PROCESS FOR MAKING SILICON NITRIDE ARTICLES

Abstract: A process for making silicon nitride articles of manufacture such as an internal combustion engine valve (2) by providing a homogeneous water-base slurry of silicon nitride powder and at least one nitriding agent and permitting the slurry to age for a time effective to enable a chemical reaction to occur and then reducing the water content sufficiently to form the article and thence forming and nitriding the article using a compound nitriding cycle.

Description: This invention relates generally to a process for making silicon nitride articles of manufacture such as engine valves and more particularly to a reaction bonding process that utilizes a homogeneous water-based slurry of silicon powder and at least one nitriding agent as a starting material and includes a chemical aging requirement as well as a compound schedule for heating the article while blanketed under a nitriding gas effective to provide a quality silicon nitride article in a rapid and economical manner. Because of its high temperature strength and creep resistance and low thermal expansion as well as its excellent oxidation and corrosion resistance, silicon nitride (Si₃N₄) has been used for many years to make critically engineered parts such as gas turbine blades. Generally, it has been the practice to form silicon nitride parts by "reaction bonding" or by "hot pressing" or by a pressureless sintering process. Reaction bonding comprises compacting silicon powder of commonly less than 400 mesh 37 µm into the part commonly at ambient temperature and then exposing the part to molecular nitrogen at about 1400°C for a period of time sufficient to convert the silicon into silicon nitride such as described in United States Patent 4,235,857. Such is also reviewed by A.J. Moulson in an article titled "Review Reaction-Bonded Silicon Nitride: Its Formation and Properties," Journal-Materials Science, 14, (1979) 1017-1051 and by Mangels in an article titled "Effect of Rate-Controlled Nitriding and Nitriding Atmospheres on the Formation of Reaction-Bonded Si₃N₄″, Ceramic Bulletin, Volume 60, No. 6 (1981), 613 in which he also describes benefits derived by using a nitriding gas mixture of nitrogen with minor amounts of hydrogen and helium. The use of a combined nitrogen-hydrogen nitriding gas in the reaction bonding of Si₃N₄ is described by Shaw and Zeleznik in an article titled "Thermodynamics of Silicon Nitridation: Effect of Hydrogen", Communications of the American Ceramic Society, November 1982, C180-C181 and the effect of temperature and nitrogen pressure on the kinetics of silicon nitridation along with the need for an activating agent such as iron is described by Atkinson, Moulson and Roberts in an article titled "Nitridation of High-Purity Silicon", Journal American Ceramic Society, Volume 59, No. 7-8, 285-289. Hot pressing involves pressing alpha-silicon nitride powder into the part at sintering temperatures of about 1700°C to about 2200°C for a prescribed period of time. In the "hot pressing" process, it has further been the practice to add "densification or sintering aids" to the silicon nitride powder to reduce porosity and improve strength. Pressureless sintering involves sintering of alpha-phase silicon nitride powder at temperatures of above 1650-1800°C to which has been added "densification or sintering aids" similar to "hot pressing." Examples of "densification or sintering aids" include monovalent metal oxides such as MgO, ZrO, NiO and divalent metal oxides such as Al₂O₃, Cr₂O₃ and Y₂O₃ such as described in United States Patent 3,950,464. It is also common to include nitriding agents in the "reaction bonding" process but those presently known to be suitable are much more limited than the "densification or sintering aids" previously described and generally include iron oxide or nickel oxide and mixtures thereof at the present time. It has also been common practice to prepare the silicon or silicon nitride powder based compounds by mixing, commonly in a ball mill, with a wetting agent. Commonly such wetting agents have been members of the alcohol family such as tertiary alcohol described in United States Patents 3,991,166 and 3,992,497. It has been less common to use water in preparing silicon or silicon nitride ceramic compounds but an example of the use of about 10% by weight water to fluidize a sinterable molding powder prior to freezing is described in United States Patent 2,869,215. United States Patent 2,268,589 describes an early use of water in forming a paste with activated silicon which is then fired at 1200°C to 1450°C. In addition to nitriding and densification agents, it has also been common practice to employ binders in the ceramic compounds to bind the silicon or silicon nitride powder particles together to enhance their forming into the parts being made. Examples of such binders include a mixture of butyl methacrylate and trichlorethylene described