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Ultralow friction behaviour of B4C-BN-MeO composite ceramic coatings deposited on steel
Surface & Coatings Technology 390 (2020) 125664
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Ultralow friction behaviour of B4C-BN-MeO composite ceramic coatings deposited on steel
T
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Evgeny V. Kharanzhevskiya, , Alexey G. Ipatovb, Mikhail D. Krivilyova, Alexey V. Makarovc,d,e, Faat Z. Gil'mutdinovf, Elena G. Volkovac a
Udmurt State University, Universitetskaya st., 1, Izhevsk, Russia Izhevsk State Agricultural Academy, Studencheskaya st., 11, Izhevsk, Russia c M.N. Miheev Institute of Metal Physics of Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya St., 18, Yekaterinburg, Russia d Institute of Engineering Science of Ural Branch of the Russian Academy of Sciences, Komsomolskaya St., 34, Yekaterinburg, Russia e Ural Federal University, Mira St., 19, Yekaterinburg, Russia f Udmurt Federal Research Center of Ural Branch of the Russian Academy of Sciences, ul. T. Baramzinoy, 34, Izhevsk 426067, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Boron carbide Coatings Sliding wear Tribofilms Adhesion
Boron based ceramic coatings were deposited on steel substrates by high-energy short pulse laser melting. Under conditions of the laser deposition, coatings consisted of boron carbide and boron nitride form a layer that acts as solid lubricant leading to a low coefficient of friction (COF). The COF could be drastically decreased by proper selection of chemical composition of boron based ceramic coatings. Ultimately low COF of 0.03 was registered during dry sliding tests of coatings with addition of different metal oxides (MgO, ZrO2, Li2O). Low COF of composite ceramics coatings was attributed to the formation of the BeO bond observed by the X-ray photoelectron spectroscopy technique. This bond as the essential component of the internal structure of coatings explains ultralow friction behaviour and remains low even during gradual deterioration of the coating. High quality adhesion of ceramics to steels was explained by reactive phase formation at the steel-ceramic interface, which was confirmed by both transmission electron microscopy and X-ray diffraction.
1. Introduction The search for new materials with an improved combination of mechanical and protective properties has become increasingly important. The largest number of parts in mechanical engineering is made of various steels and alloys containing iron. Increasing the specific load on the elements of engines or mechanisms requires invention of effective technologies of surface modification to combine the properties inherent in ceramic materials and metal alloys. Formation of ceramic coatings on steel increases hardness, abrasion resistance, heat resistance and reduces the sliding coefficient of friction (COF) [1,2]. Boron carbide (B4C) has exceptional mechanical properties [1] and tribological characteristics [2]. It has low density (2.51 g sm−3), high melting point (2350 °C), high elastic modulus (445 GPa), and hardness (37 GPa) that concede only to cubic boron nitride (c-BN) and diamond. B4C is one of the promising materials as a thin hard coating to protect surfaces from wear and improve their tribological performance [3–5]. It has been shown that frictional properties of B4C depend on the atmospheric pressure, the temperature of the interface, oxidizing
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environment and the humidity of the surrounding air [1,6,7]. A linear relationship between the natural logarithm of the wear rate and the COF was found [2]. Numerous wear test results indicated that the COF and wear rate of B4C decreased with increase of humidity in the test atmosphere [1,6,7]. Low friction tribological films may be formed insitu during tribological tests especially at high sliding velocities in a high-humidity atmosphere [1]. In certain cases, after some period of friction a sharp transition between high and low friction occurs for B4C in a sliding contact [1]. The COF usually drops from values of about 0.4–1.0 to values of 0.1–0.2 during that transition. This drop is referred to formation of a tribological film consisted of boron oxide, boric acid [1] and graphitizing layer [2]. Erdemir et al. [6] described the formation of a thin surface film on B4C with ultralow friction behaviour by special treatment. The treatment consists of an oxidation stage in a furnace at temperature above 600 °C where boron oxide layer forms. This oxidation is followed by cooling in moisture atmosphere that leads to formation of boric acid film. It has been shown that this special treatment can lead to achievement of the ultralow COF on B4C surface as a direct
Corresponding author. E-mail address: [email protected] (E.V. Kharanzhevskiy).
https://doi.org/10.1016/j.surfcoat.2020.125664 Received 29 January 2020; Received in revised form 10 March 2020; Accepted 20 March 2020 Available online 20 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 390 (2020) 125664
E.V. Kharanzhevskiy, et al.
after milling were from 0.2 to 6 μm. The obtained powder-hexane suspension was spread on the surface of the steel substrate as a thin layer, normally 60 ± 15 μm in thickness by spraying. The thickness of the powder bed was controlled via time of sputtering and was measured by the mentioned above 3D optical profilometer as a difference between the heights of covered by the powder and uncovered parts of the substrate. After drying in air until complete heptane evaporation, samples were placed into the process chamber. At the end of the laser processing, samples were evacuated from the chamber, washed in alcohol and dried in air. Multilayer coatings were produced by repeating of the whole process described above. In this case the process is similar to SLM except for the chosen type of laser.
consequence of consistent formation of boron oxide B2O3 and boric acid H3BO3 on B4C surface. Boric acid has a layered triclinic crystal structure, which resembles those of MoS2, graphite, and hexagonal boron nitride (h-BN). The atomic layers are parallel to the basal plane and consist of boron, oxygen, and hydrogen atoms. These atoms are closely packed and strongly bonded to each other by covalent, ionic, and hydrogen bonds, whereas the atomic layers are widely spaced and held together by weak van der Waals force. When this layered structure sliding against a counterbody, it shears easily, thus providing ultralow friction [6]. For practical reasons, heating parts with boron-based coatings might be unacceptable at temperatures above 600 °C in oxidizing and moisture atmosphere. This especially refers to steel machine parts due to possibility of structural defects, for example, tempering embrittlement, carbide mesh extraction, and high temperature oxidation in water-vapor media [8]. For these reasons, search for the proper composition of boron-based ceramic films is of considerable interest. The most promising are composite ceramics coatings, which initially contain boron oxide. They can easily form compounds with a layered structure, such as boric acid, thus providing instant low COF without any furnace pre-treatment. If the structure of B4C-based ceramics contains boron oxide as an essential part of its internal microstructure, it could facilitate formation of low friction tribological film during gradual wear deterioration for the whole life cycle of machine parts with coatings. Boron oxide can be formed during a deposition process by high-temperature reactions with some metal oxides, therefore the main topic of this work is devoted to study of tribological properties of ceramic coatings, which composition contains metal oxides. Thin multilayer coatings based on boron carbide and nitride can be deposited by chemical vapor deposition or magnetron sputtering techniques using appropriate targets [3]. The main limitation of all available methods is low adhesion of the coating to relatively soft substrates such as steels. Therefore, such ceramic coatings are applied onto substrates made of hard alloys and are mainly used for cutting tools [9,10]. Fundamental difficulties in formation of cermet compounds include the following factors: ionic or covalent bonds of electrons of surface atoms in ceramics, differences in thermal expansion coefficient, low thermal conductivity of ceramics, difference in melting points, formation of intermetallic phases with high fragility, lack of chemical interaction at the interface between metal and ceramics. Therefore, this paper describes microstructure, adhesion and tribological properties of composite ceramic coatings deposited on steel substrates using high-energy short laser pulses. Initial compounds for coatings deposition are boron carbide, hexagonal boron nitride, and metal oxides Li2O, MgO or ZrO2.
2.2. Laser irradiation processing The experimental facility consists of the ytterbium fiber laser with a maximum average power of 50 W and a wavelength of 1.065 μm. For laser processing a chamber with controlled atmosphere was used. The chamber was initially evacuated by a backing pump and then filled with high purity argon to pressure 1.05 bar. In this study the laser chosen for laser processing has pulse duration of about 40 ns which is only three to four times of magnitude longer than the relaxation time for energy transfer from gas of valence electrons which absorbs laser energy to lattice of metal parts [11]. Thus, action of laser energy on ceramic powder can be described mainly by a thermal conduction mechanism but the pulse time is short enough to ensure ultrafast heating and high temperature gradients in the treated zone. The pulse frequency was set to 20 kHz, pulse energy was 1 mJ, so the average power of the laser beam was 20 W. At this laser irradiation parameters, instantaneous power of the laser beam was of 25 kW. The laser beam was focused into a spot with a diameter of 30 μm, so the instantaneous power density of laser radiation was about of 3.5·1013 W/m2. The parameters of laser irradiation were selected using the calculation procedure reported in [12]. The strategy of laser beam movement is shown in Fig. 1. The laser beam was moved over the samples surface with the use of a system that consists of two mirrors as it is shown in Fig. 1. Computer controlled tilting of the mirrors allows to perform beam moving along any complex track. Very low weight of the mirrors makes it possible to stop the movement at the edges of x-axis almost immediately and start the movement in the opposite direction so the velocity is almost constant both in the x- and y-directions. The scanning speed is 120 mm/s giving about 5 laser pulses at one point. The distance between the nearest laser
2. Experimental procedure 2.1. Materials preparation All substrates for coating deposition were cut from the same batch of grade 40 carbon steel (0.4 wt% C in the Russian state standard GOST 1050–88). The steel was in the annealed state, and the samples were machined and ground to achieve the surface roughness Ra of 0.24 μm. The roughness of the samples and coatings were measured by 3D optical profilometer Wyko NT 1100 (Veeco). Conditions of laser processing are characterized by a high temperature gradient, which impose the following requirements for powder preparation. The boron based ceramic powders should be spread on the surface of the substrate as a thin layer, not thicker than 100 μm, and the powder layer should be smooth and dense. To fulfill these requirements, we used fine powders of B4C, h-BN, Li2O, MgO, and ZrO2 – with an average size about 1 μm. To thoroughly mix the powders and to reduce the average size of h-BN particles, which have shapes of disks, they were mechanically milled in a high-energy planetary mill AGO-2S in presence of hexane. Duration of the milling stage was 10 min. The particle sizes in the powder mixtures
Fig. 1. The Z-type scanning strategy. Bold arrow indicates the resulting transverse speed of laser motion. 2
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tracks was 30 μm. Analysis of the effect of laser pulse processing on the irradiated steel surface with a thin ceramic powder layer was performed computationally using the multiscale model [13]. Based on the experimental results [12] on heat conductivity of the powder bed, computer simulations of heat transport, melting and solidification were performed for actual conditions of laboratory tests. The computational domain included two subdomains, which represent the steel substrate and porous powder bed correspondingly. These computations revealed that the high energy density of the modulated laser beam leads to fast heating (at the heat rate of about 107 K/s). The short duration of laser pulses of about 40 ns leads to overheating of the powder base. The temperatures raises up to 3500 °C which is much larger than the melting point of the boron based ceramic powders (the melting point of BN – 2973 and B4C – 2350 °C) [14]. Therefore, the high temperature gradient (of about 108 K/m) and melting/solidification velocities (of about 2 m/s) are obtained in the annealed zone [13]. The results of computer simulations have shown that simultaneously with melting of the ceramic powder a thin layer of the steel substrate with a thickness of about 10 μm is also melted. Such limited melting depth occurs as a result of the high temperature gradient in the irradiated area.
