Tribology International 44 (2011) 1890–1901

Contents lists available at SciVerse ScienceDirect

Tribology International

0301-67

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Tribological studies of unpolished laser surface textures understarved lubrication conditions for use in air-conditioning andrefrigeration compressors

Surya P. Mishra, Andreas A. Polycarpou n

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, USA

a r t i c l e i n f o

Article history:

Received 18 June 2011

Received in revised form

29 July 2011

Accepted 3 August 2011Available online 12 August 2011

Keywords:

Laser surface texturing

Compressors

Starved lubrication

Wear

9X/$ - see front matter & 2011 Elsevier Ltd.

016/j.triboint.2011.08.005

esponding author. Tel.: þ1 217 244 1970.

ail address: Polycarp@illinois.edu (A.A. Polyca

a b s t r a c t

An experimental study was carried out to investigate the tribological performance of different laser

surface texture patterns, with unpolished material bulges around the dimples, under realistic operating

conditions of starved lubrication, for use in air-conditioning and refrigeration compressors. Compared

to untextured gray cast iron surfaces, the texture patterns showed significant tribological improve-

ments. Long durability tests also highlighted the long term usefulness of surface texturing. The

dominant wear mode of the texture patterns was found to be mechanical polishing and the tribological

behavior was found to be largely independent of the type of lubricant or refrigerant.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Environmental concerns have forced the refrigeration industryto move from chlorofluorocarbons (CFCs) to hydrofluorocarbons(HFCs) and more recently low global warming potential (GWP)refrigerants like R-744 (CO2) and newly-developed R-1234yf. Theimmediate impact of such a change is the load limits on criticalinterfaces within the compressors. Chlorine in CFC refrigerantcauses the formation of beneficial ferrous chloride protective layerson iron surfaces [1]. In addition, operating conditions such as loadand speed of the compressors are becoming more aggressive toimprove efficiency. Thus, there is a need for a closer examination ofthe interfaces and devise methods to make them more robust andwear-resistant. Currently, attention is focused on lubricants [2],and protective surface coatings. Hard coatings such as WC/C [1],DLC [3], WC/CþDLC and TiAlNþWC/C [4] have been successfullyapplied and shown to be beneficial under dry sliding conditions inthe presence of refrigerants such as R-134a and R-744. Researchhas also been performed on PEEK/PTFE-based soft polymeric coat-ings [5–7]. These coatings are particularly useful to further thedevelopment of oil-less compressors. However, currently compres-sors use oil for lubrication and will probably continue to do so inthe near future until robust materials/coatings that can performunder a wide range of conditions are well established. In addition,

All rights reserved.

rpou).

the state of lubrication in many critical compressor components isunknown and usually is in the boundary/mixed regime [3].

Scuffing is another tribological failure, which is of majorconcern for compressor manufactures as it occurs abruptly,leading to complete destruction of the sliding pair, thus renderingthe device non-functional [8]. Several factors affect scuffing suchas contact pressure, sliding velocity, contact temperature, lubri-cant and lubrication regime, surface topography and materials [9].Thus there is a need to also investigate solutions other than the‘‘traditional’’ approaches of protective coatings and explore inno-vative surface engineering solutions such as surface texturing.

Surface texturing involves creation of regular micro-sized featureson the surface, which serve various functions under different lubrica-tion regimes. The size and geometry of the micro-sized featuresvaries depending on the application, processing techniques andmaterials. In the literature, the most common type of geometryfound is in the form of circular/spherical micro-dimples due to theireasy fabrication and lower costs [10]. It is challenging to producesuch regular repeatable micro-structures on a material surface andtherefore, research has focused on processing techniques. Some of thetechnologies currently in use are laser surface texturing [11–13],vibromechanical texturing [14], diamond embossing [15], pulsedair arc treatment [16] and recently metal micro-casting [17,18].Laser texturing is popular because of its versatility of use, and canbe used to create different shapes and sizes with good control andrepeatability.

For a micro-dimpled surface, the three significant geometricalparameters are the diameter of the dimple, d, the depth of the

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–1901 1891

dimple, h, and the area density, A [19–21]. Researchers areworking to identify suitable optimal dimensions to maximizethe benefits from texturing. Surface micro-dimples help inimproving the tribological behavior by (1) boosting the hydro-dynamic pressure under full-film lubrication, (2) acting as lubri-cant reservoirs under boundary/mixed or starved lubrication and(3) trapping wear debris under dry sliding conditions.

Surface texturing has been used for friction and wear reduc-tion applications such as cylindrical face rings, piston rings andhydrodynamic bearings [19,20]. Both numerical and experimentalworks for such systems have been performed [22–26]. In additionto the work for practical systems, basic work to understand themechanisms behind friction reduction and to optimize the texturegeometries has also been carried out. For cases where micro-dimples act as micro hydrodynamic bearings, theoretical model-ing can be done to optimize the texturing parameters for specificoperating conditions. However, most of the work done withtexturing in the boundary/mixed or starved lubrication regimesis experimental in nature. This is because the involved phenom-ena are quite complex and can only be described analytically inlimited cases [24]. A trial and error approach based on physicalunderstanding is used to optimize the texturing dimensions,especially for the dry sliding and starved lubrication cases wheretheoretical modeling does not exist. In such cases, the tribologicalbenefits are due to the lubricant storage and wear debris trappingability of the micro-dimples.

