Thermal Stability and High-Temperature Super Low Friction of γ -Fe 2 O 3 @SiO 2 Nanocomposite Coatings on Steel

: The thermal stability of the γ -Fe 2 O 3 @SiO 2 nanocomposites and super low friction of the γ -Fe 2 O 3 @SiO 2 nanocomposite coatings in ambient air at high temperature are investigated in this paper. X-ray diffraction, scanning electron microcopy, transmission scanning electron microcopy, high-temperature tribometer, thermogravimetric analysis and differential scanning calorimetry were used to investigate the microstructure, surface morphology and high-temperature tribological properties of the γ -Fe 2 O 3 @SiO 2 nanocomposite coatings, respectively. The results show that the γ -Fe 2 O 3 @SiO 2 nanocomposite with the core–shell structure has excellent thermal stability because the SiO 2 shell inhibits the phase transition of the γ -Fe 2 O 3 phase to the α -Fe 2 O 3 phase in the nanocomposites. The temperature of the phase transition in γ -Fe 2 O 3 can be increased from 460 to 829 ◦ C. The γ - Fe 2 O 3 @SiO 2 nanocomposite coatings exhibit super low friction (0.05) at 500 ◦ C. A high-temperature super low friction mechanism is attributed to γ -Fe 2 O 3 and the tribochemical reactions during sliding.


Introduction
Solid lubricants are usually applied to minimize tribology under conditions including a simultaneous occurrence of high temperature in mechanical systems [1][2][3][4][5][6][7][8].There are multi-functional compounds (DLC films, WS 2 , h-BN, MXens, etc.) as solid lubricants offering self-lubrication at elevated temperatures [9][10][11][12][13][14].However, it is well known that it is difficult for a single solid lubricant to meet the requirements of equipment in applications that span a wide range of temperatures.Instead, it is necessary to fabricate the novel coatings with self-lubricating properties as well as high hardness and high wear resistance compactness [15,16].Nanocomposites are promising candidate materials for high-temperature lubrication [17][18][19][20][21][22][23][24].The high-performance and low-cost Fe 2 O 3 -based nanocomposites are intensively investigated for the potential applications at elevated temperature [25].For example, high temperature is a typical operating condition for aerospace [26][27][28][29][30].Our previous work reported high-temperature superlubricity due to an in situ γ-Fe 2 O 3 @SiO 2 nanocomposite [19,21].However, there are a few questions regarding the transition mechanism and transition temperature of the γ to α-phase in the γ-Fe 2 O 3 @SiO 2 nanocomposites as well as the consistency of the mechanical properties and tribological properties of the γ-Fe 2 O 3 @SiO 2 nanocomposites.Therefore, this paper investigates the thermal stability of nanocomposites.Many researchers have shown that doping improves the mechanical and tribological properties of nanocomposites [31][32][33][34].Silver is one of the doping elements in the coatings.The nanocomposite coatings exhibit a low coefficient of friction (CoF) [35,36].The research shows that the Ag element in the coatings is helpful to reduce the CoF of the Ag-Cr 2 AlC coatings from 25 to 700 • C [37].However, the mechanical properties of the coatings are relatively low due to the soft metal Ag.Excessive Ag not only reduces the hardness and elastic modulus of Ag-doped coatings but also reduces the adhesion between the coating and the substrate [38,39].Rare earth not only improves the high-temperature hardness of the coatings but also increases the bonding strength of the reinforcements [40].Ceria (CeO 2 ) is widely used in many applications, and it is a favorable material in nanocomposites based on the remarkable tribological properties [41][42][43][44][45].For example, the Cr 3 C 2 -NiCrCoMo/nano-CeO 2 composite coatings achieve high temperature and a low CoF [43].Therefore, Ag and CeO 2 are used to dope the γ-Fe 2 O 3 @SiO 2 nanocomposites, and the high-temperature tribological properties and thermal stability of the Ag and CeO 2 combined doping of γ-Fe 2 O 3 @SiO 2 nanocomposite is investigated.The sol-gel technology is useful for the preparation of the magnetic nanocomposites.Firstly, the Ag and CeO 2 combined doping of γ-Fe 2 O 3 @SiO 2 coatings was deposited on steel.Then, the tribological properties of the Ag and CeO 2 combined doping of γ-Fe 2 O 3 @SiO 2 coatings are studied at 500 • C in open air, and the high-temperature antifriction mechanism of the γ-Fe 2 O 3 @SiO 2 nanocomposite coating is also studied.The aim and novelty of the present work is to investigate the thermal behavior and stability of the high temperature and super low friction of the nanocomposite coatings with the doping of Ag and CeO 2 , enabling comparisons with our previous paper.
We used the following preparation process for the γ-Fe 2 O 3 xerogel.Fe(NO 3 ) 3 •9H 2 O was firstly dissolved in EtOH (ethanol, the content of ethanol is 99.7%).Few quantities of nitric acid (HNO 3 ) were introduced to the beaker containing the solution of the mixtures.After 10 h, the sol was packed in a PTFE tube and covered and kept in an oven at 50 • C to obtain γ-Fe 2 O 3 gel.Fe(NO 3 ) 3 •9H 2 O was dissolved in ethanol.TEOS was introduced into solution.HNO 3 was added until the pH reached 2. After 24 h, AgNO 3 solution and Ce(NO 3 ) 3 •6H 2 O were poured into sol, and a sol with Ag and CeO 2 was achieved.
