Search Results (651)

Rolling bearings in modern wind turbines operate under demanding tribological conditions that include high contact stresses, mixed lubrication, transient loads, electrical discharge, corrosion, and hydrogen-assisted damage. These factors contribute to premature failures such as micropitting, scuffing, white etching cracks (WECs), and electrical fluting, which remain major reliability challenges in both onshore and offshore turbines. Surface-engineering technologies have emerged as effective tools to mitigate these failure modes. Carbon-based coatings improve sliding performance and reduce wear under boundary-lubricated conditions; black oxide conversion layers enhance the corrosion resistance, lubricant retention, and early-life running-in behavior; and insulating ceramic coatings protect generator bearings from electrical discharge damage. Additional solutions, including polymer overlays, composite films, and hybrid ceramic architectures, offer further improvements in friction, surface fatigue resistance, and environmental robustness. This manuscript reviews the current state of coated bearing technologies relevant to wind turbine applications, synthesizing findings from tribological research, industrial practice, and emerging material developments. While only a limited number of coating suppliers provide documented evidence of coating use in wind turbine drivetrain bearings, the collective progress in surface engineering demonstrates clear potential for improving reliability and extending service life across the installed turbine base. The analysis highlights the mechanisms by which coatings enhance performance, the conditions under which they are most effective, and the gaps that remain in field validation and large-scale deployment. Coated bearings represent a promising pathway toward higher turbine availability, reduced maintenance costs, and improved drivetrain durability. Continued advances in carbon-based films, multilayer architectures, and insulating coatings, combined with better integration of lubrication strategies and condition-monitoring technologies, will play an increasingly important role in enabling the next generation of high-power wind energy systems. Full article

Polymeric biomaterials do not degrade in vivo through isolated chemical or mechanical events. Hydrolysis, oxidation, enzymatic cleavage, fatigue, wear, protein adsorption, and lipid uptake often interact across the material surface, bulk, and surrounding biological environment, producing time-dependent changes in both structure and function. Here, we propose a conceptual framework for organizing polymer degradation under biomedical conditions as a directional network of coupled mechanisms, emphasizing how established degradation pathways can influence one another and collectively contribute to time-dependent functional loss. The framework distinguishes degradation reactions from interfacial modulators and links molecular damage to functionality retention, defined according to application-specific endpoints such as mechanical strength, mass retention, dimensional stability, or interfacial performance. We introduce a qualitative interaction matrix to describe how individual mechanisms can amplify or modulate downstream pathways, and we use this matrix to identify recurrent degradation archetypes across major biomedical polymer classes, including polyesters, polyolefins, polyamides, polyurethanes, silicones, polyacrylates, polyvinyl polymers, polyimides, and natural polymers. This perspective emphasizes that material optimization strategies rarely eliminate degradation; instead, they shift the hierarchy of active pathways. By reframing biocompatibility as a dynamic, functionality-dependent property, the proposed framework provides a structured basis for comparing polymeric biomaterials, designing more realistic in vitro tests, and developing future data-driven models of long-term implant performance. Full article

This study aimed to develop polymer matrix nanocomposites reinforced with Al₂O₃ nanoparticles to enhance the mechanical properties of PMMA. The composite was fabricated via friction stir processing (FSP) to ensure the homogenous dispersion of Al₂O₃ nanoparticles in the polymer. A grid of 5 holes in a 7 × 7 mm² area was made in which the hole diameter was varied from 1.77 mm to 2.28 mm to obtain different volume fractions of reinforcement ranging from 15% to 25%. The holes were made with a depth of 3 mm in a 4 mm-thick PMMA sheets. After packing the Al₂O₃ powder in the holes, a 2 mm-thick PMMA sheet was used as a cover to prevent the sputtering of nanoparticles. A number of FSP parameters were examined. The tool rotation rates ranged from 800 to 1200 rpm, traverse speeds of 25 and 50 mm/min, and tool tilts of 1 and 2° were used. A soft paraffin (Vaseline) layer was used on the top surface to prevent severe shoulder friction with the PMMA plate, which caused severe wear and thinning on the surface. For the developed PMMA/Al₂O₃ nanocomposites, the surface quality, SEM microstructure, impact energy, and transverse hardness were investigated. Good surface quality and dispersion of nanoparticles were attained by employing adequate processing conditions. The experimental results indicated that as the nanoparticle percentage increased, impact energy, hardness, and tensile strength increased, reaching 2 kJ/m², 14.7 HV, and 52.1 MPa at a nanoparticle concentration of 25%. This means that the polymer ceramic composite’s toughness, hardness, and tensile strength are higher than those of unprocessed PMMA by 66%, 33%, and 23%, respectively. Full article

