Hyung K. Yoon

Delamination and spalling of diamond-like-carbon tribological surfaces

Abstract The material removal mechanisms accompanying the rotating contact wear of metal (Cr and W)-containing diamond-like-carbon (DLC) thin coatings on steel have been characterized by using a focused ion beam microscope. The W-DLC wears by gradual recession, in accordance with a polishing mechanism. The Cr-DLC has inferior wear resistance, associated with extensive spalling along prominent machining ridges on the steel substrate. The spalls are generated by cracks in the Cr-DLC coating that tunnel beneath the surface, along the ridges. A mechanics analysis identifies a material parameter that governs the incidence of cracking and spalling. The parameter includes the residual compression and the elastic modulus of the metal-containing DLC coating, as well as its fracture toughness. For W-DLC, this parameter is well below the critical level required for spalling, consistent with the observations. For the Cr-DLC, additional property measurements are needed before completing the spalling assessment. The implication is that the Cr-DLC is subject to either a larger residual stress or a lower toughness than the W-DLC.

Scuffing characteristics of SAE 50B38 steel under lubricated conditions

The scuffing characteristics of SAE 50B38 steel were investigated under lubricated conditions using a block-on-ring bench tester. The main purpose of this study was to better understand the failure mechanism(s) of lubricated steel contacts operating under boundary lubrication conditions. Scuffing tests were conducted with a polyalphaolefin (PAO) lubricant at various sliding speeds. A scuffing transition map, which is characterized by three regions (mild wear, microscuffing and catastrophic scuffing failure), was developed over ranges of applied load and sliding speed. To better understand the scuffing processes and failure mechanisms, the tests were interrupted during the scuffing process. The surfaces and subsurfaces of specimens obtained at various stages of scuffing were examined with SEM (scanning electron microscope), TEM (transmission electron microscope) and a nano-indentor. Based on the experimental observations, it is hypothesized that scuffing failure is caused by macroscopic adhesion at the sliding interface, which eventually leads to the plastic shearing of bulk material. It was also found that the scuffed subsurface of SAE 50B38 steel exhibits three characteristic regions (transformed layer, plastically deformed layer and bulk material). The transformed layer has a nanocrystalline structure, indicating the microstructural change during scuffing failure. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

The Microstructure and Wear Behavior of Cr- and W-DLC Coatings Sputter-Deposited onto AISI 52100 Substrates as Elucidated using Focused-Ion-Beam SEM

Magnetron sputtering has been used to deposit metal-containing, diamond-like-carbon (Me-DLC) coatings onto substrates composed of AISI 52100 steel in quenched-and-tempered condition. Coatings of two distinctly different compositions, one containing Was the metallic constituent and the second containing Cr, have been deposited in a plasma containing Ar and C 2 H 2 .Interrupted, unidirectional sliding experiments of the block-on-ring type have been conducted in a poly-alpha-olefin (PAO) lubricant at a load of 667 N for discrete numbers of cycles, N, of between 10 and 1000. Focused-ion-beam, scanning electron microscopy (FIB/SEM) has been applied to characterize the morphology of as-deposited and worn Me-DLC coatings. This technique has resulted in the determination that the Cr-DLC coating, deposited using the investigated processing parameters, fractures in a brittle manner through the formation and propagation of “tunnel cracks,” which unzip in a direction parallel to the grinding direction outside of the region of contact. Conversely, the application of specific processing parameters to deposit W-DLC produces a coating that wears by gradual recession, consistent with polishing wear. First-principles-based analysis shows that the state of residual stress is critically important in the behavior of the coating.

Insulating Structural Ceramics Program, Final Report

New materials and corresponding manufacturing processes are likely candidates for diesel engine components as society and customers demand lower emission engines without sacrificing power and fuel efficiency. Strategies for improving thermal efficiency directly compete with methodologies for reducing emissions, and so the technical challenge becomes an optimization of controlling parameters to achieve both goals. Approaches being considered to increase overall thermal efficiency are to insulate certain diesel engine components in the combustion chamber, thereby increasing the brake mean effective pressure ratings (BMEP). Achieving higher BMEP rating by insulating the combustion chamber, in turn, requires advances in material technologies for engine components such as pistons, port liners, valves, and cylinder heads. A series of characterization tests were performed to establish the material properties of ceramic powder. Mechanical chacterizations were also obtained from the selected materials as a function of temperature utilizing ASTM standards: fast fracture strength, fatique resistance, corrosion resistance, thermal shock, and fracture toughness. All ceramic materials examined showed excellent wear properties and resistance to the corrosive diesel engine environments. The study concluded that the ceramics examined did not meet all of the cylinder head insert structural design requirements. Therefore we do not recommend at this time their use for this application. The potential for increased stresses and temperatures in the hot section of the diesel engine combined with the highly corrosive combustion products and residues has driven the need for expanded materials capability for hot section engine components. Corrosion and strength requirements necessitate the examination of more advanced high temperture alloys. Alloy developments and the understanding of processing, structure, and properties of supperalloy materials have been driven, in large part, by the gas turbine community over the last fifty years. Characterization of these high temperature materials has, consequently, concentrated heavily upon application conditions similiar to to that encountered in the turbine engine environment. Significantly less work has been performed on hot corrosion degradation of these materials in a diesel engine environment. This report examines both the current high temperature alloy capability and examines the capability of advanced nickle-based alloys and methods to improve production costs. Microstructures, mechanical properties, and the oxidation/corrosion behavior of commercially available silicon nitride ceramics were investigated for diesel engine valve train applications. Contact, sliding, and scratch damage mechanisms of commercially available silicon nitride ceramics were investigated as a function of microstructure. The silicon nitrides with a course microstructure showed a higher material removal rate that agrees with a higher wear volume in the sliding contact tests. The overall objective of this program is to develop catalyst materials systems for an advanced Lean-NOx aftertreatment system that will provide high NOx reduction with minimum engine fuel efficiency penalty. With Government regulations on diesel engine NOx emissions increasingly becoming more restrictive, engine manufacturers are finding it difficult to meet the regulations solely with engine design strategies (i.e. improved combustion, retarded timing, exhaust gas recirculation, etc.). Aftertreatment is the logical technical approach that will be necessary to achieve the required emission levels while at the same time minimally impacting the engine design and its associated reliability and durability concerns.