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VOJNOTEHNIČKI GLASNIK/ MILITARY TECHNICAL COURIER, 2015., Vol. LXIII, No 4
ПРЕГЛЕДНИ ЧЛАНЦИ REVIEW PAPERS ОБОЗОРНЫЕ СТАТЬИ
MORPHOLOGY OF POWDER PARTICLES PRODUCED BY SPRAY ATOMIZATION AND OTHER PROCESSES
Mihailo R. Mrdak
Research and Development Center IMTEL
e-mail: [email protected]
DOI: 10.5937/vojtehg63-7159
FIELD: Chemical Technology ARTICLE TYPE: Review Paper ARTICLE LANGUAGE: English
Summary:
The physical and mechanical properties of coatings are heavily influenced by the technologies of powder production and by the parameters of the plasma spray process. One of the most important parameters that affects the physical and mechanical properties of coatings is the morphology of powder particles - homogeneity, granulation and granulation range, directly related to the technologies of powder production. This paper describes the technological processses of powder production most commonly used and shows the SEM micrographs of powder morphologies. Depending on the manufacturing process, powder particles have different characteristics regarding their shape, size, specific gravity, purity, etc. Since these characteristics have a significant impact on the quality and properties of deposited coatings, it is necessary to possess knowledge about the characteristics of powders in order to better control the behavior of particles in the plasma jet, in order to produce the expected characteristics of the coating.Powders have a variety of characteristics to be set for the operating parameters of the deposition in order to obtain the desired coating characteristics.
Key words: property, powders, particles, mechanical properties, coatings, characteristics. *
* Acknowledgement: The author is thankful for the financial support from the Ministry of Education and, Science and Technological development of the Republic of Serbia (national projects OI 174004, TR 34016).
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Introduction
The quality of deposited APS and VPS plasma spray coatings is influenced by several mutually dependable parameters, one of which is the powder quality, directly related to the powder production technology. Coatings of the optimal quality for respective purposes require the application of powders produced by certain technological processes. To ensure optimal properties of the coating, the powder with its chemical composition, morphology and distribution of the granulate particles must be adapted to the coating exploitation conditions. Powder manufacturers have different variations of manufacturing processes of powders in terms of morphology, density and chemical properties, which indicates that different types of powders are used for different exploitation conditions, as published in the works of authors (Vencl, et al., 2009, pp.398-405), (Vencl, et al., 2010, pp.591-604), (Vencl, et al., 2011, pp.1281-1288), (Mrdak, 2012, pp.182201), (Mrdak, 2013b, pp.68-88), (Mrdak, 2014, pp.7-22), (Mrdak, 2013a, pp.7-25), (Mrdak, et al., 2013, pp.559-567). Powder production techniques greatly influence on: powder chemical composition and homogeneity, powder particle morphology, particle size and span granulation, porosity, specific density, cleanliness, etc. (Sampath, et al., 1996, pp.629-636). Powders of the same chemical composition but produced by different manufacturing processes have different powder morphologies and characteristics. Particles of a spherical shape have a higher flowability compared to powder particles of an irregular shape. Consequently, the resulting coating properties are different, even if the deposition conditions are constant. This is due to the differences in behavior during the injection of particles into the plasma jet and the flow of molten particles to the substrate where they are deposited. Irregularly shaped particles often cause blockage in the powder inlet pipe, which leads to a reduction in the deposition rate, overheating of the substrate and the coating separation from the substrate. The situation is similar in the case of the insertion of fine particles into the plasma jet. Accordingly, it is very important to possess knowledge about the powder characteristics in order to better control the behavior of particles in the plasma jet, in order to produce the expected coating characteristics. The particle morphology, the particle size and the range of granules are the key parameters that affect: degree of melting in the plasma, deposition effect, layer density, coating stress state, and adhesive and cohesive strength. Depending on the powder production technology, the morphology and density of powder particles can be quite different, so it is necessary for their application to adjust the essential deposition parameters to obtain the desired coating properties, as described in the works of authors (Vencl, et al., 2009, pp.398-405), (Vencl, et al., 2010, pp.591-604), (Vencl, et al., 2011, pp.1281-1288), (Mrdak, 2012, pp.182-201), (Mrdak, 2013b, pp.68-88),
Mrdak, M., Morphology of powder particles produced by spray atomization and other processes, pp. 46-67
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(Mrdak, 2014, pp.7-22), (Mrdak, 2013a, pp.7-25), (Mrdak, et al., 2013, pp.559-567). In most cases, one powder production technology can be used for a number of different materials. It is also important to point out that, in the powder production process, combinations of two or more techniques of powder manufacturing are used. This is particularly benefitial when developing multi-component powders the components of which have different physical properties. By applying different techniques of powder manufacturing, significantly greater homogeneity and density of particles are achieved with a minimum share of gases. It is particularly important to note that introducing more technological methods of powder making results in a more uniform distribution of granules and a constant powder surface / volume ratio, which has a positive effect on their uniform melting in plasma and on their deposition on the substrate.
The aim of this study was to describe the technological processes of powder manufacturing used for the deposition of coatings by atmospheric and vacuum plasma spray processes. The paper presents the SEM micrographs of powder morphologies influencing the powder deposition effect, coating density, stress state of the coating and adhesive/cohesive strength of the bond. The chosen powders are those commonly used for making coatings resistant to wear, abrasion, erosion, corrosion and fatigue at low and elevated temperatures.
Powder production technologies and powder particle morphologies
Depending on powder manufacturing processes, powder particles have different characteristics relating to: microstructure, homogeneity, specific gravity, purity, morphology, particle size and particle size distribution. These characteristics have a significant impact on the quality and properties of deposited coatings in exploitation; therefore, it is necessary to possess knowledge of powder production technologies and the characteristics of produced powders in order to facilitate the control of the plasma spray parameters and the behavior of powder particles in the plasma, so as to produce high quality coatings. The most common and best-known powder production technologies are:
Agglomeration - spray drying: metals, carbides and ceramic powders; Cladding of composite powders: metals, ceramics, metal powders carbide composite powders;
Melt atomization: metals and alloyed powders;
Electric arc melting: metal powders, ceramic powders;
Agglomeration and sintering: metal powders, ceramic powders and metal - carbide powders;
Plasma treatment (Plasma remelting) to achieve the spheroidization of metal and ceramic powders;
High Temperature Synthesis - SHS, which is obtained by the reaction of a mixture of elemental powders: powders of carbide, cermet powders, composite powders, etc.
