AbstractThe purpose of the present PhD research project was to evaluate the feasibility of the electrodeposition of novel thin Ni composite coatings with lubricant particles from an additive-free Watts bath and their application as diffusion barrier layers in journal bearings for medium-speed diesel engines. Overall, the main objective was to develop thin Ni composite coatings with the following characteristics:
•Improved tribological performance.
•Good adhesion properties.
•Good ‘anti-diffusion’ performance.
Ultrasound was used in the preparation of pure Ni coatings and Ni composite coatings with lubricant particles under different conditions in order to understand how ultrasonic cavitation influences electrodeposition and characteristics of said Ni-based coatings. Two main studies were conducted related to this:
1.Study of the effect of ultrasonic power on the electrodeposition of pure Ni coatings to understand the influence of cavitation phenomena near the surface of the cathode on the properties of Ni deposits
2.Study of the effect of ultrasound on the production of Ni composite coatings to understand the influence of ultrasound in the dispersion of particles, the electrodeposition of Ni composite coatings and the properties of said Ni composite coatings.
The influence of ultrasound on the dispersion of particles in the Watts bath was evaluated by observing the visual appearance of the resulting dispersions and analysing the particle size distribution of diluted solutions by laser diffraction-based particle sizing methods. The effect of ultrasound on the characteristics of Ni deposits and Ni composite coatings electrodeposited from the additive-free Watts bath was evaluated by different material characterization techniques:
•X-Ray Diffraction (XRD) analysis was employed to observe the orientation of the Ni crystals that formed the coatings.
•Field Ion Beam – Scanning Electron Microscopy (FIB-SEM) was employed to analyse the surface morphology and microstructure of the coatings.
•Microhardness tests were performed to observe how the modification of the grain structure and the presence of particles may affect the hardness of the coatings.
•Glow Discharge – Optical Emission Spectroscopy (GD-OES) was also employed to estimate the particle content in the Ni composite coatings.
Pure Ni deposits and selected Ni/hBN and Ni/WS2 composite coatings electrodeposited under ultrasound that exhibited promising features (e.g. more fragmented/refined structure, reasonable incorporation of uniformly-distributed particles, etc.) were selected for further tribological analysis. Scratch tests performed on these coatings under lubricated (mixed-film/boundary lubrication) and non-lubricated (dry boundary lubrication) conditions showed that the Ni/WS2 composite coatings were the only newly developed Ni-based coating that exhibited enhanced tribological performance when compared with benchmark Ni coatings.
Finally, the performance of these novel Ni/WS2 composite coatings with enhanced tribological performance acting as diffusion barrier layers in real bearing overlay systems was studied:
•In terms of adhesion, similar loads before bonding failure were measured for both Ni/WS2 composite coating and the benchmark Ni deposit. No de-attachment was observed between the bronze lining and both diffusion barrier layers, and the bonding failure occurred mostly between the glue and the Sn-Cu overlay and within the Sn-Cu overlay.
•In terms of diffusion, an overall increase in diffusion phenomena (e.g. formation of intermetallic layer, reduction in thickness of the diffusion barrier layer, etc.) was observed for the Ni/WS2 composite coating when compared with the benchmark Ni deposit.
The overall increase in diffusion phenomena here reported for the novel Ni/WS2 composite coatings with enhanced tribological performance may imply the unsuitability of said coatings for their use as diffusion barrier layers in plain bearings. Nevertheless, the growth rate and nature of the intermetallic layer formed in the bearing overlay system with the Ni/WS2 diffusion barrier layer, along with the enhanced tribological performance of said composite coating, may open new possibilities in bearing overlay development where functional intermetallic phases are sought for different bearing applications.
|Date of Award||May 2015|
|Supervisor||Andrew Cobley (Supervisor), John Graves (Supervisor) & Larysa Paniwnyk (Supervisor)|
- composite coatings
- lubricant particles
- Watts bath