Phase evolution during laser In-Situ carbide coating
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I. INTRODUCTION
THE never-ending search for materials with enhanced tribological properties makes it vital to develop novel materials and simultaneously investigate techniques to modify the existing ones. This can be achieved by either modifying the microstructure (grain size) or by altering the composition on the surface of any commonly available material using one of the various surface engineering techniques. Laser surface engineering (LSE) is a versatile way of surface modification to achieve the desired properties. When the laser beam interacts with the surface, the heat energy is sufficient to melt the material in a confined area. The melt pool generated undergoes rapid solidification, which provides a unique opportunity to synthesize nonequilibrium phases.[1] The key feature of the process is localized melting and solidification within a shallow depth, which makes it possible to modify the surface layer without affecting the bulk of the as-received sample.[2] One way to improve wear properties is to introduce a hard phase in a tough metal matrix to form a composite layer on the surface. Carbide particles are frequently introduced in a tough matrix to form a particle-reinforced composite by various techniques.[3–11] Laser cladding of Fe-Cr-Mn-C produced improved wear-resistant surfaces due to the formation of complex carbide precipitates such as M7C3 and M6C.[12,13] Hard and refractory TiC particles of various shapes have been deposited on 6061 Al alloy to achieve hard and wear-resistant surfaces using LSE.[14,15] It has also been reported in the past that wear resistance improves with the amount of carbide until it reaches a steady value.[16] High-Cr white irons have high wear resistance due to the presence of high volume fraction of M7C3 carbides in a strong matrix.[17] The good wear resistance is accompanied by poor impact resistance. Composites with steel matrix and ceramic reinforcements can achieve high hardness along with sufficient fracture toughness.[17] These examples suggested that there is promise and potential in developing a carbide coating with TiC and M7C3 carbides incorporated individually. The current study was aimed at achieving a carbide coating, within an iron-based matrix, on the surface of steel via ANSHUL SINGH, Graduate Student, and NARENDRA B. DAHOTRE, Professor, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. Contact e-mail: ndahotre@ utk.edu NARENDRA B. DAHOTRE, is also with the Materials Processing Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted May 13, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
LSE. The carbide coating would be achieved by in-situ growth of the particles from Ti and Cr elemental powders. Both have carbide-forming tendencies, while Cr was also incorporated to successfully improve the toughness of the ferrite phase. Preliminary studies indicated the formation of very fine particles within the coating.[18] It was also observed that X-ray
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