Surface Preparation of Alumina for Improved Adhesive Bond Strength in Armor Applications
Surface treatments of alumina have been investigated with the aim of increasing the strength of the bond created between the alumina and a toughened epoxy adhesive. Four surface conditions have been assessed: as-fired; grit blasted; and krypton fluoride excimer laser treated under two sets of conditions. Compared with the as-fired surface, the grit blasted surface was rougher with poorer wettability, probably due to surface contamination. It was found that the laser treatments removed some of the sintering additives and caused rounding of the alumina grains, slightly increasing the surface roughness. Further, the laser treatment led to an increased surface energy and wettability, which has been linked tentatively to an observed increase in the hydroxyl groups on the surface. The adhesive bond strength was assessed by testing joints in tension and shear. It was found that the laser treated surfaces demonstrated slight improvements in bond strength, with a cohesive failure of the adhesive in tension for surfaces subjected to one of the two laser treatments, compared with failure at the interface for the as-fired, grit blasted and other laser treated samples in tension and for all samples in shear. Thus, it has been demonstrated that modifications to the surface of alumina can result in mechanical and chemical changes which affect roughness, wettability, bond strength and the locus of failure.
Enhancement of ceramic-to-metal adhesive bonding by excimer laser surface treatment
Abstract A silicon nitride (Si 3 N 4 ) based ceramic cutting material and a structural alloy steel (SAE 4340) were surface-treated using a 800 mJ KrF excimer laser with an aim to improve the ceramic-to-metal bond strength. For these two materials, the effects of laser energy density and the number of pulses upon the surface morphologies of the laser-treated surfaces to be joined were examined. Conical surface microstructures were generally observed on the laser-treated surfaces of the Si 3 N 4 material, and the number of conical features was found to be significantly influenced by the laser energy density. Moreover, the results of XPS have shown that the surface chemistry of the ceramic was altered after being laser treated. On the other hand, excimer laser treatment had caused the alloy steel surfaces be melted and resulted in some “beach-mark” features. However, the laser energy density has little effect on the overall surface morphology and the roughness of the treated surfaces. Shear tests were performed on adhesive bonded samples of the laser-treated ceramic and alloy steel. Significant improvement in adhesion shear strength was obtained for the laser-treated samples as compared with those tested in the as-received and mechanically abraded conditions. The required laser operation condition for achieving good shear joint strength was discussed.
Composite monolayer fabrication by an arc-spray process
A single layer (monotape) technique for fabricating complex high-temperature tungsten-fiber-reinforced superalloy composites is proposed. The fabrication of sheets of arc-sprayed monotape 38 cm wide and 122 cm long has been demonstrated. Composites fabricated using the method are shown to have equal tensile strength and a cleaner matrix than composites fabricated from powder metal cloth monotapes, and the present technique is less expensive than the powder metal fabrication techniques.