Micromorphology, Microstructure, and Wear Behavior of AISI 1045 Steels Irregular Texture Fabricated by Ultrasonic Strengthening Grinding Process

Abstract

In this study, the tribological properties of three AISI 1045 steel samples were investigated. Two samples were treated with ultrasonic shot peening (USP) and ultrasonic strengthening grinding process (USGP), respectively, while the other one was only treated with a polishing process. Sample properties, such as surface morphology, roughness, microhardness, elastic modulus, frictional coefficient, and phase structures were analyzed. Results show that the sample treated with USGP had the best tribological properties. It realized the highest surface roughness, microhardness, and elastic modulus. Compared with a polished sample, the roughness of the sample treated with USGP increased by 157%, and the microhardness and elastic modulus improved by 32.8% and 21.3%, respectively. Additionally, USGP provided an average frictional coefficient of 0.4, decreasing approximately 45% compared to polishing. The possible mechanisms of USGP surface texturing were discussed. The findings denote that USGP could be an efficient approach to improve the fatigue life of some mechanical components.

1. Introduction

Tribological properties are an essential factor affecting the stability and reliability of mechanical equipment, especially in the components of the automated transmission system, such as bearing rollers and gears [1]. The intense friction on a surface will lead to severe surface wear and even driving failure [2,3], which is mainly due to the rupture of oil film and lubricant failure, resulting in dry friction. Therefore, it is necessary to enhance the anti-wear properties of the contact surfaces to prevent them from experiencing dry frictional wear. Many solutions have been proposed to address this issue, such as surface alloying [4], surface coating [5,6], surface texturing [7,8], and surface nanocrystallization [9,10]. Surface texturing has emerged as a promising solution in recent decades [11,12,13,14]. The mechanical components with textured surfaces show an excellent load capacity, wear, and friction resistance. So far, various techniques have been proposed for surface texturing, such as mechanical processing [9], ion beam etching [15], and chemical etching [16]. In particular, laser technology has been widely used for surface texturing due to its benefits in efficiency and environment adaptability [17,18]. Nevertheless, the high energy effect of the pulse laser needs to be controlled carefully, since it might result in severe oxidization during the processing [19,20]. This issue will cause poor surface quality and accuracy, which will further cause non-ignorable errors and negative effects in practical application [21,22].

As an alternative, ultrasonic surface rolling processing (USRP) offers a new solution to produce textures on metallic material surfaces. USRP is a strengthening machining technique which avoids removing any additional materials. Thus, it leads to higher surface integrity and quality. The primary work mechanism of USPR is the employment of vibration and applied load simultaneously on the treated surface along a direction. Consequently, the machined surface can produce considerable residual compressive stress, high hardness, and a nano-grain deformed layer. However, the rolling process is challenging to control. It cannot process parts with poor rigidity, such as slender rods and thin wall pipe fittings, mainly due to the severe deformation that frequently occurs on their surface [23].

On the other hand, ultrasonic shot peening (USP) opens up the possibility of achieving good tribological properties on metallic material surfaces [24,25]. In USP, a dynamic loading is driven by the vibrations and applied on the treated surface, producing a lower roughness and gradient microstructure, along with the advantages of low energy consumption and high controllability of process parameters [26]. For example, Yue et al. found that an extremely fine gradient nanostructure is formed after USP treatment, significantly improving the mechanical strength [27]. Nevertheless, the hardened layer formed by USP treatment is relatively thin and uneven, which will cause surface damage when the mechanical components work in a harsh working environment [28].

Ultrasonic strengthening grinding process (USGP), which uses fused ceramic balls, brown corundum, and strengthened liquid for treating target samples is an efficient approach for obtaining high-quality surface textures. It has been poorly investigated so far. The impact and scratching of brown corundum by the ceramic ball and brown corundum on the machined surface can obtain irregular micro-dimples that are conducive to friction reduction and wear resistance [29].

