Session: 06-03-01: Advances in Aerospace Structures and Materials-1

Paper Number: 166297

Microstructure and Mechanics of High-Emissivity Oxide Coatings for Ti-6Al-4V Rocket Nozzles 

The nozzle of a hypergolic in-space thruster must withstand extreme thermal cycling, with steady-state gas temperatures in the range 1800–3750 K and typical pulse durations spanning 10 milliseconds to several seconds. The conventional material choice for this application is Nb C103 because of its high-temperature strength. However, the low-density alloy Ti-6Al-4V (Ti64) is also of interest because of its potential lightweighting and cost benefits. The main drawback of Ti64 is its relatively low maximum service temperature. In order to withstand the extreme heat fluxes during engine firing, Ti64 requires high-emissivity oxide coatings which enhance radiative cooling but are susceptible to delamination. Here we assess the delamination risk of a candidate high-emissivity oxide coating under the thermal transients expected in service.

The high emissivity coating of present interest was deposited via vacuum plasma spray on additively manufactured Ti64 substrates. We first characterized the microstructure, composition, and mechanical properties of the coating, finding a dual-phase microstructure comprising a mixture of Al- and Ti-based oxides. The splat morphology of the coating is typical of plasma spray deposits with mudcracks consistent with rapid solidification of molten droplets. The delamination behaviors of the coating were qualitatively assessed using quasi-static three-point bend testing as well as thermal shock testing. The bend tests showed strong adhesion between the coating and the substrate. The cyclic thermal shock experiments were carried out using a specialized rig that simulates the thermal transients expected in service. Samples subjected to 30, 60, and 120 cycles showed increasing damage with thermal cycles. The damage was characterized as gradual comminution of the outer layer of the coating. However, the coating remained largely intact and adhered to the substrate.

To further optimize the coating design, these experimental results were used to calibrate a parametric numerical study of coating delamination, considering a range of coating thicknesses and operating conditions (gas temperatures, hot fire durations, heat transfer coefficients). These simulations showed that the dominant contribution to the energy release rate was the thermal expansion mismatch between the coating and substrate. The coating developed large tensile stresses under steady-state operation but the peak energy release rates remained modest (of order ~10 J/m²) for coatings thinner than 20 μm. Critically, this is sufficiently thick to uniformly cover the nozzle for radiative cooling. These experimental and numerical results collectively point towards the potential of this high-emissivity coating for hypergolic thrusters. They also provide a framework to aid in the design of high-temperature coatings for space propulsion applications.

Presenting Author: Annika Vaidyanathan Massachusetts Institute of Technology

Presenting Author Biography: Annika Vaidyanathan is a third-year undergraduate student at the Massachusetts Institute of Technology pursuing a B.S. in Aerospace Engineering. For the past three years, she has been a part of the MIT Aerospace Materials and Structures Lab conducting research under Professor Zachary Cordero, where her research has focused on the development of advanced materials for use in extreme aerospace environments.

Authors:

Annika Vaidyanathan Massachusetts Institute of Technology
Macy Brennan Massachusetts Institute of Technology
Garrett Robinson Massachusetts Institute of Technology
Vibha Deshiikan Agile Space Industries
Charlie Garcia Agile Space Industries
Zachary Cordero Massachusetts Institute of Technology