CFR Metal Matrix Composites

Continuous Fiber Reinforced (CFR) Metal Matrix Composites (MMCs)

The primary motivating material system behind development of the University of Virginia's Directed Vapor Deposition (DVD) technology has been the coating of continuous fibers to be used as reinforcement in metal matrix composites. High temperature light weight load bearing aerospace structures such as compressor fan blades for the front section of aircraft engines represent one of the primary areas of application for these difficult to synthesize materials. While companies such as 3M and GE Aircraft Engines have employed techniques such as conventional e-beam evaporation and plasma spraying to create CFR MMCs, each of these methods has drawbacks. Conventional e-beam evaporation suffers from low materials utilization efficiency, with significant quantities of the expensive metal matrix material ending up on the walls of the processing chamber instead of the fibers. To capture as much vapor as possible during its conventional e-beam processing, 3M uses intricate fiber manipulation systems. These fiber manipulation systems frequently break fibers, forcing the coating run to be terminated. Plasma spray technology creates a rough surfaced, fiber containing plasma spray foil. During the hot isostatic pressing cycle which presses these layers together, significant fiber fracture often degrades the properties of the resulting material.

To explore the ability of DVD to create smoothly coated fibers efficiently, an array of 142 mm diameter SCS-6 fibers (supplied by Textron Specialty Materials, Lowell, MA) was set up in a square aluminum frame with an inside edge dimension of 5.08 cm. 25 colinear fibers were mounted vertically with a nominal center-to-center fiber spacing of 2 mm. During each run, the fiber frame itself was covered with aluminum foil to ensure that metal deposition on it was not included in the weight change measurements. For the fiber spacing used, 7.1% of the cross-sectional area of the frame interior was occupied by fibers. Thus, if a uniform flux were incident upon the fibers and deposition occurred only on surfaces that were insight of the flux, the average deposition efficiency (i.e. the efficiency achieved in a conventional e-beam system) would be 7.1%.

The observed chamber pressure/Mach number/deposition efficiency trends were similar to those recorded for flat substrate coating. The first figure below plots the relationship between Mach number, chamber pressure, and relative fiber coating efficiency (compared to that achieved by a line-of-sight deposition process). It should be noted that the deposition efficiency measured in this study is an average value over the entire surface area of the fibers. As a result, maximum deposition rates reached 50 - 100 mm per minute in certain portions of the fiber array while being limited to 1 - 2 mm per minute in other regions.

For many of the experimental conditions investigated, deposition efficiencies were well above line-of-sight levels (i.e. the area fraction occupied by fibers).

As with flat substrate coating, low chamber pressures (low carrier gas fluxes) did not entrain the vapor and transport it towards the fiber array, resulting in low deposition efficiencies. Instead, the main vapor stream crossed the primary carrier gas flow and passed between the e-beam gun and the top of the fiber array. Once the carrier gas flow was increased so that the vapor passed through the frame, deposition efficiencies increased dramatically. Increasing carrier gas flows led to a peak in deposition efficiency (at about 70 Pa) followed by a significant decrease as chamber pressure was raised. This trend suggests that the presence of the observed microwall jet (See the next figure below.) results in vapor atom redirection around the fibers. At pressures near 70 Pa this redirection led to coating around the entire fiber circumference. At higher pressures the microwall jet prevented vapor atoms from reaching the fiber surface, leading to decreased deposition efficiency in much the same manner as the larger wall shock and wall jet deflected vapor away from flat substrates.

At the point of intersection between the vapor-laden carrier gas stream and the array of fibers, a wall shock is observed, demonstrating how the presence of the fibers affects the flowfield.

The fiber coating experiments revealed a peak deposition efficiency over twice the 7.1% expected for line-of-sight deposition. Scanning electron microscopy (See the following figure.) of these samples revealed that, for conditions of maximum efficiency, vapor deposited not only on the surface of the fiber facing the incoming vapor but also on the fiber's sides and its back. The backside fiber coating phenomenon appeared to be a manifestation of atomic scattering from the flow as it passes the fiber, a phenomenon discussed previously by Hill. A demonstration program created by G.A. Bird helps illustrate how backside fiber coating occurs.

Vapor deposition was evident upon the sides (~15 mm thick) and back (~5 mm thick) of this fiber held stationary during the deposition process (M = 1.50, chamber pressure = 66.7 Pa).

Based upon the fiber coating results presented above, DVD appears to have some promise as a fiber coating system since under certain process conditions the system presents a narrow region through which rotated fibers can be passed for uniform, high rate (50 - 100 mm/min.), high efficiency vapor deposition. These results suggest that it may be possible to accomplish industrially acceptable high rate fiber coating using a fairly small, low-cost e-beam gun system, perhaps even a system the size of the experimental setup developed at the University of Virginia.

To assess more fully the ability of DVD to coat fibers in an industrial setting, research should be conducted which will demonstrate the technology's ability to deposit the correct alloy compositions desired for the metal matrix composite fiber application (e.g., Ti-6Al-4V). Single crucible evaporation of this alloy system using DVD will almost certainly experience the same stoichiometry problems observed with conventional e-beam systems. However, use of two in-line vapor targets and the gas jet could quite conceivably overcome this troublesome problem by mixing the two vapor plumes for efficient, compositionally correct deposition (See the next figure below.). Incorporation of a high rate scanning system into the current DVD configuration would allow the beam power to be precisely split between the two targets for stoichiometrically-correct evaporation. Additional research will need to be conducted to investigate the deposition rate around fibers for different elements. It should be noted that the non-line-of-sight coating efficiency of low vacuum systems is a function of the mass of the chamber gas and the vapor atoms. No data currently exists which explains how to obtain a stoichiometrically-correct, industrially-desirable Ti-6-4 deposit in a DVD system. As a result DVD research at the University of Virginia will probably need to demonstrate that the technology can create a large quantity of stoichiometrically-correct clean coating on fibers during a single processing run before industry will seriously consider the technology a viable alternative. To reach this level of processing reliability, the lifetime of the e-beam gun's filament must be extended from the current ten hour limit. In an industrial setting, a tungsten filament lifetime of 100 - 250 hours will almost certainly be required.

Stoichiometrically-correct deposition using this DVD system configuration could be much faster and more efficient than that achieved in a conventional multicrucible e-beam evaporation system. Only mixed material in region (AB) can create a compositionally correct alloy.