In this study, suspension plasma spraying is used to produce cast iron coatings that benefit from a graphite structure. In order to increase the graphite content, different hydrocarbons in the form of liquid suspension (hexane and toluene) and gas precursor (methane) were injected into the plasma stream along with iron powder. Besides promoting the formation of a soot carbon structure, liquid hydrocarbon injection also prevents in-flight particle oxidation, which is a major concern when spraying metals. In addition, it has been observed that using a shroud during spraying significantly increases the amount of soot carbon in cast iron coatings, which can be transformed into graphite by post annealing.

In this work, mechanically alloyed Al–12Si/TiB 2 /h-BN composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The results revealed that the mechanical alloying (MA) process has a significant effect on composite powder morphology and in-situ reaction intensity between the selective powders during plasma spraying. In addition, hexagonal boron nitride (h-BN) powder incorporated as a solid lubricant, which has excellent lubricating properties, decomposed into B and N and formed a solid solution after a long period of milling. More specifically, during plasma spraying a large amount of h-BN reacted with Al to form AlN. Unlubricated ball-on-disk testing ring was used to examine the anti wear performance of the coatings. The worn surfaces were examined using scanning electron and energy dispersive spectroscopy to elucidate the wear mechanisms operating at the sliding interface.

Cast iron coatings containing solid lubricant of graphite are an attractive candidate for wear resistant applications of an aluminum alloy substrate. It is difficult to generate a graphite structure in coatings sprayed with as-atomized cast iron powder which contains a few graphite owing to their high solidification rate. Although a graphite structure is remained in coatings sprayed with fully annealed cast iron powder, graphite carbon becomes lower than that in the annealed powder because of the in-flight oxidation and dissolution of graphite into molten iron. The present study is focused on an increase in graphite carbon through flying droplet diagnostic at a certain spray distance, that is, in-situ measurements of droplet temperature and velocity. Water-atomized cast iron powder which was annealed at 900°C for 3.6ks, was supplied as a spray material. The fully annealed powder was plasma-sprayed onto an aluminum alloy substrate, as well as carrying out flying droplet diagnostic. The amount of graphite carbon can be estimated by flying droplet temperature and velocity, which are controlled by spray parameters such as plasma gas flow rate and plasma current. It is confirmed that droplet velocity exhibits stronger influence on graphite carbon compared with droplet temperature. High velocity causes an increase in graphite carbon, so that it is possible to fabricate graphite-graded cast iron coatings with high amount at the surface and low at the interface.

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al–12Si and SiO 2 powder was deposited onto an aluminum substrate by atmospheric plasma spraying (APS) to obtain a composite coating consist of in-situ formed alumina reinforced hypereutectic Al-18Si matrix alloy. The effect of arc current on the corrosion behavior of the composite coating has been investigated. Corrosion resistance of the composite coatings was analyzed by using the potentiodynamic polarization scanning (PDS) technique. Surface morphology of the coatings before and after the corrosion test was examined by using metallographic methods and scanning electron microscopy (SEM). Obtained results are discussed in detail.

Graphitization behavior of water-atomized cast iron powder at each thermal spraying step, such as droplet flight, droplet impingement and splat layering, was successively examined. Both as-atomized cast iron powder and coatings sprayed with the powder contain no graphite structure owing to their rapid solidification. A short period of pre-annealing at 1173 K allows the formation of graphite structure in the cast iron powder, in which there exist precipitated graphite of 3.58 mass%. The microstructure observation exhibits that pre-existed pores in the as-atomized powder strongly affect the precipitating sites of graphite, that is, mainly inside the individual powder instead of the surface. However, marked reduction in graphite structure occurs to coatings sprayed with the pre-annealed powder because of in-flight burning and dissolution into molten iron. In-process post-annealing at 773 K for 60 s reveals the formation of graphite structure resulted from the decomposition of iron based metastable carbide in splats and coatings sprayed with the as-atomized powder. Chemical analysis demonstrates that graphitization level of post-annealed cast iron coatings is higher than that of coatings sprayed with the pre-annealed powder. Precipitated intersplat graphite structure of 1.68 mass% appears in cast iron coatings when introducing methane as a powder feeding carrier gas which is liable to decompose in plasma flame. The resultant coatings with graphite structure embedded in hard matrix are anticipated to offer superior wear resistance in comparison to centrifugally cast iron containing flaky graphite of 1.76 mass%.

