WO2014014541A2

WO2014014541A2 - Wear resistant coatings and process for the application thereof

Info

Publication number: WO2014014541A2
Application number: PCT/US2013/038630
Authority: WO
Inventors: Susie EUSTIS
Original assignee: Directed Vapor Technologies International; Hass, Derek
Priority date: 2012-04-27
Filing date: 2013-04-29
Publication date: 2014-01-23
Also published as: WO2014014541A3

Abstract

Presented are nano-composite coatings to improve wear and corrosion resistance of high-strength steel parts or other substrate including the non-line-of-sight (NLOS) regions thereof. The disclosed nano-composite coatings are shown to have a 3x to 5x improvement in wear rate compared to hard chrome. The invention includes the use of the directed vapor deposition (DVD) technique to deposit nano-composite coatings to substrates. Also presented are nano-composite coatings applied in layers of film of differing thickness and composition. Wear of the layers of film and thus, remaining life of the coating may be determined by resistance or spectroscopy.

Description

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COPYRIGHT NOTICE

[001] A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent d ocisment or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, hut otherwise reserves a!l copyright rights whatsoever.

GOVERNMENT SUPPORT

[002] Work described herein was supported by the Government support is from the

USAF SBIR: FA8650-08-M-5031 (DVTi Ref.: AF Wear SBI Phase 1) and USAF SBLR: FA8201-05-C-0064 (DVTi Ref.: AF Landing Gear SBIR Phase II). The United States government has certain rights in the invention.

RELATED APPLICATIONS

[003J None,

FIELD OF THE INVENTION

[004] The present invention relates generally to the field of applying thin film materials onto substrate.

BACKGROUND

[005] Alternative coatings and coating approaches are required to improve the wear and corrosion resistance of the interior of high strength stee! parts such as landing gear components. Interior surfaces requiring improved wear have traditionally contained electroplated chrome coatings. Electroplating is an effective coating technique for applying coatings on the exterior and interior of complex parts, Unfortunately, hexavalent chrome is used during this process. This form of chrome is very toxic and expensive to dispose. These issues have resulted in Executive Order E013148 that requires the usage reduction of this i material by 50% at the end of 2006, On regions of a substrate where wear is not an issue, electroplated cadmium layers have traditionally been employed for galvanic protection against corrosion.

[006] The driv to reduce electroplated Cr usage has led to the investigation of several deposition options for applying wear and corrosion resistant coatings for steel parts such as landing gear components. While some successes have been demonstrated on exterior surfaces using high velocity oxy-fuei spray (HVOF) to apply wear resistant coatings and ion vapor deposition (1VD) to deposit corrosion resistant coatings, components having internal surfaces that are hidden from sight are not able to be coated using these approaches. As a result the desired combination of excellent non-line-of-sight (NLOS) coating capability, high deposition rates, environmental inertness, compositional flexibility and raicrostructural control required for these NLOS applications has not heretofore been achieved.

BRIEF DESCRIPTION OF THE INVENTION

[007] In this work, the use of an advanced electron beam vapor deposition approach

(directed vapor deposition or DVD) has been investigated as a method that is uniquely qualified to vapor deposit high quality coatings at a high rate (> 10 pm/rrsin.) onto the NLOS regions of complex components. The process operates in a novel processing environment that employs a supersonic gas jet to "direct" vapor atoms into the interior regions of complex pasts and a soft vacuum (1 to 50 Pa) so that binary collisions between the vapor atoms and the background gas can "scatter" the atoms onto NLOS locations on a component. Process conditions exist which result in a significant fraction of the vapor atoms impacting the component surface at near normal incidence angles to allow coating microstructures similar to those obtained at line-of-sight positions to be obtained. This enables coatings (metals, alloys and ceramics) with a controllable thickness distribution and composition to be applied onto a wide range of components having NLOS regions. [008] To obtain the benefits of this innovative coating approach for complex parts used in landing gear applications the development of coating compositions that can achieve the required performance must also be obtained. While many wear and corrosion resistant coating compositions exist that can be vapor deposited, the critical component for the development of effective chromium and cadmium replacement coatings via physical vapor deposition is the processability (i.e. the ability to apply dense layers of a coating material onto the desired substrate using a given process) of the replacement material, For example, materials having very high melting points are difficult to apply as dense layers using PVD unless high substrate temperatures or energetic atoms (created by sputtering or plasma activation) are used. The component temperature, however, can not exceed the tempering temperature of the steel components (< 200°C) and the use of energetic particles adds to the complexity of the process and has limitations for very large parts and parts with very complex geometries. Rarely, however, is excellent processability in the vapor phase designed into the coating material employed. Thus, the development of new coating compositions optim ized for use in vapor deposition systems is a critical aspect of this application. For wear resistant coatings, nano-composite Al-Y-N coatings have been developed and demonstrated in this work. The optimized Al-Y-N coatings studied in this work have been demonstrated to have equal or better wear resistance than that of the EHC-plated samples under lubricated reciprocating sliding in metal-on-ineial contact. Al-Y-N coatings in the as-deposited condition also showed no significant damage to elastomeric seals in regions of good coating integrity. Corrosion resistant aluminum and aluminum alloy coatings that could replace cadmium have also been demonstrated. The corrosion resistance of non-chromate, conversion-coated DVD NLOS AS coatings on 300M steel alloy test bars met or exceeded the minimum performance requirements for e!ectrodeposited cadmium conforming to SAE AMS QQ-P-460, 'Type I, Classes I and 2. Additionally, the DVD NLOS AS coatings met the MIL- C-83488 for Type I, Class 3 coatings. Large parts, specifically a landing gear scaled component (an F16 bell housing), could be coated using DVD as demonstrated using Directed Vapor Deposition internationals Inc's (DVTFs) new production scaie DVD coating system.

BRIEF DESCRIPTION OF THE DRAWINGS

[009] Figure I - A) Schematic illustration showing the DVD coating setup during the experiments to identify nano-scaled wear resistant coating, B) Image of substrate heater / mask arrangement for creating combinatorial synthesis substrates. A resistive heater is located behind the substrates and mask in this view.

[010] Figure 2 - Digital images of four 1" diameter 300M coupons after deposition of Al~ Y-N alloys. In A) the coupons are still covered by a stainless steel mask with a 6x6 grid of 0.2" diameter holes. In B) the mask has been removed to reveal the individual coating pixels.

