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Processing, characterization and mechanical behavior of novel aluminum/silicon carbide metal-ceramic nanolaminates

Dissertation
Author: Danny Rao Pratap Singh
Abstract:
Nanoscale laminated composites are a novel class of materials with excellent mechanical properties like strength, flexibility, and toughness. Properties in these materials can be tailored by varying layer thicknesses, microstructure, and controlling internal stresses. Most of research to-date has focused on metal-metal and ceramic-ceramic laminates. The field of metal-ceramic laminates has remained relatively unexplored. From a mechanical structure point of view, metal-ceramic systems present a good combination of strength and hardness. Hence, there is a need for understanding and evaluating these composites in nanolaminate form. In this work, the deformation behavior of Al-SiC nanolaminates as a model system has been studied. The work has been categorized into 3 major areas of research. The first involves processing and microstructural characterization of these novel materials. Nanoscale Al/SiC layered composites were fabricated using magnetron sputtering. Samples with varying volume fractions of Al and SiC were synthesized. The second area involves quantifying the residual stresses in these materials. These are quantified by x-ray synchrotron and beam-curvature techniques. The third area focuses on understanding the fundamental mechanisms for deformation damage, and fracture under indentation and micro-compression loading. Fracture/Damage analysis is carried out using focused ion beam (FIB) and scanning electron microscopy (SEM). The systematic study has revealed the enhanced mechanical properties of metal- ceramic nanolaminates and the fundamental mechanisms governing the strength and failure of these materials. It has been found that Al/SiC metal-ceramic systems can exhibit high strength together with high toughness and flexibility. High compressive residual stresses are generated during the sputter deposition of these nanolaminates which are associated with the large number of interfaces present in the nanolaminate architecture. The mechanical properties (hardness and modulus) of such a system can be reasonable be measured using nanoindentation however the inhomogeneous stress state can lead to complicated and erroneous results. This has been resolved by using a micro-compression approach and various parameters affecting micro-compression testing are evaluated. Material properties of Al/SiC nanolaminates arc accurately determined using micro-compression testing approach.

TABLE OF CONTENTS CHAPTER Page 1. INTRODUCTION AND BACKGROUND 8 1.1 References 27 2 RESEARCH OBJECTIVES AND APPROACH 36 2.1 Processing and Micro-structure 37 2.2 Residual Stresses 37 2.3 Deformation Behavior 38 3 PROCESSING, MICROSTRUCTURE AND NANOINDENTATION BEHAVIOR OF AL/SIC NANOLAMINATES 39 3.1 Introduction 39 3.2 Experimental 40 3.3 Results and Discussion 42 3.4 Conclusions 63 3.5 References 65 4 DAMAGE EVOLUTION UNDER NANOINDENTATION LOADING IN AL/SIC NANOLAMINATES 67 4.1 Introduction 67 4.2 Experimental 68 4.3 Results and Discussion 70 4.4 Conclusion 84 4.5 References 86 VI

CHAPTER Page 5 RESIDUAL STRESSES IN AL/SIC NANOLAMINATES 89 5.1 Introduction 89 5.2 Experimental 93 5.3 Results and Discussion 94 5.4 Conclusions 108 5.5 References 110 6 MICRO-PILLAR COMPRESSION TESTING OF AL/SIC NANOLAMNATES 6.1 Introduction 114 6.2 Experimental 115 6.3 Results and Discussion 118 6.4 Conclusions 134 6.5 References 136 7 SCRATCH TESTING OF AL/SIC NANOLAMINATES 140 7.1 Introduction 140 7.2 Experimental 141 7.3 Results and Discussion 143 7.4 Conclusion 157 7.5 References 158 8 CONCLUDING REMARKS 161 8.1 Summary of Research Findings 161 8.2 Recommendations for Future Work 162 9 REFERENCES 163 vii

8 1. INTRODUCTION AND BACKGROUND Laminated composites have been used for several years. From common applications such as plywood and windshields of automobiles [1] to high end applications like the fu selage of aircrafts [2] and thermal barrier coatings, the microstructure of laminates spans multiple length scales. Laminated material systems offer significant advantages such as high strength and toughness [6]. Also, the materials can be tailored (by proper choice of constituent materials, their microstructure, and dimensions) to optimize properties for a given application. Recently, materials engineered on the nanoscale have also been shown to exhibit unique properties like high strength, toughness, fatigue resistance, and thermal resistance [7-9]. Hence, the laminated materials engineered at the nano length scale can be combined to produce materials (nanolaminates) with unique and much improved properties. These are capable of approaching near theoretical limits of strength [10]. The concept has been proved by nature as well with abalone and sea shells acquiring the na- nolaminate structure (ceramic-polymer nanolaminate), which provides high strength and fracture toughness [11-15]. Table 1 [10] shows some of the applications of nanolami nates.

