Patent Application
20150129087
The disclosure relates processes for making porous nitrogenized titanium coatings for use in medical devices.
Titanium nitride (TiN) films deposited by physical vapor deposition (PVD) can exhibit desirable properties for implantable medical device electrodes. The most desirable TiN coatings for electrode applications are typically relatively thick (2-20 um) and have a porous, columnar, microstructure which exhibits high specific surface area and high specific capacitance.
Formation of such porous, TiN columnar structures depends upon maintaining low add-atom mobility during the deposition process. Specifically, the fabrication of porous, controlled microstructures by sputter-deposition processes requires deposition process operation at relatively high working gas pressures, leading to low deposition rates. Such sputter-deposition processes result in long deposition times to achieve the desired 2-20 μm film thickness and therefore high cost.
The disclosure provides methods of treating surfaces or substrates using glancing angle deposition and nitrogen to produce substrates having porous surfaces having increased surface area and capacitance as compared to an untreated surface.
In one example, a method described in this application comprises providing a substrate having a surface, depositing a porous titanium film on the surface of the substrate using glancing angle deposition, and converting at least a portion of the porous titanium film to a porous film comprising nitrogenized surface layers using a thermal or plasma-assisted thermal gaseous nitrogenizing process.
The methods described in this application produce nitrogenized porous titanium films that are substantially free of titanium nitride (TiN).
a is a scanning electron microscope image of Example 1 after GLAD.
b is a scanning electron microscope image of a flaw shown in
c is a scanning electron microscope image showing the as deposited GLAD film morphology of Example 1 in cross-section following fracture of the film.
d is a scanning electron microscope image of Example 1 in cross section after GLAD and nitrogenizing.
Applicants have discovered that substrates having a porous titanium layer deposited onto the substrate using a glancing angle deposition process (GLAD) followed by thermal or plasma-assisted nitrogenizing of the porous titanium layer provides articles useful in medical devices, for example electrodes. The resulting article made by the above processing steps can be produced at a higher rate and can be produced at lower cost than can be made using other known processes. The nitrogenized porous titanium layer is substantially free of TiN, that is, free of detectable levels of TiN as determined by X-ray diffraction (XRD).
In general, a GLAD deposition process comprises, consists of or consists essentially of a process in which vapor deposited porous films are deposited on a substrate by directing a vapor flux at an oblique angle to the substrate. The substrate can be rotated at an axis normal to the substrate during the deposition process. The rotation can be at a set rate of slow continuous RPMs or can be paused after a partial rotation for a period of time then rotated, then paused, etc. or until a desired film thickness is obtained.
The incidence angle “α” is defined as the angle between the substrate holder and substrate surface normal 22 and the substantially line-of-sight path followed by the impinging evaporant flux from evaporant source 12 to the substrate surface. GLAD process system 10 further comprises deposition rate monitor 24 and moveable shutter 26. Deposition rate monitor 24 samples a fraction of the evaporant flux emitted from evaporant source 12 and provides feedback to evaporant source 12 such that a desired deposition rate can be established and maintained. Moveable shutter 26 can be positioned to control the fraction of evaporant flux emitted from evaporant source 12 reaching the substrate.
In the GLAD process described in this application, useful incidence angles include from about 65 degrees to about 85 degrees, including any range or value in between the range of about 65 degrees to about 85 degrees.
Useful substrates to be coated include those made from titanium, platinum, niobium, tantalum and alloys of any of them. Other materials capable of withstanding the temperatures associated with the subsequent nitrogenizing may be suitable for some applications.
In general, a thermal or plasma-assisted gaseous nitrogenizing process comprises or consists of or consists essentially of exposing a substrate within a vacuum furnace, with or without a plasma discharge, to nitrogen an elevated temperature and pressure for a time period until the desired nitridization occurs. Typically, the nitrogenizing process occurs in an oxygen and air free environment which is provided by first evacuating the vessel and then purging the vessel with nitrogen, argon or both. In this disclosure, the porous metal films are nitrogenized to form porous metal films comprising, having, consisting of, or consisting essentially of nitrogenized or nitrogen-enriched surfaces or surface layers. “Consisting essentially of” nitrogenized surfaces means surfaces substantially free of TiN.
Typically, the nitrogenizing process takes place for a time not exceeding about 2 hours at a temperature of from 650° C. to 750° C. at a pressure of less than 1×10−3 Pa. The temperature range of from 650° C. to 750° C. is intended to support any range or value between 650° C. to 750° C.
Methods described in this disclosure provide nitrogenized porous films that have increased capacitance and increased surface area when compared to untreated substrates as determined by the shift in impedance corner frequency. The shift in impedance corner frequency directly corresponds to a change in double-layer capacitance. Double-layer capacitance is directly proportional to surface area.
