SURFACE COMPOSITES - A NEW CLASS OF ENGINEERED MATERIALS

To integrate irreconcilable material properties into a single componen t, a new class of engineered materials termed ''surface composites'' h as been developed. In this engineered material, the second phase is sp atially distributed in the near surface regions, such that the phase c omposition is linearly graded as a function of distance from the surfa ce. Surface composites are different from existing engineered material s such as ''bulk composites'' and ''functionally graded materials'' (F GM). Unlike bulk composites, the surface phase in surface composites i s present only at the near surface regions. In contrast to FGM, the gr aded properties of surface composites are achieved by unique morpholog ical surface modification of the bulk phase. To fabricate surface comp osites, the initial surface of the bulk material is transformed using a novel multiple pulse irradiation technique into truncated cone-like structures. The laser induced micro-rough structures (LIMS) possess su rface areas which are up to an order of magnitude higher than the orig inal surface. The second phase is deposited on the surface using thin or thick film deposition methods. A key characteristic of surface comp osites is the formation of a three-dimensional, compositionally and th ermally graded interface, which gives rise to improved adhesion of the surface phase. Examples of various types of surface composites such a s W/Mo, silica/SiC, diamond/steel, etc. are presented in this paper. T he unique properties of surface composites make them ideal engineered materials for applications involving adherent thick film coatings of t hermally mismatched materials, compositional surface modification for controlled catalytic activity, and creating adherent metal-ceramic and ceramic-polymeric joints.