PtNiAl bond coats are diffusion aluminide coatings deposited on superalloy based turbine blades for oxidation resistance and improved adhesion between the substrate and the YSZ thermal barrier coating on top. They are deposited by pack aluminisation, which makes their microstructure inherently graded and heterogeneous as well as replete with a variety of precipitates and second phase particles. The microstructure also continuously evolves during thermal cycling, because of interdiffusion with the substrate and the continuous loss of Al to the thermally grown oxide scale on top. During service, the bond coats are exposed to impact, thermal expansion mismatch, thermo-mechanical fatigue and inter-diffusion accompanied by phase transformation, which become leading causes of their failure. The bond coats being B2 crystal structures are known to be brittle at room temperature, due to which they are expected to fail during cooling, although they undergo plastic relaxation by creep above the BDTT. Little attention has been paid to the mechanical response of the bond coats, while a number of studies focus on optimizing their composition for oxidation resistance. The fracture properties of these coatings, in particular, are not very well understood due to the several different length scales of their complex microstructure playing a part. In this context, there is an interest in determination of the fracture toughness of bond coats under different loading and temperature conditions. In the present work, the fracture properties of bond coats is measured with micron-scale resolution using edge notched doubly clamped microbeam structures positioned at individual zones of the graded bond coat, subjected to bending. In order to extract the stress intensity factor for this new configuration and to determine the stress distribution and stability of this geometry under different loading conditions, extended finite element analysis (XFEM) is carried out. After establishing the microbeam geometry as a viable fracture toughness testing configuration, the contribution of different microstructural variables to toughening at room temperature is studied using SEM based in-situ testing. Since the exact composition and structure of the coating depends principally on the elements constituting the matrix-Pt, Ni and Al content, which themselves depend on the deposition parameters, we have examined in detail, coatings aluminised at different temperatures (increasing coating thickness), varying Al content in the pack mixture and starting Pt thicknesses during electro-deposition. These parameters are by no means exhaustive and there is wide scope to investigate the effect of other processing variables as well as their synergistic effects on the mechanical behavior of these coatings. Following this, the high temperature fracture behavior of the stand-alone coatings in tension is also studied to determine their brittle to ductile transition mechanism in the presence of a notch. While this covers the average behavior of the entire coating cross-section, such a study is important to establish the BDTT unambiguously since there are chances of under-estimation of these temperatures in the absence of a notch. Also free¬standing coatings without the underlying substrate offer respite from residual stresses influencing the results of such tests. The present study essentially consists of two distinct parts, one focused on the development of the testing technique to cover multiple length scales of any graded thin film or coating and the other on the determination of fracture properties of the bond coat using these methods. The thesis reads in the following way: Chapter 1 gives an introduction to the diffusion aluminised bond coats, with a focus on the failure mechanisms associated with them while underlying the need for small scale testing in these systems. The conditions driving failure in bond coats can be vast and varied and it is extremely difficult to pin-point a single important cause and also to develop predictive capabilities regarding their failure. This is described as the motivation for the present work, with an objective of finding the variation in fracture toughness values for PtNiAl bond coats of different coating thicknesses and Pt content across the temperature range spanning the BDTT of the sample. Chapter 2 describes in detail all the available literature on thermal barrier coatings in general, and diffusion aluminide bond coats in particular, while specifically highlighting its mechanical response to loads during service. The deposition parameters during pack aluminizing and the graded microstructure which develops as a consequence of the diffusion process are described. The material’s microstructure dictates its properties, but there has been limited work on the mechanical behavior of the coatings themselves due to the difficulty in preparation and testing of free-standing films of the same. Since the base matrix is that of β¬NiAl, and there has been extensive work reported on bulk NiAl in the literature, which is discussed next. This would serve as a benchmark for comparison with the properties of the bond coats themselves, which are expected to respond differently due to their continuously evolving and complex microstructure. A summary of the known mechanical properties of the coatings themselves is given next along with the failure mechanisms that have been proposed. Since the study deals with fracture properties, a short introduction of linear elastic fracture mechanics follows before elaborating on the various small scale fracture testing geometries that have been developed. There are specific differences between testing