Sustainable machining of composite materials requires optimizing conflicting responses influenced by limited experimental datasets, trade-offs, nonlinear process variables, and response variability. This study proposes a hybrid framework (Gaussian Process Regression—Method based on the Removal Effects of Criteria—Criteria Reliability—Momentum-Based Optimization Algorithm: GPR–MEREC-CR–MOA) to address these challenges in turning composite materials (PA66, PA66 + GF30, and PA66 + MoS₂). The GPR model learns from small datasets to capture nonlinear relationships between machining variables (workpiece material, tool approach angle, tool nose radius, cutting speed, feed rate, depth of cut) and performance characteristics (surface roughness, cutting force, vibration, tool wear rate, temperature, sound pressure level, specific cutting energy, and material removal rate). The MEREC-CR method considers experimental dispersion and response variability to enhance the robustness of the multi-response aggregation model. The weighted responses determined by MEREC were optimized by exploring the operating ranges of machining variables using MOA. The GPR model accurately predicts eight performance characteristics (R² ≥ 0.973). The GPR–MEREC-CR–MOA model identified optimal conditions for PA66 + MoS₂ and composite material (tool angle = 93°, nose radius = 0.40 mm, cutting speed = 200 m/min, feed rate = 0.300 mm/rev, depth of cut = 1.08 mm), resulting in a composite performance index (CPI) of 0.9265 and a 30.2% improvement over the best experimental datasets from Taguchi L₂₇ design. The tool wear rate, specific cutting energy, and vibration have a significant impact on overall machining performance. Feed rate has the strongest influence on CPI, as confirmed by Partial Rank Correlation Coefficients analysis. Monte Carlo-driven uncertainty analysis validates the optimal solution with a 95% confidence level for CPI between 0.8859 and 0.9451. External validation with nine independent cases confirmed the GPR model’s strong generalizability (R² = 0.811–0.998). Benchmarking showed that MOA achieves solution quality comparable to GA, PSO, and GWO while reducing computational time by 66–86%, making it suitable for real-time optimization. The proposed hybrid framework provides an alternative data-driven decision support approach for evaluating sustainable machining parameters using limited experimental datasets of polymer composites. Full article

To quantify and characterize microplastic particles recovered from orthodontic appliances under simulated intraoral conditions using laser-directed infrared spectroscopy (LDIR), five maxillary orthodontic appliances were exposed to artificial saliva at 37 °C under calibrated agitation simulating nighttime (8 h) and full-time (20 h) wear over a 14-day period. A 1 mL aliquot retrieved from a 50 mL exposure volume was analyzed using LDIR to determine particle count, size, eccentricity, and polymer composition, with statistical analyses performed on an exploratory and polymer-stratified basis. Particle recovery ranged from 11–16 particles for nighttime appliances and increased substantially for aligners (SmartTrack: 341; Zendura: 1382). Multiple polymer types were identified, and no statistically significant differences in particle diameter or eccentricity were observed after multiplicity correction. These findings show that orthodontic appliances can generate recoverable microplastic particles under simulated conditions, establish a reproducible framework for automated microplastic characterization, and support future investigation into clinical exposure. Because the effective detection threshold of the LDIR workflow is approximately 20 µm, smaller microplastics and nanoplastics were not evaluated in this study. Full article

