EFFECTS OF VAPOR-PHASE ENVIRONMENTAL CONSTITUENTS ON MACRO- TO NANO-TRIBOLOGICAL BEHAVIOR
Open Access
- Author:
- Asay, David Benjamin
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 20, 2008
- Committee Members:
- Seong H Kim, Committee Chair/Co-Chair
Erwin A Vogler, Committee Member
Joan Marie Redwing, Committee Member
Darrell Velegol, Committee Member - Keywords:
- friction
adhesion
wear
silicon
MEMS
vapor phase
lubrication
adsorption
water - Abstract:
- When surfaces come into contact, a number of fascinating phenomena occur; adhesion, friction, and wear, each a consequence of material properties, environment conditions and constituents, contacting geometry, applied loads and relative velocities. Changing any of these variables can drastically alter adhesion, friction, and/or wear. Approaches to prevent wear, reduce friction, and control adhesion are determined by the application in question. In many cases, devices requiring lubrication have predetermined geometries, loads, velocities, and are made of specific materials. Effective lubrication and wear prevention then depends on engineering constituents utilized as lubricants. Typically, bulk liquid hydrocarbons are used. As devices shrink in size, bulk lubricants are not practical due to the large capillary forces they create and extreme viscous dampening forces. Microscale devices (i.e. MicroElectroMechanical Systems MEMS) require new technologies for lubrication and wear prevention. This thesis examines the capability of molecular thin films as lubricants and wear prevention films. These films are maintained by utilizing adsorption equilibrium of vapor-phase molecules. Water and various linear alcohol molecules are studied as potential vapor-phase lubricants. Because of the importance of silicon to microscale devices, tribological studies of wear, friction, and adhesion of silicon as a function of alcohol vapor pressure and chain length are studied. Additionally, the effectiveness of these molecular thin films as lubricants for single nano-asperity contact and macrosopic multi-asperity contacts is studied. Adsorption of thin water or alcohol films drastically alters adhesion, friction, and wear. In the case of water adsorption on silicon oxide surface, at low relative humidity, water preferentially adsorbs into the silicon oxide surface in an ice-like structure. As the partial pressure of water increases, liquid water structure is observed to grow on top of the ice-like water structure. As a consequence of structured water at low relative humidities, adhesion between silicon oxide nano-asperity contacts is significantly larger than predicted from capillary forces alone. During sliding in humid environments, chemical wear of silicon oxide is accelerated. Altering the surface chemistry of the silicon oxide surface to prevent water adsorption via chemisorption of self-assembled monolayers is investigated. While these hydrophobic treatments lower the total average surface coverage of water at the interface, water adsorption is not completely prevented. Adsorption of linear alcohols onto the silicon oxide surface is also investigated. In the case of alcohol vapors, monolayer coverage is observed to occur at ~10% of the saturation pressure of the alcohol. In contrast with water, alcohol is observed to drastically reduce adhesion between nano-scale contacts. The reduction in capillary adhesion is observed to decrease and is inversely proportional to the molar volume. In the case of contact asperities ranging from a single nano-scale asperity to multi-asperity macro-scale systems, alcohol vapor adsorption successfully lubricates (lowers friction) and prevents wear. Inside of the sliding contact region, high-molecular weight oligomeric species are formed via tribochemical reactions from alcohol precursor molecules forming wear protective coatings. Therefore, these oligomeric species/coatings form when and where lubrication is needed most; greatly aiding wear prevention. In the case of silicon based MEMS devices, these films completely prevent wear and increase the lifetime of these devices over 4 to 5 orders of magnitude compared with current “state-of-the-art” self-assembled monolayer coatings.