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Biomedical coatings on magnesium alloys - a review.

Tribology of electroless nickel coatings – A review

The introduction of plasma electrolytic oxidation (PEO) as a surface finishing technique has enabled a range of hard, dense oxide coatings to be produced on aluminium, magnesium, titanium and other lightweight alloy substrates. As with all surface coating technologies, successful development of PEO coatings requires adequate attention to substrate pretreatment together with careful control of electrolyte conditions and process variables. The principles and applications of the PEO coating process are considered, including the fundamentals of oxide deposition, the technology involved and the typical characteristics of the coatings. Industrial applications are considered together with their coating requirements. Plasma electrolytic oxidation coating is a specialised but well developed process. Suitable control of electrolyte and process conditions can realise a novel range of coatings having technologically attractive physical and chemical properties. The development of PEO technology over the last decade has provided coatings having controlled appearance, hardness, corrosion resistance and other tribological properties across an extending range of industrial sectors. Continuing developments are concisely reviewed and the PEO process is illustrated by the characterisation of anodised coatings on an AZ91 magnesium alloy surface.

Plasma electrolytic oxidation (PEO) for production of anodised coatings on lightweight metal (Al, Mg, Ti) alloys

Corrosion resistance and chemical stability of super-hydrophobic film deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition

Ohmic heating or Joule heating has immense potential for achieving rapid and uniform heating in foods, providing microbiologically safe and high quality foods. This review discusses the technology behind ohmic heating, the current applications and thermal modeling of the process. The success of ohmic heating depends on the rate of heat generation in the system, the electrical conductivity of the food, electrical field strength, residence time and the method by which the food flows through the system. Ohmic heating is appropriate for processing of particulate and protein rich foods. A vast amount of work is still necessary to understand food properties in order to refine system design and maximize performance of this technology in the field of packaged foods and space food product development. Various economic studies will also play an important role in understanding the overall cost and viability of commercial application of this technology in food processing. Some of the demerits of the technology are also discussed.

Technology, applications and modelling of ohmic heating: a review

Electroless Ni-plating on Mg alloy AZ91 has been studied to understand the effect of the substrate microstructure and roughness on the deposition rate, nucleation, coating microstructure, and mechanical property of the coatings. Experimental results indicate that the growth of Ni deposit in the early stage was influenced by the substrate microstructure and roughness. The electroless Ni-plating on the abrasive blasted AZ91 (rough) substrate showed a higher deposition rate than that on the finely polished one, indicating that the mechanical roughening enhances the nucleation and coalescence of Ni crystallites. Scratching tests showed that higher coating adhesion is achieved on the roughened AZ91 substrate. Wear tests, however, showed that the Ni plating on the rough substrate has a higher friction coefficient than that on the polished surface. The hardness and adhesion property of Ni coatings before and after heat treatment were also characterised.

Electroless nickel plating on AZ91 Mg alloy substrate

Magnesium (Mg) and its alloys are being used as structural components in industry because of their high strength-to-weight ratio and relatively high stiffness. A shortcoming of Mg based alloys is their high electrochemical activity and poor corrosion resistance. Therefore, coatings or surface treatment are needed for protection purpose. This paper reports our work on electroless plating of Ni on Mg alloys. Pure Mg, AZ31 and AZ91 Mg alloys were used as the substrates to investigate the deposition rate, nucleation process, chemical composition and microstructure of the electroless Ni coatings. It was found that the deposition processes are strongly affected by the alloy substrates. Ni plating on AZ31 alloy has higher deposition rates and lower surface roughness than that of other substrates. SEM investigations showed that nodular morphology of Ni coatings formed on the substrates. Chemical pre-treatment before plating increases the substrate surface roughness significantly. The plating process was strongly influenced by the substrate microstructure at the early stage. Deposition was initiated on the phase and grain boundaries due to the inter-grain galvanic coupling. Fine substrate microstructure such as that of AZ31 alloy provided a large amount of nucleation sites, benefiting the nucleation and growth processes of the electroless nickel plating. The adhesion strengths of the coatings on AZ31 and AZ91 alloys are higher than that on pure Mg. The critical load (a measure of adhesion strength) of AZ31 reached 13.1 N.

The effect of substrate on the electroless nickel plating of Mg and Mg alloys

A novel process of electroless Ni-P plating with plasma electrolytic oxidation pretreatment

Magnesium (Mg) and its alloys are being used as structural components in industry because of their high strength-to-weight ratio and relatively high stiffness. A shortcoming of Mg based alloys is their poor corrosion and wear resistance. Therefore, coatings or surface treatment are needed for protection purpose. This paper reports our work on electroless plating of Ni-P on Mg alloys. Pure Mg, AZ31 and AZ91 Mg alloys were used as the substrates to investigate friction and adhesion properties of the electroless Ni-P coatings. The maximum friction coefficient (~0.3) was found on the electroless nickel coating of pure Mg substrate. The adhesion strengths of the coatings on AZ31 and AZ91 Mg alloys are higher than that on pure Mg. The critical load (a measure of adhesion strength) of AZ31 reached 13.1 N.

SCRATCH ADHESION EVALUATION OF ELECTROLESS NICKEL PLATING ON Mg AND Mg ALLOYS

Different types of bulk and coating materials, including Ag, TiN, ZrN, Ni(P) and Ni(P)-PTFE, have been used as the electrodes for combined ohmic and plate-cooking of solid food (meat patties). The plates of the grill are electrically heated, but are also connected to a power supply to act as two electrodes, which pass an electrical current through the meat patty placed between them. It was found that complex corrosion phenomena of different levels took place on most of the metallic and coated electrodes because of the strong electrochemical and Faradaic reactions. Scanning electron microscopy (SEM) and EDX were used to analyze the corrosion mechanism. On the basis of the experimental results, a physical model is introduced to explain the corrosion mechanism under the ohmic cooking conditions. Suggestions for electrodes selection and proper operating conditions are also recommended. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Zhenmin Liu

Papers

Electroless nickel plating on AZ91 Mg alloy substrate

The effect of substrate on the electroless nickel plating of Mg and Mg alloys

A novel process of electroless Ni-P plating with plasma electrolytic oxidation pretreatment

SCRATCH ADHESION EVALUATION OF ELECTROLESS NICKEL PLATING ON Mg AND Mg ALLOYS

Corrosion mechanism of electrodes in ohmic cooking