The development of new spraying processes has increased the demand for high quality protecive coatings. Many thermal spraying processes have been developed to obtain coatings for a wide spectrum of materials and substrates. The High Velocity Oxygen Fuel (HVOF) process involves lower temperatures and higher velocities than those required by other techniques to obtain high density coatings. It is desirable to know which are the main factors that affect the corrosion behaviour of coated materials. The corrosion behaviour in chloride solution of a 34CrMo4 steel coated with different kinds of powder have been studied. The electrochemical corrosion of the coating-substrate system was characterised by corrosion potential measurements and potentiodynamic polarisations. Microscopic studies have also been performed by means of SEM. The corrosion tests were performed in synthetic marine water (ASTM D-1l41) in the presence of dissolved air. Polarisation resistances have been obtained from potentiodynamic studies. Measurements were carried out on two different (Ti,W)C+Ni coatings, as well as on the coatings obtained from a TiC+Ni-Ti powder which had been previously Ni coated using an electro less method. The best corrosion results were obtained from these last coatings.

This article investigates composite powder materials based on double chromium and titanium carbides with nickel-chromium binder produced using self-propagating high-temperature synthesis. It focuses on the hypersonic velocity oxygen fuel coatings from the synthesized powders. Laboratory tests were focused on the solid particle erosion which occur in energy production systems such as fluidized bed combustors, advanced pulverized cool boilers, and entrained coal gasifiers. Tests were conducted at elevated temperature in a blast nozzle type of tester using bed or fly ashes retrieved from the operating CFB boilers. It was observed that, when adjusting carbide phase composition and chromium content in titanium carbide solid solution, one can control the oxidation kinetic and wear resistance of the material. Fine-grained structure and high cohesion strength of the composite materials formed during synthesis provide their excellent elevated temperature erosion performance in a wide range of test conditions.

This paper presents a method for producing TiC-NiCr cermet powders with particle sizes of 40-90 μm and 15-50% volume content of ultrafine carbide inclusions. The method is based on a combination of mechanoactivation of initial components and subsequent high-temperature self-propagating synthesis. TiC-NiCr powders with different amounts of carbide content were produced and their applicability for plasma spraying is assessed.

Hardmetal-like coatings on the base of titanium carbide as a hard phase and nickel as a metal binder were prepared from agglomerated and sintered powders by plasma spray, detonation gun spray and high-velocity oxygen-fuel spray processes. The powders used in the spray experiments were plain TiC-Ni type and alloyed (Ti,Mo)C-NiCo type powders with different binder content. The coatings were characterized by optical and scanning electron microscopy, microhardness measurements, XRD analysis and in an abrasion wear test. The results showed that the sprayability of these novel hardmetal-like powders is good in all spray processes studied and the coatings deposited were found to have dense microstructures and good properties. The XRD analysis showed that the coatings have a phase structure similar to that found in the spray powder. The amount of retained carbides in the coatings was high. Some regions in which the carbides had dissolved with the metallic binder phase during spraying were also found, especially in plasma sprayed coatings. In such microstructural regions submicron size reprecipitated carbides were detected. These were clearly detectable in detonation gun sprayed coatings. HVOF sprayed coatings were found to contain a very high content of retained carbide phase. In this process the heat effect to the material seemed to be the lowest. The wear tests clearly showed the importance of alloying the hard phase and the binder phase in order to improve the wear resistance of the coatings. All studied spray processes produced coatings with nearly similar coating wear properties.

