US20140141172A1 - Metallophobic thermally applied ceramic materials - Google Patents

Abstract

Metal substrates coated with ceramics and resistant to attack by molten metal.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)
• This application is a continuation-in-part of co-pending U.S. application Ser. No. 11/891,470, filed on Aug. 10, 2007 and issuing on Jul. 2, 2013 as U.S. Pat. No. 8,475,606, together with now abandoned U.S. patent application Ser. No. 11/891,469, filed on Aug. 10, 2007, both disclosures of which are fully incorporated by reference herein.
BACKGROUND OF THE INVENTION
• This invention relates to metallophobic thermally applied ceramic materials, and more particularly, it relates to ceramic coatings used to improve heat transfer suitable for use in molten metals such as molten aluminum.
• As noted, non-metallic barrier coatings are applied to metallic substrata for the purpose of protecting the substrata from chemical reaction. Such protection depends on the continuity of the coating for protection. Discontinuities in the coating can permit the transport of chemical species or agents to the underlying substrate surface where chemical reactions may occur and result in failure of the substrate. Such discontinuities include porosity within the coating and cracks, both pre-existing and those that develop in-service.
• In the prior art, heaters used for molten aluminum are usually enclosed in ceramic tubes. Such heaters are very expensive and are very inefficient in transferring heat to the melt because of the air gap between the heater and the tube. Also, heaters such electric heaters, have very low thermal conductivity values that are characteristic of ceramic materials. In addition, the ceramic tube is fragile and subject to cracking. The same need exists for protecting metal substrates generally in contact with molten metals. Examples of such substrata include: mold components for castings, sensors, molten metal containment vessels, and flow regulation deices for molten metal.
SUMMARY OF THE INVENTION
• It is an object of the invention to provide an improved ceramic-coated material resistant to molten metal attack.
• It is another object of the invention to provide an improved ceramic-coated material for use in molten metal such as molten aluminum.
• Yet, another object of this invention is to provide an improved ceramic-coated material for use in molten metal, the ceramic coating having metallophobic properties.
• And yet, another object of the invention is to provide an improved ceramic-coated material for use in molten metal, where said material remains resistant to molten metal attack even when cracks and other discontinuities develop in the coating.
• And yet, it is a further object of the invention to provide an improved ceramic-coated material for use in electric heater assembly for use in molten metal, the electric heater assembly having a protective sleeve comprised of a material resistant to erosion or dissolution by molten metal such as molten aluminum, even with the presence of small cracks and dicontinuities that develop through cyclic heating.
• These and other objects will become apparent from the specification, drawings and claims appended hereto.
• In accordance with these objects, there is disclosed a ceramic-coated material with metallophobic properties, and an electric heater assembly for heating molten metal, the assembly comprised of a tubular sleeve suitable for immersing in molten metal, the sleeve comprised of a metal or a metal composite material and having an inside surface. A body of a copper-containing material is contained in the sleeve, the body in contact with the inside surface of the sleeve to improve heat transfer through the sleeve. The copper-containing material has the ability to flow by creep deformation at operating temperatures to eliminate air pockets between the inside surface and the copper-containing material, the body having at least one electric heating element receptacle. An electric heating element is located in the receptacle in heat transfer relationship therewith for adding heat through said body to the molten metal.
• The heater assembly may be used for a heating a body of molten metal, e.g., aluminum or other fluid media, contained in a heating bay comprising the steps of providing a body of molten metal. An electric heater assembly is projected into the molten metal. The assembly comprises a tubular sleeve suitable for immersing in the molten metal, the sleeve comprised of a metal or a metal composite material and having an inside surface. A body of a copper-containing material is contained in the sleeve, the body in contact with the inside surface to improve heat transfer through the sleeve, the copper-containing material having the ability to flow by creep deformation at operating temperatures to eliminate air pockets between the inside surface and the copper-containing material, the body having at least one electric heating element receptacle. An electric heating element is located in the receptacle in heat transfer relationship therewith for adding heat through the body to the molten metal. An electric current is passed through the element and heat is added to the body of molten metal.
• The heater assembly may be used for heating fluid material where a refractory coating is not required. Thus, the electric heater assembly comprises a tubular sleeve, the sleeve comprised of a metal and having an inside surface. A body of a copper-containing material is contained in the sleeve, the body in contact with the inside surface to improve heat transfer through the sleeve, the copper-containing material having the ability to flow by creep deformation at operating temperatures to eliminate air pockets between the inside surface and the copper-containing material, the body having at least one electric, preferably multiple, heating element receptacles. Electric heating elements are located in the receptacles in heat transfer relationship therewith for adding heat through the body.
BRIEF DESCRIPTION OF DRAWINGS
• Further features, objectives and advantages of the present invention will be made clearer in the following Detailed Description made with reference to the drawings in which:
• FIG. 1 is a cross-sectional view of an electric heater assembly in accordance with the invention.
• FIG. 2 is a cross-sectional view of an electric heater assembly showing heat transfer material and heaters containing electric heaters in accordance with the invention.
• FIG. 3 is a dimensional view showing heat transfer media and receptacles for electric heaters.
• FIG. 4 is a cross-sectional view along the line A-A in FIG. 2.
• FIG. 5 is a cross-sectional view showing electric heater elements located in direction of maximum heat transfer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
