SHS Powders for Thermal Spray Coating†

SHS Powders for Thermal Spray Coating† T. Talako*, A. Ilyuschenko and A. Letsko Powder Metallurgy Institute1 Abstract The possibilities of preparing advanced powders for thermal spraying functional coatings by the method of self-propagating high-temperature synthesis are discussed in this review. Besides important economical and ecological benefits, the method allows the formation of powders with improved or unique structure and properties in size ranges and with an external morphology suitable for different thermal spray processes. A number of novel powders and recent achievements are presented. Keywords: self-propagating high-temperature synthesis, powder, thermal spray coating ent, depending on the spraying process, the operat- Introduction ing conditions, the desired properties of the final Modern industrial technologies call for the devel- coating, etc. Besides the intrinsic material properties, opment of novel materials with improved properties, the technical requirements for the TS feedstock pow- lower costs and environmentally suitable processes. ders include good flowability and sprayability. They Surface engineering that attempts to create func- are greatly affected by the particle size, shape and tional layers on the surface is obviously the most morphology as well as particle size distribution. That economical way to provide high performance to is why thermal spray feedstock powder production machinery and equipment. Among the wide range processes must be reliable and flexible, while remain- of available methods (including varieties of atomistic ing as inexpensive as possible. and particulate deposition, bulk coatings wetting Self-propagating high-temperature synthesis (SHS) processes and surface modification), thermal spray or combustion synthesis, discovered by A.G. Mer- coatings offer the most versatile solutions. Thermal zhanov and colleagues in 1967, is known as a very spray processes form a continuous coating by melt- promising technique for processing materials (ce- ing the consumable material (feedstock) to form ramics, intermetallics, cermets, etc.) with good physi- droplets and impinging these droplets on the sub- cal and chemical properties at relatively low costs3,4). strate. The mechanism of bonding to the substrate in The main point of the process is that after localized thermal sprayings (TS) is the same as plating, both initiation, the reaction propagates as a narrow zone – mechanical interlocking and atomic interaction, with combustion wave – along a sample driven by the exo- the shear strength around 7 MPa1). The thickness of thermic reaction between components of the charge the coatings can range from 10 µm to a few millime- mixture without the application of external heating ters 2). Other advantages of thermal spraying include (furnaces, etc.). Because of the extreme conditions a practically unlimited assortment of powders to be in the SHS wave (high temperature of up to 3500℃, sprayed, high efficiency and relatively low substrate fast heating of up to 106K/s, steep temperature gradi- temperature (373-583 K), thus minimizing shape ent of up to 105 K/cm, rapid cooling in the after-burn distortion, oxidation and phase transformations in zone of up to 100 K/s, and fast accomplishment of the near-surface layer. Demanded characteristics for reaction, 0.5 s to 1 min), chemical interaction mecha- thermal spray feedstock powders can be very differ- nisms during the SHS are often non-equilibrium, resulting in the formation of materials with improved † Accepted: September 25th, 2009 1 structure and properties, especially in multi-compo- 41, Platonov str., 220005 Minsk, Belarus 3-6) nent composite systems . Moreover, a product with * Corresponding author: TEL: +375 17 293 98-27, FAX: +375 17 210 05 74, increased purity can be obtained due to evaporation E-mail: [email protected] of volatile impurities at the high temperatures of the ⓒ 2009 Hosokawa Powder Technology Foundation KONA Powder and Particle Journal No.27 (2009) 55 process. Besides the important economical benefits for TS coatings. Different kinds of plasma spraying connected with low energy requirements and short methods (air plasma spraying (APS), supersonic air reaction times, the method is promising from an en- plasma spraying (SAPS), low-pressure plasma spray- vironmental point of view7,8). Two ecological aspects ing (LPPS), underwater plasma spraying (UPS)), related to SHS are usually considered: ecological as well as detonation spraying (DS), flame spraying cleanness of the SHS technologies and possibilities of (FS) and high-velocity oxygen-fuel spraying (HVOF) industrial waste inertation, minimization and reuse in are in common use for depositing protective coatings SHS reactions. Generally, the SHS process is believed from SHS