Analysis of mechanical interlocking of as-printed LPBF surfaces

This study characterizes as-printed surface of orthodontic brackets fabricated by powder bed laser fusion additive manufacturing method in the context of the potential for mechanical interlocking with polymeric coatings. The ball-like features (BLFs) appearing on the surface of the artefact were quantitatively analyzed in terms of their size, shape, and spatial distribution using digital light microscopy (LM), scanning electron microscopy (SEM), and X-ray micro-tomography (µ-xCT). A Region-based Convolutional Neural Network (R-CNN) was implemented to extract critical parameters such as maximum diameter (D), distribution density (NA), and inter-feature distance (D*). Based on quantitative characteristic of the bracket surface, finite element simulations were conducted to estimate interlocking of metallic substrate to polymer coat. The simulation results were corroborated with a supporting mathematical model. Results indicate that as printed LPBF devices show useful mechanical interlocking, with interlocking stress values from 5 – 47 MPa. Small BLFs (dmax of 5–10 µm) and densely dispersed (D* e.g., 50 µm) maximize anchoring potential and apparent interlocking stress. Also, small value of d/D ratio increases adhesion. Conclusion is drawn that as-printed surfaces of metallic parts intended for coating with polymer may offer mechanical interlocking, which for some application may eliminate further surface post-processing.