Influence of Natural Serpentine on Tribological Performance of Phosphate Bonded Solid Coatings

Natural serpentine powders as functional fillers were incorporated to promote the anti-wear performance of phosphate-bonded solid coatings. Optimal mass percent of natural serpentine in phosphate coatings was first explored. Thereafter, to further stimulate strengthening effect of natural serpentine on tribological performance of phosphate composite coatings, harsher friction condition on the phosphate composite coatings was conducted. The experimental result indicated that the optimal incorporation of natural serpentine in phosphate coatings was 10 wt.%, through which anti-wear performance of phosphate coatings was significantly elevated. Additionally, accompanied by the increase of applied load and sliding speed, natural serpentine was gradually activated, and interfacial interactions between naturals serpentine and counterface were intensified. As a result, a continuous protective tribo-film was in situ formed on the counterface, through which serious furrows generated on the counterface were effectively self-repaired during the friction process. What’s more, further abrasion on both phosphate composite coatings and counterface was also greatly restrained. Natural serpentine as the enhanced phase was incorporated into phosphate bonded solid coatings. The experimental results indicated that when applied load and sliding speed were increased, transfer and adhesion of phosphate composite coatings on the counterface were apparently intensified, and simultaneously, serious furrows generated on the wear spot were obviously reduced. What’s more, a continuous tribo-film was in-situ formed on the counterface, through which the direct abrasion between phosphate composite coatings and counterpart steel ball was restrained, and therefore, the anti-wear performance of phosphate composite coatings was significantly promoted.

Natural serpentine, as a kind of layered silicate minerals, is usually applied as lubrication additive and exhibited superb friction-reducing and anti-wear properties in base oil. The possible reasons are mainly attributed to its special micro-structure and excellent physiochemical characteristics. Natural serpentine is mainly composed of magnesium silicate hydroxide [Mg 3 Si 2 O 5 (OH) 4 ], of which silicon oxide tetrahedron and magnesium oxide (hydroxide) octahedron (TO layer) make up the basic unit, and each unit is held together via Van der Waals forces and hydrogen bonds. Under the boundary lubrication condition, interlayer slip of natural serpentine at the friction interface can reduce the friction coefficient of base oil [28][29][30][31]. What's more, structural distortion of the MSH (Magnesium silicate hydroxide) at the friction interface also inevitably occurred, through which reactive oxygen functional groups of natural serpentine are released, such as Si-O, Si-O-Si, Mg-O, and Mg-OH. With the stimulation of friction force and local friction heat, these reactive functional groups generated complex physical and tribo-chemical interactions with tribo-pairs [28,29,[32][33][34][35][36][37][38][39]. As a result, a protective tribo-film is formed on the surface of tribo-pairs, by which further abrasion on the friction pairs is effectively abated.
Based on the special micro-structure and high chemical activity of natural serpentine, it can also be selected as enhanced phase to promote the tribological performance of phosphate bonded solid coatings. On the one hand, the chrysotile (fibrous natural serpentine) contained in natural serpentine possesses higher tensile strength, which is conducive to elevate the toughness of composite coatings. Additionally, reactive hydroxyls contained in natural serpentine will definitely generate interfacial combination with phosphate binder during the high-temperature curing process of composite coatings, through which mechanical strength of the composite coating can be promoted. On the other hand, combined with the excellent performance of natural serpentine in lubrication oil, its layered micro-structure and high reactivity are also beneficial to promote friction-reducing and anti-wear properties of phosphate coatings.
In this study, natural serpentine as enhanced phase was incorporated in phosphate bonded solid coatings. Optimal incorporation mass percentage of natural serpentine powders in phosphate coatings was firstly investigated. And then, in order to intensify strengthening effect of natural serpentine on tribological performance of phosphate coatings, tribological experiments under increased applied load and sliding speed on phosphate composite coatings were explicitly studied. Finally, the corresponding strengthening mechanism of natural serpentine on tribological performance of phosphate coatings was properly proposed based on the experimental result.

Materials
Natural serpentine was commercially purchased from Jiangsu Longteng Chemical Co., Ltd (Lianyungang, China). The phosphate cement, mainly composed of chromium magnesium phosphate, was produced by our laboratory, and synthesis process was similar with the reference of [8] and [9]. Deionized water, as the dispersive medium of sprayed paints, was also produced by our laboratory.

