Patent Application
20230340664
This invention relates to ceramic coatings for the prevention or inhibition of metal deposition, microorganism growth, and corrosion on metal substrates.
Ionic silver, added as the soluble silver fluoride (AgF) salt, is used in water distribution systems to control the growth of microorganisms in water. Due to the high antimicrobial activity of Ag+, ionic silver is an excellent approach to control the proliferation of microorganisms suspended in water. However, silver ions have limitations when it comes to long term control of biofilm growth in water treatment systems, either during operations or in dormant periods. The water-soluble silver forms tend to quickly react with surfaces such as stainless steel and is removed from the water. The low lifetime of silver ions in the water phase creates potential risks for biofilm growth, which ultimately leads to health risks and system damage. This process is a galvanic deposition process, triggering the reduction of silver ions to metallic silver/silver oxides on the surface.
This disclosure describes a stable redox insulative coating of ceramic material on the surface of a metallic substrate, as well as the coated substrates, to limit corrosion, microbial growth, and galvanic deposition of other metals (e.g., silver). Examples of suitable insulating ceramic-type two-dimensional (2D) materials coating include aluminum oxide (Al2O3, also known as alumina) and hexagonal boron nitride (hBN). Examples of suitable metallic substrates include components of biocidal water purification systems and medical devices.
Homogeneous coating of the disclosed ceramic materials on metallic substrates (e.g., stainless steel or other alloys, titanium) can be achieved by thin film deposition techniques (e.g., atomic layer deposition and chemical vapor deposition), which allow for a controllable thickness to be deposited on the surface. The insulating nature of these coatings is shown to prevent or inhibit the galvanic deposition of silver on the metallic substrate. These coatings can be applied to materials of very high aspect ratio (up to 1:2500), making them appropriate for coatings on high surface to volume ratio structures such as pipes and tubes.
In a first general aspect, a coated substrate includes a metallic substrate and a ceramic coating on the metallic substrate. The ceramic coating includes one or more layers, and a total thickness of the ceramic coating is in a range of 2 nm to 200 nm.
Implementations of the first general aspect can include one or more of the following features.
In some cases, the total thickness of the ceramic coating is in a range of 5 nm to 50 nm. In some implementations, the ceramic coating includes aluminum oxide. The ceramic coating can include hexagonal boron nitride. In some cases, the ceramic coating includes one or more of hafnium oxide, titanium oxide, and molybdenum disulfide. In some implementations, the ceramic coating includes two or more layers. The metallic substrate can include a metal or metal alloy. In some cases, the metallic substrate includes stainless steel or titanium. In some implementations, the ceramic coating inhibits or prevents the deposition of silver on the coated substrate. The ceramic coating can inhibit or prevent microbial growth on the coated substrate. In some cases, the metallic substrate includes a component of a biocidal water purification system. In some implementations, the metallic substrate includes a medical device.
In a second general aspect, coating a metallic substrate includes disposing a first ceramic coating layer on the metallic substrate and disposing one or more additional ceramic coating layers on the first ceramic coating layer to yield a laminated substrate. A total thickness of the ceramic coating layers on the laminated substrate is in a range of 2 nm to 200 nm.
Implementations of the second general aspect can include one or more of the following features.
In some implementations, the total thickness of the ceramic coating layers on the laminated substrate is in a range of 5 nm to 50 nm. In some cases, disposing the first ceramic coating layer on the metallic substrate includes contacting the metallic substrate with trimethylaluminium at a temperature in a range of 1700° C. to 1900° C. to form a layer of aluminum oxide on the metallic substrate. In some implementations, disposing the first ceramic coating layer on the metallic substrate includes contacting the metallic substrate with a mixture borazine and hydrogen and ethylene to form a layer of boron hydride on the metallic substrate. In some implementations, the second general aspect includes atomic layer deposition or chemical vapor deposition. In some cases, the metallic substrate includes a metal or metal alloy. The metallic substrate can include stainless steel or titanium. In some implementations, the metallic substrate includes a component of a biocidal water purification system. In some cases, metallic substrate includes a medical device.
