WO1994011117A1

WO1994011117A1 - Method for applying nonmetal coatings to the surface of a part

Info

Publication number: WO1994011117A1
Application number: PCT/US1993/010597
Authority: WO
Inventors: Boris Evgenyevich Paton; Vladislav Efimovich Skylarevich; Mikhail Vladimirovich Shevelev; Andrei Ivanovich Bunenko; Alexandre Timofeyevich Sanzharovskii; Sergei Dmitriyevich Bogdanov
Original assignee: Paton Tek, Inc.
Priority date: 1992-11-06
Filing date: 1993-11-05
Publication date: 1994-05-26
Also published as: AU5548294A

Abstract

A method for applying nonmetal coatings to the surface of a part involves applying to the surface of a part at least one coating layer for which absorbed energy of a millimetric-wave SHF radiation is different from energy of the same radiation absorbed by the part. The part and the coating are then simultaneously exposed to SHF radiation so that they reach different temperatures, whereby adhesion therebetween is provided..

Description

METHOD FOR APPLYING NONMETAL COATINGS TO THE SURFACE OF A PART

FIELD OF THE INVENTION

The invention relates to methods for applying a material to various surfaces and, more specifically, it relates to a method for applying nonmetal coatings to the surface of a part including a method of continuous material processing where surface and interface layers are "alloyed" with lossy chemicals and fire-disperse powders.

The invention may be used in many way including the applying of nonmetal coatings, such as a working or protective layer, to refractories and acid-resistant materials, radio ceramics and ceramic building materials, ceramic electric heaters, enameled steel and similar products. The invention may also be used for applying water-proof and heat insulating coatings on pipelines in thermal and power supply systems. The invention further may be used in the making of passive elements in integrated circuits and conductors on printed circuit boards using thick-film techniques. BACKGROUND OF THE INVENTION

Known in the art is a method for applying high-melting point and refractory coatings to products, in the form of ceramic and refractory matrices, by using a gas flame technique with the aid of gas burners. This method is, however, deficient in providing adequate adhesion of the coating to the matrix. Additionally, the method cannot be used in applying coating materials having a comparatively low melting points. Pipelines for oil and gas are often coated with various materials corrosion prevention. Known in the art is a method for applying a double-layer lined polyethylene tape to pipelines. The tape is layed over a priming coat of butyl rubber, which is first deposited to the pipelines and allowed to dry. After drying and application of the tape, the primer solvent continues to evaporate slowly during the next 200 to .300 hours through the polyethylene tape. During this time, adhesion of the coating to the tube is as low as several grams per 1 cm of tape width and, therefore, if an external load is applied, waves and creases are formed on the tape. One limitation of this method is that the use of butyl rubber, or other synthetic rubbers, in the coating limit the adhesive strength to no more than 2.0 - 2.5 kg/cm. Another limitation is that a temperature increase in. the pipeline above 60 - 70°C will result in the adhesive strength? being, substantially reduced to an unacceptable level. Another prior art technique for coating pipelines also uses double-layer tapes, e.g., tapes having a polyethylene base and a thermoplastic adhesive layer. The adhesives are generally based on copolymers of ethylene such as polyethylene in a vinyl acetate filler. This procedure includes the following steps: heating the pipe to 150 to 250^βC; pressing the tape against the pipe; allowing the coating to stay in contact with the metal during 10 to 30 minutes, and cooling the coated pipe. This method, however, requires high energy consumption since it is necessary to heat the entire pipe, which can have a wall thickness of up to 25 mm, and require a temperature of 150 - 250*C for a period of 10 to 30 minutes. Addition, with temperatures above 150^βC, not only is the adhesive layer melted, but the polyethylene base is also heated to the point that its properties are impaired. Phenomena such as burning out of a thermostablizing agent and a lowering of the heat resistance are irreversible in such a case.

When making thick-film integrated circuits, the method used involves applying pastes of materials, corresponding to conductors and various types of passive elements, to a circuit board and subsequently heating them. In carrying out this method, pastes based on borides and oxides are used. This allows single-layer circuits to be heated in a single cycle over the course of 30 - 40 minutes. This heating time is, however, comparatively long and presents the risks that the various pastes will diffuse into each other and, when metal supports are used, that the supports of the circuit board can be deformed. Addition, this heating can require temperatures as high as 850° and may require nitrogen atmospheres to protect the thick film from oxidation.

Widely known in the art is a method in which high- melting point and refractory coatings are applied to ceramic or refractory products by plasma deposition. This method involves placing a layer of a coating material on the surface of a ceramic product and melting it, as well as the material of the product surface, under the action of plasma to ensure adhesion of the coating to the product surface. However, the adhesion of the coating to the product surface with such a method is inadequate and cannot ensure the desired strength, the desired heat resistance or the desired density of the coating. Also, the partial evaporation of the electrodes between which plasma is formed causes contamination of the coating with the electrode material and impairs the quality of the coating. Since the coating material is partly blown off of the product surface by the gas flow during melting, an increase in material usage typically accompanies this method.

In view of the foregoing, it is an object of this invention to provide a method of applying nonmetal coatings to a product surface which enhances adhesion of the coating to the product surface by providing conditions which predominantly melt the materials at the interface of bonding.

A further object of the invention is to provide efficient heating and thereby reduce coating and heating time.

Yet another object is to concentrate heating in those portions requiring thermal processing without overheating other sensitive portions of the coating or the part.

A still further object of the invention is to ensure that a sufficiently high rate of physio-chemical or mechanical processes occur between the selected materials so that this high rate leads to a desirable final result.

