Kara, Levent; Boztoprak, Yalçın; Sert, Yaşar

The tribological performance of fiber-reinforced polymer composites is strongly influenced by reinforcement design, interfacial interactions, and operating environment. While carbon nanotube (CNT) reinforcement has been widely reported to improve friction and wear behavior, most existing studies have been limited to ambient atmospheric conditions, leaving its effectiveness under vacuum environments insufficiently understood. In this study, the effect of CNT addition on the friction and wear behavior of glass fiber-reinforced polymer composites were systematically investigated under three distinct environmental conditions: ambient atmosphere, low vacuum, and high vacuum. Tribological tests were conducted using a ball-on-disk configuration under varying normal loads, and the resulting frictional response, wear behavior, and dominant wear mechanisms were analyzed. The results demonstrate that the influence of CNT reinforcement is highly dependent on both applied load and the surrounding environment. While CNT addition enhances wear resistance under atmospheric conditions at higher loads through improved load transfer and crack-bridging mechanisms, it does not provide a universal improvement across all environments. Under vacuum conditions, the absence of moisture-assisted tribochemical reactions and unstable tribofilm formation can diminish or reverse the beneficial effects of CNTs. These findings highlight that CNT reinforcement in fiber-reinforced polymer composites should be evaluated within an application-specific framework rather than assumed to yield generalized tribological improvements. The present study provides critical insights into the environment-dependent role of CNTs and contributes to the informed design of polymer composite systems for vacuum and space-related applications.

Dey, Ripon Kumar; Ekinci, Huseyin; Cui, Bo

High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contact. It is also reported that a higher bias power leads to less undercut than a lower one, due to increased and more directional physical bombardment by ions.

Ekinci, Huseyin; Dey, Ripon K.; Cui, Bo

Among various grating structure fabrication techniques, potassium hydroxide (KOH) wet anisotropic etching of Si(110) wafers offers low cost and impressive aspect ratio over large areas with high etch uniformity. The aspect ratio is ultimately limited by lateral etching that constantly widens the trenches. In this paper, the authors demonstrated a method to double the achievable aspect ratio using two-step KOH etching. After first KOH etching, the grating structure was grown with a thermal oxide; and after removing the oxide from the trench bottom using reactive ion etching, a second KOH etching was carried out with the original trench sidewall protected by the thermal oxide. The authors achieved the highest anisotropy [etching rate ratio of (110) and (111)] of 247 with 50 wt. % KOH at room temperature. Using the two-step KOH etching, it is possible to increase the aspect ratio by more than a factor of 2 while keeping the trench width almost unchanged.