<title>Titanium thermodiffusional nitriding by CO2 laser: process parameters controlling the nitride growth</title>

Abstract

ABSTRACT Solid state nitriding of titanium under cw C02 laser was investigated. The nitriding kinetics were studied in the caseof C02 laser treatment in isothermal conditions, and were compared to those obtained by using a conventional heatingsystem. Identical kinetics rate constants and activation energy were obtained; therefore C02 irradiation of the titanium surfacedid not influence nitride growth. Studying nitriding treatment in non isothermal conditions showed that high heating ratesinduced modifications of the nitride morphology (compared to isothermal treatments). At last, we investigated the evolutionof the C02-surface coupling during the nitriding treatment. It was found that it depended only on the surface temperature andthe substrate surface roughness since the growth of a few micrometers thick nitride layer did not modify the coupling.Titanium, Nitriding, C02 laser, Kinetics, absorptance, thermal treatment, diffusion. I. INTRODUCTION Nitriding of titanium alloys by C02 laser was extensively studied in the case of liquid phase treatment ,2,3,4Surface melting enables to form thick nitride layers (several hundred micrometers) that are attractive for surface wear resistanceapplications. However, liquid phase nitriding treatment may also reduce the substrate fatigue resistance and in some caseinduce cracks in the nitride layers. An alternative consists in realizing the treatment by maintaining the surface temperaturebelow the melting point (solid phase treatment). This considerabily reduces the nitride thickness (tens of micrometers), but ithas been found to induce also good tribological and mechanical performances on titanium alloys5. We investigated theparamaters that influence the nitride layer growth in solid phase under cw C02 laser radiation, both under isothermal and nonisothermal conditions. For this purpose, we compared, under isothermal conditions, nitriding under C02 laser radiation to theresults obtained when the treatment is carried out using a conventional heating system (thermoresistance heating). We alsocharacterized cw C02 laser nitriding treatment in non isothermal conditions in terms of nitride layers thicknesses andmorphology depending on the process parameters. At last, investigations of the C02 -