Titanium alloys exhibit high specific strength characteristics. They are for this reason widely used by aeronautical engineers mostly to design airframes and gas turbine components. Very often, surface treatments are applied to locally increase the surface properties, especially to obtain a better resistance against wear. Thermal spraying is largely used to manufacture these coatings. This technology requires a specific surface preparation prior to the spraying stage. This surface preparation conventionally consists in surface degreasing and grit-blasting which has some drawbacks, among them a significant decrease of the components fatigue resistance due to the notch-sensitive behavior of these alloys. The PROTAL process consists in combining surface cleaning based on laser ablation to the deposition stage thus avoiding grit-blasting. This paper aims to present an overview of the possibilities offered by this process for the coating of titanium alloys; the criticality of the different processing parameters is studied and discussed.

Excepted in a few cases where metallurgical bonding occurs between deposit and substrate, thermal spray deposit adhesion generally results from a mechanical anchoring. In this case, the very first impinging particles forming the first deposited layer spread and solidify into and around the cavities of the grit-blasted surface. A palliative process to degreasing and grit-blasting prior to thermal spraying is simultaneous laser ablation; i.e., the PROTAL process. In such a case, little topographic change results from the laser-matter interaction: deposit adhesion does not derive anymore mainly from mechanical anchoring but from other types of bonding such as chemical bonding. This paper aims to clarify the bonding mechanisms of thermal spray coatings manufactured implementing the PROTAL process. The case of metallic coatings deposited on metallic substrates is especially discussed. At first, laser ablation effects on various metallic substrates are presented, from the topographic and energetic points of view. Then, the induced effects on impinged particle morphologies are discussed. The results are correlated to thick deposit adhesion.

Thermal spray deposits are built layer by layer following several passes of the spray gun in front of the part to be coated. Between each pass, dusts from the surrounding atmosphere and condensed vapors are deposited and hence blur the surface onto which the following layer will be formed; these phenomena weaken the deposit cohesion and lead in the same time to the formation of pores and others stacking defaults. To circumvent such a problem and to improve the deposit overall quality, in situ laser ablation was performed during the coating manufacturing. The ablation stage occurred between each pass. This paper presents at first the experimental device and the major considered processing parameters. Among those, energy density is one of the most critical. It then investigates the structural modifications and the induced improvements obtained for grey alumina coatings considering principally the deposit cohesion and the porosity level.