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Mechanochemical reactions of the GaN-Al2O3 interface offer a novel principle for scientific and technological merits in the micro-/nano-scale ultra-precision surface machining. In this work, the mechanochemical reactions on Ga- and N-faced GaN surfaces rubbed by the Al2O3 nanoasperity as a function of the environmental humidity were investigated. Experimental results indicate that the N-face exhibits much stronger mechanochemical removal over the relative humidity range of 20%–80% than the Ga-face. Increasing water molecules in environmental conditions significantly promotes the interfacial mechanochemical reactions and hence accelerates the atomic attrition on N-face. The hypothesized mechanism of the selective water-involved mechanochemical removal is associated with the dangling bond configuration, which affects the mechanically-stimulated chemical reactions via altering the activation energy barrier to form the bonding bridge across the sliding interface. These findings can enrich the understanding of the underlying mechanism of mechanochemical reactions at GaN-Al2O3 interface and a broad cognition for regulating the mechanochemical reactions widely existing in scientific and engineering applications. [ABSTRACT FROM AUTHOR]

The effects of counter-surface chemistry on the friction and wear of soda-lime-silica glass were investigated using a ball-on-flat tribometer with various ball materials (stainless steel, silicon nitride, and alumina) in dry and humid environments. It was found that the interfacial wear was very sensitive to environmental humidity and counter-surface chemistry. In dry conditions, soda-lime glass was damaged mechanically regardless of counter-surface materials, creating a rough and deep wear track. These results were consistent with the Archard relationship—a softer material would wear mechanically by a harder material. However, in humid conditions, the ball materials that were harder than the soda-lime glass were damaged. Thus, the wear of glass interfaces under humid conditions does not follow the Archard relationship. These results clearly show that the tribochemical reactions involving the substrate and counter-surface chemistry as well as the adsorbed water molecules are determining factors governing wear behaviors in humid environments. [ABSTRACT FROM AUTHOR]

The mechanochemical wear of multicomponent glasses was studied under controlled humidity conditions using a reciprocating ball-on-flat tribometer. For dry conditions, the surfaces were extensively damaged by scratching for all of the glasses, while for humid conditions the wear behavior varied with the glass composition suggesting a chemical effect on scratch behaviors of glass surfaces. The wear of soda lime silica (also called sodium calcium silicate) glass was suppressed with increasing humidity, while the borosilicate and barium boroaluminosilicate glasses showed an increase in wear volume with increasing humidity. The unique humidity dependence of the observed mechanochemical wear of soda lime silica glass supports the hypothesis that hydronium ion formation in the sodium-leached sites of the soda lime glass enhances its wear resistance. [ABSTRACT FROM AUTHOR]

The friction of hydrogenated diamond-like carbon (H-DLC) films was evaluated under the controlled environments of humid air and vacuum by varying the applied load. In humid air, there is a threshold applied load below which no obvious friction drop occurs and above which the friction decreases to a relatively low level following the running-in process. By contrast, superlubricity can be realized at low applied loads but easily fails at high applied loads under vacuum conditions. Further analysis indicates that the graphitization of the sliding H-DLC surface has a negligible contribution to the sharp drop of friction during the running-in process under both humid air and vacuum conditions. The low friction in humid air and the superlow friction in vacuum are mainly attributed to the formation and stability of the transfer layer on the counterface, which depend on the load and surrounding environment. These results can help us understand the low-friction mechanism of H-DLC film and define optimized working conditions in practical applications, in which the transfer layer can be maintained for a long time under low applied load conditions in vacuum, whereas a high load can benefit the formation of the transfer layer in humid air. [ABSTRACT FROM AUTHOR]

Using an atomic force microscope (AFM), the wear of monocrystalline silicon (covered by a native oxide layer) at high humidity was investigated. The experimental results indicated that tribochemistry played an important role in the wear of the silicon at different relative humidity levels (RH = 60%, 90%). Since the tribochemical reactions were facilitated at 60% RH, the wear of silicon was serious and the friction force was around 1.58 μN under the given conditions. However, the tribochemical reactions were restrained when the wear pair was conducted at high humidity. As a result, the wear of silicon was very slight and the friction force decreased to 0.85 μN at 90% RH. The slight wear of silicon at high humidity was characterized by etching tests. It was demonstrated that the silicon sample surface was partly damaged and the native oxide layer on silicon sample surface had not been totally removed during the wear process. These results may help us optimize the tribological design of dynamic microelectromechanical systems working in humid conditions. [ABSTRACT FROM AUTHOR]

