Sang Ha Leigh

Tensile toughness test and high temperature fracture analysis of thermal barrier coatings

Abstract In this paper, an effective fracture toughness test which uses interface fracture mechanics theory is introduced. This method is ideally suited for determining fracture resistance of multilayered thermal barrier coatings (TBCs) consisting of ceramic and bond layers and, unlike other fracture experiments, requires minimal set-up over a simple tensile adhesion test. Furthermore, while other test methods usually use edge cracked specimens, the present test models a crack embedded within the coatings, which is more consistent with actual TBCs where failure initiates from internal voids or defects. The results of combined computational and experimental analysis show that any defects located within the ceramic coating can significantly weaken a TBC, whereas the debonding resistances of the bond coating and its interfaces are found to be much higher. In a separate analysis, we have studied fracture behavior of TBCs subjected to thermal loading in a high temperature environment. The computed fracture parameters reveal that when the embedded crack size is on order of the coating thickness, the fracture driving force is comparable to the fracture resistance of the coating found in the toughness test. In addition, the major driving force for fracture derives from the thermal insulating effect across the crack faces rather than the mismatch in the coefficients of thermal expansion. We have also investigated the effects of functionally graded material (FGM) within TBCs and found its influences on the fracture parameters to be small. This result implies that the FGM may not contribute toward enhancing the fracture toughness of the TBCs considered here.

Evaluation of off-angle thermal spray

Abstract The effect of spray angle on the properties of plasma spray deposits has been investigated. Two different materials, NiAl and Cr 3 C 2 –NiCr, were sprayed at angles from 50 to 90° with respect to the substrate. The spray angle influenced the microstructure and the properties of the deposits. The porosity increased as the spray angle decreased, i.e. as the angle was shifted away from the perpendicular position. The surface roughness of the Cr 3 C 2 –NiCr deposit was not sensitive to the spray angle, whereas NiAl exhibited an increase as the spray angle decreased. Microhardness, tensile adhesion strength, and interfacial fracture toughness decreased with spray angle. The change of these properties with spray angle is attributed to (1) the morphology of the splats, (2) the change of the local spray angle, and (3) the change of momentum of particles impacting on the substrate or previously deposited particles. The mechanical-property variations arise from porosity changes which occur from the spray angle variations.

Microstructural Index to Quantify Thermal Spray Deposit Microstructures Using Image Analysis

The basic metallographic analysis leads to qualitative interpretation of the structural characteristics of a microstructure, for example the presence of phases, and the description of singularities such as inclusions. On the contrary, microstructural characterization which implements image analysis leads to a quantified analysis of structural characteristics. A method is described to assess thermal spray deposit microstructures using image analysis by means of a metallographic index. This index is based on the determination of several stereological and morphological parameters by primary referee to the size-shape distributions of the features, the fractal dimension of the deposit upper surface, and the Euclidean distance map of the bodies of interest. This work employs quantitative metallography on a much wider scale to provide better quality control of deposit microstructures.

A Test for Coating Adhesion on Flat Substrates-A Technical Note

The standard tensile adhesion test (TAT), ASTM C633, has been modified to perform multiple tests on flat and wide substrates. The TAT geometry, which specifies a 1-in. (25.4-mm)-diameter cylindrical substrate, has been used as the pull-off bar. Two renditions of this test were implemented and the Weibull moduli and characteristic stresses for both test methods obtained. The modified TAT, termed as the single-bar (SB) method, yields a higher Weibull modulus and characteristic strength than the other method, which is termed the double-bar (DB) method. It is believed that the different test results between the two methods arise from different stress distributions near the interface of the coating and substrate.

Acoustic emission responses of plasma-sprayed alumina-3% titania deposits

Abstract Free standing alumina-3% titania samples were prepared by gas- and water-stabilized plasma (GSP and WSP) spraying. These free standing forms consisted of metastable γ - and δ -alumina, as well as α -Al 2 O 3 . The microstructures of the materials were similar for both plasma spraying methods and typically exhibit distinct boundaries around splats, columnar grains, and inter- and intra-lamellar porosity. The modulus of rupture (MOR) and porosity were also similar for both GSP- and WSP-sprayed specimens. Four point bend tests with in situ acoustic emission (AE) were used to study the cracking behavior of these materials. Catastrophic failure was observed for GSP-sprayed samples while WSP-samples exhibited microcracking before failure. The amplitude and energy distributions were also investigated and it was thereby determined that the percentage of macrocracks for GSP-sprayed samples is much larger than for WSP-sprayed samples. The processing effects, which induced differences in microstructure, can account for the differences in AE responses.

Characterization of cracking within thermal spray deposits by an acoustic emission method: extended abstract

Four-point bend tests with in situ acoustic emission (AE) were used to study the cracking behavior of alumina-3% titania materials prepared by gas- and water-stabilized plasma technologies (GSP and WSP, respectively). Catastrophic failure was observed for GSP-sprayed samples while WSP-sprayed samples exhibited microcracking before failure. The amplitude and energy distributions were also investigated, and it was determined that the percentage of macrocracks for GSP-sprayed samples is much larger than for WSP-sprayed samples. The processing effects, which induced differences in microstructure, may account for the differences in AE responses.

Investigation of cracking mechanisms of plasma sprayed alumina-13% titania by acoustic emission

Free standing alumina-13% titania samples were manufactured using high power water stabilized plasma spraying. Heat treatment was performed at 1,450 C for 24 hours and then at 1,100 C for another 24 hours. Four point bend tests were performed on the as-sprayed and heat-treated samples in both cross section and in-plane orientations with in situ acoustic emission monitoring to monitor the cracking during the tests. Catastrophic failure with less evidence of microcracking was observed for as-sprayed samples. Energy and amplitude distributions were examined to discriminate micro- and macro-cracks. It was found that the high energy (>100) and high amplitude (say > 60 dB) responses can be characterized as macro-cracks. Physical models are proposed to interpret the AE responses under different test conditions so that the cracking mechanisms can be better understood.