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There are a number of metal finishing process which should be avoided in the manufacture of items subject to demanding specifications with regard to the presence of particles. These common processes, although employed with the best intent, result in the generation or unavoidable entrapment of particles which may be released under use conditions with potentially catastrophic results. All of these process involve either “smearing” or “upheaval” of the surface of the substrate. Consider the following examples.
Peening –
Peening is a procedure often used to increase the hardness of metal surfaces. Historically it was accomplished using a hammer (ball peen hammer) but is, today, more commonly done using small balls of metal called “shot” that are propelled against the surface being treated by a blast of air (shot peening). The problem is that metal is displaced at each impact site (much like the craters on the moon). Another impact on the raised ridge of upset metal may result in a folding process that traps scale and other surface contaminants below a skin of upset metal. The materials trapped are unstable and are often easily released if the surface is subjected to mechanical friction of any kind. The problem is intensified by the fact that the peened metal is subject to fracture due to the work hardening induced by the peening process.
Sand, Grit, and Bead Blasting –
Sand and grit blasting are processes normally used to remove paint and other surface coatings including oxides. Sand or grit blasting may also be employed to impart a cosmetically desirable appearance on some parts. Although similar to peening, in this case the impacting particles are very small and become progressively smaller as the blasting media is collected and reused. Reused media also includes remnants of materials removed from previously processed parts which, in some cases, may have been fabricated of different materials. It is bad practice, for example, to re-use grit used for sand blasting iron to subsequently sand blast stainless steel.
Blasting has much the same problems with regard to metal upheaval. The problem is that through repeated displacement of the metal surface particles of not only the substrate but the blasting material may become embedded in the surface of the substrate and held in a manner not unlike the way the prongs of the setting on a ring hold the diamond(s) in place. The problem is amplified by the fact that the blasting media is often much harder than the substrate being finished. This means that if a partially embedded particle does break loose it has the potential to do considerable damage to the substrate especially on a bearing surface.
Buffing and Polishing –
Buffing and polishing are common finishing operations usually employed to produce a cosmetically appealing surface. They involve the use of a fine abrasive which is rubbed over the surface being finished under pressure to produce a shiny appearance. Buffing wheels used with “buffing compound” and abrasive pads are examples of polishing operations. Seeing a brightly polished surface, one would not suspect it as a particle generator. Yet, I have seen numerous cases where a polished or buffed surface produced a nearly endless harvest of particles when subjected to particle testing. In the case of buffing and polishing, the metal is “smeared” by the finishing process resulting in layer upon layer of thin laminations interspersed with oxide as well as buffing or polishing compound and other contaminants. These layers, usually formed at elevated temperatures due to the friction imparted by the process, are extremely thin and brittle. The fact that they are not metallurgically contiguous with the underlying substrate makes them very unstable and subject to flake off with the slightest provocation.
Clearly, all of the above processes produce surfaces that are undesirable in cases where the presence of particles is a major concern. It is interesting that some of these finishing operations are performed primarily for cosmetic reasons and often don’t even include the real working surfaces of the part. In many cases it would be better to have a part with a little less “bling” than to have one that may shed particles in use.
– FJF –