Relationship of the structural, mechanical and tribological behavior among TiCN, BCN, and CrAlN coatings in the fatigue life of AISI 1045 steel for automotive industry

doi:10.21203/rs.3.rs-2507891/v1

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Relationship of the structural, mechanical and tribological behavior among TiCN, BCN, and CrAlN coatings in the fatigue life of AISI 1045 steel for automotive industry

https://doi.org/10.21203/rs.3.rs-2507891/v1

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published 24 Apr, 2023

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This work links the relationship between the structural, morphological, mechanical and tribological behaviors with the rotary fatigue life performances of AISI 1045 steel substrates coated with Titanium Carbo-nitride (TiCN), Chromium Aluminum Nitride (CrAlN) and Boron Carbon Nitride (BCN) through PVD. Appropriate conditions of the deposition of the coatings allowed to obtain improved properties. These coatings were deposited on AISI 1045 steel substrates. Singular layers of the different coatings were used, with a stable layer thickness of ~ 3 µm. The influence of the coating type on the crystal structure, bond energies, and microstructural properties were established by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Mechanical properties such as hardness and modulus of elasticity of the coatings were determined by nanoindentation. Tribological characterization techniques were used to determine friction coefficients. For the rotary fatigue test, the following ratios, 70%, 65%, 60% and 55% of the ultimate stress value, were chosen to determine the applied stress levels. For all three coatings an increase in fatigue strength was found: 9.625% for TiCN, 4.215% for BCN and 3.954% for CrAlN. The increase in mechanical properties together with the lower presence of residual stresses in the coatings justifies the increase in fatigue resistance.

Physical Vapor Deposition PVD

Fatigue

Residual Stress

Carbo-nitrides and nitrides transition metals (TiCN

BCN

CrAlN)

It was sought to evaluate the behavior of coatings obtained by physical vapor deposition (PVD), which would provide a better fatigue behavior. Microstructural, morphological, mechanical and tribological characterizations were carried out, as well as rotary fatigue tests with their respective fracture surface analysis. This was done with the purpose of relating the improvement of the performance and efficiency of a given mechanical component with the increase of its properties, and the influence of the coating material on the residual stresses produced. The biaxial deformation or presence of in-plane residual stresses was affected by the type of coating material. This phenomenon has been analyzed by several authors, obtaining positive results and results in which the coating layers have a lower influence (damage) on the fatigue resistance [1–7]. The four-stroke Otto engine is predominantly used as a driving machine for passenger cars and heavy motorcycles. Taking the above into account, the forces and stresses acting on the engine can be determined. A combustion gas pressure of 30 bar to 60 bar acts on the piston head; this is equivalent to forces of 15000 N to 30,000 N exerted on the piston with diameter of 80 mm, respectively [8]. Due to the action of different loads and stresses, engine components are considered to be subject to fatigue. In order to meet and satisfy increasingly stringent emission control standards, operating conditions, as well as higher fuel efficiency and durability requirements in the automotive industry, great efforts have been directed to the study and design of low friction materials and coatings in recent times. Currently, some materials are available that can provide extremely low wear rates and friction coefficients in super-lubricated sliding regimes, i.e. less than 0.01 [9]. The use of new surface treatments together with other types of coatings on various types of substrates, whether metals or ceramic materials, to study fatigue behavior has given efficient results. A complete study of a machine element must have an analysis by static failure and by variable or fluctuating load, otherwise, fundamental parameters for its correct operation in any situation could be ignored.

