Synchronous Shot Peening Applied on HVOF for Improvement on Wear Resistance of Fe-based Amorphous Coating

: Shot peening was used synchronously to improve Fe-based amorphous coating performance by delivering ZrO 2 ceramic particles into a low-temperature region of a ﬂame during the high velocity oxygen ﬂame (HVOF) spray process. The coating became denser, and its hardness became higher via the new process. Moreover, the compressive residual stress was induced by shot peening. The results from the dry friction test indicated that the coating’s wear resistance was enhanced obviously. The wear mechanism of coatings with and without shot peening is an abrasive wear combined with an oxidation wear at wear test conditions of a low load and a low frequency. The coating with the best wear resistance did not have the strongest microhardness but had the highest compressive residual stress. The compressive residual stress had a signiﬁcant positive inﬂuence on the wear resistance at a low frequency, while its e ﬀ ect is weakened at a high frequency.


Introduction
Fe-based amorphous alloys have been studied extensively due to high hardness, good wear and corrosion resistance, and relatively low cost. Meanwhile, with the development of thermal spray technology, Fe-based amorphous materials are quickly introduced into the field of surface engineering [1,2]. For improvement on Fe-based amorphous coatings performance, different kinds of methods were developed such as technology improvement [3], parameter optimization [4], hard phase reinforcement [5], and pre/post-treatment [6,7]. Most methods pay attention to the microstructures of Fe-based amorphous coatings such as density and hard phase composition. Takeshi et al. [8] studied an Fe-based amorphous composite coating with the addition of WC-12Co. When the added WC-12Co was 8%, the coating hardness increased from 660 to 870 HV 0.1 , while the wear rate decreased from 0.8 × 10 −4 to 0.1 × 10 −4 mm 3 ·N −1 ·m −1 , exhibiting significant enhancement in the hardness and wear resistance. Yugeswaran et al. [9] added TiN ceramic powder to strengthen an Fe-based amorphous coating and found that the strengthening phase decreased both the oxide content of the composite coating and the friction coefficient. However, significant attention has to be paid to the added content of the second phase, particle size, and mismatch between particles associated with Fe-based amorphous coatings in order to avoid deterioration [10].
The coating performance is related to the residual stress, which plays an important role in factories [11][12][13][14]. During the spraying technology, high velocity oxygen flame (HVOF) spraying is beneficial for preparing coatings with high density and low porosity due to the flame of supersonic speed and low temperature. For powders with a high melting point, they cannot be melted thoroughly during the HVOF spray process. Therefore, unmelted or semimelted powders impacting on the substrate or a previous lay can lead to great shot stress to induce the compressive residual stress in a coating [11,15]. Generally, the compressive residual stress is rewarding to service life, fatigue resistance, and wear resistance [16,17]. However, shot stress created by Fe-based powders is limited due to their relatively low melting points. Some literature focused on the effects of residual stress in coatings [14,16,18,19], but it is difficult to find an effective way to improve it.
Hard particle peening technology (sandblasting, shot peening, etc.) has been applied on surface engineering, where shot peening is used widely in pre-or post-treatment processes [20][21][22][23]. The purpose of pre-treatment is to increase the roughness of a substrate, so that a coating adheres to a substrate strongly [20]. Junior et al. [22] carried out shot peening on a WC-10Ni coating sprayed by HVOF and found that the fatigue strength of the coating enhanced from 750 to 850 MPa owing to the compressive residual stress caused by shot peening. Shot peening in the post-treatment is in favor of wear resistance and compressive residual stress, but the sphere of influence is limited [22,24]. Therefore, some literature turned to utilize shot peening during spraying. The earliest study on simultaneous peening during spraying was implemented by Singer [25] in 1984. In the study, they introduced an extra independent shot peening equipment into a spraying system. The result showed that the residual stress and hardness were improved and low porosity was deserved. However, independent shot peening cannot make sure all deposition areas were covered. Moreover, Oxidation was introduced by high-pressure air, and high cost was caused by an inert gas. Another method for synchronization of spraying and peening is to mix hard particles into sprayed particles, where hard particles have effects on shot peening. Liang et al. [5] mixed Al 2 O 3 particles into Fe-based amorphous powders and prepared an amorphous coating by HVOF. In the research, Al 2 O 3 particles were used as shot peening to induce compressive residual stress. However, the velocity and temperature of Al 2 O 3 particles cannot be adjusted independently, and the excessive speed and temperature can damage the coating microstructure.
In this study, hard particles ZrO 2 and sprayed powders were fed into a spraying system from different positions. The hard particles were delivered into a flame flow out of a nozzle to reduce acceleration and heating. The tribological behavior and properties of coatings were studied, and the formation mechanism was also discussed. This study provides an effective method to improve the wear resistances of Fe-based amorphous coatings.

