Effect of Different Laser Energy Nitriding on the Fretting Wear Performance of Zr Alloy
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Abstract:
The zirconium (Zr) alloy fuel cladding is one of the key structural components of a nuclear reactor and the first and most important line of defense for accommodating fission products. During the operation of nuclear reactors, Zr alloy fuel cladding is subjected to extreme harsh environments, such as high temperature, high pressure and high flow rate for a long period of time. The wear and corrosion resistance of Zr alloys is important for the safe operation of nuclear reactors. Surface modification can effectively improve the corrosion and wear resistance of fuel cladding. Compared with coating technology, nitriding technology does not have problems for bonding between the coating and the substrate. Current research on surface nitriding of Zr alloys mainly focuses on plasma nitriding and ion implantation techniques. Research on laser nitriding of Zr alloy surfaces and their fretting wear characteristics is scarce. In this study, the surface of Zr alloy was treated with laser nitriding at different laser energies. The microstructure of Zr alloy treated with different laser energies and its fretting wear performance were studied. The results showed that after nitriding with different laser energies, the surface of the Zr alloy showed a typical molten state after melting, vaporizing and cooling under the thermal effect of the laser, and this state was more obvious with the increase of the laser energy. At the same time, doping of N atoms and formation of the ZrN phase led to different cooling rates in the molten zone that produced large tensile stresses after cooling. This led to cracks on the surface of Zr alloys after laser nitriding at different energies, and the crack density increased with increasing laser energy. This also led to an increase in the surface roughness of the Zr alloy with increasing laser energy after laser nitriding treatment. Due to the presence of water in the industrial nitrogen, nitrides were generated on the surface of the sample along with some oxides. When the laser energy was 100 mJ, there was no ZrN generation, and N existed mainly as a diffusion layer within the Zr alloy substrate. ZrN generated when the laser energy reached 200 mJ and above, which increased with the increase of laser energy. Due to the generation of ZrN phase and the presence of some oxides, the surface Vickers hardness of Zr alloys after laser nitriding treatment at different energies increased by 37.5% compared to Zr alloys. After laser nitriding treatment, the wear mechanism of Zr alloys changed. For the untreated Zr alloys, the wear mechanism was dominated by delamination and spalling wear, accompanied by oxidative and abrasive wear. The phenomenon of delamination and peeling decreased with the increase of laser energy. Wear mechanisms changed to predominantly abrasive wear with oxidative wear and delamination spalling. The wear volume of sample nitriding with laser energy 400 mJ was reduced by 46.5% compared with that of untreated Zr alloy.
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Keywords:
- zirconium alloy /
- fretting wear /
- laser nitriding /
- delamination /
- laser energy
摘要:为了提高锆合金的耐磨损性能，采用脉冲激光氮化技术对锆合金表面进行激光氮化处理. 采用自主搭建的微动磨损试验机，研究了氮化处理后锆合金的微动磨损性能，利用X射线衍射、扫描电子显微镜和能谱仪等研究氮化处理后物相组成、表面微观形貌和元素分布等. 结果表明：在不同激光能量氮化后，样品表面在激光热效应的作用下呈现出熔融状，并且熔融状结构随着激光能量的增加而变得更加明显；当激光能量为100 mJ时，N主要以扩散层的形式存在于Zr合金基体内部；当能量达到200 mJ及以上时，样品表面出现ZrN峰；与锆合金的空白基体相比，硬度提高了37.5%，激光氮化处理后Zr合金的微动磨损机制发生转变；未处理锆合金的磨损机制以分层剥落磨损为主，同时伴有氧化磨损和磨粒磨损；激光氮化处理后分层和剥落现象随着激光能量的增加而减少，磨损机制转变为以磨粒磨损为主，同时伴有氧化磨损和分层剥落；与未处理的锆合金相比，400 mJ激光氮化试样的磨损量减少了46.5%.