[011] Figure 3 - Digital images showing the coating pixels tested during the initial phase of hardness and modulus testing.

[012] Figure 4 - Digital images of an A!-Y-N combinatorial coating with a reduced yttrium content. The coating pixels in this case display a range of visual differences. This included the fofmaiion of a two-phase coating (Al-A3_> B1-B3, C1-C3 , D4, E4, F4, A6), a single phase coating (at least visually) (D1-D3, D5, D6, E1 -E3, E5, E6, F1-F3, F5, F6) and a coating the immediately spailed after deposition (A4, A5, B4-B6, C4-C6), The changes are assumed to be due to the different compositions for each pixel due to the lateral compositional gradients across the substrates.

[0131] Figure 5 - Higher magnification digital images of an Al-Y-N combinatorial coating with a reduced yttrium content.

[014] Figure 6 ~ SEM image of an Ai-Y-N coating applied to a 300M alloy substrate.

[015] Figure 7 - The substrate holder / heating apparatus used for coupon coating. [016] Figure 8 - 300M substrate after coating with an A!-Y- coating,

[017] Figure 9 - Surface roughness, Ra, of the Al-Y-N coatings deposited using DVD.

[018] Figure 10 - Frietionai coeffic ient as a function of time for the samples, including hard chrome.

[019] Fig re 11 - Cross section of the wear track for pin-on-disk wear coating samples.

[020] Figure 12 - A) Set up for metal to metal testing using block-on-ring test rig. B) Detail of DVTI-coated block being tested versus cadmium-plated ring.

[021 j Figure 13 - Schematic of Block-on-Ring Test Geometry.

[022] Figure 14 - Blocks and rings to be coated for metal-to-metal wear testing.

[023] Figure IS - Digital images of a tubular substrate coating with an Al-Y-N alloy.

[024] Figure 16 - A) Apparatus used to heat and rotate wear rings during coating application and B) Wear rings coated with DVTFs wear resistant coating for metal -to-metal wear testing,

[025] Figure 17 - Digital images showing the top surface of Al-Y-N coatings applied onto block substrates for metal-to-metal wear testing. n A) the as deposited surface is given. In A) the surface following a post deposition grinding step is given,

[Θ26] Figure 18 - Cross-sectional optical micrographs of coatings after 100,000 cycles at 30-poiirsd load; amount of wear between the two samples is indistinguishable.

|027] Figare 19 - Optical photographs of wear scars on EHC (left) and DVTJ (right) coatings after 97,000 cycles under 60-pound load.

[028] Fsgyre 20 - SEM micrographs of wear scars on EHC (left) and DVTI (right) coatings after 97,000 cycles under 60-pound load.

[029] Figure 21 - Graph showing the TiN reference wear rate in red and the wear rates for best performing DVD deposited Cr-Cu-N (AFW-24) and V-Cu-N (AFW-27) in green for test conditions with 10N and 3N of force applied. Resistance is not adversely altered. [030] Figure 22 - Compositional gradient across a Qi-Cr-M combinatorial test sample as well as the resulting resistivity plot and hardness gradient. Pictures of combinatorial films with concentration gradients of B) Cr-C and c) V-Cu are also given. Note that the gradient films showed changes in the resistivity, hardness and also the color,

[031] Figsire 23 - (A) New gradient nano-composite coating applied to bushing having a high resistance and low test current flow as measured between the shaft and the bushing. (B) Worn coating where a larger number of percolated high-conduction grain paths close the measuring circuit and result in a higher test current.

[032] Figure 24 - As the gradient nano-composite coating is used and begin to wear down the there is a gradual change in resistance that will become more pronounced in order to clearly signal the end of the coating lifetime. The wear properties of the film can this way he kept optimal while balancing the requirement of a strong measurable signal of wear.

[033] Figare 25 - Schematic figure showing the gradient samples being deposited continuously with the dual source,

[034J Figure 26 - SEM micrograph showing the microstructure of a V-Cu-N coating having a through thickness compositional gradient (left). The EDS measured composition at different locations of the coating cross-section (right),

DETAILED DESCRIPTION OF THE INVENTION

[035] Introduction:

[036] The replacement of such heavy metals as chromium and cadmium along with the elimination of potentially hazardous processes used for applying corrosion protection and wear resistance to large parts such as aircraft landing gear components has been an ongoing effort for the U.S. Air Force for more than 10 years. Through the Hard Chrome Alternative Team (HCAT) program, a high-velocity oxy-fuel (HVOF), thermal-sprayed tungsten-carbide cobalt-based (WC-Co) coating has been found to be a suitable replacement for certain wear- resistant applications currently using electrodeposited hard chromium (EHC), Additionally, an ion-vapor-deposited ( D) aluminum coating has been used as an alternative to electrodeposited cadmium for providing corrosion protection to the Ime-of-sight (LOS) surfaces of specific aircraft component parts,

[037] However, both of these methods have limitations. First, neither method can be applied to NLOS surfaces. Therefore, the bore of critical cylindrical landing gear components, such as shock absorbers, cannot be protected by these methods. Further, the iVD-aluminum process has proven to be both time-consuming and sometimes troublesome— due to lengthy vacuum chamber pump-down requirements and chamber contamination/cleanup requirements.

[038] As such, there continues to be interest in novel methods of imparting corrosion protection and wear resistance coatings that have a high deposition rate, quick pump-down times, and the capability of being used on NLOS surfaces,

[039] For snetal-to-metal wear, a block-on-ring test geometry under reciprocating conditions was used to simulate the contact conditions between the gudgeon pin and spacers in the C-5 main landing gear (MLG) assembly. For assessing the aggressiveness of the coating towards sliding O-ring seals on a NLOS surface, the MLG floating cylinder from the shock strut assembly for the F-16 was selected, in both these tests, the DVT! coating performance was compared directly with that for EHC.

