Project looks to develop lightweight material for armoured vehicles
When advanced engineering materials are mentioned, ceramics may not immediatey spring to mind. Yet ceramics possess a number of attractive properties that can be utilised in a variety of applications in a number of sectors.
Lockheed Martin has recently teamed up with the University of Surrey to develop a lightweight material that could improve the protection and survivability of armoured vehicles, including those used by the British Army and Special Forces.
Ceramic materials have increasingly replaced steel in armour plating because they are extremely resistant to penetration and possess low density. This means that they offer high ballistic protection and lighter weight than more traditional methods of armour plating.
The armour is usually made up of small ceramic plates – generally around 10cm2 – that are fixed to a composite or metallic backing plate. These plates can be rectangular, circular or hexagonal. If a round hits the armour it will only take out a single plate and leave the remaining plates intact. It also makes it easy to tailor the armour to different shapes and parts of a vehicle.
The project looked at both aluminia and silicon carbide ceramic plates, which are bonded to a composite backing. This type of armour generally consists of a ceramic front face with an energy-absorbing rear face made from metal or composite materials. The brittle nature of ceramics means that mechanical fasteners are not appropriate for attachment to supporting structures, so they are usually bonded adhesively.
The efficiency of this protection method is reduced, however, by weakness in the adhesive used in bonding the ceramic plates. This project is looking specifically to improving the strength of the adhesive bond of the ceramic plates and composite backing.
Andrew Harris, an engineering doctorate research engineer at the University of Surrey, says: "Although ceramic armour has a great number of advantages over other protection methods, there are still some challenges. Our relationship with Lockheed Martin has developed a method of treating the ceramic that will considerably improve the effectiveness of ceramic armour plating."
The adhesive bond is usually achieved using polyurethane or epoxy, which encapsulates the ceramic. However, neither of these bonding materials can completely withstand the stress resulting from the plating being struck by a projectile.
When a bullet hits most armour, it transmits a massive amount of energy into the ceramic. This shock can cause the ceramic tiles to delaminate and even come off the backing material altogether. This means that ceramic armour can be substantially less effective after just one impact, which is clearly not acceptable.
"Although ceramic armour has a great number of advantages over other protection methods," says Dr Harris, "the weakness in bonding the ceramic plates to the backing has been a problem since this type of armour was first used. Our assignment has developed a method of treating the adhesive that will considerably strengthen the effectiveness of ceramic armour plating."
The work by the team at Surrey University has focused on improving the adhesive bond strength between the ceramic and the backing to improve the ballistic performance for single and multi-hit impacts.
The key to the step change in performance, proven in tests, is the pre-conditioning of the ceramic surfaces prior to bonding onto the support structure. Test results show that using this technique on alumina and silicon carbide surfaces leads to increased bond strength.
"You can get around this by over-designing the ceramic armour, making it heavier," says Dr Harris. "What we've done is improve the bond strength; we tested it and found that the armour performance is improved."
When a 14.5mm armour-piercing incendiary round was fired at the panel, it remained intact despite multiple hits without showing the classic signs of the ceramic armour falling off its supports due to poor shock resistance.
Steve Burnage, head of design at Lockheed Martin UK, says: "Our work with the University of Surrey is particularly valuable for Lockheed Martin UK as we grow our business in designing and integrating turrets for land vehicles. The reduction in weight of armoured vehicles is also increasingly important for the Army as it looks for more rapid and agile deployment into regions of conflict."
The processing and progress of ceramic armour for penetration mechanics are significant areas of ongoing academic and industrial research both in the UK and abroad. Another area of special activity pertains to personal protective vests. Large, torso-sized ceramic plates are complex to manufacture and subject to cracking in use. Monolithic plates also have limited multi-hit capacity as a result of their large impact fracture zone.
European developments in spherical and hexagonal arrays have resulted in products that have some flex and multi-hit performance. In addition, advanced ceramic processing techniques require adhesive assembly methods. One novel approach is the use of hook and loop fasteners to assemble the ceramic arrays.
Morgan Technical Ceramics is also finding and developing innovative applications for ceramics in the defence sector. It has been producing ceramic square panels which are flat packed in to a pre assembly ready to be easily and quickly integrated in to products.
It has also recently launched a range of large piezoelectric (PZT) ceramic discs developed for the defence and commercial sonar markets, thanks to a major breakthrough at the company's manufacturing facility in Ruabon, North Wales.
Using an innovative new process, Morgan Technical Ceramics can press, fire and machine discs and components up to 304mm in diameter. These are considerably larger than those previously manufactured at the site and offer a much lower radial frequency output than smaller PZT discs, allowing for improved acoustic range and drive.
These are manufactured in a range of dimensions and are available in machined thicknesses of 3mm to 30mm in PZT Navy I and Navy III types, and 3mm to 35mm in PZT Navy II, Navy V and Navy VI type piezoelectric formulations. However, Morgan says larger sizes are available on request with thickness and frequency variations also able to be tailored for individual requirements.
The discs are supplied with fired-on silver electrodes to ensure a good adhesion for soldering and bonding, and to a thickness which can safeguard durability in high drive applications. These components can also be manufactured with Wrap-Around silver Electrodes (WAE) if bonding and soldering is required on the same face of the PZT component. The design of the WAE can be made up of many configurations or to a specific design need.
Richard Carus, product sales manager for piezo components at Morgan Technical Ceramics, says: "We have a wealth of experience in both the design and manufacture of PZT ceramics. As a market leader in the field, we need to keep pushing the boundaries to see what can be achieved. This latest manufacturing-led innovation has seen not only a major breakthrough in our core process, but it has created a range of products for our defence and commercial sonar customers."
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