Metal Matrix Composites (MMCs) are seeing increased use in tribological applications where hardness, toughness and wear resistance are required. Already such qualities have been included within composite coatings by methods such as electrochemical deposition. In this study the feasibility of including diamond as the hard phase in MMC’s using thermal spraying processes has been investigated. In this work the application specifically targeted is that of hard facing for sub sea drill bits, where the coatings experience a harsh environment of high stress abrasion, erosion and corrosion. The coatings were investigated in terms of their microstructure (light microscope, SEM), their elemental composition using EDX and XRD to identify retention of diamond and phases in the coatings, their hardness and abrasion resistance. Preliminary results show that it is possible to produce a hard facing diamond composite coating with good distribution of the diamond phase and little degradation of the diamond during the spraying and that diamond MMCs (DMMC) have potential for improving durability in drill bits. Diamond/metal powder mixture is sprayed onto the surface using an oxyacetylene torch.

Standard materials like WC-Co, WC-CoCr, WC-Ni, and Cr 3 C 2 - NiCr for HVOF spraying exist in various modifications depending on chemistry, carbide size, and production method. They are widely used for wear, erosion, cavitation, and corrosion protection in many industrial fields. But there are a lot of applications in which the surface is exposed to a combination of different wear mechanisms - e.g. corrosion and erosion or abrasion - and usual state-of-the-art coatings do not fully meet the technical demands regarding lifetime under those combined attacks. New materials therefore are needed. In the present study several modified and completely new carbide based materials are investigated. A series of new cermets differing regarding carbide type, carbide size, and metallic binders are compared. The materials were sprayed with the HVOF system DJ 2600 to show the influence of their different chemistry and morphology on the microstructure and properties of the coating in comparison to standard materials. The experiments comprises microstructural examinations as well as abrasive wear, corrosion, and cavitation tests.

The paper deals with wear properties of various advanced oxide ceramic coatings deposited by plasma and HVOF spray processes. Several types of ceramic oxide coatings on the base of Al 2 O 3 , Cr 2 O 3 and TiO 2 are studied in the work. The coatings are characterised by their wear properties in slurry abrasion wear test with fine abrasives (kaolin) and in dry abrasion conditions with coarse abrasives (quartz sand). The surfaces of the wear tested coatings were studied by SEM. The results show that coatings with high homogeneity and minor porosity, especially HVOF sprayed and some plasma sprayed coatings perform extremely well in both of the wear tests used in the study. In the test with hard abrasive and high load, the coatings with lower interlamellar strengths seem to be more prone to wear than coatings with higher homogeneity, e.g. HVOF sprayed oxide ceramic coatings. The results can be explained on basis of type of the coating material and the coating microstructure.

The aim of this study was to investigate a new generation of agglomerated and sintered composite (WC, Cr 3 C 2 , Ni and Cr) powders for improved impact and wear resistance. The poor impact resistance of conventional WC cermet coatings, such as WC-20%Cr 3 C 2 -7%Ni, WC-10%Co-4%Cr and WC-12%Co, has so far hindered their use in new industrial applications. Results of this study indicated that both durability and impact resistance of the deposits, sprayed with these new powders, were significantly improved, and their wear resistance was equal or superior to conventional WC cermet coatings. This was achieved by optimizing both the spray powder chemistry, and also the HVOF spray conditions (for the new powders) to cater to the improved quality requirements for wear, corrosion, and impact resistance. It is expected that the high impact resistance of the new generation powders will open up new markets for the use of HVOF coatings for construction machinery parts such as excavating tools and shovels. For instance, it was found that excavating tools with the coating demonstrated approximately 16 times longer life than conventional tools without the coating. The mechanism for improving the impact resistance of the coating was also investigated in this study.

In this work, HVOF sprayed hard metal multi-component Cr 3 C 2 -25Ni+10 vol-% solid lubricant coatings were prepared by HVOF spraying. CaF 2 , BN, MnS and WS 2 were used as solid lubricants. Powders were prepared by the spray drying and sintering method. This method produces homogeneous and spherical powders. Powders were sprayed with DJH 2600 HVOF-system. Microstructures, phase structures and compositions of powders and coatings were analysed with optical microscope, scanning electron microscope with EDS-analysator and x-ray diffractometer. The particle size distributions of sintered powders were determined by laser diffractometer. The amount of retained solid lubricant in the sprayed coatings was evaluated by EDS and compared to that in the spray powder. This study showed that spray drying and sintering method is a preferable method for manufacturing multi-component solid lubricant containing thermal spray powders. In sintering processes, solid lubricant phases remained in powders except for the Cr 3 C 2 -25Ni+10vol-%WS 2 -powder. Also, after HVOF spraying, coatings contained solid lubricants.

