THE MECHANICAL PROPERTY ANALYSIS OF THIN DIAMOND COATED METAL SUBSTRATES JOHN THOMAS STAGON. Submitted in partial fulfillment in the requirements

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1 THE MECHANICAL PROPERTY ANALYSIS OF THIN DIAMOND COATED METAL SUBSTRATES By JOHN THOMAS STAGON Submitted in partial fulfillment in the requirements for the degree of Master of Science Thesis Advisor: Dr. Heidi Martin Department of Chemical Engineering CASE WESTERN RESERVE UNIVERSITY May,

2 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis/dissertation of John Stagon candidate for the Master of Science degree *. (signed) Heidi B. Martin J. Adin Mann John Lewandowski *We also certify that written approval has been obtained for any proprietary material contained therein. 2

3 Table of Contents LIST OF FIGURES AND TABLES... 4 ABSTRACT... 7 CHAPTER 1. INTRODUCTION, BACKGROUND, AND OVERVIEW A HISTORY OF DIAMOND FILM COATED ELECTRODES SIGNIFICANCE OF THIS STUDY CHAPTER 2. FABRICATION OF DIAMOND-FILM COATED METAL SUBSTRATES VERTICALLY GROWN SAMPLES Selection of Materials Preparation of Metal Wire Substrates Diamond Growth Procedure HORIZONTALLY GROWN SAMPLES Selection of Materials Preparation of Metal Wire Substrates Diamond Growth Procedure CHAPTER 3. TENSION TESTING AND ANALYSIS APPROACH TO TEST WIRE SAMPLES TENSION TESTING OF VERTICALLY GROWN WIRE SAMPLES Testing Procedure Analysis of Untreated and Uncoated Substrates (Direct from Manufacturer) Analysis of Diamond Coated Substrates RESULTS AND DISCUSSION HORIZONTALLY ALIGNED WIRE SUBSTRATES CHAPTER 4. FRACTURE SURFACE ANALYSIS INTRODUCTION FRACTURE SURFACE IMAGING PREPARATION IMAGES OF UNCOATED SUBSTRATES IMAGES OF DIAMOND COATED SUBSTRATES FRACTURE SURFACE ANALYSIS HORIZONTALLY GROWN SAMPLE ANALYSIS Tungsten Wire Samples Molybdenum Rhenium Samples CONCLUSIONS CHAPTER 5. FINAL CONCLUSIONS CHAPTER 6. FUTURE EXPERIMENTS FATIGUE TESTING RAMAN SPECTROSCOPY NANO-SCALE HARDNESS TESTING POST-DEPOSITION ANNEALING WORKS CITED

4 List of Figures and Tables Figure 2.1. Schematic of Vertically Oriented Wire Sample (Not to Scale) 15 Figure 2.2. Schematic of Vertically Oriented Sample in Reactor (Not to Scale) 18 Figure 2.3. Schematic of Horizontally Oriented Sample 18 Figure 2.4. Picture of Horizontally Oriented Sample in Reactor 22 Figure 3.1. Position of Specific Testing Points 23 Figure 3.2. Position of Specific Testing Points 24 Figure 3.3. Stress vs. Strain Curves for Untreated and Uncoated Materials (Direct from Manufacturer) 25 Table 3.1. Summary of Stress vs. Strain Data from the Uncoated Samples 26 Figure 3.4. Typical Stress vs. Strain Curves for the Molybdenum/Rhenium Alloy exposed to Diamondgrowth conditions, at the Specific Points of Interest (All samples were coated in the same batch) 26 Table 3.2. Summary of Stress vs. Strain Data from the Molybdenum/Rhenium Alloy exposed to Diamond growth conditions, at the Specific Points of Interest 27 Figure 3.5. Typical Stress vs. Strain Curves for the Molybdenum/Rhenium Alloy exposed to Diamond- Growth conditions at the Specific Points of Interest (All samples were grown in the same batch) 29 Table 3.3. Summary of Stress vs. Strain Data from the Molybdenum/Rhenium Alloy exposed to diamond growth conditions, at the Specific Points of Interest 30 Figure 4.1A. 750X Magnification of Uncoated Tungsten Wire Fracture Surface 32 Figure 4.1B. 1750X Magnification of Uncoated Tungsten Wire Fracture Surface 33 Figure 4.2A1. 750X Magnification of Uncoated Tungsten/Rhenium Alloy Wire Fracture Surface 34 Figure 4.2B. 1750X Magnification of Uncoated Tungsten/Rhenium Alloy Wire Fracture Surface 35 Figure 4.3A. 750X Magnification of Uncoated Molybdenum/Rhenium Alloy Wire Fracture Surface 36 Figure 4.3B. 1750X Magnification of Uncoated Molybdenum/Rhenium Alloy Wire Fracture Surface 37 Figure 4.4A X Magnification of Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 39 4

5 Figure 4.4B. 1750X Magnification of Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 40 Figure 4.5A. 750X Magnification of Diamond-Coated Tungsten/Rhenium Alloy Wire Fracture Surface Broken in 3 Point Bending 41 Figure 4.5B. 7500X Magnification of Diamond-Coated Tungsten/Rhenium Alloy Wire Fracture Surface Broken in 3 Point Bending 43 Figure 4.6A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point A) 44 Figure 4.6B X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point A) 46 Figure 4.6C X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Profile (Point A) 47 Figure 4.6D. 5000X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Outer Surface (Point A) 48 Figure 4.7A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point B) 49 Figure 4.7B. 1750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point B) 50 Figure 4.7C. 3250X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Outer Surface (Point B) 51 Figure 4.8A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) 52 Figure 4.8B. 1750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) 53 Figure 4.8C. 4000X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) 54 Figure X Magnification of 2 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 57 5