In one embodiment, the method including GLAD and nitrogenizing results in an increase in surface area (as measured by electrochemical impedance measurements, that is, the shift in impedance corner frequency) by a factor of 10 or greater as compared to the untreated geometric surface area of the substrate. In another embodiment, the method results in an increase in surface area (as measured by the shift in impedance corner frequency) by a factor of 100 or greater as compared to the untreated geometric surface area of the substrate. In another embodiment, the method results in an increase in surface area (as measured by the shift in impedance corner frequency) by a factor ranging from 10 to 100, inclusive and including any range or value within the range of 10 to 100.
The GLAD process system geometry used is shown schematically in
A grade 1 titanium sheet (25×50×0.18 mm) was prepared by cleaning in a hot deionized water/detergent mixture followed by multiple deionized hot water rinses. The titanium sheet was used as a substrate on which titanium in the form of a porous layer was deposited in a TEMESCAL electron beam evaporator (available from Temescal, Livermore, Calif.) by glancing angle deposition (GLAD). After mounting the substrate on the rotary substrate holder, the system was closed and evacuated to a pressure of <2.5 10−3 Pa. Power was then slowly increased to the electron beam evaporant source until the desired evaporation rate was established with the moveable shutter closed. Once the desired evaporation rate was established and stabilized, the moveable shutter was opened and deposition on the substrate commenced.
Each sample was rotated approximately 90° after a desired incremental film thickness was deposited (100 nm or 200 nm); the approximately 90° rotations were repeated after each incremental deposition until the desired total film thickness was obtained. The specific values of for deposition rate, incremental film thickness and total film thickness along with other process parameters are shown in Table 1. Upon achieving the desired total film thickness the evaporation source was shut down, the system was allowed to cool and was then vented to atmosphere.
The samples were then nitrogenized using either Nitrogenizing Process 1 or Nitrogenizing Process 2, described in more detail below. The nitrogenizing process temperature was 700° C. for all examples described herein. Process time is defined herein as the time from introduction of the nitrogen gas to the beginning of the furnace cooling cycle.
Nitrogenizing Process 1: Nitrogenizing process 1 was performed in a plasma-assisted thermal treatment system. As shown schematically in
Electrochemical impedance spectroscopy was used to compare the impedance magnitude as a function of frequency for uncoated titanium sheet to both as deposited and nitrogenized GLAD films. The measurements were performed using a SOLARTRON SI1287 Electrochemical Interface and SI1260 Impedance Gain-Phase Analyzer (Available from Solartron Analytical Division of Ametek, Farnborough, Hampshire, United Kingdom). The measurements were performed in phosphate-buffered saline solution. The AC excitation voltage was 100 mV, and the frequency range examined was 10,000 to 0.1 Hz. A platinum foil with an exposed surface area of approximately 14 cm2 was used as a counter electrode. Exposed surface area on the measured specimens was approximately 0.3 cm2.
Cyclic voltammetry was also performed using the identical experimental setup described for the impedance spectroscopy with the addition of a reference electrode. The reference electrode used was Ag/AgCl in 3M NaCl. Potentials were cycled from −1.5 to +1 volts relative to the reference electrode at a scan rate of 25 mV/s. Three cycles were typically performed. Changes in behavior observed for more than three cycles were minimal.
After the samples were deposited with titanium using the above described
GLAD process, all of the samples appeared flat-black in color. The samples exposed to Nitrogenizing process 1 described above were turned light brown in color.
a is an SEM image (500×) of Example 1 after GLAD but before Nitrogenizing Process 1 as viewed substantially normal to the sample surface.
b is an SEM image (10,000×) of a flaw shown in
c is an SEM image (30,000×) showing the as deposited GLAD film morphology of Example 1 in cross-section following fracture of the film as described above. The morphology comprises small, partially isolated columns which exhibit a “kinked” structure due to the stepwise rotation during deposition.
d is an SEM image (30,000×) of Example 1 in cross section after the GLAD process and Nitrogenizing Process 1 described above.
The highly porous nature of the resulting structure is evident from
Flat titanium samples were treated using GLAD as described above. The GLAD treated samples were nitrogenized using Nitrogenizing Process 2 for two hours at 650° C. (Example 8), 750° C. (Example 9), and 850° C. (Example 10). Each sample nitrogenized at one of the three temperatures was analyzed using XRD. No additional phase formation was shown at 650° C. The sample nitrogenized at 750° C. showed that the presence of Ti2N was predominating, with a small peak attributable to TiN. The sample nitrogenized at 850° C. showed more of both Ti2N and TiN as compared with the sample nitrogenized at 750° C.
X-ray diffraction (XRD) was performed on a grade 1 titanium sheet sample nitrogenized as described in Nitrogenizing Process 2. The measurements were performed using a PANalytical Xpert mpd system (available from PANalytical B.V., Lelyweg 1, 7602 EA ALMELO, The Netherlands). The XRD system employed a Cu target X-ray source operated at 40 KV, 40 mA. The incident radiation was filtered by a nickel filter and the system employed an Xcellerator multi-channel detector. The scan range was 30-80 degrees 2Θ, with a 2Θ step size of 0.02 degrees. Measurement time per step (multichannel data collection) was 163 seconds.
As shown in
Thus, embodiments of the methods of forming porous nitrogenized titanium films are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.