Coatings of Metal-Matrix-Composite (MMC) with enhanced wear resistance were created by thermal spraying. The used powders composed of iron and nickel aluminides reinforced with alumina, Cr- and Ti-carbides were produced before by self-propagating high temperature synthesis (SHS). The fused and crushed composite powders were sprayed by atmosphere plasma spraying (APS) and gas detonation spraying (D-Gun). Spray powder and coating properties like morphology, structure, microhardness, porosity, bond strength, wear and corrosion resistance are examined using optical and scanning electron microscopy, XRD analysis, pin-on-disc (wear resistance). The produced MMC coatings are compared to commercially used wear resistant coatings, e.g. high velocity oxyfuel flame sprayed Cr 3 C 2 -NiCr. The properties of a special coating type depend on the used spraying method. Plasma sprayed MMCs show a slightly higher porosity than D-Gun sprayed coatings. The chemical composition of used powders has a big influence on the coating properties. Fe- and Ni-aluminide matrix coatings reinforced additionally with carbides, especially Cr 3 C 2 , show a better wear resistance compared to coatings containing just oxides as hard material.

Crevice corrosion of metal/metal contacts in piping assemblies is a key issue for the design and the manufacturing of marine components. In this work, ceramic coatings onto alloy 625 were obtained using multi-processing CAPS facilities (Controlled Atmosphere Plasma Spraying). These coatings were sprayed in the CAPS chamber using air plasma spraying (APS, air at 100 kPa) or using high-pressure plasma spraying (HPPS, argon at 250 kPa) to achieve different coating microstructures and porosity levels. This allowed to investigate the corrosion behaviour in natural sea water of metal/ceramic contacts with different coating systems. Pure alumina or alumina-titania coatings with or without thermally-sprayed alloy 625 bond-coat were tested. Post-treatments like sealing of pores using epoxy resin were also achieved to study the resulting corrosion protection enhancement. Immersion and potentiostatic tests at +300 mV vs. SCE (Standard Calomel Electrode) tests were carried out in natural sea water at different temperature up to 60°C to expose specimens to the most severe working parameters. A beneficial protective effect of ceramic-coated alloy 625 has been clearly evidenced. Further investigations were performed using light microscopy and scanning electron microscopy to assess the corrosion behaviour and mechanical soundness of ceramic coated specimens which resulted in the determining of relevant technological solutions to prevent the risk of corrosion.

Non-oxide ceramics, such as silicon nitride, have a unique combination of high strength, toughness, wear resistance, thermal and chemical stability. However, the use of these materials as thick protective coatings on engineering components has been severely restricted by their decomposition behavior. Silicon nitride, for instance, does not melt but decomposes at ~1900oC and so thermal spraying of pure silicon nitride powder is impracticable. A limited amount of research has been carried out on depositing silicon nitride in various metallic or ceramic matrix materials but none have produced adequate coating microstructures or coating properties. This paper concerns the design of oxide matrix systems for silicon nitride composite coatings. A quantitative model is developed for the viscous flow of two-phase feedstock particles on impact with the substrate and is applied to the deposition of silicon nitride – ceramic matrix coatings. A number of matrix systems are investigated including a series of yttria-alumina and yttria-alumina -silica compositions. The research shows that the oxide matrices successfully protect the silicon nitride from decomposition but that the matrix composition and particle loading have a critical influence on splat flow and coating quality.

The advantage of cored wires, a new type of material in thermal spraying, is that it is easy to control their composition and convenient to develop new materials. This research is aimed at developing the Fe-Cr-Ni austenitic stainless steel corrosion-resistance thermal-sprayed cored wires with rare earth elements added. The experiments are conducted with arc spraying. The anodic polarization curves of the sprayed coatings of the developed wires and of the 1Cr18Ni9 solid wire are drawn by using MODEL 351 corrosion measuring system in the 0.5mol/L sulfuric and 3.5wt.% sodium chloride solution at a temperature of 20° C . For the sake of comparison, the substrate and the 18-8 wrought stainless steel are also measured. Results to date indicate good usability of thermal sprayed coatings in all the developed wires. Owing to added rare earth elements, the corrosion-resistance of coatings increases in the 0.5mol/L sulfuric solution. The wires with RE-3 added have exhibited good corrosion-resistance. Deterioration of the corrosion-resistance in the 3.5wt.% sodium chloride solution is caused by generation of crevices and inclusions between the deposited particles.