6 Figure X Magnification of 4 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 59 Figure X Magnification of 8 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 60 Figure 4.12A. 1000X Magnification of 20 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 61 Figure 4.12B. 600X Magnification of 20 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 62 Figure 4.13A. 600X Magnification of 2 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 63 Figure 4.13B. 1200X Magnification of 2 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 64 Figure 4.14A. 750X Magnification of 4 Hour Duration Diamond-Coated Molybdenum Rhenium Wire Fracture Surface Broken in Tension 65 Figure 4.14B. 1200X Magnification of 4 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 66 Figure 4.15A. 750X Magnification of 8 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 67 Figure 4.15B. 1250X Magnification of 8 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 68 Figure 4.16A. 750X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 69 Figure 4.16B. 1751X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 70 Figure 4.16C. 750X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension 71 6

7 THE MECHANICAL PROPERTY ANALYSIS OF THIN FILM DIAMOND COATED METAL SUBSTRATES Abstract By JOHN THOMAS STAGON The choice of substrate material is crucial for biosensing electrodes that use thin film diamond due to material property changes that occur during the high-temperature, chemical vapor deposition (CVD) process for diamond coating. Three materials were studied in this context: tungsten, a tungsten/rhenium alloy, and a molybdenum rhenium alloy, all of which obtained as extruded wire with a 120 µm diameter. These materials were selectively coated by hot-filament CVD with boron-doped polycrystalline diamond and the material properties studied in three different states, using tensile tests and SEM imaging of corresponding fracture surfaces. First, the inherent substrate ductility before diamond coating was determined by examining stress vs. strain data from tensile tests. Then, ductility of vertically oriented wire substrates was determined, the substrate positioning in the CVD system during the 7

8 diamond coating process similar to what has been used to selectively grow diamond micro-wire electrodes. In this case, the substrates, aligned perpendicular to the hot filaments, could be used to profile gradients in ductility along the substrate length, including in regions that would not be diamond-coated, but still exposed to the high temperature environment. Lastly, the effect of diamond growth time on ductility was studied, for a given substrate-to-filament distance; the substrates were aligned parallel to the hot filaments ( horizontally aligned) to produce uniform coatings of diamond along the wires. The process time was varied between 2 to 20 hours to produce films of different thickness. For all three states, fractured samples were examined by SEM to identify fracture modes and suggest potential embrittlement mechanisms. The molybdenum/rhenium alloy was the only substrate that retained sufficiently ductile properties to enable tension testing; all tungsten and tungsten/rhenium alloy samples became highly brittle, fracturing with any attempt to load them for testing. The molybdenum/rhenium s ductility decreased with increased diamond-growth time, the ultimate tensile stress decreasing from 2200 MPa to 1000 MPa after 20 hours at a 9 mm substrate-filament distance. Its ductility gradient was large, as varying substrate-filament distance, the ultimate tensile stress increasing from 1800 MPa to the equivalent of the untreated wire at 2200 MPa, over a 3 mm length from the point where diamond coating ceased. Tungsten and tungsten/rhenium alloy fracture cross-sections showed complete and even embrittlement. The molybdenum/rhenium alloy was embrittled only around the periphery, a distinct brittle phase visible underneath the diamond film. Ductile features such as a dimpled fracture surface remained in the core of the coated 8

9 molybdenum/rhenium alloy, even for growth times far exceeding those needed to produce a complete diamond coating. Overall, these data showed an embrittlement mechanism occurring for all three metals considered. The distance that the substrate was from the heating element in the CVD reactor and the time it was exposed to high temperature conditions both had a significant impact on its resulting ductility. Overall, mechanical testing and imaging data supported that the molybdenum/rhenium alloy is the most promising candidate for substrate material to produce ductile, implantable diamond electrodes for biomedical applications. 9

10 CHAPTER 1. Introduction, Background, and Overview 1.1 A History of Diamond Film Coated Electrodes Diamond has always been known for its alluring optical and mechanical properties. However, recent technological advances have allowed this material s unique electrochemical properties to be utilized in biosensing electrodes. Polycrystalline diamond coatings have been deposited by chemical vapor deposition (CVD) [1] which involves heating a dilute mixture of the carbon containing gas, methane, in hydrogen. As these gases pass by a heating element, the molecules become energized into free radical species and can slowly deposit as a polycrystalline diamond film on a substrate positioned within a centimeter of the heating element. By introducing a boron containing gas to the feed mixture, a small fraction of carbon atoms in the diamond lattice are replaced with boron and the diamond material becomes electrically conductive [2]. This newer electrode material shows highly promising electrode properties: a wide potential window [3], high biocompatibility [4][5], and higher signal-to-noise ratios. Additionally, recent research suggests that a diamond sensor can detect at physiological levels, important chemicals in biological systems like dopamine and adenosine [6]. Diamond films show a promising performance capability in a wide range of biomedical fields. 1.2 Significance of this Study The promise of diamond films for biomedical applications does not come without a few drawbacks, especially when considering use as a long-term (chronic) electrode implanted in the body. It has been shown that its CVD growth process can cause traditional metal 10

