Tribological Performance and Application of Antigorite As Lubrication Materials

lubricants

Review Tribological Performance and Application of Antigorite as Lubrication Materials

Zhimin Bai 1,*, Guijin Li 2, Fuyan Zhao 3 and Helong Yu 4

1 Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, University of Geosciences, Beijing 100083, China 2 Shandong Branch, China Building Materials Academy, Beijing 100024, China; [email protected] 3 Qingdao Center, Lanzhou Institute of Chemical Physics, Lanzhou 730000, China; [email protected] 4 National Key Laboratory for Remanufacturing, Army Academy of Armored Forces, Beijing 100072, China; [email protected] * Correspondence: [email protected]

Received: 22 August 2020; Accepted: 2 October 2020; Published: 11 October 2020

Abstract: Antigorite is a Mg-rich 1:1 trioctahedral-structured layered silicate mineral. In recent decades, many studies have been devoted to investigating the tribological performance and application of antigorite as lubrication materials. This article provides an overview of the mineralogy, thermal decomposition and surface modiﬁcations of antigorite powders, as well as the recent advancement that has been achieved in using antigorite to reduce friction and wear of friction pairs. The tribological performance of antigorite powders and its calcined product in diﬀerent lubricating media, such as oil, grease and solid composites have been comprehensively reviewed. The physico-chemical characteristics of surface layers of the friction pairs are discussed. Applications and mechanisms of lubricity and anti-wear of antigorite are highlighted.

Keywords: antigorite; tribological performance; lubricity; application

1. Introduction Antigorite is a Mg-rich 1:1 trioctahedral-structured layered silicate mineral of the serpentine group. Layered minerals are known as laminar structure minerals or layer lattice minerals, have layer crystal structure in which the atoms within a layer are held together by a chemical bond stronger than the bond between the layers. This provides an isotropic shear property with preferred easy shear parallel to the basal planes of the crystallites, and result in the ability of basal planes to slide easily over one another [1]. This may be the reason that layered minerals can provide low friction coefficients and be used as solid lubricants [2–4]. The earlier researches on frictional behavior of antigorite were conducted by geologists and geophysicists, with the intent of seeking information about the strength and sliding stability of natural faults containing antigorite [5–11]. In the recent decades, there has been considerable interest in using antigorite to reduce friction and wear of machinery and equipment. Significant progress has been achieved in the research and development of antigorite’s application in the lubrication of machine components. The research results indicated that antigorite micro–nano powder (AMNP) may significantly reduce friction coefficient and wear of friction pairs, and be one kind of excellent anti-frictional and lubrication materials [12–27]. This paper reviews the mineralogy, thermal decomposition, lubrication behavior and application of antigorite lubricant. The main goal is to draw attention of researchers to antigorite and to assist the researchers in better understanding the function of the antigorite lubricant and its lubrication mechanisms.

Lubricants 2020, 8, 93; doi:10.3390/lubricants8100093 www.mdpi.com/journal/lubricants Lubricants 2020, 8, x FOR PEER REVIEW 2 of 22 Lubricants 2020, 8, 93 2 of 23 2. Mineralogy and Powder Characteristics of Antigorite 2. Mineralogy and Powder Characteristics of Antigorite The theoretical chemical composition of antigorite is SiO2 43.37%, MgO 43.63%, H2O 13.00%,

6 4 10 8 4 + 3 + 3 withThe an ideal theoretical formula chemical of Mg compositionSi O (OH) . ofThey antigorite can exhibit is SiO the2 43.37%, substitution MgO 43.63%,of Si cations H2O 13.00%, by Al with, Fe + 3 + 2 + 2 + 2 + 2 + 2 + 4+ 3+ 3+ an, or ideal Cr formula, and Mg of Mgby Fe6Si4O, Mn10(OH), Ca8. They, or Ni can[28,29]. exhibit The the substitutionactual composition of Si cationsof naturally by Al occurring, Fe , orantigorites Cr3+, and exhibits Mg2+ by variation Fe2+, Mn in2+ the, Ca SiO2+, or2, MgO Ni2+ and[28,29 H].2O, The and actual minor composition proportion of of naturally Al2O3, FeO, occurring Fe2O3, 2 3 antigoritesMnO, CaO, exhibits Cr O and variation NiO are in thealso SiO present2, MgO [4,30–32]. and H2O, and minor proportion of Al2O3, FeO, Fe2O3, MnO,Antigorite CaO, Cr2O belongs3 and NiO to the are group also present of trioctahedral [4,30–32 ].1:1 layered silicates, consisting of one tetrahedral (T) andAntigorite one octahedral belongs (O) to the sheet. group The of T trioctahedral sheet is form 1:1ed layeredby the two-dimensional silicates, consisting polymerization of one tetrahedral of Si- (T)centered and one tetrahedra octahedral sharing (O) sheet. three The out T of sheet four is oxygen formed atoms by the with two-dimensional other tetrahedra. polymerization The unshared of Si-centeredoxygen atoms tetrahedra are bonded sharing to Mg three atoms out ofthat four jointly oxygen with atoms OH groups with other form tetrahedra. the octahedral The unsharedsheets (O oxygensheet). atomsThe T- areand bonded O- sheets to Mg are atoms variably that arranged jointly with and OH stacked groups one form above the octahedralanother, and sheets the (Othickness sheet). Theof the T- T-sheet and O- sheetsis thinner are than variably that arrangedof the O- sheet, and stacked resulting one is abovea subtle another, dimensional and the misfit thickness between of thethe T-sheettwo. This is thinnermisfit can than be thatreduced of the by O- substitutions sheet, resulting in the is atetrahedron, subtle dimensional which in misfit turn reduces between interlayer the two. Thisstrain misfit and canconsequently be reduced enhances by substitutions the mineral’s in the stability tetrahedron, [28,33]. which The layers in turn in reduces the structure interlayer are linked strain andby hydrogen consequently bonds enhances that form the by mineral’s pairing oxygen stability on [ 28the,33 basal]. The tetrahedral layers in surface the structure of one are layer linked with byan hydrogenOH-group bonds on the that upper form octahedral by pairing surface oxygen of onthe the layer basal below. tetrahedral These bonds surface are of generally one layer long with and an OH-groupweak but can on thebe uppermodified octahedral by the degree surface of thesubstitution layer below. [33]. These This bondscrystal arestructure generally gives long rise and to weaktypically but canplaty be or modified lamellar by along the degree（001） ofand substitution perfect cleavages [33]. This on crystal {001} structure of antigorite. gives rise to typically platyMohs or lamellar hardness along of (001)antigorite and perfect is between cleavages 2.5 to on3.5, {001} the Vickers of antigorite. microhardness are within a range fromMohs 196.3hardness to 204.9 [4]. of antigorite Its theoretica is betweenl and measured 2.5 to 3.5, densities the Vickers are microhardness 2.61 g/cm3 and are about within 2.65 a rangeg/cm3, fromrespectively 196.3 to [34]. 204.9 [4]. Its theoretical and measured densities are 2.61 g/cm3 and about 2.65 g/cm3, respectivelyFigure [34].1 shows a typical DTA/TG thermogram (Differential Thermal Analysis/ThermogravimetricFigure1 shows a typical DTA Analysis)/TG thermogram of the antigorite (Differential powders. Thermal The Analysis DTA/ Thermogravimetriccurve shows two Analysis)endothermic of the peaks antigorite and two powders. exothermic The DTA peaks. curve The shows first broad two endothermic endothermic peaks peak and in the two 70–107 exothermic °C is due to the loss of adsorbed water, the second intense endothermic peak in the 620–707 °C may be peaks. The first broad endothermic peak in the 70–107 ◦C is due to the loss of adsorbed water, the second attributed to the loss of structural water of antigorite. The first apparent exothermic peak at 793 °C intense endothermic peak in the 620–707 ◦C may be attributed to the loss of structural water of antigorite. partially overlaps the 831 °C sharp endothermic peak (the second), which indicates the formation of The first apparent exothermic peak at 793 ◦C partially overlaps the 831 ◦C sharp endothermic peak (thethe second),new minerals. which indicatesThe former the formationcorresponds of theto forste new minerals.rite formation, The former the latter corresponds corresponds to forsterite to the formation,formation theof enstatite latter corresponds + forsterite to the[4,35,36]. formation The ofTG enstatite curve shows+ forsterite outstanding [4,35,36]. weight The TG losses curve during shows 620–707 °C, which corresponds to the intense endothermic peak in the DTA curves. Apparent outstanding weight losses during 620–707 ◦C, which corresponds to the intense endothermic peak in activation energy of the reaction in the temperature range 612–708 °C was 255 kJ/mol for antigorite the DTA curves. Apparent activation energy of the reaction in the temperature range 612–708 ◦C was 255[37]. kJ /mol for antigorite [37].

FigureFigure 1.1. DTADTA/TG/TG thermogramthermogram ofof thethe antigoriteantigorite [[4].4].

Lubricants 2020, 8, 93 3 of 23 Lubricants 2020, 8, x FOR PEER REVIEW 3 of 22

Natural antigorite is commonly bladed or ﬁbrous. The antigorite as a lubrication material is Natural antigorite is commonly bladed or fibrous. The antigorite as a lubrication material is usually bladed or lamellar (Figure2), and processed into micro-nano scale powders by powder usually bladed or lamellar (Figure 2), and processed into micro-nano scale powders by powder processing equipment [4,21–23]. processing equipment [4,21–23].

Figure 2. Bladed and lamellar antigorite [4]. [4]. ( (aa,,bb)) are are different diﬀerent particles in the same sample.

Antigorite showedshowed aa positivepositive zeta zeta potential potential over over a a wide wide pH pH range range with with a pHa pH value value of of close close to to 10 10 at itsat its isoelectric isoelectric point point [38 ],[38], which which is readily is readily wetted wetted by water by water and isand said is tosaid be to hydrophilic be hydrophilic [39–41 [39–41].]. In order In toorder improve to improve the compatibility the compatibility of AMNP andof AMNP organic and lubrication organic medium, lubrication the surface medium, modifications the surface of AMNPmodifications have been of AMNP investigated have [been19,21 investigated,22]. The formation [19,21,22]. of organically The formation modified of organically antigorite ismodified carried outantigorite by applying is carried the surfactantsout by applying as boric the acid surfactants ester, sorbitan as boric monostearate acid ester, (Span60), sorbitan polysorbate monostearate 60 (Tween60),(Span60), polysorbate oleic acid, polyvinyl60 (Tween60), pyrrolidone oleic acid, (PVP), polyvinyl silane couplingpyrrolidone agent (PVP), (HK560), silane sodium coupling dodecyl agent benzene(HK560), sulfonate, sodium dodecyl trolamine, benzene stearic sulfonate, acid, sodium trolam stearateine, stearic [19,21, 22acid,,42– sodium45]. The stearate studies investigating[19,21,22,42– the45]. surfaceThe studies modifications investigating of AMNP the surface have modifica shown thattions modified of AMNP powders have shown present that superior modified dispersion powders andpresent suspension superior stability dispersion in the and nonpolar suspension solvent. stabilit It wasy in found the nonpolar that the modifiers solvent. ofIt oleicwas found acid or that Span60 the combinedmodifiers of with oleic borate acid ator 1:1Span60 (mass combined ratio) had with the bestborate modification at 1:1 (mass eff ratio)ect on had AMNP the best [44]. modification effect on AMNP [44]. 3. Tribological Performances of Antigorite 3. Tribological Performances of Antigorite The key research contents of tribological performances of antigorite are shown in Figure3. The antigoriteThe key research powders contents used forof frictiontribological and performances wear experiments of antigorite have been are shown processed in Figure into micron3. The (averageantigorite particlepowders size used 1–20 forµ frictim), oron sub-micronand wear experiments (average particle have been size 100processed nm to into 1 µm) micron or nanometer (average scalesparticle (< size100 1–20 nm). μ Them), antigoriteor sub-micron micro-nano (average powder particle (AMNP) size 100 usednm to for 1 μ frictionm) or nanometer and wear experiments scales (<100 wasnm). mixedThe antigorite with di ffmicro-nanoerent types powder of lubricating (AMNP) oil used to form for friction a suspended and wear solution experiments (Oil + wasAMNP mixed system, with suchdifferent as Table types1 of (No.1 lubricating to 46); oil or to mixed form witha suspended matrix raw soluti materialson (Oil + toAMNP make system, solid composite such as Table materials, 1 (No.1 suchto 46); as or Table mixed1 (No.47 with matrix to 66); raw or materials evenly mixed to make with so baselid composite grease to materials, make a complex such as Table grease, 1 (No.47 such as to Table66); or1 evenly(No.67 mixed to 74). with base grease to make a complex grease, such as Table 1 (No.67 to 74).

