Protective Performance of Hybrid Metal Foams as MMOD Shields

Andreas Klavzar; Maxime Chiroli; Anne Jung; Bernhard Reck

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Procedía Engineering 103 (2015) 294 - 301

Procedía Engineering

www.elsevier.com/locate/procedia

The 13th Hypervelocity Impact Symposium

Protective Performance of Hybrid Metal Foams as MMOD Shields

Andreas Klavzara*, Maxime Chirolia, Anne Jungb, Bernhard Recka

aFrench-German Research Institute of Saint-Louis (ISL), 5 rue du Général Cassagnou, Saint-Louis 68300, France bUniversität des Saarlandes, Applied Mechanics, Campus A4.2, Saarbrücken 66123, Germany

Abstract

Open cell aluminum foam core sandwich panel structures have been proven to be of interest for protecting satellites against micrometeoroids and orbital debris (MMOD). Bumpers containing aluminum foam show outstanding capabilities to induce multiple shocks to small projectiles in the hypervelocity regime. For this work the protective performance of foam cored sandwich panels with cores made from newly developed hybrid metal foams was evaluated. Therefore shots in the hypervelocity regime on the two-stage light gas gun of the French-German Research Institute of Saint-Louis were performed.

The tested targets were sandwich panels with aluminum front and rear facesheets and cores of different types of metallic foams: foams with pore densities of 10 pores per inch and 45 pores per inch were tested as pure aluminum and hybrid metallic foams. The projectiles to simulate micrometeoroids and orbital debris were aluminum spheres with a diameter of 4mm. The impact velocity was 6500m/s. It could be shown experimentally that the nickel coating of the aluminum foams leads to a decreased crater depth in the sandwich panels. However, scatter in the coating thickness leads to variations in the foam densities of the hybrid foams, making the evaluation of the increase in the protective performance difficult. Nevertheless, due to the nickel coating the influence of the pore density seems to be more significant than reported before. By reducing the coating thickness and using high performance aluminum alloys as base material for the hybrid foams, further optimization of the protective performance could be reached. Then, the complete evaluation of the ballistic limit over a broad velocity regime should be done to see the variations in the performance of the hybrid foams over the whole velocity range being of interest for MMOD shielding technologies

Peer-review under responsibility of the Curators of the University of Missouri On behalf of the Missouri University of Science and Technology Keywords: MMOD, Hybrid Metal Foams, Open Cell Aluminum Foams, Two-stage light gas gun

Nomenclature

MMOD Micrometeroid and orbital debris ppi pores per inch pH pH value

SEM scanning electron microscopy

1. Introduction

Metallic foams have remarkable properties regarding their low density and their mechanical behavior. Open cell aluminum (Al) foam core sandwich panel structures have been proven to be of interest for spacecraft micrometeoroids and orbital debris (MMOD) shielding [1]. The use of aluminum foam as a core for a sandwich panel with aluminum facesheets guarantees good specific strength and stiffness performances. At hypervelocity impact conditions an improved capability of aluminum foam compared to monolithic targets to radially disperse the particulates within the debris cloud was observed

* Andreas Klavzar. Tel.: +33-389-69-5356; fax: +33-389-69-5359. E-mail address: andreas.klavzar@isl.eu

Peer-review under responsibility of the Curators of the University of Missouri On behalf of the Missouri University of Science and Technology doi:10.1016/j.proeng.2015.04.050

[2]. Furthermore, bumpers containing aluminum foam showed outstanding capabilities to induce multiple shocks to small projectiles in the hypervelocity regime [3]. Due to the repeated shocking caused by the impact on individual foam ligaments during the penetration process, the thermal state of impacting projectiles raises, resulting in fragmentation, melting and vaporization. It is considered that optimization of these structures may lead to sizeable improvements in protective capability and efficiency [1]. Therefore a newly developed type of metallic foam was tested for our work. This so called hybrid metal foam consists of open-celled aluminum foams with a reinforcing nickel (Ni) surface coating of high hardness.

Structures built from hybrid metal foams provide higher resistance compared to pure aluminum foam structures [4], at least in quasi-static tests and dynamic tests at intermediate strain rates (5x103s-1). In this work it is investigated, if the effect of multiple shocks induced into the projectiles at hypervelocity impact conditions is also increased due to the high hardness surface of the hybrid foams. The hybrid metal foams are compared to pure aluminum foams. The target configurations and the test conditions were chosen similar to those published in the literature on tests with pure aluminum foams [1, 5].

