Patent Application: US-94362304-A

Abstract:
methods for applying an amorphous metal alloy to a substrate , comprising the steps of vaporizing an amorphous metal alloy composition , in a plasma spray gun to form a metal alloy vapor plasma plume , directing the metal alloy vapor plume onto a cooled substrate , maintained and condensing and rapidly solidifying the amorphous metal alloy composition vapor on the substrate , to form an amorphous metal layer deposit of high density and strength .

Description:
alloys and / or blends of small powder components are fed into a plasma gun to vaporize the powders to form bmg amorphous metal vapor mixtures . by condensing these vapor mixtures on a cool ( preferably metal ) substrate , the condensate can immediately cooled below its glass transition temperature before crystallizing , and can remain amorphous . it is preferable to limit or avoid oxidation , particularly when vaporizing highly reactive elements , and to control the substrate - surface - cooling temperature . desirably , if an amorphous alloy layer is to be deposited , the substrate will be maintained during the deposition at a temperature at least 50 ° c . and more preferably at least 100 ° c . below the crystallization temperature of the alloy being deposited on the substrate . if the alloy being deposited is a bulk metal glass , the substrate is preferably cooled to a temperature at least 50 ° c ., and more preferably at least about 100 ° c . below the glass transition temperature tg of the alloy being deposited . the vapor is cooled rapidly enough to a temperature below the tx or tg of the condensed amorphous or bmg alloy , that the appropriate amorphous metal alloy composition is condensed and deposited on the substrate and solidified in an amorphous state . amorphous metal coatings are strong , resist corrosion , and can be converted to hard , wear resistant surfaces upon appropriate nanocrystallization conditions . as indicated , the vaporized alloy components are condensed on the substrate . however , because different metal and metallized components of various alloys have different vaporization and condensation characteristics , depending on factors including the temperature of the substrate ( which is much lower than the vaporized alloy components ), pressure and the volume of inert gas carrying the alloy vapor , the composition of the alloy condensed on the substrate may differ from the composition of the vaporized components . in this regard , the following table lists the temperature in degrees celsius at which the vapor pressure of selected metal and metallized elements is , respectively , 1 × 10 − 2 torr , and 1 torr . element 10 − 2 torr 1 torr al aluminum 1 , 217 ° c . 1 , 557 ° c . b boron 2 , 027 ° c . 2 , 507 ° c . ba barium 610 ° c . 852 ° c . be beryllium 1 , 227 ° c . 1 , 557 ° c . c carbon 2 , 457 ° c . 2 , 897 ° c . ca calcium 597 ° c . 802 ° c . co cobalt 1 , 517 ° c . 1 , 907 ° c . cr chromium 1 , 397 ° c . 1 , 737 ° c . cu copper 1 , 257 ° c . 1 , 617 ° c . dy dysprosium 1 , 117 ° c . 1 , 437 ° c . fe iron 1 , 477 ° c . 1 , 857 ° c . ge germanium 1 , 397 1 , 777 ° c . la lanthanum 1 , 727 ° c . 2 , 177 ° c . li lithium 537 ° c . 747 ° c . mg magnesium 439 605 ° c . mn manganese 937 ° c . 1 , 217 ° c . mo molybdenum 2 , 527 ° c . 3 , 117 ° c . nb niobium 2 , 657 ° c . 3 , 177 ° c . ni nickel 1 , 527 ° c . 1 , 907 ° c . p phosphorus 185 ° c . 261 ° c . re rhenium 3 , 067 ° c . 3 , 807 ° c . sb antimony 533 ° c . 757 ° c . si silicon 1 , 632 ° c . 2 , 057 ° c . sn tin 1 , 247 ° c . 1 , 612 ° c . sr strontium 537 ° c . 732 ° c . ta tantalum 3 , 057 ° c . 3 , 707 ° c . ti thallium 609 ° c . 827 ° c . ti titanium 1 , 737 ° c . 2 , 177 ° c . w tungsten 3 , 227 ° c . 3 , 917 ° c . y yttrium 1 , 632 ° c . 2 , 082 ° c . a variety of amorphous , bmg metal alloys with their tg , tx and supercooled liquid region are listed in the following table ( with compositions given at atomic %): while this table lists certain primarily bmg alloys , other amorphous alloys may also be used . the amorphous metal powder blend may also include other components such as reinforcing and / or alloying fibers or powders . such fibers or powders may be densely consolidated within the condensed and solidified amorphous alloy layer ( s ) deposited on the substrate . if it is desired to include “ intact ” powders and / or ( short ) fibers , these components should best be substantially larger than the approximately 10 million or less metal / metallized powders which are intended to be vaporized . for example , amorphous metal alloy powders of the same or similar composition to the alloy being deposited from the vapor , but a diameter of , for example , about 45 to about 150 microns , may be introduced in to the plasma gun nozzle to be applied to the substrate with the condensing alloy vapor . amorphous alloy powders to be co - deposited on the substrate , as “ splats ” with the condensing alloy vapor should be fully melted in the plasma before implact on the substrate , and then rapidly cooled on the substrate before crystallization , if it is desired to maintain the amorphous characteristic of the “ splats ”. in such processes , the mass ratio of the vaporized small - particle alloy component to the relative large particle size component should be at least about 0 . 