Keywords

1 Introduction

In the growing environmental concerns, energy demands have led to the development of green tribology. Green tribology, an emerging field in the area of tribology, deals with environmental aspects of tribology [1]. Out of the various principles of green tribology as proposed by Nasonovskyi and Bhushan [2], self-lubrication or elimination of liquid lubricants is an important one. There is widespread involvement of lubrication in various sliding applications, and without lubrication, the life of the contact materials decreases to larger extents. The science of contact surfaces, i.e. tribology would not have been originated without lubrication. Improper lubrication has led to huge energy consumption. It has been reported by scientists and researchers that wear and friction at the contact surfaces resulted in 23% of total energy consumption. Out of this, 20% is consumed to overcome friction and rest 3% for remanufacturing. Globally, saving in this regard amounts to 1.4% of the GDP on annual basis [1]. The researchers have explored that the parts of passenger cars, e.g. brakes, engine, etc. consume one-third of the energy from fuel to overcome friction. As reported by researchers, 21.5% of the fuel energy is utilized in causing the motion of the vehicle excluding friction due to brakes. With the advent of new ways and means of lubrication, losses due to friction in passenger cars can be reduced by 18% in the short term and by 61% in the long term [2]. Moreover, in case of paper mills, the consumption of energy to overcome friction in moving parts amounts to 15–25%, and the new technological advancements in the area of lubrication will result in reducing friction losses up to 11 and 23.6% in short term and long term, respectively. Similarly, in case of heavy-duty vehicles, energy utilized to overcome friction is around 26% of the fuel energy, which can be reduced to 14% by using technological innovations [3–5]. These statistics show that lubrication is inevitable due to the continuous use of sliding parts. In view of the above discussion, it is evident that there is a need is to explore better methods of lubrication.

This paper summarizes the role of solid lubricants in minimizing frictional properties and brings forth the role of fluorides as a solid lubricant. The paper focuses on the work carried out by researchers where they have used calcium fluoride as a solid lubricant and have obtained good results.

2 Solid Lubricants

Contrary to liquid lubricants, solid lubricants are solid materials that have been used to decrease the tribological properties at the contact surfaces of the sliding parts by preventing direct contact between the materials. These lubricants can be categorized as greases, free-flowing powders, additives in various oils/coatings, etc. [6]. The contact surfaces have many asperities with peaks and valleys which cause friction and wear. These peaks and valleys are levelled by the solid lubricants resulting in reduced friction and wear. Moreover, the type of lubrication offered by solid lubricants is boundary lubrication. While comparing with liquid lubrication, solid lubrication results in least contamination, corrosion resistance, dry lubrication, etc. In addition, solid lubricants can also survive in different temperatures and in typical conditions. Contrary to liquid lubrication, solid lubrication is sustainable, as it does not require changing of the lubricant, thereby having little impact on the environment as the lubricant spilled to the environment poses serious threats. These factors have given impetus to the use of solid lubricants. Dry and clean lubrication, resistance to contamination, protection against corrosion, lubrication at various conditions and temperatures, etc. are some of the significant characteristics of solid lubricants due to which such lubrication is frequently being adopted [7]. Solid lubricants are generally classified as lamellar solids, soft metals, oxides, halides, sulphides, alkali earth metals, organic materials, etc. [8].

Various techniques have been used for incorporating solid lubricants into other materials such as stir casting, powder metallurgy, pulse laser deposition, magnetron sputtering, ion-beam mixing and ion-beam-assisted deposition [9,10,11,12].