High Velocity Oxygen Fuel (HVOF) spraying established itself as an effective method in addition to the conventional thermal spray processes within a very short period. Self fluxing nickel alloys, cermets (e.g. WC-Co / Cr3C2-NiCr) as well as oxide ceramic coatings have proved themselves suitable for wear protection applications. Weight reduction, the care of resources and the increase of efficiency for structural components leads to the substitution of customary hard particles. Titanium carbide (TiC) characterizes itself on account of the material features such as the high hardness, the high melting point, the high strength and the low density for the substitution of conventional carbides. The Self Propagating High Temperature Synthesis (SHS) is a suitable process for the production of composite powders. The powders produced by SHS show a high carbide content, which is finely distributed with an almost stoichiometric composition of the TiC inside the powder particles. The carbides are protected against dissociation and oxidation during the thermal spray process by a complete velum of matrix alloy. The current investigations deal with the wear resistance of TiC-composite coatings produced by HVOF compared to conventional wear resistant coatings. The investigations contain the analysis of the microstructure by optical and scanning electron microscope (SEM) and the measurement of the microhardness of the deposited coatings. Special attention is drawn to the interface between the hard particles and the matrix alloy. The optimized coatings are tested with different wear tests, such as Taber-Abraser test, sliding and oscillating wear test and are compared with common wear resistant coatings in order to underline the high potential for different wear applications. Moreover an additional corrosion test (salt fog test) is carried out with regard to the corrosion resistance of the different matrix alloys.

The technology of thermal spraying is approaching maturity, and in the quest to reduce production costs whilst maintaining coating quality, attention is turning increasingly to more cost-effective routes for the manufacture of the starting powders. One such route is self-propagating high-temperature synthesis (SHS), which reduces the required energy input for powder production. In this work, TiC-Ti+Ni and (Ti, W)C-Ni powders produced by the SHS process have been studied in the as-received and as-sprayed states, to evaluate the suitability of SHS powders for the production of wear-resistant coatings. The starting powders and the coatings produced by atmospheric-plasma and HVOF spraying have been characterised using analytical (XRD, EDS) and microscopical techniques (optical, SEM). The technological properties of the as-sprayed coatings have also been characterised, including hardness, wear resistance (using a Rubber-Wheel test (ASTM G-65)) and corrosion resistance (in marine water environment).

Current thermal spray chrome replacement and hardfacing materials primarily use WC or Cr 3 C 2 as hardphase materials and contain chrome. A series of cermet alternatives based on lower weight and cost ceramic particles including alumina, silicon carbide, boron carbide and titanium nitride were applied via HVOF and tested for wear and frictional properties under various loading conditions. Furthermore, based upon highly promising initial results, thermal spray particle designs were modified for lower cost and improved performance.

This paper presents the results of studies on powders used in wear-resistant thermal spray coatings. The work focuses on the influence of the shape, size, and structure of composite powders containing nickel, iron, and chromium mixed with TiC and dry lubricants such as MoS 2 and CaF 2 . The powders are analyzed using SEM and metallographic methods along with X-ray diffraction. Paper includes a German-language abstract.

Many components in helicopter dynamics systems depend on hard, wear resistant coatings for reliable performance. Component replacement times are currently limited by performance of these coatings in many cases. Thermal spray coatings have been evaluated for these applications to replace nickel and chromium electroplate. The effect of coatings on fatigue strength is quantified by a strain limit concept to reduce test requirements during development phase. Full scale bench testing and coupon wear testing were conducted to reveal large improvement in sliding and fretting wear resistance. A fine porosity network in thermal spray coatings, high hardness and high fatigue strength are shown to be the main contributors to improved wear performance. Thermal spray coatings also exhibited enhanced corrosion resistance in salt fog and crevice corrosion conditions. These data permitted implementation on several critical helicopter components.