The ceramic coatings are stable at high temperature and, due at least in part to the use of thin film deposition techniques, tightly bound to the metallic substrate surface. These characteristics prevent or inhibit coating detachment under conditions such as high water temperature, high shear forces, and extreme pH. Ceramic materials that are 40-100 times harder than parylene-C or Teflon™ AF2400 can be used, thereby limiting the risk of coating damage. The coatings are effective at a much lower thickness than polymer coatings. Results disclosed herein show <3% 7-day silver loss on 316L stainless steel coated with only 25 nm of ceramic materials. The effectiveness of the coatings at nanometer thicknesses allows them to be used on substrates with complex geometries such as SWAGELOK® fittings and flex bellows.
The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.
This disclosure describes the coating of insulating ceramic material on the surface of a metallic substrate, as well as the coated substrates, to limit corrosion, microbial growth, and galvanic deposition of other metals on the metallic substrate. Examples of suitable insulating ceramic-type two-dimensional (2D) material coatings include aluminum oxide and hexagonal boron nitride. Aluminum oxide (Al2O3, also known as alumina) is an insulating material that is stable in water and non-toxic (toxicity class 0) to humans. Hexagonal boron nitride (hBN) is an insulating material that can inhibit or prevent redox reactions on metallic substrates for corrosion prevention. Examples of suitable substrates include metals and metal alloys, such as stainless steel (SS). As described herein, 2D materials are generally understood to be materials that have a measurement in one dimension (e.g., a thickness) of less than 1 micrometer.
Uniform coating on a metallic substrate can be achieved by thin film deposition techniques such as atomic layer deposition and chemical vapor deposition, which allow for a controllable thickness of the coating on the substrate. These coatings can be applied on substrates with high aspect ratios (e.g., up to 1:2500), making them appropriate for coatings on high surface to volume (S/V) ratio structures such as cylindrical substrates (e.g., pipes, tubes, and the like).
Thin film deposition techniques can provide a continuous and uniform surface coating of controllable thickness.
The coated metallic substrates can include components of biocidal water purification systems that use silver as a biocide. The ceramic coating on the components inhibits or prevents the loss of silver ions in the water over time. Because the disclosed ceramic coatings are effective at nanoscale thickness (e.g., 2 nm to 200 nm, 5 nm to 50 nm, 10 nm to 40 nm, 20 nm to 30 nm, or 25 nm), these coatings are suitable for applications that typically require low-dimensional tolerances. Examples of substrates requiring low-dimensional tolerances that can be used as components of biocidal water purification systems include SWAGELOK® fittings and flex bellows.
Ceramic materials such as alumina or hexagonal boron nitride with a hardness 40-100 times than that of parylene-C or Teflon™ AF2400 can be used for ceramic coatings described herein, thereby limiting the risk of coating damage. These ceramic materials are also stable at high temperature and, due at least in part to the selected coating procedures (e.g., ALD), tightly bound to the substrate surface. This tight bonding can reduce or prevent coating detachment, even at elevated temperatures, extreme pH, and when exposed to high shear stresses.
Stainless steels are alloys that can be used for a wide range of applications even in aggressive environments, such as in low/high pH waters, hydroelectric energy production components, marine environment, or in contact with body tissues. In particular circumstances, stainless steels may benefit from a further improvement of corrosion protection. Ceramic coatings can possess good thermal and electrical properties and are resistant to oxidation and erosion in high temperature environments. These properties are useful in applications such as pipelines, castings, and automotive parts. The disclosed 2D ceramic coatings have mechanical properties and corrosion resistance that can be advantageous in applications such as surface passivation, gas diffusion barriers, and anti-reflection layers.
