Correlation of microstructure and wear resistance of ferrous coatings fabricated by atmospheric plasma spraying
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INTRODUCTION
DIESEL engine cylinder bores, which have been made of gray cast iron, are replaced by aluminum alloys now. This replacement is further accelerated by economic needs for reduction in vehicle weight and improved fuel efficiency. In the case of unprotected aluminum alloys, however, the compression and emission performances of a diesel engine are significantly deteriorated by the considerable wear against a piston.[1,2,3] As an alternative to solve this problem, gray cast iron liners are inserted into aluminum cylinder blocks, but this method also carries additional problems such as deformation of the cylinder bore and an increase in cylinder weight and in manufacturing cost. In order to enhance the wear resistance of cylinder bores, therefore, many automotive companies worldwide have developed new aluminum composite materials or protective coatings to aluminum alloys. In particular, development of wear-resistant ferrous coatings using thermal spraying is receiving growing attention.[1–10] Plasma spraying is most widely applied in automotive industries among thermal spraying methods because (1) it is less restricted in material selection; (2) it has higher spray rate and deposition efficiency than high velocity oxyfuel process, flame spraying, and electric arc spraying; (3) it
consumes less fuel gases, which are inexpensive and easily obtainable; (4) it requires minimum preheating and cooling during spraying; (5) technical reliability of plasma systems is well established in industrial applications; (6) spraying conditions can be easily controlled upon various applications; and (7) it is readily applicable to production lines.[1,2,8] Ferrous coatings fabricated by atmospheric plasma spraying have enhanced wear resistance because of the formation of Fe oxides when ferrous powders are oxidized during spraying in air. The oxides also demonstrate a self-lubricating performance and play a role in reducing sliding wear.[10] In addition, they are effective in simultaneously reducing wear on the surface of both diesel engine cylinders and pistons. In the present study, seven wear-resistant ferrous coatings applicable to diesel engine cylinder bores were fabricated by atmospheric plasma spraying, and the effects of variety, amount, and distribution of oxides contained in the coatings on wear properties of coatings and counterpart materials were investigated. Their microstructures, hardness, and wear resistance were investigated to understand the mechanism involved in the formation of coatings and to elucidate the correlation of microstructures with wear properties of coatings and counterpart materials. II. EXPERIMENTAL
BYOUNGCHUL HWANG, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University Science and Technology, Pohang, 790-784 Korea. JEEHOON AHN, Senior Research Engineer, is with the Materials and Processes Research Center, Research Institute of Industrial Science and Technology, Pohang, 790-600 Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang U
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