The application of thermally sprayed coatings is often limited by their porosity as well as their low adhesion. A new approach for high energy beam surface treatment of thermally sprayed coatings is presented using simultanously a lower power level for continuous remelting and pulses of higher power in order to get coatings with good corrosion behaviour and improved adhesion. This technology is applied to titanium as well as to NiCr-coatings on steel substrates. The investigations prove that remelted thermally sprayed coatings with a smooth and dense surface layer, followed by a heat treated section of the coating and a reaction zone at the substrate/coating interface can be obtained. Therefore, a change in chemical composition of the coating is avoided, while at the same time the bond strength of the remelted coatings can be influenced by metallurgical reactions at the interface zone.

Forming technology of protective properties using concentrated energy flows is based on the possibility of laser radiation firstly to provide high heat flow densities which are necessary for intensive heating on a small part of surface. When effecting metal surface laser radiation reflects from it partially and the rest flow penetrates on small depth. As far as the energy is practically absorbed fully in the surface layer, width 10 -6 to 10 -7 m, the heat source may be considered as surface one [1]. To harden steel products it is necessary to heat the surface till the temperature of phase transformations. The surface cooling takes place mainly due to heat transfer to a less heated member part. Mathematical modeling thermal processes occuring when local surface parts processing is one of effective means to study this method of materials hardering. However till now the predominant number of works of temperature fields investigation when local processing was done supposing that the ray effects semi-infinite space or the process is considered in a moving coordinates system, connected with the centre of a light spot, that enables to consider a stationary task. Such assumption are true when processing massive parts but give a considerable error when processing small parts [2]. The offered models with the aid of which there is studied the heating process for a sample of parallepiped and cylinder shape come to solving the equation of thermal conductivity with considerably nonlinear coefficients. When surface processing with continuity laser, the ray is focused perpendicularly to the sample surface. The period of light spot contact with the surface is estimated from the speed of its travel on the scanned surface. Before the test the product is subjected to special processing reducing the energy loss due to reflection.

Stainless Steels are required for many applications for ship building as well as for offshore structures such as oil exploration. AISI type 304 stainless steel is not very suitable for such applications as it has a strong tendency for pitting and crevice corrosion. Even type 316 and 317 stainless steels which have respectively 2.5 and 3.5% Mo are not very effective in these environments. Commercially available stainless steels, viz., Avesta 254 SMO is being employed for such applications because of its strong resistance to pitting and crevice corrosion. This is mainly because of high Mo concentration (6.5%). Such steels are not only costly but are prone to form deleterious phases such as delta ferrite and sigma during welding or other heat treatment operations. Hence, an alternative technique to restrict Mo at the surface is needed. In the present work, surface alloys consisting of an austenitic stainless steel with Mo content as high as 10-12% have been formed on stainless steel type 304 substrates. These steels show enhanced passivity and strong resistance to pitting corrosion.

Fusible coatings of Nickel-Chromium alloys with various amounts of Boron and Silicon commonly used for severe load applications. The coating is normally sprayed, then fused by heating to the point of liquation. The fusing process causes powder coalescence and increases density. At the same time, the high fusing temperatures creates a “brazed” bond which gives these coatings extremely high adhesive bond strengths. The improved bond strength is the result of the metallurgical bond as compared to the majority of thermal spray coatings which rely only on mechanical bonding mechanisms. The fusing operation is very sensitive, especially when a hand torch fuse is required. To circumvent these problems, a study was conducted to see if high density HVOF sprayed coatings might achieve fused quality by furnace heating to temperatures well below the liquation point. Various times and temperatures were surveyed. Bond strength tests of coatings sprayed to heavy thicknesses, hardness and impact tests, and metallography were used for evaluation. It was determined that heating as low as 1500° F for three hours could improve the properties of an as-sprayed HVOF coating to where it developed characteristics very similar to that of a fused coating.

Reducing the pore size and pore volume can lead to improved mechanical properties and enhanced corrosion resistance of plasma sprayed thermal barrier coatings. In this work, plasma sprayed 8 wt.% yttria stabilized zirconia coatings were removed from the substrate and machined to obtain 25x5x1 mm test specimens. These specimens were vacuum impregnated with alumina sol, calcined at 873 K for an hour and then heat treated at 1273 K for an hour to produce ceramic impregnated specimens. As-sprayed and impregnated specimens were investigated using optical microscopy, XRD, SEM, mercury intrusion porosimetry and electron microprobe analysis. This technique can impregnate the entire thickness of the specimens. Pores in the as-sprayed specimens were impregnated with α alumina grains, resulting in microstructural variations and reduction of the size and volume of the specimen pores.

Thermal sprayed coatings are frequently used in corrosive environments, even when their major purpose is to provide wear or thermal resistance, rather than corrosion resistance. This includes Thermal Barrier Coatings (TBC), where high porosity is a desired feature to give good thermal protection. However, as this proves to be a limiting factor in the corrosion protection, a trade off is involved. This is because the interconnected porosity in TBCs allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. This work addresses the problem of permeability of TBCs which can lead to premature delamination due to interfacial corrosion. The coatings studied were yttria-stabilized zirconia TBCs. A simple infiltration technique has been proposed using sol-gel ceramic precursors. The precursors studied include aluminum isopropoxide or pre-hydrolyzed ethyl silicate, which decomposed to alumina and silica respectively, at surface heat treatment temperatures as low as 550°C. In addition to sealing the surface, it is believed that some level of compressive stress is generated on the surface of TBCs on cooling from the processing temperature. Electrochemical tests in 3.0% NaCl have been carried out to study the effectiveness of the sealant. These potentiodynamic tests as well as permeability tests show a considerable decrease in interconnected porosity with sol-gel modifications of the coatings.

Plasma sprayed ceramic coatings usually have relatively high open porosity in order to provide a good corrosion protection. By using sealants the porosity values can be reduced. In this study atmospheric plasma sprayed (APS) aluminium oxide, chromium oxide and zirconium oxide coatings were sealed by a phosphoric acid treatment. After impregnation the coatings were heat treated at a curing temperature of 400°C. Phosphoric acid was found to react with the coating material during the heat treatment. Wear resistance was evaluated by rubber wheel abrasion tests and corrosion resistance by electrochemical potentio-dynamic polarization tests. Hardness values were also measured. Corrosion resistance and hardness values of sealed coatings were remarkable better in comparison to the unsealed coatings. Rubber wheel abrasion resistances of the sealed coatings were equal to those of Al 2 O 3 , ZTA, SiC and Si 3 N 4 sintered ceramics.

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.

Anilox rolls require dense, adherent chromium oxide coatings capable of the highest quality microfinishing and laser engraving. Process economy and high deposition rate are also essential. Increasingly, anilox roll job shops are spraying such coatings using a high power plasma system, prompting this detailed investigation of processes, powders, and finishing. This paper evaluates how the material and the process influence the finished texture and engraving quality. Criteria for a successful coating are defined. The effects of plasma process, powder type, and coating microstructure on finish and engraveability mc analyzed in detail and practical recommendations are made. The test matrix includes high power and conventional plasma spray systems, three commercial chromium oxide powders used in anilox roll spraying, and three commonly used finishing methods.