A recent study published in Engineering presents a novel method for removing thermal barrier coatings (TBCs) using soluble abrasives, addressing a long-standing challenge in the aerospace sector. TBCs, typically composed of yttria-stabilized zirconia (YSZ), are crucial for protecting aero-engine turbine blades from extreme operational temperatures. However, traditional removal methods such as grit blasting or conventional abrasive waterjets (AWJ) often result in abrasive embedment, which is unacceptable in aerospace applications due to the risk of compromising component integrity. Plain waterjets (PWJ), while eliminating contamination risks, suffer from low efficiency and uncontrollable fracture propagation when processing hard ceramic coatings.
The research, conducted by a team from the Rolls-Royce University Technology Centre (UTC) in Manufacturing and On-wing Technology at the University of Nottingham and Rolls-Royce plc, validated a new coating removal method using soluble abrasives. The study employed a 3-axis micro-waterjet system to remove TBCs with a lamellar porous structure. Common caster sugar, ground to an average diameter of approximately 100 µm, was used as the soluble abrasive. Comparative experiments were conducted between PWJ and Sugar-AWJ under pump pressures ranging from 500 to 1000 bar.
Post-process characterization using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), and Electron Backscatter Diffraction (EBSD) revealed significant differences in material removal mechanisms. The introduction of soluble sugar particles increased the material removal rate by over 58% compared to PWJ under identical operating parameters. EDX analysis confirmed that the sugar abrasives dissolved completely upon impact, leaving no residue on the bond coat, whereas PWJ often left residual coating islands due to unstable removal.
The study found that plain water removal relies on the "water hammer effect," where high compressive stress causes sudden, multi-layer fractures, resulting in large blocks of material peeling off and the formation of long, uncontrolled cracks. In contrast, the soluble particles act as cutting tools that induce lateral and radial cracks, leading to a "layer-by-layer" erosion process. This mechanism results in a more consistent trench profile and minimizes structural damage to the surrounding coating.
Furthermore, the researchers developed a mathematical model based on the Gaussian distribution of jet energy. This model utilized a "soft clipping" function to correlate the effective kinetic energy of the slurry with the machining depth, calibrated by single-trench experimental data. Validation experiments on overlapping trenches showed a maximum prediction error of 18.0%, demonstrating the model's high reliability for process control.
This research demonstrates that soluble abrasives offer a more effective solution for aerospace repair and improvement. By utilizing eco-friendly and dissolvable sugar particles, the process eliminates the risk of foreign object damage caused by abrasive embedment while maintaining high processing efficiency. The established mathematical model also paves the way for intelligent, automated remanufacturing systems, supporting the industry's transition towards greener and safer maintenance technologies.
The paper "Response of Thermal Barrier Coatings to Waterjet with Soluble Abrasives: Machining Performance and Material Removal Mechanisms," is authored by Fengrui Zhang, Zhirong Liao, Jose A. Robles-Linares, Andy Norton, Shusong Zan. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.10.014
