Analysis of the tribological performances of biodegradable hydraulic oils HEES and HEPR in the sliding of Cu–Zn/WC–CoCr alloys using the Stribeck curve

Abstract

In surface engineering, new coatings and deposition techniques for decreasing wear have been proposed. However, the tribological behaviors of these coatings under lubricated sliding with biodegradable oils are unknown. The objective of this study was to evaluate the tribological behaviors of two hydraulic biodegradable oils, namely hydraulic environmental ester oil synthetic (HEES) and hydraulic oil environmental polyalphaolefin (HEPR), with hydraulic mineral oil (HLP), using a pin-on-disk tribometer. In the sliding tests, a Cu-35Zn sphere against a flat surface is coated with WC-10Co4Cr alloy using a HVOF thermal spray. The Stribeck curve was used to evaluate the performances of the lubricants. The coefficient of friction, the contact pressure, and the film thickness were determined. In addition, the coefficient of wear of the sphere was evaluated, and the oil with the lowest value was identified, which was HEPR in this case. In long-term tests, HEPR was affected by the stick-slip phenomenon, which increased the coefficients of friction and wear. Furthermore, the mechanism of adhesion of the sphere on the disk was more evident with the use of HEES compared to HLP. The highest concentrations of Zn and P and the pressure-viscosity coefficient value, which was detected in the mineral oil, caused friction reduction and lower damage to the surfaces. Therefore, it is important to evaluate the tribological conditions of synthetic bio-lubricants for applications in hydraulic systems.

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“…It sketched out the COFs with the variable of sliding velocity, RMS roughness, contact pressure and dynamic viscosity using the Hersey number ( Hs ) via Equation (2). The RMS roughness values depend on the equivalent amplitude of roughness ( σ RMS ), which are calculated using Equation (3) [41]. $$${\sigma }_{RMS}=\sqrt{{{S}_{qball}}^{2}+{{S}_{qUHMWPE}}^{2}}$$$ where S qUHMWPE is shown in Figure 3i, and S qball was 0.04 μm.…”

“…The mean contact pressure P mean between the UHMWPE plate and 316 stainless steel ball is calculated in Equation (4) [41]. $$${P}_{mean}=\frac{{F}_{n}}{\pi {\alpha }^{2}}$$$ where a is the radius of the contact area, which is produced by the contact pressure and determined from the Hertz pressure.…”

“…It sketched out the COFs with the variable of sliding velocity, RMS roughness, contact pressure and dynamic viscosity using the Hersey number (Hs) via Equation (2). The RMS roughness values depend on the equivalent amplitude of roughness (σ RMS ), which are calculated using Equation (3) [41].…”

“…The mean contact pressure P mean between the UHMWPE plate and 316 stainless steel ball is calculated in Equation ( 4) [41].…”

“…where a is the radius of the contact area, which is produced by the contact pressure and determined from the Hertz pressure. Fn is the normal force that is applied in the tribological testing, 10 N. Figure 8 presented the variation of COFs with Hs for PE-30HA and UHMWPE in reciprocating tribological testing with reference to the changing slide speed at the normal load of 10 N. All curves have the corresponding characterisations of various lubricant regimes with reference to the typical Stribeck curve [41], that is, mixed, elastohydrodynamic and hydrodynamic, but without boundary. The divisions of the mixed, elastohydrodynamic and hydrodynamic lubrication regimes are identified based on the changes in the values of the COF, which are mainly near the upwardly concave point.…”