The crushed and rounded ferroboron (FeB) powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt %) were deposited onto an aluminum substrate by thermal spraying methods to improve its tribological properties. Hexagonal boron nitride (h-BN) powders which have excellent lubricating properties like graphite were incorporated to the iron boride powders as solid lubricant by sintering process and high energy ball milling technique which allows homogeneous distribution of solid lubricants in a hard metallic matrix to obtain protective coatings with low friction coefficient. As-sprayed coatings are composed of mainly h-BN and FeB, iron matrix supersaturated with boron owing to the rapid solidification of molten droplets flattened on a substrate. The friction and wear behaviors of each coating were evaluated using ring-on-disk type wear tester under paraffin base oil condition in air atmosphere. Preliminary results revealed that iron boride powder with h-BN powder (5 wt.%) is an applicable method to produce a protective composite coating against friction and wear.

Reactively thermal sprayed coatings based on in-process reactions during flight and after impingement offer superior properties compared with conventional coatings because it has in-situ formed fine and uniformly dispersed stable hard phases. In the present work, composite powder composed of plated nickel, fine SiO 2 particulates and Al-Si-Mg core particles with water glass binder (SiO 2 /Ni/Al-Si-Mg) was deposited onto an aluminum substrate to fabricate composite coatings by using HVOF (high velocity oxyfuel), RF (radio frequency) and DC (direct current) plasma spraying methods. The amount and constituents of phases formed during reactive thermal spraying were found to be different depending upon the methods used where in-process reactions differently proceed. Consequently, reactively sprayed composite coatings mainly consist of Mg 2 Si, MgAl 2 O 4 , NiAl 3 and Al-Si matrix through the exothermic reaction of SiO 2 and nickel with molten Al-Si-Mg alloy. The depletion of magnesium in the composite powder is responsible for the obtained lower hardness of composite coatings sprayed by RF plasma spraying which offers the highest molten droplet temperature.

Water atomized cast iron powder of Fe-2.17C-9.93Si-3.75Al (in wt%) were deposited onto an aluminum alloy substrate by atmospheric DC plasma spraying to improve its tribological properties. Pre-annealing of the cast iron powder allows to precipitate considerable amounts of graphite structure in the powder. However, significant reduction in graphitized carbon in cast iron coatings is inevitable after plasma spraying in air atmosphere due to the in-flight burning and the dissolution into molten iron droplets. Hexagonal boron nitride (h-BN) powders which have excellent lubricating properties like graphite were incorporated to the cast iron powder as solid lubricant by sintering process (1300 °C) to obtain protective coatings with low friction coefficient. The performance of each coating was evaluated using ring-on-disk type wear tester under paraffin base oil condition in air atmosphere. Conventional cast iron liner which has flaky graphite embedded in pearlitic matrix was also tested in similar conditions in order to make a comparison. Sections of worn surfaces and debris were characterized and wear behaviour of plasma sprayed coatings are discussed.

The excellent wear-resistant performance of cast iron coatings considerably depends on the formation of graphite structure with an inherent self-lubricating property. In the present study, two types of cast iron powders produced by gas- (GA) and water-atomization (WA) were deposited on an aluminium alloy substrate by atmospheric DC plasma spraying. WA powders are generally characterized by high oxygen content, irregular appearance and inexpensiveness compared with those of GA powders. Although alloying elements of silicon and aluminium work as a strong graphitizer and anti-oxidizer, graphite structures are not recognized in coatings sprayed with as-atomized high silicon and aluminium powders. Therefore, either pre-annealing of powders or post-annealing of coatings is required to achieve cast iron coatings containing graphite structure. A marked decrease in graphite occurs to the coatings with pre-annealed GA powder, since there exists precipitated graphite mainly on a GA powder surface. A short period of post-annealing is also valuable for graphitization. The weak oxide layers are observed in coating cross-sections with GA and WA powder, however, their oxidized levels are much lower than those with bearing steel powder containing low silicon and aluminium. Hence, graphitized cast iron coatings sprayed with inexpensive WA powder exhibit a splendid anti-wear performance.

Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected in-process reactions, has attracted considerable attention as a result of wide availability to in-situ composite coatings. Such inprocess reactions differently proceed in HVOF and plasma spraying because of differences in the flame temperature and speed. In the present study, composite powder of SiO 2 /Ni/Al-Si-Mg was deposited onto an aluminium substrate to fabricate in-situ composite coatings by both spraying methods. The coating hardness sprayed with Al-Si-Mg core powder increases with silicon and magnesium content, whereas the coatings by HVOF spraying show higher hardness than those by plasma spraying. In the present reactive spraying, the exothermic reaction of SiO 2 with molten Al-Si-Mg alloy leads to composite materials of MgAl 2 O 4 , Mg 2 Si and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl 3 ), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO 2 especially in HVOF spraying. A series of in-process reactions mainly proceed during splat layering on a substrate, instead of during droplet flight even in DC plasma spraying. Plasma sprayed composite coatings become much harder due to the great progress of in-process reactions.

