The present invention relates to a support body having a coating comprising at least 95% by weight titanium boride wherein said coating has an oxygen content of less than or equal to 1% by weight, a metallic impurities content of less than or equal to 0.5% by weight and a specific electrical resistance of less than or equal to 10 xcexcxcexa9xc2x7m at room temperature.
It is known that coatings on a support body often serve to increase the life of the support body in a certain application, to improve the properties for a certain application, and/or to open up new fields of use for the support material. A coating allows the possibilities for use of the support body to be improved and its resistance to be increased.
Special requirements are made of this coating. It is known that in the case of components which are employed in various technical fields, different types of stress may arise, often in combination. For use in metal melts and/or salt melts, for example, high corrosion resistance and erosion resistance are necessary. Furthermore, it is necessary to ensure good adhesion to the component and good cohesion within the coating system. In addition, the protective coatings must have low internal compressive stresses, and high hardness and load-bearing properties. In certain applications of the components, furthermore, low specific electrical resistance of the protective coating must be ensured.
DE-A-35 13 882 discloses a protective coat which consists of an adhesion layer applied to the support, an interlayer applied to the adhesion layer, and a top layer applied to the interlayer. The adhesion layer corresponds in its composition essentially to the support material, so that there is not too great a difference in the thermal expansion coefficient. The interlayer is a mixture of the materials of the adhesion layer and of the top layer. The top layer can be, inter alia, titanium diboride. This protective coat is intended to protect the support material against corrosion, oxidation, abrasion, erosion, chemical attack and radiation, insulate it electrically at the same time, and, by virtue of heat insulation, to protect it in the short term against overheating.
A coat for protection against high-temperature corrosion is described in GB-A-1 104 840 for a metallic body comprising refractory metal such as, for example, tantalum, tungsten or molybdenum. The coat consists of a mixture of from 85 to 99% by weight zirconium boride/titanium boride and from 1 to 15% by weight silicon. When the materials were selected it was taken into account that the thermal expansion coefficients of substrate and coat must be comparable.
In U.S. Pat. No. 5,368,938, protection against oxidation is achieved by specially pretreating the surface of the carbon body, applying an interlayer to this pretreated surface and then applying a top layer. The pretreatment of the carbon body consists in etching the surface and then reacting the resultant porous surface layer with boron oxide. This produces a boron carbide layer having a porosity of about 50%. The porosity of this base layer enables the glass-forming interlayer applied over it to form a connection with the base body. Then a top layer comprising refractory materials is applied to this base layer. The layers are applied by way of CVD. The complicated treatment of the substrate and the application of interlayers serve to seal cracks which occur in the outer top layer.
This aim is also pursued in U.S. Pat. No. 5,536,574, in accordance with which the protection against oxidation of a carbon substrate is intended to be achieved by way of a boron-containing SiC interlayer and a top layer of glass ceramic consisting of TiB2, colloidal SiO2 and SiC. The layers are applied by applying the various materials in paste form in succession and then sintering them.
As protection against corrosion by liquid aluminum, DE-A-23 05 281 describes a coating or a covering of melted or highly sintered, dense, refractory hard material on a cathode or a cathode element made of carbon. The hard material referred to comprises the borides, nitrides, carbides and silicides of transition metals of groups four to six of the Periodic Table. Present in addition to the hard material is a small proportion of carbon, which forms a binary system. This melt coating can be obtained either by heating at temperatures from 2200 to 2300xc2x0 C. or by plasma spraying.
Furthermore, DE-A-12 51 962 discloses a cathode or a carbon cathode with a coating that consists of a mixture of a hard material such as titanium boride or titanium carbide and at least 5% carbon. This cathode is calcined at temperatures of preferably from 1600 to 2000xc2x0 C.