In this work, the tribological properties of three AISI 1045 steel samples were investigated. Two samples experienced surface texturing by USP and USGP, respectively, while another one was only treated with polishing for experimental comparison. The texture micromorphology and its structural characteristics were analyzed. The surface roughness, microhardness, and elastic modulus, as well as the friction coefficient, were measured. These properties are further explained by phase and misorientation analysis. The results of this work indicate that USGP has the potential to be used for improving the fatigue life of some mechanical components, such as crank shaft bearing [30] and hydrodynamic journal bearing [11].

2. Materials and Methods

2.1. Samples Preparation

Three AISI 1045 steel samples were employed in this study. Their main chemical composition is reported in

Table 1. The chemical composition of AISI 1045 steel (wt %).

Elements	C	Si	Mn	Cr	S	Fe
Content	0.39	0.26	0.71	0.26	0.03	bal

, and the element ratio is based on the standard EN 10083-2. Samples with geometrical parameters of 90 mm × 45 mm × 3 mm (See Figure 1a) initially experienced a heating treatment process (See Figure 1b). First, all the samples were quenched at 830 °C using water at room temperature to obtain good surface strength and hardness. Then, the samples were treated with a tempering process of 30 min at 180 °C to maintain good hardness and wear resistance and simultaneously reduce the residual stress and brittleness of the quenched workpiece. Finally, the samples were cooled in the air. Before the experiments, the surfaces of samples were polished with 300# sandpaper to ensure the roughness of R_a < 0.1 μm and then cleaned with alcohol through an ultrasonic bath.

2.2. Surface Texture Fabrication

Two samples experienced a process of surface texture fabrication while the other one did not, for result comparison. The surface texture was fabricated using either USP or USGP treatments, both based on an ultrasonic strengthening device (See Figure 2a). In the USP, an electro-mechanical ultrasonic transducer generated ultrasonic waves, which were then applied to a workpiece, as shown in Figure 2b. An acoustically tuned resonator bar was caused to vibrate by energization with an ultrasonic transducer. The energy produced from these impulses was imparted to treat the surface through the ceramic balls. Compared with USP, the USGP uses different materials for treating target samples, which mix ceramic balls, brown corundum, and strengthened liquid, as shown in Figure 2c. The strengthened liquid is mainly made up of extrusion additive, triethanolamine, and water. The USP and USGP treatment parameters applied in this work are shown in

Table 2. USP and USGP treatment parameters.

Parameters	Value
Vibration frequency (kHz)	20
Peening distance (mm)	30
Processing time (min)	3
Diameter of ceramic balls (mm)	1
Grain size of brown corundum (μm)	15

. The sample without texture fabrication was named the polished sample, while the other two samples subjected to USGP and USP process were named sample USGP and sample USP, respectively.

2.3. Materials Characterization

Three samples with geometrical parameters of 12 mm × 12 mm × 3 mm were taken from the processed sample to collect statistics and calculate median values. The tested samples were cut using electrical discharge wire cutting, and the cross-section surfaces were mechanically polished. The scanning electron microscope (SEM, TESCAN MIRA4, TESCAN Inc., Brno, Kohoutovice, Czech Republic) was used to characterize the surface morphology of samples. The white light interferometer (Bruker Countor GT K 3D, BRUKER Inc., Karlsruhe, Germany) was utilized to obtain surface microtopography and roughness. The nano-indenter (Bruker Hysitron TI980, BRUKER Inc., Karlsruhe, Germany) was employed to measure the microhardness. The least square method was used to fit 25% to 30% of the top of the unloading curve (See Figure 3), and the contact load P was calculated by Equation (1).

$$P=a{\left(h-h_f\right)}^m$$

(1)

where m and a are the number of array and constant, respectively.

The contact stiffness S and the contact depth were obtained by Equations (2) and (3), respectively.

$$S={\left(\frac{dP}{dh}\right)}_{h=h_{max}}$$

(2)

$$h_c=h_{max}-\epsilon \frac{P_{max}}{S}$$

(3)

In Equation (3), ε is a constant related to the shape of the indenter, which is 0.75 in this work. The contact area was calculated by Equation (4).

$$A=f\left(h_c\right)$$

(4)

Finally, the microhardness H was obtained by Equation (5).

$$H=\frac{P_{max}}{A}$$

(5)

The elasticity modulus was calculated based on the Oliver Pharr method [31]. The detected distance is below the treated surface, ranging from 10 μm to 160 μm with a step of 30 μm. Those measurements were repeated three times, and the average values were calculated. The loading, holding, and unloading times were 5s, 2s, and 5s, respectively. The maximum loading force (Fn) was set at 5 mN.

