Development of Laser-Based Surface Pre-Treatments for Thermal Barrier Coating Deposition in Aero-Engine Applications: From Process Characterization to Prototype Machine Development.

High-performance machinery, operating under severe thermal and mechanical conditions, requires advanced protective coatings to improve durability and efficiency. Thermal Barrier Coatings (TBCs) are commonly applied in aerospace and automotive applications: they are coatings of different thickness and material (usually a metallic bond coat and a ceramic top), applied on the metal substrate. The lowest cost technique and the most versatile coatings deposition methods have been found in Thermal Spraying, which, in recent years, has been used to build up TBC based on alumina or zirconia. Before coating application, to ensure adequate adhesion and long-term reliability, adequate surface preparation is mandatory. Conventionally, grit blasting is used for this purpose; however, it presents several issues, such as material contamination from grit residues, reduced flexibility, surface treatment, notable environmental impact (the grit particles are hazardous if inhaled and must be disposed of properly and carefully), workers' safety problems, and long process times. The PhD thesis fits in this context and aims to investigate the feasibility of Laser Surface Texturing (LST) processes over grit blasting in prior surface preparation for Thermal Barrier Coating (TBC) deposition. Moreover, it also focuses on transferring the process from a laboratory environment (TRL=3) to an industrial one (TRL=5-6) through the development of a dedicated prototype machine. The activities have been conducted under the framework of the “ALTCAP Project” (“Sistema automatizzato per il trattamento superficiale di componenti aeronautici mediante laser”), in collaboration with industrial partners such as AVIO AERO, LASIT, OCIMA and the University of Campania Luigi Vanvitelli. The work was articulated into three main phases: optimal texture geometry and laser source identification; validation tests on LST-treated substrates and comparison with the grit-blasting process, including metallurgical, adhesion, thermal, and mechanical fatigue analyses; design and construction of a prototype machine integrating the optimised process parameters. The results demonstrated that LST can guarantee mechanical performances at least comparable with those of grit blasting, and sometimes superior, particularly concerning the component’s fatigue resistance. Furthermore, the prototype confirmed the feasibility of performing the process at an industrial scale. During the test campaign, some limitations were identified, such as the requirement of an in-line monitoring system or the adoption of different texturing strategies, to avoid the presence of texture overlap or gap, since the latter could result in a coating adhesion reduction. Overall, the thesis shows that LST represents a robust and sustainable alternative to grit blasting for TBC deposition; moreover, the development and construction of a prototype machine paves the way for further developments.