MICROSTRUCTURE AND PROPERTIES OF BRAZED COMPOSITE COATINGS

Abstract

The aim of this project centres around the usefulness of coating 80wt%AMS 4777D-20wt%SiCp composites on martensitic stainless steel, AISI 410. The coating process is achieved through brazing. Four different coatings were brazed at Tmax of 1070, 1085, 1100 and 1130 °C;. these are called 1070 coating, 1085 coating, 1100 coating and 1130 coating. In all the coatings, the major intermetallic compounds: chromium boride, nickel boride, and nickel silicide solidify together with iron as component phases in binary or ternary eutectics. Other phases include graphite, nickel carbide, iron carbide, chromium carbide and silicon carbide. It is found that further dissolution of silicon carbide in the matrix occurs as the brazing temperature increases, yielding increasing silicon and carbon content in the melt during brazing. In all the coatings, a combination of abrasive, adhesion and delamination wear mechanisms are operative during unlubricated sliding against themselves. Wear tests using a cylinder-on-cylinder test rig reveal that 1070 coating is the most wear resistant whereas 1130 coating is the least wear resistant. SEM examination reveals silicon carbide particles protruding from the wear surfaces of 1070 coating but not 1130 coating. This seems to concur with the observation made earlier that higher brazing temperature causes further dissolution of silicon carbide. The role of silicon carbide is to reinforce the much softer and ductile nickel braze, AMS 4777D. Although the amount of graphite, a solid lubricant, increases due to higher dissolution of silicon carbide at higher brazing temperature, it did not contribute to an overall decrease in the wear resistance of coatings especially those brazed at higher temperatures. It would be incomplete if the present project ignores the effects of the braze heat treatment (that yields the best wear-resistant coating) on the substrate, AISI 410. In order to investigate these changes, designated corrosion and standard mechanical tests are carried out. Results show that brazing improves the corrosion resistance, hardness and strength of AISI 410. On the other hand, post-braze heat treatment decreases hardness and strength of AISI 410. Visual examinations have shown that post-braze heat treatment has not deteriorated the surface texture of the braze coating. Microstructure studies and XRD analysis of post-braze heat-treated coating have confirmed this observation and they display similarities to that of the 1070 coating. Although these studies show that post-braze heat treatment has not significantly altered the coating's microstructure, its coating registered a drop in wear performance compared to the 1070 coating. Together with the cost of producing such coating and the results gathered from earlier work, a comparison analysis is carried out involving all the specimens: as received, brazed and post-braze heat-treated. No particular specimen stands out among the three as each has its own merit. Only after taking the actual service conditions into consideration, the brazed specimen is found to be the optimum performer. The final study involves comparing the cost of producing silicon carbide and tungsten carbide-cobalt coatings to its corresponding wear rates. The analysis concluded that tungsten carbide-cobalt coating is still more cost effective than the silicon carbide coating produced presently.