Surfaces : reactivity and protection
The research within the SURF laboratory revolves around :
1. The implementation of CVD processes to set up functional and protective coatings and films
The use of metallo-organic compounds as molecular precursors for the deposition elements (MOCVD process) makes it possible to lower the deposition temperature and to thus protect the whole microstructure and dimensions of the parts to be coated which can be made of steel, titanium alloy or polymer. The surfaces are flat or with complex geometries as preforms to be infiltrated or as powders. The operating MOCVD processes depend on the films’ physicochemical features and ultimately on the desired properties. Coatings such as multi-layer nanostructured metallic carbon nitrides, aluminium or complex metal alloys are applied on structure materials to provide wear, oxidation or corrosion resistant properties. Titanium-oxide and complex aluminium/platinum coating films are used for depositing different functions (hydrophylicity, packaging, optical uses).
2. The coatings’ characterisation using conventional techniques
(DRX, microscopy and image analysis, FTIR, thermal analyses, contact angle).
The characterisation techniques are applied to interfaces and interphases whether in the field of thin films deposited on substrates or porous/multimaterial composite materials. These processes can be supplemented by adhesion testing to analyse the mechanical characterisation of the interfaces and interphases.
3. Coating characterisation by specific techniques
a) Captive bubbles
The surface under scrutiny is submerged into water upside down. By a microsyringe an air bubble is put on the lower part of the sample. The contact angle is determined from the geometric parameters of the air bubble. Like in the Sessile drop technique, the measure is sensitive to roughness, porosity and surface heterogeneity but it is more or less sensitive to air humidity and temperature.
The zeta potential is the electric potential of the shear plane of the electrical double-layer making it possible to determine electrical surface charge. This potential is the charge the surface particles get thanks to ions around them when in solutions. The measure of the zeta potential in different pH electrolytes permits to determine the point of zero charge (pzc) which is the pH value of the solution in which the surface in question exhibits zero potential.
c) High-temperature resistivity meter
The team has designed a prototype that is being tested and approved. It permits to measure the electrical properties, especially films’ and coatings’ electrical resistivity between room temperature and a temperature in the vicinity of 1,000°. Thanks to it, a follow-up of the films’ phase changes revealed by changes in electrical properties.
d) General and local electrochemical impedances
They provide information on organic films’ barrier properties, on the corrosion inhibitors’ electrochemical effect embedded in coatings and on the inhibitors’ cicatrising effect, especially when the coating is damaged.
4. The modelling of the films’ growth stages
is carried out by quantum chemical modelling methods and DFT calculations applied to the study of gas-solid interactions. On a macroscopic scale, the growth of the films is simulated using a thermochemical and chemical kinetic approach.