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PVD – the importance of cleanliness

Such has been the rapid rise and acceptance of the hard, thin engineering coatings in many industries that they now form a part of the design specification for new parts. In the past, the use of such coatings was an exercise in research or a last attempt to solve a problem, but they are now accepted as a necessary part of achieving efficiency or reliability. While they do increase the cost of a single component, it is very often the case that such components would either not work without the coating or would be very much less durable without it.

As with many of the processes used to make a successful and highly optimised component for a race engine or transmission, there are a number of prerequisites to the application of a coating if it is to prove as effective as the customer might expect. One important aspect to consider is the preparation of the parts before coating, especially cleanliness. The difference between a component whose coating is a success and one whose nominally identical coating is unsuccessful may be down to identical preparation processes being carried out in different environments. It is not only the state of the as-machined surfaces that is important, but also the issue of surface contamination.

For the most demanding applications, some component suppliers and coatings companies prepare the parts before coating in a ‘clean room’. These are not simply rooms where no ‘dirty’ processes are carried out – the fact that you set aside a room where no grinding or fettling takes place does not make it a clean room as such – it simply means it is not as dirty as the rest of the factory.

Proper clean rooms are supplied with filtered air, and the rating of the clean room depends on the amount of particles of a certain size there are per unit volume of air. Typically ‘room air’ in an urban environment has around 35 million particles of 0.5 microns or greater per cubic metre. The first level of clean room has 10 times fewer particles of this size. Some of the standards governing the cleanest clean rooms also place maximum values on the numbers of particles down to 0.1 microns.

Clean rooms are entered by an air lock in order to control air quality, and the people who work in them wear gowns, face masks and hair nets. As a reference to how ‘dirty’ it is to have people in clean rooms, a typical person generates 100,000 particles greater than 0.3 microns in size per minute when motionless. When walking at 2 mph (3 kph), that number rises to 5 million.

Components prepared for coating in a clean room will have less surface contamination in terms of particulates on the surface, as such particles can prevent good adhesion of the coating. In many circumstances, there would be no problem with the ‘usual’ level of cleanliness, but where components operate under very demanding conditions of high instantaneous temperatures and high contact stresses, or under marginal lubrication, high levels of cleanliness improve the performance of the coatings. In race engines, such components might include valvetrain components such as valves and finger followers.

Written by Wayne Ward

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