Mohamed Kchaou

Analysis of Degradation and Failure Mechanisms that Develop in Hot Forging Die

A failed die, used to forge gas valves, was analyzed using macroscopic and microscopic equipment on some specimens. Fatigue cracks on the die were evidently observed on all the surfaces of the die. Many processes were found to be major contributors to have caused the die failure. Oxidation was observed in some areas, especially on plane surfaces and thin sections. Surface layers of tools at the tool-workpiece interface were not only exposed to high mechanical stresses but also to severe temperature cycles, which could have led to loss of strength and hardness. Metallographic analysis revealed that the oxide layers consisted of different diffusion layers which accelerated thermal fatigue mechanisms.

The Study of Disc Brake Noise on Three Different Types of Friction Materials

This work investigates the effect of road grit particles and material compositions on disc brake squeal noise. Three different Non Asbestos Organic (NAO) brake pad samples and road particles with a size range between 400 μm to 500 μm are used in the squeal experiment. The relationship between the existence of road particle with the noise occurrence were studied through the change in pressure level, friction coefficient (CoF), sound pressure level (SPL) and material composition. The material compositions were analyzed before and after squealing condition using energy dispersive X-ray analysis (EDX). Result has shown the road particles contribute to the emission of squeal noise and the increasing of some material composition of brake friction material components.

Characterisation of oxidation and wear oxidised surfaces of H13 steel/brass in dry sliding conditions

Although oxide scale has significant influence on surface quality of hot–worked products, deformation of the scale during hot working has not been understood sufficiently. The authors propose an experimental study to analyse the oxidation and the tribological behaviour of a ferrous alloy (X40CrMoV5–1, H13 steel) and non ferrous alloy (CuZn39Pb3, brass alloys) in dry sliding conditions. The tribological behaviour of the pair of materials is carried out on a pin–on disk wear tester after oxidation at 600°C for 70 h. It was found that: i) a loose oxide film wholly covered the surface of the steel; ii) Fe2O3 as the main oxide was identified except for a small amount of Fe3O4; iii) during friction, a compacted oxide film was established on the worn surface of the steel. The analysis of the worn surface showed two regions: a delamination region due to the delamination of tribo–oxide during wear and an undelamination region characterised by the dominance of Fe3O4 tribo–oxides on the worn surface.

Worn Surface Characteristics of a Friction Material during Braking Simulation Test

In this study, the chemical and microstructural changes occurring during braking simulation tests at the surface of a commercial brake lining material were investigated by scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer (X-EDS). It could be shown that patches of a third body material develop, comprising a compositional mix of all constituents of the pad and iron oxides from the disk. The majorities of particles rich in carbon which appeared dark grey remained totally or partially uncovered and did not form preferential supports of secondary plate development. It was observed that filler mineral powders, such as barite (BaSO4) and calcium carbonate, which are friable particles, easily wear and generate an important part of third body rich in C, Ca, Ba and Fe elements. Rockwool fibres and shots contributed to the formation of the bearing surface by forming primary support of development of flat plates. The major wear mechanism was delamination of filler particles from the organic binder, supported by local degradation of the phenolic resin during asperity heating.