Fig. 2. Cylinder-on-cylinder tribometer (a) and a test sample (b). The inner surface of samples (1) with a coating is pressed against a disk (2) made of WC8%Co.
2.4. Friction and wear tests To test the friction behaviour, we used the cylinder-on-cylinder tribometer. The samples were loaded with a predefined normal force against the outer cylindrical surface of a disk, as it is shown in Fig. 2a. The disk with diameter of 50 mm and thickness of 10 mm was made of WC-8Co hard alloy. The contact surface of the disk was grinded to surface roughness of Ra 0.16 μm. The choice of disk material allows to study mechanical properties of antifriction coatings at loads up to the limiting characteristics of the hard-ceramic coatings. The coatings were deposited on the inner cylindrical surface of the sample made of structural steel 40 (Fe-0.4% C) as it is shown in Fig. 2b by arrows. The loaded surface of the samples has an area of 100 mm2 (10 × 10 mm). The applied normal load on the samples ranged from 100 to 650 N. The velocity of revolutions was controlled via a servomotor providing a sliding rate of 0.9 m/s. The frictional force transferred by friction was measured by a force-measuring platform. Five identical experiments have been conducted for each coating composition to measure the evolution of the COF. All tests were performed in 60 ± 3% relative humidity (RH). The RH was controlled by an industrial dehumidifier and was set for the whole room with the test equipment. The preselected tribological testing conditions such as contact pressure, sliding rate, and RH allowed to compare the obtained results with the data reported earlier in the literature [1–7]. For example, in the 60% RH atmosphere transitions to low friction behaviour occur under different testing conditions [1,2,6].
2.3. Microstructural characterization To describe the microstructure of coatings we prepared prismatic shaped samples of 10 × 10 × 4 mm. Study of microstructure, phase and chemical compositions were performed by different methods. X-ray diffraction study was performed using DRON-3 diffractometer with FeKα radiation picking intensity during 5 s at each 2θ point with a step of 0.02°. Transmission electron microscopy was performed on an EM-125 microscope. The topography of the laser irradiated surfaces was studied by scanning electron microscopy (SEM) operating at 12 kV. The SEM was coupled with an energy-dispersive X-ray (EDX) detector. The composition of surface layers was estimated with the use of Xray photoelectron spectroscopy (XPS) on a SPECS spectrometer using MgKα-radiation (1253.6 eV) with the Phoibos-150 energy analyzer working in a constant transmission energy of 15 eV mode. The measurements were carried out at a residual pressure in the analyzer chamber of 2·10−9 Torr at room temperature (293–298 K). The calibration of the spectrometer binding energy scale was performed using the position of the Au4f7/2 spectrum line (Eb = 84.0 eV). The measuring error of the binding energy is ± 0.1 eV. Obtained data were processed using CasaXPS software. The background component (baseline) of the data was subtracted by the Shirley method. The spectrum was resolved by the least-squares method into constituent components of the spectrum, and their peaks were approximated by the mixed Gauss-Lorentz function. The half-width and the asymmetry parameters of the Zr and B peaks were predetermined by measurements of the spectra of the pure ZrO2 and B4C reference samples. Identifying of the peaks by chemical states (chemical bonds) was carried out using the spectra of the Zr, ZrO2, ZrC, B2O3 standards which spectrums were obtained on the same spectrometer at identical conditions. The obtained standard spectrums were identical to the data published in the literature. Cleaning of surfaces from adsorbed contaminants and etching of thin surface layers of samples were performed by argon ions flow with the energy of 4 keV and a current density of 30 μAcm−2 at an angle of 45° to the target surface. The etching rate 0.9–1.1 nm min−1 was previously determined on standard films with known composition and thickness. In this work, we did not carry out a deep profiling of the composition of the surface layers using ion etching, since ion bombardment itself can introduce radiation-induced changes in the elemental composition, chemical state, and structure of the analyzed layers. The effect of ion etching on the chemical composition of the surface layers was studied using the above-mentioned standard samples ZrO2, B4C, B2O3. With the chosen parameters of etching, these samples exhibit stability in the chemical compound.
2.5. Adhesion tests To test the adhesion, we performed adhesion wear tests using the tribometer described in Section 2.4. The metrological determination of the friction coefficient behavior offers a simple method for qualitative evaluation of the adhesion of the ceramic-metal joints deposited by short pulse laser radiation. In these tests, the samples with B4C-based single-layer coatings were loaded with the normal force of 650 N. The thickness of the coatings was about 10 μm. The moment of deterioration of the deposited ceramic layer in a local contact area was established by a steep increase in the COF when the ceramic layer was almost completely worn. The second way to test the adhesion is the impact testing, when the sample undergoes impact fracture in the transversal direction together with the ceramic layer.
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Table 1 Surface roughness and thickness of composite ceramic coatings depend on coating composition. N
Coating composition, wt%
Surface roughness, μm Ra
1 2 3 4 3 4 5 6 7
B4 C B4C – 40% BN B4C – 40% BN – 10% ZrO2 B4C – 40% BN – 10% MgO B4C – 40% BN – 10% ZrO2−5% Li2O B4C – 40% BN – 10% MgO – 5% Li2O B4C – 40% BN – 10% MgO – 10% Li2O B4C – 40% BN – 10% MgO – 15% Li2O B4C – 40% BN – 10% MgO – 20% Li2O
Thickness of coatings, μm
Rz
1.81 1.33 0.71 0.57 0.53
30 15 12 8 4.8
50 35 25 25 19
± ± ± ± ±
10 10 6 5 3
0.61
4.5
18 ± 3
0.20
1.3
15 ± 3
0.23
1.8
11 ± 1
0.51
3.1
8 ± 2
Fig. 4. Microsection of a steel sample with the coating B4C-40%BN-10%MgO.
selected laser processing parameters result in a relatively uniform surface of the sample, which is smooth and dense. Ceramic coatings without lithium oxide in their composition show a network of microcracks over the entire surface of the samples, see Fig. 3 a,b. All microcracks are perpendicular to the surface of the samples and are spread to the whole thickness of the coatings, terminating at the ceramicsubstrate interface, as it is shown for the coating with composition B4C40%BN-10%MgO in Fig. 4. Independently from the coating composition, there are no microcracks, which propagate parallel to the surface of the samples. As shown in Fig. 4, there are no traces of this type
3. Results and discussion 3.1. Composition and microstructure of coatings Studies of microstructure and friction behavior were performed for several compositions of three-layer coatings with different content of lithium oxide. Composition and surface roughness of prepared coatings is shown in Table 1. SEM-images of the samples with ceramic deposited by short pulse laser radiation are shown in Fig. 3. It can be seen that the
Fig. 3. SEM of the ceramic coatings with composition B4C - 40% BN (a), B4C - 40% BN - 10% MgO (b), B4C - 40% BN - 10% MgO - 10%Li2O (c). 4
Surface & Coatings Technology 390 (2020) 125664
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Fig. 5. Structure of a coating with composition B4C deposited on a steel substrate. (a) Electron diffraction pattern of the coating, (b) bright-field image of the corresponding area and (c) dark-field image in reflection (102) of B4C (positive).
