Impact of Rough Zirconia Coating on the Surface Friction Properties of Polycrystalline Zirconia

Zirconium alloys are widely used in the nuclear industry due to their excellent mechanical properties. The grid and cladding tubes in fuel rods of nuclear voltage water reactors often experience micro motion wear between equipment due to oscillations generated by coolant flowing through the pipelines. Moreover, the material is prone to oxidation and the formation of an oxide film on its surface when exposed to complex environments, such as high temperature, high pressure and corrosion for a long time, greatly reducing its service life. Although the precision of material processing is getting higher and higher nowadays, the surface roughness of materials can be achieved to be very small. However, the influence of the roughness of the two contact surfaces on friction and wear can not be ignored at the microscopic level. Therefore, the W-M fractal theory was used to establish zirconia coatings with different roughness levels (D=2.3, 2.5, 2.6, 2.7, 2.8). Molecular dynamics simulations were used to simulate the grinding process of diamond hemispheres and polycrystalline zirconia substrates with different roughness levels. Quantitative analysis was conducted on friction force, friction coefficient, wear amount, and wear depth, and combined with stress-strain, dislocation extraction analysis (DXA), and thermodynamic analysis methods to explore the plastic deformation inside the matrix. The results showed that as the roughness of the zirconia coating increased, the friction force and average friction coefficient showed a decreasing trend. The actual contact area between the grinding direction of the indenter and the coating and substrate decreased, resulting in a decrease in their interaction force. The normal reaction force of the coating on it increased, leading to a gradual decrease in the friction force on the indenter; The atomic weight of the substrate decreased and the wear depth decreased, resulting in weakened adhesion and plowing effects during the wear process. Indicating that an increase in roughness could significantly improve the surface wear resistance of polycrystalline zirconium and reduce the damage to the sub surface of the matrix. According to DXA analysis, there were fewer areas of stress concentration on the surface of the matrix, and the depth of transmission was shallower. Stacking faults were generated in the areas under the pressure of the internal pressure head of the matrix. However, the degree of sub surface damage and the internal lattice defect structure weakened correspondingly with the increase of roughness. The total potential energy showed a trend of first increasing and then decreasing, E_C-C showed an upward trend, and E_C-Zr showed a downward trend. Δ E_C-Zr showed a downward trend, Δ E_Zr-Zr showed a downward trend, Δ E_C-C had no significant change pattern. Research had shown that when designing zirconia coatings with the same coating thickness, a zirconia coating with higher roughness could enhance the wear resistance of polycrystalline zirconia matrix and reduce the degree of damage to the matrix material during grinding.