2. Investigated material

2.1. Preparation of Ni/Al hybrid metal foams

Electrodeposition is a very versatile technique for the coating of mainly metallic but at least electrically conducting substrates with metals and alloys and is based on an electrochemical deposition of metal ions from an aqueous electrolyte. Especially it is a common method to produce nanocrystalline coatings. These have the advantage that they offer higher hardness and strength than conventional polycrystalline or ultrafine grained materials. According to the Hall-Petch relationship, the hardness and strength are inversely proportional to the square root of the grain size [6, 7]. Using electrodeposition not only planar but also very complex shaped three-dimensional geometries can be coated.

For the ballistic tests in this study, open cell aluminum foams of m-pore GmbH, Dresden, Germany made from Al99% were used as substrate for the electro-deposition process. In a previous study [4, 8, 9], the mechanical properties of the hybrid metal foams were characterized on foams based on AlSi7Mgo.3. The pore densities of the foams used in the ballistic tests were 10ppi (pores per inch) and 45ppi, the ones for the mechanical characterization were 10ppi and 30ppi. The 10ppi foams had a sample size of 100 x 100 x 50mm3. Due to the production process of the metal foams by investment casting, the maximum producible thickness of the foams reduces with increasing ppi number and decreasing pore size, respectively. The sample size of the 30ppi and the 45ppi plates was 100 x 100 x 10mm3.

The aluminum foams were coated by direct current (DC) electrodeposition with nanocrystalline nickel. For the electrodeposition, a commercial nickelsulfamate electrolyte (Enthone GmbH, Langenfeld, Germany) with a metal content of 110g/l at a pH of 3.8 and a temperature of 50°C was used. According to the fact that aluminum is soluble in acids, there must be a special pretreatment of the aluminum foam samples to prevent the dissolution of the foams in the acid nickel electrolyte by finally an electroless plating with zinc. The exact pretreatment procedure can be found in a previous work [8]. In the electrodeposition process, the foams act as the negative charged electrode, the cathode. In contrast to planar electrodes, because of a strong mass transport limitation [8], for an almost homogeneous coating, the electrodeposition on such complex three-dimensional structures like open-cell metal foams requires a special cathode-anode arrangement [8, 9]. Each foam plate was placed in the center of a double-walled cage-like cube made of expanded titanium metal and filled with nickel pellets (A.M.P.E.R.E. GmbH, Dietzenbach, Germany) as sacrificial anode. The direct current electrodepositions were carried out using an average current density of 1.5mA/cm2.

The optimal coating thickness regarding the specific energy absorption capacity per density of the metal foams depends on the strut thickness of the foams and is about 30% of the strut thickness [11]. Hence, for 10ppi and 30ppi foam a coating thickness of 150^m and 75^m nickel was used, respectively. For the 45ppi foams only a coating thickness of 35^m nickel, corresponding to 25% of the strut thickness was used.

2.2. Structural and mechanical characterization of the Ni/Al hybrid foams

The microstructures of the hybrid foams were characterized by scanning electron microscopy (SEM) and light microscopy. Figure 1 shows the different sizes of the microstructure of 10ppi and 30ppi foams as well as a SEM image of the surface structure of the coating on a 30ppi foam sample.

Fig. 1. Light microscopy images of the microstructure of a 10ppi (a) and a 30ppi (b) aluminum foam and SEM images of the cross section of the 30ppi Ni/Al hybrid metal foam (c) and (d).

Whereas uncoated aluminum foams have a rough surface morphology built of bumps, the nickel coating provides a much smoother surface consisting of a fine grained cauliflowerlike structure, resulting from a radial grain growth mechanism. As seen from the SEM image (Fig. 1 (c) and (d)) the cauliflowerlike grain structure builds a closed fully covered coating layer on the struts of the aluminum substrate foams. The grain size of the nickel coating was determined by X-ray diffraction (XRD, Siemens D500, Bruker AXS, Germany) using a modified Warren-Averbach method [12, 13]. All coatings despite of the pore size of the substrate foam have a grain size of 43 + 2nm.

The microstructural sizes and morphology parameter of the aluminum substrate foams and the Ni/Al hybrid foams are summarized in Table 1. According to the coating, there is a significant increase of the density. For the 10ppi foams there is an increase of 400% and for the 30ppi foams of 425% and for the 45ppi foams of 315%. But despite of this significant increase, the density is lower than that of water and much lower than that of dense metals.

Table 1. Morphology parameters or different pore sizes.

