The present work gives and overview of the deposition, microstructure and corrosion properties of magnesium based supersaturated coatings for the corrosion protection of standard magnesium alloy components. Such protective surface layers with high corrosion resistance were produced using physical vapour deposition techniques at different particle energies, such as magnetron sputtering, ion beam sputtering and cathodic arc deposition. For all these films, a homogeneous microcrystalline morphology with a highly oriented basal texture was observed. The solubility limit of all alloying elements studied was largely extended beyond the thermodynamic equilibrium. Thus highly supersaturated single phase coatings free of localised corrosion and/or microgalvanic corrosion effects between different phases were observed, showing low corrosion rates. Furthermore, the less noble corrosion potentials of some alloying systems compared to standard cast or wrought magnesium alloy substrates allow a cathodic protection of the substrate material by the coating.

The range of applications for magnesium alloys is still limited due to their relatively poor corrosion behavior. In recent years, various new magnesium alloys were developed, some of them with improved corrosion properties, thus opening new fields of application. However, the number of alloying elements for the use in conventional cast processes is limited due to their interaction with liquid magnesium, other alloying elements or large differences in the melting temperatures. The possibilities for grain refinement by post-processing are also restricted. PVD techniques can help to produce supersaturated precipitation free and microcrystalline magnesium layers.Using ion beam and magnetron sputtering, binary or ternary Mg-Al, Mg-Ti and Mg-Sn alloy systems as well as standard alloys (AM50, AZ91 and AE42) were deposited on silicon and on magnesium substrates. The effect of the microstructure on the corrosion properties was studied by comparing as cast material and PVD coatings using potentiodynamic polarization, linear polarization resistance, and electrochemical impedance techniques.

Different PVD processes – magnetron sputtering, ion beam sputtering and vacuum arc deposition – are used to produce novel corrosion resistant Mg alloys with the average energy per deposited atom increasing from 5–10 to 15–45eV. Correspondingly, a transition from a 3-dimensional columnar growth regime towards a layer-by-layer growth at increased energies was observed with the film texture changing from a weak basal texture towards a highly oriented film with the c-axis normal to the surface. Additionally, a higher energy helped stabilizing supersaturation, especially for Mg–Ti alloys. However, only a weak influence of the morphology on the corrosion rates was observed, which were mainly governed by the chemical composition of the films.

New approaches in alloy development which promise improvements include both amorphous materials and very fine-grained alloys. Due to very high cooling rates, thin film deposition is one possible route to obtain amorphous or fine-grained material. In the present contribution, the magnesium alloys AZ91, AM50, AE42 and chemically pure (cp) Mg were used as sputter targets for ion beam sputter deposition. Argon ions with an energy between 800 and 1200 eV were used for sputtering while Si(100) served as substrate material. Very dense and microcrystalline textured films were obtained. These films exhibited improved corrosion behaviour with higher polarization resistance and an extended passive region compared to the as-cast alloys. The corrosion resistance scaled with the Al content of the films, which was lower than in the used target alloys, most probably caused by a sputter angle distribution depending strongly on the specific atomic species.

Technical Mg alloys (AZ91, AM50, AE42 and pure Mg) were used as targets in ion beam sputter deposition of corrosion resistant thin films. Due to the large number of chemical elements and the widespread concentration range between 100ppm and 90wt.%, a combination of Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA) and secondary ion mass spectrometry (SIMS) was employed for a quantitative analysis of the resulting film composition. While a lateral and vertical homogeneous layer was observed, the concentration of elements present in precipitates (mainly Al, Si and Mn) in the base material decreased by more than 50% in the deposited film. As the concentration of the other alloying constituents distributed in the Mg matrix remained constant, different angular sputter distributions of the matrix and the precipitates can be inferred from these observations.

A special designed high-temperature vacuum chamber for in situ X-ray diffraction (XRD) measurements was used to study structural phase formation and transformation kinetics in molybdenum during oxygen ion implantation and post-annealing treatment. Oxygen ions with an energy of 1.5MeV were implanted in polycrystalline molybdenum up to a fluence of 3×1018/cm2 at different temperatures (160–700°C). Subsequently, implanted samples were annealed up to 700°C for in situ study during synthesis of buried oxide layers. Complementary, transmission electron microscopy (TEM) and sputter Auger electron spectroscopy (AES) were employed to obtain depth-dependent information concerning both the crystal structure and its elemental composition. The formation of different molybdenum oxides during oxygen implantation and post-implantation annealing process was observed by in situ X-ray analysis. The XRD spectra of samples implanted at 160°C show that MoO3 and/or Mo4O11 precipitates have been formed, whereas implantation in the temperature range 300–700°C preferably leads to the MoO2 phase formation.

High fluence nitrogen ion implantation into molybdenum is compared for low energies of 30keV and below and high energies of 1MeV at a total fluence between 0.5 and 2.7×1018at./cm2. For the low energy implantation, a transformation from cubic Mo2N towards tetragonal Mo2N is observed around 580°C with ex situ X-ray diffraction (XRD), whereas high energy implantation leads to the simultaneous formation of tetragonal Mo2N and hexagonal MoN in the same temperature range, as observed with in situ XRD. Additionally, ion beam analysis was employed to measure the local atomic concentrations, which can be used for determining the phase formation threshold and the diffusion constant.

Ion beam sputtering is used to produce magnesium alloy coatings of AM 50, AZ 91, and AE 42 alloys on silicon substrates. Due to the specific process conditions, a very fine microstructure was produced with very dense and microcrystalline textured films at a growth rate between 2 and 10 mu m/h. This microstructure was compared with the microstructure of the original commercial AM50, AZ91, and AE42 target materials. The aim was to study the effect of the microstructure on the corrosion properties. Therefore, potential polarization technique was used to study the corrosion behavior of the coatings and the original materials. The corrosion behavior was indeed affected and partially improved by this modified microstructure.

An in situ X-ray diffraction study was performed to characterize aspects of the kinetics of structural phase formations in transition metals during oxygen implantation. A specially designed high-temperature vacuum chamber for X-ray measurements was installed at the high-energy implantation beamline of the 3MV-Tandetron accelerator. Titanium and molybdenum sheets were implanted with oxygen ions with an energy of 1.5MeV up to a fluence of 1.6×1018O+-ions/cm2 without sample cooling. During implantation, the phase formation was continually investigated by in situ and real-time X-ray diffraction (XRD) with a position-sensitive proportional counter system. The phase formation was studied during annealing process by in situ XRD. The obtained results were compared with ex situ XRD measurements performed by different techniques at a standard diffractometer. The analysis of XRD patterns indicates the formation of a buried MoO2 layer in molybdenum. In the Ti samples no buried oxide is formed due to significant oxygen diffusion.