Today, requirements for materials become more and more multifunctional. It is particularly true for biomedical applications. For instance, it can be required to exhibit high level of mechanical properties (in particular when material density is considered), corrosion resistance, biocompatibility etc. It is known that conventional magnesium alloys have been already identified as interesting candidates for satisfying such functionalities. Nevertheless, mechanical properties of conventional magnesium alloys are usually limited. A way to increase significantly mechanical resistance of magnesium alloys is to consider amorphous alloys. With such kind of structures, the mechanical resistance can be increased by a factor lying between 2 and 3. Moreover, very interesting elastic properties can also be obtained. Since such materials do not contain any grain boundaries, their corrosion resistance can be also particularly interesting.

However biomedical devices may have very specific requirements depending on their application field, it is then of interest to have a strategy that allows for a fine tuning of properties. A possibility to tune surface properties of metallic glasses is to pattern the surfaces and intensive efforts have been paid in the recent past to develop appropriate patterning processes. One interesting way is to use techniques derived from microelectronic such as photolithography and etching that allows to get very small patterning of the surface. This kind of technique is interesting in the case of metallic glasses since it does not cause any structural modification of the amorphous material. However, such techniques are expensive, not versatile and difficult to adapt in the case of out of plane surfaces.

Two different ways are considered in this project in order to control the surface patterning of bulk metallic glasses. The first one is to use laser processing which is a well-known technique to pattern the surface of metallic alloys. One point will be to adapt the laser process in the case of metallic glasses and to analyse the structural modifications it can cause in the specific case of metallic glasses that are very sensible to local heating. The use of pico- or femto- lasers could be of interest since they are supposed to be able to machine the surface without significant heating effects. An alternative process to pattern the surface is to use the specific thermoforming ability of metallic glasses. In that case, a mould must first be fabricated (for example using laser processing) and can then be used to replicate a pattern on the metallic glass surface. The interesting point of this technique is that this kind of patterning could be done at the same time of the forming process even though it is less versatile than the direct laser patterning.

   

Fig 1:Typical surface patterns obtained by laser (left) and by thermoforming(1)(right)

The objectives of the project are hence to develop specific surface patterning technologies adapted to metallic glasses, to compare both methods and to develop specific patterns in order to increase surface properties of those materials such as corrosion resistance or hydrophobic (or hydrophilic) properties but also to analyse the effects it can have on mechanical properties. For this last point, it is well known that magnesium based metallic glasses are brittle materials and that surface modifications can lead to some increase of the plasticity. Mechanical testing will hence be performed to deal with this specific feature of metallic glasses.

The project is based on a large experimental part including elaboration, surface patterning, mechanical testing, corrosion investigation and microstructural characterization. In particular, the effect of laser processing and thermoforming on the structure and the stability of the amorphous phase will be investigated.

The project will rely on the very complementary expertise of the two groups involved in the project. The SIMAP laboratory at Grenoble-INP has an expertise in metallic glasses since more than ten years (elaboration, alloy design, structural characterisation, mechanical properties(2-4). It has also performed studies related to thermoforming since more than fifteen years and more recently in the particular case of thermoforming of metallic glasses(5). The Laser Materials Processing Group of IST Lisbon has more than twenty years of experience in laser surface treatment for enhancing the corrosion and wear resistance of materials and published one of the first papers worldwide demonstrating the possibility of improving the corrosion resistance of metallic alloys by coating with an amorphous layer of a zirconium alloy(6). The expertise of both academic groups will be complemented by the industrial partner Amplitude Systems specialized in laser developments.

The PhD work will start with the selection of the more appropriate composition and properties specifications for the Mg based metallic glasses. In a second step, metallic glasses will be elaborated and characterised for confirming the amorphous nature of the elaborated alloys. The two techniques of surface patterning (i.e. laser and thermoforming) will be performed and processing conditions will be optimised. The geometry of the patterning will be studied in particular by appropriate microscopy investigation. For selected conditions, mechanical properties and corrosion behaviour will be investigated and compared to the properties of the glasses before patterning. For the sake of comparison, it will be also possible to evaluate the effects of laser processing and thermoforming in the case of conventional magnesium alloys.

Project Partners and their Roles

It is planned that the PhD student will spend approximately equivalent periods (1 ½ year) in IST Lisbon and Grenoble INP. Finally this PhD work should provide strategies and methodologies to fulfil complex specification requirements for materials (in particular in the case of amorphous alloys) in the context of materials for bio applications.

IST Lisbon Surface Functionalisation group has a strong experience in surface modification, in particular through laser surface processing (laser ablation, laser solid freeform fabrication, laser micromachining). Material surface group has also strong experience in characterisation of corrosion processes (electrochemical characterisation, is situ tests).

The SIMAP laboratory is internationally recognized for its work in material science and more specifically on elaboration, microstructural characterisation and mechanical behaviour of metallic alloys. Over the last ten years, an expertise in the elaboration and thermoforming of bulk metallic glasses (including magnesium based metallic glasses) has been achieved through several PhD works.

References

1. G. Kumar, A. Desai, J. Schroers, Bulk metallic glasses: the smaller, the better, Advanced Materials, 23 (2011) 461-476.
2. S. Puech, J.J. Blandin, J.L. Soubeyroux, Viscoplastic forming of Mg bulk metallic glasses in the supercooled liquid region, Materials and Metallurgical Transactions 39 (2008) p. 1874-1881.
3. S. Gravier, J.J. Blandin, P. Donnadieu, Interactions between high temperature deformation and crystallization in zirconium based bulk metallic glasses, Philosophical Magazine 88 (2008),p. 2357-2372.
4. Y. Yang, J.F. Zeng, A. Volland, J.J. Blandin, S. Gravier, C.T. Liu, Fractal growth of dense packing phase in annealed metallic glass imaged by high resolution atomic force microscopy, Acta Materialia 60 (2012), p. 5260-5272.
5. S. Gravier, G. Kapelski, M. Suéry, J.J. Blandin, Thermoplastic forming of bulk metallic glasses, International Journal of Applied Glasses Science 3 (2012) p. 180-187.
6. D. Carvalho, S. Cardoso e R. Vilar, Amorphization of Zr60Al15Ni25 surface layers by laser processing for corrosion resistance, Scripta Materialia 37 (1997) 523-527.