激光同步预热对等离子喷涂 Al₂O₃-TiO₂ 涂层组织与性能的影响

Conventional plasma-sprayed ceramic coatings usually exhibit a lamellar and porous structure due to the limited inter-splat bonding, resulting in coating bond strengths generally below 30 MPa. This significantly restricts their application in highly corrosive and heavy-load operating conditions. Studies have revealed that for specific ceramic materials, chemically bonded and fully dense ceramic coatings can be obtained when the substrate temperature exceeds the critical threshold. However, effectively preheating complex-shaped or large-scale metal structures poses technical challenges in practical production. Lasers, characterized by high power density, enable rapid heating of the substrate surface to elevated temperature within a short period without significantly increasing the temperature of the entire metal structures. Therefore, in the present work, laser-synchronized preheating was introduced into plasma spraying for depositing Al₂O₃-TiO₂ (AT13) ceramic coating of high quality on TC4 titanium alloy substrate. The AT13 coatings were prepared using a plasma spraying system with internal powder feeding. A 1 064 nm wavelength fiber laser was employed for substrate preheating. A specially designed fixture integrated the plasma torch and laser head into a system, with movement controlled by a robotic arm. To ensure full preheating coverage of the plasma powder stream area (12 mm diameter), a rectangular laser spot measuring 15 mm in length and 2 mm in width was utilized. The long edge of the laser spot was aligned tangentially to the powder stream to mitigate the rapid temperature drop. The coating microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Effects of laser power on bond strength and microhardness were investigated via Vickers hardness testing and electronic tensile testing. Impact resistance was evaluated by drop-weight impact test. Results show that at high plasma torch traverse speed of 400 mm/s, laser beam with a power over 2 kW (with the rectangular spot tangent to the powder stream) can successfully preheat the TC4 substrate surface above the critical temperature of 300 ℃. With the increasing laser power, the lamellar structure in AT13 coatings disappear and the coating reveals a sintered bulk-like characteristic. As the laser power increases from 0 to 6 kW, the porosity decreases from 4.6% to 1.2%. The coating sprayed without laser preheating primarily consists of metastable γ-Al₂O₃, minor Al₂TiO₅, and α-Al₂O₃ phases. The metastable γ-Al₂O₃ originates from rapid solidification of ceramic droplets, while α-Al₂O₃ derives from partially unmelted AT13 powder. As the laser power increases, slower cooling rates promotes gradual reduction of γ-Al₂O₃ content and the growth of α-Al₂O₃ content. At 4 kW, γ-Al₂O₃ disappears and the α-Al₂O₃ becomes the predominant phase. At 6 kW, the highest Al₂TiO₅ content is detected, which generates significant thermal stresses during cooling and inducing periodic cracks perpendicular to the coating thickness due to its much lower thermal expansion coefficient. The microstructure change highly influences the properties of the resultant coatings. As the laser power increases from 0 to 6 kW, the microhardness improves from 933HV0.1 to 1 422HV0.1, and the average bond strength increases from 35.7 MPa to 62.2 MPa. A steel ball with a diameter of 40 mm and a mass of 270 g was dropped in free fall from a height of 1.8 m to impact the flange. After 5 impacts, no cracking or delamination is observed on the coating surface, demonstrating that the coating's impact resistance exceeds 4.76 J - surpassing that of conventional plasma-sprayed ceramic coatings. The laser-synchronized preheating strategy enables the fabrication of AT13 coatings with high density and bond strength, meeting the requirements for high-performance ceramic coatings on complex-shaped metal structures.