Sliding wear map for the magnesium alloy Mg9Al0.9 Zn (AZ91

Wear 246 (2000) 106–116

Sliding wear map for the magnesiumalloy Mg-9Al-0.9 Zn (AZ91)

H. Chen, A.T. Alpas∗Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ont., Canada N9B 3P4

Received 1 November 1999; received in revised form 18 July 2000; accepted 18 July 2000

Abstract

Dry sliding wear tests were performed on a Mg-9.0%, Al-0.9%, Zn (AZ91) alloy using a block on-ring (AISI 52100 steel) configuration.Wear rates were measured within a load range of 1–350 N and a sliding velocity range of 0.1–2.0 m/s Compositions, morphologies andmicrostructures of worn surfaces and wear debris were characterised by scanning electron microscope (SEM), energy dispersive X-rayspectrometer (EDS) and X-ray diffractometer (XRD). A wear mapping approach, consisting of identification of micromechanisms of wearleading to the generation of loose debris and surface damage and determination of the range of applicability of each mechanism as a functionof load and speed, has been adopted. Two main wear regimes, namely a mild wear regime and a severe wear regime have been observed.The role of contact surface temperature on the mild to severe wear transition was investigated in detail. It was shown that the transitionfrom mild to severe wear was controlled by the contact surface temperature of the alloy and that the onset of the severe wear coincidedwith a surface temperature of 347 K. In the mild wear regime two different micromechanisms operated, i.e. two sub-wear regimes, namelyan oxidational wear regime and a delamination wear regime, were identified. Similarly, in the severe wear regime two different wearmechanisms were responsible for surface damage and debris formation. These were classified as the severe plastic deformation inducedwear and the melt wear regimes. The results of wear tests and metallographic investigations on worn surfaces have been summarised ina wear mechanism map. An empirical contact surface temperature model, which serves to predict the critical surface temperature at theonset of severe wear, has been developed. © 2000 Published by Elsevier Science S.A.

Keywords:Wear map; Wear mechanisms; Wear transitions; Magnesium alloys; Lightweight alloys

1. Introduction

The low density of magnesium, i.e. 1.79 g/cm3, which isapproximately two third that of aluminium and one-fifth ofsteel, makes it attractive for applications where weight re-duction is critical. Magnesium alloys offer good strength-to-weight ratio, machinability and corrosion resistance [1].At the present time, the most commonly used magnesiumalloy is AZ91 that contains 9.0 wt.% Al and 0.9 wt.% Zn.This alloy has good castability [2]. Its stiffness [3] andcreep resistance [4] can be improved by incorporating ce-ramic fibre reinforcements in the alloy. In certain applica-tions, lightweight alloys and composites are subjected tosliding motion including automotive brakes, engine compo-nents (piston and cylinder bores). Sliding wear is also animportant consideration in material processing by rolling,extrusion, forging etc., from the point of view of under-

∗ Corresponding author. Tel.:+1-519-253-4232-2602;fax: +1-519-973-7007.E-mail address:[email protected] (A.T. Alpas).

standing the changes in the deformation microstructure atthe work-tool interface and the associated heat generation.

Published work on the wear behaviour of magnesium al-loys is limited to a few papers [5,6,7]. Hiratsuka et al. [5]investigated dry sliding wear of pure magnesium against analumina counterface using a pin on disc apparatus. Testswere done at a normal load of 9.8 N, a sliding velocity of1.6 m/s and to a total sliding distance of 2.5 × 103 m. Theyobserved two different wear mechanisms depending on thetesting environment. Magnesium exhibited an oxidationalwear mechanism when tested in air. A transfer layer dark incolour, presumed to consist of magnesium oxide, formed onthe surface of alumina disc. When the wear tests under thesame load and velocity conditions were repeated in vacuum(2.5 × 10−4 Pa), magnesium showed metallic wear, i.e. themagnesium transfer layer on the alumina had a metallic lus-tre and the wear rate of magnesium increased significantly.Alahelisten et al. [6] have studied the role of fibre volumefraction on the wear behaviour of alumina fibre reinforcedmagnesium alloy AZ91. The wear resistance in two-bodyabrasion increased with the fibre content. But increasing the

0043-1648/00/$ – see front matter © 2000 Published by Elsevier Science S.A.PII: S0043-1648(00)00495-6

H. Chen, A.T. Alpas / Wear 246 (2000) 106–116 107

fibre volume fraction caused a reduction in solid particle ero-sion resistance. Housh and Sakkinen [7] also reported thatthe abrasive wear resistance of magnesium matrix compos-ites increased with increasing volume percent additions ofSiC particles.