SUMMARY OF THE INVENTION The invention resides in a method for applying nonmetal coatings to the surface of a part or product exposing the product and coating to millimetric-wave super high frequency (SHF) radiation until they reach different temperatures whereby maximum adhesion therebetween is ensured. As used herein, the coating includes at least one layer in which the absorbed energy of SHF radiation is different from the absorbed energy of SHF radiation in the part or other layers. Depending on the specific materials and process involved, the part surface can be heated to melting without degrading the coating, a layer of the coating can be heated to melting without excessive heating of the part, or a layer of the coating can be heated to melting without excessive heating of an overlying layer of the coating.

It is preferred that the ratio of the energy absorbed by the coating and the part be preset, either by varying the thickness of at least one material forming the part and/or the coating, by using additives in the coating, or through the selection of materials for the part and the coating.

It is preferred that a gyrotron is used as the source of SHF radiation to which the coating and the part are exposed. The radiation generated by a gyrotron at frequencies from 35 to 100 GHz has some special features, as compared to the widely used radiation of the centrimetric range generated at frequencies of 915 MHz and 2.45 GHz. First of all, the millimetric radiation is by 3 orders of magnitude more efficient in its interaction with monmetallic materials than centrimetric radiation. Besides, it is characterized by high selectivity in the heating of an irradiated material enabling its use in fast processes, processes based on short-time heating. In such cases, no insulation is needed thereby opening the way for its use in numerous continuous processes.

For example, in instances where it is desired to apply a coating to the surface of a part, according to this invention, it is preferred that the coating comprise a polymeric base and an adhesive layer, with the adhesive layer facing towards the surface of the part and containing additives increasing the absorption of SHF radiation. It is also preferred that the material of the adhesive layer has an absorption coefficient which is greater than that of the overlying polymeric base and the part. It is further preferred that the material of the adhesive layer be in the form of electrically conductive inorganic compounds (where conductance is achieved by electron flow as occurring in metals) , such as silicon carbide or molybdenum silicide, or conductive inorganic compounds and styrene-indene resin. In using the present method for the manufacture of thick-film integrated circuits, it is necessary that the coating, applied to the base or circuit board support, contain pastes of materials having different coefficients of absorption for SHF radiation. The pastes correspond to different types of passive elements in the circuit and the support should be made of a material poorly absorbing SHF radiation. When the layers of pastes for producing the electrical elements are disposed in different layers, it is preferred that the pastes be chosen in such a manner as to ensure heating of the pastes to a preset temperature, simultaneously in all layers, in a single irradiation cycle.

Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of the absorption capacity of a material for various frequencies of SHF radiation, as dependent on the size of the particles dispersed in a layer of the coating; and

FIG. 2 is a graphic representation of the absorption capacity of a material for various frequencies of SHF radiation, as dependent on the concentration of the particles dispersed in a layer of a coating.

DETAILED DESCRIPTION OF THE INVENTION

According to the broad principles of the present invention, a layer of a nonmetal coating material is applied to the surface of a part, and this layer is exposed to millimetric-wave SHF radiation. This radiation passes through the material of the coating and then through the material of the part and is partly absorbed to carry out heating. The degree of heating of each layer of the coating and of the part itself mainly depends on the coefficients of absorption for the various materials, exposure time, coating thickness and its location with respect to the radiation source.

The mechanism of heating nonmetals with a millimetric- wave SHF radiation is very efficient. As mentioned above, the gyrotron is the simplest and most efficient generator of millimetric waves and is preferably used with this invention. The gyrotron can generate waves, in the form of a Gaussian beam, having power in excess of 10 kilowatts, with greater than 30% efficiency. By means of simple metal mirrors, the radiation can be focused into a coherent beam so as to localize the radiation energy and can be given practically any desired shape, i.e. spot, strip, circle, etc.

As compared to the widely used radiation of the centimetric range, generated at frequencies of 915 MHz and 2.45 GHz, the gyrotron radiation, generated at frequencies from 35 to 100 GHz, has some special characteristics. First of all, the millimetric radiation is by 3 orders of magnitude more efficient in its interaction with nonmetallie materials than the centrimetric radiation. Importantly, the gyrotron radiation is also characterized by high selectivity in the heating of an irradiated material.

Since heating by SHF radiation occurs rapidly, and the processes associated with heat conduction do not have time to level out the temperatures between the layers of materials being irradiated. Also, substantially all materials have a coefficient of absorption which correspondingly increases with temperature rise. Thus, in the layer which is heated the most during the initial time period being increasingly heated at a rate faster than the remainder of the layers. As a result, within a short period of time, the temperature of the selected layer, preferably an internal layer, of the coating or the part can be brought to its melting point without a significant rise in the temperature of the additional layers thereby preventing their thermal degradation. The thicknesses of any overlying or non-selected layers are chosen so that the radiation energy absorbed in this layer will be of an amount insufficient to cause it to melt within the time required for the selected layer to reach its melting point. Thus, under the action of SHF radiation, the selected layer starts melting before the overlying or underlying layers so as to provide conditions for adhesion of one material to the other and so as to provide for a more favorable adhesion bond since a broad contact zone for fusion of the coating and the part is formed.

When developing such technologies, two main problems need to be solved. The first problem consists of insuring that a sufficiently high rate of physico-chemical or mechanical processes occur in the material selectively exposed to the gyrotron radiation so that his high rate will lead to the desired final result, e.g. polymerization, melting and diffusion. It had already been proven that centimetric radiation could reduce autoclave curing times of hours to microwave curing times of minutes. Now, this new practice opens the way for engineering design and development of new compositions, primers and solders which can respond adequately to the faster heating afforded by the gyrotron. The end result is technological process cycles in seconds rather than minutes. As an example further set out below, this has been observed in burning of pastes in microcircuits.

