Serge Dallaire

Synthesis and deposition of TiC-Fe coatings by plasma spraying

Abstract TiC possesses a hardness so great that it has been considered for the development of wear-resis- tant coating materials. However, its brittleness and the difficulty in dispersing it homogeneously and finely in large volume proportions within a tough metal matrix have so far limited its application in the hard surfacing industry. Because the plasma spraying of premixed powder blends or composite powders containing TiC and metallic elements does not give coatings with homogeneous microstructures, a new method was developed for producing TiC composite coatings. It consists of the simultaneous synthesis and deposition of TiC-Fe materials by plasma spraying. Solid reagants bound together to form a single particle are injected into the plasma where a reaction occurs. The reaction products are propelled onto a substrate to form a coating. Microstructural analyses comprising X-ray diffraction analysis revealed that TiC was synthesized during spraying. Dense and thick coatings containing submicron TiC particles dispersed in an iron matrix were compared with those obtained by spraying composite powders comprising TiC and iron. With their unusual microstructure, these plasma spray-synthesized (PSS) coatings may be suitable for wear resistant applications. The low cost of starting materials makes these PSS TiC-Fe coatings very attractive as substitutes for strategic materials.

Synthesis and deposition of TiB2-containing materials by arc spraying☆

Abstract Wear-resistant coatings containing TiB 2 have been previously synthesized and deposited using the plasma spray synthesis process. The technique consists of plasma spraying micropellets comprising the reagents (ferrotitanium and boron or ferroboron) and depositing the as-reacted products on a substrate. In order to facilitate the deposition of the materials, to lower the cost and to increase the spray rate, a different approach has been tried. It consists of depositing TiB 2 -containing material by using the flexible arc-spraying technology. Coatings containing TiB 2 have been synthesized and deposited by arc spraying core wires containing the reagents mentioned above. Even though the process appears simpler, it was found that the wire fabrication as well as its deposition must be carefully controlled in order to obtain the desired coatings.

Thermal shock resistance of TiC coatings plasma sprayed onto macroroughened substrates

Abstract The thermal shock resistance of TiC coatings plasma sprayed onto macroroughened substrates was studied in comparison with gritblasted substrates. Macroroughening restricts the shrinkage stresses, increases the bonding area and produces folds in the layers of coating which strengthen it. Mechanical and thermal shock tests and numerical simulations were carried out. The enhanced resistance of coatings deposited onto macroroughened substrates is probably due to the lamella folding and the particular distribution of thermal stresses.

The influence of composition and process parameters on the microstructure of TiC-Fe multiphase and multilayer coatings

Abstract Composite TiC-Fe coatings obtained by the plasma spray synthesis process can be composed of alternate TiC-rich and TiC-poor lamellae of different hardnesses. This study was undertaken to assess the influence of the fabrication parameters on the microstructure of these coatings. Both the composition of the micropellets, which lead to the synthesis and dispersion of fine crystals within the iron matrix upon spraying, and the spraying parameters were examined. Among all the variables considered it was found that the parameters related to the fabrication of micropellets, such as their size and their expected TiC volume content, and those related to the solidification of synthesized products are the main factors affecting the hardness of TiC-rich layers. Coatings obtained by spraying micropellets comprising TiC and iron powders agglomerated together are composed of large angular and hard TiC particles embedded in a matrix whose hardness is between 300 and 500 kgf mm-2. In contrast, plasma-spray-synthesized coatings are composed of alternate soft and hard layers whose hardness varies according to the fabrication parameters and is between 1000 and 1800 kgf mm-2 for hard layers and between 300 and 600 kgf mm-2 for soft ones. These plasma-spray-synthesized TiC-Fe coatings, because of their multilayer and fine-grained multiphase microstructure, are expected to be more resistant to crack propagation and, consequently, to wear.

