Amorphous SiC coatings for WC cutting tools

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  • Surface and Coatings Technology 163164 (2003) 176180

    0257-8972/03/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0257-897202.00486-3

    Amorphous SiC coatings for WC cutting toolsA.K. Costa, S.S. Camargo*

    Engenharia Metalurgica e de Materiais, Universidade Federal do Rio de Janeiro, Cx. Postal 68505, Rio de Janeiro, RJ, CEP 21945-970,Brazil

    Abstract

    In this work, SiC films were deposited by r.f. magnetron sputtering onto WC and silicon substrates from a commercial sinteredSiC target. After determining the influence of deposition parameters on the properties of the films deposited onto silicon substrates,suitable conditions were chosen to produce high quality 5 mm thick films on WC pieces. Mechanical characterization of the filmswas done by microhardness and residual stress measurements. High deposition rates (up to 40 nmymin), relatively low compressiveresidual stresses (-2 GPa) and high hardness (30 GPa) are obtained at high power levels (400 W), although a super hardmaterial (040 GPa) could be achieved at lower r.f. power (100 W). A ballcrater apparatus was used to perform both thicknessmeasurements and wear resistance tests. The wear rates of the coated pieces were found to be reduced to less than half of theuncoated ones. Coated and uncoated pieces were submitted to vacuum and atmospheric thermal annealing. SiC coatings presentedexcellent thermal stability as no reduction of hardness was observed at temperatures up to 1100 8C. Annealing in air showed thatSiC-coated WC pieces remained unaffected even at temperatures up to 700 8C. 2002 Elsevier Science B.V. All rights reserved.

    Keywords: Plasma enhanced chemical vapor deposition; Magnetron sputtering; SiC coatings

    1. Introduction

    Coatings based on hard materials are especially suit-able for the purposes of protection against metallurgicaltools wear. In cutting processes the association of prop-erties involving high hardness and thermal stability withlow wear rates and friction coefficient is the main goalto achieve. Silicon carbide is a material that presentsthese features, and therefore could be chosen as a basefor developing suitable tool coatings.Thin films of silicon carbide and siliconcarbon

    alloys are of great scientific and technological interestsince these materials present an outstanding set ofproperties like good mechanical resistance w1x, highhardness w2x and very high thermal stability w1,3,4x.Their applications may range from protective coatingsagainst corrosion of steel w5,6x to microelectronicdevices w7x and from X ray mask materials w8x toprotection of thermonuclear reactor walls w9x, among

    *Corresponding author. Tel.: q55-21-25628516; fax: q55-21-22906626.

    E-mail address: camargo@metalmat.ufrj.br (S.S. Camargo).

    others. These films can be deposited by a variety oftechniques such as laser assisted deposition w10x, dynam-ic ion mixing w11x, plasma enhanced chemical vapordeposition (PECVD) w12x, magnetron sputtering w2x andmany others. From the various possible choices, mag-netron sputtering appears to be a very attractive one dueto its relative simplicity, high attainable deposition ratesand wide acceptance by industry. At the low tempera-tures (T-500 8C) necessary for most applications SiCfilms are generally amorphous and can be producedwith hardness comparable to that of crystalline SiCw3,13,14x.The so called hardmetals such as WCCo are widely

    used as cutting tools material since their developmentin the 1920s due to their high hardness, strength andfracture toughness w15,16x. However, machining condi-tions may submit the contact area of the tools surfaceto temperatures in the range of 6001300 8C w1517x,where the mechanical properties of cemented tungstencarbide are strongly degraded mainly due to severeoxidation w16x. Therefore, SiC is a promising materialto be used as protective coating for WC cutting tools

  • 177A.K. Costa, S.S. Camargo / Surface and Coatings Technology 163 164 (2003) 176180

    Table 1Measured properties of coated and uncoated WCCo substrates

    Material Hardness Wear rate TMAX(GPa) (mm ymm N)3 (8C)

    WCCo 15.9"1.6 2.29"0.08=10y7 ;500SiCyWCCo 30.6"1.9 1.16"0.02=10y7 )700a-C:H 21.4"1.5 1.48"0.08=10y8 300y400a-SiC:H 20.0"1.6 6.96"0.34=10y8 500

    due to its outstanding thermal stability and high hard-ness. SiC films also present a minimal thermal expansioncoefficient mismatch when compared to WCCo(5.3=10 vs. 5.4=10 K ) w1x.y6 y6 y1In the present work we carried out an investigation

    of SiC films deposited by r.f. magnetron sputtering withthe aim of developing a material for application asmetallurgical and protective coatings. In previous worksmechanical (hardness and stress) and microtribologicalproperties of the films and their relation to the depositionparameters were determined w3,14x. From those resultssuitable conditions were chosen to produce films withhardness values at least equivalent to that of crystallineSiC (2628 GPa) w18x at a reasonably high depositionrates. This could be achieved at high enough applied r.f.power and low argon pressures. Additionally, under suchconditions the smoothest surfaces with the lowest fric-tion coefficients were obtained w14x.

