Titanium alloys are indispensable in aerospace and military fields for manufacturing precision components owing to their exceptional specific strength and stiffness—properties that enable weight reduction while maintaining structural integrity under high mechanical loads. However, in harsh low-temperature service environments, titanium alloys suffer from insufficient wear and corrosion resistance, necessitating surface modification. Ceramic coatings, renowned for their high hardness, excellent chemical stability, and resistance to extreme environments, are commonly applied to address this issue. Among various coating technologies, Plasma Electrolytic Oxidation (PEO) stands out due to its advantages of strong film-substrate adhesion and the ability to tailor coating composition for functionalization. Nevertheless, ceramic coatings—primarily composed of ionic or covalent bonds—exhibit inherent brittleness, which is exacerbated at low temperatures. This brittleness severely limits the service reliability and lifespan of titanium alloy components in low-temperature environments. Tetragonal zirconia (t-ZrO₂) has emerged as a promising toughening agent for ceramics by undergoing a martensitic transformation to monoclinic zirconia (m-ZrO₂) under external stress or temperature changes, accompanied by a 3%-5% volume expansion. This expansion generates a compressive stress field that inhibits crack propagation, thereby improving the coating's toughness. Critical to this effect is the martensitic transformation start temperature (M_s) of t-ZrO₂, which is strongly dependent on grain size. Optimizing the grain size of in-situ formed nano t-ZrO₂ to match specific low-temperature service windows thus becomes a key strategy for enhancing the low-temperature mechanical performance of PEO ceramic coatings.

This study aimed to systematically investigate the influence mechanism of in-situ nano t-ZrO₂ grain size on the low-temperature static and dynamic mechanical properties of Ti6Al4V-based PEO ceramic coatings, and to develop a practical process for improving the coatings' service reliability in cryogenic environments. To achieve this, the PEO process was optimized by adjusting the pulse frequency ranging from 500 Hz to 2000 Hz to prepare four sets of TiO₂/t-ZrO₂ composite ceramic coatings denoted as D1, D2, D3, and D4 samples with distinct average t-ZrO₂ grain diameters, alongside a reference sample (RS) without t-ZrO₂. The Ti6A14V substrates were first machined into two standard sizes: 10 mm × 10 mm × 1 mm for static mechanical testing and 60 mm × 6 mm × 1 mm for dynamic testing. Prior to PEO treatment, the substrates were sequentially grounded, cleaned with ethanol, and dried. The zirconium salt electrolyte used for D1-D4 samples consisted of 15 g/L K₂ZrF₆, 5 g/L (NaPO₃)₆, 5 g/L NaH₂PO₄, 10 g/L NH₄HF₂, and 1 L distilled water, and the phosphate electrolyte for RS contained 10 g/L (NaPO₃)₆, 10 g/L NaH₂PO₄, and 15 g/L NH₄HF₂. All PEO treatments were performed using a single-pulse DC power supply (MAO-20D) under constant current mode (0.2 A/cm²), a fixed duty cycle of 20%, and a treatment time of 20 min.

The t-ZrO₂ grain diameter decreased significantly with increasing PEO pulse frequency: D1 samples (500 Hz) had an average grain diameter of ~40 nm, D2 sample (1000 Hz) ~30 nm, D3 sample (1500 Hz) ~22 nm, and D4 sample (2000 Hz) ~17 nm. This trend was attributed to shorter single-pulse times at higher frequencies, which restricted t-ZrO₂ nanocrystal growth during PEO. XRD analysis revealed that all composite coatings consisted of t-ZrO₂, TiO₂, and a small amount of m-ZrO₂, with the t-ZrO₂ characteristic peak at 30.2° showing increased full width at half maximum and decreased intensity with higher pulse frequency—consistent with Scherrer formula calculations for reduced grain size. The M_s temperature of t-ZrO₂ decreased with decreasing grain diameter. XRD-derived t-ZrO₂ volume fraction (V_t) showed that V_t of D1 sample exhibited a sharp drop from 68% to 26% at 250 K, D2 sample and D3 sample at 200 K, while D4 sample showed no significant V_t change down to 150 K. This was explained by higher interfacial energy in smaller grains, which increased the martensitic nucleation energy barrier. When service temperatures were near M_s, the coatings' plastic deformation ability was significantly enhanced. Nanoindentation tests showed that D1-D4 samples had higher H and E than RS samples, with stepwise H³/E² increases corresponding to stress-induced martensitic transformation. Dynamic mechanical properties were markedly improved by t-ZrO₂ and smaller grain sizes. Dynamic Mechanical Analysis (DMA) results showed that D1-D4 samples had higher loss modulus and tanθ than RS samples across all temperatures. D1 sample peaked at 300 K, D2 sample at 250 K, D3 sample at 200 K, and D4 sample at 150 K, aligning with their M_s temperatures. Smaller grains exhibited superior damping due to increased grain boundary area, which promoted energy dissipation via grain boundary sliding and intragranular dislocation motion. MD simulations confirmed inhomogeneous strain distribution concentrated at t-ZrO₂/TiO₂ interfaces and t-ZrO₂ grains, and an inverse Hall-Petch effect in small grains. D4 sample sacrificed ~29% tensile strength compared to D1 sample to achieve a ~60% higher strain rate, enhancing ductility.

This work demonstrates that adjusting PEO pulse frequency to control t-ZrO₂ grain diameter is an effective strategy to optimize the low-temperature mechanical properties of ceramic coatings. The mechanism involving M_s temperature regulation, interfacial energy enhancement, and grain boundary sliding provides theoretical and experimental basis for the design of low-temperature vibration-resistant coatings on titanium alloys, which is of great significance for improving the service life of titanium alloy components in low-temperature aerospace environments.