• Referring to FIG. 1, there is shown a schematic of an electric heater assembly 10 in accordance with the invention. The electric heater assembly is comprised of a protective sleeve 12 and an electric heating element 14 when the heater is used for heating molten metal. A lead 18 extends from electric heating element 14 and terminates in a plug 20 suitable for plugging into a power source.
• Preferably, protective sleeve 12 is comprised of titanium tube 30 having a closed end 32. While the protective sleeve is illustrated as a tube, it will be appreciated that any configuration that protects or envelops electric heating element 14 may be employed. Thus, reference to tube herein is meant to include such configurations. A refractory coating 34 is employed which is resistant to attack by the environment in which the electric heater assembly is used. A bond coating may be employed between the refractory coating 34 and titanium tube 30.
• While it is preferred to fabricate tube 30 out of a titanium base alloy, tube 10 may be fabricated from any metal or metalloid material suitable for contacting molten metal and which material is resistant to dissolution or erosion by the molten metal. Other materials that may be used to fabricate tube 30 include silicon, niobium, chromium, molybdenum, combinations of NiFe (364 NiFe) and NiTiC (40 Ni 60 TiC), Ni—Fe (36% Ni—Fe), Ni—Co—Fe (28 Ni-18 CO—Fe)_i, particularly when such materials have low thermal expansion and low chilling power, all referred to herein as metals. For protection purposes, it is preferred that the metal or metalloid be coated with a material such as a refractory resistant to attack by molten metal and suitable for use as a protective sleeve.
• Further, the material of construction for tube 30 should have a thermal conductivity of less than 30 BTU/ft hr° F., and preferably less than 15 BTU/ft hr° F., with a most preferred material having a thermal conductivity of less than 10 BTU/ft hr° F. Another important feature of a desirable material for tube 30 is thermal expansion. Thus, a suitable material should have a thermal expansion coefficient of less than 15×10⁻⁶ in/in/° F., with a preferred thermal expansion coefficient being less than 10×10⁻⁶ in/in/° F., and the most preferred being less than 5×10⁻⁶ in/in/° F. Another important feature of the material useful in the present invention is chilling power. Chilling power is defined as the product of heat capacity, thermal conductivity and density. Thus, preferably the material in accordance with the invention has a chilling power of less than 5000 BTU²/ft⁴hr° F., preferably less than 2000 BTU²/ft⁴ hr° F., and typically in the range of 100 to 750 BTU²/ft⁴ hr° F.
• As noted, the preferred material for fabricating into tubes 30 is a titanium base material or alloy having a thermal conductivity of less than 30 BTU/ft hr° F., preferably less than 15 BTU/ft hr° F., and typically less than 10 BTU/ft hr° F., and having a thermal expansion coefficient less than 15×10⁻⁶ in/in/° F., preferably less than 10×10⁻⁶ in/in/° F., and typically less than 5×10⁻⁶ in/in/° F. The titanium material or alloy should have chilling power as noted, and for titanium, the chilling power can be less than 500, and preferably less than 400, and typically in the range of 100 to 300 BTU/ft² hr° F.
• When the electric heater assembly is being used in molten metal such as lead, for example, the titanium base alloy need not be coated to protect it from dissolution. For other metals, such as aluminum, copper, steel, zinc and magnesium, refractory-type coatings should be provided to protect against dissolution of the metal or metalloid tube by the molten metal.
• For most molten metals, the titanium alloy that should be used is one that preferably meets the thermal conductivity requirements, the chilling power and the thermal expansion coefficient noted herein. Further, typically, the titanium alloy should have a yield strength of 30 ksi or greater at room temperature, preferably 70 ksi, and typical 100 ksi. The titanium alloys included herein and useful in the present invention include CP (commercial purity) grade titanium, or alpha and beta titanium alloys or near alpha titanium alloys, or alpha-beta titanium alloys. The titanium-base alloy can be a titanium selected from the group consisting of 6242, 1100 and commercial purity (CP) grade. The alpha or near-alpha alloys can comprise, by wt. %, 2 to 9 Al, 0 to 12 Sn, 0 to 4 Mo, 0 to 6 Zr, 0 to 2 V and 0 to 2 Ta, and 2.5 max. each of Ni, Nb and Si, the remainder titanium and incidental elements and impurities.
• Specific alpha and near-alpha titanium alloys contain, by wt. %, about:
• • (a) 5 Al, 2.5 Sn, the remainder Ti and impurities.
  • (b) 8 Al, 1 Mo, 1 V, the remainder Ti and impurities.
  • (c) 6 Al, 2 Sn, 4 Zr, 2 Mo, the remainder Ti and impurities.
  • (d) 6 Al, 2 Nb, 1 Ta, 0.8 Mo, the remainder Ti and impurities.
  • (e) 2.25 Al, 11 Sn, 5 Zr, 1 Mo, the remainder Ti and impurities.
  • (f) 5 Al, 5 Sn, 2 Zr, 2 Mo, the remainder Ti and impurities.
• The alpha-beta titanium alloys comprise, by wt. %, 2 to 10 Al, 0 to 5 Mo, 0 to 5 Sn, 0 to 5 Zr, 0 to 11 V, 0 to 5 Cr, 0 to 3 Fe, with 1 Cu max, 9 Mn max, 1 Si max, the remainder titanium, incidental elements and impurities.
• Specific alpha-beta alloys contain, by wt. %, about:
• • (a) 6 A, 4 V, the remainder Ti and impurities.
  • (b) 6 Al, 6 V, 2 Sn, the remainder Ti and impurities.
  • (c) 8 Mn, the remainder Ti and impurities.
  • (d) 7 Al, 4 Mo, the remainder Ti and impurities.
  • (e) 6 Al, 2 Sn, 4 Zr, 6 Mo, the remainder Ti and impurities.
  • (f) 5 Al, 2 Sn, 2 Zr, 4 Mo, 4 Cr, the remainder Ti and impurities.
  • (g) 6 Al, 2 Sn, 2 Zn, 2 Mo, 2 Cr, the remainder Ti and impurities.
  • (h) 10 V, 2 Fe, 3 Al, the remainder Ti and impurities.
  • (i) 3 Al, 2.5 V, the remainder Ti and impurities.
• The beta titanium alloys comprise, by wt. %, 0 to 14 V, 0 to 12 Cr, 0 to 4 Al, 0 to 12 Mo, 0 to 6 Zr and 0 to 3 Fe, the remainder titanium and impurities.
• Specific beta titanium alloys contain, by wt. %, about:
• • (a) 13 V, 11 Cr, 3 Al, the remainder Ti and impurities.
  • (b) 8 Mo, 8 V, 2 Fe, 3 Al, the remainder Ti and impurities.
  • (c) 3 Al, 8 V, 6 Cr, 4 Mo, 4 Zr, the remainder Ti and impurities.
  • (d) 11.5 Mo, 6 Zr, 4.5 Sn, the remainder Ti and impurities.
• When it is necessary to provide a coating to protect tube 30 of metal or metalloid from dissolution or attack by molten metal, a refractory coating 34 is applied to the outside surface of tube 30. The coating should be applied above the level to which the electric heater assembly is immersed in the molten metal. The refractory coating can be any refractory material, which provides the tube with a molten metal resistant coating. The refractory coating can vary, depending on the molten metal. Thus, a novel composite material is provided permitting use of metals or metalloids having the required thermal conductivity and thermal expansion for use with molten metal, which heretofore was not deemed possible.
