METHOD FOR CONTROLLING THE OXYGEN CONTENT IN AGGLOMERATED MOLYBDENUM POWDER

Abstract: A method is disclosed for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, in particular into agglomerated molybdenum plasma spray powder. According to a first aspect of the invention, the method involves contacting the powder with a relatively dilute solution containing a sufficient amount of an oxidizing agent for a sufficient time to increase the oxygen content of the powder followed by removing the resulting partially oxidized powder from the resulting solution. According to a second aspect of the invention, the method involves heating the powder at a sufficient temperature for a sufficient time in the presence of water vapor, and a non-oxidizing atmosphere with the amount of non-oxidizing atmosphere being controlled to produce a partially oxidized molybdenum powder. According to a third aspect of the invention, the method involves forming a relatively uniform mixture of agglomerated powders consisting essentially of molydenum and one or more oxygen containing compounds of molybdenum wherein the mixture has an oxygen content of more than about 10% by weight and reducing the mixture at a sufficient temperature for a sufficient time to remove a portion of the oxygen therefrom and form a molybdenum powder having an oxygen content of no greater than about 10% by weight.

Description: Background of the Invention This invention relates to a method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder and in particular, into agglomerated molybdenum plasma spray powder. Flame spraying and plasma spraying are common techniques for the application of protective and wear resistant coatings of various metals, ceramics, and cermets, usually to metal surfaces (substrates). The piston ring industry commonly uses molybdenum coating on rings for internal combustion engines. In the flame spraying technique an electric arc or an oxyacetylene flame melts the end of a continuous coil of molybdenum wire and a gas propels it onto a substrate for example, the war surface of a cast iron piston ring where it splats and solidifies, forming the coating in successive layers. Because of the presence of excess oxygen either from the flame or the surrounding air, or both, the coatings produced by this technique contain large quantities of oxygen, typically from about 7% to about 8% in solution and as various molybdenum oxides. The large quantities of oxygen in the molybdenum apparently harden the coating. In the plasma spraying of molybdenum, there is usually a minimum of oxygen in the sprayed coating due to the use of an oxygen-lean plasma gas system. That is, argon, helium, hydrogen, nitrogen, or combinations of these gases, all of which are relatively free from oxygen, are used in the plasma spraying process. Hence, any oxygen in the sprayed coating is incidentally due to oxidation of the molten particles by oxygen impurity in the plasma gas and/or surface oxidation of the freshly deposited coating. In such "pure" molybdenum coatings the oxygen level is in the 1% to 2% range. Such coatings are softer than their flame sprayed counterparts. For higher hardness, therefore, a more expensive process such as the flame spray process which requires wire, or a more expensive powder such as molybdenum plus nickel-base alloy must be used. It would be desirable therefore to have a method of producing molybdenum powders of sufficiently high oxygen content to enable them to be used in a plasma spray process to produce hard coatings. US Patent 4,146,388 describes and claims molybdenum plasma spray powders and a process for producing the powders of molybdenum having an oxygen content of from about 0.5 to about 15% by weight oxygen. The process involves passing molybdenum particles through a plasma with oxygen or oxides of molybdenum to produce the oxygen containing (oxidized) powder. Summary of the Invention In accordance with a first aspect of this invention, there is provided a method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, involving contacting the powder with a relatively dilute solution containing a sufficient amount of oxidizing agent for a sufficient time to increase the oxygen content of the powder followed by removing the resulting partially oxidized powder from the resulting solution. In accordance with a second aspect of this invention, there is provided a method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, involving heating the powder at sufficient temperature for a sufficient time in the presence of water vapor, and a non-oxidizing atmosphere with the amount of the non-oxidizing atmosphere being controlled to produce a partially oxidized molybdenum powder. In accordance with a third aspect of this invention, there is provided a