25 to 1 . 00 , and preferably , at least about 0 . 5 to 1 . 0 . it should be noted that even for larger 45 - 150 micron particles , some of the surface of these particles may be vaporized in the plasma gun plume as they are heated to a temperature about the melting point . as shown in the preceding table , different elements of these surface components may have significantly different vaporization rates , which will change the composition of the molten particles . for example , magnesium and aluminum have relatively high vaporization rates in an ultrahot plasma plume , while iron and boron have relatively slower vaporization rates . however , in accordance with the present method , such differential vaporization ( and condensation rate ) may be compensated for by controlling the composition of the vapor phase to include an excess of the higher - volatility components . as indicated , many bmg alloys with a broad supercooled liquid region may be vaporized for deposition in accordance with the present disclosure . however , aluminum - based alloys tend to have marginal glass - forming ability , and do not readily form bmg alloys with a wide supercooled liquid range . a few al - based alloys have small supercooled liquid regions ( such as al85y8ni5co2 tx - tg ˜ 30 ° k . ), which have been spray formed with some degree of amorphous phase retention , but most amorphous aluminum alloys have no tg , and progressively crystallize with increasing temperature . accordingly , new aluminum - based amorphous alloys with improved amorphous properties are desirable , and are also described in accordance with the present disclosure . the new al - based amorphous alloys are msl class alloys with midsize atoms “ m ” as the majority component ( 60 - 70 at . %), small atoms “ s ” as the next - majority component ( 20 - 30 at %), and large - size atoms “ l ” as the minority component ( 10 at . %). the “ l / s ” pairs have high negative heats of mixing to stabilize the glass . the aluminum alloys based on aluminum as the midsize component and ( ca , ba )— si and / or ( zr , ti )— b as the l / s pairs . in the proposed msl alloys , the negative heats of mixing are large for enhancement of the stability of the undercooled melt . the concentration of the l atoms is from 3 - 12 , preferably about 10 at . %, and the “ s ” atom content is about 20 - 30 at . %. smaller amounts , however , can still improve the glass - forming - ability ( gfa ) properties of al - based alloys . the first l / s pair relies on calcium , strontium and / or barium as the inexpensive “ large ” atom , and silicon as the “ small ” atom component . the ca — si2 pair has a large negative heat of mixing , as does the ca — al interaction with the base aluminum “ m ” component . moreover , al and si are fully compatible in amorphous compositions , and also have a large negative heat of mixing . calcium is a relatively large atom , and very light , and ba and sr are even larger , while still having reasonably low density . the atomic size ratio of ca / al is 1 . 37 , as shown in the following table 3 . al — ca binary alloys ( and mixtures with mg , zn , fe , ga , ni and cu additions ) can be amorphized in the composition range of 9 to 11 at % ca by melt spinning . 59 amorphous mg70al20ca10 alloys with density of 1 . 80 g / cm3 can have a yield strength up to 930 mpa and a plastic strain up to 9 . 2 % 60 , which is almost twice as strong as beryllium , at approximately the same weight . new al - based compositions in which ( ca , ba )— si are added to known amorphous aluminum alloys in accordance with the present disclosure , are listed in the middle column of table 1 , above . the multinary nature of most of these compositions is favorable to amorphous property development , as most pairs have large mixing heats , and a smoother size progression is provided than with a smaller number of elements . the second l / s pair for use in msl aluminum - based bmg alloys disclosed herein relies on zr and / or zr — ti , hf blends as the large atom component , and boron and / or silicon as the small atom component . zr — b pair has a very large negative heat of mixing , and b and si are both compatible with amorphous al - alloys . the density of zr is not prohibitive for lightweight alloys in minor amounts , and the atomic size ratio of zr / al is large enough at 1 . 