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1. Introduction
The number of nuclear power plants being used is constantly increasing with the development and progress of technology. Among them, pressurized water reactors simplified PWRs are currently the mainstream type of nuclear power plant used. The fuel assembly system is one of the important components of nuclear power plants, mainly composed of cladding tubes, fuel and grids[1-3]. Zirconium (Zr) and its alloys have small thermal neutron absorption cross-sections and excellent corrosion resistance, which make them widely used as cladding tubes in the PWRs nuclear fuel[4-6]. The cladding tube not only faces the threat of high temperature, high pressure and high radiation during service, but also faces the failure problem of fritting wear between it and the grid caused by flow induced vibration (FIV) caused by the high-speed flow of heat exchange medium. The fritting wear failure between the grid and the cladding tube is one of the main reasons for the failure of PWRs[1,7-9]. Therefore, improving the fretting wear resistance of fuel cladding is highly crucial for ensuring the safe and stable operation of nuclear power plants.
The surface modification technology can effectively improve the corrosion resistance and wear resistance of the substrate without changing the substrate material[10-13]. The surface of Zr alloy cladding tubes is mainly protected by coating techniques, such as Cr coating[14-15], micro-arc oxidation coating[16-17], FeCrAl coating[18] and SiC coating[19], etc. However, coatings may introduce many types of defects during the preparation process, such as cracks, pinholes, pores, etc. This will lead to premature failure of the coating during service[20-22]. ZrN has excellent tribological properties, oxidation resistance and radiation resistance, which fully meet the requirements of cladding tubes for coating performance[23-24]. In addition to using PVD coating preparation technology, ZrN layers can also be prepared by nitriding, such as nitrogen ion implantation[25-26] and plasma nitriding[27-29]. Liu et al.[26] found that the injection of N atoms could significantly improve the oxidation behavior of Zr-Sn-Nb and cause the phase of Zr-Sn-Nb alloy to transform from monoclinic to tetragonal structure. The transformation of zirconia from a monoclinic phase to a tetragonal phase is beneficial for the formation of a dense oxide layer, thereby improving its antioxidant performance[30]. Study of Han et al.[25] showed that the wear resistance of the Zr-4 alloy was improved by four times compared to the blank substrate after injecting N ions. Zhu et al.[31] found that high temperatures during plasma nitriding could promote the formation of dense nanocrystalline layers composed of nitrides and oxynitrides, which was beneficial for improving their corrosion resistance. Reger et al.[32] investigated the importance of the effects of nitriding temperature and nitriding time on the performance of in-situ growth ZrN coatings on Zr alloy surfaces. The results showed that the nitride layer was mainly composed of ZrN and ZrO2 phases, and the effect of nitriding time on the performance of the nitride layer was weaker than that of nitriding temperature. Liu et al.[27] used a combination of laser surface texture and plasma nitriding technology to prepare a micro-textured nitriding layer on the surface of Zr-2.5Nb alloy and studied its wear resistance. They found that the formed micro-textured nitriding layer had better wear resistance than single nitriding and laser surface texture. However, currently, more research on surface nitriding of Zr alloys is focused on the medical field, and there is less research on its application in the nuclear power field. The nitriding technology used is mainly plasma nitriding and ion implantation. However, controlling the N content of the nitride layer is difficult when using the above methods for nitriding treatment. Laser nitriding not only has controllable N content, but also short nitriding time and strong metallurgical bonding between the formed nitriding layer and the substrate[33-35]. Currently, there is little research on laser nitriding of Zr alloy surfaces.
In this work, Zr alloys were nitride using different laser energies. The influence of laser energy on the surface microstructure of Zr alloys was studied. The influence of laser energy on its wear resistance performance was studied using a self-developed fretting wear testing machine and revealed the influence of laser nitriding on its fretting wear mechanism.
2. Materials and methods
2.1 Laser nitriding
The composition of the Zr alloy utilized was presented in Table 1. The size of the Zr alloy sample was 15 mm× 20 mm, which was cut by wire cutting method. The thickness of the Zr alloy was 425 μm.
