Tribological Studies on Scuffing Due to the Influence of Carbon Dioxide Used as a Refrigerant in Compressors

The refrigeration and air conditioning industry has expressed a great interest in the use of carbon dioxide (CO2) as a refrigerant. CO2 is anticipated to replace HFC refrigerants, which are known to have a negative effect on the environment. The reason behind the interest in CO2 is the fact that it is a natural refrigerant, thus environmentally acceptable. Of course, such a replacement raises concerns regarding design criteria and performance due to the different thermodynamic properties of CO2 and the very different range of pressures required for the CO2 refrigeration cycle. So far, work related to CO2 has been done from a thermodynamics point of view and researchers have made significant progress developing automotive and portable air-conditioning systems that use the environmentally friendly carbon dioxide as a refrigerant. The purpose of this work is to develop an understanding of how CO2 plays a role from a tribology standpoint. More specifically, the goal of this work is to gain an understanding on how CO2 influences friction, lubrication, wear and scuffing of tribological pairs used in compressors. Work in the area of tribology related to CO2 is very limited. Preliminary work by Cusano and coworkers showed that consistent data for tests using CO2 could not be acquired nor could a satisfactory explanation be offered for the inconsistency. Their results triggered the initiation of the work presented here. In this first attempt to understand the tribological behavior of CO2 several problems were encountered. During this work we noted that its behavior, unlike conventional refrigerants, could not always be predicted. We believe that this can be attributed to the thermodynamic properties of CO2, which cannot be ignored when studying its tribological behavior. Thermodynamic Properties such as miscibility are very important when tribological testing is performed. A limiting factor with our tester was that it was not designed for CO2 testing, but for other conventional refrigerants and therefore made previously developed testing protocols non-applicable with CO2. Through a different approach and some modifications to our tester we were able to establish a protocol for testing under the presence of CO2. CO2 was then compared to R134a and the experimental results showed that it performs equally well.Air Conditioning and Refrigeration Project 13

Project TENDR: Targeting environmental neuro-developmental risks. the TENDR consensus statement

Children in America today are at an unacceptably high risk of developing neurodevelopmental disorders that affect the brain and nervous system including autism, attention deficit hyperactivity disorder, intellectual disabilities, and other learning and behavioral disabilities. These are complex disorders with multiple causes—genetic, social, and environmental. The contribution of toxic chemicals to these disorders can be prevented. Approach: Leading scientific and medical experts, along with children’s health advocates, came together in 2015 under the auspices of Project TENDR: Targeting Environmental Neuro-Developmental Risks to issue a call to action to reduce widespread exposures to chemicals that interfere with fetal and children’s brain development. Based on the available scientific evidence, the TENDR authors have identified prime examples of toxic chemicals and pollutants that increase children’s risks for neurodevelopmental disorders. These include chemicals that are used extensively in consumer products and that have become widespread in the environment. Some are chemicals to which children and pregnant women are regularly exposed, and they are detected in the bodies of virtually all Americans in national surveys conducted by the U.S. Centers for Disease Control and Prevention. The vast majority of chemicals in industrial and consumer products undergo almost no testing for developmental neurotoxicity or other health effects. Conclusion: Based on these findings, we assert that the current system in the United States for evaluating scientific evidence and making health-based decisions about environmental chemicals is fundamentally broken. To help reduce the unacceptably high prevalence of neurodevelopmental disorders in our children, we must eliminate or significantly reduce exposures to chemicals that contribute to these conditions. We must adopt a new framework for assessing chemicals that have the potential to disrupt brain development and prevent the use of those that may pose a risk. This consensus statement lays the foundation for developing recommendations to monitor, assess, and reduce exposures to neurotoxic chemicals. These measures are urgently needed if we are to protect healthy brain development so that current and future generations can reach their fullest potential

Investigation of the Scuffing Mechanism under Starved Lubrication Conditions Using Macro, Meso, Micro and Nano Analytical Techniques

