Thermal spray is a continuous, directed, melt-spray process in which particles (e.g., 1–50 μm in diameter) of virtually any material are melted and accelerated to high velocities, through either a combustion flame or a dc or rf nontransferred thermal-plasma arc. The molten or semimolten droplets impinge on a substrate and rapidly solidify to form a thin “splat.” The deposit is built up by successive impingement and interbonding among the splats. The splats accumulate into a wellbonded deposit, generally > 10 μm thick.
The segregation of calcium to a (100) cleavage surface of an MgO crystal, with bulk calcium concentration of 200 ppm was measured in situ at T=900" to 1450°C in ultrahigh vacuum, using Auger and low-energy ion-scattering spectroscopies. A measured heat of segregation of approximately -50.3 kJ/mol (-12 kcal/mol) is in favorable agreement with a value of -58.7 kJ/mol (-14 kcal/mol) determined using solute strain energy and surface free energy criteria. The equilibrium value for the calcium segregation between 950" and 1000°C is estimated to correspond to a 20% occupation of the surface cation sites.
presented in Figs. 3(B) and (C), indicate that the phase in bright contrast in Fig. 3(A) contains both yttrium. and aluminum while the phase in dark contrast in Fig. 3(A) is rich in aluminum but devoid of yttrium.The work of Loudjani et al., ' un ublished work of McCune and Donlon, and the YZ03-A1203 phase diagram3 all suggest that the phase in bright contrast is Y3AI5Ol2. Apparently the solubility of yttrium in N203 is extremely low and the dopant was rejected to the melt during crystal growth until the local concentration was sufficient to solidify the eutectic.In summary, these results indicate that even in single-crystal alumina with an yttrium content roughly 2 orders of magnitude lower than that investigated by Loudjani et al. virtually no yttrium goes P into solution. The emission spectroscopy results indicate that the upper limit for the solubility of yttrium is
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