The magnetron sputtering (MS) method uses a momentum transfer mechanism for the atomization of the coating material. The equipment used has four guns or magnetrons (Torus − 4", 10 cm Kurt J. Lesker), facing and equidistant to the central point where the tool to be coated is positioned. The magnetrons are 10 cm in diameter each. It also has three radio frequency sources (13.56 MHz, RFX 600A), three direct current sources (MDX 500, Advanced Energy), a pressure measurement and control system (Baratron, MKS), a four-channel gas flow controller, a system for radiation heating from room temperature to 400°C (Athena 500), an in-situ thickness gauge with quartz head and two sample holder systems i.e. rotational and planetary (Figs. 1a-b). For TiCN, BCN and CrAlN coatings, titanium carbide-titanium (TiC-Ti), boron carbide-titanium (BC-Ti) and chromium-aluminum (Cr-Al) targets were used, respectively; each with a purity of 99.99%. Four silicon samples with a crystallographic orientation (100) and nine AISI 1045 steel samples were used per coating (coin type, 12.7 mm diameter and 5 mm thick) for mechanical and tribological characterization. The samples were subjected to a surface cleaning process of organic contaminants; the sequence started with a cleaning in a solution of 50% hydrogen peroxide and 50% ammonia, then drying and finally in an ultrasound system in a sequence of alcohol and acetone for each cycle. The equipment used for crystallographic structure determination of the TiCN, BCN and CrAlN coatings was an X-ray diffractometer (PANalytical X' pert pro™) using a Cu Kα radiation source with a beam wavelength of λ = 1.5405 Å, an accelerating voltage of 40 kV and a current of 50 mA. Details of the chemical composition, as well as information on the binding energies, were obtained with X-ray photo-electron spectroscopy (XPS), for which a SAGE HR100 (SPECS™) equipment was employed using a monochromatic source (Mg Kα 1253.6 eV). The ratio of areas enclosed under each peak for the different elements established the percentage of each one of them and for this purpose the CasaXPS V2.3.15dev87TM software was used. The mechanical characterization by nanoindentation was done using an Ubi1- Hysitron™ device, with a Berkovich diamond tip at varying loads. The hardness and modulus of elasticity for the three coatings were acquired with an analysis of the data obtained from the depth or penetration of the indentation when acting under increasing and decreasing applied loads (loading and unloading curves). Tribological characterization was carried out under ASTM G99-17 [10]. A Microtest MT 4001-98 tribometer was used with a 6 mm diameter 100Cr6 steel pin as the sliding pattern, an applied load of 0.5 N over a total travel length of 1000 m, and an angular velocity of 160 rpm. For this work the thicknesses, morphology, and wear tracks of the coatings were verified and determined with a JEOL Model JSM-6490LVTM (Phenom FEI) scanning electron microscope (SEM). Standard specimens of AISI 1045 steel (ASTM Standard 466 − 15) [11] were machined for fatigue tests.

The tensile test (Standard E8/E8M-13a) (see Fig. 2a) [12] was fundamental for the programming of the fatigue test protocol. The properties obtained were modulus of elasticity (E), yield strength (Sy), ultimate strength (Sut) and fracture strength (Srot). One hundred and forty specimens were prepared for the rotary fatigue tests. Calibrated AISI 1045 steel bars were used and then machined on a CNC lathe to the dimensions specified in ASTM 466 − 15 and ASTM E606/E606M – 12 (see Fig. 2b) [11, 13]. Paper grits: 240, 280, 400 and 600 were used for the sanding procedure; finally the samples were polished with diamond paste (range 2 µm − 4 µm, mesh size #8000) for the final finish. The verification of roughness in all sections of the samples was done with the purpose of ensuring a correct deposition process of the coatings (TiCN, BCN and CrAlN) on the total surface of the samples. It was established that the average roughness (Ra = 0.044 µm) was below 0.2 µm, complying with the specifications of the standard used. The rotary fatigue test simulated a tribological condition where there is interaction of the assembly (metal-ceramic) related to the metal surface and the hardened steel pin coating in a dry and lubricated medium. A real application of this condition is the piston pin assembly (piston-piston pin) inside an internal combustion engine (Fig. 2c).

