The controls for surface treatment of parts for the aerospace market are very strict. The last thing that you want to find out when you are in the air on a flight with critical parts coated is that the coating has failed. Having a good understanding of the processes and the relevant standards will save a lot of time and potentially a lot of money in avoiding errors in specifying a supplier or process to treat parts.
OEM Approvals and Qualified Supplier Lists
Before a supplier can process aerospace parts commercially they need to be included on an approved suppliers list held by the relevant prime contractor or OEM. Being Nadcap accredited for specific process commodities (e.g. chemical processing or thermal spray) is the most recognised qualification for processing aerospace parts.
Reading a Specification Designation
Most aerospace surface treatments are covered by AMS (Aerospace Material Specifications), MIL (US Military) and DEF STAN (UK Defence Standard) documents. Each of these documents specifies the process chemistry, material covered and performance criteria. For example, AMS 2480 is the specification for hard chrome plating of aircraft parts and simply referring to this number will lead you to the correct controlled document. For Aerospace Surface Treatments, see www.poeton.co.uk/about/industries/aerospace-defence.
Process Specifications Versus Performance Specifications
A process specification is a description of the steps to be taken to apply a surface treatment, such as bath temperature, current density and thickness. A performance specification describes the required characteristics of the surface finish, for example minimum hardness or hours of salt spray resistance. Aerospace surface treatment specifications, such as AMS 2480 for hard chrome, are written as a process specification and also contain performance specifications that the surface must meet.
How Drawings Drive the Standard Choice
The design authority’s drawing is the controlling document for a surface treatment process. It references the required specification (by number) and can also list alternatives that are permitted. For hazardous chemical processes used in surface treatment the supplier must also comply with COSHH (Control of Substances Hazardous to Health) in the workplace in addition to the technical standard.
Traceability and Documentation
For every treated batch a supplier will issue a certificate of conformance (or other document stating that the work has been completed in accordance with the specified requirements). This will be supported by test results for test coupons that were processed at the same time as the main job and a record of the processing of that batch of work.
Getting the correct treatment specified at the design stage is far easier than trying to rectify a non-conformance once the parts have been processed.