[040] For corrosion testing, a standard test procedure identified as the American

Society for Testing and Materials Bl 17 (ASTM Bl 17) neutral salt-spray tests was used. Tests were run with cadmium comparison samples, and the requirements of MlL-C-83488 were used to define the success of the as-deposited coating as well as that of the coating treated with a standard chromate-conversion coating (CCC). Additionally, the performance of the coating was compared to panels coated with an iVD-Ai coating. [041] DVD Application of Hard, Wear Resistant Coatings:

[042] The DVD approach lias been used to investigate potential materials and materials combinations that would give rise to the formation of nano-eomposite wear resistant coatings. The majority (or matrix) materials are selected from a group of 'nitride forming' elements such as Ti, Zr, Hf, V, Nb, ^"fa, Cr, Mo, W, A! and Si. Elements expected to have a wide miscibility gap with these materials are then be used as alloying addition. These include elements with atomic radii that differ greatly (> than 14% based on Hume-Rothery predictions) or have different crystal structures than those listed above. A large number of elements of this type exist including Ca, Sc, Ni, Cu, Y, Ag, In, Sn, La, Cr, An, Pb. Work has consisted of using Al and Ti as matrix materials, Y as an alloy addition, and nitrogen doping. Cr-Cu-N and V-Cr-N coatings have also been achieved.

[043] To be successful with this approach one must be able to deposit dense layers having precise control of the composition of the secondary element and the nitrogen. This has been achieved by using the unique aspects of the DVD approach such as multi-source evaporation and piasma activation. Multi-source evaporation was used to co-evaporate Ai and Y source rods. In initial experiments, we limited vapor source intermixing using the gas jet to create coatings with lateral compositional gradients so that ranges of coating compositions could be created. A nitrogen / helium gas jet was used to focus the evaporated flux and to incorporate nitrogen into the coating. Plasma activation was used to break the triple bond of^" the N2 molecules to enable nitrogen to bond with other coating atoms and be incorporated into the growing film, Figure 1(A). The nitrogen concentration in the gas jet was then varied to alter the nitrogen content of the coating. Optionally, mechanical masks can be used to limit the coating to specific regions on the substrate that can have their properties identified. The substrates in such a case are four 1" inch diameter 300M steel coupons placed on a resistive heater. The coupons are covered by a stainless steel mask with a 6x6 grid of 0.2" diameter holes. This will enable 36 coating pixels each having a different composition for testing, Figure 1(B), Digital images of initial Ai-Y -N combinatorial samples are given in Figure 2.

[044] I) Hardness aod Elastic Modulus Testing (Al-Y-N; Ti-Al-Y-N):

Testing of the mechanical properties of the deposited coatings was performed using a mierohardness indentor. The Micro-Hardness Tester (MHT) uses an established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a pre-set maximum value, the normal load is reduced until partial or complete relaxation occurs, This procedure is performed repetitively; at each stage of the experiment the position of the indenter relative to the sample surface is precisely monitored with a differential capacitive sensor,

[045J For each loading unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material under examination. Established models are used to calculate quantitative hardness and modulus values for such data,

[046] Six coating pixels were selected based on the criteria that they should be compositionaliy and visually different with respect to each other so the widest range coating types were initially sampled. The pixels included Al-Y-N, Ti-Al-Y-N alloys and coatings that appeared visually to be two-phased. The tested coatings are shown in Figure 3. A summary of the results is shown in Table 1 ,

Table 1 - The measured values of Hardness, Young's modalus and penetration depth for each sample are tabulated below, together with their averages and standard deviations. It presents a summary of the results, averages of 3 indents.

Note: The Young's Modulus values assume a coating Poisson ratio of 0,3

[047] Digital images of the AS-Y-N combinatorial sample 122904 are given in Figi3te 4. For convenience the pixels are labled A-F horizontally and 1-6 vertically and the quadrants are labeled I, II, ill and TV. From these images and the higher magnification images given in Figure 5, the different coating appearances can be observed. The pixeis in quadrant II have a two-phase appearance, quadrant I appears gray and single phase at low magnification, in quadrant 111 the range of coating compositions has resulted in coating spallation (and thus these compositions are undesirable) and in quadrant IV brighter gray coatings with a small fraction of the white phase in several pixels (D4, E4 and F4) is observed.

[048] The hardness values of the different pixeis ranged from a low of 238 to a high of 1545 Hv. The low end is near that of pure aiuminnm and the high end is considerably harder than electroplated hard chrome (which typically has a hardness in the range of 900-1200 Hv). Thus, the Al-Y-N compositions investigated in this work appear to have the potential to provide enhanced wear protections for landing gear components where hard chrome is currently used. The high hardness was obtained in a Al-Y-N combinatorial sample having a projected average Y at.% of 3.5 and a projected N at.% of 17.0.

30 [049J Another interesting feature of these measurements is that the pixel with the highest hardness still retained a relatively low elastic modulus of 82.65 GPa (only slightly higher than the elastic modulus of pure Ai (70 GPa) and pure Y (64 GPa)). Maintaining a relatively low modulus value is considered to he critical for wear applications where not only a high hardness is important, hut also a high resistance to plastic deformation [1 ]. The load required to initiate plastic deformation is proportional to the ratio H³/E², The B2 pixel of the Al-Y-N alloys had a H³ E" ratio of 0.643. This value is in the general range reported for nano- composite coatings that have been shown to have excellent wear performance [2], It is interesting to note that very hard TIN coatings (2500-3000 Hv, E = 600 GPa [3]) used extensively for wear protection only have a H³ E^a ratio of -0.1 , This is 6x less than the Al-Y- N alloy measured here. Hard chrome has a H7E² ratio of -0.02 to 0,03 as does WC/Co coatings [4]. Lower modulus coatings having higher toughness than ceramic coatings and a Young's modulus closer to that of the substrate are also anticipated to improve the coating adhesion and durability [1,2],

[050] The above coating properties are very exciting considering that the mechanical properties of the Al-Y-N alloys appear to be well suited for excellent wear resistance and good coating durability, aluminum has a low melting point so that applying dense layers to steel substrates at temperatures below the tempering limit should be feasible, the

compositions are environmentally "green" and high rate, non-line-of-sight deposition of aluminum based alloys using the DVD process has also been shown to be feasible.

