Zhaoru He

Evaluation systems for objects after laser removal have not been systematically established, yet they have great significance for repairing engineering coating materials. In general, this paper mainly focused on the surface quality after laser removal, which was simulated by thermal models and characterized by microscopic, residual chemical compositions, and wettability. The laser removal threshold was considered to be a key indicator to regulate removal precision. Here, the thermal model was used to explore the threshold was 15 J/cm², and the ablation P was 308 W at 100 kHz. Besides, the surface structure was observed through simulation before experiments. It also predicted the effect of laser power and overlapping on surface roughness, and therefore provided the theoretical reference for experiments. Then, two groups of samples were set up of the surface of acrylic polyurethane coatings on 2024 aluminum alloy with and without anodizing treatment, respectively. After the coatings were removed, microscopic morphologies demonstrated a wavy undulation structure with little residue. Meanwhile, the increments of roughness were less than 0.5 μm and the surfaces were with nice wettability. Evaluating the surface quality from the above aspects is meaningful and helpful for the non-destructive cleaning of aircraft skin coatings.

In the past few decades, titanium and its alloys have been widely used in the orthopaedic field. However, because titanium is bioinert and lacks antibacterial properties, infection may happen when bacteria attach to implant surfaces and form biofilms. It has been studied that some naturally existing micron-scale topographies can reduce bacterial attachment such as cicada wings and gecko skins. The aim of this in vitro study was to find an implant with good biocompatibility and antimicrobial properties by the modification of micron-scale topographies. In this paper, a femtosecond laser was used to provide microtopography coatings on Ti substrates. The surface morphology of Ti substrates was observed by scanning electron microscopy (SEM). XPS was used to fulfil the chemical compositional analysis. The surface wettability was measured by contact angle measurement system. The effect of microtopography coatings with different surface microstructures on bacterial activities and bone marrow mesenchymal stem cells (BMSC) functions was investigated. The results of in vitro study revealed that microtopography coatings restrain the adhesion of Staphylococcus aureus (S. aureus) and Staphylococcus epidermidis (S. epidermidis), which are common pathogens of orthopaedic implant infections. In addition, microtopography coatings stimulated BMSC adhesion and proliferation. Our studies suggest that a microtopography-coated sample modified by femtosecond laser showed promising antibacterial properties and favourable biocompatibility. The femtosecond laser technique provides an accurate and valid way to produce microtopography coatings with outstanding biocompatibility and antimicrobial properties, and could be widely used to modify the surface of orthopaedic metal implants with great potential.

Fatigue performance has always been an important factor affecting the application of titanium alloy. The service life of TC6 titanium alloy is easily reduced under a continuously alternating load. Therefore, there is an urgent need for a new method to improve fatigue performance. Laser shock peening (LSP) is a widely proposed method to enhance the fatigue performance. Here, through experiments and finite element simulations, it was found that LSP can prolong the fatigue life of TC6 by improving the surface stress state. In strengthening processes, the generation of residual stress was mainly attributed to the change of microstructure, which could be reflected by the statistical results of grain sizes. The content of grains with a size under 0.8 μm reached 78%, and the microhardness value of treated TC6 was 18.7% higher than that of an untreated sample. In addition, the surface residual compressive stress was increased to −600 MPa at the depth of 1500 μm from the surface. On this basis, the fatigue life was prolonged to 135%, and the ultimate fracture macroscopic was also changed. With the treatment of LSP, the fatigue performance of TC6 is highly promoted. The strengthening mechanism of LSP was established with the aim of revealing the relationship between microstructure and stress state for enhancing the fatigue performance in whatever shapes.