We investigate the sprayability of various hard metal and composite powder coatings via kinetic spray on cast iron by utilizing both powder and substrate preheat. These coatings include copper, a copper-zirconia composite, nickel, a zinc-nickel composite, and Ti6Al4V alloy. Using the kinetic spray process the coatings were applied to a cast iron substrate which was ground and sand blasted prior to spray. Analysis performed on powders and coating includes: cross-sectional microscopy, hardness of powders and substrate, substrate temperature as a function of heating and cooling times, and adhesion at the coating/substrate level. Results include spray parameters to allow for nickel and copper coatings to be developed on cast iron, adhesion strength as a function of powder hardness, porosity of nickel and Ti6Al4V coatings, and incorporation rates of zirconia in a copper matrix on cast iron. This is an attempt to spray hard powders on a hard substrate. Harder particles are more difficult to spray because they require more energy to plastically deform. Therefore the hardness of the particles plays a significant role in the deposition of a coating. Similarly, harder substrates are more difficult to spray on. This work demonstrates techniques that make spraying hard particles on hard substrates possible.

In this work, the influence of microstructure, hardness, elastic modulus, and fracture toughness on the tribological behavior of flame-sprayed Al 2 O 3 -TiO 2 and WC-NiFeCr coatings is investigated. Fracture toughness was found to be the most influential property, with higher toughness corresponding to greater wear resistance. This relationship, however, is verified only up to the point where the applied stresses cause detachment of particles from the surface. Hardness emerged as a less dominant factor than fracture toughness, elastic modulus, and porosity due to the fact that ceramic coatings, despite their high hardness, have low relative wear resistance.

Manufacturing of alloyed cast iron forming rolls is accompanied by generation of valuable solid waste as cutting and grinding swarf. The objective of this investigation was to evaluate the potential for mill debris to be used as a new source of inexpensive powders for thermal spraying of wear resistant coatings. More than 25 high-carbon iron alloy compositions are used in roll production. The structure of these cast irons usually includes from 2 to 4 phases (such as troostite, bainite, needled martensite, austenite, and graphite). The properties of powders obtained from mill debris were characterized in terms of particle size and shape, composition, structure, technological fluidity, and bulk density. The obtained powders were used for plasma spraying of wear resistant coatings. The results indicate that cutting and grinding swarf may be a feasible raw material source for economical alloyed powders, granules, and other materials for coatings. The composite plasma sprayed coatings obtained from powder mixtures of alloyed cast iron and nickel-base alloy have better tribological performance in comparison to coatings from any single powder alloy.

This work evaluates the potential of using new competitive powders of Fe/TiC system for plasma spraying of wear resistant coatings. To improve coating properties, Cr and Ni were added to the iron matrix. The results of complex investigations of plasma coatings from such materials are presented.

Obtaining dense ceramic coatings by thermal spraying still remains a challenge. Compared to metals, ceramics have a lower thermal conductivity and a larger melting enthalpy. These factors limit the heat transfer from the plasma to the particles and consequently do not necessarily allow their total melting. Problems linked to this heat transfer can be avoided, or at least limited, by using agglomerated particles made of a mixture of reactive powders yielding the ceramic material, via SHS (Self-propagating High-temperature Synthesis) reaction. In this case, the reaction can be ignited by the heat transfer at the particle surface of an agglomerate and propagate towards the centre during its flight through the plasma. The application of this process to Ti, C mixtures leads to the formation of a dense TiC based coating. The composition of the coating, influenced by the contamination of the surrounding gas entrainment during the spray process, belongs to the TiC-TiO solid solution. The influence of experimental parameters on the coating composition is discussed.

Laser cladding is a novel way to produce metal matrix composites for need of abrasion resistant coatings. There are, however, few comparative studies concerning the choice of reinforcing material and the metallic matrix material. In this study, MMC’s were formed from vanadium-, tungsten- and titanium carbides mixed with tool steel M2, Stellite 21 and NiCrBSi-alloy. The abrasion resistances were tested using rubber wheel abrasion apparatus. The wear surfaces were examined. The best results were achieved by M2 tool steel with vanadium carbides.

