US3573969A - Method for surface nitriding boron filaments - Google Patents

Method for surface nitriding boron filaments Download PDF

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Publication number
US3573969A
US3573969A US753589A US3573969DA US3573969A US 3573969 A US3573969 A US 3573969A US 753589 A US753589 A US 753589A US 3573969D A US3573969D A US 3573969DA US 3573969 A US3573969 A US 3573969A
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United States
Prior art keywords
boron
filament
filaments
coating
temperatures
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Expired - Lifetime
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US753589A
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English (en)
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Jose Camahort
Mario P Gomez
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Lockheed Martin Corp
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Lockheed Aircraft Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C47/00—Making alloys containing metallic or non-metallic fibres or filaments
    • C22C47/02—Pretreatment of the fibres or filaments
    • C22C47/04—Pretreatment of the fibres or filaments by coating, e.g. with a protective or activated covering
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S264/00—Plastic and nonmetallic article shaping or treating: processes
    • Y10S264/19—Inorganic fiber

Definitions

  • a method for surface nitriding boron filaments to make the filaments useful as reinforcement agents in composite materials involves initially forming a liquid boron oxide coating on the filament, for example, by heating the filament at temperatures of from about 560 C. to 800 C. in an oxidizing atmosphere and then converting the liquid oxide coating to a solid, continuous boron nitride coating by, for example, heating the filment at temperatures of from about 600 C. to 1100 C. in a nitrogencontaining atomsphere.
  • a typical four mil diameter boron filament made by heating a 0.5 mil diameter tungsten wire in an atmosphere of boron chloridehydrogen exhibits an average strength of 450,000 p.s.i., an elastic modulus of 60 million p.s.i. and a density of 0.095 pound per cubic inch.
  • Resin matrix boron composites are currently used as structural parts in aerospace applications. Resin matrices, however, have low mechanical strength and also fail in the area of high temperature applications since they decompose at moderate temperatures.
  • metal and metal alloy matrices that not only possess good mechanical strength but also permit the use of such composites at much higher temperatures than resin matrices due to their higher melting point.
  • boron filaments into metal and metal alloy matrices by liquid metal processing techniques.
  • structural metal matrices such as aluminum, magnesium, nickel, titanium, iron, beryllium, chromium and alloys thereof is well documented.
  • the prolonged contact of aluminum and boron at temperatures in the order of 400 C. to 500 C. results in the degradation of the boron filaments due to formation of the brittle compound aluminum boride.
  • This compound, and other brittle metal borides cause premature fracture of the boron filaments upon the application of stresses.
  • the usefulness of boron filaments in metal matrices is accordingly minimized since no substantial strength improvements are realized.
  • metal matrix composites As now fabricated by the art, must be formed at low temperatures by using cumbersome and expensive fusion bonding techniques.
  • the boron filaments are arranged with, for example, aluminum foil in sandwich fashion so that boron filaments altermate with layers of aluminum.
  • a firm bond between the aluminum and boron filaments is achieved by heating and compressing the sandwich. Since the temperature for 3,573,969 Patented Apr. 6, 1971 this process of fusion has to be held lower than 600 C. in order to avoid chemical reaction between boron and aluminum, pressures approaching 10,000 p.s.i. are required. This is rather impractical for sandwiches of say one square foot in size since forces as much as 1,500,000 pounds would be needed for compression. Further, the resulting composites are severely limited in their high temperature capabilities due to reaction between the boron and metal at temperatures in the order of 500 C. and higher and the resulting formation of brittle metal borides.
  • boron nitride should act as a highly satisfactory diffusion barrier since it is essentially inert to both boron and the metal matrices of interest at elevated temperatures.
  • boron-aluminum composite rods (0.020 inch in diameter) were prepared by identical liquid infiltration techniques using as-received boron filaments and boron filaments nitrided by the process of the invention.
  • the composite rods containing the nitrided filaments averaged about 180,000 p.s.i. in ultimate tensile strength while those rods containing the uncoated filaments averaged only about 110,000 p.s.i. in ultimate tensile strength.
  • the method of the invention involves initially forming a liquid boron oxide coating on the boron filament and subsequently converting the liquid oxide coating to a solid, continuous boron nitride coating. It has been determined that the efiicacy of the process is dependent upon this specified sequence of processing steps.
  • Formation of the liquid boron oxide coating is a prerequisite to the obtaining of a continuous boron nitride coating.
  • Applicants have found it impossible to directly produce a boron nitride coating, for example, by reacting the boron filament with nitrogen or ammonia or to replace the liquid oxide intermediate coating with a solid boron oxide coating without degrading the filament or forming a nitride film that is ineffectual in protecting the filament at elevated temperatures.
  • a further advantage accruing to the boron nitride coated filaments of the invention is that the coating is thick enough to be an effective diffusion barrier and yet thin enough to have a negligible effect on the boron filament strength. Since boron nitride is much weaker than boron, it is desirable to make the boron nitride coating as thin as possible so as not to substantially decrease the volume fraction of boron in the composite material. For example, a /6 mil thick boron nitride coating on a 4 mil diameter boron filament would constitute 14.5 volume percent of the coated filament with a corresponding loss in filament strength given by the law of mixtures:
