Bioactive Properties of Phosphate-Modified Silicon Oxycarbide Protective Coatings: Morphology and Functional Evaluation

Abstract

This article presents a study on the functional properties and morphology of coatings based on amorphous silicon oxycarbide modified with phosphate ions and comodified with aluminum and boron. The objective of this modification was to enhance the biocompatibility and bioactivity without affecting its protective properties. The comodification was aimed toward stabilization of phosphate in the structure. The coatings were prepared according to the typical procedure for polymer-derived ceramics: synthesized via the sol-gel method, deposited using the dip-coating technique, and subsequently pyrolyzed. Comprehensive analyses of the morphology, surface properties, corrosion resistance, and bioactivity were conducted to assess their functional performance. The coatings exhibited uniform and smooth surfaces, with phase separation observed in the boron-modified SiBPOC series. Surface wettability and free energy measurements demonstrated that SiPOC and SiBPOC coatings possessed moderate hydrophilicity and favorable surface free energy for cell adhesion and bone tissue mineralization. Corrosion resistance tests in Ringer's solution revealed that SiBPOC coatings provided the highest protection against ion leaching, while SiAlPOC showed decreased resistance due to surface cracks. Bioactivity tests indicated calcium phosphate precipitation on the surface of all samples with higher hydroxyapatite formation on SiPOC and SiAlPOC coatings. In vitro tests using MG-63 osteoblast-like cells confirmed the biocompatibility of the coatings, with SiPOC and SiBPOC exhibiting the best combination of bioactivity, cell adhesion, and proliferation. These findings suggest that the phosphate- and boron-modified SiOC-based coatings are promising candidates for enhancing bone integration in orthopedic implants.

Keywords: amorphous coatings; biomaterials; corrosion; polymer-derived ceramics; silicon oxycarbide.

Figure 1

SEM images of the SiPOC coatings with varying phosphate ion concentrations deposited on AISI 316L steel substrates before (top row) and after ceramization (bottom row).

Figure 2

SEM images of SiAlPOC coatings with varying phosphate ion concentrations deposited on an AISI 316L steel substrate before (top row) and after ceramization (bottom row).

Figure 3

SEM images of SiBPOC coatings with varying phosphate ion concentrations deposited on an AISI 316L steel substrate before (top row) and after ceramization (bottom row).

Figure 4

EDS point analysis of precipitates on the surface of the SiBPOC E.3 coating with the highest phosphate ion concentration [at. %] (crossing out a cell in a table indicates that the peaks of the element were out of the detection range at the accelerating voltage used).

Figure 5

SEM image and EDS maps of elements’ distribution for the A.1 SiPOC coating.

Figure 6

SEM image and EDS maps of elements’ distribution for the C.1 SiAlPOC coating.

Figure 7

SEM image and EDS maps of elements’ distribution for the E.1 SiBPOC coating.

Figure 8

Water contact angle (a) and SFE with separated polar and dispersive parts (b).

Figure 9

Exemplary plots of the open-circuit potential for the coatings and reference steel showing three characteristic behaviors of the materials at free corrosion conditions (a). Electrical equivalent circuits depicting the corrosion process for the steel substrate (b) and imperfect coating (c).

Figure 10

Representative Nyquist plots showing the impedance spectra measured for steel (a), SiPOC—A series (b), SiAlPOC—series C (c), and SiBPOC—series E (d); points obtained from fitting the EIS results to the equivalent circuits are marked with crosses.

Figure 11

Comparison of pseudocapacitance (a) and total resistance (b) of the AISI 316L austenitic steel sample and of modified SiOC coatings.

Figure 12

Linear (a,c,e) and semilogarithmic (b,d,f) PDP curves determined for a steel substrate and samples coated with SiPOC (a,b; A series), SiAlPOC (c,d; C series), and SiBPOC (e,f; E series); the linear plots do not show the recurring curve.

Figure 13

Iron (a), chromium (b), and nickel (c) concentrations in distilled water (○) and SBF (□) after 7 days of samples’ immersion.

Figure 14

SEM and EDS analysis results after the so-called Kokubo test (for EDS, given in atom %, analysis considered only the detected SBF elements).

Figure 15

MG-63 cell culture morphology after 7 days.

Figure 16

Proliferation (a), cytotoxicity (b), and cell viability (c) of MG-63 osteoblast-like cells measured for SiOC-based coatings and reference TCPS material.