Lubricants 2020, 8, 93 4 of 23

Table 1. List of tribological performance of antigorite powder.

Friction and Wear Experiments

No. System and Test Devices µc (%) wearc (%) References (1) Oil + AMNP 0.86 µm (1.0%) (FB) 19.3 16.41 [4] (2) Oil + AMNP(FB) - 18.7 [13] (3) Oil + AMNP < 10.0 µm (0.025%) (FB) - 20.5 [14] (4) Oil + AMNP < 0.5 µm (0.5%) (P-D) 68.1 - [18] (5) Oil + AMNP < 0.5 µm (0.5%) (P-F) 21.3 49.7 [19] (6) Oil + AMNP < 1.0 µm (1.5%) (R-D) 55.3 82.0 [22] (7) Oil + AMNP < 0.8 µm (1.0%) (FB) 21.7 - [24] (8) Oil + AMNP 3.0 µm (1.0%) (P-D) 9.8 23.3 [26] (9) Oil + AMNP 3.0 µm (0.1%) (P-D) - 30.4 [27] (10) Oil + AMNP 1.0 µm (1.5%) (P-D) 58.6 61.4 [46] (11) Oil + AMNP (2.5%) (D-D) 10.0 50.0 [47] (12) Oil + AMNP < 1.0 µm (1.5%) (P-F) 29.0 18.0 [48] (13) Oil + AMNP 0.3 µm (1.0%) (FB) 30.8 15.7 [49] (14) Oil + AMNP < 1.0 µm (0.5%) (P-D) 41.2 28.0 [50] (15) Oil + AMNP < 1.0 µm (0.5%) (P-D) 33.3 - [50] (16) Oil + AMNP < 0.3 µm (10.0%) (FB) 14.8 11.82 [51] (17) Oil + AMNP < 0.4 µm (1.0%) (FB) 14.8 11.6 [51] (18) Oil + AMNP < 0.5 µm (0.05%) (P-F) 16.4 56.7 [52] (19) Oil + AMNP (0.5%) (FB) 53.3 42.6 [53] (20) Oil + AMNP < 2.0 µm (0.5%) (P-F) 18.4 42.4 [54] (21) Oil + AMNP 0.19 µm (0.25%) (FB) 18.1 32.8 [55] (22) Oil + AMNP 1.0 µm (0.5%) (FB) - 79.7 [56] (23) Oil + AMNP < 0.5 µm (0.5%) (F-C) 9.7 40.7 [57] (24) Oil + AMNP < 1.0 µm (1.0%) (R-D) 50.0 - [58] (25) Oil + AMNP 0.02 µm (2.0%) (P-D) - 48.0 [59] (26) Oil + AMNP 0.2 µm (3.0%) (TW) 89.5 - [60] (27) Oil + AMNP < 0.3 µm (0.5%) (P-F) 12.3 66.7 [61] (28) Oil + AMNP 0.3 µm (0.5%) (P-F) 36.6 53.6 [62] (29) Oil + AMNP < 1.0 µm (1.0%) (F-C) 68.3 - [63] (30) Oil + AMNP < 0.5 µm (0.5%) (P-F) 15.5 50.0 [64] (31) Oil + AMNP 1.6 µm (0.5%) (P-F) 51.5 29.6 [65] (32) Oil + AMNP 300 ◦C (1.5%) (P-F) 40.0 39.0 [48] (33) Oil + AMNP 600 ◦C (1.5%) (P-F) 38.0 23.0 [48] Lubricants 2020, 8, 93 5 of 23

Table 1. Cont.

Friction and Wear Experiments

No. System and Test Devices µc (%) wearc (%) References (34) Oil + AMNP 800 C (1.5%) (P-F) 8.0 2.0 [48] ◦ − − (35) Oil + AMNP 1050 C (1.5%) (P-F) 13.0 8.0 [48] ◦ − − (36) Oil + AMNP 200 ◦C (1.0%) (FB) 33.9 17.1 [49] (37) Oil + AMNP 500 ◦C (1.0%) (FB) 27.2 11.4 [49] (38) Oil + AMNP 600 ◦C (1.0%) (FB) 27.1 11.4 [49] (39) Oil + AMNP 800 ◦C (1.0%) (FB) 26.6 8.6 [49] (40) Oil + AMNP + La(OH)2 (0.5%) (FB) 24.6 41.9 [25] (41) Oil + AMNP (0.46%) + Cu (0.04%) (P-F) 31.3 65.1 [19] (42) Oil + AMNP (0.25%) + Ce (0.25%) (FB) 43.9 50.0 [54] (43) Oil + AMNP (0.475%) + La (0.025%) (FB) 34.2 68.8 [66] Oil + AMNP (0.07%) + Ni (0.1%) + Cu (44) 37.4 34.0 [67] (0.3%) (FB) (45) Oil + AMNP (0.25%) + Mo (0.3%) (FB) 32.8 53.2 [55] (46) Oil + AMNP (0.48%) + La (0.02%) (P-F) 29.1 60.0 [64] (47) PTFE + AMNP (1%) (RF-RC) 10.0 95.6 [68] (48) PTFE + AMNP (2%) (RF-RC) 15.0 99.8 [68] (49) PTFE + AMNP (5%) (RF-RC) 10.0 99.6 [68] (50) PTFE + AMNP (10%) (RF-RC) 0.0 99.4 [68] (51) PTFE + AMNP (10%) (P-D) 9.5 - [69] (52) PTFE + AMNP (10%) (P-D) 2.7 94.4 [70] (53) Cu60Zn40 + AMNP (1.0%) (P-D) 11.1 120 [57] (54) TiAl + AMNP (7.0%) 25 ◦C (P-D) 15.0 24.6 [71] (55) TiAl + AMNP (7.0%) 200 ◦C (P-D) 8.8 24.3 [71] (56) TiAl + AMNP (7.0%) 600 ◦C (P-D) 20.8 41.9 [71] (57) TiAl + AMNP (7.0%) 800 ◦C (P-D) 8.0 11.4 [71] (58) Al88Si12 + AMNP (3.0%) (P-D) 8.6 32.7 [72] (59) NiAl + AMNP (8.0%) 100 ◦C (P-D) 8.2 40.5 [62] (60) NiAl + AMNP (8.0%) 300 ◦C (P-D) 20.9 53.1 [62] (61) NiAl + AMNP (8.0%) 500 ◦C (P-D) 39.8 62.6 [62] (62) NiAl + AMNP (8.0%) 700 ◦C (P-D) 36.7 58.7 [62] (63) NiAl + AMNP (2%) (P-D) 17.7 15.8 [73] (64) NiAl + AMNP (5%) (P-D) 31.5 25.3 [73] (65) NiAl + AMNP (8%) (P-D) 45.2 29.5 [73] (66) NiAl + AMNP (11%) (P-D) 42.7 22.3 [73] Lubricants 2020, 8, 93 6 of 23

Table 1. Cont.

Friction and Wear Experiments

No. System and Test Devices µc (%) wearc (%) References

No. System (four-ball tester) Pcr Pweld dwear µc(%) wearc (%) References (67) Grease + AMNP (0%) 549 1303 0.78 [14] (68) Grease + AMNP (1%) 588 1470 0.62 20.5 [14] (69) Grease + AMNP (2%) 40.5 72.0 [52] (70) Grease + AMNP (3%) 7.7 7.6 [74] (71) Grease + AMNP (0.75%) + Bi (2.25%) 23.3 18.2 [74] (72) Grease + AMNP (0%) 413 1232 0.73 [75] (73) Grease + AMNP (0.5%) 547 1565 0.58 20.5 [75] (74) Grease + AMNP (0.7%) 547 1565 10.3 [76] Comparison of Surface Hardness of Friction Pair Without With AMNP No. Experimental condition References AMNP (GPa) (GPa) (75) Load 50 N, 45# steel, Friction time 10 h 6.27 9.37 [19] (76) Diesel cylinder after running 16 104 km 6.26 11.37 [20] × (77) Load 10 N, 1045 steel, Friction time 1 h 3.5 5.0 [77] (78) Load 200 N, 45# steel, Friction time 2 h 3.47 6.51 [52] (79) Load 11.5 N, TiAl matrix, Friction time 0.5 h 3.69 6.15 [71] (80) Load 400 N, 45# steel, Friction time 2 h 3.85 5.22 [58] (81) Load 30 N, Tin Bronze, Friction time 1h 2.4 3.5 [61] (82) 45# steel, Friction time 160 h 9.0 15.0 [78] (83) Load 400 N, 45# steel, Friction time 8 h 3.81 4.96 [79] (84) 45# steel, Friction time 1 h 2.39 3.18 [80] (85) Cast iron, Friction time 72 h 10.14 11.17 [81] 329.9 (Hv (86) Load 38.34 N, 45# steel, Friction time 24 h 238.8 [82] Hardness) 1119 (Hv (87) Diesel cylinder after running 29.3 104 km 524 [83] × Hardness) 1185 (Hv (88) Diesel cylinder after running 50 104 km 540 [59] × Hardness) Lubricants 2020, 8, 93 7 of 23

Table 1. Cont.

Comparison of Surface Elastic Modulus of Friction Pairs Without With AMNP No. Experimental condition References AMNP (GPa) (GPa) (89) Load 50 N, 45# steel, Friction time 10 h 253.9 285.2 [19] (90) Diesel cylinder after running 16 104 km 66.5 179.0 [20] × (91) Load 10 N, 1045 steel, Friction time 1 h 210.0 235.0 [77] (92) Load 200 N, 45# steel, Friction time 2 h 214.7 236.6 [52] (93) Load 400 N, 45# steel, Friction time 2h 238.9 221.2 [58] (94) Load 30 N, Tin Bronze, Friction time 1 h 140.0 180.0 [61] (95) 45# steel, Friction time 160 h 200.0 370.0 [78] (96) Load 400 N, 45# steel, Friction time 8 h 196.5 213.3 [79] (97) 45# steel, Friction time 1 h 250.0 212.1 [80] (98) Cast iron, Friction time 72 h 208.0 296.0 [81] Comparison of Surface Roughness (Ra) of Friction Pairs original friction No. Experimental condition surface surface References (Ra/µm) (Ra/µm) (99) Load 220 N, sliding speed 0.35 m/s, run time1 h 0.046 0.036 [25] (100) Contact stress 0.33–0.7MPa, run time 27 h 0.536 0.386 [82] (101) Contact stress 0.33–0.7MPa, run time 27 h 0.369 0.260 [82] (102) Diesel cylinder after running 50 104 km 2.5 0.0267 [59] × (103) Contact stress 7.64 MPa, run time 4 h 0.742 0.207 [59] (104) Contact stress 7.64 MPa, run time 4 h 3.706 2.528 [59] (105) Contact stress 7.64 MPa, run time 4 h 1.424 1.276 [59] (106) Contact stress 5.00 MPa, run time 1 h 0.636 0.280 [84] 107) Contact stress 5.00 MPa, run time 1 h 0.229 0.155 [84] (108) load400 N, rotary speed 192 r/min, run time1 h 0.427 0.083 [80] (109) load750 N, rotary speed 200 r/min, run time100 h 0.320 0.110 [65] Lubricants 2020, 8, 93 8 of 23

Table 1. Cont.

