J. Mølgaard

A discussion of oxidation, oxide thickness and oxide transfer in wear

Abstract Wear is defined as a set of processes of which adhesion, transfer, abrasion, fatigue and oxidation may be some of the constituent processes; these effect transformations of matter from one state to another. The final transformation in wear is attrition in which free debris particles are formed. Oxidation in the wear of metals is discussed, in particular for circumstances in which an oxide layer thick enough for the oxidation rate to be inversely dependent on oxide thickness is formed i.e. the standard relationship (outside the wear situation) between the oxide thickness and elapsed time is parabolic. It is shown that the basic factors determining the wear process are the mechanical characteristics of the oxide layer and that the oxidation process, while important, is a secondary influence. Theoretical considerations show that the oxidation temperature may be lower than the contact temperature, even for oxidation nominally at the contact regions. Also, an oxidation rate in wear may decrease with increasing oxidation temperature, as is sometimes observed in practice. This may be due to an increase in the thickness to which the oxide layer can grow in wear, when the oxidation temperature increases.

The activation energy of oxidation in wear

Abstract The activation energy for oxidation in wear has been determined from sets of experimental data obtained during the dry and unlubricated wear of eutectoid steel rubbing at moderate to high sliding speeds. This value is not very different from the activation energy for the oxidation of cold-worked iron in the same temperature range. It has been observed that the calculation of activation energy is very sensitive to the assumption made for the pertinent temperature of oxidation. In the present work the appearance of wustite (FeO) has been used as an indicator of this temperature.

THE THERMAL CONTACT RESISTANCE AT GOLD FOIL SURFACES

Abstract The thermal contact resistance at gold foil surfaces has been measured over the pressure range 2·5 to 9 × 106kg/m2 and in the temperature range 50 to 300°C. Temperature had little effect on the thermal resistance, while the resistance was dependent on pressure in such a way as to suggest that both elastic and plastic deformation of the surface structure will occur in a manner determined by the sequence of pressure changes.

The dry wear of metals as a process in an open system

Abstract The wear of metals is considered as a number of possible processes occurring in an open system, containing the wear objects and the interfacial volume containing debris. It is shown for non-identical wear objects, for instance the pin and disc configuration where often only the pin wears continuously, that the net oxidation rate of the pin is not necessarily equal to the wear rate, since account must be taken of inevitable metal transfer between the two objects. Only for identical wear objects is it possible to equate the wear of each object to the net oxidation rate of that object.

Apparatus for the study of oxidative wear of unlubricated surfaces

Abstract A controlled environment apparatus designed for the study of oxidative wear of unlubricated surfaces is described and preliminary results are discussed.

Towards a general theory of tribological systems

Abstract An approach towards a general coherent description of the “function” and “structure” of tribological systems is attempted. An analysis should start with the technical function of the system together with an identification of operational variables in the relationship of the system to its environment. The tribological structure can be represented on separate conceptual planes, one for each type of quantity transmitted in the system, i.e. work, entropy and the several materials. Processes occur solely as translations on one such plane or as transformations between different types of quantities, each process being governed by certain element properties. The properties of the elements relating to a loss of useful work through friction and vibration, those properties relating to entropy transport and those relating to materials translation and transformation, i.e. processes in wear, are discussed. The interdisciplinary requirements for the study of the tribology of mechanical systems are evident.

Crushing Friction Experiments on Freshwater Ice

A unique hydraulically driven friction apparatus has been used to measure friction forces on freshwater ice specimens against a steel surface under crushing normal loads at various temperatures. The friction coefficient was found to be a decreasing function of increasing sliding speed and temperature and did not appear to be influenced by fabric, texture or the presence of bubbles. There was no correlation between normal load and the coefficient of friction. There was evidence that crushing friction has a higher coefficient than non-crushing friction, and possible mechanisms to explain this behavior are discussed. Periodic crushing was evident.

A simulative wear study critically reviewed

Abstract An example is presented of the application of systems analysis concepts and guidelines for simulative testing, described by Czichos, to the review of a simulative wear study. The investigation examined concerned the wear and the division of frictional heat in the ring-and-traveller device used in the spinning of yarn. The strong points and the shortcomings of the simulation are readily evident upon comparison with the guidelines.

Fibre Surface Replication by Rolling

A TECHNIQUE for replicating fibres by rolling has been developed in these laboratories. It is only applicable to fibres which approximate to a cylindrical shape, such as nylon, ‘Terylene’ and wool, but with these it can give replicas with sufficient resolution for the full magnification available in light microscopy. It is not known yet to what extent the replicas are suitable, either directly or in second stage form for electron microscopy.

Mechanisms of Ice Friction

Friction involving ice is reviewed in both steady and transient conditions, considering contact mechanics, adhesion, fracture, crushing, elastic and plastic deformation, recrystallization, and melting as co–operating or competing steps and mechanisms in the overall process, with reference to recent experimental work.