In the absence of detailed studies on the wear mechanisms(i.e. microscopic processes leading to the generation of loosedebris or surface damage) in magnesium alloys, it is instruc-tive to review pertinent information on the wear mechanismsof aluminium alloys. Hirst and Lancaster [8] identified twodistinct regimes of wear, namely mild wear and severe wearin 99% purity aluminium against stellite. Several investiga-tions on Al–Si alloys [8–11] have also showed that a tran-sition from mild to severe wear occurred when certain levelof load and/ or speed is exceeded. Shivanath et al. [12] asso-ciated the mild wear with the oxidational wear mechanismand severe wear with metallic wear that was characterisedby severe plastic deformation of the aluminium surface. A‘wear map’ has been developed for aluminium alloys by Liuet al. [13] who adopted the approach of Lim and Ashby [14]by summarising wear rate and wear mechanism data fromthe literature on load and sliding speed co-ordinates. Zhangand Alpas [15] constructed a wear map for a wrought alu-minium alloy 6061 Al sliding against AISI 52100 steel. Itwas shown that mild to severe wear transition that occurs ata certain combination of load, velocity and sliding distancewas controlled by a surface temperature criterion which co-incided with the threshold temperature at which thermallyactivated processes, such as dynamic recrystallisation ini-tiated. The wear mechanisms identified on the wear mapincluded delamination of oxidised surface layers, delamina-tion of subsurface layers of aluminium, severe deformationinduced wear and seizure. The same wear mechanisms werealso observed in cast aluminium alloys [16]

The present paper focuses on the sliding wear behaviourof a magnesium alloy AZ91. The experimental work wasdesigned to study the effects of the applied load and slidingspeed on the wear transitions in this alloy. The role of contacttemperature on the wear transition was also studied. A wearmapping approach has been undertaken to represent the wearregimes and the main mechanism of wear in each regime.

2. Experimental methods

The material studied was a cast Mg–Al–Zn alloy (AZ91).The chemical composition of the AZ91 alloy is given inTable 1. The Vickers hardness of the alloy, HV1 kg was66.1 ± 2.4 kg/mm2, and its density was 1.79 g/cm3. Othermechanical properties of the alloy are listed in Table 2.

Table 2Physical and mechanical properties of AZ9I at 298 K

Density (g/cm3) HV1 kg Young’s modulus (GPa) 0.2% Yield strength (MPa) Elongation at fracture (%)

1.79 66.1± 2.4 42.6 92.8 7.0

Table 1The chemical composition of AZ9I (wt.%)

Al Zn Mn Fe Cu Ni Si Mg

8.90 0.91 0.20 0.0025 <0.0007 <0.0006 0.005 Balance

Dry sliding wear tests were performed using a block-on-ring type wear apparatus. Wear test samples in the formof rectangular blocks (10 mm× 10 mm× 5 mm) were ma-chined from the cast ingots. The surface preparation proce-dure of the wear test samples consisted of grinding surfacesmanually by 240, 320, 400 and 600 grit SiC papers, respec-tively and then polishing them with 1.0 and 0.05mm aluminapowder slurry using a low-speed polishing machine. Thepolished surfaces were cleaned ultrasonically in a methanolsolution. The narrow rectangular faces of the specimens(5 mm× 10 mm) were put in contact with the slider rings.

The counterface ring was made of AISI 52100 typebearing steel (0.98–1.1 wt.% C, 0.25–0.45 wt.% Mn,0.15–0.30 wt.% Si, 1.30–1.60 wt.% Cr, the balance Fe).The width of the ring was 12 mm. The outer diameter was38 mm. The surface hardness of the ring was HV1 kg =950 kg/mm2. The counterface rings were polished to 1.0mmand cleaned in a methanol solution before each wear test.

The tests were carried out in a sliding velocity range of0.1 and 2.0 m/s and a load range of 1 and 350 N. Mass lossesfrom the surfaces of specimens were determined as a func-tion of sliding distance at each load and sliding velocity con-dition. The mass losses were calculated from the differencesin weight of specimens measured before and after the slidingtests (after removing any loose debris) to the nearest 0.l mgusing an analytical balance. Volumetric wear loss was esti-mated by dividing the mass loss by the density of the alloy.A separate specimen was used to measure the mass loss foreach sliding distance. Volumetric wear rates were calculatedfrom the average slopes of the volume loss versus slidingdistance curves. In case where the curve was not linear, thewear rate was estimated from the slope at 1.5 × 103 m.

