Effect of compound surface modification of shot peening and DLC coating on surface integrity and fatigue properties of gear steel 16Cr3NiWMoVNbE

Abstract

We conducted a study on the surface compound modification of shot peening and pure carbon DLC coating to simultaneously meet the requirements of wear resistance and fatigue resistance of spline structure. The effects of surface compound modification were investigated on the surface morphology, residual stress profile, microstructure, and nano-indentation hardness of 16Cr3NiWMovNbE gear steel, and conducted a comparative study on fatigue performance. The results show that the surface compound modification inherits the surface morphology and compressive residual stress gradient of shot peening, while the surface residual stress is slightly smaller than that of shot peening. In addition, surface compound modification still reflects the characteristics of high hardness and high fracture resistance of DLC coatings. Under the bending load based on spline tooth root, compared to the original specimen, the fatigue life after shot peening, pure carbon DLC coating, and surface compound modification is increased by 3.68, 2.35, and 3.36 respectively. Although the compound modified surface still maintains the shot peening morphology with a increasing surface roughness and stress concentration coefficient, the 100 μm-depth compressive residual stress profile and the subgrain refinement layer introduced, as well as the hard surface layer with good load-bearing capacity, have played the role of fatigue strengthening.

1. Introduction

Spline is a transmission structure extensively utilized in the machinery industry, which is mostly made of gear steel [1,2]. At present, with the development and improvement of many mechanical vehicles such as aero-engines and railway engines, higher requirements for transmission power are raised, resulting in insufficient durability and many breakdown problems. Xiao et al. [3] believe that fretting wear is the chief cause of failure from the analysis of a misaligned coupling used for helicopter and aero-engine power transmission components. Monti et al. [4] analyzed the cause of involute spline failure of a 15-ton bridge crane and concluded that wear was the main cause of failure, while surface treatment that improved the stiffness of mating parts could substantially incorporate into the reduction of the wear. According to this point of view, wear is believed to be the main cause of spline failure. Ratsimba et al. [5] and Zhao et al. [6] have developed suitable models to predict spline wear failure, which show that the wear distribution strongly depends on the friction coefficient.

Commonly, the industry utilizes the processes of nitriding [7], carburizing [8], or carbonitriding [9] to improve the hardness, to play the role of wear resistance. At present, diamond-like carbon (DLC) coatings [10] with excellent properties such as ultra-low friction [11,12], high hardness [13], and high wear resistance [14] are being developed rapidly and are also becoming a crucial choice for the mechanical industry to enhance wear resistance [15]. However, compared to the metallurgical bond of the carburized case, the bond strength of the interface between the coating and the substrate is insufficient. Therefore, many investigations have been conducted, including doping [16,17], gradient coating transfer [18], and pre/post-treatment coating [[19], [20], [21], [22]] to improve adhesion and film properties. Based on the above adhesion and performance improvement, the excellent DLC coating can be utilized as an appropriate approach to improve the wear resistance of the spline structure.

Aeroengine splines are commonly acted upon by a broad range of stresses during service life. In fact, the spline structure has the risk of fatigue failure in addition to wear. Tjenberg [23] performed extensive analysis on the service load of the spline structure and developed a simple model to calculate the approximate load and relative life on each tooth. Liu [24] analyzed the failure problem of the active bevel gear, measured the tooth thickness and root fillet, and used finite element analysis method to simulate and analyze the force on the broken teeth of the bevel gear. The results indicate that the presence of machining tool tip edges at the tooth root during the machining process of gears results in a change in the stress distribution at the tooth root, leading to stress concentration and bending fatigue failure. Based on the spline fatigue test machine, Lin et al. [25] conducted fatigue research on spline couplings under simulated service conditions, which improved the understanding of the fatigue behavior of spline structures. Hyde et al. [26] employed the test device to compare the damage mechanism of the test piece with the real spline, and considered that the test process could effectively reflect the fretting fatigue in the spline contact area and the tooth bending fatigue along the tooth angle, thus providing an economical approach for developing and testing spline couplings. Since it is expensive and time-consuming to manufacture and test spline test pieces, the simplified representative test concept is implemented. In this regard, Houghton et al. [27] utilized a simulated test piece to perform the fretting fatigue test instead of the spline, and the effect of spline rotating bending moment on the total fatigue life. Shot peening is an anti-fatigue methodology to obtain surface elastic-plastic strengthening layer by beads impact. Many investigators have proven that shot peening is capable of effectively improving bending fatigue [28] and fatigue properties [29]. Zhang et al. [30] displayed that shot peening could enhance the fatigue properties of Ti811 alloy at 350 °C, but due to residual stress relaxation, it is unable to improve its fretting fatigue properties at 500 °C. Yang et al. [31] investigated the effect of shot peening on the wear fatigue performance of TC4 alloy, which showed that the layer strengthened by shot peening could alter the main mechanism of crack initiation and substantially enhance its fretting fatigue performance.

