SLURRY FOR THERMAL SPRAYING

Abstract

To provide a slurry for thermal spraying capable of forming a favorable sprayed coating. The present invention provides a slurry for thermal spraying including spray particles including at least one material selected from the group consisting of ceramics, inorganic compounds, cermets, and metals and a dispersion medium. Here, the spray particles have an average particle size of 0.01 m or more and 10 m or less and are contained in the slurry for thermal spraying at a proportion of 10% by mass or more and 70% by mass or less. In the slurry for thermal spraying, the spray particles have a zeta potential of 200 mV or more and 200 mV or less.

Claims

1. A slurry for thermal spraying, the slurry comprising: spray particles including at least one material selected from the group consisting of ceramics, inorganic compounds, cermets, and metals; and a dispersion medium, wherein the spray particles have an average particle size of 0.01 m or more and 10 m or less, the spray particles are contained in the slurry for thermal spraying at a proportion of 10% by mass or more and 70% by mass or less, and in the slurry for thermal spraying, the spray particles have a zeta potential of 200 mV or more and 200 mV or less.

2. The slurry for thermal spraying according to claim 1, further comprising a dispersant.

3. The slurry for thermal spraying according to claim 1, wherein at least some of the spray particles include an yttrium oxyfluoride.

4. The slurry for thermal spraying according to claim 1, wherein at least some of the spray particles include a rare earth halide.

5. The slurry for thermal spraying according to claim 1, wherein the slurry for thermal spraying has a viscosity of 1,000 mPa.Math.s or less.

6. The slurry for thermal spraying according to claim 1, wherein the dispersion medium is an aqueous dispersion medium.

7. The slurry for thermal spraying according to claim 1, wherein the dispersion medium is a nonaqueous dispersion medium.

8. A sprayed coating including a thermal spray product of the slurry for thermal spraying according to claim 1.

Description

EXAMPLE 1

[0098] Preparation material A1: particles for thermal spraying

[0099] Preparation material B1: a dispersion medium

EXAMPLE 2

[0100] Preparation material A2: particles for thermal spraying and some of a dispersion medium

[0101] Preparation material B2: the remainder of the dispersion medium

EXAMPLE 3

[0102] Preparation material A3: particles for thermal spraying

[0103] Preparation material B3: a dispersion medium and optional components (additives)

EXAMPLE 4

[0104] Preparation material A4: particles for thermal spraying

[0105] Preparation material B4: a dispersion medium

[0106] Preparation material C4: optional components (additives)

[0107] When a plurality of optional components are used here, the preparation material C4 may include preparation materials C4n (n is natural numbers) of the respective optional components, for example.

[0108] In this manner, in the material for preparing a slurry for thermal spraying disclosed here, the respective components constituting a slurry for thermal spraying, such as spray particles, a dispersion medium, a dispersant, and other optional components, may be provided in separate packages each containing a single component or a mixture of two or more components. The material for preparing a slurry for thermal spraying may be mixed with other components (optionally other materials for preparing a slurry for thermal spraying) before thermal spraying to give a slurry for thermal spraying. From the viewpoint of easy transportation, it is preferred that components other than a dispersion medium be prepared in a single package as a material for preparing a slurry for thermal spraying, and the dispersion medium be prepared in another package as a material for preparing a slurry for thermal spraying (optionally another material for preparing a slurry for thermal spraying). Components other than the dispersion medium (particles for thermal spraying and optional components such as additives) can be in a powder (solid) form, for example. For example, when the dispersion medium is an easy available material such as water, a user of the slurry for thermal spraying can independently prepare such a dispersion medium. In terms of uniformity of a slurry for thermal spraying or stable performance of a coating, the slurry for thermal spraying to be subjected to thermal spraying is preferably prepared as a high concentration slurry containing spray particles at a higher concentration.

