Fabrication and characterization of cold gas-dynamic sprayed coatings

Abstract

This thesis discusses the fabrication of several kinds of coatings utilizing an emerging materials deposition process, Cold Gas-Dynamic Spray technology, or simply Cold Spray; and the characterization of these coatings to reflect the scientific background of this technology. The cold spray system used herein was imported into Hong Kong in 2004 by Hong Kong Productivity Council (HKPC) for local industries’ increasing need in repairing parts and manufacturing hardening moulds. Calibration work is done according to the dynamic principals of two-phase (particle-gas combination) fluid flow, so that the impact velocity and temperature of the feedstock particles, and some other factors facilitating deposition, can be controlled by adjusting the stagnation temperature and pressure of the carrier gas and the type of gas used. The cold spray system successfully deposited many kinds of coatings using different metal powders, and the study were focused on four kinds of pure metallic coatings, which are Aluminium, Titanium, Stainless steel, and Copper. The substrates chosen herein are Aluminium (6061) and stainless steel (316L) sheets, which are very common stuff in the manufacturing industry. Characterization of the cold sprayed coatings includes investigating microstructural characteristics using an Optical Microscope (OM), Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD); mechanical properties using a Microhardness tester and a Nanoindentation tester for hardness and by scratch testing for adhesion strength; and some affiliated properties, such as porosity and oxygen concentration. Characteristics of coating deposition are concluded from the investigations mentioned above. After comparing the results of different metallic coatings in detail, it is found that the characteristics of the coating depend greatly on the impact velocity of the feedstock particles and the hardness of both feedstock and the substrate materials. The most important part of the characterization is to evaluate the bonding mechanism of cold sprayed coating, both the coating-substrate at the interface and particle-particle bonding inside the coating. This study stands at the point that when impacting the substrate or previously deposited layers, the high velocity particle disrupts the oxide film of its own and the target, and both of them undergo extensive plastic deformation so as to make conform contact between each other, a metallic bond or metallurgical bond forms accordingly. Microstructural analysis by Transmission Electron Microscopy (TEM) gives preliminary proof of this. To further optimize the cold spray technology and exert its superior specialties of processing the feedstock of thermal-sensitive materials, this study demonstrates the feasibility of cold spraying particles of amorphous materials. One type of Magnesium-based metallic glass particles have been cold sprayed successfully onto an Aluminium (6061) substrate as the coating, and the properties of the metallic glass were maintained. It is expected that the present study can contribute to the development of cold spray technology which is still a long road both for academic researchers to uncover the fundamentals of the deposition process and for industrial engineers to control the different parameters related to its versatile application.

Date of Award	16 Jul 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ki Leuk Joseph LAI (Supervisor)