A multi-component seal coating powder material and a preparation method thereof

Abstract

The present invention provides a multi-component seal coating powder material and a preparation method thereof, belonging to the technical field of thermal spraying materials. The multi-component seal coating powder material, by mass fraction, the raw materials are 10-20 parts of titanium aluminide powder, 2-8 parts of muscovite powder, 5-15 parts of boron nitride powder, 65-75 parts of Al-based mixed powder and 0.1-5 parts of binder; the Al-based mixed powder is at least one of aluminum powder, alumina balls and alumina grinding powder. The multi-component seal coating powder material provided by the present invention has good adaptability to the plasma spraying process, and the prepared coating has high quality and stability, solving the problems of poor coating tissue uniformity, low coating bonding strength and poor abrasion resistance caused by large component fluctuations and insufficient processing process stability of traditional aluminum-based suspension coatings.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal spraying materials, and particularly relates to a multi-component sealing coating powder material and a preparation method thereof. Background Art

[0002] With the rapid development of the aviation industry, higher and higher requirements are put forward for the thrust, efficiency, and fuel consumption of aeroengines. The abradable sealing coating, as a key technology in aeroengines, can improve the overall airtightness of aeroengines without damaging the blades, which is of great significance for improving the overall efficiency of aeroengines, reducing fuel consumption, and extending the service life of components.

[0003] The components of the abradable sealing coating material are relatively complex, including metals, alloys, ceramics, inorganic non-metals, etc. The coating materials are in the forms of powders, rods, wires, or solid-liquid mixed coatings, etc. The spraying processes and equipment corresponding to different materials and forms are also different. The sealing coating prepared by coating and curing an inorganic suspension coating formulated with a binder and a metal-ceramic composite powder has been applied in key components for many years compared with conventional thermal spraying due to its advantages such as no thermal effect on the substrate and low coating processing and maintenance costs. However, the inorganic suspension coating still has the following problems: 1. The binder and metal-ceramic composite powder contained in the suspension coating have complex components, the consistency of the formulated suspension is insufficient, the viscosity fluctuates greatly, and when spraying and constructing, the liquid amount on the surface of the workpiece sprayed each time has poor consistency, the spraying process stability is insufficient, and local peeling will occur during the post-treatment of the coating after spraying, sintering, and curing, and secondary repair treatment is required, which affects the quality and processing progress of the abradable sealing coating; 2. The suspension coating is constructed by spraying with a traditional manual spray gun, and problems such as discontinuous material discharge often occur during the spraying process, and local adhesion unevenness of the coating liquid is likely to occur on the surface of the part, resulting in fluctuations in the density of the cured coating and insufficient uniformity of the coating structure, directly affecting the properties such as the hardness, bonding strength, and abradability of the coating. Summary of the Invention

[0004] In order to overcome the problems of large composition fluctuations and unstable manual spraying process existing in the existing aluminum-based inorganic suspension coating, which lead to poor coating structure uniformity, low bonding strength, and easy peeling, the present invention provides a multi-component sealing coating powder material and a preparation method thereof.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] One of the technical solutions of the present invention is a multi-component sealing coating powder material. By mass, the raw materials are 10-20 parts of titanium aluminide powder, 2-8 parts of muscovite powder, 5-15 parts of boron nitride powder, 65-75 parts of Al-based mixed powder, and 0.1-5 parts of binder; the Al-based mixed powder is at least one of aluminum powder, alumina balls, and alumina grinding powder.

[0007] The second technical solution of the present invention is a method for preparing the above multi-component sealing coating powder material, comprising the following steps:

[0008] Mix titanium aluminide powder, muscovite powder, boron nitride powder, and Al-based mixed powder, and then perform ball milling to obtain a mixed material;

[0009] Mix the mixed material with a binder evenly, granulate, then dry and screen to obtain the multi-component sealing coating powder material.

[0010] The third technical solution of the present invention is an application of the above multi-component sealing coating powder material in the preparation of a sealing coating by plasma spraying.