Despite its popularity, the effect of surface texturing has notbeen specifically studied on compressor surfaces under realisticoperating conditions. The objective of the current work is toexplore the idea of surface texturing as a surface engineeringapproach to improve the tribological performance of compressor

Table 1Details of the texture patterns.

Pattern

designation

Diameter

d (mm)

Depth

h (mm)

Diameter

to depth

ratio (d/h)

Area

density

(%)

Bulge

height

ho

(mm)

Ratio of bulge

height (ho)

over roughness

(Ra)

A1 40 10 4 5 9 32.1

B1 60 7.5 8 5 6 21.4

C1 60 4 15 5 4 14.3

A2 40 10 4 20 9 32.1

B2 60 7.5 8 20 6 21.4

C2 60 4 15 20 4 14.3

Fig. 1. Optical images of the as-received

surfaces under realistic operating conditions in the presence ofenvironmentally friendly refrigerants, particularly in the bound-ary/mixed/starved lubrication regimes.

2. Experimental

2.1. Specimens and surface texturing

Tribological experiments were conducted on a pin-on-diskconfiguration using a specialized tribometer called Ultra HighPressure Tribometer (UHPT). The pins and disks were made fromgray cast iron, a common material used in scroll compressors. Thediameters of the cylindrical pins and disks were 6.3 mm and76 mm, respectively. The initial average surface roughness (Ra) ofthe disks was 0.36 mm, while that of the pins was 0.28 mm. For thetextured pin specimens, the micro-dimples were fabricated onone of the flat sides of the pins by a commercial manufacturerusing laser surface texturing. Three types of micro-dimple pat-terns were created and two types of area densities were achievedon the surface of the pins, the details of which are shown inTable 1. In the literature, optimization of micro-dimple dimen-sions has been done by some researchers, based on hydrodynamiclubrication conditions, and usually a diameter to depth (d/h) ratioof approximately 10 is deemed ideal. However, it should be notedthat the optimization depends on the specific test conditions. Inthis work, different texture patterns were selected so that variousd/h ratios could be studied. The final set of texture patterns(Table 1) was narrowed down due to cost considerations.

The obtained texture patterns were characterized before use.Fig. 1 shows optical images of the ‘‘as-received’’ texture patternsand Fig. 2 shows representative higher magnification scanningelectron microscopy (SEM) images and a 2-D areal scan of texturepattern A2. From these images, it can be seen that the lasersurface texturing process resulted in material bulges around themicro-dimples. This was expected as laser texturing process isusually accompanied by appearance of melt material in the formof ridges or bulges around the dimples [12]. The height of thesematerial bulges (ho) were characterized through surface profilemeasurements. The ratio of bulge height (ho) over the roughnessof the pins (Ra) gives an estimate of the magnitude of the bulgesand is presented in Table 1. The material bulges were 14–32times higher than the average roughness (Ra) of the pins. Nopost-texturing polishing or lapping was performed on the pinsand ‘‘as-received’’ texture patterns were used. This is becauseinitial attempts to manually polish the material bulges led to

unpolished laser texture patterns.

Fig. 2. (a) Low magnification SEM image, (b) high magnification SEM image and (c) 2-D areal scan of the as-received laser texture pattern A2.

Table 2Summary of the experimental conditions.

Conditions Wear experiments (Set I) Durability

experiments (Set II)

Load (N) 178 178

Sliding speed (m s�1) 2.4 (0.96a, 3.84a) 2.4

Temperature Room temp. Room temp.

Refrigerant R-744 (R-134aa, R-1234yfa) R-744

Lubricantb PAG (POEa, Mineral oila) PAG

Duration One hour Three hours

a Parameters used for the additional wear experiments to investigate the

effect of sliding speed, lubricant and refrigerant type on the tribological perfor-

mance of the texture patterns.b In each case, only one drop of lubricant was used for the experiments (one

drop PAG¼22 mg).

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–19011892

a slight curvature of the pin surface. Also, it was deemedimportant to understand the role of these material bulges onthe tribological performance.

2.2. Tribological experiments

The stationary pin was held in a self-aligning holder with itsflat surface in contact with an upper rotating disk. The UHPT is acustom-designed tribometer capable of handling high environ-mental chamber pressures, which is essential for tribologicaltesting in the presence of refrigerant R-744. It can be pressurizedup to 13.8 MPa (2000 psi). Further technical details aboutthe UHPT can be found in Ref. [27]. Two types of experimentswere performed to investigate the usefulness of surface texturingunder realistic compressor operating conditions; these beingWear experiments and Durability experiments.