The contact surface temperatures were measured using achromel–alumel type thermocouple probe with a stainlesssteel sheath of diameter 0.25 mm. The probe was insertedinside the specimen through a hole drilled slightly largerthan the diameter of the thermocouple. Prior to the tests theend of the thermocouple was put in contact with the sliderring. During the wear tests, the tip of the thermocouple nor-mally remained in contact with surface of the slider ring.Occasionally the tip of the thermocouple became in contactwith lose debris particles which covered the entrance of thehole. The temperature changes were measured as a functionof sliding distances at various loads and speeds and recorded

108 H. Chen, A.T. Alpas / Wear 246 (2000) 106–116

Fig. 1. Wear rates of AZ91 as a function of applied load for tests conducted at the following speeds: (a) 0.1 m/s; (b) 0.3 m/s; (c) 0.5 m/s; (d) 1.0 m/s;(e) 1.5 m/s. Loads are rounded to the nearest integer.


by means of a digital data acquisition system installed in apersonal computer.

Microstructural investigations and chemical analyseson the worn surfaces and the loose debris particles wereundertaken using an optical microscope and a scanningelectron microscope (SEM) equipped with an energy dis-persive X-ray spectrometer (EDS). An X-ray diffractometer(XRD) with Cu Ka radiation was used to determine thecompositions of the debris.

3. Results and discussion

3.1. Effect of applied load and the sliding speed on wearrates

The volumetric wear rates of the AZ91 alloy are plottedagainst the applied loads for the tests conducted at constantsliding speeds of 0.1, 0.3, 0.5, 1.0 and 1.5 m/s in Fig. 1a–e,respectively. At all sliding speeds, the wear rates increasedwith increasing the applied load. At high sliding speeds (e.g.V > 0.5 m/s) large changes in the slopes of the wear ratecurves occurred at a certain load range. The slope changescoincided with the transition from mild wear to severe wear(Fig. 1c–e). At low sliding speeds of 0.1 and 0.3 m/s (Fig. 1aand b), the wear rates also increased with the applied load,but there was no drastic increase in the slopes of the wearrate versus load curves, i.e. the mild wear regime prevaileduntil the highest test load.

Severe wear was characterised by massive surface dam-age and production of large ‘metallic’ debris particles, whichwere readily identifiable during the test by the naked eye.The onset of severe wear transition has been determined dur-ing the course of the wear test by visual observation (ratherthan by extrapolation of the average slopes of mild and se-vere wear rates) and the differences in debris types were

Fig. 2. Transition load (mild to severe wear) vs. sliding speed.

confirmed by the SEM metallography as will be explained inSections 3.2 and 3.3. The mild to severe wear transition loadsdetermined in this way are plotted as a function of slidingspeed in Fig. 2. At the sliding speed of 0.5 m/s, the transitionload was 180 N. An increase in the test speed to 1.0 m/s re-duced the transition to 100 N and a further increase of speedto 1.5 m/s resulted in a decrease in the transition load to 80 N.

It has been observed that, in the mild wear regime, thevolume loss due to sliding wear increased linearly withthe sliding distance, indicating that wear progressed underthe steady state (i.e. constant wear rate) conditions. Rep-resentative plots that show this linear relationship betweenthe volumetric wear and sliding distance in the mild wearregime are given in Fig. 3a, for the tests performed under a

Fig. 3. (a) Volumetric wear losses vs. sliding distance at a sliding speedof 0.1 m/s. Wear at all test loads (5–222 N) is mild and wear rates areconstant (steady state wear); (b) volumetric wear losses vs. sliding distanceat a sliding speed of 1.0 m/s. Full symbols (at loads 5–45 N) representmild wear conditions. Open symbols (at loads 133–222 N) correspond tosevere wear.


constant velocity of 0.1 m/s. On the other hand, above thetransition load (to severe wear), the volumetric wear was nolonger a linear function of the sliding distance but the rateof wear increased with the sliding distance. For example,for the tests performed under a constant velocity of 1.0 m/s,as shown in Fig. 3b, accelerated volume losses with thesliding distance are displayed at the loads of 133 and 222 N.At this test speed, mild to severe wear transition occurred at80 N (Fig. 2) and that the steady state conditions prevailedat the lower loads of 5–45 N in the mild wear regime. Itshould be noted that the damage inflicted on the surface ofthe AZ91 during the severe wear was more extensive thanthe mass loss data suggested.

