The changing face of composite structures
TWI Bulletin, November/December 2007
Thermal and electrical conductivity, wear resistance, reflectivity... Whatever the surface property it can invariably be modified using a surface coating...but on composites?
Melissa joined TWI in February 2003 and is a Senior Project Leader in surface engineering involved in managing projects and providing technical expertise in the areas of thermal spraying and coating technologies. Melissa has been responsible for leading TWI's research and development in the area of thermal spray coatings for composites and managed a number of projects in this area. Prior to joining TWI, Melissa specialised in biomaterials. She is Member of the IOM3 and a Chartered Engineer.
Paul obtained a Higher National Certificate in Production Engineering with Merits and Advanced Mathematics from Cambridge College of Art and Technology. His expertise covers most engineering technologies but he specialises in composite materials and adhesives. He also has knowledge of working with plastics and ceramics, as well as design, fabrication, joining and costing of composite materials using novel production techniques for many manufacturing industries.
The use of composites as structural components in aircraft and in the automotive industry is increasing as achievable strength-to-weight ratios improve. However, the limited surface properties of composites prevent their use in applications where wear resistance, thermal management or electrical conductivity are required. To extend composite applications coatings are required to provide protection and increase functionality of the surface. The poor adhesion of most coatings onto carbon fibre reinforced polymers (CFRP) is the main technical issue hindering new applications. Ongoing work at TWI has included developing methods for coating composites and providing functional layers. Here Melissa Riley and Paul Burling discuss spraying on to composite materials to increase their use in novel applications within industries such as automotive, defence, aerospace and electronics.
Composite structures are gaining popularity in a variety of industries for a number of reasons. They possess good mechanical strength at lower densities compared to monolithic materials, as well as attractive electrical insulation properties, resistance to corrosion, increased fatigue lifetimes and ease of use. There are many types and forms of composites that have evolved over the years including glass, carbon and Kevlar reinforced materials as well as honeycomb structures created from Aramid paper (Nomex, Tyvec) or aluminium foil.
Many engineers appreciate the benefits of composites. They can see their advantages over traditional materials, but may have been put off their use because of the costs of composites and cost of change in the manufacturing procedures. One way of addressing these needs is to carry out a post manufacture coating process to provide increased functionality. TWI has evaluated thermal and cold spraying as a means of depositing these coatings. An overview of the various spraying processes is given below prior to highlighting some of the ongoing work in this area and potential applications.
Spraying processes
A number of thermal spraying processes exist including, flame, arc, high velocity oxyfuel and plasma spraying. All of these processes involve melting a consumable, either a wire or powder, and propelling the molten or semi-molten particles at a target substrate where they are deposited in layers and build up to form a coating. When coating composites the main concerns are the temperature of the spray jet, which can easily burn or melt the composite, and also the nature of the spray particles which can abrade the composite rather than deposit to form a coating. Both of these factors make coating composites technically challenging. It is the poor adhesion of most coatings onto composites which hinders exploitation of the technology. TWI has undertaken a significant amount of development work to overcome these difficulties in order to coat a variety of composite materials, such as carbon fibre reinforced polymers, glass resin materials and other polymeric and composite materials. In all cases careful control of surface preparation is required along with process choice and procedure optimisation. A variety of processes is available which vary in terms of the temperature and speed of the particles within the spray stream. These features can be used to achieve well bonded coatings which can offer increased functionality.
Flame spraying and arc spraying are perhaps the two most accessible thermal spraying processes. For flame spraying ( Fig.1) a consumable (either powder or wire) is heated by oxy-fuel combustion and propelled at a substrate to form a coating. Flame spraying is widely used where lower coating costs are desired and lower coating quality is fit for purpose.
Fig.1. Typical flame spraying gun
Arc spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes that form the spray material. Compressed gas (usually air) turns the molten spray material into fine droplets and propels them towards the substrate ( Fig.2). Arc sprayed coatings typically have superior bond strengths and lower porosity levels than flame sprayed coatings. TWI has developed arc and flame spraying parameters that can successfully achieve adherent metallic coatings on composites.
