Condition, preparation requirements and application techniques. By Andrew Courtney
While the selection process of surface coatings is crucial, what is often neglected is consideration of the substrate being treated. Without ensuring that appropriate preparation takes place, the effectiveness of the treatment and the resulting performance of the component could be compromised. This article will therefore explore surface preparation techniques and considerations, as well as alternative application methods. The result and level of preparation required can vary according to the sector, application and environment in question but there are common steps that need to be taken in all scenarios. Firstly, there is the removal of surface contamination, secondly the creation of the surface profile that is required to ensure adhesion of the coating, thirdly the application technique that will deposit the coating and fourthly, the correct curing process to produce the final properties needed.
Decontamination of the component surface
The first stage in preparing the substrate is vital, as without it the coating may not adhere to the surface properly, leaving the component at risk of damage. In most cases, it involves the removal of any substance build-up, which can include organic deposits such as oxides, oils, waxes, machining fluids, inspection compounds, release agents from mouldings or castings or rust inhibitors - the list is almost endless. They all however must be eliminated before the application process.
There are various methods of decontamination, ranging from immersion of the substrate in solvent vapours and liquids or even wiping. The same techniques can be employed using a wide range of aqueous-based degreasing agents. These can operate at a range of temperatures depending on the type and severity of the contamination. Aqueous immersion cleaners can also be used with electrolytic enhancement, involving alternating anodic/cathodic polarity of the component, which can be extremely effective in cases of severe contamination. The choice of cleaner also needs to take into account the component material itself to avoid chemicals attacking the substrate, such as strong alkalis being used on aluminium or zinc alloys.
For larger components where it is not possible to immerse the substrate in solvents, heat degradation or high pressure steam cleaning can be an effective method of decontamination.
Surface profile preparation
Often following the decontamination process and particularly in the case of electroless or electrodeposited coatings, the surface of the manufactured component is normally suitable for application after thorough cleaning and mild surface etching. This is on the basis that no serious roughening of the surface profile is normally carried out on components to be plated.
With sprayed organic and inorganic coatings however and especially high temperature sprayed ceramic and metal coatings, it is essential to create a roughened surface profile to promote adhesion. For thin coatings this can be quite minor and created by blasting the material with fine media. Aluminium oxide is popular but increasingly plastic media is employed, particularly on high performance components where fatigue strength is vital. For thicker coatings such as high build paint systems and metal sprayed coatings, a much rougher profile is required to encourage adhesion. This can be achieved using high pressure blasting with larger particle size media that is controlled to generate a coarser, measured surface profile.
As an alternative to surface roughening by mechanical abrasion, or even as an addition to that, the use of chemical surface conversion treatments are often beneficial. These can enhance the corrosion resistance of the final coating as well as providing a key for excellent adhesion. Various phosphate or chromates can be used, often as a stage in a tank-based process line linked to initial cleaning and degreasing. On large components these conversion coatings can also be produced by brush or spray methods or even using controlled pumping methods.
Once the substrate has been appropriately prepared, the coating can be applied - the method of which depends on the component and environment. Bath or tank deposition of metallic or inorganic coatings (using electrolytic methods), rely heavily on bath chemistry and can be a complex and lengthy process. A complicating factor is the varying current densities within the bath, particularly with large or unusually shaped components. Anode design and placement can normally produce acceptable thickness variation on most components.
Electroless deposition requires advanced bath chemistry and good temperature control but can make it much easier to produce even coating thickness. The compromise is that the range of coatings available is restricted compared to electrolytic coating options.
The application of metallic coatings by using selective plating provides an innovative solution to complex plating problems. It can help protect, enhance and optimise the performance of critical components and equipment quickly and efficiently. Selective plating is particularly useful for on-site applications when the component is large and difficult to disassemble or where the area being coated is small or challenging to access. However it is not typically recommended for low value, higher volume applications.
For atomised spray application of a wide range of liquid paints and coatings, the required thickness of the film guides the application technique. Small, finely atomised spray guns allow tight tolerances to be achieved, for example film thicknesses below 25 micron. A wide variety of equipment, including electrostatic spray guns can produce 25 to 100 micron film thickness and this allows multi-coat systems to be applied to fairly complex shapes. This can enable reasonable tolerances to be achieved during the final coating thicknesses of around 250 to 500 microns.
For components with larger surface areas where minimum thickness is the priority and smooth, perfect finishes are not essential, airless spray guns can provide optimum application. They can deposit thicker coatings more quickly than alternative methods on components such as large steel structures. Powder coating application is normally applied electrostatically, which ensures that it penetrates into small recesses that would not normally be accessed with the low velocity of the powder jet.
Metal or ceramic coatings applied by High Velocity Oxygen Fuel (HVOF), plasma or thermal spraying normally require the gun or the component to be moved in an even motion to prevent overheating and to achieve an even thicknesses. Post machining is often necessary to achieve the required dimensions but thermally-sprayed aluminium for corrosion protection applications is normally only treated with an organic sealer to improve performance.
Curing of the coating
The final stage of the process is curing, which ensures the coating adheres and sets to the substrate. As with the other stages of preparation and application, there are multiple methods of curing to be considered.
Metallic coatings deposited by electroless or electrolytic methods and using hot spray applications do not require curing, although the properties of a few of the deposits can be modified by heat treatments. Most organic and a few inorganic coatings however do need to be cured in order to develop their properties and achieve the required properties. The type of carrier resin for the coating determines the temperature and time needed. High temperature polymers such as PFA, PTFE, PEEK, FEP polyether-sulphones, polyphenylene-sulphones and various silicones require temperatures between 375C to 450C. Intermediate resins such as polyamide-imides, epoxies, polyurethanes, polyamides and phenolics meanwhile need curing temperatures between 180C to 220C.
Every combination of product and application type has a specific treatment prescribed in order to fully develop the coating’s properties. It is vital to take into account the mass of the component and the oven capabilities during the curing process. This is because it is the component temperature generated and the duration of the process that allows the development of the coating’s final properties. With chemically cured paint systems, the curing times are much longer in order to achieve the correct properties and also to allow solvents to escape between the layers of a multi-coat system. A four coat system can take 10 to 14 days to complete and fully harden for example, although slightly elevated ambient temperatures can reduce this time considerably.
As we have seen, it is clear that as well as selecting the most appropriate coating type, there are several stages to consider when preparing the substrate, from the method of cleaning, the application method and possible curing of the coating. To ensure that every stage of the process is achieved, it is important that your specialist surface coating partner can provide technical support and advice.
Andrew Courtney is with Surface Technology.