Scratch resistance of Al/SiC metal/ceramic nanolaminates
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Scratch resistance of Al/SiC metal/ceramic nanolaminates Danny R.P. Singh and Nikhilesh Chawla a) Materials Science and Engineering School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287-6106 (Received 6 May 2011; accepted 4 August 2011) Al/SiC nanolaminates have been shown to possess excellent combination of mechanical strength and flexibility. While metal–ceramic multilayers present a tremendous opportunity for hard coatings, the strength evaluation is usually carried out under static loading conditions such as nanoindentation and microcompression testing. In this study, we have studied the scratch resistance behavior of Al/SiC nanolaminates. These properties are then compared to monolithic films of Al and SiC. Finally, the deformation behavior under such loading was quantified by critical load, work of deformation, and postexperimental microstructural analysis by scanning electron microscopy and focused ion beam cross sections. It is shown that the combination of hard SiC and plastic Al layers provides enhanced resistance to scratch loading and makes these materials as very good candidates for wear-resistant coatings. I. INTRODUCTION Multilayered nanoscale materials have been shown to exhibit unique electrical, 1–3 magnetic, 4 optical, 5,6 and mechanical properties. 7–9 In many of these applications, these materials are subjected to a variety of dynamic loading situations 10 such as wear and abrasion that lead to degradation and failure. 11 Metal–ceramic nanolaminates have emerged as promising candidates as hard and tough coatings. 12–17 These materials exhibit a good combina- tion of strength, hardness, and toughness. 8,9,18–22 To date, an understanding of deformation in these materials has been limited to static loading such as nanoindentation 23,24 and micropillar compression. 25,26 Although such methods provide a good fundamental understanding of the me- chanical response and deformation mechanisms in these materials, a direct understanding of their deformation under wear and sliding conditions has not really been examined. Scratch testing has been used recently to probe the tribological properties of multilayered materials. 14–17 Tribological behavior on the small scale has become increasingly important with the advent of micro- and nanomechanical devices. During scratch testing, both normal and tangential forces are applied during dynamic loading. Usually, an indenter tip is moved across the sample surface with increasing normal load. As the load increases, various fracture processes can occur such as cracking, delamination, coating spalling, and buckl- ing. 27,28 The load at the onset of the fracture process is defined as the critical load. 27,29 Although the critical load is dependent on test conditions such as indenter geometry and loading rate, 27 it can be used as a good comparative tool for the evaluation of different materials tested under similar conditions. 30 In addition, scratch deformation can be quantified by of postscratch microstructural charact- erization. 10,14,30–32 In this study, we have studied the scratch resistance of Al/SiC metal–ceramic nanolaminates and compared their behavior to that of monolithic Al and SiC films. The critical load and work of deformation were quantified. Postscratch fracture analysis was carried out using sec- tions by focused ion beam (FIB) microscopy, as well as surface topography measurements from the diamond indenter itself. II. MATERIALS AND EXPERIMENTAL PROCEDURE The following three materials were compared in this study: Al/SiC multilayer (ML), Al thin film, and SiC thin film. Each of the samples was about 1-lm thick and grown on a Si (111) substrate using magnetron sputtering, as described elsewhere. 19 The Al and SiC layers were each about 50- to 60-nm thick, with a total of 21 layers. Scratch testing was performed using a commercially available nanoindenter (XP; Agilent, Chandler, AZ). Multiple scratches were performed in each of the samples under load control. A constant loading rate of 2.5 mN/s and a sliding velocity of 10 lm/s were used for all samples. Scratch analysis was carried out with the Berkovitch indenter face moving through the surface, i.e., the face of the indenter was perpendicular to the scratch direction. After scratch testing, the scratches were analyzed using scanning electron microscopy (SEM) to analyze the scratch profile and to identify the point where film failure a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.274 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012 Ó Materials Research Society 2011 278
Transcript of Scratch resistance of Al/SiC metal/ceramic nanolaminates
Scratch resistance of Al/SiC metal/ceramic nanolaminates
Danny R.P. Singh and Nikhilesh Chawlaa)
Materials Science and Engineering School for Engineering of Matter, Transport, and Energy,Arizona State University, Tempe, Arizona 85287-6106
(Received 6 May 2011; accepted 4 August 2011)
Al/SiC nanolaminates have been shown to possess excellent combination of mechanical strengthand flexibility. While metal–ceramic multilayers present a tremendous opportunity for hardcoatings, the strength evaluation is usually carried out under static loading conditions such asnanoindentation and microcompression testing. In this study, we have studied the scratch resistancebehavior of Al/SiC nanolaminates. These properties are then compared to monolithic films of Aland SiC. Finally, the deformation behavior under such loading was quantified by critical load, workof deformation, and postexperimental microstructural analysis by scanning electron microscopyand focused ion beam cross sections. It is shown that the combination of hard SiC and plasticAl layers provides enhanced resistance to scratch loading and makes these materials as very goodcandidates for wear-resistant coatings.