Powder production by spray drying
Spray drying is the technological process of agglomeration or consolidation of fine powder particles in the range of 1pm to 40pm. For agglomeration, ultra fine powder particles sized less than 1pm can also be used since they enable the production of powders of higher purity and density. Depending on the type of material and its purpose, there are various granulation powders for consolidation. By concentrating fine particles, spherical powder with an approximately constant surface area/volume ratio is produced. This procedure can also result in homogeneous single-component and multi-component powders. Fine powder particles mix with a liquid agent silicasol creating a suspension. In the process of plasma spray deposition of powders, silica sol evaporates and does not affect the properties of the deposited layers. Such suspension is sprayed with an appropriate nozzle under pressure into a chamber heated between 400 and 600 °C making dry powder particles which are agglomerated. The chamber temperature depends on the type and properties of the suspension. This process generally produces two fractions: coarse fraction collected at the chamber and the fine fraction collected in the cyclone. The fraction ratio is regulated by choosing the nozzle type and pressure in the process. The dried powder is then sintered in a gas furnace at a temperature of 1450 to 1600 °C , depending on he powder type, to enhance the density and provide uniform powder flowability in the deposition process. In this way, a range of powder systems of different quality and characteristics is produced, such as: WC/Co, WC/CoCr, WC/NiMoCr, Ni/SiC, ZrO2Y2O3,
ZrO2TiO2Y2O3, ZrO2CaOAl2O3 and powders for special purposes. Fig. 1 shows the morphologies of the powder particles of ZrO220%Y2O3 and WC17%Co produced by spray drying. The particles of ceramic powders ZrO220%Y2O3 have a spherical morphology with a range of the granulation of 16pm to 90pm, and they are used to create the upper ceramic layer of thermo - barrier coatings TBCs (Mrdak, et al., 2013, pp.559-567). WC17%Co powder particles are more porous due to larger WC and Co particles which were used for the consolidation of spray drying. The powder is of a spherical shape with a range of grain size of 11pm to 53pm (Mrdak, 2013a, pp.7-25).
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Figure 1 - (SEM) micrography of the ZrO220%Y2O3 and WC17%Co powder particles Slika 1 - (SEM) mikrografija čestica praha ZrO220%Y2O3 i WC17%Co Рис. 1 - (SEM) микрография частиц порошка ZrO220%Y2O3 i WC17%Co
Production of clad powders
Cladding is a technological process in which particles of a size of 40pm to 50pm are used as a powder core for cladding binary or multi-component composite materials. The powder particle cores can be produced by a process of spray drying or atomizing the liquid melt with an inert gas.The particle cores are coated with smaller particles of a size of 1 pm to 10 pm with the use of silica sol as a binder. Powders obtained by the cladding process prevent the segregation of individual components in deposited coatings, which can happen with a mechanical mixture of powders. Two major classes of powders produced by cladding are multicomponent composite metal powders of the self-adhesive Ni/Al, NiCr/Al and NiCr/Al/Co/Y2O3 types and soft sealant materials of the Ni/ graphite type. The goal of producing cladded Ni/Al, NiCr/Al and NiCr/Al/Co/Y2O3 composite powders is that the cladding enables the exothermic reaction between the components of the powder in the deposition process, to obtain a higher adhesive strength of the coating to the metal substrate (Mrdak, et al., 2013, pp.559-567), (Mrdak, 2013c, pp.7-22). The sealing material Ni/25%graphite is composed of a graphite corecladded with nickel. This prevents the combustion of graphite in plasma and enables a uniform and homogenous distribution of Ni and graphite in the coating (Mrdak, 2013b, pp.68-88). Fig. 2 shows the morphologies of the cross sections of Ni/25%graphite and Ni/20%Al powders. The cross section shows the graphite cores cladded with fine particles of nickel in the form of wrappers (Mrdak, 2013b, pp.68-88). The powder particles are of a granulate range of 30pm - 90pm and irregular in shape. Composite NI20%Al powder consists of Ni particle cores cladded with fine Al particles with a range of grain size of 53pm to 90pm. The powder particles are approximately spherical (Mrdak, 2013c, pp.7-22).
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Figure 2 - (SEM) micrography of the Ni/25% graphite and Ni/20%Al powder particles Slika 2 - (SEM) mikrografija čestica praha Ni/25% grafit i Ni/20%Al Рис.2 - (SEM) микрография частиц порошка Ni/25% графит и Ni/20%Al
Preparation of powders by the atomization
of a liquid melt
The dispersion of a liquid melt by an inert gas or atomization (Melt atomization) is a technological process mostly used for the production of powders of pure metals and their alloys such as: Al, Cu, Ni, CuAl,CuNi, CuNiIn, CuAlFe, FeCrNiC, FeCrNiMoSiC, CoCrNiWC, CoCrNiAlTaY, CoNiCrWC, CoMoCrSi, MeCrAlY, etc. The process consists of melting a metal or an alloy of a particular composition and the atomization of a liquid melt by the inert argon gas, where the products are powder particles of a spherical shape. The melting may be performed at atmospheric pressure or in vacuum. If the melting is performed in the presence of air, it is necessary to perform degassing of the liquid melt by a suitable inert gas prior to spraying. Powders made of elements sensitive to oxidation (such as MeCrAlY alloys containing Al and Y) are produced by melting in an induction furnace in vacuum, where the melt is degassed and sprayed by the inert argon gas of high purity. The produced powders are dense and of spherical morphology with insignificant amount of oxide impurities. Fig. 3 shows SEM micrographs of the morphology of the Ni22%Cr10%Al1%Y powder particles produced by the atomization of a liquid melt by an inert gas intended for deposition at atmospheric pressure and in vacuum (Mrdak, 2012, pp.182-201). The powder with a grain size range of 53pm to 106pm is used for the deposition of coatings at atmospheric pressure (Mrdak, 2012, pp.182-201).
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Figure 3 - (SEM) micrography of the Ni22%Cr10%Al1%Y powder particles Slika 3 - (SEM) mikrografija čestica praha Ni22%Cr10%Al1%Y Рис. 3 - (SEM) микрография частиц порошка Ni22%Cr10%Al1%Y
The powder with a grain size range of 11 qm to 37qm is used for the deposition of coatings in vacuum. The powder particles are spherical in shape and homogeneous.
