Adhesive wear coefficient

Selection of the most promising coatings and coating processes will be made after the charcicterization task. Criteria for selection will include not only performance (i.e., wear, adhesion, friction coefficient, thermal shock resistance and thermal stability) but manufacturability/cost factors as well. Using these criteria, a coating system having acceptable cost/benefit relationships will be selected. 

The four wear mechanisms described in this section all lead to Archard s law, but the physical interpretation of the wear coefficient differs. In the adhesive wear model the wear coefficient expresses the probability that an adhesive junction leads to formation of a wear particle. In the abrasive wear model the wear coefficient depends only on the geometry of the abrasive. The wear coefficient in delamination wear characterizes the critical number of cycles leading to fatigue fracture of microscopic subsurface cracks. Finally, the wear coefficient in oxidative wear includes the growth constant and the critical thickness of a surface oxide film. 

Multiwalled carbon nanotubes (MWCNTs) were proven to also contribute to the increase in the wear resistance and to the reduction of the friction coefficient in modified BMI resins (Liu et al. 2007a), especially when MWCNTs are functionalized with carboxylic groups. This is because they induce a change in the main wear mechanism—from adhesive wear in neat resin to abrasive attrition—by changing the self-lubricating property of the worn surface, the dispersion of filler in the matrix, and the interfacial adhesion between filler particles and matrix. 

Fig.l7 shows that the surface of the composite is smooth and a small amount of defect area can also be observed. The smooth surface is mainly because of the friction between the ceramic material and the steel ring. The friction chip has high surface activity and easy adheres to the material surface, forming the continual surface layer on the surface with the increase of wear time. The smooth and continue surface layer can effectively reduce the coefficient and slow the wear. With the temperature of friction area increase, wear increase because of the plastic deformation and material transfer take place, form the adhesive wear. 

The result shows that the soft layer was produced in the stage of adhesive wear process, the soft work-piece material chips were mixed the ceramic chips and coated on the hard composite ceramic surface under the friction. Next, the friction mainly between the soft layer and work-piece, this is the main reason of the low friction coefficient and wear rate. First, the soft layer form a continual smooth rubbing surface on the composite materials surface, and increases the actual friction contacted area, reduces the friction moment Second, the soft layer reduce the direct contact between the work-piece and ceramic, thus slowed down the wear of composite ceramic. 

For wear resistance and low friction, coatings of PTFE or M0S2 generally have been satisfactory. Use of low thermal expansion filler in PTFE helps minimise cracking and loss of adhesion from metal substrates with their lower coefficients of expansion. 

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material. The frictional properties of plastics differ considerably from those of metals. Even reinforced plastics have modulus values which are much lower than metals. Hence metal/thermoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed. Table 1.7 gives some typical coefficients of friction for plastics. 

Fretting at very low temperatures (4K) has been investigated. The situation appears to be similar to that in high vacuum. An oxide film does not grow at this temperature and so coefficients of friction are high due to adhesion but little wear occurs . 

Because PTFE has low adhesion, high lubricity, and low friction coefficient [14], it is not difficult to understand PTFE film has a lower friction coefficient factor and higher wear rate than PTFE/Si3N4 multilayers under the current experi-... 

When forces are transmitted between tire and road the tire is deformed to some extent. It still adheres to the road in part of the contact area, but slides locally when the ratio of tangential stress to the local pressure exceeds the friction coefficient and wear occurs. It is this partial adhesion and sliding which on the one hand allows a control over the force transmission and on the other hand leads to wear of the tire. 

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