Abstract:
A method of producing a ceramic weld, including identifying a ceramic first surface and a ceramic second surface to be bonded together, maintaining a non-oxidizing atmosphere over the first and second surfaces, and engaging the first and second surfaces to define a joint. An arc is generated between an electrode and the joint to create a liquid phase, and the liquid phase is cooled to yield a solid fusion layer, wherein the first and second surfaces are joined in the fusion layer.

Description:
TECHNICAL FIELD 
       [0001]    The present novel technology relates generally to the field of materials science and, more 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 thermal shock. 
         [0003]    Ceramic materials are typically joined 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 that will experience significant tension or flexure, since cements are also prone to brittle fracture. 
         [0004]    Further, as-formed 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 that 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 to 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 microcracks 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. 
       SUMMARY 
       [0007]    The present novel technology relates generally to materials science. One object of the present novel technology is to provide an improved method of joining two ceramic bodies. Related objects and advantages will be apparent from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a diagrammatic view of a ceramic to ceramic welding method according 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. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    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. It 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 the 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]      FIGS. 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 postheat 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 simple 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 thermophysical 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 parent bodies. 
         [0012]    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 (internal 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 bearings for high temperature applications (&gt;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 0.1% porosity). The ability to weld simple shapes into more complex structures reduces machining costs and decreases the time required to achieve a finished component. In some cases, ceramic welding is useful for improving mechanical behavior by refining grain sizes and producing thermodynamically 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 a post-welding thermal profile, as, in general, ceramic materials lack the sufficiently high thermal shock resistance and/or significant ductility below the system&#39;s melting temperature to avoid material failure from thermal shock. Alternately, the properties of the precursor pieces may be tailored to have very low coefficients of thermal expansion 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 that 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, recrystallization, 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 recrystallization occur at or above a homologous temperature (T H =T/T m ) of T H ≈0.4-0.5. For materials exhibiting grain boundary softening, microcracking, and grain boundary sliding, the pre- and post-heat treatment temperature will be 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 T H ≈0.4-0.5. 
         [0015]    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 ramp 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 typically shielded from oxidizing conditions at elevated temperatures in order to preserve the integrity of the component. 
         [0016]      FIGS. 1-2  illustrate one embodiment of the present novel technology, a method  100  for joining two (typically compositionally similar) ceramic surfaces  105 ,  110  in a weld or fusion bond  115 . As ceramics are inherently brittle materials and as such are susceptible to thermal shock damage during the rapid heating and cooling cycles, the surfaces  105 ,  110  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 a second slow ramp rate  130  is typically employed to minimize thermal shock damage to the newly welded piece  135  and the newly formed joint  115 . The application of properly selected preheat and postheat treatments assists in the joining of ceramic surfaces  105 ,  110 . 
         [0017]    Once the surfaces  105 ,  110  are at the first soak temperature  120 , the surfaces are urged together to define an interface volume  155  therebetween and additional heat  150 , such as from a plasma torch or the like, is applied at the interface volume  155  between the surfaces  105 ,  110  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  115 . More typically, the cooling rate  161  is sufficient to anneal the fusion joint  115  and adjacent surfaces  105 ,  110  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 ,  110 . Typically, the additional heat  150  is quickly ramped up at a first welding heat ramp rate  165  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  180  upon disengagement. 
         [0018]    Depending on the composition of the surfaces  105 ,  110 , the first soak temperature  120  may be from about 700 degrees Celsius to about 1100 degrees Celsius and the additional heat  150  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 1500 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 for spot welds or short (1-2 cm.) linear welds, and longer for longer linear welds. 
         [0019]    The novel joining process  100  enables the production of large, complex compound bodies  135  from precursor surfaces  105 ,  110  having the simple shapes that 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  100  can produce joints  115  that exhibit the same thermophysical and mechanical behavior as the parent material. In addition, the welded joints  115  are able to withstand the same chemically corrosive, oxidizing atmospheres, and high temperature environments as the materials of the parent surfaces  105 ,  110 . 
         [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 ,  110 , the composition of one of the constituents of one or both surfaces  105 ,  110 , or a different composition compatible with one or both surfaces  105 ,  100  so as to strengthen the joint  115 . The thin volume  187  of additive material  191  is typically provided as a pressed sheet or the like, more typically having homogeneous and predetermined thickness and composition. Typically, a second weld assistive material  191  may be added to react with the surfaces  105 ,  110  and/or the first weld assistive material  190  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 assistive material  190 . Thus, the joint  115  may be compositionally the same or similar to that of the surfaces  105 ,  110  or it may be different yet compatible with the surfaces  105 ,  110 . 
         [0021]    Weld quality may likewise be improved by providing an urging force  195  on the surfaces  105 ,  110  in the direction of the interface  155  in order to minimize drift or widening of the joint  115  during welding  100 . 
         [0022]    Weld quality may also be improved by selection of an appropriate atmosphere that may retard thermal degradation of the surfaces  105 ,  110  and/or the weld  115 , for example an oxidizing atmosphere for oxide ceramics or a nonreactive or reducing atmosphere for carbide or nitride ceramics. 
         [0023]    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 ,  110 . 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 ,  110  into more complex structures  135  reduces machining costs and decreases the time required to achieve a finished component  135 . In some cases, ceramic welding  100  is useful for improving mechanical behavior by refining grain sizes and producing thermodynamically stable grain boundaries which form from the melt in the joint region  115 . Ceramic welding  100  also enables the repair of ceramic components and composite structures. 
         [0025]    In operation, ceramic welding  100  may be accomplished by first identifying  200  a ceramic first surface  105  and a ceramic second surface  110  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  105  and the ceramic second surface  100  are held  210  at the predetermined soak temperature  120  for a predetermined first soak time  123 , and a thin volume  187  of a first weld assistive material  190  is inserted  215  between the ceramic first surface  105  and the ceramic second surface  110 . Next, the ceramic first surface  105  and the ceramic second surface  110  are urged  195  together to define an interface volume  155 . 
         [0026]    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  165  to heat the interface volume  155  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  155  to define a fusion volume  160 , and then the additional heat  170  is reduced  235  at a predetermined rate  180  upon disengagement o f the additional heat  170 . The final step is cooling  240  the fusion volume  160  at a predetermined cooling rate  130  to yield a solid fusion joint  115  and a newly welded unitary body  135 . 
       EXAMPLE 1 
       [0027]    Two SiC ceramic surfaces  105 ,  110  were positioned adjacent one another to define an interface  155 . The surfaces  105 ,  110  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 in the form of a ten mil thick pressed carbon sheet  190  was inserted into the interface volume  155  and a second additive material  190  was added to the surface of the surfaces  105 ,  110  adjacent the interface  155  so as to wick into the interface  155  during welding. The surfaces  105 ,  110  were clamped together to provide urging force  195  during welding  100 . Additional heat  155  was applied plasma welding torch current, ramped up to  55  amps at a rate  165  of five (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  115 . 
         [0028]    While the novel technology has been illustrated and described in detail in the drawings and foregoing 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 in satisfaction of the best mode and enablement requirements. It 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 the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.