[051 J II) C atiiig Microsiruciwrc stnd Composition

[0S2] To determine the microstructure of the Al-Y-N coatings in cross-section, the combinatorial samples were cut, mounted and polished. SEM images of the coating cross- sections are given in Figure 6. Note that the coating appears uniform and very dense. These samples were then sent to Evans East Inc. for wavelength dispersive spectroscopy (WDS) to be performed. This enabled a more accurate compositional analysis of the coating than was obtained from previous phase I work using energy dispersive spectroscopy (EDS) on the coating surface. The WDS results are given in Table 2.

Table 2 - WDS analysis of three pixels of a combinatorial AI-Y-N composition r duced

[053] Measurement point #2 related directly to a region of the combinatorial sample that was tested for hardness and resulted in a H E² ratio (a parameter which indicates wear performance) similar to that of chrome. The three points tested were from a region of the sample expected to have the highest amount of yttrium and thus, are believed to represent an upper bound to the Y levels of the various coating pixels. Experiments indicated that yttrium levels between 2.5 and 4.5 wt.% (0,5^■- 1.0 at.%).

|0S4| III) Application osit com os scale substrates

[0551 Once the compositions of interest were determined from the combinatorial samples, experiments were performed to enable the deposition of the Al-Υ-Ί alloys onto 1" diameter disks for wear and corrosion testing. To achieve this an A1-G.5 at.% Y source rod was evaporated. A coating was created on a 300M substrate heated to 200°C using the substrate holder / heating apparatus shown in Figure 7. A smooth adherent coating resulted, Figure 8,

[056] The wear and corrosion performance of this coating was analyzed. Hardness and wear testing (using the pin-on disc method) was performed, The testing plan included a direct comparison to hard chrome coatings. As referenced in the applicable AMS documents, the minimum thicknesses for hard chromium coatings applied by Holman Plating &

Manufacturing Incorporated to the respective test specimens was 0.0003 inch and 0.003 inch, respectively. [057] Hardness and elastic modulus values from the Al-Y-N alloy as applied to the 300M coupons are given in Table 3.

Table 3 - iardness values for DVT s Al-Y-N wear coatis lg taken at fs ve separate regions oft he 1,0" diameter 3 )0M coupon.

The testing parameters are given in Table 4,

Table 4 - Testing parameters used dtsrisig hard sess and elastic modulus testing on AS- Y-N coatings.

10

N/A

20

0

500 000 / 12%

1500 / 20%

Oliver & Pharr

0.30

Berkovich

Diamond B-H44

The coatings tested were created by evaporating a Ai-0.5 Y at.% material in a plasma environment while using a helium carrier gas jet containing 10% nitrogen. Coatings were deposited on a 1.0" diameter 300M steel substrate heated to 200°C. [058J The average hardness of this coating type was 32.04 GPa, approximately 2X that of the hardest coating found during the combinatorial study conducted. This indicated that, at minimum, the high hardness values obtained in the combinatorial assessment could be transferred onto full scale coupons, it also indicated that even harder coatings than those previously demonstrated could be obtained using our approach. The ratio riVE^* was also calculated for these samples. This value is believed to be a good indicator of wear performance. The average in this case was 1 ,02. This was considerably higher than the highest value obtained in the combinatorial study (0.6) and approximately 40x higher than literature values for hard chrome [1],

[ 1 ] hCtt?:fl*y^^

[059] Additional coatings for wear resistant coating compositional optimization were also created. These coatings were created by evaporating either a Al-0,5 Y at. or a Al-1.0 Y at.% source rod material in a plasma environment while using a helium carrier gas jet containing between 5 and 20% nitrogen. Coatings were deposited on 1.0" diameter 300M steel substrates heated to 200°C. See Table 5,

Table 5 - Test matrix showing the various test coupons created for piii-on-disc testing Microphotonks.

i reges GM ?¾>W ]

|THBo¾A^™*~^" ^"Tol l sL T20yo of tot l) j

|i 062306B i 0.5 0.50 SLM (10% of total) 1

1 06230SC j 0.5 0.25 MM (5% of total)

Γδδ2606Ο ! J .O 0.50 SLM (10% of total) S

[060] The surface roughness of these coatings was measured by DVT1 using a Dektak 6m profilometer. These results are given in Figure 8. These values are important to determine since the performance of these coatings in wear applications is in some case dependent on the coating roughness. The coatings in this case have _a values < 72

in all eases. Weight gain measurements on the substrates before and after coating indicate that can thickness of these coatings was between 2 and 20 microns. The estimated thickness is shown in Table 6.

Table 6 - Estimated thickness of the Ai-Y-N coatin s based on substrate weight g&m and cross-sectional micro ra hs f reviou l de osi oatings

It can be seen from this data that the coating roughness increased with thickness. [061] IV) Pin-on-Disk Testing (AI-Y-N):

[062] Result of Pin-on-Disk tests are given in Figure 6, the sample 062306A developed an initial constant coefficient of friction prior to settling on the final value of 0,717, Sarapie b and c on the other hand have a monotomca!ly increasing coefficient of friction reaching the steady state values in about the same time as sample a, sample b settles at 0,762 while c settles at 0,922, As can be seen sample 062606A has a rapid "settling time" reaching a steady state of 0.878 much faster. Hard chrome has a time constant somewhere in between the other films but with more noise in the reading, and settles with an average of 0.693, One possibiliiv for the noise in the signal is wear debris in the track especially since the wear rate for hard chrome is a Sot higher than for the other samples, see Table 7,

Ta le 7 - Average coefficient of friction (μ) and area of wear track as well as wear rate for all sam les.

0,695 1 558 ± 218 8.16

(75μιιι)

Note : μ is the coefficient of friction.

Figure 10 shows the cross track profile for the samples, and was used to calculate the wear rate of each sample. Wear rate of 062306A and 062306B was however difficult to evaluate since the trace left by the ball was higher than the mean height of the sample. As can be seen though, the hard chrome had the deepest wear track and thus the fastest wear rate of all the samples.

