Steven J. Simko

Deposition factors and band gap of zinc-blende AlN

Successful deposition of zinc-blende AlN films with thickness up to 1000 A was performed with plasma source molecular beam epitaxy. The films were epitaxial to the Si(001) substrate. The formation of a thin 3C-SiC layer on the Si(001) surface is one of the important factors for the formation of zinc-blende AlN. Evidence for the presence of 3C-SiC is provided by an Auger electron spectroscopy depth profile and a high-resolution transmission electron microscopy plot profile. Spectroscopic ellipsometry was used to determine the optical constants of zinc-blende AlN in the range from 1.85 to 6.5 eV. The extinction coefficient data indicates that zinc-blende AlN is an indirect semiconductor with a band gap of ∼5.34 eV.

Friction, Wear, and Surface Film Formation Characteristics of Diamond-Like Carbon Thin Coating in Valvetrain Application

The friction and wear characteristics of thin diamond-like carbon (DLC) coatings have been investigated extensively in recent years mostly in laboratory bench tests. These coatings are known to provide significant friction reduction in the absence of lubricants. In the presence of lubricants, the friction benefits of these coatings are not clearly demonstrated. The current investigation is focused on exploring the friction reduction potential of a DLC coating obtained from a supplier in laboratory bench tests and in a motored valve train test. The DLC coating was deposited on the bucket tappet. In laboratory bench tests, results showed significant friction reduction in the absence of any lubricant but not in the presence of engine oil. In motored valve train tests a significant reduction in friction torque was observed when compared against a slightly rougher uncoated bucket, but no reduction was observed when compared against uncoated bucket tappet with comparable surface finish. Under boundary lubrication conditions, no lubricant-derived surface films were present on the DLC-coated surface. However, under mixed lubrication conditions, evidence of patchy antiwear surface films could be observed on DLC-coated buckets. The antiwear film appears to be primarily composed of calcium phosphate.

Interactions of Diamond-Like Carbon Coatings with Fully Formulated Engine Oils

Diamond-like carbon (DLC) coatings have been known for their low friction and high wear resistance for many years, but only in recent years have these coatings been used in mass-produced engines due to significant improvement in coating deposition techniques resulting in good adhesion on substrate materials. The interactions of these coatings with engine oils have been a focus for further research in recent years. In this investigation, the interactions of engine oil additives with five different DLC coatings containing two different doping elements were explored using a laboratory reciprocating rig under boundary lubrication condition. Two different contact geometries were investigated: a piston ring-on-liner and cylinder-on-flat to simulate piston ring and cylinder bore and cam and tappet applications. It was observed that DLC-coated piston rings offered lower friction coefficients and lower wear of the liner compared to commonly used Mo-NiCr-coated piston rings. However, the wear of the DLC-coated rings varied. The presence of lubricant additive–derived films could be observed on the wear surfaces of three of the DLC-coated surfaces irrespective of their doping element. In addition, the thickness of the lubricant additive–derived films on DLC-coated surfaces (a) was lower than that formed on ferrous materials and (b) depended on engine oil formulations.

Exhaust Gas Recirculation Cooler Fouling in Diesel Applications: Fundamental Studies of Deposit Properties and Microstructure

This article reports on the results of experimental efforts aimed at improving the understanding of the mechanisms and conditions at play in the fouling of exhaust gas recirculation coolers. An experimental apparatus was constructed to utilize simplified surrogate heat exchanger tubes in lieu of full-size heat exchangers. The use of these surrogate tubes allowed removal of the tubes after exposure to engine exhaust for study of the deposit layer and its properties. The exhaust used for fouling the surrogate tubes was produced using a modern medium-duty diesel engine fueled with both ultra-low-sulfur diesel and biodiesel blends. At long exposure times, no significant difference in the fouling rate was observed between fuel types and hydrocarbons levels. Surface coatings for the tubes were also evaluated to determine their impact on deposit growth. No surface treatment or coating produced a reduction in the fouling rate or any evidence of deposit removal. In addition, microstructural analysis of the fouling layers was performed using optical and electron microscopy in order to better understand the deposition mechanism. The experimental results are consistent with thermophoretic deposition for deposit formation, and van der Waals attraction between the deposit surface and exhaust-borne particulate.

