Surface composites: A new class of engineered materials

To integrate irreconcilable material properties into a single component, a new class of engineered materials termed "surface composites" has been developed. In this engineered material, the second phase is spatially distributed in the near surface regions, such that the phase composition is linearly graded as a function of distance from the surface. Surface composites are different from existing engineered materials such as "bulk composites" and "functionally graded materials" (FGM). Unlike bulk composites, the surface phase in surface composites is present only at the near surface regions. In contrast to FGM, the graded properties of surface composites are achieved by unique morphological surface modification of the bulk phase. To fabricate surface composites, the initial surface of the bulk material is transformed using a novel multiple pulse irradiation techniques into truncated cone-like structures. The laser induced mirco-rough structures (LIMS) possess surface areas which are up to an order of magnitude higher than the original surface. The second phase is deposited on the surface using thin or thick film deposition methods. A key characteristic of surface composites is the formation of a three-dimensional, compositionally and thermally graded interface, which gives rise to improved adhesion of the surface phase. Examples of various types of surface composites such as W/Mo, silica/SiC, diamond/steel, ect. are presented in this paper. The unique properties of surface composites make them ideal engineered materials for applications involving adherent thick film coatings of thermally mismatched materials, compositional surface modification for controlled catalytic activity, and creating adherent metal-ceramic and ceramic-polymeric joints.