method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder. The method involves forming a relatively uniform mixture of agglomerated powders consisting essentially of molybdenum and one or more oxygen containing compounds of molybdenum wherein the mixture has an oxygen content of greater than about 10% by weight and reducing the mixture at a sufficient temperature for a sufficient time to remove a portion of the oxygen therefrom and form a molybdenum powder having an oxygen content of no greater than about 10% by weight. Brief Description of the Drawing Figure 1 is a plot of nitrogen flow rate versus weight percent oxygen in the oxidized molybdenum powder (second aspect of the invention). Detailed Description of the Invention For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above described drawing and description of some of the aspects of the invention. In particular applications in which the controlled oxygen molybdenum powders resulting from the method of this invention are preferably used, that is, in plasma spray applications, the desired oxygen content is from about 1% to about 15% and preferably from about 7% to about 10% by weight. At levels lower than this, the hardness of the plasma coating is not improved. At levels higher than this range, coating integrity or bond strength is compromised. US Patent 4,146,388 discloses a process for producing molybdenum spray powders containing oxygen by the plasma melting process or in conjunction with the plasma melting process. By the process of this invention the oxygen level in molybdenum powders is controlled in preparation for plasma coating applications. In the practice of this invention, the molybdenum powder has been previously agglomerated and sintered by well known methods. One preferred method of agglomerating the molybdenum powder is described in US Patent 3,973,948. Methods are disclosed also in a paper entitled "Properties of Oxygen-Bearing Molybdenum Coating," published in the proceedings of the Ninth International Thermal Spray Conference, The Hague, Netherlands, May 19-23, 1980. The preferred molybdenum powder of this invention is supplied by the Chemical and Metallurigcal Division of GTE Products Corporation under the designation SA-101. By the method according to the first aspect of this invention, the oxygen content of conventional molybdenum powders which preferably consist essentially of no greater than about 0.05% by weight oxygen can be increased by contacting the powder with a relatively dilute solution containing a sufficient amount of an oxidizing agent, which is preferably hydrogen peroxide for a sufficient time to raise the oxygen content to the desired level. The oxidizing agent must be present in a sufficient amount to raise the oxygen to the desired level, but not in excess amounts to cause the reaction to be uncontrolled. A dilute solution of the oxidizing agent affords better control and avoids excessive oxidation The solution contains preferably from about 1% to about 10% by weight of the oxidizing agent. By controlling the contact time, amount and dilution of the oxidizing solution, and the ratio of the molybdenum to the oxidizing agent, the oxidation can be controlled. It is the preferred practice, when using hydrogen peroxide, to have from about 2 to about 5 moles of the oxidizing agent present per mole of molybdenum. The resulting partially oxidized molybdenum powder is then separated from the resulting solution by any standard technique such as filtration. To more fully illustrate this first aspect of the invention, the following non-limiting example is presented. Example About 7 grams of molybdenum powder type SA-101 from GTE which has been spray dried and which is -200, +325 mesh and containing about 0.2% by weight oxygen is contacted with a solution consisting essentially of about 6 cc of 30% hydrogen peroxide in about 150 cc of deionized water for about 2 hours. The resulting partially oxidized molybdenum powder is then filtered off and dried. Analysis of this powder shows an oxygen content of about 1.8% by weight. By the method according to the second aspect of this invention, the oxygen content of conventional molybdenum powders which preferably consist essentially of less than about 0.05% by weight oxygen can be increased by heating the agglomerated and sintered powder at a sufficient temperature for a sufficient time in the presence of water vapor and a non-oxidizing atmosphere with the amount of the non-oxidizing atmosphere being controlled to produce a partially oxidized molybdenum powder. The heating can be done by any standard method for heating metal powders. Preferred methods involve the use of a rotary calciner or a fluidized bed. Heating temperatures are generally from about 700°C to about 900°C with from about 750°C to about 850°C being preferred. The heating