Tribological Evaluation of Various Aluminum Alloys in LubricantlRefrigerant Mixtures

Transcription

1 Purdue University Purdue e-pubs International Compressor Engineering Conerence School o Mechanical Engineering 1996 Tribological Evaluation o Various Aluminum Alloys in LubricantlRerigerant Mixtures H. Yoon University o Illinois at Urbana-Champaign T. Sheiretov University o Illinois at Urbana-Champaign C. Cusano University o Illinois at Urbana-Champaign Follow this and additional works at: Yoon, H.; Sheiretov, T.; and Cusano, C., Tribological Evaluation o Various Aluminum Alloys in LubricantlRerigerant Mixtures (1996). International Compressor Engineering Conerence. Paper This document has been made available through Purdue e-pubs, a service o the Purdue University Libraries. Please contact epubs@purdue.edu or additional inormation. Complete proceedings may be acquired in print and on CD-ROM directly rom the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 TRiOLOGICAL EVALUATION OF VARIOUS ALUMINUM ALLOYS IN LUBRICANT/REFRIGERANT MIXTURES Hyung Yoon, Todor Sheiretov, Cris Cusano Air Conditioning and Rerigeration Center Department o Mechanical and Industrial Engineering University o illinois at Urbana-Champaign Urbana, Illinois ABSTRACT The tribological characteristics o various aluminum alloys, aluminum composites and some surace treated aluminum are evaluated in lubricant/rerigerant (L/R) mixtures. All o these evaluations are based upon a specimen testing program using a High Pressure Tribometer (HPT). This research program consists o two parts. The ocus o the JrSt part is mainly on materials screening o various aluminum alloys/steel contact pairs lubricated by polyolester/r134a and PAG/R134a mixtures. The aluminum alloys are tested under the same environmental and operating conditions in order to compare their wear resistance. The results show that the lowest wear is obtained with the 39 die cast alloy. In general, the amount o wear decreases as the amount o silicon content in Al-Si alloys increases. Conventional anodizing does not improve the wear resistance o the 356 aluminum alloy under concenttated contacts. Hard anodizing and a SiC-AI composite provide very good wear resistance. However, they cause increased wear on the counterace by abrasion due to the rough, hard, suraces generated by hard anodizing processes and the hard SiC particles. From the wear results obtained, the Ester/Rl34a mixtures consistently provide better protection o the aluminum alloys compared to the P AG/R134a mixtures. I suicient amounts o R134a exists in the L/R mixture, extensive surace atigue on 356 aluminum is observed. In the second part, two Al-Si alloys (356-T61 and 39-T61), widely used in critical components o rerigerant compressors, are examined or their riction and wear behavior in dierent L/R mixtures. The UR mixtures tested include ester and PAG lubricants with Rl34a, mineral and alkylbenzene lubricants with R22, R47C and R41A, as well as an ester lubricant with both R47C and R41A. Based on the wear data obtained, the capped PAG seems to be a better lubricant or 356 alloy than the uncapped PAG. However, the lubricity o the PAG's is about the same with the 39 alloy. When the ester lubricant is used, the wear on each alloy is about the same in R134a, R47C, R41A and air environments. There is no signiicant dierence in lubricity o mineral and alkylbenzene lubricants when used with R22 and its possible substitutes R47C and R41A. INTRODUCTION Various types o aluminum alloys are continually being developed to improve their wear resistance. Among these alloys, Al-Si alloys have been ound to be beneicial in many industrial applications and considered to be appropriate substitutes or cast iron components [1-1]. The addition o silicon in aluminum alloys improves their wear, casting, machining and corrosion characteristics. Both hypoeutectic