It is established that the S-N (stress - number of cycles) curve has finite and infinite life ranges. The sloping part of the curve (design for finite life) is generally the fundamental design criterion. The region between the finite and infinite life ranges is the transition range in which the fatigue strength limit is determined, related to the number of technological cycles (Ng), which is located between 10⁶ and 10⁷ cycles in the case of steels. For this test we worked with high cycle fatigue (stresses greater than 10³ cycles) and the value of Ng = 1x10⁶ cycles was considered as the boundary between the regions of finite and infinite life [2, 14–16]. The number of samples to evaluate at each stress level was five; therefore, approximately thirty samples were used for the construction of each type of S-N curve (AISI 1045, TiCN, BCN and CrAlN steel). A percentage error of no more than 15% was established for each stress level, using a 90% confidence level. Standards ASTM 468 − 11 and ASTM 739 − 10 were used for the representation of the results [17, 18]. The points on the graphs represent the experimentally obtained data and the solid lines represent the trend lines. Coating thicknesses were obtained using a KLA Tencor D-120 profilometer; the unified thickness value for all coatings was 3 µm. Previous research established suitable conditions and helped determine parameters (deposition time) for this work [19–21]. The fatigue testing equipment HUNG TA Instrument Cia. Ltda., Model No./Serial No.: HT-8129/2031 (Figs. 3a-c), was used to test samples subjected to pure bending. The stress ratio for the machine was Re = -1. The tests were carried out at 3600 rpm, with a frequency of 60 Hz.

3.1. X-Ray Diffraction Analysis

Previous research established optimum deposition conditions so that the hardness and adhesion of the coatings to the substrate would be adequate to obtain a good performance of the coatings, which implies the presence of lower residual stresses in the three coatings [7, 19–21]. From the analysis of the X-ray diffraction (XRD) patterns or diffractograms of the TiCN, BCN and CrAlN coatings, it was established that the three materials crystallized forming a NaCl-type FCC structure with space group Fm3m [4, 5]. Table 1 shows the lattice parameters and the full width at half maximum (FWHM) of the patterns. The results obtained for the TiCN, BCN and CrAlN coatings were 4.278 Å (SD = ± 0.096), 3.622 Å (SD = ± 0.028) and 4.334 Å (SD = ± 0.137), respectively.

Table 1

Lattice parameters and FWHM values of TiCN, BCN and CrAlN coatings, calculated from diffractograms
Lattice parameter and FWHM of the three coatings
Coating type	TiCN (111)	BCN (111)	CrAlN (200)
Ángulo 2θ (°)	36,342	43,228	41,646
Ángulo θ (°)	18,171	21,614	20,823
Parámetro de red a_o (Å)	4,278	3,622	4,334
FWHM (rad)	0,8112 ± 0,0028	0,28978 ± 0,00192	0,29223 ± 0,00121

Figure 4a, shows the influence of the coating material on the FWHM; a decrease in peak width of the order of TiCN, CrAlN and BCN with values of 0.811, 0.292 and 0.289, respectively, was presented. The lateral crystallite size relationship as a function of coating material is shown in Fig. 4b. Eq. 1, called Scherrer's equation, is used to determine the crystallite size [7, 19, 22].

$${\text{D}}_{{\nu }}=\frac{\text{K}{\lambda }}{{\beta }\left(\text{cos}{\theta }\right)}$$

where D_ν is the crystallite size (Å), K is Scherrer constant that takes values between 0.81 and 1 (K = 0.9 was assumed for the present case), λ is the wavelength (1.5405 Å), β is the full width half maximum FWHM value in 2θ (rad), and θ is the Bragg diffraction angle (rad).

Based on the crystallite size results given in Fig. 4b, it can be observed that the TiCN coating has the lowest value, while the BCN coating has the highest value of lateral crystallite size; it can be established that TiCN presents greater obstacles for free crystallite growth compared to CrAlN and BCN. Previous studies determine mechanical and morphological properties of monolayers and multilayers that support these results for TiCN and CrAlN [7, 19, 20, 23, 22].

The influence of the type of coating material on the lattice deformation and biaxial deformation (biaxial stress) is shown in Fig. 5. Specifically, Fig. 5a presents the lattice deformation as a function of the coating material; Eq. 2 was used to determine this lattice deformation [24, 22].

$$\text{T}\text{tan}{\theta }=\frac{{\lambda }}{{\text{D}}_{{\nu }}\left(\text{cos}{\theta }\right)}-{\beta }$$

where T is the deformation of the lattice (Å).