I. INTRODUCTION
Multilayered nanoscale materials have been shown toexhibit unique electrical,1–3 magnetic,4 optical,5,6 andmechanical properties.7–9 In many of these applications,these materials are subjected to a variety of dynamicloading situations10 such as wear and abrasion that lead todegradation and failure.11 Metal–ceramic nanolaminateshave emerged as promising candidates as hard and toughcoatings.12–17 These materials exhibit a good combina-tion of strength, hardness, and toughness.8,9,18–22 To date,an understanding of deformation in these materials hasbeen limited to static loading such as nanoindentation23,24
and micropillar compression.25,26 Although such methodsprovide a good fundamental understanding of the me-chanical response and deformation mechanisms in thesematerials, a direct understanding of their deformationunder wear and sliding conditions has not really beenexamined.
Scratch testing has been used recently to probe thetribological properties of multilayered materials.14–17
Tribological behavior on the small scale has becomeincreasingly important with the advent of micro- andnanomechanical devices. During scratch testing, bothnormal and tangential forces are applied during dynamicloading. Usually, an indenter tip is moved across thesample surface with increasing normal load. As theload increases, various fracture processes can occur suchas cracking, delamination, coating spalling, and buckl-ing.27,28 The load at the onset of the fracture process isdefined as the critical load.27,29 Although the critical load
is dependent on test conditions such as indenter geometryand loading rate,27 it can be used as a good comparativetool for the evaluation of different materials tested undersimilar conditions.30 In addition, scratch deformation canbe quantified by of postscratch microstructural charact-erization.10,14,30–32
In this study, we have studied the scratch resistance ofAl/SiC metal–ceramic nanolaminates and compared theirbehavior to that of monolithic Al and SiC films. Thecritical load and work of deformation were quantified.Postscratch fracture analysis was carried out using sec-tions by focused ion beam (FIB) microscopy, as well assurface topography measurements from the diamondindenter itself.
II. MATERIALS AND EXPERIMENTALPROCEDURE
The following three materials were compared in thisstudy: Al/SiC multilayer (ML), Al thin film, and SiC thinfilm. Each of the samples was about 1-lm thick andgrown on a Si (111) substrate using magnetron sputtering,as described elsewhere.19 The Al and SiC layers were eachabout 50- to 60-nm thick, with a total of 21 layers. Scratchtesting was performed using a commercially availablenanoindenter (XP; Agilent, Chandler, AZ). Multiplescratches were performed in each of the samples underload control. A constant loading rate of 2.5 mN/s anda sliding velocity of 10 lm/s were used for all samples.Scratch analysis was carried out with the Berkovitchindenter face moving through the surface, i.e., the faceof the indenter was perpendicular to the scratch direction.After scratch testing, the scratches were analyzed usingscanning electron microscopy (SEM) to analyze thescratch profile and to identify the point where film failure
a)Address all correspondence to this author.e-mail: [email protected]
DOI: 10.1557/jmr.2011.274
J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012 �Materials Research Society 2011278
took place. The deformation under the scratch surfacewas analyzed by cross-sectional analysis using a dualbeam FIB.
III. RESULTS AND DISCUSSION
The microstructure of the Al/SiC nanolaminate isshown in Fig. 1. A total of 21 alternating layers of Aland SiC were grown on a Si (111) substrate. The Al
thickness was 556 4 nm, and SiC layers were 616 2 nmthick. This corresponds to a volume fraction of Al of about48%. A small degree of roughness is associated with theindividual layers and is attributed to the columnar grainstructure of the Al layers.
A schematic for typical scratch experiments is shownin Fig. 2. In the first segment, the reference surface of thesample is quantified by rastering the indenter over thesurface at a very small load (100 lN). This is followed bya load-ramp scratch segment performed near the profilingtrace. After the scratch segment, the indenter traces backthe scratch length with a small load (100 lN) to estimatethe residual deformation in the scratch. This measures thepermanent deformation at any point in the scratch. Finally,in the fourth segment, a cross profile is carried outperpendicular to the scratch direction (100 lN) to obtainthe width and profile of the scratch track. The cross profilewas carried out on the scratch at a depth before the criticalfracture event.
Figures 3–5 show representative scratch depth andapplied load versus scratch distance for Al, SiC, and thenanolaminate. A cross profile for each of the representativescratches is also shown. Figure 3 shows the scratch andrecovered profile for the Al film. Very little elasticrecovery takes place and majority of the deformation isplastic. The critical load occurs at a load of about 125 mN.In the case of Al, this critical load represents the point atwhich the diamond indenter has penetrated through thefilm and makes contact with the Si substrate. Figure 4shows a similar comparison for SiC. The film showssignificant elastic recovery after scratch. Delamination isobserved at relatively low critical loads, and the filmdebonds from the substrate easily. The scratch in thenanolaminate exhibits beneficial properties of the plasticand elastic layers, as shown in Fig. 5. A significant amountof elastic recovery takes place due to the SiC layers. Theoverall scratch depth is between that of pure SiC and Al.
A significant amount of quantitative information canbe obtained from the scratch experiments. These include
FIG. 1. (a) Microstructure of Al/SiC nanolaminate on a Si (111)substrate (b) layer thickness distribution for Al and SiC.
FIG. 2. Schematic of scratch testing procedure used in this study.
