GB2171039A - Providing an oxidation resistant refractory coating on a graphite mold - Google Patents

Abstract

The coating is deposited on surfaces of the mold of an essentially uniform thickness in the range of 20 microns up to 80 microns. The coating is deposited by chemical vapour deposition at temperatures in the range of 900 to 950 DEG C. Sources of titanium and boron are mixed to produce titanium diboride which is deposited onto the graphite mold surface within a reactor, maintained at the reaction temperature. Alternative materials are borides of zirconium, aluminium chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminium, hafnium, tungsten and silicon; and nitrides of titanium, zirconium, aluminium, boron, and hafnium. Coating conditions can be optimized by using a nozzle (92) and adjustable baffle assembly (102) comprising plates with mis-aligned apertures (110). The assembly can be positioned at various heights up the nozzle tube (24) to control movement of gases in the deposition chamber towards an exhaust port (100). <IMAGE>

Description

SPECIFICATION Chemical vapour deposition of titanium borides on graphite mold This invention relates to the vapour deposition of refractory coatings on graphite molds which are used in the casting of metals.

Graphite molds for casting of non-ferrous metals are desirable because of their relative economy for low volume production needs and the facility with which the graphite materials may be machined to reproduce detail in the desired cast products. However, graphite is susceptible to oxidation during the metal casting process which places critical limitations on the mold life and introduces loss of definition in the casting details. United States patents 3,180,632; 3,515,201 and 3,684,004 disclose the use of graphite molds in the casting of metals and in their use as crucibles in vacuum induction, melting and casing of metals. The graphite molds of these patents are coated with a variety of refractory compositions, particularly silica and alumina compositions to add durability to the graphite molds. The coatings are normally applied manually and by drying and firing readied for use.

Other techniques for coating of carbon materials to improve their tensile strength and resistance to oxidation are disclosed in United States Patents 3,264,135; 3,286,312; 4,234,630; 4,264,803 and 4,405,685. A variety of metal carbides, alkaline earth metal florides, and silica are used in the coating of the carbon bodies. The principal difficulty with these coatings is in their complexity of depositing the coatings of the graphite substrates. The substrates, as coated, are normally used in strengthening the fibres for subsequent use in reinforcing metal materials and the like.

Other forms of coating for graphite compositions include titanium diboride, such as disclosed in United States patents 3,369,920; 3,860,443; 4,082,864 and 4,175,611. However, none of these patents contemplate the use of a titanium diboride coating on graphite molds for use in metal casting. United States patent 3,369,920 discloses the chemical vapour deposition of pyrolitic coatings on carbon fibres which may include titanium boride compositions. The process is carried out at extremely high temperatures in the range of 1,300 to 2,100"C and at pressures considerably below one atmosphere in the range of 5 to 100 mm of mercury. The process for this patent discloses coating carbon and graphite filaments or fibres to improve their tensile strength and oxidation resistance.The process is carried out at the very low pressures to provide very thin coatings on the fibres. Such coatings are less than 8 microns and more suitably in the range of 1 to 2 microns. Because the materials are used as fibres in woven materials, it is important that a minimum coating thickness be provided on the fibres, to avoid such coatings rendering the fibres too rigid for use in fabrics.

Accordingly, the invention provides refractory titanium boride coatings applied to the machined surface of a graphite mold by way of a chemical vapour deposition process. The coating provides resistance to oxidation for at least the machined surface of the graphite mold and has sufficient adhesion to the graphite surface to prevent separation of the refractory coating from the mold when metal parts are cast and removed from the mold.

None of the prior art suggests the coating by chemical vapour deposition techniques of a graphite mold with a titanium boride coating. It is has been surprisingly found that such coatings are superior to known coatings for graphite molds in which molten metal is cast to take on the shape of a machined surface in the graphite mold. It has been discovered that such coatings are sufficiently adhered to the graphite surface to provide for a casting mold which has considerable longevity compared to other known forms of graphite molds.

According to an aspect of the invention, the graphite mold has an oxidation resistant refractory coating of a member selected from the group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, tungsten, hafnium and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium. The coating is deposited on at least surfaces of the mold which are contacted by molten metal. The coating is of essentially uniform thickness and has a thickness selected from the range of 20 up to 80 microns.