The compound modification of shot peening and DLC gradient coating in spline is a potential approach for spline surface modification. First, the residual compressive stress field is introduced to enhance the surface integrity of gear steel to improve the fatigue resistance, and then the DLC gradient coating with high surface hardness is utilized to achieve wear reduction and lubrication, where no literature reports have been seen before. However, there is no consensus as to whether the combination of shot peening and hard film could be incorporated into the enhancement of the fatigue performance. Zhang et al. [32] conducted research on the combined modification of shot and TiN hard film of titanium alloy and realized that the fretting fatigue performance of composite modified titanium alloy was lower than that of shot alone due to the residual stress relaxation aroused from thermal effect during the preparation of TiN hard film. However, Yang et al. [33] showed that for titanium alloys, fatigue properties are improved after shot peening and soft CuNiIn coating compound treatment. By comparing the above results, the authors first believe whether the capabilities of the compound surface modification in fatigue strengthening are closely related to the properties of the outer film. Additionally, bending fatigue is also a spline fatigue failure mode [26], and due to gradient loading, it is most sensitive to surface conditions. For this reason, herein, the DLC gradient coating with various properties and the shot peening strengthening process have been combined and modified, and a comparative study on the modified layer structure and bending fatigue performance has been conducted. The main goal is to examine the application of this kind of compound surface modification in the spline structure and provide technical guidance for reliable anti-fatigue and wear resistance services of the spline structure.

2. Materials and method

2.1. Materials and sample

16Cr3NiWMoVNbE gear steel, the most extensively utilized steel of the spline, is the subject material of the present investigation. The components and mechanical properties of the steel bar are presented in Table 1, Table 2, respectively, which come from the test results of the manufacturer. Before processing the samples, the bar was subjected to the following heat treatment: 910 °C/1h, oil-cooled, and 330 °C/2.5h, air-cooled.

Table 1.

The chemical components of 16Cr3NiWMoVNbE gear steel.

Material	C	Mn	Si	W	Cr	Ni	Mo	V	Fe
Wt/%	0.16	0.58	0.81	1.14	2.81	1.25	0.48	0.45	Bal.

Table 2.

The mechanical properties of 16Cr3NiWMoVNbE gear steel.

Temperature (°C)	Rm (MPa)	Rp0.2 (MPa)	A (%)	Z (%)
25	1400	1190	14	62

After the heat treatment, the wire cutting sampling was performed on the bar material, and then rough turning, finishing turning, and grinding were completed on the CY6140 lathe (turning tool G8YD15), TK36S CNC lathe (turning tool RinYouk), and cylindrical grinder (R36 type with 150-grit grinding wheel) to ensure that the final surface roughness of the sample is close to the spline tooth surface, with the cutting depth 0.01 mm and the rotation speed 350r/min. The drawing of the fatigue sample has been presented in Fig. 1a, and the peened and coated specimens are shown in Fig. 1b and c Fig. 2.

Fig. 1.

Fatigue sample: (a) drawing of a fatigue sample, (b) shot-peened samples, (c) compound-surface-treated samples.

Fig. 2.

The drawing of the internal (left) and external (right) spline simulators.