FIG. 3. (a) Scratch test on Al film showing the scratch penetration and residual morphology. The critical load corresponds to onset of film fracture.(b) Cross profile of scratch taken at a precritical scratch load of 10 mN. Significant pile-up is observed.
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the critical load and displacement for failure (e.g., due toblistering or delamination), the work of elastic defor-mation, work of plastic deformation, width of scratch,and penetration depth of the scratch. It should be notedthat careful postexperiment characterization of the scratchsurface needs to be conducted to obtain a valid correla-tion to features on the depth/load versus scratch distancecurves.
The critical loads were determined as the first discon-tinuity in the depth versus scratch distance curves.Average values of the critical load, for the three materialstested, are shown in Fig. 6. The nanolaminate materialsexhibited higher critical loads than either Al or SiC. Thenanolaminate benefits from strong and stiff SiC layerswhich are sandwiched between the highly plastic Allayers. This unique combination has also been shown tobe quite flexible.19 The elastic and plastic work done onthe samples during scratch, prior to film failure (criticalload), was also quantified by measuring the area under theload versus displacement, as shown in Fig. 10. As canbe seen from the graph, the work done for the ML is inbetween that of Al and SiC. Even with a high content ofSiC, the nanolaminate can accommodate deformation byplastic deformation of the Al phase before delamination
occurs. This gives the nanolaminate added scratch re-sistance over the monolithic SiC films. Also, unlike the Alfilm, the scratch does not penetrate through the entire filmthickness, as shown in Fig. 7. All of these factorscontribute to the improved scratch resistance of the ML.The scratch width and depth, Fig. 8, are also compared
FIG. 4. (a) Scratch test on SiC thin films showing the scratch penetration and residual morphology. (b) Cross profile of scratch taken at a precriticalscratch load of 10 mN.
FIG. 5. (a) Scratch test on Al/SiC nanolaminate showing the scratch penetration and residual morphology, with significant amount of elasticrecovery. (b) Cross profile of scratch taken at a precritical scratch load of 10 mN.
FIG. 6. Comparison of critical load for the various samples. TheAl/SiC multilayer exhibits the highest critical load.
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below critical load (10 mN) as indicators of scratchresistance. The composite response of the nanolaminateis observed here as well, as the scratch width andpenetration are between that of Al and SiC.
Postdeformation analysis was conducted by surfaceexamination by SEM, as well as cross sections made byFIB. Representative surface images and cross sections forAl, SiC, and nanolaminate are shown in Figs. 9–11. In Al,Fig. 9, no obvious delamination of the film takes placealthough significant macroscopic plastic deformationtakes place. Deformation proceeds by ploughing thro-ugh and dragging of material.33 The critical load takesplace a few hundred nanometers before hitting the Sisubstrate. The cross profile indicates that significant pile-up occurs on both sides of the scratch. No significantdelamination was observed in Al films. In SiC, failure
appears to be catastrophic as large chunks of the film fractureand are removed from the substrate, as shown in Fig. 10. Asthe loading further progresses, film chipping is also observedat higher loads. Higher elastic recovery and low pile-up wasobserved in the hard SiC films. The scratched surface andcross section in the nanolaminate is shown in Fig. 11. At andbeyond the critical point, film blistering is observed which isrelated to the deformation of the delaminated film. At higherloads, the center of the scratch path remains bonded to thesubstrate implying a strong compressive stress, whereas theouter edges experience a combined local tensile/shear loadthat leads to film delamination. Besides film delamination,fracture of the individual layers and plastic deformation ofAl were also observed.
IV. SUMMARY
In this study, we have evaluated the mechanicalresponse of metal–ceramic nanolaminates under scratchloading conditions. Al/SiC nanolaminate possesses betterscratch resistance than its monolithic SiC and Al counter-parts. Although Al thin films are too soft and the indenterpenetrates through the film at small loads, SiC tends todelaminate from the Si substrate due to poor adhesion. TheML structure provides a compromise between the twomaterials by having a good interface and providing bothstrength and toughness against dynamic loading. Criticalloads for fracture are highest for the multilayered structure.
FIG. 8. Comparison of scratch resistance of Al, SiC, and the nano-laminate in terms of scratch width and scratch penetration measured ata scratch load of 10 mN.
FIG. 9. Postscratch morphology of scratches in Al thin film imagedusing scanning electron microscopy (SEM).
FIG. 10. Postscratch morphology of scratches in SiC 1000-nm thinfilm with imaged using SEM. Clear delamination is visible in earlystages of the scratch process.
FIG. 7. Comparison elastic work and plastic work of deformation forAl, SiC, and Al/SiC nanolaminate, obtained from the load–depthbehavior during scratch testing. Note the high amount of recoverablework in the nanolaminate.
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It can be concluded that metal–ceramic nanolaminatespresent significant potential to produce tough coatings forwear applications.
ACKNOWLEDGMENTS
The authors are grateful for financial support forthis research from the National Science Foundation(DMR-0504781, Drs. A. Ardell, H.D. Chopra, and B.A.MacDonald, Program Managers). We also acknowledgethe use of characterization facilities at the Center forSolid State Science at Arizona State University.
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