Microstructure and tribology of laser mixed Fe/Ti/C multilayered films on AISI 304 stainless steel
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An excimer laser was used to mix Fe/Ti/C multilayered films on 304 stainless steel substrates. The samples were processed at both 1.1 and 1.7 J/cm2 with the number of pulses at each position varied between 1 and 10. Composition, microstructure, phase evolution, and tribological properties were observed to correlate with total laser fluence. Increases in Fe concentration from substrate interdiffusion and loss of C content were observed with increasing total laser fluence. The best tribological properties were observed in films possessing a combination of an amorphous or Fe3C phase plus fine grain TiC following processing at low and intermediate total laser fluence. At higher total fluences a combination of a-Fe, Fe3C, and fine-grain TiC was observed along with degradation in the wear and friction properties. Under optimum laser processing conditions the modified surface had a friction coefficient under dry sliding conditions reduced by a factor of 2 relative to uncoated 304 stainless steel and was significantly more wear resistant. These improvements in wear and friction appear to be related to the reduced chemical reactivity of the amorphous and carbide phases and to the influence of microstructure on improved mechanical properties.
I. INTRODUCTION
It has been well established that improvements in the tribological properties of steel surfaces can be obtained by the dual ion implantations of Ti and C.1"3 These improvements have been attributed to the chemical changes and amorphous structure which evolve during the implantation process. While ion implantation is quite versatile in that almost any element can be combined with any material, high concentration alloying is constrained by sputtering effects and long implantation times. An alternative beam technique which does not suffer from these limitations and is capable of producing similar tribological improvements is ion beam mixing. Hirvonen et al.4 showed that a Xe-ion mixed Fe/Ti surface layer on 304 stainless steel possessed optimum friction and wear characteristics when the composition of the layer was near Fe5OTi5o. This ion mixed layer also possessed an amorphous structure. To include C in such a sample would require either a second processing step of C ion implantation or the single step process of ion mixing an Fe/Ti/C multilayer. While the first of these techniques has been successfully applied to produce Fe/Ti-carbide with good tribological properties,5 attempts at ion mixing Fe/Ti/C, Fe/C, and Ti/C samples have proven less successful.6'7 It appears that C and/or metal-carbides formed during early stages of ion mixing are very effective ion mixing barriers.6^8 In the present paper we present the first application of excimer laser mixing of Fe/Ti/C multilayers on 304 stainless steel substrates. We will show that excimer
laser mixing overcomes barriers imposed by the presence of C layers and is also capable of producing a modified surface with tribological properties comparable to those obtained using ion beam techniques. Phase formation, microstructure,
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