Fabrication of Phosphate/Natural Serpentine Composite Coatings
Before spraying the composite coatings, the pre-sprayed paint was firstly configured. For the sake of exploring the optimal mass percent of natural serpentine in phosphate coatings, a various of paint systems that contained with 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, and 25 wt.% natural serpentine were properly prepared, respectively. Additionally, to guarantee that every component was homogenously dispersed in the paint system, the pre-sprayed paint system was stirred by superhigh speed blender for at least 5 min. The pre-sprayed substrate (Inconel 718 round block, diameter 24 mm, thickness 7.8 mm) surface was roughened by sand-blasting to a roughness about 1.5 μm (measured by non-contact three-dimensional surface profiler, ISO 25178), which was conducive to the effective combination of substrate with sprayed phosphate coatings. After that, the substrate blocks were cleaned with acetone in an ultrasonic cleaner for at least 20 min. An artificial spraying gun was employed to spray composite coatings. After the spraying was finished, the wet coatings were placed into a constant temperature drying oven, in which the temperature was firstly rising to 120 ℃ keeping for about 2 h, and then rose to 340 ℃ lasting for about 30 min. After curation, the sickness of the composite coatings was about 25 ± 5 μm when measured by the mini gauger (MINITEST 1100 microprocessor coating thickness gauge). The fabricated phosphate/ natural serpentine bonded solid coating was noted as PNS composite coating, and when the mass percent of natural serpentine was 10%, the corresponding coating was noted as PNS-10 composite coating.

Performance Characterization
A high-temperature CSM ball-on-disk tribometer (CSM Instrument, Switzerland) was employed to study the tribological performance of the PNS composite coatings. The commercialized counterpart steel balls (AISI 52,100 bearing steel ball, E = 210 GPa, HRC 58-63) with a diameter of 6 mm were used as counterface to abrade with the PNS composite coatings. All tribological experiments were conducted under room temperature about 25 ℃ and with a relative humidity about 35%. Friction coefficient of PNS composite coatings was timely recorded by the tribometer. Wear rate (w) of PNS composite coatings was speculated by the formular of w = V L /(L·F), which represented specific wear volume (V L ) of unit frictional distance (L) and unit applied load (F). Wear volume (V L ) of each PNS composite coating was measured by the non-contact three-dimensional surface profiler (KLA-Tencor MicroXAM-800, America). Each friction experiment was repeated three times to reduce error, and the averaged values were finally reported.

Analytical Methods
The morphologies of natural serpentine and composite coatings were observed by field emission scanning electronic microscopy (FESEM, Japan Electronics JSM-6701F). The crystal phase and composition of natural serpentine were detected by high-resolution X-ray diffractometer (HR-XRD, Bruker D8Discover25, Germany). Fourier transform infrared spectrometer (FTIR, Bruker V70, Germany) was used to detect the functional groups of natural serpentine. 3D morphologies as well as contour profiles of wear tracks were observed by non-contact three-dimensional surface profiler (KLA-Tencor MicroXAM-800, America). Surface morphologies and element distribution of the wear spot were tested by field emission scanning electronic microscopy (FESEM, FEI Quanta 650, America) and the affiliated energy dispersive X-ray spectrometer (EDS), separately. X-ray photoelectron spectrometer (XPS, PHI5000, America) was adopted to analyze the chemical composition of wear spot. The specific morphology and composition of the tribo-film section were observed by transmission electron microscopy (TEM, FEI TECNAL G2 S-TWIN F20, America) and spherical aberration corrected transmission electron microscopy (STEM, Titan Cubed Themis G2 300, America). Figure 1a and b showed XRD spectrum and FTIR spectrum of natural serpentine. The main absorption peaks in XRD spectrum were exactly corresponding to the standard card of PDF#00-002-0094, which depicted the monoclinic magnesium silicate hydroxide Mg 3 Si 2 O 5 (OH) 4 , and specific crystal face indexes were properly noted in Fig. 1a. However, there also appeared some weak absorption peaks that belonged to the impurities. XRD analysis indicated that the main component contained in the natural serpentine was magnesium silicate hydroxide, but some unknown mineral impurities were inevitably contained in it and difficult to purify. FTIR spectrum demonstrated the characteristic functional groups of The morphologies of the finely grind natural serpentine powders were exhibited in Fig. 2. It was obvious that natural serpentine powders contained with various morphologies, such as fibrous natural serpentine in Fig. 2a and b, schistose natural serpentine in Fig. 2c, and granular natural serpentine in Fig. 2d, respectively. With regard to fibrous natural serpentine, its diameter was about 50 nm, and aspect ratio can achieve two orders of magnitude. Furthermore, fibrous natural serpentine possessed with high tensile strength and it was conducive to reduce the self-brittleness of phosphate coatings. Diameter of the schistose natural serpentine powders was about 3 μm, at the same time, with small particles and sticks agglomerated onto their surface. Additionally, diameter of small granular natural serpentine was about 50 nm to 800 nm.