13 to facilitate bmg formation . al - based compositions in which ( zr , ti )— b are combined with known amorphous aluminum alloys in accordance with the present disclosure are listed in the middle column of table 1 , above . in both the ca , ba compositions , and the zr , ti compositions , a mixture of b and si can be beneficial in fostering larger - cluster formation , thereby increasing viscosity and reducing diffusion of al and smaller atoms . in the apparatus of fig1 , small metal / metalloid particles are vaporized in a hot plasma . preferably , the particles have a uniform bulk metal glass composition . however , powder blends of different alloy components may also be used . the metal / metalloid vapor is condensed on a suitable metal substrate to form an intimate bond with the substrate . preferably , the substrate is cooled to enhance the solidification of the deposited vapor , and keep the surface below the glass transition temperature of the amorphous metal vapor composition being condensed on the surface . conventional , larger (˜ 25 - 100 micron ) metal thermal spray particles can be included , which may have an enhanced bond in the coating because of the condensing vapor . a reducing plasma can be used to keep the metal substrate clean and even help reduce a thin oxide surface layer to assist formation of a good , preferably metallurgical , bond . by forming appropriate vapor compositions , the vapor - condensed layer can be an amorphous metal composition , and preferably a bulk metal glass composition . the condensed - deposited amorphous ( including partially nano - crystallized ) layer can be retained as is , or can subsequently be treated to further crystallize or nanocrystallize it , to obtain specific properties , such as increased hardness . the illustrated apparatus 100 comprises a conventional plasma gun 102 , and a conventional inert gas shield 104 . as shown in fig1 , the plasma gun 102 produces an ultrahigh temperature plasma plume 106 which is directed toward a suitable substrate 108 , such as cooled steel , stainless steel , aluminum or titanium alloy substrate . as shown in the cross - sectional view of fig2 , the plasma gun 102 has an axially aligned cathode 110 , typically of a refractory metal with a low work function such as tungsten or thoriated tungsten , and a water cooled anode 112 through which powdered metals to be vaporized and deposited on the substrate 108 may be introduced through appropriate passageways 114 , 116 . if desired , the thermal spray apparatus 100 ( fig1 ) may be enclosed in a controlled - atmosphere chamber or vacuum chamber 118 if it is desired to exclude reactive gas such as oxygen or nitrogen , and / or to operate in subatmospheric pressure . in operation , an inert gas such as an argon - helium mixture 103 , which may include a small amount of hydrogen , is introduced into the plasma gun 102 , while direct - current is applied from a suitable power source to the cathode and anode of the gun to heat the inert gas to a temperature of 10 - 30 thousand degrees kelvin to form a high - velocity plasma plume 106 . a selected amorphous metal alloy , which is preferentially a bulk metal glass alloy , having a largest dimension of about five microns or less , is introduced into the plasma gun 102 through conventional introduction port 114 , with an inert gas and / or hydrogen . the more volatile elements such as magnesium , boron and aluminum can be in relatively larger particles , while refractory elements such as zr are heat volatilized from a smaller particle form . the metal powder may be a homogenous alloy of the specific alloy which is desired to be deposited on the substrate 108 , or may be a mixture of alloy powders or elemental metals which are proportioned to form the desired alloy upon vaporization and deposition . because of their small size , and the intensity of the plasma thermal spray operation , the metal powder is substantially fully vaporized in the plasma plume 106 . in operation , a shielding gas 109 such as argon or helium may be introduced into the zone 104 surrounding the plasma plume 106 , in order to protect the metal components of the plasma plume from external reactive atmosphere components such as oxygen and nitrogen . preferably , however , the entire