The surface chemical evidence of the tribofilms is measured by electrical contact resistance and SEM/EDS surface chemical analysis []. Theory of Boundary Lubrication Boundary and extreme-pressure regimes of lubrication often coexist in tribological situations where hydrodynamic, mixed and elasto-hydrodynamic lubrication are ineffective. In this case, the occurring lubrication mechanisms include adsorption, sacrificial films, amorphous layers etc. on the contacting surfaces. These interacting mechanisms ensure that the machinery is kept in operation. Tribologists continue to develop the understanding of boundary lubrication mechanisms because of their practical importance in automotive contacts such as piston liner, and steel gears. Also, metal-working tools employ at least one of these mechanisms for prevention of seizure, severe wear and high coefficients of friction []. The additives in the lubricant control the boundary lubrication mechanisms. The cost of the additives is usually very low compared to that of the machinery being lubricated. Therefore, the additives can be sacrificed for large tribological benefits. The additives form protective layers that reduce coefficients of friction and wear loss. The lubricating mechanisms are related to the load capacity and interfacial frictional temperature. The principle of boundary lubrication is explained, based on coefficient of friction []. The coefficient of friction, COF or µ, is defined as the ratio of the frictional force ‘F’ and the normal load ‘W’ applied to the surface. µ = Equation 34 Interfacial contacts occur at asperities peaks which are somewhat dry. Therefore, the true total contact area is sum of the individual contact areas between the asperities. If the adhesion component of friction is the main friction mechanism between the asperities, and that other effects like ploughing are low; then the frictional force is: F = Equation 35 Where: F is the frictional force, N At is the true contact area, m2 is the effective shear stress of the material, Pa The applied load can be expressed in terms of contact area, i.e.: cylinder
tests. Figure 5-3 (b) shows the flat sample machined from EN-GJL-250 material. The is one of the most widely used ferrous materials for manufacture of automotive engine cylinder liners. Figure 53: Pictorial view of (a) ball with rigid holder and (b) flat friction coupon. All dimensions in mm. Table 51: Material properties of the ball and flat samples Property Ball Sample Flat Sample Name AISI 52100 EN-GJL-250 Manufacture As-supplied Machined, Ground and Polished Surface roughness Mirror-finish Test position and motion Upper and reciprocating Lower and stationary Dimensions, mm Radius, 3 L x B x H = 66 x 25 x 4 Young’s modulus, GPa 210 92.3 Hardness (range), Hv 700-900 224-271 Tensile strength, MPa 2240 250 Poisson’s ratio 0.3 0.211 This is understandably due to its lower cost compared to other Fe-based automotive materials and an array of desired operational advantages. These qualities include []: thermal conductivity, vibration damping, wear resistance and mechanical property. The machined flat samples were ground and polished to an average surface roughness, less than 0.30. Grinding and polishing reduce machining effects on the surface such as flaws, l6ay and texture. The Lubricants: Rapeseed Oil and Mineral Oil The lubricants used are rapeseed oil as a biolubricant and mineral oil. The rapeseed oil is selected due to its availability in the UK. Also, the plain biolubricant is tested without any other additive to show the lubricating mechanisms of the NPs. Typical mid-market fully formulated engine oil is selected as control for this investigation. Its properties as presented by the manufacturer are shown in Table 5-2. Rapeseed oil is commercially abundant and has been proposed as alternative base oil for automotive crankcase lubricant []. Typical lubrication properties of unmodified rapeseed oil as presented by [] and [] are as shown in Table 5-3. The extra virgin cold pressed rapeseed oil was selected and procured for this investigation.
Equation 51 Where: is the radius of ball in the ball-on-flat contact, in meters (m). The ball radius is 6 mm or 6E-3 m The reduced Young’s modulus, Equation 52 Where: and are the Poisson’s ratios of the contacting bodies ‘A’ and ‘B’ respectively; and are the Young’s moduli of the contacting bodies ‘A’ and ‘B’ respectively (Pa). The Poisson’s ratio for steel, is 0.3 and its Young’s modulus, is 2.1 x 1011 Pa. The Poisson’s ratio [] for grey cast iron (GCI), is 0.211 and its Young’s modulus, is 92.30 GPa. The radius of the contact area, a, is given by: Equation 53 Where: is the radius of the contact area, m; is the normal load, N; is the reduced radius of curvature, m; is the reduced Young’s modulus, Pa. The maximum, and average, contact pressures, in MPa, are expressed as follows: Equation 54 Equation 55 The maximum deflection, δ, is given by the expression:
(a) Rapeseed oil and (b) Rapeseed oil with NPs at RT The contact lubricated with mineral oil and NPs additives at RT show lower wear than that of mineral oil only as shown in Figure 5-18. The NPs reduce wear in the tribo-contact by up to 37.4 % (WSW) compared to mineral oil only. NPs can fill the asperity valleys and form tribofilm to support load. More than one NP lubricating mechanism could co-exist in the contact. Figure 519: Optical micrographs (x20) of wear scars on flat friction coupon, after tests with: (a) Mineral oil and (b) Mineral oil with NPs additives at RT Wear Mechanisms on the Ball Sample Although the major wear mechanisms are expected to be observed on the flat sample, the ball also shows surface damage. The optical micrograph of ball after test with a lubricant is therefore necessary. As usual, the harder material in the tribo-pair would experience material transfer, while the softer experience surface material removal. Some ploughed and transferred materials from the softer would bond to the harder sample. These images show some differences in the surfaces of balls with respect to lubricant. The ball in the tribo-pair lubricated with rapeseed oil has some material transfer layer, as shown in Figure 5-19. The material removed from the flat sample, are transferred to the ball. However with rapeseed oil-NPs mixture, the layer spalls off and seem as third bodies, thereby promoting abrasion. This partly explains the slightly higher mean WSW with rapeseed oil-NPs mixture. Figure 520: Typical optical micrographs (x10) of balls with lubricant: (a) Rapeseed oil and (b) Rapeseed oil-NPs mix at 100oC Figure 521: Typical optical micrographs (x10) of ball with lubricant: (a) mineral oil and (b) Mineral oil-NPs mix at 100oC
tribofilm on ferrous material surface under RT sliding. This is shown in Figure 6-7(c) by the absence of the elements of the ZDDP – Zn, S and P respectively on the wear surface. The abrasive lines on the wear surface on the flat sample lubricated with rapeseed oil and the NPs is evidence of abrasive wear action of the NPs on the contact as shown in Figure 6-7(b). But the wear track of the flat sample lubricated with rapeseed oil, ZDDP and the NPs mixture has the evidence of formation of the tribofilms made of both the NPs – Si and O, as well as the ZDDP – Zn, S, and P. This agrees with observations of similar synergy between ZDDP and NPs additives [, , ]. Figure 67: Wear surface morphology and chemistry for tests at RT with the lubricants: (a) Rapeseed oil only (b) Rapeseed oil with the NPs additives, (c) Rapeseed oil with ZDDP and (d) Rapeseed oil with both ZDDP and the NPs additives. The wear surface lubricated with mineral oil and the NPs additives has the elements, Si and O from the NPs present on the wear surface along with the elements, P, S and Zn from the ZDDP usually present in fully formulated engine lubricant, SAE 15W40. This is shown in Figure (b). The surface lubricated with mineral oil only has the elements, P, S, and Zn from the ZDDP as shown in Figure 6-8 (a). Figure 68: Wear surface morphology and chemistry for tests at RT with the lubricants: (a) Mineral oil, (b) Mineral oil and the NPs additives.