CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority to copending U.S. Patent Application Se No. {4/444,409, filed on July 28, 2014.

TECHNICAL FIELD

[0001] The present novel technology relates generally to the field of materials science and, mote particularly; to a method for welding ceramic bodies together..

BACKGROUND

[0002] Ceramics are inherently brittle materials. While very strong under compression, ceramic materials are typically weak under tension and torsional stresses. Thus, while ceramic materials generally exhibit high elastic moduli values, they are prone to brittle fracture and themiai shock.

10003] Ceramic materials are typically joi ned together through the application of a cement at the interface- between two bodies. While this technique works well for joining two ceramic materials together, it is less useful for joining a ceramic to another material, such as a structural metal body, that has a substantially different coefficient of thermal expansion. Further, cements are less useful for joining materials thai will experience significant tension or flexure, since cements are also prone to brittle fracture.

[0004] Further, as-tbrmed ceramic bodies are typically limited to simple shapes, both because it is difficult to cast or form ceramic materials directly into complex shapes and it is equally difficult to machine brittle bodies into complex shapes after they are Formed. Attempts have been made to produce ceramic bodies having complex shapes, such as by cementing or otherwise fastening the simple bodies together. Only limited success has been achieved to date using cements, due to their likewise inherent brittleness. Glues likewise do not offer sufficient bond strength to connect ceramics into more complex, shapes. The use of fasteners, such as screws or bolts, is likewise limited because drilling holes through brittle ceramics introduces cracks thai act as stress concentrators, thus giving rise to failure mechanisms in the ceramic bodies. Further, the fasteners themselves become focal points for stress concentration.

[0005] Welding ceramic bodies o themselves or to non-ceramics has thus far met with little success. The welding process typically includes the application of heat to the ceramic, thus introducing mienxraeks through thermal shock. Such ceramic welds have been hard to form, and those that have been formed have had very low bond strength.

[0006] Thus, there remains a need for a method of welding ceramic bodies together and/or to non-ceramic bodies, without experiencing detrimental thermal shock or other damage at and around the weld site. The present invention addresses this need. [0007] The present novel technology relates generally to materials science. One object of the present novel technology is to provide an improved method of jo ning two ceramic bodies.

Related objects and advantages will be apparent from the following description.

10008] FIG. 1 is a diagrammatic view of a ceramic to ceramic welding method accordin to one embodiment of the present novel technology.

[0009] FIG. 2 is a photomicrograph of a welded body including two SiC pieces joined with a fusion weld according to the embodiment of FIG. I ,

[001.0] For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same, ft will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of .he principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates.

[0011] FlGs. 1 -2 illustrate a first, embodiment of the present novel technology, a method for joining electrically conductive ceramics and ceramic composites by arc welding 10. Ceramics are inherently brittle materials that are susceptible to thermal shock during the rapid heating and cooling cycles encountered during fusion welding. The application of properly selected preheat and posiheat treatments enables the joining of conductive ceramics and ceramic composites to themselves as well as to metal structures. The novel joining process enables the joining of components of varied size from hot pressed, PVD, sputtered, CVD, plasma deposited, arc cast, sintered, and the like, ceramics, cermets, and ceramic matrix composites . Ceramic welding enables the production of large, complex compound forms 20 from precursor bodies having the sim le shapes that are common of sintered and hot pressed ceramics . while retaining the strength and toughness inherent in the starting materials. The novel welding process can produce joints that exhibit the same thermophysica and mechanical behavior as the parent material, in addition, arc welded joints are able to withstand the same chemically corrosive, oxidizing atmospheres, and high temperature environments as the materials of the pars.-!)t bodies. 10012] Some potential, uses include joining of thermal protection systems (TPS) to structural components, producing exotic thermocouples, repairing and producing hybrid ballistic armor systems, joining of wear resistant or heat resistant surfaces to load bearing components such as those found in engines (interna! combustion, Stirling, and turbine), joining refractory solar-absorptive ceramic surfaces to structural components for concentrated solar thermal applications, joining of wear resistant components to refractory alloys to produce beatings for high temperature applications (>1000°C), and the like. Ceramic welding enables the production of complex shapes from simple hot pressed and sintered shapes. The precursor bodies are- typically nearly theoretically dense, more typically at least about 98% dense (no more than 2% porosity), still more typically at least 99% dense (no more than 1 % porous}, yet more typically at least 99.5% dense (no more than 0.5% porosity), and still more typically at least about 99.9% dense no more than 0. i % porosity). The ability to weld simple shapes into more complex structures reduces machining costs and decreases the time required to achieve- a finished component, to some cases, ceramic welding is useful for improving mechanical behavior by refining grain sizes and producing therrnodyna ically stable grain boundaries which form from the melt in the joint region. Ceramic welding also enables the repair of ceramic components and composite structures.

