H.R. Pasaribu

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.

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.

A wear-resistant zirconia ceramic for low friction application

A high wear-resistant ceramic/ceramic couple is described associated with low friction. By adding a small amount CuO to yttria-doped tetragonal zirconia (Y-TZP) the (dry) coefficient of friction against alumina is only 0.2 during a sliding distance of 3-5 km after which the coefficient drastically increases and a transition from mild to sever wear occurs. Pure Y-TZP exhibits a coefficient of friction of 0.7 under the same experimental conditions but wear remains mild during the test (upto 10 km of sliding distance). These small amounts of CuO also strongly influence the densification behaviour. Sintering of this system occurs in several steps where amng other things dissolution of CuO in the Y-TZP matrix as well as liquid phase sintering takes place. Non-uniform shrinkage of the CuO-doped system resulting in relative large microcracks in the ceramic can explain its sudden drastic increase in coefficient of friction and wear rate after 3-5 km of operation.

Deterministic Contact Model of a Rough Surface Against a Flat-Layered Surface

The effective mechanical properties of a layered surface vary as a function of indentation depth and the values of these properties range between the value of the layer itself and of the substrate. In this paper, a layered surface is modelled like a solid that has effective mechanical properties as a function of indentation depth by assuming that the layer is perfectly bounded to the substrate. The normal load as a function of indentation depth of sphere pressed against a flat layered surface is calculated using this model and is in agreement with the experimental results published by El-Sherbiney (1975), El-Shafei et al. (1983), Tang & Arnell (1999) and Michler & Blank (2001). A deterministic contact model of a rough surface against a flat layered surface is developed by representing a rough surface as an array of spherically shaped asperities with different radii and heights (not necessarily Gaussian distributed). Once the data of radius and height of every single asperity is obtained, one can calculate the number of asperities in contact, the real contact area and the load carried by the asperities as a function of the separation.Copyright