*3.1. Surface Analysis*

During service, 3D printed polymer structures are typically exposed to aggressive environmental conditions that may lead to the initiation and propagation of surface damage and eventually failure of the entire structure under loading conditions. Therefore, investigation of the surface microstructure of 3D printed parts is necessary to evaluate their performance in outdoor environments.

The digital microscopy images of the surface of uncoated ABS samples before and after 1200 h of exposure to UV radiation and moisture are given in Figure 2a,b. The yellowing of the surface areas due to UV radiation and the creation of surface microcracks can be observed in Figure 2b. The cyclic temperature variation inside the environmental chamber produces thermal stresses in the specimens, which facilitates the creation and propagation of microcracks on the UV-exposed surfaces. Figure 3a,b present the surface microscopy images of ASA-coated ABS specimens before and after environmental aging. It can be observed that the ASA coating provides good protection for the underlying ABS specimen and no microcracking was observed on the specimen's surface.

**Figure 2.** Digital microscopy images of the surface of uncoated acrylonitrile-butadiene-styrene (ABS) specimen (**a**) before and (**b**) after 1200 h of exposure to UV radiation and moisture.

**Figure 3.** Digital microscopy images of the surface of acrylic-styrene-acrylonitrile (ASA)-coated ABS specimen (**a**) before and (**b**) after 1200 h of exposure to UV radiation and moisture.

To better assess the damage in the specimens after environmental aging, scanning electron microscopy (SEM) images of the surface area of uncoated and ASA-coated ABS samples before and after environmental exposure are presented in Figures 4 and 5. It can be seen that an extensive propagation of microcracking occurred on the surface of the uncoated specimen after 1200 h of environmental aging. The UV-induced microcracks and a high-temperature environment facilitate the ingression of moisture into 3D printed polymer structures that can have detrimental effects on the long-term durability of structures.

**Figure 4.** SEM images of the surface of uncoated ABS specimen (**a**) before and (**b**) after 1200 h of exposure to UV radiation and moisture.

**Figure 5.** SEM images of the surface of ASA-coated ABS specimen (**a**) before and (**b**) after 1200 h of exposure to UV radiation and moisture.

The environmental exposure has no noticeable effects on the surface microstructure of ASAcoated ABS specimens, as illustrated in Figure 5b. No observable microcracks were generated on the structural surface and the ASA-coated ABS specimen retained its structural integrity throughout the exposure.

A robust interfacial adhesion between the coating and the substrate is essential for the durability of 3D printed structures created by multi-material additive manufacturing. The strength of the adhesion bond between the ASA coating and ABS substrate before and after 1200 h of environmental aging was examined according to ISO 2409 standard. The adhesion test was performed on ASA-coated ABS 3D printed samples with dimensions of 60 mm (length) × 60 mm (width) × 3.2 mm (thickness). Since the thickness of the ASA coating is 0.25 mm, six cuts with 3 mm spacing were created along the length and width directions following the standard.

No separation of ASA coating was observed after the removal of adhesive tape from the surface of both environmentally aged and unexposed specimens. The adhesion of ASA coating to the ABS substrate was classified as grade 0 based on the ISO 2409 standard, demonstrating the highest level of resistance of the coating to the separation from the substrate that remained unaffected during the environmental aging.