This study investigated the structure and properties of flame-sprayed composite coatings based on ultra-high molecular weight polyethylene (UHMWPE) modified with Al₂O₃ and SiC ceramic fillers. The results of the composite coatings showed that the filler particles were preserved within the polymer matrix, covered by polymer fibers, and strongly bonded to the matrix. The X-ray diffraction analysis showed that for the samples containing Al₂O₃, the degree of crystallinity decreased from 77% to 64%, while for the SiC-containing samples, it remained in the range of 71–76%. The maximum microhardness was achieved in the coating with 15 wt.% Al₂O₃, reaching 8.15 ± 0.5 HV0.03, which represents a 50.9% increase compared with the initial UHMWPE coating. In the case of the SiC filler, the maximum microhardness was achieved at a content of 20 wt.%, reaching 7.40 ± 0.4 HV0.03. The abrasive wear test results demonstrated that both fillers enhanced the wear resistance of the coatings. The coatings containing 15–20 wt.% SiC exhibited the highest wear resistance, with a mass loss of approximately 0.02 g. In the case of the Al₂O₃ filler, the minimum mass loss was achieved at a content of 20 wt.%, reaching approximately 0.032 g. Full article

The nanoscale surface topography and microhardness of additive and subtractive dental polymers were evaluated following exposure to environmental challenges. The study examined two 3D-printed resins (Saremco and Voco) and two milled CAD/CAM blocks (Vita Enamic and Tetric). Specimens were allocated to control or experimental groups and subjected to hydrothermal ageing (thermocycling), in vitro mechanical wear, or a combined protocol involving wear followed by thermal ageing. Surface microtopography was analysed both quantitatively and qualitatively using atomic force microscopy (AFM), while structural stability was assessed through surface microhardness testing. Statistical significance was determined using matrix comparisons (p < 0.05). Milled monolithic blocks demonstrated a dense, uniform baseline topography, whereas 3D-printed resins exhibited structural heterogeneity attributed to their layer-by-layer photocuring process. Saremco maintained polymer network stability under thermal stress (p = 0.1878), while Voco was highly susceptible to hydrothermal swelling and early matrix plasticization (p = 0.0084). The combined protocol of wear and thermal ageing resulted in advanced structural breakdown in all groups (p < 0.001). Industrial subtractive blocks exhibited greater resistance to oral environmental stresses. The ceramic framework of Vita Enamic limited polymer domain collapse, whereas Tetric experienced accelerated inter-layer delamination and embrittlement. The combined protocol of wear followed by thermal ageing resulted in significant and uniform degradation of surface microhardness and topographic roughness in all tested groups. Nevertheless, the additively manufactured resins demonstrated substantial structural integrity and exhibited low volumetric wear rates. Full article

Agricultural soil contact components, including ploughshares, rotary blades, and furrow openers, form the active interface between machinery and complex field media. Operating in multiphase environments, these tools face severe abrasive wear, impact fatigue, and interfacial adhesion. These destructive forces irreversibly alter edge geometry and drastically degrade macroscopic operation quality. This review integrates tillage and precision seeding components into a unified tribological framework. It highlights the nonlinear relationship between microscopic material removal and geometric edge retention. Profile degradation is heavily dictated by soil texture, where sandy soils cause micro-cutting, clay soils induce severe adhesion, and gravelly soils produce impact fracture. To predict these complex wear behaviours accurately, coupled multiphysics numerical simulation using the discrete element method for particle flow dynamics and finite element analysis for transient contact stress provides a highly robust methodology. Mitigating these failures requires a functionally zoned surface engineering approach. Carbide hardfacing offers localized abrasion resistance, while polymer composite layers and bionic nonsmooth structures effectively interrupt continuous liquid films in wet cohesive soils. Ultimately, integrating online multidimensional sensing with full life cycle digital-twin models represents the future trajectory for developing adaptive and highly durable agricultural equipment. Full article