The sol was prepared on the steel surface by a spray photoresist coating device (EVG101CS, EV Group, Salzburg, Austria).The steel is high-speed tool steel.The sol was vibrated by an ultrasonic machine (YL-31s, Taizhou, China).And then, it was dropped on the surface of the steel.After that, the coatings were dried in a vacuum dry cabinet at 50 • C for 4 h, at 60 • C for 2 h and finally at 80 • C for 2 h, respectively.The coated steel was treated at 200 • C for 1 h and finally at 400 • C for 2 h.The thicknesses of the coatings are 2.2 µm for γ-Fe 2 O 3 @SiO 2 nanocomposite coatings, 2.9 µm for γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings and 3.3 µm forγ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings.
The phase and microstructure of the nanocomposites and the nanocomposite coatings were investigated by D8-Advance X-ray diffraction (XRD) (D8-Advance, Bruker, Saarbrucken, Germany) in the range of 2θ = 10 • ~90 • , using a Cu target (Kα) ceramic tube.The wavelength is adjusted to λ = 0.15406 nm, where the maximum power of the X-ray generator is 3 KW, the voltage is 60 KV, and the rated current is 60 mA.The scanning speed is 8 • /min.Subsequently, XRD analysis software (Jade software, 6.0) was used to determine the phase composition of the crystal by searching the JCPDS-Joint Commeette of Power Diffraction Standards.The crystal size can be calculated from Scherrer's formula: D = 0.89λ/B cos θ where D-crystal size/nm; λ-wavelength of incident X-ray/λ = 0.15406 nm; θ-diffraction Angle/ • ; B-corrected diffraction peak half-height width/rad.The microstructure of the nanocomposite coatings was measured by Raman spectroscopy (HR800, Horiba Jobin Yvon, Pairs, France) using a laser with a wavelength of 532 nm.The morphology and composition of the coatings were studied by scanning electron microscope (SEM, S-3000N, Hitachi, Tokyo, Japan) with an energydispersive spectroscopy (EDS) system and transmission electron microscope (TEM, G2F300, FEI, Hillsboro, OR, USA).The mechanical tests of the prepared composites are carried out to understand the mechanical properties of the nanocomposites.In this paper, the hardness of the nanocomposite coating was measured by a micro-Vickers hardness tester.The test conditions are as follows: 4.9 g of load was selected, and the force loading time was 15 s.Five different points were selected on the sample for testing, the microhardness was calculated and the average value was taken as the microhardness of the sample.The hardnesses of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings, γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings and γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings are about 808 HV, 547 HV and 953 HV.
Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were used to conduct synchronous thermal analysis.The relevant information of the change in sample weight with temperature can be obtained from the TG curve, and the weight loss rate of the sample in different temperature ranges can be obtained.The thermal information of the samples at different temperatures can be obtained from the DSC curve.The melting and crystallization processes, glass transition, phase transition, solidification and oxidation stability of samples can be investigated by DSC.By using the same sample in one measurement, synchronous thermal analysis can simultaneously obtain relevant information such as sample mass change with temperature and heat absorption and release.In this study, the TG-DSC curve of the sample was determined by a STA449F5 synchronous thermal analyzer produced by Netsch, Germany.The temperature rise rate was 10 • C/min in atmosphere air, and the test temperature range was from 30 to 1000 • C.
The tribological behaviors of coatings were investigated at 500 • C in open air.The number of the tested samples is three for each group testing.The disc was slid against a ball with the diameter of 9.5 mm at a speed of 0.05 m/s and load of 2 N. The worn surfaces of the samples were studied by optical microscopy.XRD was also applied to observe the microstructure of the worn surfaces of the disc.The material of the ball is ZrO 2 and the sliding distance is about 150 m during friction coefficient measurement.

