IMPROVED MOULDING TOOL AND METHOD

Abstract: A coating and method of depositing it is provided. The wear resistant coating comprises a thin layer of disordered boron and carbon applied to a substrate. The coating has good adherence and provides good resistance to wear so that it is useful as a tool coating. In a more specific embodiment, the coating is disordered boron carbide. The coating can be made by sputtering boron and carbon onto a substrate. A bias voltage may be applied to the substrate during sputtering. The disordered boron and carbon coating can have a com­ position B x C 1-x where x is from about 0.60 to about 0.90. An improved molding tool (10) is provided that includes a disordered carbon coating on at least a portion of the working surface of the molding tool (10). The coating provides im­ proved release characteristics, longer tool life and results in molded articles having smoother finishes and improved detail.

Description: This invention relates to a mold and to a method of molding an article by using said mold. In the past, tools and articles that are subject to wear have been fabricated to achieve various hardness, lubricity and wear characteristics by controlling certain parameters. For example, tools for working and shaping unhardened steels may be fabricated from steel containing enough carbon to form very hard martensite. In more complicated compositions, varying the carbon content and alloy content makes possible non-deforming steels, shock-resistant steels, hot-work steels, or high-speed steels. In some of these steels, alloying elements such as titanium, vanadium, molybdenum, tungsten and chromium are used. These are elements which have a great affinity for carbon and form hard, wear-resistant metallic carbides. However, in many cases, it is desirable to provide a tool having a coating on the surface thereof to improve the hardness and/or lubricity of the tool. This is especially the case where it is desired to lengthen the tool life or increase productivity, or where it is necessary to shape and work hardened steel, for example. However, many types of wear resistant coatings require high temperatures for application, thereby making them impractical for use on many types of substrate materials, since the properties of the substrate may change signifcantly under such temperatures. Other types of coatings do not adhere sufficiently to the substrate under working conditions. Thus, a need exists for a wear resistant coating that can be applied at relatively low temperature to avoid significant change of substrate properties. A need exists for a wear-resistant coating for molds to provide increased life. A need also exists for a wear-resistant coating that has improved adhesion properties and resistance to fracture. In a more specific application, it would be desirable to provide an improved mold tool to lengthen mold tool life, improve release characteristics, and improve the surface finish and detail of molded items. Boron-carbon coatings of substrates are described in Patents Abstracts of Japan (Kokai 54-15481) and Swiss patent application CH-A-534744. European patent application EP-A-0075316 describes metal/non metal coatings, which, in the context, are described as disordered. The present invention provides a mold as defined in claim 1 and a method of molding an article as defined in claim 5. The coating comprised in the mold according to the present invention, a method of coating a substrate and a machine tool comprising a coating having the same features as that comprised in the mold of the present invention are claimed in the copending European application 84306467.6 (EP-A-0136161). The mold according to the invention comprises a coating that exhibits excellent resistance to wear. As used herein, resistance to wear includes resistance to erosion, oxidation, corrosion, wear from frictional contact and similar phenomena. In accordance with the invention molds which are subject to wear, such as a result of contact with other surfaces, are coated with the disordered boron and carbon material to increase the useful life of the tool or article. The coating can be disordered boron carbide or a mixture of disordered boron and carbon. The coating is formed as a thin layer on at least a portion of the working surface of a mold and preferably comprises boron carbide. The mold can include a coating or coatings other than a coating of the invention. Molds coated in accordance with the invention utilizing disordered boron and carbon generally have excellent hardness and lubricity characteristics which result in increased lifetimes. The disordered coating can be amorphous, polycrystalline (and lacking long range order), microcrystalline or a mixture of any combination of those phases. By the term "amorphous" is meant a material which has long range disorder, although it may have short or intermediate order or even contain at times more crystalline inclusions. The composition of the coating is: B x C 1-x where "B" represents boron, "C" represents carbon and "x" and "1-x" represent the relative amount of boron and carbon respectively, present