B. Kerkwijk

Friction behaviour of solid oxide lubricants as second phase in alpha-Al2O3 and stabilised ZrO2 composites

The influence of metal oxide additives within alumina (?-Al2O3) and yttria-stabilised tetragonal zirconia (Y-TZP) matrices was studied with respect to the tribological behaviour of the composites. The solid lubricants CuO, ZnO, MgO, MnO2 and B2O3 were added in sufficiently small quantities (1 or 5 wt.%) to preserve the mechanical properties of the matrix phase without increasing the specific wear rate. The additives selected were an attempt to provide the materials with a self-lubricating mechanism and therefore decrease the coefficient of dry-friction. The ceramics were prepared by means of wet-ball milling powders, compaction by isostatic pressing and densification by sintering. Near-full densities (98?99%) were obtained for all ceramics except that containing CuO (92%). Tribological behaviour of the various composite compositions was tested on a pin-on-disk assembly using both ?-Al2O3 and ZrO2 as counter surface. Friction reductions of almost 40% for the CuO composite was obtained while the influence of the other oxide additives were less pronounced.

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.

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].