From Fig. 5a, it can be concluded that all the deformations in the coatings were positive, producing tensile type stresses; a larger deformation was present in the TiCN coating, while a smaller lattice deformation was exhibited by the BCN coating. This agrees with the small shifts of peaks (111) and (200) to lower 2θ angles [5, 22]. Similarly, Fig. 5b presents the biaxial deformation (stress) in relation to the coating material, the deformation was determined by Eq. 3.

$${\epsilon }=\left(\frac{{\text{C}}_{\text{f}\text{i}\text{l}\text{m}}-{\text{C}}_{\text{b}\text{u}\text{l}\text{k}}}{{\text{C}}_{\text{b}\text{u}\text{l}\text{k}}}\right)$$

Where ε is the biaxial deformation (stress), C_film (a_o real) is the constant or lattice parameter of the layer in orientation (111) or (200), given in the international index files (JCPDF) (Å) and C_bulk (a_o calculated) is the lattice parameter corresponding to the monolayers grown with the different types of materials (Å).

The data obtained in the international index files (JCPDF) are considered to have no biaxial stress; under this consideration the biaxial stress produced in the deposited coatings can be calculated. By analyzing the results of Fig. 5b it can be established that the biaxial deformation in the planes varied according to the type of coating material. The values obtained are tensile type stresses in the following order: ε = − 3.1794x10^− 3 for TiCN, ε = − 0.0110 for BCN and ε = -0.0447 for CrAlN.

3.2. X-Ray Photoelectron Spectroscopy

Table 2 shows the related X-Ray Diffraction and X-Ray Photoelectron Spectroscopy (XPS) results for all coatings [22]. For the XPS analysis, complete spectra were obtained for all three materials (see Fig. 6) [22]. The presence of high intensity peaks in each of the spectra allowed locating and defining the respective binding energies, facilitating the verification of the chemical composition and stoichiometry of the ternary layers. For TiCN: Ti (2p_1/2) = 458.4, N (1s) = 396.80, C (1s) = 284.8 and Si (2p) = 61.6; for BCN: N (1s) = 400, C (1s) = 285.6 and B (1s) = 192.8; and for CrAlN: Cr (2p) = 475.99, N (1s) = 396.97, Al (2s) = 119 and Al (2p) = 74. Small amounts of oxygen can be evidenced in the graphs of the three coatings due to contamination from the presence of impurities. However, the low amount did not alter the stoichiometry and therefore did not affect the characteristics of the three coatings. Thus, the binding energy values found in the coatings together with the XRD results agree with the information reported in the references, confirming the formation of the three ternary compounds. [20, 25–27]. Finally, the stoichiometry for the coatings was determined: Ti_32.45-C_35.83-N_31.72, B_48.63-C_31.22-N_20.15 and Cr_40.27-Al_38.01-N_21.72.

Table 2

Stoichiometric ratio and crystallographic characteristics for the three types of coatings TiCN, BCN and CrAlN
Coatings	Stoichiometric relation (XPS)	Crystalline structure (XRD)	Spacial group (XRD)	Lattice parameter a_o (Å) (XRD)
TiCN	Ti₃₂C₃₆N₃₂	FCC	225-Fm3m	4,278
BCN	B₄₉C₃₁N₂₀	FCC	227-Fd3m	3,622
CrAN	Cr₄₀Al₃₈N₂₂	FCC	225-Fm3m	4,334