• When the electric heater assembly is to be used for heating molten metal such as aluminum, magnesium, zinc, or copper, etc., a refractory coating may comprise at least one of alumina, zirconia, yittria stabilized zirconia, magnesia, magnesium titanite, or mullite or a combination of alumina and titania. While the refractory coating can be used on the metal or metalloid comprising the tube, a bond coating can be applied between the base metal and the refractory coating. The bond coating can provide for adjustments between the thermal expansion coefficient of the base metal alloy, e.g., titanium, and the refractory coating when necessary. The bond coating thus aids in minimizing cracking or spalling of the refractory coat when the tube is immersed in the molten metal or brought to operating temperature. When the electric heater assembly is cycled between molten metal temperature and room temperature, for example, the bond coat can be advantageous in preventing cracking, particularly if there is a considerable difference between the thermal expansion of the metal or metalloid and the refractory.
• Typical bond coatings comprise Cr—Ni—Al alloys and Cr—Ni alloys, with or without precious metals. Bond coatings suitable in the present invention are available from Metco Inc., Cleveland, Ohio, under the designation 460 and 1465. In the present invention, the refractory coating should have a thermal expansion that is plus or minus five times that of the base material. Thus, the ratio of the coefficient of expansion of the base material can range from 5:1 to 1:5, preferably 1:3 to 1:1.5. The bond coating aids in compensating for differences between the base material and the refractory coating.
• The bond coating has a thickness of 0.1 to 5 mils with a typical thickness being about 0.5 mil. The bond coating can be applied by sputtering, plasma or flame spraying, chemical vapor deposition, spraying, dipping or mechanical bonding by rolling, for example.
• After the bond coating has been applied, the refractory coating is applied. The refractory coating may be applied by any technique that provides a uniform coating over the bond coating. The refractory coating can be applied by aerosol, sputtering, plasma or flame spraying, for example. Preferably, the refractory coating has a thickness in the range of 0.3 to 42 mils, preferably 5 to 15 mils, with a suitable thickness being about 10 mils. The refractory coating may be used without a bond coating.
• In another aspect of the invention, boron nitride may be applied as a thin coating on top of the refractory coating. The boron nitride may be applied as a dry coating, or a dispersion of boron nitride and water may be formed and the dispersion applied as a spray. The boron nitride coating is not normally more than about 2 or 3 mils, and typically it is less than 2 mils.
• The heater assembly of the invention can operate at watt densities of 40 to 120 watts/in².
• The heater assembly in accordance with the invention has the advantage of a metallic-composite sheath for strength and improved thermal conductivity. The strength is important because it provides resistance to mechanical abuse and permits an ultimate contact with the internal element. Intimate contact between heating element and sheath inside diameter provides for substantial elimination of an annular air gap between heating element and sheath. In prior heaters, the annular air gap resulted in radiation heat transfer and also backs radiation to the element from inside the sheath wall which limits maximum heat flux. By contrast, the heater of the invention employs an interference fit that results in essentially only conduction.
• In conventional heaters, the heating element is not in intimate contact with the protection tube resulting in an annular air gas or space there between. Thus, the element is operated at a temperature independent of the tube. Heat from the element is not efficiently removed or extracted by the tube, greatly limiting the efficiency of the heaters. Thus, in conventional heaters, the element has to be operated below a certain fixed temperature to avoid overheating the element, greatly limiting the heat flux.
• The heater assembly of the invention very efficiently extracts heat from the heating element and is capable of operating close to molten metal, e.g., aluminum temperature. The heater assembly is capable of operating at watt densities of 10 to 350 watts/in². The low coefficient of expansion of the composite sheath, which is lower than the heating element, provides for intimate contact of the heating element with the composite sheath.
• In another feature of the invention, a thermocouple (not shown) may be inserted between sleeve 12 and heating element 14. The thermocouple may be used for purposes of control of the heating element to ensure against overheating of the element in the event that heat is not transferred away sufficiently fast from the heating assembly. Further, the thermocouple can be used for sensing the temperature of the molten metal. That is, sleeve 12 may extend below or beyond the end of the heating element to provide a space and the sensing tip of the thermocouple can be located in the space.
• Packed particulates (i.e., MgO) are commonly used as a heat transfer medium within an electric resistance heater. MgO is selected in part because of its relatively high thermal conductivity, i.e., about 8 BTU/ft-hr° F. at 1000° F. This value applies to MgO as a homologous material. In a dense pack particulate form, however, the thermal conductivity of MgO decreases by an order of magnitude to approximately 0.5 BTU/ft-hr° F. Heaters incorporating MgO as a heat transfer medium are therefore limited to relatively low heat flux unless high internal temperature gradients can be tolerated.
• Heat transfer in a packed bed occurs by a combination of conduction and radiation. Conduction is the governing mechanism for intra-particle heat transfer, and this is influenced by the thermal conductivity of the particulate material.
• Importantly, however, inter-particle heat transfer occurs predominantly by radiation, which limits the maximum effective thermal conductivity of a packed bed at temperatures under 2000° F.
• The limitations of interparticle heat transfer are illustrated in the data below wherein substantial increases in intraparticle conductivity do not result in significant increases in overall bulk heat transfer.