9 Table 1: Applications of Nanolaminates [10] Ultra High Strength Materials High Performance Tribological Coatings Coatings For Gas Turbine Engines Advanced Coatings For Medical Applications High Performance Capacitors For Energy Storage Capacitor Structures For Programmatic And In dustrial Applications Integrated Circuit Interconnects Magneto-Optic Read/Write Memory Magnetic Transducers (GMR) EUV, Soft X-Ray And X-Ray Optics Spectroscopy Imaging And Microcircuit Lithography Develop ment Advanced Coatings For Medical Applications New Engineered Smart Materials For Sensors New Materials And New Devices Based On The Chemical & Structural Control Available With Engineered Multilayer Materials Basis For New Manufacturing Strategies Nanolaminates have been studied in various forms like metal-metal [16-24], ceramic- ceramic [25-30], and metal-ceramic [31-40] systems. To date, metal-metal systems seem to be the most widely studied and there is limited literature on the mechanical potential of metal-ceramic systems. Hence, there is a need to systematically understand the funda mental properties of these materials and understand their microstructure and deformation behavior.

10 Nanoscale laminates can be processed by a variety of techniques like chemical vapor deposition (CVD), physical vapor deposition (PVD), sol-gel synthesis, electroplat ing, etc. Sputter deposition (a form of PVD) is one of the most versatile and widely used fabrication techniques for nanoscale multilayers [19,20,26,28,32,35]. The wide accep tance of this technique stems from its ability to deposit a wide variety of materials (both conducting and non-conducting elements and alloys can be sputtered) together with good process control (required to control layer thickness on the nano-scale) and deposition rates (ability to manufacture thick laminates) [41]. We first review the literature on nanoscale metal-metal and ceramic-ceramic sys tem in terms of their mechanical properties and deformation behavior. We then review the behavior of metal-ceramic nanolayered system literature to give a foundation in un derstanding the Al/SiC nanolayered system. Metal-Metal nanolaminates have been grown with varying layer thicknesses from sub-nanometer range (Cu/Nb) to 100s of nanometers [42]. Misra et al. [43] reviewed the possible deformation mechanisms in this system. They found that these materials are capable of attaining ultra high strengths (close to their theoretical strength) and the deformation mechanisms responsible for these exceptionally high strengths are dependent on the layer thickness and grain size. It was found that the laminates follow the Hall-Petch kind of strengthening by dislocation pile-up for layer thicknesses of 100 nm and larger. At smaller thicknesses the mechanism changes since the grains are unable to support large pileups of dislocation.

11 Single dislocations models have been proposed, where deformation occurs by single dislocation bowing (Orowan bowing) or through transmission of the dislocation from one layer to the other against the interface barrier (when the elastic modulus differ ence between the two materials is large-Koehler image stresses [44]) Coherency stresses at the interface can also impede dislocation activity in the cases where coherent or semi- coherent interfaces are present [45]. The strengthening in this regime (small layer thick nesses) is sometimes explained by the Confined Layer Slip [42,46,47] model wherein, it's thought that the slip remains confined to a single layer and motion of a threading dis location occurs by increasing the interfacing dislocation length (Orowan Bowing). At even smaller grain sizes, the grain sixes is so small that confined layer slip is not possible and softening has been observed [42,43,46].A schematic for deformation mechanisms operable in nanolaminates is shown in Figure 1 [48]. In another study by Foecke et.al. [49], Cu-Ni nanolaminates were deformed in situ in a TEM. Early stages of deformation were characterized by CLS followed by an unstable crack due to fracture of interfaces and layers with a long plastic zone ahead of crack tip. In Ag/Cu nanolaminates it was also seen that the yield strength increases with decreasing layer thickness, however at ex tremely small layer thickness ductility is lost and brittle fracture is encountered [50]. Ceramic-Ceramic Nanolaminates usually used for high temperature application, have also been shown to have improved strength [51] and wear resistance [52]. The major systems studied under this category are the nitrides ( TiN/NbN [51], AIN/CrN [53], TiN/ZrN [25],

Interface Cross in ii3c.»r lacattzattort Uniform reduction to Issrge plastic strains M P i ^ Confined layer slip (l.MU'JMjllUllWlMllBWW ^ jg^rf, tens of nm, Sub-microns i to microns nn- T, — ' , Layer thickness deformation m t . „-, T" ,4 :?..-] Figure 1: Deformation Mechanisms in Metallic Nanolaminates [43] K>

