One of the more promising techniques for the production of metal matrix composites uses a physical vapor deposition step for applying metals and alloys to the reinforcing ceramic fibers. A patented new vapor deposition technique called jet vapor deposition (JVD) employs an inert carrier gas at low vacuum (~10 Torr) to entrain and accelerate evaporated metal. The carrier gas and metal vapor form a jet which impinges on a substrate, where the metal deposits. This process holds the potential to deposit metal at high rates, high efficiency, and with inexpensive equipment. This study examined JVD onto ceramic fibers. As a model system, collinear Al₂O₃ fiber arrays were coated with copper using helium as the carrier gas. The fibers were coated at a 4 Torr chamber pressure with jets of five velocities, from M=0.6 to M=1.5. The mass capture efficiency of the fibers was found to be greater than the cross-sectional area occupied by the fibers. The fibers, which were held perpendicular to the flow, were found to have thorough coating around their entire circumference; copper deposited on the fibers at a location directly opposite the evaporant source. On the systematically investigated samples, the downstream coating was only 1-3 mm compared to 10-60 mm on the front. However, a single sample coated at a higher working distance, chamber pressure, and evaporation rate displayed 10-20 mm downstream coating, with 50-150 mm on the front. The coating thickness was also found to be very dependent on the radial position of the fiber in the jet. At high jet velocities, the maximum thickness was found near the center of the jet. At subsonic jet velocities, the maximum thickness was located 2-3 nozzle radii away from the jet center. The deposited coating was found to have two characteristically different coating morphologies. Type I, found near the jet center, had a smooth, elliptical cross-sectional profile. Type II, found in outlying regions, had an abrupt change in coating thickness -- a coating "treeline" -- near the fiber equator. Both displayed a columnar microstructure consistent with the structure-zone model of growth, although Type II was considerably more porous.

This study employed several analysis methods to suggest explanations for the above findings. Computational fluid dynamics calculations were used to evaluate the flow patterns of both the helium jet and flow around the cylindrical fibers. Lateral diffusion across streamlines was proposed to account for the high mass efficiency and the thorough coating. A residence-time model, also based on a diffusional deposition mechanism, was established which assumed stagnation-point deposition rates were inversely proportional to velocity. This model was able to qualitatively predict the observed coating thickness distributions, including the maxima located outside the jet at subsonic jet velocities. The structure-zone model and aspects of inertial deposition suggested that the treeline of Type II coating morphology was due to a non-negligible inertial component of deposition. Vapor-phase condensation of clusters was proposed as a mechanism for that inertial component, and theoretical calculations indicated that cluster formation would be pronounced only in the low-velocity jet fringes, where Type II morphology was found.