Peculiarities of the Structure and Phase Composition of Hydroxy Apatite Coatings Sprayed by Laminar Plasma Jet and Treated by Electron Beam

The covering of titanium implants by means gas-thermal spraying of hydroxyapatite powders is an actual scientific, technical and medical problem. Application of hydroxyapatite for these purposes is more preferable. However, the problem of its structural and cyclic strength under conditions of bioenvironment response determines of application areas of such coatings and reliability of them usage. Structure and phase composition of hydroxyapatite coating under plasma spraying on titanium substrates and their changing, caused as conditions of forming coating on its increasing, so and conditions of spraying an laminar and turbulent plasma streem were studied. Exact belief about the crystalline structure and phase composition of coating is obtained by methods electronic microscopy and X-ray analysis. Changing of coating structure after sintering in the vacuum and electron beam melting in the vacuum is discussed.

Reactive Plasma Spraying of Wear Resistant Coatings of Ti Composites

In this investigation reactive plasma spraying was used to produce wear resistant coatings of Ti-carbides/titanium or Ti-nitrides/titanium composites. Ti-powders with different powder size distributions were used as raw materials. Methane and nitrogen were used as reactive gases to form carbides and nitrides. A reactor was adapted to the plasma gun F4 of a Sulzer Metco vacuum plasma spraying equipment to increase the degree of the expected reactions. Phase analysis and micrography of the coatings reveal that the Ti-hardphases were synthesized during spraying and embedded in the titanium matrix. The in situ synthesized hardphases show different forms and sizes. Most of them are non-stoichiometry. Compared to the titanium coating the coatings produced by reactive plasma spraying are much harder and more resistant against both sliding and abrasive wear.

Mathematical Modeling of Flattening Process on Rough Surfaces in Thermal Spray

Thermal spray layers are formed on rough surfaces; however, the flattening process on rough surfaces has not yet been clarified. A mathematical flattening model which takes into account the roughness of the substrate or previously coated layers is proposed in this paper. As a result of surface roughness, the flattening degree and the flattening time decrease with increasing surface roughness in this model. In addition, the characterization of surface roughness is introduced for the flattening model. Several calculated cases of the flattening model are shown.

Microstructure and Properties of WC-CO-Cr Coatings

WC-Co-Cr represents an important composition for hardmetal-like coatings which is appHed when simuhaneous wear and corrosion resistance is required. In this paper five commercially available spray powders obtained by various production techniques (sintered and crushed as well as agglomerated and plasma-densified) of the composition WC-10%Co- 4%Cr have been thoroughly characterized and were sprayed by DCS, HVOF (CDS process) and APS. The microstructures of the coatings were characterized and their wear behaviour was investigated by means of an abrasion wear test. For the best of these powders the wear resistance was nearly equal for the DGS and HVOF coatings. Other powders show significant differences with respect to their processabilities in these spray processes. APS coatings from all powders, obtained with an Ar/H2 plasma showed inferior microstructures and significant lower wear resistance. The spray powder compositions, grain sizes and structures were found to determine the processability of the powders and the microstructure and properties of the coatings. COMPOSITE MATERIALS of the type hard phase - metallic binder with WC and CoCr as constituents are widely used for the preparation of hardmetal-like coatings. The chromium addition to the metallic binder is thought to improve its corrosion resistance in comparison with pure WC-Co. This has led to many applications of WC-CoCr coatings where simultaneous wear and corrosion resistance is required. Despite of its significant practical importance only a limited number of publications is devoted to detailed questions of structure and properties of WC-CoCr coatings (1-3). In some comparative studies such coatings have been investigated together with WC-Co and Cr3C2-NiCr coatings (4-8). However, systematic investigations of spray powder compositions and morphologies as well as investigations of the influence of different thermal spray processes on coating structures and properties which have repeatedly been provided for WC-Co (for example (9, 10)) are missing for WC-CoCr. In this paper a short survey of literature on the phase relationships in the WC-CoCr system and the effect of chromium additions on the properties of sintered parts and thermally sprayed coatings compared to WC-Co is given. In the experimental part a systematic study of the influence of the preparation process on composition and morphology of commercially available WC-10%Co-4%Cr spray powders was provided. These powders have been sprayed by DGS, HVOF and APS and the microstructure and basic properties of the coatings have been studied.

