The synthesis and characterization of model interface couples for inorganic matrix composite applications. Ph.D. Thesis
The fracture toughness of fiber reinforced composites may be optimized without unwarranted loss of transverse strength through control of the interface strength between reinforcing material and matrix. These design considerations, including limits on the permissible interface strengths, have been delineated by others. This present work examines the effect of carbon interlayers on the tensile strength of flat interface couples representing the interface between fiber and its coating. Polished single crystal sapphire wafers, 1.0 inch in diameter and 0.5 mm thick, were substituted for the fiber; relatively thick, 2 micron, SiC coatings are substituted for the thin coatings used in composite materials; and 2000 A carbon interlayers, being the weakest material present, act as mechanical fuses to control the overall strength of the entire system. As the SiC coatings had to be relatively thick, homogenous, and nearly stress-free, a study of the deposition of SiC by plasma enhanced chemical vapor deposition (PECVD) was conducted. Coatings with a nearly zero residual stress were deposited at 275 C, 50 micron pressure, and 30 watts applied power with the lower electrode self-biased at +3 volts. Uniform deposition was ensured through the use of hydrogen as a dilution gas. The carbon interlayers processed by PECVD at 400 C deposited on sapphire were found by high resolution TEM to be amorphous or microcrystalline carbon depending on the applied rf power. Carbon deposited on microcrystalline SiC by low pressure CVD (LPCVD) between 1100 and 1500 C was determined by x-ray diffraction to be highly oriented in structure with the degree of preferred orientation increasing from a Bacon anisotropy factor of 4.5 for material deposited on SiC at 1100 C to 6.8 for material deposited at 1500 C.
- Research Organization:
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
- OSTI ID:
- 264022
- Report Number(s):
- N-96-23756; NIPS-96-08274; TRN: 9623756
- Resource Relation:
- Other Information: TH: Ph.D. Thesis; PBD: Jan 1994
- Country of Publication:
- United States
- Language:
- English
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