[0011] The present invention discloses the following technical effects:

[0012] The present invention uses aluminum-based metals such as aluminum powder, alumina grinding powder, and alumina balls, oxides (aluminum oxide) or intermetallic compounds (titanium aluminide powder) as the skeleton components, which have high specific strength, specific elastic modulus, good oxidation resistance, creep resistance, excellent high-temperature strength, stiffness, and low density. It can improve the oxidation resistance and high-temperature strength of the coating and effectively reduce the coating density; hexagonal boron nitride is used as the abradable lubricating phase, which can increase the lubricity and abradability of the coating; muscovite is hard, has high mechanical strength, can withstand high temperatures and rapid temperature changes, and has good physical and chemical properties such as acid and alkali resistance. It can increase the acid and alkali resistance and electrical insulation performance of the coating, improve the frost resistance, corrosion resistance, toughness, and compactness of the coating, reduce the gas permeability of the coating, and prevent spots and cracks. Combined with the use of composite processes such as "ball milling and mixing, agglomeration stirring and granulation", the prepared multi-component aluminum-based sealing composite powder has good adaptability to the plasma spraying process, high coating quality and stability, and solves the problems of poor coating tissue uniformity, low coating hardness and bonding strength caused by large composition fluctuations and insufficient processing process stability of traditional aluminum-based suspension coatings.

[0013] After the multi-component sealing coating powder material of the present invention is plasma-sprayed, the obtained sealing coating has uniform tissue, significantly improved bonding strength and abradability. The IDR value is 4.5 - 9.8%, the bonding strength is 9.6 - 12.3 MPa, and the coating hardness is 57.8 - 62.2 HR15Y.

[0014] The coating material in powder form prepared by the present invention has the advantages of good melting and deposition effect, precise control of powder feeding rate, multiple types and strong applicability, convenient and fast transportation and storage, etc. The powder material has good adaptability to the spraying process, high coating quality and stability. Description of the Drawings

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0016] Figure 1 Morphology photos of the multi-component seal coating powder materials prepared in Example 1;

[0017] Figure 2 Microscopic morphology photos of the seal coating obtained by plasma spraying the multi-component seal coating powder materials prepared in Example 1;

[0018] Figure 3 Morphology photos of the seal coating obtained by plasma spraying the multi-component seal coating powder materials prepared in Example 1 after counter-grinding and scraping;

[0019] Figure 4 Morphology photos of the blade after counter-grinding and scraping of the seal coating obtained by plasma spraying the multi-component seal coating powder materials prepared in Example 1;

[0020] Figure 5 Microscopic tissue morphology photos of the seal coating obtained by coating and curing the multi-component aluminum-based seal coating materials prepared in Comparative Example 1;

[0021] Figure 6 Morphology photos of the seal coating obtained by coating and curing the multi-component aluminum-based seal coating materials prepared in Comparative Example 1 after counter-grinding and scraping;

[0022] Figure 7 Morphology photos of the blade after counter-grinding and scraping of the seal coating obtained by coating and curing the multi-component aluminum-based seal coating materials prepared in Comparative Example 1;

[0023] Figure 8 Morphology photos of the seal coating obtained by plasma spraying the multi-component aluminum-based seal coating materials prepared in Comparative Example 3 after counter-grinding and scraping;

[0024] Figure 9 Morphology photos of the blade after counter-grinding and scraping of the seal coating obtained by plasma spraying the multi-component aluminum-based seal coating materials prepared in Comparative Example 3. Detailed implementation manners

[0025] The various exemplary implementation manners of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0026] It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0028] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are merely exemplary.

