Development of nano composite coatings for enhanced performance in non-conventional lubricants under sliding contact.

Abstract

A refrigerant is the most critical ingredient of any air-conditioning, refrigeration or cooling system. Increasing world population and global warming has increased the usage of air- conditioning and refrigeration devices considerably. This has resulted in the development, formulation and use of various refrigerants in the thermodynamic cycle. Most of the refrigerants that are used in domestic appliances, commercial buildings, small scale industrial applications and automobiles are artificially formulated. Although artificially formulated refrigerants have excellent thermodynamic properties, but the use of man-made chemical compounds have damaged the earth’s atmosphere either by depleting the ozone layer or by increasing the global warming potential. Some naturally occurring compounds on the other hand have also shown good properties as refrigerants but have various inherent issues associated with them such as toxicity and flammability which limits and restricts their use. To overcome the issue of ozone depletion and to control the issue of global warming, the refrigeration industry has recently developed and suggested new environment friendly refrigerants. These refrigerants should not only have the thermodynamic properties matching their predecessors but should also have a lower negative impact on the environment. The thermodynamics, physical and chemical properties of the newly developed refrigerants have been the main focus of study by various researcher worldwide. The extensive investigation of material wear and friction properties of the next generation of refrigerants used in mechanical applications is incomplete. The in-use durability performance of these products have to be accessed from the viewpoint of sustainable development. Tribo-performance of refrigerants applied in refrigeration, air-conditioning and energy systems directly influences the durability, reliability and cost effectiveness of the system. The past studies concerning friction, wear and the overall tribological properties of the last generation of refrigerants have shown that the overall life and performance of a mechanical system especially a compressor is highly influenced by the type of refrigerant used. This research project has evaluated the effects on the mechanical performance of a system utilizing the next generation of refrigerants along with assessing the feasibility and reliability of electrodeposited Nickel based composite coatings for refrigerant-lubricated tribological systems. For this study a micro-friction tribo-meter has been modified to incorporate the testing specimens and refrigerants under various operating conditions. Hydrofluoroethers (HFEs) are within the family of newly developed environmentally friendly refrigerants with a wide range of application areas. HFE-7000 is a replacement solution for the existing harmful refrigerants and thermo-fluids having a broad range of application areas including usage in green energy, low carbon technologies, in aerospace and automotive industries. This study is divided into two parts. In the first part of the study a detailed investigation has been performed on uncoated steel contacts to assess friction and wear performance of HFE-7000 (HFE-347mcc3). HFE-7000 has been employed as lubricants. In the second part of this study five different types of coatings namely, Ni-Al2O3, Ni-ZrO2, Ni-SiC, Ni-Graphene and Nickel-only have been used to investigate the wear and friction performance of these coatings in systems based on HFE-7000 refrigerant. Extensive experimentation has been performed on these coated contacts using the modified pressurized lubricity tester by changing the refrigerant temperature and the applied normal load in an attempt to enhance the tribological performance of interacting machine parts employing HFE-7000. Experimental results indicate the formation of tribo-films on the topmost surfaces. Energy-Dispersive X-ray Spectroscopic (EDS) and X-ray Photoelectron Spectroscopic (XPS) analyses on the tested samples revealed significant presence of oxygenated and fluorinated anti-wear tribo-films. These oxygen and fluorine containing tribo-layers prevent metal to metal contact and contribute to the reduction of friction and wear. All coatings presented an improvement in the micro-hardness and in hardness to elastic modulus ratio compared to uncoated steel. The results of friction and wear of coated samples were compared to uncoated steel as well. The results show an improvement in wear and friction at most of the operating conditions by applying nickel based coatings on a steel substrate in the presence of HFE-7000. Friction and wear performance of nickel based coatings does drop for some of the coatings at particular testing conditions which leads to conclude that a careful selection of the coatings has to be made depending on the operating refrigerant temperature and applied load.