4.1. Nanoindentation Test

Figure 7 shows typical indentation load-displacement load curves of the coatings using a Berkovich-type indenter. The Oliver and Pharr method was used to obtain the hardness (H) and elastic modulus (E). The influence of the ternary coatings on the hardness and elastic modulus can be seen, as well as the material with the lowest values of mechanical properties, which was TiCN (H = 28 GPa, E = 224 GPa), followed by CrAlN (H = 30 GPa, E = 335 GPa) and the highest values corresponding to BCN (H = 33 GPa, E = 251 GPa). According to the above, it can be established that the elastic modulus (E) was related to the type of coating material, but not to the microstructure of the material. Under this condition, the values of (E) depended on the crystalline structure and the lattice parameter [5]. On the other hand, the hardnesses obtained from the three coatings were due to the different crystallographic directions determined from the XRD analysis. It was possible to determine that the TiCN, BCN and CrAlN coatings revealed a preferential orientation in the (111), (111) and (200) planes, respectively. It is emphasized that the hardness, due to the presence of interstitial nitrides in transition metals, depended on the crystalline orientation. In general, the (111) and (200) orientations presented high hardness values. Therefore, BCN coatings show a higher hardness in comparison to TiCN and CrAlN.

4.2. Pin on Disk Technique: Non-lubricated and Lubricated Environments

Figure 8a shows the friction coefficient values of the tribological pair AISI 1045 steel substrates with TiCN, BCN and CrAlN coatings and the 100Cr6 steel counterpart, in lubricated and non-lubricated environments. There are two typical characteristic stages in the Pin-on-Disk test. Stage I, known as the start-up period, is related to the interference of the friction mechanism due to the initial surface contact associated with the counterparts. In this stage, high values of friction coefficient were produced. In stage II, the friction coefficient presents a settling period known as running-in, producing fracture of the roughness on the ball surface, which together with the failure of the coating can generate wear particles called debris. Figure 8b shows the different friction coefficients for steel and all TiCN, CrAlN and BCN coatings as a function of unlubricated and lubricated environments, respectively, with the BCN coating showing a lower friction coefficient. For the non-lubricated environment, different values of friction coefficient of the tribological pairs were obtained such as Steel - Steel = 0.82, Steel - TiCN = 0.74, Steel - CrAlN = 0.66 and Steel - BCN = 0.60. On the other hand, the coefficient of friction values obtained from the tribological pairs in a lubricated environment were Steel - Steel = 0.256, Steel - TiCN = 0.235, Steel - CrAlN = 0.231 and Steel - BCN = 0.208. Using the latter results, Archard's surface interaction model states that the material with the best elastoplastic characteristics (hardness H, elastic modulus E) along with lower roughness has the lowest coefficient of friction. Comparing the results, a decrease in the friction coefficient was observed for the pairs in lubricated environments relative to pairs in non-lubricated environments. Analyzing the uncoated steel, TiCN, CrAlN and BCN coated steel, the values were 68.8%, 68.2%, 65% and 65.3% lower, respectively [5].

5.1. Tension and Hardness Tests

In order to plan the fatigue test and verify the material state of supply, tensile tests were carried out to determine certain mechanical properties of the substrate, as well as to determine hardness and surface roughness on cylindrical samples. All tests were carried out at room temperature. First, the tensile test was performed on AISI 1045 steel specimens (Standard E8/E8M-13a) (see Fig. 2a) [12]. The results obtained in the tensile test and Rockwell C (HRC) and Vickers (HV) hardness are shown in Table 3 [28]; three specimens were machined with the calibrated AISI 1045 steel (provision condition). The yield stress was Sy = 787.783 MPa and the ultimate stress was Sut = 871.624 MPa. The Sy/Sut = 0.904 ratio for the material helped us to determine the stress ratios for the fatigue test, which had to be less than or equal to the established ratio; in this way we ensured that the load levels with which we worked in fatigue remained within the linear-elastic regime.

Table 3

Tensile and hardness test results for calibrated AISI 1045 steel (supply condition)
Samples	E (GPa)	S_y (MPa)	S_ut (MPa)	S_rot (MPa)	Hardness HRC	Hardness HV
AISI 1045	204.563	787.783	871.624	717.045	21.913	403.375
Standard Deviation	± 29.662	± 13.625	± 22.260	± 10.691	± 3.517	± 21.725

