Tribological Properties of Magnesium Composite Alloy

with In-situ Synthesized Mg2Si Dispersoids

Katsuyoshi Kondoh1, Hideki Oginuma2 and Tatsuhiko Aizawa3

1Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan2Faculty of Engineering, Musashi Institute of Technology, Tokyo 158-8557, Japan3Center for Collaborative Research, The University of Tokyo, Tokyo 153-8904, Japan

The tribological properties of the magnesium composite alloys reinforced with solid-state synthesized Mg2Si or Mg2Si/MgO dispersoidsare evaluated under wet conditions (in engine oil lubricants) by using pin on disc type wear test equipment. Every composite shows adependence of the friction coefficient on the applied load, which corresponds to Sribeck diagram based on elasto-hydrodynamic lubrication(EHL). The increase of Mg2Si content causes the increase of both the friction coefficient and the total wear by plowing, because Mg2Sidispersoids protruded from the surface are much harder than S35C mild steel counter material. When including MgO dispersoids formed indeoxidizing SiO2 particles by magnesium, the friction coefficient reduces to 0.01–0.02 in oil lubricant. This is due to the ‘‘mild offensive effect’’by MgO dispersoids which ease the attacking or plowing phenomenon on the counter material, because they are not so hard.

(Received November 13, 2002; Accepted February 10, 2003)

Keywords: magnesium, silicon, SiO2, Mg2Si, MgO, composite, tribology, abrasive, synthesis

1. Introduction

Magnesium alloys are effective on the weight reduction ofthe components because of its lower density than otherindustrial metals such as aluminum and titanium alloys. Inparticular, from a viewpoint in saving the energy consump-tion, the application of the lightweight materials to somesliding or moving components is very favorable because theycontribute to the reduction of the inertia. However, themagnesium alloys generally are poor for the wear resistance,and easily cause the sticking or seizure phenomenon with thecounter materials. For the improvement of the wear resis-tance and tribological properties, the surface treatments orsome coatings on the magnesium alloys are employed, suchas the anodizing treatment, diamond like carbon (DLC) andNi–P coatings, etc. There are some issues to supply thesetechnologies; for example, a bonding strength at the interfaceis poor due to the surface oxide layer of the magnesium(MgO) and the economical problem by the additionallyexpensive cost. One of the favorable material designs toimprove the mechanical, physical and tribological propertiesis the use of metal matrix composite (MMC). Somecharacteristics of dispersoids, such as particle size, distribu-tion uniformity, properties, bonding strength with matrix, andcost, must be considered into the material and processdesigns for MMC. Magnesium silicide (Mg2Si) synthesizedfrom the elemental magnesium and silicon mixture powdervia solid-state reaction is available for the dispersoids,because it shows low density of 1.91 kg/m3, high meltingpoint of 1358 K, 120 GPa Young’s modulus, high microVicker’s hardness of 600–700 Hv, and a low coefficient ofthermal expansion of 7:5� 10�6 K�1.1,2) Also it has a goodcoherence with the magnesium matrix.3) In the previousworks, the magnesium matrix composites reinforced withsynthesized Mg2Si has been fabricated from the elementalmagnesium alloy and silicon mixture powder.4) In employingsilica (SiO2) particles instead of silicon, the magnesiumcomposite with Mg2Si and MgO dispersoids on route of

deoxidization and reaction process in solid-state.5)

In this study, the tribological properties of these magne-sium composite alloys are evaluated under wet conditions,that is, wear test is carried out in the oil lubricants. The effectsof the dispersoids on the friction coefficient and wearbehavior, in particular, the ‘‘mild offensive effect’’ by MgOparticles to reduce the abrasive wear phenomenon arediscussed.

2. Experimental Procedure

In-situ solid-state synthesis and hot forging are employedto fabricate magnesium composite alloys with Mg2Sidispersoids as shown in Fig. 1. Pure magnesium (Mg)powder, having a mean particle size of 111 mm and 99.9%purity, is used as a matrix raw material. Silicon (Si) and silica(SiO2) powder with that of 22 mm and 21 mm, respectively,

Raw powder

Mg powder (mean particle size; 111.5µm, Purity; 99.9%)Si powder (mean particle size; 22.3µm, Purity; 99.99%)SiO2 powder (mean particle size; 21µm, Purity; 98.5%)

Pre-mixing

Mg-2.5, 5, 10mass%Si, Mg-6, 8mass%SiO2

Cold compaction

Pressure; 600MPa, Die diameter; 34mmφ

Pre-heating

853K for 240s (N2 gas), Heating rate; 1K/s

Hot forging

Pressure; 800MPa, Die diameter; 35mmφDie temperature; 523K

Evaluation

“Pin on Disc” type wear test under wet conditions

Fig. 1 Flowchart for fabrication of magnesium matrix composite rein-

forced with Mg2Si or Mg2Si/MgO dispersoids via solid-state synthesis.