Application of AMNP Lubrication Materials in Industrial Equipment No. Equipment type and application result References (110) Gearing average power consumption of driving motor reduced by 5.0% [4] temperature of lubrication oil reduced by 9.7% [4] average power consumption of driving motor reduced by 13% [53] vibration amplitude of reduction gears reduced by 48% [85] (111) Air compressor average power consumption of driving motor reduced by 4.5% [4] temperature of lubrication oil reduced by 15.8% [4] consumption of lubrication oil reduced by 94.5% [59] average power consumption of driving motor reduced by 9.12% [59] (112) Automobile engine cylinder burst pressure increased by 3.9% [86] fuel consumption of automobile engine reduced by 7.0% [86] CO emissions of automobile engine reduced by 39.5% [86] CH emissions of automobile engine reduced by 29.5% [86] cylinder burst pressure increased by 11% [59] fuel consumption of automobile engine reduced by 1.2–6.0% [59] (113) Locomotive engine diesel consumption of engine reduced by 2.5% [87] lubricating oil consumption of engine reduced by 14.3% [87] cylinder burst pressure of engine increased by 2.7% [59] diesel consumption of engine reduced by 2.2% [58]

AMNP -antigorite micro-nano powder; Pcr = critical load (N); P = welding load (N); µc = (µ of without AMNP µ of with AMNP) 100/ µ of without AMNP; Wearc = (wear of without weld − × AMNP/wear of with AMNP) 100/wear of without AMNP; Dwear = arithmetic mean diameter of wear spots on bottom balls(mm); PTFE = polytetrafluoroethylene; FB = four-ball tester; P-D = pin-on-disk tester; P-F =× pin-on-flat tester; R-D = ring-on-disk tester; RF-RC = rectangular flat on rotating cylinder tester; D-D = disk-on-disk tester; TW = thrust washer tester; F-C = flat-on-cylinder tester. Lubricants 2020, 8, 93x FOR PEER REVIEW 89 of of 22 23

Figure 3. Schematic diagram of the tribological performance study. AMNP-oil system:Figure tests 3. Schematic were applied diagram to the of the various tribological AMNP-oil performance systems study. in an effort to determine the tribological performances of antigorite. The principal devices used were the four-ball tester (FB), pin-on-diskAMNP-oil tester system: (P-D) tests and were pin-on-flat applied tester to the (P-F); variou however,s AMNP-oil ring-on-disk systems testerin an effort (R-D), to rectangular determine theflat tribological on rotating performances cylinder tester of (RF-RC),antigorite. disk-on-disk The principal tester devices (D-D), used thrust were washerthe four-ball tester tester (TW) (FB), and pin-on-diskflat-on-cylinder tester tester (P-D) (F-C) and havepin-on-flat also been tester used (P-F); [12 however,–27]. AMNP ring-on-disk used for tester the study (R-D), had rectangular a particle flatsize on less rotating than 3 cylinder microns, tester and most (RF-RC), of them disk-on-disk were less tester than 1 (D-D), micron. thrust The washer amount tester of AMNP (TW) addedand flat- to on-cylinderthe liquid lubricating tester (F-C) medium have also was been<10%, used and [12–27]. most ofAMNP them used were for<2% the (Table study1 ).had The a particle results showedsize less thanthat the3 microns, friction and coe mostfficient of them (µ) of were oil with less than AMNP 1 micron. was reduced The amount by 9.7–89.5% of AMNP (Figure added4 to), the liquid wear lubricatingdecreased by medium 11.6–82% was (Figure <10%,5 ),and the most temperature of them ofwere the lubricating<2% (Table medium1). The results reduced showed by 35.6–44.3% that the (Tablefriction1 ,coefficient No.1 to 46; (μ Figure) of oil6 with). It wasAMNP found was that reduced AMNP by with9.7–89.5% an average (Figure particle 4), the sizewear of decreased less than by 2 11.6–82%microns all (Figure show 5), the the eff ecttemperature of reducing of theµ, while lubricating AMNP medium with an reduced average by particle 35.6–44.3% size of (Table more 1, than No.1 5 tomicrons 46; Figure significantly 6). It was increasefound that the AMNPµ. The with general an average rule is that particle the smaller size of less the AMNPthan 2 microns particle all size, show the thelower effect the frictionof reducing coeffi cientμ, while and wearAMNP [44 ].with The an eff ectaverage of AMNP particle on reducingsize of more the friction than 5 coe micronsfficient significantlyand wear can increase be summarized the μ. The in general the following rule is that four the aspects smaller [4 the]. First,AMNP the particle existence size, of the AMNP lower and the frictionmechanical coefficient attachment and ofwear AMNP [44]. toThe the effect metal of surface AMNP could on reducing prevent grossthe friction metal contactcoefficient but alsoand shearwear caneasily. be Secondly,summarized the in mechanical the following spacing four e ffaspectsect where [4]. AMNPFirst, the serve existence as spacers of AMNP between and friction mechanical pairs attachmentto prevent bare of AMNP metal-on-metal to the metal direct surface contact. could Thirdly, prevent the gross colloidal metal with contact the AMNP but also effect shear where easily. the presenceSecondly, of the AMNP mechanical moves spacing substantial effect amounts where ofAM fluidNP toserve the boundaryas spacers layers, between changing friction the pairs elastic to propertiesprevent bare of themetal-on-metal fluid. Last, the direct interaction contact. of T AMNPhirdly, withthe colloidal a metallic with surface the resultedAMNP effect the formation where the of presencea smooth of transfer AMNP film. moves substantial amounts of fluid to the boundary layers, changing the elastic propertiesCalcined of the antigorite-oil fluid. Last, system:the interaction based on of theAMNP dehydration with a metallic and phase surface transition resulted characteristics the formation of ofantigorite a smooth at transfer high temperature film. [4,35,36], the tribological performances of AMNP calcined at different temperaturesCalcined antigorite-oil were evaluated system: [24,48 based,49] (Table on the1, dehy No.32dration to 39). and It phase was shown transition that characteristics the tribological of propertiesantigorite at of high AMNP temperature under 300 [4,35,36],◦C were the better tribol thanogical those performances of the uncalcined, of AMNP thosecalcined with at different calcined temperaturestemperature between were evaluated 300 ◦C to [24,48,49] 600 ◦C were (Table similar 1, No.32 to those to of39). uncalcined It was shown samples, that both the of tribological which can propertiesreduce the frictionof AMNP coe underfficient 300 and °C wear were of thebetter friction than pairs.those Theof the effect uncalcined, of reducing those friction with coe calcinedfficient temperatureand wear of frictionbetween pairs 300 still°C to exists 600 °C in were the AMNP similar with to those the calcination of uncalcined temperature samples, ofboth 600–800 of which◦C, can but reduceit is obviously the friction worse coefficient than that and of the wear uncalcined of the friction AMNP. pairs. In sharp The effect contrast, of reducing the friction friction coeffi coefficientcient and andfriction wear pair of wearfriction of thepairs calcined still exists AMNP in the at aAM temperatureNP with the higher calcination than 800 temperature◦C are higher of 600–800 than that °C, of butthe baseit is obviously oil and of worse uncalcined than that AMNP. of the It uncalcined is found that AMNP. the di ffInerence sharp ofcontrast, tribological the friction performances coefficient of AMNPand friction calcined pair atwear diff oferent the temperaturescalcined AMNP is closelyat a temperature related to higher the high than temperature 800 °C are dehydroxylationhigher than that of the base oil and of uncalcined AMNP. It is found that the difference of tribological performances

LubricantsLubricants 2020 2020, 8, ,8 93x, xFOR FOR PEER PEER REVIEW REVIEW 109 9of ofof 22 23 22 ofof AMNPAMNP calcinedcalcined atat differentdifferent temperaturestemperatures isis closelyclosely relatedrelated toto thethe highhigh temperaturetemperature dehydroxylationanddehydroxylation phase transition and and of phase antigoritephase transition transition [24,48 ].of of The antigorite antigorite layered [24,48]. structure[24,48]. The The of AMNPlayered layered that structure structure calcined of of belowAMNP AMNP 600 that that◦C calcinediscalcined not damaged, below below 600 600 and °C °C thereis is not not isdamaged, damaged, no phase and and transition, there there is is no sono phase itsphase tribological transition, transition, performanceso so its its tribological tribological is similar performance performance to that isofis similar uncalcined similar to to that that AMNP. of of uncalcined uncalcined The AMNP AMNP. AMNP. that The calcinedThe AMNP AMNP at that 600–800 that calcined calcined◦C lost at at 600–800 its600–800 hydroxyl °C °C lost lost group its its hydroxyl hydroxyl water and group group the waterinter-layerwater and and slidingthe the inter-layer inter-layer capacity sliding becamesliding capacity worse,capacity resulting became became inworse, worse, the increase resulting resulting of thein in frictionthe the increase increase coeffi cientof of the the and friction friction wear coefficientofcoefficient the friction and and pair. wear wear It of isof the interestingthe friction friction pair.to pair. note It It is thatis interesting interesting calcined to AMNPto note note that that at acalcined temperaturecalcined AMNP AMNP higher at at a atemperature thantemperature 800 ◦C higherchangedhigher than than into 800 enstatite800 °C °C changed changed and forsterite into into enstatite enstatite with highand and hardnessforsterite forsterite andwith with high high high density hardness hardness [4], and whichand high high is thedensity density internal [4], [4], whichfactorwhich ofis is the the significantinternal internal factor factor increase of of the the of significant frictionsignificant coe increa ffiincreacientsese andof of friction friction friction coefficient coefficient pair wear. and and friction friction pair pair wear. wear.

FigureFigure 4. 4. ChangesChanges Changes of of friction friction coecoefficient coefficientﬃcient inin in four-ballfour-ball four-ballfriction friction friction test test test [ 4[4]. ].[4]. BO—without BO—without BO—without antigorite antigorite antigorite micro–nano micro–nano micro–nano powderpowder (AMNP);(AMNP); (AMNP); An—with An—withAn—with AMNP. AMNP. AMNP.

FigureFigure 5. 5. SEMSEM SEM images images images of of of wear wear wear tracks tracks tracks morphology morphology morphology [4]. [4 [4].]. ( (A A(A,,BB,B,,EE,)—withoutE)—without)—without AMNP; AMNP; AMNP; ( C (C,D,D,F,)—withF)—with AMNP; AMNP; (A(A–D–D) were) were carried carried out out using using an an four-ball four-ball friction friction tester. tester. ( E(,EF,)F )were were carried carried out out using using a a ring-on-disk ring-on-disk tester.tester. All All friction friction friction tester tester tester are are are rotation rotation speed speed of of 1200 1200 r/min, r /r/min,min, loading loading capacity capacity of of 392 392 N, N, and and test test duration duration ofof 60 60 min min ( A (A–D–D) and ) and 480 480 min min (E (,EF,).F ).

Lubricants 2020, 8, x FOR PEER REVIEW 10 of 22 LubricantsLubricants2020 2020, 8,, 938, x FOR PEER REVIEW 1110 of of 23 22

Figure 6.6.Changes Changes of lubricatingof lubricating oil in oil friction in friction test. (The test. figure (The is unpublishedfigure is unpublished data by the corresponding data by the Figure 6. Changes of lubricating oil in friction test. (The figure is unpublished data by the authorcorresponding of this article). author of this article). corresponding author of this article). Antigorite composite powders-oil system: a a few few studies studies have reported on the tribologicaltribological Antigorite composite powders-oil system: a few studies have reported on the tribological performances of the composites of AMNP and other functional components (Table (Table 11,, No.40No.40 toto 46).46). performances of the composites of AMNP and other functional components (Table 1, No.40 to 46). For example, the composite powders containing AM AMNPNP and nano-metal compounds such as Mo, La, La, For example, the composite powders containing AMNP and nano-metal compounds such as Mo, La, Cu and Ce, have a lower friction coecoefficientfficient and wear volume thanthan thatthat ofof AMNP alone.alone. It is believed Cu and Ce, have a lower friction coefficient and wear volume than that of AMNP alone. It is believed that the addition of nanonano metalmetal compoundscompounds is conducive to the formation of tribofilmtribofilm through ion that the addition of nano metal compounds is conducive to the formation of tribofilm through ion exchange, while rare earth elementselements (La andand Ce)Ce) mainlymainly actact catalystscatalysts [[19,54,55,64,66].19,54,55,64,66]. The AMNP exchange, while rare earth elements (La and Ce) mainly act catalysts [19,54,55,64,66]. The AMNP (0.1%) + Zinc dialkyl dithiophosphate (ZDDP, (ZDDP, 1.0%) has a lower wear volume of friction pair than (0.1%) + Zinc dialkyl dithiophosphate (ZDDP, 1.0%) has a lower wear volume of friction pair than that of AMNPAMNP alonealone (Figure(Figure7 7).). ItIt isis foundfound thatthat AMNPAMNP andand ZDDPZDDP actact togethertogether toto formform tribofilmstribofilms that of AMNP alone (Figure 7). It is found that AMNP and ZDDP act together to form tribofilms containing AMNP and Zn polyphosphate,polyphosphate, ZnS, from the decomposition of ZDDP on the surface of containing AMNP and Zn polyphosphate, ZnS, from the decomposition of ZDDP on the surface of friction pair, whichwhich isis aa filmfilm thatthat hashas aa lowerlower frictionfriction coecoefficientfficient andand wear.wear. Polyphosphates are the friction pair, which is a film that has a lower friction coefficient and wear. Polyphosphates are the essential ingredients of tribofilmstribofilms andand AMNPAMNP isis anan antiwear-additiveantiwear-additive [[27].27]. essential ingredients of tribofilms and AMNP is an antiwear-additive [27].

