Development of High-Flexural-Strength Titanium/Hydroxyapatite Biocomposites via Cold Spray Deposition with Titanium and Niobium Bond Coats

Achieving rapid, scalable fabrication of bone implants with a high flexural strength remains a major challenge. The persistent issues are mainly attributed to the incompatibility of the biomaterials (e.g., titanium (Ti) and niobium (Nb)) with the processing methods. The implants fabricated from solution-based or extrusion-based additive manufacturing often require the use of linkers and have disadvantages, including inferior adhesion, susceptibility to damage, implant loosening and leaching, and surrounding bone damage during their long-term use in biological environments. Furthermore, thermal spray-based methods involve melting the powders, which typically degrade the biomaterial's characteristics. Among several additive manufacturing methods, cold spray deposition is a promising fabrication method that directly forms coatings from powder feedstocks at room temperature. However, cold spray deposition of metal-matrix composites comprising ceramics remains challenging due to the inability of the primary ceramic particles to deform. Here, we report the deposition of Ti-hydroxyapatite (HAP) biocomposites with excellent adhesion and mechanical properties, utilizing intermediate Ti and Nb bond coat layers. While the Ti-HAP directly deposited on a Ti substrate exhibits a higher Young's modulus (135.9 ± 51.6 GPa) than pure Ti coating (98.0 ± 50.2 GPa), the cold spray-induced defects such as pores and voids lead to a lower flexural strength (219 ± 25 MPa) than that of a Ti coating (318 ± 32 MPa). Interestingly, the interfacial bond coat layers increase the flexural strength of the Ti-HAP layer from 219 ± 25 to approximately 310 MPa, which significantly exceeds those of natural bone and commercial dental restorative biocomposites. Thus, cold spray deposition enables the rapid and scalable fabrication of biocomposites with tunable mechanical properties, making it a promising technique for biomedical applications. Moreover, the strategy can be applied more generally to ceramic metal-matrix composites with structural and mechanical features similar to those of natural bones.