5.2. Fatigue Experimental Data

The fatigue test used the service life model, called S-N curve or the Wöhler diagrams [1, 29, 30] in which the y-axis Sa represents the maximum alternating stress, and the x-axis N is the number of cycles to failure. The fatigue tests were performed on a HUNG TA model rotary bending machine. The equivalent load diagram (W) for the fatigue machine was used to establish the shear force (V) and bending moment (M) diagrams necessary for the determination of the maximum bending moment (see Figs. 3a-c). Eq. 4 was derived from the bending normal stress equation, also considering that the critical cross-section of the moment of inertia of was circular with a diameter ds = 6.35x10^− 3 m. This analysis, together with the applied stress levels, was necessary to define the loads used for the fatigue test. 70%, 65%, 60% and 55% of the ultimate stress value were selected as the applied stress levels [1, 2, 14–16, 29]. The following was established as the protocol to perform the fatigue test: the tests were considered valid when exceeding the number of technological cycles established in 1x10⁶ (for infinite life) or when the sample failed before reaching this number of cycles, Ng (for finite life). On the other hand, as a criterion for approval of a test, it was established that those samples in which the failure took place in the critical section would be validated, otherwise they would be discarded.

$\text{S}=\frac{\left(\text{M}\right)\left(\text{c}\right)}{\text{I}}$ → $\text{S}=\frac{\left(\frac{\text{W}\text{a}}{2}\right)\left(\frac{\text{d}\text{s}}{2}\right)}{\frac{{{\pi }\left(\text{d}\text{s}\right)}^{4}}{64}}$ → $\text{S}=\frac{16\left(\text{W}\text{a}\right)}{{{\pi }\left(\text{d}\text{s}\right)}^{3}}$ → $\text{W}=\frac{{\pi }\left(\text{S}\right){\left(\text{d}\text{s}\right)}^{3}}{16\text{a}}$ (4)

Where S = normal bending stress (N/m²), M = maximum bending moment (Nm), c = distance of the element furthest from the neutral axis (m), I = moment of inertia of the area with respect to the reference axis (m⁴), W = applied weight (N), a = fixed distance (0.2 m), and ds = diameter of the cross section (6.35x10^− 3 m).

All curves for the uncoated steel and with the three coatings (TiCN, BCN and CrAlN) are presented in Fig. 9 [28]. In all three cases an increase in fatigue strength was found. The TiCN coating presented the highest improvement, followed by BCN and the lowest was CrAlN. This could be supported by the presence of titanium in the buffer layers where titanium carbide (TiC) and titanium (Ti) were used respectively; as well as in the final monolayer of TiCN, providing good adhesion in the coatings and the improvement in fatigue resistance. Therefore, it is possible to affirm that the biaxial deformation in the planes varied according to the type of coating. The values obtained showed the existence of tensile type stresses, the lowest of those calculated corresponding to the TiCN coating [5]. The analysis of the XRD patterns determined that these materials formed a NaCl-type FCC structure, therefore, Ti and B atoms tended to be located at the Wyckoff 4a site, while C and N atoms were randomly located at the Wyckoff 4b site. The presence of titanium with a high atomic radius (r_a = 2Å) and boron with an intermediate atomic radius (r_a = 1.17Å) in both structures influenced the increase of mechanical properties such as hardness and elastic modulus, as well as the improvement of fatigue resistance. The increase of these mechanical properties accompanied by an excellent adhesion of the coatings to the substrate, secondarily caused a delay in the process of nucleation and propagation of fatigue cracks [5, 31, 32]. For the CrAlN coating, titanium was not present, both in the intermediate layer and in the final layer, so it had intermediate mechanical properties compared to the other coatings, which is linked to its low fatigue resistance.