    2. Experimental

    Silicon carbide films were deposited with a r.f. mag-netron sputtering system (US Gun II) from a 3-inchsintered, commercial grade SiC target using pure argonas sputtering gas. Commercial polished WCCo cuttingtools and single crystalline Si (1 0 0) were used assubstrates and placed on an unheated sample holder atapproximately 7 cm from the target. Hardmetal sub-strates were cleaned by alkaline degreasing followed byultrasonic bath and in situ sputtering. Selected depositionconditions were argon pressure Ps5.5=10 Pa, r.f.y2power P s400 W, substrate bias V sy30 V and noRF Bintentional substrate heating. Films were produced withthickness of approximately 5 mm on WCCo substratesand 2.5 mm on reference Si substrates. After depositioncoated and uncoated hardmetal samples were submittedto thermal annealing in ambient air for 30 min attemperatures up to 800 8C. Samples deposited on siliconsubstrates were annealed for the same time in vacuum(;10 Pa) at temperatures up to 1100 8C.y5A precision ballcrater apparatus was used to perform

    both thickness measurements and abrasive wear resis-tance tests. It helped also to give a visual evaluation ofadhesion of the coatings, since poorly adhered coatingsproduced detached grains that left deep scratches on thespherical craters. Tests were performed with a 30-mmdiameter chromium steel bearing rotating at 50 rpm witha 0.1-mm diamond aqueous suspension as abradingmedium. Indentation of the samples was done with aVickers diamond micro-indenter with 0.5 N (2.5-mmthick films) and 1 N (5-mm thick films) loads duringapproximately 20 s keeping the indentation depthapproximately 20% of the sample thickness. Hardnesswas obtained by measuring the indentation diagonals onan optical microscope using the differential interferencecontrast technique. In all cases, hardness was calculated

    from the average of a series of nine different indenta-tions. Residual internal stress was obtained by thesubstrate bending method, using a Dektak IIA stylusprofilometer.

    3. Results and discussions

    In Table 1 one can see the average measured valuesof hardness and abrasive wear rates for coated anduncoated WCCo substrates. It is noteworthy that theSiC hardness value is about twice as high as the one ofuncoated hardmetal and is even higher than the onereported for crystalline SiC w1,18x. It has been demon-strated w3x that the hardness of the films could be furtherincreased (40 GPa or higher), provided sufficiently lowr.f. power levels (of ;100 W) were used. Since thisprocedure involves a great reduction in the depositionrates, in the particular case of coatings for hardmetaltools it may not be very attractive from the practicalpoint of view given the required time increase indeposition process.Also seen in Table 1, the average abrasive wear rate

    of SiC-coated pieces was reduced by a factor of 2 whencompared to uncoated ones. This could be in principlea direct consequence of the increase in surface hardnessw17,19x, although other phenomena like friction reduc-tion may take place. In fact, comparing these results tovalues obtained from a-C:H and a-SiC:H PECVD film(Table 1) we observe that pure amorphous carbon filmsshow wear rates almost one order of magnitude lowerthan sputtered SiC films and with hardness of approxi-mately 20 GPa. Friction (and so wear rate) reductionfor amorphous carbon films has been explained by awear-induced graphitization mechanism with the for-mation of a transfer layer w20x. Preliminary results onsilicon incorporated carbon films showed intermediatewear rate values between those of amorphous carbonand SiC. Therefore, one could try to improve the wearresistance of SiC coatings by depositing a-SiCya-C:Hcomposite film. Experiments in this direction are cur-rently under way by adding a hydrocarbon gas to thesputtering atmosphere.In order to investigate direct effects of thermal anneal-

    ing upon mechanical properties of SiC films with aminimal influence of oxidation, samples deposited ontosilicon substrates were annealed in vacuum. As shown

  • 178 A.K. Costa, S.S. Camargo / Surface and Coatings Technology 163 164 (2003) 176180

    Fig. 1. Hardness and compressive residual internal stress as a functionof annealing temperature of SiC films deposited on silicon substrates.