3 Fluorides

High-temperature lubrication has been of great concern for scientists and researchers. Various lubricants could not perform at high temperatures such as graphite and molybdenum disulphide. Researchers have always looked for lubricants, which could perform at both room and high temperature. Fluorides such as calcium fluoride (CaF2) and barium fluoride (BaF2) have played an important role, especially for high-temperature applications, i.e. for temperatures above 500 °C (1000 °F). Amongst fluorides, CaF2 has been used extensively and still is being explored either alone as a solid lubricant or with other lubricants/fluorides. The adhesiveness of CaF2 has made it a suitable candidate for solid lubrication [13, 14]. The thermal stability of various fluorides of calcium, barium, magnesium, aluminium, etc. has been studied at temperatures ranging from 100 to 1000 °C. The purpose of the study was to explore the possibilities of using fluorides as solid lubricant at different temperatures. The experiments carried out in air and hydrogen atmosphere have revealed that the thermal stability of zinc sulphide at temperatures from 500 to 1000 °C is higher than that of barium, calcium and magnesium fluorides. It has also been observed that sulphides of zinc decompose fully in air at 500 °C and in hydrogen and water vapour at 900 °C [15]. The hardness of fluorides of barium and calcium (CaF2 and BaF2) has been studied by researchers at temperatures ranging from 25 to 670 °C. The findings have shown fluoride eutectic as a good lubricant for high temperatures as it undergoes brittle to ductile transition at about 400–500 °C [16].

CaF2 has a strong chemical, physical and microstructural influence on the tribological behaviour of contact surfaces. The structure of CaF2 is same as that of graphite, molybdenum disulphide, i.e. it has a lamellar structure with hexagonal pattern in every plane, and these planes are connected by weak forces that shear easily at certain temperatures and thus lubricates. The efficacy of liquid lubricants in certain applications involving high temperatures or vacuum environments becomes very low, and thus, in those applications high-temperature solid lubricants like CaF2 play an important role [17].

The low solubility of CaF2 in water and its capability to resist radiations have placed it amongst the safe materials to be used in radiation exposure also. CaF2 has shown good compatibility and enhanced tribological properties with many matrices. However, researchers have also reported decrease in mechanical properties, and some have confirmed increased hardness due to addition of CaF2 [18,19,20].

The improvement in tribological properties against decrease in mechanical properties had made study of CaF2 of great significance. CaF2, being high-temperature solid lubricant, has also performed even at room temperature. This makes study of these materials most important as researchers are trying hard to find solid lubricants which could perform both at room as well as at high-temperature applications [21]. There is not much work being carried out with CaF2 as solid lubricant. The succeeding section presents the research carried out using CaF2 as solid lubricant with metals and ceramics.