[064] These results indicated that the DVTI developed wear coatings exhibit a 3 to 5X improvement in wear rate when compared to that of hard chrome. Thus, the coatings being developed in this work appear suitable for use as a hard chrome replacement from the stand point of wear resistance,

[065] V) Deposit wear coating oisto components for testing:

[066] Testing Approach: The optimized wear coating composition determined during this program was next applied to test components as required to perform wear tests suitable for the evaluating coating performance. Laboratory bench tests to assess the tribological performance of DVTI-deve loped and deposited coatings in comparison with that of electrodeposited Hard Chromium (EHC) were performed,

[067] The objectives sought through the triboSogica! assessment were:

1) Demonstrate that the performance of DVTI-deveioped and deposited coatings is at least as good as that of the currently used EHC process in metal-to-metal wear,

2) Demonstrate that the directed vapor deposition (DVD) technique is viable for non- line-of-sight (NLOS) surfaces, where the hard chrome-alternative high velocity oxy- fuel (iTVQF) process cannot be used. [068] Both the metal-to-metal wear resistance and the aggressivity of the coating against the elastomeric seal were assumed to be characteristics of the coating only. The substrate was assumed to be largely irrelevant as long as the surface chemistry was comparable with that of the actual component to which the coating is to be applied for the purposes of coating adhesion. As such, assessment of the tribologieal characteristics of the DVTI coating could be accomplished using readily available and/or easily fabricated materials for the substrates provided the chemistry at the interface is comparable to that on which the coating would be applied,

[069 J Components of interest for enhanced wear coatings include:

1) The main landing gear (MLG) outer floating cylinder from the F- 16, and

2) The Gudgeon pin OD from the C-5.

[070] The MLG outer, pressure-equalizer "floating" cylinder, also referred to as a bell housing, is a LOS component. The ID of the floating cylinder contacts a seal on the OD of the main shock strut. The seal is an o-ring of T-seal design, manufactured by Greene Tweed. It is made from ] 60 Nitriie Compound, with backup rings of PTFE. The seal is

approximately 4.5 inches in diameter, and serves to balance the pressure when the main shock is active, The relative motion of this component is axial sliding (piston in cylinder), with the seal on the aluminum piston. The concern with the coating is for wear of the seal, as well as leakage due to possible porosity of the coating. The bore of the cylinder is currently EHC plated to exact size, minimum 0,0005" in thickness, and has a requirement of better than 16 μΐη after plating. The drawing specifies 8 μι^'η before plating, with no grinding after plating. If HVOF were used, the roughness must be better than 8 ίη, and grinding is required, although HVOF cannot be used on this NLOS component.

[071] The Gudgeon Pin has mechanical contact with two tight clearance conformal spacers, which are made from 4130 steel tubing per Mil-T-6736 Type I, physical condition HT-150. Earlier designs used imcoated titanium spacers, but these are no longer produced. The current 4130 steel spacers are cadmium plated, and have rust preventative oil applied. The relative motion is reciprocating rotational sliding, with an amplitude of +/- 5 to 7 degrees (10 to 14 degree total angle of rotation). The gudgeon itself is heat treated 300M alloy steel, plated with 0.006" of EHC. The concern is for metal-on-metal wear between the gudgeon pin and the spacers,

[072] Metal-to-metai tests were conducted using a block-on-rmg geometry, with samples corresponding to the standard for ASTM G-77. A reciprocating motion of +/- 14 degrees was used to simulate the motion of the gudgeon pin and spacer assembly on the C-5 main landing gear. Tests were run under two different loads, 30-pounds and 60 lbs, up to approximately 100,000 cycles. The ring was cadmium coated, according to MIL-Spec QQ-416F Class 1, and the interface was lubricated prior to testing with Nye Rheolube 374A, per the procedure used during assembly. A photograph of the set-up is shown in Figure 8(A), with a detail of a test in progress is shown in Figure 12(B).

|073J The block-on-ring test geometry was chosen for the metal-on-metal wear test in order to span a range of stresses. With this set-up, the contact configuration could start off as high stress line contact, but as wear occurs, the contact becomes conformal and the stress level is reduced, similar to that of the actual pin/spacer interface. This provided the opportunity to monitor the wear rate over a range of contact stresses, ail in a reciprocating mode. The contact stress calculated for the C~5 gudgeon pin/spacer interface, computed using an elastic contact analysis, ranges from 4 to 19 ksi. Elastic analysis provides a conservative maximum upper bound, and wear rapidly reduces these computed stresses. Our experience shows a practical stress range to be about a quarter of the elastic maxima, on the order of ί to 5 ksi.

[074] Using a standard 1.375-inch diameter ring, a scar width of 0.047" (readily observable and measureable in a microscope) occurs at a wear depth of only 5 microns (0.0002"). Thus,

3 8 the early stages of coating wear could be observed. For a plating thickness of 0,006", the wear scar will be greater than 0,25" before the coating was completely penetrated. Both friction and wear could also be monitored in-situ using the block-on-ring test configuration, as Shown in Figure 13.

[07S] Samples for this set-up were standard rings and blocks for the LFW-1 rig, Figure 1.4, The rings were modeled after outer races for a Timken tapered roller bearing, typically made from either 52100 steel or 4620 steel carburized and hardened to Rockwell C 55-60. This was a similar strength level as the hardened 300M alloy (HRC 55 ~ 300 ksi). For DVD- coating, the rings could he stacked on a shaft that can he rotated in the coating chamber.

[076] In preparation for the coating performance analysis, deposition of the baseline Al-Y-N coatings onto the interior surface of a tube and onto rings and block was performed. Initial coating deposition onto a tubular substrate was performed using a 3" diameter, 3" long tube. This coating was created by evaporating a Ai-0,5 Y at.% source rod material in a plasma environment while using a helium carrier gas jet containing 10% nitrogen, The substrates were heated to 200^ϋ€ during the coating operation, Coating could be visually observed on the interior of the tube. The resulting coated substrate is shown in Figure 1.5,

[077] A heating and manipulation apparatus for the components in the metal-to-metaJ wear tests was also designed and constructed. The apparatus for coating multiple wear rings is shown in Figure 16(A). This set-up was used to coat three rings with DVTi's wear resistant coatings, Figure 16(B), In this case, three l -in.-i.d., 1 ¾-in.-o.d._s 0.35-in. -wide rings (one Timken TS4148, two Falex USA F-S10-A) were grit-blasted at 35 psi, ultrasonically cleaned in isopropanol, and mounted on a shaft rotating at 4 rpm. Heating elements around the shaft were maintained at a temperature between 250 and 3O0°C, while a ½~m^' . AI Y1 source was evaporated below and carried by 10 slm of He. 1 slm of N? was also introduced, resulting in nitrogen doped coatings with the aid of DVTPs hollow-cathode plasma unit. After 53 min, of deposition, the rings dispiayed uniform coatings of the expected appearance and weight, ranging from 0.012 to 0.020 g.