High velocity oxy-fuel spraying of WC-12Co was performed using a feedstock in which the WC phase was either principally in the micron size range (conventional) or was engineered to contain a significant fraction of nanosized grains (multimodal). Three different HVOF systems and a wide range of spray parameter settings were employed to study the effect on both the in-flight particle characteristics and coating properties. A process window with respect to particle temperature was identified for producing coatings with the highest resistance to dry abrasion. Although the presence of a nanosized WC phase produced harder coatings, there was little difference in the abrasion resistance of the best-performing conventional and multimodal coatings. However, there is a potential benefit in using the multimodal feedstock because of the possibility of higher deposition efficiencies and a more robust thermal spray process.

Nanocrystalline WC-Co coatings were deposited by high velocity oxy-fuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermo-chemical and gas-dynamical properties of gas and particles flows within the combustion flame were identified in various operating conditions by CFD simulation. Significant improvements of coatings mechanical properties were evidenced: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.

PyroGenesis Inc. has been conducting a program on the development of coatings prepared from nanostructured ceramic and cermet materials using atmospheric (APS), vacuum plasma spraying (VPS), and high velocity oxy-fuel spraying (HVOF). In the work presented in this paper, APS and VPS coatings from nanostructured or sub-micron Al 2 O 3 - 13TiO 2 , Cr 2 O 3 -5SiO 2 -3TiO 2 , and TiO 2 feedstock materials were developed and optimized for abrasion wear resistance. They were subsequently tested for sliding wear resistance. The resulting wear properties are discussed in terms of coating microstructure, and compared to those obtained from conventional microstructured feed materials. It is found that the starting powder and the spraying conditions play a major role in the resulting coating characteristics. VPS applied coatings from nanostructured powder were found to generally offer the best performance, most notably under sliding wear conditions.

In this paper Al-Al 2 O 3 -CrC nanostructured composite coating was presented, fatigue and fracture of the composite coating were investigated by nanoindentation, and in situ experiments performed in a scanning electron microscope to permit examination of freshly exposed surfaces. Crystallographic and morphological texture was characterized and the fracture resistance measured using fracture-mechanics. A CrC layer may improve the fracture resistance of an oxide aluminum layer. A CrC layer produced by pyrolitic deposition effectively heals the pores and defects of an oxide aluminum layer. It results in high load rating of the coating. Experiments revealed that in all cases, detection of an acoustic signal corresponded to an appearance of circular cracks seen on the surface; in a very few cases, examination of the surface after detection of a signal revealed the presence of two ring cracks. The degree of toughening associated with crack healing is determined by the number of healed defects and the effectiveness of the individual healing. Macroscopically, a crack path in the oxide aluminum appears to be straight, propagating along pores and internal voids. However, microscopically, a crack path exhibits a high degree of intergranular fracture. Because cracks generally deflect at small angles in the oxide aluminum layer, a crack path moves through pores and internal voids that usually concentrate internal stresses. The crack path is primarily intergranular at all velocities.

The present work describes abrasion wear tests that were carried out on low carbon steel test wheels coated with a series of plasma sprayed and HVOF sprayed Al 2 O 3 -SiC coatings with varying SiC contents. These were compared to a pure Al 2 O 3 coating applied by HVOF and to uncoated steel. The HVOF sprayed coatings exhibited consistently superior abrasion wear resistance compared to plasma sprayed coatings of equivalent composition and uncoated steel. Overall, the HVOF Al 2 O 3 -10%SiC nanocomposite coating showed the best abrasion resistance of all the coatings tested. They were worn very slowly by a micro-abrasion process. The poor performance of the plasma sprayed coatings was attributed to a low cohesive strength which made them particularly vulnerable to grain pull-out.

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.

Under marine and coastal conditions, the degradation by corrosion of low-alloyed steels is generally observed. In order to overcome such important corrosion problems, the use of thermal spray coatings made of noble materials may be an attractive solution. 316 stainless steel thermal spray coating, an iron alloy coating, is often considered for corrosion protection because of its low material cost. Also, the high velocity oxy-fuel (HVOF) is often the selected coating process because it is known to provide coatings with a very low porosity level preventing the corrosive media to reach the substrate. The present paper compares the corrosion behavior of wrought 316 stainless steel with sprayed coatings made of the same alloy on 1020 mild steel. The corrosion behavior of materials is studied under salt fog conditions and with electrochemical techniques in brine simulating the marine environment. The coatings have been sprayed by HVOF under usual conditions. The results of this study demonstrate that the material behavior with regard to corrosion is process dependent . The HVOF sprayed stainless steel coating is much more sensitive to corrosion than wrought stainless steel. Corrosion product appearing on the samples is not only linked to the corrosion of the substrate by diffusion of the corrosive solution through pores but is also generated by intrinsic corrosion of coating itself. An enhanced sensitivity of the coating with regard to corrosion is attributed to the surface of particles or droplets, which are most likely degraded during the spraying process. However, thermal spray coatings having performances as good as wrought stainless steel can be obtained. In the present work, it is demonstrated that coatings obtained using vacuum plasma spray (VPS) have similar corrosion properties than wrought stainless steel in simulated marine environment. The industries considering corrosion protection of their components in marine environments by the use of stainless steel coatings must be aware of the reliability of their coatings. During the usual HVOF spray process, particles or droplets of stainless steel 316 are subject to important modification leading to a loss of performance against corrosion. Oxidation of alloying elements necessary to obtain a good stainless steel most likely occurs. However, the use of vacuum sprayed stainless steel coatings results to efficient protection against corrosion in marine environment.