time depends on the temperature and on the type of equipment used. For example, the slope of the calciner tube can be adjusted to vary the length of time that the powder remains in the calciner. Water is introduced into the furnace to provide the necessary moisture for the process. The preferred non-oxidizing atmosphere is nitrogen. By controlling the amount of non-oxidizing atmosphere in the ambient atmosphere of the furnace, the degree of oxidation or the oxygen content of the molybdenum powder is controlled. The amount of the non-oxidizing gas can be controlled by controlling the flow rates. It has been found, for example, that when the flow rate of the non-oxidizing atmosphere, for example, nitrogen, is decreased, the oxygen content of the resulting partially oxidized molybdenum is increased. This will be apparent in the example that follows. The resulting partially oxidized molybdenum powder is made up of essentially spherical particles. The oxygen content of this powder ranges from about 3% to about 15% by weight. X-ray analyses of the partially oxidized power generally show molybdenum trioxide. Undesirable molybdenum trioxide can be eliminated by using ammonia solution to dissolve it without disturbing other desirable properties of the powder. Also, in the event that the oxygen content is too high, the powder can be subjected to standard reduction methods to reduce the oxygen content. To more fully illustrate this second aspect of the invention, the following non-limiting example is presented. Example Molybdenum powder type SA-101 from GTE which has been spray dried and which is -200, +325 mesh (44 to 74µm) is fed at the rate of about 12 pounds (5.4 kg) per hour into a 6" (152.44 mm) diameter rotating calciner at a temperature of about 800°C under a nitrogen atmosphere. Water is fed by a separatory funnel at the rate of about 20 to 30 cm³/min to provide the necessary moisture for the oxidation process. The resulting powder is in the form of essentially spherical brown particles with a bulk density of from about 2.4 to about 2.8 g/cm³. The above procedure is carried out with the flow rate of the nitrogen being varied. The oxygen contents of the resulting powders are given below along with the flow rates of the nitrogen. A plot of the nitrogen flow rate versus weight percent oxygen in the resulting powder is shown in Figure 1. It can be seen that a direct correlation exists between the flow rate of the nitrogen and the percent oxygen in the oxidized powder. As the nitrogen flow rate decreases, the degree of oxidation as shown by the weight percent oxygen increases. With regard to the third aspect of the present invention, the oxygen containing compounds of molybdenum can be molybdenum dioxide, molbydenum trioxide, or ammonium paramolybdate. It is to be understood that any mixture of molybdenum and oxygen containing compounds of molybdenum can be used as long as the oxygen content of the agglomerated mixture is greater than about 10% by weight. A typical composition of the mixture of molybdenum powders to be agglomerated consists essentially of in percent by weight about 40% molybdenum, about 50% molybdenum dioxide, and about 10% molybdenum trioxide, with the oxygen content being about 15.8% by weight. The mixture of molybdenum and the oxygen containing compound or compounds can be agglomerated by any of several methods well known in the art. Within the third aspect of the invention, a preferred method involves generally forming a slurry of water, ammonia or ammonium hydroxide, molybdenum trioxide, molybdenum dioxide, and ultrafine (about 1 to 3 microns in diameter) molybdenum particles. Ammonium paramolybdate is formed from the molybdenum trioxide and ammonium hydroxide and acts as the binder in this system. The resulting slurry is then spray dried to remove the water and form the relatively uniform agglomerated mixture which consists of essentially spherical particles. The above method of forming the agglomerated mixture is described in US Patent 3,973,948 which is hereby incorporated by reference. Another method of forming the relatively uniform agglomerated mixture is by first forming a slurry as described above. The water is allowed to evaporate while the slurry is being continually stirred to break up the material. The resultant coarse moist powder is then forced through a 100 mesh screen and collected. The powder is then further dried with gentle agitation. The final agglomerated mixture is then screened from this dried mixture. Some preferred methods of agglomerating the molybdenum powder are disclosed in the above-mentioned paper entitled "Properties of Oxygen-Bearing Molybdenum Coatings," published in the proceedings of the Ninth International Thermal Spray Conference, The Hague, Netherlands, May 19-23, 1980. In accordance with