[Θ78] Coating set-ups for the blocks were also designed and constructed. Using similar coating conditions as for the ring coatings described above the blocks were coated with wear resistant Al-Y-N coatings. The resulting coating in both the as-deposited and post deposition ground state are given in Figure 17. Processing parameters used to deposit AL-Y-N coatings are shown in Table 8.

Table 8 - DVD processisig parameters used to deposit Al-Y-N coatings onto ring, block zrnd tubs shaped substrates for wear testing,

Gas

Date Suhstrate Source atia

99.5%

L 3-C- Al 0.0106; 10 He/0.5

001 2 rings 0.5% Y 0.011 1.1493 0.14 7,1 50

99.5% Coating were

LS-C- Al black and

002 1 ring 0.5% Y MA 6.7 60 deiaminating

2 rings 99.5%

LS-C- and 2 Ai 10 He/0.5

003 blocks 0.5% Y 0,0464 2.2S31 2 0.22 4.4 60

0.0215;

2 rings 0.0232;

m-c- (1,2) and 2 99% Ai 0.0027; 10 He/0.5

004 biocks{3,4) 1% Y 0.0025 1.1713 N2 0,15 6.2 60

LG-C- 0.0026;

005 3 blocks 0.0031;

(1,2,3) and 0.0035;

2 99% A! 0.022; 10 Ηε/0.5

rings(4,5} 1% Y 0.0187 2.2326 H2 0.16 5.4

US-C0.0676;

OOS 0,064;

0.0811;

99.5% 0.0733;

AS 0.0843; 10 He/0.5

6 blocks 0.5% Y 0.0672 7,4684 H2 0.16 5,9 120

LG-C- Electron

007 beam stability

problems,

magnetic tube

99.5% may have

large sled Ai caused some

i e 0.5% Y NA 4.7 74 interference

LG-C- 2 blocks 99% Ai HA unknown 10 0.15 5.8 90 008 1% Y He/0, 25

H2

LG-C- 99.5% 10

009 Al 0,0319; He/0.25

2 blocks 0,5% Y 0.0265 5.5891 N2 0,089/0.16 4.8/5,3 60 IS-C- 99.5%

010 Al

2 blocks 0.5% Y NA 8.462 Variable 0.077/.14 6.2/6,3 70 LG-C- 99.5%

Ai 0.0294;

2 blocks 0.5% Y 0.024 1.9642 Variable variable variable 33

[079] In the long-term block-on-ring wear tests, both the EHC- and DVTI-coated blocks showed initial transfer of the cadmium from the ring to the block. This is expected to be the sarne process as that which occurs in the gudgeon pin and spacer interface. Tests were run without stopping to the same ultimate number of cycles. For the 30-pound load test, after 100,000 cycles of +/- 14-degree reciprocating sliding, the difference in wear between the DVT coating and the EHC plating was indistinguishable, Six measurements of each coating thickness were made on polished cross-sections, with the average wear depth at the center of the wear scar being approximately 25 μηι (0.00I inches). The wear coefficient for both materials in this lower load test was approximately 9.1 x 10^"7 mnr/Nm. A cross-sectional micro-graph of the coatings alter completion of the wear tests is shown in Figure 1 8.

[080] In the higher load tests, wear could be assessed by the width of the wear scar, so cross-sectional metallography was not required. However, some SEM observations were made to verify that the wear did not penetrate the coating at maximum depth, in the 60- pound test, after 97,000 cycles, the primary wear scar for the DVTI sample was 1.3 mm wide, while thai tor the EHC sample was 1.7 mm wide, indicating somewhat better wear resistance for the DVTI coating. However, outside of the primary wear scar was a region that appeared "frosted" (see Figure 19 below), The total width of this region was 2.7mm for the DVTI sample, but only 2.3 mm for the EHC sample. The wear coefficient for the DVTI-coated sample m the 60~poimd load test, tjsing the 2,7 mm scar width, was approximately US s iO^"6 mnrVNm, which is comparable to t e valise calculated from the Sower-load tests, mdicatiiig no change in wear mechanism under the two loadi g conditions.

[081] SEM analysis showed that the thinner EHC-plated sample had worn through at the center of the wear sear, while the DVTI sample had not (Figure 20 below). This complicates the above results somewhat. However, it cars be stated that in the two tests, the wear resistance of the DVTI coating was comparable to that of the EHC-p isg. Since limited samples were available for repeat testing, these observations should be verified.

[082] VI) Conclusions for AI-Y-N Wear Testing:

* The DVTI-eoated samples had equal or better wear resistance than that of the EHC- plated samples under lubricated reciprocating sliding in metal-on-metal contact,

* DVTI coatings in the as-deposited condition showed no significant damage to

elastomeric seals in regions of good coating integrity,

* Multi-layered coatings are effective at improving the coating adhesion.

The tribology data for this project can be found in Battelle Laboratory Record Book No. 51492. The final DVD processing conditions for the application of wear resistant Al-Y-N coatings are given in Table 9.

Table 9 - Final DVD processing conditions for the application of wear resistant AI-Y-N

tsiiSK ^: Two blocks pieced with one face towards the source

60 amps of cathode current and a -130 VDC bias

33 minutes _

Temperature; j _. 2 0°C before g!¾s,tn¾ turned on

^' Carrier gas 5 shn for 5 minutes, 8 sim for 9 minutes, 10 slm remainder of Carrier Gas flow: I run.