Wear experiments were designed as one hour long starvedlubricated constant load experiments. One hour long tests wereplanned so as to take care of any aggravated running-in due to thematerial bulges around the micro-dimples resulting from surfacetexturing. Starved lubrication was simulated by directly applyingone drop of lubricant at the interface. The amount of lubricant wasquantified in terms of the mass of the lubricant at the interface,which for PAG lubricant was 22 mg. With such a minute amountof lubricant at the interface, the operating regime is closer to theboundary than mixed regime and thus termed starved. All theexperiments were performed at a sliding speed of 2.4 m s�1 inthe presence of R-744 at a chamber pressure of 1.93 MPa atambient laboratory conditions of 16–20 1C and 40–50% RH. Initi-ally, comparison between the untextured surfaces and texturepatterns was performed at 89 N normal load. It was found that theuntextured surface could survive the entire duration of testing ofone hour without failure. Thus, for more aggressive conditions,further tests were performed at a normal load of 178 N. The initialcontact between the disk and pin occurred at the top of thematerial bulges, around the micro-dimples. So, for a normal loadof 178 N, the initial contact pressure varied between 23 and

107 MPa depending on the type of texture pattern. The contactpressure is calculated by approximating the contact patch area,considering the material bulges as truncated semi-circles, which isexplained later (Section 3.3). The contact pressure depends on thetype of texture pattern under study as the size of the materialbulges around the micro-dimples that determine the contact patcharea are different for different patterns. These experimentalconditions are summarized in Table 2 as Set I. Prior to theexperiments all samples were ultrasonically cleaned in a pool ofacetone for 10 min followed by rinsing with propanol.

Durability experiments were designed to evaluate the abilityof the textured specimens to withstand extended periods ofoperation under starved lubrication conditions. These tests wereconducted for three hours with one drop of PAG lubricant, addeddirectly to the interface before initiating the test. At a slidingspeed of 2.4 m s�1 it amounted to a sliding distance of 25,920 m.Such tests would clearly identify the benefits of using surfacetexturing to improve the durability of compressor components.Only texture patterns A1, B1 and C1 were used for this part of thestudy and the test conditions are listed in Table 2 as Set II. This isbecause these texture patterns showed superior performance in

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–1901 1893

the wear tests and only limited number of the other texturepatterns were available for testing.

To investigate the effectiveness of surface texturing underdifferent operating conditions, selected wear experiments werealso performed at different sliding speeds namely 0.96 m s�1,2.4 m s�1 and 3.84 m s�1, with different lubricants, namely PAG,POE and Mineral oil, and in the presence of different refrigerants(R-744, R-134a and R-1234yf).

2.3. Surface profilometry

Surface profile measurements of the texture patterns beforeand after experiments were performed using a Tencors P-15contact profilometer. The surface profiles were particularly usefulin calculating the amount of wear of the texture patterns. Surfaceprofile measurements of the wear tracks on the disk countersurface were also performed to ascertain the extent of surfacewear (or burnishing) on the gray cast iron disks. Typically, 1-Dline scans of 0.8 mm length were performed for the texturepatterns while scans of 10 mm length were done for the diskwear tracks. Prior to the profilometric measurements, all sampleswere ultrasonically cleaned in a pool of acetone for 10 minfollowed by rinsing with propanol to ensure they are clean andfree from any wear debris and/or residual oil to obtain an accuratemeasure of the surface profiles.

2.4. Scanning electron microscopy/Energy dispersive spectroscopy

Morphological changes on the texture pattern surfaces beforeand after testing were analyzed using an optical microscope and ascanning electron microscope (SEM). In some cases, energydispersive spectroscopy (EDS) was also used to obtain the spectraat different locations on the pin surfaces after testing, to inves-tigate any elemental chemical changes that might have occurred.A JEOL 6060LV SEM at 20 kV coupled with an EDS detector, whichmeasures the relative abundance of emitted X-rays versus theirenergy, was used for this purpose.

3. Results and discussion

3.1. Wear experiments

Fig. 3 shows typical wear experiments, comparing the perfor-mance of an untextured pin with texture patterns A1, B1 and C1

Fig. 3. Comparison of friction coefficient performance between untextured sur-

faces and texture patterns A1, B1 and C1. Test conditions Set I, Table 2 (PAG,

R-744).

under identical conditions. The friction coefficient of the untex-tured surface increased sharply shortly after the test wasinitiated, along with significant vibration at the interface, indicat-ing scuffing behavior. Untextured surfaces are nominally flat andhave only limited lubricant storage ability determined by thelevel of roughness. Thus, it is envisaged that as the test pro-gressed, the interface ran out of lubricant and metal/metalcontact occurred leading to the observed unstable behavior,which is consistent with the literature [8,9]. To verify that it isindeed the case, experiments with the untextured pin wererepeated three times and similar performance was observed (onlyone of the experiments is shown in the figure for clarity).