Characterization of WC-Co Coatings Using HP/HVOF Process

To maintain surface roughness of process rolls in cold rolling steel plants, WC-Co coatings have been known to be effective ones. In this study, a high pressure/high velocity oxygen fuel (HP/HVOF) process was used to obtain WC-Co coatings. To get the best quality of coatings, WC-Co coatings are sprayed with numerous powders made by various processes. These powders include agglomerated sintered powders, fused-crushed powders, extra high carbon WC-Co powders and (W2C, WC)-Co powders. After spraying, properties of coatings such as hardness, wear resistance. X-ray diffraction, and microstructures were analyzed. For coatings produced by agglomerated-sintered powders, hardness of the coating increased as power levels and the number of passes were increased. In case of the coatings produced by fused-crushed powders, a very low deposition rate was obtained due to a low flowablity of the powders. In addition, the WC-Co coatings sprayed with extra carbon content of WC-Co did not show improved hardness and wear resistance. Also, some decomposition of WC was observed in the coating. Finally, the coatings produced by (W2C, WC)-Co powders produced higher hardness and lower wear resistance coating.

Stainless Steel Coatings for Corrosion Protection of Steel Rebars

Steel reinforcement corrosion is one of the most serious causes of the premature deterioration of North American bridges and parking garages. Carbon steel rebars are very vulnerable to corrosion in salt contaminated concrete from deicing or coastal environment since the chloride ions induce severe corrosion as they reach the reinforcing steel rebars and depassivate the carbon steel. This paper evaluates the potential of using stainless steel coatings as a means to protect steel rebars from corrosion, especially in a salt contaminated concrete environment. The 316 L stainless steel coated coupons and rebars were prepared using Arc-sprayed and HP/HVOF processes. The corrosion performance of coatings were evaluated using linear polarization, a.c. impedance and salt spray techniques. Metallographic examination was also performed to characterize the coating microstructure.

Studies of the Flame Spraying of Polymers

Experimental studies of the subsonic combustion process have been conducted in order to determine the quality and economics of polyester, epoxy, urethane, and hybrid polyester-epoxy coatings. Thermally sprayed polymer coatings are of interest to several industries for anti-corrosion applications, including the infrastructural, chemical, automotive, and aircraft industries. Classical experiments were conducted, from which a substantial range of thermal processing conditions and their effect on the resultant coating were obtained. The coatings were characterized and evaluated by a number of techniques, including Knoop microhardness tests, optical metallography, image analysis, and bond strength. Characterization of the coatings yielded thickness, bond strength, hardness, and porosity.

An Investigation of the Electrical Behavior of Thermally Sprayed Aluminum Oxide

Electrical properties of plasma-sprayed aluminum oxide coatings were measured at temperatures up to 600 °C. High purity (>99.5 wt% pure Al2O3) alumina powders were plasma-sprayed on stainless steel substrates over a range of power levels, using two gun configurations designed to attain different spray velocities. Key electrical properties were measured to evaluate the resultant coatings as potential insulating materials for electrostatic chucks (ESCs) being developed for semiconductor manufacturing. Electrical resistivity of all coatings was measured under vacuum upon heating and cooling over a temperature range of 20 to 600 °C. Dielectric constants were also measured under the same test conditions. X-ray diffraction was performed to examine phase formation in the coatings. Results show the importance of powder composition and careful selection and control of spray conditions for optimizing electrical behavior in plasma-sprayed aluminum oxide, and point to the need for further studies to characterize the relationship between high temperature electrical properties, measured plasma-spray variables, and specific microstructural and compositional coating features.

Cavitation Erosion of HVOF Coatings

The proposed paper reports a series of experiments to investigate the cavitation erosion mechanism of HVOF coatings. Vibratory cavitation erosion tests according to ASTM G 32 have been carried out with several HVOF coatings including cermets, oxides and metallic alloys. The steady state erosion rate for each coating was determined and the effect of coating composition and microstructure on the erosion rate was investigated. The morphology and microstructure of the various coatings before and after cavitation testing were analyzed by means of light optical and scanning electron microscopy in order to study the erosion mechanism. The results demonstrate that HVOF coatings of NiCrFeBSi, WC-17Co, Cr3C2-25NiCr and Cr2O3 can exhibit a rather high resistance against cavitation erosion and should be considered for application as a protective surface layer against cavitation. Furthermore, it is shown that cavitation testing can provide a useful tool to study and characterize the bond strength between individual splats as well as the brittleness of the individual phases present in the coating.

Surface Modification of Thermal Sprayed Coatings

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.