The influence of pre-oxidation technology on NiAl laser cladding coating is deeply discussed, including the key performance indicators such as the microstructure and hardness, friction coefficient, and wear resistance of the coating. The experimental results show that after the pre-oxidation technology treatment, the NiAl laser cladding coating has a new Al2O3 phase in addition to the NiAl main phase, and the coating hardness is increased by about 20% on the basis of the NiAl coating, the friction coefficient is reduced by about 23%, and the wear resistance is significantly improved. It can be concluded that the NiAl laser cladding coating has achieved a benign transformation from microstructure to performance through pre-oxidation technology, and has a high practical application value.

In the field of modern materials science and engineering, NiAl laser cladding coatings have received widespread attention due to their excellent high temperature resistance, oxidation resistance and wear resistance. Such coatings are widely used in aviation, aerospace, automotive and energy industries, and play a key role in improving the service life and reliability of various engineering components.

As an effective surface treatment method, pre-oxidation technology has been successfully applied to a variety of metal materials in recent years to optimize the microstructure and macroscopic properties of materials. Therefore, this paper aims to deeply explore the influence of pre-oxidation technology on NiAl laser cladding coating, including the key performance indicators such as the microstructure and hardness, friction coefficient, and wear resistance of the coating. The results of this study can help to gain a deeper understanding of the potential application of pre-oxidation treatment in the field of materials science and engineering, and can also provide theoretical guidance and reference for the optimization of NiAl laser cladding coating in actual production, which has important academic and practical value.

1 Experimental materials and methods

1.1 Material and coating preparation
In this experiment, Q235 steel commonly used in industry was selected as the base material, and its size was specified as 80 mm×80 mm×10 mm. In order to obtain high-quality NiAl cladding coating, spherical NiAl powder was selected as the cladding raw material. The particle size of this NiAl powder is distributed in 80~250 meshes, ensuring that the coating has uniform texture and good bonding with the base material. First, the NiAl powder was pre-oxidized using a KT1800 high-temperature box furnace. When the furnace temperature reaches 750 ℃, the NiAl powder is evenly spread in the crucible and then placed in the furnace for oxidation. The pre-oxidation holding time is set to 25 min, and then the furnace is naturally cooled to room temperature. In order to ensure that there are no defects caused by impurities or moisture during the preparation of the cladding coating, the pre-oxidized NiAl powder is placed in a vacuum drying oven at 110 ℃ for 45 min to effectively remove the moisture that may exist in the powder.

During the coating preparation stage, the RF-LCD-3500 laser cladding equipment was used to coat the surface of Q235 steel. Two different NiAl coatings were prepared during the coating preparation process: one is the coating directly clad by the original NiAl powder, and the other is the coating clad by the NiAl powder after pre-oxidation.

1.2 Experimental method
The prepared NiAl laser cladding coating was cut into the required test size by high-speed electric spark wire cutting technology, and then finely ground and polished to ensure that the surface to be tested was flat and defect-free. The surface morphology of the coating was observed by JM-4KT optical microscope and VHX-2000E ultra-depth microscope. The detailed structure and wear marks were observed by ZEISSUltra55 scanning electron microscope and AxioScope.A1 Zeiss microscope. The XpertPro X-ray diffractometer was used for phase analysis of the coating. At the same time, in order to verify theoretically, the expected phase composition of the coating was calculated by Thermo-Calc software and compared with the experimental data. The size of the dendrite was measured by ImagePro Plus software, and the hardness test from the coating surface to the substrate was performed by HXS-2000Z microhardness tester. Finally, the coating was subjected to a reciprocating friction test using a Bruker UMT-8 friction and wear tester. The friction object was a SiO2 ball with a diameter of 5 mm, the load was set to 6 N, and the friction time was set to 1 h. The wear traces of the coating were observed in detail using the Bruker ContourElite-X three-dimensional surface morphology analyzer, and the coating wear volume was accurately calculated based on this. The experimental results provided solid data support for subsequent performance analysis.

2 Experimental results and analysis

2.1 Changes in coating morphology
The surface morphology of the coating is shown in Figure 1. As can be seen from Figure 1, the surface structure of the coating is clear, without obvious cracks or other defects. Using an ultra-depth of field microscope, it is found that the coating has good tightness and no obvious defects. The convex and concave parts on its surface are caused by the surface tension of the liquid molten pool. After solidification, a protruding part is formed in the center area of ​​the coating, and a sunken part is formed at the junction.

2.2 Phase composition of coatings
X-ray diffraction analysis was performed on both the NiAl coating and the pre-oxidized NiAl coating, and the specific data are shown in Figure 2. From the data in Figure 2-1, it can be seen that both the original NiAl coating and the pre-oxidized NiAl coating are mainly composed of NiAl. The reason is that in the binary phase diagram of Ni-Al, NiAl has a wide range of components and is easily formed through certain reactions. Therefore, the dominant phase of both coatings is NiAl.

However, compared with the original NiAl coating, the pre-oxidized NiAl coating has more diffraction peaks of Al2O3, indicating that some oxygen was introduced during the pre-oxidation process. At the same time, under laser irradiation, the powder undergoes a melting and solidification process, resulting in the stable existence of Al2O3 in the coating. In addition, the calculation data of the Thermo-Calc software also verified the existence of the Al2O3 phase in the coating. The specific calculation results show that the original NiAl coating is mainly a single phase of NiAl, as shown in Figure 2-2, while the coating after pre-oxidation treatment contains Al2O3 phase in addition to NiAl phase, as shown in Figure 2-3.

Further analysis and certification show that the M element in the coating is Al. Comparing the data results, it can be determined that in the coating treated with pre-oxidation technology, in addition to the main phase of NiAl, Al2O3 phase is also added. This discovery can provide an important theoretical basis for subsequent coating performance research.

2.3 Coating performance analysis

2.3.1 Microhardness test
In order to comprehensively evaluate the effect of pre-oxidation technology on the hardness of NiAl coating, the HXS-2000Z microhardness tester was used to perform hardness tests from the coating surface to the substrate. The measurement results are shown in Table 1.

As can be seen from Table 1, the hardness of the NiAl coating after pre-oxidation treatment is significantly increased compared with the hardness of the original NiAl coating, which is increased by about 20%. This shows that the NiAl coating after pre-oxidation treatment has a stronger hardness and better technical effect in actual use.

2.3.2 Friction and wear performance analysis
The reciprocating friction test was carried out by Bruker UMT-8 friction and wear tester, and the friction and wear performance of the two coatings were compared and analyzed according to the results, as shown in Table 2.

As can be seen from Table 2, the friction coefficient of the NiAl coating after pre-oxidation treatment is reduced by about 23% compared with the original NiAl coating. At the same time, its wear volume is also reduced from 2.5 mm3 to 1.9 mm³, showing its superior wear resistance.

3 Conclusion
1) The NiAl composite powder is prepared into a metallurgical coating by laser cladding technology. The coating is well formed and has no microscopic defects such as cracks or pores.

2) The phase analysis results of the coating show that the pre-oxidation treatment does introduce Al2O3 phase into the coating, but the dominant phase is still NiAl.
3) Compared with the original coating, the hardness of the pre-oxidized NiAl coating increased by about 20%, the friction coefficient decreased by about 23%, and the wear performance was significantly improved. This provides a new perspective for the practical application of NiAl coating and reflects its higher application value.