  • the invention may be more easily understood by reference to the drawing which on coordinates of ultimate tensile strength and time in minutes is a semi-log plot showing mechanical strength degradation of boron filaments as a function of time of exposure of the filaments in molten aluminum maintained at 700 C.
  • Curve 1 of the drawing shows the mechanical strength degradation of boron filaments protected by the boron nitride coating of the invention.
  • Curve 2 of the drawing shows the mechanical strength degradation of as-received boron filaments which were not coated in accordance with the method of the invention. As evidenced by these curves, a rapid deterioration of a mechanical strength was exhibited by the unprotected filaments.
  • coated boron filaments showed significantly less degradation of mechanical strength.
  • the data depicted by this drawing was obtained by immersing both coated and uncoated boron filaments in the molten aluminum maintained at 700 C. for A2, 1, 3, 5 and minutes and then tensile testing the filaments. All coated boron filaments were made by the technique hereinafter described in conjunction with Example 1 of the specification.
  • the method of the present invention involves the formation of a continuous adherent boron nitride coating on boron filaments, which filaments are then uniquely suitable for incorporation into metal and metal alloys matrices for use as structural composite materials.
  • the process of the invention for nitriding the surface of boron filaments results in a boron nitride coating which protects the filaments from chemical attack by the metal and metal alloy matrices and further minimizes diffusion between the filament and the matrix at elevated temperatures.
  • the method is carried out by initially forming a uniform liquid boron oxide coating on the boron filament and subsequently converting the liquid oxide coating to a solid continuous boron nitride coating.
  • the liquid boron oxide coating is readily formed by several techniques.
  • One preferred technique involves heating the boron filament surface in an oxidizing atmosphere such as air, oxygen and mixtures of oxygen with nitrogen or other inert gases. Minimum temperatures in the order of 560 C. are required to form the liquid oxide coating required by the invention.
  • oxidizing atmosphere such as air, oxygen and mixtures of oxygen with nitrogen or other inert gases.
  • Minimum temperatures in the order of 560 C. are required to form the liquid oxide coating required by the invention.
  • the softening range for this oxide is between 560" C. and 630 C. Below about 560 C., the viscosity of the oxide is sufficiently high as to make the coating, for the purpose of the invention, a solid oxide coating.
  • the liquid boron oxide coating becomes less viscous and tends to flow under the influence of gravity. Up to about 800 C. this fiow, however, is negligible. Above 800 C., the flow is such as to cause reduction in the thickness of the coating. For temperatures in the order of 800 C. to 1,000 C. and higher, evaporation of the boron oxide coating tends to further reduce the coating thickness. Since, in accordance with the invention, it is necessary to form an essentially continuous oxide coating, maximum temperatures in the order of 800 C. are preferred. However, it has been found that a continuous coating results when the coating thickness is in the order of 0.1 micron.
  • a preferred temperature range is from 5 60 C. to 800 C. with an optimum range being between about 560 C. to 650 C. At a temperature of 650 C., a reaction time of about thirty seconds has been found adequate for forming a continuous liquid boron oxide coating. Since temperatures and time are interdependent, higher temperatures will require shorter times and lower temperatures will require longer reaction times.
  • a liquid boron oxide coating also may be formed on the filament by pulling the filament through molten boron oxide or by passing the filament over an evaporating boron oxide melt.
  • the liquid boron oxide coated filament is then heated in a nitrogen containing atmosphere such as ammonia and an ammonia-nitrogen mixture to convert the liquid boron oxide coating to a solid, essentially continuous boron nitride coating.
  • a nitrogen containing atmosphere such as ammonia and an ammonia-nitrogen mixture
  • nitridation begins at temperatures in the order of 350 C.
  • temperatures in the order of 800 C. and higher are preferred since the resulting increased reaction rate improves diffusion of the nitriding gases into the boron oxide layer and causes formation of a higher purity boron nitride coating.
  • temperatures in excess of 1100 C. are to be avoided, however, since degradation of the boron filament at such elevated temperatures becomes significant. For temperatures in the range of about 800 C. to 1100 C.
  • a preferred temperature range is from about 600 C. to 1100 C. with an optimum range being between about 800 C. to 1100 C. Within these ranges, it has been determined that a reaction time of about thirty seconds at 1060 C. and about two minutes at 900 C. is satisfactory in converting the oxide coating to a continuous boron nitride coating. Since temperatures and times are interdependent,
  • Nitrogen atmospheres have been found to be unsatisfactory since they require temperatures in the order of 1600 C. to convert the oxide coating to the nitride coating, and, as previously discussed, such elevated temperatures cause serious degradation of the underlying boron filaments. Consequently, nitrogen-containing atmospheres which are sufficiently reactive to cause nitridation in accordance with the preceding discussion without attack of the underlying boron filament or formation of undesirable by-products are utilized. Such atmospheres are considered within the skill of the art.
  • a continuous boron nitride coating results when the minimum coating thickness is in the order of 0.1 micron.
