Future Work

Future work on the evaluation of the TBC thermo-mechanical performance could include:

 Application of a thermal gradient across the TBC using a burner rig flame testing apparatus. In a typical test, an equilibrated natural gas/oxygen flame heats the front of the TBC, while compressed air cools the back side (Figure 88). The front temperature is monitored using a pyrometer, and the substrate temperature is monitored by a thermocouple placed inside a hole drilled radially into the substrate. During cooling, the cooling air nozzle is brought in front of the TBC, and the heating-cooling process is repeated.

The conditions simulated in this test are closer to actual conditions in an engine, as compared to the conventional furnace test without thermal gradient.

Figure 88 - Schematic diagram of thermal-gradient cyclic testing of TBCs during (A) heating and (B) cooling cycles

Figure 89 - Thermal shock test rig

Figure 89 shows a typical thermal shock rig used to test the top coat

material. It consists of a rotating wheel with eight positions: four are heated with a burner flame and the other four cooled with compressed air on the back side. The wheel is indexed to achieve a minimum of 75 s on the hot station and 75 s on the cold position. With proper burner gas flow

adjustments, approximately 1100 ºC surface temperature on the hot side can be achieved. The sample backside is usually kept at 450 ºC by cold air coolers.

This type of thermal-gradient cyclic testing is essential for improving the accuracy of the experimental analysis of TBC performance, because the use of just conventional furnace testing can lead to underestimation of TBC performance. The phenomenological reason for this is that in a gradient test the TBC surface cools through a larger temperature range than the

substrate, which produces tensile stresses in the TBC. These tensile stresses partially cancel the overall compression produced in the TBC, if both the TBC and the substrate were cooled from the same temperature.

This results in a net reduction in the driving force for fracture;

 The oxidation behaviour and the resistance to hot corrosion of the bond coat is a crucial requirement of any TBC system. Therefore, a more careful determination of the oxidation kinetics will use a thermal gravimetric apparatus (TGA) comprising a digital recording microbalance and a vertical tube furnace. Samples are suspended from the balance into the furnace by means of a platinum wire and a platinum sample holder. Mass gain is attributed to the formation and subsequent growth of the TGO and the oxidation kinetics is expected to follow the equation:

Δ𝑤 𝐴⁄ = �𝑘p𝑡�^{1 2⁄}

Equation 40

Δ𝑤 𝐴⁄ : weight gain per unit surface area 𝑘p: rate constant

𝑡: hot time

 Investigate the mechanical properties and porosity of the EB-PVD coatings by means of the nanoindentation experimental method and confirm the measured elastic modulus with the values obtained via laser-ultrasonics.

The choice for this ultrasonic technique is based on the fact that the elastic modulus measured by this approach represents the overall coating

microstructure. The elastic modulus is calculated from the ultrasonic velocity measured using a laser-ultrasonic experimental set-up (Figure 90) usually equipped with a short-pulsed Nd:YAG laser employed to generate the surface acoustic waves. For the detection, a long-pulsed Nd:YAG laser is coupled to an interferometer by optical fibers (303).

Figure 90 - The laser-ultrasonic setup (303)

In addition, compare the measurements with finite element simulation of the elastic-plastic indentation process in order to provide a better

understanding of the plastic response of several thermal barrier coating systems. The indentation process includes both loading and unloading, so the simulation steps should define the exterior surface of the indenter as rigid body, then give a little displacement in order to establish a proper and balanced contact between the indenter and the coating, followed by the full displacement that causes plastic deformation of the coating system, and finally make the indenter return to the original position allowing the elastic recovery to occur.

The above ultrasonic methodology is only able to determine the averaged values for elastic moduli, which is somehow inadequate to describe the direction-dependent thermoelastic behaviour of anisotropic systems such as EB-PVD coatings. Resonant ultrasound spectroscopy (RUS) could offer a good alternative, as it is a dynamic and non-destructive method which can be applied to determine the components of the elastic stiffness/compliance tensor of materials. RUS involves the study of the mechanical resonance of solids in terms of the frequency spectrum, which is a rich source of

information on the elastic and damping properties of materials (Figure 91).

The ability to calculate the entire stiffness matrix from a single spectrum is a distinct advantage of this method. Another advantage of employing the RUS technique for coatings is that small amplitude excitations are adequate for the measurements (304);

Figure 91 - Schematic of the RUS experimental setup (a) and example of a typical RUS spectrum where every frequency peak corresponds to an individual

deformation or excitation mode (b)

 The performance of these ceramic coatings in operation at high

temperature, in a corrosive atmosphere, and under mechanical loading depends largely on their microstructure, and particularly on the anisotropic distribution of pores, cracks, and interfaces that govern the thermal and mechanical properties of the coatings. Thus, a quantitative characterization of the ceramic top coat microstructural parameters such as the component volume fractions and the size, shape, and orientation distributions of the void networks will be done using the small-angle scattering method, by passing a monochromatic X-ray or neutron beam though the heterogeneous TBC sample. Part of the incident beam is scattered out of the incident beam direction when it crosses heterogeneities (e.g., voids) within the sample (305). The scattered intensity profile is then used to determine the three-dimensional volume fraction (porosity) and size distribution of the

scattering features (pores, cracks, voids), by analysing the one-dimensional scattering data measured in several azimuth directions. In addition, the small-angle scattering studies will elucidate the complex processing-microstructure-property relationships, as the thermal conductivity and elastic modulus anisotropy for the top coat correlate well with the corresponding microstructural anisotropy determined by this method;

 Employ an appropriate experimental methodology to determine the interfacial strength of the TBC system and its evolution during thermal cycling. Both indentation and three-point bend tests (Figure 92) are considered good candidates. The interfacial strength is a stochastic parameter that requires a statistical treatment. It is found that the

interfacial bond strength has a characteristic scatter around a certain mean value. To evaluate this characteristic behaviour, Weibull (306) has described a statistical distribution function of fracture events using a parameter known as the Weibull modulus, which expresses the statistical scatter of fracture events. A high modulus indicates a low scatter. Based on a series of interfacial strengths obtained experimentally, after ordering them from the smallest to the largest, the interfacial strength can be estimated by means of Weibull distribution analysis;

Figure 92 - A schematic view of the three-point bend test setup

 Study other mechanisms of TBC degradation, like the impact of particles ingested into the gas stream (erosion) or CMAS attack, as illustrated in Figure 93. This can be done by simultaneously injecting particles or CMAS into the flame during the heating cycle.

Figure 93 - CMAS dust particles are preferentially captured on the leading edge and pressure surfaces (left) of turbine airfoils (118)