2.1. Preparation of the 3D-printed PEEK/n-TiO2 composite

First, the TiO₂ nanopowder (Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) was mixed with PEEK powder (VESTAKEEP^® 2000FP; Evonik, Essen, Germany) to obtain mixed powders with different n-TiO₂ contents (0 and 30 wt%). The powders were then processed using a co-rotating twin-screw extruder (PolyLab HAAKE Rheomex OS PTW16, D = 16 mm, L/D = 40; Thermo Fisher, USA) to obtain continuous PEEK/n-TiO₂ filaments (Φ = 1.75 ± 0.10 mm). The parameters of the two types of powder are listed in Table 1.

Parameters of PEEK and TiO₂ powders

Next, 3D models of the PEEK and PEEK/n-TiO₂ composite samples (Φ = 25 mm, H = 2 mm) were designed using CAD modeling software (Mimics; Materialize, Belgium). Subsequently, the filaments were assembled using a 3D PEEK printer (Medvance, Shanghai, China), melted at 450°C, injected into nozzles (Φ = 0.4 mm), and deposited layer-by-layer (0.1 mm) following the designed program. The printing speed was 10 mm/s, and the plate and chamber temperatures were maintained at 260 and 220°C, respectively. Several hours later, PEEK (tensile modulus = 6.15 GPa) and a PEEK/n-TiO₂ composite (30 wt%, tensile modulus = 4.15 GPa) were successfully manufactured.

The tensile moduli of the samples were determined using the standard tensile test method. First, a standard tensile test sample (ISO527-2:1993 1BA) (n = 1) was manufactured using a 3D printer with the aforementioned printing parameters. Tensile testing was conducted by the Weipu Technology Group (Shanghai, China) following the standard tensile test method (ISO527-2:1993). An electronic universal testing machine (Instron 5969, Canton, MA, USA) was used for tensile testing. The test speed was 1 mm/min, and the maximum loading force was 50 kN. The tensile modulus was calculated according to a previous study [22].