Fig. 4. (a) SEM micrograph of wear debris in oxidation dominated wear regime (speed= 0.3 m/s and load= 9.8 N). Wear occurred by spallation ofoxidised surface layers; (b) EDS spectrum of wear debris corresponding to areaA in Fig. 4a; (c) XRD spectrum of wear debris in oxidation dominatedwear regime (speed= 0.3 m/s and load= 9.8 N). The numbers in parentheses are the Miller indices of the planes corresponding to intensity peaks. Thenext numbers are the percentages of the intensities of MgO and Mg in their unmixed states.

3.2. Wear mechanisms operating within the mild wearregime

Metallographic observations of mild wear processes re-vealed that generation of loose debris in the AZ91 alloy wasdue either to delamination (spallation) of the oxidised sur-face layers or delamination of the subsurface material adja-cent to contact surface. For example, for the tests conductedat the sliding speed of 0.3 m/s (see Fig. 1b for wear rates),when the applied loads were less than 30 N, the wear processproduced loose debris particles that were black in colour tothe naked eye. The average size of individual debris parti-cles was usually less than 5mm. The morphology of wear


debris, generated under an applied load of 9.8 N and a slid-ing speed of 0.3 m/s, shown in Fig. 4a is typical. The EDSanalysis (Fig. 4b) indicated the presence of a strong oxygenpeak in the debris in addition to the magnesium peak. Alu-minium and iron (from the AISI 52100 steel counterface)were also present implying that the wear debris consisted ofa mixture of magnesium, iron, magnesium oxides and pos-sibly iron oxides. The presence of metallic Mg and MgO inthe debris was confirmed by the XRD spectrum shown inFig. 4c. XRD did not show presence of any iron from thesteel counterface, which suggested that the concentration of

Fig. 5. (a) Optical micrograph of counterface ring (AISI 52100 bearing steel) surface after a sliding distance of 1000 m in the mild wear regime(speed= 0.3 m/s and load= 67 N). The region marked as ‘contact area’ is covered with material transferred from AZ91 sample; (b) SEM micrographof wear debris produced by delamination wear (speed= 0.3 m/s and load= 67 N); (c) EDS spectrum of delamination wear debris, taken from the areamarked asA in Fig. 5. b; (d) XRD spectrum of wear debris produced by delamination wear (speed= 0.3 m/s and load= 67 N).

iron (either in elemental form or in the form of iron oxide)was below the detection limit of XRD (i.e.<3%). This wearmechanism is referred as the oxidational wear.

SEM and EDS observations have shown that, as the loadwas increased in the mild wear regime, a gradual transitionin the wear behaviour of the alloy occurred from an oxida-tional wear to a delamination wear. For example, under thetest conditions of 0.3 m/s and 67.7 N, the wear process wastypically characterised by the material transfer from theAZ91 alloy to the steel counterface. The transferred magne-sium formed a thin film on the counterface (Fig. 5a). In this


wear process, the typical wear debris had a shiny metallicappearance and a plate-like shape (Fig. 5b). The plate-likedebris had the same composition as the Mg-9% Al-0.9%Zn alloy (Fig. 5c), suggesting that the debris was possiblyformed as a result of detachment of the subsurface layersfrom the bulk material (delamination wear). The presenceof an oxygen peak of low intensity in the EDS spectrum ofFig. 5c implied that the surface of the debris was slightly ox-idised, probably when the freshly produced surfaces of thedetached particles became in contact with the air. No ironcould be detected. X-ray diffraction data (Fig. 5d) verifiedthe existence of MgO but again metallic magnesium fromthe base alloy was the main component of the delaminationwear debris. A similar delamination mechanism has beenobserved in aluminium alloys [10,17]. A thermo-mechanicalmodel of delamination wear has been given Wilson and Al-pas [16] The transition from oxidational wear to delamina-tion wear occurred gradually and did not have a noticeableeffect on the mild wear rates. Steady state wear conditionswere maintained throughout the mild wear regime.