Figure 7. 7. ComparisonComparison of ofwear wear scar scar width width [27]. [ 27BS]. RT—without BS RT—without AMNP AMNP and ZDDP and at ZDDP 25°C; atSB 25 RT—C; Figure 7. Comparison of wear scar width [27]. BS RT—without AMNP and ZDDP at 25°C; SB RT—◦ SBwith RT—with AMNP at AMNP 25°C ZDD at 25 RTC ZDDwith RTZDDP with at ZDDP 25°C; atSZDDP 25 C; RT—with SZDDP RT—with AMNP and AMNP ZDDP and at ZDDP25°C BS at with AMNP at 25°C ZDD◦ RT with ZDDP at 25°C; SZDDP◦ RT—with AMNP and ZDDP at 25°C BS 25100—without◦C BS 100—without AMNP and AMNP ZDDP and at ZDDP 100°C; at SB 100 100—◦C; SBwith 100—with AMNP AMNPat 100°C at ZDDP 100 ◦C 100—without ZDDP 100—without ZDDP 100—without AMNP and ZDDP at 100°C; SB 100—with AMNP at 100°C ZDDP 100—without ZDDP ZDDPat 100°C; at 100SZDDP◦C; SZDDP100—with 100—with AMNP AMNPand ZDDP and at ZDDP 100°C. at 100 ◦C. at 100°C; SZDDP 100—with AMNP and ZDDP at 100°C.

Lubricants 2020, 8, 93 12 of 23

Solid composites containing AMNP: effective improvement of the tribological properties of polytetrafluoroethylene (PTFE) or Al matrix composites is the key to prolong their service life and performance. Some research works on the evaluation of tribological performances of composite materials made by mixing AMNP with polymer or metal-base alloys has been carried out in the last decade [50,62,68–71,73,88–90] (Table1, No.47 to 66). Sleptsova et al., (2009) found that the friction coefficient and wear of PTFE with 5% AMNP decreased by 10–15% and 95.6–99.8% compared with ≤ those without AMNP, as well as their rupture strength, elongation at rupture and modulus of elasticity at rupture were similar to that without AMNP [68]. The investigators hypothesized that the key role in the wear resistance of the polymer composites with AMNP or vermiculite is played by the origin of the Mg contained in the mineral fillers. The Mg not only influences the character of the oxidation processes, which occur in accordance with the radical mechanism, but can also primarily affect the processes of structure formation [68]. TiAl and NiAl or SiAl alloys with 11% AMNP have lower friction coefficient ≤ and wear than those without AMNP at 25 to 800 ◦C (the friction coefficient and wear are reduced by 8–45.2% and 11.4–62.6% respectively). NiAl matrix composite filled with 8 wt.% AMNP has a dense and homogeneous microstructure, and good tribological properties [73]. Specifically, the friction coefficients and wear rates are fairly low when the sliding speeds are at a low level. As the sliding speed is increased to a high level, the friction coefficients and wear rates show a distinct increase. The findings indicate that the variations in friction coefficients and wear rates are similar with an increase in the applied load [73]. Moreover, SiAl alloys with AMNP have higher compression strength than those without AMNP. Because AMNP prevent the generation of dislocations on single or multiple slip planes and their pile up at the grain boundaries, the compressive strengths of AMNP-reinforced composite were better. It is found that SiAl alloys with AMNP have a thick and dense structured layer uniformly coated on the substrate, showing great anti-friction ability [71]. These results imply that AMNP has a promising application as an additive to PTFE and aluminum-based engineering materials. AMNP-grease system: the grease is a semisolid lubricant. The majority of traditional greases are composed of petroleum and synthetic oils thickened with metal soaps and other agents such as clay, silica, carbon black and polytetrafluoroethylene [90]. In recent years, some researchers have added AMNP by 0.5–3.0% to a semisolid lubricant to evaluate its tribological performances [14,52,74–76]. The results showed that those types of grease with AMNP have higher critical load and welding load and have lower friction pairs wear (Table1, No.67 to 74). For example, the critical load of grease with 1% AMNP is increased by 7.1–13% compared with that of without AMNP, welding load is increased by 12.8–25.7%, mean diameter of wear spots is reduced by 19.1–20.5% [14]. Those can be explained by AMNP stronger retention in the friction zone by the structural lubricating medium frame during the run-in period and the adhesion of the protective film to the substrate [14]. The higher critical load and welding load of grease with AMNP indicated that AMNP could be a good application prospect as anti-wear and extreme pressure additive for industrial lubricating grease. Comparative study of antigorite and other minerals: aside from the evaluating the tribological properties of AMNP, the comparative study on tribological performances between AMNP and other silicate minerals such as talc, kaolinite, vermiculite, sepiolite, attapulgite has also been carried out. For example, the studies of Lyubimov et al. [14] and Yang et al. [75] indicate that grease containing AMNP or kaolinite or talc powders can reduce the friction coefficient and wear of friction pair, but the grease with kaolinite or talc has higher critical load, welding load and diameter of wear spots than that of with AMNP [14,75]. Friction coefficient and mass wear rate of PTFE with 2% AMNP or 2% vermiculite are lower than that of the without, but AMNP is more effective than vermiculite in reducing friction coefficient and wear, which may be related to the higher reactivity of AMNP than vermiculite. By comparing the tribological properties of AMNP, sepiolite and attapulgite powders dispersed in base oil, it is found that their friction coefficient and wear volume of friction pair are lower than that of base oil. It is interesting to note that the effect of sepiolite and attapulgite in reducing the friction coefficient and wear of friction pair is slightly better than that of AMNP. It is concluded that the better frictional performance of sepiolite and attapulgite under a high load may be related to their TOT LubricantsLubricants2020 2020, ,8 8,, 93 x FOR PEER REVIEW 1213 ofof 2322 Lubricants 2020, 8, x FOR PEER REVIEW 12 of 22 be related to their TOT crystal structure (an octahedral layer (O) is sandwiched between two becrystaltetrahedral related structure to layers(T)), their (an TOT octahedral hi ghercrystal reactivity layer structure (O) [91]. is (an sandwiched octahedral between layer (O) two is tetrahedral sandwiched layers(T)), between higher two tetrahedralreactivity [91 layers(T)),]. higher reactivity [91]. 4. Application of Antigorite Lubricating Additive 4. Application Application of of Antigorite Antigorite Lubricating Lubricating Additive The application of AMNP in industrial equipment has been studied mainly in locomotive and automobileThe application engines, ofair AMNP compressors in industrial and gearing. equipment The has evaluation been studied of the mainly application in locomotive effect mainly and automobilefocused on engines, the cylinder air compressors burst pressure and of gearing. engine, Thefuel The evaluationconsumption evaluation of of theof the the application application engine, temperature effect effect mainly of focusedlubrication on the oil, cylinder CO and burst CH emissionspressure ofof engine, the engine, fuel consumptionpower consumption of the engine, of the temperature driving motor, of lubricationvibration amplitude oil,oil, CO CO and and of CH gears,CH emissions emissions engine of consumption the of engine,the engine, power of lubricatingpower consumption consumption oils of (Table the driving of 1, theNo.110 motor,driving to vibration113). motor, The vibrationamplitudemajor conclusion amplitude of gears, is ofthat engine gears, adding consumption engine AMNP consumption to of traditional lubricating of lubricating lubricating oils (Table oils medium1, (Table No.110 could 1, toNo.110 113).improve to The 113). cylinder major The majorconclusionpressure conclusion (increased is that addingis thatby 2.7–11%) adding AMNP AMNP to[85,86], traditional toreduce traditional lubricating the consumption lubricating medium ofmedium could engine improve couldfuel (by improve cylinder 2.5–7%), pressurecylinder reduce pressure(increasedconsumption (increased by 2.7–11%) of lubricating by [852.7–11%),86 oil], reduce (by [8 5,86],14.3–94.5%) the reduce consumption [58,59 the consumption,86,87], of engine reduce fuel of the(byengine temper 2.5–7%), fuelature reduce(by 2.5–7%),of lubricating consumption reduce oil consumptionof(9.7%) lubricating (Figure of oil8) lubricating [4] (by and 14.3–94.5%) reduce oil (by the 14.3–94.5%) [emission58,59,86,87 of [58,59], CO reduce (39.5%),86,87], the reduceand temperature CH the compounds temper ofature lubricating (29.5%) of lubricating [86], oil as (9.7%) well oil (9.7%)(Figureas reduce (8Figure)[ power4] and8) [4] consumption reduce and reduce the emission ofthe driving emission of motor CO of (39.5%)CO (9.12–13%) (39.5%) and and CH(Figure CH compounds 9)compounds [4,54,59]. (29.5%) Of (29.5%) importance [86 ],[86], as wellas is well also as asreducethat reduce lubricating power power consumption consumptionmedium containing of drivingof driving AMNP motor motor (9.12–13%) could (9.12–13%) obviously (Figure (Figure 9prolong)[ 4 9),54 [4,54,59]., 59the]. Ofservice importanceOf importance life of the is also frictionis also that thatlubricatingparts lubricating of the medium equipment medium containing and containing improve AMNP AMNP th coulde service could obviously efficiency obviously prolong [83,87,92,93]. prolong the service the service life of the life friction of the partsfriction of partsthe equipment of the equipment and improve and improve the service the eservicefficiency efficiency [83,87,92 [83,87,92,93].,93].

Figure 8. The change of temperature of lubrication oil over time in air compressor (left) and in gear Figure 8. The change of temperature of lubrication oil over time in air compressor (left) and in gear Figure(right) 8. [4]. The BO—without change of temperature AMNP; An—with of lubrication AMNP. oil over time in air compressor (left) and in gear (right)[4]. BO—without AMNP; An—with AMNP. (right) [4]. BO—without AMNP; An—with AMNP.

FigureFigure 9.9.The The changechange of of power power consumption consumption of of driving drivin motorg motor over over time time in in air air compressor compressor (left (left) and) and in Figuregearin gear (right 9. ( rightThe)[4 ].change) [4]. BO—without BO—without of power AMNP; consumption AMNP; An—with An—with of drivin AMNP.g AMNP.motor over time in air compressor (left) and in gear (right) [4]. BO—without AMNP; An—with AMNP. 5. Physico-Chemical Characteristics of a Friction Pair Surface 5. Physico-Chemical Characteristics of a Friction Pair Surface 5. Physico-ChemicalAs already noted, Characteristics AMNP as lubricant of a Friction medium Pair additive Surface has participated in the friction reaction As already noted, AMNP as lubricant medium additive has participated in the friction reaction process. The study of chemical composition, phase composition, structure and physical properties of process.As already The study noted, of chemical AMNP as composition, lubricant medium phase composition,additive has participatedstructure and in physical the friction properties reaction of the surface layer of the friction pairs could reveal the essence of the interaction between the friction process.the surface The layerstudy of of the chemical friction composition, pairs could revealphase composition,the essence of structure the interaction and physical between properties the friction of pair and the AMNP, which was the focus of tribology. thepair surface and the layer AMNP, of the which friction was pairs the focuscould of reveal tribology. the essence of the interaction between the friction pair and the AMNP, which was the focus of tribology.