Preparation of Coatings
The sprayed powder was a self-made Fe 69 B 22 Si 5 Nb 4 (at.%) Fe-based amorphous powder, prepared by vacuum gas atomization. Then, powders with a diameter less than 45 µm were sieved. The micromorphology is shown in Figure 1a. The typical satellite structure was found from the enlarged picture in the upper left corner of Figure 1a, due to the fact that small melts adhered to the surface of large droplets easily during the faster cooling process. The diameters of hard particles ZrO 2 were in a range of 150-180 µm, as shown in Figure 1b. Figure 2 shows the schematic diagram of the HVOF process assisted with shot peening. The sprayed powder was delivered near a combustion chamber, while ZrO 2 particles were delivered out of a gun. The incident position of ZrO 2 particles can be adjusted to control shot peening levels. Lower thermal conductivity and larger size can keep ZrO 2 particles with a low temperature, high elastic energy, and large collision kinetic energy, which is in favor of shot peening. The substrate was composed of 1045 steel and was pretreated through oil removal, rust removal, and sandblasting. For comparative analysis, three kinds of coatings were sprayed, and the related parameters are listed in Table 1, where the coatings are C0, C1, and C2 for short, respectively.

Measurement and Analysis Method
The phase structures of powders and the coating were analyzed using D8 XRD (karlsruhe, Germany), equipped with a Kα ray with Cu target (λ = 0.154060 nm). The diffraction angle range, diffraction speed, and step length were 30-90 • , 2 • /min, and 0.02 • , respectively. The microstructures of powders (Fe-based amorphous powders and ZrO 2 powders) and coatings were observed using a Nova Nano 650 SEM field emission scanning electron microscope (Nova Nano 650, Hillsboro, OR, USA), equipped with an energy-dispersive spectrometer. The microhardness in coatings was measured by a Vicker hardness tester (Micromet-6030, Buehler, Lake bluff, IL, USA). The applied load was 100 g for 10 s. The tribological performance was measured using a wear tester (UMT-3, CETR, Campbell, CA, USA), and the dry friction model was the reciprocating ball-to-surface contact. The friction ball pair consisted of Si 3 N 4 balls with the diameter Φ = 4.0 mm, and the coating surface was polished with 1000-mesh sandpaper before the test. The test conditions were as follows: load, 5 and 10 N; frequency, 5 and 10 Hz; time, 30 min; and slide distance, 4.0 mm. A three-dimensional interference surface profiler (phase shift MicroXAM-3D, ADE Corporation, Westwood, MA, USA) was used to measure worn volume and observe the morphology of the shot peening trace. The worn surface was also respected by SEM mentioned above. The residual stress on the coating surface was monitored by an XRD stress tester (X-350A, Handan, China). Usually, the tester cannot be used to obtain the residual stress value of pure amorphous materials due to no crystal phase [19]. However, there was an α-Fe phase in the coating by the result of XRD. The residual stress can be evaluated qualitatively by characterizing the lattice constant of the α-Fe phase in the coating [26,27]. Therefore, the stress can be measured by the Cr-based Kα radiation in a diffraction crystal plane of (211) at 2θ = 158 • , where the scanning rate of the tester was 0.1 • /min, and three measurements were performed at 2Ψ of 0 • , 30 • , and 45 • , respectively. Here, the residual stress value was used for vertical comparison. Figure 3 shows similar XRD patterns in powders and coatings, indicating the existence of the dominant amorphous phase and a small amount of the α-Fe phase. There were no characteristic peaks of ZrO 2 in C1 and C2 compared with that in C0, showing that ZrO 2 particles neither reacted with spray-coated powders chemically nor were embedded into coatings during the spraying and coating processes. The diffraction peak intensities of the α-Fe crystal phase of C1 and C2 grew higher, which may result from impact-induced damage of peening particles on the rather brittle material [28].  Figure 4 shows the residual stress values in C0, C1, and C2. It can be seen that the residual stress changed from the residual tensile stress (4 MPa) to the compressive residual stress (−180 and −100 MPa) after auxiliary of ZrO 2 particles. The largest compressive residual stress was induced when the peening particle rate was 50 g/min, showing that the exceeding ZrO 2 rate was harmful to the compressive residual stress. Heavier shot peening could lead to coating damage like microcracks, which relaxed the compressive residual stress [29].

