Der Artikel geht auf die tribologischen Eigenschaften von Si3N4-Binärbeschichtungen mit einem stöchiometrischen Verhältnis von 57 / 43 ein und hebt ihren Einsatz in industriellen Anwendungen aufgrund ihrer hohen mechanischen Festigkeit, Temperaturbeständigkeit und hervorragenden Verschleißfestigkeit hervor. Die Studie konzentriert sich auf die Evolution des Reibungskoeffizienten und der Verschleißmechanismen über unterschiedliche Gleitabstände, wobei ein Magnetron-Sputtersystem mit mehreren Zielen zur Abscheidung und ein Tribometer Microtest MT 4001-98 zur tribologischen Bewertung verwendet werden. Die Forschung zeigt, dass die Beschichtung unterschiedliche Verschleißmechanismen aufweist und sich der Reibungskoeffizient signifikant ändert, wenn der Gleitweg zunimmt, wobei die Beschichtung schließlich bricht und das Substrat einem erhöhten Verschleiß aussetzt. Die Ergebnisse zeigen, dass die Si3N4-Beschichtung die tribologischen Eigenschaften des Substrats verbessert und es zu einer vielversprechenden Schutzschicht für technische Geräte macht, die hohen Reibungsbedingungen ausgesetzt sind.
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Abstract
Friction coefficient depends on various factors or surface characteristics during tribological testing, and this friction coefficient can be modified by altering the properties of one of the two contacting surfaces. It is crucial to monitor the friction coefficient continuously, not only at the conclusion of the test. This research examined the evolution of friction coefficient of silicon nitride (Si3N4) coating and H13 steel over different sliding distances (250, 500, 750, 1000 m). The study assessed surface wear and oxidation through three-dimensional profilometry and SEM/EDX. The findings indicated a reduction in friction coefficient by 22%, a decrease in wear rate by 88%, and a reduction in wear volume by 87% when comparing the silicon nitride coated steel to the uncoated steel. Furthermore, the changes in friction coefficient provided insights into the timing of the complete fracture of the hard coating.
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Hinweise
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1 Introduction
Currently, hard coatings have been implemented in a variety of industrial applications, which has led to their different properties being studied. In the case of tribological properties, they have been an important area in the study of hard coatings, as they are implemented as protective coatings on engineering device, which are exposed to drastic operating conditions that can cause wear processes or mechanisms, significantly reducing the useful life of these devices.
Among the most used coatings within the metal-mechanic industry is (Si3N4) silicon nitride, which is characterized by its high mechanical strength, higher hardness, resistance to high temperatures (> 1000 °C), good wear resistance, and high chemical stability, which is generated by a stable Si–N bond. These characteristics have caused this binary coating to be used as a protective lining on engineering devices subjected to processes of high friction [1‐4]. Authors such as Barshilia et al. [5] have determined the friction coefficient (COF) of this binary system (Si3N4), obtaining a value of approximately 0.4, when in contact with a tungsten carbide counterpart (WC) and a load of 2 N. Similarly, Eslami et al. [6], study of the tribological properties of this same coating, showing a value of approximately 0.5 in the friction coefficient, under a load of 3 N and against an AISI 52,100 steel counterpart. These investigations determine the tribological characteristics such as the COF and the wear rate after the tribological test. However, authors like Meylan et al. [7], have determined that the friction coefficient is not a linear phenomenon and that it can be modified during the tribological test by some alteration of the surface properties of either of the two surfaces in contact. Furthermore, Walker et al. [8], analyzed the change in the COF, wear rate in terms of mass loss, and growth of the wear track of an Al-Si alloy subjected to tribological tests, showing a significant change in these characteristics as the tribological test progressed. Research conducted by Ma et al. [9], presented a detailed study of the evolution of the COF of the TiN coating deposited by magnetron sputtering, similar to our research. The researchers detailed the change in the friction coefficient and the wear rate as a function of the duration of the tribological test. From these results, it was possible to show that the wear mechanisms depend on multiple interactions, as well as the conditions of the test, since the tribological properties are extrinsic factors of the coatings. In addition, the researchers were able to detail and relate atypical alterations of the friction coefficient due to microfractures or partial detachment of the coating. Therefore, in this research it has been determined that the tribological response is not linear phenomenon, on the contrary, they are atypical phenomena, since during the test, constant modifications are generated to the wear mechanisms, and these modifications influence important characteristics such as: Friction coefficient, wear rate, main wear mechanism and surface deformation. So, conventional tribological studies will not give much information about these interactions mentioned above, and it is necessary to analyze in stages the tribological response of the different materials in order to have a good analysis of their behavior.
