The University of Southampton
University of Southampton Institutional Repository

Electrodeposited nanocrystalline Ni-Co and Co-Ni-P coatings for hard chromium replacement

Electrodeposited nanocrystalline Ni-Co and Co-Ni-P coatings for hard chromium replacement
Electrodeposited nanocrystalline Ni-Co and Co-Ni-P coatings for hard chromium replacement
This thesis describes the preparation and characterisation of environmentally friendly and low-cost nanocrystalline Ni-Co coatings and Co-Ni-P coatings to replace hard chromium coatings for anti-wear and anti-corrosion applications. nanocrystalline Ni–Co coatings with different cobalt contents were electrodeposited.The investigation on the role of tribofilms and wear debris in the tribological behavior sliding against AISI-52100 stainless steel under unlubricated conditions shows that the tribofilms containing iron from the counterparts were formed on the worn surface of the coatings (less than 60 at.% Co), which exhibited high coefficients of friction and wear rates. In contrast, no tribofilm or iron transfer from the pin was found on the Co-rich coatings (more than 70 at.% Co), which exhibited a dramatic friction reduction of 50 % and improved wear resistance. The wear debris contains a mixture of face-centred cubic (fcc) metallic phase and fcc oxidised phase, irrespective of the coating composition. The oxidised debris cannot form an efficient lubricative film to promote separation of the sliding surfaces. Ni-Co coatings exhibited the active-passive polarisation behaviour in 3.5 % NaCl solution. The corrosion resistance of Ni-Co coatings needs to be further improved in order to replace hard chromium for anti-corrosion applications. A new approach to fabricate single-layer Ni-Co coatings with high cobalt content onto mild steel substrates has been developed by optimising of additives (saccharin and 2- butin-1,4-diol (BD)). The present method is more feasible in industry with a competitive cost compared to other techniques, e.g. developing graded coatings and applying pulse current waveforms. The effect of saccharin and BD on the properties of the coatings were investigated, including surface morphology, grain size, crystalline texture, hardness and tribological performance against a steel counterpart. The coating microstrain could be manipulated from tensile to compressive and the fibre texture could be modified from the (10¯10) for hexagonal close-packed (hcp) structure to (0002)hcp / (111)fcc. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via grain size and texture analyses. The coating from the bath with an optimised additive content had high hardness (500 HV) due to its reduced grain size (11±2 nm) and improved tribological properties due to the high proportion of hcp structure. The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Ni-Co coatings with different porosities were obtained by controlling the concentration of nickel sulphate and nickel chloride within electroplating baths. The coatings with the grain size in the range of 11-23 nm had varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect is taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Ni-Co coatings with 77±2 at% cobalt. The present model has been applied to other porous nanocrystalline coatings, and the Hall-Petch relationship is maintained. In order to further improve the microhardness, wear resistance and corrosion resistance of Ni-Co coatings to match the properties of hard chromium, a new Co-Ni-P coating has been developed by combining the precipitation hardening of Ni-P alloys with the lubricity of cobalt-rich Ni-Co coatings. The evolution of composition and microstructure, hardness, thermal stability and tribological properties have been investigated. The local pH near the cathode played an important role in the change of the microstructure from nanocrystalline to amorphous along the growth direction as the phosphorus content increased from 7 at.% to 26 at.%. The highest microhardness (980 HV) and the lowest wear rate (an order of magnitude lower than that of hard chrome coatings under the same dry sliding conditions) were achieved by annealing the coatings at 400 °C facilitating precipitation hardening. Furthermore, the coefficients of friction of both the as-deposited Co-Ni-P coating and the heat-treated samples were approximately 0.3, only half of that of hard chrome coatings. The roll-like debris found on the worn surfaces of the coating annealed at 500 °C were oriented perpendicularly to the sliding direction. The Co-Ni-P coating annealed at 400 °C exhibited improved anti-corrosion properties, which can be attributed to the formation of a protective oxide layer.
Ma, Chao
dda2ade4-9c21-4689-882c-1dfa8c63284c
Ma, Chao
dda2ade4-9c21-4689-882c-1dfa8c63284c
Wang, Shuncai
8a390e2d-6552-4c7c-a88f-25bf9d6986a6

(2013) Electrodeposited nanocrystalline Ni-Co and Co-Ni-P coatings for hard chromium replacement. University of Southampton, Faculty of Engineering and the Environment, Doctoral Thesis, 204pp.