• Master Summary - 2″ Kl Heater Keff Data - Corrected

  Keff

  ID

  OD

  Gap

  EHL

  (BTU/hr-

  Material ID

  (in)

  (in)

  (ft)

  Volts

  VRMS

  Amps

  Power

  T1

  T2

  T3

  T4

  DT

  (in)

  Rw

  Ft-° F.)

  Graphite	0.75	1.38	0.026	40.6	68.9	2.66	183.3	815	827	682	677	141.5	7.5	10.4	0.69
  Cement
  Sodium	0.75	1.38	0.026	40.5	68.7	2.64	181.5	847	860	716	716	137.5	7.5	10.3	0.70
  Silicate/SiC
  Aremco	0.75	1.38	0.026	41.1	69.8	2.69	187.9	847	847	731	742	110.5	7.5	10.6	0.90
  Al2O3
  Aremco SiC	0.75	1.38	0.026	41.6	70.7	2.79	197.4	857	857	750	761	101.5	7.5	11.2	1.03
  SiC Mixes	0.75	1.38	0.026	41.8	71.1	2.79	198.4	1051	1066	765	760	296	7.5	11.2	0.36
  Cu Powder	0.75	1.38	0.026	42.1	71.6	2.777	198.5	839	854	760	BAD	79	7.5	11.2	1.33
  Carbon	0.75	1.38	0.026	41.3	70.2	2.75	193.0	917	896	690	686	218.5	7.5	10.9	0.47
  Powder
  Cast 954	0.875	1.38	0.021	89.9	158.4	1.96	310.5	889	892	845	832	52	5.5	20.5	3.23
  Cast 954 Rep	0.875	1.38	0.021	91.5	161.3	2.02	325.8	890	894	845	831	54	5.5	21.6	3.26

• Regardless of particle composition, radiation inter-particle heat transfer limits close packed beds of particles to an effective thermal conductivity of less than 1 BTU/ft-hr—at temperatures under 2000° F.
• In situations where dielectric properties are unimportant, copper-containing materials may be used as a heat transfer medium. The alloy must have high thermal conductivity and resist oxidation at elevated temperatures. Aluminum bronze and copper-chromium alloys are excellent candidates for this service. Such alloys can be used either as machined components or cast directly into the internal spaces of a heater.
• In the present heater design, the internal heat transfer medium will operate in the vicinity of 1800° F. internal (or core) temperature. The table below depicts the solidus temperatures of a range of copper alloys, indicating that only a 100° F.-200° F. temperature difference exists between the service temperature and solidus. Copper alloys operated within this range of temperatures will exhibit softness and flow by creep deformation due to gravity. Such flow will result in an intimacy with the internal components of a heater and substantially reduce interfacial heat transfer resistance. Machined components, used in the construction of a heater, will therefore creep deform at service temperature and flow to occupy interstitial spaces. The intimacy that results can resemble a casting, without the difficulties of feeding and gas expulsion. The proper clearance to avoid hoop stress development in the envelope within the heated region during heat-up must be used. Further, alloy creep will result in the loss of this clearance during subsequent heat and cooling cycles. The insertion of thin walled “crush tubes” can be used to accommodate internal stress development during heating.
• Further, the service temperature is sub-solidus and therefore provides higher thermal conductivity than would be otherwise obtained with a liquid. A solid metal is far less reactive with other metals in the heater. Reactivity is an important consideration because most molten metals are reactive with the atmosphere and will solubilize other metals that are present.
• This improvement consists of a solid metallic internal heat transfer medium that has high thermal conductivity and resistance to oxidation and scaling at service temperature. Such service temperature is 100° F.-500° F. below the solidus of the metal. Preferably, it is capable of flowing to occupy available interstitial space within the heater during operation.
• Such a metal is substantially un-reactive with other materials used within the heater. Copper alloys with aluminum and chromium that are capable of forming stable coherent and protective oxides at service temperature are excellent candidates for heat transfer media. Strength is not a consideration for this application.
• Internal interfaces also inhibit heat transfer. The effective thermal conductivity of a solid-solid planar intimate interface has been cited in the literature is approximately 102 BTU/hr-ft° F. Establishing a chemical bond between the heat transfer surfaces can eliminate such resistances. In the case of a steel sheathed heat producing element in a copper alloy heat transfer medium, the sheath of the heater can be aluminized to a thickness of 3-5 mils, inserted into the copper alloy, and heated to a temperature sufficient to melt the aluminum (approximately 1220° F.). The aluminum will alloy with the copper and form a contiguous interface.

• Heater Heat Transfer Alloy Candidates

  		K, BTU/
  Alloy	Liq/Sol, ° F.	ft-hr-° F.	A, ×10−6 in/in° F.

  91 Cu—9 Al	1908/1890	35
  95 Cu—5 Al	1940/1920	48
  97.7 Cu—1.5 Si	1940/1890	33	9.9
  96 Cu—3 Si	1880/1780	21	10.0
  88 Cu—9 Al—3 Fe (9A)		34
  89 Cu—10 Al—1 Fe (9B)		36
  85 Cu—11 Al—4 Fe (9C)		41
  91 Cu—7 Al—2 Fe	1940/1910	44	9.0
  91 Cu—7 Al—2 Si	1840/1800	26	10.0
  97.9 Cu—1.9 Be—0.2 Ni	1587/1750	34-68	9.3
  Cu	198111949	226
  30 Cu—67 Ni	2460/2370	15