Prevention or inhibition of microbial growth on medical devices can reduce or eliminate infections and other complications, such as the rejection of the medical device or a medical implant by the body. Coated nanoscale thick laminated films have been prepared with combinations of metal oxides and with accurate control over the film thickness and composition by ALD. These anti-biofilm nanolaminated coatings diminish the number of bacteria on surfaces compared to that of untreated substrates (e.g., catheters) by several orders of magnitude. These methods of 2D ceramic coating can be applied to the coating of medical devices with materials such as alumina, which is FDA approved for a variety of applications. Medical devices suitable for ceramic coating include stents, catheters, pacemaker enclosures, artificial joints, bone fixators, spinal fixators, and medical implants (e.g., dental implants).
A method of coating a metallic substrate (e.g., stainless steel or other alloy, titanium) includes disposing a first ceramic coating layer on the metallic substrate, and disposing one or more additional ceramic coating layers on the first ceramic layer to yield a laminated substrate. A total thickness of the ceramic coating layers on the laminated substrate is in a range of 2 nm to 200 nm (e.g., 5 nm to 50 nm). In one example, disposing the first ceramic coating layer on the metallic substrate includes contacting the metallic substrate with trimethylaluminium at a temperature in a range of 1700° C. to 1900° C. to form a layer of aluminum oxide on the metallic substrate. In one example, disposing the first ceramic coating layer on the metallic substrate includes contacting the metallic substrate with a borazine and hydrogen mixture and ethylene to form a layer of boron hydride on the metallic substrate. In some examples, disposing the ceramic coating on the metallic substrate includes atomic layer deposition. In certain examples, disposing the ceramic coating on the metallic substrate includes chemical vapor deposition. Suitable examples of metallic substrates include biocidal water purification system components and medical devices (e.g., medical implants).
316L stainless steel (SS) was used as a model surface for treatment. The 316L SS coupons were obtained from McMaster-Carr (Elmhurst, TL) and cut into a 4-inch diameter size to be coated with Al2O3 or hexagonal boron nitride (hBN). For Al2O3 coating, a trimethylaluminium (TMA, (CH3)3Al) precursor was used to grow the alumina layer on SS. The thickness was varied from 5 nm to 50 nm by varying the number of growth cycles. The growth rate was 0.1 nm per cycle at a temperature of 1700° C. to 1900° C. A Woollam ellipsometer was used to measure average film thickness (21 point measurement) with the average film index being 1.6490 at 632.8 nm. The ellipsometry measurement suggested a uniformity of variation of <1% on flat surfaces. Similarly, 2D material hBN was deposited by chemical vapor deposition (CVD). Through the CVD method, hBN was synthesized on the metal surface (serving as the catalyst) using borazine (B3N3H6) and H2 mixed with ethylene. Table 1 shows the elemental composition of the 316L SS surface before and after coating with either Al2O3 or hBN. The disappearance of the peaks associated with 316L SS (e.g., Fe, Cr, Mn) in the Al2O3 coated sample indicates that the coating was uniform and achieved complete surface coverage.
The morphology of the coated and uncoated surface was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The AFM imaging was performed by tapping mode on a Multimode 8 Bruker AFM. The SEM micrographs were collected on an ESEM-FEG XL-30 (Philips Hitachi), which was equipped with an energy dispersive X-ray spectrometer (EDX).
A thin film deposition method was applied to generate ceramic coatings of different thicknesses ranging from 5 nm to 50 nm on 316L SS. AFM images showed that the surface morphology of the pristine 316L SS and the same material coated with 10 nm and 50 nm of alumina did not appear to be altered, indicating that the thin alumina layer was conformal (e.g., followed the surface topography to coat the surface). No gaps were found in the coating applied on the SS surfaces.
Similar conclusions were obtained from SEM imaging.
Change in the surface morphology was also analyzed by surface roughness calculations using AFM images. The change in root mean square (RMS) surface roughness is shown in
Step-height measurements were performed on SS 316 coupons functionalized with 10 nm and 50 nm Al2O3 using atomic layer deposition (ALD). The thickness was measured by placing a semiconductor tape in the middle of a virgin wafer and then placing the SS samples close to the surroundings of the center of the wafer. Since all the coupons were coated on both sides, the thickness was doubled. Therefore, by dividing the thicknesses in half, 50 nm Al2O3 and 10 nm Al2O3 samples showed 50 nm and 11 nm thicknesses, respectively. These values verify the accuracy of the ALD process with regard to coating thickness.
