US 2890971 A
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June 16, 1959 w. ARNOLD ETAL COATING METHOD Filed May 2, 1956 0047A METAL J6 BASE WITH A WET M/XTURE IIVCLl/D/f/G ALU/H/AWM OXIDE AND A Bl/YDER DA 7'// 00A TED 34815 IN l/[ATED AIR JMMERJE //v A 8477/ comm/mm;
A4 u/w/vum N/TAA TE P466 THROUGH A HEA TED A'DUCl/VG ATMOSPHLAE WALTER L. ARNEILD CI IN VEN TOR$ Runen'r K. PEARCE United States Patent 2,890,971 COATING ME'I'HOD Walter L. Arnold, New Providence, and Robert K. Ifearce, Maplewood, NJ., assignors to Radio COI'POIEltlOIl of America, a corporation of Delaware Application May 2, 1956, Serial No. 582,243
A 9 Claims. (Cl. 117- 65) One of the common types of heaters for indirectly heated cathodes comprises a tungsten wire coated with aluminum oxide, the oxide coating forming an insulating covering for the heater so that when the heater is inserted into a cathode sleeve the heater and sleeve are insulated from each other. Consequently, it is desirable to provide the heater with a strongly adherent coating to prevent the coating from being stripped from the metallic base of the heater during normal handling. However, the ends of the heater must be bared in order that the metallic base of the heater be exposed for welding to lead-in wires. Consequently, the coating must also be soft enough so that it can be easily chipped or broken away from the metallic base at the places to be bared. In
addition, the portions of the base so bared must be relatively free of oxidation so that they may be in a proper condition for welding.
It is therefore an object of the invention to provide an improved method for making a coated heater having an insulating coating, such as aluminum oxide, which is relatively strongly adherent to its base material and which at the same time has bared base material portions adapted to be welded to lead-in wires.
It is another object of the invention to provide an improved method of coating a metallic base and wherein the hardness of the coating is controllable without oxidizing the base.
The foregoing and related objects are achieved in a method wherein a base to be coated is first subjectedto one or more coating steps. Each coating step includes the passing of the base through a mixture including aluminum oxide and a binder, followed by a drying of the coated base in air at a temperature high enough to dry the coating but insufficient to oxidize the coated base.
Then the base thus coated is immersed in a bath containing aluminum nitrate of a predetermined concentration. Finally, the coated base is subjected to heat in a reducing atmosphere, as by being passed through a hydrogen furnace. The stock thus made has an insulating coating which is hard enough to withstand normal handling and which may be selectively, relatively easily removed, as by a chipping or cracking of the coating.
. One example of a method of the invention involves a I heater wire comprised of a metallic base, which may be tungsten, to which an aluminum oxide coating is applied. As indicated in the sole figure of the drawing, the first operation is to coat a bare metallic wire with wetted aluminum oxide. A preferred method of applying this aluminum oxide coating is to pass the wire through a slurry containing aluminum oxide and a binder in a liquid vehicle then to dry the coating thus produced in an air Patented June 16, 1959 oven. The desired thickness of aluminum oxide coating is achieved by passing the wire successively through the slurry and the air oven through a number of cycles until the desired coating thickness is built up on the wire. For example, if a bare tungsten wire 1.24 mils in diameter is to be coated with aluminum oxide to a coating thickness of 5 mils, about 12 passes of the wire through the slurry and oven are usually required.
As a relatively hard coating of aluminum oxide is desired, the temperature of the air oven in the above dryingstep has, in the prior art, usually been from about 680 C. to about 800 C. However, a heating to such a temperature causes the base material of the wire, in this case tungsten, to oxidize. The oxide layer thus formed cannot be practicably removed in order to carry out a desired welding step. It has been found that the temperature to which the aluminum oxide coated wire can be subjected, during an air drying operation, without forming an oxide layer which cannot be practicably removed, is a temperature of about 600 C., and pref erably a temperature of not exceeding 550 C. However, when a temperature of less than about 680 C. is used to dry the aluminum oxide coating as it is formed, the coating is too soft to withstand the handling operations encountered before the wire is mounted within a cathode sleeve.
The invention provides a means for overcoming the above-described problem of excessive oxidation at the temperature required to provide the desired coating hardness in the air drying steps. i
As indicated in the drawing, it has been found that the additional steps of immersing the aluminum oxide coated wire in a bath containing aluminum nitrate, followed by a passage of the wire through a reducing furnace, hardens a coated wire which has been subjected to an air drying step at a temperature of less than 600 C. and also substantially removes any residual oxide layer which may have formed in any of the preceding steps.
A preferred aluminum oxide coating slurry used in practicing the invention is comprised of the following. To make a half gallon slurry, mix 360 cc. methyl alcohol and 285 grams of 900 mesh aluminum oxide powder. This mixture is ball milled for about three hours. This ball milling operation appears to break up any agglomerates which are present in the 900 mesh powder. After the ball milling there is added to the milled mixture 2,625 grams of 500 mesh aluminum oxide powder, 326 cc. of distilled water, and 490 cc. of a saturated solution of aluminum nitrate in water for service as a binder.
