| Thermal barrier coatings (TBCs) are commonly used in commercial gas turbine engines where the technological benefit of a TBC is derived from its ability to sustain high thermal gradients while maintaining structural integrity in an extremely hostile environment. One of the key components in a thermal barrier coating system is the intermetallic bond coat. There are two major types of commercial bond coats: diffusion aluminide (Ni,Pt)Al bond coats and plasma sprayed MCrAlY overlay bond coats. The thermal cyclic durability of a TBC relies strongly on the physical and mechanical properties of the bond coat layer, but the mechanical properties of diffusion bond coats have not been previously measured because of limitations related to bond coat geometry.; In this thesis, high temperature microsample tensile testing has been employed to characterize the mechanical properties (coefficient of thermal expansion (CTE), Young's modulus (E), yield strength (σy) and stress relaxation) of three platinum modified nickel aluminide bond coats at different stages of their cyclic lives in the temperature range of 25 to 1150°C. The mechanical properties of (Ni, Pt)Al bond coats are found to be temperature-dependent and to alter dynamically in accordance with the microstructural evolution and chemistry changes that occur with thermal cycling.; The Young's modulus of (Ni, Pt)Al bond coats is found to be in the range of 120-200GPa at room temperature, which is in good agreement with the NiAl results. The elastic modulus of (Ni, Pt)Al decreases slightly with temperature in a nearly linear manner and may change as a result of thermal cycling. The CTE of (Ni, Pt)Al bond coats has been measured to increase slightly from 15 to 17 ppm °C−1 in the temperature range of 400 to 850°C.; Above 600°C, the elevated temperature strength is found to decrease rapidly with increasing temperature, reaching values of 10∼25 MPa at 1150°C. Thermal cycling has a profound effect on the strength of the platinum aluminide bond coats. The intermediate temperature yield strength is found to increase dramatically as a result of thermal cycling, which is related to a martensitic transformation and the formation of gamma-prime. By contrast, the high temperature strength is found to remain approximately the same, resulting from the restoration of the B2 parent phase at high temperatures. Differences in the mechanical behavior of nominally identical (Ni, Pt)Al bond coats have been attributed to differences in stoichiometry, Pt content and run-to-run variation in the fabrication processes. The addition of Pt and the formation of γ ′ also appear to strengthen the bond coat. Attempts to strengthen the bond coat through the introduction of Hf by the use of Hf-rich substrates were undermined by the fact that the Hf was not incorporated into the β-phase of the bond coats.; Power-law creep was found to govern the elevated temperature stress relaxation behavior of diffusion aluminide (Ni, Pt)Al bond coats, and empirical equations for as-fabricated and thermally cycled (Ni, Pt)Al bond coats have been derived. Variations in the activation energy for creep and the creep resistance point to changes in the creep mechanism, which may be related to the emergence of martensite phase, the formation of γ′ during thermal cycling, stoichiometry and Pt content. Comparison with creep strengths of two MCrAlY bond coats suggest that the (Ni,Pt)Al bond coats generally possess a higher creep strength than MCrAlY bond coats. |
