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A P23 weld metal showed a severe intergranular cracking at elevated temperatures. This reaction results from the strong segregation of impurities at the grain boundary carbide interfaces and the decrease in the bulk content of W which arises from the partitioning of W into carbides. Meanwhile, the other P23Mo weld metal containing 0.5 wt.% Mo showed a considerable ductility under the same conditions. The primary reasons for this result are the intrinsic grain boundary strengthening effect of Mo and the suppression of both the Fe 2 W Laves phase formation and the partitioning of W into carbides.

The P92 steels were aged at 632°C for 500hrs and 1,000hrs, and creep ruptured at 650°C~625°C with stress of 120~110 MPa. The creep rupture life (CRL) of the aged samples was decreased with the aged time. The microstructure of the P92 steel was observed as fine tempered lath with dispersion of Cr-rich M23C6 along various grain boundaries. Upon aging and creep, recovery of lath and precipitation of coarse W-rich Laves phase were characterized as the main microstructural change. The M23C6 is relatively stable upon the short-term aging and creep. Decrease of the CRL of the aged specimens is considered as the degradations of microstructure such as the recovery of lath due to the dislocation annihilation and precipitation of coarse Laves phase.

The bonding strength of top coat in thermal barrier coatings (TBCs) with different bond layers has been measured with thermal fatigue and its fracture behavior has been investigated considering the effects of a thermally grown oxide (TGO) layer and resintering of the top coat. Two kinds of TBCs with different thicknesses of approximately 80 and 280 μm in bond layer were prepared by two different methods of air plasma spray (APS) and high velocity oxygen flow (HVOF). Top layer (3Y-TZP) was coated on both bond layers using the APS process. Thermal fatigue tests were conducted at temperature of 1100oC with dwell time of 10 hr, which tests were done till 20 cycles. The HVOF process provides a relatively homogeneous microstructure in the bond layer, compared with the APS process. The formation of TGO layer is influenced by the exposed time, independent of the bond layer thickness and the bond layer species. TBCs with the bond layer prepared by the APS process show higher bonding strength than those by the HVOF process in as-prepared TBCs (before thermal fatigue), showing 15 – 18 MPa for the APS process and 13 – 16 MPa for the HVOF process. Whereas the fracture in TBCs with the HVOF bond coat are originated at the interface between the top coat and the bond coat, the fracture within the top coat in TBCs with the APS bond coat However, after the thermal fatigue, the bonding strength value is increased to 18 – 20 MPa in both cases even though the TGO layer is formed, showing the fracture paths within the top coat in both cases.