geometries, stress states as well as in the instrumentation between small scale and bulk fracture toughness tests, which are highlighted. Since these configurations are material and device specific, each group has worked out its own instrument capabilities and mechanics required to extract the mechanical properties of interest from these testing techniques. Due to these differences in addition to the differences in the size scales of the samples tested, the reported properties show a wide variation. Lack of standards add to the difficulty in interpretation of the data; moreover add to the controversy on whether a size effect exists for fracture, as it does for strength. All the non-standard small scale testing configurations require modeling and simulation to extract the desired properties from them, and the present study applies the XFEM to determine the stress distribution and calculate the stress intensity factors corresponding to the fracture loads recorded from experiments. An introduction to the XFEM method is given in the last part. Chapter 3 gives all the experimental and simulation procedures that were carried out in the present work. Since the bond coat properties need to be compared with their bulk counterparts, both the samples are characterized. The exact material compositions chosen for the study were plain NiAl, 2PtAl and 5PtAl among the pack aluminized coatings and bulk arc-melted PtNiAl samples with varying concentrations of Ni and Pt which matched the bond coat matrix compositions. The choice of the three coatings was made depending on the previously known information regarding their microstructure. The deposition conditions, temperature and times of annealing are listed, followed by a brief summary of the general characterization techniques used to study the microstructure of the bond coats before and after fracture testing. Since the micro-beams under bending were fabricated using a focused ion beam, and the micro-tensile specimen were machined by electro-discharge machining, both the micro-machining procedures are described. At such small length scales, conventional testing methods cannot be used and several modifications were incorporated to the testing geometries which are described in the next section which covers two principal fracture testing methods-microbeam bending and mini-tensile testing, along with the advantages and limitations of each. Modeling is an indispensable tool for determining stress distributions in such new geometric configurations involving material property variations, and details of the exact XFEM procedure that was implemented in ABAQUS is given in the last part of this chapter. Chapter 4 summarises the microstructure and indentation properties of the bond coat and bulk NiAl samples characterised using X-ray diffraction, electron microscopy and nanoindentation. XRD was used for phase identification, texture and determination of lattice parameters of the specimen, which confirmed β-NiAl (with no texture) as the matrix with the lattice parameter varying as a function of composition. The SEM-EPMA combination was used for probing the compositional and microstructural gradients, grain size and precipitate distribution across the coating cross-sections. The bond coat was found to have 4 distinct zones with the Ni:Al ratio gradually rising across its thickness. In addition to this, the four zones had very different grain sizes, precipitate type and distributions. Hardness and modulus values were reported from nanoindentation measurements across the coating thickness over a temperature range from 25 to 400˚C and were seen to follow the composition gradients in different ways based on the effect of the off-stoichiometric defects on these properties. The hardness was found to be a minimum for the zone with stoichiometric composition, as was the case in the bulk sample, while the modulus dropped continuously with increasing Ni content in the matrix. These are important to develop a one-to-one correlation with the fracture properties and to understand the micro-mechanisms of the same. Chapter 5 gets on with the specifics of the testing geometry. Since most of the variables of the testing technique were studied using simulation procedures, a large part of this chapter deals with the results from the modeling technique using XFEM. The XFEM is introduced in detail and its applicability in modeling of cracks and discontinuities and advantages over conventional FEM are explained. The material properties are taken from the nanoindentation data and the modeling assumes linear elastic fracture mechanics. As a validation measurement, a conventional three point beam is modeled in bending and the results compared with analytical solutions of the same. The three point beam bending geometry is also used as a benchmark to study the stability of the new geometry, now with fixed boundaries in place of a free ends. This is followed by the results from the modeling for different variables like mesh density, notch root radius, loading offsets, beam dimensions and crack length (a)/specimen width (W) ratios where both the stress distribution as well as KI are captured in 3-D for stationary cracks while crack trajectories are obtained for propagating cracks. The notch root radius is seen to not affect KI below ~300 nm and such notch radii are easily machinable in the FIB at lower currents. The crack trajectory from the experiments is seen to follow the direction of maximum tangential stress, which is also modeled very well in the XFEM. The contribution of KII to the measured stress intensity factor with increasing offsets is also calculated from the model. Stable cracking is seen for the