Microstructure of Coatings
Figure 1 shows the TEM image and element composites of γ-Fe 2 O 3 @SiO 2 nanocomposites.The core-shell structures were found in the nanocomposite.The profile of the nanocomposite is close to a cotton-like shape.It can be seen that the color of the outer edge of the composite particle is light, while the inner color is dark.This is because SiO 2 and γ-Fe 2 O 3 magnetic nanoparticles do not have the same degree of electron absorption, so there is a relatively obvious contrast and boundary between light and dark.The γ-Fe 2 O 3 nanoparticles are scattered in the amorphous structure of SiO 2 in the γ-Fe 2 O 3 @SiO 2 nanocomposite.This can also indicate that the nanocomposite forms a typical cladding structure with amorphous shells covering nuclei.There are a few elements, including Fe, Si and O, according to the analysis results.The chromium present in the EDS results may come from the substrate of the high-speed tool steel.Figure 2 shows the XRD and Raman spectra of the γ-Fe2O3@SiO2 nanocompos coatings.There are three obvious bands at 675 cm −1 , 1330 cm −1 and 1585 cm −1 .Raman sults show that there is SiO2 and γ-Fe2O3.There are peaks of 35.63°, 43.28°and 53.7 which are all typical bands of γ-Fe2O3 [46].Two characteristic peaks at 43.28° and 51. were observed that can be indexed to (400) and (421).Moreover, the XRD peak at corresponds to amorphous SiO2 shells surrounding γ-Fe2O3 nanoparticles.There is γ-Fe2O3 and SiO2 in the γ-Fe2O3@SiO2 nanocomposite.We found that there is no ot impurity phase in the coatings.The particle sizes of γ-Fe2O3 are between 1 and 100 n Figure 2 shows the XRD and Raman spectra of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings.There are three obvious bands at 675 cm −1 , 1330 cm −1 and 1585 cm −1 .Raman results show that there is SiO 2 and γ-Fe 2 O 3 .There are peaks of 35.63 • , 43.28 • and 53.73 • , which are all typical bands of γ-Fe 2 O 3 [46].Two characteristic peaks at 43.28 • and 51.69 • were observed that can be indexed to (400) and (421).Moreover, the XRD peak at 25 • corresponds to amorphous SiO 2 shells surrounding γ-Fe 2 O 3 nanoparticles.There is Fe, γ-Fe 2 O 3 and SiO 2 in the γ-Fe 2 O 3 @SiO 2 nanocomposite.We found that there is no other impurity phase in the coatings.The particle sizes of γ-Fe 2 O 3 are between 1 and 100 nm, which belong to the nano-level particles, and the average particle size is about 14.2 nm, so the prepared composite coatings belong to the nanocomposite coatings.Figure 2 shows the XRD and Raman spectra of the γ-Fe2O3@SiO2 nanocompos coatings.There are three obvious bands at 675 cm −1 , 1330 cm −1 and 1585 cm −1 .Raman sults show that there is SiO2 and γ-Fe2O3.There are peaks of 35.63°, 43.28°and 53.7 which are all typical bands of γ-Fe2O3 [46].Two characteristic peaks at 43.28° and 51. were observed that can be indexed to (400) and (421).Moreover, the XRD peak at 2 corresponds to amorphous SiO2 shells surrounding γ-Fe2O3 nanoparticles.There is γ-Fe2O3 and SiO2 in the γ-Fe2O3@SiO2 nanocomposite.We found that there is no oth impurity phase in the coatings.The particle sizes of γ-Fe2O3 are between 1 and 100 n which belong to the nano-level particles, and the average particle size is about 14.2 n so the prepared composite coatings belong to the nanocomposite coatings.Figure 3 shows the Raman spectra of the Ag and CeO2 doped γ-Fe2O3@SiO2 nan composite coatings.The XRD measurement results show that there is SiO2 and γ-Fe2 The nanocomposite coating contains γ-Fe2O3, Ag and amorphous SiO2.According Scherrer's formula, the average particle sizes of γ-Fe2O3 and Ag in the coating are 1 nm and 19.8 nm, respectively, which are slightly different from the average particle s of the γ-Fe2O3@SiO2 nanocomposite coatings.The results show that Ag doping nanocomposite coatings slightly affects the particle size of γ-Fe2O3.The average parti sizes of the γ-Fe2O3 grains and Ag in CeO2 nanocomposite coating are 9.5 nm and 1 nm.Comparing with the γ-Fe2O3@SiO2-Ag nanocomposite coatings, the average parti sizes of γ-Fe2O3 and Ag decrease, indicating that CeO2 is useful to refine the gra [47,48].Figure 3 shows the Raman spectra of the Ag and CeO 2 doped γ-Fe 2 O 3 @SiO 2 nanocomposite coatings.The XRD measurement results show that there is SiO 2 and γ-Fe 2 O 3 .The nanocomposite coating contains γ-Fe 2 O 3 , Ag and amorphous SiO 2 .According to Scherrer's formula, the average particle sizes of γ-Fe 2 O 3 and Ag in the coating are 14.4 nm and 19.8 nm, respectively, which are slightly different from the average particle size of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings.The results show that Ag doping the nanocomposite coatings slightly affects the particle size of γ-Fe 2 O 3 .The average particle sizes of the γ-Fe 2 O 3 grains and Ag in CeO 2 nanocomposite coating are 9.5 nm and 14.2 nm.Comparing with the γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings, the average particle sizes of γ-Fe 2 O 3 and Ag decrease, indicating that CeO 2 is useful to refine the grains [47,48].

Thermal Stability Analysis of Nanocomposite Coatings
Figure 4 shows the TG/DSC thermal analysis curves of γ-Fe2O3 xerogel a γ-Fe2O3@SiO2 xerogel, respectively.The thermal stability of γ-Fe2O3 is low, and it is in metastable state.The γ-Fe2O3 is usually transformed into stable α-Fe2O3 when the te perature exceeds 400 °C.A layer of amorphous SiO2 is coated on the surface of γ-Fe2O3 improve the thermal stability of γ-Fe2O3 [49,50].The effect of amorphous SiO2 on t phase transition temperature of γ-Fe2O3 is investigated, and γ-Fe2O3 xerogel a γ-Fe2O3@SiO2 xerogel were analyzed by TG/DSC.