[0013] Ceramics generally exhibit high elastic moduli values and are susceptible to brittle fracture and thermal shock. In order to minimize mechanical failure arising from thermal shock of large components during the fusion welding process, the precursors are subjected to a preheating thermal profile and the compound structures so formed are subjected to & post-welding thermal profile, as, in general ceramic materials lack the sufficiently high thermal shock resistance and/or significant ductility below the system's melting temperature to avoid material failure from thermal shock. Alternately, the properties of the precursor piece may be tailored to have very low coefficients of .hernial e ansi n and/or sufficiently high ductility to offer superior thermal shock resistance. The temperature and duration of pre- and post- heating treatments are different for each material In order to predetermine the pre-heat and the post-weld profiles, the minimum temperatures required to plastically relieve stresses are investigated. Each ceramic, ceramic particle composite, ceramic matrix composite, or cermet system is characterized by its ability to relieve stresses mat accumulate during the novel welding process. Processes lending to stress relief at high temperature include microcracking, grain boundary sliding or softening, dislocation motion, twinning, grain growth, .recrystailization, combinations thereof, and the like, The pre- and post-heat treatment profiles, are influenced by the temperatures at which appreciable stress relief occur by the aforementioned mechanisms.

[0014] In general, dislocation motion, twinning, grain growth and reeryst alligation occur at or above a homologous temperature (Tn ~TiJ _m ) of 7)r=O.4-0.5. For materials exhibiting grain boundary softening, microcracking, and grain boundary sliding, the pre- and post-heat treatment temperature will he largely influenced by precursor body composition and material processing before welding. To minimize the variability of the high temperature plasticity found in ceramics, it may be useful to conduct characterization (such as mechanical testing, neutron or x-ray diffraction, or the like) studies of the materials to be welded at high temperature prior to welding to identify the proper pre- and post- heat conditions for the specific component bodies. These studies will be unnecessary if it is possible to conduct welding trials and/or if plastic deformation occurs at temperature slightly above TfptSA- 0.5,

10915] in general, large component bodies are preheated to higher temperatures to prevent warping and cracking. More typically, for larger precursor bodies lower heating and cooling rarop rates are chosen for the preheat and post-weld thermal profiles. Further,, conductive ceramics often are susceptible to oxidation at high temperature, so conductive ceramic precursor bodies are typicall shielded from oxidizing conditions at elevated temperatures in order to preserve the integrity of the component.

10016] FIGs, 1-2 illustrate one embodiment of the present novel technology, a method 100 for joining two (typically compositionaily similar) ceramic surfaces 105, i 10 in a weld or fusion bond 1 15. As ceramics are inherently brittle materials and as such are susceptible to thermal shock damage daring the rapid heating and cooling cycles, the surfaces 105, 1 10 are typically preheated to a first elevated soak temperature 120 and held there for a first soak time 123. The first ramp rate 125 from ambient to the first elevated soak temperature 120 is typically slow enough so as to avoid or minimize thermal shock damage, likewise, a post welding slow ramp down to ambient temperature at second slow ramp rate 130 is typically employed to minimize thermal shock damage to the newly welded piece 1 5 and the newly formed joint 1.15, The application of properly selected preheat and postheat treatments assists in the joining of ceramic surfaces 105, 1 10.

