**Jamie Cook, Steven Rhyans, Lou Roncase, Garth Hobson and Claudia C. Luhrs**

**Abstract:** This manuscript summarizes the failure mechanisms found in inorganic fullerene-type tungsten disulfide (IF-WS2) nanoparticles treated with diverse pressure loading methods. The approaches utilized to induce failure included: the use of an ultrasonic horn, the buildup of high pressures inside a shock tube which created a shock wave that propagated and impinged in the sample, and impact with military rounds. After treatment, samples were characterized using electron microscopy, powder X-ray diffraction, energy dispersive X-ray spectroscopy, and surface area analysis. The microstructural changes observed in the IF-WS2 particulates as a consequence of the treatments could be categorized in two distinct fracture modes. The most commonly observed was the formation of a crack at the particles surface followed by a phase transformation from the 3D cage-like structures into the 2D layered polymorphs, with subsequent agglomeration of the plate-like sheets to produce larger particle sizes. The secondary mechanism identified was the incipient delamination of IF-WS2. We encountered evidence that the IF-WS2 structure collapse initiated in all cases at the edges and vertices of the polyhedral particles, which acted as stress concentrators, independent of the load application mode or its duration.

Reprinted from *Inorganics*. Cite as: Cook, J.; Rhyans, S.; Roncase, L.; Hobson, G.; Luhrs, C.C. Microstructural Study of IF-WS2 Failure Modes. *Inorganics* **2014**, *2*, 377–395.

#### **1. Introduction**

The first inorganic fullerene-like nanoparticles of WS2 were discovered by Tenne *et al.*, working at the Weizmann Institute of Science in 1992 [1]. Using the diffusion-controlled sulfurization of metal oxides, they were able to empirically prove the existence of inorganic compounds based on WS2 with structures that were believed to only exist for carbon-based materials. Shortly thereafter, the discovery of WS2 nanotubes and fullerene-like structures led to the establishment of a new field of inorganic chemistry; one dealing with closed-hollow nanomaterials [2–5].

Originally, carbon fullerenes, made of concentric layers of carbon, were thought to provide outstanding tribological properties, and under low loads and high velocities they do [6,7]. However, due to their phase transition from graphite to diamond at high temperatures and pressures, they cannot entirely live up to the expectations that their structures suggest [8]. IF-WS2 particles have numerous properties similar to the carbon fullerenes, making them excellent solid lubricants; their extremely small size (in the nanometer scale) gives them the ability to fill imperfections in the lubricated material to effectively smooth the surface and prevent degradation. Due to their spherical shape, IFs are also said to act like nano-ball bearings, feature that allows them to roll rather than slide, performing better than other solid lubricants [4].

Currently, IF-WS2 particulates are recognized for their potential not only as lubricants but also as structural nanocomposites and shock absorbers [9–15].

The impressive shock absorbing performance of IF-WS2 and IF-MoS2 structures were first reported by Zhu *et al.* [16,17]. Using uniaxial shockwave pressures up to 30 GPa and studying the microstructural features of samples treated at diverse pressure settings, it was found that these cage-like particulates have superior performance than their carbon counterparts. Those reports also identified a lattice reduction for the samples treated at the higher pressures and provided a glimpse of the material breakage mechanisms. Moreover, those journal articles introduced the concept that smaller, more spherical IF particulates might be less prone to sustain damage than larger particles, a key feature to have in mind when designing highly resistant nanoparticles.

By studying the structural characteristics of a material that has fractured one can understand how materials fail and then make changes to the design and prevent, to a certain extent, the encountered failure modes. Fracture mechanics principles allow us to predict maximum working stress for a given material, establish relationships between materials properties, stress levels, crack producing flaws and conditions for the cracks propagation that ultimately will result in catastrophic breakdown of structures.

Here, we induced the breakage of IF-WS2 particulate structures employing three different setups: (i) the use of an ultrasonic horn operated at diverse amplitudes and periods of time with the sample immersed in a solvent; (ii) pressure waves created inside a shock tube; and (iii) impact with military rounds. The ultrasonic horn was used at diverse amplitudes and treatment times and produced cyclic loading conditions similar to the ones used for fatigue studies of macroscopic objects. The shock tube and the military rounds produced a single uniaxial impact event over only fractions of a second.

The objective of the experiments was to study the microstructural characteristics of the materials postmortem and based on those observations identify failure modes—fracture mechanisms. Then establish relationships between those and the type and duration of each treatment method employed, identifying the most important variables for the particulates failure.

Our results demonstrated that the effects observed when applying pressure loads to the IF-WS2 nanometric particles are a particular case of the more general principles established by fracture mechanics to explain macroscopic crack propagation: stress concentrators at the particle surface play a much larger role in the material failure than the directionality or duration of methods used to apply the loads.