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In relation to the above, this research focuses on studying the evolution of the tribological behavior of the Si3N4 coating with a stoichiometric ratio of 57/43 as a function of the sliding distance. Therefore, 4 stages or 4 stops corresponding to the distances of 250, 500, 750, and 1000 m, respectively, will be carried out. To determine these characteristics, the friction coefficient, volume and wear rate, as well as the growth of the wear track will be evaluated at each of the proposed distances. These results will allow us to understand how the coating will behave during the course of the tribological test, and how microfractures on the hard coating can even cause an increase in the wear of these coatings.
2 Experimental Details
2.1 Materials
For this research, two types of substrates were used: H13 steel metallic substrates, which were used for the studies of surface, mechanical, and tribological properties, and silicon substrates with preferential orientation (100), which were used for structural, compositional, and morphological characterization. These metallic substrates have a cylindrical geometry with a thickness of 5 mm and a diameter of 1/2 inch, which were superficially prepared with silicon carbide (SiC) abrasive paper with a grain size of 80–1200 μm. Subsequently, they were subjected to a polishing process using a metallographic polisher and an alumina solution with a particle size of 0.3 μm. Finally, both substrates (silicon and steel) were cleaned with ultrasound in an isopropyl alcohol solution using a Rio Grande UD50SH-2 L device with the purpose of removing residues on their surface.
2.2 Deposition Parameters
The Si3N4 binary coating with a stoichiometric ratio of 57/43 was deposited using a multitarget magnetron with a radio frequency (r.f) source of 13.56 MHz on H13 steel substrates and monocrystalline silicon (100) in a controlled atmosphere of 99.99% argon. During the deposition process, a power of 500 W was used for the Si3N4 target, with a bias voltage of − 20 V. Additionally, a temperature of 300 °C within the chamber was used during the deposition process with an Argon flow of 50 sccm, a working pressure of 6.1 × 10−3 mbar, and a target-substrate distance of 7 cm. Finally, all coatings had a thickness of 2.0 μm, which was determined by 2D profilometry and cross-sectional SEM.
2.3 Characterization Techniques
The structural study of the coating was carried out using an X’Pert PRO X-ray diffractometer with a Bragg–Brentano (θ/2θ) configuration in a high-angle range, using a Cu-Kα radiation source (λ = 1.5406 Å). For this study, a counting time of 1 s per step with a step size of 0.001 was used, and the phase identification was performed using an ICCD database through the X'pert High Score software. X-ray photoelectron spectroscopy (XPS) was performed using a SAGE HR 100 (SPECSTM) with a monochromatic source (Mg Kα 1253.6 eV) after etching the surface to remove surface contamination, and the chemical composition was obtained from the peak areas using Casa XPS V2.3.15dev87TM software. Surface analysis was performed with a KLA Tencor D-120 profilometer, which produced 3D profiles consisting of 10 individual profiles, each profile with a length of 2 mm and a spacing of 0.1 mm. Mechanical properties such as hardness (H) and modulus of elasticity (E), as well as load-penetration curves of the surfaces, were obtained by the nanoindentation technique using a Ubil–Hystron with a Berkovich type indenter with variable load. The tribological behavior of the Si3N4 coating was studied under the ASTM G99-17 standard [10]. Using a Microtest MT 4001-98 tribometer with a 6 mm diameter WC pin as the counterpart, applying a load of 10 N with a travel of 250, 500, and 750 and 1000 m, an angular velocity of 160 rpm. The study of wear mechanisms was carried out by scanning electron microscopy (SEM) using the JSM 6490LV JEOL equipment.
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3 Results and Discussion
3.1 Structural Analysis by XRD
In Fig. 1a, the XRD diffraction pattern for the Si3N4 system deposited on silicon substrates was observed. From these results, Bragg peaks were identified located on the crystallographic planes (111), (220), (311), (400), (511), (440), (533), situated at the angles 2θ = 20.22°, 33°, 39.26°, 47.22°, 63°, 69.16°, 81.75° corresponding to a hexagonal structure. Additionally, a preferential growth on the crystallographic plane (311) was possible to identify, with a space group 176-P63/m, indexed with the international file ICDD 00-009-0259, research conducted by Ortiz et al. [11] have reported this same structure. Subsequently, Fig. 1b presents an enlargement of the peak of maximum intensity (311), where it was possible to evidence a shift of the peak towards higher 2θ values, this shift is associated with the presence of residual stresses within the crystalline structure which are produced during the deposition process (intrinsic stresses) and stresses caused by the substrate/coating interface (extrinsic stresses) [12, 13].