Shot Peening Traces
For a clear observation of shot peening, ZrO 2 -assisted spraying was operated only once at a very high traveling velocity. The micromorphologies of traces are shown in Figure 5. Hard particles impact and amorphous particles deposition are random during the spray process. Splats 1 and 4 were not influenced by shot peening, as shown in Figure 5a,b, respectively, presenting a typical "fried-egg" shape [30]. However, there was not a clear thin disk in the center of the splat, which illustrated that amorphous powders were melted completely and flattened well. During the deposition process, the droplet edge flattened along with the previous layer surface at a high speed in the tangential direction [31,32]. Therefore, compared with the center of the splat, the adhesion of the edge to the other layer was weak. It is obvious the edge of splat 2 ( Figure 5a) adhered to the substrate firmly while there was a gap between the edge of splat 5 and the substrate. It was due to the different sequences of shot peening and deposition. For splat 2, deposition was first, and then shot peening made the splat edge pressed into the substrate. Interestingly, close adhesion between splats 2 and 3 proved further that shot peening can compact the coating structure. For splat 5, shot peening caused a crater firstly. When a droplet impacted onto a crater, its edge escaped from the crater along with the crater surface. The edge hanged in the air after the droplet solidification, and a big gap was formed, as shown in Figure 5b. The effect of shot peening was negative in this kind of situation, but the following shot peening or deposition still had the chance to counteract the adverse effect. Figure 5c shows another situation that hard particles impacted on the flattening droplet. The droplet was still very soft even in the liquid state. As a result, a deep and coarse crater remained after shot peening. In Figure 5d, there was an evident crater on the splat 7 surface, where cracks were observed. This suggested that exceeding shot peening can damage deposited splats.

Micromorphologies and Mechanical Properties
As shown in Figure 6, all coatings were typical thermal-sprayed layered structures. An inter-splat interface formed as the deposited splat's boundary, where pores existed. After shot peening, obvious saucer-shaped lines were seen in Figure 6b,c. Compared to the unpeened coating in Figure 6a, the shot peening-assisted coating in Figure 6b was of lower porosity, and the inter-splat interface became narrow. It suggested that shot peening was beneficial for the coating density. However, as shown in Figure 6c, more pores and inclusions appeared due to serious impact-induced damage caused by exceeding shot peening. In Figure 7, by increasing the mass rate of ZrO 2 particles, the porosity decreased first from 2.3% to 1.5% and then increased to 1.7%, while the microhardness went up progressively (C0: 669 HV 0.1 , C1: 717HV 0.1 , and C2: 865HV 0.1 ). Porosity and microhardness of both C1 and C2 were improved compared with those of C0. More interestingly, C2 with a higher mass rate of ZrO 2 particles possessed higher microhardness but worse porosity than C1, which proved that exceeding shot peening damaged the microstructure of C2. The speed of ZrO 2 particles depended on many factors such as injection velocity, diameter, and mass rate [33]. Particles with higher speeds and smaller diameters probably led to more serious plastic deformation, which broke deposited particles easily as shown in Figure 5d. It is generally known that plastic deformation caused by peening is in favor of coatings performance, but it cannot increase infinitely via the peening method [34]. Excessive shot peening caused splat damage, inclusion generation, and more pores existence around the boundary, as shown in Figure 6c. These defects can play a role in defect hardening, so that the microhardness was increased further [35].

