Some substrate materials are inherent particle generators and may never be cleaned to the point of there being no particles present. With these materials, particles are a way of life and can not be avoided. This, of course, makes cleanliness testing using particle analysis a difficult if not impossible task. The difficulty is exacerbated by the fact that progressively aggressive cleaning techniques generate even more particles. A part that is, technically, particle free after spray washing may produce another harvest of particles when it is cleaned with a brush for millipore testing and yet another when cleaned using ultrasonic techniques. Finally, there are those materials that create an endless harvest of particles even without using progressively aggressive cleaning techniques. Consider the following examples – –
Cast iron is a popular substrate used in a wide variety of applications. The advantage, of course, is that the material can be cast into complex shapes eliminating the need for metal removal processes that might otherwise be required to produce the desired part profile. Due to its metallurgy, the structure of cast iron nearly always includes free carbon disbursed through its structure. The presence of carbon gives the cast material desirable properties of strength and durability. Many cast iron parts find applications where the inevitable release of carbon particles is of no consequence. In some cases, in fact, the carbon is beneficial in the function of the part as it is released over the life of the part and acts as a lubricant. The real problem cases are those in which a cast iron part is subsequently machined to produce critical surfaces for devices including valves and bearings where the elimination of particles is a critical concern. The machining process may expose surfaces which can’t help but shed particles.
Sintered metal parts are fabricated by compressing powdered metals in molds using high temperature and tons of pressure. Under the influence of pressure and heat, the metal particles fuse together to form a solid objects. Like cast iron, this process allows fabrication of complex shaped objects without excessive metal removal operations. Depending on the application, the thoroughness of compression and fusion may vary. The particles in sintered metal filters, for example, are fused only sufficiently to maintain a given profile while still allowing liquids or gasses to pass through the structure. “Self-lubricating” bearings are often made of sintered metal which has been vacuum impregnated with oil after the sintering process. The oil escapes over the life of the part to lubricate bearing surfaces. Like many other cast parts, sintered metal parts often undergo secondary operations to produce precision surfaces. This process may expose surfaces that may release particles.
Ceramics are also sintered but are usually comprised of a mixture of materials which may include metals, metal oxides and other constituents, not just metal. Again, the powdered materials are fused together under pressure and heat but are subject to the release of unbonded or poorly bonded particles.
Carbon and Lead
Parts fabricated of carbon or graphite and those fabricated of lead are notorious particle generators. No amount of cleaning prevents subsequent particle release of the substrate itself. Again, depeding on the application, this release may be design – both carbon and lead are good lubricants. But in cases where the elimination of particles is critical, these materials pose a significant problem when it comes to cleanliness evaluation. The question is when do you stop removing contaminants and start removing a portion of the substrate itself?
Is there a solution? Well, maybe, but it requires some thinking. We generally look to a cleaning procedure that is more aggressive than the production cleaning procedure to harvest particles for the evaluation of cleanliness. In the above cases, however, the answer is often to utilize a less aggressive cleaning procedure for cleanliness testing. A part that is cleaned ultrasonically, for example, may not release particles when cleaned with a brush for millipore testing. The ultrasonic cleaning process has already eliminated any unstable particles. The “harvest” using a brush will yeild only particles that are true contaminants and may cause problems. The logic is that if the part will not see service conditions that exceed the lesser aggressive cleaning procedure used for cleanliness evaluation it should be good to go.
In summary, the use of materials that that have the potential to release particles should be undertaken with the full knowledge that cleanliness testing is not always capable of insuring “cleanliness” over the long term. The potential for the incidental release of particles in use always exists.
– FJF –