Measurements of residual stress and bond strength of plasma sprayed laminated coatings

Abstract

Laminated coatings of Al 2O 3-NiCrAl and Al 2O 3 + 13 wt% TiO 2-NiCrAl were deposited on low carbon steel substrates by plasma spraying. The bond strengths of the coatings were measured by pull-off test, and the residual stresses in the outermost surface layers were measured by X-ray diffraction. The effect of additive SiO 2 on bond strength and stress of the coatings in the two systems was compared with the results of our previous investigation of the ZrO 2 system. The nature of liquid phase sintering of additive SiO 2 in ceramic coatings is elucidated. The bond and transition layers in the laminated coatings can largely overcome the inherent mechanical and thermal incompatibilities between the oxide coating and metal substrate, therefore the residual stress can largely be relaxed and the coatings will possess good bond strength. However, when an appropriate additive is doped into refractory ceramic coatings, contact between ceramic and substrate, as well as within the coating, would be developed. Thus, the bond strength and stress can be further improved. Introduction of additive results in liquid phase sintering. The effect of additive of lower melting point would be very obvious in a system which possesses a higher melting point, such as the SiO 2-doped ZrO 2 coating. However, this effect would not be so apparent in ceramic coatings which possess a lower melting point, such as Al 2O 3 and Al 2O 3 + 13 wt% TiO 2 systems. Because the molten droplets can stay long enough for spreading and full flattening, the contact can reach rather a good state. As a result, a suggestion is put forward that when the melting point of a ceramic coating is high, an additive should be introduced; and when the melting point of a sprayed coating is low, then no additive is necessary.