Cold spraying: A low temperature variant of thermal spray techniques to deposit metallic materials

Cold spraying is a novel material deposition process in which micron size particles are accelerated to supersonic velocity on to a metallic substrate to obtain thick and dense coatings. Unlike other thermal spray coatings, the bonding mechanism is completely different. In conventional thermal spray techniques, melting and solidification upon impact dominates the bonding mechanism. In cold spraying, Plastic deformation induced adiabatic shear instability governs the bonding process in which adiabatic temperature rise, plastic strain at interface and flow stress collapse play a crucial role. Variety of material including pure metals, alloys, composites and cermets have been deposited using cold spraying for variety of applications. In this article, a brief introduction about the bonding mechanism and potential applications of cold spraying is being discussed

The influence of powder particle velocity and microstructure on the properties of cold sprayed copper coatings

No literature exists on the influence of the velocity of Cu powder particles (during cold spraying) on the properties of the coatings like porosity, hardness, electrical conductivity, residual stress, and microstructure. Similarly, though the occurrence of dynamic recrystallization in the Cu powder particle during its deformation under the cold sprayed process has been well documented, its influence on the properties of the coating has not been studied. In this study, it is shown that while porosity of the coating is determined directly by the powder particle velocity, the other properties like hardness, electrical conductivity, and residual stress are determined not only by the particle velocity which depends on stagnation pressure and preheating temperature of the air (process gas) but also additionally on preheating temperature of air per se. The extent of powder particle fragmentation and dynamic recrystallization as a function of particle velocity has been quantified in terms of the grain boundary area fraction and the influence of the latter parameter on the coating properties has also been presented

Influence of grit blasting on the roughness and the bond strength of detonation sprayed coating

The process of roughening the surfaces by grit blasting prior to coating them using thermal spray techniques is very important to obtain consistently high tensile bond strength between the coating and the substrate. The available literature on the influence of grit-blasting parameters in the case of detonation spray or HVOF coatings is quite limited. The present study aims to study the influence of grit-blasting pressure and alumina grit size on the roughening of the mild steel substrate, the resulting effect on the roughness of Cu, Al2O3, and WC-12Co coatings deposited by detonation spray coating and also on the tensile bond strengths of these coatings. Toward the above purpose, the velocity of the alumina grits have been experimentally measured using a high-speed imaging system and the tensile bond strength of the coatings have been experimentally obtained using the pin type test. The results from the above experiments point to the importance of not only the roughness of the grit-blasted mild steel substrate but also the roughness of the coatings subsequently deposited in determining the magnitude of the bond strength

Evaluation of parameters for assessment of inter-splat bond strength in cold-sprayed coatings

The quality and performance of cold spray coatings are largely determined by the extent and strength of bonding between the adjacent splats. Usually, the extent of inter-splat bonding is only qualitatively estimated by examining the polished and etched sectioned surfaces of the coated sample. Thus, there is a clear need for indirect techniques to quantitatively estimate the extent of inter-splat bonding so that they can serve as quality control tools. In this study, elastic modulus, electrical conductivity, and critical load for inter-splat debonding as determined using a scratch test are considered as possible parameters for the estimation of the extent of inter-splat bonding using four different cold spray coatings (Ag, Cu, 316 Stainless Steel, and Zn) as model coatings. It is demonstrated that all the three parameters are capable of quantifying the extent of inter-splat bonding

The influence of process parameters and heat treatment on the properties of cold sprayed silver coatings

Cold spray coating technology is a promising low temperature variant of thermal spray technology which can deposit pure, dense and thick coatings at a rapid rate. Unlike the other thermal spray coating techniques, cold spray is especially suitable for depositing coatings with high electrical and thermal conductivity as the integrity of the feedstock is maintained during the coating process. In the present study, the effect of process parameters and heat treatment on the properties of silver coatings has been investigated. An attempt has been made to correlate the powder particle velocity with the properties and microstructure of the coating. The effect of heat treatment temperature and atmosphere on the properties of the coatings, especially electrical conductivity, has been studied in detail in conjunction with a thorough analysis of the evolution of microstructure of the coatings

National Mental Health Survey of India, 2016 - Rationale, design and methods.

Understanding the burden and pattern of mental disorders as well as mapping the existing resources for delivery of mental health services in India, has been a felt need over decades. Recognizing this necessity, the Ministry of Health and Family Welfare, Government of India, commissioned the National Mental Health Survey (NMHS) in the year 2014-15. The NMHS aimed to estimate the prevalence and burden of mental health disorders in India and identify current treatment gaps, existing patterns of health-care seeking, service utilization patterns, along with an understanding of the impact and disability due to these disorders. This paper describes the design, steps and the methodology adopted for phase 1 of the NMHS conducted in India. The NMHS phase 1 covered a representative population of 39,532 from 12 states across 6 regions of India, namely, the states of Punjab and Uttar Pradesh (North); Tamil Nadu and Kerala (South); Jharkhand and West Bengal (East); Rajasthan and Gujarat (West); Madhya Pradesh and Chhattisgarh (Central) and Assam and Manipur (North East). The NMHS of India (2015-16) is a unique representative survey which adopted a uniform and standardized methodology which sought to overcome limitations of previous surveys. It employed a multi-stage, stratified, random cluster sampling technique, with random selection of clusters based on Probability Proportionate to Size. It was expected that the findings from the NMHS 2015-16 would reveal the burden of mental disorders, the magnitude of the treatment gap, existing challenges and prevailing barriers in the mental-health delivery systems in the country at a single point in time. It is hoped that the results of NMHS will provide the evidence to strengthen and implement mental health policies and programs in the near future and provide the rationale to enhance investment in mental health care in India. It is also hoped that the NMHS will provide a framework for conducting similar population based surveys on mental health and other public health problems in low and middle-income countries