powders. The first attempt to systematize to be ecologically clean. However, for some large- results on using self-propagating high-temperature scale production, gas emission of volatile impurities synthesis for thermal spraying was made recently by during synthesis can be an environmental threat. A.L. Borisova and Yu.S. Borisov82). The compositions The major achievements in the exploitation of SHS of the most widely reported thermal sprayed SHS for environmental protection include: the fixation and powders as well as methods of their deposition and consolidation of high-level radioactive wastes; the properties of the coatings have been listed. However, treating and recycling of a highly toxic solid waste only general information about some of the SHS pow- from electrolytic zinc plants; the recycling of silicon ders and their processing is presented in this review, sludge from semiconductor industries, aluminum while some promising powders and recent achieve- dross produced by aluminum foundries, nonferrous ments were not mentioned. The aim of this work is to metal cutting and grinding waste, Fe3O4 dross and summarize relevant up-to-date literature and results dead catalysts; the degradation of chlorinated aromat- of the author’s research on producing advanced SHS ics7,8). powders for thermal spraying functional coatings. The first works available in the literature that take advantage of SHS materials for TS were devoted to 1. Fundamentals of Powder SHS Technology plasma spraying (PS) of carbides, intermetallics and carbide-based composite powders9-18). It was noted As stated above, the main condition for implemen- that at equal coating properties, SHS powders pro- tation of the SHS process is a high enough exother- vide 20-25-fold energy savings and half the labor input micity of the charge mixture. According to experi- as compared with conventional methods of powders mental data, SHS normally takes place when the preparation. Since then, a great number of research- adiabatic temperature is higher than 1800 K83) or at ers have been engaged in investigating thermal spray a ratio of reaction heat to a specific thermal capacity 19-82) coatings from SHS powders . Most of them dealt of a product at room temperature ∆Н298/С298 > 2000 with wear-resistant coatings formed by different К84). thermal spray methods from‘ titanium carbide/metal The powder SHS technology is based on burning binder’ compositions. It was conditioned by the high green powder mixtures in special reactors in the exothermal effect of TiC formation (giving full scope environment of an inert or reacting gas, and also in for the choice of binder material acting as diluents in vacuum or in the open air85). The chemistry of the the synthesis process) and good coating properties SHS process is versatile. Materials can be produced (comparable with or even better than that of conven- from elements and using compounds, mineral raw tionally used WC/Co and Cr3C2/NiCr coatings). The materials and industrial waste as reactants. Combus- chemical and morphological features of different SHS tion products are usually porous sinters which are ex- powders were described, and their technical char- posed to subsequent processing to produce powders acteristics of flowability and sprayability were out- of a different function. The general technological con- lined 30-38). Wear performance and hardness test re- figuration of SHS powder production includes the fol- sults for various TS coatings were also reported lowing operations: (1) preparation of a green mixture: 21-23, 25,26, 30-39, 42,43). Further progress was connected with sieving, milling, drying of components (if necessary) widening sprayed materials including improvement and mixing; (2) filling of a reactor with a green mix- of TiC-based powders (forming double (Ti, Cr)C, (Ti, ture and gases; (3) synthesis after a short-term ther- W)C and (Ti, Mo)C carbides, complex alloyed bind- mal initiation and (4) subsequent processing of the ers, introducing additional solid lubricant phases, synthesized products. In the case of synthesis from etc.), as well as the development of other composi- elemental powders, the subsequent processing of the tions44-81). At present, more than 90 different SHS pow- synthesized product includes only mechanical treat- ders have been investigated as feedstock materials ment: scraping, crushing, milling and classification. 56 KONA Powder and Particle Journal No.27 (2009) Some additional operations can be necessary in the in carbide and oxide density occurs under gravita- case of the more complicated routes. For example, tional or centrifugal forces. A chromium carbide pow- magnesiothermy synthesized product after crushing der is hereby produced from a mixture containing is exposed to acid

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