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Failure Analysis Project
Broken Gear
Luke Plummer
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Gears such as the broken gear in this failure analysis are fabricated using a technique called Powder Metallurgy. In powder metallurgy, finely powdered metal is compacted under high pressure and heat treated to produce a more dense object. In the process, diffusion bonding and sintering is involved.
As the powder is compacted, the powder particles coming into contact with each other flatten, producing a high atom to atom contact area. As the heat treatment or sintering is applied, atoms diffuse across the grain boundaries, producing a denser solid and eliminating voids.
Gears produced by this method are usually fabricated in this way to produce ?self lubricating gears?. The gears are fabricated to have enough porosity to allow lubricating oils to penetrate the pores.
If a gear is not compacted and sintered properly and the porosity is too high, problems arise in the strength of the gear, creating weakness which will eventually lead to failure.
Copper is added to Fe ? C powdered gears due to the copper having a known high seizure resistance against steel. Hence the microstructure resembles an iron predominant matrix containing dispersed free copper phases. |
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| Broken Gear | Possible Impact Point | ||
| Once the initial photographs were taken, 2 samples from the gear were then cut with a hacksaw in 2 orientations, one vertically and one horizontally. The sample was then mounted in Perspex and ground and polished using grinding and polishing wheels. Once polished, the samples were etched to reveal the underlying microstructure. A light microscope was then used to observe this microstructure. Using the camera attachment, the following micrographs were taken. |
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Micrograph Images of Etched Sample |
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| 5x | 20x | ||
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| 50x | 100x | ||
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Picture A Shows the high porosity present. This section of the sample has approximately 16% porosity and 7% copper phase, determined using the image analyser program attached to the microscope. Picture B & D Shows the free copper phases present in the matrix. Picture C Shows the lamella pearlite eutectoid structure of the Fe-C. |
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| To determine the gears
chemical composition and examine the gear at a higher magnification,
it was taken to the Microstructural Analysis Unit and put in a
Scanning Electron Microscope. Following are various micrographs at different magnifications. |
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Scanning Electron Micrograph (SEM) Images |
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| It can be seen from the SEM micrographs above, the extent of the porosity and the copper phase present in the Fe-C matrix. | |||
| Elemental Analysis using SEM | |||
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| The first elemental
analysis was detected from the central region of the broken gear.
The second elemental analysis was detected from the edge of the
broken gear. It can be seen from the analysis, that the central region contained a lot less copper than the edge of the gear. |
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| Vickers Hardness | |||
| The hardness of the gear was determined on the polished etched sample using a Shimadzu Microhardness Tester No1919. The diamond hardness indentor, using 300g/force, resulted in a Vickers reading of 125. | |||
| Conclusion | |||
| On the inside groove of the gear, there was a small triangular impact mark, as seen in the "possible impact point" picture above. This impact point may have been the root cause of the failure. The combination of the impact, high porosity of the gear and also the high concentration of free copper phases at the surface, caused the failure of the gear. | |||
| References | |||
| Materials Science and
Engineering - An Introduction 6th Edition, William D. Callister Jr., 2003 Hitachi Powdered
Metals Technical Report Vol.1 (2002) |
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