addition to reflections of the h-BN phase, weak reflections of MgO and Li2O are present, they partially coincide with the rings of the h-BN phase. A dark-field image in Fig. 7c shows multiple nanocrystalline hBN particles and individual MgO and Li2O particles shining in the (002)BN+MgO+Li2O reflection. Large rounded particles in dark-field images do not shine, apparently this is due to the large thickness of these particles. X-ray photoelectron spectrum of the sample with the B4C–40% BN–10%ZrO2 coating is shown in Fig. 8 in the range of binding energy from 176 to 200 eV. This range includes the spectrum of zirconium (Zr3d) and boron (B1s). The spectrum (0) was taken on the surface without etching. This spectrum consists of boron peaks in two nonequivalent states BeO (192 eV) and BeC (~188 eV) and of zirconium peaks, where zirconium is in predominantly oxidized state Zr4+ (ZrO2). BeC appears to be observed through a thin surface oxide layer. After ion etching for 1 min to depth of approximately 1 nm, the intensity of the BC component increases, and to the left, in the region of large binding energy, two B1s components are registered, one of them (192.5 eV) can be attributed to B2O3, and the second peak in the region of 191 eV is attributed to the presence of nitrogen along with oxygen in the near-field region of boron. It should be noted that the binding energies of the BeO peaks on the surface itself and after ion etching are different (192 eV and 192.5 eV, respectively). Thus, rather complex multicomponent compounds have been forming in the surface layer with the participation of all observed elements (B, C, O, N, Zr) with nonstoichiometric composition, that is variable with depth. Upon further etching for up to 3 min on depth of ~3 nm, the BC component becomes dominant, but components of boron oxides and/or oxynitrides still remain (192.5 and 190.2 eV), and the Zr3d spin doublet corresponding to ZreC bonds increases (179.8 and 182.1 eV). Appearance of the carbon-bonded zirconium and of the boron oxide components indicates the presence of the chemical reaction between zirconia and boron carbide. Based on thermodynamic assessment of oxidation and reduction reactions [16], it was previously shown that such reactions can occur at elevated temperatures. Similar behavior is found for all compounds of the ceramic coatings with metal oxides (Li2O, MgO,
cracking or peeling of the coating at the substrate-coating interface. The lithium oxide addition in the composition of the ceramic coatings effectively eliminates microcracks appearance (Fig. 3c), so it has a positive effect on reducing tensile stresses in the coatings as described in [15]. The results of TEM of ceramic coatings are shown in Figs. 5–7 for various coating compounds. The structure of the coating made from pure B4C powder is shown in Fig. 5. All reflections in the electron diffraction pattern (Fig. 5a) belong to the B4C phase. The rings in the pattern are diffuse, which is characteristic for a structure with precipitations of nanocrystals in an amorphous matrix. The amorphous structure is shown in the bright-field mode in Fig. 5b. In a dark field mode (Fig. 5c), nanoparticles of the B4C phase are shown scattered in an amorphous matrix. The most of the crystals have a size from 3 to 7 nm. Fig. 6 shows the structural features of the B4C + 40% BN coating. The coating has an amorphous and partially nanocrystalline structure. A bright-field image in Fig. 6b shows alternation of light and dark gray regions with a contrast that is characteristic for the amorphous state. The microdiffraction pattern in Fig. 6a contains diffuse rings related to the h-BN phase, shown by arrows. A dark-field image in (002)BN reflections in Fig. 6c depicts individual nanocrystals of the h-BN phase. The size of the nanocrystals does not exceed 10 nm. Initial powder for coatings deposition contained h-BN particles with an average size of 1 μm, so such structure could be formed only by melting of ceramic powder and subsequent high-speed crystallization of the ceramic melt on a steel substrate. Boron nitride has a higher melting point in comparison to boron carbide, so it crystallized from an undercooled melt in forms of spherical nanocrystals of the h-BN phase. Fig. 7 shows structure of the B4C + 40% BN + 10% MgO + 10% Li2O coating. A bright-field image in Fig. 7b shows thin light areas with a contrast characteristic for an amorphous state. Areas with sharp alternation of dark gray and light contrast are observed, which indicates presence of nanocrystals. In addition, the image contains individual dark particles of a rounded shape. The microdiffraction pattern in Fig. 7a indicates rings of h-BN phase, which are shown by arrows. In
Fig. 6. Structure of the B4C + 40% BN coating. (a) An electron diffraction pattern, (b) bright-field image of microstructure, (c) dark field image in (002)BN reflection. 5
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Fig. 7. Structure of the B4C + 40% BN + 10% MgO + 10%Li2O coating. (a) Electron diffraction pattern, (b) bright-field image of microstructure, (c) dark field image in (002)BN reflection.
ZrO2). Regardless of a metal valency in oxide, boron‑oxygen bonds are registered by XPS through the analyzed thickness of the coatings. Occurrence of them is a result of the chemical interaction between the metal oxide and boron carbide at elevated temperatures during shortpulse laser deposition. The results of boron content analysis with XPS for different coatings are shown in Table 2. we think that results are considerably accurate even after etching by argon ions flow taking into account the high affinity of boron to oxygen. A short (1–3 min) etching by low-energy argon ions does not introduce significant changes in the chemical bonds of BeO. The same applies to zirconium and magnesium oxides.
dry sliding tests was stable until almost complete deterioration of the coating. This stability of the friction coefficient over a test period means that no delamination process is occurred. The SEM image of a sample with the B4C coating after the adhesion wear test is shown in Fig. 9a. Worn surface remains parts of the coating, see a dark area in Fig. 9. The EDX measurements show that the chemical composition of the worn surface depends on its spatial position. While the Area 2 (black) in Fig. 9a is almost completely composed of B4C (73.7 wt% of boron and approximately 23% of carbon), the Area 1 contains Fe-85.3%, B – 7.2% and C – 5%, and the Area 3 contains Fe-52.5%, B – 33.4% and C – 13%. This interface Area 3 was formed during the laser deposition as a result of the high-temperature chemical reaction between the steel substrate and the boron carbide. According to the EDX data, we can conclude that the inner zone of area 3 in Fig. 9a mostly consists of the FeB phase, while the outer fields belong to the Fe2B phase. The SEM view of the fractured surface with the single-layer B4C coating is shown in Fig. 9b. It is explicitly seen that under impact no delamination occurs at the steel substrate – ceramic interface. To explain the high level of adhesion between the ceramic layer and steel substrate, we performed X-ray study of the sample after laser short-pulse welding. The diffraction pattern of the single-layer B4C coating shows the presence of B4C lines, α-Fe (ferrite and martensite), iron borides (FeB, Fe2B), and carbide Fe3C. Active chemical interaction at the interface between the metal matrix and the ceramic layer helps obtaining the desired properties of ceramic composite materials. A quantitative criterion of the wetting of the solid phase by melt under the equilibrium conditions is the contact angle θ, determined by the Young equation [17]. As a rule, liquid metals practically do not wet ceramics [18] due to the fact that the surface energy at the liquid – solid phase boundary is relatively small. The criterion for wetting of a solid surface by a liquid (with a contact angle of θ < 90°) is the work of adhesion W, defined as the work required to separate a liquid-solid pair, which must exceed the surface energy at the liquid-vapor boundary γlv. The work of adhesion is determined by physical and chemical work during chemical reactions proceeding at the melt-solid interface. The typical values of the contribution of physical work to the adhesion work do not exceed 600 mJ/m2, whereas γlv for liquid metals and alloys with a high melting point Tm is significantly higher [19]. This means that only chemical reactions can increase W and provide wettability in the process of thermally activated reactive wetting [20]. FeB and Fe2B phases registered in the samples by XRD study after laser processing indicate intense chemical reactions and admit the reactive wetting. In this case, the connection between the metal matrix and the ceramic coating are significantly strengthened.
3.2. Adhesion of coatings
3.3. Schematic representation of microstructure
Fig. 8. X-ray photoelectron spectrum in the region of boron (B1s) for a sample with the B4C + 40% BN + ZrO2 + 10%Li2O coating. 0 – XPS of free surface without etching, 1 – etching for 1 min (depth ~1 nm), 3 – etching for 3 min (depth ~ 3 nm).
The results of the adhesion tests show that the measured COF during
The samples with ceramic coatings have a common pattern of 6
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Table 2 Layer-by-layer XPS analysis of atomic fractions of boron bounded to oxygen, calculated as a fraction of the total amount of boron in the coatings. Composition of the coatings
Duration of etching (min)
Total amount of boron (at. %)
Including amount of oxidized boron (at. %)
B4C + 40%BN
1 3 1 3 1 3 1 3
5.4 6.4 11.7 16.6 13.3 19.0 13.3 16.6
0.3 0.0 3.0 0.7 5.1 4.6 8.2 6.6
B4C + 40%BN + 10%ZrO2 B4C + 40%BN + 10%MgO B4C + 40%BN + 10%ZrO2 + 10% Li2O
Fig. 9. Worn surface after adhesion wear tests (a) and fractured surface (b) after an impact test that was performed in the transverse plane in relation to the B4C ceramic layer.
microstructure for all studied compositions B4C-BN, B4C-BN-ZrO2-Li2O, B4C-BN-MgO-Li2O. All ceramic coatings consist of boron carbide, boron nitride, metal oxides, and boron oxide, that is formed by high-temperature interaction with metal oxides during short-pulse laser deposition of the coating. Boron carbide included in the amorphous matrix of the ceramic coatings, in which nanoscale crystallites of h-BN and MeO are precipitated. The crystallites of h-BN have a spherical shape with a size of spheres < 10 nm. At the boundary between the coating and the steel substrate, the film contains borides FeB, Fe2B, and carbide Fe3C. The appearance of boride and nitride phases explains good adhesion of the ceramic coatings to the steel substrate, where the reactive wetting between the molten ceramics and the metal substrate plays an important role. Suitable conditions for the appearance of boride and nitride phases are provided by a high temperature (about 3500 °C) in the area of short-pulse laser treatment, which is much higher than the melting temperature of boron carbide and boron nitride. Because of the high temperature gradient, the depth of the molten zone of the steel substrate under a ceramic coating is about 10 μm, which reduces the thickness of the interface layer. Fig. 10. Variation of the coefficient of friction (COF) with the sliding distance for (1) B4C, (2) B4C-40%BN, (3) B4C-40%BN-10%MgO, and (4) B4C-40%BN10%MgO-5%Li2O ceramic coatings in 60% relative humidity. After each 700 m of the sliding distance the load increases sharply by 100 N.