13 Ti/VN [54], TiAlN/CrN[55]) grown by reactive sputtering. Other systems such as oxides (Al203/Ti02 [52], Y203/Zr203 [56], Al203/Zr203 [57]) and carbides [58] remain lesser explored. The increase in hardness in these materials is often associated with the im proved microstructure (negligible porosity), changes in crystal structure and residual stresses associated with growth mechanism [51,52]. Similar dislocation strengthening mechanisms as metallic multilayers (interface barrier to dislocation motion, image and coherency stresses) are also used to explain strengthening in these systems. In case of na- no-grained systems where one phase is amorphous, and amorphous/crystalline interfaces are abundant, deflection of nano cracks and dislocations can provide strengthening [52]. Carvalho et. al. [59] studied the deformation of TiN/(Ti-Al)N nanolaminates under inden tation loading. They found that deformation under indentation proceeded by 2 mechan isms: 1) deformation of the layers through grain rotations under low load conditions 2) brittle cracking initiating at the substrate-multilayer interface and channeling through the columnar grain boundaries due to shearing of adjacent columns. This mechanism pre vented any delamination along the substrate ML interface. One of the mechanisms responsible for strengthening in multilayer is the one proposed by Koehler [44] operating in multilayer system with the two materials having large differ ences in elastic moduli. The metal-ceramic systems usually satisfy this condition. Some early researchers realized the potential of this mechanism and measured high strength in some micron to submicron metal-ceramic multilayers [60,61]. In Ti/TiN nanolaminate system it was found that strength of the multilayer was a strong function of

14 the layer thickness and the system achieved highest hardness when the modulation length (thickness of a bilayer) was about 35-60 nm. Farhat [62] also found that AI/AI2O3 and Ti/TiN multilayers showed improved hardness and wear resistance. The hardness in creased as metallic layer thickness was decreased, and since the layer thicknesses were in the 100s of nanometer range, a Hall-Petch kind of relation of strength was observed. The wear resistance also improved as metal layer thickness was reduced. A1/A1N [63] were fabricated with bi-layer periods of 10s of nanometers. These composites showed excep tional strength and toughness characteristics compared to their monolithic counterparts. The increase in toughness was attributed to energy dissipation in the ductile phase and the fracture of interfaces [63,64] In another study of Ni/A3A1, deformation was seen through the CLS mechanism. It was also found that samples exhibited a ductile ((001) orientation) or a brittle ((111) orientation) mode of fracture depending on the grain orientation in the multilayer sample. Also, smaller layer thickness samples exhibited more ductility owing to a semi-coherent interface that allowed easier transmission of dislocations from one layer to other. Finally, also consider the case of multilayers of Cu/Cu-Zr glass developed by Wang et. al. [65]. Amorphous materials usually have negligible ductility and deforma tion occurs by shear transformation zones [66], hence in bulk form they can be consi dered brittle. However, in Cu/Cu-Zr system, significant increases in ductility were ob tained by use of a laminated architecture. They showed that at sufficiently small thick nesses amorphous materials can act as sinks for dislocations and are no longer affected by shear banding instability [66]. In an attempt to study damage

15 under indentation loading, Abadias et. al. cross-sectioned indented multilayers of Cu/TiN and found that the material deformed by extensive plastic deformation of Cu and bending (elastic or plastic- uncertain) of the ceramic TiN phase. Deformation by rotation of Cu grains was also seen. One of the challenges in studying the mechanical behavior of nanolaminates is that they are usually grown on a substrate in a thin film form. Nanoindentation has emerged as a widely used technique to measure mechanical properties of thin film mate rials [67-77]. This is largely due to the ability of the instrument to simultaneously meas ure extremely small loads (~|u.N) and displacements (~nm) during indentation. Nanoindentation has its origins rooted in traditional indentation (macro-micro) testing like Brinell hardness testing [78], wherein, a hard tip of known mechanical prop erties is pressed into a sample of unknown properties. The indentation is carried out to a specific load after which the indenter is retracted back and the area of the impression is measured using optical microscopy. Hardness (H) is then calculated from Peak Load (P) and Area (A) as follows: p H - — Equation 1 The major disadvantage of traditional indentation testing is that the area is meas ured optically after the indentation has been made. As the indentation size decreases, the errors in area measurement become larger. To overcome this limitation, depth sensing instrumentation was proposed by Doerner & Nix [79], wherein indentation impression area is determined continuously as a function of indenter penetration. This method not