Fig. 1
a Diffraction patterns for the Si3N4 coating, b enlargement of the peak of maximum intensity
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3.2 Compositional Analysis by XPS
The depth XPS spectrum for the Si3N4 coating is presented in Fig. 2a. From this spectrum, it was possible to identify the presence of high-intensity signals or peaks corresponding to O (1s), N (1s), C (1s), Si (2s), and Si (2p), located at 530.8 eV, 396.8 eV, 284.8 eV, 150 eV, and 102.8 eV, respectively. Additionally, the stoichiometric Si/N ratio, which was 57/43, was identified. It should be noted that the C (1s) peak is related to the adventitious carbon used for calibration. The O (1s) peak corresponds to a superficial oxidation of the sample, which has been observed in other research [14, 15]. Thus, the main signals of Si (2p) and N (1s) are related to the coating, and to perform a more detailed analysis, deconvolutions have been carried out to determine possible internal signals [16, 17]. Following the depth spectrum, the Fig. 2b and c present the high-resolution spectra for the Si (2s) and N (1s) signals. Thus, Fig. 2b shows the high-resolution spectrum for the Si (2p) signal, which was adjusted using two Gaussian functions to determine the internal signals of the coating, identifying bonds corresponding to Si–O and Si–N, located with binding energies of 101.77 eV and 104.88 eV respectively, corresponding to a surface oxidation and the predominant bond with N, these internal signals have been reported in other research [4, 15, 18]. In addition, Fig. 2c presents the high-resolution spectrum for the N (1s) signal. This signal was adjusted using three Gaussian functions, where internal signals corresponding to the N–Si, N–O, and N–X bonds were evidenced, located with binding energies of 400.51 eV, 396.96 eV, and 394.4 eV, these signals have been reported in other research [15, 17].
Fig. 2
a Depth XPS spectrum for the Si3N4 coating and High-resolution XPS spectrum for the signals: b Si (2p)-N and c N(1s)-S
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3.3 Cross-Section by SEM
In Fig. 3, the cross-sectional micrograph taken by scanning electron microscopy (SEM) for the Si3N4 individual coating with a stoichiometric ratio 57/43 deposited on a silicon substrate is observed, in which it was possible to evidence a contrast difference between the coating and the substrate. This difference is associated with the electronic density, crystallinity, and atomic weight of each Surface [19, 20]. Thus, the coating presents a clear contrast associated with a high electronic density compared to the substrate that presents a dark contrast [4]. On the other hand, Fig. 3b presents the EDX spectrum, obtained from the Si3N4 coating, in which the presence of elements such as Si and N, belonging to the coating, and a low signal attributed to oxygen, due to a surface contamination of the coating, could be observed. In addition, Fig. 3c shows the EDX spectrum of the substrate, where the presence of the Si element can be observed, with a low oxygen signal due to oxidation. This oxidation of the substrate (silicon 100) has been reported by other authors [16, 21, 22]. Finally, the cross-sectional micrograph presents the value of the coating thickness close to 2.0 ± 0.05 μm.
Fig. 3
a Cross-sectional SEM of the Si3N4/Si coating deposited on silicon, b EDX spectrum for the Si3N4 coating, and c EDX spectrum for the substrate
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3.4 Surface Study by Profilometry
With the purpose of identifying the surface characteristics of the coating and the metallic substrate (H-13 steel) prior to the tribological tests, a surface study was conducted by means of 3D profilometry. For this, the 3D profiles presented in Fig. 4a and b corresponding to the substrate and the coated substrate (H13 Steel/Si3N4), respectively, were obtained. These height profiles were made up of 20 individual profiles. Additionally, these profiles show a color difference where the red color indicates areas of greater height and the blue shows areas of lesser height. Thus, the metallic substrate presents a greater difference in tonality, indicating high roughness, and several oriented scratches were also observed due to the surface preparation process using abrasive paper. On the other hand, Fig. 4b shows the 3D profile for the coated substrate, where a surface variation was evidenced, presenting a greater tonality, indicating less surface irregularity (roughness), and the oriented scratches decreased significantly, as the incorporation of the coating on the substrate generates a more homogeneous surface. Subsequently, Fig. 4c shows the quantitative roughness values for the Si3N4 coating and the uncoated metallic substrate, obtained by profilometry. These results corroborate the trend evidenced by the 3D profiles, indicating an approximate roughness of 0.92 ± 0.05 μm for the substrate, and a roughness of 0.47 ± 0.05 μm for the Si3N4 coating, which corresponds to a 49% reduction. These results show that the coating thickness of ~ 2 μm exceeds the roughness value of the substrate of ~ 0.92 μm, therefore, by incorporating the coating onto the surface, the irregularities are filled, directly modifying the quantitative value of the roughness. Moreover, this surface modification will alter the tribological behavior of the system (Si3N4/H13 Steel), since this new surface is more homogeneous and dense, which will increase the real area of contact and positively influence the mechanical and tribological properties, as reported by various authors [3, 6, 23‐25].