• A heater in accordance with the invention is illustrated in FIG. 2. Heater 40 comprises a tube 42. In the embodiment shown in FIG. 2, tube 42 is comprised of a metal or metalloid layer 46 and a molten metal protective layer 48. The molten metal protective layer is only necessary when the heater is used for heating molten metal such as molten aluminum, which would attack the metal layer 46.
• Referring further to FIG. 2, there is shown a cross-section of four heating elements 70 and 72. These heating elements extend substantially the length of the heater. Electrical wires 80 and 82 extend to an electrical power source for energizing the electrical resistance heating element.
• Metal layer 46 can be comprised of any metal. However, when a refractory or protective layer is applied, it is preferred to use a metal or metalloid having a low coefficient of expansion such as referred to herein. Also, molten metal protective layer or refractory 48 may be the same as referred to herein. Further, protective layer 48 may be applied as described herein.
• In the embodiment shown in FIG. 2, an end cap 50 is used to protect the end of the heater tube. End cap 50 may be comprised of a refractory or carbon material.
• The heater of the invention illustrated in FIG. 2 employs heat conduction material comprised of a copper base or copper-containing material, as noted herein. FIG. 3 is an example of body 60 of heat conduction material for use with a cylindrical-shaped heater. It will be noted that body or member 60 has an outer circle 62 and an inner circle 64 defining a circular wall 66 having heating element cavities 68 which in the embodiment shown are circular.
• Also, shown in FIG. 3 are holes 84 and 86 used for thermocouple probes (not shown) that may be used to regulate the temperature of the heaters.
• Heater elements 70 and 72 that can be used in heater assembly 40 are any heater element that produces sufficient heat. Typically, such heating elements have a metal shell, which is not reactive with body 60. For example, such heaters may have an Inconel® metal shell or stainless steel shell or shells of similar materials.
• FIG. 4 is a cross section along the line A-A of the heater assembly of FIG. 2, showing heaters in receptacles 68 in body 60 contained in metal shell 46 that has a refractory coat 48. As noted earlier, pockets of air within the heater assembly are pockets of resistance to heat transfer, and therefore, such pockets should be minimized. Thus, it has been found advantageous to use a thin coating of aluminum between the outer surface 63 of body 60 and the inside of protective tube 42 to aid eliminating pockets of resistance. At temperature of about 1220° F., the aluminum will melt flowing into voids to provide a continuous path for heat conduction from the heating elements. In addition to aluminum, any low melting substantially nonreactive metal can be used.
• With reference to FIG. 5, there is shown another embodiment of the heater of the invention. Tubular resistance heaters produce heat 360° F. around the envelope. However, there is often a need to direct heat transfer in a specific direction. In FIG. 5, there is shown six heat producing elements spaced on 40° radials to provide a preferred heat distribution over a 240° arc.
• The benefit of such geometry is that heat flux can be concentrated in areas of greatest heat transfer. When an array of direct immersion cylindrical heaters is immersed in a flowing stream of aluminum, for example, for the purpose of heating the stream, the local heat transfer coefficient varies as a function of circumferential position relative to the approach direction of the flowing stream.
• Heat transfer occurs at a greater rate on the approach side of the heater rather than on the trailing surfaces. Thus, this design provides greater heat flux on the approach side to exploit improved heat transfer.
• This method is useful also in heating molten metal flowing in a trough where it is desired to direct the heat towards the molten metal and away from the outside walls of the trough. This embodiment of the invention is illustrated in FIG. 5 where molten metal is shown flowing towards the heater assembly. Heating elements 71 are shown arranged to transfer heat in the direction of the advancing metal for most efficient heat transfer.
• It will be understood that the heaters may be used without the refractory coatings, and such is contemplated within the purview of the invention.
• Because the refractory coatings on the heater assembly are important, it is necessary to ensure that the coatings are free of cracks and other like flaws which would permit molten metal or metal vapor to reach metal layer 46. Thus, a method to nondestructively evaluate a heater envelope refractory coating for defects is required for heater envelope use. As noted, such defects include cracks and interconnected porosity that extends from the top or refractory coat surface through to the top coat and/or bond coat interface or beyond. Thus, there is a great need for a method to evaluate the refractory coatings. A first method that has been found to be satisfactory is a potentiostatic method. That method involves an electrical discharge between an electrode and a metal refractory coated envelope within a reduced pressure environment in the presence of an ionization gas. The metallic substrate of the envelope is electrically conductive, while the refractory topcoat, e.g., yttria stabilized zirconia, is not, except for surface charging. Such an envelope is placed in a chamber, whereby the refractory coated exterior surface of the tube or envelope projects from a surface of the chamber wall that is electrically insulating. The chamber, which has been evacuated, is backfilled with an ionization gas. An electric potential is applied between the metallic substrate or interior of the envelope and an electrode placed within the chamber. In the absence of a coating defect extending through to the conductive metallic substrate of the envelope, surface charging will result in a corona forming that is substantially uniform around the refractory coated surface of the envelope. If a crack or porous network allows the ionization gas to contact the conductive metallic substrate, however, local ionization will occur due to charge concentration and high current density. This will be visible as a bright spot. Only defects extending to the conductive substrate of the envelope, or to an area of coating so thin that the local dielectric properties are breached, will behave in this manner.
• The purpose of evacuating the chamber prior to the introduction of the ionizing gas is to evacuate any defects in the coating and permit ionization gas to enter. At low absolute pressure, Knudsen diffusion will control diffusion of the ionizing gas.