[0109] The above material for preparing a slurry for thermal spraying may include information for preparing a slurry for thermal spraying. The information can also be understood as the preparation method for preparing a slurry for thermal spraying by using the material for preparing a slurry for thermal spraying. For example, information about the procedure of mixing components in separate packages or about materials required in addition to the material for preparing a slurry for thermal spraying is displayed. Although the material for preparing a slurry for thermal spraying is so constructed as to give a feeding performance index If of 70% or more, information for further improving the If value may be displayed. Such information may be displayed on the containers of components or on a covering material or the like in which such a container is stored. Alternatively, a paper sheet or the like on which information is described may be set (packed) in the container of a component. The information can be so constructed as to be available by a user having the material for preparing a slurry for thermal spraying through the Internet or the like. Accordingly, the material for preparing a slurry for thermal spraying disclosed here can be used to more easily and certainly form a sprayed coating at high efficiency.

[0110] [Coating Formation Method]

[0111] (Substrate)

[0112] In the method for forming a sprayed coating disclosed here, the substrate on which a sprayed coating is formed is not limited to particular substrates. For example, any substrate made from various materials can be used as long as the substrate is made from a material that can have an intended resistance when the substrate is subjected to the thermal spraying. Examples of such a material include various metals and alloys. Such a material is specifically exemplified by aluminum, aluminum alloys, iron, steel, copper, copper alloys, nickel, nickel alloys, gold, silver, bismuth, manganese, zinc, and zinc alloys. Of them, substrates made of steels typified by various SUS materials having comparatively high thermal expansion coefficients in general purpose metal materials (optionally what is called stainless steel), heat-resistant alloys typified by inconel, low-expansion alloys typified by invar and kovar, corrosion-resistant alloys typified by hastelloy, and aluminum alloys typified by 1,000 series to 7,000 series aluminum alloys that are useful as lightweight structural materials and the like are exemplified.

[0113] (Coating Formation Method)

[0114] The slurry for thermal spraying disclosed here can be subjected to a thermal spraying apparatus based on a known thermal spraying method and thus can be used as the material for thermal spraying in order to form a sprayed coating. When the slurry for thermal spraying is allowed to stand for a certain period of time typically for storage, the spray particles can start to sediment and precipitate in a dispersion medium. Hence, the slurry for thermal spraying in the technique disclosed here can be so prepared as to give a feeding performance index If of 70% or more, which is determined by the above procedure, when the slurry is subjected to thermal spraying (for example, in the preparation step for feeding the slurry to a thermal spraying apparatus). For example, a slurry for thermal spraying in a storage state before thermal spraying (also called a precursor liquid) can be prepared as a high concentration slurry containing spray particles at a higher concentration.

[0115] As the thermal spray method of appropriately, thermally spraying the slurry for thermal spraying, a thermal spray method such as plasma spraying and high velocity flame spraying can be preferably adopted, for example.

[0116] The plasma spraying is a thermal spray method that uses a plasma flame as a thermal spraying heat source for softening or melting a thermal spraying material. Between electrodes, arc is generated, and the arc functions to convert a working gas into plasma. Such a plasma flow is ejected from a nozzle as a plasma jet at high temperature and high speed. The plasma spraying generally encompasses coating techniques in which a material for thermal spraying is introduced to the plasma jet, then heated and accelerated, and deposited on a substrate to form a sprayed coating. The plasma spraying can be atmospheric plasma spraying (APS) that is performed in the atmosphere, low pressure plasma spraying (LPS) in which thermal spraying is performed at a lower pressure than the atmospheric pressure, or high pressure plasma spraying in which plasma spraying is performed in a pressurized container at a higher pressure than the atmospheric pressure, for example. In such plasma spraying, by using a plasma jet at about 5,000 C. to 10,000 C. to melt and accelerate a thermal spraying material, the spray particles can be hit against a substrate at a speed of about 300 m/s to 600 m/s and deposited, for example.

[0117] The high velocity flame spraying can be high velocity oxygen fuel (HVOF) thermal spraying, warm spray thermal spraying, or high velocity air fuel (HVAF) flame spraying, for example.

