Key Engineering Materials Vol. 1061

Abstract: Direct measurement of the friction of elastomers is challenging due to the complex mechanical behavior of the rubber and the limited accessibility of the often-cylindrical counter-surface of typical sealing systems. This study proposes an indirect approach to estimate the frictional behavior of sealing assembly by analyzing the friction of more accessible reference front surfaces. The apparent coefficient of friction between a rubber seal and an aluminum die-cast component was evaluated through surface roughness analysis, adhesion and direct friction measurements. Power spectral density functions were used to describe the counter-surface topography and to estimate friction using a simplified rubber friction model. Adhesion was characterized by contact angle measurements. The results reveal a moderate correlation between surface roughness and friction, demonstrating that indirect estimation may provide useful input for assembly analysis without destructive testing of the actual interface.

Abstract: This paper presents the experimental results of volume loss and wear rates achieved through severe abrasion (gouging) of A-36 structural steel and 1018 cold-rolled steel as tested materials, using CA1215 and TX10-T machinery grade steels as calibration and base materials, respectively. These results position A-36 steel as the material with the best performance under severe abrasive gouging, due to its low hardness, which provides suitable characteristics for resisting the microfracture wear mechanism while simultaneously preventing plastic deformation on its surface.

Abstract: This study investigates the tribological performance of a dry-running mechanical face seal composed of a graphite rotating ring and a silicon carbide stationary ring under zero spring preload (SP0, 0 MPa), both in clean conditions and in the presence of Martian regolith simulant (MGS-1, particle size < 80 µm). Without preload, the contact between the seal faces relied solely on the intrinsic flatness of the components and the minimal assembly force. Under clean operation, the frictional torque exhibited an initial transient phase followed by a stable regime at approximately 0.017 Nm, indicating a low and steady frictional state characteristic of graphite–SiC pairs. After around 17 hours, the torque slightly increased to ~0.021 Nm, suggesting gradual surface adaptation or mild wear, yet maintaining stable operation throughout the 24-hour test. When exposed to the fine MGS-1 regolith, the torque increased markedly to ~0.070 Nm initially and gradually decreased to ~0.064 Nm, reflecting a polishing effect of entrained particles under minimal contact pressure. Scanning electron microscopy confirmed negligible damage on the SiC surface and only shallow grooves and embedded particles on the graphite ring, evidencing mild three-body abrasion. These results demonstrate that, although zero preload reduces sealing pressure and potential sealing integrity, it enables highly stable and low-energy operation with minimal wear when particulate ingress is limited. The findings suggest that under Martian-like dusty conditions, a near-zero preload configuration may provide a favorable balance between friction stability and material preservation in low-pressure sealing applications.

Abstract: This study investigates the abrasive effects of Lunar Highland regolith simulant (TH100) and the particle size effect on stainless steel/PTFE rotary shaft–lip seal assemblies. A custom-built sand-pot testing rig filled with Nitrogen gas was used to expose EN-1.4404 stainless steel shafts paired with spring-loaded natural PTFE seals to fine (≤0.125 mm) and coarse (1.400–2.000 mm) Lunar Highland regolith particles. Each rotating pair was operated for 0.25, 0.5, and 24 hours at sequential shaft positions. Wear progression was assessed through shaft surface roughness measurements and scanning electron microscopy of the seals before and after testing. All tests under fine particles were completed without failure, whereas the sealing system exposed to coarse particles failed during the final shaft position and test was stopped. Fine particles primarily induced micro-cutting on the shaft surface, while coarse particles produced a combination of micro-cutting and micro-ploughing. These findings reveal the pronounced influence of particle size on wear mechanisms in lunar dust-exposed rotary sealing systems, with implications for the durability of moving components in extraterrestrial environments.

Abstract: This paper investigates a quasistatic frictional electro-viscoelastic contact problem with damage for a body in contact with a conductive foundation. The model incorporates normal compliance, frictional contact, piezoelectric coupling, and an internal damage variable describing adhesion degradation. The resulting system is formulated as a set of convex subdifferential variational inequalities, ensuring monotonicity and analytical tractability. A fully discrete scheme based on an implicit time discretization and conforming finite element approximations is proposed. Existence, uniqueness, unconditional stability, and convergence of the discrete solution toward the weak solution of the continuous problem are established.

