Michael J. McNallan

Carbon coatings on silicon carbide by reaction with chlorine-containing gases

Carbon films have been produced on the surface of β-SiC particles by reaction with Ar–Cl 2 and Ar–Cl 2 –H 2 gas mixtures at atmospheric pressure and temperatures of 600–1000 °C. The structure and composition of the carbon films have been investigated using XRD, SEM, EDS, TEM, FTIR and Raman spectroscopy. BET and TG were also used for measuring the amount of carbon formed in the reaction. Uniform nanoporous carbon films with surface area exceeding 1000 m 2 g -1 were obtained by reactions with Ar–Cl 2 gas at 600–1000 °C. Based on Raman spectroscopy and electron diffraction data, these films were identified as nanocrystalline graphite. An addition of hydrogen to the gas mixtures results in the etching of graphitic carbon. Traces of diamond were found along with amorphous carbon after treatment in Ar–Cl 2 –H 2 gas mixtures at temperatures above 900 °C.

Nucleation, growth, and graphitization of diamond nanocrystals during chlorination of carbides

Synthesis of nano- and microcrystalline sp3-bonded carbon (diamond) with cubic and hexagonal structure by extraction of silicon from silicon carbide in chlorine-containing gases has been reported recently. This process is attractive because it can produce diamond at ambient pressure and temperatures below 1000 °C. No plasma or other high-energy activation is required, thus providing an opportunity for large-scale synthesis. However, the mechanism of diamond formation has not been previously analyzed. This work reports on the formation mechanisms of diamond as well as the transformation of diamond to graphite and onionlike carbon upon heating. Study of SiC/carbon interfaces showed that direct epitaxial growth of diamond on SiC is possible, in agreement with previous molecular-dynamics simulation. However, random nucleation of diamond from amorphous sp3-bonded carbon produced as the result of extraction of Si from SiC has also been demonstrated. It has been shown that the presence of hydrogen in the environ...

Carbon coatings produced by high temperature chlorination of silicon carbide ceramics

Abstract Carbon coatings are widely used to modify surfaces of materials and improve their tribological properties. In this work, carbon layers were formed on various types of sintered and CVD silicon carbide (SiC) using a novel technique involving a reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 °C. Following the work done on powders and fibers, this method successfully produced adherent coatings on SiC ceramics, which could be grown to thickness above 200 µm. Highly disordered carbon with contributions from nanocrystalline graphite was identified by Raman spectroscopy, x-ray diffraction, and energy dispersive spectroscopy. The kinetics of the chlorination reaction at 1000 °C for different gas mixtures fit to a linear reaction rate equation. Coatings produced in pure Cl2 are graphitic and demonstrate a low hardness (1.8 GPa), Young’s modulus (18 GPa), low wear rate, and a friction coefficient of ∼0.1, which is almost constant for any testing conditions in dry or humid air. Coatings produced in Cl2/H2 mixtures have microhardness up to 50 GPa and Young’s modulus up to 800 GPa. Although the presence of hydrogen had little effect on the Raman spectrum of the carbon layers, its presence changed the structure and permeability of the carbon film.

Effect of Humidity on the Tribological Properties of Carbide-Derived Carbon (CDC) Films on Silicon Carbide

The effect of humidity on the tribological behavior of carbide-derived carbon (CDC) films prepared by high-temperature chlorination of silicon carbide was examined. Pin-on-disk tribological tests indicate that CDC, unlike graphite or glassy carbon, performs better in dry nitrogen (less than 0.05 friction coefficient at 0% humidity) than in humid air. This versatility is explained by the onion-like structure of the nanoporous CDC coating.

Tribological Properties of Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide

The tribological properties of highly disordered graphitic carbon layers formed on silicon carbide (SiC) substrates by reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 °C were studied. Si was selectively removed from the near surface of SiC by chlorine gas, leaving behind a layer of carbon having high structural density and strong bonding characteristics. Tribological tests showed that the carbon films were highly adherent and able to reduce friction coefficients of the base SiC by factors of up to seven. There was little or no change in the factional behavior of carbon layers when sliding velocity and load were increased. Low friction coefficients (∼0.1) could be obtained under wet, dry, polished, and rough conditions. The initially rough carbon surface underwent plastic flow producing a smooth, self-adjusting carbon layer. Structural morphology and the amount of disorder in the carbon layers were correlated with the friction and wear performance of the resultant films. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

