Abstract:
A slurry for joining metallic or ceramic surfaces or for coating metallic or ceramic and refractory surfaces, comprising a liquid suspending medium and at least two constituents in particulate form suspended in the medium, the constituents being so selected and proportioned as to undergo combustion synthesis when ignited. A method for coating or joining metallic or ceramic surfaces comprises applying such a slurry to a surface to be coated or to surfaces to be joined, and igniting the slurry to cause combustion synthesis.

Description:
This is a divisional of application Ser. No. 07/787,091, filed Nov. 4, 1991, which is a continuation-in-part of application Ser. No. 07/753,216 filed Aug. 30, 1991, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The present invention relates to a slurry which undergoes exothermic reaction by combustion synthesis when ignited, and methods for the use thereof in joining metallic or ceramic surfaces or for coating metallic, ceramic and refractory surfaces. Although not so limited, slurries of the invention have particular utility in joining stainless steel, aluminum, carbon steel and copper workpieces and in forming coatings on porous silica or aluminum oxide substrates which resist chemical attack, ingress and attack by molten alloys having high melting temperatures. 
     2. Prior Art 
     Joints between metal pieces are conventionally formed by welding or brazing, by means of a wide variety of techniques using an oxyacetylene torch, or the like. Brazing compositions for joining metal pieces by fusion are also well known. To the best of applicant&#39;s knowledge there have been no suggestions in the prior art to use a slurry for joining metallic pieces, the constituents of which are so selected and proportioned as to undergo exothermic reaction by combustion synthesis when ignited, thereby welding the metal pieces together. 
     Ceramic materials such as silica, alumina, zirconia and the like are used for containing molten metals, in furnaces for melting metals, in armor for missiles and the like, as infrared sensors, in electronic packaging, in engine materials, and in related applications. Such ceramic materials are frequently porous because they are made from powders which are sintered. Although porosity may be beneficial in imparting low density and specific toughness, porosity is detrimental from the standpoint of chemical attack, ingress into the ceramic or refractory material and resistance against attack by molten alloys at high temperature. Expensive fabrication is necessary to obtain a ceramic material of the above type in a fully densified condition. The prior art has resorted to coating ceramic surfaces in order to eliminate ingress or diffusion of material such as molten metal into the ceramic. No generally suitable technique is known to applicant to solve the problems inherent in coating ceramic or refractory surfaces. 
     Despite the existence of a wide variety of welding and brazing techniques and compositions, and despite the prior art suggestions for coating porous ceramic or refractory surfaces, a genuine need exists for alternative compositions and methods for joining metallic or ceramic surfaces or for coating metallic, ceramic and refractory surfaces. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a slurry for joining metallic or ceramic surfaces or for coating metallic, ceramic and refractory surfaces, by the use of constituents which undergo exothermic reaction by combustion synthesis. 
     It is a further object of the invention to provide methods for joining metal or ceramic surfaces and for coating metallic, ceramic and refractory surfaces by means of slurries which undergo exothermic reaction by combustion synthesis. 
     According to the invention there is provided a slurry for joining metallic or ceramic surfaces or for coating metallic or ceramic and refractory surfaces, the slurry comprising a liquid suspending medium and at least two constituents suspended in particulate form in the medium and selected from the group consisting of titanium dioxide, boron oxide, aluminum oxide, aluminum, silicon, titanium, boron, nickel, phosphorus, chromic oxide, carbon, niobium, zirconium, tantalum, molybdenum, hafnium, and vanadium, the constituents being so selected and proportioned as to undergo exothermic reaction by combustion synthesis when ignited. 
     Liquid suspending mediums are monoaluminum phosphate, colloidal silica, methyl cellulose, alcohols, acetone, water, sodium silicate, and combinations of common acids and bases. 
     Optionally, the slurry may also contain diluents or reinforcing agents, which do not undergo combustion synthesis, in particulate form, including but not limited to metals such as aluminum, iron, copper and silver, and non-metallics such as borides, carbides, nitrides, oxides and silicides of titanium, niobium, and chromium. 