According to another aspect of the invention a process for chemical vapour deposition coating of a graphite mold with an oxidation resistant refractory coating of a member selected from the coating group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, tungsten, hafnium and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium, comprises introducing in vapour form a first source of a metal selected from the group consisting of vapours of titanium, zirconium, aluminum, chromium, molybdenum, tungsten, hafnium and silicon and a second source selected from the group consisting of vapours of borides, nitrides and carbides to a reaction chamber at elevated temperature to achieve chemical reaction of the selected first and second sources to yield a solid refractory coating of said coating group which is deposited on exposed surfaces of a graphite mold positioned in the reaction chamber, a nitride being selected from said second group when boron is selected from said first group. A pressure of approximately one atmosphere or more is maintained in the reactor.The graphite mold is removed from the reactor when a thickness selected from a range of 20 up to 80 microns has been deposited on the mold.

According to another aspect of the invention apparatus for the chemical vapour deposition coating of a graphite mold with an oxidation resistant refractory coating of a member selected from the coating group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, tungsten, hafnium and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium comprises a reactor chamber into which an uncoated graphite mold is introduced. The reactor chamber includes means to support a graphite mold. Means heats the chamber to an elevated temperature and means is provided for controlling the heating means to maintain a desired elevated reactor chamber temperature at which chemical vapour deposition takes place. A nozzle is supported in the reactor chamber and positioned proximate a graphite mold.Means is provided for delivering a first source of a metal selected from the group consisting of vapours of titanium, zirconium, aluminum, hafnium, chromium, molybdenum, tungsten, boron and silicon and a second source selected from the group consisting of vapours of borides, nitrides and carbides to the nozzle within the reactor chamber. The delivery means provides for mixing of the selected first and second sources to produce a solid refractory coating of said coating group deposited onto a graphite mold.

Preferred embodiments of the invention are shown in the drawings, wherein: Figure 1 is a photograph of a graphite mold illustrating the deposition pattern of titanium diboride on the machined surfaces of the mold; Figure 2 is a photograph of a machined surface of a graphite mold with improved deposition pattern; Figure 3 is a section through the hot wall furnace for the chemical vapour deposition of refractory coatings on the graphite mold; Figure 4 is a schematic of the apparatus of the chemical vapour deposition system; Figure 5 is a side elevation of the baffle arrangement used within the furnance; Figure 6 is a photograph of a complex mold shape; Figure 7 is a line drawing of a section through the mold of Fig. 6; Figure 8 is a photograph of a graphite mold having fine detail machined therein;; Figure 9 is a photograph of the molded medallion obtained from the mold of Fig. 8 showing the fine detail thereof along with a complex casting from another mold; Figure 10 is a photograph of a graphite mold not having the refractory coating according to this invention illustrating degradation of the mold after exposure to air at high temperatures; and Figure 71 is a graph of weight loss versus time by investigating the degradation of coated and uncoated graphite molds.

Graphite is an excellent mold material for many low melting point metals. Graphite molds compete effectiveiy with conventional iron permanent molds, particularly for parts of medium volume production of approximately 500 to 20,000 per year. The advantages of graphite molds include a suprior as-cast surface finish, superior as-cast tolerance, low machining costs and high thermal conductivity. These advantages have made graphite molds attractive to zinc casting foundries.To overcome the major disadvantage of graphite molds with respect to their susceptability to oxidation, according to this invention a refractory coating is provided by the use of chemical vapour deposition techniques to protect the machined surfaces of the graphite mold from degradation during use, particularly when casting non-ferrous alloys including bronze and aluminium castings. it is appreciated that the machined molds must be made from grades of graphite suitable for casting and machining, as well as its adaptability to chemical vapour deposition processing.

Chemical vapour deposition produces refractory coatings of high integrity at several hundred degrees below their melting point by interaction of vapour species of appropriate elements at the high temperature mold surface. This is in contrast to processes such as plasma spray, which requires a melting of the species of the bonding to take place. Integrity and good adhesion of chemical vapour deposited coatings are a result of the basic mode of growth which is diffusion controlled at the substrate/vapour as weil as coating/vapour interface. The basic reaction for boride coating is:

elevated temperature where the reactants are MCI, the metal chlorides (generally) of a refractory metal such as Ti, Zr, Hf and X, the vapourizable chemical compound of boride. It is appreciated that many other types of standard refractory coatings are usable such as those including the borides which are selected from the group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, tungsten, hafnium and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium. The refractory coatings are made using known components such as sources for titanium, zirconium and hafnium which are normally the chlorides and sources for silicon which are normally the silanes such as SiHCI3. The source of the carbide is normally methane and the source for the nitrides may be ammonia.

