Effect Of Carbon Content On Properties Of Magnetron Sputtered Cr/C Coatings

Cr/C coatings were deposited on high speed steel and silicon wafers substrates by Magnetron sputtering,using sputtering graphite target as the mainly source for carbon element.The chemical composition and microstructures of the coatings were analyzed by XPS and XRD. The hardness,toughness and adhesion of the coatings were measured using a Vickers micro-hardness tester and a scratch tester,and friction and wear behavior of the coatings were studied by pin-on-disk tester and optical microscope.The thermal stability and corrosion resistance were evaluated by heat treatment at different temperatures and polarization curves respectively.Then mechanisms which carbon content effect on properties of Cr/C coatings were discussed.The results indicate that Cr/C coatings consist of pure carbon,chromium and Cr₃C₂.With the carbon content increased,the pure carbon increases,metallic chromium decreases,but chromium carbide in the coatings reaches its maximum when carbon content is 51.93 at%. Metallic Cr coating shows a crystalline structure,but when carbon is introduced into coatings,a crystalline→amorphous structure transformation occurred.With the carbon content increased, the hardness of coatings increase,whereas the toughness are weakened,all coatings excluded coating with 17.45 at%carbon content show good adhesion.Coating with 42.37 at%,51.93 at% carbon content shows the best mechanical properties.When the carbon content increased,the friction coefficients and wear rotes of coatings decrease,and the wear mechanism undergoes a adhesive wear→abrasive wear transformation.Cr/C coatings show excellent thermal stability after a 600℃heat treatment,excluding failure of coating with 75.08 at%carbon content at this temperature.All Cr/C coatings protect substrates from corrosion by 1mol/L NaCl solution. Their corrosion resistance increase with carbon content decrease,while Metallic Cr coating(0% carbon content)is inferior to coating with 17.45 at%carbon content which exhibit the best corrosion resistance.The difference among the chemical statuses of Cr and C elements in coatings with various carbon content influence the formation of adhesive layer at the initial wear stage,and adhesion of adhesive layer to coatings,then the variety of friction coefficients and wear rates are obtained.The effect that carbides could slow down the oxidation and the effect that carbon element could be oxidated at high temperature could speed up the formation of porous structure, which are competitive reactions,that's why the coating obtain a better thermal stability with carbon content increase,but when carbon content increases beyond certain extend,failure of coating occurs at a lower temperature.The varieties of crystal structure and passivation films cause the difference in corrosion resistance of Cr/C coatings.