The second problem which needs to be solve is the need to concentrate heating in only those layers or volumes that need thermal processing, without overheating other sensitive components. This can b solved by the increasing in absorbing capacity of the specific material of the layers being selectively processed. It can be achieved by different methods: e.g. by adding absorbing agents, by creating resonant electromagnetic systems or by selecting the workpiece components. An example of the latter is set out below with respect to the deposition of anticorrosion coatings on oil and gas pipelines.

When used for applying a tape, consisting of a polymeric base and a primer or adhesive layer, to the surface of a metal part or pipe by pressing the tape against the metal surface being coated, it is necessary to increase the absorptive characteristics of the primer relative to the polymer base so that desirable characteristics of the base would not be destroyed.

To solve the problem of increasing the absorbing capacities of the primer relative to the base, calculations were made assuming that radiation in the form of the plane electromagnetic wave fell on the three-layer medium in the free space and consisting of the successive layers of polyethylene, the absorbing layer (primer) and the metal layer. The equations for the amplitudes of electric field, E, and temperature T, :

e(χ,D-l^4πσ(X'^r) E=0 ω

where: ω is the frequency of the microwave radiation, e is the effective apart of dielectric permeability, σ is the conductivity, Cp, P, a are the heat capacity, the density and the thermal conductivity of the heterogeneous medium, c is the velocity of light in vacuum, were solved numerically for the following conditions: losses of the microwave radiation in the polyethylene layer are small, according to our measurements, σ = 1.1 ^■ 10 ^J (Ohm cm) . For metal -σ - ». Conductivity of the primer layer was set equal to σ - 1.8 • 10^"^ (Ohm cm)" and its dependence on the temperature could be describe by equation σ = σ_Q exp [β (T -

As a result of the calculations, it was found that the primer could be heated for a short time up to the temperatures of 600^βC without any destruction of polyethylene since it remained at a low temperature. ϊn accordance with the calculations performed, special end primer was developed having a factor of absorption for the gyrotron radiation of 0.33 cm^*1 provided by selection of resins, polyolefins and other components.

The polymeric base is placed outboard of the part and the adhesive or primer layer faces towards the surface being coated. When exposed to SHF radiation, the SHF acts upon the polymeric base to pass therethrough with only a small portion being absorbed to heat the base material. As a result of irradiation of the tape with this primer, adhesion of a coating under the elevated temperature and higher humidity does not practically change with time in service, and this allows to considerably increase the life of pipelines.

The method of the present invention with the selection of specific components can be used to solve the problem of depositing a ceramic coating on refractory items. The refractory item is coated with ceramic having a predetermined absorption for the SHF radiation and a predetermined thickness. During irradiation, the underlying surface of the refractory item begins to melt before there is a significant heating of the ceramic coating. After the surface of the refractory item has melted, the electric conductance of this layer increases and the depth of penetration of SHF radiation materially decreases (to several millimeters) . Reflection of SHF radiation from the molten layer thus materially increases causing the SHF radiation to again pass through the ceramic coating, accelerating its melting. This results in the formation of a wide transient zone and, hence, a thick heat resistant ceramic coating.

If coating thickness should be greater than the thickness obtained in melting a layer of a preset thickness, the process can be repeated.

According to experimental results, the thickness of the coating (d_) should be chosen as follows:

wherein α^ and c_o are the SHF radiation absorption coefficients of the coating and the part material, respectively; and wherein Ti and Υ^ are the melting points of the coating and the part material, respectively.

If thickness of the coating is greater than d_c, this material is the first to melt and it will screen penetration of the SHF radiation deep into the material thereby hindering the formation of a broad contact zone for fusion as mentioned above. If a lower frequency of radiation is used, e.g., a centimeter-wave radiation, the efficiency of heating of the materials drops sharply. With the lower heating efficiency, heat conductance processes have time to average out temperatures between the layers of the part and coating, and consequently the predominant initial melting of the surface of the part cannot be ensured. If radiation at higher frequencies is used (e.g. , optical wavelength radiation) , the coating material will be first to melt at any minimum practically admissible thickness and, again, a broad contact zone for fusion with the part will not be formed.

The output of the coating process in accordance with the present invention is increased when gyrotron radiation is used. As such, the SHF radiation can be three- dimensionally concentrated in an area adjacent to the surface being treated and high power levels can be used (even greater than 100 kilowatts) . In a majority of cases, however, it is not possible to realize the selectivity advantages of the gyrotron radiation just simply by selecting materials for the layers of the treated component. For this we often have to use absorbing agents, mainly in the form of powders, added to an irradiated material. In fact, fine-disperse metal powder turns out to be most efficient for this purpose since the metal surfaces substantially completely reflect the SHF radiation.

It is known that microwave radiation only slightly interacts with metal, since its reflection factor exceeds 99%. But with powder, due to multiple reflection from each particle, the total absorbing capacity actually increases. A pronounced enhancement is observed when the powder is a mixture of metal and dielectric powders. For practical purposes, there is a great interest in finding the absorbing capacity of such mixtures as they depend on a) the size of the particles; b) their electrophysical characteristics; c) their concentration; and d) the radiation wavelength.