These refractory compounds are usually hard inert materials with their attendant characteristics of abrasion resistance and chemical stability in many aggressive environments. The thermal expansion characteristics of the refractory coating must be matched with those of the machined surfaces of the graphite mold to provide endurance in the graphite mold for use in mold casting of various types of metals. In this respect the chemical vapour deposition technique offers distinct advantages over the thermal spray techniques because of the ease with which graded compositions, tailored to alter the expansion coefficient gradually from the substrate to the final coating can be applied.

According to a preferred aspect of the invention, the selected coating applied by the chemical vapour deposition technique is that of titanium diboride of the titanium borides. It is a hard coating with a typical hardness of abdout 3300 VPH compared to titanium carbide which has a hardness of 3100 VPH. VPH is the standard symbol of Vickers Pyramid Hardness. The oxidation resistance of graphite is increased considerably by a coating of chemical vapour deposited titanium diboride to provide for extended use of the graphite mold where many castings can be formed in the mold and extracted therefrom without loss of definition of the fine features within the mold.

In this respect, it has been found that titanium diboride provides a refractory coating which releases the cast metal from the mold without any destruction of the mold features.

Principles of the invention will become understood during the following discussion of a preferred embodiment of the invention in the coating of graphite molds with titanium diboride, although it is appreciated that the process and apparatus is also applicable to applying refractory coatings of other materials. A variety of graphite grades, including the extra high density nuclear grade, is useful with respect to chemical vapour depositing titaniun diboride to machined graphite surfaces. Acceptable grades of graphite are those sold under the trademark 2020 by Canadian Stackpole Limited, which is a high density, fine grain (0.043 mm/0.0017") general purpose graphite. Various shapes can be machined from this grade of graphite for the evaluation of castability and anti-oxidation properties.The machined specimens of graphite are polished, for example, with a 600 grit paper to provide for a smooth surface within the mold. This provides the desired definition within the cast product.

With reference to Fig. 1, a complex mold pattern is shown having a series of annular rings with a central post portion. The process and apparatus according to the invention is capable of coating such complex surfaces with a titanium diboride coating. The section of Fig. 2 illustrates the very smooth finish of the titanium diboride coating on a planar machined surface of graphite.

Various materials can be used in the chemical vapour deposition process as sources for the titanium and boron atoms. Preferably titanium tetrachloride is used which is available from many sources. Titanium tetrachloride is a relatively safe chemical to use and is relatively non-toxic. It can be obtained as a purified liquid from Fisher Scientific. Boron trichloride is a liquid which is usually transported in pressurized containers and may be readily obtained from suppliers such as Matheson Gas Supply. A reductant is used in the reaction of the chemical vapour deposition process to produce the titanium borides which are deposited on the graphite surfaces. It is appreciated that a variety of reductants may be used; however, hydrogen is preferred and may be introduced to the system in accordance with the following discussion of the apparatus.Other inert gases may be used as carriers, such as argon and nitrogen.

According to a preferred embodiment, the apparatus includes a furnace 10 of the type shown in Fig. 3. The chemical vapour deposition coating reactor 12 is based on the "hot wall" principle. The walls 14 of the reaction chamber 16 are at a temperature equal to or higher than the work piece temperature. This condition is obtained by surrounding the reaction chamber walls 14 with two semi-circular segments 18 and 20 of electrical heating elements. With reference to Fig. 4, the reactants are delivered into the reaction chamber 16 through the top section 22 via the gas inlet tube 24 shown in more detail in Fig. 5.For convenience as shown in Fig. 3, the gas inlet tube may be replaced temporarily with a device 26 for calibrating the temperature of the reaction chamber and correlating it with the control temperature sensor 28 (thermocouple) which is located outside of the reaction vessel. Once the system is calibrated, the temperature sensor within the reaction chamber is removed and the gas inlet tube 24 of Fig.

5 reconnected to the supply of reactants.

The furnace 10 comprises insulative material 30 surrounding the perimeter of the reactor chamber 16' with furnace brick 32 and 34 provided at the upper and lower portions of the insulative material. Such insulative material and furnace brick provide the necessary insulation to assist in maintaining the desired temperature within the reaction chamber 16. A controlling device (not shown) operates in conjunction with the thermocouple 28 to maintain the desired temperature within the reaction chamber, based on the calibration accomplished with the temperature sensing device 26.

