Research Projects

Thermal barrier coatings (TBCs) are widely used in the hot sections of gas turbine systems, as they are remarkably efficient in insulating the underlying superalloys, leading to higher operating temperatures and therefore enhancing the performance of the engines. However, this benefit is only possible if the integrity of the TBC under aggressive thermo-mechanical environments is ensured. Delamination is a common but hard to detect failure mode. We present, in this work, supported by experimentation, the implementation of a modeling approach applying the Kubelka-Munk theory to provide numerical quantification of luminescence contrast and intensity due to top coat delamination in TBCs. The method relies on the drastic change in reflectivity when a delamination forms, exploiting it for high-contrast luminescence mapping. Two distinct TBC configurations containing an erbium-doped yttria-stabilized zirconia (YSZ:Er3+) layer for delamination sensing were used to validate this model. A delamination zone induced by Rockwell indentation was successfully tracked by measuring an increase of the intensity of the erbium emission line at 562 nm. Luminescence-based methods for delamination detection can provide a revolutionary non-invasive technique, with potential for both offline and online engine monitoring.

Read our paper in Surface and Coatings Technology.

Calcium-magnesium-alumino-silicate (CMAS) particulates enter the aero-engine in a sandy environment, melt and infiltrate into 7 wt% yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs), reducing their lifetime. This leads to chemical degradation in 7YSZ accompanied by tetragonal to monoclinic phase transformation upon cooling. In this work, electron-beam physical vapor deposition coatings were infiltrated with a synthetic CMAS. Synchrotron X-ray diffraction measurements show that CMAS infiltration at 1250 °C has about 43% higher monoclinic phase volume fraction (PVF) at the coating surface compared to 1225 °C and remains consistently higher throughout the coating depth. Additionally, the increase in annealing time from 1 to 10 h results in a 31% higher monoclinic phase at the surface. Scanning electron microscopy revealed the presence of globular monoclinic phases corresponding spatially with the above findings. These results resolve the impact of time and temperature on CMAS infiltration kinetics which is important for mitigation.

Read our paper in Journal of Materials Research.

Thermal barrier coatings (TBCs) are used to protect turbine components from extreme environments and to allow for the turbine system to operate at temperatures beyond the melting point of the underlying superalloy blade. Existing in situ temperature measurement methods for high-temperature evaluation have inherent uncertainties that impose important safety margins. Improving the accuracy of temperature measurements on the materials in operating conditions is key for more reliable lifetime predictions and to increase turbine system efficiencies. For this objective, phosphor thermometry shows great potential for non-invasive high-temperature measurements on luminescent coatings. In this work, a phosphor thermometry instrument has been developed to collect two emission peaks simultaneously of an erbium and europium co-doped yttria-stabilized zirconia TBC, enabling an extended temperature range and high precision of the in situ temperature assessment. The luminescence lifetime decays and the intensity variations of both dopants were captured by the instrument, testing its high sensitivity and extended temperature range capabilities for accurate measurements, up to operating temperatures for turbine engines. The results open the way for the applicability of portable phosphor thermometry instrumentation to perform effective temperature monitoring on turbine engine materials and support the advancement of innovative sensing coatings.

Read our paper in Measurement Science and Technology.

Chromium-doped alpha-alumina is naturally photo-luminescent with spectral properties that are characterized by R-lines with two distinct peaks known as R1 and R2. When the material is subjected to stress, shifts in the R-lines occur, which is known as the piezospectroscopic (PS) effect. Recent work has shown that improved sensitivity of the technique can be achieved through a configuration of nanoparticles within a polymer matrix, which can be applied to a structure as a stress-sensing coating. This study demonstrates the capability of PS coatings in mechanical tests and investigates the effect of nanoparticle volume fraction on sensing performance. Here, measurements of spectral shifts that capture variation in stress of the coating during mechanical testing and in the region of substrate damage showed that stress contours are more noticeable on a soft laminate than hard laminate. It was found that the 20 % volume fraction PS coating showed the most distinct features of all the coatings tested with the highest signal-to-noise ratio and volume fraction of alpha-alumina. Post-failure assessment of the PS coatings verified that the coatings were intact and peak shifts observed during mechanical testing were due to the stress in the substrate. The results suggest the ability to design and tailor the “sensing” capability of these nanoparticles and correlate the measured stress variations with the presence of stress and damage in underlying structures. This study is relevant to nondestructive evaluation in the aerospace industry, where monitoring signs of damage is of significance for testing of new materials, quality control in manufacturing and inspections during maintenance.

Read our paper in AIP Advances.

Phosphor thermometry is a promising non-destructive method for accurate temperature measurement using phosphor elements that emit temperature-dependent luminescence. The method relies on the intensity and decay of luminescence arising from the phosphor elements upon excitation by an incident laser. In this work, the classical Kubelka–Munk model has been utilized and modified to model the luminescence emitted from phosphor elements that are added into thermal barrier coatings (TBCs) to enable temperature sensing using phosphor thermometry. The collectible luminescence and its time-decay behavior emerging from a tailorable multilayer TBC configuration have been predicted for different rare-earth dopants: Dy, Er, and Sm within an yttria-stabilized zirconia (YSZ) host, and with an operational gradient of temperature acting through its depth. The configurations have been designed by varying the position and thickness of the doped layer into the coating. The decay constant of the collectible luminescence has been used to determine the position in the coating from where the luminescence decay is the same as the decay of the collectible signal. This subsurface position indicates the location at which the temperature measurement is performed using phosphor thermometry under realistic operating conditions. It has been determined that YSZ:Dy provides the highest intensity of the collectible luminescence among the three dopant materials. In the TBC configuration with a fully doped coating, using YSZ:Er as a sensor enables temperature measurement from a more in-depth position in the coating. It has been shown that this position can be tailored by adjusting the geometrical configuration of the TBCs, varying the position and thickness of the doped layer. Due to the sensitivity of the dopants to temperature, the decay behavior of the emerging luminescence is demonstrated to change for different TBC configurations. The model can be used in screening the dopants to design multilayered TBCs for their suitability in temperature sensing by phosphor thermometry.

Read our paper in Applied Optics.