Specimen Current Imaging of Delamtnation in Ceramic Films on Metal Substrates in the SEM
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at. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
of the rim during spreading resulting in fingerlike projections. This effect shows a gradual transition with increasing substrate temperature. Table 1: Plasma Spray Parameters Gun - Power
PTF4: 650A, 69 V, 45 kW
Spraying gases
Ar 40 slpm*, H2 10 slpm
Spraying distance
120 mm
Powder
crushed and fused YSZ (7 wt.% Y2 0 3 )
Powder Size
+10 - 62 [tm
Powder feed rate
2g/minute
Carrier gas
Ar/N2 at 2.5 slpm
*standard liter per minute
RESULTS Scanning electron microscopy has been used extensively in this investigation for characterizing the cracking of quenched ceramic splats. The secondary electron images exhibit regions of bright contrast at the cracked edges (Fig. 1). Closer inspection (Fig. 1 inset) indicates a gradual increase in brightness over a wider region in the periphery of every cracked piece of the ceramic splat. The effect has been enhanced in the inset by posterizing using an image processing operation that assigns singular gray scale values to blocks of gray ranges.
Fig. 1. Secondary electron image of a typical YSZ splat on stainless steel showing charging at cracked edges. Enhanced inset shows the variation of brightness within a piece of cracked film The ceramic film gets charged in the SEM as it is a good insulator, but because of its small thickness (2 lim) and close proximity of a conducting substrate this does not create distortions in the imaging of the splats at accelerating potentials of 15 and 30 keV for the secondary electron and the backscattered electron imaging modes. However, presence of charge on the surface of insulators is known to affect emission of electrons from the surface used in imaging [5]. The charging of the insu112
lating ceramic in the splat seems to vary from regions in contact with the substrate to delaminated regions. The regions of the ceramic film in contact with the conductive substrate can loose some charge to ground through the specimen stage, whereas the delaminated regions of the thin film that are not in contact with the conductive substrate must hold more of the charge. Images of the splats obtained using a specimen current amplifier (Fig. 2) capturing current flowing to earth record a contrast that is complementary to the secondary electron images. The central regions of the ceramic splat pieces appear bright as they are in contact with the substrate and conduct the electrons as signal to the specimen current amplifier. The periphery of these thin films appears dark as the charge is not conducted to the substrate. Thus we have a technique to delineate regions of delamination in insulator thin films on conducting substrates.
Fig. 2 Specimen current image of YSZ splat on stainless steel substrate showing inverse of secondary electron image contrast.
We have obtained a clear indication of increasing contact area with increasing substrate temperature. Fig. 2 shows a splat obtained on a substrate with a thin oxide film obtained during a preheat, at 100*C. The low area of contact is s
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