Dirk J. Schipper

Calculation of Stribeck curves for line contacts

A mixed lubrication model is presented by which Stribeck curves can be calculated. By means of the Stribeck curve the transitions from boundary lubrication to mixed lubrication and the transitions from mixed lubrication to elasto hydrodynamic lubrication can be predicted and subsequently the lubrication regime in which a particular contact operates can be predicted. Calculations are restricted to line contacts, because of the wide range of applications of line contacts or of very wide elliptical contacts.

Deformation of Rough Line Contacts

The influence of surface roughness on the bulk deformation of line contacts is studied. The model of Greenwood and Tripp (1967) will be extended to line contacts. It is found that the central pressure is a very good parameter to characterize the pressure distribution of rough line contacts. Function fits of the central pressure, the effective half width, the real area of contact, and the number of contacts are made. Comparison is made with the work of Lo (1969) and Greenwood et al. (1984).

Friction reduction by adding copper oxide into alumina and zirconia ceramics

The friction and wear of alumina and zirconia ceramics doped with various weight percentages (0.5, 1 and 5 wt.%) of CuO was studied. Dry sliding tests by using a pin-on-disc tribotester were conducted on these materials against commercially available Al2O3, ZrO2, SiC, and Si3N4 ceramic balls. The results show that CuO give a significant reduction of friction only when the alumina and zirconia doped with CuO were sliding against Al2O3 balls. The coefficient of friction of CuO doped in alumina sliding against Al2O3 balls reduces from 0.7 to 0.4 and hardly depends on the normal load and the velocity. On the other hand, CuO doped in zirconia can reduce the coefficient of friction (when sliding against Al2O3 balls) from 0.8 to a value of about 0.2 and 0.3 depending on the normal load. SEM pictures taken from the wear track showed that smooth patchy layers were formed. These smooth patchy layers, which carry the normal load, are responsible in reducing the coefficient of friction.

Tribological properties of nanoscale alumina–zirconia composites

The tribological properties of zirconia (Y-TZP), alumina and their composites, alumina dispersed in zirconia (ADZ) and zirconia-toughened alumina (ZTA), were investigated. These ceramics are made by colloidal processing methods such that well-defined, homogeneous microstructures with submicron grains and few defects are obtained. Dry sliding tests against alumina balls were performed on a pin-on-disc tribometer using varying test conditions. It was shown that, with initial Hertzian contact pressures up to 1 GPa and sliding velocities up to 0.5 m/s, the specific wear rate was the highest for Y-TZP, 10?6 mm3/(N m), and the lowest for ZTA, 10?9 mm3/(N m). For both single-phase zirconia and alumina ceramics, it was found that addition of a harder (alumina) or a tougher (zirconia) phase, respectively, leads to an improved wear resistance. Depending on the test conditions, the wear mechanisms are abrasion, delamination and polishing. The coefficients of friction were as high as 0.8 for Y-TZP and as low as 0.45 for ZTA. The main conclusion of this work is that ZTA composites manufactured and tested in this study have a superior wear resistance and a relatively low coefficient of friction under dry sliding conditions.

Friction and wear behaviour of ceramic-hardened steel couples under reciprocating sliding motion

The friction and wear behaviour of ZrO2-Y203, ZrO2-Y203-CeO2 and ZrO2-A1203 composite ceramics against hardened steel AISI-52100 were investigated using a pin on plate configuration under reciprocating motion. The reproducibility of the results was examined in this configuration. Wear characteristics were separated into system and material contributions. Under the conditions used, all the ceramic components exhibited rather low wear rates (less than 10-6mm3N-1m-1). The frictional behaviour of ceramic-metal couples depended on a metallic layer transferred from the steel plate to the ceramic pin. A relation was determined between surface hardness and friction of ceramic-metal pairs. It was shown that the affinity for the transfer of the metal towards ceramic surfaces depended on the physical properties of the materials. In the tribosystems investigated, the effect of the hardness of the ceramics on friction and wear behaviour is found to be more important than that of toughness of the ceramics.