3.4. Tribological performance Fig. 10 shows the COF behaviour of WC-8%Co counter-disk during sliding against ceramic coatings with different compositions. The starting load on samples with all coatings was 100 N. After each 700 m of the sliding distance, the load was increased by 100 N without stopping the revolution of the counter-disk. Duration of each sliding stage with a constant load was 13 min, giving 78 min for the whole test duration of each sample, except of the sample with the B4C coating composition (Fig. 10, curve 1). This coating does not withstand lengthy tests with load of > 500 N and subjected to fast deterioration in these conditions. According to the wear tests, the COF of coatings showed a runningin characteristics, the feature of which depends on the composition of the coating and the load, see Fig. 10. As illustrated by line 1 in Fig. 10, a
pure boron carbide coating requires the longest time to achieve the steady state of the COF, especially at high loads. At the loads of 400 and 500 N, the COF does not reach the steady state for the whole test period. In contrast, the composite B4C-BN and B4C-BN-MeO coatings reach the steady state COF in a considerably shorter period of time. This is especially visible for coatings containing metal oxides. Addition of the h-BN phase in microstructure of coating reduces the COF considerably, see line 2 in Fig. 10. Hexagonal boron nitride has the layered-crystal structure, that resembles those of MoS2 and graphite. Introducing h-BN nanoparticles in the amorphous matrix of ceramic has 7
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a direct consequence in formation of a slippery film with low COF. It is interesting to note that for this coating after a load of 300 N, each step of increasing the load is accompanied by a short-term increase in the COF. It means that under action of the increased load the slippery tribofilm begins to wear more intensively. This results in increase in the COF, thereby accelerating heat generation. Heating of the contact area leads to intensification of chemical transformation of the surface. Temperature intensification of the surface chemical reactions with oxygen and water vapor in the atmosphere releases more boron oxide and boric acid per time. Thus, after an increase of the load towards the sample with the ceramic coating it takes some time to reach a new steady-state value of the COF. The underlying mechanism for the stabilization of the COF has been discussed widely in the literature [1]. The required time and sliding distance to reach the low steady-state friction level increases with increasing load and depends on the sliding velocity and RH [2]. Thereby, transition to a new steady-state friction takes a relaxation time, that is necessary for consistent formation of boron oxide B2O3 and boric acid H3BO3 on B4C surface under new sliding conditions [6]. The relaxation time of the transition to low friction is considerably shorter for coatings with different metal oxides, because their microstructure already has the boron‑oxygen bonds necessary for formation of the low friction tribofilm, see lines 3 and 4 in Fig. 10. Because these bonds are the parts of the composite microstructure, they are instantly ready to form a new low friction tribofilm on the surface as the wear process progresses. Addition of Li2O in the composition of a ceramic coating sharply decreases the COF in the whole range of loads, see line 4 in Fig. 10. Thus, the addition of Li2O in the composition of ceramics facilitates the ultralow friction behavior by intensification of boron‑oxygen bonds formation during the deposition process. The smallest value of the COF registered at a load of 200 N for coating with composition B4C-40%BN-10%MgO-5%Li2O was 0.03 in dry sliding conditions, see line 4 in Fig. 10. As it was mentioned, boron oxide plays an important role in formation of a low friction tribofilm. Usually, this tribofilm has been formed in situ as a result of reactions facilitated by heating during sliding friction of a boron carbide ceramic against a counterpart. The detected reactions at the sliding interface involve oxidation of B4C into B2O3, which is transformed into H3BO3 by the reaction with water [1–6]. Material loss is caused by delamination of the hydrated film [1]. An analysis with XPS of samples before the wear test indicates the presence of the boron‑oxygen bonds in the microstructure of ceramic coatings deposited with the metal oxides, see Table 2. Therefore, the composition of these ceramics has a strong effect on the friction. The coatings with metal oxides initially contain the essential elements, that contribute to low friction behaviour. As for the ultralow friction mechanism of ceramics with addition of the Li2O, we propose the following explanation. It is well known that lithium oxide does not interact with hydrogen, oxygen, carbon and carbon monoxide even when heated, but intensively reacts with water vapor to form lithium hydroxide [14]. At high temperatures, Li2O reacts with most metals [21]. With a number of metal oxides gives double, and triple oxides. The XPS data in Table 2 indicate an intense interaction of lithium oxide and boron carbide during laser short-pulse deposition of coatings, thus releasing considerable amount of boron‑oxygen bonds. The remaining part of the unreacted lithium oxide emerging on the surface could contribute to ultralow friction as well. When releasing on the surface, it instantly reacts with water vapors in the atmosphere with the formation of hydroxide by reaction Li2O + H2O → 2LiOH. An important property of lithium hydroxide is its good reactivity with fatty acids, for example, with stearic acid. The product of this reaction is the basis for the formation of lithium greases, known for their good lubricity [22]. Thus, unreacted during deposition process lithium oxide is an important component for ensuring self-lubrication of friction units in condition of insufficient lubrication. Based on the chemical analyses presented in Table 2, we believe that the ultralow friction coefficient of
Fig. 11. Minimum registered values of the COF during wear tests, depending on the Li2O content in the composite ceramic coatings.
ceramic coatings with addition of Li2O is directly related to the formation of B2O3 during deposition process through whole thickness of the coating. Boron oxide itself has been reported to be able to provide very low friction because at some point of generation of heat the B2O3 based films become viscous [1]. This explains the formation of very low COF of the coating with addition of Li2O even at low load of 100 N, see line 4 in Fig. 10. When the load increases the generation of heat become sufficient to activate the second-step reaction of the B2O3 with water vapors, and B2O3 is transformed into the slippery H3BO3 giving the ultralow COF of 0.03 to the coatings. Coatings without metal oxides require both reactions to form low friction films. Firstly, B4C reacts with oxygen in air to form B2O3, and, secondly, boron oxide reacts with water. Both reactions are thermodynamically feasible even at room temperature [23] but if the coatings initially contain boron oxide, the heat flow is directed only to the formation of H3BO3 films with ultralow friction. The disadvantage of lithium oxide is its low strength, and it cannot be used as a constructional material. The coatings with the Li2O content of > 15% have high wear rate and their COF is increased, see Fig. 11. 4. Conclusion The microstructure of composite ceramic coatings based on B4C and h-BN with addition of different metal oxides consists of an amorphous layer with precipitation of nanocrystals of h-BN phase. Low friction behaviour registered in composite ceramic coatings with addition of hexagonal boron nitride and different metal oxides. Metal oxides in the composition of the coatings significantly reduce the coefficient of friction through the formation of boron‑oxygen bonds during the shortpulse laser deposition process. Among other metal oxides, addition of the lithium oxide into the composition provides the lowest coefficient of friction, which is stable for a long period during gradual deterioration of the ceramic coating in dry sliding conditions. Li2O in the composition of ceramics facilitates the ultralow friction behaviour by intensification of boron‑oxygen bonds formation during the deposition process. The boron‑oxygen bonds are the essential part of the coating microstructure, which are necessary to form ultralow friction behaviour. Adhesion of the ceramic coatings with steel substrate is facilitated by the reactive wetting at the steel-ceramic interface with formation of FeB and Fe2B phases. CRediT authorship contribution statement Evgeny V. Kharanzhevskiy:Conceptualization, Methodology, 8
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Investigation, Resources, Writing - original draft, Project administration.Alexey G. Ipatov:Investigation, Resources.Mikhail D. Krivilyov:Writing review & editing.Alexey V. Makarov:Supervision.Faat Z. Gil'mutdinov:Investigation, Resources.Elena G. Volkova:Investigation, Resources.
[7] K. Umeda, Y. Enomoto, A. Mitsui, K. Mannami, Friction and wear of boride ceramics in air and water, Wear 169 (1993) 63–68. [8] P.J. Meschter, E.J. Opila, N.S. Jacobson, Water vapor–mediated volatilization of high-temperature materials, Annu. Rev. Mater. Res. 43 (2013) 559–588. [9] J. Yu, Z. Zheng, H.C. Ong, K.Y. Wong, S. Matsumoto, W.M. Lau, Thermal stability of cubic boron nitride films deposited by chemical vapor deposition, J. Phys. Chem. B 110 (2006) 21073–21076. [10] K. Jagannadham, T.R. Watkins, M.J. Lance, L. Riester, R.L. Lemaster, Laser physical vapor deposition of boron carbide films to enhance cutting tool performance, Surf. Coat. Technol. 203 (2009) 3151–3156. [11] J.P. Girardeau-Montaut, M. Afif, C. Girardeau-Montaut, et al., Aluminium electronphonon relaxation-time measurement from subpicosecond nonlinear single-photon photoelectric emission at 248 nm, Appl. Phys. A Mater. Sci. Process. 62 (1996) 3–6. [12] S.N. Kostenkov, E.V. Kharanzhevskii, M.D. Krivilev, Determination of characteristics of laser radiation interaction with nanocomposite powder materials, Phys. Met. Metallogr. 113 (2012) 93–97. [13] M.D. Krivilyov, E.V. Kharanzhevskii, V.G. Lebedev, D.A. Danilov, E.V. Danilova, P.K. Galenko, Synthesis of composite coatings using rapid laser sintering of metallic powder mixtures, Phys. Met. Metallogr. 114 (2013) 799–820. [14] D.R. Lide (Ed.), CRC Handbook Chemistry and Physics, CRC Press, 2008-2009. [15] E.V. Kharanzhevsky, A.G. Ipatov, T.A. Pisareva, F.Z. Gilmutdinov, Graphite saturation of the steel surface during laser processing with short pulses, Mater. Sci. 11 (2013) 38–43 (in Russian). [16] M. Krivilyov, E. Kharanzhevskiy, S. Reshetnikov, L.J. Beyers, Thermodynamic assessment of chrome-spinel formation in laser-sintered coatings with Cr2O3 particles, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 47B (2016) 1573–1582. [17] P.G. De Gennes, Wetting: statics and dynamics, Rev. Mod. Phys. 57 (1985) 827–863. [18] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Pergamon Press, Amsterdam, 1999. [19] Y.N. Zhou, Microjoining and Nanojoining, Woodhead Publishing, 2008. [20] B.J. Keene, Review of data for the surface tension of pure metals, Int. Mater. Rev. 38 (1993) 157–192. [21] G.F. Kessinger, A.R. Jurgensen, D.M. Missimer, J.S. Morrell, The high-temperature chemical reactivity of Li2O, Nucl. Technol. 171 (1) (2010) 108–122. [22] Tharwat F. Tadros, An Introduction to Surfactants, De Gruyter, 2014. [23] K. Sonber, P.K. Limaye, T.S.R.Ch. Murthy, et al., Tribological properties of boron carbide in sliding against WC ball, Int. J. Refract. Met. Hard Mater. 51 (2015) 110–117.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study is financially supported by Russian Science Foundation, project 19-79-20012. References [1] P. Larsson, N. Axen, S. Hogmark, Tribofilm formation on boron carbide in sliding wear, Wear 236 (1999) 73–80. [2] S. Bhowmick, G. Sun, A.T. Alpas, Low friction behaviour of boron carbide coatings (B4C) sliding against Ti –6Al –4V, Surf. Coat. Technol. 308 (2016) 316–327. [3] H.-S. Ahn, P.D. Cuong, K.-H. Shin, Ki-Seung Lee, Tribological behavior of sputtered boron carbide coatings and the influence of processing gas, Wear 259 (2005) 807–813. [4] Yu.G. Gogotsi, A.M. Koval’chenko, I.A. Kossko, Tribochemical interactions of boron carbides against steel, Wear 154 (1992) 133–140. [5] Y. Tamura, H. Zhao, C. Wang, A. Morina, A. Neville, Interaction of DLC and B4C coatings with fully formulated oils in boundary lubrication conditions, Tribol. Int. 93 (2016) 666–680. [6] A. Erdemir, C. Bindal, G.R. Fenske, Formation of ultralow friction surface films on boron carbide, Appl. Phys. Lett. 68 (1996) 1637–1639.