[0017] Once the surfaces 105, 1 10 are at the first soak temperature 120, the surfaces are urged together to define an interlace volume 155 therebetween and additional beat 150, such as from a plasma torch or the like, is applied at the interface volume 155 between the surfaces 105, 1 1 to form an at least partially liquefied fusion volume 160, which is then cooled (typically at a predetermined cooling rate 161 to a predetermined end temperature 163) to define a fusion joint 1 15. More typically, the cooling rate 16.1 is sufficient to anneal the fusion joint 1 15 and adjacent surfaces 105, 1 10 of thermally induced stresses. The additional heat 150 is sufficient to liquefy the fusion volume 160 but is not sufficiently great -and/or of such duration to significantly decompose the ceramic surfaces 105, 1 10. Typically, the additional heat 150 is quickly ramped up at a first welding heat ramp rate 1 5 to a nominal welding level. 170, held at the nominal welding level 170 for a predetermined second -soak time 175, and ramped down at a second welding rate I SO upon disengagement.

[0018] Depending on the composition of the surfaces 105, 1 10, the first soak temperature 120 may be from about 700 degrees Celsius to about 1 100 degrees Celsius and the additional heat 1.50 may be represented by a welding current of between about 25 Amperes and 75 Amperes with a duration 155 of between about 5 seconds and about 20 seconds. In other words, depending on the composition of the surfaces 105, 110, the first soak temperature .120 may be from about 700 degrees Celsius to about 1 S00 degrees Celsius and the additional heat 150 may result in final, near surface temperature of 1400 degrees Celsius to 3500 degrees Celsius with a duration 155 of between about 5 seconds and about 20 seconds to spot welds or short (1-2 cm.) linear welds, and longer for longer linear welds,

[00191 The novel joining process 100 enables the production of large, complex compound bodies S 35 from precursor surfaces J 05, 1 0 having the simple shapes thai are common of sintered and hot pressed ceramics, while retaining the compressive strength, toughness and chemical durability inherent in the starting materials. The novel welding process 1 0 can produce joints 1 15 that exhibit the same thennophysieai and mechanical behavior as the parent .material. In addition, the welded joints 1.15 are able to withstand the same chemically corrosive, oxidizing atmospheres, and high temperature environments as the materials of the parent surfaces 105, 1 10,

1.0020] During the welding process 100, some material decomposition may occur and it may be advantageous to provide a thin volume 187 of filler or additive material 190 at the interface 155 having a composition that may offset or otherwise minimize the thermal decomposition effects. The filler or additive material 190 may have the same composition as one or both surfaces 105. 1 10, the composition of one of the constituents of one or both surfaces 105, 1 10, or a different composition compatible with one orbolh surfaces 105, 100. so as to strengthen the joint 1 15. The thin volume 187 of additive material 191 is typically provided as a pressed sheet or the like, more typically having homogeneous and retermitted thickness and composition. Typically, a second weld assistive material 1 1 /nay be added to react with the surfaces 105, 1 10 and/or the first weld assistive material 1 0 while the fusion volume 160 is at least partially liquefied. The second additive material 191 is typically introduced to the interface 155 as a powder, or as a separate pressed film or sheet, or as a constituent of the pressed sheet introducing the first weld assisti ve material 190. Thus, the joint 1 5 may be compositionaily the same or similar to thai of the surfaces 105, 110 or it may be different yet compatible with the surfaces 105, 1 10.

100211 Weld quality may likewise be improved by providing an urging force 195 on the surfaces 105, 1 10 in the direction of the interface 1.55 in order to minimize drift or widening of the joint 1 15 during welding 1 0.

[0022] Weld quality may also be improved by selection of an appropriate atmosphere mat may retard thermal degradation of the surf ces 105, 1 10 and/or the weld 1 15, for example an oxidizing atmosphere for oxide ceramics or a nonreaciive or reducing atmosphere for carbide or nitride ceramics.