Thermal Stability Analysis of Nanocomposite Coatings
Figure 4 shows the TG/DSC thermal analysis curves of γ-Fe2O3 xerogel a γ-Fe2O3@SiO2 xerogel, respectively.The thermal stability of γ-Fe2O3 is low, and it is in metastable state.The γ-Fe2O3 is usually transformed into stable α-Fe2O3 when the te perature exceeds 400 °C.A layer of amorphous SiO2 is coated on the surface of γ-Fe2O3 improve the thermal stability of γ-Fe2O3 [49,50].The effect of amorphous SiO2 on t phase transition temperature of γ-Fe2O3 is investigated, and γ-Fe2O3 xerogel a γ-Fe2O3@SiO2 xerogel were analyzed by TG/DSC.It can be seen from Figure 4 that there are five endothermic peaks in the DSC curv of (a) and (b).The positions of the four endothermic peaks are almost the same.The fi exothermic peak appears around 84 °C in Figure 4a and around 94 °C in Figure 4b.T endothermic peak is produced by the exothermic heat of organic reagents such as ethan in the gel network.The second exothermic peak appears around 147 °C in Figure 4a a around 137 °C in Figure 4b.This endothermic peak is caused by disengagement of t desorption water in γ-FeOOH•xH2O.The third endothermic peak is shown around 1 °C in Figure 4a and around 206 °C in Figure 4b.This endothermic peak is caused by t decomposition of nitrate radicals in the gel system.The fourth endothermic peak appe around 263 °C in Figure 4a and around 271 °C in Figure 4b.This endothermic peak c responding to the dehydroxylation of γ-FeOOH to form γ-Fe2O3, and both absorpti peaks are wide.There is also a significant weight loss step corresponding to the TG cur The weight loss temperature ranges are 217~308 °C and 202~350 °C, respectively.T position of the fifth endothermic peak of the absorption is very different from Figure 4a The fifth endothermic peak appears around 460 °C in Figure 4a and around 829 °C Figure 5 shows the XRD of γ-FeOOH, γ-Fe 2 O 3 and α-Fe 2 O 3 under different heattreated temperatures, respectively.Figure 5a shows an XRD pattern of a powder sample dried at 50 • C in a drying cabinet.There are two diffraction peaks appearing at 36.39 • and 60.67 • , which indicates the characteristic peak of γ-FeOOH.The diffraction peak is wide, indicating that the crystallinity of γ-FeOOH is low.Figure 5b shows the XRD pattern of the samples after heat treatment at 300 • C for 2 h.The values of the diffraction peak on the spectrum are 2θ = 24.02• , 30.21  , which is consistent with the values on the PDF card JCPDS 39-1346, indicating that the samples are decomposed to obtain γ-Fe 2 O 3 .The average particle size is given as the geometric mean diameter, expressed in mm or microns (µm), and the range of variation is described by geometric standard deviation [52].We used the Scherrer equation to calculate the average particle size of the crystals.The Scherrer equation relates to the diffraction peak [53,54].The instrumental broadening and physical broadening of the sample were measured through the full width half maximum.By utilizing the correction of physical broadening, it will be possible to conduct a follow-up calculation on the crystal size with the Scherrer equation.
The average particle size of the γ-Fe 2 O 3 crystal is 13 nm, which is calculated according to the Scherrer formula at this temperature.The heat treatment temperature increases to 500 • C, and the diffraction peak values on the XRD pattern are 2θ = 24.13, respectively, consistent with the values on the PDF card JCPDS 33-0664, indicating that γ-Fe 2 O 3 has completely transformed into α-Fe 2 O 3 , and the particle crystal form is a hexahedral structure.The Scherrer formula calculates that the average particle size of α-Fe 2 O 3 crystal is 17.1 nm.With the increase in heat treatment temperature, the Fe 2 O 3 particles prepared by the sol-gel method are converted from γ-Fe 2 O 3 to α-Fe 2 O 3 , which is also consistent with the thermal analysis results of Fe 2 O 3 .In addition, the average particle size of α-Fe 2 O 3 is also larger than that of γ-Fe 2 O 3 .and 64.16°, which is consistent with the values on the PDF card JCPDS 39-1346, indicat that the samples are decomposed to obtain γ-Fe2O3.The average particle size is given the geometric mean diameter, expressed in mm or microns (μm), and the range of var tion is described by geometric standard deviation [52].We used the Scherrer equation calculate the average particle size of the crystals.The Scherrer equation relates to diffraction peak [53,54].The instrumental broadening and physical broadening of sample were measured through the full width half maximum.By utilizing the correct of physical broadening, it will be possible to conduct a follow-up calculation on crystal size with the Scherrer equation.The average particle size of the γ-Fe2O3 crysta 13 nm, which is calculated according to the Scherrer formula at this temperature.T heat treatment temperature increases to 500 °C, and the diffraction peak values on XRD pattern are 2θ = 24.13°,33.08°, 35.59°, 40.86°, 49.41°, 54.06°, 57.46°, 62.42°, 64.0 71.86°, and 75.29°, respectively, consistent with the values on the PDF card JCP 33-0664, indicating that γ-Fe2O3 has completely transformed into α-Fe2O3, and the parti crystal form is a hexahedral structure.The