The composition of the coatings at different thicknesses was evaluated by X-ray photoelectron spectroscopy (XPS) elemental analysis for samples before and after AgF exposure. As provided in Table 1, the addition of an Al2O3 layer masks the signal from the 316L SS substrate, indicating complete surface coverage. After AgF exposure, the alumina-coated sample did not show any peak associated with Ag, although a fluoride (F) peak is observed in XPS spectra and quantified in Table 2. This suggests that either silver does not interact with the alumina-coated sample or that its reaction does not occur on the alumina layer. In comparison, pristine 316L SS exposed to AgF shows the appearance of an Ag peak in the XPS analysis, listed in Table 2. The absence of Ag peaks in alumina-coated samples is consistent across all coating thicknesses.
1Samples measured after 7 days exposure to 0.4 ppm AgF.
Capacitance-voltage (C-V) measurements can be employed for a number of purposes such as determining the properties of a dielectric material. This approach was used to characterize the insulating properties of the 2D alumina coatings. For an ideal metal-dielectric-metal structure, there should be little or no capacitance variation with frequency or applied bias. Based on C-V measurement data for both 10 nm and 25 nm Al2O3-coated SS samples, the capacitance is small (˜10 pF) and fixed across the voltage range. This indicates that there was no aluminum oxide dielectric leak in the SS samples. In comparison, a 10 nm Al2O3-coated SS sample that was intentionally damaged by cutting the sample to expose the SS surface at the edges shows much higher capacitance (˜46 pF), and this value varied across the voltage range. This contrast between the intact and damaged samples demonstrates the effectiveness of the alumina coating in insulating the metallic surface.
SS coupons coated with different thicknesses (5 nm-50 nm) of Al2O3 were placed in a solution of deionized water with 0.4 ppm of silver added as AgF. Samples were exposed to the AgF solution at a surface to volume (S/V) ratio of 2. The samples were immersed in the AgF solution in sealed tubes on an orbital shaker at 50 rpm for 7 and 14 days. Once the coupons were removed from the tubes, the remaining liquid was acidified to 2% HNO3 using trace metal grade HNO3, and the concentration of silver remaining in the solution was measured using inductively coupled plasma mass spectroscopy (ICP-MS).
ICP-MS results for the 7-day experiments indicate that a minimum thickness is preferable for good retention of silver in solution when in contact with Al2O3-coated SS. Silver loss decreased with increasing coating thickness.
Prolonging the exposure to 14 days resulted in higher silver loss compared to 7 days. For the optimal coating thickness of 25 nm, the silver concentration decreases from 98.8% of the initial 0.4 ppm concentration at 7 days to 83.2% of the initial concentration at 14 days, shown in
XPS analysis of the elemental composition of the alumina-coated SS samples after exposure to AgF for 7 days is shown in Table 2. The silver loss assays suggest that silver was reacting with the alumina-coated sample, especially at a coating thickness lower than 25 nm, evidenced by the decrease in silver concentration observed in the AgF solution. However, no Ag peak was observed even on the 10 nm alumina-coated SS sample. These two results suggest that the reaction of silver ions with the alumina-coated SS samples does not occur with the alumina coating itself. At low coating thickness, silver ions are hypothesized to be able to penetrate the coating through small defects and react with the SS substrate underneath the alumina layer. This diffusion of silver ions beyond the coating can explain the silver loss observed from the AgF solution and the absence of Ag peak in the XPS spectra.