[0118] The HVOF thermal spraying is a flame spraying that uses a combustion flame prepared by burning a mixture of a fuel and oxygen at high pressure, as the heat source for thermal spraying. By increasing the pressure in a combustion chamber, a continuous combustion flame is ejected from a nozzle at high speed (optionally supersonic speed) as a high temperature gas flow. The HVOF thermal spraying generally encompasses coating techniques in which a material for thermal spraying is introduced to the gas flow, then heated and accelerated, and deposited on a substrate to form a sprayed coating. In the HVOF thermal spraying, for example, by feeding a slurry for thermal spraying to a supersonic combustion flame jet at 2,000 C. to 3,000 C., a dispersion medium can be removed (optionally burned or evaporated, hereinafter, the same applies) from the slurry. Concurrently, the spray particles can be softened and melted, then hit against a substrate at a high speed of 500 m/s to 1,000 m/s, and deposited. The fuel used for the high velocity flame spraying may be a hydrocarbon gas fuel such as acetylene, ethylene, propane, and propylene or may be a liquid fuel such as kerosene and ethanol. As a thermal spraying material has a higher melting point, the temperature of the supersonic combustion flame is preferably higher. From this viewpoint, a gas fuel is preferably used.

[0119] Alternatively, a thermal spraying method called warm spray thermal spraying to which the HVOF thermal spraying is applied can be adopted. The warm spray thermal spraying is typically a technique in which thermal spraying is performed in a condition where the combustion flame in the HVOF thermal spraying is mixed with a cooling gas including nitrogen or the like at around room temperature to reduce the temperature of the combustion flame, thereby forming a sprayed coating. The thermal spraying material when subjected to thermal spraying is not necessarily, completely melted, but may be partially melted or may be in a softened state at a temperature not higher than the melting point thereof, for example. In the warm spray thermal spraying, for example, by feeding a slurry for thermal spraying to a supersonic combustion flame jet at 1,000 C. to 2,000 C., a dispersion medium can be removed (optionally burned or evaporated, hereinafter, the same applies) from the slurry. Concurrently, the spray particles can be softened and melted, then hit against a substrate at a high speed of 500 m/s to 1,000 m/s, and deposited.

[0120] The HVAF thermal spraying is a thermal spraying method in which air is fed in place of oxygen as a combustion support gas in the HVOF thermal spraying. By the HVAF thermal spraying, the thermal spraying temperature can be lowered as compared with the HVOF thermal spraying. For example, by feeding a slurry for thermal spraying to a supersonic combustion flame jet at 1,600 C. to 2,000 C., a dispersion medium can be removed (optionally burned or evaporated, hereinafter, the same applies) from the slurry. Concurrently, the spray particles can be softened and melted, then the spray particles can be hit against a substrate at a high speed of 500 m/s to 1,000 m/s, and can be deposited.

[0121] In the invention disclosed here, when the slurry for thermal spraying is preferably subjected to high velocity flame spraying or plasma spraying because a material for thermal spraying even having a comparatively large particle size can be sufficiently softened and melted, a slurry for thermal spraying including spray particles even at a high content can be thermally sprayed with good flowability, and a dense sprayed coating can be efficiently formed.

[0122] Although not critical, the slurry for thermal spraying is fed to a thermal spraying apparatus preferably at a flow rate of 10 mL/min or more and 200 mL/min or less. When the slurry for thermal spraying is fed at a rate of about 10 mL/min or more, the slurry that is flowing in a device for feeding a slurry for thermal spraying (for example, a slurry feed tube) can be made in a turbulent flow state, and the extrusion force of the slurry can be increased. In addition, the spray particles can be prevented from sedimenting. Such a condition is thus preferred. From such a viewpoint, the flow rate when the slurry for thermal spraying is fed is preferably 20 mL/min or more and more preferably 30 mL/min or more. Meanwhile, when the feeding rate is excessively high, the amount of the slurry may exceed the amount of a slurry that can be thermally sprayed from a thermal spraying apparatus, and thus such a condition is unfavorable. From such a viewpoint, the flow rate when the slurry for thermal spraying is fed is appropriately 200 mL/min or less, preferably 150 mL/min or less, and more preferably 100 mL/min or less, for example.

