METHOD OF ANNEALING AN APERTURE SHADOW MASK FOR A COLOUR CATHODE RAY TUBE

Abstract: An aperture mask comprised of a nickel-iron alloy having a low thermal expansion for a colour CRT is produced by combining a first annealing to produce a low coercive force and mechanical properties desirable for a warm forming operation during which a desired profile of the mask is produced with minimized springback and a second annealing after forming to provide a stress free low coercive force mask also having an oxidized high thermal emissivity surface. The process requires control of the atmospheres and oxidizing potentials in the two annealing steps.

Description: This invention relates to a method of manufacturing an aperture mask for a colour cathode ray tube and more particularly to a method of producing an aperture mask comprised of an iron-nickel alloy and as incident to forming the blank with a desired final contour an annealing process is carried out to produce a tightly adhered black oxide coating on the surface of the metal while at the same time relieving forming stresses and restoring to a low value the coercive force properties of the alloy metal. U.S. Patents 4,210,843; 4,427,396; 4,609,412; and 4,536,226 disclose various processes for the manufacture of aperture masks for a colour cathode ray picture tube. In the manufacture of an aperture mask for a colour cathode ray tube, one material commonly selected for the aperture mask is a low carbon steel. Typically a 1008 grade of aluminum killed or rimmed carbon steel is used as the mask material. Recent requirements for higher image quality have brought about the desire to utilize an iron-nickel alloy, such as Invar, because of a low thermal expansion characteristic of this material which reduces the effect commonly known in the industry as doming. Doming is a deflection of the aperture mask in a direction toward the phosphor screen due to a thermal input produced by electron beams impinging on the mask material. Thermal compensating clips are usually provided to adjust the spacing between the mask and the phosphor screen according to the thermal input. Such clips, however, are not adequate to compensate for non-uniform heating of the mask material as, for example, when a limited area of the mask receives a greater thermal input than other areas of the mask. For example, such a limited area can be a white area on the screen displaying an image representing the playing surface of a hockey game. As is well known, a white area displayed by a colour CRT tube is produced by an excitation of all three colour phosphor deposits by impingement of all three electron beams through an aperture opening in the mask. When this occurs all three electron beams also impinge on the mask material immediately adjacent to the aperture opening. U.S. Patent Nos. 2,806,162 and 4,528,246 both disclose benefits gained through the use of an Invar type nickel-iron alloy material in the manufacture of an aperture mask for a colour CRT. In U.S. Patent No. 4,536,226 there is disclosed a method of manufacturing a shadow mask wherein a sheet of nickel-iron alloy such as Invar is treated by perforating a number of apertures after which the apertured sheet is annealed at a temperature of between 900 and 1200°C (1652 degrees F and 2192 degrees F) for a period of ten minutes in vacuum. The annealed sheet is pressed to form a shadow mask while the sheet is kept at a forming temperature of about 182°C (360 degrees F). The elevated temperature of the shadow mask during the forming operation is effective to reduce the yield strength of the alloy material. NL 8,600,141 discloses a method for manufacturing an apertured mask consisting of a nickel-iron alloy, including the steps of first annealing in a reducing atmosphere, warm forming the mask at a temperature of between 150°C and 250°C and finally annealing in an oxidising atmosphere. It has been discovered that by using the high temperature anneal and the warm forming of the shadow mask, it is possible to at least theoretically reduce or eliminate spring back when the mask blank is formed into an aperture mask. The present invention provides a method for processing an apertured mask said method including the steps of: selecting a nickel-iron alloy mask blank having apertures therein; a first annealing of the mask at a temperature within the range of 900 to 1200°C (1652 to 2192 degrees F) to provide a D.C. coercive force of less than 79.6 Amperes per metre (1.0 Oersted) as measured from 1 Tesla (10 kG) and coincidentally to provide a material having low springback characteristics; warm forming the mask blank to a desired curved profile for the aperture mask; and second annealing the formed aperture mask at a temperature of at least 788°C (1450 degrees F) in a controlled oxidizing atmosphere for a period of time sufficient to produce a tightly adhered black oxide coating on the surface of the metal comprising the aperture mask, and to remove strain from the forming operation. By the method of the invention an apertured mask is produced comprised of an iron-nickel alloy material such as that known as Invar by utilizing a low thermal expansion characteristic of this material to limit localized doming of an aperture mask made therefrom. It is a second characteristic of the method of the invention to produce an aperture mask the material of which has a D.C. magnetic coercive force of lower than 79.6 Amperes per metre (1.0 Oersted) as measured from 1 Tesla (10 kilogauss) (kG) to render the CRT tube insensitive to deviations of flux in the Earth's magnetic field and to stray magnetic fields generated in the normal environment during operation of the display tube or colour television. A third characteristic of the method of the invention is to produce a blackened surface on the mask material to an extent sufficient to at least provide high thermal emissivity to thereby lower the potential for doming