11 substrates, like tungsten, beneath the diamond to become extremely fragile. Halpern reported [7] that various metal wire substrates, like tungsten, subjected to the diamond film growth process over a time period of 20 hours became capable of bending only a maximum of 15º before the wire would fail and fracture. This study aims to analyze material properties of metal wire substrates before and after the diamond growth process, as a means to evaluate potential new substrates for bendable and resilient electrodes. This will be performed by first selectively growing diamond films onto the tips of metal substrate wires (see Chapter 2). Then, tension tests (Chapter 3) and a fracture surface analysis (Chapter 4) will be performed to discover where along its length, the metal substrate the strongest or weakest is. Furthermore, this study will attempt to use these experiments to draw conclusions on the modes of embrittlement as well as suggest ways to improve the strength of the metal substrate (Chapter 5). Finally, this study will suggest some useful future experiments to perform in order to analyze which substrate material and diamond growth conditions will be most promising in the fabrication of a chronically implantable electrode. 11

12 CHAPTER 2. Fabrication of Diamond-Film Coated Metal Substrates In this chapter, the production of the diamond coated samples will be discussed beginning with the selection of materials and describing through the diamond coating process. It will include the directions to produce vertically and horizontally aligned samples. These directions determine whether the diamond coating is uniform along the length of the wire (horizontal orientation) or if the diamond coating produced changes along the length of the wire (vertical orientation). 2.1 Vertically Grown Samples The following diamond film synthesis procedures are described in great detail as small discrepancies in the film growth parameters can immensely affect the properties of the metal substrate and the completeness/uniformity of the diamond coating. This specific method described below was used on all of the vertically grown wires to produce the least amount of deviation in the material properties (see Chapter 3.3 for details on these changes) Selection of Materials The selected wire material consisted of three separate metals and alloys. These metals included tungsten (Goodfellow Metals 99.95% purity), a tungsten/rhenium alloy (Rhenium Alloys Inc 75% W, 25% Re), and a molybdenum/rhenium alloy (Rhenium Alloys Inc. 47.5% Mo, 52.5% Re). All the metal substrates were extruded wires and µm in diameter. These studies seek to expand upon preliminary data found for these materials that were previously used by researchers like Dr. Halpern to construct diamond-coated metal electrodes [7]. Rhenium was an obvious choice due to its extremely high melting point and common usage in high temperature applications. 12

13 2.1.2 Preparation of Metal Wire Substrates The diamond films were deposited by Hot Filament Chemical Vapor Deposition (HFCVD). To prepare the vertically-grown wire samples, the end of the wire was shaped into a hook with a pair of tweezers. This produced a semicircular bend with a diameter of approximately 1mm. This specific geometry was chose as it could be used to properly attach itself to specific nerve cells in former in vivo experiments [8]. A simple straight wire would easily slip out of place and no longer obtain the data from the proper place. After the specific geometry was established with the wire, the hooks were held inverted within a sytrofoam holder and suspended in a diamond slurry bath in which the samples were sonicated for a period of 45 min; only the hook regions were submerged in the slurry. This treatment enhanced the nucleation of the diamond films by abrading the surface and producing nucleation sites. 13

14 Figure 2.1. Schematic of Vertically Oriented Wire Sample (Not to Scale) Then still inverted, the samples were sonicated in an ethanol bath for a minimum of 15min to ensure the surface was clean. Once clean, the wire samples were loaded into 7.5 cm quartz capillary tubes with the tip of the wire hook extending out of the tube by 3 mm. The capillary tube was a necessary piece because it could provide support to the wire to ensure the sample would not fold over. Also, this tube would crudely mask the extent of the diamond depositing down the length of the wire. These samples were then loaded vertically into the HFCVD (See Figure 2.2). These samples were oriented perpendicular to the filaments leaving an 8 10 mm gap between the samples and the closest point on the filaments. This distance was adjusted as precisely as possible, and a 9 mm gap was targeted for each of the growths. 14

15 Figure 2.2. Schematic of Vertically Oriented Sample in Reactor (Not to Scale) Diamond Growth Procedure Once the samples were properly loaded, the reactor was sealed and the pressure reduced to a vacuum (approximately 0.1 torr) for at least 8 hours. Previous growth condition analysis can be seen in Halpern s work [7]. This allowed air to be evacuated from the HFCVD. Then, a specific ratio of hydrogen, methane, and trimethyl boron (TMB) gases was fed into the HFCVD. The gas flow rates used in these experiments were 196 sccm hydrogen, 1.2 sccm methane, and 4.0 sccm TMB. It is important to note that the TMB is heavily diluted in hydrogen and the process is not overwhelmed by the boron containing gas. The dilute presence of TMB ensured a small amount of boron doping in the diamond, allowing the diamond film to become semi-metallic as performed in earlier 15

16 work [2]. As the gas streams in, an automatic exhaust throttle valve controls the reactor pressure to stay constant at 20 torr. After the gas flow and pressure were established in the chamber, a potential capable of ramping to higher magnitudes controlled by the operator was applied to the filament assembly of four filaments arranged in series. By increasing the voltage across the filaments, a high temperature is produced in the reactor allowing for the gases fed into the reactor to break down into reactive species that then deposit on the surface of the metal substrates. The maximum filament temperature was determined by pointing a pyrometer at the hottest point on the hot filaments. Using this method to gauge the desired temperature, the filament voltage was increased until the pyrometer reported ºC. For 30 minutes, this temperature was held constant allowing the diamond to begin to nucleate on the metal substrate surface. Then, the voltage was further increased until the filament temperature was ºC. This temperature was held constant for 20 hours to deposit a diamond coating on the hooks. During this time, the metal substrates undergo certain material properties changes that are the focus of this study. The growth process was ended by slowing ramping down the voltage to the filaments and removing the methane and TMB gas feeds. The HFCVD was cooled as hydrogen was fed to the chamber. After the samples were sufficiently cool at nominally hours, the hydrogen flow was stopped and the gases evacuated from the chamber using a vacuum before venting the system with nitrogen and removing the samples. 2.2 Horizontally Grown Samples This section describes how the horizontally grown wire samples were created. There are only a few differences in the preparation and growth parameters between the two 16

