Comments on process of duplex coatings on aluminum alloys

Vol.11 No. 3J. CENT. SOUTH UNIV. TECHNOL.Sep. 2004Article ID: 1005 - 9784(2004)03 - 0239 - 07Comments on process of duplex coatings on aluminum alloys'Samir H. A.，QIAN Han cheng(钱翰城)，XIA Bo-cai(夏伯才), WU Shi-ming(吴仕明)(School of Mechanical Engineering, Chongqing University, Chongqing 400044, China)Abstract: Despite the great achievements made in improvemnent of wear resistance properties of alumninum alloys,their applications in heavy surface load- bearing are limited. Single coating is insufficient to produce the desired com-bination of surface properties. These problems can be solved through the duplex coatings. The aim of the presentstudy is to overview the research advances on processes of duplex coatings on aluminum alloys combined with microplasma oxidation process and with other modern processes such as physical vapour deposition and plasma assistedchemnical vapour deposition and also to evaluate the performance of micro plasma oxidation coatings in improving theload-bearing, friction and wear resistance properties of aluminum aloys in comparison with other coatings. Where-in, a more detailed presentation of the processes and their performances and disadvantages are given 8as well.Key words: aluminum alloy; duplex coating; micro plasma oxidation; physical vapour deposition; plasma assistedchemical vapour deposition; tribological propertyCLC number: TG178Document code: A1 INTRODUCTIONmethod of forming ceramic coatings on aluminumalloys. In MPO，the aluminum material is im-Mass saving materials such as aluminum andmersed as an anode in an aqueous solution contai-its alloys have become increasingly important andning modifying elements, and voltage greater thanhave attracted increasing attention in the recenthigh voltage of the original oxide film (400 - 600 V)decades, specially in the automotive， aerospace,is applied between the anode and the cathode. Durchemical industries, and electrical devices becauseing oxidation， many visible sparks or microarcof their high strength-to-mass ratio, high electricalspots move rapidly on the metal surface[9.10]. Theand thermal conductivities, good abilities for recy-excellent properties of these coatings are of parti-cling and processing and good resistance to degra-cular interest to the components of textile ma-dation in some corrosive environmentsl-.2. In thechine, aerospace and engineering equipments, bio-automotive industry, there is desire to applymedical devices and machine building". Thisaluminum alloys more widely and extensively irstudy introduces research advances on processes oforder to achieve better energy efficiency andduplex coatings on aluminum alloys combined witheconomical profit, and more durable and recyclablemicro plasma oxidation process, and with othergoods, ie, the aluminum alloys are used for manymodern processes such as physical vapour deposi-engine parts in toady's cars such as pistons fotion(PVD) and plasma assisted chemical vapoursmall automotive and internal combustion enginesdeposition(PACVD). Also this study evaluates theand parts working at high temperatures and stres-performance of micro plasma oxidation coatings inses[3.0]. Unfortunately, the light metals mentionedabove have poor wear resistance. Specially, thecomparison to other coatings.poor surface hardness of aluminum and its alloys2 DUPLEX SURFACE ENGINEERINGand their relatively low yield strength compared tosteel and other materials give these materials poorAlthough great developments have been madewear resistance in contact situations, and thus re-duce life time and their applications in heavy sur-in the surface-engineering technologies, the tradi-face load- bearing conditions5-7]. The micro plasmational coating technologies are expected to continueoxidation ( MPO) has recently been studied as ato dominate the market. One reason lies in the rel-novel and effective technique to provide thick andatively high cost of the advanced processes. Thultra-hard ceramic coatings with excellent proper-other reason lies in the usual unsuitability of ad-ties，such as load support, corrosion, electricaland thermal properties on light alloy materials,as 1中国煤化工oy1i2-14]. There areparticularlyonaluminum alloys[8].matHCNMH G existing in indus-unconventional plasma chemical- electrochemicaltreatmeris insuf-①Received date: 2003- 12 - 08; Accepted date: 2004 -03 - 01Correspondence; Samir H. A.