Patent Application
20130011578
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The present invention relates generally to the field of directed vapor deposition and more specifically to the deposition of materials onto non-line of sight areas.
Substrates can be coated by reactive or non-reactive evaporation using conventional processes and apparatuses known as physical vapor deposition (PVD).
An improved process and apparatus for vapor depositions on a substrate in a vacuum has been developed and is known as directed vapor deposition (DVD).
The present invention improves the DVD process by the development and incorporation of advanced methods and apparatus, which enable materials to be effectively applied at high rate with the desired composition and microstructure onto complex components having non line-of-sight (NLOS) regions.
The present invention provides for a method and apparatus for the directed vapor deposition (DVD) of materials onto non-line of sight (NLOS) portions of a substrate. The method and apparatus includes evaporating a first material for deposition onto the substrate, the evaporation generating a plurality of vapor molecules. The method and apparatus therein provides for the insertion of a carrier gas and the direction of the vapor molecules to be deposited in NLOS regions of the substrate. The present invention provides for varying embodiments incorporating different aspects for improving the NLOS DVD usable individually or in combination.
The present invention includes varying embodiments for the NLOS DVD including in one embodiment utilizing plasma activation to ionize the vapor particles to create charged vapor molecules. This embodiment further includes biasing the substrate to attract the charged vapor molecules onto the NLOS portion of the substrate. In one embodiment, the plasma activation may include a hollow-cathode plasma unit.
The present invention includes another embodiment for the NLOS DVD including utilization of an inert gas as the carrier gas. The inert gas as a carrier gas provides for a specific density and velocity, so the energy of the carrier gas enhances the NLOS affect. For example, the inert gas may be Helium or Argon, such that the insertion of the inert gas as the carrier gas provides for the deposition of the vapor molecules in the NLOS region.
The present invention includes another embodiment for NLOS DVD including pre-heating the carrier gas prior to its insertion into the deposition chamber. Varying embodiments may be utilized to heat the carrier gas prior to its insertion into the chamber, such that upon insertion therein, the carrier gas provides for improved NLOS DVD on the substrate.
The present invention includes another embodiment in which the carrier gas nozzle is modified to enable the formation of gas conditions in which enhanced NLOS DVD coating is obtained. The modified nozzle enables the co-evaporation from multiple crucibles which allow for the area coated to be increased.
The invention is illustrated in the figures of the accompanying drawings which are meant to be exemplary and not limiting, in which like references are intended to refer to like or corresponding parts, and in which:
a-d are images of vapor deposition on a stainless steel tube;
a illustrates one embodiment of a gas pre-heater and
a illustrates one embodiment of resistive tubing configuration and
a illustrates one embodiment of a gas heating module and
This application describes a process for applying materials at high rate having the desired composition and microstructure onto complex components having NLOS regions. Processing conditions for coating on NLOS substrate regions are described that enable:
Improved coating growth rate in NLOS regions of a component beyond prior DVD techniques.
Combinations of high NLOS growth rates and high deposition rates that will reduce the production costs including use of inert gas, such as but not expressly limited to Argon.
The use of advanced gas jet properties along with a plasma activated DVD process that can ionize vapor molecules to further optimize the NLOS growth rates achieved using the DVD approach.
The use of plasma activated directed vapor deposition (PA-DVD) to expand the range of process conditions which result in higher NLOS growth rates and/or high deposition efficiencies than current baseline NLOS coating conditions.
The modifications of the carrier gas velocity, chamber pressure and pressure ratio to increase the coating growth rate in the NLOS regions with similar coating microstructure and crystallinity as in line-of-sight regions.
The identification of DVD process conditions which result in a both excellent NLOS region growth rates and effective performance characteristics (such as a high thermal barrier coating lifetimes, suitable oxidation resistance in a oxidation protection coating or excellent corrosion resistance or environmental protection in a environmental protection coating).
The pre-heating of carrier gas used to create a supersonic gas jet in a DVD approach, enabling an increased velocity (or kinetic energy) in the jet and promote vapor infiltration into NLOS regions of substrate.
The design of a NLOS coating apparatus in which a carrier gas pre-heating capability is included.
One approach to improve the NLOS coating growth rate is the use of plasma activated directed vapor deposition (PA-DVD). In this case, plasma activation is used to ionize the vapor molecules and pulsed substrate biasing is used to attract the charged molecules onto NLOS surfaces, as illustrated in
NLOS coating in the DVD process is a result of the collisions between carrier gas and vapor molecules that can be used to control the transfer of the vapor molecules from the source to a substrate. These collisions enable the vapor molecules to be swept generally along the streamlines that are established by the carrier gas expansion into the chamber and therefore be transported into internal NLOS regions of substrates/component.