In a general case, the task is to find a solution for the electrodynamic problem in order to determine absorption of the microwave radiation by a multicomponent medium with a pronounced frequency dispersion. Another issue involved the solution to such problems is frequency dispersion. The main issue in solving such problems is the correct description of the multicomponent system by so called effective characteristics of the medium. For this it is necessary to take into account the multiple dissipation of the microwave radiation at particles and to statistically average the result of sizes, shapes and orientation of the particles. The most correct calculation of the effective electrodynamic parameters is possible only within the theory of multiple dissipation of waves as presented by Twersky and Prishivalko et al. However, in this case, the result will not reflect the entire picture of interaction between the microwave radiation and the fine-disperse medium as it depends on the radiation frequency, the means sizes of the particles and their concentration. Therefore, when solving the problem within the scope of classical electrodynamics and for the system "metal-dielectric", the solution was chosen based on the method of the effective medium by Stroud. At the high degrees of filing the medium with the metal component (of the order of 35%) , it is also necessary to take into consideration the dipole-dipole interaction between the particles, this being possible when using the approximated diagram of self-consistent mean field (method of coherent potential) .

In a general case for the spherical particles, on the basis of the methods and the assumptions made, one obtains for ε and μ:

3Λ(e-_<a)

2(g+ε)-.f(e-g)

3 p(μ-μ μ=μ_J ^m _D 2(μ+μ)-/(μ-μ) where: ε, μ are the dielectric and magnetic permeabilities of the dielectric, e_m, μ_m are the dielectric and magnetic permeabilities of the metal, determined on the basis of the Mie theory. The accurate solution for the problem of dissipation of the pane electromagnetic wave at the spherical conducting particle was suggested by Van de Hulst. Using this solution we determine e_m and μ_m as functions of the particle sizes (T), the frequency of the incident radiation (ω) and the conductivity (σ) .

By way of example, assume the particular parameters of the two-component system polymeric composition-copper with the following values: e_ _c = 3.7, μ_ _c = 1, σς_u - 5 • 10 'S^~

, € _u = 2 and the variable values of sizes of the fine particles (a) and the volume concentration (f) , as will as consider the law of absorption of the plane-polarized microwave which is known to have the form 1 (x) = lg ^eχP (~^a ^■ x) , where 1 (x) is the intensity along the line of the wave propagation; α - 2ω / c-x, where ω is the radiation frequency:

As a result, we have the possibility to calculate the dependencies of the absorbing capacity P, on the radius of the copper particles for various frequencies (Fig. 1) and on the concentration of the copper particles in the system (Fig. 2). In both Figures 1 and 2, line w represents a frequency of 10¹¹ H_z, line 2 represents a frequency of

10i^Λn^W H_z, and line 3 represents a frequency of 10Q* H_z. It can be seen that there is the clearly defined maximum of the absorbing capacity of the mixture, depending on the particle sizes, which varies with the frequency of the radiation and the concentration of the copper particles.

This research has allowed optimization of a number of technologies associated with heating of a mixture of the metal-dielectric powders.

The method according to the invention may be used for applying to the surface of a metal part a tape consisting of a polymeric base and an adhesive layer by pressing the tape against the metal surface to be coated. The outer side is exposed to electromagnetic SHF-range radiation which acts upon the material to pass therethrough and which is partly absorbed to heat the material. The degree of absorption, hence of heating for a specific frequency of radiation depends on electrical and physical properties of a substance and is characterized by its coefficient of absorption.

Similar to that discussed above, the base for a tape as the coating material according to this invention is polymeric, and preferably made of polyolefins which are transparent to radio waves over the entire SHF range. Therefore, the base of the tape is not substantially heated during irradiation. The adhesive layer, however, is provided with an additive of a material absorbing SHF radiation and a portion of the SHF radiation is absorbed by the adhesive layer as such. It should be noted that because of the three-dimensional character of the absorption mechanism for SHF radiation, heating occurs substantially instantaneously and the conductive loss of heat into the base of the tape and into the part being coated is very low. Repeated passage of SHF radiation reflected from the part surface further contributes to heating efficiency of the adhesive. Therefore, selective heating of the adhesive layer only is ensured when the incoming energy is spent for carrying out the physical and chemical processes of adhesive interaction between the tape and the metal of the part. This allows the adhesive layer to be heated to temperatures above those destructive to the base of the tape. This, in turn, reduces the time needed for the formation of the adhesive contact and increases the number of adhesive bonds per unit of surface area. All these factors enhance adhesion, retain quality of the base, and prevent temperature-induced changes in the metal parts being coated with the tape so as to further improve coating quality. The direct heating of the adhesive layer, without significant heating of the part being coated and the base of the tape, as well as reduction in the time required for the formation of the adhesive contact, decreases the time required for the coating process and lowers power consumption requirements.

Additives used in the adhesive layer are in the form of conductive inorganic substances, such as silicon carbide (SiC) or molybdenum silicide (MoSi2) , having high absorption coefficients (on the order of 0.5 to 5 mm^"1) which roughly remains unchanged within the temperature range from 0 to 700-900^βC. This allows the adhesive layer to be heated to these temperatures within a time during which thermooxidation processes will not have time to impair the quality of the coating. The high absorption coefficients of the conductive inorganic substances at temperatures of up to 700-900"C ensures a high rate of heating of the adhesive layer, hence, a reduced process time for completing the coating process.

