Thermal spray coatings are very effective in combating wear and corrosion in many applications. New thermal spray processes and coating compositions continue to be developed with concomitant improvements in the performance of the coatings and their use in new applications. Nonetheless, the thermal spray coatings are not without competition from other coating and overlay processes and materials. This brief review considers the microstructures and the wear and corrosion resistance of a number of alternative coatings to thermal spray coatings, including physical vapor deposition, chemical vapor deposition, electroplating, autocatalytic, and laser cladding.

Amorphization induced by sliding wear and consequent wear resistance have been investigated in relation with the microstructure of Fe-Cr-B alloy spray coatings. The Fe-Cr-B spray coated layer exhibited much higher wear resistance and significantly lower friction coefficient in comparison with that of low carbon steel substrate thanks to the amorphous surface film formed during the dry sliding wear. Electron microscopy on the cross-section of the coated layer exhibited intra-particle segregation associated with rod-shape Cr rich (Cr,Fe)xB particles in the matrix of Fe-Cr solid solution phase. From the observations using TEM and EDS, Fe-Cr solid solution phase with super-saturated B and Si content was confirmed to be the phase which mainly contribute to the crystalline-to-amorphous transition induced by sliding wear. The formation of oxide inclusions seems to impede the crystalline-to-amorphous transition by lowering the solute content in Fe-Cr solid solution phase. Keywords : microstructure, wear-resistance, Fe-Cr-B alloy, amorphization, detonation gun

An alternative production route to obtain thermally sprayed self-lubricating ceramic coatings is proposed and microstructural, friction and wear properties of these coatings are discussed. A preliminary powder treatment shows to induce a higher degree of homogeneity compared to conventionally deposited composite coatings, offering some perspectives for a further improvement of the lubricating and wear properties. This approach is illustrated using CaF 2 as a solid lubricant dispersed in a Cr 2 O 3 matrix, where the high melting point of the latter (Tm{Cr2O3} = 2460°C) necessitates plasma spraying as deposition technique. The microstructure of the coatings is evaluated by a metallographic study of sections perpendicular and parallel to the substrate surface. Wear and friction behaviour is evaluated using reciprocating wear test Wear tracks are analysed by means of laser profilometry. The results are discussed in relation to test and production parameters. Test temperature as well as solid lubricant concentration predominantly affects the wear and friction properties. Using a surface temperature of 400°C, a friction coefficient of 0.25 and a wear volume of 37 x 103 µm3 after 10,000 cycles are obtained for a coating containing 16 vol.% of CaF 2 .

This paper presents the results of a study on the scuffing behavior of thermally sprayed ceramic coatings. The progress of building up of seizure is related and evaluated for a set of ceramic coatings embodying a diversity of granulations of Al 2 O 3 TiO 2 and Cr 2 O 3 plasma sprayed on steel, the IHI8N9T steel and carbon-graphite when tested on a roller-block machine. The greatest antiscuffing resistance was recorded for Cr 2 O 3 /carbon-graphite pair. An analysis of regression approximating friction force was carried through.

The use of cored wires for thermal spraying is a relatively new development that is being rapidly utilized for arc spray in a wide variety of applications. This paper will discuss the existing applications and industries in which cored wire coatings are used. Additionally, this presentation will cover the effect cored wires have had on the use of other types of thermal spray equipment. The paper will close with a trend analysis that discusses the applications of the future.

Aluminium silicon alloys have shown favourable properties when used as the matrix for abradable coatings in low pressure compressors of gas turbines [1 and 2]. This paper aims to describe the wear mechanisms found in aluminium silicon based abradables. To this end three thermally sprayed coatings are investigated. Aluminium silicon polyester, aluminium silicon-graphite and the most recently developed, aluminium silicon-hexagonal boron nitride (hBN) examined here are amongst a few of these materials. To be able to design materials to function in as wide a parameter range as possible, a test ng simulating engine mechanisms is required. Tests were conducted using titanium blades at velocities ranging from 250 - 450 m/s, temperatures of ambient to 450°C and controlled incursion rate of 5, 50 and 500 µm/s. The data obtained from these tests is best interpreted in the form of wear maps which characterise the seal performance and therefore are of use to engine and material designers.

