Advanced Materials Research Lab|College of Engineering and Applied Science

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Carl Frick, Ph.D.
University of Wyoming
Department of
Mechanical Engineering
Dept. 3295
1000 E. University Ave.
Laramie, WY 82071
Phone: (307)766-4068
Email: cfrick@uwyo.edu
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Research

Current projects primarily focus on linking mechanical behavior of materials with the underlying microstructure. Specific investigations include:


Ductility Mechanisms in B2 Intermetallics
Nickel-Titanium (NiTi) Shape-Memory Surfaces
Porous Spinal Fusion Cages
Structured Shape-Memory Polymer Anti-fouling Surfaces
Shape-Memory Polymers with High Strength

Ductility Mechanisms in B2 Intermetallics

Over the last several decades, a substantial amount of research has been performed on B2 intermetallics, in large part because of their relatively large stiffness & strength which remains stable at elevated temperatures. They have been proposed for a wide range of high-temperature applications including turbine blades, thermal coatings, and converging-diverging nozzles. Unfortunately, very few of these applications have been realized because of the extremely brittle nature of most high strength B2 intermetallics. Recently, an emerging group of newly developed B2 intermetallics (e.g. CoTi, CoZr, and a large family of rare earth elements) exhibit relatively large ductility. To date, no widely-accepted theory has been developed to explain their behavior. The purpose of this proposed research is to investigate the microstructural mechanisms responsible for the ductility in B2 intermetallics. Instead of relying on involved procedures to fabricate bulk single crystal specimens, this approach utilizes a focused ion beam (FIB) to manufacture small-scale compression pillars on the surface of a more conventional polycrystal. Pillars can be cut from individual grains, allowing for over a hundred “single crystal” compression tests from one polycrystalline sample surface. Compressive testing is performed using a nanoindenter equipped with a flat punch. Representative pillars will be sectioned and transmission electron microscopy (TEM) will be used for direct investigation of the microstructure. Results will shed insight into the microstructural dislocation mechanisms which dictate this enhanced ductility in select B2 intermetallics, and further open the possibility of developing new B2 intermetallics with optimized mechanical properties. For example, shown below is an in situ compression test of β-brass which illustrates a kinking of the pillar. This represents a relatively unique behavior not observed in bulk β-brass. TEM shows that dislocations accumulate at the boundary where the pillar forms a kink in its shape.


Compression of a brass micro-pillar imaged in situ via scanning electron microscopy (SEM).
(Click image above to download full resolution video)

Nickel-Titanium (NiTi) Shape-Memory Surfaces

NiTi shape-memory alloys are capable of undergoing a reversible thermo-elastic martensitic solid-state phase transformation. In essence, NiTi experiences relatively large amounts of inelastic deformation and subsequently recovers the deformation after load removal or upon the application of heat, due to a rearranging of its atomic lattice structure. It is therefore possible to accomplish a relative shape change that is entirely reversible via this stress-induced martensitic phase transformation. In addition, through an appropriate combination of deformation processing and/or heat treatment, the shape recovery may be “trained” to elicit a two-way shape-memory effect (TWSME). This effect is characterized by the alloy’s memorization of both a high and low temperature shape, allowing for spontaneous change between the two shapes as a function of cycling temperature. The figure below depicts room temperature topographical measurements of a TWSME surface, with a prescribed roughness upon heating, and relatively smooth surface after cooling. This effect is stable over many cycles, resulting in a robust, thermally-induced, switchable topography. Use of shape-memory alloys as high performance materials has attracted a great deal of interest from the biomedical and aerospace communities, among others, because of their unique thermo-mechanical and structural properties. Unfortunately, the use of the TWSME in applications is greatly under-utilized. This is due to the relatively complex microstructural interactions which govern TWSME behavior. The objective of our research is to establish a fundamental understanding of the relationship between underlying deformed microstructure and the martensitic phase transformation in shape-memory alloys. Using the insights gained, we will build upon this, opening the possibility of switchable surface structures which may be used for their adhesive, tribological, or reflective properties.


Images illustrating indentation-induced TWSME surface. Laser interferometer measurements performed at room temperature (a) after heating above 100°C and (b) after cooling with liquid nitrogen. Surface created by spherical indentation followed by planarization.

