Airbus A380 Landing
Surface engineering has been labelled a critical technology underpinning the competitiveness of the UK industry. In 2010, the UK engineering coatings industry was valued at £10.8 billion, affecting manufactured products worth over £140 billion in industrial sectors worth just under £250 billion. The construction, electronics, automotive, and aerospace sectors were found to be the biggest users of coatings.
Functional coatings in the aerospace industry are mainly for the protection of corrosion, erosion, oxidation and damage due to high temperatures. They are known to be backed by decades of data proving their capabilities. Therefore, the challenge lies in the ability to prove that the new coating will perform as well as the traditional solutions – for 60,000 landings over a 30-40 year lifetime.
Corrosion is a particular function that can significantly decrease the lifetime of a component, and lead to catastrophic failure if left untreated. The airframe, systems & equipment and powerplant are specific areas susceptible to corrosion due to the vast amount of metal (e.g. steel, aluminium & titanium) exposed to corrosive environments. Bimetallic corrosion (e.g. between steel and aluminium), stress corrosion cracking and hot corrosion (powerplant) are most common within an aircraft, and for decades, the use of protective coatings containing cadmium or chromium has dominated. However, the introduction of REACH legalisation banning the use of cadmium and chromium for a variety of critical components has prompted the industry to look for compliant and effective alternatives.
Although the demise of the workhorse surface engineering solutions is imminent, the focus should be on the opportunity to establish an alternative that can reduce cost and improve performance simultaneously. A reduction in wear, improved corrosion resistance and fatigue are all factors that can contribute towards enhancing the performance of components. The discovery of surface engineering solutions that comply with legislations, and are significant improvements of their predecessors could result in faster overhauls and longer periods between maintenance, resulting in more revenue earning up-time.
The demand for increased efficiency in manufacturing and operation has led to the development of more sophisticated coatings. These include coating processes with faster deposition rates that are more durable in the long term. Reliable and durable coatings allow manufacturers to make suitable lifetime predictions for components. In this way, maintenance can be scheduled accordingly and less unscheduled downtime is expected.
High-velocity oxyfuel thermal spray coatings of tungsten carbide based materials have been validated as a hard chrome replacement on several parts of the aircraft such as the landing gear. Electrodeposited coatings (functional Cr3+ and nCoP), in addition to electro-spark deposition and laser cladding are gaining more popularity as hard chrome alternatives, depending on application. For cadmium replacement in aerospace, aluminium based coatings are preferred for their corrosion resistance, although they are more expensive. IVD Al is preferred for larger components and electrodeposited (ED) Al for smaller complex parts and internal diameters. Unlike some other electrodeposited coatings, hydrogen embrittlement does not occur as a result of ED Al, and the materials used are not under threat by health & safety regulations.
The best way to find an alternative surface engineering solution for a particular application is to adopt a systems designed approach. This involves analysing the key performance variables (e.g. fatigue, wear, corrosion resistance, stress, strain) and aligning them with the properties exhibited by the coatings. In this way, a solution can be chosen which closely matches or even supersedes the requirements for that component.
This broad topic is explored in more detail in NAMTEC’s latest technical insight titled ‘Surface Engineering for Corrosion Mitigation’. Visit the NAMTEC website (www.namtec.co.uk) to download it.