AN INVESTIGATION ON THERMAL SPRAY WEAR RESISTANT NANOSTRUCTURED COATING

Abstract

The main aim of this research work was to develop a high performance wear resistant coating using thermal spray process for combating against most common wear conditions. It has been seen that high velocity oxy-fuel (HVOF) sprayed WC-Co based cermet coatings are very effective in minimizing the surface related problems, viz. wear and corrosion. Currently the use of nanomaterials for various applications is getting popular due to their improved properties. In the present research work, WC-Co-Cr based conventional and nanostructured feedstock powder materials were first selected and then sprayed by HVOF process to deposit high performance coatings. The coatings were deposited by using TAFA Praxair JP-5000 kerosene fuelled HVOF spray gun, which was assisted by in-flight particle temperature and velocity measurement system (Accuraspray, Tecnar, CANADA). The size of primary WC particles in conventional WC-Co-Cr powder was 2-5 μm and 100-500 nm in nanostructured powder. One of the main problems in WC-Co based coatings is the decomposition of WC particles which provides a necessary wear resistance. This decomposition mainly occurs when feedstock powder particles come in contact with hot gaseous products of combustion. This phenomenon is more severe in case of nanostructured feedstock powder material due tohigh surface area-to-volume ratio of nano-size WC particles, enabling them to interact largely with the heated products of combustion. In the present research work efforts have been made to minimize the decomposition of WC grains in nanostructured powder by optimizing the HVOF process parameters. Firstly, the ranges of main spray parameters(kerosene flow rate, oxygen flow rate, spray distance, and the powder feed rate)were obtained with the help of in-flight particle diagnostic study. Secondly, Taguchi L9 orthogonal array (design matrix) was selected for optimization purpose and spray parameters were taken as input factors. Thereafter, nine types of coatings were deposited by nine different parameter combination sets, listed in Taguchi experimental plan.Important coating properties such as micro-hardness, percentage porosity, and as-sprayed surface roughness were measured.Surface morphology and microstructure of these coatings were characterized by the examination of scanning electron microscopy (SEM) images. X-ray diffraction (XRD) analysis was performed to study the phase composition of coatings. As Taguchi optimization technique is commonly used for minimizing the number of experiments but it has a limitation of handling only single objective function. It has been seen that generally iii the wear resistance of coating is represented in terms of weight loss during the wear test. Therefore, the total average weight loss during the solid particle erosion, jet-type slurry erosion, and reciprocating abrasive wear testing was taken as objective function for each set of parameters listed in Taguchi experimental plan. The optimization was performed to obtained the process parameters setting that should produce a highly wear resistance coating, which means that the weight loss should be minimum. After Taguchi analysis, it was observed that the optimized spray parameters obtained were same for different wear conditions. The optimization of HVOF process parameters has resulted in a highly wear resistant nanostructured coating with improved properties as compared to conventional coating. Lastly, it has been seen that in most of the wear conditions,the loss of material takes place from WC-Co based coating surface is mainly due to the removal of binder matrix leading to the dislodging of WC particles. It has been also observed that cohesive strength between the splat boundaries plays an important role in the wear resistance of the coating, as mostly the failure or propagation of cracks takes place along the splat boundaries due to the presence of defects and detrimental phases in these areas. So these problems can be minimized by providing a proper reinforcement to the binder matrix which can increase the hard-phase/binder interface integrity and cohesive strength between the splats. It has been reported in the thermal spray literature that the performance of WC-Co based coatings can be increased by adding chromium (Cr) to soft metallic cobalt (Co) binder matrix,however, there is a scarcity of work related to the effect of varying the chromium content especially in nanostructured WC-Co based materials. Therefore in the present research work these issues werealso studied and has been resolved in case of depositing the nanostructured WC-Co-Cr based coatings by adding chromium (4-16 wt %) as alloying element and 2 wt % multi-walled carbon nanotubes (MWCNT) as reinforcement in the nanostructured coating. The introduction of chromium and MWCNT had greatly influenced the coating properties,specially the hardness and fracture toughness.A highly wear resistant HVOF sprayed nanostructured WC-Co-Cr coating has been developed which is the final outcome of this present research work.