and hypereutectic Al-Si alloys are widely used in a variety o applications, including automotive [3-8, 1] and related equipment, air-conditioning equipment and home electrical appliances [7, 9, 11]. A large number o studies in recent years have been devoted to investigate the riction and wear behavior o Al-Si alloys. However, most o these studies were conducted under dry sliding conditions. The ew studies that have been conducted under lubricated sliding conditions were mainly concerned with the eect o silicon content and silicon morphology on lhe riction and wear resistance o these alloys. Also, lhe riction and wear tests or these studies were conducted mostly in an air environment. Al-Si alloys are widely used or critical components in rerigerant compressors, especially connecting rods in reciprocating compressors and swash plates in automotive air-conditioning compressors. The successul operation o compressors used in air-conditioning and rerigeration systems is mainly governed by the tribological behavior at the critical contacts within the compressors. It is known that the environmental conditions around these contacts have a signiicant impact on their tribological perormance. Thus, lhe riction and wear characteristics o Al-Si alloys in a rerigerant and in an air environments might be quite dierent. In this study, the High Pressure Tribometer (HPT) is used to evaluate riction and wear characteristics o various Al Si alloys, aluminum composites and some surace treated aluminum alloys in rerigerants environments. Most o the riction and wear data obtained are or 356-T61 coupon mold and 39-T61 permanent mold aluminum alloys. A;waratus and Contact Geometty DESCRIPTION OF THE EXPERIMENTS All tests in this study were conducted in the High Pressure Tribometer (HP1) which provides accurate control o bolh test environment and operating conditions. A schematic o the HPT's pressure chamber and the in-line viscometer is shown in Fig. 1. The in-line viscometer provides in-situ inormation about the viscosity changes o the L/R mixture. The detail description o lhe HPT is given in the previous paper [12]. A pin on disc geometry was used to obtain the riction and wear data. A cylindrical pin having a diameter o 6.35 m.m (.25 in.) and a length o 6.35 m.m (.25 in.) was slid over an 139

3 aluminum alloy disc. The wear track on the disc is a circular band with an average radius o 1 mm (.384 in.). The lower specimen is secured in place by a specimen holder and the upper specimen is attached to the rotating spindle. Viscometer Fig. 1 Schematic o the HPT's Pressure Chamber with an In Line Viscometer Table 1 Materials Tested Material Treatment Fabrication Hardness Desij!;nation HRB HV3 T6 T6 Extrusion 46 HV3 T4 T4 Extrusion 44 HV4.T61 T61 Squeeze Cast 51 C278 T4 T4 Extrusion 47 39DC T6 T6 Die Cast 74 39PM T61 T61 Pennanent Mold DC NT Not Treated Die Cast PM-T61 T61 Permanent Mold CM-T61 T61 Coupon Mold CM-NT Not Treated Couwn Mold CM-AN Anodizing_ Col!Q_on Mold CM-HC Hardcoat Coupon Mold 6HV 356CM-NF Nitu Coupon Mold 6HV SiC-Al Die Cast 41 Hardcoat and Nitu are proprietary hard anodizing orocesses Materials or the Test Specimens Inormation about the various alloys tested, the heat or surace treatment, the method o abrication, and the surace hardness is provided in Table 1. The HV3-T4, HV3-T6, HV4-T61 and C278-T4are proprietary alloys provided by one o the sponsors o this work. The C278 alloy is very similar to the 432 alloy, except or the presence o small amounts o Bi and Pb. The chemical composition o the aluminum alloys tested is given in Table 2. For all tests conducted, the pin material was 118 carburized steel with a surace hardness o 773±78 HV and a surace roughness o.25±.6 j.tm Ra a e - enuc e umtnum All T d Allov Alloying_ Elements, % by Weight Si Fe Cu Mn M_g Cr Ni Zn Bi Pb Ti T bl 2 Ch. al C th AI OmPQ_SltlOn oys este HV HV C Test Conditions