Scherrer formula calculates that the avera particle size of α-Fe2O3 crystal is 17.1 nm.With the increase in heat treatment tempe ture, the Fe2O3 particles prepared by the sol-gel method are converted from γ-Fe2O3 α-Fe2O3, which is also consistent with the thermal analysis results of Fe2O3.In additi the average particle size of α-Fe2O3 is also larger than that of γ-Fe2O3.In Figure 3a, a peak appears near 2θ = 25°, indicating the presence of amorpho SiO2 in the sample.In addition, a weak diffraction peak of γ-FeOOH is shown in Fig 5b .After the sample was heat treated at 300 °C for 2 h, the diffraction peaks appeared In Figure 3a, a peak appears near 2θ = 25 • , indicating the presence of amorphous SiO 2 in the sample.In addition, a weak diffraction peak of γ-FeOOH is shown in Figure 5b.After the sample was heat treated at 300 • C for 2 h, the diffraction peaks appeared at 2θ = 30.26, which coincided with the values on the PDF card JCPDS 39-1346, indicating that the sample contained γ-Fe 2 O 3 , and the average crystal was calculated according to the Scherrer formula.The particle size is 11.9 nm. Figure 5c shows the XRD pattern of the sample after heat treatment at 500 • C for 2 h.The position of the diffraction peak is the same as that of Figure 5b, but the peak intensity becomes slightly high, indicating that the sample has no phase change in γ-Fe 2 O 3 .According to Scherrer's formula, the average crystal grain size is calculated to be 13 nm, indicating that as the temperature increases, the crystal grains are further grown, corresponding to the enhancement of the peak intensity of the XRD pattern.When the heat treatment temperature is 500 • C, the phase change in γ-Fe 2 O 3 in the sample does not occur, which is also consistent with the thermal analysis result of γ-Fe 2 O 3 @SiO 2 .
The rare earth element Ce has strong chemical activity and large atomic radius, which can be used as a heterogeneous nucleation point to inhibit grain growth.CeO 2 was added to improve the thermal stability of γ-Fe 2 O 3 in γ-Fe 2 O 3 @SiO 2 nanocomposites.Thermal analysis of the CeO 2 -doped γ-Fe 2 O 3 @SiO 2 -Ag composite dry gel is studied.Figure 6 shows a TG-DSC curve of γ-Fe 2 O 3 @SiO 2 gel doped with Ag and CeO 2 .There are also four stages from Figure 6  also four stages from Figure 6 during the thermal decomposition process.The weight loss temperature range of the first stage is 30~103 °C.The weight loss rate is 17%, which corresponds to the DSC curve showing an exothermic peak around 74 °C, which is caused by the combustion exotherm of organic reagents such as ethanol in the gel network.The weight loss interval in the second stage is 103~167 °C.The weight loss rate is 6%.In this temperature zone, the DSC curve has an endothermic peak near 161 °C, which corresponds to the decomposition of nitrate in the sample.The third stage weight loss interval is 167~281 °C, and the weight loss rate is 8%, which corresponds to the dehydroxylation of iron oxyhydroxide (FeOOH) producing γ-Fe2O3 in the sample.The fourth stage is 281 °C, where the TG curve becomes almost horizontal, indicating that the sample has almost no weight loss after this temperature.The endothermic peak at 725 °C corresponds to the conversion of γ-Fe2O3 to the stable α-Fe2O3.CeO2 is helpful to refine the grains of the nanocomposites.The average particle size of γ-Fe2O3 in γ-Fe2O3@SiO2-Ag-CeO2 nanocomposite coatings is smaller than that of undoped CeO2 nanocomposite coatings.After when the γ-Fe2O3 grain reduces, the surface energy increases, the energy of the phase change decreases, and the phase transition temperature decreases.It is concluded that the phase transition temperature of γ-Fe2O3 in nanocomposites decreases from 829 to 725 °C after doping CeO2.

High-Temperature Tribological Properties of Nanocomposite Coatings
Figure 7 shows the CoF of the nanocomposite coatings in ambient air at 500 • C. The CoF of the steel substrate is 0.34 initially and then fluctuates around 0.35 along with the sliding time.The CoF of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings is about 0.44 at the initial stage and then decreases slightly, although CoF increases finally.It is found surprisingly that at the stable stage, the CoF of the nanocomposite coatings is about 0.05, which exhibits super low friction.The CoF of the γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings increases to the maximum value and then decreases.The applied test time to study the CoF values is about 3000 s, because the CoF of the friction pair becomes stable at about 1000 s.The CoF of the γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings is about 0.39 at the initial stage and then decreases to 0.05 after 1500 s; finally, CoF increases slowly to the end of the tribotest.The CoF of the γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings is about 0.3 at initial stage; then, the CoF decreases slightly to 0.06 at the stable stage.During the sliding, there are few periods in the CoF values.CoF firstly increases and then decreases.From the previous work [19,21], it is found that the suggested composite can have a good CoF at high temperature, especially at 500 • C, not only at high temperature.
