DEVELOPMENT OF NANOSTRUCTURED WEAR RESISTANT SURFACES THROUGH MICROWAVE CLADDING

Abstract

Environmental sustainability and reduced life cycle costs are important considerations in industry today. The material selection for engineering components, their design and processing are, therefore, often influenced by combination of engineering as well as business criteria. The performance of the components, on the other hand, is greatly dependent on its surface and the environment in which they are operating. Therefore, tailoring the functional surfaces of the engineering components for improved performance with a composite design approach appears to be a pragmatic solution. Microwave heating is well known for uniform and volumetric heating of materials at the molecular level. This volumetric and uniform nature of heating reduces processing time significantly and results in better microstructure and properties of the materials compared to conventionally processed materials. Stainless steels are known for their excellent strength and corrosion resistance. However, in wear conditions they undergo surface and subsurface plastic deformation making them candidates for possible surface modification. Microwave cladding, as a technique for surface modification, has proven its potential for various materials. The present work, therefore, was aimed at developing engineered surfaces on austenitic stainless steel through 'Microwave Cladding' for wear resistant applications. The clad layer was developed using popular wear resistant material 'WC-12Co' in two size ranges – micrometric (MM) and nanometric (NM). The MM and NM clads of thickness of approximately 1 mm were developed on stainless steel substrate using the clad powders in the range of 30-45 μm and 100-200 nm. The cladding parameters were optimised through trial and error using exhaustive experimental trials and simulations using COMSOL Multiphysics tool. The simulation studies were carried out in order to determine the electric field distribution, temperature profile, temperature gradient, resistive losses in the clad layer during the microwave exposure. The simulation results largely agreed with the confirmatory experimental data.” The MM and NM clads were evaluated for their metallurgical, mechanical and tribological characteristics through various techniques. The metallurgical characterisation of the MM and MM clads was carried out in terms of phase analysis, microstructure, porosity analysis, elemental distribution and distribution of carbide volume. The X-ray phase analyses of the MM and NM clad layers reveal the presence of various intermetallic carbides such as WC, W2C, Co6W6C, Co7W6 and Co3W9C4. The normalised intensity ratio (NIR) results indicate that presence of the beneficial Co6W6C phase was higher in the NM clad as compared to the MM clad. The microstructure of the MM WC–12Co microwave iv clads consists of skeleton structured carbides distributed in the metallic matrix, while the NM WC–12Co consists of uniformly distributed clusters of nanocarbides in the clad layer. The carbide volume fraction in the NM clads was about 48% higher as compared to the MM clad. It was further observed that the WC–12Co MM and NM clads were free from interfacial cracks owing to uniform heating associated with microwave hybrid heating; the average porosity in the clads was less than 1%.” The mechanical aspects of the developed clads were analysed in terms of microhardness, flexural strength and residual stresses. The average Vickers's microhardness of the NM clad (1564±53 HV) was about 1.37 times the microhardness of the MM clad (1138±90 HV). The higher microhardness of the NM WC–12Co microwave clad is attributed to the presence of uniformly distributed nanocarbides and increased carbide volume fraction in the clad layer. The flexural strength of the MM and NM clads was evaluated using a three-point bend test. The average flexural strength of the MM clad was of 587±22 MPa, while the flexural strength of the NM clad is approximately 1.14 times higher (671±28 MPa) than the MM clad. The NM clad exhibited more ductile deformation during flexural loading. The deformation index (DI) of the NM clad is nearly six times higher than the MM clad indicating the fact that the clad DI is a good indicator of the flexural properties; higher DI indicates more ductile-like deformation characteristics. The residual stresses were found compressive in both, MM and NM clads. The magnitude of residual stresses in NM clad was observed to be approximately 1.68 times higher than the MM clads. The presence of higher residual compressive stresses influences the flexural properties of the microwave clads. The flexural strength increases with the magnitude of the residual stresses in the clad layer. The tribological aspects of the MM and NM clads were evaluated in three different conditions, viz. dry sliding, rolling abrasion (three-body abrasion) and solid particle erosion. In dry sliding conditions, the NM WC–12Co microwave clad exhibits higher wear resistance than its MM counterpart. The average weight loss for the NM microwave clad is reduced by 54% as compared to the MM microwave clad. The enhanced sliding wear resistance of the NM microwave clads is attributed to the uniform distribution of the nanocarbides and enhanced microhardness of the clad layer. The abrasive wear of NM clad was reduced by approximately 1.6 times as compared to the MM clad. Uniformly distributed nanocarbides increase the flow stress in the clad structure, which enhances the wear resistance of the NM clads. Material loss due to erosive wear was reduced to approximately to one third for the NM clad compared to the MM clad. This is attributed to the low mean binder path and higher carbide volume fraction in the NM clads. At low v impact angles, material was removed by microcutting of the relatively soft binder, followed by the loosening and carbide pullout. However, at high impact angles, material loss took place due to fatigue induced carbide fracture as a result of repetitive impacts of the erodents. Removal of the matrix through flaking, carbide fracture and pullout were the main wear mechanisms during erosive wear. However, the presence of clusters of nanocarbides having a low mean free path effectively arrests cracking. A mathematical (regression) model was developed to estimate the wear (cumulative weight loss) based on the erosion wear data. The wear prediction was also carried out through an artificial neural network model. The model was developed using three significant in using a three-layered configuration. It was found that both the models were reasonable in estimating the wear for the respective cases. Overall, the work demonstrates the capability of the microwave cladding process for engineering stainless steel surfaces with NM materials. The superiority of the nanostructured surfaces with respect to different wear resistance characteristics has been established with evidences and explanations of the mechanisms. The work also creates opportunities for further research in terms of economic analyses, investigations in different material systems and different wear regimes