Lubricants 2020,, 88,, 93x FOR PEER REVIEW 1314 of of 2322

The chemical composition of friction pair surface layer is commonly studied using energy dispersiveThe chemical X-ray compositionspectroscopy of(EDS) friction coupled pair surface with layerscanning is commonly electron studied microscopy, using energyX-ray dispersivephotoelectron X-ray spectroscopy spectroscopy (XPS) (EDS) and coupled X-ray with absorp scanningtion near electron edge microscopy, structure (XANES), X-ray photoelectron while the spectroscopyphase composition (XPS) and X-raystructure absorption characteristics near edge are structureusually observed (XANES), through while the transmission phase composition electron andmicroscopy structure (TEM) characteristics and atomic-force are usually microscopy observed (AFM). through Numerous transmission studies electron have reported microscopy that (TEM) the Si and atomic-forceMg contents microscopy of most friction (AFM). Numerouspairs treated studies with have AMNP reported (Figure that 10, the Simiddle and Mg image) contents were of mostsignificantly friction pairshigher treated than that with of AMNP without (Figure AMNP 10, [14,21–23,26,27,49–53,55,56,61,69,72,77,85,91,94–99]. middle image) were significantly higher than that of withoutSome AMNPfriction [14pair,21– 23,surfaces26,27,49– 53treated,55,56,61 ,69with,72 ,77AMNP,85,91,94 –only99]. Somedetected friction pairSi surfacesbut no treated Mg with[17,25,54,55,64,66,74,78,79,84,86,91,95], AMNP only detected Si but no Mg or [ 17only,25 detect,54,55,ed64 ,Mg66,74 but,78 no,79 Si,84 [18,60,100],,86,91,95], oror onlyneither detected Si nor Mg butwere no detected Si [18,60 ,[57,82,83].100], or neither According Si nor Mgto Zhao were detected[101], the [57 friction,82,83]. pair According surface towithout Zhao [101 AMNP], the frictionmainly paircontains surface Fe, withoutO and C, AMNP while in mainly addition contains to Fe, Fe, O and O and C, C,Mg while and Si in appear addition on to the Fe, surface O and C,with Mg AMNP. and Si appearAccordingly, on the Fe, surface O, C, with Mg, AMNP.Si, Zn, P Accordingly, and S appeared Fe, O, simultaneously C, Mg, Si, Zn, P on and surface S appeared with AMNP simultaneously + ZDDP on(Figure surface 10). with XANES AMNP spectra+ ZDDP clearly (Figure show 10 ).that XANES the tribofilms spectra clearly produced show by that the the lubricating tribofilms oil produced adding byAMNP the lubricating and ZDDP oil have adding a glassy AMNP appearance and ZDDP containi haveng a glassy antigorite appearance and Zn containingpolyphosphate, antigorite ZnS [27]. and ZnOf particular polyphosphate, significance ZnS [ 27is ].the Of distribution particular of significance Si and/or Mg is the on distributionthe friction pair of Si surface and/or treated Mg on with the frictionAMNP pairis not surface uniform, treated and most with AMNPof them isare not spots uniform, or patches and most rather of than them continuous are spots or layers, patches which rather is thanconfirmed continuous by TEM layers, and XANES which is [26,27,77,84,95]. confirmed by TEM Zhao and [101] XANES showed [26 that,27,77 the,84 tribofilm,95]. Zhao with [101 maximum] showed thatthickness the tribofilm of 240 nm with and maximum 458 nm are thickness formed on of the 240 friction nm and pair 458 surface nm are with formed AMNP on and the with friction AMNP pair surface+ ZDDP with (Figure AMNP 11). andA number with AMNP of studies+ ZDDP have (Figure shown 11that). Athe number phase ofon studiesthe friction have pair shown surface that thetreated phase with on theAMNP friction is very pair surfacecomplex, treated including with AMNPnanocrystalline, is very complex, amorphous including and mineral nanocrystalline, cellular amorphousskeleton [26,27,77,84,95,101]. and mineral cellular Figure skeleton 12A [26 ,27demons,77,84,95trates,101 ].a Figurebright 12 Afield demonstrates transmission a bright electron field transmissionmicrograph of electron the cross-section micrograph of of the the tribofilm, cross-section showing of the tribofilm,the thickness showing was about the thickness 500–600 was nm. about The 500–600film is uniform nm. The with film a issmooth uniform surface with aand smooth a few surface internal and pores. a few The internal diffraction pores. pattern The di shownffraction in patternFigure 13B–D shown clearly in Figure indicates 13B–D some clearly crystallinity indicates some of the crystallinity particles since of the diffraction particles rings since diareff ractionclearly rings are clearly visible. The rings were indexed and found to be the complex compounds: FeSi, AlFe visible. The rings were indexed and found to be the complex compounds: FeSi, AlFe and Fe3O4 and Fe O complexes (Figure 12B); Fe O and FeSi complexes (Figure 12C) and amorphous FeSi and complexes3 4 (Figure 12B); Fe3O4 and FeSi3 4 complexes (Figure 12C) and amorphous FeSi and SiO2 SiOcomplexes2 complexes (Figure (Figure 12D) 12 [77].D) [It77 can]. It be can inferred be inferred from fromthe characteristics the characteristics of composition of composition and phase and phase that thatthe AMNP the AMNP adsorbed adsorbed or adhered or adhered to tothe the friction friction surface surface forms forms an an amorphous amorphous or nanocrystallinenanocrystalline components structurestructure layer layer with with a discontinuous a discontinuous distribution distribution under theunder influence the influence of frictional of dynamicsfrictional anddynamics thermal and energy. thermal energy.

Figure 10.10. EDS analysis of frictionfriction pairpair surfacesurface [[101101].]. Left—without AMNP; Center—with AMNP; Right—with AMNP and ZDDP.

Lubricants 2020, 8, x FOR PEER REVIEW 13 of 22

The chemical composition of friction pair surface layer is commonly studied using energy dispersive X-ray spectroscopy (EDS) coupled with scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES), while the phase composition and structure characteristics are usually observed through transmission electron microscopy (TEM) and atomic-force microscopy (AFM). Numerous studies have reported that the Si and Mg contents of most friction pairs treated with AMNP (Figure 10, middle image) were significantly higher than that of without AMNP [14,21–23,26,27,49–53,55,56,61,69,72,77,85,91,94–99]. Some friction pair surfaces treated with AMNP only detected Si but no Mg [17,25,54,55,64,66,74,78,79,84,86,91,95], or only detected Mg but no Si [18,60,100], or neither Si nor Mg were detected [57,82,83]. According to Zhao [101], the friction pair surface without AMNP mainly contains Fe, O and C, while in addition to Fe, O and C, Mg and Si appear on the surface with AMNP. Accordingly, Fe, O, C, Mg, Si, Zn, P and S appeared simultaneously on surface with AMNP + ZDDP (Figure 10). XANES spectra clearly show that the tribofilms produced by the lubricating oil adding AMNP and ZDDP have a glassy appearance containing antigorite and Zn polyphosphate, ZnS [27]. Of particular significance is the distribution of Si and/or Mg on the friction pair surface treated with AMNP is not uniform, and most of them are spots or patches rather than continuous layers, which is confirmed by TEM and XANES [26,27,77,84,95]. Zhao [101] showed that the tribofilm with maximum thickness of 240 nm and 458 nm are formed on the friction pair surface with AMNP and with AMNP + ZDDP (Figure 11). A number of studies have shown that the phase on the friction pair surface treated with AMNP is very complex, including nanocrystalline, amorphous and mineral cellular skeleton [26,27,77,84,95,101]. Figure 12A demonstrates a bright field transmission electron micrograph of the cross-section of the tribofilm, showing the thickness was about 500–600 nm. The film is uniform with a smooth surface and a few internal pores. The diffraction pattern shown in Figure 13B–D clearly indicates some crystallinity of the particles since diffraction rings are clearly visible. The rings were indexed and found to be the complex compounds: FeSi, AlFe and Fe3O4 complexes (Figure 12B); Fe3O4 and FeSi complexes (Figure 12C) and amorphous FeSi and SiO2 complexes (Figure 12D) [77]. It can be inferred from the characteristics of composition and phase that the AMNP adsorbed or adhered to the friction surface forms an amorphous or nanocrystalline components structure layer with a discontinuous distribution under the influence of frictional dynamics and thermal energy.

Lubricants 2020, 8, x FOR PEER REVIEW 14 of 22 LubricantsFigure2020 ,10.8, 93EDS analysis of friction pair surface [101]. Left—without AMNP; Center—with AMNP;15 of 23 Right—with AMNP and ZDDP.

Lubricants 2020, 8, x FOR PEER REVIEW 14 of 22 Lubricants 2020, 8, x FOR PEER REVIEW 14 of 22 Figure 11. Focused Ion Beam/SEM images of the tribofilm [101]. Above—with AMNP; Below—with AMNP + ZDDP.

Figure 12. Transmission electron microscopy (TEM) images of the tribofilm with AMNP [77]. Figure 11. Focused Ion Beam/SEM images of the tribofilm [101]. Above—with AMNP; Below—with FigureAMNPIn the 11.comparison+ ZDDP. Focused Ion of BeamBeam/SEMthe surface/SEM images roughness of the (Ra) triboﬁlmtribofilm and [hardness[101].101]. Above—with of the friction AMNP; pair, Below—with we found that AMNP + ZDDP. the roughnessAMNP + ZDDP. of the friction pair treated with AMNP is significantly lower than that without AMNP

(Table 1, No.99–109; Ra of the friction pair with AMNP reduced by 40.2–72.1%), while the surface hardness is significantly higher (Table 1, No.75–88, the hardness increased by 1.8–94%). Figure 13 and Table 2 show the typical load-depth curves, nano-hardness (H) and elastic modulus (E) of normal worn surface (without AMNP) and the tribofilm (with AMNP) tested with a maximal applied load of 15 mN. It is found that the maximum indentation depth for the tribofilm (with AMNP) is much smaller than those of normal worn surface (without AMNP). Correspondingly, tribofilm has a higher hardness and elastic modulus than normal worn surface, indicating the tribofilmFigure was a 12. hard–thin TransmissionTransmission layer. electron microscopy (TEM) images of the triboﬁlmtribofilm with AMNP [[77].77]. Figure 12. Transmission electron microscopy (TEM) images of the tribofilm with AMNP [77]. In the comparison of the surface roughness (Ra) and hardness of the friction pair, we found that the roughnessIn the comparison of the friction of the pair surface treated roughness with AMNP (Ra) and is significantly hardness of lower the friction than that pair, without we found AMNP that (Tablethe roughness 1, No.99–109; of the frictionRa of the pair friction treated pair with with AMNP AMNP is significantly reduced by lower 40.2–72.1%), than that while without the surfaceAMNP hardness(Table 1, No.99–109;is significantly Ra ofhigher the friction (Table 1,pair No.75–88, with AMNP the hardness reduced increased by 40.2–72.1%), by 1.8–94%). while the surface hardnessFigure is significantly13 and Table higher 2 show (Table the 1, typical No.75–88, load-depth the hardness curves, increased nano-hardness by 1.8–94%). (H) and elastic modulusFigure (E) 13of normaland Table worn 2 surfaceshow the (without typical AMNP) load-depth and thecurves, tribofilm nano-hardness (with AMNP) (H) tested and withelastic a maximalmodulus applied(E) of normal load ofworn 15 mN. surface It is (without found that AMNP) the maximum and the tribofilm indentation (with depth AMNP) for testedthe tribofilm with a (withmaximal AMNP) applied is much load smaller of 15 mN. than It those is found of normal that the worn maximum surface (withoutindentation AMNP). depth Correspondingly, for the tribofilm tribofilm(with AMNP) has isa muchhigher smaller hardness than and those elastic of normal modulus worn thansurface normal (without worn AMNP). surface, Correspondingly, indicating the tribofilm washas aa hard–thinhigher hardness layer. and elastic modulus than normal worn surface, indicating the tribofilm was a hard–thin layer.

Figure 13. Typical load–depth curves ofof normalnormal wornworn surfacesurface andand thethe triboﬁlmtribofilm [[21].21].

Figure 13. Typical load–depth curves of normal worn surface and the tribofilm [21]. Figure 13. Typical load–depth curves of normal worn surface and the tribofilm [21].

Lubricants 2020, 8, 93 16 of 23

In the comparison of the surface roughness (Ra) and hardness of the friction pair, we found that the roughness of the friction pair treated with AMNP is significantly lower than that without AMNP (Table1, No.99–109; Ra of the friction pair with AMNP reduced by 40.2–72.1%), while the surface hardness is significantly higher (Table1, No.75–88, the hardness increased by 1.8–94%). Figure 13 and Table2 show the typical load-depth curves, nano-hardness (H) and elastic modulus (E) of normal worn surface (without AMNP) and the tribofilm (with AMNP) tested with a maximal applied load of 15 mN. It is found that the maximum indentation depth for the tribofilm (with AMNP) is much smaller than those of normal worn surface (without AMNP). Correspondingly, tribofilm has a higher hardness and elastic modulus than normal worn surface, indicating the tribofilm was a hard–thin layer.

Lubricants 2020Table, 8, x FOR 2. Nano-mechanicalPEER REVIEW properties of normal worn surface and the triboﬁlm [21]. 15 of 22

Max Depth (nm) Plastic Depth (nm) H (GPa) E (GPa) H/E( 10 2) The H/E ratio of nano-hardness (H) and elastic modulus (E) is usually introduced as× a −main Normal surface 438.80 59.70 393.50 54.47 3.45 0.85 215.53 32.10 1.60 parameter to estimate the ±relative wear resistance± of materials.± Figure 14 demonstrates± H/E tested Tribofilm 342.55 42.41 289.65 37.57 6.68 0.65 238.52 29.65 2.80 with different maximal indentation± depth on± a normal worn± surface (without± AMNP) and the tribofilm (with AMNP). The ratio varies between 1.61 × 10−2 and 1.37 × 10−2 for a normal worn surface withThe different H/E ratio maximal of nano-hardness indentation de (H)pths, and while elastic it modulusdecreases (E)from is usually3.41 × 10 introduced−2 and 1.78 as × a10 main−2 for parametertribofilm with to estimate the increasing the relative maximal wear resistance indentation of materials. depth. The Figure result 14 demonstratesindicates the Hwear/E tested resistance with dipropertyfferent maximal of the tribofilm indentation (with depthAMNP) on is a superior normal wornto the surface normal (withoutworn surface AMNP) (without and the AMNP). tribofilm (with AMNP). The ratio varies between 1.61 10 2 and 1.37 10 2 for a normal worn surface with Besides, it is interesting to note that the elastic× − modulus of× the− surface of the friction pair treated different maximal indentation depths, while it decreases from 3.41 10 2 and 1.78 10 2 for tribofilm with AMNP does not show the same regularity as hardness and× roughness− (Table× 1,− No.89–98). In withother the words, increasing the elastic maximal modulus indentation of the surface depth. Theof th resulte friction indicates pair treated the wear with resistance AMNP propertyis higher ofor thelower tribofilm than that (with of AMNP)the one without is superior AMNP. to the normal worn surface (without AMNP).