Abstract: With an atomic force microscopy, the humidity-dependent nanotribology behaviors of Si(100) against SiO2 microsphere were investigated while the relative movement translated from stick to slip. The relative humidity RH of air exhibits a strong effect on the motion behavior of Si(100)/SiO2 pair. With the increase in RH, relative movement of Si(100)/SiO2 pair is easier to keep into stick state, namely, the relative slip becomes more difficult to occur in a higher humidity range. The adhesion F a will increase with the increase in RH in the given humidity range. In the low RH range (<20%) where the adsorbed water layer forms the ‘solid-like’ structure, due to the absence of water meniscus, F a increases very slowly. However, in relative higher RH range (>20%), F a increases very sharply once ‘liquid-like’ adsorbed water layer forms, because it increases the capillary force. The initial friction forces F t of Si(100)/SiO2 pair also increase with the increase in RH in the given humidity range. However, different from F a, it increases sharply in the low RH range (<30%) and slightly in the higher RH range (>30%). During the cyclic friction process, under the higher RH, relative stable tangential force is easier to be observed at higher displacement amplitude, here, the relative movement usually keeps into stick state. With the increase in RH, the surface damage of Si(100) transforms from mechanical deformation (forming hillock) to tribochemical wear (material removal). The tribochemical wear is sensitive to the absorbed water film with ‘solid-like’ structure, here, the wear volume increases drastically in this RH range (<20%); further increase of wear is small in higher relative humidity regime where the ‘liquid-like’ water layer is formed. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis verifies the water molecules participate into tribochemical reaction to rupture the Si bonds on Si(100) substrate. [Copyright &y& Elsevier]

Abstract The role of water in the material removal of single crystal calcium fluoride (CaF 2) was studied using a scratch tester with a diamond tip. Water can facilitate the sliding wear of CaF 2 , and the degree strongly depended on the amount of water molecules involved in the wear process. CaF 2 wear increases with the increase of relative humidity (RH). When the sliding wear tests were conducted in dry condition, the wear rate increases linearly with applied load, obeying the Archard wear equation. When water participated in the wear process, the Archard wear equation was failure and a exponential dependence of wear rate on contact stress was given under water condition. The transformation of the applied load (or contact stress) dependence indicates the change of wear mechanism in water condition. In dry condition, the surface damage of CaF 2 is mainly attributed to mechanical wear where the stress-induced plastic flow under the activation of the slip systems results in a relatively smooth worn surface. In humid nitrogen or water condition, the plastic wear of CaF 2 weakens due to the stress-associated dissolution though the wear volume increases compared to that in dry case. Both the water-associated dissolution itself and induced delamination promote the sliding wear of CaF 2 in water. The results may help elucidate the water-associated wear mechanism of CaF 2 and optimize the processing parameters in the ultra-precision machining to reduce the residual surface/subsurface defects. Highlights • Linear dependence between load and CaF 2 wear in dry N 2 obeys Archard's equation. • Load dependence of CaF 2 wear changes to exponential relationship in water. • Stress-induced plastic flow dominates CaF 2 wear in dry N 2. • CaF 2 wear originates from stress-induced dissolution and delamination in water. • Water weakens the residual plastic wear of CaF 2 due to stress-induced dissolution. [ABSTRACT FROM AUTHOR]

Nanowear tests of bare silicon against a SiO 2 microsphere were conducted in air (relative humidity [RH] = 0%–89%) and water using an atomic force microscope. Experimental results revealed that the water played an important role in the tribochemical wear of the bare silicon. A hillock-like wear trace with a height of 0.7 nm was generated on the bare silicon surface in dry air. As the RH increased, the wear depth increased and reached the maximum level in water. Analysis of frictional dissipated energy suggested that the wear of the bare silicon was not dominated by mechanical interactions. High-resolution transmission electron microscopy detection demonstrated that the silicon atoms and crystal lattice underneath the worn area maintained integral perfectly and thus further confirmed the tribochemical wear mechanism of the bare silicon. Finally, the role of water in the tribochemical wear of the bare silicon may be explained by the following three aspects: the hydroxylation by hydroxyl ions auto-ionized in water, the hydrolytic reaction of water molecules, and the dissolution of the tribochemical product SiO m H n in liquid water. With increasing RH, a greater water amount would adsorb to the Si/SiO 2 interface and induce a more serious tribochemical wear on the bare silicon surface. The results of this paper may provide further insight into the tribochemical removal mechanism of bare monocrystalline silicon and furnish the wider reaction cognition for chemical mechanical polishing. [ABSTRACT FROM AUTHOR]