A tungsten wire is passed through the slurry above described and then passed through an air oven heated to a temperature of between from about 500 C. to about 600 C. At any event the temperature should not exceed 600 C. else an oxide layer will be formed which cannot be readily removed by later operations. A preferred temperature is about 550 C., a lower temperature giving a softer coating and a higher temperature giving a harder coating. The wire is passed through the slurry and air oven in a suflicient number of cycles to build up the coating to the desired thickness. Then the coated wire is immersed in a bath containing aluminum nitrate in water. The aluminum nitrate concentration is preferably from about 30 percent to about percent of a Thewireis then passed through a reducing furnace at a temperature of between about 800 C. and 1000 C. The wire remains in this furnace for a length of time suflicient to dry it. The atmosphere of the reducing furnace 'ispreferablyone which consists essentially of hydrgen. The'hardness ofthecoating is softened by increasing oven temperatures] (This softening with increasing temperatures'holds true up to sinteringtempera- 'ture, that is, roughly about'l550 C.). When the wire *hasbeen subjected to an air oven temperature of about "550 C., and to an aluminum nitrate bath comprised of a substantially saturated solution of aluminum nitrate in water; the desired reducing furnace temperature is about '1000" C. This providesa coating having a desired hardness for good adherence of the coating and at the same time permits easy removal of the coating at'the portions tobe involved in welds. The passage of the wire through the reducing furnace, however, does not affect the hardness of the coating as much as the air oven heating and the aluminum nitrate bath. Consequently, the greatest amount of control over the coating hardness can be exercised by a control of the air oven temperature and of the concentration of the aluminum nitrate in the bath.
The wire produced by the aforedescribed steps is then wound into heater form vand finally cut 011? to desired lengths for use as heaters in electron tubes. -.The portions of the wire .whichare to be welded to. lead-in wires are subjected to any of the well knownchipping or cracking .operations which break off. the coating only at these wire portions. It-will be noted that u-p to this point the wirehas not been subjected to a temperature as high .as sintering tem- ..pera-ture. However, after. the heater is mounted within an. electron tube andwduring the usual bake-out .and evacuation step in the .manufacture of the tube, the ,heateris subjected to atemperature at least as high as v.thesintering point. (at least about 1550 C.) of thecoating. This last, heating sinters the. aluminum oxide of the acoatingrand rids the-coating of substantially all other matter.
It is seen, from the foregoing-thatvthe method of the invention provides a means for controlling the'hardness of an: aluminum oxide coating on a metallic base. The hardness of -the.coating-may becontrolled by adjusting .-the' temperature of; the air oven, by adjusting theconcentration' of the aluminum .nitrate inthe aluminum nitrate wbath, or by adjusting the temperature of the reducing ifurnace. vThe method of the invention thus provides a 1.3163115} for making improved cathode heaters having an insulating coating-which is relatively strongly adherent to the base material1and'which atthe. same time has --,bared base material portions adapted to .be. welded to #lead-inzwires.
. :What is claimed is:
1. Agcoating method .comprising-theusteps of-successively coating an oxidizable metallic base with .a mixture including aluminum. oxide and an aluminum nitrate: bind- -.=er:to form acoating on said base, dryingin air'the coating thus formedatia temperature highenough to dry the "coating but insufiicient to oxidize the'coated base to a =;degr ee.wherein the oxide resists chemical removal, wherebyvthe coating is relatively soft, then immersingthe coatedi base in a bath consisting essentially of-aluminum nitrate and a solvent for said nitrate, and then heating "the coated-base in a reducing atmosphere to a temperatureof from' about 800 C. to about 1000 C. for simultaneously increasing the-hardness of the coating for substantia-llyeasy removal of selected portions thereof, 'and removing chemicallyanyoxide formed on said base duringsaid air drying.
2. A coating method comprising the steps of succes- -sive1y-.co'ating.:a1.n1'etallic base with a mixture including rain-minum oxide-and an. aluminum oxide-binder to pro- 4, vide a coating on said base, drying said coating in air heated to atemperature between room temperaturenand about 600 C., whereby said coating acquires a first and relatively low order of hardness and a chemically removable oxide is formed on said base, immersing the coated base in a bath consisting essentially of aluminum nitrate and a solvent for said nitrate, and then heating the coated base in a reducing atmosphere at a temperature higher than about-600 C.-'and up to 1000 C. for simultaneously removing said oxide .and imparting a second and relatively high order of hardness toz said coating, whereby a metallic base is provided which is substantially oxide-free, and which has an aluminum oxide coating which is relatively strongly'a'dherent to said base and which is adapted to be selectively, relatively easily removed therefrom.