Pore size (ppi) and foam type Coating thickness (^m) Pore size (mm) Strut thickness (mm) Density (g/cm3) Porosity (%)

10ppi Al - 4.5 0.50 0.154 94.3

10ppi Ni/Al 150 4.5 0.80 0.621 94.3

30ppi Al - 1.8 0.26 0.168 93.7

30ppi Ni/Al 75 1.8 0.41 0.713 93.7

45ppi Al - 1.2 0.12 0.245 90.9

45ppi Ni/Al 35 1.2 0.19 0.764 90.9

Based on field scans of the magnetic flux density distribution of homogeneously magnetized Ni/Al hybrid foams, a coating homogeneity of 85% between the coating thickness of the outer surface of the 10ppi foams and the foams center was measured [8, 14]. Due to the lower thinness, the coating homogeneity in the 30ppi and 45ppi foams must be higher.

The mechanical characterization of the coating and the foams itself was done by nanoindentation (NI) [15] and quasi-static [9] as well as dynamic compression tests [16]. Keep in mind, that the mechanical characterization was done on the 10ppi and 30ppi hybrid metal foams based on AlSi7Mg0.3, whilst the ballistic tests were done on 10ppi and 45ppi hybrid metal foams based on Al99%. Still, the results of the mechanical characterization are given here in order show the improvement of the mechanical properties due to the nickel coating.

Nanoindentation on Ni/Al hybrid foams was performed to determine the hardness of the aluminum substrate foam and of the nickel coating. The NI measurements were performed using a Berkovich indenter (Hysitron TriboIndenter TI950 NI)

with an indentation load of 1200 ^N. The hardness was determined from the force displacement curves using the Oliver and Pharr method [17]. The hardness of the nickel coating is with 4.23GPa about four times the hardness of aluminum with 0.98GPa for an indentation depth of 63nm. Due to the fact, that the mechanical behavior of foams is based on the buckling and bending of the struts of the foams, the hard nickel coating improves the bending stiffness of the struts and hence the global mechanical properties of the Ni/Al hybrid foams. As the highest compressive and tensional stresses arise during bending at the outer fiber of a strut, the hardness increase of the nanocrystalline nickel coating leads to a weight optimized composite strut, whereas the heavier nickel is only needed in the outer regions of the strut cross section.

The quasi-static compression tests were performed using a universal testing machine (INSTRON 4204) at a strain rate of 5x10"V\ The dynamic compression tests were carried out using the Split-Hopkinson Pressure Bar method (SHPB) at a strain rate of 5000s-1. Table 2 shows the specific energy absorption capacity per volume, which is equal to the integral of the stress vs strain curve of the foams for the 10ppi and 30ppi foams. The 45ppi samples were not tested in this case, still, the general behavior should be similar to the one of the 30ppi foam.

Table 2. Specific energy absorption capacity per volume as function of the pore density and strain rate.

Pore density (ppi) and foam type Coating thickness (^m) Strain rate Specific energy absorption capacity per volume (MPa)

10ppi Al - 5x10-3 1.05

10ppi Ni/Al 150 5x10-3 9.23

150 - -

30ppi Al - 5x10-3 0.98

- 5000 1.12

30ppi Ni/Al 75 5x10-3 7.25

75 5000 9.53

It can be seen from Table 2 that there is a significant increase of the energy absorption capacity per volume by the coated foams in comparison to the pure aluminum foams. Furthermore, there is no strain rate sensitivity for the uncoated foams, whereas there is a pronounced strain rate sensitivity for the nickel coated foams. At higher strain rate, there is an increase in the energy absorption capacity.

3. Ballistic tests

3.1. Tested target configurations

The foam samples have a cross-section of 100mm*100mm and a thickness of 50mm. Four different types of targets were prepared for the tests, with different porosities and different types of foam materials: Pure aluminum foams (Al) and hybrid metal foams (Ni/Al). We further used two different porosities: 10ppi and 45ppi. The 45ppi foam samples were realized by stacking 5 plates of 10mm thickness together. The foams were covered by 0.25mm thick aluminum alloy sheets at the front and rear faces, resulting in a sandwich panel with a metal foam core. The front- and rear facesheets of the sandwich panels were made from an Al97Mg3 aluminum alloy produced by Goodfellow Ltd, with a thickness of 0.25mm and an areal density of 0.005g/cm2 each.

The sandwich structure was held in place by an aluminum frame. A steel witness plate of 2.0mm thickness was added at a distance of 150mm behind the target (Fig. 2) to visualize debris ejecting from perforated sandwich panels.

