This paper overviews the relatively new surface engineering discipline of plasma electrolysis, the main derivative of this being plasma electrolytic deposition (PED), which includes techniques such as plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES) processes such as plasma electrolytic nitriding/carburizing (PEN/PEC). In PED technology, spark or arc plasma micro-discharges in an aqueous solution are utilised to ionise gaseous media from the solution such that complex compounds are synthesised on the metal surface through the plasma chemical interactions. The physical and chemical fundamentals of plasma electrolysis are discussed here. The equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined. AC-pulse PEO treatment of aluminium in a suitable passivating electrolyte allows the formation of relatively thick (up to 500 μm) and hard (up to 23 GPa) surface layers with excellent adhesion to the substrate. A 10–20 μm thick surface compound layer (1200HV) and 200–300 μm inner diffusion layer with very good mechanical and corrosion-resistant properties can also be formed on steel substrates in only 3–5 min by use of the PEN/PEC saturation techniques. Details are given of the basic operational characteristics of the various techniques, and the physical, mechanical and tribological characteristics of coatings produced by plasma electrolytic treatments are presented.

Plasma electrolysis for surface engineering

Progress in preparation, processing and applications of polyaniline

Atomic-scale control and manipulation of the microstructure of polycrystalline thin films during kinetically limited low-temperature deposition, crucial for a broad range of industrial applications, has been a leading goal of materials science during the past decades. Here, we review the present understanding of film growth processes—nucleation, coalescence, competitive grain growth, and recrystallization—and their role in microstructural evolution as a function of deposition variables including temperature, the presence of reactive species, and the use of low-energy ion irradiation during growth.

Microstructural evolution during film growth

The gas-phase reaction between MoO3-x and H(2)S in a reducing atmosphere at elevated temperatures (800 degrees to 950 degrees C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS(2). A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions.

http://science.sciencemag.org/content/sci/267/5195/222.full.pdf

High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes

Sol–gel coatings on metals for corrosion protection

Thin films of MoS2 have been grown on stainless steel substrates by pulsed laser evaporation. Analysis by X-ray photoelectron spectroscopy indicates that films grown at substrate temperatures up to 300 °C have the same stoichiometry as bulk MoS2; films grown at 450 °C are sulfur rich. The laser-deposited films have a granular structure and exhibit none of the dendritic structures typically observed in sputter-deposited films. Analysis of the films by X-ray diffraction indicates some preferred orientation. The coefficients of friction were measured in laboratory air and ranged from 0.09 to 0.25; the majority of values were between 0.16 and 0.20. These friction coefficients are in the appropriate range for a solid lubricant and indicate that pulsed laser evaporation is clearly a feasible technique for growing lubricious MoS2 films.

Deposition and properties of MoS2 thin films grown by pulsed laser evaporation

The tribological properties of diamond-like carbon (DLC) coatings produced by pulsed laser deposition (PLD) are investigated. Films are grown onto steel substrates to 0.5 μm using a 248 nm laser to ablate graphite and polycarbonate targets in high vacuum. Chemical bonding is studied with Raman, XPS and EELS techniques; mechanical and tribological properties are evaluated using microindentation and ball-on-disk friction tests. Coatings grown from graphite targets are amorphous DLC (a-C), while those grown from polycarbonate targets are amorphous hydrogenated carbon (a-C:H). The hardness of the a-C coatings is 55–70 GPa and the hardness of the a-C:H coatings is 12–20 GPa depending on the substrate bias. Friction coefficients of the coatings against steel and sapphire balls are determined in several environments: in air as a function of relative humidity, in dry nitrogen, and in 10 Pa vacuum. For a-C coatings, the friction coefficients are typically below 0.1 and are as low as 0.03 in dry nitrogen. In wear tests, a critical contact pressure of 1.4 GPa led to catastrophic adhesive failure of a-C coatings, whereas failure of a-C:H coatings is by wear-through after 5 x 10 3 cycles. Extremely low wear rates of 10 −9 mm 3 N −1 m −1 are found for a-C coatings at the contact pressure of 0.8 GPa.

Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

Investigation of corrosion protection performance of sol–gel coatings on AA2024-T3

The prevention of pitting corrosion in aerospace aluminum alloys by the application of protective sol–gel coatings requires a thorough understanding of pit formation kinetics and morphology developments in such surface coating systems. This study reports results of chemical and electrochemical methods of pitting corrosion tests for bare and sol–gel coated Al 2024-T3 alloy. Specific attention is focused on the characterization of pitting in samples coated with vinyl-silicate and epoxy-silicate sol–gel coatings. Specimens were exposed to a variety of chemically aggressive environments, based on 3–5% NaCl solutions with addition of HCl and H2SO4, including a standard CASS solution. The exposure of bare samples to these environments produced extensive surface corrosion, but pits were not observed for sol–gel coated samples. Anodic polarization tests with potentials above that required for pitting in bare samples were used to initiate pitting corrosion in sol–gel coated samples. A corrosion current monitoring test provided a method of controlling the pit formation process, which provides well-defined pits in terms of spatial density and geometry. A two-stage kinetic phase in pit development was observed and correlated with pit morphological developments in sol–gel coatings. An initial low current stage was associated with pit penetration through the coating to the surface and the secondary high current stage was associated with an active growth stage which grew in sub-coating surface interface regions. Results of this research provide a basis for designing and improving corrosion protection systems based on the application of sol–gel coatings.

M.S. Donley

Papers

Deposition and properties of MoS2 thin films grown by pulsed laser evaporation

Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

Investigation of corrosion protection performance of sol–gel coatings on AA2024-T3

Characterization of pitting corrosion in bare and sol–gel coated aluminum 2024-T3 alloy

Deposition of stoichiometric MoS2 thin films by pulsed laser evaporation