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Vapour phase coatings
J. AUBRETON, J. DESMAISON, I and J.L. JAUBERTEAU, C. TIXIER, P. TRISTAN
SPCTS – Science des Procédés Céramiques et Traitements de Surface
Université de Limoges / Faculté des Sciences, Limoges (F)

During the last 20 years the ever increasing service and performance requirements demanded of materials by the mechanical and electronic industrial sectors has led to significant developments in the field of ceramic coatings. At the same time, environmental pressures (air and water quality, waste reduction demands) have favoured the development of « dry » processes including many coating techniques. Control of the coating steps « coating parameters – diagnostics – modelling – properties – applications » allows high performance systems to be developed which guarantee quality coatings, good yield and protection of the environment.

In the normal chemical vapour deposition (CVD) process, also known as thermal CVD because the reactions are thermally activated, reactive gases decompose allowing substrates to be coated with a range of materials including TiB2, Si3N4 and SiC. The applications for such coating are varied but in many cases resistance to wear, friction, corrosion or oxidation (thermostructural composites such as C/SiC, SiC/SiC, or porous non oxide ceramics such as Si3N4 or SiC) is required. In order to meet the industrial demands for lower deposition temperatures, it is possible to compensate for the lower thermal energy available by using organometallic precursors (OMCVD) or with the aid of a plasma (PECVD).

The role of the plasma in PECVD is to make the chemically active species (ions and free radicals) available. These species are produced by electron-molecule collisions in the gaseous phase or by impacts with the solid surface of ions, electrons or photons, produced in the plasma. Although the electron temperature is greater than 20000K, the gas temperature remains between 25 and 350°C. An advantage of this technique is that coatings may be deposited on delicate or sensitive substrates such as polymers. The process is extremely flexible as it is possible to heat or polarise the substrate independently of the plasma.

Research currently underway in the Laboratory of « Science of Ceramic Processes and Surface Treatment » at the University of Limoges in France deals particularly with surface treatment and coatings assisted by microwave plasmas.

With the ultimate goal of optimising the PECVD process to allow coatings of uniform thickness to be deposited on substrates of simple and complex shape, the aims of the research are to understand each of the process steps :

  • influence of the coating application parameters,
  • gaseous phase diagnostics,
  • modelling of reaction phenomena in homogeneous and heterogeneous phases,
  • characterisation and properties of the coatings.
  • Specifically the research programmes in progress deal with :

  • deposition of ceramics (SiO2, Al2O3, Si3N4, …) by microwave PECVD,
  • surface treatment (oxidation, nitridation) of metallic alloys in an expanding plasma (O2, N2, …) activated by a microwave.
  • The aim of these research programmes is to adapt the properties of the coatings to those required in the final application, such as oxidation resistance, corrosion resistance, wear resistance and electric insulation.

    The academics, research engineers and postgraduate students are working in the laboratory have a range of specialist equipment available to them including :

  • 2 thermal CVD reactors,
  • 4 microwave PECVD reactors,
  • plasma characterisation equipment: mass spectrometers, optical emission and absorbtion spectrometers, electrostatic sound,
  • equipment measurement of thickness (profilometer), density, surface roughness,
  • IR and Raman spectrometers, X-ray diffraction,
  • optical microscopes, scanning electron microscopes, atomic force microscope,
  • mechanical characterisation (elastic modulus, 4 point flexure, scratch tests).
  • Research, carried out in collaboration with industrial partners, is specifically aimed at :

  • technology transfer of microwave PECVD for deposition of alumina,
  • feasibility of microwave PECVD for deposition of hydrogenated amorphous carbon,
  • study of the plasma and the processes for deposition of thin coatings.
  • The research themes are constantly evolving towards development of multi-technique processes (laser ablation-PECVD, PECVD-thermal spraying), modelling of gas flow, electromagnetic coupling of microwaves and plasma and modelling of the interactions between the plasma and the material surface.

    For PVD coatings, the metallic element is introduced in solid form and then converted into a vapour phase by a physical method (thermal evaporation, ionic or electronic bombardment). Two processes are well developed: ionic deposition and vacuum electric arc deposition. The principal applications for these techniques are to use hard, wear resistant ceramic coatings (TiN, CrN) for the improvement of cutting tool and mould tooling lifetimes. Many industrial suppliers claim significant lifetime improvements for such coatings.

    In Limoges, the local industrial development agency (DRIRE), in collaboration with local industries, have established a Centre for Advanced Surface Treatments and Coatings (CITRA). The aims of the centre are to establish pilot plants that will allow research on a semi-industrial scale to be carried out and will also provide technical assistance to industry.

              
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