eJournals International Colloquium Tribology 23/1

International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231

Tribological behavior study of elastomer - hard substrate contact in marine environment

125
2022
Claire Robin
Ahmad Al Khatib
Jean-Marie Malhaire
Jean-François Coulon
ict2310479
23rd International Colloquium Tribology - January 2022 479 Tribological behavior study of elastomer - hard substrate contact in marine environment Claire Robin, Ahmad Al Khatib ECAM RENNES, Bruz, France Corresponding author: ahmad.al-khatib@ecam-rennes.fr Jean-Marie Malhaire ECAM RENNES, Bruz, France Jean-François Coulon ECAM RENNES, Bruz, France 1. Introduction and state of art A lot of machines and equipment (pumps, open hydraulic drive system and blades of ships, etc.) are used in seawater environment for several applications like transportation, offshore naval missions and oil drilling. Thus, friction and wear studies under seawater conditions is an important topic in tribology [1]. In this environment, seawater plays the role of lubricant and modify the tribological behavior between the mechanical parts. However, these parts should be resistant for high corrosive nature of seawater. For this reason, recent studies involve testing the friction and wear behavior of several materials with anti-corrosive and anti-wear properties like some polymers, ceramics metals and metal alloys in marine environment [2, 3, 4]. The Nitrile butadiene rubber (NBR) is known as an elastomer with good chemical and corrosive resistance for seawater [5]. It is commonly used in industry and especially for marine applications like seals on boats’ propeller shafts. So, we are focusing in this work on the study of tribological behaviour between NBR and hard substrate in marine environment. In the case of elastomers, the phenomenon of friction can be defined as the sum of four main components [6]. We note that the friction force F f as being: F f = F adhesion + F hysteresis + F viscosity + F cohesion . F adhesion represents the frictional force required to break the molecular chains between the surface of the elastomer and the surface of the substrate. It is proportional to the real contact surface. F hysteresis represents the dissipation of energy due to the viscoelasticity of the elastomer. It defines the friction force due to the pressure difference between charge and discharge [7]. F viscosity appears in the case of lubricated friction. It results from the viscosity of the lubricant. Finally, F cohesion corresponds to the initiation of cracks and the wear of the elastomer. These four components are not independent of each other, which complicates the analysis. In lubricated conditions, several researchers show the link between wettability and friction coefficient. Wettability is the ability of a substrate to be wetted by a liquid when they come into contact. It is not characteristic of the substrate or the liquid, but rather of the combination of the two. This is characterized by the shape of the gout of liquid in the materiel surface [8]. In their study, Pawlak et al. (2011) [9] studied the link between the wettability of the material and the resulting coefficient of friction in the case of hydrophilic-hydrophilic, hydrophilic-hydrophobic and hydrophobic-hydrophobic tribological couples with water lubrication. For a hydrophilic-hydrophilic couple, the coefficient of friction increases significantly when the wettability increases. The water then plays a bonding role. For a hydrophobic-hydrophobic couple, the coefficient does not change if the wettability increases. In the case of hydrophilic-hydrophobic contact, the coefficient of friction decreases significantly with wettability. The wettability of the surface of material can be modified by several treatments and coatings (Plasma, DLC). Kim et al. (2011) [10] studied the relationship between the decrease in the wettability of the substrate and the friction coefficient in the case of plasma treatments on NBR. The tribological tests were carried out with grease lubrication. Decreasing the contact angle from 100° to 50° leads to a decrease in the coefficient of friction from 0.25 to 0.15. Our state of the art leads us to focus our study of master degree on the link between the wettability and the tribological behavior of NBR in friction against a hard substrate in a marine lubrication environment. The NBR samples will be in friction with a glass plate but also with a stainless steel plate, this to bring us closer to industrial reality. 2. Material and methods NBR samples (60 ±7 Shore A; Ra=0.6μm) are cut in the form of pellets with a diameter of 6mm and a thickness of 1.5mm. The used hard substrates are a glass plate (80mm×40mm×3mm; Ra =0.06 μm) and tow stainless 480 23rd International Colloquium Tribology - January 2022 Tribological behavior study of elastomer - hard substrate contact in marine environment steel plates with tow roughness (80mm×40mm×3mm; Ra 1 = 2μm; Ra 2 =0.03μm). A sliding tribometer developed in our university is used. The speed of sliding is of 2mm/ s. We set the normal load on NBR samples during the tests at 140 grams. The tests are realized in three conditions: a) dry condition; b) demineralized water lubrication condition; c) artificial seawater lubrication condition. For the third condition, it is difficult to have repeatable measurements with natural seawater. Indeed, the composition of this water is varying significantly depending on the geographical area but also on temperature and sea currents. For this reason, we decided to use so-called artificial seawater by adopting a seawater synthesis using the standard D1141 - 98 [11]. In addition to the standard concentration seawater, we used doubled concentration seawater in order to study the coefficient of friction as a function of the salinity of the water. To modify the wettability of NBR samples, atmospheric-pressure plasm is used. The plasma treatment is carried out at a height of 20mm from the sample, the speed is 300 mm/ s. The torch is set at a frequency of 200 kHz and the flow rate is 40 l/ min. These parameters are determined to not exceed the temperature of deterioration of the elastomer while having a treatment that allows a significant change in the contact angle. To measure the wettability of different surfaces, a contact angle test is performed using the Owens-Wendt method [8]. The liquids used to measure the contact angle are demineralized water, diazomethane as well as artificial seawater. 