Characterization of the flame sprayed and furnace fused NiCrBSiC alloy coatings with two different carbon contents and 15–45 wt.% WCCo addition is described in terms of microstructure, microhardness and differential thermal analysis. Microstructural development of these coatings before and after fusing treatment is discussed to identify the precipitates in the coatings. Optimum fusing conditions (time and temperature) for wear-testing sample were investigated in terms of microhardness and porosity of the coatings. Wear performance of these coatings was also investigated by two- and three-body abrasive and dry sliding wear experiments. The coating of 35% WC with NiCrBSiC is showing the best quality (the highest hardness and the lowest porosity). However, 25% WC addition is showing the best wear resistance for Sugaru abrasive wear test, while 40% WC addition is showing the best wear resistance for DSRW (dry sand rubber wheel) abrasive wear test. It is also shown that the dry sliding wear resistance of the 20% (and/or 30%) WCNiCrBSiC composite coating is almost 10 times better than that of the quenched and tempered JIS SUJ2 bearing steel.

Assessment of wear performance of flame sprayed and fused Ni-based coatings

An experimental study of the erosion-corrosion behavior of plasma transferred arc MMCs

Microstructural development in the hardfacing alloys NiCrBSi and Stellite 6 resulting from the commercially available ‘open arc’ and ‘spray and fuse’ coating techniques was compared with that resulting from the plasma transferred arc (PTA) weld surfacing process. Denser eutectic structures were observed in the case of PTA weld surfaced layers of Stellite 6 in comparison with the open arc weld surfacing process. The shapes of both carbides and borides in the 16C alloy (NiCrBSi type) coated by PTA processing were observed to have coarse morphology, while the spray and fuse layers showed needlelike morphology with finer distribution. The possible thermal history during each coating process is discussed. Based on microstructural observation, the hardness, wear resistance, and corrosion behaviours are reported. As expected, the alloy properties are directly related to the microstructural constituents.

Wear and corrosion resistance of PTA weld surfaced Ni and Co based alloy layers

Electroless deposition is a very simple process which can be used to obtain amorphous metallic coatings of uniform thickness on metallic or non-metallic substrates. In this work, nickel-boron coatings were deposited by electroless plating on WC and VC powders and sintered substrates. Two electrolyte solutions for the deposition of Ni-B coatings were developed: an alkaline and an acidic bath, both based on nickel chloride. Coating characterization was carried out by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD). Both electrolytes produced continuous firms of uniform thickness on sintered carbide substrates; the acidic bath showed a higher deposition rate. Furthermore on the carbide powders it produced crack-free Ni-B continuous coatings. Ni was detected by EDS and the coating purity was confirmed by AES. XRD proved that amorphous coatings were obtained in all cases.

Electroless nickel-boron coatings on metal carbides

The Ni base alloy (EWAC 1004EN) was used to develop coatings on the mild steel substrate by flame spraying process. The Ni base alloy powder was modified by adding tungsten carbide (10 and 20 wt-%) and CeO2 to study their effects on the microstructure, microhardness and abrasive wear behaviour of unmodified and modified coatings. All the coatings were subjected to remelting using tungsten inert gas arc welding. The abrasive wear behaviour of all the coatings in different conditions was studied using different normal loads (5, 10, 15 and 20 N) against 120 and 600 grit size abrasive medium. Scanning electron microscopy analysis of the worn out surfaces was carried out to understand the wear mechanisms. It was observed that the remelting of the coatings increased the hardness of WC and CeO2 modified flame sprayed coatings by ∼35%, while the abrasive wear resistance increased by 1·5- to threefold.

Studies on remelting of tungsten carbide and rare earth modified nickel base alloy composite coating

The crystallization of complex nickel-base hardfacing alloys is studied with reference to the ternary Ni-Si-B and quaternary Ni-Cr-B-Si systems. Quenched interrupted, directional solidification is employed to establish the solidification sequence. The transformation temperatures are measured by differential thermal analysis. Solid-state transformations occurring during the cooling process are described. A model of the solute exchanges during solidification is proposed in order to predict the amount of liquid remaining at each stage in the process, particularly at the last stage when all the eutectic liquid is frozen in.

S. Lebaili

Papers

The metallurgical structure of as-solidified Ni-Cr-B-Si-C hardfacing alloys