Wear of ceramics due to thermal stress: a thermal severity parameter

Wear of ceramics under dry sliding conditions is, besides mechanical overload, often governed by frictional heating. On the basis of this study a new thermal severity parameter is introduced and applied to an existing wear map as a function of mechanical and thermal loading introduced by Adachi et al. [Wear 203/204 (1997) 291]. The new map is valid for a wide range of material properties and operating conditions. In this paper, a method is presented to predict the boundary between the mild wear regime and the severe wear regime due to frictional heating. To obtain this, the work of Bos [Frictional heating of tribological contacts, Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1995] was used to calculate contact temperatures. This model is essentially a fit on numerical calculations, making it possible to include heat partitioning between dissimilar materials while retaining closed form expressions. The stress resulting from the contact temperature can be compared with material properties to obtain the thermal severity of a contact. A new dimensionless thermal severity number, TS, is introduced to determine the thermal severity of a contact. The model has been experimentally justified using measurements of pins of different materials sliding against two types of tetragonal zirconia polycrystal discs. In these experiments it was observed that if the specific wear rate exceeds the value of 3×10?6 mm3/N m, the sliding surfaces were rough and the wear was regarded as severe. This criterion is closely related to the criterion based on the ratio of process surface roughness over mean grain size Adachi et al. [Wear 203/204 (1997) 291].

Micro-EHL in lubricated concentrated contacts

In sliding friction experiments, performed with lubricated concentrated contacts macroscopically operating in the lubricants’ liquid-state regime, the existence of micro-EHL has been shown. With the measuring method used, the lubricants’ limiting shear stress τ 1 or the ratio of the limiting shear stress with pressure, τ 1 /p, can easily be obtained.

Application of a deterministic contact model to analyze the contact of a rough surface against a flat layered surface

In this paper, a rough surface is modeled as an array of asperities represented by spheres with different radii and heights to be able to calculate the deformation (elastic, elastic-plastic, and plastic) at each asperity in contact. The total contact area and the total load carried are calculated by summarizing respectively the contact area and the load carried by each individual asperity (deterministic model). This model will diminish the assumption of an average asperity radius and enable one to calculate the contact of non-Gaussian surface more precisely. Further, in this paper, the deterministic model is used to analyze the contact behavior of a rough surface against a flat layered surface by modeling the flat layered surface as a solid that has effective mechanical properties as a function of indentation depth.

Thermally induced wear of ceramics

Wear of ceramics under dry sliding conditions is often governed by frictional heating. In this paper, a method is presented to predict the boundary between the mild wear regime and the severe wear regime due to frictional heating, and to a lesser extent, the prediction of the wear rate. To obtain this, the work of Bos was used to calculate contact temperatures. This model is essentially a fit, based on numerical calculations, making it possible to include heat partitioning between dissimilar materials while retaining closed form expressions. The stress resulting from the contact temperature can be compared with material properties to obtain the thermal severity of a contact. A dimensionless thermal severity number, TS, is introduced to determine the thermal severity of a contact. The model has been experimentally justified using alumina pin on zirconia disc measurements.

The Influence of Plastic Bulk Deformation on Surface Roughness and Frictional Behavior during Deep Drawing Processes

In Sheet Metal Forming (SMF) processes, friction does play an important role. This with respect to the increase of product quality demands and the ability of predicting these processes by for instance finite element simulations. The existing simulation models do not contain an adequate friction model. In SMF processes different contact situations can be distinguished. As a result different coefficients of friction are locally present, which influences the forming process. Experiments are performed on a testing device by which it is possible to simulate the operational conditions as present in SMF processes. This test rig is a combination of a tensile tester and a friction measuring device, by which it is possible to measure the coefficient of friction as a function of the operational conditions (velocity and contact pressure) and deformation (elastic or plastic) in a well controlled way. Friction is presented in a generalized Stribeck-curve in which the different lubrication regimes can be distinguished, i.e. Boundary Lubrication (BL) and Mixed Lubrication (ML), which are also occurring during SMF processes. In SMF processes the sheet material deforms elastically and plastically and therefore the surface roughness will change and as a consequence will influence the frictional behavior between sheet and tool. In this paper, the influence of plastic deformation on A) the surface microgeometry and B) as a consequence of that on the frictional behavior of the sheet-tool system is studied. With the aid of a 3D-surface interference microscope, the microgeometry of the deformed material has been analyzed. The result of this investigation is that the CLA-roughness due to the deformation first decreases and then increases with increasing deformation. Furthermore, friction is hardly influenced due to the change in surface roughness. No change in the shape and the level of the generalized Stribeck curve is found.