In contrast to the untextured interface, all three texturepatterns were able to operate successfully without failure forthe entire duration of one hour testing, which corresponds to asliding distance of 8,640 m. Similar experiments were performedwith texture patterns A2, B2 and C2. Fig. 4 summarizes thefriction performance of all the untextured and texture patternstested (the reported friction coefficient values were averaged overthe last 20 min, i.e., from 5,760 m to 8,640 m of sliding distance).It is clear that all the texture patterns perform better than theuntextured surfaces, which failed early during testing. Texturepatterns A2, B2 and C2 exhibited higher friction coefficient values,compared to patterns A1, B1 and C1. The texture geometries,namely diameter and depth of the micro-dimples, are the samefor these two sets of patterns, the difference being the areadensity of the micro-dimples. As the area density increases, thenumber of micro-dimples increases, which means that the weardebris generated will be more when sliding occurs. This is furtherconfirmed from optical images wherein texture patterns A2, B2and C2 appear to have worn out more than patterns A1, B1 andC1. This is possible with more wear debris acting as third bodyabrasives.

Among the two sets of texture patterns, the trend in frictioncoefficient behavior appears to be the opposite. That is, thefriction performance of pattern C1 is the best among A1, B1 andC1 while pattern C2 is the worst among A2, B2 and C2. In bothcases, however, patterns A1, B1 and A2, B2 exhibited similarperformance. Although the lubricant storage ability of pattern C1is the lowest among A1, B1 and C1, as it has the shallowest micro-dimples, the wear debris at the interface would also be the lowest,as it has the smallest material bulges. It should be noted that thewear experiments were conducted with one drop of PAG lubri-cant (22 mg) applied directly at the interface before initiating anexperiment, which is sufficient to completely fill all the micro-dimples on the surface, as shown in Fig. 5(a). Fig. 5(b) and (c),depicting the texture pattern C1 after the wear and durability

Fig. 4. Comparison of friction coefficient values for the untextured and texture

patterns tested under identical conditions Set I, Table 2 (PAG, R-744).

Fig. 5. Pin photographs, texture pattern C1: (a) before test with 22 mg PAG lubricant, (b) after wear experiment (Set I, Table 2, PAG, R-744) and (c) after durability

experiments (Set II, Table 2). Note that the pin diameter is 6.3 mm.

Fig. 6. Friction coefficient performance of texture patterns A1, B1 and C1 under

durability test conditions Set II, Table 2 (PAG, R-744).

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–19011894

experiments, respectively. Even after the experiments, traces ofPAG lubricant can be seen on the surface in addition to a viscousblack residue, which is a mixture of the excess lubricant and weardebris. Thus, the friction performance in such constant loadexperiments could be attributed to the trade-off between thelubricant storage ability of the micro-dimples and the wear debrisgenerated during the sliding experiments, which in turn dependon the area density of the micro-dimples.

It appears that below some critical area density, the frictionperformance is dominated by the material bulges around themicro-dimples, while for higher area density the lubricant storageability is the dominant factor. This would be particularly true fortexture patterns with unpolished material bulges around themicro-dimples. Kovalchenko et al. [28] conducted studies withLST samples and without post-texturing lapping to remove thematerial bulges around the micro-dimples. They reported thatsamples without the lapping displayed higher friction coefficientand recommended removing the bulges before optimizing thetexture geometries for best performance. Thus, the effect ofremoving the material bulges before the tests on the tribologicalperformance under compressor realistic conditions should beexplored in the future through further experimental studies andanalysis. As mentioned previously, attempts to manually polishthe texture patterns led to a slight curvature of the surface, whichwas not always consistent, and thus not desirable. Thus, it isimperative that a robust automated polishing process be put inplace before any such studies can be done as precise removal ofmaterial is necessary, which might be difficult with standardmachining techniques if the dimples are shallow (such as texturepatterns C1 and C2 used in this study).

3.2. Durability (life) experiments

The results of the durability or long term life experiments werealso encouraging, as shown in Fig. 6. It is of particular importancebecause compared to the untextured surfaces, which failed afterapproximately 10 min, all the texture patterns lasted for theentire test duration of three hours, which is a very significantimprovement. Furthermore, the friction behavior was very stable,reaching steady-state values less than 0.05.

The ability of textured surfaces to sustain low friction over longintervals of time was also demonstrated by Wakuda et al. [29]. Asin the case of wear testing, durability testing was conducted underidentical lubricant conditions (one drop, 22 mg of PAG). Borghiet al. [30] showed the long term performance of textured surfacesunder ‘‘single drop’’ lubrication, similar to what is used in thiswork. As can be seen in Fig. 6, the friction coefficient values werelow for all the texture patterns with pattern C1 performing thebest. Similar results were obtained for the wear experiments

although the friction coefficient values were slightly higher.The average friction coefficient for the durability experimentswas 32–43% lower than those for the wear experiments. Sincethe wear experiments were of 60 min duration, this indicates thata stable sliding interface is established after 60 min under thetested starved lubrication conditions.