Characterization of Natural Serpentine and PNS Composite Coatings
The surface morphologies of the fabricated PNS composite coatings were demonstrated in Fig. 3. Mass percent of natural serpentine in Fig. 3a and b was 0%; mass percent of natural serpentine in Fig. 3c and d was 10%. Although, the two kind of phosphate coatings both appeared different degrees of cracks, obvious disparities between them can still be easily discovered. For coatings without incorporation of natural serpentine, there emerged serious micro-cracks and the width can achieve 1 μm to 5 μm. Some cracks even ran through the total figure and extended to the further place of phosphate coatings. Along with the water evaporation during the curing process of phosphate coatings, volume of the phosphate coatings was rapidly decreased, and therefore, micro cracks were inevitably generated. When it came to the coatings incorporated with 10 wt.% natural serpentine, such situation was significantly improved. Owing to fibrous natural serpentine possessed with high fracture toughness, when it was incorporated into phosphate coatings, width of the micro cracks was conspicuously narrowed. As a result, the defects of PNS-10 composite coatings were obviously decreased. However, for the reason that the proportion of fibrous natural serpentine was relatively small and some fibrous natural serpentine may have been broken during the grinding process. Therefore, some micro cracks still appeared on the surface of PNS-10 composite coatings. More significantly, reactive hydroxyl groups of natural serpentine would certainly generate interfacial combination with the reactive groups of phosphate cement during hightemperature curing process, by which effective combination between natural serpentine and phosphate cement was promoted. As a result, mechanical strength of the composite coatings can be elevated, and anti-wear performance of the composite coatings can be subsequently promoted.

Tribological Performance of PNS Composite Coatings
The tribological performance of phosphate composite coatings was demonstrated in Fig. 4, of which the mass percent of incorporated natural serpentine was increased from 0 to 25%, with an increasing rate of 5%. Figure 4a exhibited the specific friction coefficient and wear rate of each coating, and Fig. 4b represented concrete friction curve of each coating. All experiments were conducted under the same friction settings. Specifically, the friction coefficients of PNS composite coatings were slightly ascended with the increase of natural serpentine, and when the mass percent of natural serpentine was 20%, the friction coefficient of the composite coatings was in maximum. The same conclusion can also be verified in Fig. 4b. However, when it referred to wear rates of PNS composite coatings, its variation trend emerged distinct discrepancy to that of friction coefficients.
With the mass percent of natural serpentine increased from 0 to 25%, wear rate of each coating was firstly decreased and then rapidly increased when content of natural serpentine exceeded 10 wt%, which illustrated that with incorporation of 10 wt% natural serpentine, phosphate composite coatings owned the most excellent anti-wear property, and anti-wear performance was promoted about 55.8% compared with phosphate coating without incorporation of natural serpentine. When mass percent of natural serpentine was less than 10%, content of natural serpentine in phosphate coatings was relatively small, and its strengthening effect was insufficient.
In contrast, when the mass percent of natural serpentine was exceeded 10%, the bonding strength between phosphate cement was largely wakened by excessive natural serpentine. As a result, mechanical strength of the corresponding phosphate composite coatings was greatly decreased, and deficient anti-wear performance was exhibited. Only when the mass percent of natural serpentine was 10%, anti-wear performance of phosphate coating can be mostly promoted. The 3D morphologies and contour profiles of wear tracks were exhibited in Fig. 5, of which the mass percent of natural serpentine in Fig. 5a and b was 0%; in Fig. 5c and d was 10%; in Fig. 5e and f was 25%. It was obvious that when phosphate coating was incorporated with 10 wt% natural serpentine, its wear track in Fig. 5c was relatively flat, and simultaneously, the abrasion extent was apparently mitigated compared with Fig. 5a and e. The same conclusion can also be confirmed by the specific contour line of each wear track. Especially for PNS-10 composite coating, both width and depth of the wear track were simultaneously achieved the minimum value. The experimental results same proved that with incorporation of 10 wt% natural serpentine can significantly promote anti-wear performance of phosphate coatings.
Although anti-wear performance of phosphate coatings was effectively promoted by incorporation of 10 wt% natural serpentine powders, friction coefficient of the composite coatings was still relatively high. Especially, accompanied by the increase of natural serpentine, friction coefficient of the composite coatings also slightly increased. Frictionreducing property of natural serpentine was not reflected in the phosphate boned solid coatings. Apart from the size uniformity of grinded natural serpentine was relatively poor and some big particles were contained in it. Another possible reason was may attributed to that small friction force may be insufficient to completely overcome the interlayer force of natural serpentine, and interlayer slip of natural serpentine during the friction process was relatively difficult, which Based on the above considerations, performance of PNS-10 composite coatings under different applied load and sliding speed was investigated, and specific result was shown in Fig. 6. Figure 6a was the tribological performance under different applied load from 2 to 14 N, in which sliding speed was 8 cm/s, and sliding distance was 150 m. Figure 6b was the tribological performance under different sliding speed from 4 to 16 cm/s, in which applied load was 11 N, and   Figure 6c and d reflected the specific friction curves during the friction process. When applied load and sliding speed was increased, layer slip in natural serpentine was boosted, and friction coefficient of the composite coatings was firstly reduced. But in the later stage of friction experiment, friction coefficient of the composite coatings was gradually increased. As a result, friction-reducing ability of natural serpentine was finite in phosphate composite coatings throughout the total friction process. What's more, more obvious impact also generated on the wear rate of composite coatings. Accompanied by the increase of applied load, wear rate was conspicuously declined, which indicated that the anti-wear ability of composite coatings was significantly promoted when the applied load was increased. In Fig. 6b, accompanied by the increased sliding speed, wear rate of composite coatings was firstly decreased and then increased.
Superhigh sliding speed of 16 cm/s may exceed the speed resistant capacity of the composite coatings and serious abrasion was imposed on the composite coatings, Therefore, increased wear rate was finally exhibited. Figure 7 exhibited the optical morphologies of wear spots under various friction condition, and the specific experimental settings was noted on each picture. An obvious variation law can be perceived was that accompanied by the increasing of applied load and sliding speed, rubbing degree appeared on the wear spot was oppositely alleviated. And simultaneously, transfer and adhesion of composite coatings on the surface of counterpart steel ball was obviously intensified. What need to be pointed out was that the wear spot area in Fig. 7f was minimum. Under this friction condition, the applied load was 11 N and the sliding speed was 8 cm/s. Combined with the result of Figs. 6 and 7, it can be inferred that excellent anti-wear performance of the composite coatings under harsh friction condition may be related to the adhesion and transfer of composite coatings onto the counterface.