Reactive gas off for first 14 minutes, on at 0.25 sin) for 10, off for 5 minutes, and on the rest

0.084 mbar to 0, 16 mbar

4,9 to 5.5

Preparation; j Grit blast using 220_. Α1₂0₃ grit Vli) Hardness and Wear Testing (C ~Cr~ ; V-Cr-N):

Deposition conditions for some Cn-Cr- and V~Cr~N coatings are shown in Table

Table 10: DVD processing conditions for the application of wear resistant Cu-Cr-N 8¾d

[084] DVD Cr-Cis-N and V-Cu-N Pin-on-Disc: Pin-on-disk testing results obtained on DVD deposited coatings are given in Figure 21 below for both ION and 3N test conditions (see Table 11), The wear performance of the best performing Cr-Cu-N was very similar for the two testing conditions, 0.13 (10^'5 mm'/ m) for 10N and 0, 17 (10^"5 mm³ Nm) for 3N. Many different test conditions were investigated by Micro Photonics in order to find conditions which produced measureable wear in this sample, The two conditions reported here were chosen due to the measurable change in the coating at the end of the test, The wear rate of the best performing V-Cu-N composition was 1 ,33 (10^"s mm³ m) for 10N and 0,42 (10^"" mm^' Nra) for 3N. The Cr-Cu-N based coating had a coefficient of friction of 0,59 at 1 ON and 0,52 at 3N, and for the V-Cu-N based coating it was 0.47 at 10N and 0.65 at 3N (see Table 12). The test conditions were identical to the conditions for the reference sample, with:

[085] The environmental conditions:

« Lubricant: none,

* Atmosphere: Air,

» Temperature: 23C (room temperature),

• Humidity: 35%.

[086] During the test, the maximum depth of the wear track was only 0.78 μτη at 10 N and 2.07 μπΐ at 3 N for the Cr-Cu-N coating AFW-24, and 4.08 am at 10 N and 3.19 μιη at 3 N for the V-Cu-N coating AFW-27 (i.e. much less than the -20 μχη for the reference coating). The area of the track cross section for Cr-Cu-N was: 65.1 μαι² at 10 N and 183.5 μη ^' at 3 N, and for V-Cu-N 660 μηι² for 10N and 449.2 μη ' at 3 N, again much less than the -7500 μηι² for the reference coating.

Table 11: Ex erimental conditions for piit-en-disc testing

Ball material oxide steel Ta le 12: Wear rate and coefficient of friction summary data for psrs-on-dise testing.

Sam le Wear Rate

[087] The Cr-Cu-N coating was very difficult to wear. The microsiructure of this Cr-Cu-N coating appeared to change from a granular structure to a smooth coating after the conclusion of the test as shown below. This was in sharp contrast to the reference TiN coating which was slowly removed through the test while compressing the substrate. The behavior of the V-Cu-N was in-between that of the Cr-Cu-N coating and the reference TiN coating. The coating was observed to wear away, where most of the coaling was gone in the 10 N test, while slight compression of the substrate was experienced, as observed from the 4 μηι depth of the wear track as compared with 20 μηι in TiN, More of the coating remained in the 3N test and less compression was observed, suggesting that this coating is more suited to applications with smaller loads.

[088] SEM EDS results of coatings used in wear testing: The compositions of DVD deposited Cr-Cu-N and V-Cu-N films were determined using EDX analysis, The composition of AFW-24 was found to be dominantiy chromium with 90.6 wt ¾, with a smaller amount of copper, 8.8 wt % and nitrogen, 0.6 wt % incorporated into the film. Sample AFW-27 was a vanadium compound, with a much larger amount of copper of 61.9 wt % followed by 37,6 % vanadium and a similar amount of nitrogen, 0,5 wt %. These results are shown in Table 13.

Table 13: Weight percent of the compounds in the wear films

[089] VIII) Smart, Wear Resistant Coalings

[090] in some wear coating applications the ability to accurately determine the lifetime remaining from the protective coatings can be critical in maximizing the coating lifetime and aircraft readiness. "Smart" coating concepts are therefore also being developed in this work in which the remaining coating life can be assessed from the non-destructive measurement of a property of interest. One such "smart" coating strategy consists of coating components with a multi-layered (or compositional graded) films where the composition of the coating is varied through the thickness of the coating. This results in a measurable change in coating properties (such as resistance or color) which could eventually be used to indicate when the failure condition of the coated component is near. Results from DVD deposited wear coatings. Figure 22, indicate that at least two different strategies, resistivity or reflectivity measurement, exist for the measurement of smart properties, hi principal, the coating architecture required for both smart coating concepts is similar requiring a coating in which the composition varies through the thickness across a desired compositional range.

[091] Electrical resistance to detect remaining coating life: In general, the resistance R of a material is dependent on the length and cross sectional area of the conducting path and its resistivity p. In a normal metal (near or above room temperature) the resistivity varies linearly with the temperature. However, in a nano-granular material of high and low resistivity phases the resistance will be highly dependent on whether the low resistivity grains are connected or percolated thro light the matrix material, in such a system close to the percolation point the resistance can change dramatically with only small changes in composition. As a result, if the proper compositional range can be identified, a large change in the measurable resistance could be created across a relatively small compositional change (which would also likely represent only a small change in the relative coating hardness (and wear resistance). Thus, it appears feasible to create a wear resistant nano-granular material where a large change in the measurable resistance would occur with a relatively small compositional change and, thus, the remaining coating lifetime could be assessed. Figure 23 depicts a scenario with a gradient nano-composite matrix film where in (A) it is a new (thick) film and the resistivity measured is high, in (B) the coating has been worn down to a thickness where the conducting networks connect a component (such as a bushing) with a shaft in more points, thus allowing a higher current to flow through the test instrument, 1092] Ideally the composition and material choices shall be such that the optimal composition range for wear protection is not significantly deviated from when the large resistance change occurs, hi Figure 24 the time in use relates to the thickness left of the wear protective film. And the theoretical graph indicates an ideal scenario with a replacement resistance easily detectable prior to any significant change in the wear properties.

[093] In certain applications, it is anticipated to be possible to arrange a maintenance testing in such a way that both the shaft and the bushing are electrically separated during the measurement phase and that a simple resistance test could be used to determine the remaining life of the coating.