Comparison of the Effects of Biodiesel and Mineral Diesel Fuel Dilution on Aged Engine Oil Properties

Dilution of engine oil occurs when fuel is injected late in the combustion cycle to regenerate the diesel particulate filter used for trapping particulate emissions. Fuel dilution reduces oil viscosity and the concentration of engine oil additives, potentially compromising lubricant performance. Biodiesel usage may compound these issues due to its oxidative instability, and its higher boiling point compared to mineral diesel potentially causes it to concentrate more in the oil sump. In this work, different amounts of mineral diesel and biodiesel (soy methyl ester, SME) were combined with 15W-40 CJ-4 diesel engine oil in laboratory oil aging experiments. Fuel was added and oil samples were withdrawn at periodic intervals. The oils were analyzed using typical oil analysis procedures to determine their condition, and wear evaluations under boundary lubricating conditions were determined using a high-frequency reciprocating rig (HFRR). Results showed that fuel dilution accelerated engine oil degradation, with biodiesel having a larger effect. However, friction remained unchanged with dilution, and wear actually decreased for fuel-diluted oils after 48 h of aging compared to aging without fuel dilution. Examination of the tribofilms by ultraviolet (UV) and visible Raman spectroscopy as well as Auger electron spectroscopy showed that additional carbon-containing components were present on tribofilms formed from fuel-diluted oils. These fuel-derived components may be responsible for the decreased wear observed.

Abrasion of Steel by Ceramic Coatings: Comparison of RF-DLC to Sputtered B4C

The abrasion rates of steel balls sliding against a very smooth diamond-like carbon (DLC) coating and a rough boron carbide (B4C) coating are compared. The initial abrasiveness of the B4C coating is about 2 orders of magnitude greater than that of the DLC coating. Both coatings exhibit a rapid decrease in their abrasiveness with sliding distance, but the details of the abrasion kinetics of these coatings are quite different. The abrasiveness of B4C falls according to a simple power law, while the abrasiveness of the DLC remains constant for a duration that depends on the load and then switches rather suddenly to zero. An explanation for this different behavior is proposed. During the abrasion process the asperities on the B4C are smoothed to a startling extent.

Wear control in a lubricated contact through externally applied Electric Current

It has been reported earlier that when an electric current is externally applied between two sliding surfaces in a lubricated contact, an electrochemical cell is produced with the lubricant acting as an electrolyte and two sliding surfaces acting as electrodes. In this paper, the effect of an externally applied direct current on friction and wear of a sliding steel pair lubricated by engine oils has been investigated using a ball on disk machine. The current was passed through the contact by connecting the ball and disk specimens to a current source. It has been observed that the magnitude and the direction of current can significantly alter wear of sliding surfaces but does not greatly impact friction coefficients. The wear of the cathode surface decreased while that of the anode surface increased compared to the condition when no current passed through the contact. It was also observed that wear on anode and cathode surfaces was influenced by lubricant chemistry. The wear reduction on cathode surface is believed to be related to the modification of elemental composition of lubricant derived surface films due to the passage of an electric current.

Valvetrain Friction and Wear Performance of Polyalkylene Glycol Engine Oils

ABSTRACT The application of polyalkylene glycol (PAG) as a base stock for engine oil formulation has been explored for substantial fuel economy gains over traditional formulations with mineral oils. Various PAG chemistries were explored by varying the feedstock material. All but one formulation have the same additive package. The friction performance of these oils was evaluated in a motored valvetrain rig with current production engine hardware in the temperature range 40–100°C and in the speed range 300–2,500 rpm. PAG formulations showed up to 48% friction reduction over GF-5 SAE 5W-20 oil depending on temperature, speed, and oil chemistry. The wear protection capability was evaluated using a radiotracer technique on another motored valvetrain rig where only one cam lobe rotated against a bucket tappet. The wear trend of some PAG oils was equal to or better than that of GF-5 SAE 5W-20 oil, whereas others showed high initial wear. The wear rate of the PAG oils was not significantly different from that of GF-5 oil. The bucket tappet surfaces were analyzed using scanning electron microscopy, auger electron spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectroscopy to characterize the tribofilm formed and to help explain the friction and wear performance.

Friction and Wear Reduction Mechanism of Polyalkylene Glycol-Based Engine Oils

ABSTRACT Polyalkylene glycols (PAG) have been explored as a possible base stock for engine oil formulation. The friction, wear, and load-carrying capacity of five different PAG chemistries were evaluated either as a base stock or as formulated oils in pure sliding and sliding-rolling conditions using various laboratory bench test rigs operating under boundary and mixed lubrication regimes. The results were compared against GF-5 SAE 5W-20 and a mineral-based oil. The wear surfaces were also characterized using various surface-sensitive techniques for analysis of tribofilms to understand the mechanism of friction reduction. The results indicated that PAG oils show lower friction/traction coefficients and improved load-carrying capability, depending on the formulation, than those of the GF-5 SAE 5W-20 and mineral-based oil. The adsorption of PAG molecules on the surface appeared to be responsible for the lower friction characteristics.