and Lubricant') All tests were conducted at an environmental temperature (T) o 38 C (1 F) and an environmental pressure (P) o.86 MPa (125 psig). With the exception o the hard anodized 356CM alloys (Hardcoat and Nitur in Table 1) and a SiC particle reinorced composite, a contact load o 111 N (25 lb) was used. In order to get a measurable wear, a much higher load o 667 N (15 lbt) was used or the hard anodized alloys and the SiC composite. The initial maximum Hertz contact stresses or 111 N (25 lb) and 667 N (15 lb) correspond to 322 MPa (46.7 ksi) and 1929 MPa (28 ksi), respectively. The sliding velocity o.29 m/s (41.3 pm) was used or all tests conducted. Again, this sliding speed was chosen to get a measurable wear or the limited time o test duration. 14

4 Various lubricant/rerigerant mixtures were used in this experimental program. Two esters which have similar viscosity were used with R134a, R47C orr41a rerigerants. Two PAGs which also have similar viscosity were used in Rl34a environment. A mineral oil and alkylbenzene were used with R22, R47C or R41A rerigerants. Data or the lubricants used are given in Table 3. Table 3 - Data or the Lubricants Tested Designation Lubricant Type Family Additives Density Viscosity, cs g/ml at4 oc at 1 oc Esterl Polvolester Pentaerythritol No Ester 2 Polyolester Pentaervthritol Antioxidant PAG 1 Polyalkylene glycol u.. No PAG2 PolyaJkvlene _glycol Caooed No AB Alkylbenzene No Mineral Mineral oil No Test Procedure and Data Obtained All tests were conducted in the pressure chamber o the HPT under copious lubrication condition (the contact pair was completely submerged in the L/R mixture) or the test duration o one hour. Whenever a test was conducted under a rerigerant environment, the rerigerant was charged into the pressure chamber and allowed to dissolve in the lubricant or one hour prior to initiating the test. The coeicient o riction was monitored and recorded continuously throughout the test by a computer-based data acquisition system. The riction data presented are average values. The wear on the aluminum specimens (discs) was obtained via a stylus surace proilometer by taking surace proiles and measuring the average wear scar depth. The wear on the steel pins was obtained by measuring the wear scar with an optical microscope. The wear scars on the pins were also examined with an optical microscope or material transer rom the aluminum counterace. The test specimens were also examined with an X-ray photoelectron spectroscopy (XPS) and a Secondary Ion Mass Spectroscopy (SIMS) to analyze the worn suraces o the aluminum alloys. RESULTS AND DISCUSSION Materials Screening Tests The riction and wear data or the aluminum alloys lubricated by Esterl/R134a and PAG1/RI34a mixtures are given in Fig. 2. For each condition, two tests were conducted using the HPT. The riction data given represent average values. The wear result shows average as well as the scatter o the test data. It should be noted that there is more scatter with 356 alloys. This is mainly due to the relatively severe surace atigue (or delamination) on these alloys which could not be controlled rom test to test. The surace atigue phenomenon will be discussed in more detail later in this section. The load used or these tests was insuicient to cause measurable wear on the SiC-Al composite and on the specimens with hard anodized coating. Thereore, these materials were tested at six times higher load. The base material (356CM-T61) was also tested at higher load or comparison purpose. The riction and wear results rom these higher load tests are given in Table Ester! I RI34a PAG1/R134a =.2 [ 12.:::J u ii 1 i.15 a <:: 8 t -s =! u '!l:i -' ld 1!: ! (%) (I) E-< 1 E-< 1 i! >a,... i!... - E-<... \ z z E-< \ z \ -\ a!, <}-, <}-, E7 E-;< u u... E-<... ::;,_ QO ::; -.;J. C) Cl >!;; ::E 1 >a D ::r: >a l C) N '\...,.,.., C) 5: > \n...,.,..,.,.., ::r: 9 '\..., \ \ \...,...,..., on lr)...,.,..,.,.., on.,.., on..., \ Fig. 2- Friction and Wear Results or Various Aluminum Alloys P =.86 MPa, T = 38 C, Contact Load = 111 N 141

