Magnetron sputtering of nanolaminates with materials designed and engineered properties

Nanolaminates with individual layer thickness below 100 nm have been widely investigated because of their outstanding mechanical properties, which arise due to the existence of high-density interfaces and nanoscale grains. The nanolaminates exhibited 1/3 to 1/2 strength of the theoretical strength or even reach to the theoretical strength at a few nanometers individual layer thickness. Besides layer thickness, constituents, type of interfaces, grains, nanotwins and microstructural defects are significantly important in determining the strength of nanolaminates. Though nanolaminates show high strength with decreasing the individual layer thickness, often it possesses low ductility. So, studies have been focused on the nanograin structures of the constituents and their work hardening capabilities at this length-scale to avoid the softening effect at larger strain. Particularly, investigation at the length-scale of a few nanometers is given priority because of the anomalous strengthening effect that mostly related to the microstructural variations at this length-scale. Apart from strengthening of nanolaminates, it is essentially needed to investigate the deformation behavior with a wide range of individual layer thickness which relates to the plasticity at the interfaces and grain boundaries, dislocation pileup at the stress concentrated regions and nanotwin structures. It is generally believed that the cubic crystal structured (i.e., face-centered cubic (fcc) and body-centered cubic (bcc)) metals have more slip systems compared to the hexagonal closed-pack (hcp) metals. To date, research efforts on metallic nanolaminates are mainly focused on fcc/fcc and fcc/bcc crystal structured nanolaminates and the nanolaminates consisted of at least one hcp structured constituent received less attention. The presence of fewer slip systems in hcp crystal structured metals compared to cubic crystal structured metals may provide an opportunity to develop nanolaminates with enhanced strength and at present, research attention has turned to design and develop high-strength nanolaminates by taking hcp crystal structured metals as constituents; so that softening at a few nanometers length-scale can be eliminated. In addition to microstructural characterization and mechanical property evaluation, it is also important to investigate the tribological performances of nanolaminates to ensure their applications in various structures, actuators and micro/nano electromechanical systems (MEMS/NEMS), where friction and wear are involved. Chapter 1 describes the overview of nanolaminates and discuss various factors that control the property of nanolaminates. The designing and manufacturing of nanolaminates were highlighted in this chapter. The scope, research aims, objectives, significance and structures of this thesis are discussed in a brief. Later on in Chapter 2, an overview of high-strength nanolaminates, advantages and disadvantages of commonly used manufacturing techniques of nanolaminates, influencing factors in determining their mechanical and physical properties and corrosion resistance properties were discussed in detail. Designing concept of high-strength nanolaminates for future research directions were also highlighted based on the analysis of the up-to-date research efforts. In this thesis, considering the influencing factors in determining the strength of nanolaminates, two different nanolaminate systems were designed by carefully choosing the constituent elements. A physical vapor deposition method, i.e. magnetron sputtering was used to deposit nanolaminates in this research because of its good control over the layer composition and thickness, high quality and low impurity films, and higher deposition rate. Two different sources were alternately used to deposit alternate layers during nanolaminates fabrication. Due to the high impact velocity of ionized gas onto the target, atoms, molecules, or clusters of atoms are loosened and ejected from the target. These ejected particles hit the substrate from one direction, without losing their high energy in collisions. As a result, a thin layer of the target material is formed on the substrate. Thus, the deposition of the two different materials were altered many times to fabricate the nanolaminate samples. At first, fcc/fcc nickel/aluminum (Ni/Al) nanolaminates were designed and manufactured, where the constituent Al was the soft constituent between hard Ni layers. Fabrication, microstructural characterization and mechanical properties evaluation of Ni/Al nanolaminates were discussed in detail in Chapter 3 and Chapter 4. Magnetron sputtering was utilized to fabricate the Ni/Al nanolaminates because of its ease of