High Velocity Oxy Fuel (HVOF) spraying uses higher flow rates and pressures than conventional flame spraying achieving particle speeds greater than either arc or flame spray, but with lower particle temperatures ( Fig.3). However, due to the nature of the process, it can often be difficult to deposit coatings directly on to composites using the HVOF process due to the high velocity of the process and its tendency to cause erosion of the substrate. TWI has, however, developed methods that allow the deposition of HVOF coatings on to composites.
Plasma spraying uses a DC electric arc to generate a stream of high temperature ionised plasma gas, which acts as the spraying heat source (Fig.4). The coating material, in powder form, is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate. Due to the high temperature and thermal energy of the plasma jet, materials with high melting points can be sprayed eg ceramics such as alumina and zirconia. One of the main applications of plasma spraying is zirconia-based thermal barrier coatings (TBCs) for gas turbines. The process has also been used to produce TBCs on composites.
Fig.4. SG-100 Plasma spray system and plasma spraying process (image courtesy of Praxair Inc)
Finally, cold spraying is a relatively new process akin to thermal spraying ( Fig.5). Particles are propelled at supersonic speeds by a fast moving helium or nitrogen gas stream so that when they impact with the substrate they plastically deform and build up to form a coating. Unlike thermal spraying no melting of the powder consumable occurs although the gas may be heated to reach a critical velocity which enables the particles to bond with the substrate and form a coating. If the velocity is too slow, the particles bounce off the substrate without forming a deposit. Cold spraying has the advantage that the powder is not melted and oxidation of the powder does not occur during the spraying process allowing very pure coatings to be produced. The lower heat input also minimises thermal damage to substrate materials. When attempting to coat composites by this method the spraying parameters must be carefully controlled as the high velocity of the particles can abrade the substrate. Cold spray coatings are also very dense, so for certain applications the process has significant advantages over thermal spraying.
Fig.5. Cold spraying equipment
Coating composites surface preparation
The surfacing team at TWI has evaluated each spraying process for depositing coatings on to composites. The work has focused on developing well adhered coatings which can be used as-sprayed or as bond coats or primers for subsequent coating layers. One of the main factors influencing the success of sprayed coatings is surface preparation. In some cases poor surface preparation can prevent a coating being deposited or will result in a poorly adhered coating. As well as being influenced by surface finish, the type of resin and filler used in the composite plays a significant role in coating adhesion (due to how it reacts to the preparation), combined with selection of the spraying process.
The adhesion of thermally sprayed coatings on composites is significantly lower than similar coatings on metallic substrates, because it is much easier to form a mechanical key by grit blasting metallic substrates. However, TWI has developed surface preparation and coating techniques for depositing coatings on composites with adhesion values in the range of 5-10MPa. These compare favourably with the adhesion of paints on steel and thermally sprayed aluminium on steel ( Fig.6). As a result designers and engineers are becoming increasingly aware that coated composites may be suitable for a number of new applications.
Fig.6. Typical coating adhesion values for coatings on CFRP composites, (the adhesion values of paint an aluminium on steel are shown for comparison)
Metallic coatings
As well as evaluating various surface preparation techniques and thermal spraying processes for coating composites TWI has produced a number of samples to demonstrate some of the concepts and potential applications for the technology. Metallic coatings such as aluminium can be deposited on flat composites, including honeycomb structures ( Fig.7), using processes such as flame and arc spraying as well as HVOF and cold spraying in some cases. Spraying also offers the capability of coating curved surfaces such as the section of motorbike wheel ( Fig.8). Thermal spraying is a line-of-sight process so only certain geometries can be coated but some processes can be operated manually, which gives greater versatility. Two of the main applications of these metallic coatings are for reflective or conductive layers, where they can protect the underlying substrate by improving thermal management ( ie radiation reflecting) or for electromagnetic shielding. Other applications of metallic layers include use as primers for subsequent paint layers or functional thermal spray coatings, or as bearing surfaces.
