The Effect Of Lubricating Oil And Grease On The Freeting Wear Of Steel And The Study Of Their Action Mechanism

As one of main factors of failure of matching components in many industry departments, fretting damage has been known as the cancer of modern industry. Fretting wear, one of the mechanisms of fretting damage, has resulted in precision failure, component occlusion and looseness, power loss, noise and pollution. In industry, the fretting wear exists usually because the vibration source can't always be avoided, and different kinds of methods have been developed to alleviate fretting wear. Among them, oil and grease lubrication as one of the effective method has been highly emphasized by the researcher.The fretting behavior has been performed on high precision PLINT fretting device with the GCr15 bearing steel ball/GCrl5 bearing steel flat and GCrl5 bearing steel ball/ 45 steel flat using different types of oils and urea grease lubrication. The displacement amplitude, frequency, number of cycles, normal load, temperature and relative humidity were 5～80μm,1～5 Hz, 1～3×104, 50～100 N,15～20℃and 30～70%, respectively. The wear scar was studied by using optical microscope (OM), laser confocal scanning microscopy (LCSM), AMBIOS XP-2 surface profilometer, MVK-H21 micro-hardness tester, scanning electron microscopy (SEM) and EDX. Surface layer of the wear scar was observed by etching the worn surface in a solution containing 4% HNO3 in ethanol. The mechanism of palliating fretting wear of different types of synthetic oils and urea grease made in the laboratory was analyzed, and the main factors which affect the palliating effect of oils and greases were investigated. Main conclusions are drawn as follows:1. The fretting wear under oil lubrication is related closely to the fretting displacement amplitude. At lower displacement amplitude, the fretting is in the partial slip regime. The oil is ejected out of the contact surface, which can only be found in the contact edge and forming oil shield. Due to isolation between air and contact surface by it, the oxidation effect decreases and adhesion increases, so the steady state coefficient of friction is higher than that under dry condition and the partial slip regime range is increased. The border line between the partial slip regime and the mixed regime moves towards the right in the running condition fretting map compared to that of dry condition. The micro-slip area is very small, and the oil is hard to penetrate into the contact surface again after discharging. As a result, little difference in wear topography is observed under different types of oil lubrication. At intermediate displacement amplitude, the fretting is in the mixed regime. Similar to the partial slip regime, the oil shield results in the steady state coefficient of friction being higher than that under dry condition. The oil penetrates into the micro-cracks of worn surface, and accelerates the detachment of particles owing to the extrusion effect of oil during fretting. The fretting mark is deeper than that under dry condition. At higher displacement amplitude, the fretting is in the slip regime. The oil penetrates into the friction interface to form the mixed lubrication regime and reduces the oxidation corrosion, so the steady state coefficient of friction and wear are remarkably reduced.2. The digging effect at intermediate displacement amplitude and the palliating effect at higher displacement amplitude of different synthetic oils are related closely to the chemical and physical properties of synthetic oil. At intermediate displacement amplitude, the ability of penetration into the cracks of silicone oil Silicone 68 is the biggest because of the lowest surface tension and the adsorption of polyalkylene glycol PAG 68 and synthetic ester PETO to the metal surface of the cracks is stronger owing to polarity, so they form deeper wear cavity than that in Polyalphaolefin PAO 10 and mineral oil 350SN lubrication. At higher displacement amplitude, the palliation effect of silicone 68 is the poorest due to the lowest surface tension and pressure-viscosity, as well as the maximum compressibility resulting in the thinnest lubricating film and the maximum direct contact area between metals. The polar PAG 68 and PETO may form the adsorption film at the metal contact surface, as a result the maximum and steady state coefficient of friction is lower than those under PAO 10 and 350SN lubrication. The high hardness white layer formed at initial stage of fretting under PAO lubrication leads to the lowest wear. The wear depth and volume for the PAO 10 lubrication are less than half of them for the Silicone 68 lubrication.3. There isn't apparent relation between wear area and wear volume for fretting. At the mixed regime of fretting, the wear scar under oil lubrication exhibited smaller wear area but higher wear depth comparing to the dry condition. At the slip regime of fretting, the wear area under PAO 10 lubrication is larger than that under other oil lubrication, but the wear volume is the smallest.4. At higher displacement amplitude, the fretting is in the slip regime. The evolution of coefficient of friction with the number of cycles is closely related to the forming, the breaking and the forming again of lubrication film under grease lubrication. At the early stage of fretting, the coefficient of friction increases to the maximum value owing to grease ejectment from the contact zone which results in the breakdown of grease film, and then the high shear stress gives rise to the formation of wear cavity and white layer on the wear scar surface. Base oil separated from the grease penetrates into the wear cavity, and then the mixed lubrication regime is formed, thus the coefficient of friction fluctuates down to a low steady state value, only about 1/6 of that under dry condition. Under urea grease lubrication, wear volume is only about 1/24 of that under dry condition, and the wear occurs mainly at the initial stage of fretting. The more base oil separated from the grease, the better palliation effect for wear. Besides, both the high hardness white layer which hasn't been destroyed on a large scale because of the low steady status shear stress and the reducing of oxidation corrosion under grease lubrication conduce to suppressing wear. The bubbles have little influence on the fretting wear.5. In industrial practice, oil and grease lubrication is a very good method to palliate fretting wear if the fretting is at higher displacement amplitude. For palliation effect, the chemical and physical properties of oils and the oil separation of grease should be taken into account at first. In addition, the effection of oil and grease lubrication on the friction behavior of steel is different for fretting and sliding condition.