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Title: Laser surface modification of HVOF coatings for improvement of corrosion and wear performance
Author: Rakhes, Mohsen Mohamed
ISNI:       0000 0004 2741 0093
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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Metal Matrix Composite (MMC) coatings, comprised of a hard ceramic phase embedded in a metallic matrix, are increasingly being applied for many industrial applications to provide cost effective protection against wear and corrosion. Such coatings are commonly produced by thermal spray. Although the most advanced thermal spray techniques, such as high-velocity oxy-fuel (HVOF), produce MMC coatings with total porosity levels lower than 1%, due to the nature of thermal spray MMC coatings, corrosion still takes place. The corrosion processes are dominated by the complex microgalvanic and interfacial mechanisms, as well as by porosity, due to the existence of various defects in HVOF MMC coatings. As a result, HVOF coatings do not ultimately meet the requirements in certain service conditions in operating environments. Therefore, there is a need to find a method of modification of coatings, with significantly reduced microstructural defects and improved cohesive and adhesive strength so that the service life of the coated components can be increased. This work aims to investigate the effects of laser surface treatment on the corrosion and wear performance for Tribaloy 800 (T800), and T800-based WC HVOF-sprayed MMC coatings onto 316L stainless steel substrate. Laser surface treatments have been carried out using a 1.5 kW high power diode laser. Laser operating windows for various coatings have been established for the relationships between the laser operating conditions and melt pool dimensions, in the consideration of formation of cracks and porosity within laser-treated surface layers. Microstructural analysis of the powders, and various coatings before and after laser treatments has been conducted by means of optical and SEM (with EDX) microscopy, electron probe micro-analysis (EPMA), white-light interferometery, and X-ray diffraction, to characterise morphology, chemical composition and phase. Corrosion performance of various coating was evaluated using immersion testing in 3 M H2SO4 at pH ~ 1.27 at room temperature for different periods of time (including 24, 48, 72, 96 and 168 hours), followed by Inductivity Coupled Plasma-optical emission spectrometer (ICP-OES) technique, potentiodynamic polarisation in 0.5 M H2SO4, and electrochemical impedance spectroscopy studies in 0.5 M H2SO4 solution after 1, 3, 6, 12, 24, and 48 hours. Inaddition, dry sliding wear behaviour measured by pin-on-disk and microhardness test of various coatings before and after laser treatment were evaluated.The results indicated that it was possible to achieve full control of melt depth and the degree of melting, particularly full or partial melting of WC particles by proper selection of the laser processing parameters while preventing dilution. Significant improvement of corrosion and wear resistance has been achieved after laser treatment as a result of the elimination of discrete splat-structure, removal of microcrevices and porosity, as well as the reduction of microgalvanic driving force between the WC and the metal matrix by formation of new phases at the interfaces. The degree of melting of WC particles controls the corrosion properties of the laser-treated HVOF coatings. Moreover, the results also suggested that partial melting of WC had positive effect on wear resistance of the coatings.
Supervisor: Liu, Zhu Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Laser ; HVOF ; Corrosion ; Wear