MEASUREMENTS OF RESIDUAL-STRESS AND BOND STRENGTH OF PLASMA-SPRAYED LAMINATED COATINGS

Laminated coatings of Al2O3-NiCrAl and Al2O3+13 wt% TiO2-NiCrAl were d eposited on low carbon steel substrates by plasma spraying. The bond s trengths of the coatings were measured by pull-off test, and the resid ual stresses in the outermost surface layers were measured by X-ray di ffraction. The effect of additive SiO2 on bond strength and stress of the coatings in the two systems was compared with the results of our p revious investigation of the ZrO2 system. The nature of liquid phase s intering of additive SiO2 in ceramic coatings is elucidated. The bond and transition layers in the laminated coatings can largely overcome t he inherent mechanical and thermal incompatibilities between the oxide coating and metal substrate, therefore the residual stress can largel y be relaxed and the coatings will possess good bond strength. However , when an appropriate additive is doped into refractory ceramic coatin gs, contact between ceramic and substrate, as well as within the coati ng, would be developed. Thus, the bond strength and stress can be furt her improved. Introduction of additive results in liquid phase sinteri ng. The effect of additive of lower melting point would be very obviou s in a system which possesses a higher melting point, such as the SiO, -doped ZrO2 coating. However, this effect would not be so apparent in ceramic coatings which possess a lower melting point, such as Al2O3 an d Al2O(3) +13 wt% TiO2 systems. Because the molten droplets can stay l ong enough for spreading and full flattening, the contact can reach ra ther 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, th en no additive is necessary.