The microstructural composition was measured by an X-ray diffractometer (Rigaku + Ultima IV, Rigaku Corp, Akishima, Tokyo, Japan) with Cu Kα radiation. The phase change and kernel average misorientation distributions of the polished, USP, and USGP samples were investigated by electron back-scattered diffraction (EBSD Oxford Symmetry, Oxford Instruments, Abingdon, UK). The 5 mm × 5 mm × 3 mm blocks were prepared; the cross-sectional surfaces were mechanically polished, chemically etched, and ultrasonically cleaned with alcohol. The detected areas were 200 µm × 200 µm at a depth of 10 µm below the treated surface, and the step size was set at 0.25 µm.

2.4. Frictional Wear Tests

The frictional wear properties tests were performed on the ball-on-disk reciprocating friction sliding tester (MQP-5H, Jinan Heng Xu Testing Machine Technology Inc., Jinan, China) at a temperature of 20 °C and an ambient humidity of ~35%. GCr15 steel balls with a diameter of 7.85 mm and a hardness of 700 ± 20 HV were utilized in the tests. It was used to slide reciprocally against the fixed samples at a speed of 20 mm/s, a weight of 20 N, and a 10 mm linear stroke. The frictional force in the sliding plane was measured continuously, and the friction coefficient was calculated. The wear tracks were detected by SEM micrographs and white light interferometer, respectively.

3. Results and Discussion

3.1. Characteristics of Surface Texture

3.1.1. Surface Morphologies of Different Samples

SEM and white light interferometer maps were employed to analyze the morphology of the polished, USP, and USGP samples. The micrographs of the polished sample are depicted in Figure 4. In general, the machined surface of this sample is relatively smooth, along with some irregular tracks, micro-bulges, and micro-dimples distributed on the surface (Figure 4a). These micro-bulges and micro-dimples are mainly accumulated near the tracks. The tracks are not oriented in one direction and typically have an irregular micro shape; the largest width of the track region is over 10 μm (See Figure 4b–d). This can be explained by considering the surface plastic deformation that occurred under the effects of micro-stretch and vibratory shock in the polishing process.

Figure 5 reports the micrograph of sample USP. The peaks and dimples on the surface are visible, and the height between peaks and valleys reaches approximately 24.13 μm (Figure 5a). It is noted that the peaks are oscillating down in the direction of length (Figure 5b), accompanied by different micro-dimple shapes. These irregular micro-dimples can be observed in Figure 5e, which are characterized by a diameter ranging from 2 to 5 μm. Surprisingly, some nano-particles are observed in Figure 5f, which have a diameter of approximately 100 nm. These can be attributed to the fact that a micro-dimple can store the abrasives in a frictional wear process [32].

A significant difference is observed in the USGP sample, where many granular bumps along with micro-dimples appear on the surface (Figure 6). The surface texture is relatively uniform, and the maximum value between peaks and valleys drops compared with that of the USP sample. This is because the mixed abrasives consist of ceramic balls, brown corundum, and strengthened liquid, which lead to less severe plastic deformation. The micro-cutting and extrusion of these mixed materials further reduce protruding regions. Such a texture matches various shapes of abrasive particles, making it more reliable to absorb and prevent them from rolling on the friction surface.

3.1.2. Surface Roughness of Specimens

Arithmetic mean height Ra of all the samples was measured, and the obtained results are reported in Figure 7. The polished sample shows the lowest roughness of 0.5485 μm, while the roughness of the USGP and USP samples is apparently higher, realizing a value of 1.412 μm and 1.3475 μm, respectively. This could also be explained considering that the irregular micro-grooves in the USGP sample and the polished sample increased the roughness. In the USGP process, the mechanical micro-cutting removed part of the material, and such a condition will not occur in a USP treatment. Therefore, the roughness of the USGP sample is slightly increased compared with the USP sample.