An adhesive composition for a double-layer polymeric coating was also formed containing 75-88 parts by weight of polyethylene, 7-15 parts by weight of styrene-indene resin, and 5-10 parts by weight of conductive inorganic compounds. With the exposure of this composition to SHF radiation, because of the high absorption of coefficient of the inorganic compounds, selective heating of the adhesive layer occurs. The inorganic compounds are capable of retaining their high absorption coefficient under heating up to 700-900*C. This property ensures not only rapid heating of the inorganic compounds, but also of the styrene-indene resin to 700-900^βC. The high absorption coefficient of the resin proper and the much greater heat resistance of this resin in comparison with resins generally used for this purpose (such as indene-coumaric resin) also ensures the rapid heating. The use of styrene- indene resin in the composition lowers the viscosity of the composition so as to enhance the wetting of the metal surface and filling of microrelief. The use of the styrene-indene resin contributes to the distribution of the inorganic compounds in the polyethylene component of the composition. Free radicals are formed at 700- 900"C and their amount is much greater than in the case where a known composition, having maximum heating temperature not exceeding 200°C, is used. As a result, the number of adhesive bonds per unit of surface area increases thereby enhancing the adhesion and moisture resistance of the coatings. Achieving these high temperatures in the adhesive layer, apart from increasing the number of free radicals, creates conditions for generating new mechanisms of adhesive bonding which ensure an unexpectedly high moisture resistance.

The use of more than 10 parts by weight of inorganic compounds and more than 15 parts by weight of styrene- indene resin has been found to impair adhesion because of porosity and the greater water absorption capacity of the adhesive layer. The use of less than 5 parts by weight of inorganic compounds and less than 7 parts by weight of styrene-indene resin in the adhesive composition has been found to result in an increase in the heating time during the exposure to SHF radiation, and therefore, a lower maximum possible heating temperature to ensure that the thermooxidation process will not have time for its destructive effect. Lower heating temperatures and decreased amounts of resin can also materially reduce the number of adhesive bonds and thereby decrease adhesion.

Low-density polyethylene of various grades may be used in the composition according to the invention, and inorganic compounds with conductivity characteristics similar to methods may be used in the form of various carbides or suicides, such as silicon carbide and molybdenum silicide.

Accordingly, the invention allows the SHF radiation to be used for applying a protective coating and ensures a rapid and selective heating of compositions while ensuring a high adhesive capacity. The method according to this invention can also be carried, out in the following manner, during the manufacture of thick-film integrated circuits. Screen printing is used to apply pastes of materials onto an integrated circuit support board. The pastes of materials correspond to preset passive elements such as resistors, conductors, insulators, and capacitance elements. Materials with different absorption coefficients for SHF radiation are chosen for each type of element. The support board is made of glass-reinforced plastic, a material poorly absorbing SHF radiation. The main point of the process is selective heating of the paste deposited on the surface of glass-reinforced plastic. By selecting the sizes of the copper of other types of pastes, powder particles and the radiation wavelength, it is possible to achieve the excess of the paste absorbing capacity over the substrate material , capacity by more than an order of magnitude. This, in its turn, allowed selective heating of the paste up to 240^βC. Here, the temperature of the glass-reinforced plastic did not exceed 60-80*C. The technology is fast and continuous and provides the required properties of the conductors.

The support with the pastes applied thereto is exposed to millimetric-wave SHF radiation. The radiation acts upon the pastes and the support materials and is partially absorbed by them causing heating; > The amount of absorbed SHF radiation energy per unit of time mainly depends on coefficients of absorption of the materials of the pastes and support. Millimetric-wave SHF radiation is very effectively absorbed by nonmetals, and this absorption occurs substantially simultaneously within the whole body of the material exposed to the radiation. When the SHF radiation is applied, the heating prevails over the heat removal processes associated with heat conductance.

The choice of pastes having different coefficients of absorption of SHF radiation and the use of a support made of a material which does not absorb this radiation ensures that the pastes are heated to preset temperatures, with the support remaining practically unheated. The number of pastes having different coefficients of absorption may be large enough so that multiplanar circuits can be made in a single heat treatment cycle. This might be accomplished by using pastes made of materials with different coefficients of absorption for making passive elements of one type disposed in different layers of the circuit. These materials are chosen in such a manner that pastes are heated to a preset temperature simultaneously in all layers during a single radiation exposure cycle. Again, the higher the desired temperature, the greater the coefficient of absorption should be.

Additionally, power losses through absorption in top layers should be taken into account. For this reason the pastes used in the bottom layer of the circuit should have greater absorption coefficients. The choice of pastes having different coefficients of absorption is made by way of experiments.

The support material of the circuit board may be in the form of various insulators having a low coefficient of absorption for millimetric-wave SHF radiation. Such support materials include beryllium, silicon oxide, boron nitride, fiberglass plastic, and the like, as well as metals which poorly absorb SHF radiation because of a high reflectivity (over 99.9%). When metal supports are used, the speed of the method according to the invention increases because of the repeated passage of SHF radiation, reflected from the support, through the pastes.

Coefficients of absorption of pastes may be calculated by using reference data, or they can be determined by way of experiments. The use of gyrotron radiation allows the speed of the process to be increased and lowers power requirements in carrying out the method according to the invention because of the three-dimensional concentration of the radiation and the greater power efficiency of the generator.

The method according to the invention makes use of the millimetric-wave radiation which is most efficient for ensuring selective heating of the paste materials. If a radiation with a greater wavelength is used, the absolute values of the absorption coefficients of the paste materials will be much lower and the process of energy absorption (heating) will be comparable, in term of duration, with heat conductance processes. If a shorter radiation wavelength is used, e.g., with a laser radiation, it would be substantially impossible to ensure a significant difference between coefficients of absorption of pastes and the support material.