Table 1. The composition of the Zr alloyElement Nb Sn Fe Zr Mass fraction/% 1.0 1.0 0.3 Bal Before laser nitriding, the Zr alloy was cleaned for 15 min in the anhydrous ethanol by ultrasonic. Then the Zr alloy was placed into the irradiation chamber, and the pressure of the irradiation chamber was pumped to 1 Pa. During the laser nitriding process, the laser irradiated the surface of Zr alloy under a 10 Hz repetition frequency. The laser with a pulse width and wavelength were 25 ns and k=248 nm, respectively. Fig. 1(a) showed the schematic diagram of laser nitriding. The spot was generated by a laser beam through a convex lens with 1 mm × 2 mm. Using the thermoelectric energy detector to track the laser’s out energy and adjust the energy density. In the irradiation chamber, the 2D motorized was installed to allow for a larger treatment area. In the X direction, the laser scan move speed was 0.1~1.28 mm/s. In the Y direction, the irradiation was moved 0.5 mm. The Zr alloy’s whole surface was irradiated by repeating these two steps. The preparation parameters of laser nitriding was presented in Table 2.
Table 2. The preparation parametersSample Scanning rate/(mm/s) Laser energy/mJ Nitrogen pressure/105 Pa Untreated ̶ ̶ ̶ 100 mJ 0.6 100 3 200 mJ 0.6 200 3 300 mJ 0.6 300 3 400 mJ 0.6 400 3 2.2 Fretting wear test
The self-developed fretting wear equipment, as shown in Fig. 1(b~c) was used to study the fretting wear performance of different laser energy nitride Zr alloys. The fretting wear equipment used a voice coil motor to achieve tangential displacement of the specimen. Real time displacement signal acquired by grating ruler displacement sensor. Collection of friction force during the experiment obtained by PCB force sensor. Loading normal force acquired through guide rails and weights. Displacement amplitude and fretting frequency controlled by means of a control system. The displacement resolution ≤1 μm, tangential displacement amplitude range of 5~500 μm, force acquisition resolution ≤ 0.2 N, normal force range of 5~100 N, maximum operating frequency of 15 Hz. During the fretting wear test, the grinding pair used a Si3N4 ball with a diameter of 8 mm. And the sliding distance, cycle number and normal force were 100 μm, 10 000 and 10 N, respectively.
2.3 Material characterization
White light interferometer (WLI, SuperView W1, Chotest) was used to characterize the 3D morphology of the sample surface and wear scar. The surface roughness, cross-sectional profile of the wear scar, and wear volume could also be obtained from the 3D analysis. The super-depth field microscope (VXH-6000, Keyence) was used to study the optical microscope of the sample and wear scar. X-ray diffraction (XRD, Empyrean, Malvern Panalytical) was used to characterize the phase of samples. The scanning range was from 20° to 90°, with a scanning step size of 0.02°. X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD) was used to determine and analyze the chemical composition and chemical state of samples. Scanning electron microscope (SEM, Tescan Mira 3 XH) was used to characterise the microscopic morphology of the sample surface and wear scar. The element distribution of wear scar was characterized by an energy dispersive spectrometer (EDS, Aztec X-MaxN 80).
3. Result and discussion
3.1 Microstructure and phase analysis of sample
Fig. 2 showed the optical microscope micrographs, 3D morphology and surface roughness of Zr alloy treated with different laser energy. Different from the untreated Zr alloy, the surface of Zr alloys treated with laser nitriding at different energies exhibited a molten morphology. As the laser energy increasing, the molten morphology became more pronounced. This was mainly due to the re-solidification of the sample surface after melting under the action of the laser during laser nitriding treatment. As the laser energy increasing, more heat was generated during the nitriding process, which increased the melt region on the surface of the Zr alloy, resulting in the formation of a more pronounced molten morphology upon cooling[36]. It could be seen from Fig. 2(e) that the surface roughness of Zr alloys treated with different laser energies increased with the increase of laser energy, which was also consistent with the result of the optical microscope and 3D morphology.
