Figure 1 is the microstructure of the sample, including the cross-sectional morphology, surface morphology, surface hardness statistics and surface roughness statistics of the sample. From the cross-sectional morphology, with the increase of (CH3CO3)3Ce·xH2O addition, the thickness of the layer increases, and the micropores and discharge channels are gradually filled. Considering that (CH3CO3)3Ce·xH2O itself is a rare earth organic compound, in the process of layer preparation, on the one hand, it has a strong affinity for non-metals, on the other hand, it has the effect of hindering grain growth and refining grains, so some micropores and discharge channels are filled. Because there is a large amount of sodium phosphate in the electrolyte system, it is speculated that there may be a new phase formation. From the surface morphology of the sample, with the increase of (CH3CO3)3Ce·xH2O, the surface of the sample appears bright ' flake ' morphology. From the three-dimensional map, it can be seen that the raised ' small mountain peaks ' are gradually flushed with the green quasi-plane, and some light blue ' pits ' appear. Combined with the cross-sectional morphology and surface roughness of the layer, it can be concluded that with the increase of (CH3CO3)3Ce·xH2O addition, the thickness of the prepared layer gradually increases and the surface flatness gradually decreases. Ten points were randomly selected on the surface of the layer to test its hardness. It was found that when the addition amount of (CH3CO3)3Ce·xH2O was 7.5 g/L, the maximum hardness value was 961.97 HV, which was 1.8 times that of the unadded (CH3CO3)3Ce·xH2O and 8.36 times that of the substrate. Therefore, it can be inferred that the addition of (CH3CO3)3Ce·xH2O makes a new phase appear in the layer, which further improves surface hardness.
Through the tribological test of the sample, Fig. 2 is obtained, which includes the wear morphology, dynamic friction coefficient, average friction coefficient and wear amount of the layer surface. From the wear trajectory, when the addition amount of (CH3CO3)3Ce·xH2O is 7.5 g/L, the wear trajectory is the most uniform, and the motion trajectory is narrower than that without (CH3CO3)3Ce·xH2O. It can be seen from the variation of friction coefficient with time that when the addition amount of (CH3CO3)3Ce·xH2O is 7.5 g/L, the friction coefficient changes the least, and the overall is maintained at 0.05–0.07. Through the statistics of the average value of friction coefficient and wear amount, it is concluded that when the addition amount of (CH3CO3)3Ce·xH2O is 7.5 g/L, the average friction coefficient and average wear amount are the lowest. Therefore, it can be inferred that the formation of new phase has a significant effect on improving the wear resistance of the sample.
It can be seen from Fig. 3 that when the addition amount is 0 g/L, the surface of the layer shows a sprayed ' volcano ' morphology, which is ejected from the pores and accumulated around; when the addition amount is 7.5 g/L, the pores on the surface of the layer are filled in the form of scales. The porosity was calculated by Image J software, and the porosity was reduced from 5.4–3.69%. From the enlarged wear trajectory, the wear scar morphology is more uniform and on the same plane when the addition amount is 7.5 g/L, which is in stark contrast to the wear trajectory when the addition amount is 0 g/L.
It can be seen from the Fig. 4 that the newly formed phase is mainly Ce(PO)4 and the amorphous phase produced by the diffraction angle at 21.6 ° − 38.6 °. There are obvious three strong diffraction peaks of Ce(PO)4 at the position of the diffraction peak of the amorphous. It can be seen from the EDS diagram that the Al and O elements are the most obvious, and the main phases formed by the layer are α-Al2O3 and γ-Al2O3. Considering that the layer has good non-conductivity, Au element was sprayed on the sample. By adding NaS2 to the electrolyte to form MoS2 in situ, the presence of Mo and S elements was also detected from EDS. The formation of Al2O3 and MoS2 is beneficial to improve the wear resistance of the layer [9–12]. Followed by the addition of 7.5 g/L (CH3CO3)3Ce·xH2O, it can be seen in the EDS diagram that there is a clear presence of Ce elements. Therefore, it is concluded that the newly generated phase is mainly Ce(PO)4 and amorphous phase with more Ce elements.
In order to further determine the friction and wear phase between the participating metals, the valence state of the elements at the wear site was detected by XPS, and Fig. 5. During the preparation of micro-arc oxidation composite ceramic layer, the electrolyte and metal materials are ionized under the action of high-voltage electric field. Electrons free from metal materials move directionally under the guidance of external electric field to form current. In the preparation process of the ceramic layer, the external power supply is alternating current, the substrate is connected to the anode/cathode of the circuit, and the electrolytic cell is connected to the cathode/anode of the circuit. The charged ions carry the charge energy to continuously impact the surface of the substrate. According to the law of conservation of energy, part of the energy disappears in the form of heat energy, sound energy and light energy. Another part of the energy is absorbed by the matrix. The appearance of thermal energy makes the surface of the substrate produce high temperature, and the surface appears molten material, which is mainly composed of Al. Affected by external factors such as frequency, duty cycle, pulse number, and external cooling system, the molten material undergoes thermochemical and ionic chemical reactions during this process and produces solidification and accumulation. The addition of (CH3CO3)3Ce·xH2O and Na2S effectively avoided the aggregation of Na+ with PO3− and MoO42−. Combined with XRD, EDS and XPS test results, it is concluded that the in-situ accumulation phase of the ceramic layer contains Al2O3, MoS2 and Ce (PO4). According to the friction test, it can be clearly seen that the development of this composite ceramic layer will open up new areas to improve the wear resistance of metals and alloys.