[0029] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0030] With the development of thermal spraying technology, the sealing coating materials are mainly powders. The coating materials in powder form have advantages such as good melting and deposition effect, precise control of powder feeding rate, various types and strong applicability, convenient and fast transportation and storage, etc.; Plasma spraying is a more advanced thermal spraying technology at present. Plasma spraying utilizes the principle of generating a high-temperature arc in the ionized state of gas to heat and accelerate the coating materials. The central temperature of the plasma flame can reach up to 30000 °C and the particle flight speed can reach up to 400 m / s, which can significantly improve the density and bonding strength of the coating structure, etc. It can generate sufficient energy to melt almost all powder coating materials, and can spray various powder materials such as metal powders, ceramic powders, cermet powders, composite powders, etc., and can prepare various functional coatings such as sealing coatings, anti-ablative coatings, thermal barrier coatings, and wear-resistant coatings. At the same time, the degree of automation of the equipment is high, closed-loop control can be achieved, the process is stable, and the coating quality is consistent. The plasma spraying process of powder materials has good adaptability, and the coating quality and stability prepared are high. In order to prepare high-quality coatings, the present invention provides a multi-component sealing coating powder material suitable for plasma spraying.

[0031] In the first aspect of the present invention, a multi-component sealing coating powder material is provided. By mass, the raw materials are 10-20 parts of titanium aluminide powder, 2-8 parts of muscovite powder, 5-15 parts of boron nitride powder, 65-75 parts of Al-based mixed powder, and 0.1-5 parts of binder; the Al-based mixed powder is at least one of aluminum powder, alumina balls, and alumina grinding powder.

[0032] In a preferred embodiment of the present invention, by mass, the raw materials are 10-15 parts of titanium aluminide powder, 3-6 parts of muscovite powder, 8-12 parts of boron nitride powder, 68-73 parts of Al-based mixed powder, and 0.1-3 parts of binder; the Al-based mixed powder is at least one of aluminum powder, alumina balls, and alumina grinding powder.

[0033] In a preferred embodiment of the present invention, by mass, the raw materials are 12-15 parts of titanium aluminide powder, 4-6 parts of muscovite powder, 8-11 parts of boron nitride powder, 70-73 parts of Al-based mixed powder, and 0.1-2 parts of binder; the Al-based mixed powder is at least one of aluminum powder, alumina balls, and alumina grinding powder.

[0034] In a preferred embodiment of the present invention, in the Al-based mixed powder, the mass ratio of aluminum powder, alumina balls, and alumina grinding powder is 6.5-7:1.4-1.6:1 (more preferably, 6.5:1.5:1).

[0035] In the present invention, by mass percentage, the components of the multi-component sealing coating powder material are: Ti 2-9%, boron nitride 5-15%, SiO2 1-4%, binder 0.1-5.0%, and the balance is Al and other inevitable impurities. In the present invention, the Ti element in the multi-component sealing coating powder material is provided by the titanium aluminide powder, SiO2 is provided by the muscovite powder, the Al-based mixed powder provides the remaining required aluminum element, and the impurities in the multi-component sealing coating powder material are brought in by the inevitable impurities in the raw materials. For example, in addition to SiO2, the muscovite powder also contains potassium oxide, magnesium oxide, etc.

[0036] In a preferred embodiment of the present invention, the binder is prepared by dissolving CrO3 in an aluminum dihydrogen phosphate solution; the mass concentration of the aluminum dihydrogen phosphate solution is 50%-80%; the concentration of CrO3 in the binder is 65-75 g / L.

[0037] In a preferred embodiment of the present invention, the particle size of the titanium aluminide powder is 10 - 180 μm; the particle size of the muscovite powder is 10 - 180 μm; the boron nitride powder is hexagonal boron nitride powder; the particle size of the hexagonal boron nitride powder is 5 - 20 μm; the particle size of the aluminum powder is 10 - 180 μm; the particle size of the alumina grinding powder is 35 - 75 μm; the particle size of the alumina balls is 10 - 180 μm.

[0038] The second aspect of the present invention provides a preparation method of the above multi-component seal coating powder material, comprising the following steps:

[0039] Mix the titanium aluminide powder, muscovite powder, boron nitride powder, and Al-based mixed powder, and then perform ball milling to obtain a mixture.