Fig. 2. Target configuration with aluminum alloy front and rear facesheets, 10ppi Ni/Al hybrid metal foam and witness plate

The densities of the tested foam cores vary with the foam material. The mass of each tested foam core and the resulting areal density of the sandwich panel are summarized in Table 3 together with the test results. For the pure aluminum foams tested in this work the density increases with decreasing pore size. Further on, the densities of the coated foams vary due to varying coating thicknesses, as shown in Table 3.

3.2. Test conditions

The tests were done with the two-stage light-gas-gun of the French-German Research Institute of Saint-Louis (ISL). The launch tube utilized for the tests has a caliber of 11.0mm. The projectiles to simulate the impact of MMOD were aluminum alloy spheres made from Dural (AlCu4Mg1) with a diameter of 4mm and a mass of 0.09g. They were embedded in sabots. The total mass of projectile and sabot is 1.6g.

The performance of the target structures, simulating MMOD shields, is usually defined by the ballistic limit curve over the velocity range of interest. The velocity range is separated into three regimes: low, intermediate (or shatter), and hypervelocity. At low velocities, pressures that are generated during impact are insufficient to fragment the projectile. Transition to the shatter regime occurs when the pressures are large enough to induce projectile fragmentation. Increasing impact velocity within the shatter regime leads to increased projectile fragmentation and therefore to a finer dispersed debris cloud of smaller particles. The transition from shatter to hypervelocity impact regime is defined when the failure mechanism of the rear wall switches from cratering to that of an impulsive blast wave [18].

The transition velocities from the low to the intermediate regime and, respectively from the intermediate to the hypervelocity regime, have to be evaluated by several tests for each regime. For similar test conditions as used in this work, Ryan et al. [1] defined the transition limits of the hypervelocity-impact regime for open-cell aluminum foam bumpers at 4000m/s.

In the case of our hybrid metal foams the transition velocity from the shatter to the hypervelocity regime should be different. An impact velocity of 6500m/s was chosen to ensure impact conditions in the hypervelocity regime. The impact conditions chosen ensure furthermore comparable conditions to test results published in the literature. The impact angle was always 0° to the target surface normal.

3.3. Test results

The test conditions are summarized in Table 3 together with the results of the different shots. We used the following definitions: perforation (P), if the rear facesheet of the sandwich panel is perforated by one or a few pin-sized holes. Significant perforation (P>) is considered if the size of the holes in the rear facesheet is in the range of a few millimeters, severe perforation (P>>) if the rear facesheet is perforated by a hole in the centimeter range and significantly damaged and deformed. No perforation (NP) means that the rear facesheet is totally intact. In the case of no perforation slightly below the failure threshold (NP<) the rear facesheet is not perforated but small traces of fragments are visible.

Shot number 2527 is listed in Table 3 without the impact velocity. In this case, no measurement value was available, probably because the sabot was not separated properly. The result of this shot is therefore not taken into account.

The cross-sections of four different target types with the impact crater in the foam core and the corresponding rear facesheets are shown in Figure 3.

Andreas Klavzar et al. / Procedía Engineering 103 (2015) 294 - 301 Table 3. Summary of test results

Sample type Shot number Impact velocity Mass of foam core Total areal density Test result

(m/s) (g) (g/cm2)

10ppi Ni/Al 2518 6454 206.0 2.19 P>

10ppi Ni/Al 2527 - 152.7 1.66 P>>

10ppi Ni/Al 2531 6506 162.3 1.75 P>>

10ppi Ni/Al 2533 6621 209.6 2.23 P>

45ppi Ni/Al 2519 6384 384.0 3.97 NP

45ppi Ni/Al 2528 6363 370.5 3.84 NP

45ppi Ni/Al 2530 6156 263.8 2.77 NP<

45ppi Ni/Al 2535 6345 391.1 4.04 NP

10ppi Al 2520 6435 94.0 1.07 P>>

10ppi Al 2529 6556 85.3 0.98 P>>

45ppi Al 2521 6421 129.0 1.42 P>

45ppi Al 2532 6489 118.7 1.32 P

Fig. 3. Test results showing impact craters in metal foam cores and rear facesheets of sandwich panels: a) 10 ppi Ni/Al: P>, b) 45 ppi Ni/Al: NP, c) 10 ppi Al: P>>, c) 45 ppi Al: P>

