Damage Mechanisms Of Functionally Graded Thermal Barrier Coatings Under Thermo Mechanical Cycle Loading

Damage Mechanisms Of Functionally Graded Thermal Barrier Coatings Under Thermo Mechanical Cycle Loading

In a high temperature environment such as those in a gas or steam turbine and in low temperature applications like drill pipes, coatings are essential to safe guard base structures. Thermal loads coupled with fatigue causes gas/steam turbine components, particularly at the first stage nozzle and turbine blades reduces the service life. Thermal barrier coatings with functionally graded concept provide better life than conventional thermal barrier coatings. From the previous publication [ref 1], it is seen that the spallation life of functionally graded coatings is six times better than that of conventional barrier coatings. However, due to the porosity effect of ceramic coating, early crack nucleation and cracks that connect the voids, service life will still be reduced. The type of loading (out-of-phase and in-phase) both thermal and mechanical will also influence the life of coatings.

This paper considers the effect of loading cases for both thermal barrier coatings and functionally graded thermal barrier coatings and the influence of loading cases on damage mechanisms. This paper also makes an attempt to simulate the damage model of functionally graded thermal barrier coatings.

Introduction

Surface engineering is an emerging discipline that has led to the expansion of a wide range of functional physical, chemical, electrical, electronic, magnetic, mechanical, wear resistance, tribological properties, and corrosive resistance properties at the sub-structure. All types of coating combinations are possible with the help of emerging technologies. Performance of these coatings can be enhanced with an appropriate coating design which uses functionally graded concept (FGM).

In recent years, there is a clear paradigm shift in extensive applications of surface engineering from conventional electroplating to processes such as vapor deposition, diffusion and thermal spray. In many high and low temperature applications, the essential function of coating is to protect the base structure from severe loading conditions such as thermal-fatigue coupled, or environmental loads like corrosion. The quest for improved, reliable coatings is still in process.

Ceramic-metal FGM has been attracting attention as thermal barrier coatings (TBC) for aerospace structures, gas turbines and aircraft engines. These are being considered for working under high temperatures and thermal gradients. FGM is a relatively new concept that involves:

o Tailoring the internal microstructure of composite materials to specific applications
o Producing a microstructure with continuously varying thermal and mechanical properties at the continuum or bulk level

TBC has continuous variation of material properties from one surface to another; this alleviates stress discontinuities. Hence, they are ideal for applications involving severe thermo fatigue loadings which minimize the effect of stress discontinuities in spallation life. A comparative study on TBC and FTBC has been made in previous publication (ref 1) which shows that the spallation life of Functionally Graded Thermal Barrier Coatings (FTBC) gives six times better spallation life than that of conventional TBC.

This paper mainly focuses on the in-phase (maximum strain at maximum temperature) and out-of-phase (maximum strain at minimum temperature) thermal cyclic loading on both TBC and FTBC and also the damage mechanism of the coatings. These two types of thermal cyclic loading phase may reproduce many of the mechanisms that develop under thermo-mechanical fatigue. Figure 1 shows the stress versus strain under thermo mechanical fatigue loading for both OP and IP loading cases.

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