Bifunctional hairy silica nanoparticles as high-performance additives for lubricant

Bifunctional hairy silica nanoparticles (BHSNs), which are silica nanoparticles covered with alkyl and amino organic chains, were prepared as high-performance additives for lubricants. Compared with hairy silica nanoparticles covered by a single type of organic chain, binary hairy silica nanoparticles exhibit the advantages of both types of organic chains, which exhibit excellent compatibility with lubricants and adsorbability to metal surfaces. Nanoparticles with different ratios of amino and alkyl ligands were investigated. In comparison to an untreated lubricant, BHSNs reduce the friction coefficient and wear scar diameter by 40% and 60%, respectively. The wear mechanism of BHSNs was investigated, and the protective and filling effect of the nanoparticles improved because of collaboration of amino and alkyl ligands.

. The average size of HSNs dispersing in PAO, the, blue column is the particle size in freshly prepared HSNs-PAO lubricants and the orange column is the particle size after 4 months standing.

Viscosity-temperature properties of BHSNs-PAO lubricants
The viscosity index (VI) is an important measure for the change of viscosity with temperature. The higher the VI, the smaller the change of viscosity with temperature. The VI was calculated as following: V indicates the viscosity index, U the oil's kinematic viscosity at 40 °C (104 °F), and L & H are values based on the oil's kinematic viscosity at 100 °C (212 °F). L and H are the values of viscosity at 40 °C for oils of VI 0 and 100 respectively, having the same viscosity at 100 °C as the oil whose VI we are trying to determine. These L and H values can be found in ASTM D2270. The VI of different types of HSNs was shown in Figure S4(b).

Surface roughness of wear surface and surface adsorption
The surface roughness of the wear scar was characterized by laser scanning confocal microscope (LSCM). The surface average roughness (SRa) and surface root mean square roughness (SRq)of the wear scar surface was shown in table S1. Average roughness is the arithmetic average of the absolute values of the profile height deviations recorded within the evaluation length and measured from the mean line. Root mean square roughness is the root mean square average of the profile height deviations taken within the evaluation length and measured from the mean line (Standard ASME B46.

The wear surface of A-HSNs and O-HSNs
The wear surface of A-HSNs and O-HSNs were examined by SEM and shown in Figure S6 (a, c) and (b, d) respectively. It could be found from the marked region in Figure S6 (a) that large clusters of A-HSNs are found on the wear surface. Grooves are found near the large nanoparticle clusters, which could be due to the ploughing effect of nanoparticles. With amino functional groups on the surface, nanoparticles are easy to aggregate and form large clusters. Those clusters would lead to three body abrasion. FigureS6 (b) shows the wear surfaces of O-HSNs, it could be found that nanoparticles are monodispersed and the wear surface is smoother than the wear surfaces of A-HSNs. However, comparing with A-HSNs which large amount of nanoparticles adsorbed on the surface, only few numbers of O-HSNs are found on wear surface, which could be due to the bad adsorption of alkyl ligands. Nano-grooves were found from Figure S6(c). However, A-HSNs were found to form large cluster nearby instead of filling into the grooves. This could be attributed to the bad dispersion of HSNs that prevent nanoparticles from filling into nano-grooves (the diameter of the grooves is smaller than the diameter of nanoparticle cluster). Although A-HSNs have good adsorption on metal surface, nanoparticles could only adsorbed beside the nano-grooves. With good dispersion, O-HSNs are found mono-distributed on the wear surface, but only few of the nanoparticles are adsorbed on the surface due. With better adsorption and dispersion, BHSNs are found to exhibit better filling effect which monodispersed nanoparticles fill into the grooves and anchored in the grooves.  Figure S7. The nanoparticles in the groove of (a, b) before and after ultrasonic cleaning, (c) start of the lubrication process and (d) during the lubrication process.

Four-ball tribometer
Chuck (Collet), makes top ball turn with motor shaft.
Test-lubricant cup, clamps three lower balls together and holds test lubricant.
Friction arm, prevent test oil cup from turning.
Spacer, permits bringing test-lubricant cup into place under top ball.
Test bearing ball.
ThrusT bearing, permits three lower balls to align with upper ball and allows frictional torque on test-lubricant cup to be measured.
Force from lever-arm system Figure S8. The section view of four-ball tribometer The section view of four-ball tribometer was shown in Figure S8. Test procedure was list as following: Place the three test balls in the test-lubricant cup. Place the lock ring over the test balls and screw down the nut securely. Pour the lubricating fluid to be tested over the three test balls until they are covered. Press one ball into the ball chuck and mount the chuck into the chuck-holder Install the test-lubricant cup assembly on the test apparatus in contact with the fourth ball. Place the spacer between cup and thrust bearing. Place the weight tray and sufficient weights on the horizontal arm in the correct notch for a base test load. Release the lever arm and gently apply the test load to the balls, making certain the cup assembly and spacer are centered. If the optional friction-measuring device is used, connect the calibrated arm on the test-lubricant cup to the indicator spring by means of the clip and wire.