Advanced Nanoindentation Testing for Studying Strain-Rate Sensitivity and Activation Volume

Recent Advances in Nanomechanical Measurements and Their Application for Pharmaceutical Crystals

Mechanical behavior of pharmaceutical crystals directly impacts the formulation development and manufacturing of drug products. The understanding of crystal structure-mechanical behavior of pharmaceutical and molecular crystals has recently gained substantial attention among pharmaceutical and materials scientists with the advent of advanced nanomechanical testing instruments like nanoindentation. For the past few decades, instrumented nanoindentation was a popular technique for measuring the mechanical properties of thin films and small-length scale materials. More recently it is being implemented to investigate the mechanical properties of pharmaceutical crystals. Integration of correlative microscopy techniques and environmental control opened the door for advanced structure-property correlation under processing conditions. Preventing the degradation of active pharmaceutical ingredients from external factors such as humidity, temperature, or pressure is important during processing. This review deals with the recent developments in the synchronized nanomechanical measurements of pharmaceutical crystals toward the fast and effective development of high-quality pharmaceutical drug products. This review also summarizes some recent reports to intensify how one can design and control the nanomechanical properties of pharmaceutical solids. Measurement challenges and the scope for studying nanomechanical properties of pharmaceutical crystals using nanoindentation as a function of crystal structure and in turn to develop fundamental knowledge in the structure-property relationship with the implications for drug manufacturing and development are discussed in this review. This review further highlights recently developed capabilities in nanoindentation, for example, variable temperature nanoindentation testing, in situ imaging of the indented volume, and nanoindentation coupled Raman spectroscopy that can offer new quantitative details on nanomechanical behavior of crystals and will play a decisive role in the development of coherent theories for nanomechanical study of pharmaceutical crystal.

Nano-Mechanical Behavior of Ti6Al4V Alloy Manufactured Using Laser Powder Bed Fusion

The microstructure of Ti6Al4V alloy, manufactured using laser powder bed fusion (L-PBF), is affected by process parameters and heat treatment. However, their influence on the nano-mechanical behavior of this widely applicable alloy is still unknown and scarcely reported. This study aims to investigate the influence of the frequently used annealing heat treatment on mechanical properties, strain-rate sensitivity, and creep behavior of L-PBF Ti6Al4V alloy. Furthermore, the influence of different utilized L-PBF laser power-scanning speed combinations on mechanical properties of annealed specimens has been studied as well. It has been found that the effect of high laser power remains present in the microstructure even after annealing, resulting in increase in nano-hardness. Moreover, the linear relation between the Young's modulus and the nano-hardness after annealing has been established. Thorough creep analysis revealed dislocation motion as a dominant deformation mechanism, both for as-built and annealed conditions of the specimens. Although annealing heat treatment is beneficial and widely recommended, it reduces the creep resistance of Ti6Al4V alloy manufactured using L-PBF. The results presented within this research article contribute to the L-PBF process parameter selection, as well as to understanding the creep behavior of these novel and widely applicable materials.

Phase transition and nanomechanical properties of refractory high-entropy alloy thin films: effects of co-sputtering Mo and W on a TiZrHfNbTa system

Refractory high-entropy alloys (RHEAs) that consist of multiple principal refractory elements have attracted significant attention due to their many interesting and useful properties for structural applications. However, so far, a vast majority of reports on RHEAs focused on a few well-known compositions such as NbMoTaW, NbMoTaWV, and TiZrHfNbTa. The discovery of new RHEAs with enhanced mechanical properties has been highly desirable. Here we produce two new RHEA thin films - TiZrHfNbTaMo and TiZrHfNbTaW, by co-sputtering Mo or W on a previously studied TiZrHfNbTa RHEA system. The TiZrHfNbTaMo and TiZrHfNbTaW thin films exhibit an amorphous state, while the TiZrHfNbTa one shows a nanocrystalline structure. Using the nanoindentation method, we show that the addition of Mo or W in the TiZrHfNbTa during the co-sputtering process increases the hardness while resulting in comparable elastic moduli. Through the strain rate sensitivity tests of the thin films, we obtain their activation volumes and discuss their deformation mechanisms in the nanoindentation tests.

Nanomechanical testing in drug delivery: Theory, applications, and emerging trends

Mechanical properties play a central role in drug formulation development and manufacturing. Traditional characterization of mechanical properties of pharmaceutical solids relied mainly on large compacts, instead of individual particles. Modern nanomechanical testing instruments enable quantification of mechanical properties from the single crystal/particle level to the finished tablet. Although widely used in characterizing inorganic materials for decades, nanomechanical testing has been relatively less employed to characterize pharmaceutical materials. This review focuses on the applications of existing and emerging nanomechanical testing methods in characterizing mechanical properties of pharmaceutical solids to facilitate fast and cost-effective development of high quality drug products. Testing of pharmaceutical materials using nanomechanical techniques holds potential to develop fundamental knowledge in the structure-property relationships of molecular solids, with implications for solid form selection, milling, formulation design, and manufacturing. We also systematically discuss pitfalls and useful tips during sample preparation and testing for reliable measurements from nanomechanical testing.