Preparation of powders by electric arc melting
Electric arc melting is a general method of producing powders, in which, after melting the powder components and their casting into blocks, the molten melt is rapidly cooled to room temperature. The cooled blocks are ground. After grinding, the powder is classified by a desired particle size distribution. The advantage of this method is pre-alloying and the natural homogenization of powder particles. The consequence of grinding is a typical irregular and angular shape of powder particles. This technological process is quite frequent in the production of ceramic powders: Al2O3, TiO2, ZrO2, Al2O3TiO2SiO2, Al2O3TiO2, Al2O3SiO2, Cr2O3, Cr2O3TiO2, Cr2O3SiO2TiO2, ZrO2CeO2Y2O3, ZrO2Y2O3, ZrO2MgO,
CeO2Y2O3 and other ceramicsas well as in the production of metal and alloy powders. Fig. 4 shows the morphology of the Al2O340%TiO2 and ZrO224%MgO powder particles. Fast sub-cooling of the molten melt allows the formation of a suitable structure in polymorphous ceramic materials. In the structure of the Al2O340%TiO2 ceramic powder, the a Al2O3 hard phase is present in a greater proportion while there is an insignificant proportion of the y - Al2O3 softer phase, which is very suitable for the production of coatings resistant to wear. The Al2O340%TiO2 powder is in a range of particle granulation of 15qm 45qm and it is used to protect surfaces from friction, abrasion and erosion of particles (Mrdak, 2014, pp.7-22).
Figure 4 - (SEM) micrography of the А12Оз40%ТЮ2 and ZrO2 24%MgO powder particles Slika 4 - (SEM) mikrografija čestica praha Al2O340%TiO2 i ZrO2 24%MgO Рис. 4 - (SEM) микрография частиц порошка Al2O340%TiO2 i ZrO2 24%MgO
Quick subcooling of the ZrO224%MgO ceramic melt also provides a small fraction of the monoclinic phase (below 5%) which is not desirable in the deposited layers. The ZrO224%MgO powder has a range of the granulate of 10^m - 53^m and is applied for the preparation of the upper ceramic layer of thermal - barrier coatings (TBCs) (Kakaš, et al., 2005, pp.335-340).
Preparation of powder agglomeration - sintering
Agglomeration - sintering is a technique in which agglomerated powder particles are condensed by a suitable heat treatment (gas or plasma) below the melting point of the components. This technology is used for the production of metal, metal - carbide and ceramic powders. Agglomeration and sintering do not produce completely alloyed materials, which is why this technique is used only for powders for the given purpose. Fig. 5 shows the morphologies of the agglomerated and sintered powder particles of Al2O313%TiO2 and Mo. The Al2O313%TiO2 powder particles consist of fine agglomerated and sintered particles of Al2O3 and TiO2. Because of the fine granulation of the initial components of Al2O3 and TiO2 powders, theagglomerated and sintered particles are quite dense, homogeneous and spherical.
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Figure 5 - (SEM) micrography of the Al2O313%TiO2 and Mo powder particles Slika 5 - (SEM) mikrografija čestice praha Al2O3l3%TiO2 i Mo Рис. 5- (SEM) микрография частиц порошка Al2O3l3%TiO2 i Mo
The powder is produced with the distribution of granules from 15qm to 45qm. The Mo agglomerated powder particle is more porous compared to the Al2O313%TiO2 powder particle because it consists of bigger particles of sintered and agglomerated Mo. The powder is produced with a granulation of the distribution of 16qm to 45 qm and is used for producing coatings with increased resistance to wear (Mrdak, et al., 2005, pp.235-239). The powder particles have quite a regular shape, taking into account that sintered components have quite different melting temperature and other physical properties.
Preparation of powders by the plasma treatment
The process of plasma arc remelting has been developed as a new method to improve the existing quality of powders and to provide the best possible reproducibility of coatings with optimum properties. This primarily refers to the shape of particles, homogeneity, density, size, and the oxygen content as compared to the starting powder prior to the treatment with plasma. Remelting of plasma powders can result in powders with completely different specific properties. A constant powder flow is not the only parameter that affects the reproducibility of the coating quality. Since the melting process must be reproduced, it is also necessary to control the powder reactivity. All powder particles should have a constant ratio of surface/volume without gas content. The grain size uniformity can be achieved by careful screening and classification of powders by granulation. Most commercial powders are produced with different surface area/volume ratios, causing non-uniform melting of powder particles. This has led to the development of a new
technique of powder production, which also applies plasma energy in the production. This technique can produce dense and spherical particles with a constant ratio of surface area/volume. Powders of different grain sizes can be easily sieved and classified in accordance with the desired granulation. This is not the case in conventional methods of powder production that produce powders with different morphologies and variable surface/volume ratios. The production of spherical particles by the powder remelting process in the plasma has great advantages. The spherical shape and a constant surface/volume ratio also provide a constant flow of powder in the plasma deposition process. The gas content in the powder particles is significantly reduced and in the deposition process there is no evaporation of particles as in the case of angular grains. The process of plasma arc remelting is carried out in a furnace with controlled temperature and atmosphere. The chamber must not contain oxygen. The most commonly used gas for protection is inert gas argon of high purity. The furnace is protected from noise with a material on the furnace outer part. A required quantity of powder to be treated is uniformly injected into the plasma jet of defined characteristics. Powder particles injected into the plasma zone are remelted and, at the exit of the jet, they are undercooled quickly in some of cryogenic liquids. The treated powder particles of different granulation pass through cyclones to be collected in the powder chamber. After the treatment, the powder is classified in accordance with grain sizes. If the desired effect has not been obtained during the initial treatment, ie. If all the particles are not spherical, the process is repeated. The starting materials are powders of different chemical compositions and morphologies of different particle types produced by different technological processes. Multi-component powders cannot be produced by other technological methods. In the process, a number of particles of different powders of chemically determined granulation are mixed and then micro pelleted into a new particle. Thus formed particles are used as a starting material for the plasma treatment and for the preparation of multicomponent particles with novel properties (Hansz and Tourenne, 1988, pp.35-40). Typical examples of multi-component powders are WC-TiC-NbC-TaC and TiC-TiN-MoSi2-Cr3C2 powder. The produced powders are dense and homogeneous without a share of impurities and gases. Because of a high density and purity that can be achieved in powder particles, this procedure is widely used for thickening powder particles of metals susceptible to oxidation. Fig. 6.1 shows the morphology of a multicomponent sintered particle of WC-TiC-NbC-TaC before the plasma treatment and the morphology of multicomponent powder particles after the plasma treatment (Lugscheider, 1988, pp.23-48).
