Microstructure and Phase Composition of Composite Coatings Formed by Plasma Spraying of ZrO 2 and B 4 C Powders
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JTTEE5 19:816–823 DOI: 10.1007/s11666-010-9479-y 1059-9630/$19.00 ASM International
Microstructure and Phase Composition of Composite Coatings Formed by Plasma Spraying of ZrO2 and B4C Powders P. Karuna Purnapu Rupa, Prashant Sharma, R.M. Mohanty, and K. Balasubramanian (Submitted April 22, 2009; in revised form January 18, 2010) The effect of addition of 5 to 30 wt.% boron carbide (B4C) on structure and hardness of plasma sprayed zirconia (ZrO2) coating has been studied in this paper. The coatings have exhibited a uniform porous microstructure. A reaction between B4C and ZrO2 resulted in the formation of a diboride (ZrB2) phase. The presence of ZrB2 in the coatings has been confirmed through x-ray diffraction studies. In order to study the effect of critical processing parameters, the coatings have also been deposited under increased hydrogen flow rate (11.8 SLM). This increased the abrasion integrity of the coatings. A high yield of ZrB2 was observed in the case of 15 wt.% B4C addition. Hardness of the coatings have been influenced by the porosities, additionally generated by the formation of ZrB2. Under increased hydrogen flow rate, a composite coating of ZrO2-ZrB2 was obtained from the ZrO2-B4C powder mixture.
Keywords
boron carbide, composite coatings, plasma spray, zirconia, zirconium boride
1. Introduction Controlled atmospheric plasma spraying (APS) is being used as a coating process to enhance the performance of substrate materials at extreme environments by the surface modification. In this process, powder particles are melted, accelerated in the plasma and are deposited on the substrates as thin lenticular features known as splats. With time the continuous deposition of splats results in the formation of a coating with a distinct microstructure and properties. The materials for plasma spray include oxides ceramics such as Al2O3 and ZrO2, metals and alloys such as nickel-chromium and intermetallics such as NiAl, AlN, etc. In APS parameters, such as particle size, particle size distribution, particle density, electric power, primary and secondary gas flow rates, particle in-flight time and standoff distance all play an important role in obtaining a coating with desired set of properties. For this, process maps have been developed as a guide to obtaining the desired coating properties and also to improve the reproducibility and reliability (Ref 1, 2). These have been extensively studied for single phase metals/alloys and ceramics. Selected composites, such as WC-Co (Ref 3) and Al2O3-TiO2 (Ref 4) coatings, also have been obtained P. Karuna Purnapu Rupa, Prashant Sharma, R.M. Mohanty, and K. Balasubramanian, Non Ferrous Materials Technology Development Centre, Kanchanbagh, Hyderabad 500058, India. Contact e-mails: [email protected] and mohantyrm@ yahoo.com
816—Volume 19(4) June 2010
using APS. The number of process variables increases as two or more powders with different thermo-physical properties interact with the plasma. A variant of composite coatings is reactive plasma spray (RPS) coatings w
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