clamped beam geometry, with KI dropping off beyond a critical a/W ratio. This was true even for a model assuming homogeneous, elastic properties with a flat R-curve under load control. This makes the clamped beam structure require higher loads for continued propagation of cracks. This critical ratio is dimension dependent, making a shorter thicker beam stable in comparison to a longer, slender one. This is unusual, especially in comparison to the three point bend geometry which shows stable cracking only in displacement control, specifically for large a/W ratios alone. Also superimposition of the load-displacement curves from simulations with those of experiments gives a good-fit. The experimental results are shown next to back¬up the claims made on geometric stability of such clamped structures. Digital Image Correlation is introduced as a means for direct measurement of crack opening displacements (COD) and fracture toughness without the aid of KI formulations. This also served as a cross¬check on the assumptions of linear elastic fracture mechanics (LEFM) made in the simulation and a good correlation is seen between the CODs measured experimentally and that obtained from the FEM analysis. Fracture toughness measurements of brittle materials with known KIC values, like fused silica glass and single crystal Si film from this proposed geometry are reported as additional validation of this geometry. Further the capabilities of in-situ testing using this geometry to measure R-curve and fatigue properties along with the initiation KIC values are shown via results from monotonic and cyclic loading under different conditions. Chapter 6 returns to address bond coat fracture at room temperature, which is the main objective of the present study. Fracture toughness is evaluated both ex-situ and in-situ, using clamped microbeam bending experiments across individual zones of the 5PtAl bond coat and for different initial Pt contents in the zone 2. KIC is seen to rise sharply with increasing Ni content of the matrix in the former case, from 5 to 15 MPam1/2 which is attributed to the change in defect chemistry with changing stoichiometry. Al rich NiAl is found to be more brittle due to vacancy hardening while Ni rich NiAl is known to increase the metallic character of the NiAl bond. Both Ni rich and Pt rich (Pt,Ni)Al give higher toughnesses among the coatings studied while the crack trajectories and toughening mechanisms distinctly depend on the precipitate morphology in individual zones. Alloying additions are seen to add to the complexity of the fracture behavior of bond coats by strengthening the matrix or by improving its ductility. Micro-kinking, grain boundary and precipitate bridging are seen in the crack wake as contributing factors to partial closure of the crack on unload. The influence of each of the microstructural variable on the fracture mode is dissected in detail before coming to an overall conclusion. The microbeams show controlled, stable cracking, which enable following of the crack trajectories across micron-length scales and make R-curve measurements possible. Both 2PtAl and 5PtAl compositions show a rising R-curve within the length scale of an individual microbeam tested. Size and geometric effects on real vs apparent R-curve behavior are discussed at the end of the chapter. Chapter 7 addresses a different area of high temperature fracture of bond coats, which becomes relevant in terms of determination of brittle to ductile transition temperature (BDTT) in notched specimen and in evaluating topography after failure across this temperature range. This set of tests is designed to measure fracture toughness and study the fracture mode along the temperature scale to exactly identify the BDTT for a given bond coat composition and strain rate, below which the coating undergoes brittle catastrophic fracture and beyond which it creeps and relaxes plastically at very low stresses. Notched free¬standing bond coat specimens are pulled in uni-axial tension to fracture and the stress at failure is used to calculate the average fracture toughness of the bond coat. The stress-strain curve shows linear elastic behavior upto the BDTT of the bond coat as expected, beyond which it becomes increasingly plastic. The KIC is seen to rise marginally upto 750˚C beyond which it showed a significant increase, from which the BDTT was calculated to be ~775˚C for notched samples. The KIC is not reported beyond the BDTT due to irrelevance of LEFM after macroscopic plasticity sets in. Fracture mode is seen to change from transgranular cleavage below the BDTT to void coalescence and ductile rupture beyond it. The experimental challenges, differences in the through thickness KIC’s obtained from tensile tests vis a vi bend tests (due to changing stress states and size scales), as well as mechanisms of ductile to brittle transition in the context of previously available literature are discussed. Chapter 8 gives the closure and important conclusions from the present work. It summarises the key results from the testing technique and highlights the proposed mechanisms which bring about a rising fracture toughness with both increasing Ni:Al ratio across the bond coat cross-section and across individual micro-beams themselves. Some new techniques and geometries which can be adopted for fracture property determination, on which work was initiated but not complete, are also proposed. The last part of the chapter deals with the future implications of the results found and some open threads and challenges on bond coat optimisiation for different properties, which are yet to be dealt with.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.