16 only enables the measurement of area without the use of a microscope, but also much smaller areas can be measured. Also, since depth and load were now being measured si multaneously a complete history of deformation can be recorded, which enables the mea surement of other mechanical properties besides hardness, like elastic modulus [79], creep properties [80], fracture toughness [81], yield strength [82], strain hardening expo nent [83], stress-strain curves [84] and residual stresses. The ability to perform this load- displacement experiment on the nanometer scale constitutes nanoindentation. Principle: In nanoindentation, a form of depth-sensing indentation, an indenter of known geometry is pressed into a sample with precise control, monitoring and recording of load and displacement data in-situ during the test. A schematic (and image of commercial MTS nanoindenter) of the instrument used for nanoindentation are shown in Figure 2.The recorded load-displacement data for a typical nanoindentation test with a Berkovich indenter is shown in Figure 2. The instan taneous area during indentation for various indenter shapes used in nanoindentation is shown in Figure 3. Since area is known instantaneously at each depth, hardness can be monitored continuously as a function of indenter penetration.

Machine compliance Displacement ,h .To electronics To electronics Displacement sensor n TJ CO o - i P=Ch2X ^ S ^ P=k(h-h,^// 7_ dhL - • Displacement h m Figure 2: Schematic of a Nanoindentation Instrument with Typical Load-Displacement Data from a Nanoindentation Test - j

Berkovich a=65.3° Area=24.5d2 Vickers a=68° 24.56 d2 Conical c=70.3° 7t.(d.tan(c))2 Spherical NA 7i. (2.R.d-d2)05 Figure 3: Various Common Indenter Shapes and their Area Functions as a function of Inden tation Depth (d) {www.microstartech.com) 00

19 Also it is possible to measure the modulus of the sample by measuring the slope of the unloading curve at maximum displacement which is a measure of the stiffness (S) of the material at any given displacement (Eq. 2). dP\ S = dh Equation 2 The modulus of the material (E) can then be easily derived using Sneddon's contact equa tions [85] which yields the reduced elastic modulus (Er) according to Eq. 3 E = P=T Equation 3 r 2p42 — = 1 Equation 4 Er E E, The reduced elastic modulus is related to modulus of the sample (E) and the in- denter (Ej) as given in Eq. 4, where v is the poisson's ratio of the sample and Vj is the in- denter's poisson's ratio. As can be seen from the above analysis, elastic modulus is measured at peak displacement unlike hardness which can be monitored continuously as a function of depth. In 1992, Oliver and Pharr introduced the Continuous Stiffness Mea surement technique (CSM) [86] [87] wherein stiffness and hence the modulus could be measured continuously as a function of depth (Figure 4).

T3 ro o P=Posexp(icot) h(co)=h0exp(icot+(j)) Displacement Figure 4: Continuous Stiffness Measurement (CSM) Technique O

21 CSM employs a high frequency harmonic load oscillation superimposed on the loading signal. The resulting displacement phase and amplitude is measured and is direct ly related to the stiffness of the material at that depth according to Eq 5 and Eq. 6. S = 1 cos^-(A^s -mco2) •X? h(co) Equa tion 5 co.C h(to) sin(^) Equation 6 Ever since the introduction of CSM, nanoindentation has found increased and innovative applications in measuring properties at the small length scale. However, many challenges still remain like , Pile-Up and Sink-in effects during Nanoindentation, Thermal Drift, Machine Compliance, Tip Rounding, Initial Penetration Depth determination, Specimen Preparation and Indentation Size Effects [88]. Some of these issues are discussed below. Pile-up and Sink-in effects affect a wide variety of materials in terms of extracting me chanical properties accurately. The schematic below (Figure 5) demonstrates these ef fects. Pile-up results in an underestimation of contact area while Sink-in results in overes- timation. All elastic materials undergo sink-in while elastic-plastic materials will show pile-up depending on the degree of plasticity. Elastic Sink-in can easily be corrected by using a modified equation for contact depth (Eq. 7) P /z„ =h-e: Equation 7

Figure 5: (a) Elastic (Sink-in) (b) Elastic-Plastic (Pile-up)