Pastes of semiconductor materials, which are most widely used, frequently contain a substantial amount of a metal powder having a high coefficient of reflection of SHF radiation. When applied, the thickness of such pastes is much smaller than the radiation wavelength. For this reason they are characterized by a comparatively low efficiency in absorbing of the SHF radiation. This makes it necessary to prolong the "burn-in" time for the pastes which in turn causes heating of the support through heat conductance occurring during this period. In certain applications this can bring about the destruction of support boards made of polymer-based materials. To increase the choice of support board materials and to make it possible to use support boards which are cheaper than ceramic support boards, such as support boards made with polymeric fillers having comparatively low temperatures of destruction, it is desirable to reduce burn in time for semiconductor pastes by increasing their heating efficiency. With this aim in mind, a standing wave mode of SHF radiation is generated in the material of the support board, with the peak of power of the radiation at the surface of the board or within the paste material. This can be achieved if the support board thickness is a multiple of one quarter of the wavelength of the SHF radiation in the support material. According to the above, the SHF radiation which has passed through a layer of paste, in which its energy is partly absorbed, and through the support board made of a material poorly absorbing this radiation, is then partly reflected back from the opposing surface of the board, in other words, the support board-air interface, and passed back through the support board material to the paste material. The radiation is then partly absorbed by paste material and is partly reflected back into the support board material at the boundary between the support board and paste. During further propagation of this radiation it is again reflected from the support board-air interface and is further partly absorbed in the paste material, and so on.

Since the support board thickness is chosen so as to be a multiple of one quarter of the wavelength of the SHF radiation, a standing wave mode is generated in the support board during continuous exposure to the radiation with its power peak being on top of the support, i.e., within the paste material. As a result, the paste heating efficiency is substantially increased so that process time materially decreases because of the reduced time required for paste heat treating. The reduced paste heat treatment time also lowers the degree of heating occurring in the support board because of heat conductance from the paste. This further enlarges the choice of paste materials facilitating the above mentioned goal. The resonance thickness of a support board is determined as follows:

where: λ is the wavelength of SHF radiation in the air; ∑_j is the relative permeability of the support board material as determined by experimentation or from reference data; and K is a coefficient equal to 1, 3, 5, etc.

If a screen having a surface which reflects SHF radiation is placed on the support board at the support board-air interface, that side opposite to the side exposed to SHF radiation, paste heating is efficiency is increased. If there is a reflecting surface, e.g., a metal surface, reflection of SHF radiation will be complete, and energy of standing wave increases. If the thickness of the support board is set up in advance and/or cannot be equal to a resonance thickness as determined by the Equation immediately above, an air gap or a gap filled with an insulator can be provided between its lower boundary and the reflecting surface of the screen. The gap thickness is determined as follows: λ. ∑i

'2 wherein ∑2 is the relative permitivity of the gap material. If the support board thickness is not equal to the resonance thickness, the standing wave mode is not created in the support board, and the peak of SHF radiation power is not repeatedly passed at the top boundary of the support, to which the paste is applied, so that efficiency of the paste heating will be much lower. In making a choice of the support board thickness multiple difference than one-quarter of the wavelength, a minimum of SHF radiation power will be absorbed at the top boundary of the support to which the paste is applied, and the paste will not be practically heated.

The results of the researches conducted are equally suitable for creating powders of inorganic materials having high conductivity. Examples are carbides. One process developed is the heating of silicon or boron carbide in nitrogen-oxidation environment by the gyrotron radiation. At the temperature of 1400 - 1600"C carbon is oxidized and remaining boron or silicon form nitride. The peculiar feature of this process is that as soon as the nitride particle is formed, its heating automatically stops, because tangent of the loss angle of nitride in the millimetric range is by two orders of magnitude smaller than that of carbide. As a result, the thermal process will last until transformation of all carbide powders into nitride ones is completed.

Examples in support of carrying out the method according to the invention are set out below.

EXAMPLE 1 This example involves applying nonmetal coatings to ceramic parts. Gyrotron radiation at 83 GHz with a radiation power of up to 10 kW was used in carrying out the method.

A sample part made of corundum refractories containing 97% of AI O3 were used. Refractory samples were of a size 60x60x30 mm with an open porosity of 12 to 14%. Coating materials were in the form of powders of Zr0₂ and a mixture of MgO and AI2O3 in a ratio of 60:40 % by mass. A coating powder was applied to the working surface of the refractory sample of a size 60x60 mm with a layer thickness of 1.7 mm for Zrθ2 and 3.9 mm for the mixture.

Tests 3, 4 were carried out with a metal reflecting plate and tests 1, 2 were carried out without such plate. A coating was deposited to the samples by plasma spraying (tests 5, 6) for the sake of comparison. Thickness of the contact zone of the refractory was determined in slices by using a microscope. The results are given in Table 1. TABLE 1

It can be seen from Table 1 that when the method of coating application according to the invention is used, the present coatings outperform plasma-applied coatings in all aspects. Also, the use of a metal reflecting plate allows power requirements to be reduced and production output to be increased.

The petrographic analysis of samples with coatings applied in accordance with the method of the invention showed that a transition from the composition of the refractory to the surface layer of the coating occurs gradually through solid solutions with a corresponding change in the coefficient of thermal stress. With plasma deposition, there is a clear-cut interface between refractory and coatingJ The significant difference between thermal expansion coefficients of the refractory and coating in such a case may bring about destruction of the coating under sudden temperature fluctuations.