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Figure 2. Optical microscopic morphology and roughness of Zr alloy surface treated with different nitriding processesFig. 3 showed the XRD pattern and surface hardness of Zr alloy treated with different laser energy nitriding processes. Fig. 3(a) showed the XRD pattern of the laser nitriding Zr alloy with different laser energies. The physical phase of the untreated Zr alloy exhibited a Zr peak unique to Zr alloys. XRD patterns of 100 mJ nitride Zr alloy samples showed no significant difference compared to untreated Zr alloys. Unlike the Zr alloy treated with 100 mJ, a weak ZrN peak appeared at position 39° when the laser energy was 200 mJ. When the laser energy reached above 300 mJ, new peaks of the ZrN phase appeared at 34° and 68° compared to the Zr alloy treated with 100 and 200 mJ laser energy. This indicated that at low laser energies, N existed mainly as a diffuse layer in the Zr alloy substrate. As the laser energy increasing, the melting and vaporization regions on the surface of the Zr alloy increased, and the region of reaction with N also increased, which also promoted the formation of the ZrN phase. As the laser energy increasing, the intensity of the Zr peak at around 35° decreased compared to the untreated Zr alloy. The generation of ZrO2 was mainly due to the inevitable presence of O2 and water vapor in the industrial pure N2. And compared to N2, the O2 was more easily combined with Zr alloy to form ZrO2. At the same time, the ZrN could react with oxygen, the N atoms were replaced by the O atom and form ZrO2[37-38]. So during the nitriding process, an oxy-nitriding composite layer was formed. The hardness of Zr alloy before and after treatment was shown in Fig. 3(b). After the laser nitriding, the hardness increases with the increase of laser energy. At a laser energy of 400 mJ, a hardness of 267HV0.2 was achieved, which increased by 31.2% compared to the Zr alloy substrate. This result was mainly due to the formation of nitride and oxide ceramic phases. The lattice distortion of Zr caused by N could also improve the surface hardness of the treated sample [27,37], which was also consistent with the XRD result.
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Figure 3. XRD pattern and Surface hardness of Zr alloy treated with different nitriding processesTo better investigate the composition and element present, the XPS of Zr alloy with different laser energy treatments was characterized. The Fig. 4 showed the XPS survey spectrum and spectral analyses of different constituent elements. Fig. 4(a) showed the XPS survey spectrum which consisted of C 1s, Zr 3d, N 1s and O 1s of samples treated with laser nitriding. The peaks of C corresponded to C introduced during character rather than carbides generated on the surface. At the wide range of XPS spectra, the element valence state couldn’t be detected. To better analyze the element’s chemical states, the Zr, N and O three elements were conducted in narrow scans. Fig. 4(b) showed the narrow scans of the Zr 3d peak. The Zr 3d exhibited four peaks with binding energy were about 178.62~178.95, 180.37~181.10, 181.79~182.00, and 184.18~184.36 eV. Among them, the binding energy of the characteristic peak of metal Zr ranged from 178.62 to 178.95 eV[39]. Two peaks with binding energy ranged 181.79~182.00 and 184.18~184.36 eV were ascribed to Zr 3d5/2 and Zr 3d3/2 of ZrO2, respectively. The distance between the binding energies of Zr4+ 3d5/2 and Zr4+ 3d3/2 was approximately 2.4 eV[31,37,40]. The binding energy ranging 180.37~181.10 eV was ascribed to Zr 3d5/2 of ZrNxOy. This was also consistent with the results of previous reports[31,41]. The narrow sans of N 1s consisted of three peaks, which with binding energy ranged about 395.38~395.63, 396.53~396.88 and 399.28~399.41 eV. Among them, the binding energy ranging 395.38~395.63 and 396.53~396.88 eV was consistent with ZrNx. The binding energy ranging 399.28~399.41 eV was consistent with ZrNxOy[31,42]. The O 1s intensity peak was composed of two distinct peaks with binding energies of 530 and 531.56 eV. These binding energies corresponded to the valence state of O2– in metal oxides and O2– in water vapor or OH-[37,43].
The Fig. 5 showed the SEM micrographs of morphology of Zr alloy before and after treatment under different laser energies. After laser nitriding, the surface of the samples presented a corrugated molten structure. The increase in laser energy made the corrugated molten structure more obvious[44]. This was consistent with the previous results of optical morphology micrographs. From the enlarged area, it could be seen that the number of cracks on the surface of the Zr alloy increased with the increase of laser energy. The cooling rate and the coefficient of thermal expansion of Zr alloys after melting under laser action were changed due to the incorporation of N atoms and the formation of ZrN phase, which led to the formation of cracks on the surface of Zr alloy after laser nitriding[36]. To observe the variation of N element with laser energy, point element scanning was performed at different positions. The distribution of N element on the surface of the sample was uneven, but overall, it increased with the increase of laser energy.