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Ultralow friction behaviour of B4C-BN-MeO composite ceramic coatings deposited on steel
T
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Evgeny V. Kharanzhevskiya, , Alexey G. Ipatovb, Mikhail D. Krivilyova, Alexey V. Makarovc,d,e, Faat Z. Gil'mutdinovf, Elena G. Volkovac a
Udmurt State University, Universitetskaya st., 1, Izhevsk, Russia Izhevsk State Agricultural Academy, Studencheskaya st., 11, Izhevsk, Russia c M.N. Miheev Institute of Metal Physics of Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya St., 18, Yekaterinburg, Russia d Institute of Engineering Science of Ural Branch of the Russian Academy of Sciences, Komsomolskaya St., 34, Yekaterinburg, Russia e Ural Federal University, Mira St., 19, Yekaterinburg, Russia f Udmurt Federal Research Center of Ural Branch of the Russian Academy of Sciences, ul. T. Baramzinoy, 34, Izhevsk 426067, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Boron carbide Coatings Sliding wear Tribofilms Adhesion
Boron based ceramic coatings were deposited on steel substrates by high-energy short pulse laser melting. Under conditions of the laser deposition, coatings consisted of boron carbide and boron nitride form a layer that acts as solid lubricant leading to a low coefficient of friction (COF). The COF could be drastically decreased by proper selection of chemical composition of boron based ceramic coatings. Ultimately low COF of 0.03 was registered during dry sliding tests of coatings with addition of different metal oxides (MgO, ZrO2, Li2O). Low COF of composite ceramics coatings was attributed to the formation of the BeO bond observed by the X-ray photoelectron spectroscopy technique. This bond as the essential component of the internal structure of coatings explains ultralow friction behaviour and remains low even during gradual deterioration of the coating. High quality adhesion of ceramics to steels was explained by reactive phase formation at the steel-ceramic interface, which was confirmed by both transmission electron microscopy and X-ray diffraction.
1. Introduction The search for new materials with an improved combination of mechanical and protective properties has become increasingly important. The largest number of parts in mechanical engineering is made of various steels and alloys containing iron. Increasing the specific load on the elements of engines or mechanisms requires invention of effective technologies of surface modification to combine the properties inherent in ceramic materials and metal alloys. Formation of ceramic coatings on steel increases hardness, abrasion resistance, heat resistance and reduces the sliding coefficient of friction (COF) [1,2]. Boron carbide (B4C) has exceptional mechanical properties [1] and tribological characteristics [2]. It has low density (2.51 g sm−3), high melting point (2350 °C), high elastic modulus (445 GPa), and hardness (37 GPa) that concede only to cubic boron nitride (c-BN) and diamond. B4C is one of the promising materials as a thin hard coating to protect surfaces from wear and improve their tribological performance [3–5]. It has been shown that frictional properties of B4C depend on the atmospheric pressure, the temperature of the interface, oxidizing
⁎
environment and the humidity of the surrounding air [1,6,7]. A linear relationship between the natural logarithm of the wear rate and the COF was found [2]. Numerous wear test results indicated that the COF and wear rate of B4C decreased with increase of humidity in the test atmosphere [1,6,7]. Low friction tribological films may be formed insitu during tribological tests especially at high sliding velocities in a high-humidity atmosphere [1]. In certain cases, after some period of friction a sharp transition between high and low friction occurs for B4C in a sliding contact [1]. The COF usually drops from values of about 0.4–1.0 to values of 0.1–0.2 during that transition. This drop is referred to formation of a tribological film consisted of boron oxide, boric acid [1] and graphitizing layer [2]. Erdemir et al. [6] described the formation of a thin surface film on B4C with ultralow friction behaviour by special treatment. The treatment consists of an oxidation stage in a furnace at temperature above 600 °C where boron oxide layer forms. This oxidation is followed by cooling in moisture atmosphere that leads to formation of boric acid film. It has been shown that this special treatment can lead to achievement of the ultralow COF on B4C surface as a direct
Corresponding author. E-mail address: [email protected] (E.V. Kharanzhevskiy).
https://doi.org/10.1016/j.surfcoat.2020.125664 Received 29 January 2020; Received in revised form 10 March 2020; Accepted 20 March 2020 Available online 20 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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after milling were from 0.2 to 6 μm. The obtained powder-hexane suspension was spread on the surface of the steel substrate as a thin layer, normally 60 ± 15 μm in thickness by spraying. The thickness of the powder bed was controlled via time of sputtering and was measured by the mentioned above 3D optical profilometer as a difference between the heights of covered by the powder and uncovered parts of the substrate. After drying in air until complete heptane evaporation, samples were placed into the process chamber. At the end of the laser processing, samples were evacuated from the chamber, washed in alcohol and dried in air. Multilayer coatings were produced by repeating of the whole process described above. In this case the process is similar to SLM except for the chosen type of laser.
consequence of consistent formation of boron oxide B2O3 and boric acid H3BO3 on B4C surface. Boric acid has a layered triclinic crystal structure, which resembles those of MoS2, graphite, and hexagonal boron nitride (h-BN). The atomic layers are parallel to the basal plane and consist of boron, oxygen, and hydrogen atoms. These atoms are closely packed and strongly bonded to each other by covalent, ionic, and hydrogen bonds, whereas the atomic layers are widely spaced and held together by weak van der Waals force. When this layered structure sliding against a counterbody, it shears easily, thus providing ultralow friction [6]. For practical reasons, heating parts with boron-based coatings might be unacceptable at temperatures above 600 °C in oxidizing and moisture atmosphere. This especially refers to steel machine parts due to possibility of structural defects, for example, tempering embrittlement, carbide mesh extraction, and high temperature oxidation in water-vapor media [8]. For these reasons, search for the proper composition of boron-based ceramic films is of considerable interest. The most promising are composite ceramics coatings, which initially contain boron oxide. They can easily form compounds with a layered structure, such as boric acid, thus providing instant low COF without any furnace pre-treatment. If the structure of B4C-based ceramics contains boron oxide as an essential part of its internal microstructure, it could facilitate formation of low friction tribological film during gradual wear deterioration for the whole life cycle of machine parts with coatings. Boron oxide can be formed during a deposition process by high-temperature reactions with some metal oxides, therefore the main topic of this work is devoted to study of tribological properties of ceramic coatings, which composition contains metal oxides. Thin multilayer coatings based on boron carbide and nitride can be deposited by chemical vapor deposition or magnetron sputtering techniques using appropriate targets [3]. The main limitation of all available methods is low adhesion of the coating to relatively soft substrates such as steels. Therefore, such ceramic coatings are applied onto substrates made of hard alloys and are mainly used for cutting tools [9,10]. Fundamental difficulties in formation of cermet compounds include the following factors: ionic or covalent bonds of electrons of surface atoms in ceramics, differences in thermal expansion coefficient, low thermal conductivity of ceramics, difference in melting points, formation of intermetallic phases with high fragility, lack of chemical interaction at the interface between metal and ceramics. Therefore, this paper describes microstructure, adhesion and tribological properties of composite ceramic coatings deposited on steel substrates using high-energy short laser pulses. Initial compounds for coatings deposition are boron carbide, hexagonal boron nitride, and metal oxides Li2O, MgO or ZrO2.