The prior art described above shows by way of example the problems of applying a coherent, jointless coating, preferably of titanium boride, to a support body with good adhesion when the coating and the support body have different thermal expansion coefficients. The approach described in GB-A-1 104 840, namely to use only materials having a similar expansion coefficient, in practice entails too great a restriction in the selection of material. In many cases, interlayers are applied in order to harmonize the different thermal expansion coefficients of coating and support body, the material of said interlayers having to be selected such that there is gradual adaptation of the expansion coefficients between support body and top layer. One disadvantage of this procedure is a complex coating, which becomes relatively expensive and also requires the presence of suitable materials having the correct thermal expansion coefficients.
In order to seal the support material effectively against the external medium, the coating must be so impermeable that it has no open pore channels which connect the support body with the environment outside the coating. This means that the coating must have a sufficiently low porosity and a certain thickness. Pure titanium diboride, as a high-melting material (Tm at approximately 2900xc2x0 C.) compacts very poorly on sintering. In order to obtain compaction at more readily achievable temperatures, additives are frequently added in order to reduce the sintering temperature. Titanium diboride and carbon, for example, form a binary eutectic for which the eutectic temperature for the composition 85% TiB2 and 15% C is about 2287xc2x0 C. The addition of the low-melting silicon, which lowers the sintering temperature by more than 1000xc2x0 C., must be seen from the same standpoint. The addition of other metals and alloys, such as Fe, Ni, Cr and Mo, for example, produces the same effect. Likewise, with oxygen, a very low-melting boron-containing phase (glass phase) is formed. The addition of additives is advantageous for the greater ease of compaction of the titanium diboride; however, the formation of low-melting secondary phases is accompanied to the same extent by a reduction in the high-temperature resistance of the material. A further disadvantage of adding additives is the change in a number of material properties of the sintered body, such as, for example, the specific electrical resistance. An oxygen content of a few percent by weight leads to an increase in the specific electrical resistance by a factor of from 100 to 1000. The oxygen content of the coating is preferably less than 0.6, in particular less than 0.3, % by weight.
The formation of vitreous or glass-ceramic interlayers allows for compensation of the stresses which occur on heating owing to the different thermal expansion coefficients, since these amorphous layers are relatively flexible. However, even these glass-ceramic layers have the disadvantage that their high-temperature stability is low.
The majority of the coatings known to date exhibit poor adhesion to the support body, especially at relatively high temperatures such as, for example, at 900xc2x0 C. if heating and cooling are conducted in alternating cycles. In many cases cracks are formed, and the coating begins to become detached. Some coating compositions, and their production methods, are highly complex and therefore of no economic interest.
The object of the invention was to provide a support body having a jointless coating principally comprising titanium boride which is able to protect the support body effectively at high temperatures both against corrosion by liquid metals, e.g., against the penetration of aluminum and/or sodium from melts comprising aluminum and/or sodium and/or compounds thereof and against erosion, said coating having a high level of adhesion to the support body, especially to a support body comprising carbon and/or graphite, and being electrically at least as conductive as the support material. For a particular application, the coating should be able to withstand the melt of an aluminum fusion electrolysis cell under operating conditions for at least one year.
This object is achieved with a support body which carries a coating comprising at least 95% by weight titanium boride, wherein said coating has an oxygen content of less than or equal to 1% by weight, a metallic impurities content of less than or equal to 0.5% by weight, and a specific electrical resistance of less than or equal to 10 xcexcxcexa9xc2x7m at room temperature.
Titanium boride is a material suitable in principle for coatings of support bodies which are intended to be protected against chemical corrosion by metal melts at high temperatures and against erosion. Owing to the aggressive nature of metal melts at high operating temperatures, the titanium boride should preferably be of high purity and should be virtually free from oxide phases and soluble metallic impurities, since these can be leached out of the metal melt, as a result of which the microstructure becomes unstable because of grains escaping. The properties of the coating, in accordance with the invention, can be best achieved by preparing the coating by means of plasma spraying. However, in the case of reactive spray powders, such as TiB2, for example, there is a change in the chemical composition of the spray coating prepared from them. Atmospheric oxygen can diffuse into the plasma flame and lead to oxidation of the reactive spray powder and, consequently, to altered properties of the coating as well, such as chemical stability and electrical resistance, for example. Therefore, the coating of the invention is applied in a substantially or totally oxygen-free atmosphere. The metallic impurities are preferably xe2x89xa60.35% by weight, with particular preference xe2x89xa60.2% by weight and, in particular, xe2x89xa60.12% by weight. In the process of the invention, no metallic powder is added to the spray powder.