[0040] Mix the mixture evenly with a binder, granulate, and then dry and screen to obtain the multi-component seal coating powder material.

[0041] In a preferred embodiment of the present invention, the ball milling time is 10 h - 14 h, and the rotation speed is 40 rpm - 60 rpm. After ball milling, use a 30-mesh sieve to collect the obtained mixture.

[0042] In a preferred embodiment of the present invention, the drying temperature is 110 - 130 °C, and the time is 4 h - 6 h.

[0043] In a preferred embodiment of the present invention, the granulation specifically is: add the mixture into a granulator, then add a binder, and perform agglomeration stirring granulation. The stirring rotation speed is 40 rpm - 80 rpm, and the stirring time is 1 - 3 h to obtain a granular multi-component seal coating powder material.

[0044] In a preferred embodiment of the present invention, the screening is performed using 60-mesh and 325-mesh sieves respectively. After screening, a multi-component seal coating powder material with a particle size of 45 - 300 µm is obtained.

[0045] The third aspect of the present invention provides an application of the above multi-component seal coating powder material in the preparation of a seal coating by plasma spraying.

[0046] The technical solutions of the present invention, unless otherwise specified, are all conventional solutions in the art. The reagents or raw materials used, unless otherwise specified, are all purchased from commercial channels or are publicly available.

[0047] To better understand the present invention, the content of the present invention will be further clarified below in conjunction with embodiments. However, the content of the present invention is not limited to the following embodiments only.

[0048] In the examples, the particle sizes of the respective materials are as follows: the particle size of titanium aluminide powder is 10 - 180 μm; the particle size of muscovite powder is 10 - 180 μm; the boron nitride powder is hexagonal boron nitride powder; the particle size of hexagonal boron nitride powder is 5 - 20 μm; the particle size of aluminum powder is 10 - 180 μm; the particle size of alumina grinding powder is 35 - 75 μm; the particle size of alumina balls is 10 - 180 μm.

[0049] Example 1

[0050] A multi-component seal coating powder material is prepared according to the following steps:

[0051] Step 1: Put 2.64 kg of aluminum powder, 0.38 kg of alumina grinding powder, 0.60 kg of alumina balls, 0.75 kg of titanium aluminide powder, 0.29 kg of muscovite powder and 0.53 kg of hexagonal boron nitride powder into the ball mill barrel.

[0052] Step 2: Add 3 kg of zirconia grinding balls into the ball mill barrel, with a ball milling speed of 50 rpm and a ball milling time of 12 h.

[0053] Step 3: Screen the powder obtained in Step 2 using a 30-mesh sieve conforming to the GB T 5330-2003 standard and collect the mixed material.

[0054] Step 4: Measure 1200 ml of aluminum dihydrogen phosphate solution with a mass concentration of 70%, add 84 g of CrO3 powder, and stir evenly to completely dissolve it to obtain an aluminum-chromium phosphate binder.

[0055] Step 5: Add the mixed material obtained in Step 3 into the granulator, then add the binder obtained in Step 4, start stirring, with a stirring speed of 70 rpm and a stirring time of 2.5 h until the powder becomes granular.

[0056] Step 6: After the stirring stops, put the granular material obtained in Step 5 into the oven for drying, at a temperature of 120 °C and a drying time of 4 h.

[0057] Step 7: Screen the granular material obtained in Step 6 using a 160-mesh sieve and a 325-mesh sieve conforming to the GB T 5330-2003 standard. The powder with a particle size below 60 mesh and above 325 mesh is the multi-component seal coating powder material. The morphology photo of this multi-component seal coating powder material is as Figure 1 shown.

