domingo, 7 de marzo de 2010

TRIBOLOGY OF FILMS ANO COATINGS



Tribology is the study of phenomena associated with interacting surfaces in relative motion. Adhesión, hardness, friction, wear, erosión, and lubrication are among some of the scientific and technological concerns of this important subject. From an engineering standpoint three sepárate categories of behavior can be distinguished based on the relative magnitudes of friction and wear :
1. Friction and wear are both low. This is the case in bearings, gears, cams, and slideways.

2. Friction is high but wear is low. This combination of properties is desired in devices that use friction to transmit power, e.g., clutches, belt drives, tires.

3. Friction is low and wear of one contacting body is high. This is the situation that prevails during material removal processes such as machining, cutting, and grinding. In these operations plástic deformation is involved, whereas in the first two categories the contacting bodies are generally elastically stressed.
We now explore the concepts of friction and wear in more detail.

 Friction

AJÍ engineering surfaces are rough and characterized by a density of projec-tions or asperities with some distribution of heights. Surfaces may be thought of as producing contact with each other at the summits of the asperities. The sum of these local contact áreas represents the real área of contact, which may only be but a fraction of the apparent geometric contact área. In the majority of contacts surface films are present with properties different from the underlying bulk material. Surface films may be gases, fluids (e.g., oil), or deposited solid layers such as graphite, metáis, or ceramic coatings. In we consider three cases of contact between rough surfaces with an intervening film layer . In the first, the film thickness is less than the roughness so that some asperities pierce the film enabling contact between bulk bodies . the film (2) is still thin but of the order of roughness dimensions. All contacts are now between the upper body and the film . the film is thicker, and all contacts are also of 1/2 type. Surface roughness is not expected to play a marked role here.

The dependence of frictional effects on the film thickness is of considerable technological importance. Consider a soft solid film such as lead on steel. As the film thickness increases, / decreases from fa (steel-steel) to fb (lead-steel), reflecting the changing nature of contact. Beyond this thickness, T essentially stays the same but P, which is related to the effective film hardness changes. Thin lead films are harder than thicker ones because of the support provided by the steel. Therefore, P decreases as the film thickness increases, leading to an increase in fb or /. This causes a friction variation in film thickness that passes through a mínimum. The same effect occurs with thin fluid films used to lubrícate metal surfaces in contact. The well-known Stribeck curve then displays a minimum in the coefficient of friction as a function of thickness or equivalent quantity (viscosity • speed/pressure). As the film thick¬ness increases, boundary, quasi-hydrodynamic and hydrodynamic lubrication regimes are successively operative.

An important set of applications involves the dry lubrication of moving parts where fluid lubrication is not possible, at high and low temperaturas or pressures. Thin, low-friction solid films composed of chalcogenides, oxides, fluorides, or carbón are used instead. In particular, MoS2 is a favored lubricant and has been deposited by reactively sputtering MoS2 targets in an H2S ambient.

 Wear Mechanisms

Wear may be defined as the progressive removal of material from surfaces that are under load in relative motion. Several different mechanisms have been identified to characterize contact wear, and these are briefly described below.

Adhesive Wear.

Adhesive wear occurs when applied tangential forces cause fracture between surfaces bonded at asperities. One option is for the fracture path to follow the original microwelded interface. Other paths lie above or below the interface when the strength of the bonds between asperities exceeds the cohesive strength of the bodies in contact. The result is material transfer, usually from the softer to harder body. During subsequent surface motion cycles, these particles may eventually be removed by fatigue fracture. In more severe cases of adhesive wear, smearing, galling, and seizure of surfaces may occur.
Abrasive Wear.

Abrasive wear is a form of cutting wear where the material is removed by hard wear particles, by hard asperities, or by hard particles entering the interface from the environment.

Fatigue Wear.

Fatigue wear occurs in situations where there is repeated loading and unloading of surfaces in contact. Failure may initiate at both surface flaws or cracks or at subsurface inhomogeneities. Crack growth eventually results in detached wear particles.

Fretting Wear.

Fretting wear may be viewed as a type of fatigue wear that occurs under conditions of oscillatory movement of small amplitude (in the range of 1-200 ¿un), but relatively high frequency. Many sequential damage processes occur during fretting, including breakup of protective films, adhesión and transfer of material, oxidation of metal wear particles, and nucleation of surface cracks.

Delamination Wear.

Delamination wear takes the form of regu¬lar detachments of thin platelike particles from wearing surfaces due to the influence of high tangential (friction) forces in the surface contact zone. During cyclic loading the cracks that develop propágate parallel to the surface at a depth governed by the material properties and coefficient of friction.

 Oxidation Wear.

Oxidation wear arises from the continuous rub-bing and removal of surface films produced by reaction with the environment. Wear damage is modest in this case because oxide regrows soon after it ís lost. However, at high temperatures where chemical reactions are accelerated oxidation wear is aggravated.


Ronellys Flores---CRF---libro the materials science of thin films



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