Engineering topics

Farmer, S. C.

Publications and source records attributed to Farmer, S. C..

7 records

NASA NTRS2005

Wear of Selected Oxide Ceramics and Coatings

The use of oxide ceramics and coatings for moving mechanical components operating in high-temperature, oxidizing environments creates a need to define the tribological performance and durability of these materials. Results of research focusing on the wear behavior and properties of Al2O3/ZrO2 (Y2O3) eutectics and coatings under dry sliding conditions are discussed. The importance of microstructure and composition on wear properties of directionally solidified oxide eutectics is illustrated. Wear data of selected oxide-, nitride-, and carbide-based ceramics and coatings are given for temperatures up to 973K in air.

Miyoshi, K.↗

NASA NTRS1994

Elevated-Temperature Deformation Properties of a HfC Modified Ti-48Al-2Mn-2Nb Matrix Particulate Composite

Rapid solidification techniques in combination with HIPing have been used to produce Ti-48Al-2Mn-2Nb and a Ti-48Al-2Mn-2Nb+15 wt% HfC composite. While the composite does contain several second phases within the gamma + alpha(sub 2) matrix, none was identified to be HfC. The elevated-temperature properties were determined by constant velocity compression and constant load tensile testing in air between 1000 and 1173 K. Such testing indicated that the elevated temperature strengths of the HfC-modified aluminide was superior to those of the unreinforced matrix with the best 1100 K temperature slow strain rate properties for both materials being achieved after high-temperature annealing prior to testing. Examination of the microstructures after deformation in combination with the measured stress exponents and activation energies suggest that creep resistance of the HfC-modified form is due to solid-solution strengthening from carbon and hafnium rather than the presence of second phases.

Whittenberger, J. D.↗

NASA NTRS1993

Sliding wear of self-mated Al2O3-SiC whisker-reinforced composites at 23-1200 C

Microstructural changes occurring during sliding wear of self-mated Al2O3 SiC whisker-reinforced composites were studied using optical, scanning electron microscopy and transmission electron microscopy. Pin-on-disc specimens were slid in air at 2.7 m/s sliding velocity under a 26.5 N load for 1 h. Wear tests were conducted at 23, 600, 800 and 1200 C. Mild wear with a wear factor of 2.4 x 10 exp -7 - 1.5 x 10 exp -6 cu mm /N per m was experienced at all test temperatures. The composite showed evidence of wear by fatigue mechanisms at 800 C and below. Tribochemical reaction (SiC oxidation and reaction of SiO2 and Al2O3) leads to intergranular failure at 1200 C. Distinct microstructural differences existing at each test temperature are reported.

Farmer, S. C.↗

NASA NTRS1991

Silsesquioxane-derived ceramic fibres

Fibers formed from blends of silsesquioxane polymers were characterized to study the pyrolytic conversion of these precursors to ceramics. The morphology of fibers pyrolyzed to 1400 C revealed primarily amorphous glasses whose conversion to beta-SiC is a function of both blend composition and pyrolysis conditions. Formation of beta-SiC crystallites within the glassy phase is favored by higher than stoichiometric C/Si ratios, while carbothermal reduction of Si-O bonds to form SiC with loss of SiO and CO occurs at higher methyl/phenylpropyl silsesquioxane (lower C/Si) ratios. As the carbothermal reduction is assumed to be diffusion controlled, the fibers can serve as model systems to gain understanding of the silsesquioxane pyrolysis behavior, and therefore are useful in the development of polysilsesquioxane-derived ceramic matrices and coatings as well.

Hurwitz, F. I.↗

NASA NTRS1991

Mechanical properties of monolithic and particulate composites of L12 forms of Al3Ti

Preliminary studies of simple compressive properties of particulate composites with L1(2)-modified Al3Ti's as the matrices were performed as functions of matrix composition, amount of TiB2, processing technique, temperature, and strain rate. At and below about 900 K the introduction of TiB2 particles overwhelms minor chemical effects, and extremely high-yield strengths (about 2000 MPa at the 20 vol pct TiB2 level) were obtained. As L1(2)-modified Al3Ti has about half the density of the Ni-base superalloys, such values translate to strength/density ratios approximately four times those for superalloys. This advantage is not maintained by the Al3Ti-based composites above about 1000 K, particularly under slow strain rate plastic flow conditions, and it is proposed that this behavior could be due to the dissolution of intermetallic second phases as well as grain boundary creep mechanisms.

Whittenberger, J. D.↗

NASA NTRS1989

On the orthorhombic phase in ZrO2-based alloys

During TEM observation, a tetragonal (t) to orthorhombic (o) phase transformation often occurs in thin portions of ZrO2-containing foils. This transformation is stress-induced and in some senses artifactual, in that the reaction product is actually a high-pressure phase, relative to monoclinic (m) ZrO2, that can form from metastable t-ZrO2 in the TEM because its density is intermediate between t- and m-ZrO2. Examples of the formation of o-ZrO2 in a number of different systems are given.

Heuer, A. H.↗

NASA NTRS1988

Transmission electron microscopy of composites

Since interphase-interfaces are often both the structurally weakest and chemically least stable regions of a composite material, they are critical determinants of such macrostructural characteristics as tensile strength and fracture toughness. Attention is presently given to the use of TEM for the study of interfaces between dissimilar materials; electron-diffraction, analytical, and high-resolution forms of TEM are employed, for the cases of both structural and semiconductor composites. The materials studied are SiC/Si, GaP/Si, and SiC fiber- and whisker-reinforced Si3N4.

Pirouz, P.↗