operation and turnaround, and good control over the thickness. The microstructures of Ni/Al nanolaminates with varied individual layer thickness from 5 nm to 100 nm confirmed the existence of randomly oriented Ni (111) and Al (111) textures perpendicular to the growth direction. Surprisingly, nanotwin structures were evident in both thick and thin layered Ni/Al nanolaminates, which ultimately contributed to enhance its mechanical properties. With reducing the individual layer thickness from 100 nm to 5 nm, the microstructures were changed. Thick layered Ni/Al nanolaminates had larger grains with thick grain boundaries and the trenches between islands were appeared as preexisting cracks in the constituent layers; while the thin layered Ni/Al nanolaminates were composed of smaller grains with stress concentrated broken interfaces and valleys between the islands. The nanohardness reached to a peak of ~5.07 GPa with a decrease in individual layer thickness from 100 nm to 10 nm, and after that the nanohardness reached a plateau. The corresponding yield strength also reached a peak of ~1.87 GPa for 10 nm thick layered nanolaminate. Moreover, the elastic modulus reached a peak of ~134.65 GPa with reducing the individual layer thickness from 100 nm down to 20 nm; after that, the elastic modulus started to decrease monotonically with further reduction in individual layer thickness. The estimated specific strength (~300.75-327.24 GPa cm3/kg) of the 10 nm alternately layered Ni/Al nanolaminate is significantly greater than those of available steel (130 GPa cm3/kg), Al-6061 alloy (110 GPa cm3/kg) and Ti-6Al-4V (240 GPa cm3/kg). The classical Hall-Petch strengthening mechanism fits well from 100 nm down to 20 nm individual layer thickness, whereas the confined layer slip (CLS) strengthening mechanism applied below 20 nm and the strengthening mechanism became independent of the individual layer thickness when it decreased to 5 nm. The nanowear performance was tested for 10 nm and 50 nm alternately layered Ni/Al nanolaminates, where the wear resistance increased with a decrease in individual layer thickness. The wear rate of the 10 nm alternately layered Ni/Al nanolaminate was 3-3.5 times less than that of the 50 nm alternately layered Ni/Al nanolaminate. In Ni/Al nanolaminates, the work-hardening was observed on the worn surfaces of both nanolaminates during wear testing, and the subsurface hardening was more significant for the 10 nm alternately layered nanolaminate than 50 nm alternate layers despite its low plastic deformation. Moreover, the deformation behavior of Ni/Al nanolaminates were studied using micropillars under uniaxial compression tests that facilitated a homogeneous stress state and it is discussed in Chapter 5. The deformation of the Ni/Al micropillars at the individual layer thickness of 20-100 nm was determined by the competing effect between preferential thinning of the soft Al layers between the Ni layers and the nucleation and propagation of cracks at the pre-existing cracks and grain boundaries of the Ni layers. In contrast, at smaller individual layer thickness (≤ 10 nm), deformation of the Ni/Al micropillars occurred with plastic barreling due to layer shearing with a competing effect of the initiation and propagation of cracks at the stress-concentrated broken interfaces of the Ni and Al layers, and the valleys between the islands of constituents. The flow strength of the Ni/Al micropillars increased gradually with a decrease in individual layer thickness and reached the peak of ~2.35 GPa for 10 nm thick layer at 30% plastic strain. Strain hardening rate increased with a decrease in individual layer thickness from 100 nm down to 20 nm, afterwards it started decreasing and a significant strain softening was observed at 5 nm thick individual layered micropillar. Microstructures and mechanical properties evaluation and deformation behavior of Ni/Al micropillars with varying individual layer thickness confirmed the designing of lightweight and high-strength Ni/Al nanolaminate and to tailor its mechanical properties. In the next step, bcc/hcp tantalum/cobalt (Ta/Co) nanolaminates were designed and fabricated. Here hcp crystal structured Co was introduced as an alternating layer which offered fewer slip systems compared to the bcc crystal structured Ta. Fabrication, microstructural characterizations and mechanical properties evaluation were discussed in detail in Chapter 6 and Chapter 7.  