The world’s first plant for manufacture of stainless steel clad structural steel is now operational in the USA. The process consists of coating round steel billets with a spray of stainless steel. A metallurgical bond is achieved so that the billets can be reheated and hot worked into long products while retaining the integrity of the coating. The process consists of teeming stainless steel from a ladle into a spray chamber and atomizing the emerging stream with jets of nitrogen to form a spray of semi-liquid particles. The spray is directed onto a 140mm diameter preheated carbon steel billet to form a thick coating (4 – 5mm). The spraying rate of 50Kg/minute produces clad billet at the rate of 15tonnes/hr. Billet is then hot rolled in a conventional bar mill to make corrosion resistant clad steel sections such as rebar and dowel pins. Coating thickness after rolling is in the range 0.5 – 1.0 mm depending on the final section. Clad products have a life expectancy of 75 – 100 years in high chloride environments such as tidal zones, bridge decks and highways treated with de-icing salts. The spray coating process is described together with mechanical properties of the clad bar and results of corrosion tests. The economics of stainless clad steels vs. other corrosion resistant materials are reviewed.

This work reports research concerning the production of powder, suitable for reactive HVOF spraying, produced by mechanically alloying Ni(Cr), Ti and C elemental powder constituents. Powder mixing was achieved using a high-energy Uni-Ball II Mill and optimisation of the milling parameters are reported. The composition of the powder, at 50wt.%NiCr-40wt.%Ti-10wt.%C, was such that the application of heat has the potential to cause a SHS (Self propagating High Temperature Synthesis) reaction to take place. The utilisation of SHS reactions to produce TiC particles within metallic matrices is well known in bulk systems. However, this work describes carrying out this reaction in individual powder particles on exposure to the high temperature within the HVOF gun. The powder having undergone the SHS reaction during the spray process was deposited onto mild steel substrates to form a dense, coherent coating. The coatings thus formed were shown to contain nanoscale TiC in a Ni(Cr) matrix, indicating a SHS reaction had taken place. This TiC is much finer than that produced in conventional SHS reactions, which is typically ~5ìm. The percentage of TiC formed, and retained in the coating, was lower than expected from the constituent proportions and explanations for this observation are proposed. The microstructure of the coating is described and compared with a Ni(Cr)-TiC cermet coating sprayed using conventional SHS powder generated from reacted compacts which were crushed, sieved and classified to give sprayable feedstock powder.

The thermal spray committee of the Japan Association of Corrosion Control (JACC) has been conducting a marine corrosion test of thermal sprayed Zn, Al and Zn-Al coatings since 1985. Twelve kinds of sprayed coating were deposited onto steel pipes by arc- and flame-spraying to varied thickness and subjected to various post-spray treatments. The samples were set vertically into seawater at a port 80 km south from Tokyo. Corrosion performance of these coatings has been inspected annually by recording the appearance and coatings’ thickness at sea air-, splash- and tidal-zones. No significant changes were observed for five years exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, Al and Zn-Al coatings still exhibit superb corrosion performance. The test will continue till 2006 to complete the test period of 20 years. In order to place the test into a proper perspective, we also conducted a survey on the corrosion prevention by thermal spray technology in 2001, collecting more than 170 published reports and asking experts to contribute reviews on various aspects of the technology. This paper first describes several topics from the survey report to explain the past and the present situation of thermal spraying for corrosion prevention in Japan. Then, the corrosion performance of sprayed coatings in the exposure test during 15 years will be summarized.

For 6 years, we have developed corrosion resistant coatings for the marine structural steels by using a thermal spray technique. Such a coating requires primarily impermeability and secondarily cleanliness. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. In addition, some new methods were designed to evaluate the sprayed particle’s state and coating properties with high accuracy and sensitivity. An inert gas shroud system was attached with the commercial HVOF apparatus and this attachment enabled inflight spray particles to be accelerated over 750 m·s-1 and simultaneously to avoid to be oxidized. The coating of HastelloyC nickel base alloy by this process had zero through porosity and 0.2 mass% of oxygen content, leading to be comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.