3.3. Wear mechanisms operating within the severe wearregime

It was observed that two types of material removal mech-anisms operated in the severe wear regime as the slidingspeed and applied load conditions changed. At a slidingspeed of 1.0 m/s, for example, in the range of applied loadsfrom 90 to 140 N, extensive surface damage as a result ofplastic deformation of the material layers adjacent to thecontact surface was the main wear mechanism. The severelydeformed material layers extruded along the sliding direc-tion and out of the contact surface of the sample. Theselayers often reached 1–3 mm in length before fragments offlat debris were detached. Fig. 6a shows the morphology oftypical debris produced by this local extrusion process. TheEDS analysis (Fig. 6b) showed that the debris had the samecomposition as the AZ91 alloy and did not show evidencefor oxidation.

During the wear tests at the highest applied loads, i.e.above 140 N, there was evidence that the local temperatureof the contact surfaces exceeded the melting temperature ofthe alloy. The friction-induced surface melting appeared tobe confined to a depth of about 10–20mm in thickness. It ispossible that the liquid layer formed due to the local metingsupported the normal load by providing hydrodynamic lu-brication. Consequently, although the wear rates were higherthan other mechanisms, they did not increase dramaticallyas a result of surface melting. The molten material spreadout of the contact surface in the sliding direction as wellas by moving sideways. During the sliding wear, the solid-ified material formed thin layers of approximately constantthickness of 10–20mm. The new layers were continuouslygenerated over the previously formed layers as the previ-ously molten material extruded out of the contact surface.Consequently, a multilayered debris structure was generated

Fig. 6. (a) SEM micrograph of wear debris produced as a result ofextrusion of severely deformed surface layers (speed= 1.0 m/s andload = 133 N); (b) EDS spectrum of wear debris corresponding to areaA in Fig. 6a.

(Fig. 7a). These layers were ductile and curved up uponthemselves. They had the appearance of the continuous ma-chining chips and reached lengths longer than the lengthof the wear sample itself (>10 mm). According to the EDSanalysis (Fig. 7b) the multilayered debris consisted mainlyof magnesium whose surface was slightly oxidised.

3.4. Wear rate and wear transition maps

The test results presented in Sections 3.1 to 3.3 revealthat the sliding wear behaviour of the alloy AZ91 can beclassified into two main wear regimes, namely into a mildwear and a severe wear regime. In each of these regimes twosub-regimes exist: In the mild wear regime, these are oxida-tional wear and delamination wear. The sub-regimes in thesevere wear are the severe plastic deformation induced wear(surface extrusion) and melt wear. These wear mechanismscan be summarised by plotting the transition boundaries be-tween the wear regimes on the diagrams, which have theco-ordinates of applied load, and sliding speed. Fig. 8a is


Fig. 7. (a) SEM micrograph of wear debris produced as a result of frictioninduced surface melting (speed= 1.0 m/s and load= 224 N); (b) EDSspectrum of wear debris corresponding to areaA in Fig. 7a.

such a diagram (a wear rate map) that shows the wear ratesof the AZ91 alloy in dry sliding and the wear regimes. Thiswear rate map has been constructed using the wear rate datapresented in Fig. 1a–e. The metallographic observations aswell as the compositional information on the debris (Figs.4–7) were utilised to draw the lines separating the wearregimes and the sub-regimes. When reading the wear ratesit should be kept in mind that in the severe wear regime theactual scale of surface damage inflicted to the alloy duringsliding is more extensive than that depicted by the wear ratedata shown on this map. This is because the volumetric wearrate data do not take into account the effect of massive sur-face deformation and local melting during the severe wearthat produce extensive surface damage but not necessarilylarge amounts of loose debris.

Fig. 8b is the wear transition map for the AZ91 alloy. Thewear rate data is omitted but the emphasis is on the wearmechanism regimes and the transitions between them. Thetransition from mild to severe wear is indicated by the solidline BB (which is reproduced from Fig. 2). As indicated inSection 3.1, the transition boundary BB between mild andsevere wear has been established according to the visual

Fig. 8. (a) Wear rate map for AZ91 in dry sliding wear. Each point onthe map represents a wear rate measurement made at a given set of loadand sliding speed. To find the actual wear rates in mm3/m multiply eachnumber by 10−3. The continuous line represents the boundary betweenthe mild and severe wear. The transitions between the sub-regimes aremarked by dashed lines; (b) wear transition map for AZ91 showing theregion of dominance of wear mechanisms and the transition boundariesbetween them: BB: mild wear to severe wear transition; AA: transitionbetween the oxidational wear (s) and delamination (e) wear in the mildwear regime; CC: transition between severe deformation induced wear(4) and melt wear ( ).