With reference to Fig. 4, the reactor 12 has the reactants fed thereto through a conduit 36 which is wrapped with a heating tape 38. The sources of the reactants, namely the vapour sources of boron and titanium, are obtained as follows. The boron vapour is, according to this preferred embodiment, obtained from liquid boron trichloride which is in a gas-tight vessel 40.

The gas in vessel 40 forces the boron trichloride vapour through the flow meter 42 to the "T" junction 44 which intersects at junction 46 for the feed of the titanium vapour.

The titanium vapour is obtained by bubbling through a vessel 48 of titanium tetrachloride a carrier gas which includes a reductant. The reductant gas, which may be hydrogen is obtained from the pressurized cylinder 50 and one or more carrier gases including argon in vessel 52 and/or nitrogen in pressurized vessel 54 may be selectively used as controlled by the valves 56, 58 and 60. The flow of the gases is monitored by the flow meter 62, and is introduced to the liquid titanium tetrachloride through conduit 64. The bubbles 66, as they rise upwardly through the liquid of titanium tetrachloride, are removed through conduit 68 which includes heating tape 70. The vessel containing the liquid titanium tetrachloride is closed, as shown, so that all gases are removed through conduit 68.The carrier gases including the reductant hydrogen are combined at junction 46 with the boron trichloride, and commence mixing along conduit 36 before entry to the reaction chamber 16 through the delivery tube 24. in order to maintain the desired temperature of the titanium tetrachloride solution, a bath or other form of temperature maintaining material 72 is provided exterior of the closed vessel 74 for the titanium tetrachloride solution.

The temperature of the titanium tetrachloride bath is adjusted to deliver the required quantity of titanium vapour to correspond to the amount of boron vapour delivered to the vessel. The hydrogen flow rate is adjusted to deliver approximately, according to this embodiment, 34 mL/minute of titanium tetrachloride vapour to the vessel together with 6.9 mL/minute of boron trichloride. The temperature of the reaction chamber 16 is preferably in the range of 900 to 950"C. Once a temperature within the furnace is established, it is maintained at a constant level, such as the preferred temperature of 930"C for depositing the reaction products at the above noted flow rates for the reactant gases.

Pressure gauges are located at places noted in Fig. 4 to monitor unwanted pressure increases which would indicate constriction of passages caused by the accumulation of solid particles.

The reaction products consist mostly of hydrogen and hydrogen chloride gas, various amounts of unreacted titanium tetrachloride, boron trichloride gases and the reaction product, titanium diboride, which is primarily deposited on the heated surfaces of the graphite mold placed in the reaction chamber. As shown in Fig. 4, the graphite mold 76 is positioned at the base of the reaction chamber 16 beneath the reactant delivery tube 24. As the reaction product, titanium diboride is formed at the hot surfaces of the mold, it is deposited on such surfaces of the graphite mold 76. The remaining reaction products, which are in gaseous form, are exhausted from the reaction vessel 16 via outlet 78. The exhaust gases are passed through conduit 80 to a filter 82 to remove crystalline substances, and then through a cold trap 84 to condense any remaining gases.The materials then pass through a subsequent filter 86 to remove any additional particulate material. A dryer 88 is employed in combination with a water scrubber 90 to neutralize any remaining gases before discharge of the reaction products to atmosphere.

Within the furnace of Fig. 4, the gas delivery tube 24 has a nozzle 92. The nozzle height may be adjusted with respect to the position of the graphite mold 76 within the furnace. The specimen is centered at the bottom of the reactor chamber 16. The flow distribution pattern of the reactant gases as they emerge from then nozzle determines to a certain extent the uniformity with which the titanium diboride coating is vapour deposited on the heated surfaces of the graphite mold. Ideally, the positioning of the nozzle relative the the graphite mold is such to ensure laminar flow of the reactant gases towards the surface of the graphite mold. Once the position of the nozzle relative to the graphite mold is determined within the furnace, the system is sealed and pressure tested-with nitrogen at 4 psig for two minutes.The system is then flushed with nitrogen at a rate of approximately 130 mL/minutes. The nitrogen is then replaced by hydrogen flowing at a rate of approximately 890 mL/minutes and the furnace is turned on.