2.2. Shot peening and coating-deposition method

After cleaning, the samples were shot-peened via a KXS-3000 P CNC shot peening machine, AZB300 ceramic beads(Nominal size is 0.3 mm) with a peening intensity of 0.15 mmA, and a surface coverage of more than 150 %. Before DLC coating deposition, Ti atom coating is performed to reduce the internal stress. Pure carbon DLC coatings by magnetron sputtering using graphite targets (target current: 8 A, bias voltage: 200 V, duration: 30 min) as the source of thin coating materials were prepared. After the above treatment, the following four process states were formed, which are presented in Table 3.

Table 3.

The combination of process states.

Process state	Pre-treatment		Film
	Grinding	Shot peening	Pure carbon DLC coating
AR	✓	×	×
SP	✓	✓	×
FC	✓	×	×
SP + FC	✓	✓	✓

2.3. Characterization method

Surface morphology was mainly characterized using a Bruker NPFLEX white light interferometer to obtain surface profile statistical data, including average roughness (S_a), root mean square roughness (S_q), and maximum peak valley height (S_z), the calculation method of which is given in Ref. [34]. The maximum valley depth (H) and valley width (D) were measured in the cross-sectional morphology obtained and the surface stress concentration coefficient (K_st) was calculated according to Ref. [35]. The residual stress profiles were obtained by alternating X-ray diffraction method (PROTO LXRD diffractometer using co-tilt method with CrKα target) with electrochemical thinning via saturated salt water solution. The test result "+" value represents the tensile residual stress and vice versa. The microstructure analysis was completed using a JSM-7900F thermal field emission SEM equipped with an Oxford Nordlenano EBSD attachment with a test voltage of 20 KeV and a tilt angle of 70°. Based on the EBSD results, we performed normal distribution statistics on grain size at four depths (0–20 μm, 20–40 μm, 40–80 μm, and 80–200 μm) to arrive at the influence of process methods on the grain size distribution in terms of the depth. The nano-harnesses of gear steel in the presence of various surface modification processes were tested on a TI-950 nanoindentation tester using a three-point Berkovich indentation for each testing state to test the nano-hardness (H) and elastic modulus (E) by quasi-static approach. The fracture toughness indexes H/E and H³/E² are evaluated to characterize the elastic fracture resistance and plastic deformation, respectively.

To study the influence of alternating load on the bending fatigue performance at the bottom of the spline teeth as shown in Fig. 2, we established a spline pair model, as presented in Fig. 3, by INVENTOR 3D modeling software. During the simulation by COMSOL commercial software, only part of the spline teeth was preserved, and other features were removed to simplify the calculations, and the physical parameters of the spline are provided in Table 4. A fixed constraint was imposed at the left end face of the inner spline, 200Nm torque was applied to the right side of the outer spline end face and displacement constraints were imposed on both ends of the outer spline shaft to achieve a 20′ tilt angle. A torque of 200 Nm and a tilt angle of 20′ represent both extreme service conditions to understand the fatigue performance.

Fig. 3.

A spline pair model employed in the numerical simulation.

Table 4.

The physical parameters of the spline during numerical simulations.

Parameter	Magnitude/level
Number of teeth	19
Modulus	1.5
Normal pressure angle (°)	30
Tooth orientation tolerance (mm)	0.010
Tooth shape tolerance (mm)	0.021
Accumulated error tolerance of weekly intervals (mm)	0.033
Minimum diameter of involute termination circle (mm)	30.30

On this basis above, the rotary bending fatigue tests were completed on the QBWP-1000 cantilever fatigue testing machine at 200 °C, with a speed of 5000 r/min, a stress ratio of R = −1, and a control parameter of load.

3. Results

3.1. Surface topography

The surface morphology and line profile in the presence of four process states are presented in Fig. 4, and the calculated surface roughness parameters and surface stress concentration coefficients K_st are calculated form the data of Table 5.

Fig. 4.

Surface morphologies and line profiles of grinding, pure carbon DLC [35], shot peening, and compound surface modification.

Table 5.

Surface roughness parameters and surface stress concentration coefficients of grinding, pure carbon DLC, shot peening, and compound surface modification.

