PhD Oral Exam - Aravind Kumar Thoutam, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The evolution of gas turbine technology for aerospace applications is critically dependent on the advancement of thermal barrier coating (TBC) systems, which enable increased operating temperatures and enhanced efficiency. At the core of these TBC systems lies the NiCoCrAlY-based bond coat, which serves as both an oxidation-resistant barrier and the primary interface for adhesion between the ceramic topcoat and metallic substrate. The performance and service life of a TBC are fundamentally governed by the microstructural characteristics and oxidation resistance of the bond coat, which are in turn determined by the deposition technique employed.

This work investigates a comprehensive exploration of high-velocity air-fuel (HVAF) thermal spray technology for the deposition of NiCoCrAlY bond coats, addressing its comparative advantages over established techniques such as high-velocity oxy-fuel (HVOF) and plasma spraying. The HVAF process achieves coatings with notably dense microstructures and minimal inflight particle oxidation, a result of lower operational temperatures and higher particle velocities. Systematic exploration within this research focuses on HVAF hardware configurations, such as variations in combustion chamber design, nozzle geometry, and powder injector technology, to understand their effects on coating density, microstructure, inflight particle oxidation and oxidation behavior at elevated temperatures. Advanced diagnostic tools, including AccuraSpray 4.0 for real-time particle temperature and velocity monitoring, and post-deposition characterization via Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and focused ion beam (FIB) imaging, the work dissects the relationships between process control, feedstock properties, and resultant coating microstructures.

In this dissertation, individual splat analysis was initially performed with varying deposition rates to demonstrate the inherent capabilities of HVAF in producing partially molten and fully deformed splats. These splats exhibited minimal inflight particle oxidation at the nanoscale level and preserved aluminum reservoir within the NiCoCrAlY matrix, essential for the generation of a continuous α-Al₂O₃ scale and long-term oxidation resistance during thermal cycling.

A comprehensive understanding of the influence of hardware configurations, with particular emphasis on four distinct convergent-divergent secondary nozzle configurations and variable air-fuel pressures were also explored in this work. Longer nozzles enabled enhanced dwell time, leading to improved peening and denser microstructure, and that partially molten particles, often originating from feedstock containing crowns and satellite morphologies, contributed to splat boundary-based porosity. Minimal in-flight oxidation is confirmed by elemental mapping, with only nanoscale oxide formation at particle interfaces within coatings.

Comparative studies of HVAF and HVOF techniques show that HVAF-deposited NiCoCrAlY coatings experienced less in-flight oxidation and provided superior resistance to high-temperature degradation. A short-term oxidation test at 1000 °C for 4 h was conducted to evaluate early-stage oxide formation and the initial coating response. These samples, together with the as-sprayed condition, were subsequently subjected to long-term oxidation at 1000 °C under ambient atmosphere for 12.5, 25, 50, and 100 h to examine progressive oxidation behaviour.

In summary, this research demonstrates the fundamental relationships between HVAF process parameters, powder composition, and the microstructural integrity of NiCoCrAlY coatings, contributing to the long-term reliability and sustainability of advanced gas turbine engines. The findings provide a foundation for future developments in bond coat materials, including compositional enhancements and process-modeling strategies for next-generation aerospace coatings.