3. A coating method comprising the steps of successively. coating a metallic base with ,a mixture including aluminum oxide and an aluminum nitrate'binder,'-'drying the coated'base thus formed in hot air, at a temperature high enough to dry the coating but insufficient tojoxidize the. coated base,.immersing.the coated'lbase in'a bath consisting essentially of aluminum nitrate and a solvent for said nitrate, and then heating the coated baseto a .temperatureof from about 800 C. to about'lOOO C.
in a reducing atmosphere consisting essentially of ,hydrogen, whereby a metallic base is provided which is 'substantial'ly oxide-free, and which has an aluminum oxide coating which is relatively strongly adherent to saidbase and which is adapted to be selectively; relatively easily removed therefrom.
4. A coating method comprising the steps of successively coatings. tungsten base with a mixturelin'cluding aluminum oxideand an aluminum nitratebinder, drying the coated base thus formed in air heated to. a tempera- .tureof between about 500 C. and about' 60,0 C., immersing the coated base in a bath consisting essentially of aluminum nitrate and a solvent for said nitrate, and. then heating the coated base toa temp eratureof betweenabout .800" C. and about 1000 C. in a substantially dryhydrogen atmosphere, whereby a tungsten baseis provided whichis substantially oxide free, and whichhas .an aluminum oxide coating which is relatively strongly adherent tosaid base and which is adapted to beselective 1y, relatively easily removed therefrom.
5. A coating method comprising the ,steps of successively coating a tungsten basewith amixture including aluminuinoxide and an aluminum nitrate binder,. drying the coated base thus. formed in an air -oven. at1a.tem-
.perature of between about 500 C. and about;- 600. C.,
immersing the coated base in a bath consisting "essentially of a substantiallysaturated solution of aluminum nitrate in water, andthenheating the coated -base toa temperature of between about 800 C.-and about-W -C. i-n a substantially dry hydrogen atmosphere;'.-wher eby atungsten base is provided which is substantiall'yroxide free, .and-1whieh has an aluminum oxide coating-.which 1s relatively strongly adherent to said base and. which is.adapted to be selectively, relativelyeasily-removed therefrom.
6. A coating method comprising the steps ofsuccessively coating a tungsten base with a mixturej including aluminum oxide and a saturated aqueous solutionof aluminum -nitrate,.,drying the coated base: thus formed in an .air -oven at a temperature ofbetweenw about 500 C. and about 500 ,C., immersing the coated ahase .in a bath consisting essentially of aluminum. nitrate:;and .al-solvent for said nitrate, and then heating theqcoatedybase toa temperatureof between about 800 C- and'about 1000" C. in a substantially dry hydrogenatmosphererfor :a period of time sufficient to reduce the base material to substantially oxide-free tungsten, whereby a tungsten;.base isprovided which is substantially oxide free, ;and'..;which has an aluminum oxide coating which is relatively strong- -ly adherent to said base and which is adapted to be selectively, relatively easily removed therefrom.
7. A coating method comprising the steps of first coating a metallic base by subjecting said base to a plurality of operations each comprising a pass through a slurry of aluminum oxide in an aqueous solution of aluminum nitrate followed by a drying of the base in air heated to a temperature of between about 500 C. and about 600 C., then immersing the base thus coated in a bath consisting essentially of a 30 to 100 percent saturated solution at ordinary room temperatures of aluminum nitrate in water, and finally heating the base to a temperature of between about 800 C. and about 1000 C. in substantially dry atmosphere consisting essentially of hydrogen and for a period of time sufficient to reduce the base material to a substantially oxide free form, whereby a metallic base is provided which is substantially oxide free, and which has an aluminum oxide coating which is relatively strongly adherent to said base and which is adapted to be selectively, relatively easily removed from said base.
8. A coating method comprising the steps of first coating a metallic base by subjecting said base to a plurality .of operations each comprising a pass through a slurry of aluminum oxide in an aqueous solution of aluminum nitrate followed by a drying of the base in an air oven at a temperature of about 550 C., then subjecting the base thus coated to a bath consisting essentially of a substantially saturated solution at ordinary room temperatures of aluminum nitrate in water, and finally subjecting the base to heat at a temperature of about 1000 C. in a substantially dry atmosphere consisting essentially of hydrogen and for a period of time sufficient to reduce the base material to a substantially oxide free form, whereby a metallic base is provided which is substantially oxide free, and which has an aluminum oxide coating which is relatively strongly adherent to said base and which is adapted to be selectively, relatively easily removed from said base.
9. The product of the process of claim 1.
References Cited in the file of this patent UNITED STATES PATENTS 2,008,733 Tosterud July 23, 1955 2,043,720 Wagenhals June 9, 1936 2,179,453 Palmateer Nov. 7, 1939 2,722,491 Anderson Nov. 1, 1955 2,743,192 White Apr. 24, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noa 2,890,971 June 16, 1959 Walter Lo Arnold et a1a It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 4, lines 66 and 6'7, for "about 500 0., and about 500 0.,"
O O rs'ad about 3500 (Lu and about 600 C Signed and sealed this 23rd day of February 1960a (SEAL) Attest:
KARL H AXLINE 7 ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents