فصلنامه علوم و مهندسی سطح ایران
پیاپی 25 (پاییز 1394)

Ex-PAN carbon fibers are considered as vital reinforcements in production of aluminum based cast composites due to their unique characteristics. In the present study, the effect of polyvinyl alcohol (PVA) addition, as a low cost alternative surfactant, to an acidic Ni-P electroless bath on the coatability of ex-PAN carbon fibers was investigated. Scanning electron microscopy (SEM) results showed that with PVA addition, deposition rate of fine Ni-P particles on the carbon fibers and its uniformity improved. The surface finish of the coated layer significantly improved when the PVA concentration reached 0.5g/lit. The energy dispersive X-ray analysis (EDX) confirmed that the phosphorous content of the Ni-P deposit was about 5% at PVA addition of 0.5g/lit.

The aim of this research is evaluation of tribological properties of Ni(5Al)-Al2O3-MoS2 composite coating deposited by atmospheric plasma spray process on the CK45 steel substrate. For this purpose, three powder materials composed of 60wt%Ni(5Al)-40wt%Al2O3, 60wt%Ni(5Al)-35wt%Al2O3-5wt%MoS2 and 60wt%Ni(5Al)-25wt%Al2O3-15wt%MoS2 were prepared by mechanical milling and then spray drying processes. Plasma spray treatment was done by a plasma spray apparatus equipped with a gun of PS50 model. The coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), wear and coefficient of friction tests. In order to study the tribological properties of the coating, wear tests were performed by using a high temperature pin on disk machine. The tests were performed at temperatures of 25, 300 and 6000C, under 5N load and pin rotation speed of 0.1m/s. The wear test results at 25oC showed the presence of brittle phase of NiAl2O4 and lack of MoS2 lubricate in the 60wt%Ni(5Al)40wt%Al2O3 coating led to delamination wear. In this case, coefficient of friction decreased from 0.98 to 0.80. Significant weight loss was not observed during the wear test at 25oC in the coating contains 15wt% lubricant, due to the presence of Ni solid solution and absence of brittle phase of NiAl2O4. Due to the simultaneous presence of NiO and MoO3, 15wt%MoS2 coating showed the best lubricant performance at 600oC, thus, coefficient of friction was reduced from about 0.95 to 0.25.

In the present study, the effect of friction stir welding (FSW) on the corrosion behavior of lean duplex stainless steel was investigated. The alloy was welded using a rotation speed of 800 rpm and welding speed of 150 mm/min. The microstructures of the base and weld metals were evaluated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion behavior of the specimens was investigated using open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and zero-resistance ammeter (ZRA) tests in 0.1 M sulfuric acid. Results revealed that FSW decreased the grain size of the α and γ phases and improved the corrosion resistance of the stir zone (SZ). This was attributed to the grain refinement and recrystallization during the FSW that resulted in improving the stability of the passive film.

Usual techniques such as PVD and CVD are not used for coating heavy industrial dies due to their size, and to increase their wear resistance and lifetime, conventional Hard chromium plating is used. The substitution of this technique with safer and more efficient ways has been recently studied and performed due to environmental pollution and restrictions for using Cr(VI) compounds, which are known to be carcinogenic. The aim of this research is to increase the life time of Dies by replacing Hard chromium with Thermal Spraying process. In order to this replacement, HVOF Thermal Spraying and 75Cr3C2-25NiCr cermet were selected as coating process and material respectively. Also, the Thermal Spray process was optimized by using The S/N ratio analysis approach and selecting four Kerosene Flow Rate (KFR), Feed Rate (FR), Spray Distance (SD) and Substrate Surface Roughness (SSR) parameters. Pin and Disc wear tester was used to compare wear rate of these coatings. The results showed significant decrease of wear rate on the thermally sprayed samples. The results Also showed a desirable improvement of wear rate by optimizing parameters. Eventually, it is expected that the life time of dies should increase a lot.

In this study, the effect of surface processing of CoNiCrAlY coatings on oxidation behavior of this coating has investigated. The HVOF method used for deposition of CoNiCrAlY coating on the Inconel 617 substrate. The surface of coatings polished with grand paper No. 40, For investigation effect of surface processing and removing of oxide layer that formed during thermal spray on oxidation behavior of CoNiCrAlY coating. Then the samples oxidized in 1050 °C for 100 hrs. The Scanning Electron Microscope (SEM), X-ray Diffraction and surface profilometer used for microstructure and topographical investigation. Results show that surface polishing of coatings caused to developing of oxidation behavior of CoNiCrAlY coatings. The formed oxide layer on the surface of this coating after 100 hrs oxidation is α-Al2O3 that prevented of more oxidation of coating. The oxidation rate of coating with surface polishing calculated about 0.1×10-13 (10-10 gr2 cm-4 h-1)that is 16 times lower than coating without surface polishing. The oxide layer morphology formed on polished coating has globelar-like morphology that researchers believed that this morphology related to α-Al2O3 phase but in the non-polished coating, the surface morphology of oxide scale formed of polyhedral particles. This morphology related to spinel, CoO and NiO oxides.

In the present work, microstructure and slurry erosion properties of the satellite 6 cladding produced by TIG on AISI 316 and AISI 410 stainless steels was investigated. Optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to study the microstructure and erosion mechanisms. X-ray diffraction method was also employed to identify phases formed in the coating. The results showed that the microstructure of coating consisted of carbides embedded in a Co-rich solid solution. The higher erosion resistance was observed at coated samples compare to bare steels. Also, a marked corrosion influence was identified on erosion behavior of coating. In this case, the higher erosion resistance of coating was obtained for AISI 316 due to its better response to corrosive attack. A brritle mode of erosion behavior was observed for stellite coating.

Microstructural characteristics of stellite6-B4C composite clad layer fabricated by GTAW were investigated on steel substrate. Mixtures of stellite6 with different B4C weight percentages were pasted on substrates. X-ray diffractometery (XRD) was used for structural evaluation of clad layers. Microstructural investigations of clad layers were conducted by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometery (EDS). Hardness profiles of clad layers were obtained by microhardness testing method. According to the results the microstructure of composite clad layers contained a hypoeutectic structure. X-ray diffraction analysis showed the existence of cobalt-rich matrix together with eutectic carbides such as Cr7C3 and Cr23C6. It was suggested from the investigations that during the cladding process, boron carbides which were added to clad layer, melted completely and solidified again. B4C was found to have a preventing effect against the growth of dendrites tending to refinement of microstructure. The hardness increased with increasing B4C content up to 30 wt.%, but increasing B4C amount (40 and 50 wt.%) resulted in degradation of the hardness of composite clad layers.

In this study, ferritic ductile iron was coated by high chromium (36%wt.) nickel base alloy using gas tungsten arc welding (GTAW). The microstructure of the clad layer and the interface were studied using optical microscope (OM) and X-ray diffraction (XRD). The wear behavior of clad layer and base metal was compared through reciprocating wear test, and the wear track and wear debris were observed by scanning electron microscope (SEM). Results revealed that the microstructure of the clad layer consisted of carbide phases such as Cr23C6 and Cr7C3 embedded in Ni-rich solid solution dendrites. Moreover, ledeburite structure was observed in the partially melted zone. Results showed that wear resistance of the clad layer was considerably higher than that of the base metal. SEM images proved that predominant wear mechanism is delamination in the substrate and the clad layer.

In this research, the wear behavior of commercial grade of WC-10wt%Co (H10F), WC-40vol%Co and WC-40vol%FeAl-B composites with different amount of boron from zero to 1000ppm was investigated by the pin on disk test method. The wear tests were done in ambient temperature, under load of 40 N and a distance of 100 m. Wear surfaces were examined by scanning electron microscopy (SEM). The results showed that among samples, the WC-40vol%FeAl composite without boron has the lowest wear resistance due to higher brittleness and lack of suitable continuity between iron-aluminide matrix and tungsten-carbide particles. This behavior leads to the separation of the reinforcer particles from matrix. In the presence of boron up to 500 ppm in iron-aluminide matrix, the wear behavior of these composites improved and the wear mechanism of these composites changed in to abrasive state. In ambient temperature the wear resistance of WC-40vol%FeAl-500ppmB composite was close to commercial WC-10wt%Co although has lower WC percent.