Impact and erosion resistant coatings - double auto-expanding polymer foam and hybrid CVD/PVD laminated hierarchical multilayered approaches

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment


Recent research has greatly increased the knowledge and understanding of biological materials and how their unique mechanical and physical properties arise from their structures. New fabrication techniques have allowed the modification of biological materials or the synthesis of novel materials and structures that are based on established biological principles. The aim is to use modern surface engineering techniques to develop hard and tough lightweight coatings for impact resistance and develop expandable foam to create intelligent airbags to minimise behind armour blunt trauma. These systems would be compatible lightweight body armour with impact resistant coating externally and intelligent airbags internally. This propsal will look at two novel and biomimetically inspired strategies to accommodate impact energies from erodents, foreign objects and ballistics to enhance damage tolerance of surfaces to high strain events: (1) hard and tough multi-layered CVD based coatings matched to their substrates that will dissipate impact energy through sub-critical nano/micro crack propagation and elastic responses; (2) plastically deforming and auxetic (negative Poisson's ratio) foam based coatings that foam under high-strain to act as a double crumple zone for damage resistance and self-repair and self-protection. Energy would be dissipated via elastic and plastic deformation and auxetic performance.


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Description The presence of internal coatings produced a measurable improvement in energy absorption for multi-layer carbon composites.

We have developed a rig able to measure the energy transmitted through a sample during impacts at speed between 40 and 150 m/s. This rig was then used to understand how the impact energy was affected by adding internal and external layers to chosen substrate materials. It seemed that hard surface coatings would increase the energy going through the samples, while internal coatings would decrease it.
The original objective of the project have been partially met as the multilayer structure produced is partially bio-inspired and it reduces the impact energy. The aim of the project was to use hard and tough surface coatings to improve impact resistance, but these kinds of coatings did not make the effect we were expecting.

More studies could be carried out to see how different coating materials would change the properties of the multilayer structure produced.

Auxetic foams were of particular interest due to their negative Poisson's ratio and hence the possibility of improved indentation resistance. It was found that the presence of a negative Poisson's ratio had little effect upon the crater depth and energy absorption. Furthermore, through compressive testing there was evidence to suggest that auxetic foams do not exhibit greater energy absorption than their conventional counterparts due to their negative Poisson's ratio, but instead because of the increase in density that they obtain from the fabrication process.

The 3D printed auxetic foams were compared to classically manufactured auxetic foams using digital volume correlation. Under quasi-static loading, 3D printed foams were found to exhibit not only the same behaviour as commercial foams but also demonstrated the same increased resistance to indentation as the classic auxetic foams, as demonstrated by the presence of a negative Poisson's ratio.

Studies of nature demonstrated that a protective strategy for body armour can be achieved by producing a structure with numerous distinct but complementary layers, each one with its exclusive deformation and energy dissipation mechanism. It was proposed that the combination of hard (CVD ceramic) coatings interlaced with softer (PVD metallic) coatings or a similar hard/soft structure made from soft (CF) composite material with internal layers of hard (metal/ceramic) sheets or coatings should be investigated.
A laborious process involving the production and inspection of hybrid coating combinations was undertaken to identify which multi-layer coating systems would successfully adhere to a substrate and remain cohesive. This work revealed that currently very few combinations (using CVD/PVD alternating techniques) are possible commercially and that the number of layers deposited are limited (maximum of four was achieved in this work). Single layer and multi-layered coatings, with thickness of 45 µm in total, were successfully adhered to metallic, composite and ceramic substrates. The coating combinations included tungsten carbide/nickel and amorphous carbon/nickel.

The need to measure the performance of the armour systems, and the desire to achieve this using a quantitative (rather than qualitative) method, led us to design a purpose built high-speed (low mass) impact test rig. The rig was based on a half Hopkins bar arrangement, which allowed the energy passing through the armour sample to be recorded via a shock accelerometer. To allow (safer) fundamental impact research to take place, a spherical projectile travelling at 50-100 m/s was selected over 'bullet' shapes traveling at ballistic speeds.

The size of the first peak of the acceleration-time plot was shown to be related to the compressive energy wave generated at impact, and this parameter was used to rank the performance of test samples. A test armour sample producing a lower first peak of acceleration value than the comparative 'blank' sample could be said to be demonstrating an improvement in impact performance, i.e. it reduced the level of compressive energy reaching the sub-structure below the armour, which for body armour could be human tissue.
Testing of coating solutions revealed that where the substrate was metallic there was no measurable change in impact performance with the coating systems present. However, when a composite (CF) substrate was coated the peak accelerations (shown to equate to the compressive energy wave) were increased.
A 2 µm thick commercial erosion resistant coating was supplied onto high-speed steel. Testing showed that it too increased the peak acceleration compared to the blank material. These results were important for two reasons: (1) they proved that the impact rig was sensitive enough to measure the effect of a 2 µm coating; (2) they showed that surface coatings can have a detrimental effect on the impact properties in terms of shock wave transference to the subject.

Testing with ceramic substrates was inconclusive because there was so much scatter in the results for both 'blank' or coated samples. For example two test runs under identical conditions produced widely differing results. In one the projectile was almost stopped and the energy dissipated, whereas the other resulted in much higher transmitted energy readings, and the ball rebounded with speed. This highlighted the randomness of the brittle fracture mechanism of these ceramic materials. This is an interesting result, as most modern military body armour systems use brittle ceramics as main components.

Amorphous carbon coatings were applied to pre-preg CF composite 8 ply sheets; these were then stacked and glued to produce a composite structure with thin, hard internal layers. Under test conditions these layered samples displayed a measurable improvement over surface coated CF samples, and also 'blank' CF samples. Thicker interlayer materials were also researched. It was found that sheet (0.5 mm thick) of aluminium or titanium increased the transmitted energy. This might be related to the observation that the ball was stopped by the first metal layer it struck. When a rubber interlayer (0.5 mm thick) was used, the transmitted energy was reduced.

Foams studied in the PhD work package were added to blank CF samples to examine their behaviour as backing materials. It was found that a 3D printed foam, based on a honeycomb structure, performed exceptionally well. The transmitted compressive energy was reduced by 93%, the subsequent shear and Rayleigh energy waves were removed, and most importantly the results were consistent.

From the above key findings we can confirm that having taken inspiration from nature, we have measurably improved the resistance of protective systems to high strain rate events. We have shown that surface coatings such as those traditionally used for erosion and wear resistance produce large shock waves when subjected to impact events. However, internal layers (hard coatings) within a CF composite structure reduce the transmitted shock energies associated with high strain rate events. We have also highlighted that digitally created foams (3D printing) inspired by nature's honeycomb can be successfully used as an under-armour backing material. Obviously, all these solutions need optimising; however, this current body of work has shown where the potential research avenues lie, and equally importantly where dead-ends have been found.
Exploitation Route At the moment the findings are just fundamental work but have the potential of lead to improved future armour solutions for British military forces if future funding can be sourced and ideas taken fourth. The high strain rate rig developed has now been modified to study liquid droplet erosion of surfaces and is used by a follow on PhD student funded by University DTP funding
Sectors Aerospace, Defence and Marine,Education,Energy

Description How have your findings been used? Please provide a brief summary Final report sent to DSTL. MoD/DSTL Workshops attended and dissemination of key findings given by poster and oral presentations. Due to the sensitivity of the project, we might not know what, how and if they will be used.
Sector Aerospace, Defence and Marine,Education,Energy,Transport
Impact Types Societal,Economic