23 Equation 7 has been derived from elastic-contact analysis. The value of s depends on the geometry of the indenter and is 0.72 for Berkovich and conical indenters [86,89]. The issue of pile up is more complicated and usually involves determining actual contact area through direct inspection (SEM/AFM) [90] or by finite-element simulations [91,92]. During nanoindentation testing, thermal expansion and contraction of the instrument can lead to errors in depth measurement. This can be particularly deleterious to property mea surement in tests that span over relatively long times and small displacements (slow strain rate tests). It is hence customary, to measure the drift rate of the indenter system by loading the indenter to a fixed small load. The resulting slope of the displacement vs. time plot at fixed small loads is a measure of drift rate and the resulting displacement data can be corrected based on this value. However, some materials show time dependent in dentation behavior (creep). Under such circumstances the drift obtained from the above procedure will include time dependent effects from the sample and it's virtually impossi ble to separate the two effects [87,93]. Most sharp indenters used in nanoindentation are not perfectly sharp and have rounded tips. For most indentations, the area is usually calculated based on a perfectly sharp geometry. The area however will be in large errors if indentation depths are small and tip radius is of the order of indentation depth. This is usually taken care of by cali brating the tip area for small depths [87,93]. Despite all these challenges nanoindentation continues to be an invaluable tech nique due to its ease, robustness and its ability to provide host of invaluable mechanical

24 property data and works particularly well for isotropic bulk solids. However, for complex thin films like nanolaminates, another complication arises. Due to the heterogeneous and anisotropic nature of nanolaminates, the stress state under the indenter can be extremely complicated and an average property number is obtained that cannot be correlated with the directional behavior of anisotropic materials. Also, strain gradient effects are preva lent in conventional nanoindentation testing. This problem led to a recent advancement in testing in form of micro-compression developed by Uchic et. al. [94]. The technique involves fabricating micron to submicron sized pillars in a thin film and indenting it with a flat punch (Figure 6Figure). The tech nique leads to close to uniaxial stress state in the pillar, leading to better estimation of constitutive properties. Since the inception of this technique in 2003 by Uchic, a host of researchers have used this technique to measure material properties on the small length scale [94-99]. Some of the concerns associated with this technique can be fabrication of perfect vertical pillars (taper free), alignment of the the pillars with respect to the indenter testing axis, compliance of the base supporting the pillar, friction between the pillar and indenter tip, microstructural damage during pillar fabrication (as in case of FIB). [100,101]

sdL I 11 -a- Figure 6: Schematic of Micro-compression Test t o

26 In summary, ample evidence exists that point to the potential of nanolaminates for mechanical applications. However, there seems to be a lack of understanding of mechani cal properties and deformation characteristics of nanolaminates, especially in the metal- ceramic form. Also, testing these novel structures at the nanometer scale presents a chal lenge in understanding these materials. Hence, we propose to take up the field of processing and characterization of metal-ceramic nanolaminates by using a model system of Al/SiC nanolayered material for fundamental understanding of their mechanical beha vior.

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Full document contains 188 pages
Abstract: Nanoscale laminated composites are a novel class of materials with excellent mechanical properties like strength, flexibility, and toughness. Properties in these materials can be tailored by varying layer thicknesses, microstructure, and controlling internal stresses. Most of research to-date has focused on metal-metal and ceramic-ceramic laminates. The field of metal-ceramic laminates has remained relatively unexplored. From a mechanical structure point of view, metal-ceramic systems present a good combination of strength and hardness. Hence, there is a need for understanding and evaluating these composites in nanolaminate form. In this work, the deformation behavior of Al-SiC nanolaminates as a model system has been studied. The work has been categorized into 3 major areas of research. The first involves processing and microstructural characterization of these novel materials. Nanoscale Al/SiC layered composites were fabricated using magnetron sputtering. Samples with varying volume fractions of Al and SiC were synthesized. The second area involves quantifying the residual stresses in these materials. These are quantified by x-ray synchrotron and beam-curvature techniques. The third area focuses on understanding the fundamental mechanisms for deformation damage, and fracture under indentation and micro-compression loading. Fracture/Damage analysis is carried out using focused ion beam (FIB) and scanning electron microscopy (SEM). The systematic study has revealed the enhanced mechanical properties of metal- ceramic nanolaminates and the fundamental mechanisms governing the strength and failure of these materials. It has been found that Al/SiC metal-ceramic systems can exhibit high strength together with high toughness and flexibility. High compressive residual stresses are generated during the sputter deposition of these nanolaminates which are associated with the large number of interfaces present in the nanolaminate architecture. The mechanical properties (hardness and modulus) of such a system can be reasonable be measured using nanoindentation however the inhomogeneous stress state can lead to complicated and erroneous results. This has been resolved by using a micro-compression approach and various parameters affecting micro-compression testing are evaluated. Material properties of Al/SiC nanolaminates arc accurately determined using micro-compression testing approach.