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Figure 5. SEM micrographs of surface morphology of Zr alloy treated with different nitriding processes3.2 Fretting wear performance and mechanism of samples
Table 3 showed the parameters of fretting wear test. Through the fretting wear test, we could acquire Ft-D-N curves and gleaned valuable kinetic information. During the fretting wear progress, the fretting wear could be divided into three states: partial slip regime (PSR), mixed slip regime (MSR) and gross slip regime (GSR)[45-46]. The different mechanisms of fretting wear were also closely related to their failure modes. Fig. 6 showed the Ft-D-N and friction coefficient of Zr alloys before and after being nitrided with different laser energy. From Fig. 6(a~e) could be seen that the fretting states of all samples presented a parallelogram curve, indicating that they belong to the GSR. The Ft-D-N curves of all samples had no obvious changes with the cycle increasing when the cycle exceeded 1×103. This meant during the full fretting wear progress, the samples were always in a state of material loss, and this would result in significant wear volume and wear depth. At the beginning of fretting wear, the Ft-D-N curves were different from that in the later stage. This was related to the sample surface’s initial state, such as surface pollutants, cracks, etc. Fig. 6(f) showed the variation of the friction coefficient of Zr alloy with the number of cycles before and after nitriding treatment. The friction coefficient was calculated using the following formula:
Table 3. The parameters of fretting wear testNumber of cycles Displacement amplitude/μm Load/N Frequency/Hz 10 000 100 10 10 ![]()
Figure 6. The Ft-D-N curve and friction coefficient of Zr alloy treated with different nitriding processes$$ \mu =\frac{E}{4\times F\times D} $$ Where μ was the friction coefficient, E was the dissipated energy, F was the normal load, D was the displacement amplitude.
The dissipated energy of different cycles could be obtained by integrating the area enclosed by the corresponding Ft-D-N curve[47-48]. The friction curve could be divided into two stages: rapidly rise and stable stage. As the number of cycles increasing, the friction coefficient increased rapidly at the first stage and at this time, the sample surface contacted with the grinding pair, surface layer and the pollutants of the sample were removed. After the initial running, the friction coefficient entered a stable stage. At the stable stage, there was no significant change in the friction coefficient curve of Zr alloy before and after laser nitrided with different energy levels.
The wear scar optical microscopy micrographs of morphology of Zr alloy before and after nitriding treatment were shown in Fig. 7. It could be seen that the wear area of Zr alloy was larger than that of Zr alloy treated with laser nitriding. The wear area of Zr alloy treated with different laser energy nitriding was decreased with the increase of laser energy. The wear scar surface of all samples showed black and this might be caused by oxidation during the fretting wear process. Later, we would conduct a detailed analysis by combining SEM and EDS results. Compared to the central contact area, the damage caused by fretting slip in the edge area was slight and the central contact area was accompanied by obvious peeling.
The 3D morphology of the wear scar was shown in Fig. 8. It could be seen that there was a slight accumulation of debris at the edges of the wear scar on all samples. The main reason was that the debris generated during the fretting wear process was gradually pushed around the wear scars due to reciprocating motion. At the edge of the wear scar, traces of plow furrow could be seen, indicating the presence of abrasive wear during the wear progress. This was mainly due to the hard debris generated during the fretting wear process. Concurrently, all samples exhibited peeling in the contact region at the center of the wear scar. Compared with untreated Zr alloy, the peeling area at the center of the wear scar was reduced after laser nitriding treatment. The peeling area decreased with the increase of laser energy. This meant a reduction in wear volume.
Fig. 9 presented the cross-section profile, wear area and wear volume. From Fig. 9(a), it could be seen that the deepest depths of the wear scar on untreated, 100, 200, 300 and 400 mJ samples were 21.0, 21.0, 22.7, 20.2 and 9.0 μm, respectively. The deepest depth of the wear scar was consistent with the 3D morphology. When the laser energy reached 400 mJ, there were no obvious deep peeling pits. Compared with the untreated sample, the wear area of different laser energy treatment samples decreased with the laser energy increase, as shown in Fig.9(b). After laser nitriding with different energy levels, the wear volume also decreased compared to the untreated sample. When the laser energy reached 400 mJ, the wear volume was the smallest. However, the wear volume at 200 mJ was larger than that at 100 mJ, which might be due to the deeper peeling pits at 200 mJ.
