Abstract 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.

Abstract This paper compares the coating characteristics of two HVOF processes: air-cooled converging-nozzle Diamond Jet (DJ) spraying and Hybrid 2600 air/water-cooled converging/diverging technology. WC-Co coatings were deposited on steel substrates using different combinations of spray parameters, gas flow ratios, and cooling gas types. The coatings were then examined and tested to determine the extent to which microstructure, hardness, surface roughness, wear resistance, and deposition efficiency can be controlled. In addition to investigating process relationships, the paper also addresses the issues of practicality and cost.

Abstract Hardmetal-like coatings of the TiC-Ni system are potential for use as wear, corrosion and heat resistant coatings in various operation conditions. Our previous works [1-12] have shown that these materials are well sprayable using different thermal spray processes such as plasma, D-Gun and HVOF spraying. Since HVOF spraying is today the most important process used to apply carbide based coatings, this study was carried out in order to evaluate more systematically the sprayability of these novel spray powders and the influence of HVOF spray parameters on some coating properties. Coating samples were prepared by using DJ Hybrid gun with propane as a fuel gas, and a CDS gun with hydrogen fuel gas. Oxygen flow rate was varied in both cases for changing the flame temperature. Microstructure, phase composition, hardness, and abrasion wear resistance of the coated samples were investigated. The results showed that both HVOF processes used give satisfactory coating properties and that the use of high oxygen flow rates is beneficial for improving the wear resistance of the coatings. Powders with fine particle size are beneficial in the DJ Hybrid process; the use of coarse powders results in coatings with somewhat higher wear rates. The optimum spray condition for the TiC-Ni system powders differs from that typically used for conventional WC-Co and Cr3C2-NiCr powders by a higher flame temperature.

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 This paper presents the results an experimental study on ferroalloy-base flux-cored wire coatings. The work conducted shows that it is possible to improve the structure and properties of coatings by adding aluminum to the ferrochromium powder charge and rare-earth elements and calcium to ferroboron. This reduces the oxygen content and porosity of coatings by a factor of 1.5-2. It also reduces residual tensile stresses in outer coating layers and improves coating-substrate adhesion strength as well as abrasive wear resistance.

Abstract A ball-milled mixture of glass and alumina powders has been plasma sprayed to produce alumina-glass composite coatings. The coatings have the unique advantage of a melted ceramic secondary phase parallel to the surface in an aligned platelet composite structure. The alumina raises the hardness from 300HV for pure glass coatings to 900HV for a 60wt% alumina-glass composite coating. The scratch resistance increases by a factor of three and the wear resistance by a factor of five. The glass wears by the formation and intersection of cracks. The alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reached a plateau at 40-50vol% alumina, which corresponds to the changeover from a glass to a ceramic matrix. Keywords: glass composite coatings, wear, thermal spraying

Abstract 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.

Abstract 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.

Abstract Oxide-bonded silicon nitride (OBSN) powders have been developed to address thermal spray problems associated with high temperatures. This paper examines how such powders perform when applied via detonation gun (DGS) and atmospheric plasma spraying (APS) with axial powder injection. All coatings were characterized using optical microscopy and X-ray diffraction with additional tests being performed on DGS coatings. For the first time, relatively dense Si3N4-rich coatings with an oxide binder phase were produced, and some of the DGS coatings were found to be sufficiently wear resistance for industrial use.

Abstract The aim of this study was to improve the properties of atmospheric plasma sprayed WC-Co-Ni coatings by post treatment with induction heating. The spray powders used were WC-Co-Ni composite powders, produced by adding Ni-P alloy to WC-Co powder. Induction heating applied to the coating caused fusion of the Ni-P alloy with the WC-Co-Ni coating and a strong metallurgical bond with the steel substrate. An innovation in recent experiments was the use of an antioxidant paste applied to the outer surface of the coating under treatment. This eliminated the need for working under the restrictions of a hydrogen gas environment. The properties of the treated coating were investigated regarding microstructure, hardness, adhesive strength, abrasion resistance and porosity.

