PLASMA-SPRAYED ALUMINUM AND TITANIUM ADHERENDS - II - DURABILITY STUDIES FOR WEDGE SPECIMENS BONDED WITH POLYIMIDE ADHESIVE

The durability of plasma-sprayed metals bonded with a polyimide adhesi ve has been studied. Metal adherend surfaces were prepared for adhesiv e bonding by plasma-spraying inorganic powders on aluminum and titaniu m. The plasma-sprayed materials included Al2O3, AlPO4, MgO, and SiO2 o n aluminum, and TiO2, TiSi2, MgO, and SiO2 on titanium. The coatings w ere sprayed at two different thicknesses. Durability studies of sample s prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to t he conditions; low temperature, -20 degrees C; relative humidity at el evated temperature, 70% RH at 66 degrees C; elevated temperature (160 degrees C) in air, high temperature (160 degrees C) in vacuum (130 tor r, 0.2 atm.), and room temperature. Crack growth rate and mode of fail ure were determined. The results of the durability tests indicate that thin coatings (25 mu m) of plasma-sprayed materials perform better th an thicker (150 mu m) coatings. The crack growth rate for thin coating s (25 mu m) of Al2O3, AlPO4, SiO2, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarl y, the durability performance for titanium samples prepared with a 25 mu m-thick TiO2, TiSi2, and SiO2 plasma-sprayed coatings was equivalen t to that for a Turco(R) -prepared titanium surface. Although the eval uation of durability as a function of surface chemistry was an objecti ve of the study, it was not possible to evaluate the effect, since mos t failures occurred within the adhesive (cohesive failure) during the environmental tests. That Failure occurred in the adhesive indicates t hat the coating-adherend and the coating-adhesive interactions are suf ficiently robust to pre vent interfacial failure under the experimenta l conditions investigated.