Surface treatment of co-cr based alloys using plasma carburization

Abstract

The present invention relates to a method of modifying a surface characteristic (e.g., wear resistance and/or corrosion resistance) of a cobalt-chromium based alloy article. The method comprises plasma treating the article at a temperature in the range of from 300° C. to 700° C. and at a pressure of from 100 Pa to 1500 Pa for 1 hour to 50 hours in an atmosphere comprising at least one carbon-containing gas, whereby to introduce carbon into a surface region of said article. The present invention also resides in a surface-hardened cobalt-chromium based article producible by the method of the invention. The article is characterised by having a surface region comprising a supersaturated solid solution of carbon in cobalt or a surface region comprising a supersaturated solid solution of carbon in cobalt and chromium carbides. Surface hardened articles producible by the method of the invention include medical implants and engineering components.

1 . A method of improving surface hardness wear resistance or fatigue strength of a cobalt-chromium based alloy medical implant without loss in corrosion resistance, the method comprising:
 plasma treating the implant at a temperature in the range of from 300° C. to 550° C. and at a pressure of from 100 Pa to 1500 Pa for 1 hour to 50 hours in an atmosphere comprising at least one carbon-containing gas, whereby to introduce carbon into a surface region of said implant, to produce a supersaturated solid solution of carbon in cobalt.   
     
     
         2 . The method of  claim 1 , wherein the medical implant is a joint or knee prosthesis.
 D. The method of  claim 1 , wherein the plasma treating is carried out at a temperature in the range of from 350° C. to 550° C.   
     
     
         4 . The method of claim  3 , wherein the plasma treating is carried out at a temperature in the range of from 400° C. to 500° C. 
     
     
         5 . The method of  claim 1 , wherein the treatment pressure is in the range of from 400 Pa to 600 Pa. 
     
     
         6 . The method of  claim 1 , wherein the duration of said treatment is in the range of from 5 hours to 30 hours. 
     
     
         7 . The method of  claim 1 , wherein the carbon-containing gas is selected from a hydrocarbon, carbon dioxide, and carbon monoxide. 
     
     
         8 . The method of  claim 1 , wherein the plasma treatment is carried out in the presence of at least one other gas selected from hydrogen, helium, argon or other noble gas. 
     
     
         9 . The method of  claim 1 , wherein the plasma treatment is carried out in the presence of at least one additional gas to be incorporated into the surface region of said implant in addition to the carbon. 
     
     
         10 . The method of  claim 9 , wherein the additional gas is a nitrogen containing gas. 
     
     
         11 . The method of  claim 9 , wherein said additional gas constitutes from 0.5% to 10% by volume of the total atmosphere. 
     
     
         12 . The method of  claim 8 , wherein the at least one other gas is hydrogen or a mixture of hydrogen and argon, and the carbon-containing gas is methane. 
     
     
         13 . The method of  claim 1 , wherein the carbon-containing gas constitutes from 0.5% to 20% by volume of the total atmosphere. 
     
     
         14 . The method of  claim 1 , wherein said plasma treatment is effected in the absence of oxygen. 
     
     
         15 . The method of  claim 1 , further comprising a medical implant cleaning step prior to the plasma treatment to remove oxide scale. 
     
     
         16 . The method of  claim 15 , wherein cleaning is effected by sputter cleaning. 
     
     
         17 . The method of  claim 15 , wherein said cleaning step is effected at or below the subsequent plasma treatment temperature in an atmosphere of one or more gases selected from hydrogen, helium, argon or other noble gas. 
     
     
         18 . The method of  claim 1 , wherein the medical implant is cooled after the plasma treatment. 
     
     
         19 . The method of  claim 18 , wherein the rate of cooling is from 0.1° C./min up to 1000° C./min. 
     
     
         20 . The method of  claim 18 , wherein the cooling is achieved by relatively slow cooling in the plasma treating atmosphere or by relatively fast cooling by quenching in a liquid. 
     
     
         21 . The method of  claim 1 , wherein at least one of a passivation step and a polishing step is effected after completion of the plasma treatment. 
     
     
         22 . The method of  claim 1 , wherein the cobalt-chromium based alloy includes one or more other alloying ingredients selected from molybdenum, nickel, tungsten, titanium and carbon. 
     
     
         23 . A surface-hardened cobalt-chromium based alloy medical implant producible by the method of  claim 1 , said implant characterised by having a surface region comprising a supersaturated solid solution of carbon in cobalt. 
     
     
         24 . The medical implant of  claim 23 , wherein said surface region has a thickness in the range of from 3 μm to 50 μm. 
     
     
         25 . The medical implant of  claim 23 , wherein the cobalt-chromium based alloy includes one or more other alloying ingredients selected from molybdenum, nickel, tungsten, titanium and carbon. 
     
     
         26 . The medical implant of  claim 25 , wherein carbon is present as an alloying ingredient in an amount of from 0.04 wt % to 1.6 wt %. 
     
     
         27 . The medical implant of  claim 26 , wherein carbon is present as an alloying ingredient in the range of from 0.04 wt % and 0.4 wt %. 
     
     
         28 . The medical implant of  claim 23 , wherein the implant includes at least one of a conventional hip or knee joint prostheses, a metal-on-metal advanced bone conservation prostheses, or a dental implant.