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Production of more efficient surgical implants from PM 2000 alloy coated in situ with alumina by thermal oxidation

The Fe-20Cr-5Al alloy (PM 2000), coated with alumina by thermal oxidation, could replace current biomaterials used for the fabrication of surgical implants.

General advantages with respect to current biomaterials are:

- Absence of Ni (allergenic and carcinogenic element) and Co (geostrategic element).
- Possibility of developing an outer alumina layer of about 5 microns by thermal oxidation.
- A reduction of about 20% in density with respect to that for Co-Cr alloys.
- Mechanical properties of the final product are equal or superior to those of biomaterials in use.
- Superior corrosion resistance and very low ion release, even under friction conditions, especially in the preoxidised condition.
- Good biocompatibility, often approaching the behaviour of materials well known for their biological acceptance, e.g. alumina
- Wear properties could approach that of bulk alumina, assuming that a final mechanical polishing yields the roughness required at the ISO standard for ceramics (20 nm).

General advantages with respect to other biomaterials coated with bioinert coatings are:

- High level of compressive residual stresses at the coating without compromises the scale adherence. This implies a high fatigue limit (about 550 Mpa) and a high level of deformation of the coating without cracking (about 1%).

General advantages with respect to other surface modification techniques are:

- Thermal oxidation is a low cost and non-contaminant process, easy to perform.
- Coating is homogenous in thickness irrespective the shape of the component.
- Dimensional tolerance of the component does not change during preoxidation.

General advantages with respect to bulk alumina are:

- High versatility when designing (shape, wall thickness) the components.
- The alloy is easier to machine due to the less hard nature, which results in an increase in service life of tooling and no need to use expensive tools.
- Components can be elaborated by the producer of current metallic implants with their own facilities.

The only technical disadvantage, that however could be useful for certain medical devices, is that the alloy exhibit a soft ferromagnetic behaviour which obviously prohibits the use of medical control techniques based on strong magnetic fields (NMR). Therefore, alternative control techniques should be used.

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