Abstract 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.

Abstract The gas tunnel type plasma spraying enables to produce high quality ceramic coatings. Moreover a high hardness coating was obtained at a short spraying distance in the case of alumina coating. A high hardness zirconia (ZrO 2 ) coating could also be obtained at an atmospheric pressure by using the gas tunnel type plasma spraying. The Vickers hardness of the ZrO 2 coating at a short spraying distance was very high: a high hardness of more than Hv = 1200 was achieved at the surface side of the coating. In this study, the characteristics of these high hardness zirconia coatings produced the gas tunnel type plasma spraying were investigated by the measurement of the Vickers hardness of the coating. The microstructure of the obtained high hardness zirconia coatings were also investigated by the microscopic method and by the X-ray diffraction method.

Abstract During grinding of thermally sprayed WC-Co, the grinding ratio G ( ratio of volume of work removed to the volume of wheel consumed) is usually low and the finish produced sometimes is inadequate. Improvement in surface finish accompanies increase in grinding ratio. The objective of this investigation is to study the effect of type of abrasive, table speed, and depth of cut on the surface finish and hardness of WC-Co. Thermally sprayed WC-12 wt % Co and WC-17 wt % Co produced using the high velocity oxygen fuel (HVOF) process, have been ground using silicon carbide and diamond wheels under different operating conditions. The surface profile reveals the significant role played by the above parameters on the surface finish. The grinding ratio, G in case of diamond grinding was found to be larger than silicon carbide grinding however, the quality of the surface finish produced by silicon carbide was better than the diamond. The surface structure of the ground WC-Co was examined by SEM. Surfaces ground using a silicon carbide wheel exhibited extensive plastic flow, while surfaces ground with diamond wheels are highly fractured with localized flow which suggests two different mechanisms of material removal. The surface hardness after grinding, was found to depend on the type of abrasive and table speed. Silicon carbide grinding has shown higher hardness and better surface finish than diamond grinding.

Abstract Results concerning the relationship between deposition efficiency and mechanical properties with coating roughness are presented. A commercial partially stabilized zirconia powder was plasma sprayed at different power levels, H2/Ar ratio and spray distance. Coatings sprayed at high deposition efficiencies demonstrated improved mechanical properties. The deposition efficiency is also proportional to the coating roughness. When prepared with a high plasma power and a high H2/Ar ratio, and at short spray distances, these coatings exhibited the highest deposition efficiency and coating roughness. The high coating roughness associated with a high deposition efficiency arises because coarse powder particles are likely to be melted at higher plasma power. However, opposite trends were also observed within different regimes of powder size distribution and torch power. Another objective of this work was to demonstrate that a surface profilometer can be used to understand the deposition efficiency and mechanical properties of thermal spray coatings. Key words: Roughness, Mechanical Properties, DE.

Abstract 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.

Abstract High-velocity oxyfuel (HVOF) spraying is a viable alternative for hard chrome plating because of its excellent coating properties and substantially lower environmental pollution. Initial applications in ball valves, journals, and hydraulic cylinders on earthmoving equipment have proven successful. This paper discusses the advantages and disadvantages of HVOF spraying as an alternative to hard chrome plating for aircraft components and presents some of the early results of laboratory and in-service testing.

Abstract Segregating the convoluted effects of particle size, impact temperature and velocity on deposition behavior and adhesion is of utmost interest to the cold spray field. The current study aims to associate the particle impact behavior and adhesion to its in-flight characteristics by studying and decoupling the influence of particle size, temperature and velocity for single particle impacts and full coatings. Experimental results reveal that in-situ peening processes contribute to the adhesion at low impact temperature while particle velocity controls the adhesion/cohesion at increased particle impact temperatures. The benefits of both bonding mechanisms are discussed in terms of measured adhesion/cohesion, bend-to-break fracture surfaces, pseudoplasticity, deposition efficiency and critical velocity. Computational fluid dynamics (CFD) results provide individual particle trajectory, size, temperature and velocity, of successfully deposited particles, which have led to the observed signs of metallurgical bonding.

Abstract In cold spray, high adhesion of soft materials on hard substrates is commonly achieved by using helium as the propelling gas. This is the case of copper coatings on steel where adhesion may reach values as high as 60 to 80 MPa (glue failure), however, helium is a limited, expensive natural resource, and the use of more abundant nitrogen gas is preferred in an industrial setting. Unfortunately, when using nitrogen gas, little to no adhesion is obtained. In order to eliminate the use of helium gas we studied how laser assisted cold spray could lead to an improvement in adhesion of nitrogen sprayed copper coatings. In this work, several laser parameters (e.g., power and spot size) and process parameters (traverse speed, relative position laser spot vs. gas jet) were varied at a coupon level. Upon optimization, an equivalent adhesion to the coatings prepared with helium was obtained. Furthermore, the cross section of the coatings showed that the copper particles penetrated the steel, similar to what is observed when using helium gas. Optimization of these parameters for application to large diameter (~559 mm) cylinders was also performed. A discussion on the mechanisms which contribute to achieving high adhesion considering the use of helium versus laser assistance is provided.

