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Title: A study of bond coat cracking in TBCs for turbine aerofoil applications
Author: Fox, M. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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This dissertation describes work on the thermomechanical fatigue (TMF) behaviour of coated single crystal, nickel-based superalloys. During the course of study, the TMF facility at Cambridge was developed to enhance the reliability of the test technique and to enable the testing of thin ceramic shell TBC test pieces. Benchmark TMF tests were carried out under strain control on a series of different coatings to evaluate the performance of two new coat systems developed at Rolls-Royce that incorporate a platinum surface modification. Cyclic life to failure suggested that the surface modification had minimal effect on TMF performance. The results compared favourably with data from different research centres as well as the lifting model used at Rolls-Royce to predict high temperature coating mechanical integrity (the Tcrit model). From this model, the effect of different TMF cycle directions was considered. Experimental tests on aluminide and overlay coated specimens did not support the Tcrit predictions in terms of cyclic life to failure, although crack density analysis would seem to provide a more accurate correlation between the TMF cycle experienced and the coating performance. A separate finite element (FE) approach to predict the stress/strain history in an overlay coated specimen was developed in order overcome some of the limitations of the Tcrit method. Incorporating developments in the experimental technique, TMF tests on the modified overlay bond coat with and without a thin shell ceramic top coat, were performed. The FE model was extended to include this ceramic in order to investigate the constraint of the bond coat. The level of constraint generated does not seem to support the effect that a thin shell ceramic has on suppressing the surface undulations and crack densities observed in TMF testing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available