[0123] The slurry for thermal spraying is fed to a thermal spraying apparatus preferably by an axial feed system. In other words, the slurry for thermal spraying is fed preferably in the same direction as the axis of a jet flow generated in a thermal spraying apparatus. For example, when the slurry for thermal spraying of the present invention in a slurry state is fed by the axial feed system to a thermal spraying apparatus, the thermal spraying material in the slurry for thermal spraying is unlikely to adhere to the inside of the thermal spraying apparatus because the slurry for thermal spraying has good flowability. Consequently, a dense sprayed coating can be efficiently formed. Such a condition is thus preferred.

[0124] When a common feeder is used to feed the slurry for thermal spraying to a thermal spraying apparatus, the feed amount varies periodically, and thus stable feeding may be difficult. When the feed amount of the slurry for thermal spraying oscillates due to the periodic variation of the feed amount, the thermal spraying material is unlikely to be uniformly heated in a thermal spraying apparatus, and an uneven sprayed coating can be formed in some cases. In order to stably feed the slurry for thermal spraying to a thermal spraying apparatus, a two-stroke system, or two feeders may be used in such a manner that variable periods of the feed amounts of the slurry for thermal spraying from both the feeders have opposite phases to each other. Specifically, the feeding system can be controlled to give such periods that when the feed amount of one feeder increases, the feed amount of the other feeder decreases, for example. When the slurry for thermal spraying of the present invention is fed to a thermal spraying apparatus by the two-stroke system, a dense sprayed coating can be efficiently formed because the slurry for thermal spraying has good flowability.

[0125] As the means for stably feeding a material for thermal spraying in a slurry form to a thermal spraying apparatus, the slurry sent from a feeder may be once stored in a storage tank provided just before the thermal spraying apparatus, and the slurry may be fed from the storage tank to the thermal spraying apparatus by using natural drop. Alternatively, the slurry in the tank may be forcedly fed to the thermal spraying apparatus by using a means such as a pump. When the slurry is forcedly fed by a means such as a pump, a thermal spraying material in the slurry is unlikely to adhere to the inside of a tube that connects the tank and the thermal spraying apparatus. Such a condition is thus preferred. In order to uniformize the distribution state of components in the slurry for thermal spraying in the tank, a means of stirring the slurry for thermal spraying in the tank may be provided.

[0126] The slurry for thermal spraying is fed to a thermal spraying apparatus preferably through a metal conductive tube, for example. When a conductive tube is used, static electricity can be prevented from generating, and thus the feed amount of the slurry for thermal spraying is unlikely to vary. The inner surface of the conductive tube preferably has a surface roughness Ra of 0.2 m or less.

[0127] A thermal spraying distance is the distance from the tip of a nozzle of a thermal spraying apparatus to a substrate and is preferably set to 30 mm or more. When the thermal spraying distance is excessively small, the time for removing a dispersion medium in the slurry for thermal spraying or for softening/melting spray particles may be insufficiently secured, or a thermal spraying heat source is excessively close to a substrate, and thus the substrate may deteriorate or be deformed. Such a condition is therefore unfavorable.

[0128] The thermal spraying distance is preferably about 200 mm or less (preferably 150 mm or less, for example, 100 mm or less). Such a distance allows spray particles sufficiently heated to reach to a substrate while the temperature is maintained, and thus a denser sprayed coating can be produced.

[0129] For thermal spraying, a substrate is cooled preferably from the side opposite to the side undergoing thermal spraying. Such cooling can be water cooling or cooling with an appropriate refrigerant.

[0130] (Sprayed Coating)

[0131] By the technique disclosed here, a sprayed coating including a compound having the same composition as spray particles and/or a degradation product thereof is formed.

[0132] The sprayed coating is formed by using a slurry for thermal spraying in which spray particles have an absolute zeta potential of 200 mV or less and are satisfactory dispersed. Thus, spray particles are maintained in an appropriate dispersion state and a flow state in the slurry for thermal spraying, are stably fed to a thermal spraying apparatus, and form a uniform sprayed coating. The spray particles are not hit by a flame or a jet but can be efficiently fed to the vicinity of the center of a heat source and sufficiently softened or melted. Hence, the softened or melted spray particles densely adhere to a substrate and to each other with good adhesiveness. Accordingly, a sprayed coating having good uniformity and adhesiveness is formed at an appropriate coating forming speed.

