Los artículos Noveno, Décimo y Décimo Primero del Presente Decreto

The presence of a lubricant film between two mating surfaces greatly reduces friction and wear. As a matter of fact, the lubricant prevents direct contact between the asperities, reducing the average shear stress at the junctions.

Influid film lubrication, no wear is thus recorded because of the absence of any contact between the asperities. However, in case of mixed or, specially, boundary lubrication, adhesive wear may take place where metal-to-metal contacts are established. Since lubricant greatly reduces the availability of oxygen in the contact areas, wear by tribo-oxidation is somewhat difficult, although its contribution cannot be excluded. Wear in lubricated sliding is really difficult to model. A promising approach is provided by a refinement of the model presented in Sect.3.5, where parameterδ, i.e. the contact area ratio, has been introduced. δ represents the fraction of real area of contact at which boundary lubrication occurs. In the present model, the parameter α′ is introduced, which represents the fraction of boundary film that is defective, i.e., the fraction of real area of contact that is metallic [11]

Table 5.3 Characteristics and operating limits for a number of materials used for dry bearings (taken from [10] and other sources in the literature; the sintered bronze impregnated with oil operate under boundary or mixed lubrication)

Material Friction coefficient (against steel)

Specific wear rate against steel, Ka(m2/N) Maximum pressure (MPa) Maximum sliding speed (m/s) Maximum temperature (°C) Nylon 6,6 (filled with PTFE) 0.2 2.4× 10−16 10 – 200 PTFE 0.03–0.15 4× 10−13 3.4 0.3 250 Filled PTFE (15 % glass fibre) 0.1 1.4× 10−16 17 5 250 Acetal resin (filled with PTFE) 0.07–0.1 4.9× 10−16 – – 110 Carbon- graphite 0.06–0.15 1.4× 10−15 4.1 13 400 Sintered bronze (filled with oil) 0.05–0.15 4× 10−17 6.1 28 80

(α′ is often called the fractional film defect, and it is smaller than δ). Equation4.1 can thus be rewritten for lubricated sliding wear:

W ¼ a0 Kad

FN

H ¼ Klub FN

H ð5:5Þ

where Klubis the coefficient for adhesive wear in lubricated conditions.

Therefore, Klubdecreases as the Λ factor is increased and also depends on the lubricant quality. In mixed lubrication, Klub typically ranges from 10−10 to 10−6, while in boundary lubrication it typically ranges between 10−6 and 10−5[12]. In case of boundary lubrication, the following relation can be used as a first approximation:

Klub¼ x Kad

where:

x = 10−1 for poor lubrication (α′ is quite high; a poor lubricant, such as water, is used);

x = 10−2 for average lubrication (α′ is low; a common mineral oil is used); x = 10−3 for excellent lubrication (α′ is very low; lubricants with EP additives are

employed)

The evaluation ofα′ is rather difficult. In general, α′ decreases as [10]: • the lubricant heat of adsorption is increased (if the lubricant adsorption on the

metal surface is strong, the desorption during sliding can be more difficult); • the surface contact temperature is decreased (desorption is due to the attainment

of a critical contact temperature; in addition, temperature also limits the mechanical performances of the secondary boundary layerthat is formed when EP additives are used);

• the sliding speed is decreased (in this way a shorter contact time at the asperities is allowed).

Two important aspects have to be considered when dealing with lubricated sliding wear: running in, and scuffing (i.e., the failure of boundary lubrication). Figure5.4shows the wear evolution of a heat-treated Ni-Cr-Mo steel (H: 420 kg/ mm2) tested in a pin-on-disc and in a like-on-like configuration under lubricated conditions [13]. The applied load was 200 N (p0= 6.3 MPa) and the sliding speed 2 m/s. A quite long running in stage (see Fig.5.1a) was recorded, characterised by rapid wear, followed by a steady-state stage. The calculated Klubduring the steady- state stage is equal to 1.3× 10−8. This value is representative of mixed lubrication as confirmed by the recorded friction coefficient that was quite low, around 0.025. During running in, Klubwas between one and two order of magnitudes larger than during steady state. Adhesive wear was mainly caused by the elimination of initial misalignments between the mating surfaces and the highest peaks in the roughness profiles. The correct execution of the run-in stage is thus of paramount importance

in lubricated tribological systems, since it will increase theΛ factor and decrease the subsequent risk of scuffing. This fact introduces the necessity of a careful operation at the beginning of service of lubricated machinery (like gears, bearings and engines). A commonly adopted strategy is to operate with mild loading con- ditions in freshly assembled surfaces, and gradually increase the loads to the design levels [11]. In some cases, small abrasive particles are introduced between the surfaces to help surface polishing, and therefore attain a very low roughness after running in (such particles have to be washed out before entering service). Most operations are still based on experience, and there is a continuous industrial need to optimise the run-in procedures, including a reduction in the run-in times.

If the operating conditions are severe, lubricant failure by scuffing can occur during lubricated sliding in boundary conditions. The phenomenon of scuffing has been treated in Sect.3.4. As seen, there is no universally accepted procedure to predict the conditions leading to scuffing. The most common approach is to eval- uate the average surface temperature, and make sure that it is less than the critical value, which is around 150 °C for mineral oils. Following the arguments high- lighted in the previous paragraph, a design procedure based on the PVlimitconcept can be possibly adopted. In order to minimise the risk of scuffing, it is useful to select a heat resistant lubricant or to adopt lubricants with EP additives, and/or to modify the materials at the surface, by using surface treatments that reduce friction and/or are able to act as oil reservoirs (examples include chromium plating, molybdenum coating and gas nitriding, see also Chap.7). It may be also useful to favour a high value of the Λ factor (by properly optimising the surface micro- geometry and the running in stage); a low lubricant temperature (by properly cooling the lubricant); a high thermal conductivity of the mating materials (which favours the achievement of low contact temperatures).

Fig. 5.4 Wear evolution in a Ni-Cr-Mo steel during sliding at 2 m/s speed and 200 N applied load in lubricated conditions (modified from [13])