Kenneth C. Ludema

Wear models and predictive equations: their form and content

Abstract Most wear models and equations in the literature were analyzed as to origin, content and applicability. No single predictive equation or group of limited equations could be found for general and practical use. The reasons include the perpetuation of erroneous and subjective expressions for the mechanisms of wear, the slow pace of translation of microscopic observations into macroscopic models of the wearing processes and the paucity of good experiments to verify proposed models.

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

Abstract 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.

Surface damage of structural ceramics: Implications for wear modeling

Summary The reason for the failure of the fracture toughness approach to wear modeling for ceramics is that the assumed linking of the radial and lateral cracks does not often occur. There is indeed wear in sliding but this is due to the fracture and fragmentation of material nearer the surface than the radial and lateral cracks. This scale of fracture is of the order of grain sizes whereas the radial and lateral cracks are of the order of the size of indentations and are usually below the indentation. These conclusions were reached by observing the damage to four commercial ceramic materials, after indentation and after sliding. The four ceramic materials were hot-pressed Si3N4, hot-pressed Sic, sintered A120s and sintered ZrO,. Tests were done in air, water, aqueous acid and aqueous base. The surfaces of the specimens were “finished” by four different abrasive operations before the test, thereby providing four different states of surface roughness and stress state. All of the materials showed both large scale (macro) and small scale (micro) damage. There were major differences in micro-damage but only minor or negligible differences in macro-damage. The latter provide the information from which hardness and fracture toughness are calculated.

Mechanism of transfer film formation during repeat pass sliding of ceramic materials

The formation of transfer film and the consequent effects on the friction and wear behavior of ceramic materisIs during repeat sliding contact were studied. This was done using four structuraI ceramics, namely silicon nitride, silicon carbide, ahunina and zirconia, with a cylinder-on-flat test configuration. The transfer film consists of reattached fme wear debris particles, the film, whenever formed, is strongIy attached, enough to resist being wiped off by the slider. Cahxdations suggest that the Sne particles are attached primariIy by van der Wsals forces and to a lesser extent by electrostatic attractive forces. As a consequence, the formation of transfer ti leads to a decrease in the wear rate because of the ‘protecting’ role of the SIm. The presence of the ti at the contact interface also results in high friction. The presence of a liquid environment and/or surface active species reduces the particle adhesive forces and hence can inhibit the formation of a transfer f%n.

Mechanisms of formation of polymeric transfer films

Abstract The role of the transfer film in reducing wear of polymers is discussed. It is shown that the transfer film forms more readily on roughened surfaces and that it can exist in a solid state and in a low viscosity or fluid state. Each state controls friction and wear of the polymer in a different way.

A model of the dynamics of boundary film formation

Abstract The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dialkyldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the boundary film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

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

Abstract 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.

A rheological mechanism of penetrative wear

A model is proposed which explains the penetrative wear of a soft material by a harder one. Three distinct modes of penetration are present depending on the applied load. During the most severe penetration plate-like wear debris is ejected at the leading edge of the slider. A series of slip line fields is presented to approximate this debris formation process. Plastic constraint is seen to be an important factor in wear particle formation.

Dynamics of dual film formation in boundary lubrication of steels Part I. Functional nature and mechanical properties

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.

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

Abstract 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°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.