Fig. 4
3D profiles for a H-13 steel substrate and b Si3N4 coating, and c roughness values (Color figure online)
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3.5 Mechanical Analysis by Nanoindentation
To evaluate the mechanical properties of the coating, Fig. 5a presents the load-unload curve for the Si3N4 individual coating with a stoichiometric ratio of 57/43, which allows determining the mechanical response of the coating. This curve shows the load trend that goes from 0 to 9 mN, where the greatest penetration on the coating was obtained, corresponding to a depth of ~ 150 nm. Subsequently, the path that the curve takes in the unloading part is shown, evidencing a different path to the loading curve, due to the elasto-plastic response that our material had, which is deforming and hardening, modifying its mechanical properties such as hardness and modulus of elasticity. It should be noted that this nanoindentation test was carried out according to the model of Oliver and Pharr [26], which establishes that the maximum indentation penetration should not exceed 10% of the total thickness of the coating (~ 200 nm), with the purpose that there is no contribution from the substrate in the values [26‐28]. Thus, Fig. 5b shows the quantitative values of the mechanical properties for the Si3N4 coating, indicating a reduced modulus of elasticity and a hardness of 271.3 GPa and 29.3 GPa, respectively. These results are associated with the structural and surface properties of the coating, as having an ordered and textured structure, and having less surface roughness, increasing the real area of contact, presents a greater resistance to deformation, as has been determined in different investigations, where this coating has exhibited high mechanical properties [1, 29].
Fig. 5
a Load-depth curve for Si3N4 and b mechanical properties: hardness and reduced elastic modulus
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From the nanoindentation results, the resistance to plastic deformation (H3/E2) and elastic recovery (R) were determined. The elastic recovery of the coating was calculated using Eq. 1:
where δmax is the máximum displacement and δP is the residual displacement. The data from Eq. 1 were taken from the load-depth curves presented in Fig. 5a. Figure 6a and b show the values obtained for resistance to plastic deformation (H3/E2) and elastic recovery (R). These determined high values for resistance to plastic deformation (H3/E2) and elastic recovery (R) for the Si3N4, coating, which is mainly due to its mechanical properties such as hardness and reduced modulus of elasticity [30‐32].
Fig. 6
Mechanical properties of the Si3N4 layer: a Resistance to plastic deformation and b elastic recovery
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3.6 Tribological Evaluation
3.6.1 Evolution Friction Coefficient as a Function of Sliding Distance
In order to evaluate the tribological behavior of the coated substrate (Si3N4/H-13 Steel) and the uncoated substrate (H-13 Steel), the evolution of the friction coefficient (µ) as a function of sliding distance was analyzed. For a deeper study of the friction coefficient, this tribological test was carried out in four characteristic stop stages, which were evidenced by a drastic change in the friction coefficient, due to the interruption and resumption of the tribological test (Fig. 7). From these results, it was possible to identify a startup period and four characteristic stages for both surfaces. In the startup period, a rapid increase in the friction coefficient at low sliding distances was observed for both surfaces, which is attributed to the initial interaction of the surface irregularities (roughness) with the counterpart (WC), causing a large number of abrasive particles on the surface increasing the interferential friction in this area and rapidly increasing the friction coefficient [3]. Subsequently, approaching Stage I (250 m), a stabilization of the friction coefficient was evidenced for both the uncoated substrate (H-13 steel) and the coated substrate (H-13/Si3N4). This behavior of the friction coefficient is associated with the surface properties of each material. Thus, H-13 steel, which has low hardness and high plasticity, presents a faster and easier surface deformation, leading to the elimination of roughness (asperities) and producing a large number of abrasive particles. This wear mechanism is competitive, where the abrasive particles are dispersed and expelled from the wear track while new abrasive particles are simultaneously generated. Moreover, these particles deform and harden, which may increase the friction coefficient, as well as generate new wear mechanisms such as adhesive wear. On the other hand, the tribological behavior of the coating is different due to its surface properties (higher hardness, lower plasticity, and more homogeneous surface). In this case, the coating will gradually fracture, creating larger irregular areas, which will cause partial delaminations of this coating. Similarly, these fractures will generate wear particles, which will be located on the wear track creating a three-body wear mechanism (WC/abrasive particles/coating). These tribological mechanisms for both surfaces will increase as a function of the sliding distance, as has been reported in various investigations [33, 34].
Moreover, the four specific stages of the tribological test correspond to the sliding distances at which the interruptions were made: 250 m (Stage I), 500 m (Stage II), 750 m (Stage III), and 1000 m (Stage IV). These interruptions allowed for the observation of changes in the friction coefficient for the coated substrate. From these results, it was observed that the steel maintained a stable friction coefficient after Stage I, corresponding to the tribological behavior of H-13 steel. However, for the coated substrate, there was a drastic increase in the friction coefficient after Stage III (750 m), which is associated with a possible total fracture of the coating, leaving the steel completely exposed and increasing the friction in this area due to the change in surface properties of the new contact surface (low hardness, high plasticity, and a large number of abrasive particles).