    Fig. 2. Aspect of hardmetal cutting tools: (a) as-deposited; (b) and (c) SiC coated after 30-min annealing in ambient air at 700 and 800 8C,respectively; (d) TiN coated after 30-min annealing in ambient air at 800 8C.

    in Fig. 1, the internal stress is reduced to essentiallyzero by annealing at high temperatures without anyappreciable effect upon the material hardness. Indeed,the curvatures of the SiC-covered Si substrates couldnot be distinguished by profilometry from a bare Siwafer after annealing the samples at temperatures equalto or higher than 900 8C. Therefore, these results arepresented as zero-stress in Fig. 1. Thus, one can validatethe intrinsic nature of the hardness of this material, i.e.

    the compressive stress does not play a significant rolein the hardness measurements, what is quite frequent forsome PVD deposited coatings w21x.From a practical point of view, results of Fig. 1 are

    very interesting since the material presents an excellentthermal stability to bear the high temperatures usuallyfound in machining conditions w1517x. Furthermore, inthe case of a coating submitted to elevated temperaturesat work, one must take into account the hot hardness ofthe material, which follows the relationship HsH eyaT0w1x. Taking the experimental values for H and a w1x,0one can predict that SiC coatings would present aremarkably low decrease in hardness at elevated tem-peratures. Indeed, at 700 8C the hardness of SiC is still93% of its room temperature value (in case of TiN itwould be dropped down to 19%). Therefore, SiC coat-ings seem to be a good choice, for it is expected tomaintain its properties during and after the machiningprocess.Hardmetal samples were annealed at high tempera-

    tures in air and inspected in an optical microscope.Loose oxidation products (a greenish powder) wereobserved on uncoated WCCo surfaces for annealingtemperatures of 600 8C increasing in thickness for highertemperatures. In Fig. 2a and b, photographs of as-deposited and annealed (700 8C) samples, respectively,are shown. It is evident from these pictures that no signsof oxidation appear on coated surfaces. However, at 8008C the coatings peel off from the substrates (Fig. 2c).It must be noted, however, that this is due to the growth

  • 179A.K. Costa, S.S. Camargo / Surface and Coatings Technology 163 164 (2003) 176180

    Fig. 3. Hardness of uncoated and SiC-coated WCCo surfaces as afunction of annealing temperature (annealing time of 30 min in air).

    Fig. 4. Wear rates of uncoated and SiC-coated WCCo surfaces as afunction of annealing temperature (annealing time of 30 min in air).

    of the oxide under the SiC films, starting from the edgesof the samples, rather than a real destruction of thecoatings. Indeed, large self-sustained SiC films could befound all over the samples. Therefore, it is expectedthat if the whole piece can be coated, living no placefor oxidation to start, the protection of the substrate willbe further enhanced. For sake of comparison, annealingof a TiN CVD-coated commercial tool at 800 8C resultedin strong oxidation and total consumption of the coating(Fig. 2d).Fig. 3 shows hardness values for coated and uncoated

    samples after thermal annealing. In close agreementwith the results obtained for silicon substrates, hardnessof coated surfaces was found to be unaffected byannealing. On the other hand, the hardness of theuncoated tools is strongly degraded at temperatureshigher than approximately 500 8C, reaching to one-sixthof the original values at 700 8C. The observed degra-dation of mechanical properties of uncoated WCCotools is due to severe surface oxidation of this materialthat results in an oxide layer with reduced hardness.Abrasive wear rates of annealed coated and uncoated

    samples are presented in Fig. 4. One can see thatmeasured wear rates of coated surfaces were almostunchanged due to annealing, while that of bare WCCo started to increase at approximately 500 8C. Again,this is a deleterious effect of surface oxidation startingat this temperature. Taking these results (hardness andwear resistance) into account we included in Table 1estimated maximum work temperatures (T ) for SiC-MAXcoated and uncoated WCCo tools. One should note,however, that the value for coated substrates was limited

    by the adhesion failure due to the growth of oxide layerbeneath the film as explained before.

    4. Conclusions

    High quality silicon carbide films were successfullydeposited on tungsten carbide cutting tools. Coatedsubstrates were found to be twice as hard and wearresistant as uncoated ones. Upon thermal annealingsevere oxidation was found to occur on the surface ofuncoated WCCo at temperatures over 500 8C leadingto degradation of its hardness and wear resistance. Nosigns of oxidation were found to occur in case of coatedtools so that mechanical properties remained unchangedup to 700 8C. At higher annealing temperatures, how-ever, the WCCo oxidation caused the films to peel offfrom substrates. Annealing of films deposited on siliconsubstrates showed, however, that the coatings propertiesmay remain unaffected up to 1000 8C.

    Acknowledgments

    This work was supported by CAPES and CNPqBrazilian agencies. The authors are also grateful toSandvik do Brasil which kindly supplied the WCCocutting tools.

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    Amorphous SiC coatings for WC cutting toolsIntroductionExperimentalResults and discussionsConclusionsAcknowledgementsReferences