Studies to better understand scuffing began approximately sixty years ago, when it was postulated that scuffing occurs when a critical temperature of the surface is reached, at which desorption of surface films occurs. An extension to this hypothesis was proposed suggesting interaction of chemically active species with the metal surfaces, notably oxygen and thus scuffing would be associated with the rate of oxide formation and destruction. In the 1950???s another major development in the field of tribology was started which recognized and understood elastohydrodynamic lubrication (EHL) became the focus of study and it was assumed that a system is in danger of scuffing when the thickness of the fluid film between the surfaces. Recent fundamental scuffing studies related to air conditioning compressors under dry sliding conditions were performed by Sheiretov, who proposed a process leading to subsurface failure, which eventually led to scuffing. Further studies carried by Yoon focusing on scuffing under starved lubrication conditions (typical conditions for air conditioning compressors), suggested that scuffing was related to shear failure of the bulk material which was caused by formation of macroscopic adhesions at the sliding interface. In these recent studies the effect of surface topography on scuffing has not been treated thoroughly. Further more their approach was based on macro -tribological experiments and meso-to-micro analysis methods. This was intentional since the surface roughness of typical engineering surfaces is large with Ra values from 0.1 to 1 mm, thus the previous researchers were seeking average effects in their studies. Also earlier studies did not examine the progression leading to scuffing. In the present study we investigate, first; the progression of surface topography and respective change in surface parameters with tribological testing leading to scuffing; second, changes in chemical composition of the surface and near surface layers of the sample at the micro and nano-meter scales, and investigate its relation to scuffing, this study differs from the ones carried out by its predecessors (Sheiretov, Yoon) in that it tracks topographical and chemical changes of the interface surface of the (softer) aluminum disc as it progresses form its virgin state to its scuffed state. Furthermore the analyses are carried out using macro to nanometer scales and techniques. To our knowledge this is the first such comprehensive study that will investigate the use of techniques that are typically used in semi -conductors and MEMS applications, whose surfaces are by several orders smoother than the engineering surfaces that will be used in this study. A shoe-on-disc geometry, which is used to carry out the experiments consists of a steel shoe on aluminum disc and in an approximate simulation of a swash-plate/shoe contact in an automotive swash--plate compressor. The High Pressure Tribometer (HPT) is used to simulate the progression of wear conditions. The refrigerant used in this case is R410A with a POE lubricant. Once the experimental samples have been tested on the HPT, two sets of analytical tests are run, the first to measure the surface roughness and extract the statistical parameters, and the second to analyze chemical composition of the surfaces and the nano-meter range sub-surfaces. Based on the study it is suggested that the use of the micro to nano meter range scales in analyzing engineering surfaces is appropriate and correlates well to the macro to meso scale wear results. Thus the goal of this thesis Is to investigate the evolution of both the topographical and chemical characteristics as the surface of an aluminum alloy is worn out in a swash plate compressor using macro to nano-scales.Air Conditioning and Refrigeration Project 12

Experimental, Analytical and Finite Element Studies of the Nanoindentation Technique to Investigate Material Properties of Surface Layers Less Than 100 Nanometers Thick

Scuffing as a phenomenon has been studied for many years, however, the mechanism underlying scuffing remains unexplained. Recent findings suggest that the most significant changes occur in the top 50 ??? 100 nm of the surface, not at the micron level as previously suggested. The goal of this study is to identify different layers and their material properties on Al390-T6 disk surface and incorporate them into a thermomechanical Finite Element model to compare with tribological testing in a High Pressure Tribometer that simulates the contact in actual air conditioning compressor surfaces. Experimental, analytical and Finite Element studies of the nanoindentation technique are developed and used to investigate the material properties of surface layers less than 100 nm thick. All the methods are first verified in simple cases such as homogeneous materials and deposited thin film, then applied to rougher engineering Al390- T6 sample. The thin surface layers and the corresponding properties obtained from the above studies are then integrated into a thermomechanical FEM model to study the scuffing mechanism for the Al390-T6 disk and steel shoe sliding contact condition experienced in the High Pressure Tribometer that simulates realistic tribological contact in air conditioning compressor surfaces. It is shown that the FEM for nanoindentation is very useful as it is able to obtain additional properties and quantify properties of layers. It is found that a simple thermomechanical macro model does not provide sufficient information about the cause of scuffing. A FEM asperity-based micromodel is then built and successfully shows that the local contact stress and temperature increase could be extremely high under the critical loading. Also, it shows that scuffing should be a combination effect of stress and temperature increase.Air Conditioning and Refrigeration Project 12

Fundamental Investigation on the Tribological Failure Mechanisms of Compressor Surfaces, Scuffing: Detailed Roughness Analysis of Al390-T6

