Surface Engineering

The present communication re-examines the basis of a previous model (E. S. Puchi-Cabrera: Surf. Coat. Technol., 2002, 160, 177 – 186) for the computation of the composite hardness of coated systems as a function of the relative indentation depth (RID), applicable to monolayer coatings. The results initially derived from simple geometrical considerations are compared with those obtained taking into account the volumes of the plastic zones of the coating and substrate under the indenter, as proposed by several models already published. A similar expression for the rate of change of the substrate volume fraction with the RID is obtained from both approaches. The consideration of the validity of a linear law of mixtures for the determination of the composite hardness is also reexamined, assuming that the composite hardness could be given by a harmonic law of mixtures. Both the previous model and the one developed from the introduction of a harmonic law of mixtures are extended to deal with multilayer coatings. In this case, the composite hardness as a function of the RID is calculated in terms of the volume fraction and hardness of each of the layer materials which compose the multilayer coating, and also two constants which characterise the behaviour of the coating. The results derived from the new model based on the harmonic law of mixtures are then compared with those obtained from the previous model, employing a number of systems involving both monolayer and multilayer coatings deposited onto different substrates.

The fatigue properties of a quenched and tempered (Q&T) 4140 (NiCrMo) steel coated with a hard Cr deposit, ∼7 μm thick were investigated. The fatigue properties of the coated specimens were evaluated in the as deposited condition. The results obtained showed that both fatigue life and fatigue limit are substantially reduced in comparison with the uncoated material. The reduction in fatigue life was computed from the Basquin parameters of the materials tested under different alternating stresses. It was shown that the fatigue life of the steel substrate can be reduced by up to 88%, whereas the fatigue limit can be reduced by, ∼21%, as a consequence of the coating. SEM observations showed that the fatigue fracture of the coated specimens initiates at the substrate–deposit interface and that the coating remains well adhered to the substrate during fatigue testing. The fracture surfaces of the specimens tested showed multiple crack initiation sites which led to the conclusion that the coating acts as a crack source for the steel substrate.

Composite layers were obtained by electrochemical deposition of cobalt or nickel from a plating bath with cerium oxide as dispersed particles. The changes in microstructure and microhardness of the composite were studied in correlation with the number of oxide particles incorporated. It was found that the presence of cerium oxide (CeO₂) particles has an influence on the codeposition process. The CeO₂ included particles lead to structural modifications of the metal matrix and have an effect on the mechanical as well as the surface properties of the composite.

Laser surface alloying (LSA) of AISI 316L stainless steel with various compositions of Ni and SiC for different laser parameters was investigated. The hardness of the LSA layer was found to be a strong function of silicon content. The maximum hardness achieved was 733 HV. Columnar dendrites, equaxial dendrites and cellular structure were observed in the resolidified microstructure. XRD and EPMA studies were used to find different phases and elemental concentration, respectively.

The synthesis of monodispersed ZnS nanoparticles (∼1.2 nm) stabilised with thioglycerol molecules which have been attached to functionalised silica particles is reported. The coupling agent used was 3-aminopropyltrimethoxysilane. The chemical bonding was studied by Fourier transform infrared spectroscopy. TEM of these particles clearly shows a uniform coating of ZnS on silica particles. These SiO₂@ZnS core shell particles were also studied using characterisation techniques such as optical absorption spectroscopy, X-ray diffraction and EDAX.

Pure ZnO films and ZnO nanoparticle dispersed polyvinylpyrrolidone (PVP) films were prepared on a Pyrex glass substrate by sol–gel dip coating using a zinc acetate precursor. The thin film is extensively characterised for its surface morphology, chemistry, thickness and nanocrystallite size, using various advanced analytical techniques such as SEM, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Under the given processing conditions, ZnO semiconductor thin films with nanocrystallite size 20–30 nm were obtained, and the ZnO nanoparticle size in the PVP matrix increased with increase in ZnO content.

Cost modelling provides a unified framework for optimising industrial processes by simultaneously considering all the key metrics that characterise the performance of a process. The broad scope of cost modelling is to correlate the process parameters with the plant performance metrics and, in turn, relate these to the overall process cost. In the present paper, the concept of cost modelling will be elaborated in a case study on the gas carburising operation. The cost model for the case carburisation process relates the controllable parameters (carbon potential and T–t set points) to the relevant cost drivers (energy, productivity, gas consumption, emissions) and quality parameters (case depth, grain size and distortion). These cost drivers and quality parameters are transformed to obtain the overall normalised production cost using appropriate cost functions. The cost model is subsequently used to optimise the carburisation cycle by the exhaustive search method for boost stage optimisation and by the differential evolution method, a genetic algorithm based technique, for the entire cycle optimisation, with the objective of minimising the production cost. This approach resulted in significant energy reduction (14%) and productivity enhancement (20%) in an industrial carburising operation.

Thin films of molybdenum oxide (MoO₃) were deposited by a physical vapour deposition method, i.e.the electron beam evaporation technique, using Corning 7059 microscopic glass and SnO₂:F as substrates. The effects of annealing temperature on the structural, surface morphological and optical properties of the films were studied and the results are discussed in detail.

The effects of pressure on the surface roughness of silicon carbide (SiC) film were investigated with variations in other process parameters, including radio frequency source power, bias power, O₂ fraction and gap. The SiC films were etched in a C₂F₆ inductively coupled plasma. The surface roughness, measured by atomic force microscopy, was examined in three particular situations: generated at minimum parameter level (case 1), at high source power (case 2); and at high bias power (case 3). For all variations in the process parameters except the O₂ variation, smoother surfaces were observed at lower pressures in all cases. Both surface roughness and dc bias due to the pressure variation were highly correlated in case 1. In case 3, they were inversely related. It is noticeable that in cases 1 and 2 the surface roughness was little affected by the pressure, particularly at high bias power. This feature can be used not only for tight control of surface roughness, but for optimising other etch outputs.