5 Table 4 -Friction and Wear Data or Hard Anodized Aluminums and a SiC-Al Composite P 86 MP T 38 C C L N L V. 48 S) a = a. =, on tact oad=667 Ester 1 ubricant, iscosiry o the LIR DUX.ture =. 3 c Material Treannent Average Disc Wear Pin Wear Scar Wear Scar Roughness Beore Friction Depth [umj [nun] Roup;hness [Jl.Dl Ra] Test [u.m.ral 356CM-T61 T CM-HC Hardcoat CM-NF Nitu SiC-Al From the wear results given in Fig. 2, it is seen that the Ester/Rl34a mixtures consistently provided better protection o the aluminum disc specimens compared to the PAG/RI34a mixtures, even though the viscosity o the Ester/R134a mixture is lower than that o the PAG/R134a mixture (Table 5). This eect is probably due to the ability o the esters to orm bidentate bonds with aluminum [13, 14]. Note that the wear result o 356DC-NT in PAG1/R134a is not given in Fig. 2. The 356DC-NT has approximately one order o magnitude higher wear than the other materials tested. Conventional anodizing (356CM-AN) does not improve the wear resistance o the 356 alloy under concentrated contacts. The bard layer cracks under the high contact stress causing an increase in wear. The lowest wear is obtained with the 39DC alloy which also has the highest hardness. In general, the amount o wear decreases as the amount o silicon content increases. This trend, however, is complicated by the presence o other alloying elements and the dierent treatment processes. A comparison between the HV3 and 356, which have similar Si contents, indicates that the HV3 consistently provided better wear resistance. The reason or the improved perormance o HV3 can be either the higher Cu content, or the presence o Bi. Similar conclusions can be drawn when comparing the results or C278 and HV 4, which also have dierences in these two elements. While copper is known to provide better wear resistance, it is still not clear what is the eect o bismuth. From the wear results given in Table 4, it can be seen that hard anodizing and SiC particle reinorcement provide very good wear resistance. However, they cause increased wear on the counterace due to the rough, hard, suraces generated by hard anodizing processes and the bard SiC particles on the surace o a SiC-Al composite. Friction and Wear o 356 and 39 Alloys Under Various Conditions Two Al-Si alloys (356CM-T61 and 39PM-T61), widely used in critical components o rerigerant compressors, were examined or their riction and wear behavior in dierent L/R mixtures. The results or the wear depth on the aluminum discs with these mixtures are plotted in Fig. 3. Changes in the viscosity o the L/R mixtures were monitored inside the pressure chamber o the HPT or one hour beore initiating the test. The viscosity o the L/R mixtures measured beore and ater one hour o rerigerant saturation is given in Table 5. e :l..,; 1 8. is 6. g 4. c. lo [::;: 2. L/R Mixture Beore Ater..,.. - ' ;;;; <:'> cj..,.,.., ' ;;z CM-T61 39PM-T I 356CM-T61 '.....,.. u N 3. ' ' a 2.5 ' ;;;;..,.... u <... s... c...!i < 2.,....c.._,... 'E.....,..... Q 1.5 III cp + e 3., u N ; - 1' < :::: + i (a) (b) Fig. 3- WearResultso356CM-T61 and39pm-t61 in VariousL/RMixtures P =.86l\1Pa, T = 38 C, Contact Load == 111 N. (a) Ester and PAG in R134a, R47C and R41A (b) Alkylbenzene and Mineral Oil in R22, R47C and R41A :: u [3..,.. -; = ::E t 39PM-T611 Table 5 - Viscosity_(cS) o the L/R Mixtures Beore and Ater One Hour o Reri2emnt Saturation PAGl PAG2 Ester2 Esterl Ester1 Ester I AB AB AB Mineral Mineral Mineral R134a R134a R134a R134a R47C R41A R22 R47C R41A R22 R47C R41A <..,.... -E., = 142