17 orientations. Therefore, this section will discuss solely the differences in the procedure. This orientation was chosen in the time dependent diamond growth runs because it would most clearly depict how the material under a complete diamond film is being affected (see Chapter 3.4 for details on the changes in the material properties) Selection of Materials The metal substrates used in the horizontally prepared wire were the same Molybdenum/Rhenium alloy and Tungsten wires as described in Section 2.1 for the vertically grown samples. As these tests were performed after the results of the vertically grown samples were found, these two materials were chosen because the Molybdenum/Rhenium alloy best demonstrated a ductile fracture behavior while the Tungsten best demonstrated a brittle fracture behavior. Therefore, these materials would best show a transition in the material properties as the time the samples are exposed to the coating process was varied Preparation of Metal Wire Substrates The preparation for the horizontally arraigned wire resembles the vertically grown samples except for the geometry and loading of the specimens. Instead of a hook shape, the wire was bent at 90º on each of the edges leaving a 4.2 cm bridge in the middle, much resembling a long staple (See Figure 2.3). 17

18 Figure 2.3. Schematic of Horizontally Oriented Sample After using the same sonication process used above, the long bridge between the two bends was selectively suspended into the slurry and ethanol baths. Then, these samples were removed and loaded into the metal holders so the long bridge part of the sample was parallel with the filaments when loaded into the reactor (see Figure 2.4). The gap between the filaments and the samples was held constant at 9 mm. Figure 2.4. Picture of Horizontally Oriented Sample in Reactor 18

19 2.2.3 Diamond Growth Procedure The growth process was very similar to the procedure used for the vertically grown samples with the only difference being the growth time. Thus, the only difference in this part of the process was the length of time in which the samples were subjected to the 2000 ºC filament temperature. Several time intervals were compared including 2, 4, 8 and 20 hours. Using these time intervals, the metal substrates material properties would evolve as a thicker diamond film is deposited. 19

20 CHAPTER 3. Tension Testing and Analysis In this chapter, the tension tests performed on the samples are discussed. The setup and procedure are described as well as the results obtained from the tests. In this section, both the vertically aligned and horizontally aligned samples were tested and the results in the form of stress vs. strain curves are presented. A few conclusions about the best material that could be used to make the electrode are made at the end. 3.1 Approach to Test Wire Samples Tension testing was used to quantitatively determine the mechanical properties of the metal substrate before and after the diamond film growth process. Testing in tension at a low rate of strain while monitoring and recoding the subjected load and displacement allows the experimenter to identify which materials are more ductile through analysis of stress-strain plots. Because a flexible and ductile substrate is desired to create the implantable electrodes, a highly ductile substrate material after the diamond-coating process is crucial for the implementation of such a device. 3.2 Tension Testing of Vertically Grown Wire Samples The tension testing was performed on an Enduratek Axial Manipulator. This device consists of a pneumatic arm that can pull samples in one direction with a high degree of precision (to about.001 mm). It was used to create the stress-strain plots for analyzing the material properties of the wire samples. The stress-strain plots were constructed based on the engineering stress and engineering strain definitions. The engineering stress is defined by the equation: 20

21 Where σ is the engineering stress, P is the load (or force) acquired by the load cell, and A c is the initial cross sectional area of the sample. Furthermore, the engineering strain is defined as: Where e is the engineering strain, Δl is the change in the sample length, and l o is the initial sample length. Additionally, there were three main points along the vertically-grown wire samples that were of interest for testing (See Figure 3.1). The first area of interest was located at the point on the wire that was adjacent to the end of the capillary tube. This was chosen as the diamond film at this point would be complete and uniform, mimicking the properties of the exposed hook portion. This point will be called Point A. Next, another point was chosen located 1.5 mm below Point A. This point is called Point B and was determined to be in a transition zone and have an incomplete diamond film. Finally, Point C was also identified as being 1.5 mm down the length of the wire from Point B; this point should have minimal diamond growth. A goal was to test these points for each of the materials selected. 21

22 Figure 3.1. Position of Specific Testing Points Testing Procedure The setup for the tension tests, as shown below in Figure 3.2, included the Enduratek Axial Manipulator connected to a parallel plate steel clamp where one side of the wire sample can be loaded. The other clamp of the same style was attached to a 50 lbs. load cell which would relay the force data to a computer. This load cell was connected in turn to a stationary mount. Because the wire samples diameter was very small relative to the design specifications of the tester, extreme attention was given to the clamp positions to ensure that they were level and flush with each other. This would help reduce premature fracture due to additional forces being placed on the sample by being loaded in different stress states. Additionally, two pads were placed inside of the clamps that allowed the samples to be held firmly in place without being deformed by the pressure applied by the clamps. Without these pads in place, the samples would repeatedly break at a low ultimate tensile strength when compared to test performed with the pads. The pads consisted of a thin layer of epoxy sandwiched by two sheets of paper. 22

