About the change of flow factors during the run-in process of lubricated contacts

Purpose This paper aims to increase the knowledge of the run-in process of lubricated contacts in hydraulical pumps. When performing EHL simulations of tribological contacts, the surface influence needs to be taken into account. This experimental measurement wants to investigate the amount of change of the flow factors in the first hour of run of tribological contacts. Design/methodology/approach An experimental test bench is used to run-in several samples. After several minutes of running, samples are removed and the surface structure is captured using a digital microscope. With the measured data, flow factors are calculated. Findings The findings were clear that flow factors are highly direction-dependent, especially shear flow factors in radial directions experience almost no change. Overall, influence of the surface structure of up to 30% compared to a flat surface can be registered. Originality/value This paper helps to choose application-oriented values for the simulation of tribological contacts.

Friction and contact temperature in dry rolling-sliding contacts with MoS2-bonded and a-C:H:Zr DLC coatings

Gearboxes are usually lubricated with oil or grease to reduce friction and wear and to dissipate heat. However, gearbox applications that cannot be lubricated with oil or grease, for example in the space or food industry, are commonly lubricated with solid lubricants. Especially solid lubricants with a lamellar sliding mechanism like graphite and molybdenum disulfide (MoS2) or diamond-like carbon (DLC) coatings can enable very low coefficients of friction. This study investigates the friction and temperature behavior of surface coatings in rolling-sliding contacts for the application in dry lubricated gears. In an experimental setup on a twin-disk test rig, case-hardened steel 16MnCr5E (AISI5115) is considered as substrate material together with an amorphous, hydrogenated, and metal-containing a-C:H:Zr DLC coating (ZrCg) and a MoS2-bonded coating (MoS2-BoC). The friction curves show reduced coefficients of friction and a significantly increased operating area for both surface coatings. Due to the sufficient electrical insulation of the MoS2-BoC, the application of thin-film temperature measurement-known from lubricated contacts-was successfully transfered to dry rolling-sliding contacts. The results of the contact temperature measurements reveal pronounced thermal insulation with MoS2-BoC, which can interefere the sliding mechanism of MoS2 by accelerated oxidation. The study shows that the application of dry lubricated gears under ambient air conditions is challenging as the tribological and thermal behavior requires tailored surface coatings.

Molecular Dynamics Simulation Study of Mechanical Effects of Lubrication on a Nanoscale Contact Process

Using molecular dynamics simulation, we study the effect of a lubricant on indentation and scratching of a Fe surface. By comparing a dry reference case with two lubricated contacts—differing in the adsorption strength of the lubricant—the effects of the lubricant can be identified. We find that after an initial phase, in which the lubricant is squeezed out of the contact zone, the contact between the indenter and the substrate is essentially dry. The number of lubricant molecules confined in the tip-substrate gap increases with the lubricant adsorption energy. Trapped lubricant broadens the tip area active in the scratching process—mainly on the flanks of the groove—compared to a dry reference case. This leads to a slight increase in chip height and volume, and also contributes to the scratching forces.

Distinguishing between rheophysical regimes of fluid-saturated granular-flows using dilatancy and acoustic emission measurements

Dry granular flows provide an ongoing challenge to physics and under saturation the multiphase physics is even more difficult to disentangle. A rich literature has elucidated the possible regimes achieved, however, the nonlinear nature of the multiphase process makes predicting the appropriate dynamic regime difficult. In this study, we introduce a new experimental strategy to identify the appropriate dynamical regimes by combining traditional methods with acoustic emission measurements. We sheared natural granular materials under dry, water and oil-saturated conditions while recording mechanical, acoustic and visual data. By applying alternate low and high velocity steps we respectively obtained quasi-static and inertial granular flow regimes. Dilation was observed for all high-velocity flows but its amount varied as did the degree of acoustic emission. At high velocities, the water-saturated flow dilated less and had reduced acoustic emissions relative to the dry case. In contrast, the oil-saturated flow dilated more while having even less acoustic emissions. This difference in trends of the dilation and acoustic emissions with increasing fluid viscosity suggests that oil and water granular flows achieved distinct dynamical regimes. Damping of granular pressure by reducing grain collisions and Dilatancy due to fully lubricated contacts are two competing processes influence the saturated shear physics and theoretically expected, but distinguishing between the regimes is difficult to anticipate. The acoustic emissions provide an extra piece of information that allows us to distinguish the physical regimes and determine the competition between processes that control the physics of saturated granular flows in the granular inertial regime.

BUBBLE DYNAMICS-BASED MODELING OF THE CAVITATION DYNAMICS IN LUBRICATED CONTACTS

Cavitation is a common phenomenon in fluid machinery and lubricated contacts. In lubricated contacts, there is a presumption that the short-term tensile stresses at the onset of bubble formation have an influence on material wear. To investigate the duration and magnitude of tensile stresses in lubricating films using numerical simulation, a suitable simulation model must be developed. The chosen simulation approach with bubble dynamics is based on the coupling of the Reynolds equation and Rayleigh-Plesset equation (introduced about 20 years ago by Someya).Following the basic approach from the author’s earlier papers on the negative squeeze motion with bubble dynamics for the simulation of mixed lubrication of rough surfaces, the paper at hand shows modifications to the Rayleigh-Plesset equation that are required to get the time scale for the dynamic processes right. This additional term is called the dilatational viscosity term, and it significantly influences the behavior of the numerical model.

Coupling Molecular Dynamics and Micromechanics for the Assessment of Friction and Damage Accumulation in Diamond-Like Carbon Thin Films Under Lubricated Sliding Contacts

Diamond-like carbon (DLC) coatings have proven to be an excellent thin film solution for reducing friction of tribological systems as well as providing resistance to wear. These characteristics yield greater efficiency and longer lifetimes of tribological contacts with respect to surface solutions targeting for example automotive applications. However, the route from discovery to deployment of DLC films has taken its time and still the design of these solutions is largely done on a trial-and-error basis. This results in challenges both in designing and optimizing DLC films for specific applications and limits the understanding, and subsequently exploitation, of many of the underlying physical mechanisms responsible for its favorable frictional response and high resistance to various types of wear. In current work multiscale modeling is utilized to study the friction and wear response of DLC thin films in dry and lubricated contacts. Atomic scale mechanisms responsible for friction due to interactions between the sliding surfaces and shearing of the amorphous carbon surface are utilized to establish frictional response for microstructure scale modeling of DLC to DLC surface contacts under dry and graphene lubricated conditions. Then at the coarser microstructural scale both structure of the multilayer, substrate and surface topography of the DLC coating are incorporated in studying of the behavior of the tribosystem. A fracture model is included to evaluate the nucleation and growth of wear damage leading either to loss of adhesion or failure of one of the film constituents. The results demonstrate the dependency of atomistic scale friction on film characteristics, particularly hybridization of bonding and tribochemistry. The microstructure scale modeling signifies the behavior of the film as a tribosystem, the various material properties and the surface topography interact to produce the explicitly modeled failure response. Ultimately, the work contributes towards establishing multiscale modeling capabilities to better understand and design novel DLC material solutions for various tribological applications.