Fig.7. Reflective coating on a honeycomb sandwich, showing two types of surface finish (boundary shown by two arrows).
Fig.8. Coated wheel section showing that coatings can be deposited on a variety of surface profiles.
As well as depositing continuous layers, patterned coatings or tracks can also be deposited onto CFRP as conductive tracks for electronic or electrical applications and lightning strike dissipation. They may also be used as resistor tracks for de-icing and in radar applications or other screening applications. Masking tapes or metal masks can be used to mask the substrate before spraying. Once spraying is completed the mask is then removed to reveal the pattern and discreet areas of coating. One advantage of this technique, eg for lightning strike protection, is that the metal coating can be added post manufacture, compared to aluminium mesh which is incorporated into the composite lay-up manufacturing process ( Fig.9). For applications where high electrical conductivity is required, cold spraying offers significant advantages over thermal spray processes as no oxidation of the consumable occurs during spraying. Flame, arc and HVOF spraying can also be used providing the spraying parameters are carefully controlled to minimize oxidation and porosity in the coating.
Fig.9. Patterned coatings on CFRP
Graded and multilayer coatings for increased functionality
In addition to depositing single metallic layers on composites, TWI has also developed graded and multilayer coatings for increased functionality. In many applications there is a need for some form of thermal protection for composite substrates. Where reflective coatings are not suitable, thermal barrier coatings may offer additional protection. In some cases, reflective layers and thermal barrier coatings can be combined to offer maximum protection of the underlying composite. TWI carried out some initial trials to deposit a multilayer coating for Team Jota to protect the floorplan of their Le Mans car from exhaust heat ( Fig.10). The work has opened up new ideas on further process development to deposit graded and multilayer coatings and the development of improved thermal barrier coatings using the plasma spraying process.
Fig.10. Team Jota multilayer thermal barrier coating
Methods of depositing wear resistant coatings have also been investigated and coatings have been deposited on CFRP tube and flat samples ( Fig.11). Coatings can be deposited as multilayer coatings or as graded coatings and a typical graded coating is shown in Fig.12.
Fig.11. Coated tube incorporating a thermally sprayed metal coating and a WC-CoCr top coating for wear resistance.
Fig.12. Graded coating with wear resistant top layer.
Optimisation and performance
The examples above demonstrate that single and multilayer coatings can successfully be deposited onto composite materials. However, the correct surface preparation is essential to achieve well bonded coatings and must be tailored to each particular composite. TWI has significant experience in developing the most suitable surface preparation methods for a variety of composites combined with experience in selecting the most appropriate coating materials and spraying process for a particular application. No single process is best for all applications and TWI has used its wide range of equipment to develop the best procedure for a particular application in order to maximise performance.
The main considerations are coating adhesion, mechanical performance and also environmental performance. Promising bend and fatigue test results have been obtained for complex multilayer coatings such as the graded wear resistant coating. Test samples were subjected to four point bend tests and a simple fatigue test and coatings were shown to survive 0.5million cycles in 4-point bending without delamination of the coating from the composite. In other cases coatings have been subjected to environmental tests, eg cyclic humidity and temperature tests, where they have also exhibited good performance.
Summary
Industry is continually looking for ways to improve their products, either increasing their function or reducing costs and their effect on the environment. The ongoing work at TWI is expanding the use of composites into new areas by providing functional coatings for a number of applications, some of which are now in production. In addition to automotive, aerospace and defence industry sectors, which use composites in a wide range of applications, construction and engineering, medical, marine and white goods, can also benefit from the use of multifunctional materials.
TWI has experience of developing coatings for many of these applications. Ongoing work at TWI has allowed composites to be considered in new applications and is helping to reduce the need for more expensive composite systems. The application of functional coatings after manufacture of the component offers a significant step without the need to complicate the manufacturing process and increase cost. Perhaps this technology can be used in your industry? To find out more about the technology and suitability for your application why not contact the team: Melissa Riley, Paul Burling or Dave Harvey.
