Experimental Aspects of Nanoindentation

1. Experimental Aspects of Nanoindentation I. Why, What and How Krystyn J. Van Vliet 3.052 Spring 2003 February 11, 2003

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

3. Why nanoindentation? 1. Materials engineering P 10 g Johannes Brinell (1880s) stress = s = P/A strain = e = [Lf-Li]/Li P = mg H = P/[pD2/4] Units of stress, but NOT stress, NOT a physical measure Indicates resistance to plastic deformation A L sy n D E = s/e Uniaxial tension/ compression Hardness testing

4. Why nanoindentation? 1. Materials engineering • Indent to several depths to build stress-strain relation: • = a/R sa P/pa2 R Multiaxial, NOT uniaxial stress state Requires plastic deformation to see indentation a

5. Why nanoindentation? 1. Materials engineering Microelectronic thin films (early 1980s) Mechanical properties important: electromigration Questions: Is E a function of tf? Is sy a function of tf? Is n a function of tf? Use nanoindentation to measure thin film props Consider sharp indenters to mitigate substrate effects tf 10 nm – 1 mm

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

7. Why nanoindentation? 3. Material physics Ceramics: Fracture initiation Metals: Dislocation nucleation, material flow 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)

8. What is nanoindentation? P h 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

9. 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

10. What is nanoindentation? dP/dh Show variables in response: C = loading curvature dP/dh = unloading stiffness Pmax, hmax Wp = plastic work of indentation Wt = total work of indentation C We Wp

11. What is nanoindentation? 35 mN Load 2 mN SiC (ceramic) 100 500 nm 16 nN 500 PE (polymer) 100 nm Depth

12. 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 B. Indenter considerations Ei >> Es: Ideally, deformation occurs only in sample material Primarily diamond (hardest natural material) Spherical: * Elastic, THEN plastic * Difficult to machine diamond Sharp: * Immediately elastic AND plastic * Can facet diamond into pyramids, not cones Vickers Berkovich

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

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

15. How does nanoindentation work? Capacitance of Parallel Plates: 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: • As plates move closer, d decreases and V, C increase • Temperature/noise insensitive displacement meter

16. How does nanoindentation work? Inductance of a solenoid Inductance is the change in B(V) created by a change in I For fixed A, a change in current I leads to: Magnetic field of a solenoid: Thus 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)

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

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

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

20. How does nanoindentation work? light microscope transducer 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 P t controlled positioning stage

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

22. How does nanoindentation work? • Results in thin films • Substrate affects indentation response • Complicated relationship due to n • Empirical rule: hmax < 10% tf for continuum analysis to hold Ni coating on Pb-Sn solder: tf = 5 mm coating + substrate coating decreasing hardness constant hardness

23. h hf hs How does nanoindentation work? • Results in thin films • Substrate affects indentation response • Complicated relationship due to n • Empirical rule: hmax < 10% tf for continuum analysis to hold FILM BULK Load [mN] Depth [nm]