Chemical aspects of ceramic tribology

Effect of Humidity on Friction and Wear—A Critical Review

The friction and wear behavior of materials are not intrinsic properties, but extrinsic properties; in other words, they can drastically vary depending on test and environmental conditions. In ambient air, humidity is one such extrinsic parameter. This paper reviews the effects of humidity on macro- and nano-scale friction and wear of various types of materials. The materials included in this review are graphite and graphene, diamond-like carbon (DLC) films, ultrananocrystalline diamond (UNCD), transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), boric acid, silicon, silicon oxide, silicates, advanced ceramics, and metals. Details of underlying mechanisms governing friction and wear behaviors vary depending on materials and humidity; nonetheless, a comparison of various material cases revealed an overarching trend. Tribochemical reactions between the tribo-materials and the adsorbed water molecules play significant roles; such reactions can occur at defect sites in the case of two-dimensionally layered materials and carbon-based materials, or even on low energy surfaces in the case of metals and oxide materials. It is extremely important to consider the effects of adsorbed water layer thickness and structure for a full understanding of tribological properties of materials in ambient air.

Influence of Environmental Humidity on the Wear and Friction of a Silica/Silicon Tribopair Lubricated with a Hydrophilic Ionic Liquid

In this study, the tribological behavior of silica/silicon surfaces lubricated with the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM] EtSO4) was investigated. Tests were carried out in the presence of either humid air (45-55% relative humidity) or in a nitrogen atmosphere, and the results were compared with those obtained using pure water as a lubricant. The cross-sectional analysis of the contact area performed by focused-ion-beam scanning electron microscopy indicated the presence of cracks in the subsurface region, showing that brittle fracture contributed to wear. Sliding promoted the formation of a third body, the presence of which was indicated by optical and secondary electron microscopy. X-ray photoelectron spectroscopy showed that the third body was mostly composed of silicon oxides. The accumulation of the debris was controlled by the presence of water: in the presence of a nitrogen atmosphere, particles were trapped between the sliding surfaces, whereas in the case of humid air, the debris was progressively removed from the contact. Notably, the presence of trapped particles was associated with higher values of wear coefficients of both disks and pins. In addition, a lower roughness was observed along the direction of sliding in the case of water-containing ionic liquid. The observed trends in wear and the combined results of the various techniques, as well as the comparison with tests carried out in the presence of pure water, all point to the characteristic tribochemical reactions of water with silicon-based materials, namely, the formation of a sacrificial layer of hydrated oxide and the dissociative adsorption of water at crack tips of SiO2. In the absence of water, the lack of a tribochemical mechanism forming a sacrificial layer leads to a microfracture-dominated wear mechanism over the entire duration of the test, thus leading to more severe wear. The possible occurrence of stress-induced phase transformation of silicon during sliding is also discussed.

Tribo-electrochemical surface modification of tantalum using in situ AFM techniques

A scanning-probe-based technique to observe tribo-electrochemically stimulated surface was demonstrated. The configuration consists of an electrochemical cell attached to an atomic force microscope (AFM) scanner. Under an applied electrical potential and in various chemical environments, the surface morphology, roughness, skew, bearing ratio, as well as surface adhesive forces between probes were measured, and the effects of mechano-electrochemical stimuli were evaluated. The effects of mechanical, electrochemical, and mechano-electrochemical stimuli were found to compete during AFM sliding process. Their effects do not follow a linear relationship, implying that the mechanical stimulus promotes electrochemical reactions. Similarly, electrochemically enhanced mechanical removal of surface materials is possible.

Targeted functionalization of nanoparticle thin films via capillary condensation

Capillary condensation, an often undesired natural phenomenon in nanoporous materials, was used advantageously as a universal functionalization strategy in nanoparticle thin films assembled layer-by-layer. Judicious choice of nanoparticle (and therefore pore) size allowed targeted capillary condensation of chemical vapors of both hydrophilic and hydrophobic molecules across film thickness. Heterostructured thin films with modulated refractive index profiles produced in this manner exhibited broadband antireflection properties with an average reflectance over the visible region of the spectrum of only 0.4%. Capillary condensation was also used to modify surface chemistry and surface energy. Photosensitive capillary-condensates were UV-cross-linked in situ. Undesired adventitious condensation of humidity could be avoided by condensation of hydrophobic materials such as poly(dimethyl siloxane).

AFM study of forces between silica, silicon nitride and polyurethane pads

Interaction of silica and silicon nitride with polyurethane surfaces is rather poorly studied despite being of great interest for modern semiconductor industry, e.g., for chemical-mechanical planarization (CMP) processes. Here we show the results from the application of the atomic force microscopy (AFM) technique to study the forces between silica or silicon nitride (AFM tips) and polyurethane surfaces in aqueous solutions of different acidity. The polyurethane surface potentials are derived from the measured AFM data. The obtained potentials are in rather good agreement with measurements of zeta-potentials using the streaming-potentials method. Another important parameter, adhesion, is also measured. While the surface potentials of silica are well known, there are ambiguous results on the potentials of silicon nitride that is naturally oxidized. Deriving the surface potential of the naturally oxidized silicon nitride from our measurements, we show that it is not oxidized to silica despite some earlier published expectations.

Wear of mica under aqueous environments:direct observation of defect nucleation by AFM

The generation of defects at surfaces in sliding contacts is the catalyst for the eventual wear of the materials. Here, the wear of muscovite mica has been investigated under aqueous environments using atomic force microscopy (AFM). Through concomitant acquisition of topography, friction, and adhesion data under controlled pH conditions, defect nucleation on the atomic scale prior to gross wear may be directly observed. Nucleation is found to present itself initially as charging of the surface due to stress-induced tribochemical bond scission as OH- breaks open the surface terminating Si-O-Si or Si-O-Al bonds. As the surface bonds are continually cleaved, an ensemble of defects (e.g., Si-OH/Al-OH and Si-O-) contribute to a crystal lattice reconstruction (from approximately 5.2 to approximately 3 A), as observed in AFM topographic and frictional force micrographs. Following lattice restructuring, displacement/abstraction of mica surface materials ensues, yielding readily discernible wear scars ranging from approximately 2 to 10 A in depth. The environmental OH- concentration profoundly affects the efficacy of this sequence of events leading to wear and is illustrated by the acceleration or inhibition of wear with adjustment of pH under identical load and scan conditions.