Abstract A novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotary tube furnace has been investigated as a means to improve the cold sprayability of H13 tool steel powder. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature, cooling rate, and atmosphere), cold spray of H13 powder went from virtually no deposition to the production of thick dense deposits with a deposition efficiency of 70%. Powder agglomeration, surface state, microstructure evolution, and softening are identified as key factors determining powder deposition efficiency and resulting deposit microstructure.

Abstract Exposed to particle erosion environments, metal-sprayed coatings are damaged by micro-machining and ploughing at low impact angles. The generation and propagation of subsurface lateral cracks at high impacting angles damage single-phase ceramic coatings. Therefore, multicomponent coatings deposited by high-energy processes have been widely used to provide wear protection in most of the applications. As commercial arc-sprayed coatings have been used to a limited extent in applications involving erosion and abrasion wear, developing attractive wear resistant arc-sprayed coatings has been found necessary. A cored wire formulation, referred to as Alpha-1800, has been developed to produce tailored arc-sprayed coatings that are tough enough to resist particle impacts at 90° and sufficiently hard to deflect eroding particles at low impact angles. Typical 1 mm-thick coatings composed of ductile and hard phases with Knoop hardness reaching 1800 kg/mm2 were easily produced by arc spraying the cored wire with air. Coatings were: 1) erosion tested at 25°C and higher temperatures at impact angles of 25° and 90° in a gas-blast erosion rig, 2) slurry erosion tested at impact angles of 25° and 90°, 3) abrasion wear tested using the ASTM G-65 test procedure. Results show that coatings produced with the new cored wire are at least 5 times more erosion resistant and 10 times more abrasion resistant than coatings produced by arc spraying commercial cored wires. The performance of the new arc-sprayed coating can be compared with that of high-energy WC-based coatings. Being thermally stable up to 850°C, arc-sprayed coatings produced with the new cored wire are attractive for applications in many industrial sectors up to high temperatures.

Abstract Naval ship components are subject to extreme operating conditions while at sea, either during regular sailing periods and/or during training exercises. The Chilean Navy has employed thermal spray technology since the early 1960 decade. By setting up a joint cooperative program between the Chilean Navy industry and suppliers, it has been possible to start regaining confidence in thermal spraying and positioning the same as a "must" technology in regular naval maintenance work. This article presents a description of the Naval Components Reclamation Program and discusses the processes involved in hard chrome plating replacement in naval components. It provides a review of characteristic thermal spray applications developed by the Joint Naval Components Reclamation Program.

Abstract The one problem of zirconia (ZrO 2 ) coatings is control of the porosity, because the functionality of wear resistance, corrosion resistance, and so on are required for their application in the extreme environment. In this study, a high hardness and dense ZrO 2 coatings is formed by a gas tunnel type plasma spraying under the condition of a short spraying distance. The properties of this ZrO 2 coating formed are investigated about coating qualities, such as Vickers hardness characteristics, wear characteristic, etc. The graded functionality of this coating is clarified by the coating qualities.

Abstract A CoNiCrAlY-hBN/Polyester material has recently been developed for clearance control applications in gas turbine compressors that use titanium alloy blades. While engine tests serve as the final evaluation of the coating performance, quality assurance laboratories and production shops would rely upon the more readily available coating hardness values to predict performance. This paper will focus on the reproducibility of coating macrohardness with a plasma spray process. It is shown that plasma spray parameters affect the hardness of CoNiCrAlY-hBN/Polyester coatings by changing the level of polyesters retained in the coating and the volume percentage of metallic matrix. The correlation between hardness, retained polyester level and microstructure of these coatings is captured in a coating hardness map from which desired microstructure and polyester entrapment are determined. Based on the understanding of the correlation between coating hardness and microstructural features, the use of additional criteria other than hardness such as retained polyester level and non-metallic portion of the coating is recommended in order to assure the quality of the coating more effectively.

Abstract During the last decades, improved understanding of tribological behavior of different material combinations led also to an intensified development of thermal spray applications. In the field of e.g. hard chromium replacement by thermal spraying, significant amount of work has been done and published world wide, however, the authors manly focused on only one tribological aspect like friction, abrasion, erosion, cavitation or corrosion, respectively. In real applications, often more than one of those factors influence the successful use of these coatings. Besides the bulk properties of the materials, the coating micro structure, which is strongly spray system dependent, needs to be considered and investigated. Higher functionality and reliability than conventional competitive coatings still has to be proved at laboratory scale and under field conditions for thermally sprayed coatings. This paper describes the state of the art of thermally sprayed coatings as alternatives for other coatings. Published literature data and a wide range of own tribological investigations and field tests, reveals the potential for other applications.