     A slurry for coating metallic or ceramic and refractory surfaces, in accordance with the invention, comprises a liquid suspending medium and particulate constituents suspended in the medium capable of undergoing exothermic reaction by combustion synthesis when ignited, the constituents being selected from the group consisting of titanium and crystalline carbon; titanium and amorphous carbon; titanium and aluminum; aluminum, titanium dioxide, and boron oxide; aluminum oxide, titanium, and amorphous carbon; and mixtures thereof. 
     A slurry for joining metallic or ceramic surfaces, in accordance with the invention, comprises a liquid suspending medium and particulate constituents suspended in the medium capable of undergoing exothermic reaction by combustion synthesis when ignited, the constituents being selected from the group consisting of titanium dioxide, boron oxide, aluminum, titanium, boron, nickel and phosphorus; titanium and boron; titanium, boron, nickel and phosphorus; chromic oxide, carbon and aluminum; and mixtures thereof. 
     A method of coating metallic or ceramic and refractory surfaces in accordance with the invention comprises the steps of providing a slurry having a liquid suspending medium and particulate constituents suspended in the medium capable of undergoing exothermic reaction by combustion synthesis when ignited, the constituents being selected from the group consisting of titanium and crystalline carbon; titanium and amorphous carbon; titanium and aluminum; aluminum, titanium dioxide and boron oxide; aluminum oxide, titanium, and amorphous carbon; and mixtures thereof; applying the slurry to a metallic, ceramic or refractory surface so as to form a layer of uniform thickness thereon; and igniting the layer to cause combustion synthesis thereof; thereby forming an adherent coating of titanium carbide; titanium aluminide; a titanium boride and aluminum oxide composite; or an aluminum oxide, titanium dioxide and titanium carbide composite. 
     The invention further provides a method of joining metallic or ceramic surfaces, which comprises the steps of providing a slurry having a liquid suspending medium and particulate constituents suspended in the medium capable of undergoing exothermic reaction by combustion synthesis when ignited, the constituents being selected from the group consisting of titanium dioxide, boron oxide, aluminum, titanium, boron, nickel and phosphorus; titanium and boron; titanium, boron, nickel and phosphorus; chromic oxide, carbon and aluminum; and mixtures thereof; applying the slurry to metallic or ceramic surfaces to be joined, placing the metallic or ceramic surfaces in abutting relation with the applied slurry therebetween; and igniting the slurry to cause combustion synthesis thereof at a temperature sufficient to fuse the metallic or ceramic surfaces together. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment of the invention adapted for coating ceramic and refractory surfaces, preferred particulate constituents comprise, in weight percent based on the total weight of constituents, the following groups: 
     from 75% to 85% titanium, and 15% to 25% crystalline carbon; 
     from 75% to 85% titanium, and 15% to 25% amorphous carbon; 
     from 60% to 68% titanium, and 32% to 40% aluminum; 
     from 35% to 41% aluminum, 30% to 36% titanium dioxide, and 27% to 31% boron oxide; 
     from 50% to 97.5% aluminum oxide, 2% to 25% titanium, and 0.5% to 25% amorphous carbon. 
     A preferred liquid suspending medium for the slurry is monoaluminum phosphate and/or colloidal silica. Both these liquids are reactive with the reactants, which is desirable since they become an integral part of the final coating. 
     Water and acetone were also tested as liquid suspending mediums and found to be unsatisfactory since the low evaporation temperatures thereof resulted in cracks and distortion in the coatings during drying. 
     In the embodiment of a slurry adapted for joining metallic surfaces, preferred particulate constituents comprise, in weight percent based on the total weight of the constituents, the following groups: 
     from 28% to 32% titanium dioxide, 25% to 27% boron oxide, 32% to 35% aluminum, 3% to 4% titanium, 1.5% to 2.0% boron, 3.8% to 4.5% nickel, and 0.7% to 1.1% phosphorous; 
     from 60% to 70% titanium, and 30% to 40% boron; 
     from 30% to 35% titanium, 15% to 20% boron, 38% to 44% nickel, and 8% to 10% phosphorus; 
     from 12% to 20% titanium dioxide, 10% to 16% boron oxide, 15% to 20% aluminum, 0 to 10% iron, 15% to 17.5% titanium, 8% to 9.5% boron, 19% to 22% nickel, and 4% to 5% phosphorus; 
     from 67% to 70% chromic oxide, 6.5% to 8% carbon, and 23% to 26% aluminum. 