• Typical operating parameters are: ionizing gas—neon, potential—1000 to 5500 VAC, initial vacuum—5 mm Hg, ionizing gas backfill and operating pressure—45 mm Hg.
• In a second method of evaluation of the refractory coat, an aqueous conduction method subjects the envelope to a low (<25 V) potential in a conducting liquid. Such a liquid can consist of water and potassium chloride, or water and other ionic compound solutes with a high ionization potential.
• The envelope to be evaluated is placed in the conducting liquid, with or without a surfactant and vibration, e.g., ultrasonic vibration, is applied to promote liquid intrusion into small defects. A potential is established between the envelope and a second electrode. If a defect exists, and the conducting liquid intrudes it, current will flow. Quantification of the current flow at a particular potential can yield information regarding the size of the defect.
• The second electrode is preferably an inert material, for example, carbon or platinum, and alternating current is preferred to direct current. A defect consisting of a single crack will produce a current flow of approximately 80 mill amperes at a potential of 6 volts.
• Failure of the refractory coating material will occur when a discontinuity exists in the top coating that permits aluminum, for example, to contact and chemically react with elements within the bond coating and/or substrate material. Such reaction produces a volume change that ultimately leads to delamination and exfoliation of the top refractory coating. A point defect arises in situations where a localized reaction occurs without delamination, either of which comprises the coating to the extent that failure results.
• Interconnected porosity or as sprayed cracks (discontinuities) constitutes a diffusion path for aluminum. Unless discontinuities are on the order of several tens of mils in width, capillary counter-pressure prevents liquid aluminum from intruding such a discontinuity. Washburn Equation gives the magnitude of this counter pressure:
• 
  P(r)=−2σ cos θ/rg,
• where:
• • P=capillary intrusion pressure
  • σ=surface tension of fluid
  • θ=contact angle fluid/solid
  • r=capillary radius
• For example, in the case of a discontinuity in a yttria stabilized zirconium coating submerged in aluminum at an immersion depth of 12 inches. The metallostatic pressure exerted by the melt is capable of intruding a crack with an effective diameter on the order of 135μ (0.0053 in.) or greater. Most cracks have been measured to be much smaller than 0.005 in. Since the crack or pore is “blind”, the added complication of air displacement makes intrusion by molten aluminum even less likely.
• Alternatively, aluminum vapor is capable of both ordinary and Knudsen diffusion in small discontinuities. The capillary counter-pressure intrusion criterion does not apply. If a chemical sink reaction exists for diffusing aluminum vapor species, transport of aluminum is maintained and a failure results. Such a reaction can occur between bond coat species and/or the substrate to form the respective aluminides.
• In-service cracks may form due to thermally induced mechanical stress resulting from non-uniform heating and differential thermal expansion. Thus, there is a great need for a solution to this problem. It has been discovered that an as-sprayed tube can be thermally cycled to intentionally induce cracking Such cracking results in a relaxation of stress. At some point, insufficient stress exists for the nucleation or growth of cracks, and repeated thermal cycling fails to contribute to additional cracking. This stress level will be the crack saturation/propagation inhibition parameter.
• Tubes can be thermally cycled to induce cracks. In a sufficiently oxidizing environment, a protective oxide can form that prevents aluminum vapor diffusion. Alternatively, a chemically stable compound can be made to form in the crack that accomplishes the same diffusion arrest effect, which is referred to as the crack/fill mechanism. This may be accomplished by intentionally forming cracks in the refractory coating.
• Cracks may be formed by cyclic heating and cooling of a refractory coated tube from within to lower stress. The temperatures may range between 500 to 2300° F. The cracks then may be filled by the use of gas phase oxidizing environment to oxidize the bond coating at the base of the crack. This may be accomplished by use of steam or N₂O. Electrochemical oxidation of the bond coat at the base of the crack can be used to fill cracks. Solid oxidants are SiO₂, for example, carbon based material (hydrocarbon intrusion), siloxane, sputter coating (Mg,C) or ALD (Argonne National Lab Atomic Layer Deposition process) may be used. In yet another embodiment, cracks are allowed to form from intentional pre-service thermal cycling, followed by one of the following post crack treatments:
• a. oxidation of the bond coat using high temperature air or oxidizer;
• b. electrochemical or chemical oxidation of the bond coat;
• c. mechanical intrusion of sufficiently small particles, i.e., boron nitride;
• d. intrusion of magnesium vapor, followed by oxidation to MgO;
• e. intrusion of carbon into pores (may react in-situ to form Al₄C₃);
• f. use of atomic layer deposition to intrude metal oxides;
• g. use of sputter coating to intrude metals or carbon;
• h. incorporation of “reducible oxide” into pores/cracks to form in-situ Al₂O₃.
• As noted herein, yttria stabilized zirconia (YSZ) is used as a topcoat material because it has a coefficient of expansion value that is numerically compatible with the titanium substrate. This property of YSZ maintains low top coat/substrate shear stress during heating, and is partially responsible for satisfactory topcoat adhesion to the substrate. In the case of an immersion heater for use in molten aluminum, the topcoat must also be essentially unreactive with aluminum.
• In situations where a YSZ-Ti heater envelope is exposed to a substantially quiescent aluminum melt, it was found that stability of the YSZ topcoat was satisfactory. When the heater envelope was presented with a flowing stream of molten aluminum, however, YSZ degradation was found to occur on certain areas of the envelope surface. It was further discovered that heater envelopes in contact with quiescent aluminum developed a thin layer of alumina powder on the surface, which is not present in cases involving envelopes exposed to a flowing stream.
• The following chemical reactions apply to a situation where YSZ is in contact with molten aluminum at 1000° K (1340° F.):
• 
  3/2ZrO₂→ 3/2Zr+ 3/2O₂ΔG°=+323 kcal
• 
  2Al+ 3/2O₂→Al₂O₃ΔG°=−322 kcal
• The net reaction is:
• 
  2Al+ 3/2ZrO₂→ 3/2Zr+Al₂O₃ΔG°=+1 kcal
• The corresponding equilibrium constant (k) for the net reaction is:
• 
  k=(h ^3/2Zr^aAl₂O₃)/(a ²Ala ^2/3ZrO₂)=exp [−1/(1.99)(1000)]=0.999
• where: a=Raoltian activities and h=Henrian activities.
• Due to the near equilibrium conditions exhibited by the alumina/zirconia reaction, the removal of the alumina reaction product would have the effect of driving the zirconia reduction reaction forward, favoring the continued decomposition of zirconia.
• This was in fact occurring with the flowing metal stream. In quiescent or near quiescent situations, it is believed that integrity of the alumina layer was maintained and the reduction reaction was essentially terminated. Alumina is essentially passivating or inhibiting the reduction reaction.
• YSZ was selected as the topcoat material of choice on the basis of thermal expansion compatibility with the substrate titanium alloy and thermal shock resistance. Alumina's thermal expansion coefficient is only approximately 50-75% of YSZ. Further, it is well known that the thermal shock resistance of alumina is quite poor. Although alumina has excellent chemical stability in contact with aluminum, thermal expansion and thermal shock considerations disqualifies it as an acceptable topcoat candidate. A layer of alumina, sufficiently thick to provide mechanical robustness, would not be adherent to the Ti substrate, particularly when subjected to cyclic heating/cooling.
• Thin layers of otherwise brittle material coatings can withstand the elastic and plastic deformation of more flexible substrata. Paint on a metal surface is an example. In the particular situation of a heater envelope, an intentionally applied coherent and adherent layer of sufficiently thin alumina could protect the YSZ from reaction with molten aluminum. Since the wettability of alumina by aluminum is very poor (contact angle>110°), chemical reaction is further inhibited due to poor interfacial contact.
• It was found that the air plasma spray (APS) application of an alumina vainer on the surface of a YSZ topcoat was sufficient to protect the heater envelope at high melt velocities. The thickness used was 0.001″-0.0015″. However, the thickness can range from 0.0003″ to 0.006″. Such envelopes do not exhibit evidence of chemical reaction with a flowing aluminum stream.
• Thus, it has been found that the use of an unreactive and thin vainer applied to a YSZ topcoat protects the topcoat from chemical reaction with an aluminum melt. It has been discovered that a chemically stable thin layer (vainer) of material can protect an underlying and thicker layer of material where the ticker material has thermal expansion and elastic modulus compatibility with the substrate and is used to protect the substrate. Thermal expansion compatibility is not a primary requirement because the vainer is thin and therefore compliant to deformation. The essential requirements for this vainer are: thermodynamically stability in contact with aluminum, ability to be applied and adhere to the main layer of coating (YSZ), and reasonable mechanical robustness.
• Preferably the vainer material is not wetted by the aluminum melt. In this invention, the alumina vainer can be 0.001″ to 0.0015″ in thickness, and the main coating layer (YSZ) can be 0.008″ to 0.010″ in thickness. A 10:1 main coat to vainer thickness ratio is reasonable. Alumina or yttria may be used; however, the cost of yttria is very high. Magnesium oxide, magnesium aluminate, magnesium zirconate, equilibrium fired mullite, and various combinations of these oxides with and without yttria, can also be used.
• U.S. Pat. No. 5,963,580 is incorporated herein in its entirely, as if specifically set forth.
• As noted, in molten aluminum systems, thermally sprayed (plasma deposited) refractory coatings protect metallic substrata from reaction with molten aluminum. In many cases, the substrate is titanium or an alloy of titanium. Aluminum and titanium are reactive and form titanium aluminide compounds of various stoichiometric ratios. If therefore, aluminum and titanium are placed in contact with each other, aluminide formation reactions occur that destroy the protective integrity of the plasma deposited coating. Such reactions result in a volume change that provokes exfoliation of the refractory coating due to a volume change that accompanies the formation of the aluminides. For example, the density of the cubic polymorph of zirconia is 6.27 g/cm³. Titanium aluminide densities range from 4.1 to 4.7 g/cm³, representing approximately 30% more volume than an equivalent mass of zirconia. Commonly encountered aluminides include: TiAl₃, TiAl, and Ti₃Al.
• Further, various alloys of nickel, chromium, aluminum, cobalt, and yttrium are used a bond coatings, as noted earlier, to improve, among other things, the adhesion of the plasma deposited refractory matrix coating with the substrate. Chemical reactions between aluminum and elements in these bond coatings also produce intermetallic compounds with similar detrimental effects on the integrity of the refractory coating.
• Continuous porosity, cracks, and other discontinuities in the refractory matrix coating itself are frequently responsible for allowing contact between the titanium substrate and surrounding aluminum alloy.
• Such cracks provide a transport path for infiltrating molten aluminum to contact and react with underlying metallic species to form intermetallic compounds.
• The prerequisite for the penetration of the overlay refractory matrix coating and subsequent transport of chemical species to a reactive surface under the coating is surface contact or wetting. Capillary conduction is the transport mechanism in situations involving cracks or interconnected porosity. Capillary conduction becomes operative when the intrusion pressure exceeds the capillary counter-pressure, via:
• 
  P _i >P _c or P _i(r)=−2σ_l-v cos θ/rg
• where
• • P_c=capillary counter-pressure
  • P_i=intrusion pressure
  • σ_l-v=liquid-vapor “surface tension”
  • θ=contact (wetting) angle
  • r=pore radius
  • g=gravity constant
• For a coating submerged in a molten metal situation, the intrusion pressure is directly proportional to immersion depth and the expression becomes:
• 
  h=−2σ_l-v cos θ/rρg,
• where:
• • h=immersion depth
  • σ=molten metal density
• Alternatively, if Pc exceeds Pi as represented by the metallostatic pressure associated with a particular value of h, penetration of a pore of radius, r, will not occur. The capillary counter-pressure will, in essence, “protect” the refractory overlay coating from intrusion. Evaluation of this expression for pure molten aluminum at its melting point and at various contact angles provides values for effective discontinuity size that will remain non-intruded. Two immersion depths are used as shown in the following tabulations:

