Influence of Structural Disorders on the Tribological Behavior of Phosphate-Intercalated Layered Double Hydroxide Additives in Polyalphaolefin

Abstract

In this work, several phosphate-intercalated Mg–Al layered double hydroxides (LDHs) were synthesized and evaluated as solid lubricant additives in polyalphaolefin (PAO-4) by means of tribotesting on coupled GCr15/cast iron contacts. The effects of test parameters such as normal loads, additive concentrations, and substrate surface roughness were investigated, while the LDH crystal structure received considerable attention. Several types of structural disorder after anion exchange were identified based on X-ray diffraction (XRD) analysis. The unstable structures promote feasible shearing during sliding to improve friction and wear. In addition, antiwear properties correlate well with the anion charge number or the quantity of anion in the interlayer region. Overall, the tribological performance increased in the order HPO4 2–-LDH < PO4 3–-LDH < P2O7 4–-LDH < (PO3)6 6–-LDH. (PO3)6 6–-LDH demonstrated the best antiwear performance with a reduction of 69% of the ball volume loss compared to PAO-4 oil due to the synergy of the disordered stacking LDH sheets and flexible ring structure of the (PO3)6 6– anion. Furthermore, on polished surfaces, the coefficient of friction (COF) of the (PO3)6 6–-LDH sample dropped significantly by 26%, while the wear loss reduction of more than 80% was also substantial compared to the base oil sample. A performance comparison between the best-performing LDH additive was also conducted against popular nanomaterials, such as hexagonal boron nitride (BN), graphene nanoplatelets (GNPs), and titanium oxide (TiO2). The performance of (PO3)6 6–-LDH was close to that of GNPs.

Tribochemical synergy between phosphate-intercalated layered double hydroxide additives and super high oleic safflower oil on sliding contacts

“…The tribological performance of the LDH additives was evaluated in SHO safflower oil, addressed as the OA base. Compared to the performance of the conventional polyalphaolefin-4 (PAO-4) oil base under the same testing conditions in another study, 20 the SHO safflower oil can significantly lower friction by 35% and wear by 82%. Worn surfaces were thoroughly examined while the complex interaction between base oil and LDHs was also discussed when elaborating the lubrication mechanism.…”

“…As a benchmark, same testing conditions were applied for the PAO-4 sample, a popular engine oil base. 20 The steady state COF of PAO-4 was 0.143 AE 0.005 on ground cast iron. Meanwhile, OA significantly reduced steady state COF to only 0.093 AE 0.001, which is 35% lower than that of PAO-4.…”

Intercalation vs Adsorption Strategies of Myo-Inositol Hexakisphosphate into Zn–Fe Layered Double Hydroxide: A Tiff between Anion Exchange and Coprecipitation

“…4 The interlayer spacing calculated from eq 2 is highest for Z1 sample followed by Z2 > Z3> Z4> Z5 (Table 3). The basal repeat distance "c" and supercell parameter "a" are calculated as c = 3d 003 and a = 2d 110 48 for the samples and are listed in Table 3. A broadening of peak corresponding to the (01l) plane around mid-two theta angles refers to stacking faults occurring due to the simultaneous formation of LDH and rearrangement of the bulky phytate anion.…”

Surface-Modified LDH Nanosheets with High Dispersibility in Oil for Friction and Wear Reduction

“…However, pristine LDHs are prone to agglomeration and sedimentation in lubricating oil, resulting in abrasion of the friction pair interface. Depending on the availability of different anionic types, numerous studies have inserted long monovalent and inorganic anions into the LDH interlayer, such as C 12 H 23 O 2 Na, dodecanoic acid, and phosphate. − As a consequence, interlayer interaction forces are reduced, which greatly reduces friction. Moreover, surface modification can be used to enhance the dispersion and tribological performance of LDHs .…”