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Figure 6.1 - (SEM) morphology of the WC-TiC-NbC-TaC powder particles before and
after the treatment with plasma
Slika 6.1 - (SEM) morfologija čestica praha WC-TiC-NbC-TaC pre i posle tretmana sa plazmom Рис. 6.1 - (SEM) морфология частиц порошка WC-TiC-NbC-TaC до и после
плазменной обработки
The process of powder plasma remelting is suitable for the production of powders of zirconium oxide stabilized with other types of oxides. High remelting temperature helps to improve prealloying zirconium oxide, which significantly improves the oxide stabilization. This technological process results in systems of ceramic powders such as ZrO2CeO2Y2O3, ZrO2Y2O3, ZrO2MgO, CeO2Y2O3 and other ceramic powders. Fig. 6.2 shows the particle morphology of the ZrO225%CeO23%Y2O3 (Mrdak, et al., 2013, pp.559-567) and ZrO28%Y2O3 ceramic powders treated by plasma in order to spheroidize powder particles with a constant surface area/volume ratio. The spherical powder of ZrO225%CeO23%Y2O3 is fully prealloyed with particles ranging from 11 pm to 90 pm. The figure also shows the morphology of the powder particles of ZrO28%Y2O3 with a grain size of 15 pm to 45 pm. The powder particles have a regular spherical shape.
Figure 6.2 - (SEM) morphology of the ZrO225%CeO23%Y2O3 and ZrO28%Y2O3 powder particles Slika 6.2 - (SEM) morfologija čestica praha ZrO225%CeO23%Y2O3 i ZrO28%Y2O3 Рис. 6.2 - (SEM) морфология частиц порошка ZrO225%CeO23%Y2O3 и ZrO28%Y2O3
Mechanical alloying
The technological process of making powders by mechanical alloying -MA aims at producing homogeneous powder particles. It produces very homogeneous coatings with a fine microstructure. Mechanical alloying (MA) is described as a high - energy grinding process in which powder particles are subjected to repeated cold-welding, breaking and re-welding. The transfer of mechanical energy to powder particles results in the introduction of stresses in powder particles through the creation of dislocations and other defects that act as fast diffusion paths. In addition, grains are filtered and grain size is processed, which decreases the diffusion distance. During grinding, there is a slight increase in powder temperature. All these effects lead to the alloying of the powder mixture components, during the grinding process (Suryanarayana, 2004, p.466). Since mechanical alloying can lead to the formation of intermetallic phases, which are often difficult to form even at high temperatures, plasma spray powders prepared by high energy grinding are a great alternative to the formation of deposits of this type of composite phases. The versatility of mechanical alloying enables the production of composite powders of hydroxyapatite - HA reinforced with zirconium oxide - ZrO2 stabilized with yttrium oxide Y2O3, Ni/Al/Mo, Cu/Al, Cu/Al2O3, TI/Al/Si3N4, etc. (Fukumoto and Okane, 1992 pp.595-600). In the case of systems with an explosive material such as aluminum powder with a very fine particle size of < 3 pm, short-term mechanical alloying reduces the reactivity of the powder due to the inclusion of hard phase particles (Al2O3 and / or SiC) within Al particles or fine Al2O3 and SiC particles sticking on the surface of aluminum (Bach, et al., 2000, pp.299-302). Fig. 7 shows a (SEM) morphology of the Ni/5.5%Al/5%Mo and Cu/10%Al powder particles produced by mechanical alloying. The Ni/5.5%Al/5%Mo powder particles have a spherical shape which allows an even flow of the powder in the plasma jet. The range of granulation of the powder particles is in the range of 45 pm to 90 pm.
Figure 7 - (SEM) morphology of the Ni/5,5%Al/5%Mo and Cu/10%Al powder particles Slika 7 - (SEM) morfologija čestica praha Ni/5,5%Al/5%Mo i Cu/10%Al Рис. 7 - (SEM) морфология частиц порошка Ni/5,5%Al/5%Mo и Cu/10%Al
The powder particles of Cu/10%Al are also of a spherical shape in the granulate range from 45 pm to 106 pm.
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Sintering, agglomeration by high-temperature synthesis
For the production of carbide and cermet powders, different technological processes are used, eg: high-temperature synthesis - SHS, agglomeration and sintering, agglomeration and densification of powder by plasma and mechanical mixing of carbide and bonding alloys - MA. Fig. 8 shows the SEM - photomicrographs of the morphology of the TiC (Garrett, et al., 2012) and TiB2 powder particles (Logan and Villalobos, 2006, pp.249257) with the grain size of 1 pim to 5 pim, produced by the high temperature synthesis process - SHS. Carbide and cermet powders are often produced by the high temperature synthesis - (SHS), through the reaction of a mixture of elemental powders of Ti and C or B, which comprises a binder phase of Ni,Fe(Cr) and a hard cermet phase of TiC or TiB2. This process uses an exothermic reaction of a certain reactive mixture of flammable Ti and C or Ti and B powders. Because of high temperatures that can be reached during the reaction (~1200 - 6000 °C), impurities can evaporate and improve the bond strength between the hard carbide phase and the alloy used as a binder (Smith, et al., 1995, pp. 1121-1126).
Figure 8 - (SEM) morphology of the TiC and TiB2 powder particles (Garrett, et al., 2012), (Logan and Villalobos, 2006, pp.249-257)
Slika 8 - (SEM) morfologija čestica praha TiC i TiB2 (Garrett, et al., 2012),
(Logan and Villalobos, 2006, pp.249-257)
Рис. 8 - (SEM) морфология частиц порошка TiC и TiB2 (Garrett, et al., 2012), (Logan and Villalobos, 2006, pp.249-257)
This technological process allows the production of powders with a particle size of 1 ^m to 5 ^m in diameter and smaller than 100nm.