Poly(methyl methacrylate) (PMMA) remains a clinically important denture base polymer because of its favorable processability, aesthetics, repairability and long-term prosthodontic use; however, its limited surface hardness and susceptibility to sliding-induced degradation constrain surface durability. This study establishes the structure–chemistry–microstructure–tribology relationships and composition-window predictive behavior of heat-polymerized PMMA reinforced with titanium diboride (TiB₂). PMMA/TiB₂ composites containing 1, 3 and 5 wt.% TiB₂ were prepared and compared with unreinforced PMMA. X-ray diffraction confirmed preservation of the broad amorphous/semi-amorphous PMMA response, while TiB₂-related crystalline features became increasingly detectable with reinforcement content. ATR-FTIR showed retention of the characteristic C–H, ester C=O and C–O/C–O–C vibrations. SEM/EDS demonstrated progressively greater particle-related surface contrast and local Ti/B-associated elemental signatures. Vickers microhardness increased from 20.0 ± 0.7 to 35.0 ± 1.39 HV0.03, corresponding to a 75.0% improvement at 5 wt.% TiB₂. After 1000 m of dry sliding, total mass loss decreased from 32.4 ± 0.5 to 14.9 ± 0.4 mg (54.0% reduction), overall coefficient of friction decreased from 0.58 to 0.35, and representative wear-track width decreased from 481.4 to 101.8 µm. A parsimonious distance–composition interaction model retained strong grouped leave-one-composition-out performance for cumulative wear (R² = 0.9622; RMSE = 1.53 mg), while a linear composition model provided the most robust hardness prediction (R² = 0.9317; RMSE = 1.50 HV0.03). More complex nonlinear models did not improve prediction for held-out compositions. Within the investigated 0–5 wt.% window, 5 wt.% TiB₂ provided the most effective combination of matrix preservation, surface hardening, wear suppression and frictional stabilization. Full article

This study investigates the tribological performance of chromium nitride (CrN)-coated polyether ether ketone (PEEK) polymers. CrN coating was deposited by physical vapor deposition (PVD) onto two commercially available substrates: neat PEEK and a modified PEEK. The suitability of the coated substrates for tribological applications was evaluated using ball-on-disc tests against stainless-steel (SS) counterparts. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) confirmed the high thermal stability of the PEEK substrates, ensuring compatibility with low-temperature coating deposition. Microstructural analysis revealed dense and continuous CrN coatings with an average thickness of ~1.5 µm on both substrates. Tribological results of selected PEEK-based polymers showed that under the applied load of 2 N, uncoated PEEK substrates exhibited lower coefficients of friction (COFs) and smoother wear tracks compared with coated samples. In contrast, under the applied load of 4 N, CrN-coated PEEK substrates demonstrated reduced friction and improved stability relative to uncoated PEEK. This behavior is attributed to load-induced tribo-oxidation and the formation of a chromium-oxide-rich tribolayer that stabilized the sliding interface and suppressed adhesive wear. Overall, the results demonstrate that CrN coatings significantly enhance the load-bearing capacity and tribological performance of selected PEEK substrates under applied load of 4 N, highlighting their potential for advanced lightweight engineering applications requiring improved wear resistance. Full article