as well as improve the functioning of the article as a shadow mask in conjunction with returning the material to the state of low coercive force. The present invention provides a solution for a requirement for an aperture mask namely that, through proper heat treatment, the mask blanks can be easily formed into aperture masks while providing a low mechanical spring back characteristic, and have the advantageous low D.C. coercive force. The present invention thus provides a method for manufacturing apertured masks for colour CRT tubes which achieves attributes of the mask of low thermal expansion, low mechanical spring back, low D.C. coercive force, and high thermal emissivity. The present invention will be more fully understood when the following description is read in light of the drawing which is a flow chart diagram of a process for producing an apertured mask according to the present invention. In the present invention a shadow mask material comprises a nickel-iron alloy such as Invar having a composition in weight percent of carbon from 0.050 to 0.120; manganese from 0.4 to 0.7; phosphorus of 0.03 maximum; sulfur of 0.03 maximum; silicon from 0.10 to 0.30; and nickel from 35.0 to 42.0; the remainder being principally iron. This mask material has different mechanical properties than the currently employed 1008 grade aluminum killed or rimmed carbon steel. In particular, the yield strength of the nickel-iron alloy is higher and Young's modulus is lower than the carbon steel. As a result, the spring back ratio is larger for the nickel-iron alloy than for the carbon steel. Accordingly when forming the apertured mask, problems are encountered with spring back at the corners of the formed mask. It is also desirable to have an apertured mask with a blackened surface to provide a high thermal emissivity to dissipate heat generated when the material is bombarded by electrons in the picture or display tube. At the same time, the apertured mask must exhibit a D.C. coercive force of lower than 79.6 Amperes per metre (1.0 Oersted) as measured from 1 Tesla (10 kilogauss) to render the entire picture tube insensitive to deviations of flux in the earth's magnetic field as well as to stray magnetic fields generated near the working environment of the picture tube. The low coercive force enables effective demagnetization by degaussing coils built into the CRT. In accordance with the present invention the flow chart identifies an initial step of selecting a mask blank having apertures therein. A strip of nickel-iron alloy is selected having a desired thickness and width from which aperture blanks are taken in a manner per se well known in the art. Before blanks are taken from the strip, the strip is processed in a manner well known in the art to form apertured openings by, for example a photo resist process. Examples of such a photo resist process to provide patterns of apertures in metal stock material can be found in U.S. patents 4,427,396 and 4,210,843. The flat mask blanks are then heat treated by first annealing the flat mask blanks to a temperature of at least 900°C (1652 degrees F) and preferably not higher than 1200°C (2192 degrees F) for a period of at least 5 minutes at temperature in a reducing gas or gas mixture to impart desired tensile properties to the nickel-iron alloy whereby the yield strength of the material is reduced, and coincidentally to provide a D.C. coercive force of less than 79.6 Amperes per metre (1.0 Oersted) as measured from 1 Tesla (10 kilogauss) as a consequence of proper selection of the annealing atmosphere, time and temperature. The reducing atmosphere during the annealing process of the flat apertured masks can be hydrogen, nitrogen or combinations of these gases, having a dew point sufficiently low to prevent oxidation of the iron or nickel contents of the Invar alloy. The temperature at which the annealing process is carried out can be as low as 900°C (1652 degrees F) and up to 1200°C (2192 degrees F). The soaking time during annealing may be as long as 4 hours where desired, however, annealing times such that the mask material is within the above temperature range for as short as 5 minutes is sufficient. However, the longer time at temperature and the higher the temperature the better the D.C. coercive force property attained. The annealed mask blanks are then formed to a desired curvature having the same form as the glass picture tube screen containing the phosphor dots. The forming operation is preferably carried out at a temperature of about 93°C (200 degrees F) during which the blanks are elastically deformed to the desired curvature. This step of the present invention provides a process which eliminates the spring back problem heretofor associated with the warm forming operation as well as providing an aperture mask having a low thermal expansion coefficient as well known in the art. According to the present invention shown as in the flow diagram the formed masks are given a second annealing treatment in a manner to combine the functions of stress relief annealing and blackening wherein the masks are heat treated by annealing at a temperature of at least 788°C (1450 degrees F) in an atmosphere having a controlled oxidizing potential for a period of time sufficient to form an adhered black oxide coating which is an oxide of the nickel-iron alloy material. The total time for the annealing process can be as short as 90 seconds but longer times can be employed as desired. Some alteration of the time - oxidizing potential relationships will be required for longer anneal times to prevent excessive oxidation. Normally this anneal can be completed in any inert or reducing gas such as nitrogen, argon, or hydrogen, or a mixture thereof, to which a controlled amount of moisture is added. The inert gas atmosphere, 100% nitrogen, to which moisture is added to achieve a dew point of 32°C (+90 degrees F), provides an adequate