Fig. 7 shows the optical images of the disks and pins after thedurability tests. From the optical images of the disks, no sig-nificant wear occurred during testing. In some cases, the weartracks on the disks were distinct but only mild burnishing wasobserved. For texture pattern A1, some smooth polished regionswere observed, which probably meant that some mechanicalrubbing occurred between the pin and the disk. It should beemphasized that even under such extended periods of testing thewear of the pins and disks was insignificant.

3.3. Wear quantification

From the optical micrographs it was observed that afterextended periods of testing the micro-dimples were still present.However, the material bulges around the micro-dimples weregetting polished. To quantify the amount of this polishing or mildwear, profile measurements were carried out on the surfaces ofthe textured pins. Fig. 8 shows representative surface profilemeasurement on texture pattern A1 before testing and after wearand durability testing. Note that even after extended periods oftesting, there is no appreciable change in the dimple depth, whichindicates that only wear of the material bulges occurred.

Fig. 7. Optical images of the disks and texture patterns A1, B1 and C1 after durability test conditions Set II, Table 2 (PAG, R-744). Horizontal double arrows indicate the

direction of machining marks on the disks and vertical arrows indicate direction of sliding.

Fig. 8. Surface profile measurements of texture pattern A1: (a) before, (b) after wear and (c) after durability experiments under test conditions Set II, Table 2.

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–1901 1895

It is difficult to calculate the exact wear volume from thesurface profile measurements, as the material bulges are notidentical in shape and it would require measuring each of thematerial-pileups, which is impractical. Even then, the resultswould not be accurate with the 1-D line scans and full 2-D areal

scans (as shown in Fig. 2) would be necessary. Thus, to calculate thewear volume and hence wear rate, a simple geometrical representa-tion of the material bulges as a truncated semi-circle was used.Note that the wear rate calculated following such a procedureleads to an ‘‘average’’ wear rate. Fig. 9 shows a schematic of the

Fig. 9. (a) Geometrical representation of the material bulges around the micro-dimples on the textured surfaces as truncated semi-circles and (b) schematic of the wear of

the material bulges.

Table 3Average height of material bulges before and after wear

and durability experiments.

Texture pattern ho (mm) hw (mm) hd (mm)

A1 9 3 0

B1 6 3 1

C1 4 2 1

Fig. 10. Calculated average wear rates for texture patterns A1, B1 and C1 after

wear and durability experiments, Table 2 (PAG, R-744).

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–19011896

proposed geometric model for average wear of the materialbulges.

Based on the proposed geometric average wear model and thesurface profile measurements of the texture patterns after thewear and durability experiments, the heights of the materialbulges before testing ho, after wear testing hw and after durabilitytesting hd for the three texture patterns are summarized inTable 3. Fig. 10 shows the comparison of the calculated averagewear rate for texture patterns A1, B1 and C1 for the wear anddurability experiments. The wear rates for the durability experi-ments were 45–50% lower than those of the wear experiments.This implies that most of the wear of the material bulges aroundthe micro-dimples occurred during the initial contact and hencewear rate is not uniform. This is because during the initial contactthe localized contact pressure is higher, leading to higher wear.However, as the test progresses, the material bulges get flattenedout leading to an increase in the contact area, which in turndecreases the contact pressure and hence the amount of wear.

Overall, the simple geometric representation of the materialbulges around the micro-dimples enabled the determination ofaverage wear rates for the experiments done under different condi-tions in this work. Despite small differences, the overall wear rate ofthe texture patterns, which is typically less than 10�8 mm3 N�1 m�1

under starved lubrication conditions, is encouraging and comparableto the wear rate of protective surface coatings. For example, Nunezet al. [6] tested different PEEK-based polymeric coatings understarved lubrication conditions in the presence of R-744 and reportedthe wear rates to be �10�6 mm3 N�1 m�1. Wear rate of ATSP/PTFEblends tested under unlubricated conditions has been reported to be�10�5 mm3 N�1 m�1 [31] while that for WC/CþDLC hard coatingunder unlubricated conditions in the presence of different refrigerantenvironments has been reported to be �10�9 mm3 N�1 m�1 [4].

3.4. SEM/EDS

Fig. 11 shows the SEM images of single micro-dimples ontexture patterns A1, B1 and C1 after wear experiments (TestConditions Set I, Table 2, PAG, R-744). From the figure, it is clearthat the material bulges around the micro-dimples are not fullyremoved. During the course of testing, these material bulges getflattened out (worn) and form a stable tribocontact with therotating counter disk. Also shown in Fig. 11 are the SEM images ofsingle micro-dimples on texture patterns A1, B1 and C1 afterdurability experiments (Test Conditions Set II, Table 2). From theoptical images and wear profile measurements, it could be seen

that for texture pattern A1 some contact between the pin surfaceand disk occurred, which is also confirmed with the SEM image.Compared to texture patterns B1 and C1, there appears to besignificant polishing or smooth regions on the surface of thetexture pattern A1. This is possible when there is significantmechanical rubbing between the two surfaces such that thebulges are completely removed. This confirms that under starvedoperating conditions, the effect of mechanical rubbing on thelaser texture bulges is dominant.