3.1.3. Microhardness (H) and Elastic Modulus (E) of Samples

The load–displacement curves of the sub-surface for each sample are shown in Figure 8. The USGP Sample offers the highest hardness H value of 5.35 GPa, which is approximately 30% larger than that of the USP sample. The lowest H value of 4.03 GPa is found in the polished sample, as expected. It can also be seen that the elastic modulus E value behaves in a similar way and also follows the rank: E_USGP > E_USP > E_Polished, although they are very close in the USGP and USP samples. These results reveal that USP and USGP treatments will enhance the microhardness and elastic modulus of the samples. On the other hand, a higher microhardness and elastic modulus will result in higher stiffness, which may further improve wear resistance.

The sample microhardness as a function of the distance between the sub-surface and the substrate is reported in Figure 9. The microhardness of the polished sample initially drops rapidly and then gradually reaches a stable value of approximately 2.5 Gpa, while the microhardness of the USGP and USP samples behaves a little differently, decreasing slowly with the increase in the distance from the surface. However, in general, a similar tendency is observed: the microhardness decreases as the distance increases. When the distance reaches 130 μm, the microhardness of the three samples is almost equal. The higher microhardness in the sub-surface can be attributed to the thermal, polishing, USP, and USGP treatments. In addition, since the mixed abrasives, which were used in the USGP treatment, have higher-impact kinetic energy and lower elastic energy than those ceramic balls, the work-hardening capacity of the USGP is better than that of the USP. Therefore, the microhardness of the USGP sample is higher than that of the USP sample.

The ratio of microhardness H to elastic modulus E represents the elastic strain properties of a material, which is an important index for evaluating the wear resistance of metals. It is also employed to predict the friction behavior and to explain the mixture of elastic and plastic deformation characteristics. The previous study demonstrated that a higher H/E value leads to higher toughness and better tribological properties [19]. As shown in Figure 10, the H/E value of the USGP sample is slightly higher than that of the polished sample and the USP sample. Consequently, the USGP sample may possess a better tribological performance.

3.1.4. XRD Diffraction Patterns of Surface Textures

The X-ray diffraction patterns of the top surface for each sample are shown in Figure 11. The {110}, {200}, and {211} crystal planes could be found in all the samples, which indicate a ferrite. It can be observed that some new diffraction peaks appeared in the USGP sample. This could be explained by phase transition and Fe₃C phase precipitating. In the USGP treatment, severe plastic deformation occurred on the samples’ surface layer owing to the impact of mixed abrasives. A subsequent dislocation multiplication and stress concentration occurred in the distorted grain boundaries, which made it possible for phase transformation and element diffusion [27,33], followed by the decomposition of the ferritic and Fe₃C phase formation. Fe₃C is further confirmed by the following EBSD analysis. A shift of diffraction peaks of {110} and {200} crystal planes is also observed in the USP and USGP samples, which could be attributed to the lattice distortion and grain refinement caused by the generation of dislocations [34].

3.1.5. Distributions of Phase and Kernel Average Misorientation

EBSD analyses were performed to observe the distributions of phase and kernel average misorientation on the samples’ cross-section (See Figure 12). As presented in Figure 12a–c, mass alpha iron accompanied by a small amount of gamma iron and Fe₃C could be seen. The amount of Fe₃C in the USGP and USP samples is more considerable than that of the polished sample. This is because the phase change occurred during the USP and USGP processes, and higher efficiency of phase transformation was found in USGP. The Fe₃C phase is cementite, which shows the properties of high hardness and high brittleness; therefore, a harder surface layer can be obtained in the USGP sample. The kernel average misorientation of each sample is shown in Figure 12d–f. Obviously, many misorientations are distributed around the alpha iron in the USGP sample, which forms a network of enhanced layers. At the same time, a small amount of misorientation is found in the polished and USP samples.