the third aspect of the invention, the resulting agglomerated mixture is then reduced at a sufficient temperature for a sufficient time to remove a portion of the oxygen and form a molybdenum powder having an oxygen content of no greater than about 10% and preferably from about 7% to 10% by weight. The reduction can be done in a standard furnace in a dry hydrogen atmosphere. The preferred temperatures are from about 700°C to about 1000°C. The time depends on the temperature and on the nature of the equipment. However, typical times are from about 2 hours to about 4 hours. The reduction can be done in a fluidized bed or rotary calciner. The reducing conditions are adjusted to give the final desired oxygen content. The advantages of using a fluidized bed or rotary calciner over the above described static bed reduction are that a bed depth problem is avoided resulting in a more uniform reduction than in the static bed furnace. In a fluidized bed or rotary calciner the reduction of the agglomerate takes place from the outside of the agglomerates to the inside resulting in the metal phase being on the outside of the agglomerates. This results in a more efficient melting of the agglomerates in the plasma application and therefore produces a harder coating. The resulting powders with controlled oxygen levels are used in plasma spraying applications to produce coatings such as on piston rings. To more fully illustrate the third aspect of this invention, the following non-limiting example is presented. All parts, portions, and percentages are on a weight basis unless otherwise stated. Example An aqueous slurry is made up consisting essentially of about 85% solids, the solids consisting essentially of about 11% molybdenum trioxide, about 52% molybdenum dioxide, and about 36% molybdenum, the oxygen content of the solids being about 16.6%, and ammonia in an amount equal to about 87% of the molybdenum trioxide. The slurry is dried in a conventional dryer to produce a relatively uniform agglomerated mixture consisting of particles which are essentially spherical in shape. The mixture is reduced in dry hydrogen at about 800°C for about 2 hours resulting in a free-flowing molybdenum spray powder having an oxygen content of from about 7% to about 8%. While there has been shown and described what are at present considered the preferred embodiments of the three aspects of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, said method comprising a) contacting said powder with a relatively dilute solution containing a sufficient amount of an oxidizing agent for a sufficient time to increase the oxygen content of said powder; and b) removing the resulting partially oxidized powder from the resulting solution. A method of claim 1 wherein said powder is contacted with a solution consisting essentially of in percent by weight from about 1% to about 10% of said oxidizing agent with from about 2 moles to about 5 moles of said oxidizing agent being present per mole of molybdenum. A method of claim 2 wherein said oxidizing agent is hydrogen peroxide. A method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, said method comprising heating said molybdenum powder at a sufficient temperature for a sufficient time in the presence of water vapor, and a non-oxidizing atmosphere with the amount of said non-oxidizing atmosphere being controlled to produce a partially oxidized molybdenum powder. A method of claim 4 wherein the temperature is from about 700°C to about 900°C. A method of claim 5 wherein said non-oxidizing atmosphere is nitrogen. A method of claim 6 wherein the oxygen content of said partially oxidized molybdenum powder is from about 3% to about 15% by weight. A method for introducing a controlled level of oxygen into agglomerated molybdenum metal powder, said method comprising: a) forming a relatively uniform mixture of agglomerated powders consisting essentially of molybdenum and one or more oxygen containing compounds of molybdenum, wherein said mixture has an oxygen content of greater than about 10% by weight; and b) reducing said mixture at a sufficient temperature for a sufficient time to remove a portion of the oxygen therefrom end form a molybdenum powder having an oxygen content of no greater than about 10% by weight. A method of claim 8 therein said mixture consists essentially of molybdenum, ammonium paramolybdate, and molybdenum oxides. A method of claim 8 wherein the reducing temperature is from about 700°C to about 1000°C. A method of claim 10 wherein the oxygen content of the reduced molybdenum powder is from about 7% to about 10% by weight.

Inventors: HOUCK, DAVID L., KENNEY, FRANK D., PORT, DAVID J., RITSKO, JOSEPH E.
Applicants: GTE PRODUCTS CORPORATION
Application year: 1987
Main classification: C22C32
IPC codes: C22C1, B22F1, C23C8, C23C4