3. Results and discussion In the case of NBR/ glass friction (Figure 1), seawater allows a reduction in the coefficient of friction which drops from 0.62 for demineralized water to 0.305 for standard seawater. This reduction is less important for doubled concentration seawater. The reduction of friction coefficient is valid for treated and untreated NBR. This result is in agreement with the work of Wang et al (2009) [12]. In fact, seawater is a lubricant that significantly reduces the coefficient of friction. However, this depends on the concentration of salts in seawater. In the case of NBR/ stainless steel (Ra: 2µm) friction (figure 2), this difference is less significant. This can be explained by an influence of roughness more important. Indeed, the figure (3) shows that the friction coefficient increases when the roughness of the steel plate is less important. This is explained by the effect of the real contact surface between NBR and hard substrate. Thus, the NBR sample does not fully fit into the interstices created by the increase in roughness as shown by the work of et Ido al., (2019) [7]. The plasma treatment, as we carried out, made it possible to reduce the contact angle for the three lubrication conditions (demineralized water, standard seawater and doubled seawater). Reducing the contact angle by plasma treatment leads to a significant increase in the coefficient of friction for both standard concentration seawater and doubled concentration seawater. We find this effect for NBR/ glass and NBR/ steel frictions. However, we couldn’t find this effect with demineralized seawater NBR/ glass friction. If we reconsider the equation developed by Kummer [6], F cohesion can be identified by tensile tests but also by a numerical simulation that we carried out on Abaqus commercial solution (figure 4). By comparing the results of stress from the simulation with tensile strength, it can be considered that wear does not exist. Between demineralized water and seawater, the difference in viscosity is 0.059 centipoise at room temperature. However, we have not investigated the role of such a small difference in viscosity on F viscosity and consequently on the friction coefficient. F hysteresis is due to energy dissipation due to the viscoelasticity of the elastomer which does not change between the different conditions. However, when the roughness changes, the hysteresis is also modified. Finally, F adhesion was related to the couple of material in friction but also to the plasma treatment. Figure 1: coefficient of friction on glass with and without plasma treatment for different lubrication conditions 23rd International Colloquium Tribology - January 2022 481 Tribological behavior study of elastomer - hard substrate contact in marine environment Figure 2 : coefficient of friction on steel with and without plasma treatment for different lubrication conditions Figure 3: coefficient of friction of steel for two roughness Figure 4 : finite elements simulation for friction NBR/ hard substrate References [1] Haq, M. I. U., Raina, A., Vohra, K., Kumar, R., & Anand, A. (2018). An assessment of tribological characteristics of different materials under sea water environment. Materials Today: Proceedings, 5(2), 3602-3609. [2] Dong, C. L., Bai, X. Q., Yan, X. P., & Yuan, C. Q. (2013). Research status and advances on tribological study of materials under ocean environment. Tribology, 33(3), 312-320. [3] Wang, D., Li, Z. Y., & Zhu, Y. Q. (2003). Lubrication and tribology in seawater hydraulic piston pump. Journal of Marine Science and Application, 2(1), 35-40. [4] Shan, L., Wang, Y., Li, J., Li, H., Wu, X., & Chen, J. (2013). Tribological behaviours of PVD TiN and TiCN coatings in artificial seawater. Surface and Coatings Technology, 226, 40-50. [5] ZHAI, Z. S., ZHONG, X., ZHANG, Y. P., & HU, G. (2011). Predicting the service lifetime of O-rings in sea water. Synthetic Materials Aging and Applicationg, 6, 39-43. [6] Kummer, H. W. (1966). Unified theory of rubber and tire friction. Engineering research bulletin, 94, p. 152. [7] Ido, T., Yamaguchi, T., Shibata, K., Matsuki, K., Yumii, K., & Hokkirigawa, K. (2019). Sliding friction characteristics of styrene butadiene rubbers with varied surface roughness under water lubrication. Tribology International, 133, 230-235. [8] Owens, D. K., & Wendt, R. C. (1969). Estimation of the surface free energy of polymers. Journal of applied polymer science, 13(8), 1741-1747. [9] Pawlak, Z., Urbaniak, W., & Oloyede, A. (2011). The relationship between friction and wettability in aqueous environment. Wear, 271(9-10), 1745- 1749. [10] Kim, J. H., Kim, S. S., Choi, S. G., & Lee, S. H. (2011). The friction behavior of NBR surface modified by argon plasma treatment. International Journal of Modern Physics B, 25(31), 4249-4252. [11] ASTM (2003). Standard Practice for the Preparation of Substitute Ocean Water D1141-98. ASTM international. [12] Wang, J., Yan, F., & Xue, Q. (2009). Tribological behavior of PTFE sliding against steel in sea water. Wear, 267(9-10), 1634-1641.