George A. Graves

Interfacial characteristics of composites fabricated by laminated object manufacturing

Abstract This paper examines interfacial issues that arise when fabricating ceramic (SiC/SiC) and polymer matrix (glass/epoxy) composites using a novel, fully automated rapid prototyping method called Laminated Object Manufacturing (LOM). Discussed in this report are the three types of interfaces that are of concern in composites fabricated with the LOM system: interlaminar, particle–matrix in particulate composites, and fiber–matrix in fiber-reinforced composites. In order to obtain good interlaminar adhesion, it was necessary to address three issues: material preforms, LOM processing parameters, and high-temperature post-processing techniques. Developments in each of these areas led to the successful production of high-performance composites using the LOM process.

Development of a curved layer LOM process for monolithic ceramics and ceramic matrix composites

A novel rapid prototyping technology incorporating a curved layer building style was developed. The new process, based on laminated object manufacturing (LOM), was designed for efficient fabrication of curved layer structures made from ceramics and fiber reinforced composites. A new LOM machine was created, referred to as curved layer LOM. This new machine uses ceramic tapes and fiber prepregs as feedstocks and fabricates curved structures on a curved‐layer by curved‐layer basis. The output of the process is a three‐dimensional “green” ceramic that is capable of being processed to a seamless, fully dense ceramic using traditional techniques. A detailed description is made of the necessary software and hardware for this new process. Also reviewed is the development of ceramic preforms and accompanying process technology for net shape ceramic fabrication. Monolithic ceramic (SiC) and ceramic matrix composite (SiC/SiC) articles were fabricated using both the flat layer and curved layer LOM processes. For making curved layer objects, the curved process afforded the advantages of eliminated stair step effect, increased build speed, reduced waste, reduced need for decubing, and maintenance of continuous fibers in the direction of curvature.

PHYSIOCHEMICAL CHARACTERISTICS OF ALUMINO-CALCIUM PHOSPHOROUS-OXIDE (ALCAP) CERAMICS

ABSTRACT Ceramics composed of aluminum, calcium and phosphorous oxides (ALCAP) were incubated for seven days in human plasma at 37°C. Incubation of ceramics in plasma did not induce morphological changes in the starting particle sizes of the ceramics. ALCAP ceramics significantly decreased the hydrogen ion concentration of plasma and the pH shifted from 7.34 ± 0.15 to 8.27 ± 0.03 by the end of the seven day incubation period. Significant amounts of calcium and phosphorous were transferred from the ceramic to the plasma. Incubation of ceramics in plasma significantly changes the aluminum content of the plasma. Results of this investigation suggest that ALCAP ceramics are composed of two oxide phases, one more reactive (resorbable) than the other.

LANTIRN infrared window failure analysis

Analysis of 68 condemned LANTIRN navigation pod FLIR windows was undertaken to determine the nature and extent of damage to these windows. Visual and low-magnification examinations using reflected and transmitted light conditions were performed, as well as profilometry and scanning electron microscopy (SEM) examination on selected specimens. A number of primary modes were found which accounted for the majority of the failures seen in this population of windows. These modes were: high energy impacts due to large objects such as hail, birds, and runway debris; interaction of the residual stress state at the interface of the bulk ZnSe/ZnS coating with rain and bug strikes; and opacification due to sand erosion and atmospheric etching. Machining damage and misoriented window installation were also found. Windows which had seen appreciable hours of service were almost completely devoid of AR coating on the forward face. A navigation pod, which houses the window, was also obtained to determine if the window installation contributed to the causes of failure. Suggestions to improve the reliability of the present window material were listed.