The role of porosity on the wear behaviour of sintered iron was investigated. Sintered iron was used in preference to steel because the various alloy additions and matrix heterogeneity in the latter might affect the microwear behaviour and the role of porosity. The wear tests were performed in a pin on disc test bed under 10-40 N loads at 0·56 m s-1 sliding speed in air. For the given tribological conditions, it was found that the wear mechanism is delamination or mechanical wear, which is basically similar to that of wrought materials according to subsurface crack generation and crack propagation processes. However, the open pores on the surface act as a site for generation and collection of wear debris. Furthermore, the pores affect the stress distribution and the metal deformation of subsurface layers during dry sliding. Accumulation of the plastic shearing on the subsurface results in nucleation of cracks that further grow by shearing: the fracture process. The propagation of subsurface cracks...

Effects of porosity on delamination wear behaviour of sintered plain iron

In this study the idea of spur gear teeth with circular instead of the standard trochoidal root fillet is introduced and investigated numerically using BEM. The strength of these new teeth is studied in comparison with the standard design by discretizing the tooth boundary using isoparametric Boundary Elements. In order to facilitate the analysis the teeth are treated as non-dimensional assuming unitary loading normal to the profile at their Highest Point of Single Tooth Contact (HPSTC), so that non-dimensional stress vs. Contact ratio diagrams are plotted. The analysis demonstrates that the novel teeth exhibit higher bending strength (up to 70%) in certain cases without affecting the pitting resistance since the geometry of the load carrying involute is not changed. The circular fillet design is particularly suitable in gear with a small number of teeth (pinions) and these novels gears can replace their existing counterparts in any mechanism without any alterations. Finally the geometry of the generating tool (i.e. rack) is determined in order to be able to cut these teeth using a generating method (i.e. hobbing).

Increasing the Strength of Standard Involute Gear Teeth with Novel Circular Root Fillet Design

Effect of porosity on dry sliding wear of Al-Si alloys

Ashby's map for dry sliding wear of wrought steels has been used as a guide to interpret the dry sliding wear behaviour of PM materials. It has been shown that this map is useful in understanding the acting wear mechanisms and also the experimental wear rates. For given tribological conditions, in terms of normalised pressure and sliding velocity, the sliding wear resistance of PM materials is similar to that of wrought steels, although a closer look at the experimental results highlights the peculiar role of porosity and of a heterogeneous microstructure. In particular, materials with a porosity content of about 10% and with an homogeneous microstructure display the best performances. Since mild wear in metals can be obtained through the formation of protective oxide glazes, steam treatment turned out to be a natural way of increasing the sliding wear resistance of PM ferrous materials. The ‘surface durability’ of steam treated materials was thus shown to depend on the quality of the layer, the a...

Dry sliding wear of ferrous PM materials

Influence of residual porosity on the dry and lubricated sliding wear of a powder metallurgy austenitic stainless steel

Details of a commercial rolling contact fatigue test bench are given; its design permits the evaluation of the rolling contact fatigue behavior of different materials, and makes it suitable for testing porous materials. Without additional machining the original as-sintered surface is tested. Elastohydrodynamic lubrication is applied, which is essential for testing porous material under realistic service conditions. Using the test bench, material behavior under variable loading conditions can be studied, allowing the life-time calculation for a given load-time history. Following a brief overview of rolling contact fatigue, results obtained with the test bench on wrought, cast, and P/M materials are given and analysed. The change from open to closed porosity in sintered steel influences significantly endurance limit and this impacts the development of new, high loading structural parts such as planetary gears and camshaft lobes.

J. A Rice

Papers

Testing P/M materials for high loading gear applications