**1. Introduction**

Metal materials have excellent comprehensive mechanical properties (high strength, toughness, fatigue resistance) and good processing and forming ability [1,2]. Thus, medical metal implants have been widely used in the field of orthopedics. Degradable metals not only have excellent comprehensive mechanical properties, but also the degradation products can be absorbed by the human body [3,4]. As a new type of medical implant, it is able to be completely degraded and absorbed in the human body after service in vivo, avoiding the pain of patients' secondary operation. Among the several representative degradable metal, iron (Fe) has gained intensive attention recently [5,6]. It is an essential nutrient element and participates in a variety of metabolic processes, which is able to maintain the normal function of bone cells. However, its degradation rate is too slow, which will hinder the growth of new bone as an implant.

Destroying the passive film is an effective way to accelerate its degradation of metallic matrix. Owing to the multivalent character of sulfur (S), sulfide is able to generate various S-containing species, which can attach on the metal surface and induce severe damaging effect on the passive film [7]. Previously, some scholars studied the S-induced corrosion of Fe-based metal material and indicated that the adsorbed S catalyzes the metal dissolution, thereby resulting in a decreased dissolution activation energy and accelerated anodic dissolution process [8]. It was also reported that the S-containing species changed the film properties including the semiconductivity, doping concentration, film dissolution rate, and

**Citation:** Zhou, Y.; Xu, L.; Yang, Y.; Wang, J.; Wang, D.; Shen, L. Microstructure and Corrosion Behavior of Iron Based Biocomposites Prepared by Laser Additive Manufacturing. *Micromachines* **2022**, *13*, 712. https:// doi.org/10.3390/mi13050712

Academic Editor: Kunal Mitra

Received: 10 April 2022 Accepted: 28 April 2022 Published: 30 April 2022

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film composition. Particularly, the synergistic effect of S species and chloride (Cl) was also confirmed by previous research [9].

Among S-containing species, zinc sulfide (ZnS) possessed relatively good biocompatibility and water solubility [10,11]. Basing on the above consideration, in this work, ZnS was incorporated into Fe implants aiming to accelerate the corrosion of Fe matrix. Meanwhile, Zn ion, as a trace element, could promote cell proliferation and differentiation, which was expected to improve the biocompatibility of Fe implant [12]. The Fe based implant was fabricated by the laser powder bed fusion (LPBF) technique. LPBF is a powder bed melting technology. The focused laser beam selectively melts the powder layer by layer to produce the required geometry. Since LPBF meets the requirements of high melting point, high dimensional accuracy, high performance and design flexibility, it has become the main additive manufacturing technology of metal implants [13–15]. The microstructure, corrosion behavior, and biocompatibility of Fe/ZnS composite fabricated by LPBF were investigated. Additionally, the corrosion mechanism was deeply studied.

#### **2. Materials and Methods**

#### *2.1. Original Materials and Laser Powder Bed Fusion (LPBF) Process*

Sphere Fe powder (mean particle size 35 µm) and ZnS powder (5–10 µm) were utilized in this work. Fe and ZnS (9 wt %) were mixed by a miniature planet ball mill (PUL-VERISETTE 6, Fritsch, Germany). The ball mill was operated at a rotation speed of 220 rpm for 2 h, with a 15 min pause every half an hour. During operation, high purity argon (99.9%) was offered to reduce the oxidation.

The mixed powder was adopted to fabricate Fe/ZnS biocomposite using LPBF system, which was consisted of a fiber laser and a computer control system. A series of pilot experiments were carried out before the LPBF experiments to obtain an optimized processing parameter and as follows: laser power 210 W, scanning rate 80 mm/s, hatching space 50 µm and layer thickness 50 µm.

#### *2.2. Microstructural Characterization*

The LPBF-processed parts were grounded and polished using SiC paper. The microstructure was characterized using a scanning electron microscopy (SEM, Zeiss, Oberkochen, Germany) equipped with an energy dispersive spectroscopy (EDS). The phase composition was determined using X-ray diffractometer (XRD, D8 Advance, Bremen, Germany) with Cu Kα radiation at 45 kV and 40 mA. The scanning range was 20–90◦ , and the scan rate was 8◦/min.