6 From the wear results shown in Fig. 3, it is seen that, with the exception o MineralJR22, the wear depths on 39PM-T61 alloy were consistently smaller than those on 356CM-T61 alloy. The capped PAG (PAG 2) seems to provide better lubricity or 356CM-T61 alloy than the uncapped lubricant (PAG 1). However, or the 39PM-T61 alloy, the lubricity o the PAG's is about the same. When Ester!lubricant is used, the wear on the discs or both alloys is about the same in R134a, R47C, R41A and air environments. The R47C and R41A used with alkylbenzene lubricant provide similar (with 39PM-T61) or slightly better (with 356CM-T61) wear resistance when compared to R22. The lower wear obtained with the 356CM-T61 alloy is probably due to the higher viscosity o alkylbenzene/r47c or R41A mixtures compared to the alkylbenzene/r22 mixture. However, the relative wear dierences between these mixtures are small, thereore, more tests need to be conducted to examine their lubricity dierence, i any exists. The m,ineral/r22 mixture produces less wear (with 356CM-T61) than the other mixtures, even though the viscosity o the mineral/r22 mixture is smaller than that o the other mixtures. However, with the 39PM-T61, it is the mineral/r47c mixture which provides slightly better wear resistance compared to the other mixtures. Again, the relative wear dierences between these mixtures are not signiicant. The Eect or134a on Surace Fatigue As previously indicated, or the 356CM-T61 and 356PM-T61 alloys tested under a R134a rerigerant environment, extensive surace atigue (delamination) was present, as shown in Fig. 4. Surace atigue was not observed when the 356 alloys were tested in an air, R41A, or R47C environments. Surace atigue or cracks were observed on the suraces tested under R134a with high environment pressure. Accelerated cracking due to the presence o the rerigerant may be a possible reason or the observed atigue behavior. To explore this possibility, a set o tests at various environmental pressures was conducted with R134a. To exclude any eects due to the viscosity o the L/R mixture, the viscosity was kept approximately the same by changing the environmental temperature. The results rom these tests are given in Table 5. T bl 5 Eti th E. W M Fig. 4 - SEM Micrographs o the Worn Suraces o 356CM-T61 Alloy Tested Under (a) R134a and (b) Air Environments a e - ecto e nvlonmen talpr essureo R 4 s Environmental Pressure, MPa TPsii.17 (25).52 (75).69 (1).86 (125) Environmental Temnerature, oc ( F) 121 (25) 1 (212) 8 (176) 38 (1) Approx. Amount ori34a bv Weicllt 3% 8% 13% 5% Viscosity o the LIR Mixture, cs Surace Fatigue None None Small Lar.ee 13 a on urace F 3 ati2:ue o 56CM T61 Alloys From the table, it is evident that at an environmental pressure o.69 MPa or higher, surace atigue occurs. The suraces o the test specimens were examined with a Secondary Ion Mass Spectroscopy (SIMS) and an X-ray photoelectron spectroscopy (XPS) to try to identiy the chemical compounds ormed. From the SIMS prolles, it was observed that both hydrogn and luorine were present to a signiicant depth on the worn surace. The amount o these elements, at larger depths, 1S orders o magnitude higher than on the virgin surace. Additional study o the swace with XPS revealed that the 143

7 luorine was in the onn o AIF3. A reaction between reshly exposed aluminum and R134a. leading to the ormation o AIF3, may be a reason or the enhanced brittleness. However, the actual causes and mechanisms or the surace atigue on the 356 alloys are not clear at this time. It is hypothesized that the presence o bydrogen causes surace atigue due to bydrogen embrittlement, wbile luorine may cause corrosion cracking. No direct evidence or either o these possible mechanisms has been ound so ar. However, an important conclusion that can be drawn rom these studies, is the act that Rl34a attacks the aluminum surace and enhances its brittleness. It should be noted that the indications o surace atigue, although much less severe, were also observed with some o the other alloys