Magnetron sputtering was utilized to deposit alternately layered Ta and Co layers. Columnar structured nanograins of Ta and Co layers were very smooth and pre-existing cracks/thick grain boundaries were absent in the stacked layers. The nanohardness of Ta/Co nanolaminates were monotonically increased with reducing the individual layer thickness from 100 nm to 5 nm and reached a peak of ~7.20 GPa and the corresponding maximum yield strength was ~2.67 GPa. The Co3Ta and Co2Ta intermetallic phases appeared irrespective of individual layer thickness and they may work as a strong barrier at the interfaces and the increased amorphous layers at h=5 nm may have hindered the spread of single dislocations across the amorphous layers and caused the yield strength to a maximum. The yield strength of the nanolaminates was compared with the estimated values based on classical strengthening mechanisms, where Hall-Petch strengthening mechanism fits very well from the individual layer thickness of 100 nm down to 25 nm. The CLS strengthening mechanism operated at 10 nm thick layer and the strengthening mechanism became independent of individual layer thickness when it decreased to 5 nm. The deformation behavior of Ta/Co micropillars with varying individual layer thickness from 5 nm to 100 nm were systematically investigated under a homogeneous stress state using micropillars under uniaxial compression testing. Three different deformation behaviors were observed in Ta/Co nanolaminates. Firstly, at the individual layer thickness of 5 nm, the Ta/Co micropillar showed homogeneous deformation with plastic barreling near to the top pillar surface. Deformation mechanism of Ta/Co micropillar at 5 nm individual layer thickness involved with the expansion of grain boundaries and interfacial sliding, and dislocations can be trapped at the increased amorphous/intermixed interface constraints. Secondly, with increasing individual layer thickness from 10 nm to 50 nm, the micropillar experienced a major shearing at higher strain. The grain boundary expansion and interfacial sliding may be limited prior to major shear. Finally, with the increase of individual layer thickness to 100 nm, the Ta/Co micropillars showed preferential thinning of Co layers with partial dislocations and extrusion of soft Co layers between the hard Ta layers during deformation. Moreover, the expansion of grain boundaries in Ta and Co layers and cracking of hard Ta layers were observed at the stress concentrated grain boundaries at larger strain. A competing effect of strengthening (CES) between the grain boundary and interface activated plasticity may play a crucial role for the improved strength at a few nanometers length-scale. The Ta/Co micropillar at 5 nm individual layer thickness exhibited a flow strength of ~2.54 GPa with minimal deformation and excellent ductility. The flow strength of Ta/Co micropillar at 5 nm length-scale is significantly higher than the yield strength of nanolaminates with at least one hcp constituent reported in the literature, to date. The exceptionally high flow strength of the newly developed Ta/Co micropillar at a few nanometers length-scale suggested a new design concept in nanolaminates¿ research, where the plasticity at the grain boundaries and amorphous/intermixed interfaces with nanograins could facilitate us to tailor its mechanical properties towards high-strength and excellent ductility. Chapter 8 reports the nanoindentation and nanotribological properties including nanoscratch and nanowear resistances of Ta/Co nanolaminates with varying individual layer thickness from 5 nm to 100 nm. The nanohardness of the nanolaminates was increased gradually with the decrease of individual layer thickness, while the elastic modulus also increased gradually with a sudden decrease at h=25 nm. The normal load at the critical point of delamination was increased gradually with a decrease in h under a ramped load during nanoscratch testing. The post-scratching microstructures after delamination showed clear film chipping/crack formation and propagation of cracks/breakdown of grains at larger h; while no film chipping/crack/breakdown of grains was observed in the nanolaminate with h=5 nm. The nanolaminate with h=5 nm showed the lowest wear rate with minimal plastic deformation during nanowear testing. Overall, the Ta/Co nanolaminates with individual layer thickness at a few nanometers length-scale exhibited high-strength and excellent scratch and wear resistances with minimal plastic deformation under nanoscratch and nanowear tests. The major findings of this thesis have been summarized and discussed in Chapter 9. Directions for future studies have also been explained and provided at the end of this chapter. In conclusion, both Ni/Al and Ta/Co nanolaminates exhibited ultrahigh strength with the plasticity at the grain boundaries/pre-existing valleys between the islands and interfaces played a crucial role in determining their strength. Moreover, the ultrahigh-strength and excellent tribological performances of Ni/Al and Ta/Co nanolaminates may allow their use in high performance protective coating applications.