The Strain Rate Sensitivity and Creep Behavior for the Tripler Plane of Potassium Dihydrogen Phosphate Crystal by Nanoindentation

As an excellent multifunctional single crystal, potassium dihydrogen phosphate (KDP) is a well-known, difficult-to-process material for its soft-brittle and deliquescent nature. The surface mechanical properties are critical to the machining process; however, the characteristics of deformation behavior for KDP crystals have not been well studied. In this work, the strain rate effect on hardness was investigated on the mechanically polished tripler plane of a KDP crystal relying on nanoindentation technology. By increasing the strain rate from 0.001 to 0.1 s⁻¹, hardness increased from 1.67 to 2.07 GPa. Hence, the strain rate sensitivity was determined as 0.053, and the activation volume of dislocation nucleation was 169 ų. Based on the constant load-holding method, creep deformation was studied at various holding depths at room temperature. Under the spherical tip, creep deformation could be greatly enhanced with increasing holding depth, which was mainly due to the enlarged holding strain. Under the self-similar Berkovich indenter, creep strain could be reduced at a deeper location. Such an indentation size effect on creep deformation was firstly reported for KDP crystals. The strain rate sensitivity of the steady-state creep flow was estimated, and the creep mechanism was qualitatively discussed.

Extended Applications of the Depth-Sensing Indentation Method

The depth-sensing indentation method has been applied for almost 30 years. In this review, a survey of several extended applications developed during the last three decades is provided. In depth-sensing indentation measurements, the load and penetration depth data are detected as a function of time, in most cases at controlled loading rates. Therefore, beside the determination of hardness and Young's modulus, different deformation mechanisms and many other dynamic characteristics and phenomena, such as the dynamic elastic modulus, load-induced phase transition, strain rate sensitivity, etc. can be studied. These extended applications of depth-sensing indentation measurements are briefly described and reviewed.

On the Size Effect of Strain Rate Sensitivity and Activation Volume for Face-Centered Cubic Materials: A Scaling Law

In a recent experimental study of indentation creep, the strain rate sensitivity (SRS) and activation volume v* have been noticed to be dependent on the indentation depth or loading force for face-centered cubic materials. Although several possible interpretations have been proposed, the fundamental mechanism is still not well addressed. In this work, a scaling law is proposed for the indentation depth or loading force-dependent SRS. Moreover, v* is indicated to scale with hardness H by the relation ∂ln(v*/b3)/∂lnH=−2 with the Burgers vector b. We show that this size effect of SRS and activation volume can mainly be ascribed to the evolution of geometrically necessary dislocations during the creep process. By comparing the theoretical results with different sets of reported experimental data, the proposed law is verified and a good agreement is achieved.

Testing Effects on Shear Transformation Zone Size of Metallic Glassy Films Under Nanoindentation

Room-temperature creep tests are performed at the plastic regions of two different metallic glassy films under Berkovich nanoindetation. Relying on the strain rate sensitivity of the steady-state creep curve, shear transformation zone (STZ) size is estimated based on the cooperative shear model (CSM). By applying various indentation depths, loading rates, and holding times, the testing effects on the STZ size of metallic glasses are systematically studied. Experimental results indicate that STZ size is greatly increased with increased indentation depth and shortened holding time. Meanwhile, STZ size is weakly dependent on the loading history. Both the intrinsic and extrinsic reasons are discussed, to reveal the testing effects on the nanoindentation creep flow and STZ size.

Non-Newtonian Flow to the Theoretical Strength of Glasses via Impact Nanoindentation at Room Temperature

In many daily applications glasses are indispensable and novel applications demanding improved strength and crack resistance are appearing continuously. Up to now, the fundamental mechanical processes in glasses subjected to high strain rates at room temperature are largely unknown and thus guidelines for one of the major failure conditions of glass components are non-existent. Here, we elucidate this important regime for the first time using glasses ranging from a dense metallic glass to open fused silica by impact as well as quasi-static nanoindentation. We show that towards high strain rates, shear deformation becomes the dominant mechanism in all glasses accompanied by Non-Newtonian behaviour evident in a drop of viscosity with increasing rate covering eight orders of magnitude. All glasses converge to the same limit stress determined by the theoretical hardness, thus giving the first experimental and quantitative evidence that Non-Newtonian shear flow occurs at the theoretical strength at room temperature.