Porous Spinal Fusion Cages

Primary care physicians report that back pain is the second most predominant complaint next to the common cold. To alleviate pain associated with chronic low-back pain, spinal fusions are a commonly performed procedure. Approximately 440,000 spinal interbody fusions are performed annually, with an average sales price ranging from $2,250 to $4,700, resulting in a $1 billion medical device market. Most typically, interbody fusion techniques involve combining a stabilization rod with an interbody fusion cage meant to maintain the spacing between vertebral endplates. Unfortunately, it is estimated that these procedures have an average complication rate of 36.4%. Existing designs of cages do not evenly distribute forces across the vertebral endplates which can lead to micro-fractures and subsidence (i.e. loss of disc height). Subsidence combined with the supra-physiological stiffness values of the fused constructs can contribute to adjacent-level disease (i.e. the next level of the spine will need to be fused). We believe that a porous interbody fusion cage would reduce complications and improve clinical outcomes by (1) spatially tailoring the modulus of the implant to the endplate, (2) more evenly distributing stresses across the entire endplate, (3) lowering the overall construct stiffness value, and (4) better allowing for osteointegration of bone into the implant. As an initial approach, we are in the process of developing and characterizing porous poly(para-phenylene) (PPP) constructs. PPP represents an emerging class of thermoplastic polymer that offers great potential as a load-bearing biomaterial. Due to its high strength and stiffness, which approach an order of magnitude greater than traditional biomedical grade polymers, PPP can be specifically tailored to match the modulus of trabecular bone while optimizing the porosity necessary for successful osteointegration. Our research has been the first to develop a hot-press powder sintering technique for manufacture of tensile and compression samples, and to characterize the basic mechanical behavior of PPP over a large range of porosity and pore size, and also characterize the fatigue behavior of both porous and bulk PPP. Future research will characterize the biocompatibility of porous PPP in the form of cellular interaction studies, subcutaneous implant studies, and the study of segmental bone defect repair models in live rats. The capstone to this research will be the development of a novel lumbar interbody fusion cage made from porous PPP.

X-ray micro-computed tomography image of 75 vol.% porous PPP scaffold with an enlarged cutout view.

Structured Shape-Memory Polymer Anti-fouling Surfaces

Interest in, and the use of, membrane distillation for desalination applications is growing in areas like Wyoming that are grappling with dwindling freshwater supplies and the large volumes of saline water that are generated from the development of our energy resources. Realizing the full potential of membrane distillation hinges on the development of new membrane materials that are tailored for the unique requirements of this process. The overarching goal of this research is the synthesis, characterization, and testing of a new membrane surface coating whose properties can be changed in response to environmental stimuli. The objective is to create a structured surface capable of switchable hydrophobicity for improving the performance of membrane distillation processes in order to make it viable for desalination applications. It is our central premise that a biologically inspired micro-patterned surface manufactured through conventional photolithography techniques using shape-memory polymers, can create a highly hydrophobic surface when erect, while demonstrating dramatically less hydrophobicity when in a relaxed state, as a result of the relationship between surface roughness and hydrophobicity. Such a surface would facilitate easier cleaning of the membrane by backwashing, while maximizing the separation efficiency and permeate flux rate through the membrane. Because the shape-memory effect in polymers relies on polymer structure and processing, mechanical properties can be tailored through variation of the molecular constituents. Based on previous testing, which illustrates the feasibility of (meth)acrylate networks as shape-memory polymers, we have developed such a material with tailored material properties. For this application it is critical that the polymer systems must exhibit good shape-memory properties targeted for an onset temperature of approximately 30-40 °C under submerged conditions, appropriate high & low temperature mechanical properties including strain-to-failure, and the ability to be photopolymerized into a structured surface. For an initial approach we created micron-sized cylindrical vertical pillars. Fouling testing will provide insight into the effectiveness of the proposed method and will also help provide insight as how to optimize the structured surfaces size and geometry.

Contact angle representations a structured shape memory surface displaying the hydrophobicity under (a) normal operating conditions, (b) deformed state, (c) and its return to the normal operating conditions.


Shape-Memory Polymers with High Strength

Shape-memory polymers have the ability to recover to a pre-determined shape after mechanical deformation. Most typically, the polymer is heated above a critical temperature, such as the glass transition temperature and mechanically deformed into a temporary shape, a process known as shape storage. The polymer will remain in the stored shape until it is reheated in the vicinity of this critical temperature, upon which it will experience shape recovery. Shape-memory polymers have been proposed for a myriad of actuator and sensor applications due to their ability to recover a pre-defined shape upon heating; however, utilization of shape-memory polymers in commercial applications remains extremely limited. In comparison to alternative smart materials and actuators, they are relatively weak due to their low modulus, strength, and toughness. These drawbacks are unfortunately inherent to most polymer materials due to their carbon backbone molecular structure. Alternatively, poly(para-phenylene) (PPP) consists of directly linked repeating phenyl units (benzene rings) as their backbone. The direct linkage of repeating benzene rings confers exceptional mechanical strength to the polymer by providing a strong anti-rotational aryl-aryl bond. Furthermore, the steric hindrance caused by the –R groups along the background help limit chain mobility. Other polymer systems composed primarily of phenyl units such as polyphenylene sulfide (PPS) or polyetherether ketone (PEEK) are considered the gold standard for applications where high strength, or stability at elevated temperatures is required. However these systems have inferior strength/stiffness values relative to PPP. To date, no polymer networks composed of phenyl units have been suggested for use as shape-memory polymers. Preliminary research has indicated that PPP may have excellent shape-memory properties despite being an amorphous thermoplastic. As PPP is a relatively new material, its mechanical properties have yet to be thoroughly characterized. The focus of this study will be to firmly establish the mechanical properties of this material, and to draw a fundamental understanding on the effect of molecular structure on shape-memory behavior.

Shape-memory cycles for poly(para-phenylene) illustrating above 90% shape recovery.

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