Plasma-Sprayed Aluminum and Titanium Adherends: II. Durability Studies for Wedge Specimens Bonded with Polyimide Adhesive

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Abstract
The durability of plasma-sprayed metals bonded with a polyimide adhesive has been studied. Metal adherend surfaces were prepared for adhesive bonding by plasma-spraying inorganic powders on aluminum and titanium. The plasma-sprayed materials included Al2O3, AlPO4. MgO, and SiO2 on aluminum, and TiO2, TiSi2, MgO, and SiO2 on titanium. The coatings were sprayed at two different thicknesses. Durability studies of samples prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to the conditions; low temperature, − 20°C; relative humidity at elevated temperature, 70% RH at 66°C; elevated temperature (160°C) in air, high temperature (160°C) in vacuum (130 torr, 0.2 atm.), and room temperature. Crack growth rate and mode of failure were determined. The results of the durability tests indicate that thin coatings (25 μm) of plasma-sprayed materials perform better than thicker (150 μm) coatings. The crack growth rate for thin coatings (25 μm) of Al2O3, AlPO4, SiO2, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarly, the durability performance for titanium samples prepared with a 25 μm-thick TiO2, TiSi2, and SiO2 plasma-sprayed coatings was equivalent to that for a Turco®-prepared titanium surface. Although the evaluation of durability as a function of surface chemistry was an objective of the study, it was not possible to evaluate the effect, since most failures occurred within the adhesive (cohesive failure) during the environmental tests. That failure occurred in the adhesive indicates that the coating-adherend and the coating-adhesive interactions are sufficiently robust to prevent interfacial failure under the experimental conditions investigated.

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