in the coating, "x" being from about 0.60 to a about 0.90. Most preferably, the coating is disordered boron carbide (B₄C). Thus, included in accordance with the present invention are coatings which are non-stoichiometric as well as those that are. The coating comprised in the mold of the present invention is disordered when formed. It is believed that a disordered wear resistant coating performs better than a single phase crystalline coating. A disordered coating may be more susceptible than a single phase crystalline coating to diffusive bonding between the substrate surface and the coating, resulting in better adherence. Disordered materials also lack extended lattice planes through which fractures can propagate and in general can withstand relatively high deformation forces without fracture. Such materials are generally less susceptible to adverse chemical attack, such as corrosion and oxidation, than a single phase crystalline material. It is believed that the foregoing advantages are more fully realized with an amorphous or substantially amorphous coating. In accordance with the invention, a nonstoichiometric disordered wear resistant coating is provided in which the amount of boron and carbon present in the coating composition can be varied to achieve desired characteristics while avoiding the formation of extended lattice planes present in non-disordered materials which could adversely affect the adherence, wear resistance or other properties of the coating. In accordance with the invention the coating is formed by dc magnetron sputtering. Since sputtering can take place at a relatively low substrate temperature (generally about 200°C or less, for example), the coating can be formed while avoiding significant changes in the properties of the substrate material while providing a surface that has increased resistance to wear and excellent lubricity. Accordingly, the invention is particularly useful for coating materials such as tool steel and tungsten carbide, for example, since the processing temperature does not degrade the properties of these materials. Sputtering at low substrate temperature also allows formation of the coating in a disordered state. The coating can be applied to the substrate surface as a continuous thin layer without significantly changing the dimensions of the mold since the thickness of the coating can be relatively thin and can be closely controlled. After a mold, with or without a coating, has been in use, a coating in accordance with the invention can be applied thereto, to achieve a desired tolerance or otherwise replace material that has been worn from the tool. Thus, the invention makes possible the reclamation of molds that would otherwise be discarded. The coating comprised in the molds of the invention can be applied directly over a substrate surface while achieving excellent adherence for use on molds that are subjected to wear or friction, for example. No adherence or layer is required, as the coating of the invention can provide excellent adherence. Preferably, the coating is continuous. The disordered boron and carbon coating of the invention can be further characterized as being extremely inert and stable, with good resistance to degradation as a result of exposure to humidity and heat, for example. It has been discovered that the coating in accordance with the invention provides longer mold life, improved release characteristics and better surface detail and smoother finishes on the resulting molded parts. The coating of the invention can be used in on all types of molds or dies where a moldable or castable material is to be formed. For example, the coating may be used in connection with molds and dies for molding, injection molding and die casting. The terms "mold" and "moldable" are used in a broad sense and are meant to include "die" and "castable". In accordance with a more specific embodiment of this aspect of the invention, a molding tool is provided that includes a disordered coating of boron and carbon as previously described on at least a portion of the working surface of the mold. Generally, the entire working surface will be coated with the disordered coating. Use of such coatings on molding tools have been found to provide significant advantages compared to uncoating molding tools. These advantages include longer mold life, improved release characteristics, and molded articles having greater detail and smoother finishes. In addition, cleaning of the mold after use is facilitated. The smoothness of the substrate surface to which the coating is applied generally has an effect on the gloss of the coating. Usually, providing a smoother surface results in a coating having a higher gloss. Accordingly, where a high gloss coating is desired, the substrate surface should be relatively smooth. Preferably, the coatings essentially contain only carbon and boron, although it is to be understood that minor amounts of impurities will, almost invariably be present. Further, other elements or compounds other than carbon may be present, as long as they do not interfere with the desired color, gloss or wear resistance. The