     All constituents should have an average particle size of less than 53 microns (i.e., passing 270 mesh screen). Materials are generally available having average particle sizes of -300 or -325 mesh screen. 
     The amount of liquid suspending medium is not critical and is selected to provide a viscosity suitable to permit application by painting, spraying or dipping, while at the same time avoiding excessive run-off. When coating metallic or ceramic or refractory surfaces, it is preferred to form a layer of slurry having a thickness of about 0.2 to 0.3 mm. In general, from about 1 to about 12 parts by weight of the powdered constituents are dispersed uniformly in from about 2 to about 5 parts by volume of the liquid suspending medium, although these limits can be varied depending upon the particular mixture of constituents. 
     Tests have been conducted on slurries for coating ceramic surfaces. For test purposes five examples of powdered constituents were prepared as follows: 
     Example 1 - titanium 80%, crystalline carbon 20% 
     Example 2 - titanium 80%, amorphous carbon 20% 
     Example 3 - titanium 64%, aluminum 36% 
     Example 4 - titanium dioxide 33%, boron oxide 29%, aluminum 38%. 
     Example 5 - aluminum oxide 95-97.5%, titanium 2-4%, amorphous carbon 0.5-1%. 
     The percentages above are by weight. The titanium, carbon, and aluminum powders were obtained from ALFA or from Johnson Matthey Electric and ranged in purity from 99.0% to 99.5%. The crystalline and amorphous carbon powders were -300 mesh particle size, while the titanium and aluminum powders were -325 mesh. Titanium dioxide and boron oxide were obtained from Fisher, grade designations LO-904811 and LO-904641, respectively. Aluminum oxide was obtained from Alcoa Chemicals, grade designation A-17. Slurries were prepared by mixing uniformly 1 part by weight of the powders of the above examples with 2 parts by volume of each of four different liquid suspension media, viz., water, colloidal silica, monoalunimum phosphate and acetone, for Examples 1-4. As indicated above, preliminary tests indicated that water and acetone were unsuitable, and hence were not subjected to further tests. Monoaluminum phosphate (MAP) was found to be the preferred suspension medium. 
     For Example 5, the liquid suspension media were MAP 90% and colloidal silica 10% by volume, and MAP 95% and colloidal silica 5% by volume. The colloidal silica was REMET-SP30. Slurries were prepared by mixing uniformly 1 part by weight of the powder with 1.5 part by volume of suspension medium. 
     Fused silica substrates were coated by painting to form a layer of 0.25 mm (250 microns) thickness with slurries of each of the above examples 1-4 in MAP. After coating, reaction was initiated either by a wave propagation mode or thermal explosion mode of reaction in air. 
     In the wave propagation mode, reaction was started from one part of the sample with a butane flame torch, and the reaction then self-propagated across the sample. Since each of examples 1-4 had a different heat of reaction, it was necessary to heat samples to different initial temperatures to sustain the combustion synthesis. 
     In the thermal explosion mode reaction was started simultaneously in all parts of the sample subsequent to a heating and drying step. Samples of Examples 1-4 were heated from room temperature to 1000° C. and then cooled slowly in a furnace to avoid possible surface cracks in the coatings. The thermal explosion mode in air is the preferred method. 
     Examples 1-4 reacted by combustion synthesis as follows: 
     1. Ti+C (crystalline → TiC 
     2. Ti+C (amorphous) → TiC 
     3. Ti+Al → TiAl 
     4. 10/3 Al+TiO 2  +B 2  O 3  → TiB 2  +5/3 Al 2  O 3   
     The surfaces resulting from reactions 1 and 2 above had lower porosity than those produced by reactions 3 and 4. 