Conclusion
This paper describes the technological procedures commonly used for the production of powders intended for thermal spray processes, as well as the morphologies of the powder particles tested by scanning electron microscopy - SEM.
The technological procedures commonly used for the production of powders are: agglomeration spray drying, powder cladding, spraying the liquid melt, electric arc melting, agglomeration and sintering, powder spheroidization by plasma remelting and high-temperature synthesis.
This paper has shown that various technological processes can produce powders with different morphologies of powder particles whose characteristics are directly related to the structure and physical -mechanical properties of deposited coatings.
Each technological procedure gives some specific characteristics to the produced powders, on the basis of which the powders are selected for protecting functional surfaces in the production of new machine parts, for repairing surfaces worn by mechanisms of wear, cavitation and corrosion at low and high temperatures as well as for special purposes.
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Mrdak, M., 2013b, Characterization of nickel graphite sealing coatings in the system with the nickel-aluminum bonding coating, Vojnotehnički glasnik/Military Technical Courier, Vol. 61, No.1, pp.68-88.
Mrdak, М., 2013c, Structure and properties of plasma sprayed APS - Ni20Al coatings, Vojnotehnički glasnik/Military Technical Courier, Vol.61, No.2, pp.7-22.
Mrdak, M., Vencl, A., Nedeljkovic, B., Stanković, M., 2013, Influence of plasma spraying parameters on properties of the thermal barrier coatings, Materials Science and Technology, Vol.29, No.5, pp.559-567.
Mrdak, M., 2014, Characterization of the wear resistant aluminum oxide 40% titanium dioxide coating, Vojnotehnički glasnik/Military Technical Courier, Vol. 62, No.1, pp.7-22.
Sampath, S., Matejicek, J., Berndt, C.C., Herman, H., Leger, A.C., Vardelle, M., Vardelle, A. & Fauchais, P., 1996, Plasma Sprayed Zirconia: Relationships Among Particle Parameters, Splat Formation and Deposit Generation - Part II: Microstucture and Properties, in Thermal Spray: Practical Solutions for Engineering Problems, Ohio-USA, ISBN-10: 0871705834, pp.629-636.
Smith, RW., Mohanty, M., Stessel, E., and Verstak, A., 1995, Self Propagating High Temperature Synthesis (SHS) of Thermal Spray Powders, International Thermal Spray Conference, Kobe, Japan, pp. 1121-1126.
Suryanarayana, C., 2004, Mechanical Alloying and Milling, Materials Science and Engineering, pp.466.
Vencl, A., Mrdak, M., Banjac, M., 2009, Correlation of microstructures and tribological properties of ferrous coatings deposited by atmospheric plasma spraying on Al-Si cast alloy substrate, Metallurgical and materials transactions A, Vol.40, No.2, 2009, pp.398-405.
Vencl, A., Manić, N., Popovic, V., Mrdak, M., 2010, Possibility of the abrasive wear resistance determination with scratch tester, Tribology Letters, Vol.37, No.3, pp.591-604.
Vencl, A., Arostegui, S., Favaro, G., Zivic, F., Mrdak, M., Mitrović, S., Popovic, V., 2011, Evaluation of adhesion/cohesion bond strength of the thick plasma spray coatings by scratch testing on coatings cross-sections, Tribology International, Vol. 44, No.11,
pp.1281-1288.
ТЕХНОЛОГИЯ ИЗГОТОВЛЕНИЯ ПОРОШКОВЫХ ИЗДЕЛИИ И МОРФОЛОГИЯ ЧАСТИЦ ПОРОШКА
Михаило Р. Мрдак
Центр исследований и разработок АО„ИМТЕЛ Коммуникации", г. Белград, Республика Сербия
ОБЛАСТЬ: химические технологии ВИД СТАТЬИ: обзорная статья ЯЗЫК СТАТЬИ: английский
Резюме:
На физические характеристики и механические свойства покрытий большое влияние оказывают, применяемая технология изготовления порошка и параметры, используемые в процессах плазменной обработки.
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Ключевыми параметрами, влияющими на физические и механические свойства покрытий, являются такие производственнотехнологические параметры порошковых изделий, как: морфология частиц порошка, однородность, грануляция порошка и диапазон грануляции, являющихся неотъемлимой частью технологического процесса при изготовлении порошковых изделий.
В данной статье освещены технологические процессы производства порошковых изделий, приведена морфология частиц порошка (ведущих произодителей порошковых изделий), изученная методом электронной сканирующей микроскопии - SEM.
Во многих публикациях объясняется корреляционная зависимость между морфологией частиц и технологическими процессами, применяемыми в производстве порошковых изделий, а также параметрами нанесения покрытия. Различные характеристики частиц, такие, как форма, величина, размер, густота, чистота и пр. непосредственно зависят от производственного процесса при изготовлении порошковых изделий.
Так как от перечисленных характеристик зависят качество и свойства нанесенного покрытия, для лучшего конроля частиц в плазменной струе и обеспечения желаемых результатов покрытия, необходимо владеть знаниями о характеристиках и свойствах порошка.
При нанесении покрытия необходимо учитывать, различия характеристик порошковых изделий.
Ключевые слова: технологии, порошки, морфология, микроструктура.
TEHNOLOGIJE IZRADE PRAHOVA I MORFOLOGIJE ČESTICA PRAHOVA
Mihailo R. Mrdak
Istraživački i razvojni centar IMTEL Communications a.d., Beograd
OBLAST: hemijske tehnologije VRSTA ČLAnKa: pregledni članak JEZIK ČLANKA: engleski
Sažetak:
Fizičke osobine i mehanička svojstva prevlaka su pod velikim utica-jem tehnologije izrade prahova i parametara plazma sprej procesa. Je-dan od veoma bitnih parametara koji utiče na fizička i mehanička svojstva prevlaka su morfologije čestica praha, homogenost, granulacija praha i raspon granulacije, koji su u direktnoj vezi sa tehnologijama izrade prahova. U ovoj studiji opisani su tehnološki procesi proizvodnje prahova koji se najviše koriste i prikazane su morfologija čestica prahova renomi-
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ranih proizvođača praha, koje su ispitane skening elektronskom mikro-skopijom - SEM. Postoji mnogo studija koje su objavile korelaciju izme-đu morfologije i tehnologije izrade prahova sa parametrima depozicije prevlaka. U zavisnosti od procesa proizvodnje praha, čestice poseduju različite karakteristike koje se tiču oblika, veličine, specifične gustine, či-stoće, itd. Pošto ove karakteristike imaju značajan uticaj na kvalitet i svojstva deponovanih prevlaka, potrebno je posedovati znanje o karak-teristikama praha radi bolje kontrole ponašanja čestica u mlaz plazme, kako bi se proizvele prevlake očekivanih karakteristika. Prahovi imaju ra-znovrsne karakteristike za koje treba podesiti operativne parametre depozicije kako bi se dobile prevlake željenih osobina.