coatings comprised in the molds of the invention can be further characterized in being extremely inert and stable, with good resistance to color change as a result of exposure to ultraviolet radiation, for example. The coatings also exhibit good resistance to chemical attack. Fig. 1 illustrates in sectional view mold tool implements in accordance with the present invention; Fig. 2 illustrates in sectional view a blow mold tool in accordance with the present invention; and Fig. 3 illustrates in perspective view the mold tool implements of Fig. 1. The disordered coating comprised in the mold of the invention is formed by dc magnetron sputtering. The preferred type of sputtering is dc magnetron sputtering with a bias voltage. Sputtering allows the coating to be applied at relatively low temperatures and is less likely to affect the substrate properties than other techniques which require relatively high temperatures. While sputter depositing techniques are generally known to those skilled in the art, to maximize the benefits of the invention, it is advantageous to form the desired coating with a sputtering technique that is adapted to the particular geometry of the surface to be coated. Usually, a dc or rf bias is applied to the substrate during application of the coating by sputtering. The bias may improve adhesion of the coating formed on the substrate, reduce stress in the coating and increase the density of the coating. The substrate geometry in part determines the most desirable sputtering technique for a particular application. Prior to sputter depositing, it is important to provide an atomically clean surface on the portion of the substrate surface that is to be coated. This facilitates the formation of a uniform coating which adheres to the substrate surface. There are several methods known to those skilled in the art for providing an atomically clean surface for sputtering and any such method may be utilized. The following surface preparation method is provided by way of example only and is not to be construed as a limitation upon the present invention. In accordance with one method for providing an atomically clean substrate surface, the substrate is degreased with a chlorinated hydrocarbon degreaser. Thereafter, the substrate is rinsed in methanol and is then subjected to either plasma or dry chemical etching. When plasma etching is utilized, preferably a fluorinated carrier gas, such as carbon tetrafluoride is utilized The carrier gas decomposes and provides fluorine which cleans the substrate surface. The final step for providing an atomically clean surface for the coating is sputter etching in an argon plasma. After an atomically clean surface has been provided on the substrate or at least on that portion of the substrate which is to be coated, the coating can be applied. Where a high gloss coating is desired, usually for decorative purposes, the surface will preferably be relatively smooth, since this is believed to facilitate the formation of a high gloss coating. Some surfaces will already be smooth and no treatment is,required. Where a smoother surface is desired, any suitable means can be utilized. For example, the surface may be polished. Alternatively, the surface may be precoated with a layer or layers of polymer materials such as a thermoplastic or thermosetting polymer resin or other material which can provide a relatively smooth surface. Epoxy and acrylic resins are examples of such polymer materials. A combination of polishing and applying a precoat layer can also be utilized. The preferred sputtering conditions depend on surface geometry and the type of microstructure desired. Generally, however, it is desirable for the surface of the coating to be smooth, especially for many wear-related applications. The internal microstructure of the disordered coating may be columnar or non-columnar. For some applications, a columnar surfact of the coating can be desirable. One technique for producing a columnar microstructure applies sufficient bias voltage to the substrate to cause formation of the columnar microstructure. For some coating materials and/or substrate geometries, a columnar microstructure may not be formed, even with a high bias voltage. As is known to those skilled in the art, bias sputtering is the process of maintaining a negative bias voltage on the substrate during deposition. By applying a bias voltage to the substrate, the density, purity, adhesion and internal stress of the coating can be controlled. Generally, application of a bias voltage tends to increase the density, purity and adhesion and also tends to decrease the internal stress of the coating. The bias voltage applied to a substrate during sputtering may be varied in a desired sequence. The preferred bias sequencing depends on the substrate geometry and the desired microstructure. For complex shapes, or for surfaces having a relatively high (about 2.0 or greater) aspect ratio (which is the ratio of the macroscopic depth to the width of a surface, e.g. the aspect ratio of a planar surface is 0 and the aspect ratio of a surface having a depression whose depth