Fig. 7
Friction coefficient versus sliding distance
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3.6.2 Statistical Analysis of Friction Coefficient Evolution
For the purpose of interpreting the tribological test data, Fig. 8 presents a Gaussian statistical analysis of the friction coefficient values for both (coated substrate and uncoated substrate). So, this analysis is based on histograms generated from the number of data collected, using a Gaussian fit to represent the distribution of friction coefficients. Therefore, the uncoated substrate (H13 steel) has significant variability in the background level of the data set, with a baseline shift (y0) of 359.63779 and an uncertainty of ± 108.37468. These significant changes in the number of data taken are associated with the surface characteristics (high roughness and low hardness), which generates a high friction and a significant change in the height of Z-axis of the counterpart dur to the surface irregularity, obtaining a large amount of data. In addition, the statical analysis indicated a peak center position (Xc) of 0.90905, suggests that this is the most frequent or probable value of the observed friction coefficient, with remarkable accuracy. The peak width (w) of 0.09807 reflects a standard deviation and variance of 0.0096, while the area under the curve (A) of 3030.68973 indicates the intensity and consistency of the peak.
In contrast, the coated substrate (Si3N4/H13) shows a baseline shift (y0) of 519.09281 with an extremely high uncertainty of ± 375.98391 which could be associated with a rapid increase in the friction coefficient during the initial stage of the tribological test. On the other hand, these statical results evidenced an Xc value of 0.70318 with low uncertainty and a peak width (w) of 0.06455 suggests more consistent and less scattered measurements, with a variance of 0.0042. The model fit indicators, such as Reduced Chi-Sqr, R-Square (COD) and Adj. R-Square, confirm an excellent fit of the Gaussian model to the experimental data. In addition, it was determined that the presence of the coating decreased 22.22% in the coefficient of friction compared to the uncoated substrate during the stabilization stage of the test. This implies that the implementation of the Si3N4 coating has a more defined value and presents a lower variation of the friction coefficient during the test, being associated to a better tribological response. Finally, the implementation of a statistical analysis to the tribological tests, allows to determine that the Si3N4 coating not only provides more accurate and consistent measurements, but also improves the tribological response by reducing the coefficient of friction against the uncoated substrate. This highlights the relevance of Si3N4 coating in engineering applications [3, 35, 36].
Fig. 8
Gaussian analysis of the evolution of the friction coefficient
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3.6.3 Wear Track Analysis by 3D Profilometry
With the purpose of identifying the progressive deformation of the surfaces during the tribological test, Fig. 9 presents the 3D profiles corresponding to the substrate and the coated substrate (H13 Steel/Si3N4). From these results, it was possible to identify, through the variation in coloration, the presence of surface deformation caused by the constant passage of the counterpart (wear track). Through Fig. 9a and b, corresponding to Stage I, it is evident that the uncoated substrate showed a much more severe wear track compared to the coated substrate. This is attributed to the modification of the surface properties due to the incorporation of the coating on the surface, providing greater hardness, less roughness, which leads to a reduction in interferential friction. This implies that the coated substrate has a greater capacity to withstand the cyclic loads of the counterpart. Therefore, the energy required to produce wear on the coated substrate is greater than that on the uncoated substrate, reflecting a lower generation of abrasive particles.
Subsequently, Fig. 9g presents the 3D profile at Stage IV (1000 m) for the uncoated substrate (H-13 steel). At this stage, a wear track with high depth and amplitude was observed, due to the formation of abrasive particles during the tribological test, which accumulate on the wear track. These abrasive particles are constantly deforming and hardening, causing an increase in wear in this area. In addition, the high friction in this area generates an increase in temperature in the actual contact area, causing a decrease in the surface properties of the substrate, leading to greater deformation on the surface (wear). On the other hand, Fig. 9h shows the 3D profile of the coated substrate (H13 Steel/Si3N4) at Stage IV, where a lesser depth and amplitude in the wear track were evidenced compared to the uncoated substrate. This is due to the increase in the surface properties of the substrate granted by the coating, thus producing a smaller number of abrasive particles, thanks to the reduction in roughness and the increase in hardness, allowing it to withstand a greater number of cyclic loads than the uncoated substrate, indicating less material loss and less surface deformation, demonstrating an improvement in the tribological properties of the substrate [37].