thermal spray process is carried out in a controlled atmosphere or vacuum zone 118 , in which reactive gases such as oxygen and nitrogen are substantially excluded . as indicated , the amorphous metal coating is deposited on the selected substrate 108 . in this regard , it is important that the substrate be maintained at a temperature below the glass transition temperature or the crystallization temperature of the amorphous metal composition which is being deposited . preferably , the substrate will be maintained at a temperature of at least 25 degrees kelvin , and preferably at least 100 degrees kelvin below the half crystallization temperature of the amorphous metal composition being applied thereto . in order to facilitate the formation of a strong , metallurgical bond between the metallic substrate 108 and the amorphous metal layer being deposited on the substrate , a pulsed power supply 120 may be provided in electrical contact with the plasma gun anode 112 and the substrate 108 . while a continuous current , which is a substantial fraction of that between the cathode 110 in the anode 112 , could tend to heat the surface of the metal substrate in deposited layer , a short duty - cycle discharge as optionally provided in accordance with the present methods can be utilized to enhance surface bonding , while limiting surface heat generation . in this regard , a capacitively - pulsed power supply , with a capacitance of 100 , 000 microfarads is charged to a dc voltage of 100 - 220 volts , and in connection with its positive ( cathodic ) voltage terminal to the substrate and its negative ( anodic ) voltage terminal to the cooled metal anode 112 of the plasma gun 102 . the pulsed power supply is periodically discharged at a rate of above once per second and a duty cycle of about 0 . 1 to 1 percent ( 1 to 10 - milliseconds per pulse ) during the initial pass of the amorphous metal vapor phase over the substrate , to enhance the bond with the substrate by removing any oxide surface layer . by a cathodic pulse to its electrically - conducting substrate 108 , cations from the plasma plume 106 are accelerated to impact and clean the substrate surface . in addition , the substrate tends to be “ cooled ” by the evaporation of electrons compared to being heated by their impact if connected to the anode . an iron - based bmg having a composition of ( fe66mn29cr5 ) 68zr4nb4b24 ( atomic percent ) is vaporized in the plasma gun 102 , and condensed and solidified on a steel substrate 108 which is actively cooled to − 10 ° c . by a glycol cooling stream and refrigeration unit . the feed rate of the alloy powder having a particle size of less than 3 microns is 1 pound per hour with 5 - 10 scfh of argon 103 . argon is fed to the plasma spray gun 102 at a rate of about 75 scfh , and dc power is fed to the plasma spray gun to produce a plasma temperature of over 20 , 000 ° k . the vapor plasma plume is moved along the substrate at a rate of about 2 meters / second at a distance from the end of the gun to the substrate of 5 - 15 cm . the deposition is carried out in a vacuum in the chamber of approximately 0 . 01 to 0 . 1 atmosphere . a slight excess ( e . g ., 5 atom percent ) of the more volatile boron component may be included in the small diameter powder , to produce the desired bmg stoichiometry in the condensed vapor deposit on the substrate . this may be determined empirically . the alloy vapor condenses on the substrate and rapidly solidifies to form a bmg coating of substantially full density with good adherence to the substrate . in this example , an aluminum - based alloy from table 2 having a composition of ( al85y8ni5co2 ) 70ba8ca210si20 or ( al85y8ni5co2 ) 70 ( zr , ti ) 10b18si2 ( atomic percent ) is applied to a cooled , clean copper sheet as described in example 1 , with a similar result . excess ca and al may be used in the input powders , as empirically determined , to obtain the desired atomic ratio in the deposit . in this example , a copper - based bmg having a composition cu40ti30ni15zr10sn5 ( atomic percent ) is applied to a cooled copper sheet as described in example 1 , with a similar result in this example , a nickel - based bmg having the composition ni60nb20ti12 . 5hf7 . 5 is applied to a rotating , cooled steel mandrel , as generally described in example 1 , with a similar result . in this example , a zirconium - based bmg alloy , zr65al7 . 5ni10cu17 . 5 is applied to a cooled steel alloy substrate as in example 1 . excess aluminum may be vaporized , as indicated , to achieve the desired deposit ratios . in this example , a titanium - 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