The influence of metallic particle size on the mechanical properties of polytetraflouroethylene-Al–W powder composites

Abstract

The dynamic mechanical properties of reactive materials (e.g., high density mixtures of polytetraflouroethylene (PTFE), aluminum (Al) and tungsten (W) powders) can be tailored by changing the morphology of the particles and porosity. Cold isostatically pressed PTFE-Al-W powder composites with fine metallic particles and a higher porosity exhibited higher ultimate compressive strength than less porous composites having equivalent mass ratios with coarse W particles. The mesoscale force chains between the fine metallic particles are responsible for this unusual phenomenon. We observed macrocracks below the critical failure strain for the matrix and a competition between densification and fracture in some porous samples in dynamic tests. 2Mixtures containing polytetrafluoroethylene (PTFE) and aluminum (Al) are known to be energetic under dynamic and/or thermal loading [1][2][3][4][5][6] . They are similar in composition to thermites 7 , a subgroup of the class of pyrotechnics, and are formulated to generate a large quantity of heat during the reaction driven by mechanical deformation in the bulk material. This paper describes the mechanical behavior of the PTFE-Al-W composites with varying W particle size and porosity. Varying cold isostatic pressing conditions introduces different porosities and component configurations within the samples. The quasi-static and dynamic ultimate compressive strength of the composites were measured to identify influence of their mesostructure on mechanical properties and fracture. The unusual phenomenon of increased strength of porous composites with reducing size of metallic particles was observed. A two-dimensional Eulerian hydrocode is used to numerically model the composite systems to explain experimental phenomenon.Tailoring the mechanical and chemical properties of reactive materials is important for various applications. For example, varying particle size and morphology in pressed explosives (HNS) 8 or layer thicknesses in laminate 9 can be used for tailoring of shocksensitivity and the rate of energy release. The stress/force chain formation in granular energetic materials can be related to ignition sites within composite energetic materials under a compressive load 10 . Bardenhagen, Brackbill 11 and Roessig, Foster, and Bardenhagen 12 examined the localized stress propagation due to force chains and effects of binder in two dimensional particle bed under static and dynamic loading.Cold Isostatic Pressing (CIPing) was used to prepare specimens from a mixture of 17.5 wt% PTFE, 5.5 wt% Al, and 77 wt% W powders with different porosities. The initial powders had the following average sizes: Al: 2 μm (Valimet H-2); coarse W powder: < 44 μm (Teledyne, -325 mesh) and fine W powder with particle sizes < 1μm 3 (Alfa-Aesar); PTFE: 100 nm (DuPont, PTFE 9002-84-0, type MP 1500J). The mixed powders were ball milled in the SPEX 800 mill for 2-10 minutes using alumina balls with a 1:5 mass ratio of balls to powder to break down the agglomeration of powders. Table I shows the density of vari...

Role of material properties and mesostructure on dynamic deformation and shear instability in Al-W granular composites

“…This behavior is similar to observations in the PTFE-Al-W mixtures and in the experiments with the Al-W powder mixture. 11,12,[16][17][18] The global shear localized zones and the subsequent cracks developed at an approximate 45 degree angle but was kinked by the rigid W particles resulting in the initial shear band forming in a range of angles from 36 to 50 degrees (see Fig. 9).…”

“…Multimaterial Eulerian hydrocode simulations of the dynamic tests for the various types of samples were used to elucidate the observed experimental results. 11,16,18 The complexity of the dynamic behavior of granular/porous materials is caused by the significant role of the mesoscale parameters including the particle sizes of components, the formation of force chains, the morphology of particles, and the bonding between particles. In these materials, shear localization and fracture can be delayed by strain hardening mechanisms such as compaction which results in porosity reduction.…”

“…6 Additionally, mesoscale features like force-chains in granular energetic materials may also serve as ignition sites. [7][8][9] Quasi-static, Hopkinson bar, and drop-weight experiments were performed for PTFE-Al-W and for Al-W composites [10][11][12][13][14][15][16][17][18] with different particle sizes of W, porosity, and morphology. W particles were used to increase sample density and generate the desirable mode of disintegration of the sample into small sized debris.…”

Mechanics of particulate composites with glassy polymer binders in compression

“…These studies have shown that particle size [15,19,21,22], shape [23] and concentration [24] and properties of the constituents can affect mechanical properties. In Al 2 O 3 particle-filled epoxy (Epon 828/Z), increasing the particle concentration and decreasing the particle size is found to increase the stress corresponding to 4% plastic strain [25].…”

“…Increasing volume fraction of particulates in the composite resulted in decreased strength. particle size, loading fractions, particle type and the adhesion between the particulate and the matrix [15,[17][18][19][20].…”