Friction Coefficient and Wear Rate
Figure 8a-c shows the coefficient of friction (COF) curves of C0, C1, and C2 at different test conditions. The profiles of COF curves in Figure 8a,c looked similar. In Figure 8b, the bread peak existed in the curves of C1 and C2 marked as arrows. It illustrated that the stable oxide layer formed in the initial period of wear tests. Figure 8d shows the COF curves of C0, C1, and C2 under all test conditions. It can be seen that the COF decreased slightly with the increasing mass rate of ZrO 2 particles. It should be attributed to the good deformation resistance for coatings with high microhardness. However, the COF did not change significantly with a fluctuation between 0.62 and 0.68 at a frequency of 5 Hz. When the frequency was increased, more energy was generated between the friction pair and the coating surface. The amorphous areas in the coatings were softened easily due to their higher temperature sensitivity. Strong adhesion between the asperity of these areas and the friction pair formed by "spot" welding, which dramatically increased the COF.  Figure 9 shows the wear rates of coatings under different test conditions. The wear rate was calculated by the equation [36]: W = V m / (N × S), where W is the wear rate (mm 3 ·N −1 ·m −1 ), V m is the wear volume (mm 3 ), N is the load (N), and S is the sliding distance (m). The wear rates of both C1 and C2 were lower than that of C0 at any test condition, showing that wear resistance was improved via the process of simultaneous shot peening. The wear rate of C1 with a load of 10 N and a frequency of 5 Hz (2.19 × 10 −5 mm 3 ·N −1 ·m −1 ) was almost a third of that of C0 (6.03 × 10 −5 mm 3 ·N −1 ·m −1 ). Generally, high microhardness is in favor of improvement in wear resistance. Interestingly enough, C1 had the best wear performance, although its microhardness was lower than that of C2, illustrating that microhardness was not the only factor in improving wear resistance. It is well-known that wear rates are directly influenced by residual stresses [37]. Compared with the wear rates obtained with a load of 10 N and a frequency of 5 Hz, the wear rates for C0 and C2 decreased, while the wear rate for C1 actually increased slightly, when the test conditions were a load of 10 N and a frequency of 10 Hz. The reason will be discussed in detail later.  Figure 10 shows the worn surface profiles of coatings. More black chipping, spalling holes, and shallow grooves were observed on the C0 surface, presenting the occurrence of abrasive wear. The black chippings were identified as oxides through energy-dispersive spectroscopy (EDS) analysis of area A, as shown in Table 2, illustrating that an oxide layer was formed. The layered structure of the sprayed coating inevitably led to a rough and uneven convex-concave surface. When a friction ball pair contacted with the convex-concave area, higher shear stress was generated at the edge of the asperity, resulting in larger plastic deformation. When the plastic deformation increased to a certain extent, cracks occurred and expanded along a sliding line. After a certain cycle, the pieces were separated along the propagation trajectory of cracks, forming wear debris. Under the repeated grinding and the frictional heat, the wear debris underwent the dynamic oxidation and microzone hot pressing and then formed an oxide layer covering the worn surface [38]. However, the oxide layer cannot exist on a surface with low microhardness, and it was broken into debris again and embedded into the surface [39,40]. The oxide layer could exist steadily, only when its forming rate was higher than the breaking rate. The oxide layer also existed on the C1 surface in Figure 10b, by the result of EDS of area B, as shown in Table 2, which protected the coating from the wear. Therefore, the wear mechanics for all coatings is an abrasive wear combined with an oxidation wear. Compared with those on the C1 surface in Figure 10b, more chipping and holes existed on the C2 surface in Figure 10c. It suggested delamination occurred easily on the C2 surface. As the load was increased to 10 N, more spalling holes appeared on the worn surface, as shown in Figure 10d-f. The morphology of C0 obtained with a load of 10 N and a frequency of 5 Hz was similar to that obtained with a load of 5 N and a frequency of 5 Hz, which suggested the same wear mechanics. Evident plastic deformation is presented in Figure 10e. The plastic deformation denoted the adhesive wear appearance [41]. In this situation, the ball pair surface contacted with the friction surface steadily. Therefore, its COF curve displayed the minimum fluctuation as shown in Figure 8b. In Figure 10f, many spalling holes were observed, illustrating severe brittle fracture occurred [42]. The bottom of the spalling holes was actually the fracture surface, where cracks were generated and propagated. Stress concentrated on the inter-splat interface and defects like pores during the wear test. When the stress exceeded the bonding strength, cracks were generated from the region of stress concentration and propagated along the inter-splat interface. Defects such as pores found in the bottom of spalling holes were usually the source of cracks as shown in Figure 10f. As the load and frequency were up to 10 N and 10 Hz, respectively, the amount of chippings decreased dramatically, and a lot of spalling holes existed on the C0 worn surface, as shown in Figure 10g. No evident black chipping illustrated that the wear mechanics was just an abrasive wear. Apparent grooves appeared in the center of the C1 worn surface as shown in Figure 10h, and only a few spalling holes were seen. In Figure 10i, some traces of plastic deformation were observed besides delamination on the C2 surface, showing the wear mechanics is an abrasive wear combined with an adhesive wear. As shown in Figure 10f, the spalling holes became small. High heat inside the contact surface softened the surface and then reduced the brittle fracture, which promoted the opportunity of an adhesive wear and improved the wear resistance at some level. Therefore, the wear rate of C2 decreased, when the frequency increased from 5 to 10 Hz, as shown in Figure 9.