Three steps are utilized to obtain efficient NLOS coating on the interior of a complex engine component (such as a doublet vane). Step 1 is focusing of the vapor flux to create a high density flux of vapor molecules. Step 2 is infiltration of the focused vapor flux into the interior of the component. And Step three is de-focusing of the flux and deposition of the vapor molecules onto the substrate surface.
It has been demonstrated in the past using prior DVD NLOS conditions that the key properties of the gas jet (i.e. its density, velocity) strongly affect the NLOS growth rate and NLOS coating microstructure. This work has demonstrated that additional improvements to the NLOS coating capability of the baseline DVD technique (Version 1.0) may be achieved through the development of novel concepts to enable more optimal processing conditions. Of particular interest are modifications to the gas jet composition, density and velocity. The gas jet properties affect the infiltration of the vapor flux into the interior NLOS regions of complex components. The gas jet velocity can be increased through the incorporation of higher gas jet pressure ratios which can be achieved either through increased chamber pumping efficiency, the use of novel gas jet nozzle designs and/or the use of carrier gas pre-heating, which are described in further detail below.
Further, novel carrier gas conditions and compositions that modify the momentum of the gas jet atoms can further enhance NLOS coating growth rates. Alternate processing conditions using both He and Ar carrier gases are therefore envisioned. The use of any inert gas such as He, Ne, Ar, Kr and/or Xe and combinations of these are also envisioned as is the use of N2, Air and O2 or additions of these into the inert gases. Processing conditions are identified in which improved NLOS growth rates are obtainable using inert gases as the carrier gas (or combinations of the above gases) where the volume fraction and type of the gas are carefully controlled.
The identified processing conditions were studied to determine the rotation rates, temperatures and plasma activation conditions (if any) which enhanced the coating quality. The process conditions explored are given in Table 2.
As used in Table 2, A1, A2, B1 and B2 represent the prior art baseline techniques (Version 1.0), whereas A3, B3, C1-C3, D1-D3 and E1-E3 represent varying embodiments of multiple versions of DVD described herein.
The results in Table 3 below indicate significantly improved infiltration of vapor flux and hence the NLOS growth. The coating thickness ratio from coupon A to coupon C and coupon B to coupon C were the lowest of any mock-up box coating condition to date using He carrier gas compositions. It was also observed that reduced gas jet pressure ratios reduced the effectiveness of coating the mock-up boxes (see conditions C2 and B3). This was due to a reduction in the infiltration of the vapor flux into the box.
As with Table 3 (C), B1 relates to baseline NLOS processing conditions and E2, E3 and E3-M illustrates results on improved DVD NLOS processing conditions.
These results indicate that higher gas jet pressure ratios should provide a further opportunity to improve the NLOS coating conditions. Such conditions can be obtained using the PS-DVD system following a modification to the nozzle/crucible apparatus used in this system. Heating of the carrier gas may also produce enhanced effects without the need to alter the pumping rate or gas jet nozzle geometry. It is well known that high gas jet velocities can be enhanced by pre-heating the gas jets because the gas jet velocity, U, is proportional to its temperature, T. The gas jet velocity is given by the expression U=M(γRT)1/2 where M is the Mach number, γ is the ratio of specific heats, R is the specific gas constant and T is the temperature. Thus, increasing the gas jet temperature prior to expansion in the gas jet nozzle will increase the gas jet velocity.
Mock-up box coating was also performed in the work using an Argon carrier gas flow. It is understood that the higher mass of the Ar carrier gas allows focusing of the vapor flux and improved infiltration into NLOS regions of substrates using reduced gas flows and lower velocities. The use of reduced gas jet velocities while still enabling vapor flux infiltration into the NLOS regions results in more effective NLOS coatings having enhanced NLOS growth rates and properties in part based on the enhancement effects of the inert gas to facilitate vapor deposition. The mock-up box coating data for conditions E1, E2 and E3 are given in Table 3(b). Conditions E2 and E3 gave greatly improved coating uniformity with E2 yielding significantly improved NLOS coating conditions over those obtainable using the baseline approach.
The use of the E2 condition and a variable rotation pattern resulted in greatly enhanced coating uniformity of the mock-up box and indicates very promising processing conditions for use in component coating.
The results from the carrier gas modification work are summarized in
A 12× improvement is achieved through the combination of the enhanced conditions and variable rotation rate techniques.
The invention further provides for the enhanced NLOS deposition conditions based on the DVD approach using a gas injection apparatus. Multiple embodiments are described in detail below.
The design of a first embodiment of a gas pre-heater is based on a resistively heated tube through which the carrier gas could be flowed. The design was aided by an experimental investigation of heating Ar gas flowing through a tube, such as an Inconel tube. As described herein, the embodiments refer to an Inconel tube, but it is recognized that suitable tube may be utilized and the invention is not expressly limited to an Inconel tube.