  • it is desirable to utilize thin boron nitride coatings on the boron filaments since as the volume fraction of the boron nitride coating on the boron filament increases, the ultimate tensile strength of the boron nitride coated filament decreases.
  • a coating thickness of 0.7 micron to be a maximum, practical thickness in view of the preceding.
  • thickness above 0.7 micron can be utilized if the corresponding loss in filament strength is not a significant factor in the use of the filament.
  • the thickness of the boron nitride coating depends on the thickness and flow and evaporation characteristics of the liquid boron oxide coating, which varies as discussed herein with the reaction times and temperatures utilized during the oxidation and nitridation steps.
  • nitriding temperatures in the order of 1,000 C., some evaporation of the boron oxide coating occurs prior to its conversion to boron nitride.
  • a continuous nitridation process has been devised whereby one or more boron filaments were pulled through two reactors containing respectively, air and ammonia.
  • a small amount, for example, five volume percent of nitrogen or hydrogen gas is preferably added to the ammonia atmosphere to prevent excessive cracking of the ammonia at elevated temperature.
  • Flow rates of ammonia in excess of the stoichiometry amount are preferably maintained in the nitriding reactor. Suitable reaction times are easily achieved by varying the filament take-up speed. The following reactions are assumed to take place:
  • the resulting nitrided boron filaments are uniquely suitable for incorporation into structural metal and metal alloy matrices by liquid infiltration techniques or metal casting methods.
  • a preferred composite fabrication process involves pulling a number of the nitrided boron filaments through the desired molten metal bath to produce a composite wire or rod wherein the filaments are arranged and aligned properly.
  • the filaments enter the molten metal bath through separate holes on an entrance disk which insures sufiicient space between the filaments so that all filament surfaces are wetted.
  • the filaments then exit together through an orifice of any desired size and shape.
  • the resulting composite rods or wires are then used to produce larger composite bodies, such as sheets and tapes by passing the rod or wire through a second molten bath of the same metal or a lower melting metal or alloy.
  • a 0.020 inch diameter boronaluminum composite rod was produced by pulling 16 nitrided boron filaments through a molten aluminum bath. Ten of these composite wires were then pulled through a second molten aluminum bath to produce a 0.20 inch wide by 0.020 inch thick composite tape, The boron nitride coating formed on the boron filaments by the process of the invention protected the filament from chemical attack by the molten aluminum during this liquid infiltration process.
  • Example 1 A single boron filament was pulled continuously through a reactor containing air at 650 C. and then through a second reactor containing ammonia at 900 C. Reaction times of about two minutes in each reaction were maintained by a filament take-up speed of 0.5 feet per minute through the two 12 inch long quartz reactors. An ammonia flow rate of about two cubic feet per hour was maintained in the nitriding reactor. Chemical analysis and optical microscopy showing a boron nitride coating of about 0.3 to 0.4 micron thickness on the boron filament, This coating was an effective diffusion barrier in aluminum, aluminum alloy, nickel, titanium and glassy carbon.
  • Example 2 Sixteen boron filaments were passed continuously through two successive one inch diameter twelve inch long stainless steel reactors containing air at 650 C. and an ammonia-5% nitrogen atmosphere at 1060 C. respectively. A filament take-up speed of about two feet per minute resulted in reaction times of about thirty seconds in each reaction. Gas flow rates were two cubic feet per hour of ammonia and 0.1 cubic feet per hour of nitrogen in the nitriding reactor. Optical microscopy showed a 0.1 to 0.2 micron thick boron nitride coating on the filaments. This coating showed excellent protective action in molten aluminum during liquid infiltration experiments,
  • Example 3 An experiment similar to that described in Example 2 was performed with the oxidizin reactor containing air at about 540 C. and the nitriding reactor containing a mixture of ammonia-5% nitrogen at about 1,000 C. No. effective boron nitride coatings were obtained under these conditions using filament take-up speeds of from about 0.5 feet per minute to about 2 feet per minute. Such filaments were attacked by molten aluminum during subsequent composite fabrication resulting in an average ultimate tensile strength of only 100,000 to 110,000 p.s.i. In contrast, utilizing a temperature of 650 C.
  • a method for surface nitriding boron filaments consisting essentially of the steps of:
  • liquid oxide coating is formed by heating the filament in an oxidizing atmosphere at a temperature of at least about 560 C.
  • a method for surface nitriding boron filaments to make the filaments useful as reinforcement filaments in composite metal and metal alloy materials consisting essentially of the steps of:
  • liquid oxide coating is formed by heating the filament in an oxidizing atmosphere at a temperature of from about 8 560 C. to 1000 C. and said oxide-coated filament is heated at a temperature of from about 350 C. to 1100 C.
  • boron filaments are suitable for incorporation into metal and metal alloy matrices by liquid infiltration and metal casting techniques and are characterized by having an adherently bonded, essentially continuous surface coating of boron nitride at least about 0.1 micron thick.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Inorganic Chemistry (AREA)
  • Manufacture Of Alloys Or Alloy Compounds (AREA)
  • Inorganic Fibers (AREA)
  • Chemical Or Physical Treatment Of Fibers (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Ceramic Products (AREA)
US753589A 1968-08-19 1968-08-19 Method for surface nitriding boron filaments Expired - Lifetime US3573969A (en)

