The impact of cryogenic cyclic aging on the thermal, mechanical, and tribological properties of polyetheretherketone (PEEK) and its composites was investigated to reveal the mechanisms and factors affecting the durability of high-performance materials under prolonged exposure to extreme environments. Cryogenic cyclic aging involved twelve cycles, each consisting of 6-day aging in liquid nitrogen (−196 °C) and 1-day in oven at 40 °C in normal ambient (relative humidity ∼30 %), mimicking operational conditions of cryogenic systems. Tribological tests were conducted using a ball-on-disk configuration with a stainless steel (SS 316L) ball counterface. Tests were performed at 25 °C in air and high vacuum (10⁻⁵ Pa). The thermal stability of the materials was slightly affected. Thermal stresses developed during aging led to polymer embrittlement. Fracture toughness decreased by approximately 22 % after cyclic aging. Abrasive wear increased significantly for unreinforced PEEK, while PEEK composites showed a rise in adhesive wear with prolonged aging. In air, the wear rate increase reached 510 %, whereas in vacuum, some compositions exhibited a wear rate reduction of up to 56 %.

The strive to reduce harmful emissions from transport has resulted in an increased emphasis on minimising friction in lubricated contacting components to improve the energy efficiency of automotive engines. In this sense, it is of particular interest to investigate whether a synergistic tribological performance could be achieved by combining two or more friction modifier additives with nanoparticles. This study conducts a comprehensive investigation into the tribological characteristics of lubricant formulations enriched with nanodiamonds (NDs), combined with organic (Glycerol Monooleate, GMO) and inorganic (molybdenum dithiocarbamate, MoDTC) friction modifiers and a low-concentration anti-wear additive (Zinc dialkyl dithio-phosphate, ZDDP). The interaction between NDs and MoDTC has been evaluated using reciprocal sliding tests at two different temperatures. The outcomes of the tribological experiments revealed that the interaction of NDs and MoDTC can enhance the friction and wear performance of steel pairs. However, this enhanced performance is shown to highly depend on other additives present in the lubricant mixture. Analysis of wear scars using High-Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM) and Raman spectroscopy reveals that when NDs are fully entrapped into the formed tribofilm that contains the MoDTC-derived MoS2 layer, the lowest friction coefficient can be achieved.

The sustainable utilization of polymers depends on efficient recycling and the ability to retain their critical physical properties for further processing. In this study, high-density polyethylene (HDPE) nanocomposite properties were enhanced by the integration of carbon dots (CDs), in terms of processability and optical traceability during recycling. HDPE composites with varying CDs loadings were prepared to assess their effects on optical and mechanical properties over three consecutive recycling cycles. The composite containing 0.5 wt% CDs demonstrated a 17% increase in tensile strength after recycling, with a maximum strain of 11%, significantly outperforming the neat HDPE while preserving its crystalline structure. Additionally, incorporating 0.1 wt% CDs reduced the wear rate by up to 98%, highlighting a substantial improvement in durability. Improved processability of the recycled material was confirmed by producing 3D-printed specimens at each CDs concentration. Notably, composites containing 0.1 wt% CDs exhibited excellent printability even after three recycling cycles. CDs have also been utilized as luminescence tracers. This study revealed that the quenching of the blue phosphorescence associated to the carbonyl groups of the polymer backbone was highly dependent on the CDs content. Importantly, nanocomposites with 0.1 wt% CDs exhibited progressive luminescence changes corresponding to the number of recycling cycles, enabling quick and reliable traceability and sorting using standard mobile phone cameras. These findings are highly promising, paving the way for rapid, automated, and scalable HDPE recycling. This innovation offers significant potential for advancing the circular economy of HDPE and enhancing the sustainability of polymer materials.

This study investigates the effect of test-environment, temperature, and cryogenic aging in liquid nitrogen for 5 months on mechanical and tribological performance of polyimide (PI) and PI composites. Tribological tests were conducted at 25 °C in air, and at both 25 °C and −100 °C in a high vacuum (10-5 Pa) environment. Impact of cryogenic aging, testing condition on PI-based materials varied depending on the polymer structure and composition. Cryogenic aging led to embrittlement, increasing the coefficient of friction and wear rate up to 77 % and 165 %, respectively. Some polyimides exhibited the lowest coefficient of friction (0.04) in vacuum at 25 °C, while the temperature reduction to −100 oC in vacuum generally decreased the tribological performance, increased contact stresses and abrasive wear of the materials.

Spurred by the growing trend towards sustainability and use of green material alternatives, Carbon Fiber Reinforced Polymer paired with Titanium alloys (CFRP-Ti stacks) are employed in aerospace industry for their high strength-to-weight ratio and good galvanic corrosion resistance. However, production of holes for bolt or rivet connections, imperative for components assembly depicts unmatched difficulties in their machinability due to superior mechanical characteristics and distinct nature of stack components, causing rapid tool wear and compromised hole quality. In this work, in-depth tool wear analysis of Diamond-like-Carbon (DLC) coated drills were analysed periodically to evaluate tool wear growth and associated wear mechanism while drilling of CFRP-Ti stacks. Qualitative analysis was performed after drilling set number of holes by two DLC coated drills (DLC-Ar and DLC-Bn), and results compared with uncoated solid carbide (WC) drills tested under similar dry machining conditions. The wear progress was observed by SEM and EDX; and for extensive wear mechanism investigation, Focused Ion Beam (FIB) cross-sections were analysed. Moreover, morphological examination of etched drills after drilling experiment was performed to observe active cutting regions for overall damage extent, gain insight about coating reminiscences and underlying wear mechanism. The results demonstrate that DLC-Bn drill showed best performance with reduced thrust forces and torque, reduced wear of cutting edges and no fracture/chipping of drill corner compared to DLC-Ar and WC drill. Although, DLC-Ar drill does alleviate corner chipping but suffer premature coating failure, undergoes intense tribo-chemical wear, excessive removal of carbide grains and generation of highest thrust forces compared to WC drill.

Additive manufacturing (AM) is growing as a resource-efficient and economical processing technique for polymer-based materials. In recent years, substantial advancements have been made in the fused filament fabrication (FFF) of high-performance polyether-ether-ketone (PEEK). However, there is a notable lack of information in the existing literature on the 3D printing of nanoparticle-filled PEEK composites. In this study, PEEK-based composite filaments filled with nanoscale silicon dioxide (SiO2) and microscale short carbon fibers (SCF) were successfully fabricated using melt compounding and 3D printing using FFF. The addition of 2 wt% nano-SiO2 significantly enhanced interfacial bonding, reduced internal porosity, and improved the microstructure of SCF-PEEK composites. Tomography and microstructure analysis revealed a uniform distribution of fibers. Thermal and structural analysis confirmed that the chemical integrity of the PEEK matrix remained intact during the filament processing and 3D printing. Nano-SiO2 enhanced the thermal decomposition temperatures and improved the crystallization behavior of SCF-PEEK. Multiscale composites exhibited up to 40 % and 11 % increments in stiffness compared to neat PEEK and SCF-PEEK, respectively. Overall, SiO2 improved the microstructure, thermal properties, and dynamic modulus of printed SCF-PEEK composites. The findings in this study demonstrate that nano-SiO2 is a promising filament filler for 3D printing of PEEK composites.

Polyetherketoneketone (PEKK) (60/40 TERE/ISO) is characterized by a lower processing temperature and a higher glass transition temperature (Tg) compared to polyetheretherketone (PEEK), making it a promising material for 3D printing. However, it remains in amorphous state post-printing due to its slow crystallization rate. In this study, isothermal annealing is conducted on 3D printed pseudo amorphous PEKK at various times and temperature. Thermal analysis techniques reveal that both annealing time and temperature play a pivotal role in enhancing crystallinity, with levels reaching up to 27% when annealed between the Tg and the melting temperature (Tm). However, thermal stability decreased as the annealing temperature approached Tm. X-ray diffraction studies demonstrate that annealing between Tg and Tm promotes the development of stable form II crystals, while annealing at higher temperatures encourages the formation of form I crystals. Furthermore, dynamic mechanical analysis indicated a 44% increase in mechanical stiffness following annealing, and compressive testing shows improved yield strength, comparable to that of other polyaryletherketone (PAEK) materials. These findings suggest that controlled annealing of 3D printed PEKK enables precise tailoring of its crystallinity and mechanical properties, making it adaptable for a wide range of applications, such as biomedical devices that require patient-specific customization.