Gas Heater version 1: In this case, the Inconel tubing was wound as a coil and voltage applied directly to each end of the tube so that tubing itself became the resistive heater (
Gas Heater Version 2:
a and 9b show the gas heating tube configuration. The full design of the gas heating module (GHM) and its location in the PS-DVD coater is shown in
Following the initial proof of concept of the heater design, the gas heater was designed and constructed next for the PS-DVD coater having varying wall thickness and/or length of the resistively heat gas carrier tube. In the PS-DVD design, (a) the heater was enclosed in a sealed and insulated container to reduce heat loss to the surroundings and (b) the gas exiting the heated coil flowed over the outer coil walls first before exiting the container. The electrical connections were welded to the coil and a thermocouple was added to monitor coil surface temperature for this iteration.
Table 4 shows the results from the tests performed with the PS-DVD gas heater (version 1 and 2) using a range of gas flow conditions and various heater configurations. Note that the carrier gas temperature was observed to be a function of the power applied to the gas heater and the heater configuration.
Gas Heater version 3: To reach gas pre-heat temperatures up to ˜800° C., additional embodiments provide for further optimization of the gas heater design. In this embodiment, the wall thickness, length of the resistively heated gas carrier tube, or both, were altered to improve the reliability and to obtain the high gas temperature. The gas heater v3 consists of single coil, enclosed in a sealed and insulated container to reduce heat loss to the surroundings. The electrical connections in this case were welded to the coil and a thermocouple was added to monitor coil surface temperatures.
To test the v3 heater configuration and determine stable operating conditions, two sets of experiments were designed based on the theoretical calculations concerning the capacity of this heater in terms of applied load and the possible achievable temperature:
Gas flow was kept constant at 20 slm and the working chamber pressure was maintained at 10 Pa. The power was increased slowly and kept constant at 480 W. The gas temperature and the heater coil temperatures were measured as a function of time.
Gas flow was kept constant at 20 slm and the working chamber pressure was maintained at 10 Pa. The power was increased slowly to 920 W and the variation of gas temperature and coil temperature was monitored.
Based on the above experimental data, it was concluded that with the above-noted embodiments readily provided for controllably pre-heating the gas temperature in the range of 300-400° C.
Gas Heater version 4: further embodiments of the pre-heating device provide for reaching higher temperatures. One approach included increasing the gas per-heat temperature by incorporating the longer heating tube and installing the heater inside the nozzle/crucible apparatus inside the PS-DVD coater.
To test the v4 gas heater the same protocol was followed as in previous case (20 slm of gas at 10 Pa). Because of the long length of tube, it was possible to apply a higher power (up to 5 kW) and the gas temperature and the heater coil temperature were measured as a function of time.
Efforts were also made to further improve the process robustness by adding additional thermocouples into the gas pre-heater set-up for continuous monitoring of the gas heater temperatures. Using Labview software a data log was configured in which the temperature was recorded every 10 seconds of both the gas temperature and heating coil temperature to enable the temperature stability during deposition to be monitored as shown in
Coatings were deposited using DVD NLOS conditions with or without gas preheating onto test coupons placed in LOS and NLOS regions of a mock-up geometry/box. Table 6 summarizes several of these runs. The change in weight ratio of the coated coupon was measured as a function of process conditions. At the highest gas pre heat temperature, a significant reduction in the thickness ratio of LOS to NLOS regions (A to C) was observed when compared with a no gas pre-heat condition. This clearly demonstrates the fact that with the increase in the carrier gas pre-heat temperature, vapor flux penetrates deeper into NLOS regions thereby resulting in an increase in thickness of coating in those regions.
In another embodiment, nozzle design variations further provide for improved direct vapor deposition in NLOS regions. Novel converging-diverging nozzle design was explored further to improve the NLOS efficiency and to aid in the development of fully scaled crucible/nozzle apparatus for use during full production scale coating application.
The main concept, shown in
Another design of the single source linear nozzle is given in
Using the installed single source linear nozzle, several test runs were performed. The nozzle opening area and the area of the diverged section were altered by systematically varying the spacing between nozzle plates and the crucible.
The processing conditions explored using the above configurations using ambient temperature carrier gas are given in Table 9. Letter and numbers were assigned to each process condition based on the approach used. Using these configurations along with an Argon carrier gas, chamber pressures ranging from 9 to 15 Pa and, in some cases, gas pre heating, optimization of the gas jet conditions required for NLOS coating onto doublet vane mock-up structures were performed The DVD NLOS process conditions are categorized with symbols F, G or H (Table 9) to distinguish runs performed using either the baseline (F), RANC#1(G) or RANC#2 (H) condition. Each nozzle set-up had a distinct range of pressure ratios. The inclusion of the letters M or P in the process condition code represented the sub conditions of rotation and gas pre heat, respectively
A mock up box run was also performed using the linear nozzle configuration to determine LOS to NLOS thickness ratio for this set-up. For condition 13 the pressure ratio as high as 27.6 could be achieved. Mock-up box runs were performed using the linear nozzles. The results indicated that using this nozzle design with a moderate pressure ratio (11.0) and chamber pressure (process condition I3) resulted in good NLOS coating efficiency, Table 11. Note that the A to C ratio was as good as with the linear nozzle conditions and thus, this nozzle geometry appears to be well suited for scale the crucible/nozzle apparatus to full production scale dimensions.