23 The samples were then prepared for testing in the Enduratek Axial Manipulator. The wire was cut down in length so that it may fit inside the two parallel plate steel clamps with the added pads. A 0.5 mm gap was left between the clamps, and the screws were tightened on the clamps to apply the needed pressure. The same amount of torsion was applied on each of the screws that held the clamps together to ensure that the samples were not being deformed to an additional degree. It should be noted that the desired fracture point was always loaded at one end of the clamp causing the desired fracture point to fail at this point repeatedly (this always occurs with the vertically aligned samples since the weakest point was found to be the point that was closest to the filaments). The pressure applied was just enough so that the wire samples did not slide out of the clamps when the steady displacement was applied. The strain rate was held constant for all tests at mm/sec. The samples were pulled until fracture occurred; then the displacement and resulting load data could be used to create the stress-strain curves. Figure 3.2. Position of Specific Testing Points 23

24 3.2.2 Analysis of Untreated and Uncoated Substrates (Direct from Manufacturer) The untreated and uncoated wire material straight from the manufacturer was tested in tension to determine the initial, baseline material properties. Then, these data can be compared to the samples that were subjected to the diamond growth conditions. Multiple, at least 6, straight sections of samples of the same diameter were pulled in tension to acquire more precise data. Figure 3.3. Stress vs. Strain Curves for Untreated and Uncoated Materials (Direct from Manufacturer) From the typical stress-strain curve, as shown in Figure 3.3, The ultimate tensile strength, the elastic moduli, yield strength, and strain to failure are determined. The samples shown closely mimic the curve produced from other trials of the same materials One can easily see from these tabulations (see Table 3.1) that the Tungsten/Rhenium Alloy, as 24

25 received from the manufacturer, is the strongest material by having a very large ultimate tensile strength. All produced ultimate tensile stresses over the 2000 MPa mark. Also, the elastic moduli of all of the materials tested, within the measurement error, were the same. W Metal Wire W/Re Alloy Wire Mo/Re Alloy Wire Ultimate Tensile Stress (MPa) Elastic Modulus (GPa) 0.2% Yield Strength (MPa) 2500 ± ± ± ± 4 29 ± 5 26 ± ± ± ± 300 Strain to Failure 0.20 ± ± ± 0.05 Table 3.1. Summary of Stress vs. Strain Data from the Uncoated Samples Analysis of Diamond Coated Substrates After the hooks of three different materials were coated in diamond, it was immediately apparent that only one was capable of withstanding the forces needed to test the wires in tension. The tungsten and the tungsten/rhenium alloy both became so brittle that they could not be loaded into the clamps without immediate fracture. Multiple attempts to carefully load these diamond-coated materials ended in failure, so it was concluded that tension data from these materials could not be obtained through this method. The molybdenum/rhenium samples, however, did manage to remain ductile enough to withstand the pressure applied by the clamps. Thus, tension test data could then be 25

26 obtained for the three points of interest, with testing of multiple samples at each point. By creating an array of data at each point, the mechanical properties at each fracture point can be most accurately reported. Figure 3.4. Typical Stress vs. Strain Curves for the Molybdenum/Rhenium Alloy exposed to Diamondgrowth conditions, at the Specific Points of Interest (All samples were coated in the same batch) By analyzing the stress vs. strain curves presented in Figure X to tabulate mechanical properties (Table 3.2), one can see that the most ductile location was point C, the point furthest from the filaments. On the contrary, the most brittle point was point A, the point closest to the filaments, at the end of the capillary tube during diamond growth. Point B s properties fell between Points A and C, as would be expected. It should be noted the ultimate tensile strength (Table 3.2) increases down the wire length as the 26

27 distance between the filaments and the fracture point is increased. Also, the area under the elastic region of the fracture point locations can be used to crudely estimate the energy that can be stored from elastic deformation; using this estimate (not shown explicitly), Point C is the most resilient, while point A is the least resilient. From these data, it can be determined with confidence that wire remained more flexible with increasing distance from the filaments during the diamond coating process. Point A Point B Point C Ultimate Tensile Stress (MPa) Elastic Modulus (GPa) 0.2% Yield Strength (MPa) 1800 ± ± ± ± 8 47 ± 7 26 ± ± ± ± 100 Strain to Failure 0.06 ± ± ± 0.04 Table 3.2. Summary of Stress vs. Strain Data from the Molybdenum/Rhenium Alloy exposed to Diamond growth conditions, at the Specific Points of Interest 3.3 Results and Discussion The data obtained from this series of tension tests displayed a few crucial trends. First, the diamond-growth process causes some degree of embrittlement in each of the materials reviewed, even in regions that are somewhat masked from the reactive diamond growth region. The tungsten-based samples both transformed from flexible and ductile wires to brittle wires that would easily fracture upon a minute, applied bending force. 27

28 The material that is the most promising to be used as a chronically implantable electrode, due to its better maintenance of ductility, is the molybdenum/rhenium alloy. However, to fully profile the hook tip s material properties a new test must be created. Therefore, the horizontally aligned wire samples were tested. 3.4 Horizontally Aligned Wire Substrates Since the molybdenum/rhenium alloy was the only substrate that retained its ductile properties after exposure to the diamond CVD environment, the next step was to obtain a profile of the top of the hook. At this location on the vertically-oriented wire samples, the film should be at its thickest and completely uniform. However, the length on one side of the loop was not long enough to be placed into a clamp. Therefore, a separate set of wire samples was constructed in a way that the entire length of the wire would be at a constant distance away from the filaments emulating the same conditions that the top of the loop on the vertically-oriented samples would experience. This was accomplished by as described in Section 2.2. In this way, the diamond-coated regions of the horizontallyarranged wires could be exposed to the same conditions as the hooks of the vertically oriented wires. In addition to the 20 hour growth runs for both the vertically and horizontally arranged samples, other horizontally oriented wire samples were subjected to 2, 4 and 8 hour growth runs to investigate how these material properties are changing with respect to time, with a constant sample-filament gap (9 mm). Tungsten and molybdenum/rhenium alloy samples were coated, representing, based on the tension tests of the vertically-grown samples, the most brittle and ductile samples after the diamond film coating process. 28