The SEM micrographs of microstructure and elemental distribution of the wear scar on the untreated and different laser energy-treated Zr alloys were shown in Fig. 10. From the microscopic morphology of the wear scars, it could be seen that there were plow furrows at the edges of all samples, and the center position of the wear scars was mainly characterized by delamination and peeling. This was also consistent with the previous 3D morphology and optical microscope morphology results. At the same time, there were many cracks at the edge of the wear scar. Based on the previous Ft-D curve (Fig. 6), we concluded that all samples were at GSR. This would result in a large amount of debris generation. According to the previous 3D morphology (Fig. 8), it could be seen that some of the debris were discharged to the edge of the wear scar during the reciprocating motion of fretting wear. However, more debris was repeatedly crushed by the grinding pair during reciprocating motion, forming a third layer between the substrate and the grinding pair. The formation of the debris layer avoided direct contact between the grinding pair and the substrate, which was beneficial for reducing the wear volume. During the fretting wear process of GSR, the normal force transformed into tangential stress during reciprocating motion. The central area of the wear scar experienced the most intense reciprocating tangential stress. This would result in the formation of layered cracks in the debris layer under the action of alternating tangential stress, ultimately leading to peeling off[49-51]. Therefore, all samples had obvious peeling marks at the center of the wear scar. In the edge area of the wear scar, due to the relatively small alternating tangential stress, the formed debris layer only produced cracks without significant peeling. Compared with untreated samples, the peeling phenomenon of the abrasive layer formed by laser nitriding treatment was weakened. From the EDS analysis of the wear scars, it could be seen that there was a significant enrichment of O and Si elements on the wear scars of all samples. This indicated that oxidative wear occurred during the fretting wear process, accompanied by material transfer. In summary, it could be concluded that there was no change in the wear mechanism of the samples before and after laser nitriding treatment with different energy levels. The wear mechanism was mainly characterized by delamination wear, accompanied by oxidation wear and abrasive wear. This was also consistent with the type of wear mechanism in the GSR[45]. Through laser nitriding, the delamination of the sample could be mitigated, thereby improving its wear resistance.
In order to better analyze the wear mechanism, the cross-section of the wear scar was polished and its surface microstructure and element distribution using SEM and EDS were analyzed, as shown in Fig. 11. The SEM micrographs of cross-sectional morphology of the wear scars could provide a more intuitive understanding of the peeling and delamination phenomena that occurred during the wear process. Only the peeling pits and delamination generated during the wear process could be seen on the cross-section SEM micrographs of the untreated sample wear scars. Cracks were observed in the wear scar cross section of the Zr alloy treated with laser nitriding at different laser energies compared to the untreated samples. Combining the previous 3D morphology (Fig. 8) and SEM microstructure micrographs (Fig. 10), it could be seen that this was mainly due to severe peeling of the sample in the center area of the wear scar during fretting wear. After laser nitriding treatment, it was beneficial for the formation of a debris accumulation layer and reduced the peeling phenomenon. During the fretting wear process, the central area bears severed alternating tangential stress, which led to partial peeling of the debris accumulation layer[49-51]. Part of the unpeeled debris layer also developed cracks between the substrate under the action of alternating tangential stress. From the distribution of elements in the cross-section micrographs of the wear scars, it could be seen that it was consistent with the surface of the wear scars, mainly enriched with O and Si elements above the wear scar.
