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 Title: Processing and characterization of laser-synthesized overcoats for surface engineering Author(s): Nagarathnam, Karthikeyan Doctoral Committee Chair(s): Komvopoulos, K. Department / Program: Mechanical and Industrial Engineering Discipline: Mechanical Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Engineering, Mechanical Engineering, Metallurgy Engineering, Materials Science Abstract: Surface modification of engineering materials using lasers by means of alloying and cladding have opened up numerous applications in recent years to selectively improve the tribological and high temperature corrosion properties without affecting the bulk-properties of the substrate. The present investigation has been focused on developing such laser-synthesized cost-effective coatings. The major emphasis has been given to identify the effects of key laser processing parameters such as laser power, process speed, and powder feed rate on the microstructure, compositional homogeneity, coating geometry, mechanical properties, tribological characteristics and high temperature oxidation behavior. The coatings include Fe-Cr-W-C and Fe-W-Co-Cr-V-C alloy systems on low carbon steel substrate for wear resistance applications at low to medium temperatures and Fe-Cr-Al-Y coatings on stainless steel substrates for high temperature applications ($>$1000$\rm\sp\circ C).$ The Fe-Cr-W-C coatings were found to solidify via hypoeutectic (primary fcc-$\gamma$ phase and interdendritic $\gamma$ and Cr-rich $\rm M\sb7C\sb3$ phase (hypoeutectic) or hypereutectic (primary Cr-rich carbides and eutectic network) solidification modes depending on the alloy composition. In-situ TEM study of the Fe-Cr-W-C coatings (at $570\rm\sp\circ C)$ provided systematic understanding of the fundamental phase change characteristics with fcc-$\gamma$ phase changing to a bcc-$\alpha$ phase and stability of Cr-rich $\rm M\sb7C\sb3$ carbides with adequate hardness after about 3.2 hours. The tribological tests revealed that the major friction mechanism is due to plowing and the wear resistance is attributed to the fine grained carbide network in a strained $\gamma$-matrix. The high speed tool coatings were found to contain primary $\gamma$ phase and $\alpha$-ferrite in the matrix, with V-rich (MC) and W-rich $\rm (M\sb6C$ and $\rm M\sb2C)$ carbides in the grain boundaries and in the eutectic and cause secondary hardening after thermal treatment at $550\sp\circ\rm C$ for 2 hours due to martensitic and secondary carbide precipitation reactions. Fe-Cr-Al-Y coatings were found to provide excellent high temperature oxidation protection at $1200\sp\circ\rm C$ for about 100 hours with the formation of $\alpha$-$\rm Al\sb2O\sb3$ and thermally stable $\alpha$-phase comprising Fe, Cr and Al in the matrix regions. Based on several experimental findings, Laser Processing Maps for Surface Engineering Applications (LPMSEA) which are useful for engineers in the industry and academic researchers were developed. Issue Date: 1994 Type: Text Language: English URI: http://hdl.handle.net/2142/22062 Rights Information: Copyright 1994 Nagarathnam, Karthikeyan Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9512495 OCLC Identifier: (UMI)AAI9512495
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