Process state	Sa (μm)	Sq (μm)	Sz (μm)	$\bar{H}$ (μm)	$\bar{D}$ (μm)	Kst
AR	0.549	0.699	6.768	1.69	58.52	1.040
SP	1.296	1.634	18.682	3.19	81.23	1.059
FC	0.068	0.088	2.872	0.05	39.55	1.001
SP + FC	0.808	1.015	13.762	1.39	40.82	1.050

From the graphs, the plotted results reveal as follows. Firstly, the surface morphology of the coated sample is similar to that of grinding with machine tool marks. However, after the DLC coating of pure carbon, the surface is smoothly undulated, resulting in a reduction of the surface roughness parameters. The depth and width of the pit vary from 1.69 μm to 58.52 μm–0.05 μm and 39.55 μm, respectively, which leads to a lessening of the surface stress concentration coefficient from 1.040 to 1.001. Secondly, after shot peening, the surface roughness value increased significantly, the depth and width of the pits also rose simultaneously, and the surface stress concentration coefficient exhibited an ascending trend from Kt1.040 in the grinding state to Kt1.059. Thirdly, when plating the pure carbon thin film after shot peening (later called compound surface modification), the surface morphology is similar to shot peening, which indicates that the crater marks still exist and the surface of the carbon film has a smooth fluctuation of crater marks as the same coating after grinding. The modification of the compound surface simultaneously led to the lessening of the depth and width of the pits, but the degree of depth reduction was greater, and as a result, the surface stress concentration coefficient decreased from 1.059 to 1.050.

From this point of view, the pure carbon coating could alleviate the fluctuations of the surface and reduce the coefficient of surface stress concentration, which has a certain increasing effect in improving the fatigue performance caused by surface morphology changes.

3.2. Residual stress profile

The residual stress plays a pivotal role in the fatigue process and the residual stress gradient in four states, as demonstrated in Fig. 5.

Fig. 5.

Residual stress profiles of grinding, pure carbon DLC, shot peening, and compound surface modification.

The residual stress under the AR state exhibits a monotonic decreasing curve, with surface residual stress around −255MPa and an influence layer depth of 20 μm. The residual stress field of the FC state still exhibits a monotonic reduction, with the surface residual stress increasing to −527 MPa and the depth of the stress field slightly increasing to 40 μm. After shot peening, the residual stress on the surface increases significantly to −800MPa and the depth of the stress field increases by 120 μm. Finally, after modification of the compound surface, the residual stress gradient demonstrates an inverted hook-shaped curve, with an inflection point appearing at a depth of 40 μm. The residual stress at locations greater than this depth essentially corresponds to the shot peening state, while those below this depth show a monotonically increasing pattern with a surface residual stress of −600 MPa, which is lower than the shot peening state and higher than the coating state after grinding. Compared to shot peening, the reason for the reduction of residual stress on the surface after the compound surface modification should be related to the annealing during the coating process. However, after grinding and coating, the compressive residual stress of the surface rises, which may be attributed to the subtle tensile deformation of the surface during the carbon film coating process.

Based on the residual stress field profiles in the above four process states, the compressive residual stresses after shot peening and compound surface modification are both higher than those of the grinding and coating at the same depth. It should be noticed that some investigations have revealed that surfaces of stainless steel 316L [36] and aluminum alloys 4140 [37] exhibit compressive residual stress after DLC coating, while our study indicates that the surface of gear steel also exhibits compressive residual stress after DLC coating, which should be beneficial for fatigue performance.