Record type: Thesis (Doctoral)

Abstract

This thesis describes the preparation and characterisation of environmentally friendly and low-cost nanocrystalline Ni-Co coatings and Co-Ni-P coatings to replace hard chromium coatings for anti-wear and anti-corrosion applications. nanocrystalline Ni–Co coatings with different cobalt contents were electrodeposited.The investigation on the role of tribofilms and wear debris in the tribological behavior sliding against AISI-52100 stainless steel under unlubricated conditions shows that the tribofilms containing iron from the counterparts were formed on the worn surface of the coatings (less than 60 at.% Co), which exhibited high coefficients of friction and wear rates. In contrast, no tribofilm or iron transfer from the pin was found on the Co-rich coatings (more than 70 at.% Co), which exhibited a dramatic friction reduction of 50 % and improved wear resistance. The wear debris contains a mixture of face-centred cubic (fcc) metallic phase and fcc oxidised phase, irrespective of the coating composition. The oxidised debris cannot form an efficient lubricative film to promote separation of the sliding surfaces. Ni-Co coatings exhibited the active-passive polarisation behaviour in 3.5 % NaCl solution. The corrosion resistance of Ni-Co coatings needs to be further improved in order to replace hard chromium for anti-corrosion applications. A new approach to fabricate single-layer Ni-Co coatings with high cobalt content onto mild steel substrates has been developed by optimising of additives (saccharin and 2- butin-1,4-diol (BD)). The present method is more feasible in industry with a competitive cost compared to other techniques, e.g. developing graded coatings and applying pulse current waveforms. The effect of saccharin and BD on the properties of the coatings were investigated, including surface morphology, grain size, crystalline texture, hardness and tribological performance against a steel counterpart. The coating microstrain could be manipulated from tensile to compressive and the fibre texture could be modified from the (10¯10) for hexagonal close-packed (hcp) structure to (0002)hcp / (111)fcc. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via grain size and texture analyses. The coating from the bath with an optimised additive content had high hardness (500 HV) due to its reduced grain size (11±2 nm) and improved tribological properties due to the high proportion of hcp structure. The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Ni-Co coatings with different porosities were obtained by controlling the concentration of nickel sulphate and nickel chloride within electroplating baths. The coatings with the grain size in the range of 11-23 nm had varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect is taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Ni-Co coatings with 77±2 at% cobalt. The present model has been applied to other porous nanocrystalline coatings, and the Hall-Petch relationship is maintained. In order to further improve the microhardness, wear resistance and corrosion resistance of Ni-Co coatings to match the properties of hard chromium, a new Co-Ni-P coating has been developed by combining the precipitation hardening of Ni-P alloys with the lubricity of cobalt-rich Ni-Co coatings. The evolution of composition and microstructure, hardness, thermal stability and tribological properties have been investigated. The local pH near the cathode played an important role in the change of the microstructure from nanocrystalline to amorphous along the growth direction as the phosphorus content increased from 7 at.% to 26 at.%. The highest microhardness (980 HV) and the lowest wear rate (an order of magnitude lower than that of hard chrome coatings under the same dry sliding conditions) were achieved by annealing the coatings at 400 °C facilitating precipitation hardening. Furthermore, the coefficients of friction of both the as-deposited Co-Ni-P coating and the heat-treated samples were approximately 0.3, only half of that of hard chrome coatings. The roll-like debris found on the worn surfaces of the coating annealed at 500 °C were oriented perpendicularly to the sliding direction. The Co-Ni-P coating annealed at 400 °C exhibited improved anti-corrosion properties, which can be attributed to the formation of a protective oxide layer.

PDF
Chao Ma-PhD thesis-July 2013.pdf - Other
Download (41MB)

More information

Published date: 1 June 2013
Organisations: University of Southampton, Faculty of Engineering and the Environment

Identifiers

Local EPrints ID: 355979
URI: http://eprints.soton.ac.uk/id/eprint/355979
PURE UUID: 6e3d712e-d83c-48cc-9c1f-ef960ddd8752

Catalogue record

Date deposited: 19 Nov 2013 14:45
Last modified: 18 Jul 2017 03:42

Export record

Contributors

Author: Chao Ma
Thesis advisor: Shuncai Wang

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×