Magnesium alloys are used or considered for lightweight structures and biodegradable implants, but high electrochemical activity, limited wear resistance, and localized corrosion still limit their service reliability. Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), forms an adherent ceramic scaffold. Discharge channels, interconnected pores, thermal cracks, and a mechanically weak outer layer mean that the as-formed coating is rarely a complete protective system. This review examines advanced PEO/MAO-based composite coatings through a process–structure–function lens and develops an application-oriented design framework. The discussion covers PEO/MAO process-window control, electrolyte and particle engineering, sol–gel and polymer sealing, layered double hydroxide/inhibitor systems, self-healing reservoirs, superhydrophobic and slippery interfaces, Ca-P/hydroxyapatite and polymer biofunctionalization, and duplex coatings for wear, electrical, and thermal functions. Emphasis is placed on how these modules regulate defect connectivity, mass transport, interfacial stability, damage response, tribocorrosion, and biodegradation, as well as on the evidence needed to support each claimed function. The analysis indicates that coating performance is governed not by multilayer complexity alone, but by the compatibility among the ceramic scaffold, functional module, dominant failure mode, and service-specific validation protocol. Chloride-exposed structures require durable pore sealing and active inhibition; wear-critical components require coupled corrosion–wear assessment; and biodegradable implants require a degradation window that balances corrosion moderation, cytocompatibility, biofunctionality, and residual mechanical integrity. Remaining challenges include interfacial durability, finite inhibitor reservoirs, wetting-state instability, process reproducibility, scale-up, and life-cycle impacts. The proposed process maps and validation criteria are intended to support modular, testable, and application-specific PEO/MAO surface systems for magnesium alloys. Full article

In recent years, the quest for advanced materials has intensified, driving the exploration of innovative coating systems to enhance material properties. Polymeric materials, renowned for their adaptability, have found extensive use across various industries. However, their inherent limitations in mechanical strength, wear resistance, and thermal stability have prompted researchers to seek enhancement avenues. Hybrid polymeric–metallic coatings have emerged as a promising solution, employing thermal spray techniques to metalize polymeric substrates. This amalgamation utilizes the versatility of thermal spraying methods, ranging from cold spraying, flame spraying, arc spraying, to plasma spraying, to achieve robust adhesion between metal and polymer layers. These techniques yield durable composite structures, fortifying surfaces against corrosion and wear while enabling dimensional restoration. Cold spraying, in particular, stands out among thermal spray methods due to its effectiveness in metalizing various materials. This comprehensive review delves into recent advancements in hybrid polymeric–metallic coatings via thermal spray processes. Emphasis is placed on analyzing critical factors influencing coating properties, including various thermal spray parameters. Furthermore, the paper scrutinizes the challenges and future potentials inherent in thermal spray techniques, with a focused exploration of cold spray technology. Understanding these methodologies is pivotal for optimizing the design and durability of structural materials made of polymers or composites. Full article

Carbon fibre-reinforced polymer (CFRP) composites represent promising lightweight materials for automotive powertrain systems, where increasing demands for weight reduction, energy efficiency, and emission reduction are driving the replacement of conventional metallic components. However, automotive powertrain environments expose CFRP materials to elevated temperatures, cyclic mechanical loading, chemical exposure, and tribological interactions, creating complex degradation conditions that significantly influence long-term durability and reliability. This review systematically analyzes CFRP composites for automotive powertrain applications, focusing on the relationship between operational requirements, material selection, reinforcement architecture, manufacturing technologies, and degradation mechanisms. High-performance thermoplastic systems such as CF/PEEK, CF/PPS, and CF/PEKK are critically compared with conventional thermoset composites. CF/PEEK systems demonstrate superior thermomechanical stability, maintaining significant mechanical performance at temperatures approaching 250 °C and tensile strengths of approximately 1400–1600 MPa, whereas CF/PPS composites provide a more economically efficient compromise between thermal resistance, chemical stability, manufacturability, and recyclability for medium-temperature applications. The review further analyzes dominant degradation mechanisms, including creep deformation, fatigue damage, delamination, fibre–matrix interface degradation, and tribological wear. CFRP degradation is shown to result from the interaction of multiple coupled mechanisms rather than from isolated material failure modes. Tribological wear rates typically range from 10⁻⁶ to 10⁻⁵ mm³/(N·m), while creep–fatigue interactions may reduce component lifetime by up to 40–60% under combined thermomechanical loading. Advanced design strategies, including fibre orientation optimization, laminate architecture tailoring, thickness gradation, and hybrid metal–composite structures, are evaluated together with major manufacturing technologies such as injection moulding, compression moulding, overmoulding, automated fibre placement, and additive manufacturing. The presented review establishes an integrated framework linking material systems, operating conditions, manufacturing processes, and durability requirements for automotive powertrain applications. The analysis demonstrates that no universal CFRP system exists for all powertrain components and that optimal material selection requires balancing thermal stability, fatigue resistance, tribological performance, manufacturability, recyclability, and economic constraints according to the specific operating conditions of each component category. Full article

