James C. Sung

The tribology of nano-crystalline diamond

Nano-crystalline diamond is the hardest diamond-like carbon (DLC) with properties very close to true diamond. Among such properties is the low frictional coefficient and high wear resistance. Cemented WC disks were coated by nano-crystalline diamond deposited by cathodic arc. These disks were rubbed cyclically by pins made of aluminum–silicon alloy (4032), carbon steel (52100), and alumina ceramics (Al2O3). It was found that frictional coefficient, after the period of “break-in”, was significantly reduced when compared with that of uncoated carbides. However, the frictional coefficient (0.2) with steel was more than twice that with Al alloy (0.09) and alumina (0.08). This abnormal high frictional coefficient is possibly due to the chemical interaction between surface atoms of iron and carbon. The wear mechanisms include mechanical attrition, chemical adhesion, and fatigue. The wear loss was the highest for the softer Al alloy, and lowest for 52100 steel. However, the wear of nano-crystalline diamond was the highest when it was rubbed against Al alloy. It was postulated that diamond was worn down primarily by the hardened layers of Al alloy and oxygen formed in situ during the rubbing action.

The CVD growth of micro crystals of diamond

Hot filament CVD method can be used to grow micron-sized diamond crystals of several millions per square centimeter. These crystals possess fully developed facets. Such euhedral diamond crystals of uniform size may not scratch workpieces. Moreover, the smooth surfaces can minimize the adhered impurities, therefore the contamination of products may be avoided. They are the ideal superabrasives for polishing high valued materials such as semiconductors.

Thermally activated electron emission from nano-tips of amorphous diamond and carbon nano-tubes

The sp2 character of graphite can carry electricity like a metal and the sp3 character of diamond can emit electrons in vacuum like an insulator. In this research, we have studied two ways of combining the electrical conductance and electron emission properties in carbon materials. In one case, the two characters are mingled atomistically to form a rather uniform mixture of amorphous diamond. Alternatively, graphite basal planes are wrapped around to form nano-tubes that exhibit a slight diamond character. Both amorphous diamond and carbon nano-tubes (CNTs) contain emission tips of nanometer sizes. When they are connected to a negative bias, they can emit electrons in vacuum toward an anode at very low turn-on field. However, when the cathode material is heated up, the responses of electron emission in vacuum are dramatically different between the two types of carbon materials. At a temperature of 300 °C, amorphous diamond can emit 13 times more electrons than it did at room temperature; but CNTs show no response to thermal agitation. The thermally sensitive emission of amorphous diamond indicated that electrons climbed up an energy ladder to reach the vacuum level for efficient emission. The energy ladder contained minute but discrete energy levels that were created by distorting the tetrahedral bonds of carbon atoms to different degrees. On the other hand, the CNTs are substantially graphitic so the energy gap between their conduction band and valence band overlap and are continuous in energy. In this case, electrons could not acquire energy to reach the vacuum energy unless the temperature could be sufficiently high. Hence, it confirms that CNTs emit electrons primarily by enhancing the applied field on nano-tips. This is in contrast to amorphous diamond that emits electrons by combining graphitic carbon atoms and diamond-like carbon atoms of various states.

Field emission of micro aluminum cones coated by nano-tips of amorphous diamond

Abstract Nano-tips of amorphous diamond films were deposited on Al micro cones to form a metal/diamond interface structure. The amorphous diamond films were deposited by cathodic arc at a temperature less than 150 °C. The turn on applied field was 4.5 V/μm at 10 μA/cm2 for the Al/diamond structure. The turn on applied field is approximately 10 times lower than that for Al alone. The high emission current was obtained (50 mA/cm2) due to metal/diamond structure and the presence of defect band within band gap of amorphous diamond that allowed electrons to pass through with little hindrance. Moreover, the stability of electron emission was monitored at electrical field strength of 7 V/μm for 100 h. The fluctuations of long-term emission current were at ±4% during the entire duration of monitoring. The fluctuation may be due to the absorption and desorption of molecules by the dangling bonds of carbon atoms on surface of nano-tips of amorphous diamond in a vacuum chamber during electron emission. The high stability of electron emission form micro Al cones by nano-tips amorphous diamond coatings was observed.

Electron emission from nanotips of amorphous diamond

Amorphous diamond can be deposited with a high-density (4×1010 emitters/cm2) of nano-sized emitters. The turn on applied field strength was reduced by increasing aspect ratio of amorphous diamond nanotips. Moreover, the field emission was highly sensitive to the aspect ratio of tips, and relatively inert to the sp3/(sp3+sp2) ratio. The lowest turn on applied field strengths was 4.6 V/μm at the current density of 10 μA/cm2; and 11 V/μm at the current density of 10 mA/cm2. High reproducibility of field emission was also observed in this study.

Nano-tip emission of tetrahedral amorphous carbon

Abstract Various forms of tetrahedral amorphous carbon were deposited on n-type silicon (100) substrate. Tetrahedral amorphous carbon is an excellent electron emitter for field emission array (FEA) applications. The negative electron affinity (NEA) of diamond surface was believed to facilitate the electron emission for diamond-like carbon. However, in this research, we have demonstrated that the geometric enhancement factor may be even more effective to promote electron emission. Moreover, we have succeeded in depositing high-density (4×10 10 emitters/cm 2 ) of nano-sized emitters. The electrons are emitted under both the effect of enhanced field due to the sharp tips, and the optimized distortion of tetrahedral bonding of carbon atoms. The sp 3 /(sp 3 +sp 2 ) ratio of C–C bonded tetrahedral amorphous carbon films was measured by ESCA and Raman; and the geometry of emitters by AFM. The lowest turn on applied field strength of 4.6 V/μm was found at the current density of 10 μA/cm 2 . High reproducibility of field emission was also observed in this study.

Development of a novel cooling system-assisted minimum quantity lubrication method for improvement of milling performance

This paper presents a novel lubrication method for milling processes that employs cooling system-assisted minimum quantity lubrication (CSMQL) using a thermoelectric cooling system. The CSMQL method improves the cooling effect in the cutting area and enhances processing quality, in addition to reducing energy consumption. Four different coolant strategies including CSMQL, dry, minimum quantity lubrication (MQL), and wet methods were compared in processing mill die steel (SKD11), which is widely used in industry. Different aspects of the milling performance (e.g. surface roughness, morphology, milling temperature, and milling forces) were investigated using these coolant strategies. The experimental results show that not only is the surface roughness of steel milled using CSMQL better than that of steel milled using dry and MQL methods, but CSMQL also produces fewer tool marks on the workpiece surface. In addition, it was found from observations of chip color that using the CSMQL method reduced the cutting temperature by 27% and the cutting force by 22%, compared with dry machining. In summary, the use of CSMQL can not only improve the surface roughness and reduce the cutting force and cutting temperature, but also promote processing quality. This study will help researchers develop more efficient cooling strategies in the future.

Growth of single crystal silicon carbide by liquid phase epitaxy using samarium/cobalt as unique solvent

An approach for synthesizing single crystal silicon carbide at low temperature using liquid phase epitaxy is proposed. A mixture of samarium and cobalt (Sm:Co = 64:36 at.%) was used as a unique solvent in this synthesis process. Electron microscopy indicates the epitaxial growth of single crystal silicon carbide with a thickness of 4 µm over a silicon wafer followed by the formation of polycrystalline silicon carbide and silicon carbide whiskers. Some growth mechanisms are proposed to explain the formation of silicon carbide. It is hypothesized that the single crystal silicon carbide grew from the liquid phase, whereas polycrystalline silicon carbide whiskers grew via the vapor–liquid–solid process.

Euhedralmicrodiamond crystals grown by CVD method

Hot filament CVD method can be used to grow micron-sized diamond crystals in the quantity of several millions per square centimeter. These crystals are polyhedral with fully developed facets. Such uniform-shaped crystals can be sized easily with tight distribution. When they are used to polish workpiece, they may not cause conspicuous scratches. Moreover, the smooth surfaces of these whole crystals can minimize the amount of adhered impurities, so that the contamination of workpiece may be avoided. The micro-diamond crystals so grown are the ideal superabrasive for polishing high-valued materials such as semiconductors and other electronic devices.

Liquid phase synthesis of diamond in hydrogen atmosphere

Graphite can be converted to diamond under high pressure by the catalytic influence of certain molten metal. The transition is reversed at low pressure. However, if hydrogen atoms are available in the atmosphere, graphite may be gasified to form methane, and the latter deposited on the molten metal as diamond.