Several recently published studies have shown remarkable improvements in dry abrasion resistance and corrosion resistance of aluminum phosphate sealed oxide coatings when compared to unsealed ones. There are numerous applications in chemical industry where a corrosive environment is accompanied with abrasive or erosive particles. In this study the wet abrasion resistance and slurry erosion resistance of aluminum phosphate-sealed and unsealed oxide coatings were studied and compared to their dry abrasion resistance. In wet abrasion tests kaolin and water mixture was used as the abrasive. In slurry erosion tests several abrasives in water with various pH values was used as the erosive medium. The coatings were characterized for microstructure and their wear mechanisms were analyzed using SEM. The results from wear tests are reported and correlated with coating properties. The influence of coating quality to the relative improvement achieved by sealing is presented and discussed.

The elevated temperature erosion resistance of experimental amorphous thermal spray coatings was determined in a laboratory elevated temperature erosion tester. Test conditions attempted to simulate the erosion conditions found at the combustor waterwall tubes in fossil fuel fired boilers. Erosion tests were conducted on four experimental amorphous thermal spray coatings, using the bed ash retrieved from an operating coal fired boiler. An experimental arcspray process was used to spray coatings. These results were compared with erosion test results of two common structural materials, two commercially available arc-sprayed coatings, and a proprietary HVOF coating. Test results indicated that the Duocor coating had the highest erosion resistance among the four experimental coatings, it showed equal resistance to the HVOF coating (DS-200). Compared to AISI 1018 steel, both Duocor and DS-200 coatings reduced material wastage by 26-fold. Other test results indicated that the XJ-16, 60T and Armacor M coatings had equal erosion resistance reducing material wastage approximately 7-fold, while Armacor CW reduced by 10-fold. Only slightly better than the unprotected 1018 steel, the X-20 coating performed poorly on erosion tests. The high erosion resistance of Duocor and DS-200 coatings was attributed to their high densities and fine splat structures.

A high energy recovery of dc plasma torch has been developed and applied to the deposition of >10 mm thick polymer composites for abrasion resistant protective surfaces. The injection of low cost fillers such as alumina or silica in the hot plasma zone can absorb a lot of energy and cool down the plasma whereas polymer powder is injected downstream in a much cooler zone. Indeed, the energy absorbed by the fillers can then be transferred inside the polymer matrix coating allowing an energy recovery mechanism. The result is a composite polymer/ceramic with the following benefits: The shrinking phenomena due to the polymer recrystallisation is eliminated allowing a good coating adhesion, a high polymer/filler throughput can be achieved and the risk of the in-flight polymer combustion is largely reduced. The fillers addition decreases the overall cost of the coating and the type of filler can influence the composite properties. Abrasion resistant composites have been produced with alumina fillers. Medium density polyethylene (MDPE) sprayed with 45 wt % reinforcement as tested on the modified ASTM G-65 apparatus has shown abrasion resistance as good as ultra high molecular weight polyethylene (UHMWPE), which is one of the highest abrasion resistant polymer. In order to understand the abrasion resistance mechanisms, variables in the coating process such as: size of the fillers, polymer injection angles, polymer degradation and composite microstructure have been analyzed.

