The mechanical response of plasma-sprayed coatings is studied by recourse to instrumented spherical micro-indentation. It is shown that the elastic moduli of the coatings can be determined in a more reliable and reproducible manner with instrumented spherical indentation than prior (Knoop) indentation methods, while at the same time minimizing damage to the coating during indentation. The results show that the values of the elastic moduli, E, of air plasma-sprayed (APS) materials are much lower than those of vacuum plasma-sprayed (VPS) materials. The VPS deposits are more homogeneous at a microstructural level as splat formation, microcracking and the development of oxide scales are suppressed. Such homogenization increases the value of E to the point where it becomes similar to that of fully dense materials. Larger amounts of inelastic flow occur in APS deposits as compared with VPS deposits. This flow is associated with fracture and sliding of splats.
Dramatic effects of S on the adhesion and fatigue resistance of interfaces between ␥ -Ni(Cr) and ␣ -Al 2 O 3 , have been explicitly demonstrated and quantified. This has been achieved by using two bonding conditions: one involving solid-state diffusion (SSDB) and another through a liquid phase (LPB) that forms at temperatures above a eutectic that releases a S-rich liquid. Upon SSDB, there is no significant S excess at the interface, whereas LPB forms a thin interphase with a high local concentration of S. The SSDB materials have interfaces with such high toughness (above 300 J m Ϫ2 ) and fatigue resistance that mode I cracks divert into the Al 2 O 3 , rather than propagate at the interface. Conversely, the LPB materials delaminate at the interface, solely as a result of the residual stresses from thermal expansion misfit, with an interface toughness in the range 2 to 7 J m Ϫ2 .
Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina interfaces. The LPB interfaces contain voids and dendritic FeAl 3 precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both interfaces are strong and tough. Upon cyclic loading, mode l cracks in both systems grow alternately along the interface and within the Al. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the Al rather than along the interface. Conversely, for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the Al. Correlation between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting.
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