[0058] The multi-component seal coating powder materials prepared in this example were subjected to plasma spraying. The parameters of plasma spraying were as follows: the main gas flow rate was 25 - 45 NLPM, the auxiliary gas flow rate was 5 - 10 NLPM, the carrier gas flow rate was 3 - 6 NLPM, the current was 500 - 600 A, the spraying power was 30 - 40 kW, the powder feeding rate was 20 - 50 g / min, the spraying distance was 80 - 120 mm, and the gun traversing rate was 3 - 8 mm / s. During the plasma spraying process, the powder feeding was smooth and uninterrupted, and the prepared seal coating had a uniform structure, and the bonding strength and abradability were significantly improved. Figure 2 It is a micrograph of the coating morphology after plasma spraying. Under the simulated working conditions of high temperature and high speed of 300 °C, 300 m / s, and 50 μm / s and the abrasion and scraping conditions against the titanium alloy blade, the wear scar of the coating was smooth and flat ( Figure 3 ), and there was no wear at the tip of the blade ( Figure 4 ), and the abradability was significantly improved, and the IDR value was 6.3%. The bonding strength of the plasma-sprayed coating was 10.9 MPa; the coating hardness was 59.8 HR15Y.

[0059] Comparative Example 1

[0060] A multi-component aluminum-based seal coating material was prepared according to the following steps:

[0061] Step 1: The same as Step 1 of Example 1.

[0062] Step 2: The same as Step 2 of Example 1.

[0063] Step 3: The same as Step 3 of Example 1.

[0064] Step 4: Measure 3800 ml of aluminum dihydrogen phosphate solution with a mass concentration of 30%, add 84 g of CrO3 powder, and stir evenly until it is completely dissolved to obtain a suspension coating (i.e., the multi-component aluminum-based seal coating material).

[0065] The suspension coating of this comparative example was used to prepare a seal coating by coating and curing. The coating structure was uneven ( Figure 5 ). Under the simulated working conditions of high temperature and high speed of 300 °C, 300 m / s, and 50 μm / s and the abrasion and scraping conditions against the titanium alloy blade, the coating was hardly scraped ( Figure 6 ), and the blade was severely worn and deformed ( Figure 7 ), and the IDR value was 76%, and the abradability was poor. The bonding strength of the coating prepared from the suspension coating was 4.5 MPa, and the hardness was 52.2 HR15Y.

[0066] Comparative Example 2

[0067] A multi-component aluminum-based seal coating material was prepared according to the following steps:

[0068] Step 1: The same as Step 1 of Example 1.

[0069] Step 2: The same as Step 2 of Example 1.

[0070] Step 3: The same as Step 3 of Example 1.

[0071] Step 4: Measure 1000 ml of aluminum dihydrogen phosphate solution with a mass concentration of 70%, add 170 g of CrO3 powder, and stir evenly until it is completely dissolved to obtain an aluminum-chromium phosphate binder.

[0072] Step 5: The same as Step 5 of Example 1.

[0073] Step 6: The same as Step 6 of Example 1.

[0074] Step 7: The same as Step 7 of Example 1.

[0075] Compared with Example 1, in this comparative example, the addition amount of the binder is too much, resulting in uneven particles of the multi-component seal coating powder material prepared, too large particle size, and poor adaptability to plasma spraying.

[0076] Comparative Example 3

[0077] The difference from Example 1 is only that 2.64 kg of aluminum powder is replaced by 3.2 kg of aluminum powder, and other steps and parameters are the same as those of Example 1.

[0078] Perform plasma spraying on the multi-component seal coating powder material of this comparative example, and the plasma spraying parameters are the same as those of Example 1. The hardness of the obtained coating: 39.5 HR15Y, and the bonding strength is 5.7 MPa; under the simulated working conditions of high temperature, high speed and abrasion with a titanium alloy blade at 300 °C, 300 m / s, and 50 μm / s, there are gullies on the coating surface ( Figure 8 ), and obvious adhesion marks on the tip of the blade ( Figure 9 ), and the IDR value is -15.5%, and the abradability is poor. That is, compared with Example 1, the addition amount of aluminum is too much, resulting in a decrease in the hardness and bonding strength of the coating and a decrease in abradability (negative IDR value, adhesion at the tip of the blade).