Plasma spraying of carbon-coated TiC powders in air and inert atmosphere

Abstract An important carbon loss occurs during the plasma spraying of TiC in air and, to a lesser extent, in an inert atmosphere. Moreover, an increase in the oxygen content is observed in coatings sprayed in air. In the present study, different carbon coatings were applied to TiC powders by an agglutination coating technique in order to control both the carbon stoichiometry and the oxygen content within sprayed coatings. The effects of the carbon crystalline form (graphite or carbon black) and of the coating weight percentage deposited on TiC particles on the chemical composition and microstructure of plasma-sprayed coatings were investigated. An increase in both combined and free carbon content and a decrease in the oxygen contamination were observed in coatings sprayed in air with the carbon-black-coated TiC powders. On the other hand, graphite coatings applied to TiC particles had a less significant effect on the composition of coatings sprayed in air or in the inert atmosphere. Severely decarbonized and oxidized zones were found within coatings sprayed in air.

Investigation of selected plasma-sprayed coatings for bonding glass to metal in hermetic seal applications

Abstract A study was carried out to investigate the feasibility of joining glass to 304 stainless steel by using selected coatings applied by plasma spraying in the fabrication of hermetically sealed components. After adapting the plasma spraying technique to coat the small bore of a ring, it was found possible to deposit two-layer coatings on prototypes. Among the coating systems tested, the thin Al 2 O 3 -TiO 2 /Ni-Cr-Al-Y two-layer coating was selected. Following the bonding with glass which was performed at 1000°C in air atmosphere, the sealed components were found to be leak proof.

Zinc-nickel coatings for improved adherence and corrosion resistance☆

Abstract Plasma-sprayed ceramic coatings may spall catastrophically in service because corrosion products are developed at the coating-substrate interface. This phenomenon is likely to occur with ceramic coatings on steel substrates even when the substrates have first been coated with metallic bond coats. This work was aimed at developing zinc-nickel thermal-spray powders for cathodic protection of steel substrates. Powders were produced from zinc-30 wt.% nickel alloy with a particle size distribution of 90 - 150 μm and a study of their spraying parameters on steel substrates was carried out. The performance of these zinc-nickel coatings was determined by subjecting plasma-sprayed alumina and chromia coatings with and without a zinc-nickel underlayer to aqueous corrosion for various periods of time according to the ASTM B117-85 test. The effect of degradation by corrosion was measured by the ASTM C633-79 procedure. A drastic loss of bond strength, as a function of corrosion time, was noticed in ceramic coatings deposited directly on steel substrates. Similar behavior was also observed for ceramic coatings plasma-sprayed on to nickel-chromium bond coat. Results of electrochemical, salt-spray and bond strength tests showed that zinc-30 wt.% nickel coatings provide a cathodic protection. This cathodic protection confers an active corrosion protection on the substrate and is not affected by the presence of porosity in the ceramic coating. The adhesion strength of these zinc-30 wt.% nickel coatings is in the neighborhood of 40 MPa.

Technical note: Agglutination process for coating ceramic particles

Summary In the fabrication of high performance metal-ceramic composites and plasma-sprayed coatings containing dispersed ceramic particles, the use of metal-coated ceramic powders can be expected to ensure better bonding between constituents and provide a more uniform microstructure than the mixing of ceramic and metal powders could do. Various coating methods have been used to coat ceramic powders. However, these methods are limited in terms of coating materials and thicknesses. A new inexpensive method based on an agglutination process is proposed to coat different kinds of ceramic particles (40 – 100 pm) with various metallic elements. The process variables are discussed with examples concerning Al 2 O 3 and SiC. It was found that there is no limitation in terms of ceramic cores and metal coatings and that it is possible to produce thick coatings depending upon the nature of metal powders and ceramic particles.

Aqueous corrosion behaviour of plasma sprayed coatings

ABSTRACT The aqueous corrosion degradation of ceramic and metallic coatings plasma-sprayed onto steel substrate were assessed by measuring their bond strengths as a function of salt spray exposure time. The bond strength-corrosion time dependence of Cr 2 O 3 , AlO 3 .TiO 2 , Zr0 2 .MgO, NiCr and Cr 2 0 3 /NiCr coatings were measured. The severity of the aqueous corrosion attack depends more upon the difference in the electrochemical potentials between the coating and the substrate than in the amount of the open porosity. For large differences in the electrochemical potentials, as in the case of Cr 2 O 3 and Cr 2 0 3 /NiCr, a severe drop in bond strength is observed. For small differences in the electrochemical potential, the corrosion is less severe and a reduction in bond strength with corrosion time is only observed after longer period of corrosion or this reduction is more gradual.