Gobinda C. Saha

Micromechanical Thermoelastic Model for Sandwich Composite Shells made of Generally Orthotropic Materials

Analytical solutions based on the two-scale asymptotic homogenization method for thermoelastic problem pertinent to general thin composite shells are obtained. The model allows the determination of both local fields and effective elastic and thermal expansion coefficients of composite sandwich shells made of generally orthotropic materials. Orthotropy in the material characteristics leads to a significantly complex set of thermoelastic local problems and is considered for the first time in the present article. First, a 3D-to-2D general asymptotic homogenization composite shell model based on a set of four unit-cell problems is derived. Secondly, the expansion of the model is continued to the further derivation of formulae for the forces, moments, displacements, strains, stresses, and effective thermoelastic coefficients that are representatives of the sandwich shell. Finally, the theory is illustrated by examples pertaining to thin composite sandwich shells with hexagonal honeycomb, hexagonal-triangular, and star-hexagonal cellular cores of orthotropic materials.

Strain Hardening of Heat Treated Nanostructured WC-17Co Coatings and Their Sliding Wear Behavior

Grain size, their distribution and geometry are important to study the dislocation behavior and grain boundary sliding of ceramic reinforced metallic composite materials. Grain size reduction has been shown to lead to significant improvements of the wear resistance in nanostructured materials. As the grain size decreases from polycrystalline to nanocrystalline range, abrasive wear resistance increases considerably from the increased hardness and volume loss following Archard’s law of wear. Further, the heat treatment effect on the content of the metallic binder in a ceramic-metallic (cermet) material is thought to increase the hardness with decreasing crystalline size, thereby improving the sliding wear behavior of materials. In this study, the high velocity oxy-fuel (HVOF) thermal spraying of nanostructured WC-17Co coatings with engineered ‘duplex outer coating’ is conducted. The microhardness and sliding wear studies of the coatings and their heat-treated counterparts are performed. The nanostructured coatings showed a significant increase in the microhardness and wear resistance when compared with those of the conventional microstructured coatings of the same composition. It is believed that the improved performance is related with the work hardening as well as dispersion hardening of the nanostructured grains in the deposited coatings.Copyright

Mechanical property changes in HVOF sprayed nano-structured WC-17wt.%Ni(80/20)Cr coating with varying substrate roughness

Thermally sprayed coatings developed by use of high velocity oxy-fuel (HVOF) process are known for their superior wear characteristics. In many industrial applications, new parts as well as repaired and refurbished parts coated with WC-Co microstructured coatings have shown enhanced erosion-corrosion and abrasive resistant properties when compared with other surface modification technologies such as chrome replacement, fusion welding, and cladding. This research has been further directed towards the development of HVOF technique to deposit dense nanostructured ceramic-metallic composites. The mechanism of plastic deformation, which determines the strength and ductility of materials, in nanostructured materials are different, thereby leading to novel mechanical properties. Various parameters can influence these properties, but the substrate surface preparation by grit blasting before thermal spraying is one critical parameter. The grit blasting process generates a surface roughness, which ensures mechanical anchoring between the coating and the substrate surface. In this work, the sliding wear behavior and microhardness of WC-17wt.%Ni(80/20)Cr cermet coatings deposited onto carbon steel substrates are examined as a function of three different surface roughness values under different loads. The results show that as-prepared surface with different blasting profiles have a direct influence on the surface roughness and wear performance of the coatings. The sliding wear resistance of the coatings increased as the substrate surface roughness increased. The wear depth decreased with increasing surface roughness.

Development of Novel ‘Duplex NiCr-Coated’ Nanostructured HVOF WC-17Ni(80/20)Cr Coatings for Severe Wear Applications

Thermally sprayed coatings have long been used to develop engineered surfaces for protection from severe degradation due to abrasive/erosive wear. High Velocity Oxy-Fuel (HVOF) thermal sprayed cermet composite coatings based on WC-Co systems offer better wear resistance and greater application flexibility compared with the traditional surface treatment techniques such as hardfacing. Recently, the development of nanostructured surfaces based on HVOF deposition of nano-grain WC reinforced in a variety of alloy matrix based cermet systems have gained research focus thanks to their initial performance results, including high hardness and wear properties without concomitant loss of ductility or fracture toughness in the sprayed coatings. In this research, the novel design and manufacturing of the ‘duplex Co-coated’ nanostructured WC-17Ni(80/20)Cr cermet powder is developed. The spraying of the feedstock is carried out using a diamond jet DJ2600 HVOF spray gun. In this study the mechanical properties of the novel coating are investigated and compared with the industry-standard microstructured WC-10Ni-5Cr coating.Copyright

Tribological Performance Study of HVOF-Sprayed Microstructured and Nanostructured WC-17wt.%Co Coatings

Abrasive and erosive wear of components and machinery is an ongoing challenge in the oil sands industry in northern Alberta, Canada. To improve the wear resistance by increasing surface hardness of steels, heat treatments and deposition of hard layers of metal alloys (such as stellite) by fusion welding techniques are traditionally used. However, these deposition techniques are not applicable to all shapes and add considerable weight to the final component. Thermal spraying techniques such as the use of high velocity oxy-fuel (HVOF) composite coatings based on WC-Co cermet system offer better wear resistance and greater flexibility in applications. This study presents work on two feedstock powders, namely nanocrystalline and microcrystalline WC-Co cermets, with identical matrix phase content: WC-17wt.%Co. The novelty of the research is that an engineered duplex Co coated WC-17wt.%Co cermet particle designed to withstand coating spalling under elevated loads as well as to limit abrasive debridement during wear is introduced for the first time to produce a more homogeneously-dispersed coating microstructure. The engineered particle has 6wt.% of the ductile phase material mixed into the core to insure that the reinforcement WC phase is discontinuous to limit the debridement during wear, while remainder (11wt.%) of the Co is applied as a coating on the particle to improve the ductility. The mechanical properties of the overall particle are further improved by controlling the size of the reinforcing phase (WC) in the matrix (Co). This resulted in a WC-17wt.%Co particle containing a characteristic WC grain in the order of 350 nm in the core with the Co outer coating of 1–2 μm thick, making the powder particle as nanocrystalline. HVOF deposited coatings of the nanocrystalline and microcrystalline powders were examined for microhardness, fracture toughness, sliding abrasion (ASTM G133-05) and dry-sand rubber wheel abrasion (ASTM G65-04) wear performance. The wear rate under various loads and sliding distances was studied. In both the coatings, it was found that the wear rate increased with increasing applied loads, while it decreased with increasing sliding distances. 3D surface analysis of the wear tracks using atomic force microscopy (AFM) revealed two distinctive mechanisms associated with the two coatings after abrasive wear. The improved wear resistance was attributed to the higher hardness value of the nanostructured WC-17wt.%Co coating. It was also found that the nanostructured WC-17wt.%Co coating has about twice the toughness of the conventional microstructured coating counterpart. The extent of the WC decarburization and the dissolution of Co in the coatings were also studied.Copyright