FigureFigure 14.14. DependencyDependency ofof HH/E/E onon maximalmaximal indentationindentation depthdepth [[21].21].

Besides,Table it is interesting 2. Nano-mechanical to note that properties the elastic of normal modulus worn of surface the surface and the of tribofilm the friction [21]. pair treated with AMNP does not show the same regularity as hardness and roughness (Table1, No.89–98). In other Max Depth (nm) Plastic Depth (nm) H (GPa) E (GPa) H/E (×10−2) words, the elastic modulus of the surface of the friction pair treated with AMNP is higher or lower Normal surface 438.80 ± 59.70 393.50 ± 54.47 3.45 ± 0.85 215.53 ± 32.10 1.60 than thatTribofilm of the one without 342.55 AMNP. ± 42.41 289.65 ± 37.57 6.68 ± 0.65 238.52 ± 29.65 2.80

6. Mechanism Study A study on the lubrication mechanism of antigorite is of paramount importance in the understanding of the AMNP with friction pairs interaction, which can lead to the optimization of the subsequent preparation process and application technology of lubricity. The mechanistic studies have been carried out with either the assistance of surface composition and structure characterization of friction pairs, or the postulation from comprehensive experimental observations on antigorite mineralogy and friction characteristics. The internal relationship among antigorite mineralogy— surface features of friction pairs—application performance is shown in Figure 15. Pogodaev et al. [15] deduced from the structure, composition and phase transition that antigorite as a lamellar serpentine variety is more stable and durable to an external (mechanical) effect and to high temperature drops during its modification. Antigorite favors the formation of low dielectrically resistant, stronger, and wear-resistant protective layers on friction surfaces. In the presence of lubricating compositions with AMNP, the formation of superfinishing of friction surfaces may be due to the original availability of abrasive particles in powder like antigorite and to the abrasive effect of secondary particles present in the compositions after the AMNP decompose (forsterite, fayalite, oxides) [15].

Lubricants 2020, 8, 93 17 of 23

FigureFigure 15.15. SchematicSchematic diagram diagram of theof relationshipthe relationship among among antigorite antigorite mineralogy-friction mineralogy-friction pairs-application. pairs- application. Pogodaev et al. [15] deduced from the structure, composition and phase transition that antigorite as a lamellarDolgopolov serpentine et al., found variety that is morethe friction stable andpair durable treated towith an externalAMNP has (mechanical) good wear eff resistanceect and to [16,17].high temperature They thought drops that during good itswear modification. resistance Antigoriteis due to the favors formation the formation of a two-level of low dielectricallystructure on theresistant, rubbing stronger, surfaces. and This wear-resistant structure is a mineral protective skeleton layers with on a friction developed surfaces. surface In and the a presencelayer made of uplubricating of the products compositions of friction-p with AMNP,olymerization the formation of the oflubricating superfinishing materials of friction [16]. The surfaces friction may process be due involvingto the original AMNP availability is accompanied of abrasive by substitution particles in of powder magnesium like antigorite atoms of andthe mineral to the abrasive structure eff ectby ironof secondary atoms from particles the friction present surface. in the As compositions a result, recombination after the AMNP of the decompose crystalline lattice (forsterite, of the fayalite, initial mineraloxides) [and15]. the formation of pseudo-mineral structures similar to the plane SiO compounds occur [17]. DolgopolovThey found etthat al., foundantigorite that theis capable friction pairof fo treatedrming witha transfer AMNP film has with good tended wear resistance crystals [16upon,17]. friction.They thought It is assumed that good that wear under resistance the friction is due of tosilicates, the formation two substructures—silicate-oxygen of a two-level structure on the and rubbing iron- oxygen—aresurfaces. This formed structure in the is atransfer mineral film skeleton [99]. with a developed surface and a layer made up of the productsJin’s research of friction-polymerization findings on generation of the mechanisms lubricating materialsof reconditioning [16]. The layer friction on cylinder process involvingbore of a locomotiveAMNP is accompanied diesel engine by indicate substitution that the of magnesiumstrongly diffusing-in atoms of of the oxygen mineral atom, structure ion and by ironfree atomswater fromfrom the the metal friction surface surface. by Ashigh a result,chemical recombination activity of antigorite of the crystalline results in lattice oxidation of the of initial alloy mineralcomponent and the formation of pseudo-mineral structures similar to the plane SiO compounds occur [17]. They found (Fe3C) of Fe-based metal. This is a special internal oxidation process quite different from the high temperaturethat antigorite internal is capable oxidation of forming which a plays transfer a crucial film with role in tended the formation crystals uponof the friction. reconditioning It is assumed layer. Sequentially,that under the the friction deformed of silicates, refinement two substructures—silicate-oxygen and strengthening of the internal and iron-oxygen—areoxidation structure formed under in reciprocatingthe transfer film movement [99]. of the fiction pair bring about the reconditioning protective layer with excellentJin’s behaviors research findings[102]. on generation mechanisms of reconditioning layer on cylinder bore of a locomotiveBai et al. diesel [4] thought engine that indicate the flaked-bladed that the strongly AM diNPffusing-in and their of dehydrated oxygen atom, products ion and could free waterserve asfrom spacers the metal and surfacepolishing by media high chemical between activity asperities of antigorite to eliminate results direct in oxidationmetal-on-metal of alloy contact component and adhesion,(Fe3C) of Fe-basedand polish metal. the rubbing This is pa a specialrtners with internal rough oxidation surface. processAbundant quite structural different water from provides the high interplanartemperature lubrication internal oxidation and weakens which playsantigorite’s a crucial structure. role in theThermal formation phase of thetransformation reconditioning of antigoritelayer. Sequentially, and ion-exchange the deformed (Fe2 + or refinement Fe3 + -Mg2 + ) and between strengthening dehydrated of theantigorite internal and oxidation iron base structure friction pairs induced the formation of ceramic-like film on the surface of the metals, which can greatly decrease friction and wear on friction surfaces [4]. Recently, Li et al., developed a comprehensive model to describe AMNP participating in the friction process and interaction of AMNP with friction pair (Figure 16) [44]. It is concluded on the basis of the summary of a large number of previous research results, which may be useful for understanding the mechanism of friction and wear of AMNP as an additive of lubricating medium. The model explains the mechanism of action of AMNP as a lubricating material from three aspects: the physical change of AMNP during the friction process, the chemical change during the interaction between AMNP with friction pair and the catalysis of antigorite to the formation of tribofilm. First of all, AMNP adsorbed on the surface of the friction pair is the basis for its particle size to gradually decrease and participate in the friction reaction so as to reduce the friction coefficient and wear of the

Lubricants 2020, 8, 93 18 of 23 under reciprocating movement of the fiction pair bring about the reconditioning protective layer with excellent behaviors [102]. Bai et al. [4] thought that the flaked-bladed AMNP and their dehydrated products could serve as spacers and polishing media between asperities to eliminate direct metal-on-metal contact and adhesion, and polish the rubbing partners with rough surface. Abundant structural water provides interplanar lubrication and weakens antigorite’s structure. Thermal phase transformation of antigorite and ion-exchange (Fe2+ or Fe3+ -Mg2+) between dehydrated antigorite and iron base friction pairs induced the formation of ceramic-like film on the surface of the metals, which can greatly decrease friction and wear on friction surfaces [4]. Recently, Li et al., developed a comprehensive model to describe AMNP participating in the friction process and interaction of AMNP with friction pair (Figure 16)[44]. It is concluded on the basis of the summary of a large number of previous research results, which may be useful for understanding the mechanism of friction and wear of AMNP as an additive of lubricating medium. The model explains the mechanism of action of AMNP as a lubricating material from three aspects: the physical change of AMNP during the friction process, the chemical change during the interaction between AMNP with friction pair and the catalysis of antigorite to the formation of tribofilm. First of all, LubricantsAMNP adsorbed 2020, 8, x FOR on PEER the surface REVIEW of the friction pair is the basis for its particle size to gradually decrease17 of 22 and participate in the friction reaction so as to reduce the friction coefficient and wear of the friction frictionpairs. Furthermore, pairs. Furthermore, the dehydrogenation the dehydrogenation and phase an transitiond phase transition of antigorite of antigorite under the under action the of friction action ofheat friction and kinetic heat and energy kinetic are energy the key are factors the key for thefact surfaceors for oxidationthe surface of oxidation iron-base of friction iron-base pair andfriction the pairformation and the of compositeformation gradient of composite structure gradient layer. Instructure addition, layer. the abundant In addition, surface the unsaturatedabundant surface charge unsaturatedof antigorite charge powder of andantigorite the catalytic powder eff andect ofthe trace cata elementslytic effect such of trace as Ni elements and Mn such on the as Ni interaction and Mn onof lubricatingthe interaction oil—AMNP—friction of lubricating oil—AMNP—friction pair are the motivating pair factors are the for motivating the formation factors of tribofilm. for the formationThe combined of tribofilm. action of The the abovecombined factors action leads of toth thee above formation factors of leads tribofilm to th withe formation high hardness, of tribofilm high withelastic high modulus hardness, and high low frictionelastic modulus coefficient and and low wear. friction coefficient and wear.

Figure 16. Schematic diagram of the antigorite powders–friction pair interaction [44]. Figure 16. Schematic diagram of the antigorite powders–friction pair interaction [44]. 7. Conclusions 7. Conclusions Lubricity and anti-wear of antigorite may be related to its lamellar morphology, medium hardness, abundantLubricity hydroxyl and wateranti-wear and surfaceof antigorite unsaturated may be charge, related strong to its ion lamellar exchange morphology, capacity, easy medium to slide hardness, abundant hydroxyl water and surface unsaturated charge, strong ion exchange capacity, easy to slide between layers and high temperature phase transition. Under the action of friction kinetic energy and heat energy, the bladed AMNP forms a friction reaction layer having low roughness and high hardness on the friction pair surface by abrasive polishing and ion exchange, which effectively reduces the friction resistance and wear of the friction pairs. As a lubricant additive, AMNP should have a bladed morphology with a blade diameter less than 2 microns, and should be subject to surface modification of organic matter. The AMNP can be mixed with different types of lubricating oil to form a suspended solution, or evenly mixed with base grease to make a complex grease, or mixed with matrix materials (such as PTFE and Al alloys) to make solid composite materials. AMNP used as a lubricant additive for PTFE can significantly reduce the wear of PTFE, which can extend the service life of the PTFE friction seal components. As already noted, the tribological properties of calcined AMNP under 300 °C are better than that of the uncalcined AMNP. The AMNP used as a lubricating material should not be calcined at temperatures >600 °C. The application of AMNP in industrial equipment could reduce friction wear and improve cylinder pressure; reduce the consumption of engine fuel and lubricating oil, as well as the temperature of lubricating medium and the emission of pollution, obviously prolong the service life of the friction parts of the equipment, extend the overhaul period of the equipment and improve the service efficiency.

Lubricants 2020, 8, 93 19 of 23 between layers and high temperature phase transition. Under the action of friction kinetic energy and heat energy, the bladed AMNP forms a friction reaction layer having low roughness and high hardness on the friction pair surface by abrasive polishing and ion exchange, which effectively reduces the friction resistance and wear of the friction pairs. As a lubricant additive, AMNP should have a bladed morphology with a blade diameter less than 2 microns, and should be subject to surface modification of organic matter. The AMNP can be mixed with different types of lubricating oil to form a suspended solution, or evenly mixed with base grease to make a complex grease, or mixed with matrix materials (such as PTFE and Al alloys) to make solid composite materials. AMNP used as a lubricant additive for PTFE can significantly reduce the wear of PTFE, which can extend the service life of the PTFE friction seal components. As already noted, the tribological properties of calcined AMNP under 300 ◦C are better than that of the uncalcined AMNP. The AMNP used as a lubricating material should not be calcined at temperatures >600 ◦C. The application of AMNP in industrial equipment could reduce friction wear and improve cylinder pressure; reduce the consumption of engine fuel and lubricating oil, as well as the temperature of lubricating medium and the emission of pollution, obviously prolong the service life of the friction parts of the equipment, extend the overhaul period of the equipment and improve the service efficiency. Antigorite as a lubricant has not yet formed a set of standards and specifications, which is a technical obstacle that restricts the large-scale application of AMNP in industry. In published papers, some authors indiscriminately refer to “antigorite” as “serpentine”. In fact, the “serpentine” is a group of minerals that contains lizardite, antigorite and chrysotile [102]. Although the three minerals share the same composition (Mg6Si4O10(OH)8), they have different structures. Lizardite shows an ideal layer topology, whereas antigorite is a modulated layer and chrysotile is a bent layer. Of the three minerals, antigorite is currently the only additive lubricant, while fibrous chrysotile is not used because of its potential environmental hazards. The authors suggest that “antigorite” rather than “serpentine” should be used as a lubricant. Some papers refer to antigorite as a “self-repairing” material based on its anti-wear properties [23,44,73]. The word “self-repairing” means that the object itself has a self-repairing function. In fact, antigorite was not repaired during the friction process, but the friction pair using AMNP was repaired. So, this paper argues that “self-repairing” may be an inappropriate term to describe the effects and functions of antigorite, and so should be abandoned.