Fig. 9
3D profiles after the tribological test for the uncoated substrate (H-13 steel), and the coated substrate (Si3N4/H-13 Steel) as a function of sliding distance: a, b 250 m (Stage I), c, d 500 m (Stage II), e, f 750 m (Stage III), and g, h 1000 m (Stage IV)
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Figure 10a and b show the 2D profiles of the wear tracks for the uncoated substrate (H13 steel) and the coated substrate (H13/Si3N4) according to the characteristic stages during the tribological test. These profiles allow us to evidence the progressive evolution of the depth and amplitude of the wear tracks, where the steel showed a maximum depth of 8000 nm in the first stage and a final stage depth of 26,000 nm, indicating greater wear. In contrast, the coated substrate (H13 Steel/Si3N4) showed a lesser wear depth throughout the tribological test, with a maximum depth in the first stage of 1050 nm and in the final stage of 4200 nm, indicating better wear behavior. However, it can be seen that after 750 m (Stage III), the depth of the wear track exceeds the total thickness of the coating (2000 nm), indicating that during this stage, there was a total fracture of the coating and the contact surface was modified, completely altering the tribological response at this point. Finally, in Stage IV, the depth and width of the wear track increased exponentially, because at this stage, the counterpart (WC) is already in direct contact with the substrate (H13 steel), which is a surface with lower hardness, higher plasticity, and a large number of abrasive particles from the coating on the surface, causing mainly abrasive, adhesive, and plowing wear mechanisms. It should be noted that, although there was a total fracture of the coating, the complete system (H13 Steel/Si3N4) showed less wear compared to the uncoated substrate. These results determined that the incorporation of this coating on engineering devices will significantly increase the service life in high-friction applications [3, 6, 24, 25, 35, 38].
Fig. 10
2D profiles of the wear track a uncoated substrate (H-13 steel) and b coated substrate (H-13/Si3N4)
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3.6.4 Wear Mechanisms by SEM
To study the different wear mechanisms generated during the tribological test, Fig. 11 shows the SEM micrographs for the surfaces as a function of sliding distance, for the uncoated substrate (H13 steel), and the coated substrate (H13 steel/Si3N4), in the first (250 m) and fourth stage (1000 m). In addition, a compositional study inside and outside the wear track was conducted using EDX, where some variations in oxygen content were observed, attributed to possible surface oxidation.
Thus, Fig. 11a presents the SEM micrograph for the uncoated substrate at Stage I (250 m). From this, two characteristic zones were identified. Zone I correspond to the undeformed substrate, and Zone II corresponds to the deformed area (wear track), which is generated by the constant passage of the counterpart over the surface. On the other hand, the EDX analysis shows that in Zone I, elements such as Fe, Cr, C, Mo, and V were obtained, corresponding to the substrate (H-13 steel) [39]. In Zone II, corresponding to the wear track, the presence of elements such as tungsten (W) was observed, which is attributed to the transfer from the counterpart, due to adhesive wear. Additionally, an increase in oxygen content was observed, which is due to surface oxidation because this area has been deformed and the metallic nature of the substrate makes it more susceptible to a corrosive process. Similarly, Fig. 11b presents an enlargement within the wear track, where it was possible to identify that the predominant wear mechanism was abrasive. This wear mechanism is associated with the surface characteristics of the substrate (high plasticity, low hardness, high roughness, and susceptibility to corrosive processes); thus, during the tribological test, a large number of abrasive particles are generated on the surface which deform and harden, increasing surface deformation and causing elevated corrosive processes [40, 41].
On the other hand, Fig. 11c shows the SEM micrograph of the coated substrate at Stage I (250 m). In this micrograph, it was possible to identify a contrast change, which is associated with the difference in electronic density, crystallinity, and atomic weight of each surface. Thus, two zones could be distinguished: Zone I, corresponding to the coated surface (dark contrast), and Zone II, a clear contrast attributed to some partial fractures of the coating leaving the steel exposed. Subsequently, the EDX analysis in Zone I showed elements such as Si and N, belonging to the coating, and compositional analysis corroborates the stoichiometric ratio obtained by XPS of Si57N43. While in Zone II, new elements such as Fe, Cr, Mo, V, O, W, and C were identified, which are associated with the exposed steel due to partial delamination of the coating [39]. In addition, the presence of W, and an increase in C, is the result of material transfer from the counterpart to the surface [34]. Likewise, the presence of oxygen is due to partial oxidation of the surface during the tribological test. Figure 11d shows a ×250 resolution of the SEM micrograph, which confirms the predominant adhesive wear mechanism, observing micro-welds that subsequently detach from the material, originated by the accumulation of abrasive particles and the load exerted by the counterpart [42]. However, this mechanism is less aggressive than that of the uncoated substrate, due to the difference in mechanical and morphological properties, showing a better response of the coated substrate, indicating greater resistance to deformation and less material loss [22, 25, 43].
To analyze the evolution of the wear track, Fig. 11e presents the SEM micrograph of the uncoated substrate at Stage IV (1000 m). It was possible to observe a significant increase in the amplitude of the wear track, compared to Stage I (Fig. 11a), due to the increase in sliding distance, which resulted in a significant increase in external loads by the contact between surfaces (counterpart-H-13 steel). Figure 11f shows the SEM micrograph at a resolution of x250, where variations in the wear mechanisms on the surface were identified compared to Stage I. From these micrographs, an increase in abrasive particles on the surface (abrasive wear) was evidenced. This phenomenon is associated with the constant deformation and hardening of the initially generated particles. This caused a scratching (plowing) process on the surface of the substrate. In addition, the increase in deformation from the tribo-mechanical loads led to an increase in the accumulation of abrasive particles that adhered to the surface, generating a greater number of micro-welds (adhesive wear). These wear mechanisms altered the surface of the material, increasing friction and the wear rate [44].