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US (1) US3573969A (enrdf_load_stackoverflow)
JP (1) JPS4937098B1 (enrdf_load_stackoverflow)
DE (1) DE1942194B2 (enrdf_load_stackoverflow)
FR (1) FR2016973A1 (enrdf_load_stackoverflow)
GB (1) GB1241104A (enrdf_load_stackoverflow)
NL (1) NL144666B (enrdf_load_stackoverflow)
SE (1) SE354639B (enrdf_load_stackoverflow)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3976444A (en) * 1974-10-25 1976-08-24 United Technologies Corporation Production of improved boron abrasives
US4605588A (en) * 1985-03-14 1986-08-12 The Boeing Company Barrier coated ceramic fiber and coating method
US4948662A (en) * 1985-03-14 1990-08-14 The Boeing Company Boron nitride coated ceramic fibers and coating method
CN117900467A (zh) * 2024-03-19 2024-04-19 天津大学 氮化硼纳米片及其制备方法

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2441260B2 (de) * 1974-08-28 1976-12-02 Laboratoire Suisse de Recherches Horlogeres, Neuenburg (Schweiz); Gutehoffnungshütte Sterkrade AG, 4200 Oberhausen Verfahren zur herstellung einer als trennmittel und diffusionsbarriere wirkenden schicht
CA1085596A (en) * 1977-03-02 1980-09-16 Robert S. Hamilton Bn bonded bn fiber article from bn fiber
US4130631A (en) * 1977-03-02 1978-12-19 The Carborundum Company BN Bonded BN fiber article from boric oxide fiber
DE3025636A1 (de) * 1980-07-07 1982-02-04 Alfred Teves Gmbh, 6000 Frankfurt Gegossenes werkstueck

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3976444A (en) * 1974-10-25 1976-08-24 United Technologies Corporation Production of improved boron abrasives
US4605588A (en) * 1985-03-14 1986-08-12 The Boeing Company Barrier coated ceramic fiber and coating method
US4948662A (en) * 1985-03-14 1990-08-14 The Boeing Company Boron nitride coated ceramic fibers and coating method
CN117900467A (zh) * 2024-03-19 2024-04-19 天津大学 氮化硼纳米片及其制备方法

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Publication number Publication date
SE354639B (enrdf_load_stackoverflow) 1973-03-19
JPS4937098B1 (enrdf_load_stackoverflow) 1974-10-05
NL6912631A (enrdf_load_stackoverflow) 1970-02-23
DE1942194B2 (de) 1971-06-09
DE1942194A1 (de) 1970-05-27
FR2016973A1 (enrdf_load_stackoverflow) 1970-05-15
NL144666B (nl) 1975-01-15
GB1241104A (en) 1971-07-28

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