3.3. Surface microstructure

Since the 16Cr3NiWMoVNbE steel itself is a fine-grained material, it is a challenging issue to distinguish the effects of the four states on the surface structure from the densely distributed grain boundary map provided by the EBSD. Herein, we utilized the kernel average misorientation (KAM) of four cross-sectional specimens in the presence of various states to analyze the strain distribution of the processed cross-section, as illustrated in Fig. 6a–d. The KAM value is the average value of the misorientation angle between the data point at this location and the data point around it, especially suitable for explaining the strain distribution of crystalline materials after deformation, where red, green, and yellow represent larger, smaller, and intermediate values, respectively. Comparing the KAM diagrams of cross-sectional specimens in four different states, we found that all shot peening, DLC coating, and surface modification would noticeably affect the KAM values of the surface layer, which means that these processes induce microscopic plastic strain. After the pure carbon DLC coating, the upper thickness of 5 μm is the DLC coating, which produces a 10 μm thick micro-plastic deformation layer. The surface appears to have a severe plastic deformation layer with a thickness of approximately 3 μm after shot peening, and the depth of the plastic deformation-influencing layer is more than 20 μm. We also observed that the DLC coating with a thickness of nearly 5 μm and a damaged layer with a depth of 20 μm was slightly smaller than the shot peening, which may be related to the slight relaxation at the temperature of 200 °C during the modification process.

Fig. 6.

Results obtained from the cross-sectional samples: (a) KAM of grinding, (b) pure carbon DLC, (c) shot peening, (d) compound surface modification.

According to the grain size distribution with depth, as presented in Fig. 7, for gear steel in the ground state, there is a fine grain layer at a depth above 20 μm. After covering the pure carbon film, a grain refinement layer with a statistical size of about 3 μm was observed on the surface, and the grain size returned to the matrix state at a depth of 80 μm with increasing depth. The degree of grain refinement degree is the highest, that is, the surface grain size is statistically 2 μm at the surface, and when the depth rises to 80 μm, the grain size is still 3.5 μm. The trend of grain size variation of compound surface modification is similar to that of shot peening, while the grain size at the same depth is slightly larger than that of shot peening. It is generally believed that the grain size is an indicator of the loading capacity: the smaller the grain size, the more obvious the grain effect of the process and the higher the loading capacity. From this point of view, both shot peening and DLC coating have an enhancing effect on fatigue performance.

Fig. 7.

Statistical grain size distribution profile of grinding, pure carbon DLC, shot peening, compound surface modification obtained by EBSD results.

3.4. Nano-hardness

The nano-hardness test results (Table 6) indicate that the deposition process of pure carbon DLC coating remarkably improves the nano-hardness and modulus of the surface, regardless of whether the substrate is shot peening or grinding. Achieving high hardness and wear resistance is the primary goal of pure carbon film in spline structures. However, a further comparison of the above results shows that the hardness and modulus of the coated carbon film after shot peening is higher than that of the coated carbon film after grinding, which is essentially ascribed to the enhancement of the surface hardness by shot peening, allowing the surface has a higher bearing capacity during the nano-indentation process. In the nano-hardness testing process, the carbon layer is supported by a harder substrate underneath, exhibiting a higher nano-hardness.

Table 6.

Nano-hardness test results of grinding, pure carbon DLC, shot peening, and compound surface modification.

Process state	H	E	H/E	H3/E2 (GPa)
AR	3.722	212.436	0.0175	0.00114
SP	6.875	226.765	0.0303	0.00632
FC	35.837	337.952	0.1060	0.40299
SP + FC	36.818	364.370	0.1010	0.37592

By comparing two indicators of resistance to elastic fracture and plastic deformation (i.e., H/E and H³/E²), the results showed that both indicators have improved after compound surface modification compared to shot peening, while the indicator of resistance to plastic deformation after compound surface modification is slightly reduced compared to the grinding coating.

3.5. Fatigue performance

3.5.1. Fatigue load calculation

Richardson's extrapolation was utilized to test for the grid's irrelevance with an error margin of 5 %. After a thorough review, a grid size of 1.6 mm was chosen to appropriately meet the requirements. The calculation results are presented in Fig. 8. We dissected the outer spline shaft to analyze the stress distribution inside the shaft, which shows that the maximum von Mises stress in the spline shaft reaches about 533 MPa when the torque is 200 Nm and the inclination angle is 20'.

Fig. 8.

Numerical simulation calculation results of the fatigue load.

A specified load margin was included in the fatigue performance test of this study because the numerical simulation process does not consider factors such as surface stress concentration. Based on the above-mentioned numerical simulation results of the service state of the spline structure, the designed fatigue load would be 800 MPa (considering a load margin of 1.5 times), and the tested fatigue life data is processed statistically.

3.5.2. Fatigue lives and fractures

The fatigue life of the samples prepared in four different states is shown in Table 7 and Fig. 9. To obtain the real fatigue life of the population in four states with a small margin of error (less than 5 %) and high confidence (more than 95 %), we conducted statistical analysis on the test results in each state. By default, the geometric mean value of fatigue life, i.e., the average of logarithmic fatigue life, is set equal to the logarithmic fatigue life of the population when the geometric mean value of fatigue life conforms to a normal distribution. Based on the above objectives and statistical principles, we perform t-tests on fatigue data by combining the mean, number, and dispersion of sub-samples and population to obtain Eq. (1).

$$t_{\gamma }=\frac{\bar{x}-E_x}{\sigma }$$ (1)

where $\bar{x}$ and $E_x$ represent the logarithmic mean of fatigue life logarithm of subsamples and the population in each state, respectively, and $\sigma$ is the population standard deviation. The factor $t_{\gamma }$ denotes a standardized statistic, which is a function of the number of sub-samples (5 in this study) and the confidence level (95 % in this study), which is utilized to signify the discrepancy between the mean of the subsamples and the population. In the current investigation, $t_{\gamma }$ is set as 2.776. According to statistical principles, $\sigma$ can be evaluated in Eq. (2):

$$\sigma =\frac{s}{\sqrt{n}}$$ (2)

where n denotes the number of fatigue samples tested in each state, and s represents the standard deviation of the logarithmic mean of subsamples' fatigue lives. To control the error limit, herein, ${\delta }_{\mathit{max}}$ is limited to 5 %, i.e., $\frac{\bar{x}-E_x}{\bar{x}}<5\%$. Therefore, we can arrive at the inequality given in Eq. (3):

$$\sqrt{n}>\frac{s}{\bar{x}}\times \frac{t_{\gamma }}{{\delta }_{\mathit{max}}}$$ (3)

Table 7.

Rotary bending fatigue lives of four process states in the presence of 800MPa/200 °C conditions.

Process state	Sample No.	Fatigue life	Logarithmic life	Average logarithmic life s	Geometric mean $\bar{\mathit{x}}$	Standard deviation	$\frac{\mathit{s}}{\bar{\mathit{x}}}$
AR	AR-26	3.66 × 104	4.56	4.59	3.9 × 104	0.065	0.0141
	AR-27	4.47 × 104	4.65
	AR-28	3.46 × 104	4.54
	AR-29	4.71 × 104	4.67
	AR-30	3.43 × 104	4.54
SP	SP-26	1.86 × 105	5.27	5.26	1.83 × 105	0.132	0.0251
	SP-27	2.85 × 105	5.45
	SP-28	1.67 × 105	5.22
	SP-29	1.22 × 105	5.09
	SP-30	1.92 × 105	5.28
FC	FC-26	1.48 × 105	5.17	5.12	1.31 × 105	0.136	0.0266
	FC-27	1.18 × 105	5.07
	FC-28	2.11 × 105	5.32
	FC-29	9.29 × 105	4.97
	FC-30	1.13 × 105	5.05
SP + FC	SP + FC-26	1.81 × 105	5.26	5.23	1.71 × 105	0.043	0.0082
	SP + FC-27	1.47 × 105	5.17
	SP + FC-28	1.68 × 105	5.22
	SP + FC-29	1.69 × 105	5.23
	SP + FC-30	1.92 × 105	5.28

Fig. 9.

Fatigue fractures of the ground sample (a, b, c, No. AR-26, 3.66 × 10⁴ cycles) and the shot-peened sample (d, e, f, SP-30, 1.92 × 10⁵ cycles) in the presence of 200 °C/800 MPa: (a,d) overall view of the fracture; (b,e) fatigue source morphology; (c, f) fatigue source tilting view.