equals its width is 1), it is desirable to initially sputter the coating material onto the substrate at a relatively low bias voltage (for example, about -100 to -200 volts) to insure complete coverage. Thereafter, the bias voltage is increased to a relatively high bias voltage (for example, about -1000 to -2500 volts). The biasing voltage can be gradually increased (ramp increased) or step increased. Utilizing such a bias voltage tends to promote a more dense, purer coating having greater adhesion, less internal stress and also tends to promote columnar growth. It is believed that a columnar microstructure generally results in better adherence, possibly as a result of mechanical anchoring to the substrate. For the exterior coating surface applied to a surface with a high aspect ratio, the bias voltage can be applied as for the underlying portion of the coating, except that if a smooth surface is desired, towards the end of the deposition the bias voltage is lowered (for example, generally to about -100 to 200 volts) or eliminated, which tends to allow formation of a smooth surface. For a surface having an aspect ratio of about 0.5 to about 2.0, the coating is preferably sputtered at essentially a constant bias voltage, generally between -500 and -1000 volts. A higher voltage can be used. For the exterior portion of the coating, the bias voltage should be adjusted such that a relatively smooth surface is provided, if this is desired. For surface having a relatively low aspect ratio (between 0 and about 0.5), preferably the bias voltage initially is higher (about -1000 to -2500 volts) and can be decreased to low voltage (about -100 to -200 volts) in either step or ramp fashion, or eliminated. Again, the decrease or elimination of bias voltage usually applies towards the end of the deposition of the coating. A decreasing or relatively low bias voltage also tends to promote a relatively smooth surface which generally results in a more lubricious surface, which can be desirable in many cases. Since sputtering can take place at relatively low substrate temperatures (generally about 200°C or less, for example), the coatings can be formed while avoiding significant changes in the properties of the substrate material while providing a surface that has increased resistance to wear and excellent lubricity. Accordingly, the invention is particularly useful for coating materials such as tool steel, aluminum, brass, chrome, nickel, tungsten carbide and cemented carbides, graphite, glass natural and synthetic polymers and other substrates that can be adversely affected by elevated temperature, for example, since the processing temperature does not degrade the properties of these materials. Sputtering at low substrate temperatures also allows formation of the coatings in a disordered state. The invention is also suitable for coating precisely dimensioned substrates, regardless of substrate composition. To produce sputtered disordered coatings, generally the sputtering will take place at substrate surface temperatures of less than 200°C and usually at about 100°C or even less, to facilitate formation of disordered coatings. Thus, the coatings comprised in the mold according to the present invention can be formed at relatively low temperatures. The target generally is also cooled to prevent evaporation, melting or other unwanted degradation of the target. As a result, the coating is applied to a surface, for example, without significantly altering physical properties of the tool, such as the dimensions, hardness and transverse rupture strength. Generally, substrate temperatures, target compositions, deposition rates and gas pressures which tend to prevent the formation of disordered coatings should be avoided. For wear resistance, it is usually desirable to form a coating that is between about one (1) and about eight (8) micrometers in thickness, with a thickness of about 2.5 micrometers usually being a good thickness for use on tools. Coatings having a thickness in excess of about eight (8) micrometers may not be particularly desirable in applications where high tolerance must be maintained since the geometry and/or size of the tool and parts resulting therefrom may be altered. The sputtering technique can be chosen in accordance with the guidelines previously set forth relating to relatively simple and complex substrate surface geometries. It is to be understood that the coatings and methods described herein can be utilized on molds that have been subjected to use, either with or without the coatings described herein. For example, after a mold having a coating or coatings in accordance with the invention has been in use, and is either worn or outside of the desired tolerance range, the same type of coating or another type of coating in accordance with the invention can be applied to the mold, resulting in an increased mold life. Also, a coating can be applied to molds which did not previously have a coating of the invention thereon. Thus, molds which would otherwise be discarded can be reclaimed. Referring now to Figs. 1-3, several molds are illustrated which can be coated