ABSTRACT
Surface coating technique plays an important role in prompting surface properties of a metal component and prolonging service lifetime. Particularly, wear resistance of metal matrix could be improved markedly by incorporating ceramic particles to form a composite coating, which is a hot research topic in wear-resistance coating field. Different kinds of composite coatings consisted of reinforcement and metal matrix have obtained. Among many surface modification techniques, plasma surfacing has an extensive application due to its high efficiency,lower cost and good quality. Recently, plasma surfacing has been employed in fabricating WC reinforced iron based composite coating and gotten good research achievement. But the price of WC is very high and this limits the coating’s application. In this paper, attempts have been taken that SiC with lower price was used to replace WC to strengthen iron based coating and the surface combination of SiC with iron based matrix was studied.
In order to improve SiC stability, PIRAC pre–coat process was conducted prior to fabrication to set up a protective layer on its outer part. By reaction with metallic powders (Cr, Ti) in a condition of high temperature and vacuum, reaction layers consisting of carbides (Cr7C3, Cr23C6/TiC, Ti8C5) and silides (Cr3Si/Ti5Si3)were created on the surface. The carbides and silides in the layers distribute regularly.
Coatings which were bonded closely to substrate and had advantages of plane interface, good processing and macro crack–free were obtained by plasma surfacing process. Microstructure of iron-based coating is consisted of α-(Fe, Cr) primary phase and a few eutectics of α+carbides. Growth morphology shows planar grain at interface and dendrite above. In plasma surfaced coating with synchronous feeding model of iron based alloy powder and Cr-SiC particles, no particle was retained.This shows coating SiC by Cr powers is no use to protect the particle at this circumstance. Due to Cr-SiC decomposition, the amount of eutectic within microstructure is increased. Additionally, grain growth morphology is influenced.
When a content of Cr-SiC addition is 2.5%, dendrite is formed apparently. For the content increasing to 5%, the microstructure type changes from hypoeutectic to hypereutectic. However, with back feeding for Cr-SiC particles, particles were not decomposed completely and retained within microstructure. It is shown that SiC particles within coating surface are more and keep integral. Due to large difference in density between Cr-SiC and iron alloy, the particle only reaches superficial zone of the coating and the amount there is decreased greatly. Due to Cr-SiC decomposition which influenced pool regions to varying degrees, the matrix microstructure is complicated and heterogeneous. It varies from upper part to bottom of the coating and shows similarity in the microstructure of coatings in the manner of synchronous feeding with different contents of Cr-SiC addition. It
consists of cellular grain for bottom part, dendrite for middle part and hypereutectic for upper part. With synchronous feeding model for mixture of iron alloy powder and Ti-SiC particles, SiC was not destroyed and retained within coating structure. It indicated that SiC particles were in good protection during processing due to Ti coat. Since carbides in coated layer around SiC particles were dissolved inevitablely, matrix microstructure is more complicated and contains cellular and dendrite grains. Interface layer between SiC and matrix is thick, which is the retained coated layer dissolving partly during plasma surfacing. But the layer contains Fe and Cr elements besides Ti. It is shown that Fe and Cr diffused into the coated layer during processing.
Hardness for upper part of coating with Cr-SiC back feeding is higher than that of iron alloy coating and the maximum value reaches 930HV0.2. Below, it is consistent for the two coatings. Under a load of 50Kg, wear resistance of coating with Cr-SiC back feeding is 1.4 times than that of iron alloy coating and the wear mechanism is slight abrasive wear. Under load of 100Kg, wear resistance of coating with Cr-SiC back feeding is 1.5 times than that of iron alloy coating and the wear mechanism is severe adhsive wear. Owing to lower content of SiC particles,properties of the coating with Cr-SiC back feeding are not strengthened greatly. The improvement in wear resistance results mainly from solution and dispersion strengthening due to SiC decomposition.
Keywords: coated SiC; iron alloy; composite coating; microstructure;properties
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