about 1000 s.The CoF of the γ-Fe2O3@SiO2-Ag nanocomposite coatings is about 0.39 the initial stage and then decreases to 0.05 after 1500 s; finally, CoF increases slowly to end of the tribotest.The CoF of the γ-Fe2O3@SiO2-Ag-CeO2 nanocomposite coating about 0.3 at initial stage; then, the CoF decreases slightly to 0.06 at the stable stage.D ing the sliding, there are few periods in the CoF values.CoF firstly increases and th decreases.From the previous work [19,21], it is found that the suggested composite have a good CoF at high temperature, especially at 500 °C, not only at high temperatu Figure 8 shows the wear morphology of the nanocomposite coatings and ball.the γ-Fe2O3@SiO2 nanocomposite coatings, a smooth and continuous transfer film pears on the ball, corresponding to the micro-plough on the wear surface of γ-Fe2O3@SiO2 nanocomposite coating.According to the wear morphology and frict curve, it can be concluded that the CoF is high at the beginning of friction, and th γ-Fe2O3 becomes soft.In addition, there is the formation of the lubrication film wit continuous oxide film on the wear surface under the combined action of ambient te perature and friction heat.Because γ-Fe2O3 is easy to shear at high temperature, the C of the nanocomposite coating is significantly reduced to 0.05.For the γ-Fe2O3@SiO2- Figure 8 shows the wear morphology of the nanocomposite coatings and ball.For the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings, a smooth and continuous transfer film appears on the ball, corresponding to the micro-plough on the wear surface of the γ-Fe 2 O 3 @SiO 2 nanocomposite coating.According to the wear morphology and friction curve, it can be concluded that the CoF is high at the beginning of friction, and then γ-Fe 2 O 3 becomes soft.In addition, there is the formation of the lubrication film with a continuous oxide film on the wear surface under the combined action of ambient temperature and friction heat.Because γ-Fe 2 O 3 is easy to shear at high temperature, the CoF of the nanocomposite coating is significantly reduced to 0.05.For the γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings, t the lubrication films on the wear marks of the nanocomposite coatings are smooth, there is only a small amount of wear debris on the wear surface, and the wear mechanism is adhesive wear.The transfer films are distributed continuously on the wear spots of the ball, which plays a good lubricating role on the wear surface and effectively reduces the CoF of the nanocomposite coatings.For the γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings, a non-continuous lubricating film appears on the wear surface of the nanocomposite coating, and the worn surface of ball is almost smooth, resulting in high CoF relatively.The volume wear rate of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings, γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings and γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings are about 15.02 × 10 −5 mm 3 (Nm) −1 , 4.32 × 10 −5 mm 3 (Nm) −1 and 8.65 × 10 −5 mm 3 (Nm) −1 .
of the abrasive chips is transferred to the ball to form the initial transfer film.As the fri tion and wear process continues, under the combined action of friction and high tem perature, γ-Fe2O3 is formed on the friction surface on the ball, forming a layer of lubric tion film with low shear strength, and a continuous smooth transfer film is formed on th surface of the ball, so the friction between the friction pairs can be effectively reduce Therefore, the γ-Fe2O3@SiO2 nanocomposite coating has good high-temperature lubric tion performance.During the sliding, there are a few periods in the CoF values.The CoF firstly increases and then decreases.From the previous work [19,21], it is found that the suggested composite can have a good CoF at high temperature, especially at 500 • C, not only at high temperature.CoF decreases slightly with the increase in sliding time.In the initial stage of friction, the nanocomposite material is broken to form abrasive chips distributed on the friction surface, and the friction surface becomes rough, which may result in the high CoF, and the CoF firstly increased and then decreased.At this time, a small amount of the abrasive chips is transferred to the ball to form the initial transfer film.As the friction and wear process continues, under the combined action of friction and high temperature, γ-Fe 2 O 3 is formed on the friction surface on the ball, forming a layer of lubrication film with low shear strength, and a continuous smooth transfer film is formed on the surface of the ball, so the friction between the friction pairs can be effectively reduced.Therefore, the γ-Fe 2 O 3 @SiO 2 nanocomposite coating has good high-temperature lubrication performance.