[0133] Some examples of the present invention will next be described, but the present invention is not intended to be limited to these examples.

[0134] [Preparation of Slurry for Thermal Spraying]

[0135] As spray particles, yttria (Y.sub.2O.sub.3), alumina (Al.sub.2O.sub.3), yttrium fluoride (YF.sub.3), and yttrium oxyfluorides having various compositions (YOF, Y.sub.5O.sub.4F.sub.7, Y.sub.6O.sub.5F.sub.8, Y.sub.7O.sub.6F.sub.9) , having the corresponding average particle sizes shown in Table 1 were prepared. As dispersion media, distilled water was prepared as an aqueous dispersion medium, and a mixed solvent containing ethanol (EtOH), isopropyl alcohol (i-PrOH), and n-propyl alcohol (n-PrOH) at 85:5:10 in terms of volume ratio was prepared as a nonaqueous dispersion medium. As additives as optional components, dispersants and a viscosity modifier were prepared. As the dispersant, any of an aqueous nonionic surfactant-type dispersant (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., Noigen XL-400) and a nonaqueous special polycarboxylic acid polymer surfactant (manufactured by Kao Corporation, HOMOGENOL L-18) was used. As the viscosity modifier, an anionic special modified polyvinyl alcohol (PVOH) (manufactured by The Nippon Synthetic Chemical Industry Co., Ltd., Gohsenx L-3266) was used. Such particles for thermal spraying and a dispersion medium were prepared in different containers in such a manner that the proportion of particles for thermal spraying would be 30% by mass.

[0136] The particles for thermal spraying and the dispersion medium were mixed together with a dispersant and a viscosity modifier in accordance with the formulations shown in Table 1, giving slurries for thermal spraying of Examples 1 to 27 each having a proportion of particles for thermal spraying of 30% by mass. In the present embodiment, the amount of the viscosity modifier was constant at 0.1% by mass relative to the mass of spray particles. In Table 1, - in the viscosity modifier column means that no viscosity modifier was used. The amount of a dispersant was appropriately controlled while the dispersion state of spray particles in a slurry for thermal spraying was observed, and the amounts used are indicated in the content column in Table 1.

[0137] [Presence or Absence of Secondary Particles Formed]

[0138] The average particle size of the spray particles in each slurry for thermal spraying prepared was determined by using a laser diffraction/scattering particle size distribution analyzer (manufactured by Horiba, Ltd., LA-950). The average particle size of the spray particles in the slurry was compared with the average primary particle size of spray particles prepared for the slurry for thermal spraying. When the average particle size of the spray particles in the slurry was 1.5 or more times larger, it was determined that the spray particles agglomerate to form secondary particles in the slurry. An example in which spray particles are determined to form secondary particles is indicated by presence in the secondary particle formation column in Table 1, and an example in which spray particles are determined not to form secondary particles is indicated by absence.

[0139] [Viscosity]

[0140] The viscosity of each slurry for thermal spraying prepared was determined by using a viscometer (manufactured by Rion, Viscotester VT-03F) in a room temperature (25 C.) environment at a rotation speed of 62.5 rpm. The results are shown in Table 1.

[0141] [Zeta Potential]

[0142] The zeta potential of the spray particles in each slurry for thermal spraying prepared was determined by using an ultrasonic particle size distribution/zeta potential analyzer (manufactured by Dispersion Technology, DT-1200).

[0143] [Feeding Performance Index If]

[0144] The feeding performance index If of each slurry for thermal spraying prepared was determined by the following procedure. In other words, first, a polyurethane tube (manufactured by CHIYODA, Touch Tube (urethane) TE-8 with an outer diameter of 8 mm and an inner diameter of 5 mm) having an inner diameter of 5 mm and a length of 5 m was placed on a test table with no difference in height. To one end of the tube, a roller pump for feeding a slurry was connected, and to the other end, a slurry recovery container was connected.

