ICMCTF2006 Session E2: Friction and Wear of Coatings II: Design and Modeling

Wear is generally considered a mode of failure and at best minimised if not avoided completely. In some situations however, especially those at the wheel rail interface the minimization of wear may lead to failure by rolling contact fatigue, a potentially catastrophic fatigue failure. Recently such a failure on the UK railway network led to many fatalities and cost the railways many billions of dollars in replacing rails which were susceptible to such failures. The subsequent research has highlighted the beneficial role played by wear in (i) producing low contact pressure by generating run-in rail and wheel profiles, and (ii) reducing the growth rate of cracks developing in the rail by crack mouth truncation. The models predict that life is governed by wear as well as rolling contact fatigue. If wear rate is reduced then the life due to wear increases, but that due to fatigue decreases, the optimum occurring at a critical wear rate. A better material with a lower wear rate performs worse in this situation!

In the past decade the rail steels have become much cleaner and the wear rate has been reduced by a factor of 10 or so. The frictional traction at curves has also been reduced by better design of railway vehicles and suspension systems. Both these effects have led to an increase in the number of Rolling Contact Fatigue incidences in the UK as well as in Europe.

The talk will review the current state of the art in this area and the work underway in the UK in understanding this phenomenon and utilising it to enhance the rail life.

The high storage density required in modern magnetic recording systems can be achieved by reducing the flying height of read/write head sliders over spinning disk surfaces. This can increase the likelihood of high-velocity intermittent head/disk contacts, which generates flash temperature and stress fields at the interface. The high temperature and stress fields can degrade the domain magnetization of the hard disk, which is known as the demagnetization. Former studies were not extended to give the critical conditions for the demagnetization. In this paper, therefore, our aim is to propose a demagnetization map for the identification of the demagnetization of the disk by considering the maximum temperature and stress under head/disk sliding contact. For the purpose, a coupled temperature-displacement finite element model of a patterned disk sliding contact with a slider is introduced to calculate the transient temperature and stress distributions of the magnetic media.

As the conclusions, the magnitudes of maximum temperature and stress for the magnetic media of patterned surface disk are simulated for various sliding velocities and loading conditions under sliding contact. The demagnetization map for a patterned surface disk is developed by combining the simulated results together with available demagnetization temperature/stress, the critical temperature and stress magnitude at which occurrence of demagnetization is observed experimentally. In the map, three zones, i.e. safety zone, transition zone and danger zone were divided. When the maximum stress is lower than 1.2GPa and the maximum temperature is lower than 180K, the magnetic disk is located in the safety zone without demagnetization. When the maximum stress is higher than 1.2GPa and the maximum temperature is higher than 180K, the magnetic disk is located in the danger zone. In the region between 1.2GPa and 180K, the magnetic disk is located in the transition zone.