Figure 9 shows XRD results of the worn surfaces of the nanocomposite coatings after the tribo tests.XRD results of the nanocomposite coatings show that Fe, α-Fe 2 O 3 and γ-Fe 2 O 3 are on the worn surface of the disc.The γ-Fe2O3@SiO2 nanocomposite coatings have good tribological properties.Th CoF of the γ-Fe2O3@SiO2 nanocomposite coatings is low compared to the uncoated stee disk.The CoF of the γ-Fe2O3@SiO2 coatings decreases to the minimum value (about 0.05) The low friction behavior is attributed to oxidation and phase change accord to th analysis and results.At 500 °C, the oxides were formed on the contact area.Super low friction is obtained due to the γ-Fe2O3 and high load capacity of amorphous SiO2.The sof phase γ-Fe2O3 has a high plasticity performance at elevated temperature.The soft phas The γ-Fe 2 O 3 @SiO 2 nanocomposite coatings have good tribological properties.The CoF of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings is low compared to the uncoated steel disk.The CoF of the γ-Fe 2 O 3 @SiO 2 coatings decreases to the minimum value (about 0.05).The low friction behavior is attributed to oxidation and phase change accord to the analysis and results.At 500 • C, the oxides were formed on the contact area.Super low friction is obtained due to the γ-Fe 2 O 3 and high load capacity of amorphous SiO 2 .The soft phase γ-Fe 2 O 3 has a high plasticity performance at elevated temperature.The soft phase of γ-Fe 2 O 3 would melt under the localized heating and high pressure at the friction interface, releasing molten material.The mechanism is a major contributor to the low friction that occurred under high temperature.The solid lubricant is added to the nanocomposite coating to improve the lubrication performance of the nanocomposite coating.Silver has a face-centered cubic structure, which is prone to intergranular slip and has low shear force.And the thermochemical properties of silver are stable, and it can maintain good anti-friction properties.The γ-Fe 2 O 3 and Ag are relatively soft, resulting in the long run-in time of the Ag-γ-Fe 2 O 3 @SiO 2 nanocomposite coatings and reducing the surface hardness of the coatings.It leads to a large initial CoF, and γ-Fe 2 O 3 and Ag are transferred to the worn surface of the ball.The nanocomposite coatings with the Ag doping reach a low CoF because γ-Fe 2 O 3 has a low shear performance and plays a synergistic lubrication role with Ag in the nanocomposite coatings.The chemical activity of rare earth element Ce is strong, the atomic radius is large, and it is used as a heterogeneous nucleation particle, inhibiting the grain growth of the material, thus refining the coating grain, making the coating structure dense and uniform, and improving the compatibility of the lubricating component of the lubrication system.The microhardness and binding properties of the nanocomposite coatings become high.The addition of CeO 2 refines Ag grains, the density of nanocomposite coatings increases, and the hardness also increases.The wear resistance of the CeO 2 nanocomposite coating is improved.The average particle size of γ-Fe 2 O 3 in the γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coating is smaller than that of the undoped CeO 2 nanocomposite coating.When the particle size of γ-Fe 2 O 3 decreases, the surface energy increases and the energy required for phase transition decreases, so the structural phase transition temperature decreases.The γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings also have the characteristic peak of α-Fe 2 O 3 appearing in the wear scar.For the γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coating, Ag in the coatings mainly acts to reduce friction.The lubricating films formed on the worn surface of the coatings are non-continuous due to the addition of the CeO 2 toughening phase, the fluctuation of the friction curve reduces and the wear is reduced.Although the CoF of the nanocomposite coatings increases slightly, the wear becomes low compared with the undoped CeO 2 coating.The novelty of the work is that the thermal behavior and the stability of the high-temperature super low friction of the nanocomposite coatings are improved by Ag and CeO 2 ; the mechanism is also provided according to the experimental results.

Conclusions
The γ-Fe 2 O 3 @SiO 2 nanocomposite coatings with Ag and CeO 2 were prepared, and the tribological properties and thermal stability of the nanocomposite coatings were investigated.The conclusions of this work are summarized as follows: (1) Amorphous SiO 2 in coatings is coated with crystal particles of γ-Fe

Figure 4
Figure 4 shows the TG/DSC thermal analysis curves of γ-Fe 2 O 3 xerogel and γ-Fe 2 O 3 @SiO 2 xerogel, respectively.The thermal stability of γ-Fe 2 O 3 is low, and it is in a metastable state.The γ-Fe 2 O 3 is usually transformed into stable α-Fe 2 O 3 when the temperature exceeds 400 • C. A layer of amorphous SiO 2 is coated on the surface of γ-Fe 2 O 3 to

Figure 4 .