Scanning electron microscopy (SEM) was used for the analysis of fracture surfaces of TiCN, BCN, CrAlN coated and uncoated samples subjected to fatigue. The failure analysis of the samples' surfaces only included lower maximum alternating stresses; i.e., it was performed at a stress level of 55% of the ultimate stress value. The purpose of the study of these fracture surfaces was to explain the fracture process of the substrate-coating systems and particularly to identify or predict the development by which fatigue cracks propagated after nucleation [32]. Figures 10a-c show the fracture surfaces of the TiCN-coated substrate tested at low stresses. Theory states that at lower maximum alternating stress, the final fracture zone area is smaller than the fatigue zone, as seen in Fig. 10a, which reveals the failure surface of a specimen tested at 479.393 MPa and failed at 510838 cycles. It can be assured that the crack responsible for the fracture of the specimen had its nucleation initiation in the area marked with A, and therefore the fracture zone is located at the opposite end. A close-up view of the crack initiation site is established in Fig. 10b, in which the crack propagation direction is clearly defined by the white arrows and, analogously to the previous analysis, indicates that its origin was at the specimen's contour. In Fig. 10c, the detail of the beginning of the crack formation site, the place where the crack probably nucleated, is identified where the partial delamination of the coating can be observed.

The fracture surfaces for the BCN-coated steel tested at low stresses are presented in Figs. 11a-c. Figure 11a, shows the fracture surface of a specimen tested at 479.393 MPa and failed at 462383 cycles. A wider fatigue zone is observed compared to the final fracture zone. It can be deduced that the fracture occurred had its beginning in the nucleated crack at site A. Additionally, there is the presence of other cracks that could have helped the occurrence of failure. The location of the fracture zone is on the opposite side of the surface and there is a part with a non-flat appearance (atypical), perhaps due to the presence of some discontinuity in the sample. Detailed images of the crack initiation site in Figs. 11b and 11c show the direction of crack expansion and the origin of the crack on the surface contour.

For the substrate with the final CrAlN coating, the fracture surfaces are visualized in Figs. 12a-c, analyzed at low stresses. Figure 12a, records the fracture surface of a specimen tested at a maximum alternating stress of 479.393 MPa (low stress) and failed at 252731 cycles. Similar to the high stress case, it is observed that the fracture was the product of some cracks, whose cores are located in the contour of the section and the fracture zone is located in the center of the surface. The area of the fatigue zone is smaller compared to the fracture zone. An enlarged view of detail A is shown in Fig. 12b, where the initiation of one of the cracks is observed, the white arrows denote the direction of the cracks and the subsequent crack expansion. Figure 12c indicates the crack nucleation site and final fracture of the sample.

Finally, for comparison purposes, Figs. 13a-c show the fracture surfaces of uncoated AISI 1045 steel tested at low stresses. Under lower maximum alternating stresses, the rough-looking final fracture zone is small compared to the stable crack growth zone that presents a smooth and fine grained appearance, evidenced in Fig. 13a. The image shows the fracture of a specimen tested at 479.393 MPa and failed at 149490 cycles. The failure in the specimen appears to be the result of a single crack, which initiates in the area labeled A, and the fracture zone is displaced to the other end of the specimen. Figure 13b shows a magnified view of the crack initiation area, the white arrows indicate the direction of crack displacement, and its origin is again located at the contour of the section. Figure 13c indicates the enlarged area B, showing in detail the probable crack initiation site.

It can be stated that the results found in the fracture surfaces comply with the established theory, they had their beginning in irregularities of the material when the cyclic stress was maximum. The fracture surfaces were flat and normal to the stress axis, they followed a usual sequence: stage I where the fracture had its beginning in nucleation sites found in the periphery, stage II where micro-cracks formed parallel surfaces in the form of plateaus, which are known as beach marks, and stage III, consisting of a sudden and rapid fracture. The results are in agreement with the information and studies reported in references with similar deposition processes and types of coatings [1, 2, 14–16, 29].

Figures 14a-b show the merit indices [33] that relate fatigue strength, microstructural and mechanical properties of the coating as a function of the material type, such that the increase in fatigue strength (%), biaxial deformation, hardness (GPa) and the coating material were the components of the merit indices based on the coating type. From this correlation, Fig. 14a shows an increase in biaxial deformation and also determines a relative decrease in fatigue strength as a function of the type of coating used. The TiCN coating had the lowest tensile stress value and high fatigue strength. In Fig. 14b, a merit index linking the increase in coating hardness and a relative decrease in fatigue strength (%) for the same coating type is shown. Consequently, the TiCN coating presented a better association for mechanical and fatigue properties with excellent hardness and high ultimate alternating tensile strength (Sa). This is relevant for applications in machine elements, e.g. automotive industry with high fatigue demands in service conditions.