Stripping systems for thermal spray coatings tend to use many hazardous acids which require critical safety procedures and produce heavy volumes of environmentally unfriendly waste streams. A new technology, filed under application numbers 2009/0120804A1 and 2009/0229636A1, and additionally covered under U.S. patent numbers 8262870 and 8377324 has been created using a very safe and environmentally friendly percarbonate based chemical system with minimal DC rectified voltage. It was successfully qualified at Tinker Air Force Base (TAFB) and is currently in production use throughout the facility. It has safely stripped WCCo and CrC-NiCr coatings such as WCCo and CrC-NiCr from Ti 6-4 and IN718 substrates showing minimal stock loss in 48 hr exposure testing. Many lessons were learned during this qualification process which will be shared and documented. Both plasma and HVOF coatings can be stripped with this process in the pH 10-12 range. The system can also be combined with a refining system that purifies the waste water and condenses the coating sludge into a condensed volume, eliminating the tremendous volume of waste chemicals. Numerous case studies and actual stripping data will also be presented and discussed.

A conventional GTV K2 kerosene fuel HVOF spraying system has been modified with the aim to expand the applicable range of process conditions in order to also cover the presently existing gap between conventional HVOF and cold gas spraying. Different measures have been applied in order to reduce heat transfer and increase momentum transfer to spray particles. Increase of momentum transfer results both in reduced particle temperature as a consequence of reduced dwell time in the hot flame area and high particle velocities. In detail expansion nozzles with conical shape that provide improved expansion of combustion gases, combustion chambers with reduced critical diameters that provide increased combustion chamber pressures and the injection of coolant media water and / or nitrogen into the combustion chamber are applied. The effect of modified spraying conditions on the process characteristics and coating properties have been studied for a variety of materials. Besides copper, titanium alloys and MCrAlYs also WC/Co(Cr) and Cr 3 C 2 /Ni20Cr have been sprayed and respective coatings have been analysed concerning their microstructure, gas content, microhardness and wear resistance. In particular a moderate increase of oxygen content in dense titanium alloy coatings compared to the powder feedstock from 0.41 to 0.59 wt.-% proves the high potential of the undertaken measures to expand the application field of HVOF spraying. Thereby existing systems only need relatively small modifications to achieve this expansion.

The surfaces of machine components can be effectively protected against wear by highly resistant hardmetal-like coatings, such as WC-Co and Cr 3 C 2 -NiCr, deposited by different thermal spray processes. These composite materials are characterized by the presence of hard carbide particles embedded in a ductile metal binder matrix which have also found many applications as sintered parts (cutting tools, wear resistant parts, mining drills and others) obtained by a powder metallurgy route. Conclusions on the potentials of the different systems for coating applications can be made on the base of experiences and fundamental research from sintered hardmetals. In this paper a comparison of the properties of sintered parts and thermally sprayed coatings of the WC-Co, Cr 3 C 2 -NiCr and (Ti,Mo)C-NiCo systems is given. The structure and properties of the coatings depend strongly on the technology of spray powder preparation, the combination of spray process temperature and particle velocity, and other spray process parameters. It is shown that the TiC-Ni based system can be significantly improved by alloying. This makes the system suitable for coating applications where simultaneous high wear and corrosion resistance in combination with high temperature stability are required. This system can partially substitute the commercially introduced systems but has also the potential to explore new applications.

Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Thermally sprayed cermet powder coatings as well as bulk cermet materials sintered of carbide/metal powder blends are widely used in applications with severe abrasive wear conditions. A cost-saving alternative can be provided by using iron-based melt-atomised hard alloy powder feedstocks. Among them, commercial alloys containing high amounts of vanadium and carbon obtain outstanding wear resistance due to their high volume fraction of finely dispersed, hard vanadium carbides. However, their performance is still exceeded by cemented carbides. A further improvement of the wear properties of hard alloys basically can be attained by increasing their carbide content, concurrently considering the limitations of the melting and atomisation process regarding the melting temperature. A possible solution can be provided by alloying the basic system Fe-V-C with an additional strong carbide former like niobium. Subject of this work is the comparing investigation of the technologically important melting equilibria in the systems Fe-V-C and Fe-V-C-Nb.