Slurry-handling equipment and pipelines particularly used in coal processing and mining industries are continuously exposed to the Impact of liquid-borne solid particles, resulting in progressive damage and loss of material. Cost-effective solutions to slurry erosion in aqueous media have been mainly limited to austenitic stainless steels, although coatings have been proposed. This work was aimed at evaluating the slurry erosion resistance of arc-sprayed coatings and determining what improvement IS achieved after laser melting. Multiphase and Type 316 stainless steel arc-sprayed coatings were obtained by arc spraying in air solid and cored wires. The surface of arc-sprayed coatings was melted using a pulsed Nd-YAG laser producing 1.06 µm wavelength radiation. Arc-sprayed and laser-melted coatings were slurry erosion tested at impact angles of 25° and 90° in a laboratory slurry jet erosion device using quartz sand as erodent. The evaluation of wear damage was done with a laser profilometer. Scanning electron microscopy and X-ray diffraction analysis were used to evaluate the microstructural changes which occurred after laser surface melting. Multiphase arc-sprayed coatings were more slurry erosion resistant than Type 316 stainless steel coatings. Improvement in slurry erosion resistance, particularly at the impact angle of 90°, was achieved by laser melting multiphase arc-sprayed coatings. Although deep microstructural changes occurred within coatings upon laser melting, the removal of stringers between sprayed platelets by laser melting was found responsible for the increase in slurry erosion resistance of multiphase laser-melted coatings.

The tribological behaviour of WC-Co-Cr coatings deposited by HVOF and HVAF and WC-Co coating deposited by HVAF was investigated in pin-on-disc tests. Wear rates were determined and wear tracks on the coatings and counterbodies were investigated in SEM. The HVAF sprayed coatings showed greater wear resistance compared to the HVOF coatings. The main wear mechanism in the WC-Co coatings was adhesive wear. The cobalt matrix is lubricious, resulting in very low wear rates and low debris generation. The main wear mechanisms in the WC-Co-Cr coatings were adhesive and abrasive wear. Adhesive wear results in pull-outs that are trapped in the contact zone and act as a third-body abrasive. Particle pull-out of the coating significantly increases the wear rate of the coated specimen. The HVAF WC-Co-Cr coatings proved to have a better resistance to particle pull-out which reflected in a considerably lower wear rate than the HVOF WC-Co-Cr coatings.

Air plasma spray coatings of three different WC-12%Co powders based on the powder manufacturing methods were conducted. S&F (Sintered and Crushed) WC-12%Co and NiCrSiB mixed powder with different ratio were also sprayed. The best wear resistance of S&F WC-12%Co coating was mainly due to its high hardness related with large amount of homogeneously and uniformly distributed retained carbides. The wear resistance of blend coatings increased with increasing WC-12%Co weight percent which was associated with the fact that the wear resistance of all coatings increased with increasing hardness and decreasing friction coefficient. But the exact relation of wear resistance with cohesive bond strength could not be found. The dominant wear mechanisms of WC-12%Co coatings were adhesive wear, tribo-film formation by material transfer, fatigue crack, particle (splat) fracture and delamination (spallation of coating layers). In the case of blend coatings, when the weight percent of WC-12%Co was small plastic deformation and abrasive mark (or scratching) by asperity and/or hard particle were observed. with increased weight percent of WC-12%Co the wear mechanism was closer to that of WC-12%Co coatings where no plastic deformation and abrasive mark was observed.

The purpose of this study is to investigate the microstructure and wear resistance of plasma sprayed WC-Co-Ni coatings. WC-Co-Ni composite powders were prepared by mixing of WC powder, Co powder and a Ni-P alloy powder, followed by sintering and crushing to improve the properties of plasma sprayed WC-Co coatings. In this study, their coatings were deposited by the atmospheric plasma spraying. The evaluation of their coatings were carried out by the observation of microstructure, measuring of microhardness values, adhesion strength values and an abrasive wear test. The abrasive wear resistance of the as-sprayed WC-Co-Ni coatings was comparable with that of WC-Co coatings deposited by HVOF spraying, and besides, the properties of the post-treated WC-Co-Ni coating were comparable with those of cemented carbides.

The paper reports a series of experiments with various HVOF spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000) using different types of WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and X-ray diffraction. Carbon and oxygen content of the coatings were determined in order to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on the type of powder, the spray system and the spray parameters. Within a relatively wide range, the extend of phase transformations has only little effect on coating properties. Therefore coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen.

