ICMCTF2013 Session G5-2: Coatings, Pre-Treatment, Post-Treatment, and Duplex Technology

An atmospheric controlled induction-heating fine particle peening (AIH-FPP) treatment system was developed. Using the system the surface of structural steel with carbon content of 0.45%C was modified with Cr shot particle at room temperature, 500 C, 700 C and 900 C in atmosphere of argon and air. The treated surfaces were characterized using a scanning electron microscope (SEM) an energy dispersive X-ray spectrometer (EDX), an X-ray diffractometer (XRD) and an X ray photoelectron spectroscope (XPS). In the case of specimen treated in argon atmosphere, a relatively thick and uniform Cr diffused layer was formed at the surface. The thickness of the layer was changed with an increase in temperature, the higher the temperature, the thicker the layer. In the case of the specimen treated in air, however, an oxidized scale was formed on the treated surface instead of a Cr diffused layer. The results of the experiments prove that AIH-FPP treatment successfully creates a Cr diffused surface layer.

Nowadays with the severe competitive business environment, limited material sources and high cost of manufacturing, the importance of maintenance and repair is self-evident. One of the potential applications of cold spray coating is dimensional recovery of damaged structural parts. In most cases thick coatings are necessary for this demand. Thick coatings can be deposited in single or multiple numbers of passes giving different thermal and stress contribution to the components and coatings itself. The thermal input, the amount and the type of residual stresses (compressive or tensile) confer appreciable or depreciable characteristics to the coatings mechanical property. In this study single and multi-pass deposition of a desired thickness of Al 6082 coting on the same substrate is studied. In addition one pass deposition of different deposition thicknesses are also investigated. Micro-structural observation, micro-hardness measurements and X-Ray diffraction (XRD) measurement of residual stress were performed on all groups of sprayed material. Tubular coating tensile test tests were also carried out to characterize the cohesion of the coating in different cases. Observation of fractured surface was used to investigate the failure mechanism of the cold-sprayed materials. A critical discussion on the effects of thickness and number of passes on mechanical behavior of coated specimens is presented.

Plasma sprayed acid washed aluminum oxide coatings are prepared and their microstructural and dielectric properties are measured. The coatings are compared with that of commercial grade Aluminum oxide coatings. Coating thickness variation and use of various sealers is assessed to see their effect on improvement of the dielectric strength. Application of various sealers such as Metco AP, Metco ERS, and Dichtol and the effect of thickness variation on dielectric strength will be evaluated. Coatings will be characterized for their microstructure, dielectric properties, bond strength, porosity, and sealer percolation. Coating characteristics on various electronic substrates such as alumina, stainless steel, pure aluminum etc. will be evaluated and reported.

Boriding is carried out with the purpose of increasing the hardness, wear and corrosion resistance in engineering components for industrial applications. The boriding of different steel grades results in the formation of either a single-coating (Fe₂B) or double-coating (FeB/Fe₂B). Unfortunately, the presence of a FeB/Fe₂B coating is not desirable in industrial applications due to the difference between the thermal expansion values of both coatings. This results in cracking and spalling at the FeB/Fe₂B interface, and reduces the mechanical properties of the borided steel.

This study proposes a diffusion annealing process to increase the fracture toughness of boride coating formed at the surface of the AISI 1045 borided steel. First, the boriding of AISI 1045 steel was developed by the powder-pack method at a temperature of 1223 K in the range of exposure times of 8 – 12 h. The evolution of the FeB/Fe₂B coating as a function of the exposure time was estimated in the range of 238 to 301 microns. Vickers microindentation tests were conducted at 50 microns from the surface of the borided steels, where the indentation loads in the FeB coating ranged between 0.098 and 9.8 N for the set of experimental parameters of the boriding process. In addition, the magnitude of the indentation size, and the cracks emanated from the corners of the indentation marks were measured for the different applied loads.

Second, the diffusion annealing process was carried out on the borided samples at a temperature of 1273 K with 8 h of exposure. The borided steels were embedded in a closed-cylindrical container with a powder mixture of silicon carbide and bentonite with the absence of inert gases into the furnace. Before the process, the presence of a single Fe₂B coating was observed at the surface of the borided steels with thicknesses between 285 to 363 microns. The mechanical characterization of the borided samples exposed to the diffusion annealing process was similar to those applied in the boriding conditions.

The behavior of the hardness as a function of the indentation load for both conditions (FeB/Fe₂B and Fe₂B coatings, respectively) showed the presence of the indentation size effect (ISE) at 50 microns from the surface; the load-independent hardness value (H_o) of the boride coatings was estimated by the proportional specimen resistance (PSR) model.

Finally, considering that the hardness is independent of the applied load, the fracture toughness evaluated in the Palmqvist crack regime for the borided samples exposed to the diffusion annealing process showed a reduction of the brittleness around 30% in comparison with the estimated values over the FeB/Fe₂B coating.