By use of the temperature sensor external to the furnace and so calibrated when the reactor temperature reaches approximately 930"C, boron trichloride line is opened to the gas delivery system. Hydrogen is diverted to bubble through the titanium tetrachloride bath which is maintained at 450C and boron trichloride is then introduced at a rate of 6.9 mL/minutes. With the furnace now fully operational, coating duration may be in the range of 30 to 60 minutes where it has been found that coatings of titanium diboride were deposited in the range of 20 microns up to 80 microns and preferably in the range of 20 microns to 45 microns. On completion of the vapour deposition process, the titanium tetrachloride and boron trichloride sources are shut off and the hydrogen flow rate reduced to approximately 350 mL/minute.The system is allowed to cool and the graphite with refractory coating is removed and allowed to slowly reach room temperature.

The reaction between the gaseous reactants and the graphite occurs in the presence of a large amount of reducing agent, which, according to this embodiment, is hydrogen. It has been found that the use of hydrocarbons, such as methane, in the gaseous stream can improve adhesion.

Because of the dilution of reactants, the ratio between the reaction chamber volume and the surface area of the graphite mold to be coated can become an operative variable.

With reference to Fig. 5, the top 22 of the reactor 12 is shown in more detail. The delivery tube 24 is connected to the top 22 by bushing 94 and secured in place by lock ring 96 with bolts 98. The appropriate seals formed to ensure that there is not leakage of products through the connection of delivery tube 24 to the top 22. The exhaust nozzle for reaction products to be removed from the reaction chamber is provided at 100 and threaded to the top plate 22 to provide a gas-tight seal with the chamber. The delivery tube 24 extends downwardly into the reaction chamber 16 in the manner shown, where it terminates in a nozzle 92. Secured to the periphery of the delivery tube 24 is a baffle system 102 which has a central collar portion 104 having lock screws 106 which may be turned inwardly to fix the baffle arrangement 102 at various positions along the delivery tube 24.The baffle arrangement 102 consists of a plurality of circular discs 108 having apertures 110 provided therein. The apertures in each disc are misaligned with apertures in adjacent discs to provide a tortuous path through which the gases flow when moving towards the exhaust nozzle 100. For particular flow rates of the reactants, the baffle system may be moved along the gas delivery tube 24 to vary the volume of the reaction chamber without altering location of the nozzle relative to the surface of the graphite mold to be coated. This allows one to optimize on the ratio of the volume of the reactor chamber to the surface area of the graphite mold being coated. By moving the the baffle up and down along the delivery tube various characteristics of the process may be monitored to optimize on the ratio of the reactor chamber volume to the machined surface area of the mold being coated.

The following Table I outlines the results of tests conducted in accordance with the above parameters to demonstrate the effective coating of the titanium diboride on the graphite mold surfaces having a sufrace area of 20 cm2. From the results of the runs itemized in the following Table I, it has been found that the optimum ratio between the reactor chamber volume and the machined surface area of the mold is in the range of 8 to 10 cm.

TABLE I SUMMARY OF CVD EXPERIMENTS

SURFACE CHAMBER NOZZLE COATING COATING TRIAL PREP. VOL., L DIST mm TIME HRS. APPEARANCE 1 600 GRIT 0.2 13 1 Coated 2 600 GRIT 0.2 13 1 Coated 3 600 GRIT 0.2 13 Well coated 4 Precoated 0.2 13 Well coated 5 600 GRIT 0.2 13 1 Well coated 6 120 360 600 GRIT 0.2 13 1 Well coated To assist in providing uniform coatings across the face of the graphite mold, it has been found that for some flow rates in some situations it is desirable to provide a baffle diffuser plate at the nozzle outlet of the gas delivery tube to prevent a build-up of unnexcessarily thick coatings in the centre of the graphite mold.

During the period of deposition of approximately 30 to 60 minutes, the thicknesses of deposited materials on a complex mold shape, such as shown in Fig. 6, demonstrate thorough coating on all surfaces. With reference to Fig. 7, the numerals 1 through 13 indicate positions on the complex mold of Fig. 6 where refractory coating thickness have been measured. The results of those measurements are summarized in the following Table II.