In a similar way, Fig. 11g shows the SEM micrograph, with the evolution of the wear track of the coated substrate after stage IV (1000 m) of the tribological test. In this micrograph, it is possible to evidence a greater change in contrast, where the deformed area presents a bright contrast, due to the fact that the wear track has a greater amplitude and depth, compared to stage I (Fig. 11c). From Fig. 11h, it was possible to identify a total delamination of the coating. In addition, on the exposed steel, a lesser wear was identified compared to Fig. 11f, this reduction in wear is due to the coating absorbing a large amount of external energy applied during the tribological test, which causes the surface to have a wear track with less wear. On the other hand, mechanisms of abrasive wear, adhesive wear, and plowing were identified. These mechanisms were intensified by the plastic nature of the steel when being fully exposed to the counterpart. This caused a considerable increase in the detachment of the abrasive particles, as well as constant deformation and hardening. This originated the scratching of the surface [3, 24, 36].
Fig. 11
SEM micrographs for wear tracks: a uncoated H-13 steel track of 250 m, b H- 13/Si3N4 steel track of 150 m, c uncoated H-13 steel track of 1000 m, d H-13/Si3N4 steel track of 1000 m
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In order to determine the susceptibility to corrosive processes of the coated substrate and the uncoated substrate during the tribological test, an analysis of the variation in the percentage of oxygen content was carried out. Thus, Fig. 12 presents the percentage of oxygen as a function of the sliding distance. In this figure, it can be seen that, in stage I, the presence of the coating causes a 33% decrease in oxygen content when compared with the uncoated substrate. In stage IV, the coating causes a 47% decrease in the percentage of oxygen compared to the uncoated substrate. Therefore, the coating not only increases the wear resistance of the substrate but also decreases surface oxidation [36].
Fig. 12
Percentage of oxygen content as a function of the sliding distance
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3.6.5 Counterpart Analysis by SEM
During the analysis of wear mechanisms by SEM/EDX, it was possible to observe that different wear mechanisms occurred, such as adhesive wear, where part of the counterpart (WC) adhered to the other surface. Therefore, Fig. 13 shows the SEM micrographs of the counterparts evaluated in the tribological tests. In Fig. 13a, a micrograph of the counterpart corresponding to the metallic substrate is observed, where it was possible to evidence through the change in contrast, a significant deformation of the metal after the stipulated sliding distance. Additionally, considerable adhesive wear is presented, as corroborated in the EDX analysis where atypical elements of the counterpart were detected, such as Fe, Cr, Mo, and V, associated with the adherence of the metallic substrate to the counterpart.
Subsequently, Fig. 13c shows the SEM micrograph of the counterpart used in the tribological test with the coated substrate (H13/Si3N4 steel). From these results, it was possible to evidence significantly less wear compared to the tribological pair (WC/H13 steel), which is attributed to a greater hardness of the coating, considerably reducing the adhesive type wear.
Fig. 13
SEM micrographs of counterpart wear. a, b Used in the tribological test with the uncoated substrate at ×50 and ×250 resolution and c, d used in the test with the coated substrate at ×50 and ×250 resolution
×
3.6.6 Wear Rate Analysis
In order to perform a quantitative analysis of the wear rate, Table 1 presents the values of the wear track area, wear volume, and wear rate for the coated substrate (H-13/Si3N4) and the uncoated substrate (H-13 steel) as a function of the sliding distance. The data were obtained through profilometry (2D and 3D profiles) and the results of the tribological test, making it possible to calculate the wear volume using Eq. 2.
$$\:\text{V}=2{\uppi\:}\text{r}\text{A}$$
(2)
Figure 14a shows the wear volume as a function of the sliding distance. It was possible to demonstrate that the coated substrate (H-13/Si3N4) exhibited a low volumetric change compared to the uncoated substrate (H-13), which is attributed to a low material loss and greater wear resistance. However, after stage III (750 m), the wear volume of the coated substrate increased drastically, which is associated with the complete delamination of the coating, leaving the steel fully exposed, as corroborated in the analysis by SEM micrography (Fig. 14g). On the other hand, the uncoated substrate shows a greater volume loss compared to the coated substrate. Likewise, the wear volume behavior is linear until stage III, after which a drastic increase in wear volume was observed, which is associated with longer sliding distances, since throughout the tribological test there is a generation of abrasive particles, which accumulate, causing micro-welding between them, leading to an increase in adhesive wear, as evidenced in the SEM micrography (Fig. 14c). Indicating an increase in material loss on this surface.