To determine if a change in pH affects the stability of the ceramic coating, 316L SS as well as 10 nm and 25 nm Al2O3-coated samples were placed inside 50 mL tubes containing deionized (DI) water at various pHs, ranging from 5 to 9. Samples were then taken out after 7 days and AFM analyses were conducted on them. For the 10 nm coated SS, no change in RMS surface roughness value was noted after exposure to the different pH conditions, suggesting no alteration in the coating morphology. For the 25 nm sample, which had initially higher RMS surface roughness values, a decrease in RMS value was observed for the pH 9 conditions. However, visual and AFM images of the coating did not show coating damage, and alumina is expected to be stable at pH 9.
Nanoindentation analysis was carried out to characterize the mechanical properties of the alumina coating. The indentation modulus was found to change with the indentation depth, as the indentation probe penetrated deeper into the alumina layer and eventually into the SS substrate, which is softer than alumina. A transition zone, between 30 and 40 nm penetration depth, was found where the modulus value changed from being mostly coating-related (with a value of ˜145 GPa) to being a mixture of the properties of the coating and the substrate (resulting in a decrease in the indentation modulus). A similar trend was found for the indentation hardness. The hardness value of the alumina layer was found to be ˜11 GPa. Because of limitations related to the minimum penetration depth required for reproductive indentation data, only the 50 nm-coated sample was measured. The “weathered” samples, which were exposed to the different water chemistries, were all 25 nm coatings and could not be measured.
To evaluate the coating detachment from the SS surface under high shear forces, a water jet system was used which included a waterjet cell, pump tubing, peristaltic pump and a solution reservoir. The water jet cell was customized from a 2.5-inch PVC compression coupling (Homewerks Worldwide) to hold the coupons at a fixed location and prevent water splash. A clear plastic tube was positioned between the water jet and the location at which the water hit the coupon to provide a view of the water stream accurately coming in contact with the coupon. Samples of pristine SS 316 and SS 316 with coatings having thicknesses of 10 nm and 25 nm were placed on a circular opening and positioned to be visible in the water stream. The bottom polytetrafluoroethylene (PTFE) cushion provided support for the SS coupon, while the top gasket had a circular hole (∅=0.634 cm) to expose ˜0.32 cm2 membrane to the water jet from a vertical direction.
On the upper end of the coupling, a ⅛″ jet nozzle (gauge 16 needle) was fixed on the top of the coupling and connected to a ¼″ pump tubing. The waterjet cell was fixed on a metallic stand using two clamps to maintain a vertical position. Additionally, 1 L of deionized water was poured inside a plastic beaker which served as the water reservoir. Then, coupons were exposed to the water stream for 5, 10, 15, and 30 minute contact times. After each time interval, the system was stopped and 15 mL sample of the water in the reservoir was collected.
To characterize the surface coatings after being exposed to the high shear forces of the water jet system, optical microscopy images were taken of the SS coupons. Analysis indicated that SS coatings with 10 nm coating thickness sample had an imprint which could potentially be the circle of the seal edge, whereas the 25 nm coating thickness sample showed no abnormalities. The 25 nm coating thickness appeared to be stable under the water jet conditions.
The 15 mL water samples were characterized using ICP-MS.
To further investigate and determine the effectiveness of these 2D alumina coatings on complex large-scale SS structures used in biocidal water purification systems, complex geometries including high-purity SS tubing (SWAGELOK©) and flex bellows were functionalized with a ceramic coating by using the ALD technique.
The samples were heated to 180° C. over the course of ˜4 hrs. In-situ quartz crystal microbalance monitoring indicated a typical TMA/water growth per cycle of 36 ng/cm2/cycle, which resulted in a standard alumina ALD growth of ˜0.12 nm/cycle. No significant change in the tint on the SS flex bellows 702 was observed, which is typical for such thin Al2O3 films. Ellipsometry on the silicon wafer samples 704 yielded an alumina coating thickness of 27.4 and 27.8 nm. These results indicate that the alumina coating was deposited as intended.
Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.
Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.
Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.
This application claims the benefit of U.S. patent application No. 63/334,900 filed on Apr. 26, 2022, which is incorporated herein by reference in its entirety.
This invention was made with government support under 80NSSC20C0320 awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63334900 | Apr 2022 | US |