TABLE II COATING THICKNESS DISTRIBUTION Coating Coating Position Thickness (m) Thickness (ins) 1 10 0.00039 2 12 0.00047 3 11 0.00043 4 13 0.00051 5 21 0.00083 6 25 0.00099 7 46 0.0018 8 25 0.00099 9 16 0.00063 10 13 0.00051 11 12 0.00047 12 15 0.00059 13 12 0.00047 From the above Table II, it is apparent that all surfaces of the mold have been coated including the corner regions, whether they are obtuse or acute. Although there is a variation in the thicknesses of deposited coating, they are reasonably consistent throughout, excepting for region 7 which is directly opposite the nozzle of the gas introduction tube.

By means of X-ray defraction, the composition of the refractory coating can be identified for a preferred embodiment as exemplified in following Table III. The coating is of essentially pure titanium diboride compound with no trace of graphite or metallic contaminants. The characteristic peaks of the X-ray defraction pattern are at the known Bragg angles for titanium diboride compound.

TABLE III d (Angstrom) Intensity hkl 3.230 22 001 2.6247 55 100 2.0370 100 101 1.6145 12 002 1.5153 27 110 1.3751 16 102 With the graphite molds made in accordance with the preferred process of this invention in the system of Fig. 4, numerous casting trials were conducted in the mold using zinc/aluminum alloys. The alloys do not stick to the coating even though the temperature of the melt is above 600"C. During the casting trials, there was no appreciable loss in definition of the cast product and no deterioriation in the mold surface. This is exemplified in Fig. 8 where the high definition of features in the mold did not deterioriate as evidenced by the medallion surface as cast within the mold, as shown in Fig. 9.Furthermore a complex shape, as shown in the right hand side of Fig. 9, was also cast from a graphite mold coated in accordance with this invention to provide a casting with the desired degree of definition in a surface of the cast product.

To evaluate thermal shock resistance of the refractory coatings, the coated graphite mold was subjected to heating and air cooling cycles. Heating was accomplished by flame, or electrical or induction heating with subsequent cooling. Peeling or flaking was not observed and repeated cycling of the heating and cooling did not appear to accelerate degradation behaviour, particularly as examined under a microscope.

The oxidation resistance of the graphite molds was investigated. Graphite molds with and without the coating of titanium diboride were subjected to heating at 600"C. The weight loss of the coated and uncoated graphite samples was plotted in logarithmic form against time for a period of 24 hours, as shown in Fig. 11.

From the results as plotted in Fig. 11, the coated graphite sample is about a factor of 50 times more resistant to oxidation in air than the uncoated sample. As shown in Fig. 10, the effect of oxidation on an uncoated mold is evident. The walls of the mold crumple and loose definition very quickly when exposed to oxygen in the air at temperatures of oven 600"C. With coated molds of the type investigated and plotted in Fig. 10, the cumulative weight loss over 26 hours of air exposure is only in the range of 0.18%. However, with uncoated graphite molds approximately 40% of its original weight is lost.

The refractory coating, according to this invention as applied by chemical vapour deposition techniques, considerably prolongs the life of the graphite mold while maintaining the desired definition in the cast product. According to this invention, the chemical vapour deposition process can be conducted at relatively low temperatures in the range of 900"C to 950 C compared to the excessively high temperatures of other chemical vapour deposition techniques.

It is appreciated that for very large surfaces to be coated, the gas distribution nozzle will have to be moved over the surface of the object to scan it in an appropriate manner to achieve complete coating of the surface.

In accordance with various embodiments of this invention, by way of chemical vapour deposition techniques, refractory coatings are deposited on the machined surfaces of the graphite mold to protect same from oxidation. Titanium diboride is the preferred refractory coating, because the reactants are relatively safe chemicals to work with as compared to the silicon carbide coatings, which required the use of silanes which are spontaneously flammable in air.

Claims (22)

1. A graphite mold having an oxidation resistant refractory coating of a member selected from the group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, hafnium tungsten and silicon; and nitrides of titanium, zirconium aluminum, boron and hafnium, said coating being deposited on at least surfaces of said mold which are contacted by molten metal, said coating being of essentially uniform thickness and having a thickness selected from the range of 20 up to 80 microns.

2. A graphite mold of claim 1 wherein said refractory coating is a titanium boride.

3. A graphite mold of claim 2 wherein said titanium boride coating is essentially of titanium diboride.

4. A graphite mold of claim 2 or 3 wherein said coating is applied by chemical vapour deposition at elevated temperatures.