29 Upon coating the horizontally arranged samples for their respective growth times, it was determined that regardless of the growth time tested (2, 4, 8 and 20 hours), the tungsten samples became too brittle to endure the forces from the clamps to be tested in tension. The molybdenum/rhenium samples, on the other hand, once again retained their ductile properties for the different growth times. Thus, all of the different timed runs for the Mo/Re alloy were tested in tension and stress-strain curves obtained (Figure 3.5). Figure 3.5. Typical Stress vs. Strain Curves for the Molybdenum/Rhenium Alloy exposed to Diamond- Growth conditions at the Specific Points of Interest (All samples were grown in the same batch) The stress-strain curves (Figure 3.5) of the horizontally aligned samples reveal interesting details about the effects of the duration of the deposition process on the material properties; tabulated values of the time dependence on the various material 29

30 properties are in Table 3.3. From the curves, the linear region from all of the samples overlap at low strains, suggesting that the elastic region does not greatly differ with increased diamond deposition time. However, the ultimate tensile strength of the samples steadily decreases, as the time is extended in the reactor; the samples tend to become weaker and weaker. Furthermore, the strain-to-failure dramatically decreases from the 2- hour deposition to the 4-hour deposition process. This means that most of the flexibility was lost within the first four hours of growth. After this period of time, from a mechanical property viewpoint, the samples become less suitable for use as an implantable electrode. 2 Hours 4 Hours 8 Hours 20 Hours Ultimate Tensile Stress (MPa) Elastic Modulus (GPa) 0.2% Yield Strength (MPa) Strain to Failure 1700 ± ± ± ± ± 7 30 ± 6 30 ± 6 30 ± ± ± ± ± ± ± ± ± 0.01 Table 3.3. Summary of Stress vs. Strain Data from the Molybdenum/Rhenium Alloy exposed to diamond growth conditions, at the Specific Points of Interest 30

31 CHAPTER 4. Fracture Surface Analysis 4.1 Introduction The mode of fracture within the material can be qualitatively analyzed by examining the fracture surfaces of materials tested in tension at high magnifications. Thus, the samples that were fractured in the tests described in Section 3 were then imaged in a scanning electron microscope (SEM) in order to observe the characteristics present on the fracture plane. 4.2 Fracture Surface Imaging Preparation The samples were prepared for imaging by first blasting them with compressed air to remove surface contamination. Then, the samples were adhered strongly onto a metal holder using double-sided carbon tape. These sample mount was then rotated so that the fracture plane surface could be observed with the SEM imaging. Then, both high and low magnification pictures of these surfaces were taken in order to clearly view the mode of fracture along with a few interesting features unique to certain samples and/or specific diamond growth conditions. 4.3 Images of Uncoated Substrates The first set of images (Figures 4.1A 4.1B for tungsten, Figures 4.2A 4.2B for tungsten/rhenium alloy, and Figures 4.3A -4.3B for molybdenum/rhenium alloy.) show the samples that were uncoated and pulled in tension. The analysis of each image can be found directly following the figure. 31

32 Figure 4.1A. 750X Magnification of Uncoated Tungsten Wire Fracture Surface In this magnified image (Figure 4.1A) of the tungsten wire, some necking (around 85% of the initial cross sectional area) has occurred near the fracture. A reduction in the fracture area proves that the mode of fracture that the material failed in was a ductile mode. 32

33 Figure 4.1B. 1750X Magnification of Uncoated Tungsten Wire Fracture Surface In this highly magnified image (Figure 4.1B) of the same tungsten wire, elongated dimples in the direction of the tension axis, or along the length of the wire, support that ductile failure indeed occurred within the wire. 33

34 Separation Figure 4.2A5. 750X Magnification of Uncoated Tungsten/Rhenium Alloy Wire Fracture Surface For the tungsten/rhenium alloy wire (Figure 4.2A), necking has occurred to a lesser degree (to only about 90% of the original cross sectional area) than the pure tungsten sample. Furthermore, some separation has occurred between the weaker grain boundaries within the alloy. 34

35 Figure 4.2B. 1750X Magnification of Uncoated Tungsten/Rhenium Alloy Wire Fracture Surface At a higher magnification of this fracture plane (Figure 4.2B), elongated dimples indicate the ductile fracture mode. 35

36 Figure 4.3A. 750X Magnification of Uncoated Molybdenum/Rhenium Alloy Wire Fracture Surface In this image (Figure 4.3A) of the molybdenum/rhenium wire, a high degree (slightly less than 50% of the original cross sectional area) of necking has taken place. This reduction in cross-sectional area is much more emphasized in this alloy than the other two materials. This is consistent with the tension tests showing the molybdenum-rhenium alloy to have a larger degree of flexibility and ductile properties. 36

37 Figure 4.3B. 1750X Magnification of Uncoated Molybdenum/Rhenium Alloy Wire Fracture Surface In Figure 4.3B of the molybdenum-rhenium wire, the dimples are again visibly elongated in the direction of the applied tension, or along the length of the wire. 37

38 4.4 Images of Diamond Coated Substrates After the diamond coating process, it became clear that the tungsten based materials became too brittle to be tested in tension. Therefore, to obtain the fracture plane images, these samples were broken in 3-point bending with tweezers at the location at the intersection of the tip of the capillary tube and the wire (Point A). The tungsten materials did accurately display the brittle features one would expect from the tension testing data performed previously (see Chapter 3). 38