oxidizing potential at 788°C (1450 degrees F) to achieve the correct blackening oxidation in 90 seconds. In this step of the present invention, the process provides the aperture mask having a blackened surface of high thermal emissivity, and at the same time relieves the strains imparted by the forming operation, which is required to restore the coercive force of the mask to less than 79.6 Amperes per metre (1.0 Oersted). The oxidizing atmosphere employed in the stress relief annealing of the contoured aperture masks to achieve blackening and restoration of the coercive force to low values is preferably nitrogen, argon, or combinations of nitrogen and hydrogen to which a controlled moisture content is added. To achieve the object of the current invention the soaking time during annealing of the contoured mask can be controlled to achieve both stress relief and blackening. This annealing time can be as long or short as desired as long as the tight black oxide is formed on the surface of the contoured aperture mask. The black tightly adhering oxide desired on the metal surface is preferably less than 15.10 -8m (1500 angstroms) thick. Development of thicker oxides will cause spalling and loss of adhesion. The black oxide coating on the metal is tightly adhered and provides a thermal emissivity that is higher than a bright annealed surface. Also, the coercive forces of metal samples annealed in the manner of the present invention still have values lower than 79.6 Amperes per metre (1.0 Oersted). In the following table there is set forth magnetic coercive forces of samples annealed under various conditions for the first annealing step. Sample Annealing Atmosphere Annealing Soak Temp. Dew Pt. Annealing Soak Time D.C. Coercive Force From 10 kG (1T) A1 100% H₂ 2150 F (1177°C) -60 F (-33°C) 4 hours 0.0594 Oe (4.7269Am⁻¹) A2 100% H₂ 1850 F (1010°C) -60 F 1 hour 0.0988 Oe (7.8623Am⁻¹) A3 100% H₂ 1650 F (899°C) -60 F 1 hour 0.298 Oe (23.7141Am⁻¹) A4 80% N₂-20% H₂ 1850 F (1010°C) -60 F 1 hour 0.149 Oe (11.8570Am⁻¹) A5 80% N₂-20% H₂ 1650 F (899°C) -60 F 1 hour 0.343 Oe (27.2951Am⁻¹) A6 100% N₂ 1475 F (802°C) +90 F (32°C) 1 hour 5.35 Oe (425.7395Am⁻¹) A7 100% N₂ 1650 F (899°C) -30 F (-17°C) 5 min 0.4192 Oe (33.3589Am⁻¹) Further, the second annealing superimposed on the above sample A1 produced a coercive force of 5.2282Am⁻¹ (0.0657 Oe) after an anneal of 90 seconds in a 100% nitrogen atmosphere with a dew point of 32°C (+90 degrees F). Sample A5 was also processed through the second anneal in the 100% nitrogen 32°C (+90 degree F) dew point for 90 seconds. After this second annealing sample A6 exhibited a coercive force of 29.5232 Am⁻¹ (0.371 Oe). Both examples show that the coercive force is not deleteriously affected by the oxidizing anneal which produces the blackened surface. Preferably the reducing atmosphere maintained during the first annealing process may comprise 100% hydrogen or nitrogen or combinations of the two gasses. The annealing dew points should be maintained at a low value to prevent internal oxidation of the constituents of the nickel-iron alloy. In the case of sample A6 the oxidizing potential was excessive in the first annealing step producing an unacceptable value for the coercive force. It has been discovered, according to the present invention that both low initial tensile strengths as well as low coercive forces are obtained by the same annealing treatment of the mask blanks. It has been found that the result in the coercive forces are well below, as shown in Table I, the desirable 79.6 Am⁻¹ (1.0 Oersted) maximum obtained by low carbon steel in this application. Moreover, it has been found that by proper selection of the second annealing conditions the dual result desired of having a restored low coercive force and a blackened high thermal emissivity surface are achieved by the present invention.

Claims: A method for processing an apertured mask said method including the steps of: selecting a nickel-iron alloy mask blank having apertures therein; a first annealing of the mask at a temperature within the range of 900 to 1200°C (1652 to 2192 degrees F) to provide a D.C. coercive force of less than 79.6 Am⁻¹ (1.0 Oersted) as measured from 1T (10 kG) and coincidentally to provide a material having low springback characteristics; warm forming the mask blank to a desired curved profile for the aperture mask; and second annealing the formed aperture mask at a temperature of at least 788°C (1450 degrees F) in a controlled oxidizing atmosphere for a period of time sufficient to produce a tightly adhered black oxide coating on the surface of the metal comprising the aperture mask, and to remove strain from the forming operation. A method according to claim 1, wherein said first annealing is carried out for a period of greater than 5 minutes. A method according to claim 1 or 2, wherein said first annealing is carried out in a reducing atmosphere with a dew point sufficiently low to prevent internal oxidation of the nickel-iron alloy. A method according to any one of the preceding claims, wherein said warm forming the mask blank is carried out at a temperature of at least 93°C (200 degrees F). A method according to claim 3 or 4, wherein said reducing atmosphere for the first anneal comprises hydrogen, nitrogen, or hydrogen-nitrogen mixtures. A method according to any one of the preceding claims, wherein said oxidizing atmosphere for the second annealing comprises nitrogen, argon or hydrogen with controlled amounts of moisture as an oxidizing agent. A method according to any one of the preceding claims, wherein said oxidizing atmosphere for the second annealing comprises 100% nitrogen with a dew point of 32°C (+90 degrees F), and where the time of the annealing is 90 seconds.

Inventors: GRAY, THOMAS H.
Applicants: ALLEGHENY LUDLUM CORPORATION
Application year: 1988
Main classification: H01J9
IPC codes: H01J9, C21D9, C21D8, H01F1, H01J29, C21D6, C23C8