Strengthening Mechanism of Natural Serpentine on Tribological Performance of Phosphate Coatings
For the purpose of exploring the strengthening mechanisms of natural serpentine on tribological performance of phosphate coatings, detailed analyses on the wear spot were conducted. The selected counterpart steel ball was under the friction condition where applied load was 11 N, and sliding speed was 8 cm/s. Elemental analysis on wear spot was demonstrated in Fig. 8, of which Fig. 8a was the morphology of wear spot, Fig. 8g was the EDS result of the wear spot, and others was the mapping result of each element on the wear spot. Aside from the counterface element of Fe and C, appearance of element P, O, Mg, Si, and Cr was attributed to the transfer of PNS-10 composite coatings onto the counterface during the friction process. Among them, the mass percent of element O was in maximum, which was because both phosphate binder and natural serpentine were rich in O. Appearance of element P and Cr on the wear spot were from transfer of phosphate cement. Additionally, element Mg may come from the transfer of phosphate cement (magnesium phosphate) or natural serpentine, and element Si was from the transfer of natural serpentine.
To explicitly explore the specific role of natural serpentine during the friction process of phosphate composite coatings, XPS analysis on the wear spot was also conducted. The concrete results were shown in Fig. 9, of which Fig. 9a-d was the fine spectrums of Fe2p, P2p, Si2p, and Mg1s, respectively. Chemical bond of Fe-O-Si in Fig. 9a demonstrated that natural serpentine happened isomorphic replacement reaction with the counterface during the friction process, and at the same time, generated the microcrystalline ceramic phase of MgO and SiO 2 on the counterface [28,32,35,36]. The specific reaction equations were as follows: Because of special micro-structure and high chemical activity, natural serpentine possessed high cation exchange capacity. Especially, bigger Mg 2+ in octahedron layer can be easily replaced by Fe 3+ , Cr 3+ , Al 3+ and etc. What's more, lattice bending and structure distortion of natural serpentine at the friction interface were also inevitably happened during the friction process, through which unsaturated chemical  friction heat, natural serpentine can easily generate tribochemical reaction with the counterface. Chemical bond of Fe-O-P indicated that phosphate cement also occurred chemical reaction with the counterface. Physical adsorption and tribo-chemical reaction greatly intensified adhesion and transfer of composite coating onto the counterface, which was beneficial to the in-situ formation of tribo-film on the counterface during the friction process [42][43][44][45][46][47][48].  Fig. 9c and d may come from the transfer of natural serpentine, but also may come from the newly generated microcrystalline ceramic phase of MgO and SiO 2 .
The morphology of tribo-film formed on the counterpart steel ball was shown in Fig. 10. It was clear that a continuous protective tribo-film was generated after the friction experiment, and it was well combined with the counterface. The thickness of the tribo-film was from 73 to 146 nm.
The frictional surface of the counterpart steel ball was completely protected by the generated tribo-film reflected in Fig. 10a. However, it can be obviously discovered that numerous white and black spots were filled in the tribofilm exhibited in Fig. 10b and c. For the sake of identifying the concrete chemical composition of these unknown spots, higher resolution of STEM analysis was conducted. What should be emphasized was that the diffraction contrast between the TEM and STEM was opposite. That was to say, substance with black appearance in TEM images was turned white in STEM images, and vice versa. Figure 11 exhibited the STEM images of micro-particles captured in tribo-film and their composition analysis. Figure 11a and b were specific morphology of the particles. Particles in Fig. 