Funding: This research was funded by National Key R&D Program of China(2017YFB0310703). Acknowledgments: This project was sponsored by the National Key R&D Program of China(2017YFB0310703). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Allam, I.M. Solid lubricants temperatures for applications at elevated—A review. J. Mater. Sci. 1991, 26, 3977–3984. [CrossRef]

2. Bai, Z.M.; Wang, Z.Y.; Zhang, T.G.; Fu, F.; Yang, N. Synthesis and characterization of Co-Al-CO3 layered double metal hydroxides and assessment of their friction performances. Appl. Clay Sci. 2012, 59–60, 36–41. [CrossRef] 3. Bai, Z.M.; Wang,Z.Y.; Zhang, T.G.; Fu, F.; Yang,N. Characterization and friction performances of Co-Al-layered double-metal hydroxides synthesized in the presence of dodecylsulfate. Appl. Clay Sci. 2013, 75–76, 22–27. 4. Bai, Z.M.; Yang, N.; Guo, M.; Li, S. Antigorite: Mineralogical characterization and friction performances. Tribol. Int. 2016, 101, 115–121. [CrossRef] 5. Raleigh, C.B.; Paterson, M.S. Experimental deformation of serpentinite and its tectonic implications. J. Geophys. Res. 1965, 70, 3965–3985. [CrossRef] Lubricants 2020, 8, 93 20 of 23

6. Dengo, C.A.; Logan, J.M. Implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting. J. Geophys. Res. 1981, 86, 10771–10782. [CrossRef] 7. Reinen, L.A.; Weeks, J.D.; Tullis, T.E. The frictional behavior of serpentinite: Implications for aseismic creep on shallow crustal faults. Geophys. Res. Lett. 1991, 18, 1921–1924. [CrossRef] 8. Reinen, L.A.; Weeks, J.D.; Tullis, T.E. The frictional behavior of lizardite and antigorite serpentinites: Experiments, constitutive models, and Implications for natural faults. Pure Appl. Geophys. 1994, 143, 317–358. [CrossRef] 9. Moore, D.E.; Lockner, D.A.; Summers, R.; Ma, S.L.; Byerlee, J.D. Strength of chrysotile- serpentinite gouge under hydrothermal conditions: Can it explain a weak San Andreas fault? Geology 1996, 24, 1041–1044. [CrossRef] 10. Morrow, C.A.; Moore, D.E.; Lockner, D.A. The eﬀect of mineral bond strength and adsorbed water on fault gouge frictional strength. Geophys. Res. Lett. 2000, 27, 815–818. [CrossRef] 11. Jung, H.; Fei, Y.W.; Silver, P.G.; Green, H.W. Frictional sliding in serpentine at very high pressure. Earth Planet. Sci. Lett. 2009, 277, 273–279. [CrossRef]

12. Yang, H.; Li, S.H.; Jin, Y.S. Study on tribological behavior of Mg6Si4O10(OH)8 additive package with steel tribo-pair. Tribology 2005, 25, 308–311. 13. Tupotilov, N.N.; Ostrikov, V.V.; Kornev, A.Y. Finely disperse minerals as antiwear additives for lube oils. Chem. Technol. Fuels Oils 2008, 44, 29–33. [CrossRef] 14. Lyubimova, D.N.; Dolgopolova, K.N.; Kozakovb, A.T.; Nikolskiib, A.V. Improvement of performance of lubricating materials with additives of clayey minerals. J. Frict. Wear 2011, 32, 442–451. [CrossRef] 15. Pogodaev, L.I.; Buyanovskii, I.A.; Kryukov, E.Y.; Kuz’min, V.N.; Usachev, V.V. The mechanism of interaction between natural laminar hydrosilicates and friction surfaces. J. Mach. Manuf. Reliab. 2009, 38, 476–484. [CrossRef] 16. Dolgopolov, K.N.; Lyubaimov, D.N.; Chigarenkoc GGPonomarenko, A.G.; Chigarenko, G.G.; Boiko, M.V. The structure of lubricating layers appearing during friction in the presence of additives of mineral friction modifiers. J. Frict. Wear 2009, 30, 377–380. [CrossRef] 17. Dolgopolov, K.N.; Lyubaimov, D.N.; Kozakovb, A.T.; Nikol’skii, A.V.; Glazunova, E.A. Tribochemical aspects of interactions between high-dispersed serpentine particles and metal friction surface. J. Frict. Wear 2012, 33, 108–114. [CrossRef] 18. Zhang, B.; Xu, B.S.; Xu, Y.; Wang, X.L.; Zhang, B.S. Effect of magnesium silicate hydroxide on the friction behavior of ductile cast iron pair and the self-repairing performance. J. Chin. Ceram. Soc. 2009, 37, 492–495. 19. Zhang, B.S.; Xu, B.S.; Xu, Y.; Gao, F.; Shi, P.J.; Wu, Y.X. Cu nanoparticles effect on the tribological properties of hydrosilicate powders as lubricant additive for steel–steel contacts. Tribol. Int. 2011, 44, 878–886. [CrossRef] 20. Jin, Y.S. The effect of internal oxidation from serpentine on generating reconditioning layer on worn ferrous metal surfaces. Chin. Surf. Eng. 2010, 23, 45–50. 21. Yu, H.L.; Xu, Y.; Shi, P.J.; Wang, H.M.; Zhao, Y.; Xu, B.S.; Bai, Z.M. Tribological behaviors of surface-coated serpentine ultrafine powders as lubricant additive. Tribol. Int. 2010, 43, 667–675. [CrossRef] 22. Yu, H.L.; Xu, Y.; Shi, P.J.; Wang, H.M.; Zhang, W.; Xu, B.S. Effect of thermal activation on the tribological behaviours of serpentine ultrafine powders as an additive in liquid paraffin. Tribol. Int. 2011, 44, 1736–1741. [CrossRef] 23. Qi, X.W.; Jia, Z.N.; Yang, Y.L.; Fan, B.L. Characterization and auto-restoration mechanism of nanoscale serpentine powder as lubricating oil additive under high temperature. Tribol. Int. 2011, 44, 805–810. [CrossRef] 24. Zhao, F.Y.; Bai, Z.M.; Zhao, D.; Yan, C.M. Synthesis and tribological properties of serpentine/La composite powders. J. Chin. Ceram. Soc. 2012, 40, 126–130.

25. Zhao, F.Y.; Bai, Z.M. Tribological properties of serpentine, La (OH)3 and their composite particles as lubricant additives. Wear 2012, 288, 72–77. [CrossRef] 26. Zhao, F.Y.; Kasrai, M.; Sham, T.K.; Bai, Z.M. Characterization of triboﬁlms generated from serpentine and commercial oil using X-ray absorption spectroscopy. Tribol Lett. 2013, 50, 287–297. [CrossRef] 27. Zhao, F.Y.; Kasrai, M.; Sham, T.K.; Bai, Z.M.; Zhao, D. Characterization of triboﬁlms derived from zinc dialkyl dithiophosphate and serpentine by X-ray absorption spectroscopy. Tribol. Int. 2014, 73, 167–176. [CrossRef] 28. Caruso, L.J.; Chernosky, J.J.V. The stability of lizardite. Can. Miner. 1979, 17, 757–769. 29. Fuchs, Y.; Linares, J.; Mellini, M. Mossbauer and infrared spectrometry of lizardite-1T from Monte Fico, Elba. Phys. Chem. Miner. 1998, 26, 111–115. [CrossRef] Lubricants 2020, 8, 93 21 of 23

30. Capitani, G.; Mellini, M. The modulated crystal structure of antigorite: The m = 17 polysome. Am. Mineral. 2004, 89, 147–158. [CrossRef] 31. Uehara, S.; Shirozu, H. Variations in chemical composition and structural properties of antigorites. J. Mineral. Petrol. Sci. 1985, 12, 299–318. [CrossRef] 32. Evans, B.W.; Dyar, M.D.; Kuehner, S.M. Implication of ferrous and ferric iron in antigorite. Am. Mineral. 2012, 97, 184–196. [CrossRef]

33. Mellini, M.; Zanazzi, P.F. Crystal structures of lizardite 1T and lizardite 2H1 from Coli, Italy. Am. Mineral. 1987, 72, 943–948. 34. Anthony, J.W.; Bideaux, R.A.; Bladh, K.W.; Nichols, M.C. Handbook of Mineralogy; Mineralogical Society of America: Chantilly, VA, USA; pp. 20151–21110. Available online: http://www.handbookofmineralogy.org/ (accessed on 22 August 2020). 35. MacKenzie, K.J.D.; Meinhold, R.H. Thermal reactions of chrysotile revisited; a 29 Si and 25 Mg MAS nmR study. Am. Miner. 1994, 79, 43–50. 36. Viti, C. Serpentine minerals discrimination by thermal analysis. Am. Miner. 2010, 95, 631–638. [CrossRef] 37. Gualtieri, A.F.; Giacobbe, C.; Viti, C. The dehydroxylation of serpentine group minerals. Am. Miner. 2012, 97, 666. [CrossRef] 38. Alvarez-Silva, M.; Uribe-Salas, A.; Watersa, K.E.; Finch, J.A. Zeta potential study of pentlandite in the presence of serpentine and dissolved mineral species. Miner. Eng. 2016, 85, 66–71. [CrossRef] 39. Manser, R.M. Handbook of Silicate Flotation; Warren Spring Laboratory: Stevenage, UK, 1975. 40. Li, G.J.; Zhao, P.; Bai, Z.M. Surface characteristics of serpentine. J. Chin. Ceram. Soc. 2017, 45, 1204–1210. 41. Feng, B.; Lu, Y.P.; Feng, Q.M. Mechanisms of surface charge development of serpentine mineral. Trans. Nonferrous Met. Soc. China 2013, 23, 1123–1128. [CrossRef] 42. Cao, J.; Zhang, Z.Z.; An, S.H.; Wang, C. Surface modification and tribology properties in bass oil of ultra-fine serpentine. J. Chin. Ceram. Soc. 2008, 36, 1210–1214. 43. Cao, J.; Zhang, Z.Z.; Zhao, F.X.; Jiang, C.J.; Duan, Z.W. The Effect of different surfactants on the dispersion properties of serpentine in the alcohol solvent. Lubr. Eng. 2007, 32, 83–86. 44. Li, G.J.; Bai, Z.M.; Zhao, P. Antifriction repair function of serpentine on Fe based metal friction pairs. J. Chin. Ceram. Soc. 2018, 46, 306–314. 45. Li, G.J.; Bai, Z.M.; Huang, W.J.; Ju, Y. Research of serpentine powder surface modification. Bull. China Ceram. Soc. 2008, 6, 1091–1095. 46. Qi, X.W.; Lu Li Jia, Z.N.; Yang, Y.L.; Fan, B.L.; Shi, L. Tribological properties of serpentine nanoparticles as oil additive under different material friction pairs. Adv. Mater. Res. 2011, 199–200, 1051–1057. [CrossRef] 47. Leont’ev, L.B.; Shapkin, N.P.; Leont’ev, A.L.; Shkuratov, A.L.; Vasil’eva, V.V. Effect of the composition of mineral and organomineral mixtures on the tribological properties of friction pairs. Inorg. Mater. 2013, 49, 894–898. [CrossRef] 48. Yu, H.L.; Xu, Y.; Shi, P.J.; Wang, H.M.; Wei, M.; Xu, B.S. Tribological properties of heat treated serpentine ultrafine powders as lubricant additives. Tribology 2011, 31, 504–509. 49. Zheng, W.; Zhang, Z.Z.; Zhao, F.X.; Xu, D.D. Heat treating serpentine powder and its effect on the tribological properties of lube base oil. Pet. Process. Petrochem. 2013, 44, 71–74. 50. Wu, J.W.; Wang, X.; Zhou, L.H.; Wei, X.C.; Wang, W.R. Formation factors of the surface layer generated from serpentine as lubricant additive and composite reinforcement. Tribol. Lett. 2017, 65, 93–102. [CrossRef] 51. Zhang, Y.W.; Li, Z.P.; Yan, J.C.; Ren, T.H.; Zhao, Y.D. Tribological behaviours of surface-modified serpentine powder as lubricant additive. Ind. Lubr. Tribol. 2016, 68, 1–8. [CrossRef] 52. Yu, H.L.; Xu, Y.; Xu, B.S.; Shi, P.J.; Zhao, C.F. Research on tribological properties of No.2 tank grease improved by ultra-fine mineral micro-powder. J. Acad. Armored Force Eng. 2009, 23, 80–83. 53. Wang, X.; Wu, J.W.; Wei, X.C.; Liu, R.D.; Cao, Q. The effect of serpentine additive on energy-saving and auto-reconditioning surface layer formation. Ind. Lubr. Tribol. 2017, 69, 158–165. [CrossRef] 54. Fang, X.; Yan, Z.J.; Yan, X.L.; Liu, D.N.; Zhang, Y. Tribological properties of nano-ceria and serpentine mixtures as lubricating oil additives. Lubr. Eng. 2019, 44, 68–73. 55. Zhang, Y.; Yan, Z.J.; Zhu, X.H.; Cheng, D.; Liu, D.N.; Yan, X.L. Tribological properties of compound additives of magnesium silicate hydroxide superfine powder and MoDTC in lubricating oil additives. Lubr. Eng. 2018, 43, 18–22. Lubricants 2020, 8, 93 22 of 23