To verify that the observed surface morphological changes ofthe texture patterns after testing are due to mechanical rubbingonly and not due to any chemical changes; EDS studies were alsoconducted. EDS spectra from untested pattern A2 are presented asa baseline in Fig. 12. This was done to capture any elementalchanges due to the laser texturing process. EDS spectra were

Fig. 11. SEM images of texture patterns A1, B1 and C1 showing representative single micro-dimples after wear experiments (a,b,c) and durability experiments (d,e,f). Test

conditions Table 2 (PAG, R-744).

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–1901 1897

taken at different regions and found to be similar. In addition tothe elements such as Fe, Si, F and C detected in the untexturedsurface, traces of O were detected in the texture pattern A2. Thisindicates the formation of oxides during the laser texturingprocess. This is expected as laser texturing is a high temperatureprocess, causing melting of the surface material.

From the high magnification SEM images in Fig. 11, it appearsthat there were some surface changes for texture pattern A1 afterthe wear and durability experiments. To investigate if there wereany chemical changes, EDS studies were also performed. Thesame elements were detected as in the EDS spectra for texturepattern A2 after the wear experiments, as can be seen in Fig. 12.This meant that no chemical changes occurred and the surfacechanges are due to mechanical rubbing action only. Fig. 12 alsoshows an SEM image of texture pattern A1 after a durabilityexperiment along with a representative EDS spectrum. From theSEM image it can be observed that the micro-dimple is filled withwear debris. This is because as the material bulges are worn, themicro-dimples provide an easy escape route to the wear debris bytrapping them. Such a mechanism of wear debris trapping hasbeen known to be one of the functions of the micro-dimples,especially under dry sliding conditions [30]. This might be partlyresponsible for the improved wear resistance observed in this

case. From the EDS spectra, the chemical composition seems to besimilar to the previous cases, except that traces of O are missing,which means that the oxide layer was removed due to themechanical rubbing action.

3.5. Effects of operating conditions

3.5.1. Sliding speed

The critical sliding interfaces in some compressors operate atsliding speeds up to few m s�1. This places an extra stress on theinterface making it more prone to failure, especially when there islimited lubrication. Sliding speed also has an effect on themaximum load the interface can withstand. Scuffing resistanceof a sliding interface typically follows the relationship PV¼con-stant (where P is pressure and V velocity). This means that as thesliding speed increases, the scuffing resistance or the maximumload carrying capacity of the interface decreases. Representativeexperiments were conducted with textured patterns B1, B2 andC2, and compared against an untextured pin. B1 and B2 wereidentical in the micro-dimple geometry, the difference being inthe area density of the micro-dimples. B2 and C2 were selected toinvestigate the role of micro-dimple geometry, as they had thesame diameter (60 mm) and area density (20%) but differed in the

Fig. 12. EDS spectra at different locations on texture pattern (a) A2 before testing, (b) A1 after wear experiment and (c) A1 after durability experiment. Spectrum 1 corresponds

to marker 1 in each figure. Spectra 2 and 3 are identical as 1 and are not shown.

Fig. 13. Variation of friction coefficient values with sliding speed, test conditions

Set I, Table 2 (PAG, R-744).

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–19011898

dimple depth (with pattern B2 having a depth of 7.5 mm while C2having a depth of 4 mm). Tests were conducted at three differentsliding speeds of 0.96 m s�1, 2.4 m s�1 and 3.84 m s�1. Theremaining test conditions are as given in Table 2 (Set I). Fig. 13shows the variation of the friction coefficient values of thedifferent texture patterns and untextured surfaces with a changein the sliding speed.

At lower sliding speeds, although the texture patterns dis-played slightly lower friction coefficient values than the untex-tured surfaces, the difference was insignificant. Also, the frictioncoefficient values of the textured patterns were fairly similar,indicating that the effect of texture geometry was not significantat lower sliding speeds either. More importantly, the untexturedsurface was able to survive the entire testing duration. Thus,benefits from texturing at less aggressive lower sliding speeds arenot remarkable. However, as the speed was increased, the operat-ing conditions become aggressive and the effect of texturingbecomes clearly visible. Compared to the untextured interface,which is unable to operate under such aggressive conditions, thetexture patterns perform distinguishably well. Also, the friction

Fig. 14. Comparison of friction coefficient values for texture patterns C1 and C2

with untextured surfaces in the presence of different refrigerants and ambient air,

test conditions Set I, Table 2, PAG.

Table 5Average height of material bulges for texture pattern C1 after

testing in different refrigerant–lubricant combinations.