Microstructure and properties of plasma sprayed cast iron coatings are closely linked to the spray conditions such as substrate temperature, chamber pressure, particle size, and spray distance. Another factor is the chemical composition of sprayed particles, which affects the physical properties such as density, viscosity, and thermal conductivity of droplets. In spraying cast iron on aluminum alloy substrate the purpose is to deposit a superior wear resistant coating as an approach to improve its wear resistance. Presence of graphite in cast iron increases the wear resistance of cast iron coating because of its self-lubricant property. Graphite grows during droplet solidification and splat cooling and thus its appearance is related to the solidification rate of the individual droplets. Alloying elements such as Al and Si in cast iron materials promote the graphite formation because they act as strong graphitizers probably by creation active nuclei for graphite growing. The aim of this paper is to examine the influence of powder chemical composition on the features and properties of sprayed cast iron splat and coating by spraying three cast iron powders of different chemical compositions on Al-Si-Cu alloy. The effect of powder chemical composition on graphite formation and microstructure was investigated. In addition, the mechanical properties such as friction, wear resistance, and microhardness of sprayed coatings with those powders were examined. The influence of chemical composition of sprayed powders on the microstructure of cast iron coatings was examined by X-ray and SEM.

Superior wear resistance property of cast iron coatings is strongly linked to their microstructure. Graphite formation in plasma sprayed cast iron deposit is the main reason for this distinct phenomenon owing to its self-lubricant property. Since solidification rate affects graphite formation, optimization of spray conditions such as substrate temperature, chamber pressure, particle size and spray distance are required to slow down the solidification rate. The adhesion property of cast iron coating depends strongly on the adhesive strength between the first deposited layer and the substrate. It is important to improve the adhesive strength of splats by optimization of spray parameters. In this paper, cast iron splats were sprayed on aluminum substrate by plasma spray method using high Si and Al cast iron powders in Ar atmosphere. The effect of particle size and spray distance on splat microstructure (including graphite distribution) and its adhesive strength has been investigated. A correlation between microstructure and solidification rate was also introduced. Spraying by using large particle size leads to an increase in the number fraction of disk splats and a decrease in the splat’s adhesive strength. In contrast, the number fraction of star-shaped splats and their adhesive strength increase by spraying using small particle size. Longer spray distances lead to a decrease in number fraction of disk splats and their adhesive strength. The rear-side observation of cast iron splats sprayed with different spray distances show no distinct difference.

Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate are investigated in the present study: focusing on the effects of preheat substrate temperature (T S ) and chamber pressure (P C ) on the splat morphology, the adhesive strength of splats, the formation of a reaction layer and graphite. Splash-type splats appear at low T S but disk and star-shaped splats arise at high T S . Deformed substrate ridges, mainly due to the slight surface melting, are formed adjacent to the splat periphery at high T S . At low T S , pores are observed at the splat/substrate interface, which cause a decrease in the adhesion of splats. In contrast, a reaction layer composed of iron, aluminum and oxygen is ready to form at high T S . The amount of graphitized carbon increases in cast iron splats with T S . At a low P C of 26.3 kPa, disk-type splats are in the majority at a constant T S of 473 K. As P C increases, star-shaped splats appear along with disk splats. The flattening ratio of disk splats decreases with the increase of P C , because of a decrease in the kinetic energy and temperature of molten droplets. An interfacial oxide layer composed of iron, aluminum and oxygen is ready to form at high P C . The number of pores intensively increases with P C , which leads to a decrease in the adhesive strength of splats. The amount of formed graphite in cast iron splats slightly increases with P C , however, that of a rapidly solidified phase of Fe-Si-C decreases because of lowering of the solidification rate.

This paper presents a novel process that uses RF plasma spraying and premixed elemental powders to produce intermetallic matrix composites in situ without having to add reinforcement fibers or particles. Splats were collected on a stainless steel substrate and were analyzed to determine if nitrides had formed in metal droplets during flight and how it affected splat morphology. The typical splat morphology of impinged Ti droplets is disk-type with an outer peripheral fringe. Aluminum splats, on the other hand, are classified into two categories: a disk-type with an irregular outer periphery and a semi-massive-type. Other composites produced and examined include TiAl, AlN, and Ti2AlN.