On the other hand, Eq. 3 was used to calculate the wear rate (K), which relates the wear volume (V) with the sliding distance (S) and the applied force (5 N). Figure 14b shows the results obtained, where it was possible to identify that the coated substrate presented a low wear rate compared to the uncoated substrate. Additionally, it shows a decreasing behavior until stage III, this is attributed to a relatively constant volume loss with a continuous increase in the sliding distance, indicating less material loss and greater stability. However, after 750 m, the wear rate of the coating increased drastically, which is due to the total delamination of the coating and the direct contact of the metallic substrate with the counterpart [3, 4, 36, 37].
$$\:\text{K}=\frac{\text{V}}{\text{F}.\text{S}}$$
(3)
Finally, Fig. 14c and d show the correlation of the depth and width of the wear track as a function of the sliding distance, where it was identified that the coated substrate presented a smaller amplitude and depth compared to the uncoated substrate. Indicating that the presence of the coating on the surface improves the tribological properties of this material, which will increase the useful life of a coated engineering device compared to a conventional device [3, 4].
Table 1
Tribological test parameters
Sliding distamce (m)
H-13Si3N4
H-13
A (mm2)
V (mm3)
K (mm3/Nm)
A (mm2)
V (mm3)
K (mm3/Nm)
250
3,46E-04
0,01108
8,86E-06
1,26E-03
0,05778
4,62E-05
500
3,93E-047
0,0126
5,04E-06
3,10E-03
0,14199
5,68E-05
750
5,26E-04
0,01687
4,50E-06
4,85E-03
0,22243
5,93E-05
1000
1,54E-03
0,04935
9,87E-06
8,87E-03
0,40695
8,14E-05
Fig. 14
Quantitative values of the wear track as a function of the sliding distance: a wear volume, b wear rate, c amplitude, and d depth
×
4 Discussion
With the purpose of identifying the increase in properties for the coated steel compared to the uncoated steel, this work carried out a merit index to evaluate the response of both surfaces under different conditions. Thus, Fig. 15a presents the merit index which relates roughness, wear volume, and sliding distance. It was determined that the coated steel presented the best set of properties since it had a lower roughness, which is associated with less surface deformation, as well as less material loss, consequently, a lower wear volume, corroborating that the coating provides greater wear resistance to the metallic substrate.
On the other hand, Fig. 15b shows the merit index that relates the friction coefficient, wear volume, and sliding distance. From these results, it was observed that the coated steel also had the best set of properties in this case, which is attributed to a lower coefficient of friction, associated with a larger real area of contact and a more homogeneous surface. In this way, the surface can perform better under cyclic external loads, reducing the generation of wear particles on the surface, resulting in a lower wear volume.
Finally, Fig. 15c presents the merit index that relates the volume loss, wear rate, and sliding distance. It is possible to observe that the coated substrate demonstrated a better set of properties in the four characteristic stages of the tribological test; since it presented a lower volume and a lower wear rate, the coating reduces friction and particle detachment, which improves the tribological properties of the material and increases the useful life of the device. In this way, it was corroborated that this coating has optimal properties and great potential to be implemented as a protective coating on engineering devices and to improve their production cycle.
Fig. 15
Correlation of properties as a function of sliding distance: a roughness, wear volume, b coefficient of friction, wear volume and c wear volume and wear rate
×
5 Conclusion
By means of XRD results it was possible to determine that the Si3N4 coating with a stoichiometric ratio of 57/43 presented a hexagonal structure with a preferential growth in the crystallographic plane (311) and a displacement of the peak of maximum intensity due to the residual stresses derived from the deposition process. In addition, XPS analysis determined a stoichiometric Si/N ratio a 57/43.
The measurement of the wear tracks as a function of the sliding distances determined for the uncoated substrate (H13 steel) a maximum depth of 25 μm. But, for the coated substrate (Si3N4/H13 Steel) the maximum depth was 4 μm for 1000 m. In addition, it was possible to determine that at 750 m of sliding, the wear track presented a depth of 2.2 μm, which indicated that at this sliding distance there was a complete fracture of the coating and the tribological pair was modified, causing a significant change in the friction coefficient due to the fact that the contact surfaces after this distance was (WC counterpart/abrasive particles/H13 steel).
Finally, the trend of the wear volume, wear rate, width and depth of the wear track curves showed higher values for the substrate (H13 steel) compared to the coated substrate (Si3N4/H13 steel), indicating the significant improvements brought by the modification of the surface properties such as high hardness, high elastic modulus and low friction coefficient. In addition, the trend curves for the coated steel showed an abrupt change after 750 m of sliding distance, due to the total fracture of the coating, leaving the steel exposed to increased wear in this contact zone.
Acknowledgements
This research was supported by the Tribology, Polymers, Powder Metallurgy and Solid Waste Transformation (TPMR) research group of the Universidad del Valle; And the Universidad del Valle through project C.I 21218.
Declarations
Conflict of interest
The authors declare no competing interests.
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