The operating efficiency and reliability of modern jet engines have undergone significant improvement largely owing to the advances of the materials science over the past 60 years. The use of both superalloys and TBCs in engine components such as turbine blades has made it possible for jet engines to operate at higher temperatures, allowing an optimal balance of fuel economy and thrust power. Despite the vast improvement in high temperature capability of superalloys, the utilization of TBCs has brought the concern of coating adhesion during their usage. TBCs are prone to spallation failure due to interfacial rumpling, which is driven primarily by thermal coefficient mismatch of the multi-layered structure. Although interfacial degradation of TBCs has been widely studied by detailed numerical and analytical models, the predicted results (i.e. stress state and rumpling amplitude) often deviate from that obtained by experiments. This is largely due to the lack of consideration of the influence of bond coat and substrate chemistry on the interfacial evolution of TBC systems. It is only in recent year that more and more study has been focused on studying the role of chemistry on the interfacial degradation of TBCs. The purpose of this PhD project is to clarify how the bond coat and substrate chemical compositions dictate the mechanisms of interfacial degradation, leading to the final spallation. A cross-sectional indentation technique was utilized to quantitatively characterize the adhesion of oxide-bond coat interface among 5 systematically prepared TBC systems. The adhesion of isothermally exposed oxide-bond coat interface was then correlated with different microstructure parameters, in an attempt to identify the key parameters controlling the TBC spallation lifetime. EBSD and EPMA analyses were conducted on the bond coat near the oxide-bond coat interface, in order to understand the relationship between the key parameters and specific alloying elements. The results clearly demonstrated that the phase transformation of bond coat near the oxide-bond coat interface plays the dominant role in the degradation of interfacial adhesion. Particularly, the co-existence of gamma prime and martensitic phases, each having very different thermomechanical response under thermal exposure, can generate a misfit stress in the TGO layer, and ultimately causes early TBC spallation. In addition, the phase transformation behavior has been closely associated with the inherent chemistry of the bond coat and substrate.

The continuing quest for higher performance levels of modern gas turbine engines has been accompanied by the demand for higher engine operating temperatures. The use of Thermal Barrier Coatings (TBCs) enabled gas turbines to operate at higher temperatures by protecting the blade material (nickel superalloy) while operating in extreme environments. The TBC system typically consists of a bond coat for protection of the nickel–based superalloy against oxidation followed by a top coat consisting of a thermally insulating zirconia-yttria. In addition to the complex gradation in phases, the coatings are subjected to continuous oxidation with service exposure, mechanical loading on rotating parts, fatigue, thermal mis-match and temperature gradients. Hence, the study of failure mechanisms of TBCs become important in deciding operational reliability and service life of the coating. As there are many systems in which the operating temperatures are not high enough to warrant the use of the top coat (ceramic layer), the study of failure mechanisms in superalloys coated with only the bond coat continue to be of great interest. The present work concentrates on the fracture behavior of NiAl bond coats on nickel superalloy and seeks to evaluate the fracture toughness of the coating through the use of micro-machined samples. A review of the relevant literature indicated that while a considerable body of work exists on bulk tests of failure (spalling, splitting, etc.), not much has been reported in the open literature on the evaluation of basic quantities such as the toughness of the coating itself. The present thesis seeks to establish a protocol for the evaluation of toughness and crack propagation mechanisms in coatings through a combination of micro-sample testing that allows fracture to be correlated with location in the film and the use of an analytical model to quantitatively evaluate stress intensity factors in a bi-material system. A system of NiAl coating produced by pack aluminizing is studied for the fracture properties of the coating. Specimen geometries are optimized to enable micro-cracks to be machined and propagated in a low load testing system, such as a depth sensing indenter, so that crack lengths (and position relative to the interface) can be correlated with load. To enable linear elastic theory to be used, dimensions are determined that allow fracture before general yielding. A three point bending test using miniaturized micro-beam specimens of ~ 4 X 0.3 X 0.3 mm is found to be suitable for the above purpose. The technique is a challenging one that requires focused ion beam machining (FIB) along with careful handling and alignment of small samples. The coatings are characterized for their microstructure by electron microscopy to identify compositional variation across the thickness and to determine the thickness of the coating and inter diffusion zone (IDZ). The crack advancement is monitored with increments of loading and the stress intensity factor is evaluated using a program written in “MAPLE” for an edge crack subjected to bending in a bilayered material. Surprisingly, fracture in this system is found to be stable owing to a gradual increase in toughness from the coating surface to the interface. Such an increase from less than 2 to more than 9 MPa m0.5 may be due to the increasing Ni/Al ratio across the thickness of the bond coat. Crack branching is observed as the crack approaches the IDZ and the reasons for such behaviour are not fully understood. This work establishes the viability of this technique to determine fracture properties in highly graded coated systems and may be readily extended to more complex coating architectures and other forms of loading such as cyclic, mixed mode, etc.