Wear at sliding interfaces of siliconis a main cause for materialloss in nanomanufacturing and device failure in microelectromechanicalsystem (MEMS) applications. However, a comprehensive understandingof the nanoscale wear mechanisms of silicon in ambient conditionsis still lacking. Here, we report the chemical wear of single crystallinesilicon, a material used for micro/nanoscale devices, in humid airunder the contact pressure lower than the material hardness. A transmissionelectron microscopy (TEM) analysis of the wear track confirmed thatthe wear of silicon in humid conditions originates from surface reactionswithout significant subsurface damages such as plastic deformationor fracture. When rubbed with a SiO2ball, the single crystallinesilicon surface exhibited transitions from severe wear in intermediatehumidity to nearly wearless states at two opposite extremes: (a) lowhumidity and high sliding speed conditions and (b) high humidity andlow speed conditions. These transitions suggested that at the slidinginterfaces of Si/SiO2at least two different tribochemicalreactions play important roles. One would be the formation of a strong“hydrogen bonding bridge” between hydroxyl groups oftwo sliding interfaces and the other the removal of hydroxyl groupsfrom the SiO2surface. The experimental data indicatedthat the dominance of each reaction varies with the ambient humidityand sliding speed. [ABSTRACT FROM AUTHOR]

Previous studies demonstrated the macroscale wear behavior of soda lime silicate (SLS) glass in humid air was governed by the mechanochemical reactivity of sliding interfaces, which is a function of relative humidity (RH). In this study, it was observed that no disenible nanowear of SLS glass surface in dry conditions because the contact is elastic. In humid conditions, nanowear occurred due to water-induced mechanochemical reactions and the RH dependence above 20% was relatively weak. Nanowear of SLS glass decreased at the sodium ions enriched region, which was revealed with ReaxFF-MD simulations. Further analyses revealed that the wear behavior of SLS glass surface depends on both the mechanochemical reactions occurring at the sliding interface and the transport of the mechanochemical reaction products. [Display omitted] • No disenable nanoscale wear of SLS glass surface in dry air at elastic contact regime. • Nanoscale wear of SLS glass occurred in humid air due to water-involved mechanochemical reactions. • Nanoscale wear of SLS glass decreased at the sodium ion enriched region. • SLS wear behavior depends on the interface reactivity and transport of reaction products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nano-scale adhesion behaviors studied for various few-layered nano-sheets by AFM. • Adhesion of bi-layer Ti 3 C 2 T x and Si showed an increase-to-decrease transition vs. RH. • Graphene oxide only showed a slight monotonous increase of adhesion with RH. • Adhesion of WSe 2 demonstrated a decrease-to-increase transition as a function of RH. • Adhesion was correlated with the existing surface terminations and water adsorption. Two-dimensional materials have gained significant attention in the research community due to their unique physicochemical and mechanical properties. Regarding their applications in devices irrespective of the involved length scale, interfacial adhesion plays an important role in nanoscopic single-asperity and macroscopic multi-asperity contacts. Therefore, this study aims at studying the adhesion behaviors of few-layered Ti 3 C 2 T x , graphene oxide and WSe 2 nano-sheets as a function of the adjusted relative humidity by atomic force microscopy. Reference experiments are conducted with graphite and silicon surfaces. The experimental results for graphene oxide and WSe 2 nano-sheets show a monotonous increase of the adhesion force with increasing humidity. In contrast, silicon reference and bi-layered Ti 3 C 2 T x nano-sheets show an increasing adhesion with increasing humidity before reaching a maximum value at a critical humidity followed by a pronounced decrease. Based upon the obtained adhesion and characterization data, the measured water adsorption behavior, and the existing knowledge for silicon regarding their superficial water structures, the observed experimental trends were connected with the respective surface groups of the nanomaterials and their ability to adsorb water on their surfaces. [ABSTRACT FROM AUTHOR]