The above data is summarized in
DVD Processing conditions appropriate for applying TBC layer onto NLOS regions Have been determined to be: Temp.=950 to 1050° C., Pressure=8 to 15 Pa, Pressure ratio=>7, Carrier gas temperature: >200° C., plasma activation can optionally be used.
In one embodiment, plasma-activation in DVD is performed by a hollow-cathode plasma unit capable of producing a high-density plasma in the system's gas and vapor stream. The particular hollow cathode arc plasma technology used in DVD is able to ionize a large percentage of all gas and vapor species in the mixed stream flowing towards the coating surface. This ionization percentage in a low vacuum environment is unique to the DVD system and importantly the use of the plasma generates ions that can be accelerated towards the coating surface by either a self-bias or by an applied electrical potential. This enables some vapor species, which would otherwise not deposit onto the NLOS surface the ability to deposit, thereby increasing the NLOS growth rate.
To demonstrate this effect,
The experimental set-up for the laboratory scale DVD system (LS-DVD) required modifications to the heating set-up to enable the deposition onto the mock-up box while using the plasma activation system, as illustrated in
Results indicated that improved NLOS coating into the mock-up box was obtained including in this embodiment the usage of He gas as the carrier gas. Improvements were also noted through the addition of plasma activation and AC substrate biasing. Additional embodiments allow for the design of the plasma system and its introduction into the DVD processing environment in such a way that the plasma orientation may be aligned with the orientation of the vapor flux. The use of a heavier, Ar, carrier gas is also envisioned. This carrier gas can more effectively align the plasma direction with the direction of the vapor flux, such as illustrated in
A modified experimental set-up was also used to test the ability of the plasma system to further enhance NLOS coating efficiency. In this case, a 1″ diameter tube substrate was used and aligned in two configurations: A) the tube was aligned at 90° with respect to a source and B) the tube was aligned parallel and above the source, such as visible in
The results of
Two key results were obtained from these results: i) a plasma flux can be used to direct a vapor flux and promote its infiltration into NLOS regions; and ii) the plasma flux is estimated to be as effective as carrier gas jet for promoting the infiltration of a vapor flux into NLOS regions.
Thus, it appears that a plasma consisting in part of electrons having a scattering cross-sections may be an effective means to further promote additional NLOS coating efficiency. Thus, the combination of plasma infiltration with carrier gas infiltration will be an excellent technique to promote further NLOS coating capability
Microstructural analysis of the coated coupons of
Thermal Spallation Resistance: Optimized DVD NLOS processing conditions determined above must demonstrate as good or better thermal spallation resistance compared with conventionally applied TBC coatings with no gas (baseline condition). Under a parallel effort (DOD contract number: W911QX-07-C-0013) testing has demonstrated that robust coatings can be produced using the PS-DVD coater and processing conditions which use DVD NLOS Version 2.0, Table 6. Additional optimization and testing will be continued using the most optimal Ar gas pressure for NLOS coating and the use of gas pre-heating.
Erosion Testing (LOS and NLOS regions): TBC coated coupons placed in LOS and NLOS configurations. Low temperature erosion testing was then performed to demonstrate that as good or better erosion resistance than conventional EB-PVD TBC coatings can be obtained using the optimized DVD NLOS coating conditions.
Room temperature erosion tests were performed on coupons created using the DVD NLOS F3-M condition.
Notably, the figures and examples above are not meant to limit the scope of the present invention to a single embodiment, as other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention are described, and detailed descriptions of other portions of such known components are omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not necessarily be limited to other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, Applicant does not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.
The foregoing description of the specific embodiments so fully reveals the general nature of the invention that others can, by applying knowledge within the skill of the relevant art(s) (including the contents of the documents cited and incorporated by reference herein), readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Such adaptations and modifications are therefore intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.
The present application relates to and claims priority to Provisional Patent Application Ser. No. 61/339,126 entitled “Method for applying a coating at a high rate onto non-line-of-sight regions of a substrate” filed Jul. 7, 2010.
Work described herein was supported, in part, by the U.S. Navy under contract N68335-08-C-0322, Phase II SBIR. The United States government has certain rights in the invention. Work described herein was also supported, in part, by private funds.