11a were more concentrated and their diameter was only about 3 nm, and diameter of the particle in Fig. 11b was about 20 nm. Elemental mapping result of Fig. 11b in Fig. 11c demonstrated that the elemental composition of the particle was Ni. Simultaneously, lattice fringe spacing of this  particle was also corresponding to the spacing of (111) in cubic Ni. Obviously, because the friction condition was relatively harsh, partial substrate was exposed, and wear debris of the substrate can be easily embedded in tribo-film during the friction process. However, for extremely small diameter of particles in Fig. 11a, resolution of the elemental mapping was too poor to identify their elemental composition. But lattice fringe spacing of those particles was exactly corresponding to the spacing of (400) in magnesium silicate hydroxide, which demonstrated that particles contained in Fig. 11a was super-finely grinded natural serpentine powders. Figure 12 was composition analyses of the black spot in STEM image of tribo-film, which corresponded to the white spot in Fig. 10. Combined with the morphology in Fig. 12a and elemental mapping results of Mg, O, Ni, Cr, and Fe, it can be found that signal strength of each element was apparently reduced at the black area. Obviously, elemental composition of the bleak area was not from the phosphate cement, substrate, or counterpart steel ball. Therefore, the only result of the bleak area was a hole. Additionally, because signal of element Si was uncharacteristic, its mapping result was not contained in Fig. 12.
Based on the above analyses on wear spot and tribo-film. The strengthening mechanism of natural serpentine on anti-wear performance of phosphate coatings can be demonstrated as follows. When applied load and sliding speed were increased, under the stimulation of friction force and local friction heat, natural serpentine was activated and happened isomorphic replacement reaction with the newly friction-polished counterface, which triggered the formation of tribo-film on the counterface [32,37,39]. As the friction process continued, the abrasion and generation of tribo-film finally achieved dynamic equilibrium, and as a result, a certain thickness of tribo-film was in-situ formed on the counterface. Although, numerous micro-holes were inevitably produced during the accumulation process of tribo-film, the captured micro wear debris and super-finely grind natural serpentine powders can effectively fill in these holes, which definitely elevated the compactness of tribo-film. What's more, the newly generated microcrystalline ceramic phase of MgO and SiO 2 also definitely promoted anti-wear ability of the tribo-film. During the formation process of tribo-film, serious furrows generated on the wear spot were effectively self-repaired, and direct abrasion between the friction pairs was greatly abated. Therefore, excellent anti-wear performance of the phosphate composite coatings was exhibited.

Conclusion
Natural serpentine as enhanced phase was incorporated into phosphate coatings and its optimal mass percent was 10%, through which anti-wear performance of the phosphate coatings was significantly elevated. Additionally, accompanied by the increase of applied load and sliding speed, anti-wear performance of the composite coatings was further promoted. The possible reason was attributed to that harsh friction condition stimulated tribo-chemical reaction between natural serpentine and counterface. As a result, a continuous protective tribo-film was in-situ formed on the counterface, and simultaneously generated the microcrystalline ceramic phase of MgO and SiO 2 in the tribo-film. During this process, serious furrows generated on the wear spot were effectively self-repaired, and abrasion at the friction interface was greatly restrained. However, the friction-reducing ability of natural serpentine in phosphate coatings was finite and friction-reducing performance of the phosphate coatings need further optimization.