Experimental Aspects of Nanoindentation I. Why, What and How Krystyn J. Van Vliet Spring 2003 February 11, 2003

Why nanoindentation? 1.Materials engineering: Optimizing material composition, structure, processing for particular applications (bulk and thin films) 2.Mechanics of small volumes: Understanding whether thin films, lines, dots have different mechanical properties than bulk counterparts 3.Material physics: Measuring deformation processes such as dislocation nucleation, crack growth, etc. Definition: Mechanical probing of a material surface to nm-scale depths, while simultaneously monitoring LOAD and DEPTH.

Why nanoindentation? 1. Materials engineering Uniaxial tension/ compression 10 g Hardness testing P L A stress =  = P/A strain =  = [L f -L i ]/L i P = mg H = P/[  D 2 /4] Units of stress, but NOT stress, NOT a physical measure Indicates resistance to plastic deformation D Johannes Brinell (1880s) yy E =  /  n

Why nanoindentation? 1. Materials engineering Indent to several depths to build stress-strain relation:  = a/R   P/  a 2 Multiaxial, NOT uniaxial stress state Requires plastic deformation to see indentation R a

Why nanoindentation? 1. Materials engineering Microelectronic thin films (early 1980s) Mechanical properties important: electromigration Questions: Is E a function of t f ? Is  y a function of t f ? Is n a function of t f ? Use nanoindentation to measure thin film props Consider sharp indenters to mitigate substrate effects tftf 10 nm – 1  m

Why nanoindentation? 2. Unique mechanics of small volumes Are mechanical properties affected by constraint? Are E,  y, n f(size)? When structure dimensions approach atomic dimensions, is mechanical behavior quantized?

Why nanoindentation? By probing materials near atomic and molecular dimensions, can we learn about fundamental deformation mechanisms? Polymers: Entropic elasticity Biologicals: Ligand/receptor binding Progress due to rapid increases in microelectronics (data acquisition, signal processing) Utility due to rapid progress in computational modeling (same time/length scales) 3. Material physics Ceramics: Fracture initiation Metals: Dislocation nucleation, material flow

What is nanoindentation? Show P-h response at loading/unloading. Discuss hysteresis and what it means. Introduce concepts of indirect measurement: Actually measuring voltages Emphasize that this is a material signature, but gives no real property values P h

What is nanoindentation? Materials engineering goal: Interpret P-h data in terms of continuum scale properties That is, test a small material volume to obtain parameters for a large material volume Translate a complicated, multiaxial stress state into uniaxial properties

What is nanoindentation? Show variables in response: C = loading curvature dP/dh = unloading stiffness P max, h max W p = plastic work of indentation W t = total work of indentation C dP/dh WpWp WeWe

SiC (ceramic) mN 16 nN 100 nm Depth 500 nm mN What is nanoindentation? Load PE (polymer)

How does nanoindentation work? A. Material assumptions Continuum: No structural length scales e.g. grain size, film thickness Homogeneous: One phase Elastoplastic: Deforms via plastic yielding, rather than fracture, phase transformation Spherical: * Elastic, THEN plastic * Difficult to machine diamond E i >> E s : Ideally, deformation occurs only in sample material Primarily diamond (hardest natural material) Sharp: * Immediately elastic AND plastic * Can facet diamond into pyramids, not cones VickersBerkovich B. Indenter considerations

How does nanoindentation work? C. Instrumentation Triboindenter Hysitron, MN NanoIndenter MTS, TN NanoTest MicroMaterials, UK

How does nanoindentation work? C. Instrumentation 3-plate capacitor P, h via same device + -

Capacitance of Parallel Plates: How does nanoindentation work? Electric field between two parallel plates: E = V/d Voltage difference between two plates as +q moves from positive to negative plate: Capacitance is the amount of charge Q stored per unit voltage V applied to plates: Relevance to nanoindentation: (1)As plates move closer, d decreases and V, C increase (2)Temperature/noise insensitive displacement meter

Inductance of a solenoid How does nanoindentation work? Relevance to nanoindentation: (1) Can be used to exact motion (change in I) (2) Can be used to measure load (change in I or V) For fixed A, a change in current I leads to: Thus Inductance is the change in B(V) created by a change in I Magnetic field of a solenoid:

How does nanoindentation work? D. Calibration i. P(V) ii. h(V) Add mass to load train Determine I, V to move mass (LOAD) Calculate relationship: P =  V Determine zero point: P =  V +  Move indenter fixed, known distance Measure corresponding C, V Calculate relationship: h =  V Determine zero point: h =  V + 

How does nanoindentation work? D. Calibration iii. C m = Machine Compliance Definition: Displacement of the instrument (load train) in response to applied load Importance: Need to subtract this response: C total = C sample + C m Approach: (1.) Nanoindent standard sample Determine Cm empirically to get “correct” E, H (2.) Compress machine with flat Measure C m directly

How does nanoindentation work? E. Environmental control Acoustic isolation (< 1 Hz) Thermal control (< 0.5 C) Humidity control (< 55%) Vibration isolation (< 1 Hz) HysitronMicroMaterials

How does nanoindentation work? Procedure: 1. Prepare sample surface: Flat, Parallel faces, Smooth 2. Find region of interest on surface 3. Program load profile 4. Program spatial matrix 5. Execute experiment 6. Analyze data to obtain properties light microscope transducer controlled positioning stage P t

How does nanoindentation work? Results in bulk materials E [GPa] Results in composites 1. Multiphase alloys/Copolymers 2. Trabecular bone Benzocyclobutene (BCB) [VanLandingham, NIST] E = 3.5 GPa [Smith, MicroMaterials] [Wyrobek, Hysitron]

How does nanoindentation work? Results in thin films Substrate affects indentation response Complicated relationship due to n Empirical rule: h max < 10% t f for continuum analysis to hold constant hardness decreasing hardness coating + substrate Ni coating on Pb-Sn solder: t f = 5  m

How does nanoindentation work? Results in thin films Substrate affects indentation response Complicated relationship due to n Empirical rule: h max < 10% t f for continuum analysis to hold Load [mN] Depth [nm] BULK FILM hfhf hshs h