observations of the debris during the wear tests and are notbased on a specific wear rate criteria. Below the line BBis the mild wear regime where the wear occurs by surfaceoxidation as well as by the delamination of the bulk material.In either case the wear occurs essentially under steady state


conditions. From the practical point of view it is desirablethat a component made of this alloy operates under the loadand the velocity conditions that lie below the line BB. Inthe severe wear regime wear rates are not only higher butalso steady state conditions could not be established. Thewear rates increase with the sliding distance even thoughthe applied load and velocity are constant. The increase inthe wear rates is closely associated with the increase in thecontact surface temperatures that will be considered in aseparate section (Section 3.5).

In the mild wear regime, the transition boundary betweenthe oxidational wear and the delamination wear is shownby the dashed line AA. As indicated in Section 3.2, thetransition from the oxidational wear to the delamination wearoccurred gradually, with the proportion of the delaminationdebris increasing as the sliding speed or the load increased.Quantitative SEM metallography was conducted on the weardebris produced at different speed and load combinations.The demarcation between the fields of dominance of thetwo mechanisms is assumed to occur when the proportion ofeither oxidational or delamination debris exceeded 50% ofthe total amount of debris generated. It should be mentionedhere that the transition load between these two sub-regimesdecreases with the increase of the test speed. An increase inthe sliding speed from 0.1 to 1.0 m/s reduces the transitionload from 160 to 2 N. As the speed is further increased to1.5 m/s and above, the transition becomes displaced belowthe minimum test load of 1 N. As a result no delaminationdebris could be observed at lower loads and consequently thewear process is controlled entirely by the surface oxidation.

In the severe wear regime, the transition from the severeplastic deformation induced wear to the melt wear is markedby the dashed line CC. The transition from the severe defor-mation induced wear to the melt wear was also controlledby both sliding speed and applied load. The transition loadsand speeds were determined by visual observation of thesample surfaces and debris morphologies during the courseof the wear tests. An increase in sliding speed from 1.0 to1.5 m/s resulted in a decrease of transition load from 140to 110 N. As the speed was further increased to 2.0 m/s, thesevere wear process was controlled principally by the meltwear.

3.5. Contact surface temperature and its effect on mild tosevere wear transition

The average contact surface temperature (or bulk tem-perature) of the AZ91 alloy was measured by inserting a250mm diameter chromel–alumel type thermocouple probeinside the samples. Details of the temperature measurementmethod are given in [15]. The contact surface temperatureswere measured continuously as a function of sliding distanceand the temperature measurements shown in Fig. 9 are typi-cal. The variation of the surface temperature with the slidingdistance at a sliding speed of 0.1 m/s is shown in Fig. 9a. It

can be seen that the contact surface temperature increased atfirst with sliding distance and reached a maximum, remain-ing constant thereafter. Therefore, a thermal equilibrium wasestablished between the contact surfaces (in parallel withthe steady state wear rate conditions Fig. 3a)) which is animportant aspect of the mild wear. Fig. 9a also shows that ittakes a longer time to establish a thermal equilibrium duringthe tests performed at higher loads. It is also seen that themagnitude of the steady state contact temperature increaseswith the applied load. For example at 0.1 m/s, an increase ofthe applied load from 9.8 to 222.3 N resulted in an increasein the steady contact temperature from 298 to 309 K

The effect of load on the surface temperature of the AZ9Ialloy at a higher sliding speed of 1.0 m/s is shown in Fig. 9b.It can be seen that the thermal equilibrium at the contact sur-faces was established at applied loads 9.8 and 44.5 N, underthe steady state mild wear conditions (Fig. 3b). For the testsat the applied loads of 133.4 and 222.3 N, where severe wearprevailed, thermal equilibrium could not be attained. Con-tact surface temperature of the alloy increased continuouslywith the sliding distance. An important conclusion of thesurface temperature measurements was the observation thatthe transition from mild to severe wear occurred when thesurface temperature of the AZ91 alloy reached 347± 15 K.For the tests at 1.0 m/s this condition was satisfied at 133.4and 222.3 N as soon as the sliding wear test started.