This study extended a previously established intraoral scanning (IOS) and optical coherence tomography (OCT) dual-modality monitoring workflow for computer-aided design/computer-aided manufacturing (CAD/CAM) restorations to three additional crown material classes alongside a resin composite (RECO) reference. Four material classes were investigated ($n=8$ each): RECO, polymer-infiltrated ceramic network (PICN), lithium disilicate ceramic (LDSC), and zirconia-reinforced lithium silicate ceramic (ZLSC). Monolithic crowns were adhesively luted to standardized human molar abutment teeth and aged by cyclic loading (50500$\mathrm{N}$, 2$\mathrm{Hz}$, 37 2${}^{\circ }\mathrm{C}$, up to 1250000 cycles) in a mouth-motion simulator. IOS and handheld OCT were performed at baseline and after every 250000 cycles under phantom-head conditions; correspondence was assessed using Spearman’s rank correlation coefficient (exploratory, uncorrected for multiple comparisons). OCT consistently showed higher defect extents than IOS across all material classes and timepoints. While no significant IOS-OCT associations were found for RECO and the PICN, OCT detected full-thickness vertical subsurface damage propagation from the earliest timepoint in LDSC and ZLSC, with IOS-derived surface wear remaining markedly lower. Surface-based monitoring alone did not reliably reflect subsurface damage propagation, a dissociation most pronounced in the vertical dimension and silicate-based materials. Intraoral OCT may provide complementary, non-invasive subsurface information to support individualized recall scheduling and minimally invasive repair decisions. Full article

This study investigates the tribological performance of gum arabic (GA)-reinforced epoxy (EP) overlays on EN8 steel. Four GA concentrations (0.25, 0.5, 1, and 3 wt.%) were incorporated into the epoxy matrix to prepare overlays designated as EPGA1–EPGA4. Tribological performance was evaluated using a reciprocating tribometer under varying loads (5–20 N), sliding frequencies (1–2.5 Hz), and temperatures (40–70 °C). An L16 orthogonal array based on the Taguchi method was used to design the experimental matrix, and multi-criteria decision-making using the TOPSIS technique was employed to identify the optimum tribological condition based on minimum coefficient of friction (COF) and specific wear rate (SPWR). The optimum condition was obtained for the EPGA3 overlay (1 wt.% GA) at 5 N, 2 Hz, and 60 °C, which exhibited the lowest COF of 0.0567 ± 0.0021 and negligible wear. In contrast, the pure epoxy overlay showed severe adhesive wear, catastrophic delamination, a high COF of 1.15 ± 0.0023, and a wear rate of 163 × 10⁻⁸ mm³/Nm. Thermal characterization showed that GA improved the thermal stability and thermal transition behaviour of the epoxy matrix. Thermogravimetric analysis revealed an increase in onset degradation temperature from 320 °C for pure EP to 342 °C for EPGA4, while differential scanning calorimetry showed that EPGA3 exhibited the highest glass transition temperature (~118 °C), indicating improved interfacial interactions and restricted polymer-chain mobility. Nanoindentation and pull-off adhesion tests further confirmed the improved mechanical integrity and interfacial adhesion of the GA-reinforced overlays, demonstrating its potential as a sustainable reinforcement for tribological coating applications. Full article