，Doctoral candidate; Tel; + 86-23-65102418; Fax: + 86-23-65102418; E-mail: samirhamidaw@ yahoo. comJournal CSUT Vol.11 No.3 2004ficient to produce the desired combination of sur-face engineering, prior to PVD ion plating, LM13face properties, or their market penetration is lim-aluminum piston alloys were locally melted over aited despite their excellent performances in impro-small area of a few hundred square millimeters byving surface properties. These challenges can be o-two-stages alloying method using an oscillatingvercome by duplex surface engineering. It is re-beamof 3 kW power. During such procedures, va-cently developed that surface modification methodrious elements (Ni, Si) were alloyed into the sur-can improve the tribological performance and theface. By PVD ion plating, thin layers of 5 μm thickoad capacity of industrial parts and formingof copper, nickel and chromium were deposited ontools[15]. It has shown some beneficial effects onthe nickelalloyed materials. Pin-on disc wear testsadhesion of coating to substrate and the perform-and hardness test show the expected improvementsance of some cutting tools.' The first attempt to ap-in wear performance in nickel and silicon alloyedply the duplex surface treatment was carried out inmaterials over the aged LM13 substrates due to the198016]. Duplex coating can be defined as the se-higher hardness (1 400，800 and 140 HV, respec-quential applications of two or more establishedtively) achieved by refining the structure of thsurface technologies to produce a surface compositesubstrates during surface melting. Also, the high-with combined properties which cannot be achievedthrough any individual surface technologyl2.12.er peak surface hardness recorded throughout theTwo general groups are identified. In the firstrange of applied loads in the duplex coating provesgroup, two individual processes complement eachthe load bearing ability of the subsequent PVD me-other and the combined effects result from bothtallic layers. Wear test results show that the du-processes. In the second group, one process sup-plex coatings have a significant impact on impro-plements and reinforces the other, acting as a pre-ving the wear rate of Al-based materials as shownor post-treatment, and the resultant properties aren Fig. 1[17]. However, these techniques requirerelated to one process. Some examples of duplexrelatively high substrate temperature to providesurface engineering are listed in Table 1.adequate coating. Whereas, an oscillating beamwas used to create a wider track, and the optimumTable 1 Examples of typical duplex surfaceresults were achieved by two-stages alloying. Fur-engineering technologies[12]thermore, melting to obtain good adherence be-Group 1Group 2tween the alloying elements and the substrate wasPVD coating of pre nitrided Energy beam meling of over-carried out prior to the second treatment, necessary tosteellay coatingsobtain good homogeneity to an appropriate depth withPlasma nitriding of energy Nitriding of pre-laser alloyeduseful surface finish. Other issues are the relativelybeam alyed Tihigh costs and the process technology is required forPVD coating of Ni/Cu diffu- Nitriding of pre-laser harden-the existing surface treatment processes.sion treated substratesed steelsNitriding of pre-carburised Sprayed MoSz on electrolessr 1X 105-LM13- Ni aloyingsteelsNi coatingsCu CoatingE 1X106-Cu duplexPVD coating of electroplatedsiC intermediate layer foCr coatingdepositsdiamond-like carbon ( DLC )二NiCoptincoatings1x107曲一Ni duplex旨1X103 TECHNIQUES OF DUPLEX COATINGS ONALUMINUM ALLOYS1x 109Applied load/NIn accordance with the above classification,the main research advances on the processes of du-Fig.1 Wear test results for Al alloy LM13 withplex coatings on aluminum alloys combined withPVD coated and duplex treatedmicro plasma oxidation and vapour depositionmaterials at load of 1 N[17](PVD and PECVD) processes were presented.3.中国煤化工ating3.1 PVD coating on energy beam alloyed metalliceby Musil et al ascoating.MYHCNMH Gex coating tech-Energy beam melting and alloying treatmentsniques18.19]. It differs in the order of individualsuch as plasma nitriding, are shown to be suitablesteps from the duplex coating combining withpre-treatment techniques17.18]. In this duplex sur-plasma nitriding of the substrate prior to the .Samir H. A.，et al; Comments on process of duplex coatings on aluminum alloys，241●coating deposition. In the technique， Ti filmvacuum heat treatment (eg, OvHT = 600 C). In-(5 μm thick) was deposited by the magnetrondeed, it can be seen that the nitriding time tn wassputtering ion-plating process onto aluminumconnected with the thickness of Ti coating, where(99. 