Next, we proceed with combining Eq. (3) and exploiting the fatigue performance data of Table 7 for statistical analysis. On one hand, in terms of statistics, the $\frac{s}{\bar{x}}\times \frac{t_{\gamma }}{{\delta }_{\mathit{max}}}$ values for grinding, shot peening, DLC coating, and compound surface modification states are 0.783, 1.396, 1.477, and 0.454, respectively, which are all less than the root 5 (i.e., number of samples in each state), indicating that the geometric mean fatigue lives of five sub-samples in each state can be performed to represent the overall fatigue lives of the samples in each state with greater than 95 % confidence and an error limit of less than 5 %.

On the other hand, using grinding as the baseline in Table 7, the average fatigue life of shot peening, DLC coating, and compound surface modification increased by 3.68, 2.35, and 3.36 times, respectively. The fatigue test results reveal that the compound surface modification is capable of significantly improving the rotary bending fatigue life of gear steel materials in the presence of large stress at the tooth base and high ambient temperature, and the shortest fatigue life of the compound-modified samples (i.e., 1.47 × 10⁵ cycles) is also higher than the longest fatigue life of the ground samples (i.e., 4.71 × 10⁴ cycles).

We observed the fatigue fracture of the typical specimen in each state. For the states without DLC coating as shown in Fig. 9a–c, under rotary bending fatigue mode, the fractures of both ground and shot-peened samples exhibit multi-source fatigue modes with 3 and 2 sources at both ends of the fracture, respectively in Fig. 9a/b and d/e. By observing the specimen tilted at 45°, the fracture of the ground sample initiated from the discontinuous position of the grinding tool marks in Fig. 9c, whereas the fracture of the shot-peened sample originated from the crater (Fig. 9f). For the DLC coating states in Fig. 10, the fatigue fractures of both the grinded and DLC-coated and compound-surface-modified specimens belonged to a single source state of surface origin and were observed to originate on the cracked DLC coating surface as shown in Fig. 10a/b and d/e. The research of Bai et al. [38] shows that DLC coatings undergo an irreversible sp3-sp2 transformation of carbon atoms in the presence of external loading, which leads to a lessening in film strength. The regions with high sp2 content are prone to deformation due to their weak strength. Herein, the main reason for the coating fracture at this location must be the occurrence of severe strain localization during fatigue loading (Fig. 10c/f).

Fig. 10.

Fatigue fractures of DLC-coated sample (a, b, c, No. FC-26, 1.48 × 10⁵ cycles) and compound-surface-modified sample (d, e, f, SP + FC-27, 1.47 × 10⁵ cycles) in the presence of 200 °C/800 MPa: (a,d) overall view of the fracture; (b,e) fatigue source morphology; (c,f) fatigue source tilting view.

4. Discussion

From Table 7, we rationally conclude that the fatigue lives have been improved during shot peening, DLC coating, and compound surface modification, and that the fatigue effect of shot peening is better than DLC-coating and slightly higher than that of compound surface modification. Based on the results of surface integrity analysis in conjunction with numerical simulation methods, we analyze the evolution of fatigue performance with process conditions.

On the one hand, as a mechanical anti-fatigue surface treatment, there is no need to explain more about the reasons for increased fatigue caused by shot peening. Previously, our work [35] successfully could predict the location of critical sections subjected to rotary bending loads and explained that shot peening with suitable parameters could achieve fatigue strengthening because of over 100 μm-in-depth compressive residual stress profile and exceeding 80 μm-in-depth subgrain refinement layer although with the surface stress concentration. On the other hand, the main purpose of magnetron sputtering DLC coating is generally to enhance wear resistance and reduce friction coefficient, which is not applied to fatigue resistance. However, the results of this investigation revealed that the presence of DLC coating has a beneficial effect on fatigue performance for several reasons as explained in the following.