2.2. Laser irradiation processing The experimental facility consists of the ytterbium fiber laser with a maximum average power of 50 W and a wavelength of 1.065 μm. For laser processing a chamber with controlled atmosphere was used. The chamber was initially evacuated by a backing pump and then filled with high purity argon to pressure 1.05 bar. In this study the laser chosen for laser processing has pulse duration of about 40 ns which is only three to four times of magnitude longer than the relaxation time for energy transfer from gas of valence electrons which absorbs laser energy to lattice of metal parts [11]. Thus, action of laser energy on ceramic powder can be described mainly by a thermal conduction mechanism but the pulse time is short enough to ensure ultrafast heating and high temperature gradients in the treated zone. The pulse frequency was set to 20 kHz, pulse energy was 1 mJ, so the average power of the laser beam was 20 W. At this laser irradiation parameters, instantaneous power of the laser beam was of 25 kW. The laser beam was focused into a spot with a diameter of 30 μm, so the instantaneous power density of laser radiation was about of 3.5·1013 W/m2. The parameters of laser irradiation were selected using the calculation procedure reported in [12]. The strategy of laser beam movement is shown in Fig. 1. The laser beam was moved over the samples surface with the use of a system that consists of two mirrors as it is shown in Fig. 1. Computer controlled tilting of the mirrors allows to perform beam moving along any complex track. Very low weight of the mirrors makes it possible to stop the movement at the edges of x-axis almost immediately and start the movement in the opposite direction so the velocity is almost constant both in the x- and y-directions. The scanning speed is 120 mm/s giving about 5 laser pulses at one point. The distance between the nearest laser
2. Experimental procedure 2.1. Materials preparation All substrates for coating deposition were cut from the same batch of grade 40 carbon steel (0.4 wt% C in the Russian state standard GOST 1050–88). The steel was in the annealed state, and the samples were machined and ground to achieve the surface roughness Ra of 0.24 μm. The roughness of the samples and coatings were measured by 3D optical profilometer Wyko NT 1100 (Veeco). Conditions of laser processing are characterized by a high temperature gradient, which impose the following requirements for powder preparation. The boron based ceramic powders should be spread on the surface of the substrate as a thin layer, not thicker than 100 μm, and the powder layer should be smooth and dense. To fulfill these requirements, we used fine powders of B4C, h-BN, Li2O, MgO, and ZrO2 – with an average size about 1 μm. To thoroughly mix the powders and to reduce the average size of h-BN particles, which have shapes of disks, they were mechanically milled in a high-energy planetary mill AGO-2S in presence of hexane. Duration of the milling stage was 10 min. The particle sizes in the powder mixtures
Fig. 1. The Z-type scanning strategy. Bold arrow indicates the resulting transverse speed of laser motion. 2
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tracks was 30 μm. Analysis of the effect of laser pulse processing on the irradiated steel surface with a thin ceramic powder layer was performed computationally using the multiscale model [13]. Based on the experimental results [12] on heat conductivity of the powder bed, computer simulations of heat transport, melting and solidification were performed for actual conditions of laboratory tests. The computational domain included two subdomains, which represent the steel substrate and porous powder bed correspondingly. These computations revealed that the high energy density of the modulated laser beam leads to fast heating (at the heat rate of about 107 K/s). The short duration of laser pulses of about 40 ns leads to overheating of the powder base. The temperatures raises up to 3500 °C which is much larger than the melting point of the boron based ceramic powders (the melting point of BN – 2973 and B4C – 2350 °C) [14]. Therefore, the high temperature gradient (of about 108 K/m) and melting/solidification velocities (of about 2 m/s) are obtained in the annealed zone [13]. The results of computer simulations have shown that simultaneously with melting of the ceramic powder a thin layer of the steel substrate with a thickness of about 10 μm is also melted. Such limited melting depth occurs as a result of the high temperature gradient in the irradiated area.
Fig. 2. Cylinder-on-cylinder tribometer (a) and a test sample (b). The inner surface of samples (1) with a coating is pressed against a disk (2) made of WC8%Co.
2.4. Friction and wear tests To test the friction behaviour, we used the cylinder-on-cylinder tribometer. The samples were loaded with a predefined normal force against the outer cylindrical surface of a disk, as it is shown in Fig. 2a. The disk with diameter of 50 mm and thickness of 10 mm was made of WC-8Co hard alloy. The contact surface of the disk was grinded to surface roughness of Ra 0.16 μm. The choice of disk material allows to study mechanical properties of antifriction coatings at loads up to the limiting characteristics of the hard-ceramic coatings. The coatings were deposited on the inner cylindrical surface of the sample made of structural steel 40 (Fe-0.4% C) as it is shown in Fig. 2b by arrows. The loaded surface of the samples has an area of 100 mm2 (10 × 10 mm). The applied normal load on the samples ranged from 100 to 650 N. The velocity of revolutions was controlled via a servomotor providing a sliding rate of 0.9 m/s. The frictional force transferred by friction was measured by a force-measuring platform. Five identical experiments have been conducted for each coating composition to measure the evolution of the COF. All tests were performed in 60 ± 3% relative humidity (RH). The RH was controlled by an industrial dehumidifier and was set for the whole room with the test equipment. The preselected tribological testing conditions such as contact pressure, sliding rate, and RH allowed to compare the obtained results with the data reported earlier in the literature [1–7]. For example, in the 60% RH atmosphere transitions to low friction behaviour occur under different testing conditions [1,2,6].
2.3. Microstructural characterization To describe the microstructure of coatings we prepared prismatic shaped samples of 10 × 10 × 4 mm. Study of microstructure, phase and chemical compositions were performed by different methods. X-ray diffraction study was performed using DRON-3 diffractometer with FeKα radiation picking intensity during 5 s at each 2θ point with a step of 0.02°. Transmission electron microscopy was performed on an EM-125 microscope. The topography of the laser irradiated surfaces was studied by scanning electron microscopy (SEM) operating at 12 kV. The SEM was coupled with an energy-dispersive X-ray (EDX) detector. The composition of surface layers was estimated with the use of Xray photoelectron spectroscopy (XPS) on a SPECS spectrometer using MgKα-radiation (1253.6 eV) with the Phoibos-150 energy analyzer working in a constant transmission energy of 15 eV mode. The measurements were carried out at a residual pressure in the analyzer chamber of 2·10−9 Torr at room temperature (293–298 K). The calibration of the spectrometer binding energy scale was performed using the position of the Au4f7/2 spectrum line (Eb = 84.0 eV). The measuring error of the binding energy is ± 0.1 eV. Obtained data were processed using CasaXPS software. The background component (baseline) of the data was subtracted by the Shirley method. The spectrum was resolved by the least-squares method into constituent components of the spectrum, and their peaks were approximated by the mixed Gauss-Lorentz function. The half-width and the asymmetry parameters of the Zr and B peaks were predetermined by measurements of the spectra of the pure ZrO2 and B4C reference samples. Identifying of the peaks by chemical states (chemical bonds) was carried out using the spectra of the Zr, ZrO2, ZrC, B2O3 standards which spectrums were obtained on the same spectrometer at identical conditions. The obtained standard spectrums were identical to the data published in the literature. Cleaning of surfaces from adsorbed contaminants and etching of thin surface layers of samples were performed by argon ions flow with the energy of 4 keV and a current density of 30 μAcm−2 at an angle of 45° to the target surface. The etching rate 0.9–1.1 nm min−1 was previously determined on standard films with known composition and thickness. In this work, we did not carry out a deep profiling of the composition of the surface layers using ion etching, since ion bombardment itself can introduce radiation-induced changes in the elemental composition, chemical state, and structure of the analyzed layers. The effect of ion etching on the chemical composition of the surface layers was studied using the above-mentioned standard samples ZrO2, B4C, B2O3. With the chosen parameters of etching, these samples exhibit stability in the chemical compound.
2.5. Adhesion tests To test the adhesion, we performed adhesion wear tests using the tribometer described in Section 2.4. The metrological determination of the friction coefficient behavior offers a simple method for qualitative evaluation of the adhesion of the ceramic-metal joints deposited by short pulse laser radiation. In these tests, the samples with B4C-based single-layer coatings were loaded with the normal force of 650 N. The thickness of the coatings was about 10 μm. The moment of deterioration of the deposited ceramic layer in a local contact area was established by a steep increase in the COF when the ceramic layer was almost completely worn. The second way to test the adhesion is the impact testing, when the sample undergoes impact fracture in the transversal direction together with the ceramic layer.
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Table 1 Surface roughness and thickness of composite ceramic coatings depend on coating composition. N
Coating composition, wt%
Surface roughness, μm Ra
1 2 3 4 3 4 5 6 7
B4 C B4C – 40% BN B4C – 40% BN – 10% ZrO2 B4C – 40% BN – 10% MgO B4C – 40% BN – 10% ZrO2−5% Li2O B4C – 40% BN – 10% MgO – 5% Li2O B4C – 40% BN – 10% MgO – 10% Li2O B4C – 40% BN – 10% MgO – 15% Li2O B4C – 40% BN – 10% MgO – 20% Li2O
Thickness of coatings, μm
Rz
1.81 1.33 0.71 0.57 0.53
30 15 12 8 4.8
50 35 25 25 19
± ± ± ± ±
10 10 6 5 3
0.61
4.5
18 ± 3
0.20
1.3
15 ± 3
0.23
1.8
11 ± 1
0.51
3.1
8 ± 2
Fig. 4. Microsection of a steel sample with the coating B4C-40%BN-10%MgO.
selected laser processing parameters result in a relatively uniform surface of the sample, which is smooth and dense. Ceramic coatings without lithium oxide in their composition show a network of microcracks over the entire surface of the samples, see Fig. 3 a,b. All microcracks are perpendicular to the surface of the samples and are spread to the whole thickness of the coatings, terminating at the ceramicsubstrate interface, as it is shown for the coating with composition B4C40%BN-10%MgO in Fig. 4. Independently from the coating composition, there are no microcracks, which propagate parallel to the surface of the samples. As shown in Fig. 4, there are no traces of this type
3. Results and discussion 3.1. Composition and microstructure of coatings Studies of microstructure and friction behavior were performed for several compositions of three-layer coatings with different content of lithium oxide. Composition and surface roughness of prepared coatings is shown in Table 1. SEM-images of the samples with ceramic deposited by short pulse laser radiation are shown in Fig. 3. It can be seen that the
Fig. 3. SEM of the ceramic coatings with composition B4C - 40% BN (a), B4C - 40% BN - 10% MgO (b), B4C - 40% BN - 10% MgO - 10%Li2O (c). 4
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Fig. 5. Structure of a coating with composition B4C deposited on a steel substrate. (a) Electron diffraction pattern of the coating, (b) bright-field image of the corresponding area and (c) dark-field image in reflection (102) of B4C (positive).