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 The results of low stress, pin-on-disc and high stress grinding abrasive wear tests on coatings produced by plasma and oxy-acetylene flame spraywelding are presented. FNil5A and FNiWC35 Ni-based self-fluxing alloys were selected as typical spraywelding materials for abrasive wear resistance. The abrasive wear resistance mechanisms of welded overlays produced by various materials and processes were also characterized by hardness tests, microstructural and compositional analyses, and through analysis of the effect of different kinds of abrasive on the wear resistant of Ni-base self-fluxing spraywelding overlays. Results showed that FNiWC35 overlays exhibited improved resistance under low stress abrasion, but the relative wear resistances of FNiWC35 and FNil5A still depended primarily on the type and hardness of the abrasive medium used. For the same material, the abrasive wear resistance of oxyacetylene flame sprayed overlays was higher than that produced by plasma spraywelding. The wear resistance of the plasma spraywelding overlays depended not only on the material, but also strongly on the spraywelding process parameters.

Abstract Protective coatings adapted for titanium alloys which come into frictional contact with one another are described. Among the many coatings investigated, the best ones are cobalt based. The coatings are sprayed only on one of the rubbing surfaces. During rubbing, a small part of the coating transfers to the unprotected titanium alloy surface. The rubbing pair is thus essentially composed of two cobalt alloy-based surfaces. This leads to low coefficient of friction and little or no damage to the rubbing surfaces. The coatings find particular application in the protection from adhesive and fretting wear, galling and seizure of gas turbine and jet engine parts or the like made from titanium alloys.

Abstract A new concept of high energy recovery of dc plasma torch has been developed and applied to the deposition of polymer composites. The heat transferred to low cost fillers, such as alumina or silica injected into the high temperature zone of the plasma, is recovered in the applied polymer coating, which is injected downstream in a much cooler zone of the plasma. The result is a polymer/ceramic composite spraying process having the following benefits: Shrinkage phenomena due to polymer recrystallisation is eliminated, providing a good coating adhesion, a high spray material throughput rate is achieved and die risk of coating polymer combustion is greatly reduced. The polymer composite form decreases the overall costs of coatings and the type of filler employed influences the properties of the composite. Abrasion resistant composites have been produced with alumina and silica fillers (50 wt.%). Medium density polyethylene (MDPE), sprayed with 50 wt.% reinforcement, have shown abrasion resistance as high as UHMWPE. Polymer degradation, abrasion mechanisms and microstructures of these composite coatings are explained.

Abstract Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.

Abstract Cermet (WC-Co) coatings have been produced on steel substrates by plasma spraying in vacuum and in air. These have been examined microstructurally and characterised in terms of porosity content, stiffness, microhardness and abrasion resistance. Particular attention has been paid to the phase constitution, as revealed by X-ray diffraction and scanning electron microscopy. High precision densitometry has been used to study porosity levels. Coatings with three different metal contents (9, 12 and 17wt.%Co) have been examined. There is a strong tendency for chemical reactions to occur within the plasma plume, particularly for spraying in air. These reactions can result in the formation of various carbides and even of metallic tungsten. Thermodynamic and kinetic aspects of the reactions involved are briefly examined. Such reactions are strongly promoted by the presence of oxygen, and are much less marked during vacuum plasma spraying. Plasma power and substrate temperature have secondary effects on the degree of reaction which occurs. A marked correlation was observed between degree of reaction and resistance to abrasive wear. This is consistent with the reaction products being brittle and causing poor interfacial cohesion. It was also found that wear resistance was greater for the coatings with lower metal contents. This behaviour can be attributed to the wear occurring predominantly by ploughing of the metallic phase and consequent release of ceramic particles. This occurred more readily when the metal content was higher. In coatings which had undergone pronounced chemical reaction, however, metal had been replaced by reaction products which conferred poor cohesive strength, leading to poor wear resistance.