Development of innovative nanocomposite coatings for magnesium castings protection

TEKNIKER has set-up during a testing procedure to evaluate the friction procedure of the magnesium coatings by means of the use of a Nanotribometer. The utilization of this nanotribometer will allow giving values about the coating properties minimising the effect of the substrate. The testing procedure has been presented in 3 workshops and conferences. The write procedure for the calibration of cantilever should be further studied in the future.

Pre-treatment (cleaning, activation) and coating of magnesium by means of atmospheric plasma process to create protective oxidic surface layers. Know how in investigation of coating - substrate interface. Know how in investigation of corrosion mechanism, monitoring of corrosion and simulation of corrosion process Effect of substrate composition and topography on coating performance.

TEKNIKER has evaluated and compared the environmental lifecycle analysis of the magnesium coated (Keronite and PAPVD) in relation to aluminium to apply to pistons. The coated magnesium reduces the environmental impact of the aluminium pistons and Keronite coating give a bit lower environmental impact than the CrN coating. The environmental impact of the use is much higher than the environmental impact of the production process.

TARABUSI has designed a new piston in magnesium alloy, aimed at being assembled in a reciprocating compressor, to be applied in the braking system of commercial vehicles. The overall design made include cast and finished part, casting mould and machining tooling and devices. Piston prototypes made in magnesium alloy MRI 201s were produced with Keronite coating and with NCr PAPVD coating for a specific compressor used in air brake systems of commercial vehicles. These prototypes were tested in the compressor test bench of TARABUSI for the mentioned automotive application with promising results after more than 1000 hours of running test.

Thin films of SiO2 have been deposited on AZ91D (as cast, sand blast and machined) and AZ31 (rolled and machined) magnesium alloys substrates by PA-PVD process using SiO2 target at different experimental parameter. The deposited films have been tested for adhesion (measured by scratch test), hardness (measured by Nanohardness tester). The microstructure and elemental composition were evaluated. The corrosion measurements of the SiO2 films by PA-PVD indicated that the AZ91D machined samples have no pitting on the surface after 96h exposure to slat spray test compared to the as cast and sand blast samples.

The results refer to the development of a PECVD process for depositing SiOx coatings on Mg-alloys by applying frequencies higher than the conventional 13.56 MHz i.e 27.12, 40.68 and 54.24 MHz. This approach allows the deposition of SiOx coatings at higher growth rates, with better adhesive properties and with a rather high deposition and power usage efficiency. This is basically the result of the increase of the precursor¿ s vapour dissociation due to the higher plasma density and more resistive character of the discharges. Thickness non-uniformities that can be raised from the higher frequencies has been minimised using a special reactor configuration.

A process for depositing a compact SiOx thin film to coat metallic and polymeric surface for different application such as protection from corrosion of metals, protection from tarnish of silver containing surfaces, gas and liquid barrier, etc. has been obtained utilizing RF plasma fed with hexamethyldisiloxane, oxygen and Ar mixtures. Magnesium alloys surface are contaminated by carbon containing species and by Mg-oxides and hydroxides. These contaminations affect negatively the surface properties of the metal and in particular the adhesion of coatings of different nature. It has been demonstrated that this problem can be solved by means of a pre-treatment in an RF - H2 plasma at low power density and high pressure.

Thermal Spray process to deposit metallic, ceramic layers or multilayers metal/ceramic respectively on top of magnesium substrates to increase wear properties.

In order to characterize the chemical composition of the exhaust gas of a plasma process performed in organosilicon plasmas within the project it has been studied and optimized a sampling and analysis procedure, which allow the identification and quantification of unreacted monomer as well as of plasma by-products. Sampling is performed with a LN2 cold trap, while the analysis is performed by means of Gas Chromatography with mass spectrometric detection.

KERONITE is an environmentally friendly electrolytic oxidation coating for Magnesium. The treatment is an effective substitute to the traditional chromating process. KERONITE is a corrosion resistant, hard ceramic (400 - 600HV), which enables to use Mg in functional applications. Thickness of KERONITE ranges from 10 to 50 microns. The layer is completely uniform and well bonded to the Magnesium substrate. Coating of magnesium alloy by the KERONITE process involves creation of a plasma discharge around a component immersed in electrolyte that results in oxidation of the component surface as well as elemental co-deposition from the electrolyte solution. The low concentrated alkali electrolyte does not contain any toxic or aggressive elements and is no more hazardous than water in a domestic washing machine. The KERONITE layer is the complex oxide ceramic consisting mainly of spinel (MgAl2O4 or MgO-Al2O3). Such composition provides KERONITE with a combination of high hardness and wear resistance together with corrosion resistance and resistance to fretting. KERONITE on Magnesium Alloys is characterized by following properties: - High Corrosion resistance: Magnesium alloy AZ91D with KERONITE coating can survive 1000 hours in a salt spray environment without significant visual evidence of corrosion attack. - Galvanic corrosion: polymer sealed KERONITE reduced galvanic corrosion with stainless steel in saline solution from current density 10-2 A/cm2 for bare magnesium down to 10-11 A/cm{2}. - High hardness and wear-resistance: microardness of KERONITE on Magnesium ranges from 350 to 600HV (Rc 36 - 55) depending on the alloy. The KERONITE surface can be used as a matrix for composite coatings, when two coating systems were used in conjunction, in order to create superior properties for the combined coating. The external coating penetrates partly into the porous top layer of KERONITE and partly remains on the surface.