8%) substrate. Then, the precoated substratea longer tn (several hours) was necessary for thic-was plasma nitrided or similarly vacuum heat-trea-ker Ti coating to intermix Ti and Al and to produceted in different treatment temperatures and time.a broad diffused interface.The as-deposited and then plasma nitrided or vacu-um-heat-treated Ti coating / Al substrate couple3.3 Plasma spraying on PVD or PACVD coatingwas characterized by elemental depth profilesIn order to meet the steady demand for surfacemeasured by glow discharge optical spectroscopy.coatings with solid lubricant ability and dry frictionIt was shown that both the plasma nitriding andGadow et al[7] investigated the friction and wearvacuum heat treatment processes could stimulate aproperties of aluminum alloys ( AlMg3 ) substratesstrong interdiffusion between Ti and the substrateunder dry sliding conditions. The duplex coatingselements. This converse duplex coatings，providedconsisted of wear resistant Al2O3 ceramic primarythe following main advantages compared with thelayer, applied by atmospheric plasma spraying. Aclassic duplex technique ( deposition of coating onsecondary layer with dry solid lubrication abilitynitrided and/ or implanted substrate surface).was applied to provide a low friction coefficient.1) Efficient interdiffusion of elements betweenThe secondary layers include polymer coating( con-the coating and the substrate with a dramatic redis-taining microscale polytetrafluroethylene (PTFE)tribution of the elements in the coating due to theparticles) deposited by pneumatic air sprayingabsence of diffusion barrier which creates on the(30-50 μm thick), DLC and titanium interlayersubstrate surface in the case of conventional duplexcontaining diamond like carbon (Ti-DLC) coatingcoating, makes it possible to modify and /or im-(2 μm thick) deposited by plasma enhanced chemi-prove the surface properties of the coating.cal vapour deposition ( PECVD) technique and2) lntermetallic compounds will be formedMoSz coating deposited by cathode sputtering (di-when the temperature is sufficiently high. A sur-rect current magnetron sputtering). The resultsface modification of aluminum can be seen iindicate that without protection coatings the alumi-Fig. 2, showing that the vacuum heat treatmentnum alloy substrates show high friction coefficients(VHT) of a5 μm thick Ti coating deposited on aand sever wear in pin-on disk testing. The tribo-99. 8%Al substrate at 600 C for 5.5 h results inlogical properties cannot be improved by the directthe formation of a very broad (about 20 μm) dif-deposition of various solid lubricant coatings onfused interface and the formation of an Al-Ti inter-aluminum alloy substrates, while the deposition ofmetallic compound. The surface properties strong-Al2O3 ceramic primary layer by thermal sprayingly depend on the structure of the as-deposited Tileads to the improved wear resistance, compressivecoating. In order to develop a strong interdiffusionstrength and load bearing capacity of the sub-a certain threshold value of nitriding temperatureshould be exceeded (eg, 0.= =520 C for Ti coating/strates. The additional application of solid lubri-Al substrate couple) and the same temperature ofcant films by duplex surface engineering results in1.[(a)100)A0.s品0.5-T|co、c0一182430-o中国煤化工本3σDepth/ umHCNMHGFig.2 Comparison of depth profiles of Ti, Al, C and O in as deposited (a) andvacuum heat treated (VHT) Ti coating/99. 8% Al substrate couples (b)(Sputteringof Ti: Vs=-100 V, 0g=200 C, PAr= 0.6 Pa; VHT : OvHτ=600 C)●242●Journal CSUT Vol.11 No.3 2004.2-1.080.6(d0.4-(e)~2X 1044X 1046X 1048X 10410X 104Number of oscillationFig. 3 Dry sliding behavior of different duplex coatings on AlMg3 aluminum alloy against100Cr6 bearing steel bal[7](a)一Al2O, coating; (b)- Duplex Al2O2 polished/lubricating varnish coatings;(c)- Duplex Al2O3 as sprayed/ lubricating varnish coatings; (d)- MoS2 coating; (e)- Ti-DLC coating; (f)- DLC coatinglow friction coefficient (Fig. 3) and a good tribo-the friction coefficient and improve the endurancelogical behavior under dry sliding conditions. It islife of 2024 Al alloy in vacuum (Fig. 4(a)).well known that the plasma spray techniques ofHowever, wear track analysis after 1X10* cy-Al2O3 require a relatively high substrate tempera-cles for the rotation disc in the ball-on- disc test de-ture to provide adequate coating adhesion of highnotes a lubrication failure of duplex- treated 2024contact loads0. Other issues concerned are theAl alloys. Whereas, it can be seen in Fig. 4(b)relatively high costs of Al2Os and PTEF particlesthat failure is attributed to the generation of theand that the adhesion of thermally sprayed layerscracks in the anodic oxide coating induced by nor-mainly depend on the pre-controlling of surfacemal load combined with friction force ( shearingroughness'7. Besides, the surface roughness is tooforce) and the crack propagation mainly caused byhigh after thermal