Surface treatment of co-cr based alloys using plasma carburization

Abstract

The present invention relates to a method of modifying a surface characteristic (eg. 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 to 700° C. and at a pressure of from 100 to 1500 Pa for 1 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 modifying a surface characteristic of a cobalt-chromium based alloy article, comprising plasma treating the article at a temperature in the range of from 300 to 700° C. and at a pressure of from 100 to 1500 Pa for 1 to 50 hours in an atmosphere comprising at least one carbon-containing gas, whereby to introduce carbon into a surface region of said article.  
     
     
         2 . The method of  claim 1 , wherein the surface characteristic to be modified is one or more of hardness, wear resistance, corrosion resistance and fatigue strength.  
     
     
         3 . The method of  claim 1 , wherein the article whose surface characteristic is to be modified is a medical implant, such as a joint or knee prosthesis.  
     
     
         4 . The method of  claim 3 , wherein the plasma treating is carried out at a temperature in the range of from 350 to 550° C.  
     
     
         5 . The method of  claim 4 , wherein the plasma treating is carried out at a temperature in the range of from 400 to 500° C.  
     
     
         6 . The method of  claim 1 , wherein the article whose surface characteristic is to be modified is an engineering component, such as a knife, valve, blade or shaft.  
     
     
         7 . The method of  claim 6 , wherein the plasma treating is carried out at a temperature in the range of from 450 to 700° C.  
     
     
         8 . The method of  claim 7 , wherein the plasma treating is carried out at a temperature in the range of from 600 to 650° C.  
     
     
         9 . The method of  claim 1 , wherein said treatment pressure is in the range of from 400 to 600 Pa.  
     
     
         10 . The method of  claim 1 , wherein the duration of said treatment is in the range of from 1 to 50 hours.  
     
     
         11 . The method of  claim 10 , wherein the duration of said treatment is in the range of from 5 to 30 hours.  
     
     
         12 . The method of  claim 1 , wherein the or each carbon-containing gas is selected from a hydrocarbon, carbon dioxide and carbon monoxide.  
     
     
         13 . The method of  claim 1 , wherein the plasma treatment is carried out in the presence of at least one unreactive gas selected from hydrogen, helium, argon or other noble gas.  
     
     
         14 . The method of  claim 1 , wherein the plasma treatment is carried out in the presence of at least one reactive gas.  
     
     
         15 . The method of  claim 14 , wherein the reactive gas is a nitrogen containing gas.  
     
     
         16 . The method of  claim 15 , wherein the plasma treatment is effected at a temperature in the range of from 300 to 500° C.  
     
     
         17 . The method of  claim 14 , wherein said reactive gas constitutes from 0.5 to 10% by volume of the total atmosphere.  
     
     
         18 . The method of  claim 13 , wherein the unreactive gas is hydrogen or a mixture of hydrogen and argon, and the carbon-containing gas is methane.  
     
     
         19 . The method of  claim 1 , wherein the or each carbon-containing gas constitutes from 0.5 to 20% by volume of the total atmosphere.  
     
     
         20 . The method of  claim 1 , wherein said plasma treatment is effected in the absence of oxygen.  
     
     
         21 . The method of  claim 1  which includes an article cleaning step prior to the plasma treatment to remove oxide scale.  
     
     
         22 . The method of  claim 21 , wherein cleaning is effected by sputter cleaning.  
     
     
         23 . The method of  claim 21 , wherein said cleaning step is effected at or below the subsequent plasma treatment temperature in an atmosphere of one or more of gases selected from hydrogen, helium, argon or other noble gas.  
     
     
         24 . The method of  claim 1 , wherein the article is cooled after the plasma treatment.  
     
     
         25 . The method of  claim 24 , wherein the rate of cooling is from 0.1° C./min up to 1000° C./min.  
     
     
         26 . The method of  claim 24 , wherein the cooling is achieved by relatively slow cooling in the plasma treating atmosphere or by relatively fast cooling by quenching in a liquid.  
     
     
         27 . The method of  claim 1 , wherein a passivation and/or polishing step is effected after completion of the plasma treatment.  
     
     
         28 . A surface-hardened cobalt-chromium based article producible by the method of  claim 1 , said article characterised by having: 
 (i) a surface region comprising a supersaturated solid solution of carbon in cobalt or,    (ii) a surface region comprising a supersaturated solid solution of carbon in cobalt and chromium carbides.    
     
     
         29 . The article of  claim 28 , wherein said surface region has a thickness in the range of from 3 to 50 μm.  
     
     
         30 . The method of  claim 1 , wherein the Co—Cr based alloy includes one or more other alloying ingredients selected from molybdenum, nickel, tungsten, titanium and carbon.  
     
     
         31 . The article of  claim 30 , wherein carbon is present in an amount of from 0.04 to 1.6 wt %.  
     
     
         32 . The article of  claim 31 , wherein the article is a medical implant and carbon is present in the range of from 0.04 to 0.4 wt %.  
     
     
         33 . The article of  claim 31 , wherein the article is an engineering component and carbon is present in the range of from 0.4 to 1.6 wt %.  
     
     
         34 . The article of  claim 28  which is a conventional hip or knee joint prostheses, a metal-on-metal advanced bone conservation prostheses, a dental implant or other implant device, a knife, blade for chemical or food processing industrials, a turbine blade, a valve or pump in the chemical and power industries, a bushing, steel mill equipment, a die, punch or mould.