Lingqian Wang

Effects of additives on microstructure and properties of electrodeposited nanocrystalline Ni–Co alloy coatings of high cobalt content

Abstract Saccharin and 2-butin-1,4-diol (BD) were investigated as electrolyte additives to electrodeposit high quality nickel-cobalt alloys (78±2 at-%Co) as a protective surface coating on steel for tribological applications. The additives facilitated controlled electrodeposition of nanocrystalline coatings. The properties of the coatings investigated included surface morphology, grain size, crystalline texture and hardness. Tribological performance against a steel counterpart was studied via a reciprocating ball-on-disc apparatus. The coating microstrain could be manipulated from tensile to compressive and texture could be modified from (100) for hexagonal close packed (hcp) structure to (0002)hcp/(111) for face centred cubic (fcc) structure. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via analysis of grain size and texture. The coating from the bath with an optimised additive content had a high hardness (500 HV) due to its reduced grain size (11±1 nm) and improved tribological properties due to a high proportion of hcp structure.

Corrosion behaviour of plasma electrolytic oxidation coated AZ91 Mg alloy: influence of laser surface melting pretreatment

Plasma electrolytic oxidation (PEO) was performed on a laser surface melting (LSM) modified AZ91 Mg alloy. The effect of LSM pre-treatment on the long-term corrosion resistance of the PEO coated AZ91 alloy was evaluated. Results showed that the LSM pretreatment had a negligible effect on the phase composition and microstructure of the PEO coatings. However, after LSM pretreatment, the long-term corrosion resistance of the PEO coated AZ91 alloy revealed a large enhancement. This was mainly ascribed to the improved corrosion resistance of the substrate resulting from the change of microstructure characteristics induced by the LSM treatment. This provided an alternative approach to improve the long-term corrosion resistance of the PEO coated Mg alloy by appropriate surface modified pretreatment of substrates.

Microstructure and corrosion behaviour of laser surface melting treated WE43 magnesium alloy

An attempt has been made to improve the corrosion behavior of WE43 magnesium alloy by laser surface melting (LSM) using a 10 kW continuous-wave CO2 laser. The microstructure evolution of WE43 alloy after LSM treatment was analyzed by using scanning electron microscopy, energy-dispersive spectroscopy and metallographic microscope. The corrosion resistance of specimens was assessed by electrochemical and immersion tests. Results showed that the LSM treated WE43 alloy presented a uniform microstructure with refined grains, enriched alloying elements and redistributed intermetallic compounds. The LSM treatment effectively improved corrosion resistance of WE43 alloy, which was mainly associated with enrichment of alloying elements in α-Mg matrix and uniform distributions of the refined Mg14Nd2Y phase.