One-pot synthesis of SIB@ZIF-8 with enhanced anti-corrosion properties and excellent lubrication properties

Highlights

• SIB was successfully encapsulated in ZIF-8 with one-pot synthesis, which was designated as SIB@ZIF-8.

• SIB@ZIF-8 exhibited better anti-corrosion and higher anti-wear property than SIB and mechanically mixed SIB/ZIF-8.

• SIB@ZIF-8 provides a highly promising extreme-pressure additive for future industrial applications.

Abstract

Sulfurized isobutylene (SIB) is widely used for extreme-pressure additive. However, the high sulfur content of SIB may cause excessive metal corrosion. Here, we present a strategy for the one-pot encapsulation of SIB in zeolitic imidazolate framework-8 (ZIF-8). The tribological properties of SIB@ZIF-8 as an oil additive was studied by a ball-on-disk reciprocating tribometer. In comparison to SIB and mechanically mixed SIB/ZIF-8, SIB@ZIF-8 displayed better inhibition of metal pairs corrosion, and exhibited higher anti-wear property and load-carrying capacity. This may be attributed to slow release of SIB from SIB@ZIF-8 during the friction process. This study broadens the tribological application of ZIF-8 and also provides a highly promising extreme-pressure additive for future industrial applications.

Introduction

Friction is one of the major sources of energy dissipation, and high wear will shorten the service life of mechanical equipment [[1], [2], [3]]. Therefore, friction and wear reduction technologies have received extensive attention, and good lubrication practices will bring significant economic benefits and energy conservation [[4], [5], [6]]. With the development of technology, mechanical equipment is increasingly required to perform under more severe working conditions, such as heavy load, high-speed, or high-temperature operation conditions. Under these harsh conditions, the natural protective oxide films are removed from the surface of friction pairs, and oil alone cannot provide a reliable protective film. Thus, extreme-pressure (EP) additives have been designed to react with the nascent metal surface to form a protective film, preventing metal–metal adhesion or seizure. Seizure involves severe adhesive wear, and it may cause catastrophic damage to the equipment [7].

EP additives are among the most important ingredients for industrial lubricant formulations [8]. Sulfur element usually is the main composition element in EP additives. Sulfurized olefins (SIB) has been the most widely used and the most cost-effective EP additive [9]. Their mode of action is mainly through thermal decomposition, whereby sulfur reacts with the metal surface to form an intermediate film of iron sulfide, which prevents the interface from undergoing seizure. Sulfide layers can retard the seizure of moving metal surfaces. Although the high percentage of active sulfur in SIB (30–32%) increases the likelihood of reaction with the metal pairs, imparting its anti-seizure property, it can also cause excessive metal corrosion, resulting in high wear of the metal surface [[10], [11], [12], [13]]. Encapsulation of SIB in a porous solid material was envisaged as a possible effective way of overcoming this issue.

Zeolitic imidazolate framework-8 (ZIF-8) belongs to a unique class of crystalline porous metal–organic frameworks (MOFs), the members of which not only show high chemical and thermal stability [14,15], but also flexibility of structure [16]. Therefore, ZIF-8 have been widely used in a variety of applications, including gas adsorption [17,18], separation [19,20], catalysis [21,22], sensing [23,24] and encapsulation [25,26]. It should be noted that the elastic modulus of ZIF-8 is very similar to that of the solid lubricant additive polytetrafluoroethylene (PTFE) [[27], [28], [29]], and like PTFE, ZIF-8 is also hydrophobic [30,31]. Motivated by these merits, our group has studied the use of ZIF-8 as a solid lubricant additive [32,33]. It has been found that ZIF-8 can improve the load-bearing capacity and wear reduction properties of base oil and grease [[32], [33], [34]]. In addition, there has a number of reports about drugs encapsulated into MOFs to stabilize drug against degradation, allowing drugs to be slowly released to produce good therapeutic effect [35,36]. ZIF-8 has large cages (11.6 Å), allowing it to accommodate a number of molecules of the guest extreme-pressure SIB (9.5 × 6.4 × 7.8 Å³). Thus, ZIF-8 was regarded as an ideal candidate as the mother framework. Herein, we report a lubricant additive consisting of organosulfur compound SIB encapsulated in zeolitic imidazolate ZIF-8, which is designated as SIB@ZIF-8. In fact, SIB is not readily adsorbed because its molecule is too large to diffuse through the pore opening (3.4 Å), although the pore opening is relatively flexible. SIB can, however, be readily encapsulated in situ by carrying out the synthesis of ZIF-8 in one pot. The tribological properties of SIB@ZIF-8 as an oil additive have been evaluated with an SRV tester operated in reciprocating mode. In order to ascertain the impact of SIB@ZIF-8 on the tribological behavior, the contact surfaces of friction pairs have been characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), three-dimensional (3D) optical profiling, and X-ray photoelectron spectroscopy (XPS). The results verify that our method is favorable for encapsulating SIB within ZIF-8. The obtained composite exhibited good performance in terms of anti-wear, load-bearing capacity, and extreme-pressure properties.