5. A graphite mold of claim 2 or 3 wherein said coated mold surface is machined to provide desired mold surface shapes prior to coating with said titanium boride.

6. A graphite mold of claim 2 or 3 wherein said coating has a hardness in the range of 3300 VPH.

7. A graphite mold of claim 2 or 3 wherein said coating is deposited directly onto machined surfaces of said graphite mold.

8. A process for chemical vapour deposition coating of a graphite mold with an oxidation resistant refractory coating of a member selected from the coating group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, tungsten, hafnium and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium, comprising introducing in vapour form a first source of a metal selected from the group consisting of vapours of titanium, zirconium, aluminum, chromium, molybdenum, tungsten, boron, hafnium and silicon, and a second source selected from the group consisting of vapours of borides, nitrides and carbides to a reaction chamber at elevated temperature to achieve chemical reaction of said selected first and second sources to yield a solid refractory coating of said coating group which is deposited on exposed surfaces of a graphite mold positioned in said reaction chamber, a nitride being selected from said second group when boron is selected from said first group, a pressure of approximately one atmosphere or more being maintained in said reactor, said graphite mold being removed from said reactor when a thickness selected from the range of 20 to 80 microns has been deposited on said mold.

9. A process of claim 8 wherein said refractory coating is a titanium boride and said source of metal is titanium and said second source is a boride.

10. A process of claim 9 wherein said elevated temperature is in the range of 900" to 950"C.

11. A process of claim 9 or 10 wherein said graphite mold is machined to provide a desired mold shape in which molten metal is cast.

12. A process of claim 9 wherein said source of boron is a vapour stream of boron trichloride and said source of titanium is titanium tetrachloride, a reductant source for said chemical reaction being included.

13. A process of claim 12 wherein said titanium tetrachloride vapour is carried in a stream including hydrogen as said reductant and boron trichloride is introduced as a gaseous pressurized stream, maintaining the correct ratio of each to produce dense protective coatings.

14. A process of claim 13 wherein said streams of titanium tetrachloride and boron trichloride are combined to yield titanium diboride and introduced to said reactor via a nozzle, said nozzle being directed onto said mold to deposit the titanium diboride onto said graphite mold.

15. A process of claim 14 wherein reactor chamber volume to surface area qf mold to be coated in a ratio appropriately adjusted to obtain uniform essentially pure titanium diboride coatings.

16. A process of claim 14 wherein said streams are introduced at flow rates which provides an essentially laminar flow of reaction products onto said graphite mold.

17. Apparatus for the chemical vapour deposition coating of a graphite mold with an oxidation resistant refractory coating of a member selected from the coating group consisting of: borides of titanium, zirconium, aluminum, chromium, molybdenum, tungsten and hafnium; carbides of titanium, zirconium, aluminum, hafnium, tungsten and silicon; and nitrides of titanium, zirconium, aluminum, boron and hafnium, said apparatus comprising a reactor chamber into which an uncoated graphite mold is introduced, said reactor chamber including means to support a graphite mold, means for heating said chamber to an elevated temperature and means for controlling said heating means to maintain a desired elevated reactor chamber temperature at which chemical vapour deposition takes place, a nozzle supported in said reactor chamber and positioned proximate a graphite mold for delivering a first source of a metal selected from the group consisting of vapours of titanium, zirconium, aluminum, hafnium, chromium, molybdenum, tungsten, boron and silicon, and a second source selected from the group consisting of vapours of borides, nitrides and carbides to said nozzle within said reactor chamber, said delivery means providing for mixing of said selected first and second sources to produce a solid refractory coating of said coating group deposited onto a graphite mold.

18. An apparatus of claim 17 wherein said refractory coating is a titanium boride and said source of metal is titanium and said second source of anion is boride.

19. An apparatus of claim 1 7 wherein said reactor chamber has a volume relative to surface area of a graphite mold to be coated to provide a volume to surface area ratio of 8 to 10 cm.

20. A graphite mold substantially as hereinbefore described with reference to Fig. 1, Fig. 2, Figs. 6 and 7 or Figs. 8 and 9 of the accompanying drawings.

21. A process for chemical vapour deposition coating of a graphite mold substantially as hereinbefore described.

22. Apparatus for the chemical vapour deposition coating of a graphite mold substantially as hereinbefore described with reference to Fig. 3 or Figs. 4 and 5 of the accompanying drawings.