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

Analytical approximations in modeling contacting rough surfaces

A critical examination of the analytical solution presented in the classic paper o

Extracting summit roughness parameters from random Gaussian surfaces accounting for asymmetry of the summit heights

ABSTRACT The random Gaussian surface model proposed by Nayak is important to many statistical summit-based micro contact models. A Gaussian distribution is usually assumed for the summit heights as many surfaces have a Gaussian distribution of surface heights. In this work, based on Nayak's model, the skewness and kurtosis of the summit heights distribution are derived as a function of the bandwidth parameter α. The correctness of these two equations is verified using a numerical scheme that generates random Gaussian surfaces with various α values. Also, practical contact simulations are performed to demonstrate the significance of the proposed equations and also to show the error of using a Gaussian distribution versus a correct asymmetric distribution for the summit heights

Tribology of Unfilled and Filled Polymeric Surfaces in Refrigerant Environment for Compressor Applications

Recent changes in environmental laws requiring the transition from known ozone depleting chlorofluorocarbon (CFC) refrigerants such as R-22 to safer alternative refrigerants such as fluorocarbon based R- 134A and R-410A, have necessitated other changes in the refrigeration systems as well. Refrigeration compressor oils have also been replaced in order to be miscible with the alternative refrigerants. These alternative refrigerants and oils have changed the tribological characteristics of compressor critical contacts, and in some cases have led to an increase in failure rates. Much research has been conducted on the compressor contacts with alternative refrigerants and oils to understand the tribological impacts. Polymers have seen very limited study related to compressor tribocontacts in the presence of refrigerants. Polymers have a self lubricating effect by transferring material to the metal counterface. This is an important group of materials for tribological applications, especially blended polymers, which often have enhanced mechanical and low friction properties. Most of the literature on polymer tribology is conducted at speeds and loads significantly lower than typical compressor conditions. At these low speeds and applied loads, varying degrees of polymer transfer films are reported on the metal counterfaces. It is postulated that coherent transfer films are necessary for reduced wear. The current study looks at the tribological response of polymer/metal contacts in the presence of refrigerant versus ambient air under conditions simulating refrigeration compressors. Ten different polymers are employed as potential compressor bearing materials; four unfilled polymers and six blended polymers. Friction coefficient, wear, and surface topography were evaluated at a 60??C system temperature, 25 psi R-134A atmosphere (or ambient air), 2.4 m/s sliding velocity, and 45 or 225 N applied loads. Polymers were tested against cast iron disks of roughness 0.3 to 0.5 ??m Rq. Experiments conducted in R-134A show slightly favorable friction and wear characteristics to experiments conducted in ambient air. All blended polymers have good tribological characteristics. PEEK and polyimide in both unfilled and blended forms exhibit minimal wear and do not adversely affect the metal disks. These polymers show promise for compressor bearing materials. Representative testing in starved lubricant conditions shows decreased polymer friction and wear. This study also shows that although coherent, uniform films are not produced under compressor-like conditions, as evidenced by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX), the tested polymers still have favorable tribological properties.Air Conditioning and Refrigeration Project 14

Tribology of Protective Hard Coatings for Oil-Less Compressors

Compressors are being required to operate at increasingly severe conditions in order to increase efficiency, and with the transition from CFC to HFC and natural refrigerants, natural ferrous chloride layers on iron surfaces have been eliminated. To recover lost wear resistance with the transition to oil-less operation, greater protection is sought through the application of hard coatings with enhanced tribological properties. Controlled shoe-on-disk experiments simulating the interface in a swashplate compressor were performed using a High Pressure Tribometer under unlubricated conditions. Specimens used for these experiments included 52100 steel shoes and coated aluminum A390-T6 disks. Coatings were provided by two leading manufacturers and consisted of two different single layer WC/C coatings (one from each manufacturer), multi-layer WC/C + DLC and multi-layer TiAlN + WC/C. To help increase load bearing ability on the relatively soft aluminum, a CrN underlayer was deposited on half of the disks. The performance of the coatings was evaluated using surface profilometry and scanning electron microscopy. In some cases, the CrN underlayer increased friction due to increased roughness, while it also significantly increased the load bearing capability in most applications. One of the CrN + WC/C multilayered coatings exhibited friction coefficients similar to lubricated experiments with scuffing loads roughly ten times larger than uncoated, unlubricated tests. Controlled, reciprocating pin-on-disk experiments imitating the wrist pin-connecting rod interface were also performed under unlubricated conditions. Specimens used for these experiments included coated, cylindrical 52100 steel wrist pins and uncoated cast iron disks. Coatings used were the same as those for the swashplate simulation tests but without the CrN underlayer. Analysis of experiments investigating temperature effects, frequency variation, and performance in various refrigerants including R134a, R410a, and R600a, was completed using scanning electron microscopy, energy dispersive x-ray microanalysis, and surface profilometry. It was found that steady-state friction coefficients decreased with increasing temperature while friction coefficients increased during the running-in period. Also, wear decreased from testing in room temperature up to 80??C and beyond that, increased due to unstable running-in. Tests conducted in R410a produced the lowest wear, while those in R600a had the lowest friction coefficients. Tests in R134a performed the worst, but better than tests conducted in dry nitrogen. Based on the research presented in this work, hard coatings have the potential to replace oil in future compressors.Air Conditioning and Refrigeration Project 17

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

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