Materials Transactions, Vol. 44, No. 4 (2003) pp. 524 to 530Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, II#2003 The Japan Institute of Metals

are employed for the in-situ formation of Mg2Si or Mg2Si/MgO via solid-state reaction and/or deoxidization. Theseelemental mixture powder, having the silicon content of 2.5and 5 mass% and silica content of 3, 6 and 8 mass%, areconsolidated in the die with a diameter of �34mm by theconventionally cold compacting at 600 MPa. The solid-statesynthesis to form Mg2Si or Mg2Si/MgO dispersoids occursduring pre-heating each green compact at 853 K for 240 s innitrogen gas atmosphere. The pre-heating temperature isdecided on the DSC thermograms with the exothermic heatdue to the above reaction. After pre-heating the greencompact, it immediately is consolidated into the full dense ina heated die at 523 K, which diameter is �35mm, by hotforging at 800 MPa. The hot forged magnesium compositesare machined to fabricate ‘‘Pin type’’ wear test specimenswith a diameter and length of �7:8mm and 20 mm,respectively. The roughness of the sliding surface of thepolished specimen is less than 1 mm in Ra scale, provided inJapanese Industrial Standards (JIS) B0601-1994.

Figure 2 shows an appearance of the ‘‘Pin on Disc’’ typewear test equipment used in this study (a), and the illustrationof the installation of pin and disc specimens in this wear test(b). The load and sliding speed are controlled by a personalcomputer. A friction coefficient between the pin and discspecimens is calculated automatically by using the frictionforce monitored by a torque sensor. The change in thedisplacement, corresponding to the total wear of the pin anddisc specimens, is also monitored in process. The countermaterial is S35C mild steel and its surface roughness is alsoless than 1 mm in Ra scale. The wear test is carried out underthe wet condition, that is, the specimens are sunk in theengine oil lubricant (SJ/GF-II 10W30) with the dynamicviscosity over 4.1 mms�1 at 373 K. The sliding speed isconstant of 1.0 m/s, however the step-wise load with a decentof 1 N/s is applied up to 500 N as shown in Fig. 2(c).

3. Results and Discussion

3.1 Microstructure analysisFigure 3 shows X-ray diffraction (XRD) patterns of the hot

forged magnesium composite alloys, in employing the rawmixture powder of Mg–5 mass%Si (a) and Mg–8 mass%SiO2(b). No peak of Si and SiO2 is detected of the composite, thatis, Si and SiO2 raw materials are completely reacted withmagnesium powder to synthesize Mg2Si and Mg2Si/MgO viasolid-state reaction. Figure 4 reveals optical microstructuresof the hot forged magnesium composite with Mg2Si/MgOdispersoids (a), employing Mg–8 mass%SiO2 powder mix-ture, and that with Mg2Si dispersoids (b) in using Mg–5 mass%Si mixture. A relative density of each composite isabout 97–98%. As shown in Fig. 4(a), Mg2Si and MgOcompounds are distributed uniformly at the primary particleboundaries (PPB) of the magnesium matrix. The particle sizeof Mg2Si and MgO dispersoids with 23 mm and 18 mm,respectively, is almost same as that of SiO2 raw particlebecause of the no coarsening of the dispersoids on route ofthe solid-state synthesis from the elemental magnesium andSiO2 mixture powder. In employing 8 mass% SiO2 powder asthe raw material, the theoretical volume fraction of thesynthesized Mg2Si and MgO particles is 10.0% and 5.7%,respectively.