Likewise, Figs. 15a-b show the merit indices linking fatigue resistance and the tribological properties as a function of the material type, such that the increase in fatigue resistance, coefficients of friction with and without lubrication, and coating type (material) were the constituents of the merit indices. Figures 15a-b present a reduction in fatigue strength and a decrease in the coefficient of friction found in both cases without and with lubrication, respectively, as a function of the type of coating used. In this case there was a decrease in fatigue resistance and an improvement in tribological properties. The points of intersection of the curves determined the coating that presented a relative increase in fatigue resistance and relatively low coefficients of friction (without and with lubrication). For both cases as shown in Figs. 15a-b it is observed that the titanium carbo-nitride (TiCN) coating had high fatigue resistance with low coefficients of friction.

Adequate preparation and deposition conditions were determined to obtain TiCN, BCN and CrAlN coatings. By means of diffraction analysis (XRD), chemical composition results (XPS), tribological characterization and nanoindentation tests, it was determined that the crystallographic structure, chemical and stoichiometric ratio and wear tests had an impact on the improvement of the mechanical properties (hardness and elastic modulus). Therefore, the increase in the mechanical properties of the coatings (TiCN, BCN and CrAlN) indirectly caused a delay in the fatigue crack nucleation process, thus improving the fatigue resistance.

There was an increase in fatigue strength for all coated samples in the following order: 9.625% for TiCN, 4.215% for BCN and 3.954% for CrAlN, determined for N = 1x10⁶ cycles, while microstructural characterization results showed tensile type stress values in the following order: ε =- 3.1794x10^− 3 for TiCN, ε = -0.0110 for BCN and ε = -0.0447 for CrAlN; which indicates the lower presence of residual stresses in TiCN, followed by BCN and CrAlN. The analysis of this correlation justifies the increase in fatigue strength in the defined sequence.

The analysis of the fracture surfaces in the tested specimens (both coated and uncoated), served to reveal the development by which the cracks diffused after nucleation producing fatigue failure. It was determined that the nucleation points appeared at the periphery of the surface, these acted as crack growth centers and in general the fracture zone was located at the opposite end of the nucleation point.

In applications with high stresses due to dynamic loading under service conditions, the study of coatings additionally requires the examination of fundamental properties such as fatigue resistance. Improvements in microstructural, tribological and mechanical properties in these materials will be useful for their application in coatings for substrates used in industrial applications that work under these requirements (e.g. automotive industry).

Funding

This research was supported by Universidad Militar Nueva Granada, Bogotá, Colombia; Asistencia Técnica a la Industria del Servicio Nacional de Aprendizaje (CDT-ASTIN-SENA), Cali, Colombia; Universidad de las Fuerzas Armadas ESPE, Sangolquí, Ecuador y Universidad Técnica de Ambato, Ambato, Ecuador.

Competing Interests

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.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Juan Francisco Correa Jácome and Julio César Caicedo Angulo. The first draft of the manuscript was written by Juan Francisco Correa Jácome and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Relationship of the structural, mechanical and tribological behavior among TiCN, BCN, and CrAlN coatings in the fatigue life of AISI 1045 steel for automotive industry

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Journal Publication

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Abstract

Figures

1. Introduction

2. Material And Experimental Procedure

3. Microstructural Characterization

3.1. X-Ray Diffraction Analysis

3.2. X-Ray Photoelectron Spectroscopy

4. Mechanical And Tribological Characterization

4.1. Nanoindentation Test

4.2. Pin on Disk Technique: Non-lubricated and Lubricated Environments

5. Fatigue Tests

5.1. Tension and Hardness Tests

5.2. Fatigue Experimental Data

6. Fracture Surfaces

7. Correlation Between Microstructural, Mechanical, Tribological And Fatigue Properties

8. Conclusions

Declarations

References

Status:

Journal Publication

Version 1