addition to reflections of the h-BN phase, weak reflections of MgO and Li2O are present, they partially coincide with the rings of the h-BN phase. A dark-field image in Fig. 7c shows multiple nanocrystalline hBN particles and individual MgO and Li2O particles shining in the (002)BN+MgO+Li2O reflection. Large rounded particles in dark-field images do not shine, apparently this is due to the large thickness of these particles. X-ray photoelectron spectrum of the sample with the B4C–40% BN–10%ZrO2 coating is shown in Fig. 8 in the range of binding energy from 176 to 200 eV. This range includes the spectrum of zirconium (Zr3d) and boron (B1s). The spectrum (0) was taken on the surface without etching. This spectrum consists of boron peaks in two nonequivalent states BeO (192 eV) and BeC (~188 eV) and of zirconium peaks, where zirconium is in predominantly oxidized state Zr4+ (ZrO2). BeC appears to be observed through a thin surface oxide layer. After ion etching for 1 min to depth of approximately 1 nm, the intensity of the BC component increases, and to the left, in the region of large binding energy, two B1s components are registered, one of them (192.5 eV) can be attributed to B2O3, and the second peak in the region of 191 eV is attributed to the presence of nitrogen along with oxygen in the near-field region of boron. It should be noted that the binding energies of the BeO peaks on the surface itself and after ion etching are different (192 eV and 192.5 eV, respectively). Thus, rather complex multicomponent compounds have been forming in the surface layer with the participation of all observed elements (B, C, O, N, Zr) with nonstoichiometric composition, that is variable with depth. Upon further etching for up to 3 min on depth of ~3 nm, the BC component becomes dominant, but components of boron oxides and/or oxynitrides still remain (192.5 and 190.2 eV), and the Zr3d spin doublet corresponding to ZreC bonds increases (179.8 and 182.1 eV). Appearance of the carbon-bonded zirconium and of the boron oxide components indicates the presence of the chemical reaction between zirconia and boron carbide. Based on thermodynamic assessment of oxidation and reduction reactions [16], it was previously shown that such reactions can occur at elevated temperatures. Similar behavior is found for all compounds of the ceramic coatings with metal oxides (Li2O, MgO,
cracking or peeling of the coating at the substrate-coating interface. The lithium oxide addition in the composition of the ceramic coatings effectively eliminates microcracks appearance (Fig. 3c), so it has a positive effect on reducing tensile stresses in the coatings as described in [15]. The results of TEM of ceramic coatings are shown in Figs. 5–7 for various coating compounds. The structure of the coating made from pure B4C powder is shown in Fig. 5. All reflections in the electron diffraction pattern (Fig. 5a) belong to the B4C phase. The rings in the pattern are diffuse, which is characteristic for a structure with precipitations of nanocrystals in an amorphous matrix. The amorphous structure is shown in the bright-field mode in Fig. 5b. In a dark field mode (Fig. 5c), nanoparticles of the B4C phase are shown scattered in an amorphous matrix. The most of the crystals have a size from 3 to 7 nm. Fig. 6 shows the structural features of the B4C + 40% BN coating. The coating has an amorphous and partially nanocrystalline structure. A bright-field image in Fig. 6b shows alternation of light and dark gray regions with a contrast that is characteristic for the amorphous state. The microdiffraction pattern in Fig. 6a contains diffuse rings related to the h-BN phase, shown by arrows. A dark-field image in (002)BN reflections in Fig. 6c depicts individual nanocrystals of the h-BN phase. The size of the nanocrystals does not exceed 10 nm. Initial powder for coatings deposition contained h-BN particles with an average size of 1 μm, so such structure could be formed only by melting of ceramic powder and subsequent high-speed crystallization of the ceramic melt on a steel substrate. Boron nitride has a higher melting point in comparison to boron carbide, so it crystallized from an undercooled melt in forms of spherical nanocrystals of the h-BN phase. Fig. 7 shows structure of the B4C + 40% BN + 10% MgO + 10% Li2O coating. A bright-field image in Fig. 7b shows thin light areas with a contrast characteristic for an amorphous state. Areas with sharp alternation of dark gray and light contrast are observed, which indicates presence of nanocrystals. In addition, the image contains individual dark particles of a rounded shape. The microdiffraction pattern in Fig. 7a indicates rings of h-BN phase, which are shown by arrows. In
Fig. 6. Structure of the B4C + 40% BN coating. (a) An electron diffraction pattern, (b) bright-field image of microstructure, (c) dark field image in (002)BN reflection. 5
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Fig. 7. Structure of the B4C + 40% BN + 10% MgO + 10%Li2O coating. (a) Electron diffraction pattern, (b) bright-field image of microstructure, (c) dark field image in (002)BN reflection.
ZrO2). Regardless of a metal valency in oxide, boron‑oxygen bonds are registered by XPS through the analyzed thickness of the coatings. Occurrence of them is a result of the chemical interaction between the metal oxide and boron carbide at elevated temperatures during shortpulse laser deposition. The results of boron content analysis with XPS for different coatings are shown in Table 2. we think that results are considerably accurate even after etching by argon ions flow taking into account the high affinity of boron to oxygen. A short (1–3 min) etching by low-energy argon ions does not introduce significant changes in the chemical bonds of BeO. The same applies to zirconium and magnesium oxides.
dry sliding tests was stable until almost complete deterioration of the coating. This stability of the friction coefficient over a test period means that no delamination process is occurred. The SEM image of a sample with the B4C coating after the adhesion wear test is shown in Fig. 9a. Worn surface remains parts of the coating, see a dark area in Fig. 9. The EDX measurements show that the chemical composition of the worn surface depends on its spatial position. While the Area 2 (black) in Fig. 9a is almost completely composed of B4C (73.7 wt% of boron and approximately 23% of carbon), the Area 1 contains Fe-85.3%, B – 7.2% and C – 5%, and the Area 3 contains Fe-52.5%, B – 33.4% and C – 13%. This interface Area 3 was formed during the laser deposition as a result of the high-temperature chemical reaction between the steel substrate and the boron carbide. According to the EDX data, we can conclude that the inner zone of area 3 in Fig. 9a mostly consists of the FeB phase, while the outer fields belong to the Fe2B phase. The SEM view of the fractured surface with the single-layer B4C coating is shown in Fig. 9b. It is explicitly seen that under impact no delamination occurs at the steel substrate – ceramic interface. To explain the high level of adhesion between the ceramic layer and steel substrate, we performed X-ray study of the sample after laser short-pulse welding. The diffraction pattern of the single-layer B4C coating shows the presence of B4C lines, α-Fe (ferrite and martensite), iron borides (FeB, Fe2B), and carbide Fe3C. Active chemical interaction at the interface between the metal matrix and the ceramic layer helps obtaining the desired properties of ceramic composite materials. A quantitative criterion of the wetting of the solid phase by melt under the equilibrium conditions is the contact angle θ, determined by the Young equation [17]. As a rule, liquid metals practically do not wet ceramics [18] due to the fact that the surface energy at the liquid – solid phase boundary is relatively small. The criterion for wetting of a solid surface by a liquid (with a contact angle of θ < 90°) is the work of adhesion W, defined as the work required to separate a liquid-solid pair, which must exceed the surface energy at the liquid-vapor boundary γlv. The work of adhesion is determined by physical and chemical work during chemical reactions proceeding at the melt-solid interface. The typical values of the contribution of physical work to the adhesion work do not exceed 600 mJ/m2, whereas γlv for liquid metals and alloys with a high melting point Tm is significantly higher [19]. This means that only chemical reactions can increase W and provide wettability in the process of thermally activated reactive wetting [20]. FeB and Fe2B phases registered in the samples by XRD study after laser processing indicate intense chemical reactions and admit the reactive wetting. In this case, the connection between the metal matrix and the ceramic coating are significantly strengthened.
3.2. Adhesion of coatings
3.3. Schematic representation of microstructure
Fig. 8. X-ray photoelectron spectrum in the region of boron (B1s) for a sample with the B4C + 40% BN + ZrO2 + 10%Li2O coating. 0 – XPS of free surface without etching, 1 – etching for 1 min (depth ~1 nm), 3 – etching for 3 min (depth ~ 3 nm).
The results of the adhesion tests show that the measured COF during
The samples with ceramic coatings have a common pattern of 6
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Table 2 Layer-by-layer XPS analysis of atomic fractions of boron bounded to oxygen, calculated as a fraction of the total amount of boron in the coatings. Composition of the coatings
Duration of etching (min)
Total amount of boron (at. %)
Including amount of oxidized boron (at. %)
B4C + 40%BN
1 3 1 3 1 3 1 3
5.4 6.4 11.7 16.6 13.3 19.0 13.3 16.6
0.3 0.0 3.0 0.7 5.1 4.6 8.2 6.6
B4C + 40%BN + 10%ZrO2 B4C + 40%BN + 10%MgO B4C + 40%BN + 10%ZrO2 + 10% Li2O
Fig. 9. Worn surface after adhesion wear tests (a) and fractured surface (b) after an impact test that was performed in the transverse plane in relation to the B4C ceramic layer.
microstructure for all studied compositions B4C-BN, B4C-BN-ZrO2-Li2O, B4C-BN-MgO-Li2O. All ceramic coatings consist of boron carbide, boron nitride, metal oxides, and boron oxide, that is formed by high-temperature interaction with metal oxides during short-pulse laser deposition of the coating. Boron carbide included in the amorphous matrix of the ceramic coatings, in which nanoscale crystallites of h-BN and MeO are precipitated. The crystallites of h-BN have a spherical shape with a size of spheres < 10 nm. At the boundary between the coating and the steel substrate, the film contains borides FeB, Fe2B, and carbide Fe3C. The appearance of boride and nitride phases explains good adhesion of the ceramic coatings to the steel substrate, where the reactive wetting between the molten ceramics and the metal substrate plays an important role. Suitable conditions for the appearance of boride and nitride phases are provided by a high temperature (about 3500 °C) in the area of short-pulse laser treatment, which is much higher than the melting temperature of boron carbide and boron nitride. Because of the high temperature gradient, the depth of the molten zone of the steel substrate under a ceramic coating is about 10 μm, which reduces the thickness of the interface layer. Fig. 10. Variation of the coefficient of friction (COF) with the sliding distance for (1) B4C, (2) B4C-40%BN, (3) B4C-40%BN-10%MgO, and (4) B4C-40%BN10%MgO-5%Li2O ceramic coatings in 60% relative humidity. After each 700 m of the sliding distance the load increases sharply by 100 N.