39 Figure 4.4A X Magnification of Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending Once exposed to diamond-growth conditions and/or coated in diamond, the tungsten displays a much different method of fracture (Figure 4.4A). It no longer displays necking and the fracture plane is predominantly flat. These characteristics indicate that this material has become extremely brittle with very little elastic energy being able to be stored in the material. Also, a layer of diamond can be seen around the surface of the wire s circumference. 39

40 Crack Initiation Figure 4.4B. 1750X Magnification of Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending At an increased magnification of the brittle surface (Figure 4.4B), the absence of dimples characteristic of a ductile material is evident. Instead, the surface consists of series of flat surfaces that tend be aligned toward the point at which the fracture started (near the top left corner). 40

41 Figure 4.5A. 750X Magnification of Diamond-Coated Tungsten/Rhenium Alloy Wire Fracture Surface Broken in 3 Point Bending (Region in Red Box is Magnified in Figure 4.5B) Instead of a macroscopically flat surface like the tungsten in Figure 4.4A, the tungsten/rhenium alloy clearly shows (Figure 4.5A) two planes separated by a ledge. This interruption between the two planes is caused by the formation of a neutral plane when the sample is bent. On the right, the sample is under tension and fractures, then the crack continues to extend into the material until it reaches the fracture plane, or the site of the ledge. The uneven surface on the left side suggests that the material then fractured in 41

42 overload. The region within the red box is magnified and presented below in Figure 4.5B. 42

43 Direction of Fracture Lines Separation Figure 4.5B. 7500X Magnification of Diamond-Coated Tungsten/Rhenium Alloy Wire Fracture Surface Broken in 3 Point Bending This image magnifies a feature outlined in red from the previous picture. It shows some intergranular separation as well as a few lines on the surface oriented in the direction of the crack. 43

44 The next set of images of show the Mo/Re alloy at the specific locations. Figure 4.6A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point A) The diamond coated molybdenum/rhenium alloy samples show a much different type of fracture mode compared to the tungsten-based samples; Figures XX to XX show characteristics for fracture at three different points, as was described in Section XX. In a thin layer around the wire s surface, a brittle transformation occurred, resulting in sharp 44

45 inward cracks on the surface (which can be seen in Firgure 4.6C). However, these cracks are blunted once they reach the ductile core of the material. 45

46 Figure 4.6B X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point A) In the magnified view (Figure 4.6B), the dimples are present in the core; however, they appear less elongated than for the uncoated molybdenum-rhenium pulled in tension. 46

47 Figure 4.6C X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Profile (Point A) In Figure 4.6C is shown the profile view of the same wire so that the cracks can be clearly seen penetrating from the outer surface. The cracks begin to separate when the ductile region begins to plastically deform. Further magnification is shown below in Figure 4.6D. 47

48 Figure 4.6D. 5000X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Outer Surface (Point A) In this image (Figure 4.6D), a crack is highly magnified showing the transition from the brittle material to the ductile material in the wire s core. 48

49 Figure 4.7A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point B) As fracture is shifted down the length of the wire, corresponding to increasing distances from the filaments during the diamond growth, the mode of fracture begins to again resemble that of the uncoated molybdenum/rhenium alloy. In Figure 4.7A is the image for fracture at Point B which is 1.5 mm below Point A, which has already been discussed. For this fracture, necking and the dimples are again more prevalent. 49

50 Figure 4.7B. 1750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point B) A higher magnification image of the fracture surface (Figure 4.7B) shows the elongation of the dimples consistent with a ductile mode of fracture. 50

51 Figure 4.7C. 3250X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Broken in Tension Outer Surface (Point B) Another profile image of the wire (Figure 4.7C) shows how the scales on the outside of the surface from a brittle surface layer are smaller in size compared to those seen for fracture at Point A, resulting in crack that penetrates to a smaller amount into the surface around the outside of the wire. 51

52 Figure 4.8A. 750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) Fracture of the molybdenum rhenium alloy at Point C, which is 3 mm below Point A, is shown in Figure 4.8A. This wire s fracture characteristics closely resemble those of the uncoated specimens. Necking and dimples are present, with small traces of a brittle transition on the surface (seen in 4.8C). 52

53 Figure 4.8B. 1750X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) The dimples show (Figure 4.8B) that the fracture mode is clearly ductile in nature. 53

54 Ductile Center Brittle Surface Figure 4.8C. 4000X Magnification of Diamond-Coated Molybdenum/Rhenium Alloy Wire Fracture Surface Broken in Tension (Point C) For this image (Figure 4.8C) of molybdenum-rhenium fractured at Point C, small regions are visible where the brittle transformation has occurred. These small scales show the brittle outside part of the wire quickly transitioning to the ductile material found in the core of the wire. 54

55 4.5 Fracture Surface Analysis These fracture surfaces provided several conclusions about the diamond coating process. First, the diamond coating process embrittled all of the materials, with the extent of the embrittlement varying with the selection of the material. The tungsten and tungsten/rhenium samples were clearly the most affected by the CVD process as their fracture planes showed signs of a brittle fracture mode throughout the bulk of the wire, including a flat surface and river lines pointing to the point of initiation of the failure. The molybdenum/rhenium samples performed much differently. The fracture surface images showed ductile fracturing at all three points of interest. Point C was clearly the most ductile and its microstructure closely resembled features found on the uncoated sample of the same material. In contrast, Point A began to show signs of the diamond coating process starting to weaken the material by creating a brittle region surrounding the core of ductile material. Finally, being at a position between the two previous extremes, point B shows a mix of characteristics from both points A and C, but more closely resembling point C rather than A. 4.6 Horizontally Grown Sample Analysis The horizontally-grown samples were constructed to study how the duration of growth time in the reactor during the diamond coating process would affect the material properties of the subjected wires; the various growth times compared were 2, 4, 8 and 20 hours. Only the tungsten wire and the molybdenum/rhenium alloy wire were tested. This decision was made because the molybdenum/rhenium alloy had shown the most ductile characteristics and the tungsten had shown the most brittle characteristics after the diamond growth process. By subjecting these wire to varying amounts of time where the 55