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Kvaliteti deponovanih APS i VPS plazma sprej prevlaka su pod uticajem više međusobno uslovljenih parametara, a jedan od njih je kvalitet praha koji je u direktnoj vezi sa tehnologijom proizvodnje prahova. Da bi se obezbedila optimalna svojstava prevlake, prah sa svo-jim hemijskim sastavom, morfologijom i raspodelom granulata čestica mora biti prilagođen eksploatacionim uslovima prevlake. Od tehnike proizvodnje prahova umnogome zavisi: hemijski sastav i homogenost praha, morfologija čestica praha, veličina čestice i raspon granulacije, poroznost, specifična gustina, čistoća i dr. (Sampath, et al., 1996, pp.629-636). Za proizvedene prahove istog hemijskog sastava sa razli-čitim procesima izrade dobijaju se različite morfologije i karakteristike prahova (Kubel, 2000, pp. 12-32). Čestice sfernog oblika imaju bolju protočnost u odnosu na čestice praha nepravilnog oblika. Shodno tome, dobijena svojstva prevlaka se razlikuju, čak i ako su uslovi za de-poziciju konstantni. To je zbog razlike u ponašanju tokom ubrizgavanja čestica u^mlaz plazme i toka istopljenih čestica do podloge gde se de-ponuju. Čestice nepravilnog oblika često izazivaju blokadu u dovodnoj cevi za prah, što dovodi do smanjenja stope depozicije, pregrevanja substrata i odvajanja prevlake sa podloge. Slična je situacija i sa ubaci-vanjem finih čestica u mlaz plazme. Shodno tome, veoma je bitno posedovati znanje o karakteristikama praha radi bolje kontrole ponašanja čestica u mlaz plazme, kako bi se proizvele prevlake očekivanih karakteristika. Zavisno od tehnike izrade praha, morfologije i gustine čestica praha mogu biti sasvim različite, pa je za njihovu primenu potrebno podesiti vitalne parametre depozicije da bi se dobile prevlake željenih osobina, kako je opisano u radovima autora (Vencl, et al., 2009, pp.398-405), (Vencl, et al., 2010, pp.591-604), (Vencl, et al., 2011, pp.1281-1288), (Mrdak, 2012, pp.182-201), (Mrdak, 2013b, pp.68-88), (Mrdak, 2014, pp.7-22), (Mrdak, 2013a, pp.7-25), (Mrdak, et al., 2013, pp.559-567). Važno je naglasiti da se sa primenom više tehnoloških postupaka izrade prahova postiže ujednačenija raspodela granulata i konstantan odnos površina -zapremina čestica, što povoljno utiče na njihovo ravnomerno topljenje u plazmi i deponovanje na podlozi.
Cilj rada jeste da se opišu tehnološki postupci izrade prahova, koji se koriste za depoziciju prevlaka atmosferskim i vakuum plazma sprej proce-sima. U radu su prikazane SEM mikrofotografije morfologija prahova, od kojih zavisi efekat depozicije praha, gustina prevlake, naponsko stanje prevlake i adheziono-koheziona čvrstoća spoja. Odabrani su prahovi, koji se najčešće koriste za izradu prevlaka otpornih na habanje, abraziju, ero-ziju, koroziju i na zamor na niskim i povišenim temperaturama.
Tehnike proizvodnje prahova i morfologije čestica prahova
U zavisnosti od procesa proizvodnje praha, čestice poseduju raz-ličite karakteristike koje se odnose na: mikrostrukturu, homogenost, specifičnu gustinu, čistoću, morfologiju, veličinu i raspodelu veličine če-stica. Ove karakteristike imaju značajan uticaj na kvalitet i svojstva de-ponovanih prevlaka u eksploataciji, zbog čega je neophodno posedo-vati znanje o tehnologijama izrade prahova i karakteristikama proizve-denih prahova radi lakše kontrole plazma sprej parametara i ponašanja čestica praha u plazmi, kako bi se proizvele prevlake visokog kvaliteta. Najčešće i najpoznatije tehnologije proizvodnje prahova su:
- aglomeracija suvim raspršivanjem (Agglomeration - spray draying) metalni, karbidni i keramički prahovi;
- oblaganje prahova (Clad composite powders) metalni, keramič-ki prahovi metalokarbidni, kompozitni prahovi;
- raspršivanje tečnog rastopa (Melt atomization): metalni i legirani prahovi;
- topljenje električnim lukom (Electric arc melting): metalni prahovi, keramički prahovi;
- aglomeracija i sinterovanje (Agglomerating and sintering): metalni prahovi, keramički prahovi i metalokarbidni prahovi;
- plazma tretman (Plasma fusion) radi sferoidizacije metalnih i ke-ramičkih prahova;
- proces visokotemperaturne sinteze (High Temperature Synthesis - SHS) koja nastaje reakcijom smeše elementarnih prahova: karbidni prahovi, kermet prahovi, kompozitni prahovi i dr.