WC-Co and WC-CoCr coatings were deposited with the JP-5000 liquid fuel HP/HVOF system using various thermal spray powder types. The microstructure, microhardness, deposition rate and wear resistance of the coatings were characterized. The results show that these coatings provide significantly more protection from dry three-body abrasion than from dry sand erosion, when compared to mild steel. They also provide more advantage at low angles of erosion than at high angles of erosion. Furthermore, the coating composition was found to have a significant effect on the wear rates, with WC-CoCr providing the best wear resistance even after taking the higher cost of the powder into account. The powder manufacturing route had only a secondary effect on the wear rates, except in the case of fused and crushed powder, which produced an inferior coating.

The use of thermal spraying processes as an alternative to industrial chromium plating has significantly increased in the past years. Advanced processes, such as APS and HVOF, have proven their efficiency with thick coating in chromium plating replacement, with cost advantage for coatings thicker than 5/10 mm. This article is the synthesis of different trials made with plasma and HVOF coatings. It includes the selection of materials for trial; a description of trial's equipment; friction and wear results; properties and metallography; and the cost comparison of plasma spraying and HVOF with other processes. It has been shown that thermal spray coatings, both APS and HVOF, can be used as chromium plating alternatives, with advantages in terms of wear resistance.

For the purpose of getting high hardness and high wear-resistant coating by arc spraying technology, the arc spraying of 7Cr13 cored wire is adopted in this paper. The metallurgical process of the cored wire arc spraying is discussed. The bond strength, hardness and tribological properties of the composite coating are investigated.

Different oxides layers have been studied by friction in natural sea water medium under the same conditions (Cr 2 O 3 , Al 2 O 3 , Al 2 O 3 + Cr 2 O 3 ). The evolution of different parameters have been analyzed: friction coefficient, electrochemical potential, degradation of the layers in the contact, impedance spectroscopy. The main result observed is concerned by the cracking of the coatings under stresses, in such a way interconnection paths are rapidly present between the substrate and the sea water medium through the layers. The coating impregnation process by epoxy, before the tribocorrosion tests, improve the protection of the substrate against the corrosion.

Commercially available coating techniques such as "open arc" and "spray & fuse" methods were used to compare the microstructural development with the plasma transferred arc (PTA) coating process for the hardfacing of NiCrBSi and Stellite 6 alloys. Denser eutectic structure was observed in the case of PTA coated layers of the Stellite 6 alloys than those of open arc weld-surfacing process. The shape of both carbides and borides in the 16C alloy coated by PTA processing were also obtained to have coarse morphology of carbides and borides while the "spray & fused" layers show a needle shape with finer distributions. Possible thermal history during each coating process is discussed. Based on microstructural observation, the hardness, wear resistance, and corrosion behaviors are reported. As expected, the alloy properties are directly related to their constituents of microstructure.

The Thermal Barrier Coating (TBC) used to improve the heat barrier and wear resistant property in high temperature of the aircraft engine and the automobile engine, usually has a two layer structure. One is a ceramic top layer for heat insulation and the other is a metal bond layer to facilitate the bond strength between the top ceramic layer and the substrate. But, the coated layers can be peeled off because of the accumulation of the thermal stress by the difference of the thermal expansion coefficient between metal and ceramics in a heat cyclic environment. In this study, the intermediate layer produced by plasma spray process was introduced to reduce the thermal stress. The powders of plasma spray coating were Yttria Stabilized Zirconia (YSZ), Magnesia Stabilized Zirconia (MSZ) and NiCrAIY. The intermediate layer was sprayed with the powders of partially stabilized zirconia with 50wt% NiCrAIY between the ceramics top coat and the bond coat for the purpose of alleviating heat expansion. The high temperature wear and thermal shock test were conducted. The high temperature wear resistance of the YSZ TBC was better than that of the MSZ TBC. The wear resistance decreased with increasing temperature between 400°C to 600°C. The 3 layers TBC with YSZ top coating showed the best thermal shock resistance. This means that the intermediate layer played an important roll to alleviate the difference of the thermal expansion between metallic layer and ceramics layer. SEM and OM were examined. The bond strength, hardness test, and wear test were also studied.