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Jafari M.,Isfahan University of Technology | Jafari M.,Iranian Surface Research and Engineering Center | Enayati M.H.,Isfahan University of Technology | Enayati M.H.,Iranian Surface Research and Engineering Center | And 4 more authors.
Materials Science and Engineering A | Year: 2013

In this research, a novel WC-Co coating was deposited from electroless Ni-P coated WC-12Co powders using high velocity oxygen fuel (HVOF) process. Toward this purpose, an electroless Ni-P plating process was used to develop a uniform Ni-P layer on the surface of WC-12Co powders. The obtained Ni-P coated powders were then used as HVOF feedstock material. Microstructural characteristics of the Ni-P coated WC-12Co powders and the resultant coating, which is denoted as Ni-P modified coating, were investigated using X-ray diffractometry (XRD) and high resolution field emission scanning electron microscopy (HR FE SEM). The micro-hardness, elastic modulus and fracture toughness measurements were executed to evaluate the mechanical properties of the Ni-P modified coating. For comparison, the same experiments were performed on two conventional HVOF sprayed WC-12Co and WC-17Co coatings. The Ni-P modified WC-12Co coating showed a dense structure with extremely low porosity of ~0.3% which was much lower than that of WC-12Co and WC-17Co coatings. Besides, it was observed that the Ni-P modified coating has undergone negligible decarburization of 2.6% as compared to conventional WC-12Co and WC-17Co coatings with that of 16.3 and 17.6%. The Ni-P modified coating showed the maximum hardness of ~11.45GPa, while lower hardness values of 10.98 and 10.59GPa were measured for the WC-12Co and WC-17Co coatings. The fracture toughness of Ni-P modified WC-12Co coating was found to be 9.86MPam1/2, indicating 71.2 and 61.1% increase in comparison with WC-12Co and WC-17Co coatings, respectively. © 2013 Elsevier B.V. Source


Jafari M.,Isfahan University of Technology | Jafari M.,Iranian Surface Research and Engineering Center | Enayati M.H.,Isfahan University of Technology | Enayati M.H.,Iranian Surface Research and Engineering Center | And 4 more authors.
Surface and Coatings Technology | Year: 2013

The aim of this research is to investigate the sliding friction and wear behavior of a novel WC-Co thermal spray coating deposited from electroless Ni-P coated WC-12Co feedstock powders. The Ni-P coated powders were sprayed on ST37 steel substrate to form a coating, denoted as Ni-P modified WC-12Co coating, using high velocity oxygen fuel (HVOF) process. The X-ray diffractometry (XRD) and high resolution field emission scanning electron microscopy (HR FESEM) were used to analyze microstructural properties of Ni-P coated WC-12Co powders and the resultant coating. The sliding friction and wear behavior of Ni-P modified coating was investigated using a ball-on-disk technique under an applied load of 30N. The Ni-P modified WC-12Co coating showed extremely lower decarburization level, higher hardness and fracture toughness as compared to the conventional WC-12Co and WC-17Co coatings. The wear rate of Ni-P modified coating was found to be ~3.2×10-4mg/m indicating ~68 and 72% improvement in wear resistance, with respect to the conventional WC-12Co and WC-17Co coatings. Moreover, the Ni-P modified coating exhibited the lowest average friction coefficient of ~0.4 with minor fluctuations. The dominating wear mechanism of Ni-P modified coating was individual WC particles pull-out following extrusion of Ni (Co) binder phase. © 2013 Elsevier B.V. Source


Jafari M.,Isfahan University of Technology | Jafari M.,Iranian Surface Research and Engineering Center | Enayati M.H.,Isfahan University of Technology | Enayati M.H.,Iranian Surface Research and Engineering Center | And 4 more authors.
International Journal of Refractory Metals and Hard Materials | Year: 2013

In this study, the high temperature oxidation behavior of HVOF-sprayed WC-12Co and WC-10Co-4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500-800 C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 C for both coatings. At higher temperatures, i.e. 700 and 800 C, the oxidation mass gains of WC-12Co were found to be much higher than those for WC-10Co-4Cr coating, respectively. The higher oxidation resistance of WC-10Co-4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO4 type tungstate (M: Co and/or Cr) to tungsten trioxide (WO3) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC-12Co coating obeys the linear law within temperature range of 600-800 C with apparent oxidation activation energy of ∼ 104 kJ/mol. As for the oxidation of WC-10Co-4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC-12Co coating. The apparent activation energy for oxidation of the WC-10Co-4Cr coating was found to be ∼ 121 kJ/mol. © 2013 Elsevier Ltd. All rights reserved. Source

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