The effect of sliding speed on the surface temperatures atthe constant load tests is similar, i.e. higher contact surfacetemperatures were obtained at high test speeds during theconstant load test. The effect of sliding speed on the contactsurface temperature can bee seen in Fig. 9c and d for thetests performed at two constant loads of 9.8 and 133.4 N,respectively. For the tests at 9.8 N, the contact surface tem-peratures of the AZ91 alloy do not reach the critical transi-tion temperature of 347 K, so that the conditions favour themild wear. At 133.4 N and speeds of 0.8 m/s or higher how-ever, severe wear transition occurred within the first fewmeters of the sliding wear tests, when the surface tempera-ture exceeded 347 K. The combined effects of the load andspeed on the contact surface temperatures of the AZ9l alloyworn against the AISI 52100 steel are summarised in Fig.9e, where the 347 K isotherm demarcating the boundarybetween the mild and the severe wear regimes is shown.

A regression analysis performed on the surface tem-perature data presented in Fig. 9a–d, indicated that therelationship between the highest temperatureTmax obtainedat any given condition (either the steady state temperaturesattained or the highest temperature measured just before theseizure) of the applied load (F) and the sliding speed (V)can be expressed as follows

Tmax = T0 + aF1/4V 1/2 (1)

whereT0 = 290 K anda = 15.1 for (F l/4V 1/2) < 2.8 andT0 = −191 anda = 210.5, for (F 1/4V 1/2) > 2.8.

Fig. 10 shows the maximum surface temperaturesattained as a function of the parameter (F1/4Vl/2). The con-


Fig. 9. (a) Variation of surface temperature as a function of sliding distance at different applied load levels and at a constant speed of 0.1 m/s; (b)variation of surface temperature as a function of sliding distance at different applied load levels and at a constant speed of 1.0 m/s; (c) variation ofsurfacetemperature as a function of sliding distance at different sliding speeds and at a constant load of 9.8 N; (d) variation of surface temperature as a functionof sliding distance at different sliding speeds and at a constant load of 133.4 N; (e) surface temperatures as a function of applied load and sliding speed.The continuous line indicates the transition from mild to severe wear and can be considered as an isotherm for 347 K. The temperatures in the mild wearcorrespond to steady state conditions, in the severe wear, where steady state conditions are not established they represent the maximum temperatures.


Fig. 10. Comparison of predicted and experimentally measured contacttemperatures. The measured temperatures are marked by the symbol (s).The two continuous lines are the temperatures calculated using Eq. (6).The slope change corresponds to mid to severe wear transition.

tinuous line indicates the maximum attainable temperaturespredicted by the Eq. (1). The measured temperatures arerepresented by open circles and obey very well the Eq. (1).The important point arising from the Fig. 10 is that theslope change in the contact surface temperature data coin-cides with the mild to severe wear transition and that thistransition can be predicted using the Eq. (1).

4. Summary and conclusions

1. Dry sliding wear tests were conducted on an AZ91 mag-nesium alloy against an AISI 52100 bearing steel using ablock on ring geometry. Volumetric wear rates were mea-sured as a function of applied load and sliding velocityand presented in a wear rate map.

2. The sliding wear behaviour of the AZ91 alloy can beclassified into two main wear regimes, namely into amild wear regime and a severe wear regime. Mild wearprogressed under steady state (i.e. constant wear rate)conditions. In the severe wear regime the rate of wearincreased continuously with the sliding distance.

3. In the mild wear regime, two sub-regimes (namelyan oxidational wear and a delamination wear regime)have been observed. Unlike the transition from mild to

severe wear normally accompanied by an abrupt wearrate change, this transition occurred gradually and didnot have a noticeable effect on the mild wear rates. Inthe severe wear regime, similarly two other sub-regimes(severe plastic deformation induced wear and melt wearregimes) have been identified. The wear mechanisms ineach wear regime were summarised in the wear mech-anism map that also delineates the loads and speeds atwhich wear transitions occur.

4. The contact surfaces reached a thermal equilibrium dur-ing mild wear which produced steady state wear rates.Wear rates and contacts temperatures increased with thesliding distance during severe wear. The contact surfacetemperature was found to be a function of (F1/4V1/2).The transition to severe wear was controlled by a crit-ical surface temperature criterion, rather than a load orsliding speed criterion. Severe wear occurred when thesurface contact temperature of the AZ91 alloy exceeded347 K.

Acknowledgements

Financial support from the Natural Sciences and Engi-neering Research Council of Canada (NSERC) is gratefullyacknowledged.

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Sliding wear map for the magnesium alloy Mg9Al0.9 Zn (AZ91

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Transcript of Sliding wear map for the magnesium alloy Mg9Al0.9 Zn (AZ91