     Fused silica specimens coated with titanium carbide (Examples 1 and 2) were immersed in molten aluminum at 700° C. and kept in the melting furnace for 7 days. For comparison uncoated fused silica specimens were also subjected to the same test. At the end of 7 days, samples were cut along their cross-sections and tested. In the ceramic specimens coated with titanium carbide, diffusion of aluminum into the ceramic body was not observed. In contrast to this, aluminum penetration to a depth of 0.5 mm occurred in the uncoated specimens. 
     The slurry of Example 5 was applied by dipping silica crucibles two or three times at 5 second intervals to obtain a coating thickness of 150-200 microns. After coating, samples were dried for 10 hours. The coated crucibles were then placed in a furnace which reached 1200° C. in 2 hours and held at that temperature for 4 hours. The resulting combustion synthesis reaction (by thermal explosion mode in air) was as follows: 
     5. Al 2  O 3  +2Ti+C+O 2  →Al 2  O 3  +TiO 2  +TiC 
     Molten stainless steel was then poured into the coated silica crucibles. It was found that these crucibles could be reused for several heats of molten stainless steel. By way of comparison, an uncoated silica crucible could be used for only one heat due to extensive surface damage. 
     Numerous tests have been conducted on slurries of the invention for use in joining metallic surfaces. The following test procedures were used: 
     A - Samples in the form of rods (0.54 cm diameter, 5 cm length) were clamped in a Gleeble machine with the combustible slurry between the rod ends, using various joint geometries. (The Gleeble is a standard thermomechanical apparatus which applies both stress and temperature to a sample.) Current was then passed through each sample until the mixture reached ignition temperature and underwent combustion. 
     B - Bars of various sizes were butted together with a slurry in MAP between them. Another combustible slurry was spread around the joint and ignited using an oxyacetylene torch. After combustion the slurry outside the joint was then chipped off. 
     C - Bars of various sizes were butted together, using a butt joint, with a slurry between the abutting ends and ignited by an oxyacetylene torch. 
     D - Mild steel bars of various thicknesses were joined using a silver brazing alloy foil in the joint and a combustible slurry around the outside of the joint. 
     The compositions, in weight percent, of the powdered constituents were as follows: 
     Example 6 - titanium dioxide 30%, boron oxide 26.25%, aluminum 33.75%, titanium 3.25%, boron 1.75%, nickel 4.1%, phosphorus 0.9%; 
     Example 7 - titanium 65%, boron 35%; 
     Example 8 - titanium 32.5%, boron 17.5%, nickel 41%, phosphorus 9%; 
     Example 9 - titanium dioxide 16.67%, boron oxide 14.58%, aluminum 18.75%, titanium 16.25%, boron 8.75%, nickel 20.5%, phosphorus 4.5%; 
     Example 10 - chromic oxide 68.5%, carbon 7.2%, aluminum 24.3%. 
     Slurries were prepared of all the above examples in MAP by mixing 1 gram of the powder with 0.3 cc of MAP. Maximum reaction temperatures were determined in preliminary tests, and it was found that the highest reaction temperature was achieved by Example 6. This composition was therefore used in further tests. 
     Test data on the four procedures by which rods or bars were joined are tabulated below. While results were not uniformly successful with the slurries of the invention, comparison of joints prepared by procedure D with a conventional fusion welded joint using an oxyacetylene torch indicated that joint strength when using an optimum amount of slurry was generally comparable with joint strength obtained by conventional welding. In this connection the joint thickness was found to have a marked effect on joint strengths both for procedure D and conventional fusion welded joints, as shown in Table 5. 
