Experimental investigations on electrical field-assisted sintering technology (FAST) used for embedding optical fiber into stainless steel

A gold-coated fused-silica optical fiber was embedded in 316 L stainless steel using field-assisted sintering technology (FAST) and examined along a longitudinal section parallel to the fiber axis. SEM, EDS, EBSD, and tensile testing were used to characterize the fiber–matrix region and the surrounding FAST-consolidated SS316L. SEM showed that the fiber remained intact after processing and that the Au coating was still present between the fused silica and the steel. No obvious cracks, interfacial gaps, or debonded regions were observed at the examined magnifications, although isolated residual pores remained in the matrix. EDS maps and line scans showed distinct Si–O, Au, and Fe–Cr–Ni regions corresponding to the fiber, coating, and SS316L, with no broad reaction layer resolved by SEM-EDS. EBSD showed a broad crystallographic orientation distribution without a clear fiber-related texture. The number-weighted and area-weighted grain diameters were 4.80 ± 3.00 μm and 9.08 ± 4.83 μm, respectively. Approximately 69.0% of the measured boundaries had misorientation angles below 15°, and the indexed region consisted of 98.9% FCC and 1.1% BCC. Tensile testing of five specimens in each orientation showed higher strength in the vertical direction, with average yield strength and UTS of 455.7 ± 70.6 MPa and 652.9 ± 23.2 MPa, compared with 347.9 ± 19.7 MPa and 582.1 ± 38.2 MPa in the horizontal direction. The results characterize the longitudinal interface, local SS316L microstructure, and orientation-dependent mechanical response after FAST embedding. Interfacial strength, strain transfer, and optical sensing performance were not directly measured in the present work.