Nickel Coating on Hexagonal Boron Nitride Particles by Chemical Plating
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TRODUCTION
GAS turbine is broadly used in airplane and other applications. It consists of a rotor and stator. Perfect contact of these two parts with almost no gap in between is essential to ensure a good thrust. Usually, a sealing material is applied between the stator and rotor to form an abradable seal layer. The material should unite with the stator and, at the same time, should not damage the tips of impeller blades. The sealing material should also stand the working temperature and the friction of the rotor at high speed. Nickel and boron nitride or nickel alloy and boron nitride composite layers are members of the abradable seal layer family. Hexagonal boron nitride (BN) has graphite-like lattice structure. It is well abradable and self-lubricant. Nickel or nickel alloy may easily join with the nickel alloy substrate on stator surface. The combination of nickel and BN as composite material would be an attractive alternative of the abradable layer. The abradable layer is usually produced by the plasma or oxygen-acetylene flame thermal spray method. In these thermal spray processes, nickel-coated BN particles, as the raw material, are employed to obtain a composite abradable seal layer. Chemical plating is one of the direct methods to prepare nickel-coated BN particles. However, most of the deposit of the chemical plating produced by the existing techniques contain P or B element.[1,2,3] These elements have negative effects on the properties of the layer at high temperature.[3,4,5] FAN LI, Doctoral Candidate, is with the Department of Material Science and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] DENGJUN ZHANG, Professor is with the Institute of Process Engineering, Chinese Academy of Sciences, 100080 Beijing, P. R. China. Manuscript submitted December 23, 2005. Article published online April 5, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
Precipitation of boride or phosphide phase might be able to strengthen the mechanical properties of the materials at lower temperatures. On the other hand, the fast diffusion of P or B into the plated substrate and formation of intermetallic compounds would dramatically damage the mechanical properties and corrosion resistance of the material at about 973 K and above.[2,3] Electroless deposition of pure nickel has also been suggested.[3,6–8] However, the bath with potassium sodium tartrate, ethylene diamine tetraacetic acid, etc. as the complexing agent has been found to be unstable. Besides the high cost of the chemical bath, the waste solution would be harmful to the environment. The present study envisages the possibility of a new technique to produce nickel-coated BN particles using ammonia solution as the complexing agent, nickel sulfate as the main salt, and ammonia-ammonium sulfate as the buffer system. It aims at a theoretical consideration with respect to the thermodynamic constraints and the surface charge condition. This is followed by an experimental study of the production of nickel-coated BN particles. II.
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