3.2 Tribology property of Mg2Si/Mg composite underwet condition

Figure 5 shows the dependence of the friction coefficient(�) on the applied load of the magnesium composite with in-situ synthesized Mg2Si compounds. At the initial stage inapplying small load until about 250 N, every magnesiumcomposite indicates the decrease of � value, however itincreases again with increase in the load. This corresponds toStribeck diagram, which means the � value is proportional to

Fig. 2 Appearance of pin on disc type wear test equipment (a), schematic illustrations of setting test specimens (b) and pattern of applied

load (c).

Tribological Properties of Magnesium Composite Alloy with In-situ Synthesized Mg2Si Dispersoids 525

both the viscosity of oil lubricants and the inverse of appliedload, on the tribological behavior under wet conditions. Thereduction of � value at the initial stage is due to that the thickoil films exist between the pin and disc specimens. Thefriction torque also strongly depends on the force to breakthem, in other word, the viscosity of the oil lubricant. Inincreasing the applied load, the oil lubricant is removedbetween the contacting surfaces, and the thin thickness of the

oil film reduces the friction torque. That is, elasto-hydro-dynamic lubrication (EHL) dominates the friction mechan-ism at the initial stage in applying a small load. On the otherhand, when the applied load is over 250–400 MPa, eachfriction coefficient increases again. This is because themechanical contact between both specimens occurs after thebreakage or removal of the oil films. In particular, themagnesium composite including the silicon content of

Fig. 4 Optical microstructures of magnesium composite with Mg2Si/MgO dispersoids in employing Mg–8 mass%SiO2 powder mixture

(a), and that with Mg2Si dispersoids in using Mg–5 mass%Si mixture (b).

Fig. 3 XRD patterns of magnesium composite with Mg2Si or Mg2Si/MgO dispersoids via solid-state synthesis, in employing Mg–

5 mass%Si (a) and Mg–8 mass%SiO2 (b) powder mixtures.

526 K. Kondoh, H. Oginuma and T. Aizawa

10 mass% reveals the smaller load to increase the frictioncoefficient; for example at 250 N. It means that the breakageof the oil lubricant films easily occurs compared to the othercomposites. That is, Mg2Si dispersoids protruded from thesliding surface of the pin specimen seem to effect on thebreakage of the oil films. Furthermore, Fig. 6 indicates achanges in the � value at 500 N loading as a function of the

silicon content of the raw powder mixture. It increasesproportionally to the Mg2Si content. In general, there are twofactors to increase the � value; one is an abrasive wear due tothe attacking or plowing by he hard particle and another is anadhesive wear due to sticking or seizure phenomenon. Theobservation on the damages of the sliding surface is availableto discuss what the dominant factor is, that is, to clarify thewear mechanism. Figure 7 shows the optical observationresults on the damaged surface of each specimen after weartest. The sliding surface of every pin specimen shows a slightwear tracks by an abrasive wear. However, the damages ofthe disc specimens are quite different each other. It is clearthat the damaged area (attacked tracks by pin specimens) dueto a plowing wear increases with increase in the siliconcontent. Furthermore, as shown in Fig. 8, the in-situmonitored total wear also increases in increasing the siliconcontent. That is, the attacking or plowing by the Mg2Sidispersoids protruded from the pin specimen surface in-creases the wear of the counter materials. Considering theseresults, Fig. 9 illustrates the wear mechanism in using themagnesium composite with Mg2Si dipersoids under wetcondition. The increase of the friction coefficient at large loadis caused by the abrasive wear due to the plowing by Mg2Sidispersoids of the pin specimen under EHL conditionbecause the dispersoid, having micro-hardness of about600–700 Hv, is much harder than the S35C mild steel countermaterial with that of 100–120 Hv.

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Fig. 5 Changes in friction coefficient as a function of applied load; Si content of 0 mass% (a), 2.5 mass% (b), 5 mass% (c)

and 10 mass% (d).

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Tribological Properties of Magnesium Composite Alloy with In-situ Synthesized Mg2Si Dispersoids 527

3.3 Friction material design by ‘‘Mild offensive’’ effectFor the improvement of the abrasive wear phenomenon by

Mg2Si hard dispersoids of the magnesium composite, it isnecessary to not only supply the wear resistance to thecomposite, but also obstruct the offensive by the dispersoids.In the friction materials used for the break pads ofautomobiles or motorcycles, the oxide particles, such asCaO, Al2O3, MgO, etc., are mixed into the materials toimprove the abrasive or attacking wear of the disc rotor bythe pads, because they are not so hard compared to the rotormaterials. That is, ‘‘mild offensive effect’’ by the oxideparticles dispersed in the matrix.6–8) The same materialdesign is also applied to this magnesium composite material

with Mg2Si dispersoids. Concretely, MgO fine particles,which are produced via the deoxidizing process of SiO2 rawpowder by magnesium, are uniformly dispersed in the matrixof the magnesium composite. They have a good coherencewith the matrix as shown in Fig. 4, that is, a strong bondingstrength enough to be not removed from the matrix inapplying large load.

