Key Engineering Materials Vol. 384

Abstract: In many cases degradation of a material initiates at its surface, including wear, corrosion, fretting, etc. Such deterioration / failure modes are hence surface properties sensitive. This study is one discrete effort towards the optimization of the surface microstructure for specific properties by understanding the fundamentally unknown ‘corrosion – grain size relationship’ for magnesium. There is a special need to understand this relationship as we outline in some detail within this study. Results showed that there was a significant variation in corrosion resistance with grain size, which is a key finding; however these trends were also strongly dependent upon the specific thermomechanical processing used to prepare the specimens.

Abstract: Magnesium alloys are attractive alloys in industrial applications , where light weight and good strength structures are required. Magnesium alloys are versatile and include both cast and wrought alloys. However, they have a drawback as they are prone to corrosion especially in harsh environments. For this reason, surface treatment of magnesium alloys is of prime importance for improving their corrosion characteristics. In the present paper, a survey of the different methods for surface treatment of magnesium alloys is presented and discussed. This is followed by a review of the recent work on electroless Ni plating technique with zinc pre-treatment applied on several magnesium alloys and the effect of pretreatment and post heat treatment on the coat characteristics. The surface morphology, surface roughness, thickness of the layer, EDX analysis, adhesion, hardness and corrosion resistance are covered in this review.

Abstract: This article describes the flexibility and the potentials of the most important finish for aluminium-based materials. After a suggestion for the classification of anodizing processes, the historical development of the electrolytic anodic oxidation (EAO) and the plasma-electrolytic anodic oxidation (PAO) are presented. In the following section the focus is given on selected topics concerning the process parameters, the microstructure including the layer composition and properties. The variability of the anodizing process parameters and the used materials provide this variety of application. Furthermore, both differences and similarities of the EAO and PAO processes are shown. The conclusion of this review emphasizes that there is still some need for further research especially in the interaction of the process, the formed microstructure and the resulted properties determining the final application. Hence new possibilities for this surface treatment will be open.

Abstract: Electroplated nickel coatings provide ductility, excellent corrosion resistance and good wear resistance, which qualifies them to meet complex demands of engineering, microtechnology and microelectronics. The co-deposition of particles is a promising alternative to deposit layers with adequate microstructure and properties avoiding the rise of residual stress. The incorporation of the sufficient quantity of particles, monodisperse distribution and downsizing to nanometre scale affect the amount of strengthening by dispersion hardening. To avoid agglomeration in the electroplating bath as well as in the layer is a challenge which has been met by simple Watts nickel electrolyte with a minimum of organic additives and adequate bath agitation comprising sonication, i.e. the exposure of the bath to high-frequency sound waves. Well-dispersed hard particles (titanium oxide and silicon carbide) were incorporated in nickel films. The focus was set on the correlation between the gained microstructure of the composites with particles from micron to nanometre scale and the electrochemical and mechanical properties. Corrosion was quantified from polarisation curves and volumetric erosion measurements. Wear resistance was evaluated by scratch energy density studies, oscillating sliding wear testing and cavitation wear testing and compared to indentation hardness results. Sonication and particle downsizing result in matrix grain refinement and dispersion hardening. Incorporation of different particles with respect to different material and size proved to meet different demands. Submicron TiO2 is best for high corrosion resistance, sonicated nickel without particle incorporation is best for high abrasion resistance, nano TiO2 is best for oscillating sliding wear resistance and submicron SiC is best for cavitation wear resistance.

Abstract: Poly(tetrafluoroethylene)(PTFE) thin films were coated onto metal substrates by a spin coat apparatus, vacuum evaporator and RF sputtering, and their adhesion and friction properties evaluated. PTFE thin film coated onto nickel-titanium (Ni-Ti) substrate by spin coating showed a low friction coefficient, however pull strength between the thin film and Ni-Ti substrate was low. In order to increase the pull strength, PTFE and poly(vinyl alcohol) (PVA) composite thin films were introduced between the PTFE thin film and Ni-Ti substrate by spin coating. PTFE thin film was also coated onto SUS302 substrate by a vacuum evaporator. This PTFE thin film showed poor adhesion to the SUS302 substrate. The adhesion was enhanced by heating of the substrate during the evaporation. In addition, a PTFE and ethylene vinyl alcohol (EVOH) composite thin film showed higher adhesion strength than that of the PTFE thin film. Poly(fluorocarbon) thin films were prepared by a conventional RF sputtering with PTFE target. These thin films showed a higher friction coefficient than that of the pristine PTFE. Molecular structures of the poly(fluorocarbon) thin films prepared by RF sputtering were different from the pristine PTFE. This difference may have influenced the friction coefficient. The pull strength of metal thin films such as gold, copper, nickel and aluminum deposited on the sputtered PTFE thin films by vacuum evaporation was measured. The nickel thin film adhered to the PTFE thin film most strongly of all the thin films.

Abstract: Surface mechanical attrition treatment, an approach to fabricate nanostructured surface layer on bulk metallic materials has been extensively investigated in the past few years with respect to grain refinement mechanism, friction and wear behavior and the subsequent chemical treatments. The present paper briefly overviews the friction and wear behaviors of the surface nanocrystalline layers generated by SMAT on Cu, steels and Mg alloy with emphasis on reciprocating sliding wear behaviors. The potential applications of the present approach are also prospected.