The effect of oil pockets size and distribution on wear in lubricated sliding
Introduction
Surface texturing emerged as an option of surface engineering resulting in improvement in load capacity, coefficient of friction, wear resistance, etc. Various techniques can be employed for surface texturing including machining, ion beam texturing, etching techniques and laser texturing [1]. The oil pockets (also known as micropits, holes, dimples or cavities) may reduce friction in two ways: by providing lift themselves as a micro-hydrodynamic bearing, and also by acting as a reservoir of lubricant [2]. Holes can also serve as a micro-trap for wear debris in lubricated or dry sliding [1].
The most familiar practical examples include plateau honed cylinder surfaces in combustion engines. The two-process surface is created. The authors of article [3] obtained the proportionality between cylinder oil capacity and engine oil consumption. Santochi and Vignale [4] stated that increase of oil capacity improved engine performance. Jeng [5] found that friction coefficient under mixed lubrication condition of two-process surface was smaller than that of one process surface, when Rq parameters of two analysed surfaces were the same. Now laser surface texturing is successfully applies to cylinder liners [6], [7]. Surface texturing was observed to reduce the coefficient of friction [6], oil consumption and cylinder wear during running-in [7] compared to non-textured liners.
The benefits of applying laser surface texturing to piston rings were demonstrated theoretically and experimentally [8], [9]. The results of theoretical work showed a potential reduction of friction force of about 30% by ring surface texturing in comparison to non-textured rings under full lubrication conditions [8]. These results were confirmed experimentally [9].
Surface texturing is also successfully applied to mechanical seals resulting in increase in seal life [10]. It was found that partial laser surface texturing improved substantially load-carrying capacity of hydrodynamic thrust bearings [11]. Surface texturing is also used extensively in metal forming [2].
A majority of researchers found that surface texturing of contacting elements reduced the frictional force substantially in comparison to untextured surfaces. Surface texturing was observed to expand the range of hydrodynamic lubrication regime [12], [13], [14].
Surface texturing resulted in minimizing the surface ability to seizure [15], [16]. The dimples existence from area density of 10% improved seizure resistance of sliding pair: steel–spheroidal cast iron [16].
Textured surfaces can provide traps for wear debris in dry contacts subjected to fretting. The dimple existence could improve the fretting wear resistance [17] and almost doubled the fretting fatigue life [18].
We found little information about the effect of dimple existence on improvement of tribological properties of journal bearings, although textured bearing sleeves are produces by some firms (for example, Glacier) and are recommended to work under mixed lubrication conditions. Only a few papers were concerned with the effect of oil pockets on wear intensity.
The dimples of mainly spherical shape are usually formed on stationary surface of smaller hardness. Three dimensions characterise surface texturing: diameter, depth and area density. Extensive literature survey revealed that usually dimple depth over dimple diameter ratio range of 0.01–0.3 and area density to 30% exist for assemblies operated in lubricated sliding conditions. The laser texturing is the most popular technique in forming micropits. However other methods may be used. Impulse burnishing can be a very promising approach. In this technique special endings act as hammers to form oil pockets on metal surfaces.
Accommodation of sliding surfaces over a period of time (running-in, breaking-in, shakedown, wearing-in) causes changes of their initial surface topography. The term running-in is used more in Europe, while term breaking-in tends to be favored in the United States. The running-in process enables machines to improve surface topography and frictional compatibility. Running-in characteristics for a machine assembly are affected by its design, fitting-up during assembly, and its history of prior use.
Several criteria can be employed to characterise the running-in completion. These include stable roughness, steady wear and steady friction. The time needed to reach a steady rate of wear and that to achieve a steady-state of friction may not necessarily be equal [19].
During running-in the wear removal or plastic deformation (initial stage of running-in) can take place [20].
Past research revealed that obtaining longer life for engines relied on a suitable running-in process [21]. Surface roughness is the main factor that influence the running-in if there are no apparent surface defects.
Kragelsky et al. defined the end of running-in in terms of the number of cycles to reach the optimum load-carrying capacity of a surface, and that involved surface roughness [22].
During the ‘zero-wear’ process the wear volume or wear loss is within the limits of the original surface topography of the component and is hard to determine [23]. Initial surface topography affects running-in period, running-in wear intensity and sometimes steady wear.
The wear intensity is often proportional to initial surface height. Usually the bigger surface topography height causes bigger wear during running-in, after this period the wear intensity is constant [24]. The wear of cylinder surfaces during running-in was proportional to the initial roughness height [25].
It was found that initial cylinder surface topography affected its wear not only during running-in, but also when the wear amount was big [26]. The consequence of the removal of oil pockets from surface of cylinders is dangerous for the engine, because leads to engine failure.
Usually surface topography height decreased during running-in [19], [22], [23], [24], [25], [27]. Qualitative three-dimensional characterisation of cylinder surface wear was done by Dong and Stout [27]. They were marked changes in skewness and kurtosis. However some authors found increase of roughness height during initial period of wear. The authors of paper [28] observed the increase in roughness height during collaboration of metals of different hardness, even during lubrication.
It is believed that surface roughness obtained after running-in does not depend on initial surface height. When solid contact occurs, smooth surfaces tend to get rougher and rougher surfaces tend to get smoother (equilibrium surface roughness [22]). Some authors reported an optimum initial surface roughness (after machining). Becker and Ludema [29] obtained similar values of the Ra parameter of various cylinders tested on Cameron-Plint tribometer (duration of the test was 1 h). The authors of paper [30] found that the wear rate increased with increasing roughness though the final roughness of all specimens reached the same roughness. Whatever surface roughness begins on a surface, roughness changes to a roughness that is characteristic of the system and its running conditions. So the machined surface should be similar to worn surface (after finishing “zero-wear”).
However different results were also mentioned. For example the authors of paper [31] analysed the change in surface roughness during running-in of partial elastohydrodynamic lubricated wear. Specimen surfaces with different roughness ended up with different roughness after running-in. The larger the initial roughness, the larger the final roughness.
Section snippets
The aims and scope of the investigations
The fundamental aim of the investigations is to study the effect of dimple size and distribution on wear in lubricated sliding.
The second aim is to analyse the influence of initial wear period on tribological performance of sliding components.
The co-action between bearing sleeve and journal was simulated using block-on ring tester. Dimples were created on the stationary block surface by impulse burnishing (embossing) technique.
The test apparatus
The experiments were conducted on a block-on ring tester as shown in schematic representation of Fig. 1. The tribosystem consists of the stationary block (specimen) pressed at the required load P against the ring (counter-specimen) rotating at the defined speed. The temperature of the test block can be measured using thermocouple. The construction allows us to measure the friction force between ring and block. This tester can simulate some real practical machinery, particularly slide bearings.
Results and discussion
The results of total wear values of specimens and counter-specimens, maximum friction force after run duration (specified sliding distances), and roughness parameters before and after wear were studied. Hardness as well as the results of the microscopic observations of sliding surfaces were also analysed.
Wear values of counter-specimens were small (up to 3 μm). The results of total wear rates of analysed assemblies are displayed in Table 4. The experiment (for each series) was repeated three
Conclusions
Surface texturing of the block surface (area density between 20 and 26%) by burnishing technique resulted in significant improvement in wear resistance in comparison to a system with untextured samples. The area ratio of 26% minimised linear wear of the tested assembly by 27% in comparison to a system with a turned block. However the oil pockets area ratio should not be very big, because it could cause increase of unitary pressures and then increase of wear intensity. The smallest wear was
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