     
                       TABLE 1______________________________________Slurrycomposition      Joint(in MAP)   geometry    Result______________________________________Stainless Steel RodsEx. 8      Lap joint   Did not weld to surface                  of either rod.Ex. 8      Mortise and Rods welded but broke      tenon joint apart when hit at joint.Ex. 8 +    Mortise and Rods welded but broke33% Al-67% Fe      tenon joint apart when hit at joint.Ex. 9 +    Butt joint  Rods welded together.10% Fe                 Combustion visible.Aluminum RodsEx. 8 +    Mortise and Rods welded but broke33% Al-67% Fe      tenon joint apart when hit at joint.Ex. 8      Butt joint  Welded to surface of one                  rod.______________________________________ 
    
     
                       TABLE 2______________________________________PROCEDURE B     Slurry      SlurryBar dimensions     inside      outsidein cms.   joint       joint   Result______________________________________Stainless Steel Bars0.515 × 2 × 1.5     Ex. 9 +     Ex. 8   Bars welded but     Cu (9:1)            broke when bent                         by hand.0.515 × 2 × 1.5     Ex. 9 +     Ex. 8   Bars welded but     Cu (1:4)            broke when bent                         by hand.Aluminum Bars0.515 × 2 × 1.5     Ex. 9 +     Ex-8    Bars welded but     Cu (9:1)            broke when bent                         by hand.0.515 × 2 × 1.5     Ex. 9 +     Ex. 8   Bars welded but     Cu (3:2)            broke when bent                         by hand.Mild Carbon Steel Bars0.336 × 3 × 15     Ex. 6+      Ex. 6   Bars not welded     Ag (1:1)0.31 × 0.9.1.2     Ex. 6 +     Ex. 6   Bars welded     Ag (3:1)            together.0.266 × 1.5 × 2     Ex. 6 +     Ex. 6   Bars not welded     Ag (1:1)0.266 × 1.5 × 2     Ex. 6 +     Ex. 6   Bars welded     Ag (1:1)            together.0.266 × l.5 × 2     14% Ex. 7 + Ex. 6   Bars welded     14% Ex. 10 +        together.     Ni-31.5% Al         X-ray                         examination of                         cut &amp; polished                         section of weld                         indicated                         presence of Ni.Copper Bars0.515 × 2 × 1.5     Ex. 6 +     Ex. 6   Bars not welded     Cu-7.4% P     (1:1)0.515 × 2 × 1.5     Ex. 6 +     Ex. 6   No combustion     Cu-7.4% P           inside joint.     (7:3)0.515 × 2 × 1.5     Cu-7.4% P   Ex. 6   Bars welded                         together.                         Cu-7.4% P                         melted.0.515 × 2 × 1.5     Cu-7.4% P   Ex. 6   Bars welded                         together. More                         uniform                         melting of                         Cu-7.4% P.0.515 × 2 × 1.5     Cu-7.4% P + Ex. 6   No combustion     30% Al              inside joint.______________________________________ 
    
     
                       TABLE 3______________________________________PROCEDURE CBar dimensions         Slurryin cms.       inside joint                    Result______________________________________Copper Bars0.515 × 2 × 1.5         Ex. 7      Combusted but                    did not weld.0.515 × 2 × 1.5         Ex. 6 +    Bars not welded         Cu-7.4% P         (4:6)0.515 × 2 × 1.5         Ex. 6 +    Bars not welded         Cu-7.4% P         (7:3)Mild Carbon Steel Bars0.266 × 1.5 × 2         Ex. 10     Did not combust.0.266 × 1.5 × 2         90% Ex. 10 +                    Did not combust.         10% Ex. 70.266 × 1.5 × 2         Ex. 7      Fully combusted                    but not welded.______________________________________ 
    
     
                       TABLE 4______________________________________PROCEDURE DMild Carbon Steel Bars                           Joint strength                           of welds:Bar dimensions        Inside  Outside    (1) 40 gms slurry(in cms)     Joint   Joint      (2) 20 gms slurry______________________________________0.266 × 2.98 × 15        Ag foil Ex. 6      (1) 68 MPa                           (2) 63 MPa0.291 × 2.98 × 15        Ag foil Ex. 6      (1) 58 MPa                           (2) 38 MPa0.3365 × 2.98 × 15        Ag foil Ex. 6      (1) 52 MPa                           (2) 32 MPa______________________________________ 
    
     
                       TABLE 5______________________________________     Joint strength of welds                          Fusion weldJoint thickness       40 gms     20 gms  oxyacetylene(in cms)    slurry     slurry  torch______________________________________0.266       68         63      800.291       58         38      68 0.3365     52         32      56______________________________________