in accordance with the present invention. These are illustrated by way of example only and it is to be understood that the coating may be applied to virtually any type of molding or casting implement. Referring specifically to Fig. 1, there is illustrated in sectional view mold implements for molding, such as with a press-blow machine. The mold implements include a parison or blank mold 10, an airing valve 12, a finish mold 14, a ring 16 and a plunger 18. The same implements are illustrated in Fig. 3 in perspective view, with like reference numerals indicating like elements. The wall 20 of parison mold 10 is cylindrical and the working surface 22 of parison mold 10 is coated with a disordered boron and carbon coating in accordance with the invention. Preferably, the entire working surface of parison mold 10, finish 14, ring 16 and plunger 18 will be coated with a disordered boron and carbon coating of the invention. Usually, for applications in which the coating is utilized in a mold implement, the thickness of the coating will be the range of from about 0.08 micrometers to about 5 micrometers. The thickness of about 2.5 micrometers is an especially desirable thickness. Typical mold implements such as those shown in Figs. 1-3 may be constructed of cast iron. The disordered boron and carbon coatings comprised in the molds of the invention are especially suitable for such materials and do not require an adherence or undercoating. However, it is to be understood that the coating can be utilized on virtually any type of material. During operation of a press-blow process, finish mold 14 is placed on top of parison mold 10. Thereafter, a gob of moldable material is dropped into parison mold 10 and plunger 18 with ring 16 is introduced into parison mold 10 from above (not shown). After the parison is formed, it is removed from parison mold 10 and placed in a blow mold 24, shown in Fig. 2. Compressed air is then introduced into the interior of the parison which is then transformed into the shape defined by the interior of blow mold 24. Working surface 26 of blow mold 24 has a disordered boron and carbon coating in accordance with the invention thereon. Blow mold 24 also includes a bottom 28. The top portion of bottom 28 which forms the interior of the bottom of blow mold 24 also has a discarded boron and carbon coating in accordance with the invention. The molding implements depicted in Figs. 1-3 can be coated on their working surfaces by any suitable technique as described herein. Depending on the working surface configuration, it may be advantageous to build a specific molding section while coating the working surfaces prior to assembly of the mold. This would facilitate depositing the coating and the working surface in some cases, especially where the mold has a narrow entrance and a relatively wide cavity, such as shown in Fig. 2. Use of the coatings on molding implements and the like is especially suited for glass molding, for example, and for molding other materials, including metals and polymers, especially where the temperatures encountered are less than the crystallization temperature of the coating. Usually, for glass molding, temperatures will be less than about 1000°C. Generally, the KHN 9 (50 grams) hardness of the boron carbide coatings in accordance with the present invention is about 4,700 kg/mm² as measured on a 50 micrometers thick (boron carbide) coating that was substantially amorphous with some microcrystalline material. Since the disordered coatings are generally relatively thin, direct measurement of a typically used thickness is impractical. It is expected that thinner coatings would have about the same hardness. However, in addition to being relatively hard, the coatings of the present invention generally also exhibit excellent lubricity. As a result, tools in accordance with the present invention have increased life and the use of such tools can result in an improved surface finish on parts machined therewith. The present invention and its advantages can be more completely understood from the following tests: Test 1 A disordered coating of the type which may be comprised in the molds in accordance with the invention was dc magnetron sputtered utilizing a target that was formed by hot pressing 99% pure, crystalline B₄C powder. The target was silver epoxy bonded to the baking plate and initially broken in by depositing for six hours dc at 300 watts, then for two hours rf at 500 watts and finally brought up to 1050 watts dc over 30 minutes and allowed to preclean at the voltage for an additional 30 minutes. A coating of approximately 50 micrometers of carbon and boron was formed. X-ray diffraction data indicated that the coating was substantially amorphous with some microcrystalline material. Differential thermal analysis indicated that the crystallization temperature of the disordered boron carbide coating was about 1280°C. Test 2 An amorphous boron and carbon coating which may be comprised in the molds in accordance with the invention was DC magnetron sputtered utilizing a target that was formed by hot pressing 99% pure, crystalline B₄C