Formation of Cylindrical Sliding-Wear Debris on Silicon in Humid Conditions and Elevated Temperatures

The present study investigates the mechanics of roll formation between sliding bodies at elevated temperatures and humid conditions. Silicon is used as the model material for reciprocating linear sliding tests. The evolution of tribological rolls initially involves the rapid oxidation of silicon wear debris by water, the deformation of SiO2 particles into platelets, and then the compaction of these particles into a film deposited on the wear surface. The formation of compacted silica film requires minimum adsorption of water which enhances the adhesion between silica platelets. The stress cycle imposed on the film leads to the delamination of platelets near the sliding surface. The delaminated debris cluster into multiple aggregates that are subsequently rolled into dense cylindrical particles so as to relieve the interfacial shear stress. When the film and rolls are formed, the friction and wear rate is maintained at low steady state values.

Conversion of metal carbides to carbide derived carbon by reactive ion etching in halogen gas

The excellent tribological properties, very low friction coefficient, ~0.05, of the recently discovered carbide derived carbon (CDC) films have shown them to be excellent candidates in many applications where friction and wear are dominating issues in performance. In this work we examine the feasibility of employing a reactive ion etching process (RIE) with chlorine gas at low temperature, as opposed to the current high temperature chlorination process, in achieving the conversion of metal carbide films into amorphous carbon films. The overall goal is develop a process that is friendlier to microfabricated devices towards employing the tribological properties of CDC films in such devices. We examine this RIE processing using both bulk scale and thin film specimens. These metal-carbide specimens are subjected to a halogen containing ion plasma at reduced pressure in order to leach out the metal, resulting in an amorphous carbon film, a so-called carbide-derived carbon (CDC) process. This reactive ion etching process has been used to produce carbon layers on multiphase carbide materials containing silicon and titanium. The resulting carbon layers have been characterized using a variety of techniques. The results on the bulk scale specimens, via Raman spectrometry, indicated that RIE processing can indeed achieve conversion, while results of the thin films indicated that although conversion occurred poor adhesion of the films to the substrate resulted spallation during friction testing attempts.

Effects of length, diameter and population density of tribological rolls on friction between self-mated silicon

Abstract Tribological rolls, composed of amorphous silica, were produced by the ball-on-flat linear reciprocating sliding of self-mated silicon at elevated temperatures (200, 400, 500 and 600°C) and humid conditions (PH20=0.034, 0.064 and 0.085 MPa). Normal loads of 2.04, 3.59, 4.73 and 5.80 N were used in the tests. The changes in the coefficient of friction at various loads and environmental conditions were identical. The coefficients of friction were initially between 0.45 and 0.65 and then rapidly decreased to a steady value of about 0.13. The present study describes a model where the steady state coefficient of friction is a function of the length, diameter and population density of the rolls (number of rolls per unit area). The roll length and diameter increase, and the population density decreases with temperature. The roll length increases with load, whereas the other parameters are nearly independent of load. Humidity does not affect these parameters. The compensating effects of the variation in length, diameter and population density lead to the uniformity in the steady state coefficient of friction with the applied load and environment.

Morphology of corrosion products formed on cobalt and nickel in argon-oxygen-chlorine mixtures at 1000 K

The morphology of corrosion products formed on cobalt and nickel in argonoxygen-chlorine mixtures at 1000 K have been examined using optical microscopy, scanning electron microscopy, and X-ray diffraction. The corrosion products formed under conditions where the oxides are thermodynamically stable consist of porous oxides containing little or no chlorine. This morphology is consistent with a corrosion mechanism involving vapor phase transport of volatile metal chlorides.

Platinum Reactions with Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide

Highly disordered graphitic carbon layers were formed on various types of commercially available silicon carbide (SiC) ceramics by reaction with chlorine and chlorine-hydrogen gas mixtures at 1000°C. The carbon was produced ranging from only a few micrometers to hundreds of micrometers thick. When a platinum sample holder was employed (instead of fused silica), platinum was found dispersed in the carbon layer concentrated near the SiC/C interface. This process can be used for incorporating platinum in porous carbon films for catalytic and other applications. In addition, the platinum resulted in a smoother physical interface between the SiC and carbon sublayer. The morphology of the platinum dispersion, its effect on the carbon layer, and its proposed formation mechanism are presented in this paper.