Physical factors in tyre traction

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49006/2/ptv6i1p11.pd

The influences of roughness on film thickness measurements by Mueller matrix ellipsometry

The accuracy of measurement of the thickness of uniform thin films on solid substrates by null ellipsometry is severely limited when the substrate is rough. It is impossible to separate these two effects experimentally with the null ellipsometer, and there is no theoretical basis or generally used model available to separate these effects. Thus, a dual rotating‐compensator Mueller matrix ellipsometer has been constructed to carry out film thickness measurements on rough substrates. Measurements were made on a set of specially prepared specimens of 8630 steel, roughened by grit blasting with aluminum oxide. Grit sizes and blasting pressures were varied to produce 11 different roughness values ranging from 0.01 to 1.295 μm Ra, as measured with a stylus tracer device. Upon each of the 11 roughness groups, films of magnesium fluoride were overlaid to thicknesses of 89, 180, 254, and 315 nm. One set of specimens was left uncoated. Experimental results for film thickness measurements on rough surfaces matched the ideal (for smooth surfaces) form well for roughnesses up to 0.13 μm Ra, at most angles of incidence. For rougher specimens, significant deviations in results were observed for all but the largest angles of incidence. The nonideal data were attributed to the cross‐polarization effects of surface geometry, and apparent depolarization. The resolution of thickness measurements was 1 nm for polished specimens, and decreased continuously to 10 nm for the roughest specimens examined.  Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69775/2/RSINAK-65-9-2874-1.pd

A review of scuffing and running-in of lubricated surfaces, with asperities and oxides in perspective

The slow progress in the understanding of scuffing (scoring) and runningin of most lubricated surfaces is probably due to an inadequate understanding of the details of asperity deformation and oxide formation. The thickness and properties of oxides influence the stress states imposed on asperities as much as does the liquid lubricant, but the oxides are ignored in theories. Present theories also focus on adhesion as the cause of scuffing and they usually do not take account of the changing surface roughness during sliding. There may indeed be some evidence of adhesion in the later stages of damage but adhesion has not been demonstrated to be the initiating mechanism of scuffing. Plastic fatigue is the more likely explanation, and this can occur without atomic contact between the sliding surfaces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24617/1/0000027.pd

Dynamics of dual film formation in boundary lubrication of steels part III. Real time monitoring with ellipsometry

Effective "breaking-in" of lubricated steel surfaces has been found to be due primarily to the rate of growth of "protective" films of oxides and compounds derived from the lubricant. The protection afforded by the films is strongly dependent on lubricant chemistry, steel composition, original surface roughness and the load/speed sequence or history in the early stages of sliding. Given the great number of variables involved it is not possible to follow more than a few of the chemical changes on surfaces using the electron, ion and X-ray column analytical instruments at the end of experiments. Ellipsometry was therefore used to monitor the formation and loss of dual protective films in real time, and detailed chemical analysis was done at various stages to calibrate the ellipsometer. This work is reported in three interlinking parts: I, functional nature and mechanical properties; II, chemical analyses; III, real-time monitoring with ellipsometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29175/1/0000221.pd

Wear of Fluorapatite Single Crystals : IV. Influence of Sliding Direction on Frictional Behavior and Surface Failure

Frictional anisotropy of the basal plane of natural fluorapatite single crystals was observed ; the coefficient of friction (β) in the [2110] direction (β = 0.217) was approximately 12% higher than that in the [0110] direction (β = 0.193). The load at which the ductile-to-brittle transition occurred (Ω) was significantly higher in the [2110] direction (Ω = 77 gm) than in the [0110] direction (Ω = 15 gm). A mechanism is proposed involving the maximum normal stress and probable cleavage planes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67194/2/10.1177_00220345730520050401.pd

Dynamics of dual film formation in boundary lubrication of steels part II. Chemical analyses

Effective "breaking-in" of lubricated steel surfaces has been found to be due primarily to the rate of growth of "protective" films of oxides and compounds derived from the lubricant. The protection afforded by the films is strongly dependent on lubricant chemistry, steel composition, original surface roughness and the load/speed sequence or history in the early stages of sliding. Given the great number of variables involved it is not possible to follow more than a few of the chemical changes on surfaces using the electron, ion and X-ray column analytical instruments at the end of experiments. Ellipsometry was therefore used to monitor the formation and loss of dual protective films in real time, and detailed chemical analysis was done at various stages to calibrate the ellipsometer. This work is reported in three interlinking parts: I, Functional Nature and Mechanical Properties; II, Chemical Analyses; III, Real-time Monitoring with Ellipsometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29174/1/0000220.pd

Wear of Fluorapatite Single Crystals : VI. Influence of Multiple-Pass Sliding on Surface Failure

The influence of multiple-pass sliding on the surface failure of fluorapatite single crystals was evaluated. The surface and subsurface damage on the basal plane of fluorapatite was accelerative, rather than additive. Damage was more severe for sliding in opposite directions than for two traversals in the same direction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66476/2/10.1177_00220345730520050601.pd

The "breaking-in" of lubricated surfaces

Steel cylinders were slid against flat steel surfaces, which were prepared to various surface roughness values, in order to determine the mechanisms of "break-in" of lubricated surfaces. It was found that proper break-in of a surface is effected when a film of Fe3O4 about 400 A thick forms. The formation of Fe2O3 is to be avoided. In addition, it was found in laboratory experiments that proper breaking-in by sliding requires a specific initial surface roughness of about 0.1 [mu]m center-line average. Smoother and rougher surfaces failed quickly. It appears that the optimum surface roughness was one in which the asperities plastically deformed at a rate that was too slow for fast progression to low cycle fatigue failure but at a rate sufficient to accelerate the formation of oxides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26196/1/0000275.pd

The viscoelastic nature of the sliding friction of polyethylene, polypropylene and copolymers

According to the adhesion theory of sliding friction, the sliding force or frictional resistance to motion is due to making and breaking of adhesional bonds between the sliding bodies. Thus the sliding force F is proportional to the shear strength S of adhesional bonds and the area of contact A between contacting bodies. This paper reports the results of a study to find the proper relationship between F, A and S. The value of F is taken from a friction test over a sliding speed range of 10,000 to 1 and over a temperature range up to 150[deg]C. The values of A and S are derived from shear tests, also over wide ranges of strain rate and temperature. All data were then subjected to special viscoelastic transformation. It was found that the friction data were transformable by the same transforms applicable to mechanical property data, provided changes in polymer morphology are taken into account. This coincidence is evidence of a strong connection between A, S and F which lends support to the adhesion theory of friction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33588/1/0000092.pd

The break-in stage of cylinder-ring wear: A correlation between fired engines and a laboratory simulator

The wear of the piston ring-cylinder wall contact area in fired engines has not been satisfactorily simulated in bench testers so far. This paper reports the development of a successful test device in which the same progression of surface change occurs as in fired engines. These changes were observed by microscopy, by hardness indentations and by the use of the stylus roughness tracer. The test device uses ring and cylinder segments and it oscillates at 350 cycles min-1 over a stroke of 19 mm. It is apparently not necessary to duplicate the temperature, atmosphere, fluid film thickness and other obvious conditions in an engine to achieve simulation. At least, the materials in the engine were not affected by the conditions in the engine that were not duplicated in the laboratory tester.The purpose in developing the tester was to study the role of the honed roughness pattern found on most cylinder walls. Several engine manufacturers attempt to achieve "fplateau honing". No manufacturer polishes new cylinder walls. The reason for the honed roughness is to allow a high wear rate, without catastrophic scuffing, in locations of high stress between poorly conforming parts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26638/1/0000180.pd