Schematic models of the Zr substrate before and after the addition of N atoms were drawn using the “Visualization of Electronic and Structural Analysis” software, as shown in Fig. 12[52]. Accompanied by the addition of N atoms obviously induced changes in the crystal structure of the Zr substrate. Fig. 13 displayed a schematic representation of the wear mechanisms for untreated samples and laser-nitrided samples. According to the Ft-D-N curve (Fig. 7), it could be concluded that all samples belong to GSR during the fretting wear process[45]. This meant that the material was constantly removed during the fretting wear process, resulting in the generation of a large amount of debris. Part of the debris was discharged to the outer edge of the wear scar during reciprocating motion. Remaining of the debris was repeatedly crushed during reciprocating motion, forming a debris accumulation layer that existing between the sample and the grinding pair. For untreated samples, the interface between the debris accumulation layer and the Zr alloy substrate nucleated and growed under alternating tangential stress, forming cracks parallel to the matrix. This ultimately led to spalling of the debris accumulation layer[49-51]. The cross-section profile of the wear scar gave a V-shape in the peeling area[53]. Due to the relatively small alternating tangential stress at the edge of the wear scar, there was a small amount of debris accumulation layer present. This results in the wear mechanism of untreated Zr alloys being dominated by delamination and peeling, supplemented by abrasive wear and accompanied by oxidative wear. For the samples treated with laser nitriding, as shown in Fig. 13(b), after laser nitriding treatment, the sample could be divided into three layers: nitride layer, diffusion layer and substrate[33]. According to the previous XRD results (Fig. 3), it could be seen that a ZrN layer was only formed on the surface of the Zr alloy when the energy reached 200 mJ. This meant that the role of the nitrogen diffusion layer in the fretting wear process was much greater than that of the nitride layer. During the formation process of the nitride layer, the diffusion of N atoms would cause lattice distortion of Zr, as shown in Fig. 12, this resulted in a solid solution strengthening effect[37]. The hardness of Zr alloy also increased after laser nitriding, therefore, the presence of N atoms reduced the peeling phenomenon of Zr alloy during fretting wear and reduced the wear volume of the sample. The wear mechanism changed to predominantly abrasive wear, accompanied by delamination and oxidative wear. However, under the action of alternating tangential stress, cracks also nucleated and formed between the partially unpeeled debris layer and the substrate. Overall, as the laser energy increased, the depth of the nitriding layer and N diffusion layer became thicker and the peeling phenomenon became milder. When the laser energy reached 400 mJ, the wear depth was shallowest and the wear volume was smallest.
4. Conclusion
The fretting wear performance of Zr alloy before and after laser nitriding was investigated. The effects of laser nitriding treatment and different laser energy nitriding treatments on the fretting wear mechanism of Zr alloy were discussed. The conclusions were as follows:
a. After laser nitriding, the surface of Zr alloy exhibited a molten structure, which became more pronounced with the increase of laser energy. This led to an increase in surface roughness of Zr alloy after laser nitriding. As the laser energy increased, the crack density on the surface of the sample also increased.
b. When the laser energy was low, N existed in the form of a diffusion layer within the Zr alloy matrix. Only the laser energy reached 200 mJ and above, ZrN formation occurred on the surface of the Zr alloy. The formation of nitrides and the diffusion of N atoms caused lattice distortion of Zr helps to improve the surface hardness of Zr alloy.
c. The fretting wear state of Zr alloy before and after laser nitriding was in the GSR state without any transformation. The wear mechanism changed from a predominantly delamination and peeling to a predominantly abrasive wear, accompanied by oxidative wear. As the energy of laser nitriding increased, the peeling phenomenon also decreased. The wear volume and wear area were significantly reduced. The minimum wear volume and area were achieved when the laser energy reached 400 mJ.
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Figure 2. Optical microscopic morphology and roughness of Zr alloy surface treated with different nitriding processes
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Figure 3. XRD pattern and Surface hardness of Zr alloy treated with different nitriding processes
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Figure 5. SEM micrographs of surface morphology of Zr alloy treated with different nitriding processes
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Figure 6. The Ft-D-N curve and friction coefficient of Zr alloy treated with different nitriding processes
Table 2 The preparation parameters
Sample Scanning rate/(mm/s) Laser energy/mJ Nitrogen pressure/105 Pa Untreated ̶ ̶ ̶ 100 mJ 0.6 100 3 200 mJ 0.6 200 3 300 mJ 0.6 300 3 400 mJ 0.6 400 3 
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Table 3 The parameters of fretting wear test
Number of cycles Displacement amplitude/μm Load/N Frequency/Hz 10 000 100 10 10
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