[0145] A prepared slurry for thermal spraying was stirred with a magnetic stirrer, and good dispersion state of the spray particles was ascertained. The slurry was then fed into the tube at a flow rate of 35 mL/min. The slurry for thermal spraying that had passed through the tube was recovered in the recovery container, and the spray particles contained in the recovered slurry was weighed to give mass B. From the previously determined mass A of the spray particles contained in 800 mL of the slurry for thermal spraying after preparation and the mass B of the spray particles contained in the recovered slurry, the feeding performance index If was calculated in accordance with the following equation, and the results are shown in Table 1.

If(%)=B/A100

[0146] [Formation of Sprayed Coating]

[0147] Each slurry for thermal spraying prepared above was used and thermally sprayed by an atmospheric plasma spraying (APS) method to form a sprayed coating. The thermal spraying conditions were as shown below.

[0148] In other words, first, a SS400 steel plate (70 mm50 mm2.3 mm) was prepared and was subjected to roughening treatment, and the product was used as the substrate to be subjected to thermal spraying. For APS thermal spraying, a commercially available plasma spraying apparatus (manufactured by Praxair, SG-100) was used. As for plasma generation conditions, at atmospheric pressure, argon gas was fed at a pressure of 100 psi, helium gas was fed at a pressure of 90 psi as plasma working gases, and the plasma generation power was 40 kW. To feed a slurry for thermal spraying to a thermal spraying apparatus, a slurry feeder was used to feed the slurry at a feed amount of about 100 mL/min to a burner chamber in the thermal spraying apparatus. When the slurry was fed to the thermal spraying apparatus, a storage tank was install adjacent to the thermal spraying apparatus, the prepared slurry for thermal spraying was once stored in the storage tank, and then the slurry was fed from the storage tank to the thermal spraying apparatus by using natural drop. A plasma jet was ejected from a nozzle of the thermal spraying apparatus, and the slurry for thermal spraying fed to the burner chamber was allowed to fly together with the jet while the dispersion medium in the slurry was removed. Concurrently, the spray particles were melted and were sprayed to a substrate, and consequently a coating was formed on the substrate. The conveyance speed of a thermal spraying gun was 600 mm/min, and the thermal spraying distance was 50 mm.

[0149] [Coating Formation Efficiency]

[0150] The coating formation efficiency (adhesion efficiency) of spray particles was evaluated when the slurry for thermal spraying of each example was thermally sprayed to form a coating. Specifically, the thickness (m) of a sprayed coating formed by a single pass (which means that thermal spraying is performed once from a thermal spraying apparatus to a substrate) in the above thermal spraying conditions was determined. In the present embodiment, when the coating formation efficiency is 2.5 m or more by a single pass, the formation efficiency is evaluated as good. [Table 1]

[0151] As shown in Table 1, it was revealed that the coating formation efficiency greatly varies as slurries for thermal spraying have different zeta potentials even when spray particles have the same composition and the same average particle size and are contained in the same amount (at the same concentration) as shown in Examples 1 to 16. It was further revealed that a good coating formation efficiency of 2.5 m or more is achieved when the absolute value of the zeta potential is 200 mV or less. A higher coating formation efficiency means that a slurry for thermal spraying fed to a thermal spraying apparatus has good flowability and good feeding performance.

[0152] It was also revealed that in the slurry for thermal spraying having good coating formation efficiency, the spray particles form secondary particles. The result suggests that in the slurry for thermal spraying disclosed here, primary particles of spray particles agglomerate to give a certain size, and accordingly the agglomeration particles (secondary particles) are stably dispersed in the flowing slurry for thermal spraying. It was ascertained that as a result, a slurry for thermal spraying having an absolute zeta potential of 200 mV or less had good feeding performance, which was indicated by a feeding performance index If of 70% or more.

[0153] Specific examples of the present invention have been described in detail hereinbefore, but are merely illustrative examples, and are not intended to limit the scope of claims. The techniques described in the scope of claims include various modifications and changes of the above exemplified specific examples. For example, in the above embodiment, slurries for thermal spraying were so prepared as to have various zeta potentials while the types of the dispersant and the viscosity modifier were fixed. However, selection and use of additives such as a dispersant and a viscosity modifier suitable for controlling the zeta potential can be understood by a person skilled in the art on the basis of teachings disclosed here and common general knowledge at the time of patent application.