tested (39DC with PAG1/R134a). lbis suggests that the mechanism or this behavior is more general and not limited to the 356 T61 alloy. SUMMARY OF THE RESULTS For the operating and environmental conditions considered in this study, the results can be summarized as ollows: Among the aluminum alloys tested, the lowest wear is obtained with the 39--T6 die cast alloy. In general, the amount o wear decreases as the amount o silicon content increases. The addition o bismuth and higher amount o copper reduces wear or alloys o otherwise similar composition and heat treatment Conventional anodizing does not improve the wear resistance o the 356 aluminum alloy under concentrated contacts. Hard anodizing and SiC particle reinorcement provide very good wear resistance. However, they cause increased wear on the counterace. The Ester/Rl34a mixtures consistently provided better protection o the aluminum disc specimens compared to the PAG/R134a mixtures, even though the viscosity o the Ester/Rl34a mixture is lower than that o the PAG/R134a mixture. When Ester lubricant is used, the wear on the aluminum discs is about the same in Rl34a, R47C, R41A and air environments. There is no signiicant dierence in lubricity o mineral and alkylbenzene lubricants when used with R22 or with its possible substitutes, R47C and R41A The R134a rerigerant does not behave as an inert environment. Under the conditions o this study, a chemical reaction between the reshly exposed aluminum and R134a occurred. R134a and other HFC's, at suiciently high partial pressures, increase the brittleness o aluminum alloys. Corrosion cracking and/or hydrogen embrittlement are viewed as possible ailure mechanisms. ACKNOWLEDGMENTS The authors grateully acknowledge the support o this work by the Air Conditioning & Rerigeration Center, Mechanical & Industrial Engineering Department at Urbana Champaign. REFERENCES 1. Davis J. R., ASM Specialty Handbook, Aluminum and Aluminum Alloys, ASM International, Materials Park, OH, Somi Reddy, A., Pramila Bai, B. N., Murthy, K. S. S., and Biswas, S. K., Wear and Seizure o Binary Al-Si Alloys, Wear, 171 (1994) Sarkar, A. D. and Clarke, J., Friction and Wear o Aluminum Silicon alloys, Wear, 61 (198) Sarkar, A. D., Wear o Aluminum Silicon Alloys, Wear, 31 (1975) Torabian, H., Pathak, J. P. and Tiwari, S. N., Wear Characteristics o AI-Si Alloys, Wear, 171 (1994) Hanna, A. H. and Shehata, F., Friction and Wear o Al-Si Alloys, Lubrication Engineering, 49 (1992) Konishi, T., Klaus, E. E. and Duda, J, L., Wear Characteristics o Aluminum Silicon Alloy Under Lubricated Sliding Conditions, To be Published in Tribology Transactions, 39 (1996). 8 Davis, F. A. and Eyre, T. S., The Eect o Silicon Content and Morphology on the Wear o Aluminum Silicon Alloys Under Dry and Lubricated Sliding Conditions, Tribology International, 27 (1994), 3, Barber, G. C., Matthews, J, J. and Jary, S., Wear and Scu Resistance o Aluminum 39, Lubrication Engineering, 41 (1991) Ferrante, J. and Brainard W. A., Wear o aluminum and Hypoeutectic Aluminum Silicon Alloys in Boundary Lubricated Pin on Disk Sliding, NASA Technical Paper 1442, (1979).. 11 Tseregounis, S. I., Wear and Galling o 356 T6 Aluminum on Steel in Low Amplitude Reciprocating Sliding in the Presence o Synthetic Lubricants in HFC 134a Atmosphere, Tribology Transactions, 39 (1996), 1, Sheiretov, T. K., VanGiabbeek, W. H., and Cusano, C., Tribological Evaluation o Various Surace Treatments or M2 Tool Steel in a Rerigerant Environment, Proceedings o the 1994 International Compressor Engineering Conerence at Purdue, Vol. I, Laemm.le, J, T. and Bohaychick, J., Novel Substituted Manolic Diesters: Eectiveness obidentate Bonding o Additives on Aluminum Workpiece Suraces, Lubrication Engineering, 43 (1986), 9, Rotten, B. W., Bidentate Organic Oxygen Compounds as Boundary Lubricants or Aluminum, Lubrication Engineering, 3 (1974), 8,