spraying and has significantfriction force. Indeed, it is thought that the poreseffects on the tribological properties of most ther-of several tens of nanometers in diameter in themal spray/ polymer coatingsl21.221. Accordingly, ananodic oxide coating can postpone the propagationoptional post- treatment step is essential to removeof the cracks. .he most protruding asperities by mechanicalgrinding and polishing and to create more homoge-3.5 PACVD or PVD coating on pre-micro plasmaneous load bearing area. As it can be seen inoxidized coatingFig.3，that the duplex thermal spray/ polymerConsidering the excellent wear and load-sup-coatings perform better when the Al2O3 layers areport characteristics of MPO alumina coating, sev-polished prior to the second layers.eral attempts were made for the development ofnovel duplex coating of aluminum alloy in which3.4 PVD coating on pre anodized coatingthe alumina layer provided a more adequate under-In this field23], 2024 Al alloy was anodicallylying load support for a hard coating and combina-oxidized first using 180 g. L-1 H2 SO, solution fortion of good wear resistance and load-bearing ca-different thickness of 2.5, 5.0 and 10.0 μm; thenpacity as well as corrosion resistance. The microcoated with REMF- MoS2-Au nanocomposite filmplasma oxidation technique was employed for du-(1 pum thick) containing rare earth fluorinatedplex treatment of Al alloy, creating a layered sur-(REMF) compound by multi-arc ion plating andface中国煤化工ring oxide ceramicTi/Ag dual layer film (1 μm thick) by radio fre-subla. based coating pro-quency sputtering. The investigation on the tribo-ducedHC N M H G assisted chemicallogical properties using ball-on-disc tester in vacu-vapour deposition (RFPACVD)[24]. A reduction inum indicate that duplex treatment， especially ,friction cofficient from 0. 49-0.62 to0. 17 -0. 32REMF-MoS2-Au nancomposite film, can decreaseagainst a WC- Co ball counterface was achieved bySamir H. A.。et al; Comments on process of duplex coatings on aluminum alloys●243.Pores0.8a)0-1/Ag.INb)1T/Ag2 Ndiameter 30-50 nm芽0.6-图- -Mo'S25NPeeled ofrg 0.4-, Anodic oxidecoating/2024 Al芒0.21.5.010.0Thickness of oxide coating/ umMicro cracks-Fig, 4 Mean friction coefficients of duplex treated 2024 Al (a) and schematic diagramof lubricating failure mechanism (b)23]this top layer. Indeed, this layer is found to stabi-due to a lack of an MPO intermediate load supportlize friction behavior of the pair due to the deceaselayer. Indeed, the single layer shows the biggestof adhesion between the bodies. This attemptdiameter of all the impact tests due to the absenceurged Nie et a[25] to study the deposition of a load-of load supporting MPO interlayer. The ratio ofsupport Al2O3 and low-friction diamond-like car-C2H2 to Ar (0. 25 - 0.35) significantly influencesbon DLC coatings on Al alloys for tribological ap-the interfacial adhesion between the DLC and alu-plications using a combined micro plasma oxidationmina layers, but has no significant effect on coat-and plasma immersion ion implantation tech-ing hardness. The investigations indicate that aniques[25] to reduce the relatively high friction coef-combination of micro plasma oxidation and plasmaficient (0. 67-0. 70) in dry sliding test[26]. Theimmersion ion implantation represents a promisingplasma immersion ion implantation technique is atechnique for surface modification of the Al-alloysfeasible route for mass production of plasma assis-for tribological applications in which high contactted chemical vapour deposition[25]. In this work, aloads are anticipated.weakly-ionized, hot-filament supported low-voltage argon acetylene plasma with ratios of Cz H2to Ar from 1.00 to0. 15 was used, in combinationwith a low-frequency direct current pulse voltageplasma immersion implantation system ( 100 μs ,5 kV) at 850 Hz to deposit a low friction DLC toplayer onto MPO treated-Al alloy substrates. Mi-crohardness measurements and pin- on- disc slidingwear tests (load 10 N) were performed to evaluate上0.2/the mechanical and tribological properties. Ball-on-6)200400 600 800 T 000plate impact tests (load 400 N) were also carriedSliding distance/mout to assess coating layer adhesion and cohesion.Scanning electron microscopy was used to observeFig. 5 Pin-on- disc test results of duplexthe morphology of coating and to exarine wearAl2O:/DLC coating vs AISI 52100 balls (a)scars from pin-on-disc tests and crater scars fromand 100Cr6 bearing steel balls (b)[25]impact tests. The present work demonstrates thata hard and uniform DLC coating with low and sta-Research at Chongqing University involvingble friction coefficient in the range of 0. 10 -0. 