Mohamed Bensalem, Ph.D.

This work presents an integrated methodology combining analytical and finite element (FEM)-based viscoelastic modeling for characterizing the nanoindentation response of coatings. The proposed framework combines two complementary modeling approaches: an analytical model based on the Burgers formulation to analyze nanoindentation load–displacement data and extract rheological parameters, and a numerical FEM model implemented in ABAQUS using a 2D axisymmetric indenter–coating-substrate configuration with viscoelasticity represented through Prony series. An automated inverse optimization routine employing the Nelder–Mead simplex algorithm minimizes the discrepancy between experimental and simulated responses. The methodology is demonstrated and validated on WSe2 coatings, showing an agreement between the analytical predictions, FEM simulations, and experimental measurements. Despite the geometric simplifications, the FEM approach provides accurate predictions while maintaining high computational efficiency of time-dependent mechanical behavior. The proposed framework provides a robust tool for characterizing viscoelastic behavior from nanoindentation data while enabling access to internal stress and strain fields, thereby offering deeper insight into plastic deformation, crack initiation, and failure mechanism.

Helium accumulation in structural ceramics used in nuclear, fusion, and aerospace systems causes swelling, cracking, and early failure, yet controlling this damage has remained elusive. Here, we introduce defect landscape engineering, the deliberate creation of vacancy clusters prior to helium exposure, as a general strategy to suppress helium-induced degradation. Using alpha-SiC as a model, we combine advanced microscopy, strain mapping, helium depth profiling, positron annihilation spectroscopy, and atomistic simulations to demonstrate that tailored pre-damage transforms helium defect evolution. Instead of forming extended platelets and nanocracks, helium is trapped in stable, uniformly dispersed nanobubbles. Simulations reveal that small vacancy clusters act as dual-function sinks for irradiation-induced interstitials and preferential traps for helium, fundamentally altering the dynamics of cascade recombination. This mechanism is composition-independent and scalable, offering a new design principle for radiation-tolerant ceramics across carbides, nitrides, and oxides. By viewing defect control as a tunable parameter instead of a fixed material property, this work outlines a possible design route toward enhanced radiation tolerance in ceramics used in extreme environments.

Understanding irradiation-induced strain in silicon carbide (SiC) is essential for designing radiation-tolerant ceramic materials. However, conventional methods often fail to resolve nanoscale strain gradients, especially in polycrystalline forms. In this study, we employ nano-beam precession electron diffraction (N-PED) to perform high-resolution, multi-directional strain mapping in both single-crystal 4H-SiC and polycrystalline alpha-SiC subjected to helium and hydrogen ion irradiation. The high-resolution X-ray diffraction (HR-XRD) simulations of He + H irradiated single-crystal 4H-SiC closely match the strain profiles obtained from N-PED, demonstrating the reliability and accuracy of the N-PED method. In He-irradiated polycrystalline alpha-SiC at high temperatures, a bubble-depleted zone (BDZ) near the grain boundary (GB) reveals that GBs act as active sinks for irradiationinduced defects. N-PED further shows strain amplification localized at the GBs, reaching up to similar to 2.5 %, along with strain relief within the BDZ. To explain this behavior, density functional theory (DFT) calculations of binding and migration energies indicate a strong tendency for Si, C, and He atoms to segregate toward the GB core. This segregation reduces the availability of vacancies to accommodate He atoms and leads to local strain relaxation near the GB. Furthermore, first-principles tensile simulations reveal that Si and C interstitials mitigate He-induced GB embrittlement. Charge density and DOS analyses link this effect to the bonding characteristics between point defects and neighboring atoms at GB. These insights underscore the importance of grain boundary engineering in enhancing radiation tolerance of SiC for nuclear and space applications.

Nanoindentation is widely used to evaluate the mechanical properties of irradiated materials; however, its potential for quantifying irradiation- induced subsurface strain remains underexplored. In this work, an integrated experimental–numerical framework based on a physicsconstrained inverse modeling approach is employed to estimate the magnitude of a depth-dependent irradiation-induced strain distribution in single-crystal 4H-SiC following sequential He and H ion implantation. The approach combines depth-sensing nanoindentation, finite element modeling (FEM), and a simplex-based inverse optimization routine to calibrate a physically motivated eigenstrain profile derived from ion-damage simulations. The strain field is assumed to follow a lognormal distribution consistent with independently determined damage profiles (stopping and range of ions in matter) and is implemented in the FEM model through a depth-dependent thermal expansion formulation. By minimizing the squared error between simulated and experimental force–displacement curves, the peak tensile strain is estimated to be ∼0.91%, accompanied by an effective Young’s modulus of 310 GPa and a yield strength of 16.4 GPa. Independent validation by nano-beam precession electron diffraction confirms good agreement between the reconstructed and experimentally measured out-of-plane strain profiles in both magnitude and spatial distribution. The results demonstrate that nanoindentation, when combined with physics-based inverse modeling, can provide a practical tool for quantifying irradiation-induced strain and residual stress in nuclear ceramics. This methodology offers a complementary approach to diffraction-based techniques for assessing subsurface damage in ion-irradiated materials relevant to advanced nuclear systems.

Radiation-induced helium (He) bubble formation poses a major challenge to the structural integrity of materials in nuclear energy systems. In this study, we investigate defect evolution and He behavior in Zr/Nb nanoscale metallic multilayers (NMMs) with immiscible BCC/HCP interfaces, irradiated with 80 keV He ions at fluences ranging from 1 ×10¹⁶ to 1 ×10¹⁷ He/cm². For comparison, single-crystal Nb and polycrystalline Zr were also irradiated under identical conditions to serve as reference materials. Using cross-sectional TEM, SIMS, STEM- EELS, nanoindentation, Doppler Broadening Positron Annihilation Spectroscopy (DBPAS), Positron Annihilation Lifetime Spectroscopy (PALS), and atomistic simulations (DFT and MD), we reveal a highly asymmetric damage response across the multilayer interfaces. Zr layers exhibit larger He bubbles (1.5–2.8 nm), higher swelling (~1.2%), and greater helium retention, while Nb layers develop bubble-denuded zones (BDZs) exclusively around the interfaces, where bubble nucleation is strongly suppressed and swelling is limited to ~0.4%. This asymmetry arises from differences in atomic transport properties: DFT calculations show lower migration barriers for vacancies and He atoms in Nb (0.4 and 0.19 eV, respectively), enabling efficient defect migration and recombination at interfaces, whereas Zr retains defects due to higher migration barriers. EELS and DBS-PALS measurements confirm bubble densities of 63–96 He/nm³ and the presence of sub-nanometer open volumes. Compared to monolithic samples, the Zr/Nb multilayers exhibit ~50% lower irradiation-induced hardening and reduced He retention (11% vs. 17.5% in single-crystal Nb and 16% in polycrystalline Zr). These findings highlight the role of interfaces in driving asymmetric radiation damage and demonstrate the effectiveness of BCC Nb layers in mitigating defect growth. Overall, Zr/Nb multilayers are established as a superior alternative to conventional single- and polycrystalline materials for extreme irradiation environments.