Effect of the Compressive Residual Stress
Crystallization for amorphous materials during friction should be considered first. The flash temperature during a wear test can be presented by Archard's equation [43]: where T F is the flash temperature ( • C); µ is the COF; V is the velocity (m·s); F N is the load force (N); σ s is yield strength of coating (Pa), and K is the thermal conductivity of coating (W·m −1 ·C −1 ). σ s can be obtained from microhardness [44], and K values (5 W·m −1 ·C −1 ) were assumed the same for all coatings to compute the flash temperature. The highest calculated flash temperature was 375.81 • C, lower than the coating's glass transition temperature [45]. It illustrated that no crystallization occurred during the wear test. C2 with the highest microhardness did not have the lowest wear rate, showing that hardness was not the only factor in improving wear resistance in the present work. Compared with C2, C1 is of the lowest porosity and highest compressive residual stress. Low porosity meant high density and few defects, which restricted cracks generation and propagation. Meanwhile, precious literature has also given the point that the high compressive residue stress is beneficial for wear resistance whereas the tensile residual stress induced negative effects on wear resistance [19,46]. As shown in Figure 11a, the area from the ball was compressed andthe area behind the ball was tensed during the sliding process. Fracture was always in the tensile stress area. When the tensile stress exceeded the inter-splat bonding strength, cracks occurred. The intrinsic compressive residual stress could neutralize parts of the tensile stress, which delayed the formation and propagation of microcracks [47]. As shown in Figure 11b,c, the crack growth was prevented in C1, while the crack propagated fully in C2 and more chipping was on the C2 surface.
As the frequency was increased to 10 Hz at A load of 10 N, the wear rate of C2 decreased significantly although it was still higher than that of C1, while the wear rate of C1 increased slightly. The different variation tendencies of wear rates for C1 and C2 further illustrated that porosity and hardness were not the only influencing factors. The compressive residual stress should be an important factor that restricted crack generation and propagation and protected splats from delamination at a low frequency. At a high frequency, the quick relative motion during the wear test resulted in higher heat and larger deformation in the contact surface [48,49]. The deformation relaxed the residual compressive residual stress further [50,51]. It led to an increase in the wear rate of C1. More deformation usually occurred on the center of the sliding surface, where the influence of the compressive residual stress on the wear decreased dramatically [52]. Therefore, lots of grooves concentrated on the center of the C1 worn surface, as shown in Figure 10h.

Conclusions
Hard particles were delivered into a flame flow to achieve synchronous shot peening during the HVOF spraying process. The result showed that the porosity was reduced, the coating became denser, and its microhardness was increased significantly. Moreover, the compressive residual stress was induced by shot peening, and it played an important role in wear resistance. Shot peening of hard particles ZrO 2 was random in the spraying process. In individual cases, the peeing went against splats spreading or broke the deposited particles. Exceeding shot peening led to coating damage and relaxed the compressive residual stress.
It can be seen from the results of the wear test that the COF did not change obviously for coatings with and without shot peening. Compared with the unpeened coating, the wear resistance of the coating under proper shot peening increased three-fold at a low frequency. It was mainly attributed to the induced compressive residual stress. As the frequency of the wear test was increased, high heat generated during the sliding process relaxed some of the compressive residual stresses. However, the wear resistance of coatings obtained by the shot peening-assisted process was improved under all test conditions. Hopefully, the new process can be applied in other coating preparation.

Conflicts of Interest:
The authors declare no conflict of interest.