powder. The coating was applied to an M2 high speed single point lathe tool and to M7 high speed steel drills. To provide an atomically clean surface, the tools were dc etched. The boron and carbon amorphous coating was applied by dc magnetron sputtering with a gas pressure of 6 micrometers argon and a dc bias voltage of 500 volts for about the initial 40% of the coating thickness and thereafter a bias voltage of 250 volts dc was applied for the remainder of the coating deposition. The total coating thickness on each tool was about 3.2 micrometers. X-ray defraction testing indicated that the coatings were amorphous. The lathe tool and drills were subjected to tests to determine the life of coating tools versus uncoated tools. The lathe tools was utilized in a turning operation on cold rolled 1040 steel. The drills were subjected to a drilling test on gray cast iron plates having a Brinell Hardness Number of from 219 to 239. The Turning and drilling tests did not utilize lubrication. The lathe tool was utilized at a speed of 0.864 m/s (170 ft/min) at a feed rate of 0.203 mm/rev (0.008 in/rev.). The depth of cut was 1.3 mm (0.050 inches). The lathe tool having the amorphous boron and carbon coating exhibited a tool life that was four times as long as the life of the same type of lathe tool used under the same conditions without any coating. In the drilling tests, through holes were drilled in the gray cast iron plates which were 13 mm (1/2 inch) thick with full penetration of the point and shoulder of the drills. The feed rate was 0.211 mm/rev (0.0083 inches per revolution). The number of holes to catastrophic failure was taken as the tool life criterion. The following results were obtained in the drilling test: The foregoing results indicate a substantial improvement was obtained from the use of tools coated with coatings which may be comprised in molds in accordance with the invention compared with uncoated tools. TEST 3 A stainless steel plate was coated with an amorphous boron and carbon coating which may be comprised in the molds in accordance with the invention. The coating was applied to the stainless steel plate in accordance with the procedure set forth in Example 2. The coating had a thickness of about 3.2 micrometers. The stainless steel plate having the coating thereon was subjected to an air atmosphere having a relative humidity of about 88% and a temperature of about 33°C. The exposure occurred for more than 56 days with no apparent spalling, degradation or loss of integrity of the coating. TEST 4 A stainless steel plate was coated as set forth in Example 3. The coated stainless steel plate was subjected to the vapor of a cutting fluid that was being reflexed. The temperature of the vapor was about 100°C and the exposure occured for eight hours. The cutting fluid contained sulphur and chlorine additives of the type utilized to control the growth of bacteria. After the test, no visible change was apparent in the coating. TEST 5 A chrome water faucet was coated with a coating which may be comprised in the molds in accordance with the invention by applying a boron and carbon coating by dc magnetron sputtering. The coating had a thickness of about 1 micrometer (10,000 angstroms) and was of an attractive gray color with a high gloss. The coating could not be visibly scratched by rubbing with steel wool. Coatings of disordered boron and carbon of desired proportions can be made by dc magnetron sputtering and appropriately choosing the target composition. Also, multiple targets of different elements or compositions could be utilized.

Claims: 1. A mold having a hollow form defining a working surface for forming a moldable or castable material into a predetermined shape, said mold having a wear resistant coating adherently applied over at least a portion of the working surface, said coating being formed by dc magnetron sputtering and consisting of a combination of boron and carbon in at least one phase selected from amorphous, polycrystalline lacking long range order and microcrystalline phases, and said coating having a composition on an atomic basis of B x C 1-x where x is from about 0.60 to about 0.90. 2. The mold according to claim 1 further characterised by at least a portion of the coating being formed by sputtering while applying a bias voltage to the substrate. 3. The mold according to claim 1 or claim 2 further characterised by said boron and carbon being amorphous. 4. The mold according to any one of claims 1 to 3 further characterised by the thickness of said coating being in the range of from about 0.080 micrometers to about 5.0 micrometers. 5. A method of molding an article with a mold having a working surface that defines a predetermined shape, said method comprising: placing a moldable material in a mold according to any one of claims 1 to 4 and forming said moldable material into the desired shape; and thereafter releasing the moldable material from the mold.

Inventors: EICHEN, ERWIN, FLASCK, JAMES DOMINICK
Applicants: OVONIC SYNTHETIC MATERIALS COMPANY, INC.
Application year: 1984
Main classification: C23C14
IPC codes: C23C14, C23C30, A61C3