Scuffing that occurs at tribological contacts in mechanical components, brings about topographical, chemical, and mechanical changes mainly at the sub-micron surface. In this project, extensive studies involving the use of various engineering and scientific tools were performed to better understand the exact mechanisms behind this phenomenon. A High Pressure Tribometer (HPT) was used to simulate shoe-on-disk tribological contacts under various conditions as encountered in compressor surfaces. Specifically, Al390-T6 disk/52100 steel pin tribo-pairs are representative of typical contacting surfaces used in swash plate-type air-conditioning compressors. Once the time to scuff a sample under the test protocol was determined, subsequent HPT tests were stopped at 0.25 x Scuffing Time, 0.50 x Scuffing Time, and 0.75 x Scuffing Time intervals. The progressive change in disk topography leading up to scuffing was first observed in the 1-D roughness study [1], but more extensively captured by the 2-D Birmingham-14 roughness characterization that is also described in this report [2]. When the chemical analyses were conducted on the uppermost surfaces for depths of 120 nm, significant changes in some of the major chemical element concentration were revealed at scuffing. The major chemical compositional changes include a depletion of silicon, which was used to strengthen the aluminum alloy, and a drastic increase of oxygen component, signaling heavy oxidation at scuffing [1]. The mechanical properties of the disks undergoing tribological evolution were also investigated through various experimental hardness measurements, ranging from macro- to micro-, and to nanoscales [3]. Based on the experimental hardness results, it was found that the hardness of the material becomes higher at the micro- and sub-micro scales than the bulk, regardless of the amount of wear towards scuffing. It was also observed that there was a gradual weakening of the uppermost 60 nm. In this report, we describe in detail the surface roughness changes that occur to the Al390-T6 samples as they undergo progressively longer tribological testing, eventually leading to scuffing. [1] Patel, J.J., Polycarpou, A.A., and Conry, T.F., 2002, ???Investigation of the Scuffing Mechanism Under Starved Lubrication Conditions Using Macro, Meso, Micro, and Nano Analytical Techniques,??? ACRC TR-191, University of Illinois. [2] Suh, A.Y., Polycarpou, A.A., Conry, T.F., 2003, ???Detailed Surface Roughness Characterization of Engineering Surfaces Undergoing Tribological Testing Leading to Scuffing,??? Wear, in press. [3] Pergande, S.R., Polycarpou, A.A., and Conry, T.F., 2002, ???Use of Nano-Indentation and Nano-Scratch Techniques to Investigation Near Surface Material Properties Associated with Scuffing of Engineering Surface,??? ACRC TR-193, University of Illinois.Air Conditioning and Refrigeration Project 12

Use of Nano-Indentation and Nano-Scratch Techniques to Investigate Near Surface Material Properties Associated With Scuffing of Engineering Surfaces

Scuffing is a very complex process, without a clear understanding of the fundamental causes behind its occurrence. It is clear that there are many factors that affect this process, but it is only through obtaining an in-depth understanding of the actual conditions (i.e. chemical, topographical, mechanical, and microstructural analyses), that a fundamental cause can be determined. Most prior research has focused on examination of subsurface changes at the micron level. Recent findings suggest that the most significant changes occur in the top 50 ??? 100 nm of the surface, not at the micron level as previously suggested. The goal of this project is to substantiate this claim that the most significant changes occur in the top 50 ??? 100 nm, and to quantify the changes in material properties at this level. Microstructural analysis and nano-mechanical methods of determining thin film material properties are used to accomplish these goals. The nano-mechanical methods that will be used in this work are nano-indentation and nano-scratch techniques. These methods are routinely used in such applications as semi-conductors and magnetic storage hard disk drives. Applying these methods to engineering surfaces is anticipated to be somewhat difficult (and thus, the lack of published works in this area), due to significant roughness, non-homogeneous surfaces and inconsistent layers of unknown and non-uniform thicknesses. Through careful examination and analysis of individual data, it is shown that these methods can in fact be applied to engineering surfaces.Air Conditioning and Refrigeration Project 12

Investigation of the Scuffing Mechanism for Gray Cast Iron Undergone Tribological Testing Using Macro-, Micro-, and Nano- Analytical Techniques

Air Conditioning and Refrigeration Project 12