The above-described method for application of a coating is characterized by high adhesion of the coating and high heat resistance, both attributed to the formation of a broad contact zone of fusion because of the primary melting of the part surface and deep heating of the part material as a whole. The high production output of the process and the low material usage are ensured because of the high efficiency of the interaction between millimetric- wave SHF radiation and ceramic materials and the absence of a gas flow. The use of the method according to the invention for applying coatings to ceramic and refractory parts allows for operating properties of parts to be improved, especially under sudden temperature fluctuations, and the specific cost of a coating can be reduced. A substantial improvement of coating quality materially enhances operating properties of parts being coated. EXAMPLE 2

This Example illustrates the application of polymer- based coatings to the surface of a part. A double-layer tape was prepared which consisted of a polyethylene base and an adhesive layer. The components of the adhesive layer, styrene-indene resin and silicon carbide (SiC) or molybdenum silicide (MoSi ) , and the polyethylene were mixed between rolls at 100 to 200"C during 5 to 15 minutes, with subsequent calendaring of the composition at a roll temperature of 50 to 60°C. A double-layer tape was also prepared by coextrusion of the base and adhesive on a coextrusion line. After being applied to the part, the resulting film was exposed to SHF radiation from a gyrotron. Adhesion quality data is given in Table 2.

TABLE 2

It can be seen from Table 2 that adhesion and moisture resistance are at maximum with the following proportioning of components: 88 to 75 parts by weight of polyethylene, 5 to 10 parts by weight of inorganic compounds, 7 to 15 parts by weight of styrene-indene resin (tests 1 through 5) . A minor decrease or increase in the amount of inorganic compounds and styrene-indene resin results in a lower adhesion (tests 6 through 9) . With a further increase in the amount of absorbing additives the coating looses its protective properties (tests 10 and 11) .

The coating formation time was a maximum of 10 seconds. By comparison, this is 100 times shorter than that achieved with the prior art adhesive compositions.

Therefore, the method according to the invention ensures an enhanced quality of coating because of a differentiated and rapid heating of the adhesive layer, makes possible the addition of new and efficient modifying additives, while retaining the protective properties of the base of the coating. Production output of the process is increased and power consumption is reduced because of the selective heating of only the adhesive layer of the coating, as compared to the prior art where heating of the adhesive layer occurs through a heat conductance mechanism with energy being transfer from a heated metal part.

EXAMPLE 3 This Example illustrates the present invention in use during the manufacture of thick film integrated circuits. In making test boards, SHF radiation from a gyrotron was used at 83' GHz with a power of up to 30 kW. Pastes were applied by screen printing to support boards, made of ceramic (with a coefficient of absorption of SHF radiation 0.1 X 10^'1 cm^"1) and steel (having a reflectivity of 99.9%) . Biplanar test circuit boards were also made. For applying an insulator layer to a steel support, an insulator paste with a coefficient of absorption of 1.1 X 10"¹ cm^"1 was used. For conductors of the first level in both circuit boards, a paste having a coefficient of absorption of 3.0 X lO^cnf¹ was used. The interlayer insulation was made out of a paste with a coefficient of absorption of 0.8 X 10^" ¹cm . The conductors of the second level were made of a

1 ι paste having a coefficient of absorption of 2.1 X 10^"x cm^" .

Resistors of the second level were made of a paste having a coefficient of absorption of 1.8 X 10^"1cm^"1.

The choice of material of the pastes and supports with the above-given coefficients of absorption of the radiation ensures their heating to a preset temperature (the higher the necessary temperature, the greater the coefficient of absorption) . Additionally, the pastes for the lower levels were chosen to have a greater coefficient of absorption so as to take into account power losses through absorption of energy by pastes of the upper or top layers. The choice of pastes with different coefficient of absorption was made by way of experimentations.

The support boards, with the applied pastes, were exposed to a SHF radiation focused into a strip. The integrated circuits themselves were moved with respect to the radiation strip so as to follow the strip. At the same time, similar circuits were constructed by a conventional method (oven heating) . A comparison of the circuits made according to this invention with those of the prior art method, showed that circuits made according to the invention did not exhibit support board deformation while the breakthrough voltage of insulating layer was 1.6 times higher than conventional boards. Also, the conductors were not oxidized and scatter of resistor values was 1.3 times as low. Furthermore, the time required for making the circuits according to this invention was 100 times shorter than prior methods.

EXAMPLE 4 In this Example of thick film integrated circuit manufacturing, a radiation generator in the form of a gyrotron at 83 GHz (3.6 mm) and 37.5 GHz (8mm) was used. A copper-based conductor paste was used, having a coefficient of absorption of 6.3 X 10 cm^" , with a heat treatment temperature of 190^βC. The paste was applied by screen printing to a support of a fiberglass textolite having α = 2.67 (at a frequency of 3.7 X 10¹⁰ to 8.3 X 10¹¹ Hz) . The support boards with the applied pastes were exposed to radiation focused into a strip 300 mm wide. The circuits were moved with respect to strip of radiation at a constant speed. Circuits were also made by the conventional prior art method.

The test results are given in Tables 3A and 3B below. TABLE 3A

TABLE 3B

It can be seen from Tables 3A and 3B that the choice of thicknesses in multiples of one quarter of the wavelength, in the support board material ensures the desired characteristics of printed circuit boards (tests 1 through 5) . The output of the process is 12 sq. m/h. The absence of resonance conditions (as seen in tests 6, 7 and 9) required a decrease in the speed of printed circuit board movement (and therefore longer exposure times) which caused impaired electrical properties of the support boards. With the same speed (test 8) as in tests 1-5, the paste did not have time to harden. Therefore, the method according to this example ensures increased production due to efficient heating of the paste material, as well as the possibility of carrying out the heat treatment of circuit boards in a single cycle, including multiplanar circuit boards. The quality of circuit boards produced using the present invention is enhanced because of the prevention of paste and support materials e.from diffusing, as well as the absence of oxidation of the conductor pastes and the deformation of supports. This is a result of a shorter heat treatment cycle and minimized support board heating. In the present invention, costs are further reduced since oxidation can be avoided without the need for special shielding atmospheres such as nitrogen. The invention also provides the possibility of using metal supports to facilitate the manufacture of circuit boards and thereby making them less expensive. While circuit boards may be made of various types, sizes and configurations, the advantages of the above method become much more pronounced in making ultiplanar circuits.