3.2. Tribological Properties Characterization of Different Samples

The frictional coefficients of each sample versus time were investigated, as shown in Figure 13. The frictional coefficient of the polished sample initially increased slowly and then sharply increased at the sliding time of ~80 s. Finally, it reached a stable state with an average value of ~0.98. The frictional coefficient of the USP and USGP samples behaves slightly differently, increasing slowly at the beginning and then fluctuating at an average value of 0.81 and 0.71, respectively. At the initial phase of the friction of the polished sample, the surface is relatively smooth. As time rises, the surface starts to wear down and forms abrasive particles [35], which roll between the sliding interfaces or adhere to the frictional surface, thus increasing the frictional force. Finally, the contacted surface material is softened with the rise in frictional temperature, which results in a higher frictional coefficient. In the USP sample, the contact areas are decreased due to surface micro-dimples, which can capture the wear debris and alleviate wear [5]. Moreover, the increase in hardness benefits the friction reduction [36]. In the USGP sample, a lower frictional coefficient is represented in the first 300 s, which can mainly be attributed to the coupling effect of a higher microhardness, a larger area of micro-dimples, and a large number of misorientations on the surface.

To better understand the tribological characteristics of the samples, they were observed by SEM after sliding for 600 s (See Figure 14). The wear track width of the polished sample is about 470 μm, and the morphology exhibits a compression fissure with a mass of abrasive particles and furrows (See Figure 14a). It can be observed that furrows were dispersed along the wear track along with some abrasive particles. These abrasive particles show a diameter of approximately 0.5–2 μm. This evidence indicates that a severe adhesive existed in the contact region, which further verifies that adhesive wear occurs during abrasive wear [37]. The abrasive particles and debris began to form when the polished surface was rubbed by the steel ball because of the concentration of alternating stress, which resulted in high temperature over time. The high temperature softened the debris, making it adhere to the contact surface. Sample USP has a wear track of approximately 360 μm, severe delamination spalling, and furrows along the sliding direction are observed on the surface (See Figure 14b). Material spalling is mainly due to plastic deformation in the process of hardening the contact surface. The furrows are formed when the hardened debris is peeled off and rubbed on the surface. In the USGP sample, some chipping pits and abrasive particles were also formed (See Figure 14c). The deep dimples with different peaks and valleys decreased the abrasive wear of the material. In addition, a higher microhardness is obtained in this sample, which improved the resistance to wear. Therefore, the friction and wear performance of the USGP sample is the result of the combined effects of dimples, abrasive particles, and surface hardness.

Figure 15 reports the micromorphology and cross-sectional profiles of the samples’ wear tracks after sliding for 600 s. Both the polished sample and the USP sample have a deep wear track, exhibiting a depth of ~7 μm and ~6 μm, respectively. The wear mechanisms are primarily adhesive wear and abrasive wear and are accompanied by significant volume loss in the track center and material accumulation on the track flanks. In the USP sample, both adhesive wear and abrasive wear show a depth of ~9 μm, while in the USGP sample, slightly abrasive and adhesive wear were observed, which increased the stability of the sample in the friction and wear test.

4. Conclusions

In this paper, the AISI 1045 steel surface texturing by polish grinding, USP, and USGP treatments was demonstrated, which are named sample polished, sample USP, and sample USGP, respectively. Their tribological properties were investigated according to the analysis of their surface topography, surface roughness, microhardness, elastic modulus, phase, as well as frictional coefficient.

The polished sample had some irregular tracks, along with little micro-bulges and micro-dimples. The USP sample had many irregular and deep micro-dimples, as well as some nano-particles, while the USGP sample had more uniformly distributed micro-dimples along with granular bumps.

Compared with the other two samples, the USGP sample showed the largest surface roughness, microhardness, and elastic modulus, exhibiting a value of 1.141 μm, 5.35 GPa, and 168.99 GPa, respectively. These behaviors can be attributed to the fact that the impact and scratching of brown corundum by the ceramic ball on the machined surface resulted in the transformation of the Alpha Fe phase to Fe₃C and to more uniform irregular micro-dimples and network misorientations.

In addition, the lowest coefficient (i.e., 0.6) was obtained in the USGP sample, mainly due to the coupling effect of misorientation, cementite, and micro-dimples induced by USGP treatment, which formed a hardened layer on top of the surface and reduced the abrasive wear.