Hot coatings for composites

Tom Shelley reports on coatings that can protect composites against intense heat and provide other beneficial properties.

Carbon fibre composites are regularly being coated to protect them from the effects of hot vehicle exhaust gases, as well as to provide other beneficial properties, such as resistance to abrasion and other physical damage. A recent example is providing high-friction, wear resistant brake surfaces for upmarket bicycle wheels and to resist wear. Coatings can be ceramic or metal, and come in a wide variety of colours, for those concerned with appearance as well as performance. Zircotec in Oxfordshire specialises in coatings sprayed on using modified arc plasma torches. Technical director Dr Andy McCabe, said recently: "It has been a really good year for us in F1, but we want to take the technology into other industries." When Eureka first came across Zircotec, the company had recently spun out of AEA at Harwell, and was focussed on spraying zirconium oxide onto motorsport vehicle exhausts in order to reduce heat transfer to adjacent bodywork. It still does this, but also a lot of other things as well and has been actively filing patents to protect its IP, including its technology for coating carbon fibre composites. The purpose can be to coat a part that is hot to reduce heat transfer to something else, to coat a part to prevent it being over heated by something else that is hot, to reduce wear or increase friction. Possible coatings, as well as zirconia, include aluminium on top of zirconia, stainless steel, molybdenum, tungsten, 'Diamond black', and a 'Ceramic mix' to control both heat transfer and wear at the same time. Dr McCabe said that, before laying down the protective coat, 'there is a proprietary bond coat for all surfaces that is crucial'. Finishes are now available in 14 different colours. One of the motorsport applications for F1 teams that we were shown was a zirconia coat on part of a carbon fibre suspension wishbone to protect it from exhaust gases. This is no mean challenge since the 50kW DC arc plasma torches used work at a temperature of around 20,000°C in order to heat and melt the feedstock material. Coatings go down as a series of fine, molten splats. Dr McCabe said: "We can also make this black, or overspray it with a metal finish, so competing teams cannot see what has been done. Another F1 composite coating application is to coat diffusers. Routing hot exhaust gas through a diffuser along with ambient air to increase combined gas speed is allowed in the rules and enhances down force, reducing lap times by just over 0.5s. Composite surfaces require chemical or physical preparation before application of the bond coat. The process can also be applied to the coating of sintered nylon or even fibreglass. Exact parameters to ensure successful coating are critical. Dr McCabe says that it is essential to control the whole range of spray parameters, the 'range of materials that you put down first', as well as ensuring sufficient cooling or the part during the coating process. He considered that, "Most people who tried to do this would damage the part during pre-preparation," he claimed. It is also possible to lay down aluminium oxide coatings on carbon fibre that are highly electrically insulating as well as thermal insulating. Zircotec also has a ceramic coating which only requires a 100µm thick layer to insulate to 10kV.