in United States Patent 3,819,786 in conjunction with silicon nitride powder and a blend of polyvinyl alcohol and water and a silicon carbide and berylium oxide blend described in United States Patent 3,205,080. Up until the time of the present invention, it has been the further practice to nitride silicon powder by heating for long periods of time. An example of such is described in United States Patent 3,819,786, where a blend of silicon nitride powder and the binder mixture is heated in a stream of nitrogen from ambient to 1000°C at 50°C/hr and then held under static nitrogen for 20 hours at 1350°C and 10 hours at 1450°C with the total time more than thirty hours long. An example of a compound heating schedule for nitriding a mixture of silicon and silicon carbide powder is described in United States Patent 3,222,438, where the mixture is first compacted into a green compact and then heated in an atmosphere of nitrogen at a temperature of 1250°C for 16 hours and then at 1450°C for 3-4 hours where the first stage heating is conducted to pre-sinter the compound so that it doesn't melt at the 1450°C temperature since the melting point of silicon is about 1420°C. Finally, another example of a compound heating schedule for sintering a silicon nitride and MgO powder mixture is described in United States Patent 3,992,497, where the mixture is formed into a compact and then first heated at a temperature of about 600°C for about 60 minutes to remove volatiles and then the temperature is increased to a temperature between 1500°C and 1700°C at a rate of climb above 1450°C being about 15°C to 200°C per minute and holding it at that temperature for a period between about 5 and 30 minutes and more specifically teaching that the heating rate is immaterial until the temperature of about 1450°C is attained. Contrary to the above, the process of the present invention, although employing known binders and nitriding agents, utilizes relatively large amounts of water under prescribed conditions in providing a homogeneous slurry of silicon powder and at least one nitriding agent which is subsequently processed and formed into green stock which is then nitrided with a unique compound heating schedule incorporating a multi-component nitriding atmosphere to provide high quality silicon nitride articles in a rapid and economical manner. In accordance with the invention, there is now provided a process for making a silicon nitride article, including the steps of: (a) providing a homogeneous slurry comprising a mixture of 40-60 parts by weight silicon powder, 60-40 parts by weight water, and from 0.5 to 3 parts by weight of at least one nitriding agent; (b) comminuting the slurry of step (a) in a comminution device to activate the silicon powder and enhance chemical reaction between the silicon powder and the water; (c) aging the activated silicon slurry of step (b) to enable the chemical reaction between the silicon powder and the water to occur; (d) processing the aged silicon slurry of step (c) to reduce the water content sufficiently to form the article; (e) forming the processed slurry of step (d) into the article; (f) heating the article of step (e) from ambient temperature to a temperature of 1000°C in a hydrogen atmosphere at a rate non-damaging to the article whilst removing volatiles therefrom; (g) exposing the article of step (f) to nitrogen containing gas; (h) heating the article of step (g) from the temperature of step (f) to a temperature from 1400°C to 1450°C at a rate of from 15°C to 25°C per hour in a gas mixture of 40 to 60 parts helium and 1 to 4 parts hydrogen and the balance nitrogen whilst replenishing consumed nitrogen with fresh nitrogen so as to maintain a substantially constant nitriding gas composition while converting the silicon into silicon nitride; and (i) cooling the article of step (h) to ambient temperature. FIGURE 1 is a block diagram of an embodiment of the process of the invention being utilized to make a silicon nitride engine valve 2 for illustrative purposes. The process of the invention starts by providing a homogeneous slurry of silicon powder, water and at least one nitriding agent such as iron oxide suitable to enhance nitriding of the silicon powder. Any suitable comminutation device may be used; the slurry is preferably prepared in a ball mill type mixer well known to those skilled in the art. The slurry preferably comprises a mixture of 40-60 parts by weight silicon powder and from 60 to 40 parts by weight water and from 0.5 part to 3 parts by weight of the nitriding agent. The slurry may additionally include effective amounts of suitable organic binders in conjunction with plasticizers and viscosity modifiers well known to those skilled in the art. Commonly, such organic binders, plasticizers and viscosity agents comprise less than 5% by weight of the silicon powder. The amount of water and the amount of binder, if included, can be tailored to provide the viscosity