At an oxygen content of less than or equal to 1% by weight, the specific electrical resistance of the coating is generally less than or equal to 10 xcexcxcexa9xc2x7m at room temperature, preferably less than xe2x89xa65 xcexcxcexa9xc2x7m. For the coating of the invention it is important to select an appropriate spray powder particle size distribution. Excessively large spray powder particles produce a highly porous coating having a relatively high specific electrical resistance. The smaller the particles, the lower the specific electrical resistance. However, this is true only down to a certain limit. The specific electrical resistance of TiB2 single crystals lies between 0.066 and 0.09 xcexcxcexa9xc2x7m and thus below this limit. If the particle size is reduced further, the resistance increases again. This increase in resistance is caused by the higher proportion of oxygen in finer powders.
Similarly, it can be important that when the coating is applied to the support body surface there is no reaction of the support body material with titanium boride. The interlayer forming as a result of such a reaction could be soluble in the metal melts or could have a distinctly higher specific electrical resistance, acting as an electrical barrier layer.
Where the titanium boride coating and the support body have markedly different thermal expansion coefficients, this can readily lead to flaking of the coating under thermal loads, for example, at temperatures of up to 950xc2x0 C., if the coating is completely impermeable. Since the application of a plurality of individual layers having different properties atop one another, in order to gradually adapt the thermal expansion coefficients of the support body to those of the coating, is uneconomic, an advantageous titanium boride coating is one having a porosity of preferably not more than 10% by volume, in particular from 4 to 7% by volume, and having a specific microstructure which permits compensation of the mechanical stresses in the coating: the coating preferably has a microstructure which in the form of a transverse ground section (ground section vertically through the coating for observation under the direct-light microscope or scanning electron microscope) shows a lamellar microstructure having an average ratio of lamella length to lamella height in the transverse ground section in the range from 1.5:1 to 10:1; the individual lamellae represent cross-sections through the spray spots which are formed when the individual melted spray powder particles impinge and which become easily visible by etching with boiling 30% strength hydrofluoric acid for about 5 minutes. In the case of the coating of the invention it is advantageous if the extent of melting of the TiB2 particles is substantially or totally complete. This is advisable since without low-melting phases, e.g. metallic or oxide-like binder phases, solidification of the coating is difficult. The extent of adhesion of the jointless titanium boride coating of the invention to the support body of carbon and/or graphite is such that if the coating is stressed, for example, by impact or jolting, any crack which forms, insofar as it extends essentially parallel to the coating surface, does not run right across the coating and does not run through the interface between coating and support body, but instead runs within the support body. The uniformity of the microstructure is ensured by the uniformity of the spray powder and of the process conditions. Similarly, the measurement of the tensile adhesive strength in accordance with DIN 50160 showed that the tensile samples failed almost exclusively within the support body of carbon and/or graphite.
The coating of the support body takes place advantageously under operating parameters of the plasma burner, with a coating-chamber pressure level and with an appropriate spray powder particle size, such that a porosity of not more than 10% by volume, in particular from 4 to 7% by volume, is produced. The porosity and the thickness of the coating are advantageously within a range in which the coating exhibits no pore channels that connect the support body with the environment outside the coating. The thickness of the titanium diboride coat is preferably from 0.1 to 1 mm, although in principle it is possible to produce thicker coats. Within this thickness range, the risk of flaking on exposure to very severe fluctuations in temperature is particularly low. The impermeability of a jointless coating having a coat thickness of from 0.1 to 1 mm, which for the high-melting titanium diboride with a melting point of more than 2900xc2x0 C. can be achieved preferably by means of a controlled inert gas plasma spraying process, is a prerequisite for the coating to withstand the operating conditions in an aluminum fusion electrolysis cell for more than a year. Coherent jointless coatings are preferable for corrosive loads.