56 filaments were at the elevated temperature, the expected ductile to brittle transition was to be analyzed using high magnification images of the fracture planes. Furthermore, by subjecting the molybdenum/rhenium sample to a 20-hour growth, the sample will exhibit the same material properties as the tip of the hook in the vertically oriented samples. 56

57 4.6.1 Tungsten Wire Samples It is important to note that these tungsten samples were again broken in 3point bending, which will give a slight bias in the orientation of the features on the surface in the direction of the crack. Figure X Magnification of 2 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending 57

58 The tungsten wire (Figure 4.9) subjected to a two-hour diamond growth run already transitioned from ductile to brittle. However, the fracture surface is not macroscopically flat, suggesting that a limited amount of ductility remains in the sample. 58

59 Figure X Magnification of 4 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending A magnified view (Figure 4.10) of a tungsten wire after 4 hours of growth shows that any remaining ductility seen from the 2-hour growth sample was eliminated. The sample became purely brittle in nature. 59

60 Figure X Magnification of 8 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending Furthermore, Figure 4.11 shows the purely brittle tungsten wire after being subjected to the 8 hour diamond growth; the diamond film was thicker, as expected with a longer growth time. 60

61 Figure 4.12A. 1000X Magnification of 20 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending A magnified image (Figure 4.12A) for the 20-hour diamond growth on the tungsten wire clearly shows a brittle fracture mode. The large thickness of the diamond coating (approximately µm) is to be noted. 61

62 Figure 4.12B. 600X Magnification of 20 Hour Duration Diamond-Coated Tungsten Wire Fracture Surface Broken in 3 Point Bending This 600X magnified image (Figure 4.12B) of the 20-hour diamond growth process on tungsten wire shows the completeness of the diamond film. The uniform polycrystalline diamond had completely encased the entire length of the sample. 62

63 4.6.2 Molybdenum Rhenium Samples Figure 4.13A. 600X Magnification of 2 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension In this 2 hour growth image (Figure 4.13A) of the molybdenum /rhenium alloy, the brittle exterior transformation can clearly be seen. However, the material still displays necking and a dimpled fracture surface from the ductile core comprising the bulk of the sample. The diamond film is sporadic at this point and is not readily seen in Figure 4.13 A. 63

64 Figure 4.13B. 1200X Magnification of 2 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension After 2 hours of diamond growth (Figure 4.12B) on the molybdenum-rhenium wires, the dimples have been extended along the axis of tension. 64

65 Brittle Surface Ductile Core Diamond Growth Figure 4.14A. 750X Magnification of 4 Hour Duration Diamond-Coated Molybdenum Rhenium Wire Fracture Surface Broken in Tension After 4 hours of growth (Figure 4.13A), the brittle exterior has extended from sporadic brittle scales to a complete outer layer encasing the ductile core. The additional diamond layer can also be seen further down the wire. Some delamination of the diamond has occurred as a result from applying pressure with the clamps from the tensile test; this suggests the adhesion to the molybdenum rhenium might be an issue with increased diamond thickness. 65

66 Figure 4.14B. 1200X Magnification of 4 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension Further magnification of the fracture surface for the 4-hour growth (Figure 4.13B) shows the dimples are not as extended in the tensile direction as observed for the shorter growth times. 66

67 Figure 4.15A. 750X Magnification of 8 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension After an 8 hour growth process (Figure 4.14A), the brittle exterior (non-diamond) layer of the diamond-coated molybdenum-rhenium wire greatly penetrates into the ductile core. The necking in the ductile region has been greatly reduced due to the flatter appearance of the core. 67

68 Figure 4.15B. 1250X Magnification of 8 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension After 8 hours of growth (Figure 4.14B), dimples still exist in the ductile region, but the pores are smaller relative to those seen in the samples coated for shorter times. 68

69 Ductile Core Brittle Layer Diamond Growth Figure 4.16A. 750X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension After the 20 hour growth (Figure 4.15A), a thick (approximately 20µm) diamond film covers the wire surface. The diamond coats a brittle outer (non-diamond) layer that appears to have some cracks through the length of the wire. In the core of the fracture surface, a small brittle region can be spotted. 69

70 Brittle Surface Ductile Core Diamond Growth Figure 4.16B. 1751X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension Higher magnification (Figure 4.15B) of the 20-hour growth run displays crack formation in the brittle layer of the wire. This could be a result of hydrogen embrittlement. Hydrogen embrittlement has been known to cause cracking in brittle materials [9]. The diamond film also appears to have separated from the brittle alloy layer; this could be a similar delamination observed for growth of thick diamond layers on molybdenum substrates. 70

71 Figure 4.16C. 750X Magnification of 20 Hour Duration Diamond-Coated Molybdenum-Rhenium Wire Fracture Surface Broken in Tension This image (Figure 4.15C) displays the completeness of the polycrystalline diamond film surrounding the brittle area. 71