Nano-Micro-Structured 6%–8% YSZ Thermal Barrier Coatings: A Comprehensive Review of Comparative Performance Analysis

Beneficial properties achieved by nanostructuring effects in materials have generated tremendous interests in applications in surface engineering, especially in thermal barrier coatings (TBC). Limitations in conventional TBC processing for gas turbines and aero-propulsion systems have been exposed during past decades when rapid progress was made in nano-structuring coating research and developments. The present work is a comprehensive review of the current state of progress in nanostructured TBC (Ntbc) in reference to its microstructure, damage progression, failure mechanisms and a wide range of properties. The review aims to address the comparative performance analysis between the nanostructured and conventional (microstructured) 6–8 wt.% yttrium stabilized zirconia (YSZ) TBC systems. Oxidation resistance and sintering behavior in two TBCs are considered as the central focus of discussion. A few schematics are used to represent major microstructural features and failure progression. A performance analysis is performed for standard 2-layer, as well as functionally graded multilayer, TBC systems. A comparison of TBC characteristics processed by plasma spray and vapor deposition techniques is also made as reference. Compared to the sea of R&D efforts made for conventional TBC (Ctbc), limited experimental studies on Ntbc offers conflicting data, and prediction modeling and computational research are scarce.

Using a DOE Approach to Optimize a LVPS Bond Coat and APS Top Coat for TBC Systems

Acquisition of a new LVPS and APS coating system at Delta Air Lines necessitated optimization of the coating parameters on both systems; especially for application of bond coat (LVPS) and top coat (APS) for a TBC coating system. To expedite the coating optimization; it was determined that a design of experiments (DOE) approach would best enable the establishment of the operating window for the two systems. Samples prepared were primarily evaluated for their performance while exposed to a cyclic oxidation cycle. Samples were also evaluated for the microstructure and composition using energy dispersive spectroscopy (EDS) analysis. Samples from the ceramic coating DOE were also evaluated for their erosion characteristics. Results indicate a low correlation between the individual bond coat parameters evaluated to the furnace cycle life. However; the top coat spray parameters were found to have a greater correlation to furnace cycle life and erosion performance.

The mechanisms of controlling FCT life and failure mode of Ni-based EBPVD YSZ thermal barrier coatings via the effects of both substrate elements and reactive element doping

Electron-beam physical vapor deposition (EBPVD) of NiCoCrAlY- and Hf-modified bond coats on (1) selected polycrystalline, directionally solidified, (2) single crystalline substrate alloys and (3) an uncoated NiCrAl bond-coat surrogate substrate, all of them covered with standard EBPVD YSZ topcoats were subjected to cyclic furnace testing (FCT) at 1100 °C. The lifetime and spallation failure upon FCT were evaluated. A typical mixed layer zone (MZ) of alumina and zirconia has formed during topcoat processing above the thermally growing oxide layer. The MZ was investigated by energy-dispersive X-ray spectroscopy after intermediate lifetimes and at the end of life. Chemical composition of the MZ and lifespan data were related to each other thus accounting for rate-determining reactions which could be assigned to either cation- or anion-controlled transport mechanisms. These provide a new approach to address FCT life and failure mode of even complex TBC systems containing reactive elements (e.g. Y and Hf). The cation-controlled processes are accelerated according to their concentration by tetravalent elements of the substrates, while the anion-controlled processes are unaffected by this and only adopt a cation-dominated mode when alloying elements of a low valence (e.g. Ti+) reach a supercritical concentration.

Damage Assessment and Fracture Resistance of Functionally Graded Advanced Thermal Barrier Coating Systems: Experimental and Analytical Modeling Approach

Enhancement of stability, durability, and performance of thermal barrier coating (TBC) systems providing thermal insulation to aero-propulsion hot-section components is a pressing industrial need. An experimental program was undertaken with thermally cycled eight wt.% yttria stabilized zirconia (YSZ) TBC to examine the progressive and sequential physical damage and coating failure. A linear relation for parameterized thermally grown oxide (TGO) growth rate and crack length was evident when plotted against parameterized thermal cycling up to 430 cycles. An exponential function thereafter with the thermal cycling observed irrespective of coating processing. A phenomenological model for the TBC delamination is proposed based on TGO initiation, growth, and profile changes. An isostrain-based simplistic fracture mechanical model is presented and simulations carried out for functionally graded (FG) TBC systems to analyze the cracking instability and fracture resistance. A few realistic FG TBCs architectures were considered, exploiting the compositional, dimensional, and other parameters for simulations using the model. Normalized stress intensity factor, K1/K0 as an effective design parameter in evaluating the fracture resistance of the interfaces is proposed. The elastic modulus difference between adjacent FG layers showed stronger influence on K1/K0 than the layer thickness. Two advanced and promising TBC materials were also taken into consideration, namely gadolinium zirconate and lanthanum zirconate. Fracture resistance of both double layer and trilayer hybrid architectures were also simulated and analyzed.