• 	Im-	Discontinuity			Discontinuity
  Contact	mersion	Size	Contact	Immersion	Size
  Angle	Depth	(effective, μ)	Angle	Depth	(effective, μ)

  95	10	138	95	50	28
  100	10	276	100	50	55
  105	10	412	105	50	82
  110	10	544	110	50	109
  120	10	796	120	50	159
  125	10	914	125	50	183
  130	10	1024	130	50	205
  135	10	1127	135	50	225
  140	10	1221	140	50	244
  145	10	1306	145	50	261

• A graphical representation of these results illustrates the impact of increasing contact angle on the size threshold a discontinuity capable of being intruded by molten aluminum. Two immersion depths are shown. An increase in contact angle from 95° to 145°, for example, increases the size of an intrudable discontinuity by almost a factor of 10. Overlay refractory matrix coatings at a contact angle of 145° and immersed to a depth of 50 cm, would therefore be protected from intrusion by pure molten aluminum.
• Surfaces are spontaneously wetting, by definition, below contact angle values of 90°. The maximum allowable discontinuity size to avoid metal intrusion therefore approaches zero as the contact angle approaches 90°.
• It has been discovered that several surface-active additives can be used to protect refractories from molten aluminum. Such refractories include zirconia, yittria stabilized zirconia, calcia stabilized zinconia, magnesia stabilized zirconia, magnesia, alumina, magnesium aluminate, spinel, and combinations thereof. Various additives include BaSO₄, CaF₂, AlF₃, and combinations thereof. Additionally, BN, B₂O₃, and CaSiO₃ have demonstrated similar non-wetting effects in molten aluminum.
• Thermally sprayed coatings have been used to protect metallic substrata from chemical reaction with or attack by molten metals, such as aluminum, as disclosed herein. These coatings must have an appropriate thermal expansion coefficient, modulus of elasticity, and chemical stability in the molten metal system intended for use. Molten aluminum, and alloys thereof, are particularly aggressive and require that coatings intended for service in such alloys are chemically stable. Alternative methods of depositing a refractory matrix coating can be used, such as flame spraying, high velocity oxyfuel, and electron beam deposition.
• Normalized reaction free energy values at 1000° K. (1350° F.) for the reduction of various oxides by aluminum having desirable mechanical properties for coatings applications at are tabulated below. Values for the equilibrium constant of the reduction reactions are also shown.

• 		Normalized Free
  		Energy, Kcal/mole	Equilibrium
  	Oxide	(T = 1000° K)	Constant

  	ZrO2	1.0	6.10 × 10−1
  	MgAl2O4	13.3	1.67 × 10−6
  	MgO	29.6	1.40 × 10−13

• Zirconia, (ZrO₂) has the most desirable combination of mechanical properties for the protection of a titanium substrate exposed to molten aluminum. It also has the lowest chemical stability in this system, making it most susceptible to reaction with aluminum. Magnesium aluminate spinel (MgAl₂O₄) is more energetically favorable (stable) and magnesia (MgO) is predicted to be the least capable of reduction by aluminum, based on energetic considerations under equilibrium conditions.
• Under stagnant conditions, a small quantity of reaction products (Al₂O₃ in this case) will be produced in accordance with the value of the equilibrium constant. If the system remains undisturbed, the reaction products will inhibit further reaction and the global reaction rate essentially approaches zero.
• A flow of metal, such as aluminum, frequently occurs around the object being protected by the coating. Shear induced by the flow has the capability of transporting reaction products away from the coating surface, thus renewing new surface for reaction. This process continues until the protective coating is completely reduced by the flowing metal stream.
• It is therefore desirable to limit this reaction through kinetic considerations. A non-wetted surface essentially accomplishes this objective by discouraging inter-phase contact between the coating and surrounding molten aluminum.
• A second benefit to high contact angle is the avoidance of aluminum intrusion into pre-existing pores or cracks. Such intrusion can result in a chemical reaction between aluminum and the substrate being protected, with consequential failure of the coating.
• Further, if a high contact angle condition is maintained throughout the volume of the coating, cracks developing in-service will also resist intrusion by aluminum.
• While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass other embodiments, which fall within the spirit of the invention.
• Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims (9)

What is claimed is:

1. A method of improving the resistance to attack of a metal substrate coated with a ceramic material when submerged in a molten metal, the method comprising the steps of:

(a) providing a metal substrate;

(b) providing a ceramic powder capable of being thermally applied to said substrate;

(c) mixing a molten metal non-wetting agent with said powder to provide a mixture; and

(d) thermally applying said mixture to said substrate to provide a ceramic coated substrate having a contact angle greater than 90° when immersed in the molten metal to improve said resistance of said substrate by said molten metal.

2. The method in accordance with claim 1 wherein said metal substrate is titanium or alloys of titanium.

3. The method in accordance with claim 1 wherein said molten metal is molten aluminum or alloys of molten aluminum.

4. The method in accordance with claim 1 including plasma spraying said mixture.

5. The method in accordance with claim 1 wherein said ceramic powder is selected from the group consisting of zirconia, yittria stabilized zirconia, calcia stabilized zinconia, magnesia stabilized zirconia, magnesia, alumina, magnesium aluminate, spinel, and combinations thereof.

6. The method in accordance with claim 1 wherein said metal substrate is a metal selected from the group consisting of Ti, Kovar, and nickel base alloys.

7. The method in accordance with claim 1 wherein the molten metal is selected from the group consisting of aluminum, copper, lead, magnesium, zinc and alloys of these metals.

8. The method in accordance with claim 1 wherein said molten metal non-wetting agent is selected from the group consisting of barium sulfate, barium oxide, boron nitride, boron oxide, aluminum fluoride, calcium fluoride, sodium fluoride, magnesium fluoride and calcium silicate.

9. A method of improving the resistance to attack of a titanium metal substrate coated with a ceramic material when submerged in a molten metal, the method comprising the steps of:

(a) providing a titanium metal substrate;

(b) providing a zirconia powder capable of being thermally applied to said substrate;

(c) mixing a molten metal non-wetting agent with said powder to provide a mixture, the non-wetting agent selected from the group consisting of barium sulfate, barium oxide, boron nitride, boron oxide, aluminum fluoride, calcium fluoride, sodium fluoride, magnesium fluoride and calcium silicate; and

(d) thermally applying said mixture to said titanium substrate to provide a zirconia, magnesium aluminate, spinel, or MgO coated substrate having a contact angle greater than 90° when immersed in the molten metal to improve said resistance of said titanium substrate by said molten metal.

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