Lubricant film

The growth of solid films onto solid substrates allows for the production of artificial stmctures that can be used for many purposes. For example, film growth is used to create pn junctions and metal-semiconductor contacts during semiconductor manufacture, and to produce catalytic surfaces with properties that are not found in any single material. Lubrication can be applied to solid surfaces by the appropriate growth of a solid lubricating film. Film growth is also... 

Lubricant additives Lubricant-film bearings Lubricants... 

Antiwear Compounds. Additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, ie, when fuU lubricating films cannot be maintained. Two general classes of materials are used to prevent metallic contact. 

Bonded Solid-Film Lubricants. Although a thin film of soHd lubricant that is burnished onto a wearing surface often is useful for break-in operations, over 95% are resin bonded for improved life and performance (62). Use of adhesive binders permits apphcations of coatings 5—20 p.m thick by spraying, dipping, or bmshing as dispersions in a volatile solvent. Some commonly used bonded lubricant films are Hsted in Table 12 (62) with a more extensive listing in Reference 61. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

The molecular dipstick microscope is related to the AFM. It measures lubricant film thickness. The probe is lowered into the oil film on a surface (like the automobile engine crankcase dipstick). The tip is attracted to the surface by the surface tension of the film but repelled by van der Waal s forces from the hard substrate. By noting the height of the probe from the two surfaces as it makes contact, the film thickness can be measured with a precision of about 0.5 nm. 

Individual particle surfaces can be lubricated by an adsorbed film that produces a smoother surface and/or decreases interparticle attraction. A plasticized binder may serve this purpose. Forming surfaces can be lubricated by coating with a film of low viscosity Hquid such as water or oil. Die surfaces can also be coated with a solution of stearic acid dissolved in a volatile Hquid that rapidly evaporates to leave a lubricating film. 

One of the major advantages of the worm gear is low wear, which is due mostly to a full-fluid lubricant film. In addition, friction can be further reduced using metals having low coefficients of friction. For example, the wheel is typically made of bronze and the worm of highly finished hardened steel. 

Large generators typically use Babbitt bearings, which are non-rotating, lined metal sleeves (also referred to as fluid-film bearings) that depend on a lubricating film... 

Lubricating-film instability is the dominant failure mode for sleeve bearings. This instability is typically caused by eccentric, or off-center, rotation of the machine shaft resulting from imbalance, misalignment, or other machine or process-related problems. Figure 44.48 shows a Babbitt bearing that exhibits instability. 

Oil analysis has become an important aid to preventive maintenance. Laboratories recommend that samples of machine lubricant be taken at scheduled intervals to determine the condition of the lubricating film that is critical to machine-train operation. Typically eleven tests are conducted on lube oil samples ... 

To generate a lubricating film within a bearing, the opposed surfaces must be forced apart by pressure generated within the fluid film. One way is to introduce the fluid under sufficient pressure at the point of maximum loading, but this hydrostatic method, although equally effective at all speeds, needs considerable power and is consequently to be avoided whenever a satisfactory alternative exists. 

The oils for these engines have several functions to perform while in use. They must provide a lubricant film between moving parts to reduce friction and wear, hold products of combustion in suspension prevent the formation of sludge and assist in cooling the engine. Unless the lubricant chosen fulfils these conditions successfully. 

If the effects of friction are to be minimized, a lubricant film must be maintained continuously between the moving surfaces. Two types of motion are encountered in engines, rotary and linear. A full fluid-film between moving parts is the ideal form of lubrication, but in practice, even with rotary motion, this is not always achievable. At low engine speeds, for instance, bearing lubrication can be under boundary conditions. 

At medium speeds, contact of the high spots will cause incipient melting and surface welding. The rise in surface temperature will encourage the breakdown of the lubricant film and the resultant scoring which is distinctive will cause rapid wear. 

To avoid the instabilities of wedge-shaped oils films, a lubricating film can be maintained by the application of pressurized oil (or, occasionally, air) to the bearing. The hydrostatic bearing maintains a continuous film of oil even at zero speed, and induces a strong stabilizing force towards the center that counteracts any displacement of the shaft or spindle. Disadvantages include the power... 

Excessive end play as a result of defective thrust bearings can reduce seal performance by disturbing both the established wear pattern and the lubricating film. 

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