• (1)Compared with ground surfaces, the surface roughness is substantially reduced after pure carbon coating. In the present scrutiny, it can be clearly seen from the three-dimensional photographs of the surface morphology that the surface roughness of the pure carbon coating is very low. Greitemeier et al. [39] found that the lower fatigue limit of electron beam melted (EBM) samples compared to direct metal laser sintering (DMLS) materials could be attributed to the higher surface roughness. Liu et al. [40] explained that the reduction of surface roughness by ultrasonic surface rolling can prevent the initiation of surface fatigue cracks, which is beneficial for enhancing fatigue performance. In our opinion, under a type of surface texture, a lower roughness indicates a lower surface stress concentration coefficient. Therefore, the reduction of roughness after DLC coating leads to a fatigue-strengthening factor.
• (2)Interestingly, the surface subgrain size is slightly reduced after pure carbon coating. According to the Hall-Petch relationship, decreasing subgrain size tends to increase strength. The refinement of surface grains after coating has a positive effect on improving fatigue performance. Additionally, the surface layers after pure carbon coating and compound surface modification are in a state of compressive residual stress that is deeper than the ground sample, which indicates a strengthening mechanism for fatigue performance.
• (3)For grinding surfaces, the pure carbon film layer has high hardness, high modulus, and excellent performance. The surface nano-hardness (H) of carbon film gear steel is 35.8 GPa, while that of the grinding is only 3.7 GPa. The elastic modulus of surface (E) of carbon film is 338 GPa, which is more than 50 %, higher than 212.4 GPa after grinding. Furthermore, the H/E of the carbon film is 0.1060, which indicates that the carbon film exhibits a stronger resistance to elastic failure compared to 0.0175 of the ground H/E. Kakiuchi et al. [41] examined the effect of DLC coating on the fatigue performance of magnesium alloys and concluded that the constraint of diamond-like carbon films on the slip deformation of the matrix metal is effective in improving fatigue strength. Our study indicated that the DLC coating exhibits higher hardness and modulus through the nano-indentation method, which is a limitation in deformation.

It can be assumed that when the compound surface modification is performed on the surface of the gear steel, a damage-resistant, flat, load-bearing, and elastic “hard shell” is developed on the surface of the gear steel. Underneath the “hard shell” is a layer of compressive residual stress with special subgrains to support the “shell”. The above factors are all strengthening ones for enhancing fatigue, so, naturally, the fatigue performance of gear steel would be substantially enhanced after compound surface modification. Furthermore, we found that the hard shell may rupture in the presence of large local loads, and further research on the matching relationship between the shot peening parameters and the DLC coating based on the characteristics of the DLC coating can be doped to adjust the hardness.

5. Conclusions

We compared the surface integrity of 16Cr3NiWMoVNbE gear steel in the presence of four conditions (grinding, plating pure carbon film after grinding, shot peening, and plating pure carbon film after shot peening), and performed the rotary bending fatigue performance of the gear steel under spline loading conditions four states, and obtained the following results.

• (1)Compared with ground specimens, the geometric mean fatigue lives of shot peening, pure carbon coating, and compound surface modification specimens under 200 °C/800 MPa conditions are improved by 3.68, 2.35, and 3.36 times, respectively.
• (2)The reasons for the improved fatigue performance after pure carbon film coating include the good elastic bearing capacity of the harder surface film, the smoother surface, the surface fine subgrain effect produced, and the compressive residual stress field by the process.
• (3)After the compound surface modification, the surface still maintains the shot peening morphology. Although the surface roughness and stress concentration coefficient have enhanced, the compressive residual stress profile and the subgrain refinement layer introduced at a depth of more than 100 μm, as well as the hard surface layer with good load-bearing capacity, have played the role of fatigue strengthening.

CRediT authorship contribution statement

Bo Yu: Writing – original draft, Data curation. Chunling Xu: Writing – original draft, Methodology. Kaixiong Gao: Investigation. Wenming Yang: Software, Data curation. Xin Wang: Writing – review & editing, Supervision, Resources, Funding acquisition. Zhongwu Sun: Validation, Resources. Bin Zhang: Visualization. Zhihui Tang: Supervision.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Xin Wang reports article publishing charges, equipment, drugs, or supplies, and statistical analysis were provided by Aero Engine Corporation of China Beijing Institute of Aeronautical Materials. Xin Wang reports a relationship with Aero Engine Corporation of China Beijing Institute of Aeronautical Materials that includes: employment. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.