RefrigerantþLubricant ho (mm) hw (mm)

R-744þPAG 4 2

R-744þPOE 4 0

R-744þMineral Oil 4 2

R-134aþPAG 4 1

R-1234yfþPAG 4 2

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coefficients of the textured patterns are different from each otherat 2.4 m s�1 and 3.84 m s�1, which points to the strong influenceof the texture geometry on the performance at aggressive operat-ing conditions, with B1 performing the best under aggressivespeed conditions.

3.5.2. Lubricant type

The objective was to identify if the tribological benefit observedearlier is due to surface texturing alone, or if the type of lubricantand lubricant–refrigerant interaction plays a significant role. Notethat based on earlier EDS studies, it would appear that mechanicalwear was more significant than chemical changes and this part ofthe study would further validate this point. Three differentlubricants that are used in compressors were chosen for thispurpose: PAG, POE and Mineral oil. Details about the lubricantsare given in Table 4. Texture pattern C1 was used for the study, asit was the best performing texture pattern in the wear tests in thepresence of R-744. All the tests were conducted under identicalconditions (Set I, Table 2). The friction coefficient for the test donein the presence of POE was the highest at 0.103 (70.017). Frictioncoefficient values for texture pattern C1 in the presence of mineraloil and PAG lubricant followed next at 0.086 (70.002) and 0.068(70.003), respectively.

From the optical images of the texture pattern C1 after thetests, the best wear performance was achieved in the presence ofPAG and the disk and pins tested in the presence of PAG showedminimum surface damage. The disk tested with POE showedmore burnishing followed by the disk tested in mineral oil. Thecorresponding texture patterns followed similar trends. While thetexture pattern C1 tested with PAG showed only few minorscratches on the surface, those tested with POE and mineral oilshowed smooth polished regions indicating mechanical contactbetween the pin and disk surfaces during testing. Nunez et al. [2]conducted starved lubrication experiments and compared theperformance of PAG and POE in the presence of R-744 refrigerantenvironment on Al390-T6/SAE 52100 steel material pairs. Theyreported that the disks lubricated with PAG suffered less burnish-ing than those lubricated by POE, which is similar to what wasobserved in this work.

Under the tested conditions, the tribological performancecould be largely attributed to the effect of surface texturing andsecondarily to the type of lubricant. Comparable performance isobserved for the inexpensive mineral oil compared to advancedsynthetic compressor lubricants PAG or POE, which further high-lights the positive effect of surface texturing.

3.5.3. Refrigerant environment

To further generalize this study, additional tests were con-ducted in the presence of other refrigerants of interest, namelyR-134a and R-1234yf. R-134a is an HFC refrigerant, whichis widely used, but has a high global warming potential (GWP)and its use has been regulated by the 1997 Kyoto Agreement.R-1234yf has been recently introduced by Honeywell and DuPontand it has negligible GWP and is being intended as a directreplacement to R-134a (because the working pressure ofR-1234yf is similar to that of R-134a). R-744 (CO2) compressorsare high pressure systems compared to R-134a and R-1234yf.

Table 4Lubricant properties.

Lubricant Type Manufacturer Viscosity Mass of 1 drop

PAG Emkarate RL 300 SUS 22 mg

POE Emkarate RL 300 SUS 25 mg

Mineral oil (C-4s) Calumet lubricants 300 SUS 26 mg

So, while R-744 tests were conducted at 1.93 MPa (280 psi) ofchamber pressure, tribological studies in the presence of R-134aand R-1234yf were conducted at 0.69 MPa (100 psi) chamberpressure. Two of the texture patterns (C1 and C2) were used forthis study and compared against an untextured surface underidentical conditions (Set I, Table 2, PAG). This is because C1 wasthe best performing texture pattern in both the wear and durabilitytests carried out in the presence of R-744, and C2 was chosen toinvestigate the effect of the area density of micro-dimples.

The friction coefficient values are compared in Fig. 14. Ingeneral, the refrigerant environment had a strong influence onthe untextured surfaces. The untextured surfaces performed thebest with the lowest friction coefficient in the presence of R-134aand survived the entire duration of testing. In comparison,untextured surfaces failed in the presence of refrigerantsR-1234yf and R-744. The average friction coefficient values calcu-lated till failure were higher than those in R-134a. In the case oftexture patterns, the refrigerant did not have a very significantinfluence, with the texture patterns behaving similarly in thepresence of the different refrigerants. However, the texture geo-metry had a strong influence on the performance. As the areadensity increased, the friction performance became worse. Inaddition, tests were performed in ambient air to highlight theimportance of the refrigerant environment. Both the untexturedsurface and texture pattern C1 failed early when tested in ambientair indicating the positive influence of refrigerant on the tribolo-gical performance and the need for realistic tribotesting.

Surface profile measurements were used to calculate the wearrate of the different texture patterns. In the case of tests performedat different sliding speeds, no noticeable difference in the surfaceprofiles was observed. For the tests performed with differentlubricants and refrigerants, surface profile measurements werecarried out for texture pattern C1. Table 5 presents the averageheight of the material bulges around the micro-dimples of pattern

Fig. 15. Calculated average wear rates for texture pattern C1 under different

lubricant–refrigerant combinations, test conditions Set I, Table 2.