Abstract Frequent reporting of microhardness data for thermal spray coatings testifies to the widespread use of this technique for coatings characterization. However, inadequate reporting of microhardness procedures makes comparisons between published coatings hardness statistics difficult and it appears that both microhardness in general and its significance to characterizing thermal spray coatings in particular, are poorly understood. This paper demonstrates that though microindentation technique is a useful laboratory procedure that can be used for coatings optimization, research and quality control purposes, poor understanding often leads to worthless data and thus to erroneous conclusions. A high quality WC-12%Co coating supplied by Sulzer Metco was hardness tested on both the polished cross-section and plane surface of the coating. Contributions to the variance in results obtained and sources of significant errors are discussed and conclusions are drawn regarding the methodology and suitability of hardness testing for characterizing thermal spray coatings. The limits in repeatability and reproducibility of Vickers microhardness data for hard metal thermal spray coatings are discussed. The necessity for rigorous statistical procedures of data analysis is demonstrated. It is suggested that the technique is inherently unsuitable for characterizing hard thermal spray coatings due to poor intrinsic reproducibility.

Abstract In cutting tool technologies WC-Co based materials are increasingly replaced by composites containing TiC as hard phase and Ni or Co based metallic binders to improve life time and the performance at higher temperatures. These new light-weight materials are also promising for wear resistant coatings. However, while the production of WC-Co coatings by thermal spray techniques, especially high velocity oxy-fuel flame (HVOF) spraying, is well-established, thermal spraying of TiC-based powders did not lead to satisfactory results so far. This could be attributed to the oxidation during the spray process and the insufficient bond between hard phases and the metallic binder. Strategies to improve the properties of TiC-based coatings aim for microstructural modifications, especially by alloying additives into the thermal spray powder. By HVOF and vacuum plasma spraying (VPS), modified TiC-based coatings are produced, which globally show similar microstructures but significantly differ in their oxide contents. Investigations of mechano-technological properties and wear mechanisms demonstrate that alloying Mo into the hard phases or the metallic binder of thermal spray powders can improve the adhesion between hard phases and metallic binder of the coatings. In addition, properties of the metallic matrix can be tuned up for specific applications by solution hardening. In case of HVOF-coatings these effects are partially compensated by high oxygen contents. The overall better performance of coatings produced by VPS demonstrates that the high potential to improve properties of TiC-based composites by alloying additives can only be attained by minimizing the oxidation during the spray process.

Abstract Thermally sprayed coatings are applied to various substrates for the purpose of providing a wear-resistant, corrosion-resistant, or heat barrier surface. The materials used to coat a substrate will vary in hardness with respect to each other and with respect to the substrate. Denser coatings produce more reliable hardness data than less dense coatings due to the absence of pores and other inconsistencies in the coating. Variations in coatings will cause preparation problems such as pullout, edge rounding, or coating-to-substrate relief which effects the reliability of hardness determinations. The repeatability of hardness data is directly related to the density of the coating. This paper deals with the effect of coating characteristics (density) on the reliability of manual and automated hardness measurements.

Abstract 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.

Abstract Magnesium alloys are widely used as lightweight metals in cars and trucks, but they require special surface treatments to offset corrosion and wear limitations. One such treatment, electron beam remelting, is the focus of this present work. As described in the paper, copper and nickel powders were plasma sprayed onto magnesium in a vacuum chamber, then heated to melting temperatures with a swept electron beam. Copper and nickel were chosen because they form hard intermetallic compounds when melted with magnesium. The structure and properties of the remelted Cu- or Ni-alloyed surface layer were found to be dependent on the properties of the plasma sprayed coating and the parameters of the e-beam treatment. In the course of the investigation, the coatings were assessed based on their microstructure, the thickness of the remelting zone, occurring phases, and the determination of hardness and abrasive wear resistance.

Abstract To maintain surface roughness of process rolls in cold rolling steel plants, WC-Co coatings have been known to be effective ones. In this study, a high pressure/high velocity oxygen fuel (HP/HVOF) process was used to obtain WC-Co coatings. To get the best quality of coatings, WC-Co coatings are sprayed with numerous powders made by various processes. These powders include agglomerated sintered powders, fused-crushed powders, extra high carbon WC-Co powders and (W 2 C, WC)-Co powders. After spraying, properties of coatings such as hardness, wear resistance. X-ray diffraction, and microstructures were analyzed. For coatings produced by agglomerated-sintered powders, hardness of the coating increased as power levels and the number of passes were increased. In case of the coatings produced by fused-crushed powders, a very low deposition rate was obtained due to a low flowablity of the powders. In addition, the WC-Co coatings sprayed with extra carbon content of WC-Co did not show improved hardness and wear resistance. Also, some decomposition of WC was observed in the coating. Finally, the coatings produced by (W 2 C, WC)-Co powders produced higher hardness and lower wear resistance coating.