56. Si, Y.B.; Chang, Q.Y.; Qiao, J.F.; Cui, Y.B. Friction reduction and antiwear properties of serpentine and oleic acid as lubricating oil additives. Lubr. Eng. 2015, 40, 42–45. 57. Zhang, B.S.; Xu, B.S.; Xu, Y.; Ba, Z.X.; Wang, Z.Z. Self-restoration effect and action mechanism of tribopairs consisting of analogue shaft-bushing induced by serpentine powders. Mater. Sci. Eng. Powder Metall. 2013, 18, 346–352. 58. Yang, Q.M.; Bai, Z.M. The Material characteristic and tribological intervention behaviors of hyper-fine serpentine powder and its industrial application. Lubr. Eng. 2010, 35, 98–101. 59. Liu, J.J.; Guo, F.W. The application and analysis of a new self-repairing technology on the frictional surface. China Surf. Eng. 2004, 3, 42–47. 60. Yan, Y.H.; Xiao, H.; Yang, Y.L. Influence of the self-repairing additive concentration on tribology properties of cast iron/cast iron friction pair. China Surf. Eng. 2008, 21, 35–39. 61. Yin, Y.L.; Yu, H.L.; Wang, H.M.; Zhang, Z.; Bai, Z.M.; Xu, B.S. Effect of serpentine mineral as lubricant additive on the tribological behaviors of tin bronze. Tribology 2020, 40, 510–519. 62. Chen, M.; Xu, Z.S.; Xue, B.; Liu, Y.; Ma, W.D. Friction and wear performance of a NiAl-8 wt% serpentine-2 wt% TiC composite at high temperatures. Mater. Res. Express 2018, 5.[CrossRef] 63. Pang, N.; Bai, Z.M. The antifriction properties of ultra-fine serpentine powders as lube oil additive on iron-based friction pair. Adv. Mater. Res. 2012, 399–401, 616–619. [CrossRef] 64. Zhang, B.S.; Xu, B.S.; Xu, Y.; Ba, Z.X.; Wang, Z.Z. Lanthanum effect on the tribological behaviors of natural serpentine as lubricant additive. Tribol. Trans. 2013, 56, 417–427. [CrossRef] 65. Gao, Y.Z.; Chen, W.G.; Zhang, H.C. Super lubricity characteristics using ceramic composite mineral powder as lubricating oil additive. In Advanced Tribology; Springer: Berlin/Heidelberg, Germany, 2009; pp. 329–332. 66. Xu, Y.; Zhang, B.S.; Xu, B.S.; Gao, F.; Shi, P.J.; Zhang, B. Thermodynamic characteristics and tribological properties of lanthanum/serpentine composite lubricating material. Mater. Sci. Eng. Powder Metall. 2011, 16, 349–354. 67. Wang, L.Y.; Zhang, Z.Z.; Huang, J.J.; Zhao, F.X. Study on tribological properties of gear oil with nano-metal/serpentine powder. Pet. Process. Petrochem. 2016, 47, 98–102. 68. Sleptsova, S.A.; Afanas’eva, E.S.; Grigor’eva, V.P. Structure and tribological behavior of polytetrafluoroethylener modoifed with layered Silicates. J. Frict. Wear 2009, 30, 431–437. [CrossRef] 69. Jia, Z.N.; Yang, Y.L. Self-lubricating properties of PTFE/serpentine nanocomposite against steel at different loads and sliding velocities. Comp. Part B 2012, 43, 2072–2078. [CrossRef] 70. Jia, Z.N.; Yang, Y.L.; Chen, J.J.; Yu, X.J. Influence of serpentine content on tribological behaviors of PTFE/serpentine composite under dry sliding condition. Wear 2010, 268, 996–1001. [CrossRef] 71. Li, S.M.; Zhang, Q.X.; Zhang, J.H. Tribological properties of TiAl matrix composite with serpentine powder. J. Wuhan Univ. Technol. 2016, 38, 13–39. 72. Wu, J.W.; Wang, X.; Zhou, L.H.; Wei, X.C.; Wang, W.R. Preparation, mechanical and anti-friction properties of Al/Si/serpentine composites. Ind. Lubr. Tribol. 2018, 70, 1051–1059. [CrossRef] 73. Xue, B.; Jing, P.X.; Ma, W.D. Tribological properties of NiAl Matrix composites filled with serpentine powders. J. Mater. Eng. Perform. 2017, 26, 5816–5824. [CrossRef] 74. Wang, P.; Zhang, F.X.; Zhang, Z.Z.; Yang, J.H. Tribological properties and initial exploration mechanism of composite grease with bismuth nano-particles and ultrafine serpentine powders. Acta Pet. Snica Pet. Process. Sci. 2011, 27, 643–648. 75. Yang, H.W.; Li, Z.L.; Fei, Y.W.; Sun, S.A. Anti-wear & self-repair mechanism of magnesium silicate and talc powder. Synth. Lubr. 2006, 33, 1–3. 76. Qu, M.; Zhao, F.X.; Zhang, Z.Z.; Yang, J.H. Study on the triological properties of the grease added with ultra-fine serpentine powders. Lubr. Eng. 2010, 35, 74–76. 77. Yu, H.L.; Xu, Y.; Shi, P.J.; Wang, H.M.; Wei, M.; Zhao, K.K.; Xu, B.S. Microstructure, mechanical properties and tribological behavior of tribofilm generated from natural serpentine mineral powders as lubricant additive. Wear 2013, 297, 802–810. [CrossRef] 78. Yu, Y.; Gu, J.L.; Kang, F.Y.; Kong, X.Q.; Mo, W. Surface restoration induced by lubricant additive of natural minerals. Appl. Surf. Sci. 2007, 253, 7549–7553. [CrossRef] 79. Zhao, Y.; Xu, B.S.; Xu, Y.; Shi, P.J.; Wang, X.L.; Zhang, B. Study on the characteristics of wear resistance using silicate particles as additive on the metal friction pairs. Key Eng. Mater. 2008, 373–374, 452–455. [CrossRef] Lubricants 2020, 8, 93 23 of 23

80. Shi, P.J.; Yu, H.L.; Zhao, Y.; Xu, Y.; Xu, B.S.; Bai, Z.M. Eﬀects of in-situ tribochemical treatment on mechanical property and tribological behavior of 45 steel. Trans. Mater. Heat Treat. 2007, 28, 113–117. 81. Zhang, B.S.; Xu, B.S.; Xu, Y.; Wu, Y.X.; Zhang, B. Friction reduction and anti-wear mechanism of serpentine micro powders for spheroidal graphite iron tribopair. J. Chin. Ceram. Soc. 2009, 37, 2037–2042.

82. Yang, H.; Jin, Y.S.; Kazuhiko, Y. Experimental study of applying Mg6Si4O10(OH)8 reconditioner to simulative journal bearing. Lubr. Eng. 2006, 7, 144–146. 83. Yang, H.; Zhang, Z.Y.; Li, S.H.; Jin, Y.S. XPS Characterization of auto-reconditioning layer on worn metal surfaces. Spectrosc. Spectr. Anal. 2008, 25, 945–948. 84. Zhang, B.; Xu, B.S.; Xu, Y.; Zhang, B.S. Tribological characteristics and self-repairing effect of hydroxy-magnesium silicate on various surface roughness friction pairs. J. Cent. South Univ. Technol 2011, 18, 1326–1333. [CrossRef] 85. Chen, W.G.; Gao, Y.Z.; Zhang, H.C.; Xu, X.L.; Yu, Z.W. Investigation of the effects of lubricant oil with silicate particles as additive on the wear resistance of friction pair. China Surf. Eng. 2006, 19, 36–39. 86. An, X. The field test of magnesium silicate hydroxide as lubricating oil additive. Synth. Mater. Aging Appl. 2017, 46, 57–59. 87. Zhou, P.Y. Experiment on self-repairing metal material applied to diesel engine of locomotive. Railw. Locomot. Car 2003, 23, 13–15, 53. 88. Sleptsova, S.A.; Okhlopkova, A.A.; Kapitonova, I.V.; Lazareva, N.N.; Makarov, M.M.; Nikiforov, L.A. Spectroscopic study of tribooxidation processes in modified PTFE. J. Frict. Wear 2016, 37, 129–135. [CrossRef] 89. Zhi, X.L.; Yan, H.X.; Li, S.; Niu, S.; Liu, C.; Xu, P.L. High toughening and low friction of novel bismaleimide

composites with organic functionalized serpentine@Fe3O4. J. Polym. Res. 2017, 24, 1–8. [CrossRef] 90. Bhushan, B. Introduction to Tribology; John Wiley & Sons: New York, NY, USA, 2002. 91. Yin, Y.L.; Yu, H.L.; Wang, H.M.; Zhou, X.Y.; Song, Z.Y.; Xu, Y.; Xu, B.S. Tribological performance of phyllosilicate minerals as lubricating oil additives. J. Chin. Ceram. Soc. 2020, 48, 299–308. 92. Zhang, Z.Y.; Yang, H.; Li, S.H.; Jin, Y.S.; Zhang, W.F. Application research of auto reconditioner for worn metals on DF locomotive diesel engines. Lubr. Eng. 2004, 4, 75–80. 93. Yang, Q.M. Industrial Test of the Tribological behaviors under material intervention circumstances. Lubr. Eng. 2009, 34, 17–19. 94. Qi, X.W.; Lu Li Jia, Z.N.; Yang, Y.L.; Liu, H.R. Comparative tribological properties of magnesium hexasilicate and serpentine powder as lubricating oil additives under High temperature. Tribol. Int. 2012, 49, 53–57. [CrossRef] 95. Zhang, B.S.; Xu, B.S.; Xu, Y.; Ba, Z.X.; Wang, Z.Z. An amorphous Si-O film tribo-induced by natural hydrosilicate powders on ferrous surface. Appl. Surf. Sci. 2013, 285, 759–765. [CrossRef] 96. Gao, Y.Z.; Zhang, H.C.; Xu, X.L.; Wang, L.; Chen, W.G. Formation mechanism of self-repair coatings on the worn metal surface using silicate particles as lubricant oil additive. Lubr. Eng. 2006, 10, 39–42. 97. Xiao, Z.; Hou, G.L.; Su, X.J.; Bi, S.; Ma, H.L. Friction-reducing performance of serpentine on 45# carbon steel surfaces with different roughness. J. Chin. Ceram. Soc. 2012, 40, 1190–1195. 98. Qi, X.W.; Jia, Z.N.; Chen, H.M.; Yang, Y.L.; Wu, Z. Self-repairing characteristics of powder as an additive on steel-chromium plating pair under high temperature. Tribol. Trans. 2013, 56, 516–520. [CrossRef] 99. Dolgopolov, K.N.; Lyubimov, D.N.; Kolesnikov, I.V. Investigation of the microstructure of mineral films obtained by frictional interaction. Glass Phys. Chem. 2016, 42, 302–306. [CrossRef] 100. Yang, Y.L.; Yan, Y.H.; Zhang, R.J.; Re, J. Influence of wear time on metal wear self-repair and mechanism analysis. Chin. J. Mech. Eng. 2008, 44, 172–176. [CrossRef] 101. Zhao, F.Y. Tribological Properties of Serpentine Micro-Nanoparticles and Mechanisms of Tribofilm Formation; China University of Geosciences: Beijing, China, 2014. 102. Veblen, D.R.; Wylie, A.G. Mineralogy of amphiboles and 1:1 layer silicates. In Reviews in Mineralogy and Geochemistry; Mineralogical Society of America: Washington, DC, USA, 1993; Volume 28, pp. 61–131.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).