The functional capacity of the coating applied to the surface of carbon and/or graphite bodies is determined above all, inter alia, by the adhesive strength of the coating. The adhesion of the coating of the invention to a support body of carbon and/or graphite, especially graphitized carbon, is high: the adhesive strength of the coating is higher than the strength of the material of the support body. The outstanding adhesion is achieved by applying melted powder particles under reduced pressure and at high speeds, which result in mechanical anchoring to the surface. It is advantageous to apply the coating directly, without an interlayer, to the slightly roughened surface of the carbon bodies, as is present in the case of normal machining by means, for example, of milling or cutting. For coating, the support bodies must be clean and dry.
When the support body of carbon and/or graphite is employed in an electrolyte melt and/or metal melt, as little as possible of the metal and electrolyte melt should be able to penetrate it. The sodium formed as a result of a side reaction in the fusion electrolysis of aluminum forms, together with graphite or carbon, intercalation compounds which can lead to an increase in volume. The ex ion caused by the increase in volume results in bowing and bulging of the carbon cathode elements in the aluminum fusion electrolysis cell. The expansion of the carbon material as a result of the penetration of sodium into carbon is determined using the so-called Rapoport test.
The mechanical erosion resistance of a coating can be determined using a method adapted specifically to the in-use wear conditions: carbon, and especially graphitized carbon, since it is relatively soft, is abraded by undissolved aluminum oxide, which in the course of the operating period collects on the surface of the carbon cathode elements at the bottom of the aluminum fusion electrolysis cell. Uncoated cathode elements exhibit erosion of approximately from 5 to 10 mm per annum. This stress can be simulated by measuring the erosion resistance in accordance with the method of Liao et al. (X. Liao, H. Oye, Carbon, Vol. 34(5), 1996, 649-661). In accordance with Liao et al., the coating of the invention should exhibit little or no erosion in the course of a six-hour test in an Al2O3 slip, so that a coated sample exhibits an attrition ratio of not more than 1:2 in comparison to an uncoated sample.
In the titanium boride coating it is possible for preferably up to 5% by weight of borides, carbides, nitrides and/or silicides of the composition MxBy to be present, Mx representing levels of the transition metals of groups 4, 5 and/or 6 of the Periodic Table, or representing aluminum, and By representing the boride, carbide, nitride and/or silicide content. Particular preference is given to zirconium boride and aluminum boride. Preferred levels are from 1 to 2% by weight.
The coating of the invention for support bodies of whatever shape is particularly advantageous for the coating of carbon and/or graphite bodies of large surface area which are used as electrodes for the fusion electrolysis of aluminum. In the case of such cathode elements, usually only one face of the element is coated, that facing the anode, with the possible addition of a marginal region beyond the edges of this coated face. The coating can be applied both to bodies of fired, nongraphitized carbonxe2x80x94that is, bodies which in the course of their production have been heated to a temperature of at least 750xc2x0 C. and to not more than 1400xc2x0 C.xe2x80x94and to bodies of electrographitexe2x80x94that is, bodies which in the course of their production have been heated a temperature of at least 1600xc2x0 C., preferably to temperatures of from 2400 to 3000xc2x0 C. The coating of the invention can also be applied to support bodies of steel, other metals or alloys, refractory metals, composite materials, or ceramics. For many applications it is important that the surface of a support body is protected against attack from chemically aggressive media at high temperatures, against abrasion, erosion and/or oxidation.
Support bodies coated in accordance with the invention can be employed as electrodes, especially as cathode elements in the fusion electrolysis of aluminum, as heating elements, inter alia, for electrically heated ovens, as refractory linings, as wear-resistant elements, or as vessels, especially as crucibles, as elements in heat exchangers or nuclear reactors, as nozzles, evaporator boats and heat shields.