4 CaF2-Reinforced Metal Matrix Composites

Researchers have investigated many composites with metals as base matrix and CaF2 as solid lubricant. The results were interesting due to the improvement in tribological properties. Wear and friction characteristics of sintered steels reinforced with titanium carbide, manganese sulphide and calcium fluoride have been investigated at high temperature. The results have shown improved lubricating characteristics with CaF2 [22]. Fe–W–CaF2 composites were fabricated using powder metallurgy, and their mechanical and tribological properties were investigated. Grain refinement resulting due to addition of CaF2 along with intermetallic formations led to increased hardness. The wear and friction testing carried out at high temperature revealed decreased wear rate with increase friction at high speeds. The increase in friction at high speeds is due to the dominance of hard iron oxides over CaF2 [23]. Researchers have developed iron base composite with molybdenum (15 wt%) and CaF2 (6 and 9 wt%) as solid lubricant using powder metallurgy and have found that at high temperatures, there is an increase in mechanical properties such as hardness, impact strength and rupture strength, which is due to the high concentration of CaF2 [24]. Using compaction and sintering, Fe-based composites were fabricated with FeS, ZnS, BaF2, CaF2 and BN as solid lubricants. Amongst all these developed composites, CaF2 reinforced composite have shown low coefficient of friction and wear rate which is attributed to the decreasing shear strength of CaF2 at high temperatures [25]. Sintered Fe-Mo-based composites were developed using CaF2 as a solid lubricant, and the composites were subjected to mechanical and tribological testing at room and high temperature of 600 °C. The weight percentage of Mo and CaF2 was varied and different composites were prepared. From the results, it was observed that reinforcing CaF2 in Fe-10Mo matrix has enhanced mechanical as well as tribological properties. Composite with Fe–10Mo–8CaF2 has shown raised mechanical along with tribological properties at both room and high temperature [26]. Composites with Fe–Cu–C as base and CaF2 as solid lubricant have also been developed by the researchers and have been evaluated at room and high temperatures. Interestingly, the results have revealed that CaF2 can offer increased tribological properties at room as well as high temperatures. However, a slight decrease in some physical and mechanical properties was observed with the increase in CaF2 contents such as density and compression strength [27, 28]. Moreover, the wear and friction of the Fe–Cu–C as base matrix have been enhanced using CaF2 as a solid lubricant [29]. The authors have evaluated the composites at high speeds, and improvement in the tribological properties have been reported. Fe-based impregnated diamond bit matrix was reinforced with CaF2 and hBN, high-temperature solid lubricants. The composites were fabricated by powder metallurgy route and the volume percentage of the lubricants used was 2, 4, 6, 8 and 10. The objective was to study mechanical and tribological properties of the developed composites and draw a comparison between the usages of both high-temperature lubricants. Findings have shown that in general, increasing the lubricant content degrades the mechanical properties; however, the impact of CaF2 in decreasing mechanical behaviour is less significant as compared to hBN. A slight decrease in hardness was observed in CaF2-added composite to that of hBN-added composite. Similarly, the wear loss of hBN-added composites was more than that of CaF2-added ones. Moreover, the worn surface of CaF2-added composite was better than that of hBN-added composite [30]. The mechanical and tribological properties of nickel base bearings developed using hot isostatic pressing have also been studied. Bearings were fabricated with nickel alone and nickel with CaF2. CaF2 was used in these bearings as a solid lubricant. The bearings were developed for high-speed printing machines, and the results have shown interesting outcomes. Intense friction films were observed for bearings made from nickel and CaF2. These films formed at the contact surface resulted in low friction and enhanced wear resistance, 3.2–6 times more as compared to the bearings with nickel only [31].