S.P. Mishra, A.A. Polycarpou / Tribology International 44 (2011) 1890–19011900

C1 before and after the tests. Fig. 15 compares the wear rate basedon these surface profile measurements.

The performance of the PAG lubricant and mineral oil wascomparable in the presence of R-744, which is reflected in the wearrate values. Since R-1234yf is to be a direct replacement of R-134adue to environmental concerns, it is encouraging to see that thewear performance of the texture pattern is better in R-1234yf andcomparable to R-744 in the presence of PAG lubricant.

4. Further discussion of texture pattern performance

The tribological performance of the texture patterns was foundto be largely dependent on the lubricant storage ability of themicro-dimples. The material bulges around the micro-dimplesalso played a vital role in providing a stable tribocontact. How-ever, they also contributed to the wear debris between the slidinginterfaces, which led to higher friction and polishing type of wear.The dominant wear mechanism was mechanical rubbing causeddue to the interaction between the rotating disk and the materialbulges on the texture patterns. This mechanism was evident byobserving the optical and high magnification SEM images(Fig. 11). The material bulges were intact and only got flattenedout. Chemical interactions between the material surface, lubricantand refrigerant also play a critical role on the tribological behaviorin a compressor environment. However, as noted in Fig. 12, theanalysis done on the texture patterns after testing, in the form ofEDS, did not clearly indicate any chemical changes. The tribolo-gical performance was also found to be largely independent onthe type of lubricant or refrigerant. Moreover, limited nanoinden-tation measurements performed on texture pattern C1 showedthat the material bulges were in fact harder than the surface, withvalues of 13 GPa compared to 9 GPa for the measurements doneon the flat area between the micro-dimples.

However, it should be noted that the tribological behavior isdependent on the specific testing conditions. For example, thebehavior of the texture patterns was less predictable and notnecessarily better than the untextured surfaces when tested inunlubricated conditions, which was attributed to the large localizedcontact pressure due to the presence of unpolished material bulges.Under aggressive scuffing type tests (results not shown), it wasfound that the scuffing resistance was mainly dependent on thelubricant storage ability of the micro-dimples, with texture patternswith deeper micro-dimples performing the best. The presence of thematerial bulges did not seem to play a significant role. At constantlower loads, however, the wear experiments demonstrated that theperformance was more complicated because of the presence of the

material bulges, which were not completely worn even afterextended periods of testing. The material bulges provided a stabletribocontact on one hand while becoming a source of wear debris onthe other hand. This complex role of the unpolished material bulgesneeds further attention in the future.

5. Conclusions

A study was performed to investigate the viability of surfacetexturing for improved tribological performance under aggressivestarved lubrication compressor conditions. From the experimentsand subsequent analyses, the following conclusions could be drawn:

(a)

In comparison to untextured surfaces, which failed early, thetexture patterns successfully operated under identical condi-tions with minimal wear, up to three hours of durabilitytesting. The wear rate was extremely low and was found to beequivalent to that of protective tribological surface coatings.

(b)

Among the different texture patterns tested (which were selectedbased on the literature for optimized geometries), the differencein the performance, though not significant, was primarily drivenby the geometrical parameters of diameter, depth and areadensity of the micro-dimples. The role of the unpolished materialbulges was also critical. The tribological performance got worseas the area density increased: Area density of 5% was found to bebetter than 20%, under the tested conditions.

(c)

The texture patterns were found to have a distinct advantageover the untextured surfaces as the test conditions becameaggressive. At low sliding speeds, both the textured anduntextured surfaces behaved similarly. However, at highersliding speeds, the texture patterns outperformed the untex-tured surfaces.

(d)

The beneficial tribological behavior was found to be fairlyindependent of the type of lubricant and refrigerant, thusoutlining the main wear mechanism to be mechanical rub-bing. EDS studies confirmed no chemical changes at thesurface, ruling out the possibility of a beneficial lubricant–refrigerant interaction causing the improved tribologicalbehavior of the texture patterns.

In summary, through the experiments and analyses done inthis work, it can be concluded that surface texturing has a greatpotential for application in air conditioning and refrigerationcompressors to improve the durability and reliability of criticalcompressor components. However, further work needs to bedone, especially with respect to fully understand the role of thematerial bulges in the tribological performance.

Acknowledgements

This research was supported by the 30 member companies ofthe Air Conditioning and Refrigeration Center (ACRC), an Indus-try-University Cooperative Research Center at the University ofIllinois at Urbana-Champaign. The SEM images were performed atthe Center for Microanalysis of Materials at the University ofIllinois, which is supported by the U.S. Department of Energyunder Grant DEFG02-96-366 ER45439. Further financial supportfor Surya Mishra, in the form of a fellowship by FMC TechnologiesInc. is also acknowledged.

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