22improvement of aluminum alloys for tribological(Fig. 5)，low wear rate and a good adhesion, calapplications by deposition of duplex load-supportbe successfully deposited on the top of a aluminaAl2O2/low- friction TiN using a combined microintermediate layer. Such coating can withstandplas中国煤化工-ing technique promuch higher contact stressesthan that ofducems of wear resist-aluminum-alloy substrate materials. Whereas, in-ancefYHC N M H Gectable in considera-vestigations on both wear tracks and impact craterstion of the fact that several benefits can be gainedshows that the duplex layer is not penetrated afterby coating the surface first with a load-bearing lay-1 000 m pin-on-disc sliding distance, while a singleer and then with PVD coating[27]. In view of thelayer ( without DLC coatings) fails catastrophicallypossible applications and the observed differences●244Journal CSUT Vol,11 No. 32004between the processes in treatment regimes, testsdeformation of the substrate that results in eventu-and the resulted improvements by each process, ital coating failurel2028.29]. Such plastic deformationis hard to distinguish the novel technology amongwas observed on TiN films coated-aluminum al-the processes. But on the whole, considering theloys[30]. The adhesion properties of MPO coatingsimportant comparison factors between the proces-are superior to those of other new coating materialsses such as process complexity， performance ofdue to their limited tribological performance resul-each process in improving the mechanical and tri-ted from the poor adhesion of coating film when in-bological properties of aluminum alloy and the pos-creasing its thickness to meet some tribologicalsible costs and equipment required to continue theproperties requirementsta . .process, it is obvious that the duplex surface engi-neering combined with micro plasma oxidation4 SUMMARY AND CONCLUSIONStechnique has the superiority to the others, partic-ularly in such excellent features as the relatively1) The common goal for research develop-simple equipment required, high stability and sim-ments on duplex coatings on aluminum alloys is toplicity of its safe electrolytic solution, low temper-enhance the load-bearing capacity by the pre-ature of the process and no need to make prelimina-coating and provide low friction and good wear re-ry preparation of surface before coating and alsosistance properties by the second coating.(to some extent) after coating as compared to the :2) Considering important comparison factorscase with plasma sprayed Al2O3 coatings. Also, itbetween the processes such as process complexity,is interesting to note that the superior load- bearingperformance of each process in improving the me-property of MPO coating can put this coating in achanical and tribological properties of aluminum al-successful competition with the other pre- treat-loy and the possible costs and equipment requiredment coatings discussed in this study. For in-to apply each process, the MPO has the superioritystance, the thermal sprayed Al2O3 ( polished )to other processes.shows a higher friction coefficient (Fig. 3) than3) MPO pre-coating in the duplex coatingsthat recorded (0. 67 -0.70) for polished aluminashows its superiority to other coatings in the com-deposited by micro plasma oxidation technique. In-bination of desired properties to the surface such asdeed, microhardness values of MPO coating areexcellent load bearing, good hardness, low frictionproved to be higher than that of energy beam alcoefficient and low wear rate.loyed metallic coating even after they combine with4) MPO coating can be a promising candidatePVD metallic coatings. Hardness results can befor the design engineers to get both technical andfound in Refs. [9,17,26]. Based on lubricated re-economical achievements in surface improvement ofciprocation sliding wear tests, it is founds] that thealuminum alloys for heavy surface load-bearing ap-MPO coating can give the most durable and con-plications.sistent wear protection in a range of extreme mean5) The increased demands for heavy surfacecontact pressure (222 -2 933 MPa)，while the SiCcontact applications of aluminum alloys can openor Al2O3 particles enriched coating by laser meltingthe prospects for new research on duplex coatingsprovide wear protection when the mean contactcombining PVD or PACVD hard and thin coatingpressure is less than 1 700 MPa, because these par-on MPO pre-coating.ticles can be removed at a higher pressure and acollapse is statistical in nature and accompanies byREFERENCESan increase in friction coefficient. Micro plasma oxidation technique becomes the promising one which[1] Serope K, Steven R S. 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