desired for the particular manner in which the article is to be formed. Generally, higher amounts of water will provide lower viscosities suitable for "slip casting" articles which is essentially pouring the mixture into molds shaped to form the article as is well known to those skilled in the art. The nitriding agent comprises at least one such agent to suitably enhance the nitriding operation. Preferably, the nitriding agent comprises at least one of iron oxide and nickel oxide of which iron oxide has been determined particularly suitable. Although molecular nitrogen gas may be used for nitriding the silicon, it is preferred that the gas comprises a mixture of from 40% to 60% helium on a mole basis and from 60% to 40%, on a mole basis, of a blend comprising about 4% hydrogen and about 96% nitrogen. It has been discovered that helium greatly enhances the thermal conductivity of the gas and that hydrogen, both for purging and as a member of the nitriding gas, enhances the reaction converting silicon into silicon nitride which is believed to be a vapor phase reaction. The pressure of the nitriding gas is preferably controlled so that it blankets the article under a slight positive pressure such as an excess pressure of about 20 KPa and nitrogen gas is admitted into the furnace to make up for gas loss during the nitriding procedure. A preferred embodiment of the process of the invention will now be described in greater detail in which 45-50 Kg of silicon powder, dry ground to a size of approximately 2 F.A.D., (Fisher Average Diameter), of commercial purity, and obtained from a commercial source, is mixed with from 0.5-1.5 Kg of finely ground iron oxide, also obtained from a commercial source, and approximately 15-30 Kg of distilled water. The mixing is done in a ball mill or other comminutation device for a time necessary to effect additional grinding of the silicon and iron oxide to both further reduce the size of the silicon particles and to activate the silicon so that it will react with the water and to assure complete distribution of the iron oxide throughout the silicon slip so that a homogeneous mixture results. Organic binders in conjunction with normal plasticizers and viscosity modifiers, if employed, are preferably added to the ball mill approximately one hour before pumping the slip out of the mill. The silicon slip is aged for a time effective to enable the substantial reaction to occur between the silicon and water which characteristically appears to be at least about 24 hours. During aging, the slip appears to virtually double its volume through frothing. It is believed that the silicon is reacting with the water during aging forming at a minimum, a silicon oxyhydrate coating on the silicon particles and releasing hydrogen gas. It has been found that the reaction subsides after several days with the slip developing thixotropic properties. Although the slip could be slip cast at this point to form a green body or the water content reduced to permit extrusion of shapes, it is preferred to spray dry the slip to obtain a homogeneous free flowing powder for isopressing or dry compression forming using standard powder metal presses. The use of standard binders common to the ceramics industry produces compacts with sufficient strength to allow machining without the need for special heat treatments which partially nitride or sinter the silicon compact. The green silicon compacts are then loaded into a furnace operable to perform the nitridation of the silicon as described below. The furnace is of a type having good heat insulating characteristics and accurate temperature control. It is positioned within a vacuum chamber preferably capable of holding a positive pressure of up to about 170 KPa absolute. Gases are admitted to the furnace through a precision gas regulator. The gas regulator permits control of the pressure in the furnace from 100-150 KPa absolute and maintains a constant pressure in the furnace. Furnace atmosphere is allowed to leave the furnace preferably through a burner arrangement. The vacuum chamber is evacuated to remove oxygen from the furnace. The furnace is then backfilled with pure hydrogen and the start of the nitriding cycle is initiated. The first part of the nitriding cycle comprises heating the furnace to a temperature in the 600-1000°C range, while flowing pure hydrogen through the furnace. The heating rate during this portion of the cycle is controlled at a rate of about 500°C per hour. The pure hydrogen gas which is flowing through the furnace is burned in the burner arrangement as it exits the furnace which is also effective to remove any organic binders from the furnace as they come out of the silicon compacts. Once the furnace temperature has reached the range of 600-1000°C, the hydrogen gas is then turned off and the furnace purged with helium until a non-combustible gas mixture is left in the furnace whereupon the furnace is evacuated. The