While the above description constitutes the preferred embodiments of the present invention, it will be appreciated that the invention is susceptible of modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.

Claims

IN THE CLAIMS

1. A method for applying nonmetal coatings to a surface of a part, comprising the steps of: applying to the surface of a part at least one coating layer for which absorbed energy of a millimetric wave SHF radiation is different from energy of the same SHF radiation absorbed by the part surface and; simultaneously exposing the part surface and the coating layer to SHF radiation causing them to reach different temperatures of a magnitude whereby adhesion between the part surface and the coating layer is provided.

2. The method of Claim 1, wherein a ratio of energies absorbed by the coating layer and the part surface is preset.

3. The method of Claim 2, wherein the ratio of energies absorbed by the part surface, and the coating layer is preset by controlling the thickness of the part surface or the coating layer.

4. The method of Claim 3, wherein said thickness of said coating layer is chosen in accordance with the following formula:

wherein a_j and a2 are the coefficients of absorption of SHF radiation by the coating material and by the part material, respectively; and T_j and T2 are the melting points of the coating material and the part material, respectively.

5. The method of Claim 2, wherein the ratio of energies absorbed by the part surface and the coating layer is preset by using an additive in the coating layer.

6. The method of Claim 2, wherein the ratio of energies absorbed by the part surface and the coating layer is preset based on the choice of materials of the part surface and coating layer.

7. The method of Claim 1, wherein during said exposing step a gyrotron is used as a source of the SHF radiation.

8. The method of Claim 1, wherein said coating layer comprises a polymeric base and an adhesive layer facing towards said part surface containing additives absorbing the SHF radiation, the additives being of a material having an absorption coefficient which is more than the absorption coefficient of the materials of said polymeric base and the part surface.

9. The method of Claim 8, wherein said material of said additive is in the form of inorganic compounds having metal-type conductivity.

10. The method according to Claim 9, wherein said inorganic compound comprises silicon carbide.

11. The method according to Claim 9, wherein said inorganic compound comprises molybdenum silicide.

12. The method of Claim 8, wherein the material of said additive is in the form of inorganic compounds having metal-type conductivity and styrene-indene resin.

13. The method of Claim 8, wherein the adhesive layer is made of a composition comprising polyethylene, styrene- indene resin and inorganic compounds with metal-type conductivity, with the following proportioning of components in parts by weight: polyethylene 75-88 styrene-indene resin 7-15 inorganic compounds with metal-type conductivity 5-10.

14. The method of Claim 1 wherein said coating layer is applied to a part surface in the form of a support of a thick-film integrated circuit, the coating layer comprising pastes of materials having different coefficients of absorption of said SHF radiation corresponding to different types of passive elements of a thick-film integrated circuit after heating and wherein the support is made of a material poorly absorbing the SHF radiation.

15. The method of Claim 14, wherein, there are a plurality of layers of pastes for passive elements of one and the same type disposed in different layers insulated from one another, with pastes of materials used which have different coefficients of absorption of said SHF radiation and which are chosen in such a manner as to ensure simultaneous heating of the pastes in all layers to a preset temperature in a single cycle of exposure to said radiation.

16. The method of Claim 14, wherein the thickness of the support is chosen to be a multiple of an odd number of quarters of the wavelength of said SHF radiation.

17. The method of Claim 14, wherein a screen having a surface reflecting said SHF radiation is placed behind the support on the side opposite to the side exposed to said SHF radiation.

18. A method of Claim 14, wherein an insulating layer is provided between said support and said surface of said screen reflecting SHF radiation, the thickness of the layer being determined as follows:

λis the wavelength of SHF radiation in the air; ∑_j is relative permeability of the support material determined by way of experiments or from reference data; and

K is the odd number coefficient equal to 1, 3, 5, etc.

19. A method for applying coatings to a surface of a part, comprising the steps of: applying to the surface of a part at least one coating layer for which absorbed energy of a millimetric wave SHF radiation is different from energy of the same SHF radiation absorbed by the part surface, the coating layer having fine dispersed particles therein, said particles increasing the absorption capacity of the coating over that of the coating without said particles; simultaneously exposing the part surface and the coating layer with said particles to said SHF radiation causing them to reach different temperatures of a magnitude whereby adhesion between the part surface and the coating layer is provided.

20. The method of Claim 19 wherein said particles are a powder.

21. The method of Claim 19 wherein said particles are metal powders.

22. The method of Claim 19 wherein said particles are powders of inorganic compounds.

23. The method of Claim 19 wherein said particles are powders of an electrically conductive inorganic compounds.

24. The method of Claim 19 wherein said particles are of silicon carbide.

25. The method of Claim 19 wherein said particles are of molybdenum silicide.

26. A method for applying coatings to a surface of a part, comprising the steps of: applying to the surface of a part at least one coating layer for which absorbed energy of a millimetric wave SHF radiation is different from energy of the same SHF radiation absorbed by the part surface and; simultaneously exposing the part surface and the coating layer to SHF radiation; causing a surface of the part between the part and the coating layer to melt said SHF radiation reflecting from the melted surface of the part back into the coating layer causing the coating layer to be further heated to a temperature whereby adhesion between the part and the coating layer is provided.

27. The method of Claim 26 wherein the coating layer is ceramic.

28. The method of Claim 26 wherein the part is refractory material.