TEKNIKER has design a device to test the piston/gudgeon pin contact and the piston/cylinder liner contact, in order to compare the properties of the magnesium coating in comparison with the aluminium material currently used. The device has been fully tested. The testing procedure is available for further research project activities, and it has been detailed in a published paper.

Thin films deposited in RF plasmas from organosilicon precursors (HMDSO and TEOS, in particular), can be utilized to replace conventional primer before spray painting. Electrochemical Impedance Spectroscopy has shown that after 130 days of immersion in NaCl 3,5% the coating resistance was fixed at 106 M cm2. Any difference has been revealed respect to a substrate spray painted after the application of a conventional primer.

The results are a homogeneous and compact coating characterized by low porosity, which is able to protect Mg alloys from corrosion. The film, about 1000nm thick, is free from SiOH groups and with an organic fraction, which is below the detection limit of FTIR. The coating can be deposited both in conventional parallel plate low-density reactor as well as in high-density inductively coupled system. The protective capability has been tested by means of Electrochemical Impedance Spectroscopy. Coating resistance as high as 1,6 M? cm2 have been detected.

A process for depositing a compact SiOx thin film to coat metallic and polymeric surface for different application such as protection from corrosion of metals, protection from tarnish of silver containing surfaces, gas and liquid barrier, etc. has been obtained utilizing RF plasma fed with tetraethoxysilane, oxygen and Ar mixtures.

SiOx and multicomponent oxide coatings formed by a especially developed low temperature (less than 180 °C) sol-gel dip-coating technology has been developed based on the precursor: tetraethylorthosilicate and aluminium-sec-butoxide. These precursors have shown themselves to be promising precursors with good adhesion to the Mg alloy substrate. The corrosion measurements of the SiO2 films containing Al2O3, ZrO2 and CeO2 indicated no pitting on the surface of AZ91D and AZ31 (machined) after 96h exposure to salt spray test compared to the as cast and sand blast samples. Experiments were conducted on the development of multi-component oxides such as Al2O3, SiO2, ZrO2 as well as a highly corrosion resistant polymer layer on top of the metal oxides by sol-gel dip coating technique. The corrosion and wear resistant results of the multi-component Si-O ¿Al-O films were tested and have shown promising properties. The use of polymer layers was later abandoned due to strict requirements of a principal end-user of the project. This process has been up-scaled to an industrial process capable of coating medium to large areas.

Magnesium alloys, cast and especially wrought material, with a protective oxide coating. This coating is applied by means of a electrical discharge process in liquid electrolyte. Substrate quality is still very important for achievable corrosion properties. The coating can be additionally sealed for improved protection or integration of additional properties (e.g. stability against atomic oxygen, release properties, tribological properties). Type of sealing is e.g. impregnation and/or covering by sol-gel formulations. Substrate quality is still very important for corrosion properties.

A process for depositing a compact SiOx thin film to coat metallic and polymeric surface for different application such as protection from corrosion of metals, protection from tarnish of silver containing surfaces, gas and liquid barrier, etc. has been obtained utilizing RF plasma fed with hexamethyldisiloxane, oxygen and Ar mixtures.

Magnesium alloys, cast and especially wrought material, with a protective ceramic-like coating - e.g. SiO2 based. This coating is applied by means of low pressure PE-CVD, using various precursors. Coating is especially appropriate for surfaces with low roughness (machined and polished surfaces), where only thin coatings are applicable for tolerance reason. Results are depending on alloy composition and substrate quality as well as topography.

A first comparison is established between ANOMAG and PE-CVD taking into consideration that the first is an industrial process, while plasma depositions experiments have been performed in a lab-scale reactor. The comparison has been established taking into account only the electricity use required for covering a 1m2 of magnesium parts. As a matter of fact, during the LCA activities it is crucial the precise definition of the functional unit and a correct LCA comparison can be performed only if the functional properties are exactly the same. The obtained results showed the very low environmental impact of PECVD that render the plasma deposition technique very interesting for future industrial applications. A second comparison has been established between two industrial processes: ANOMAG and KERONITE. In this case the LCA study have been carried out taking into account the global energy requirement linked with the covering of 1 m2 of magnesium parts with a 20 µm protective layer. The results show that KERONITE coating process has a higher impact if compared with ANOMAG, but this result has to be revised taking into account differences in the corrosion protective properties and others environmental impact parameters. Moreover, due to the high impact of electrolytic magnesium production, the analysis shows that the coating processes have global energetic and environmental loads depending on the shape of magnesium part. The GER weight of the coating process is of the same order of the magnesium production and overcomes this when magnesium castings have a coil type shape. In fact, the global energy requirement shows that the specific surface area of magnesium parts is the most critical parameter in the case of coating industry, because the contribution to GER of the coating has the same weight of the alloy production at shape factor commonly reached in castings. The comparison between different coating processes is difficult due to the simplification involved if the thickness of the layer is the only considered parameter.

Chromium nitride coatings developed in this project distinguish themselves from other standard CrN coatings in that they are deposited at low temperatures (less than 180°C) and yet are well-adherent and show excellent frictional and wear properties. Thin films of CrN have been deposited on a new and in the future commercially available MRI201S magnesium alloys substrate by PA-PVD process using. The deposition process has been optimized with respect to the adhesion, uniformity, stoichiometry, hardness, and frictional and wear behaviour. Additionally, a new wet chemical method was also developed to dramatically increase the adhesion of the CrN coating to the Mg alloy substrate. This process has been upscaled to an industrial process capable of coating large areas.

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