EFFECT OF AL CONTENT ON MICROSTRUCTURE AND PROPERTIES OF AN INTERMETALLIC Ni-Ti (Al) COMPOUND/Ni GRADED COATING DEPOSITED ON COPPER SUBSTRATE

摘要

Copper and its alloys with high electrical and thermal conductivity are a group of widely used engineering materials in numerous applications. In order to improve the tribological properties of copper substrate, an electroplating nickel layer was firstly deposited on copper substrate, subsequently these electroplated specimens were treated by slurry pack cementation process at 900°C for 12 h using a slurry mixture composed of TiO2 as titanizing source, pure Al powder as aluminzing source and also a reducer for titanizing, an activator of NH4C1 and albumen (egg white) as cohesive agent. The effect of Al content on the microstructure and the properties of the coating has been studied. The results showed that an intermetallic Ni-Ti (Al) compound/Ni graded layer was formed on copper substrate after slurry pack cementation process. With the rise of Al content in slurry mixture, the microhardness of the graded coating increased and the friction coefficient decreased from 0.35 to 0.18, at the same time, the slurry pack process gradually transited from the titanizing process to an aluminizing one. Correspondingly the main phases of the coating were changed from Ni-Ti intermetallic compounds into Ni-Al ones.