Wear rate crystallinity

The third design feature is the polymer microstructure. Morphology of polymer can influence wear resistance of polymers. For example, in a semicrystalline polymer, both amorphous and crystalline phases coexist. The amorphous phase has been shown by Tanaka (8) to be weaker than the crystalline phase, thus the former wears faster than the latter. In addition to the difference in phases, the size of the spherulites and the molecular profile can also influence the wear rates. Thus, a control of the morphology through crystallization can improve the wear resistance of a polymer such as polytetrafluoroethylene (11). 

Measurements of friction and wear were made on poly (ethylene-terephthalate)(PET) sliding against a smooth steel disk. The friction was little dependent upon crystallinity, while the wear rate increased with increase in crystallinity, especially remarkable in the range above about 40%. It was found that the reciprocal of wear rate was closely related to the Vickers hardness in PET of different crystallinity. 

It is well known that the morphological and molecular structures of polymers play an important role in their wear behavior. It seems that the degree of crystallinity is also a structural factor of semicrystalline polymers important to their wear. Lontz et al. ( ) reported that the wear of poly(tetrafluoroethylene),(PTFE) decreased with the increase in crystallinity. Tanaka et al. (2 ) studied the wear of heat-treated PTFE specimens and concluded that the wear rate was affected by the width of the band in the fine structure rather than crystallinity. Recently, Hu et al. ( 3) have studied the effect of crystallinity on wear of PTFE using various heat-treated specimens. They have shown that the wear decreases with the increase in crystallinity, when molecular weight is constant. Eiss et al. ( ) reported that poly(chlorotrifluoroethylene) of a crystallinity of 65% exhib-ted higher wear than that of 45%. The results obtained by the authors mentioned above indicate that the effect of crystallinity on the wear of polymers is somewhat complicated and further investigation is needed to clarify the effect of crystallinity on polymer wear. 

In order to study the effect of the degree of crystallinity on friction and wear of PET, the friction measurements in which the steel sphere was slid on flat PET surfaces were carried out. Measurements of the friction and wear rate were also made on the PET pins sliding against a smooth steel surface at a speed 0.1 m/s under a load 10 N by means of a pin-on-disk type apparatus. The PET specimens of four different crystallinity (8, 39, 55 and 75 % ) were used in the present work. The specimen of the lowest crystallinity had a structure similar to an amorphous polymer, while two specimens of crystallinity, 39 % and 55 % had a spherulite-like structure. The highest crystallinity had a fiber-like structure. Conclusions obtained are as follows ... 

The wear rate increased with the increase in crystallinity and especially remarkable in the crystallinity range above about 40% ... 

Wang (2001) found that irradiation crosslinked UHMWPE had a significantly lower wear rate than un-crosslinked material. The radiation dose must be high to obtain the optimum effect (Fig. 15.19). Rieker et al. (2003) showed that the wear surfaces of highly crosslinked UHMWPE implants after 18 months in vivo, consisted of folds (Fig. 15.20). Such folds are also found in conventional UHMWPE, but fatigue leads to their detachment from the surface. The folds on the surface of the crosslinked polymer appear to stay in place. Crosslinking leads to a reduction in crystallinity, hence a... 

Details about the in vitro test results with Crossfire have recently been published (Kurtz et al. 2002-2003). In comparison wiOi conventional UHMWPE, Crossfire is associated with more than 90% improvement in median hip simulator wear rate across a broad range of implant designs. The crystallinity and... 

Optimum PTFE loading of 15% in amorphous and elastomeric base resins and 20% for crystalline base resins provide the lowest wear rates. Higher PTFE loadings have minimal effects in terms of further reduction in wear rate, although the coefficient of friction will continue to decrease [56]. The effects of PTFE on the wear characteristics of various engineering resins are strongly dependent on the type of resin [57]. 

The highest wear rate is for liquid crystalline polymer and the lowest wear rate is for 30% glass fibre reinforced PA 4,6. 

The specific wear rates of liquid crystalline polymers, 30% glass fiber-reinforced polyamide 4,6, and 30% glass fiber-reinforced polyphenylene sulfide ranged from 3 X 10" to 4.43 X 10- mmVNm, 1.63 x 10 to 1.1 x 10 mm /Nm, and 2.4 x 10 to 2.1 X 10" mmWm, respectively. 

Soap In-Use Properties and Recrystallization The soap bar in-use properties such as hardness, hydration and wear rate, mush layer and lather volume, etc., are influenced strongly not only by the crystalline phase structure (including the liquid crystalline phase) but also by the shape and size of the crystalline phases. These influences are strongly dependent on the formulations, the processing methods, i.e., high or low shear, and the processing temperatures. 

In view of the indirect roles of surface energetics in numerous forms of polymer wear, we can not establish a unique correlation between surface energetics and each of the above discussed mechanisms. The transfer of polymer to the counterface does not follow a certain set of rules or regularities. Pooley and Tabor concluded that the low friction and the light transfer of PTFE and polyethylenes are not affected by surface energy, e.g. or crystal texture of the polymer, or the crystallite size. The transfer is essentially due to smooth molecular profiles. On the other hand, Tanaka observed the formation of a long band on PTFE and determined the wear rate to be a function of the width of the bands rather than the crystallinity. This wear mechanism bears a similarity to that of delamin-ation ... 

MoSj, otherwise known as moly, is a solid lubricant usually used in nylon and other composites to reduce wear rates and increase PV limits. Acting as a nucleating agent, MoS creates a better wearing surface by changing the structure of nylons to become more crystalline, creating a harder and more wear-resistant surface. MoS will not lower the COFs like other modifiers, and its use is therefore confined to nylons where it has this crystallizing effect on the nylon molecular structure. 

The wear rate of ultrahigh molecular weight polyethylene is essentially independent of irradiation at levels used to sterilize medical prostheses, i.e., 5 Mrad and below [54]. At higher levels of irradiation, such as those that result in significant levels of cross-linking, wear rate is increased substantially [55]. However, if the polyethylene exceeds its crystalline melting temperature because of an increase in sliding speed or applied load, the cross-linked polyethylene will exhibit a lower wear rate than the uncross-linked material [55]. 

Rapid solidification of A1 rich alloys gives fine scale dispersion of crystalline [2002Su], quasi-crystalline [2003Ino] and amorphous [20031no] particles in (Al) matrix. Addition of Ti in Fc3Al lowers the wear rate and slightly decreases the coefficient of fiiction [2004Gua]. 

Brake Linings. Substantial amounts of crystalline flake, lump, and amorphous graphite are used in brake and clutch finings, mostly in heavier duty nonautomotive situations. The graphite has been substituted for asbestos because of health considerations. The graphite proportion of the part has risen from 2 to 15% in some instances. The graphite lubricates, transfers the heat of friction away from the fining, and lowers the rate of wear. 

Number of cavities, layout and size of cavities/runners/gates/cooling lines/side actions/knockout pins/etc. Relate layout to maximize proper performance of melt and cooling flow patterns to meet part performance requirements preengineer design to minimize wear and deformation of mold (use proper steels) lay out cooling lines to meet temperature to time cooling rate of plastics (particularly crystalline types). 

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