From the results Ryan et al. [1] have published two of the tests were done with similar conditions as the tests presented here. For sandwich panels with pure aluminium foam cores of 2.0" (50.8mm) thickness impacted by a projectile of 4mm diameter their tests resulted in perforation slightly above the failure threshold for both, 10ppi and 40ppi samples at impact velocities of 6890m/s and 6790m/s, respectively. The areal density of the 10ppi sample was 1.25g/cm2, whereas the density of the 40ppi sample was 1.20g/cm2. In our case, the areal densities of the 10ppi Al foam core samples were 0.98 and 1.07g/cm2, the ones for the 45ppi Al foam core samples 1.32 and 1.42g/cm2. The 10ppi Al foam core samples were clearly perforated in our tests with severe damage on the rear facesheets. The 45ppi Al foam core sample was also perforated, but

only slightly above the failure threshold. The different results on the Al foam core samples in our tests compared to the ones published in [1] indicate a reduced performance for our samples in the tested velocity range. A reason may be the base material to produce the foams. In our case it was a relatively weak Al99 % whilst in [1] it was a 6101 aluminum alloy.

The results also show that the hybrid metal foams have an increased protective performance compared with the foams of pure aluminum, at least with regard to the thickness of the target structure. However, the nickel coated aluminum foam has a higher density which means, that its thickness must be reduced in order to obtain an equivalent mass per protected surface. The areal density of the pure aluminum foams necessary to protect against the projectile was not determined. Therefore, the protective capabilities of the pure aluminum foams and the hybrid metal foams taking the areal densities into account cannot be compared here directly. Still, regarding the masses one can say that in the case of the 45 ppi samples, which were not perforated with the hybrid metal foam cores, the areal density increased due to the nickel coating by a factor of approx. 3 for the three samples with the higher densities (shot numbers 2519, 2528 and 2535), resp. by a factor of 2 for the lighter sample (shot number 2530). Reducing the thicknesses of these samples by a factor of 1/3 resp. 1/2 would have resulted in a clear perforation of the target, indicated by the crater depth in the foam core. So with respect to the areal density the nickel coating did not improve the ballistic performance of the foam cores, as the coating thickness is way too thick and therefore adds too much mass to the target. In order to optimize the influence of the coating on the ballistic performance with respect to the areal density a thin coating should be used or even different coating materials as e.g. copper should be tested.

Regarding the pore size one could conclude, that the protective performance of the tested metal foams increases with an increasing pore density, i.e. with a decreasing pore size. Still, in our case, the densities of the foams also increases with decreasing pore size, for the pure aluminum foams and even more for the hybrid metal foams.

In [1] it was observed that the increase in the protective performance with decreasing pore size is only minimal. An interesting result is therefore provided by shot number 2530, a 45ppi Al/Ni sample with a relatively low density compared to the other samples of the same type. The areal density of the target in this test was 2.77g/cm2, the result was no perforation, slightly below the failure threshold. Two of the tested 10ppi Al/Ni samples have slightly lower densities with 2.19 and 2.23g/cm2, but the result in this case was significant perforation. This indicates an influence of the pore density for the hybrid metal foams, even though there are still some uncertainties due to the scatter in the foam density.

4. Conclusions

Hybrid nickel coated Al-foams were tested in order to optimize the ballistic performance of metallic foams as core layers in sandwich panels in the hypervelocity regime. They were compared with non-coated (pure) Al-foam panels. The crater depths in the hybrid sandwich panels are smaller than that in pure aluminum foam panels. Still, scatter in the coating thickness leads to variations in the foam densities of the hybrid foams, making the evaluation of the increase in the protective performance difficult.

Nevertheless, due to the nickel coating the influence of the pore density seems to be more significant than observed before [1]. By reducing the coating thickness and using high performance aluminum alloys [1, 3] as base material for the hybrid foams, further increase of the protective performance could be reached. Also could other coating materials as e.g. copper be tested. Additionally the complete evaluation of the ballistic limit over a broad velocity regime should be considered to see the variations in the performance of the hybrid foams over the whole velocity range being of interest for MMOD shielding technologies.

Acknowledgements

The ballistic tests were performed at the French-German Research Institute of Saint-Louis by the group Protection-Perforation-Materials; here special thanks to Mr. Thomas Wolf and Mr. Serge Gaisser.

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Protective Performance of Hybrid Metal Foams as MMOD Shields Academic research paper on "Materials engineering"

Abstract of research paper on Materials engineering, author of scientific article — Andreas Klavzar, Maxime Chiroli, Anne Jung, Bernhard Reck

Similar topics of scientific paper in Materials engineering , author of scholarly article — Andreas Klavzar, Maxime Chiroli, Anne Jung, Bernhard Reck

Academic research paper on topic "Protective Performance of Hybrid Metal Foams as MMOD Shields"