furnace is caused to come to a temperature hold when it reaches about 1000°C until the furnace has been evacuated. Once the required vacuum is achieved at about 1000°C the furnace is backfilled to an absolute pressure of about 50 KPa with high purity helium, and then the furnace is further backfilled with a nitriding gas mixture preferably comprising from 1 to 4 parts hydrogen and the balance nitrogen to a pressure of about 120-135 KPa absolute. The inlet line to the furnace is then switched to a cylinder of high purity nitrogen which will replace the nitrogen consumed during the nitriding of the silicon. Unlike Mangels' demanding nitriding cycle previously described in which the composition of the nitriding atmosphere changes during nitriding, the preferred nitriding atmosphere of the process of the invention remains constant in chemical composition throughout the nitridation of the silicon. Once the backfilling of the furnace with the nitriding gas begins, an increasing temperature ramp of 20 to 25°C per hour is initiated to run the furnace to a temperature of approximately 1420°C. A one-hour soak period at 1420°C is allowed before cooling to room temperature. A second important difference between the preferred nitriding cycle of the process of the invention and others is that the furnace temperature is virtually continuously increasing from a temperature of approximately 1000°C to the melting point of silicon (1420°C) with the only temperature hold occurring at the end of the cycle. A significant result of the process of the invention is that it is readily adaptable to a continuous type of kiln similar to a tunnel kiln, not just batch kilns as are normally used in the manufacture of reaction bonded silicon nitride. Using the above-described process for the manufacture of reaction bonded silicon nitride, test bars have been made with modulus of rupture (MOR) strengths tested in the as-fired condition in excess of 241 MPa with typical modulus of rupture being in the range of about 172-206 MPa. Typically, a 90-95% conversion, based on optical microscopy, in parts up to 25 mm (1 inch) in cross section has been observed. Thus, by means of aging a slurry of silicon powder and water in conjunction with at least one nitriding agent and then nitriding articles formed therefrom under a compound heating schedule, a process is provided by which to make silicon nitride articles of high quality in a rapid and economical manner.

Claims: 1. A process for making a silicon nitride article, the process including the steps of: (a) providing a homogeneous slurry comprising a mixture of 40-60 parts by weight silicon powder, 60-40 parts by weight water, and from 0.5 to 3 parts by weight of at least one nitriding agent; (b) comminuting the slurry of step (a) in a comminution device to activate the silicon powder and enhance chemical reaction between the silicon powder and the water; (c) aging the activated silicon slurry of step (b) to enable the chemical reaction between the silicon powder and the water to occur; (d) processing the aged silicon slurry of step (c) to reduce the water content sufficiently to form the article; (e) forming the processed slurry of step (d) into the article; (f) heating the article of step (e) from ambient temperature to a temperature of 1000°C in a hydrogen atmosphere at a rate non-damaging to the article whilst removing volatiles therefrom; (g) exposing the article of step (f) to nitrogen containing gas; (h) heating the article of step (g) from the temperature of step (f) to a temperature from 1400°C to 1450°C at a rate of from 15°C to 25°C per hour in a gas mixture of 40 to 60 parts helium and 1 to 4 parts hydrogen and the balance nitrogen whilst replenishing consumed nitrogen with fresh nitrogen so as to maintain a substantially constant nitriding gas composition while converting the silicon into silicon nitride; and (i) cooling the article of step (h) to ambient temperature. 2. A process according to claim 1, wherein the nitriding agent of step (a) is selected from iron oxide, nickel oxide and mixtures thereof. 3. A process according to claim 1 or claim 2, wherein the slurry of step (a) includes at least one organic binder. 4. A process according to any one of claims 1 to 3, wherein the slurry of step (a) includes at least one plasticizer. 5. A process according to any one of claims 1 to 4, wherein the slurry of step (a) includes at least one viscosity modifier. 6. A process according to any one of claims 1 to 5, wherein the processing of step (d) provides a free-flowing powder suitable for forming the article. 7. A process according to any one of claims 1 to 5, wherein the processing of step (d) is spray drying. 8. A process according to any one of claims 1 to 7, wherein the heating rate of step (f) is 400°C to 600°C per hour.

Inventors: EDLER, JAMES PATRICK, LISOWSKY, BOHDAN
Applicants: EATON CORPORATION
Application year: 1988
Main classification: C04B35
IPC codes: C04B35