[0079] Comparative Example 4

[0080] The difference from Example 1 is only that 0.29 kg of mica powder is replaced by 0.52 kg of mica powder, and other steps and parameters are the same as those of Example 1.

[0081] The multi-component seal coating powder material of this comparative example was subjected to plasma spraying, and the plasma spraying parameters were the same as those in Example 1. The hardness of the obtained coating: 76.4 HR15Y; under the simulated working conditions of 300 °C, 300 m / s, and 50 μm / s with high temperature, high speed, and abrasive wear against a titanium alloy blade, there were grooves on the coating surface, and the tip of the blade was severely worn. The IDR value was 18.9%, and the abradability was poor. That is, compared with Example 1, the excessive addition of muscovite led to an increase in the coating hardness and a decrease in the abradability (the IDR value increased, and the blade wear became more serious).

[0082] Comparative Example 5

[0083] The difference from Example 1 was only that the addition of hexagonal boron nitride was omitted, and other steps and parameters were the same as those in Example 1.

[0084] The multi-component seal coating powder material of this comparative example was subjected to plasma spraying, and the plasma spraying parameters were the same as those in Example 1. The hardness of the obtained coating: 61.3 HR15Y; under the simulated working conditions of 300 °C, 300 m / s, and 50 μm / s with high temperature, high speed, and abrasive wear against a titanium alloy blade, there were grooves on the coating surface, and the adhesion at the tip of the blade was more obvious. The IDR value was -23.6%, and the abradability was poor. That is, compared with Example 1, without adding hexagonal boron nitride powder to the raw materials, the coating hardness increased slightly, the abradability decreased, and the adhesion at the tip was more obvious.

[0085] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. Application of a multi-component seal coating powder material in preparing a seal coating by plasma spraying, characterized in that By mass parts, the raw materials of the multi-component seal coating powder material are 12 - 15 parts of titanium aluminide powder, 4 - 6 parts of muscovite powder, 8 - 11 parts of boron nitride powder, 70 - 73 parts of Al-based mixed powder, and 0.1 - 2 parts of binder; the Al-based mixed powder is aluminum powder, alumina balls, and alumina grinding powder; In the Al-based mixed powder, the mass ratio of aluminum powder, alumina balls, and alumina grinding powder is 6.5 - 7:1.4 - 1.6:1; The binder is prepared by dissolving CrO3 in an aluminum dihydrogen phosphate solution; the mass concentration of the aluminum dihydrogen phosphate solution is 50% - 80%; the concentration of CrO3 in the binder is 65 - 75 g / L; The particle size of the titanium aluminide powder is 10 - 180 μm; the particle size of the muscovite powder is 10 - 180 μm; the boron nitride powder is hexagonal boron nitride powder; the particle size of the hexagonal boron nitride powder is 5 - 20 μm; the particle size of the aluminum powder is 10 - 180 μm; the particle size of the alumina grinding powder is 35 - 75 μm; the particle size of the alumina balls is 10 - 180 μm.

2. Use of the multi-component seal coating powder material according to claim 1 in the preparation of a seal coating by plasma spraying, characterized in that, The preparation method of the multi-component seal coating powder material includes the following steps: Mix the titanium aluminide powder, muscovite powder, boron nitride powder, and Al-based mixed powder and then perform ball milling to obtain a mixed material; Mix the mixed material with the binder evenly, granulate, then dry and screen to obtain the multi-component seal coating powder material.

3. The application of the multi-component seal coating powder material according to claim 2 in the preparation of a seal coating by plasma spraying, characterized in that, The time of the ball milling is 10 h - 14 h, and the rotation speed is 40 rpm - 60 rpm.

4. Use of the multi-component seal coating powder material according to claim 2 in the preparation of a seal coating by plasma spraying, characterized in that The temperature of the drying is 110 - 130 °C, and the time is 4 h - 6 h.

5. Use of the multi-component seal coating powder material according to claim 2 in preparing a seal coating by plasma spraying, characterized in that, The screening is performed using 60-mesh and 325-mesh sieves respectively.

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