[00231 The welding technique 100 may be performed as a spot weld, or may be a linear weld accomplished by moving the source of additional heat 150 along the interface 155. typically at a predetermined rate .

[0024] Ceramic welding 100 enables the production of bodies 135 having complex shapes from simply shaped precursor surfaces 105, 11.0. The precursor surfaces 105, 110 are typically nearly theoretically dense, more typically at least about 98% dense (no more than 2% porosity), still more typically at least 99% dense (no more than .1 % porous), yet more typically at least 99.5% dense {.no more than 0.5% porosity), and still more typically at least about 99,9% dense no more than 0.1 % porosity). The ability to weld 100 simple surfaces 105, 1 10 into more complex structures 135 reduces machining costs and decreases the time required to achieve a finished component 35. in some cases, ceramic welding 100 is useful for improving mechanical behavior by refining grain sizes and producing herrnodynaniicaily stable grain boundaries which form from the melt in the joint region i 15. Ceramic welding 100 also enables the repair of ceramic components and composite structures.

10025] in operation, ceramic welding 100 may be accomplished by first identifying 200 a ceramic first surface 105 and a ceramic second surface 1 10 to be joined together and then preheating 205 the ceramic first surface 105 and the ceramic second surface 100 at a predetermined first, ramp rate 125 to a predetermined soak temperature 120, Next the ceramic first surface 1 5 and the ceramic- second surface \ 00 arc held 210 at the predetermined soak temperature 120 for a predetermined first soak time 12.3, and a thin volume 187 of a first weld assistive material 1 0 is inserted 215 between the ceramic first surface 105 and the ceramic second surface 1 10. Next, the- ceramic first surface 105 and the ceramic second surface 1 10 are urged 195 together to define an interface volume 155.

10026] A second weld assistive material 191 is introduced 220 to the interface volume 155, and additional heat 150 is applied 225 to the interface volume 155 at a predetermined second .ramp rate 1 5 to heat die interface volume .1 5 to a predetermined fusion temperature 170. A fusion temperature 170 is maintained 230 for a predetermined period of time 175 to at least partially liquefy the interface volume 1 5 to define- a fusion volume 160. and then the additional heat 170 is reduced 235 at a predetermined rate- ISO upon disengagement o f the additional heat 170. The final step is cooling 240 the fusion volume 1 0 at a predetermined cooling rate 130 to yield a solid fusion joint 1 15 and a newly welded unitary body 135.

Π [0027] Example I . Two SiC ceramic surfaces I 05, 1 10 were positioned adjacent one another to define an Interface 155, The surfaces 105, 1 10 were heated at a rate 125 of about two (2) degrees Celsius per minute and maintained at a first soak temperature 120 of about one-thousand (1000) degrees Celsius, A first carbon additive material the form of a ten mi! thick pressed carbon sheet 190 was inserted into the interface volume 155 and a second additive material 190 was added to the surface of ihe surfaces 105, 1 10 adjacent the interface 155 so as to wick into the interface 155 during welding. The surfaces 105, 1 10 were damped together to provide urging force: 395 during welding 100. Additional heat . 55 was applied plasma welding torch current, ramped, up to 55 amps at a fate 1 5 of five i ^' 5) Amps per second from a pilot arc of twenty-five (25) Amps to a welding current 170 of fifty-five (55) Amps and maintained for a duration 175 of ten (10) seconds, followed by a five (5) second ramp down 180 to yield an at least partially liquid fusion volume 160. The fusion volume was cooled at a rate .161 of about 2 degrees Celsius per minute- until it reached a predetermined end temperature 163 at which point the fusion volume 160 had solidified to yield a joint 11.5.

[0028] While the novel technology has been illustrated and described in detail in the drawings and fore-going description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification i satisfaction of the best mode and enablement requirements. If is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in ihe present specification. Accordingly, it is understood that ail changes and modifications that come within the spirit, of the novel technology are desired to be- protected.