[094] Reflectance spectroscopy to detect remammg coating life: The color of the coating in many coating systems (such as Cr-Cu-N and V-Cu-N) are visibly compositionaf!y dependent (see Figure 22 above). This dependence may also be used to determine coating failure, in such a case a simple fiber optic reflectance spectrometer could be used to detect the change film composition as it is worn. As the lower layers are exposed due to wear, the color of the under layers would be displayed,

[095] Reflection spectrometry measures the light that is scattered by surface and returns to the detector. Any light that is absorbed by the material is removed from the source and not available for reflection. Since absorption of photons by a material is specific to the energy levels of the molecule, absorption spectroscopy and reflectance spectroscopy are both sensitive to the chemical composition of the material investigated. Since the reflectance depends on the angle between detector and the surface, changes in grain structure or surface roughness will also change the signal detected. Since the color changes in these systems can be detected by eye, the visible region of the electromagnetic spectrum is expected to be the most useful for this characterization. The setup will have a white light source coupled to a fiber optic with a central source fiber with a 70 μηι spot size on the coating area. The fiber optic bundle will contain multiple fibers around the central source for detection of the scattered radiation, with a fixed angle of detection. The scattered light will then travel back up the fibers to a grading spectrometer with a detector, insuring that the intensity at each wavelength is detected. A fixed geometry setup will be created to insure consistent placement of the fiber optic in relation to the coating. Changes as the coating is worn will be detected by changes in the spectrum corresponding to changes in the film chemistry due to the gradient composition deposited. Reflectance spectra of the coatings with different chemistries will be obtained to correlate to the spectrum obtained during wear testing to known chemistries,

[0961 Compositional Gradient Coating Creation: Using the "smart" coating composition profiles identified above and multi-source DVD deposition coatings having through thickness compositional variations which mimic the identified gradients will be created. These coatings will be created either by altering the beam power distribution (on each evaporation source rod) during coating deposition to create a coating composition that is a function of thickness or by translating components through a vapor flux having a lateral composition gradient as shown in Figure 25.

[097] Functionally Graded Coatings Having a Through Thickness Compositional Gradient:

[098] Functional graded wear coatings were created through by continuously altering the electron beam power applied to multiple evaporation sources (A and B), As the e-beam power applied to source A was increased the with respect to source B the composition of the coating was altered. The result was a continuous variation in the coating through the thickness of the coating. An example of this concept in given in Figure 26 where

compositional analysis of the through thickness compositional gradient coating is given. The results indicated that the coating composition could be varied from high Cu levels near the substrate to high V levels near the coating surface by using the approach described above. The resulting films were dense with no observable through-holes, This work indicated that smart coatings having through thickness compositional gradients were feasible to apply using the DVD approach.

Claims

CLAIMS What is claimed is;

1. Nano-composite wear and corrosion resistant coating compositions optimized for

deposition on a substrate, including non-1 ine-of-sight regions thereof, using the directed vapor

deposition technique comprising:

One or more matrix materials selected from the group of nitride forming elements;

One or more alloying elements having a wide rniscibility gap with the matrix material,

2. The nano-composite wear-resistant coating compositions of Claim 1, wherein the one or more alioying elements have an atomic radii 14% or more greater than the matrix material based on the Hume-Rothery predictions,

3. The nano-composite wear-resistant coating compositions of Claim 1, wherein the one or more alloying elements have different crystal structure than the matrix materia!.

4. The nano-composite wear-resistant coating compositions of Claim 1, wherein the one or more matrix materials are selected from the group of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, and Si.

5. The nano-composite wear-resistant coating compositions of Claim 2, wherein the one or more matrix materials are selected from the group of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, and Si,

6. The nano-composite wear-resistant coating compositions of Claim 1, wherein the one or more alloying elements are selected from the group of Ca, Sc. Ni, Cu, Y, Ag, In. Sn, La, Cr, Au, and Pb.

7. The nano-composite wear-resistant coating compositions of Claim 1 further comprising nitrogen doping,

8. The nano-composite wear-resistant coating compositions of Claim I having a low elastic modulus value.

9. The nano-coraposite wear-resistant coating compositions of Claim 1 having a Young's modulus within 50% of that of the substrate.

10. A method for determining the remaining lifetime of a nano-composite coating comprising:

depositing on a substrate one or more layers of film using the directed vapor deposition technique to form a coating, each film having varying thickness;

measuring the resistivity of the coating before wear;

measuring the resistivity of the coating after compositional change in the

coating resulting from wear;

comparing the measured resistivities to determine remaining life of the coating.

1 1 . A method for determining the remaining lifetime of a nano-composite coating comprising:

depositing on a substrate one or more layers of film using the directed vapor deposition technique to form the coating, each film having varying composition;

measuring the resistivity of the coating before wear;

measuring the resistivity of the coating after compositional change in the

coating resulting from wear;

comparing the measured resistivities to determine remaining life of the coating;

12. A method for determining the remaining lifetime of a nano-composite wear and corrosion resistant coating comprising: depositing on a substrate one or more layers of film using the directed vapor deposition technique, each film having varying color or reflectivity;

recording the color of each film of the coating before wear to form known

coating chemistries;

sensing changes in the spectrum of the coating caused by wear;

comparing the changes in film chemistry by correlating the changes in the

spectrum of the coating with the known coating chemistries to determine remaining life of the coating,

13. A method to form functional graded nano-eomposite coatings using the directed vapor deposition technique (DVD) comprising;

altering the electron beam power applied to one or more

evaporation sources used in making one or more layers of film to form a coating during the DVD

process,

14. The method of Claim 13, wherein the one or more layers of film vary in thickness.

.

15. The method of Claim 13, wherein the one or more layers of film vary in composition,

16. The method of Claim 13, wherein the one or more layers of film vary in color,

17. The method of Claim 13, wherein the one or more layers of film vary in reflectivity.

18. A method for directed vapor deposition of a wear or corrosion-resistant coating on a substrate, the method comprising:

providing at least one source rod material in a plasma environment; using a helium carrier gas jet containing between 5% and 20% nitrogen; depositing material from the source rod onto the substrate.

19. The process of Claim 18 where the depositing of the source rod material is deposited on a non-iine-of-sight region of the substrate.

20. A method for directed vapor deposition of a wear or corrosion-resistant coating on a substrate, the method comprising:

providing a chamber pressure in the range of 0.07 torr to 0.22 torr,

21. A method for directed vapor deposition of a wear or corrosion-resistant coating on a substrate, the method comprising:

providing a pressure ratio of between 4.4 and 9.0.

22. A method for directed vapor deposition of a wear or corrosion-resistant coating with improved adhesion on a substrate, the method comprising:

starting the DVD process with the reactive gas flow turned off for a period of time;

turning the reactive gas flow on for a period of time;

turning the reactive gas flow off for a period of time;

turning the reactive gas flow on until the completion of the process.