Abstract: Kesterite-type quaternary chalcogenides, particularly Cu2NiGeS4, are emerging as promising candidates for next-generation photovoltaics due to their earth-abundant and non-toxic constituents. However, the performance of Cu2NiGeS4-based devices is often limited by significant charge carrier recombination at the back interface. In this work, we investigate the impact of integrating a tin sulfide (SnS) layer as a Back Surface Field (BSF) to passivate this interface. By employing numerical simulations with the SCAPS-1D software, we model and systematically optimize an innovative ZnO:Al/ZnO/ZrS2/Cu2NiGeS4/SnS/Mo device architecture. The optimization of key physical parameters reveals that the inclusion of the SnS-BSF layer is critical for achieving high performance. Our final optimized device achieves a power conversion efficiency of 21.17%, with a high open-circuit voltage of 1.1009 V, a short-circuit current density of 22.89 mA/cm2, and an excellent fill factor of 83.99%. Analysis confirms that this performance enhancement is primarily attributed to the effective suppression of back surface recombination by the robust p-p+ heterojunction formed at the Cu2NiGeS4/SnS interface. These results validate the SnS-passivated Cu2NiGeS4 architecture as a highly promising pathway for developing efficient, low-cost, non-toxic and scalable photovoltaic technologies.

Abstract: Halide perovskites are promising candidates for optoelectronic applications due to their tunable band gaps and strong optical response. In this work, the structural, electronic, and optical properties of the lead-free mixed-halide series KGeBr3-xClx (x = 0–3) are systematically investigated using first-principles calculations based on density functional theory (DFT). Structural optimization reveals a progressive lattice contraction from 169.86 ų (x = 0) to 145.6 ų (x = 3), accompanied by negative formation energies (−1.527 to −1.556 eV per atom for intermediate compositions), confirming thermodynamic stability. Electronic band structure calculations show that all compounds are direct band-gap semiconductors at the R-point, with band gaps increasing monotonically from 0.57 eV (KGeBr₃) to 0.89 eV (KGeCl₃). The intermediate compositions exhibit band gaps of 0.70 eV (x = 1) and 0.80 eV (x = 2), demonstrating effective band-gap engineering through halogen substitution. Optical analysis reveals static dielectric constants ranging from 6.29 to 6.80 and refractive indices between 2.5 and 2.6. The absorption edge shifts from 0.7 eV to 0.8 eV for x = 1 and x = 2, respectively, while energy-loss maxima reach approximately 0.33 near 4.5 eV. These results establish KGeBr3-xClx as a tunable, environmentally friendly semiconductor platform suitable for infrared optoelectronic and photovoltaic devices

Abstract: In this report, we describe a multiphysics model for a multilayer perovskite solar cell consisting of a reduced graphene oxide (RGO) back contact, a Spiro-OMeTAD hole transport layer, a MAPbI3 absorber, a TiO₂ electron transport layer, and a transparent FTO electrode. This model was developed using COMSOL through the application of the finite element method (FEM) and integrates three-dimensional electromagnetic solutions with a drift/diffusion method, as well as non-radiative recombination mechanisms provided by Shockley-Reed-Hall. Optical analysis directly links spectral photo-generation to the quantity |E|² and the absorption coefficient of materials, then integrates generation between 300 and 1,000 nm. Based on the results obtained, it is clear that the field has a shallow penetration depth in the ultraviolet, followed by an intensification of the field in the visible range and deeper propagation in the near infrared. A parametric study of doping shows that increasing donor doping in TiO₂ strengthens the internal field at the perovskite/TiO₂ interface, while increasing acceptor doping in Spiro-OMeTAD decreases the field peak at the Spiro-OMeTAD/perovskite interface and strengthens the field at the rear contact.

Abstract: Thermoelectric cooling (TEC) offers compact, reliable, and eco-friendly thermal management for biomedical and medical refrigeration applications. This numerical study compares SrTiO₃ and PbTe-based TEC modules using a fully coupled electro-thermal finite-element model. Performance metrics include temperature difference-current (ΔT-I), temperature difference-heat load (ΔT-Q), and coefficient of performance under varying operating conditions. Results show that the SrTiO₃ module achieves a maximum temperature difference of ΔTmax ≈ 65.48 K, surpassing PbTe ≈ 52-58 K under similar conditions. The SrTiO₃ module also exhibits a maximum cooling capacity of Qc,max ≈ 8.79 W at ΔT = 0 K. COP analysis reveals superior efficiency for SrTiO₃, reaching ≈ 1.1 at ΔT = 20 K, compared to PbTe < 0.6. Additionally, SrTiO₃ demonstrates enhanced thermal robustness, maintaining stable cooling under increasing heat loads. The superior performance of SrTiO₃ stems from its favorable transport properties (high electrical conductivity and moderate thermal conductivity), combined with non-toxicity, environmental compatibility, and thermal durability-advantages over toxic lead-based alternatives. These findings highlight SrTiO₃ strong potential for next-generation solid-state cooling in localized therapeutic devices, medical refrigeration, and biomedical thermal management. Nomenclature: