Jean-Luc Bozet

Nanoindentation investigation of Ti/TiN multilayers films

The hardness of Ti/TiN nanolaminated films is investigated in this study. Monolithic Ti and TiN films and Ti/TiN multilayers were deposited on silicon substrates by radio-frequency sputtering. The period thickness of multilayers was decreased from 20 to 2.5 nm. Grazing x-ray reflectometry showed that the modulation of composition of Ti/TiN multilayers exists for all the period thickness considered. From nanoindentation measurements, we determined the hardness and Young’s modulus of multilayers. Hardness increased with decreasing period thickness to go beyond the rule-of-mixture value for samples with period thickness of Λ⩽5 nm. The maximum hardness, 1.6 times higher than the value obtained by the rule of mixture, is obtained for Λ=2.5 nm. Our results are compared to a dislocation-based model previously introduced by Lehoczky.

Modelling of friction and wear for designing cryogenic valves

Abstract From a tribological point of view, the selection of materials for seals for valves of cryogenic rocket engines is a critical issue for the designer. Due to the lack of comprehensive information on this topic, data have been obtained in the framework of research programmes. In the first step, two polymers potentially usable for sealing cryogenic fluids were identified. They were submitted to general test conditions to gain some fundamental understanding of their tribological behaviour when immersed in a cryogenic fluid. This fundamental understanding proved to be helpful for an efficient technical approach. This approach with the test conditions as close as possible to those met in real valves has been used in the second step. In order to obtain the best information with the minimum number of tests, the statistical method of Doehlert has been adopted. The use of such a method led to the construction of surface response for modelling friction and wear. These surfaces and their associated equation are directly and easily usable by the designer. This way of treating technical tribological problem is illustrated by an example.

Nanoindentation and scanning force microscopy as a novel method for the characterization of tribological transfer films

Abstract In conventional pin-on-disk testing of the tribological characteristics of two different materials in sliding contact, the main parameters of interest are notably the friction and wear properties of the material pair. However, when two bodies consisting of hard and soft materials respectively are subjected to such testing, the appearance of a transfer film, or third body , which can be a composite mixture of the two, is often observed. Until now the characterization of transfer films in terms of their mechanical properties has been hampered by their non-homogeneous distribution across a tested surface, their small size, low thickness and the difficulty in accurately positioning a test probe such that the film properties can be measured independently from those of the substrate. In this paper a new method is introduced, consisting of nanoindentation and scanning force microscopy (SFM), which is capable of highly localized indentation testing of a specified sample site with high resolution imaging of the area prior to and after indentation. In this way the hardness and modulus of a transfer film can be obtained, as well as valuable surface topographical information concerning the material response to the indentation. Measurements are presented for the material pair A286/polyimide after testing on a pin-on-disk tribometer in ambient air and liquid nitrogen. Distinct variations in hardness between the transfer films and their contacting bodies have been observed and correlated to the wear behaviour and testing environment.

Type of wear for the pair Ti6A14V/ PCTFE in ambient air and in liquid nitrogen

In the development of large, high pressure valves for cryotechnic rocket engines, the Tribology Department of the University of Liege had the opportunity to evaluate the pair Ti6A14V/polychlorotrifluoroethylene (PCTFE) on a pin-on-disk tribometer in ambient air and liquid nitrogen with the contact pressure and sliding speed ranging, respectively, from 3 to 9 MPa and 0.03 to 0.05 m s−1. Because of the affinity of titanium for halogen elements, such as chlorine and fluorine, which are both present in PCTFE, we noticed the formation of an abrasive compound made of chlorine, fluorine, titanium and aluminium. These elements were detected by electron microscopy. In both environments the heat produced during sliding is responsible for the decomposition of the PCTFE. The surface of the polymer is softened by the heat and embedded abrasive particles then scratch the sliding track of the Ti6A14V disk. This abrasiveness is confirmed with a three-dimensional stylus profilometer used on the Ti6A14V disks. The low temperature and neutrality of liquid nitrogen hardly inhibit the decomposition of the polymer. This is related to the poor thermal conductivity of both titanium and PCTFE: the heat produced at the contact is fully available for the chemical reaction between the chlorine, fluorine and titanium, all the more so since this reaction requires little energy to take place. A better understanding of the wear behaviour of the pair PCTFE/Ti6A14V in both air and liquid nitrogen should lead to the development of a surface treatment which prevents the formation of abrasive particles. We could then use titanium alloys as bulk materials for sliding parts in cryogenic valves and, because of the low specific mass of titanium, could benefit from the subsequent weight reduction.

Comparative analysis of the lubricating power between a pure mineral oil and biodegradable oils of the same mean iso grade

Abstract In this paper, we compare the viscosimetric and tribological properties of some vegetable oils and one synthetic ester with a pure mineral oil. The viscosimetric characteristics of lubricants show that both vegetable oils and ester have a higher viscosity index than mineral oil. The test results show that all tested oils have a somewhat dilatant behaviour.The oiliness, antiwear and extreme pressure properties of lubricants have been studied respectively with a Stribeck machine and a four-ball machine. We have noted the following: On the four-ball machine, extreme pressure results expressed in terms of pressure wear index and mean hertz load show that the higher performance is obtained with the vegetable oils. Extreme pressure results expressed in terms of weld load, do not show a big difference between the tested oils. Antiwear results expressed in terms of wear scar show that die highest performances are obtained with vegetable oils. On the Stribeck machine, oiliness results expressed in terms of friction coefficient indicate that: in the regime of fluid lubrication, tested oils have me same behaviour; in die boundary lubrication regime, both vegetable oils and synthetic, ester have a better response uian mineral oil.