Preparation and tribological behaviors of porous oil-containing polyimide/hollow mesoporous silica nanospheres composite films

Highlights

• The hollow mesoporous SiO₂ improved the oil storage performance of composites.

• Liquid paraffin realize its continuous release during friction process.

• The friction coefficient of oil-containing composites is lower and more stable.

• The wear life of oil-containing composites is longer than that of the PI oil film.

• The oil-containing composites can be used in high temperature and load condition.

Abstract

Porous oil-containing polyimide (PI)/hollow mesoporous silica nanospheres (HMSNs) composite films of PI/HMSNs were fabricated by infusing liquid paraffin into the pores under vacuum. The microstructure, oil storage capability and tribological behaviors of porous oil-containing PI/HMSNs composite films were evaluated using the porous oil-containing PI film as comparisons. As a result, the addition of HMSNs could greatly improve the oil content and oil retention, realizing the intended goals that the prepared oil-containing composites could effectively reduce the friction coefficient and extend the wear life. Besides, liquid paraffin in the porous PI/HMSNs composite films released stably under the actions of the applied load and the friction heat, which can be used in high temperature and load conditions.

Introduction

With the rapid developments of aerospace and civil equipment, the great challenges brought by the friction and wear issues raise higher demands on the quality of the mechanical elements (bearings, gears, etc.) [1]. On the one hand, the tribological properties of traditional materials can no longer meet the operational requirements under special working conditions. On the other hand, the improper usage of the lubricant oil (volatilization, climbing and moving, etc.) cause serious environment pollution and waste of resources [2,3]. Therefore, it is very promising to develop new wear-resistant materials and improve the supply mode of lubricant oil to reduce or control the friction and wear.

In recent years, researches on the lubrication application of oil-containing porous polymer materials have been advanced [[4], [5], [6]]. At present, polyimide (PI), polyamide (PA), polytetrafluoroethene (PTFE), ultrahigh molecular weight polyethylene (UHMWPE) and polyether ether ketone (PEEK) are the common porous polymer materials [[7], [8], [9], [10], [11], [12]]. Among them, the oil-containing porous PI materials have attracted attention as one kind of potential candidates in tribological systems and have been reported in some literatures [[13], [14], [15], [16]]. Yan et al. [17] studied the friction and wear properties of porous PI materials with lubricant oils of different viscosities. It was found that the lubricant oil could be exuded and form a lubricant film on the worn surface, which could effectively reduce the friction coefficient. But under the condition of high rotating speed, the friction coefficient increased rapidly due to insufficient supply of lubricant oil, and a large amount of friction heat led to the temperature increase of worn surface, which resulted in severe adhesion wear and the ultimate failure of materials. Qiu et al. [18] prepared porous PI material using cold pressing and sintering techniques, then investigated its oil storage capabilities and tribological behaviors by infusing three different lubricant oils. However, the problems of low oil content, serious wear and short life of porous PI materials still exist. It is difficult to meet the requirements of high oil storage performance, excellent friction and wear properties at the same time.

In view of the problems and challenges faced by oil-retaining porous PI materials, the preparation of organic-inorganic porous composites by introducing inorganic nanoparticles into porous polymer matrix is attracting a great deal of interest. Silica (SiO₂), as the representative of inorganic nano-additives, possesses different morphology and structure, high surface area, extremely high thermal stability and the lowest thermal expansion coefficient (about 5.0 × 10⁻⁷ K⁻¹ at room temperature) [[19], [20], [21]], which is very suitable to serve as a nano-component for enhancing the performance of the polymer materials [[22], [23], [24], [25], [26], [27]]. And the morphology diversity of SiO₂ further expands its application fields. In recent years, the research on using mesoporous channel of SiO₂ as a micro storage container for retaining lubricant oil has made a breakthrough progress. Ye et al. [28] realized the storage of liquid paraffin oil in the mesoporous SiO₂ under the reduced pressure conditions. It is also proved that the paraffin oil stored in the mesoporous SiO₂ could be released slowly and played a key role in reducing friction and wear during operation. In addition, Wang et al. [29] also found that the mesoporous SiO₂ could improve the tribological performances of the lubricating oil well when the mesoporous SiO₂ particles were used as the lubricant oil additives. Based on the above mentioned research results, it can be inferred that the hollow-structured mesoporous SiO₂ nanospheres (HMSNs) as a special structural form of silicon-based microspheres, can be introduced into the porous PI matrix to prepare PI/HMSNs composite materials, aiming at enriching the diversity of the internal pores, as well as improving the oil storage performance and tribological properties of traditional porous PI materials. Up to date, there is no related report involved on the preparation and tribological behaviors of oil-containing porous PI/HMSNs composites.

In this work, the preparation of oil-containing porous PI/HMSNs materials was carried out elaborately, which showed good performances including slow release of oil, apparent friction-reduction and wear-resistance capabilities under the special working conditions of high load and temperature.