Recent advances in the tribology of polymer composites in water-lubricated conditions have focused on achieving a synergistic effect by incorporating multiple fillers into the polymer matrix. This approach requires a comprehensive understanding of how each filler interacts with the other components, something that remains unknown for hard fillers. This study investigates the tribological performance and tribological mechanisms of a hard filler, nano-titanium dioxide, in a short-carbon-fibre-reinforced, ultra-high-molecular-weight-polyethylene (UHMWPE) composite when combined with microfillers that include graphite, hexagonal boron nitride and silicon dioxide. The results reveal that, while composites with nano-titanium dioxide and microfillers improve the friction by 30–53 % compared to basic fibre-reinforced UHMWPE, the nano-titanium dioxide reduced the friction only with boron nitride and silicon dioxide. In these cases, the nano-titanium dioxide contributes to transferred material onto the steel surface where it embeds in a relatively softer layer of material composed of other components transferred from the composite and thereby reduces the friction. In contrast, all the composites have lower wear rates than the basic carbon-fibre-reinforced composite. We present how the nano-titanium dioxide interacts synergistically with all the microfillers, with each filler contributing in its own way to improving the load-bearing capacity and hindering the fibre pullout. Surprisingly, the combination with the graphite microfiller did not reduce the friction, but had the lowest wear rate in this study of 5–8 × 10⁻8 mm³ /Nm. The impact of the contact pressure on the fillers’ interactions and the tribological performance is also highlighted in this study.

Cutting tool experiences rapid variation in thrust forces and torque during drilling of carbon fiber reinforced plastic and titanium (CFRP-Ti) stacks due to distinct materials characteristics, leading to complex wear mechanism. Existing tribotests fail to emulate the multifaceted contact situation of tool interacting with multi-materials simultaneously. This work presents a cost-effective modification using hybrid CFRP-Ti split-ring in cross-cylinder configuration to replicate contact situation during the drilling operation. The results showed a high coefficient of friction (COF) of ∼0.5 against Ti6Al4V and ∼0.21 against CFRP ring. Whereas, the test against CFRP-Ti split-ring showed cyclic COF variation, driven by complex synergistic wear mechanism that completely transformed the sliding contact. The modified tribotest results provide applicable estimation of COF variation and synergistic wear mechanism correlating well with WC-Co tool wear experienced in CFRP-Ti stacks drilling.

In this study, the tribological behaviour of different Diamond-like-Carbon (DLC) coatings sliding against titanium alloy (Ti6Al4V) was analysed in a pin-on-disc tribometer at different applied loads to study effectiveness of tool coatings in titanium alloys machining. Three different DLC coatings were deposited on cemented carbide substrate using HiPIMS (DLC-Ar, DLC-Ne) and arc (DLC-Bn) deposition techniques. A detailed analysis of the wear track and titanium countersurfaces were performed following the tribotest to develop understanding about the wear mechanism and associated variation in the friction response. The results indicated that DLC-Ar presents low friction and reduced wear of coating and respective titanium countersurface at lowest load, seemingly due to its inherent tendency to spontaneously form graphitic transfer-layer at the interface. With an increase in the applied load, the tendency to retain tribofilm decreases as shearing ensue quickly exposing the underneath substrate material. The wear performance of DLC-Ne coatings is better than DLC-Ar under highest load and friction behaviour relatively close to DLC-Ar coatings. In comparison, under increased applied loads, DLC-Bn coatings offered better wear resistance and low friction compared with DLC-Ne and DLC-Ar coatings, which would offer improved performance in machining of titanium alloys.