This paper presents an experimental study on the effects of micro-textured surfaces on lubricated nonconformal point contacts. Thus, we focus on a regime poorly investigated in literature, where the contact area and the micro-holes have a comparable size. Tribological characterization are performed on three geometrical patterns, which are textured on stainless steel polished surfaces. Experiments are carried out on a rheometer, where a steel ball slides against the surface of the samples. These samples are tested with two different viscosities of the PAO (Poly-Alpha-Olefin) as a lubricant. Results show the change in the friction with respect to the sliding velocity under different lubrication regimes due to the stress, void ratio and two different kinematic viscosities of PAO. In particular, we show that, depending on the void ratio, a significant friction reduction or, on the contrary, a deterioration of the frictional performances can affect the boundary and mixed lubrication regimes. This is due to the simultaneous occurrence of two competing effects. One is related to the stress intensification, due to the presence of the micro-hole edges on the contact topography, which leads to a consequent increase in wear and friction. On the other hand, micro-texture may play a positive role in the friction optimization given the possibility, offered by the micro-holes, to entrap wear debris and, then, to preverse a smoother interface between the contacting pairs.
Laser texturing is a viable tool to enhance the tribological performance of surfaces. Especially textures created with Direct Laser Interference Patterning (DLIP) show outstanding improvement in terms of reduction of coefficient of friction (COF) as well as the extension of oil film lifetime. However, since DLIP textures have a limited depth, they can be quickly damaged, especially within the tribocontact area, where wear occurs. This study aims at elucidating the fluid dynamical behavior of the lubricant in the surroundings of the tribocontact where channel-like surface textures are left after the abrasion wear inside the tribocontact area. In a first step, numerical investigations of lubricant wetting phenomena are performed applying OpenFOAM®. The results show that narrow channels (width of 10 μ m ) allow higher spreading than wide channels (width of 30 μ m ). In a second step, fluid transport inside DLIP textures is investigated experimentally. The results show an anisotropic spreading with the spreading velocity dependent on the period and depth of the laser textures. A mechanism is introduced for how lubricant can be transported out of the channels into the tribocontact. The main conclusion of this study is that active lubricant transport in laser textured surfaces can avoid starvation in the tribocontact.
Ultrasonic excitation of pins during a press-in process can reduce the press-in force. There are various theories describing the reason for the force reduction. In this paper, the printed circuit board (PCB) was excited to reduce the press-in force. The advantages of this process in comparison with the excitation of the pin are its suitability for already embedded pins. Another point is that several pins at the same time can be joined with the PCB. A new pin geometry was developed, and an appropriate aluminum-based alloy was chosen to show the effects of ultrasonic excitation. The influence of the amplitude of the ultrasound on the press-in force and the deformation of pin and plated through hole (PTH) were studied. With an excitation amplitude of 20 μm, the press-in force could be reduced by more than 80%. Furthermore, it is found that the force needed for plastic deformation and the friction are reduced by ultrasound. The higher the amplitude, the more deforms the PTH while the pin deforms less.
Press-fit technology is a well-known process in electrical joining and connecting. This process is combined with a subsequent ultrasonic excitation of the printed circuit board (PCB) to enhance the retention force. No additional temperature treatment is necessary because the holding force increases directly after the process. It is discussed how the amplitude and the duration of ultrasonic excitation determine the resulting retention force. A suitable set of parameters is found to increase the retention force. The materials details of the bonds are analyzed. Two models are created to explain the results. These models can be used to predict the retention force for further investigations with changed geometries or materials. status solidi physica a Press-Fit Technology www.pss-a.com
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