Figure 10 shows changes in the friction coefficient as afunction of the applied load, in employing Mg–6 mass%SiO2(a) and Mg–8 mass%SiO2 (b) mixture powder as rawmaterials to fabricate the magnesium composite reinforcedwith synthesized Mg2Si/MgO dispersoids. Compared to themagnesium composite including only Mg2Si shown in Fig. 5,they indicate remarkably low friction coefficient. Whenloading over 300 MPa, the � value does not increase again,that is, the pin specimens are not offensive to the countermaterials. Figure 11 shows the dependence of frictioncoefficient on the volume fraction of Mg2Si dispersoids ofthe magnesium composites, compared to the Mg2Si/Mgcomposite. When including the same content of Mg2Si, the

Fig. 7 Optical observations on sliding surfaces of pin specimens (magnesium composite with Mg2Si) and counter materials (S35C mild

steel); Si content of 0 mass% (a), 5 mass% (b) and 10 mass% (c). (Upper; pin specimens, Lower; Disc specimens)

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Counter material(S35C mild steel)

Magnesium composite

Mg2Si

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Oil film

Fig. 9 Schematic illustration of wear behavior between magnesium

composite with Mg2Si dispersoids under wet condition.

528 K. Kondoh, H. Oginuma and T. Aizawa

composite with both Mg2Si and MgO compounds revealsextremely lower � value than the composite without MgO.As shown in Fig. 12, there are a few damaged areas with avery slight wear tracks even including 7–10% volumefraction of Mg2Si, compared to the composites reinforcedwith Mg2Si dispersoids shown in Fig. 7. These resultssuggest that MgO dispersoids are much effective to improvethe attacking or abrasive properties of the magnesiumcomposites including Mg2Si hard reinforcements becauseof their mild offensive properties on the counter part material.

4. Conclusions

Tribological properties of the magnesium compositesreinforced with Mg2Si or Mg2Si/MgO dispersoids on routeof the solid-state synthesis are evaluated by using pin on discwear test under wet conditions. When increasing the contentof Mg2Si compounds of the composite, the friction coeffi-cient and total wear increase proportionally, because theMg2Si dispersoids, which are much harder than S35C mild

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6 mass% (Vf ¼ 7%) (a) and 8 mass% (Vf ¼ 10%) (b).

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Fig. 11 Dependence of mean friction coefficient at 500 N of magnesium

composite with Mg2Si/MgO dispersoids on Mg2Si content, compared to

that with only Mg2Si.

Fig. 12 Optical observations on sliding surfaces of S35C mild steel counter part, in employing magnesium composite with Mg2Si/MgO

dispersoids with volume fraction of 7% (a) and 10% (b).

Tribological Properties of Magnesium Composite Alloy with In-situ Synthesized Mg2Si Dispersoids 529

steel, attack the surface of the counter material. The materialdesign by ‘‘mild offensive effect’’ due to the oxidedispersoids such as MgO is suggested to improve theattacking properties. The magnesium composite with Mg2Siand MgO is fabricated from the elemental magnesium andSiO2 mixture powder on route of deoxidizing and reactionprocess. It shows a lower friction coefficient than thatincluding only Mg2Si dispersoids; for example the � value isabout 0.01–0.02 under the oil lubrication. The countermaterial also indicates very slight damaged tracks on itssliding surface.

Acknowledgments

Authors would like to express their great gratitude to Mr.Hiroshi Muramatsu, graduate student of Musashi Institute ofTechnology for experimental help. This study was financiallysupported by the matching foundation project to create

university based businesses from the METI and by the project‘‘Development of New Magnesium Alloys for WeightReduction of Social Welfare’’ from Kanagawa AcademyScience and Technology (KAST).

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530 K. Kondoh, H. Oginuma and T. Aizawa