C. S. Yust

Observations on the Sliding Wear of Ceramics

Modern design concepts for heat engines with increased thermal efficiency call for the utilization of ceramic components. In some applications, lubrication of these components will be difficult to maintain. Consequently, a need exists for an expanded understanding of the dry sliding wear behavior of ceramics. Sliding wear experiments were performed on several ceramic pairs from which wear rate data and information on the nature of the physical damage resulting from dry sliding were obtained. The wear evaluation was a pin-on-disk test. The test conditions included temperatures as high as 425°C, a surface velocity of 0.3 m/s, and a normal force of 9.0 N. Most of the specimens experienced moderate to severe damage. Damage characterized as mild wear was observed in a single room temperature test. The coefficient of friction for the damaged specimens ranged from 0.7 to 1.0, while this value was consistently 0.24 for the undamaged specimen. The implications of these results for wear mechanism are discussed. Pre...

Transmission electron microscope observations of porosity in coal

Abstract Samples of a high-volatile bituminous coal (HVab) were examined by means of transmis sion electron microscopy in a high-voltage electron microscope (1 MeV). The porosity observed within the exinite and inertinite constituents of this coal falls primarily into the mesopore size range (2–50 nm). Small particles (≈25 nm in diameter) were seen in a majority of the exinite pores and are believed to have acted as catalysts in the pore formation.

Preparation and friction characteristics of self-lubricating TiN-MoS2 composite coatings

Abstract Composite coatings consisting of discrete phases of TiN and MoS 2 were codeposited on graphite and Ti-6Al-4V substrates from Ti((CH 3 ) 2 N) 4 /NH 3 /MoF 6 /H 2 S gas mixtures. Chemical composition and microstructure of the coatings were characterized by Auger electron spectroscopy, X-ray diffraction, and transmission electron microscopy. Friction coefficients at room temperature in air were typically in the range of 0.07 to 0.3. The friction coefficients remained comparable at 573 K, but increased to 0.7 to 1.0 at 673 K. A friction coefficient value of ∼0.3 was, however, obtained from a composite coating tested at 973 K.

Structure-Property Correlations for TiB2-Based Ceramics Densified using Active Liquid Metals

The compound TiB2 has numerous exceptional properties including high hardness, high melting temperature, high electrical conductivity, and nonreactivity with various liquid metals. These make it an attractive candidate for technological applications, such as cutting tools, valve trim for erosive environments, and cathodes in Hall-Heroult cells for aluminum smelting. In general, such applications require fabrication of TiB2 into various shapes, and the latter requirement dictates the production of high-density polycrystalline ceramic bodies. The preparation and characterization of TiB2-based materials have already been the subject of numerous previous works, exemplified by the references (1–9). These include data on the phase equilibria in TiB2 containing systems,1,3,9 the wettability of TiB2 by various metals,1,3,4 and the results of property determinations on TiB2-based specimens.

Wear Characteristics of an Alumina-Silicon Carbide Whisker Composite at Temperatures to 800°C in Air

The unlubricated sliding wear of an alumina-silicon carbide whisker composite (20 volume percent whiskers) has been investigated in air at 20, 400, and 800°C. Mild wear was observed in room-temperature pin-on-disc tests at sliding velocities of 0.1 and 0.5 m/s for contact stresses up to 75 MPa. At a velocity of 0.5 m/s and a contact stress of 50 MPa. a transition to severe wear occurs after 6 to 8 km of sliding. Severe wear begins within the first kilometer of sliding at 400°C, while at 800°C mild wear results at stresses as great as 150 MPa. The severe wear debris produced at 400°C was examined by analytical electron microscopy (AEM) techniques and found to consist primarily of particles of Al2O3 and SiC in the 10- to 50-nm-size range. The mild wear process at 800°C did not produce any significant debris, but did show evidence of surface chemical reaction. Auger analysis of the surface showed the wear track to be covered by a mixture of aluminum and silicon oxides and the exposed silicon carbide whisker ...

The Friction and Wear of Lubricated Si3N4/SiC(w) Composites

The friction and wear of a 20 via silicon carbide whisker-silicon nitride matrix composite was measured at room temperature in both lubricated and unlubricated sliding. A pin-on-disk test configuration was used in which the pin member was a 9.53 mm diameter sphere. Silicon carbide and silicon nitride spheres were tested against the composite in both unidirectional and reciprocating sliding. The range of experimental parameters included normal forces from 1 to 45 N, velocities of 0.1 and 0.3 m/s and test durations of five minutes to three hours. The lubricant used in the room temperature tests was a commercial synthetic, fully formulated engine oil (5W30). In unlubricated sliding, silicon nitride produced more severe wear of the composite than did silicon carbide. In lubricated sliding, very little wear occurs in either sliding pair at even the longest test exposure. The data are presented in “wear space” co-ordinates and this form of presentation is compared to other wear mechanism map concepts presently ...

Growth and properties of Ni20.3Ti2.7B6 (τ-phase) crystals☆

Single crystals of Ni20.3Ti2.7B6 1 cm in diameter were grown from the melt by the Czochralski method at 1200–1215°C. Growth parameters and crystal properties are characterized.

Solids formed in a solvent-refined-coal pilot plant

Abstract Optical and X-ray diffraction methods have been used to study microstructures in a carbonaceous plug. The plug, which was obtained from a feedline in a solvent-refined-coal (SRC) pilot plant, was shown to consist primarily of minerals and semi-coke. The semi-coke formed the matrix phase surrounding the minerals, mineral and coke mixtures, and fusinite. These studies revealed that all the pyrite had been transformed to pyrrhotite which in turn formed shell-like structures surrounding both inorganic and organic constituents. In addition, it was observed that the slurry mixture of coal and solvent oil had been converted to semi-coke.

Czochralski growth and characterization of single-crystal akermanite (Ca2MgSi2O7)☆

Abstract High-purity, optically transparent single crystals of the tetragonal silicate, akermanite (Ca 2 MgSi 2 O 7 ), were grown by the Czochralski method. Boules were pulled in sizes up to 8 mm in diameter by 20 mm in length from stoichiometric melts in platinum crucibles at approximately 1520°C. The best quality growth was achieved in argon or nitrogen at pulling rates of 3 mm/hr or less, and spectrochemical analyses indicate a crystal purity of >99.99%. Several properties of the resulting samples are given, including X-ray diffraction, optical and compression data. The room-temperature uniaxial compressive strength of akermanite was determined to be 35 kg/mm 2 in the [001] and 5.5 kg/mm 2 in the [001]. Crystals were also grown doped with limited amounts, individually, of Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Nd 3+ , Eu 3+ , and Gd 3+ .

Tribological properties of ion beam deposited diamond-like carbon film on silicon nitride

Abstract The present article reports on the physical characterization and tribological properties of diamond-like carbon (DLC) films deposited on structural Si3N4 substrates. The films were deposited by the direct ion beam deposition technique. The ion beam was produced by plasma discharge of pre-mixed methane and hydrogen gas in a Kaufman-type ion source. The deposited films were found to be amorphous and contained about 70% carbon and 30% hydrogen. The friction coefficient of an uncoated Si3N4 ball on a DLC coated Si3N4 disc starts at about 0.2, then decreases rapidly to 0.1–0.15 with increasing sliding distance. Increasing humidity results in a slight increase in friction coefficient, but a significant decrease in wear factor. The wear factor for the tests at ≈ 60% rh (relative humidity) are about an order of magnitude smaller than the tests at 30% rh.