I'm a little disturbed at the assumption, evident in
many recent messages, that a hard surface is
necessarily more resistant to abrasion than a soft
one. This is not correct, and people on this list in
particular need to be aware of that.
When a hard abrasive particle hits a hard surface, it
loses nearly none of its kinetic energy, and in a
turbulent stream may strike the surface again and
again, each time spalling or scraping off a small
amount of material.
When it hits a soft surface, it tends to lose energy -
essentially embedding itself temporarily. This can
often result in LESS wear on a soft surface than on a
hard one. The ultimate useful application of this
property is in an industrial process called lapping,
in which a soft tool (the lap) charged with an
abrasive, is used to remove material from a much
harder workpiece.
It is therefore incorrect to assume that a plastic
turbine bucket will necessarily have less wear in it
than a metal bucket of the same profile. It MAY be so
- not all soft surfaces have the property of resisting
wear - but that must be substantiated by test. Surface
hardness is only one of the relevant parameters.
Best,
Marc de Piolenc
Iligan City, Philippines
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