Izrada prahova suvim raspršivanjem
Suvo raspršivanje (Spray drying) jeste tehnološki proces aglomeraci-je ili okrupnjavanja finih čestica praha u opsegu od 1pm do 40 pm. Za aglomeraciju se mogu koristi i ultrafine čestice praha, veličine manje od 1 pm, koje omogućavaju izradu prahova veće čistoće i gustine. Fine čestice praha vezuju se sa tečnim sredstvom silikasolom u jednu celinu, praveći pri tome suspenziju. Takva suspenzija raspršuje se sa odgovarajućom mlaznicom pod određenim pritiskom u komoru zagrejanu od 400 do 600PC, praveći suve čestice praha koje su okrupnjene - aglomerisane. Temperatura u komori zavisi od vrste i svojstava suspenzije. Ovaj proces proizvodi generalno dve frakcije: grubu frakciju skupljenu u komori i finu frakciju skupljenu u ciklonu. Odnos frakcija reguliše se izborom tipa mla-
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znice i radnim pritiskom u procesu. Osušeni prah se zatim sinteruje u ga-snoj peći na temperaturi od 1450 do 160CPC u zavisnosti od vrste praha, kako bi se poboljšala gustina i obezbedila ujednačena protočnost praha u procesu depozicije. Na ovaj način proizvodi se čitav niz sistema prahova različitog kvaliteta i svojstava kao što su: WC/Co, WC/CoCr, WC/NiMoCr, Ni/SiC, ZrO2Y2O3, ZrO2TiO2Y2O3, ZrO2CaOAl2O3 i prahovi za specijalne namene. Proizvedeni prah je sfernog oblika.
Izrada prahova oblaganjem
Oblaganje prahova (Clad composits) jeste tehnološki proces u kojem se za oblaganje dvokomponentnih ili višekomponentnih kom-pozitnih materijala, kao jezgra praha, koriste čestice veličine od 40 pm do 50 pm. Jezgra čestica praha mogu se proizvesti postupkom suvog raspršivanja ili atomizacijom tečnog rastopa inertnim gasom. Jezgra čestica oblažu se manjim česticama veličine od 1 pm do 10 pm uz upotrebu silikasola kao vezivnog sredstva. Dve najvažnije kla-se prahova izrađene oblaganjem su višekomponentni samovezujući kompozitni metalni prahovi tipa Ni/Al, NiCr/Al i NiCr/Al/Co/Y2O3 i meki zaptivni materijal tipa Ni/grafit. Cilj izrade obloženih kompozitnih prahova Ni/Al, NiCr/Al i NiCr/Al/Co/Y2O3 jeste što oblaganje omogućuje egzotermne reakcije između komponenti praha u procesu deponova-nja, kako bi se dobila što veća adheziona čvrstoća prevlake sa metal-nom podlogom (Mrdak, et al., 2013, pp. 559-567), (Mrdak, 2013c, pp.7-22). Zaptivni materijal Ni/25% grafit sastoji se od jezgra grafita obloženog niklom. Na taj način sprečava se sagorevanje grafita u plazmi i omogućuje ravnomerna i homogena raspodela Ni i grafita u prevlaci (Mrdak, 2013b, pp.68-88).
Izrada prahova atomizacijom tečnog rastopa
Raspršivanje tečnog rastopa inertnim gasom ili atomizacija (Melt atomization) tehnološki je proces koji se najviše koristi za proizvodnju prahova čistih metala i njihovih legura kao što su: Al, Cu, Ni, CuAl,Cu-Ni, CuNiln, CuAlFe, FeCrNiC, FeCrNiMoSiC, CoCrNiWC, CoCrNiAl-TaY, CoNiCrWC, CoMoCrSi, MeCrAlY i dr. Proces se sastoji od toplje-nja metala ili legure određenog sastava i atomizacije tečnog rastopa inertnim gasom argonom, pri čemu se proizvode čestice praha sfernog oblika. Topljenje se može izvesti na atmosferskom pritisku ili u vakuu-mu. Ukoliko se topljenje izvodi uz prisustvo vazduha, obavezno se pre raspršivanja vrši degazacija tečnog rastopa pogodnim inertnim gasom. Prahovi izrađeni od osetljivih elemenata na oksidaciju, kao što su legure MeCrAlY koje sadrže Al i Y, proizvode se topljenjem u indukcionoj peći u vakuumu, u kojoj se tečni rastop degazira i raspršuje inertnim gasom argonom visoke čistoće. Proizvedeni prahovi su gusti i sferne morfologije sa neznatnim udelom oksidnih nečistoća.
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Izrada prahova topljenjem električnim lukom
Topljenje električnim lukom (Electric arc melting) opšta je metoda proizvodnje prahova, kod koje se, nakon topljenja komponenti praha i livenja u blokove, istopljeni rastop brzo hladi do sobne temperature. Ohlađeni blokovi se melju, nakon čega se vrši klasifikacija praha po željenoj raspodeli veličine čestica. Prednost ove metode je predlegira-nost i prirodna homogenizacija čestica praha. Posledica mlevenja je ti-pičan nepravilan i uglast oblik čestica praha. Ovaj tehnološki postupak često je zastupljen u izradi kako keramičkih prahova: Al2O3, TiO2, ZrO2, A2O3TiO2SiO2, A2O3TiO2, A2O3SiO2, Cr2O3, Cr2O3TiO2, Cr2O3SiO2TiO2, ZrO2CeO2Y2O3, ZrO2Y2O3, ZrO2MgO, CeO2Y2O3 i drugih keramika, ta-ko i metalnih i legiranih prahova.
Izrada prahova aglomeracija - sinterovanje
Aglomeracija - sinterovanje praha (Agglomerating - Sintering) je-ste tehnika kojom se aglomerisane čestice praha zgušnjavaju pogod-nim toplotnim tretmanom (gasom ili plazmom) ispod temperature topljenja komponenti. Ova tehnologija primenjuje se za proizvodnju me-talnih, metalo-karbidnih i keramičkih prahova. Aglomeracijom i sintero-vanjem ne proizvode se sasvim legirani materijali, zbog čega se ovom tehnikom proizvode samo prahovi za datu namenu. Čestice praha ima-ju dosta pravilan oblik, s obzirom na to da se sinteruju komponente koje imaju različite temperature topljenje i druga fizička svojstva.