Figure 4. TG/DSC curve of (a) γ-Fe 2 O 3 dry gel and (b) γ-Fe 2 O 3 @SiO 2 dry gel.It can be seen from Figure4that there are five endothermic peaks in the DSC curves of (a) and (b).The positions of the four endothermic peaks are almost the same.The first exothermic peak appears around 84 • C in Figure4aand around 94 • C in Figure4b.This endothermic peak is produced by the exothermic heat of organic reagents such as ethanol in the gel network.The second exothermic peak appears around 147 • C in Figure4aand around 137 • C in Figure 4b.This endothermic peak is caused by disengagement of the desorption water in γ-FeOOH•xH 2 O.The third endothermic peak is shown around 193 • C in Figure 4a and around 206 • C in Figure 4b.This endothermic peak is caused by the decomposition of nitrate radicals in the gel system.The fourth endothermic peak appears around 263 • C in Figure 4a and around 271 • C in Figure 4b.This endothermic peak corresponding to the dehydroxylation of γ-FeOOH to form γ-Fe 2 O 3 , and both absorption peaks are wide.There is also a significant weight loss step corresponding to the TG curve.The weight loss temperature ranges are 217~308 • C and 202~350 • C, respectively.The position of the fifth endothermic peak of the absorption is very different from Figure 4a,b.The fifth endothermic peak appears around 460 • C in Figure 4a and around 829 • C in Figure 4b.Under the TG curve, there are almost no weight loss steps, which indicates that the endothermic peak corresponds to the transition of γ-Fe 2 O 3 to the more stable α-Fe 2 O 3 .It is also inferred that the thermal phase transition of the dry gel is γ-FeOOH•xH 2 O → γ-FeOOH → γ-Fe 2 O 3 → α-Fe 2 O 3 [51].Comparing Figure 4a,b, it is known that SiO 2 -coated γ-Fe 2 O 3 has relatively high thermal stability compared with single γ-Fe 2 O 3 , and the phase transition temperature of the γ-Fe 2 O 3 phase to the α-Fe 2 O 3 phase increases from 460 to 829 • C.Figure5shows the XRD of γ-FeOOH, γ-Fe 2 O 3 and α-Fe 2 O 3 under different heattreated temperatures, respectively.Figure5ashows an XRD pattern of a powder sample dried at 50 • C in a drying cabinet.There are two diffraction peaks appearing at 36.39 • and 60.67 • , which indicates the characteristic peak of γ-FeOOH.The diffraction peak is wide, indicating that the crystallinity of γ-FeOOH is low.Figure5bshows the XRD pattern of the samples after heat treatment at 300 • C for 2 h.The values of the diffraction peak on the spectrum are 2θ = 24.02• , 30.21 • , 35.70 • , 43.39 • , 53.95 • , 57.48 • , 63.04 • , and 64.16• , which is consistent with the values on the PDF card JCPDS 39-1346, indicating that the samples are decomposed to obtain γ-Fe 2 O 3 .The average particle size is given as the geometric mean diameter, expressed in mm or microns (µm), and the range of variation is described by geometric standard deviation[52].We used the Scherrer equation to calculate the average particle size of the crystals.The Scherrer equation relates to the diffraction peak[53,54].The instrumental broadening and physical broadening of the sample were measured through the full width half maximum.By utilizing the correction of physical broadening, it will be possible to conduct a follow-up calculation on the crystal size with the Scherrer equation.
during the thermal decomposition process.The weight loss temperature range of the first stage is 30~103 • C. The weight loss rate is 17%, which corresponds to the DSC curve showing an exothermic peak around 74 • C, which is caused by the combustion exotherm of organic reagents such as ethanol in the gel network.The weight loss interval in the second stage is 103~167 • C. The weight loss rate is 6%.In this temperature zone, the DSC curve has an endothermic peak near 161 • C, which corresponds to the decomposition of nitrate in the sample.The third stage weight loss interval is 167~281 • C, and the weight loss rate is 8%, which corresponds to the dehydroxylation of iron oxyhydroxide (FeOOH) producing γ-Fe 2 O 3 in the sample.The fourth stage is 281 • C, where the TG curve becomes almost horizontal, indicating that the sample has almost no weight loss after this temperature.The endothermic peak at 725 • C corresponds to the conversion of γ-Fe 2 O 3 to the stable α-Fe 2 O 3 .CeO 2 is helpful to refine the grains of the nanocomposites.The average particle size of γ-Fe 2 O 3 in γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings is smaller than that of undoped CeO 2 nanocomposite coatings.After when the γ-Fe 2 O 3 grain reduces, the surface energy increases, the energy of the phase change decreases, and the phase transition temperature decreases.It is concluded that the phase transition temperature of γ-Fe 2 O 3 in nanocomposites decreases from 829 to 725 • C after doping CeO 2 .

Figure 9
Figure 9 shows XRD results of the worn surfaces of the nanocomposite coatings aft the tribo tests.XRD results of the nanocomposite coatings show that Fe, α-Fe2O3 an γ-Fe2O3 are on the worn surface of the disc.

Figure 9
Figure9shows XRD results of the worn surfaces of the nanocomposite coatings afte the tribo tests.XRD results of the nanocomposite coatings show that Fe, α-Fe2O3 and γ-Fe2O3 are on the worn surface of the disc.

Funding:
2 O 3 due to the coreshell structures found by TEM.The microstructure of coatings can improve the thermal stability of γ-Fe 2 O 3 , and the temperature of the phase transition in γ-Fe 2 O 3 can be increased from 460 to 829 • C according to the results of TG/DSC thermal analysis.(2) At 500 • C, the γ-Fe 2 O 3 @SiO 2 nanocomposite coating can reach a stable super low friction coefficient of 0.05.The CoF of Ag coatings is about 0.05.The run-in time of nanocomposite coatings can be reduced with the doping of Ag.The CoF achieved by the γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coating is 0.06.(3) The volume wear rates of the γ-Fe 2 O 3 @SiO 2 nanocomposite coatings, γ-Fe 2 O 3 @SiO 2 -Ag nanocomposite coatings and γ-Fe 2 O 3 @SiO 2 -Ag-CeO 2 nanocomposite coatings are about 15.02 × 10 −5 mm 3 (Nm) −1 , 4.32 × 10 −5 mm 3 (Nm) −1 and 8.65 × 10 −5 mm 3 (Nm) −1 .(4) The high-temperature self-lubrication mechanism of coatings is attributed to the hightemperature soft phase of γ-Fe 2 O 3 with low shear strength and the stability of Ag and CeO 2 doping.The present work is financially supported by the Liaoning Key Laboratory of Aero-engine Materials Tribology (LKLAMTF202401) and the National Natural Science Foundation of China (51675409).