5 CaF2-Reinforced Ceramic Composites

Ceramics have been explored by many researchers in various applications and have obtained good results [32,33,34]. In order to obtain high tribological performance at elevated temperature of Titanium–Aluminium (TiAl) intermetallic alloy, CaF2 was mixed with NiCr–Cr3C2, and finally, coatings of NiCr–Cr3C2–CaF2 were fabricated on TiAl alloy using laser cladding process. From the wear and friction studies, it was found that the coating with NiCr–Cr3C2–40CaF2 has shown low friction and wear rate [35]. The authors have developed coatings of cobalt alloy, titanium carbide (TiC) and CaF2 on the copper specimen. The ratio of the coating material was fixed as Co alloy alone and variations in the proportions of Co alloy with TiC and CaF2 powders. The coatings were fabricated on Cu specimen using laser cladding. It was observed that fine spherical particles of TiC along with spherical particles of CaF2 dispersed uniformly in the Co matrix. The tribological tests revealed that introduction of CaF2 and TiC in the coating resulted in reduced friction and wear rate of the fabricated composites [36]. A wear-resistant composite was fabricated using Al2O3 and CaF2 using laser cladding. Al2O3 alone and Al2O3–30 wt% CaF2 were tested for wear resistance at room temperature, and Al2O3–CaF2 composite was found to exhibit better wear resistance than monolithic Al2O3. A unique microstructure comprising of isolated spherical CaF2 particles in plate-like framework of Al2O3 was developed, and uniform distribution of the solid lubricant was observed in the inter-framework regions [37]. There was a need to develop silicon carbide-based self-lubricating composites which should operate up to a temperature of 900 °C. The researchers have developed two composites, with SiC alone and with combination of SiC and CaF2. These composites were investigated for tribological properties and results showed raised friction and wear rate in SiC-based composites, whereas composites fabricated with SiC and CaF2 have shown reduced friction and wear rate. This reduction in friction and wear of SiC+ CaF2 up to 700 °C are due to the smearing of calcium fluoride on the contacting surfaces, and at 900 °C, CaF2 reacted with silica, and thus, improvement in wear behaviour was observed [38]. CaF2 has been tried with various ceramics to enhance their tribological properties. CaF2 has been reinforced in Al2O3/TiC composites in 5, 10 and 15 vol% using powder metallurgy route. The results have shown that addition of CaF2 has resulted in decrease of the flexural strength, fracture toughness and hardness as compared to the base matrix. Coefficient of friction also decreased with the increase in CaF2; however, the wear rate was observed to decrease up to 10 vol% CaF2, and beyond 10 vol%, wear rate increases rapidly [39]. Due to the capability of enhancing wear and friction, CaF2 was again tried by researchers with Al2O3 and TiC, but this time, the processing was slightly changed. Al2O3/TiC composites were prepared using cold pressing and sintering, and one more composite was prepared using CaF2. Layered structure was developed by cold pressing, i.e. between the layers of Al2O3/TiC, there was introduced a layer of Al2O3/TiC/CaF2. The friction and wear testing of these composites revealed low coefficient of friction and high wear resistance of the prepared Al2O3/TiC/CaF2–Al2O3/TiC laminated composite as compared to that of Al2O3/TiC ceramic. The worn surface of the Al2O3/TiC/CaF2–Al2O3/TiC laminated composites was found to improve and thus resulted in enhanced wear resistance. During the study, Al2O3/TiC/CaF2 layer in the laminated composite was also observed to be more compact than Al2O3/TiC [40]. BaF2/CaF2 was used with Ag and Ti3SiC2 in a TiAl matrix and the composites were prepared using Spark Plasma Sintering at 1100 °C under a pressure of 40 MPa in pure Ar atmosphere. The proportions of BaF2, CaF2, Ag and Ti3SiC2 were varied in the composites. The objective was to investigate the tribological behaviour against Si3Ni4 over a temperature range from room temperature to 600 °C. The investigation depicted enhanced wear and friction behaviour which was ascribed to the synergetic effect of Ag, Ti3SiC2 and BaF2/CaF2 lubricants [41]. The lubricating potential of combined CaF2 and BaF2 has been investigated by fabricating a composite coating. The coating consists of base matrix as WC-Co, and copper along with both fluorides reinforced using powder metallurgy process. The weight percentage of CaF2 and BaF2 was kept constant at the rate of 19 wt% and 31 wt%, respectively. The findings have revealed that coatings with CaF2 and BaF2 exhibited raised compactness, low coefficient of friction and low wear loss. The study also reflected the capability of these solid lubricants to prevent decarburization and decomposition of the composite [42]. On the same lines, BaF2/CaF2 has been tried with NiCr–Cr2O3–Ag–CaF2/BaF2 coatings, and good results were obtained [43]. Ceramics such as ZrO2 (Y2O3) were tried with 10 wt% of MoS2 and 10 wt% of CaF2 using powder metallurgy. The results of the study have shown the usefulness of such composites at both room as well as high temperatures, e.g. at room/low temperature, MoS2 has shown good tribological behaviour, whereas at high temperatures, an intermetallic CaMoO4 was formed which acted as an effective solid lubricant [44]. Table 1 summarizes various compositions, which have been discussed in this chapter.

Table 1 Compositions
Full size table

6 Conclusion and Future Scope

The expenditure of energy in overcoming friction and the importance of solid lubricants have been discussed in the present work. Solid lubricants play a vital role in minimizing friction in the regimes where liquid lubrication is not suitable. Apart from this, solid lubricants have also been used to enhance the lubrication capability of the liquid lubricants. The paper also focuses upon the importance of fluorides especially CaF2 as a high-temperature solid lubricant, which has also proved its worth at room temperature. Since there is not much work carried out with CaF2 as solid lubricant, the usage of CaF2 in combination with other low- and high-temperature lubricants has also not been explored much. Thus, there is a scope for further research where more materials with CaF2 can be explored. This gives a space for researchers to develop more composites with combined lubricants. In fact, CaF2 can also be tried with liquid lubricants and nano-CaF2 powders with different matrices.