Izrada prahova plazma tretmanom
Postupak pretapanja praha mlazom plazme (Plasma fusion) razvio se kao novi postupak iz zahteva da se unaprede postojeći kvaliteti prahova i obezbedi što bolja reproduktivnost prevlaka sa optimalnim svojstvima. To se, pre svega, odnosi na oblik čestica, homogenost, gustinu, veličinu i sadr-žaj kiseonika u poređenju sa polaznim prahom pre tretmana sa plazmom. Ovom tehnikom proizvode se guste i sferne čestice sa konstantnim odno-som površina- zapremina. Prahovi sa različitom raspodelom veličine zrna mogu se lako prosejati i klasifikovati po željenoj granulaciji. To se ne odnosi na konvencionalne postupke izrade prahova koji proizvode prahove različi-tih morfologija i sa promenljivim odnosom površina -zapremina. Proizvodnja sfernih čestica praha postupkom pretapanja u plazmi ima velike prednosti. Sferičan oblik i konstantan odnos površina-zapremina omogućava, takođe, konstantan protok praha u procesu plazma depozicije. Sadržaj gasa u česti-cama praha znatno je smanjen i u procesu depozicije ne dolazi do isparava-nja čestica kao što je slučaj za uglasta zrna. Proces pretapanja praha plazmom radi se u kabini sa kontrolisanom temperaturom i atmosferom. U mlaz plazme definisanih karakteristika ravnomerno se ubacuje određena količina praha koji se tretira. Ubrizgane čestice praha u zoni plazme preta-paju se i na izlasku iz mlaza brzo pothlađuju - gase u nekoj od kriogenih tečnosti. Tretirane čestice praha različitih granulacija prolaze kroz ciklone i
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skupljaju u komoru za prah. Nakon izvršenog tretmana prah se klasifikuje po granulacijama. Ukoliko prvim tretmanom nije postignut željeni efekat, tj. sve čestice nemaju pravilan sferičan oblok, proces se ponavlja.
Mehaničko legiranje
Tehnološki postupak izrade prahova mehaničkim legiranjem (Mechanical alloying - MA) ima za cilj izradu homogenih čestica praha, koji pro-izvodi prevlake veoma homogene i fine mikrostrukture. Mehaničko legira-nje (MA) opisano je kao visokoenergetski proces mlevenja u kojem su če-stice praha podvrgnute višestrukom hladnom zavarivanju, lomljenju i po-novnom zavarivanju. Prenos mehaničke energije na čestice praha rezulti-ra uvođenjem naprezanja u čestice praha kroz stvaranje dislokacija i dru-gih grešaka koje deluju kao brze difuzione staze. Pored toga, javlja se pre-čišćavanje čestica i prerada veličine zrna, a samim tim se smanjuje i raz-daljina difuzije. Tokom mlevenja javlja se blagi porast temperature praha. Svi ovi efekti dovode do legiranja komponenti mešavine praha tokom pro-cesa mlevenja (Suryanarayana, 2004, p.466). Pošto mehaničko legiranje može dovesti do stvaranja intermetalnih faza, koja se često teško formira-ju čak i na visokim temperaturama, plazma sprej prahovi pripremljeni viso-kom energijom mlevenja odlična je alternativa za formiranje depozita ove vrste kompozitnih faza.
Sinterovanje, aglomeracija visokotemperaturnom sintezom
Za proizvodnju karbidnih i kermet prahova koriste se različiti teh-nološki procesi kao što su: visokotemperaturna sinteza (High Temperature Synthesis - SHS), aglomeracija i sinterovanje, aglomeracija i zgušnjavanje praha plazmom (Plasma fusion) i mehaničko mešanje karbida i vezne legure (Mechanical alloying - MA). Karbidni i kermet prahovi često se proizvode visoko- temperaturnom sintezom - High Temperature Synthesis (SHS) reakcijom smeše elementarnih prahova Ti i C ili B, koja sadrži vezivnu fazu Ni,Fe(Cr) i tvrdu kermet fazu TiC ili TiB2. Ovaj proces koristi egzotermnu reakciju određene reaktivne sme-še zapaljivih prahova Ti i C ili Ti i B. Zbog visokih temperatura koje se postižu tokom reakcije (~1200 - 6000°C), nečistoće mogu da ispare i poboljšaju čvrstoću veze između tvrdih karbidnih faza i legure koja se koristi kao vezivno sredstvo (Smith, et al., 1995, pp.1121-1126). Ovaj tehnološki postupak omogućuje izradu prahova veličine čestica prečni-ka od 1 pm do 5pm i manjeg od 100nm.
Zaključak
U radu su opisani tehnološki postupci koji se najčešće koriste za proizvodnju prahova namenjenih za termosprej procese, kao i morfolo-gije čestica prahova koje su ispitane skening elektronskom mokrosko-pijom - SEM.
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Tehnološki postupci koji se najčešće koriste za proizvodnju pra-hova su: aglomeracija suvim raspršivanjem, oblaganje prahova, raspr-šivanje tečnog rastopa, topljenje električnim lukom, aglomeracija i sin-terovanje, sferoidizacija praha pretapanjem plazmom i proces visoko-temperaturne sinteze.
Ovaj dokument je pokazao da se sa različitim tehnološkim po-stupcima mogu proizvesti prahovi sa različitim morfologijama čestica praha, čije su karakteristike u direktnoj vezi sa strukturom i fizičko-me-haničkim karakteristikama deponovanih prevlaka.
Svaki od tehnoloških postupaka daje proizvedenom prahu speci-fične karakteristike, na osnovu kojih se vrši odabir praha za zaštitu funkcionalnih površina u proizvodnji novih mašinskih delova i za repa-raciju pohabanih površina mehanizmima habanja, kavitacije i korozije na niskim i povišenim temperaturama i za specijalne namene.
Ključne reči: tehnologije, prahovi, morfologija, mikrostruktura.
Datum prijema članka / Paper received on / Дата получения работы: 17. 11. 2014. Datum dostavljanja ispravki rukopisa / Manuscript corrections submitted on / Дата получения исправленной версии работы: 02. 01. 2015.
Datum konačnog prihvatanja članka za objavljivanje / Paper accepted for publishing on / Дата окончательного согласования работы: 04. 01. 2015.
© 2015 Autor. Objavio Vojnotehnički glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). Ovo je članak otvorenog pristupa i distribuira se u skladu sa Creative Commons licencom (http://creativecommons.org/licenses/by/3.0/rs/).
© 2015 The Author. Published by Vojnotehnički glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/rs/).
© 2015 Автор. Опубликовано в "Военно-технический вестник / Vojnotehnički glasnik / Military Technical Courier" (www.vtg.mod.gov.rs, втг.мо.упр.срб). Данная статья в открытом доступе и распространяется в соответствии с лицензией "Creative Commons" (http://creativecommons.org/licenses/by/3.0/rs/).
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Mrdak, M., Morphology of powder particles produced by spray atomization and other processes, pp. 46-67
