Jonathan Cormier

Laser Patterning Pretreatment before Thermal Spraying: A Technique to Adapt and Control the Surface Topography to Thermomechanical Loading and Materials

Coating characteristics are highly dependent on substrate preparation and spray parameters. Hence, the surface must be adapted mechanically and physicochemically to favor coating–substrate adhesion. Conventional surface preparation methods such as grit blasting are limited by surface embrittlement and produce large plastic deformations throughout the surface, resulting in compressive stress and potential cracks. Among all such methods, laser patterning is suitable to prepare the surface of sensitive materials. No embedded grit particles can be observed, and high-quality coatings are obtained. Finally, laser surface patterning adapts the impacted surface, creating large anchoring area. Optimized surface topographies can then be elaborated according to the material as well as the application. The objective of this study is to compare the adhesive bond strength between two surface preparation methods, namely grit blasting and laser surface patterning, for two material couples used in aerospace applications: 2017 aluminum alloy and AISI 304L stainless steel coated with NiAl and YSZ, respectively. Laser patterning significantly increases adherence values for similar contact area due to mixed-mode (cohesive and adhesive) failure. The coating is locked in the pattern.

Role of Powder Granulometry and Substrate Topography in Adhesion Strength of Thermal Spray Coatings

APS coating is deposited with different treated surfaces to understand the effects of surface topography and particle sizes on adhesion bond strength. Grit blasting and laser surface texturing have been used to create a controlled roughness and controlled surface topography, respectively. Coating adhesion is mainly controlled by a mechanical interlocking mechanism. Fully melted Ni-Al powder fills the respected target surface with high-speed radial flow. Pores around central flattening splat are usually seen due to splash effects. Laser surface texturing has been used to study near interface coating depending on the target shape and in-contact area. Pull-off test results have revealed predominant correlation with powder, surface topography, and adhesion bond strength. Adhesion bond strength is linked to the in-contact area. So, coating adhesion might be optimized with powder granulometry. Pores near the interface would be localized zones for crack initiations and propagations. A mixed-mode failure has been reported for sharp interface (interface and inter-splats cracks) due to crack kicking out phenomena. Coating toughness near the interface is a key issue to maximize adhesion bond strength. Volume particles and topography parameters have been proposed to enhance adhesion bond strength for thermal spray process for small and large in-contact area.

Issues related to the constitutive modeling of ni-based single crystal superalloys under aeroengine certification conditions

This paper presents a constitutive modeling approach (the Polystar model) used to compute the viscoplastic behavior and the durability of high pressure turbine blades and vanes of aeroengines during complex thermomechanical histories typically encountered during certification procedures. This model is based on internal variables representing explicitly the microstructure evolutions occurring during very high temperature non-isothermal loading (e.g., dissolution/re-precipitation of the strengthening phase, dislocation recovery mechanisms, etc.) in a crystal viscoplasticity modeling framework. This article shows that the development of such a modeling tool requires a good characterization of fast microstructure evolutions, as well as in-service-type experiments (using burner rigs) able to reproduce the complex thermomechanical loading spectra. The capabilities of the model are illustrated, as well as its potential industrial applications and further developments are commented.