Development and characterization of new coatings for improved balance of mechanical properties of titanium matrix composites

The need for improved transverse performance of titanium matrix composites (TMCs) has stimulated efforts at developing fiber–matrix coatings with a better blend of transverse and longitudinal properties. In the first part of this paper, the critical requirements for an appropriate coating are examined, based on experience gained from single-fiber and single-ply tests on TMCs. Two new coatings are then discussed: (i) the Ti-Y-Ti system, abbreviated as TYT; and (ii), the sheathed SiC system. The coatings were characterized under transverse loading using the cruciform specimen geometry, and under longitudinal tension using a single-ply dog-bone geometry. The transverse strength of the TYT system was ≈160 MPa, and needs further improvement. The fiber had poor in situ strength, and is explained based on less than expected performance of the Y layer due to oxygen pickup and columnar grain structure. The sheathed fiber offered a better blend of transverse and longitudinal properties. Under transverse loading, the sheath remained load bearing until an applied stress as high as 550 MPa, compared with only 330 MPa for SCS-6 fibers. Under longitudinal tension, while the sheath cracked at relatively low stresses, the base SiC region appeared to retain a strength that was identical to a virgin carbon-coated Trimarc fiber. The sheathed fiber appears attractive for TMC laminates, and further experimental validation is suggested.