3.4. Tribological performance Fig. 10 shows the COF behaviour of WC-8%Co counter-disk during sliding against ceramic coatings with different compositions. The starting load on samples with all coatings was 100 N. After each 700 m of the sliding distance, the load was increased by 100 N without stopping the revolution of the counter-disk. Duration of each sliding stage with a constant load was 13 min, giving 78 min for the whole test duration of each sample, except of the sample with the B4C coating composition (Fig. 10, curve 1). This coating does not withstand lengthy tests with load of > 500 N and subjected to fast deterioration in these conditions. According to the wear tests, the COF of coatings showed a runningin characteristics, the feature of which depends on the composition of the coating and the load, see Fig. 10. As illustrated by line 1 in Fig. 10, a
pure boron carbide coating requires the longest time to achieve the steady state of the COF, especially at high loads. At the loads of 400 and 500 N, the COF does not reach the steady state for the whole test period. In contrast, the composite B4C-BN and B4C-BN-MeO coatings reach the steady state COF in a considerably shorter period of time. This is especially visible for coatings containing metal oxides. Addition of the h-BN phase in microstructure of coating reduces the COF considerably, see line 2 in Fig. 10. Hexagonal boron nitride has the layered-crystal structure, that resembles those of MoS2 and graphite. Introducing h-BN nanoparticles in the amorphous matrix of ceramic has 7
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a direct consequence in formation of a slippery film with low COF. It is interesting to note that for this coating after a load of 300 N, each step of increasing the load is accompanied by a short-term increase in the COF. It means that under action of the increased load the slippery tribofilm begins to wear more intensively. This results in increase in the COF, thereby accelerating heat generation. Heating of the contact area leads to intensification of chemical transformation of the surface. Temperature intensification of the surface chemical reactions with oxygen and water vapor in the atmosphere releases more boron oxide and boric acid per time. Thus, after an increase of the load towards the sample with the ceramic coating it takes some time to reach a new steady-state value of the COF. The underlying mechanism for the stabilization of the COF has been discussed widely in the literature [1]. The required time and sliding distance to reach the low steady-state friction level increases with increasing load and depends on the sliding velocity and RH [2]. Thereby, transition to a new steady-state friction takes a relaxation time, that is necessary for consistent formation of boron oxide B2O3 and boric acid H3BO3 on B4C surface under new sliding conditions [6]. The relaxation time of the transition to low friction is considerably shorter for coatings with different metal oxides, because their microstructure already has the boron‑oxygen bonds necessary for formation of the low friction tribofilm, see lines 3 and 4 in Fig. 10. Because these bonds are the parts of the composite microstructure, they are instantly ready to form a new low friction tribofilm on the surface as the wear process progresses. Addition of Li2O in the composition of a ceramic coating sharply decreases the COF in the whole range of loads, see line 4 in Fig. 10. Thus, the addition of Li2O in the composition of ceramics facilitates the ultralow friction behavior by intensification of boron‑oxygen bonds formation during the deposition process. The smallest value of the COF registered at a load of 200 N for coating with composition B4C-40%BN-10%MgO-5%Li2O was 0.03 in dry sliding conditions, see line 4 in Fig. 10. As it was mentioned, boron oxide plays an important role in formation of a low friction tribofilm. Usually, this tribofilm has been formed in situ as a result of reactions facilitated by heating during sliding friction of a boron carbide ceramic against a counterpart. The detected reactions at the sliding interface involve oxidation of B4C into B2O3, which is transformed into H3BO3 by the reaction with water [1–6]. Material loss is caused by delamination of the hydrated film [1]. An analysis with XPS of samples before the wear test indicates the presence of the boron‑oxygen bonds in the microstructure of ceramic coatings deposited with the metal oxides, see Table 2. Therefore, the composition of these ceramics has a strong effect on the friction. The coatings with metal oxides initially contain the essential elements, that contribute to low friction behaviour. As for the ultralow friction mechanism of ceramics with addition of the Li2O, we propose the following explanation. It is well known that lithium oxide does not interact with hydrogen, oxygen, carbon and carbon monoxide even when heated, but intensively reacts with water vapor to form lithium hydroxide [14]. At high temperatures, Li2O reacts with most metals [21]. With a number of metal oxides gives double, and triple oxides. The XPS data in Table 2 indicate an intense interaction of lithium oxide and boron carbide during laser short-pulse deposition of coatings, thus releasing considerable amount of boron‑oxygen bonds. The remaining part of the unreacted lithium oxide emerging on the surface could contribute to ultralow friction as well. When releasing on the surface, it instantly reacts with water vapors in the atmosphere with the formation of hydroxide by reaction Li2O + H2O → 2LiOH. An important property of lithium hydroxide is its good reactivity with fatty acids, for example, with stearic acid. The product of this reaction is the basis for the formation of lithium greases, known for their good lubricity [22]. Thus, unreacted during deposition process lithium oxide is an important component for ensuring self-lubrication of friction units in condition of insufficient lubrication. Based on the chemical analyses presented in Table 2, we believe that the ultralow friction coefficient of
Fig. 11. Minimum registered values of the COF during wear tests, depending on the Li2O content in the composite ceramic coatings.
ceramic coatings with addition of Li2O is directly related to the formation of B2O3 during deposition process through whole thickness of the coating. Boron oxide itself has been reported to be able to provide very low friction because at some point of generation of heat the B2O3 based films become viscous [1]. This explains the formation of very low COF of the coating with addition of Li2O even at low load of 100 N, see line 4 in Fig. 10. When the load increases the generation of heat become sufficient to activate the second-step reaction of the B2O3 with water vapors, and B2O3 is transformed into the slippery H3BO3 giving the ultralow COF of 0.03 to the coatings. Coatings without metal oxides require both reactions to form low friction films. Firstly, B4C reacts with oxygen in air to form B2O3, and, secondly, boron oxide reacts with water. Both reactions are thermodynamically feasible even at room temperature [23] but if the coatings initially contain boron oxide, the heat flow is directed only to the formation of H3BO3 films with ultralow friction. The disadvantage of lithium oxide is its low strength, and it cannot be used as a constructional material. The coatings with the Li2O content of > 15% have high wear rate and their COF is increased, see Fig. 11. 4. Conclusion The microstructure of composite ceramic coatings based on B4C and h-BN with addition of different metal oxides consists of an amorphous layer with precipitation of nanocrystals of h-BN phase. Low friction behaviour registered in composite ceramic coatings with addition of hexagonal boron nitride and different metal oxides. Metal oxides in the composition of the coatings significantly reduce the coefficient of friction through the formation of boron‑oxygen bonds during the shortpulse laser deposition process. Among other metal oxides, addition of the lithium oxide into the composition provides the lowest coefficient of friction, which is stable for a long period during gradual deterioration of the ceramic coating in dry sliding conditions. Li2O in the composition of ceramics facilitates the ultralow friction behaviour by intensification of boron‑oxygen bonds formation during the deposition process. The boron‑oxygen bonds are the essential part of the coating microstructure, which are necessary to form ultralow friction behaviour. Adhesion of the ceramic coatings with steel substrate is facilitated by the reactive wetting at the steel-ceramic interface with formation of FeB and Fe2B phases. CRediT authorship contribution statement Evgeny V. Kharanzhevskiy:Conceptualization, Methodology, 8
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Investigation, Resources, Writing - original draft, Project administration.Alexey G. Ipatov:Investigation, Resources.Mikhail D. Krivilyov:Writing review & editing.Alexey V. Makarov:Supervision.Faat Z. Gil'mutdinov:Investigation, Resources.Elena G. Volkova:Investigation, Resources.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study is financially supported by Russian Science Foundation, project 19-79-20012. References [1] P. Larsson, N. Axen, S. Hogmark, Tribofilm formation on boron carbide in sliding wear, Wear 236 (1999) 73–80. [2] S. Bhowmick, G. Sun, A.T. Alpas, Low friction behaviour of boron carbide coatings (B4C) sliding against Ti –6Al –4V, Surf. Coat. Technol. 308 (2016) 316–327. [3] H.-S. Ahn, P.D. Cuong, K.-H. Shin, Ki-Seung Lee, Tribological behavior of sputtered boron carbide coatings and the influence of processing gas, Wear 259 (2005) 807–813. [4] Yu.G. Gogotsi, A.M. Koval’chenko, I.A. Kossko, Tribochemical interactions of boron carbides against steel, Wear 154 (1992) 133–140. [5] Y. Tamura, H. Zhao, C. Wang, A. Morina, A. Neville, Interaction of DLC and B4C coatings with fully formulated oils in boundary lubrication conditions, Tribol. Int. 93 (2016) 666–680. [6] A. Erdemir, C. Bindal, G.R. Fenske, Formation of ultralow friction surface films on boron carbide, Appl. Phys. Lett. 68 (1996) 1637–1639.
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