N 2 , H 2 .  
   
   
       17 . The method of  claim 15  wherein the reaction gases are Ar, N 2 , H 2 , CH 4 .  
   
   
       18 . The method of  claim 15  wherein the said material is ASTM F-75 and ASTM F-75 Modified alloy  
   
   
       19 . The method of  claim 15  wherein the process temperature is held between 400° C. to 600° C.  
   
   
       20 . The method of  claim 15  wherein the process partial pressure is held at less than atmospheric.  
   
   
       21 . The method of  claim 15  wherein the process partial pressure is held at approximately 1-10 mbars.  
   
   
       22 . The method of  claim 15  wherein the said cobalt chromium alloy is ASTM F-799, F75, and F75 modified.  
   
   
       23 . A method for enhancing the surface hardness and smoothness of a cobalt chromium molybdenum base material and with surface hardness and smoothness improved through exposure in a reaction vessel incorporating pulse plasma of reaction gases varies in on-to-off process levels.  
   
   
       24 . The method of  15  and  24  where in the pulse plasma on-to-off ratio is 2:1 to 1:10.  
   
   
       25 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating convection preheating.  
   
   
       26 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating single and multi zone cooling  
   
   
       27 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating a central anode.  
   
   
       28 . The method of  1 ,  3 , and  15  through in a reaction vessel wherein the parts are masked to prevent nitriding in unwanted areas.  
   
   
       29 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating helium as a replacement carrier gas for the hydrogen.  
   
   
       30 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating thermocouple temperature measurement of the parts.  
   
   
       31 . The method of  1 ,  3 , and  15  through exposure in a reaction vessel incorporating a sputter step to increase the surface reactivity of the workpiece.  
   
   
       32 . The method of any one of  claims 15  to  31  wherein the creation of a substantial nitrogen diffusion layer is avoided.  
   
   
       33 . The method of  claim 1  wherein the reaction vessel utilizes a plasma glow discharge as a media for the reaction.  
   
   
       34 . The method of  claim 1  wherein the reaction gas is nitrogen.  
   
   
       35 . The method of  claim 1  wherein the reaction gases are nitrogen and a carrier gas with optionally argon and optionally a carbon precursor.