Surface alloying of Cu with Ti by double glow discharge process

Vol.14 No. 3Trans. Nonferrous Met. Soc. ChinaJun. 2004Article ID: 1003 - 6326(2004)03 - 0516 - 04Surface alloying of Cu with Ti by double glow discharge process"YUAN Qing-long(袁庆龙), CHI Cheng zhong(池成忠), SU Yong-an(苏永安),XU Zhong(徐重)， TNAG Bin(唐宾)(Surface Engineering Research Institute, Taiyuan University of Technology, Taiyuan 030024, China)Abstract: The surface of pure copper alloyed with Ti using double glow discharge process was investigated. Themorphology, structure and forming mechanism of the Cu-Ti alloying layer were analyzed. The microhardness andwear resistance of the Cu-Ti alloying layer were measured, and compared with those of pure copper. The results in-dicate that the surface of copper activated by Ar and Ti ions bombardment is favorable to absorption and diffusion ofTi element. In current experimental temperature, as the Ti content increases, the liquid phase occurs between thedeposited layer and diffused layer, which makes the Ti ions and atoms easy to dissolve and the thickness of Cur-Ti al-loying layer increase rapidly. After cooling, the structure of the alloying layer is composed of CuTi, CuTi andCu(Ti) solid solution. The solid solution strengthening and precipitation strengthening efcts of Ti result in highsurface hardness and wear resistance.Key words: titanizing; CurTi aloying layer; double glow discharge processCLC number: TG174. 445; TG113Document code: A1 INTRODUCTIONproduced by the double glow discharge to driveCopper has many application in electrical, elec-metal atoms of one or more elements to deposit andtronic and heat exchanger fields because of its superiordiffuse into the substrate surface. Energetic metalelectrical and thermal conductivity and better corrosionatoms combine with atoms of the substrate to formresistance. However, the wear resistance of copper isan alloyed layer up to 300 μm thick. For example,poor. Being both soft and ductile, surface of pure cop-nickel-chromium base alloys, high speed steels andper are easily deformed and scratched, particularly atvarious alloys have been formed on the surface ofhigh temperature. Copper -based alloys are developedcommon carbon steels and other conductive materi-to improve some properties of copper. For example,als6-10]. Compared with ion implantation or laserCur Ti alloys have high tensile properties and reasona-surface alloying, this process is simple and versa-ble wear resistance, and are applied in relatively hightile in that a large number of metals and their com-wear applications such as plastic injection mouldingbinations can be diffused into the substrate.dies where the relatively high thermal conductivity ofIn the current study, pure copper substratethe copper alloy when compared with, for example,was alloyed with Ti using the Xu-Tec Process.tool steel, is damended, However, the thermal con-The distributions of Ti, hardness from surface toductivity of the bulk Cu-Ti alloy is lower than that ofsubstrate and the wear characteristics on the sur-pure copperf1-].face were measured and analyzed. The formationSurface engineering, as opposed to bulk allo-mechanism of Cu-Ti alloying layer was discussed.ying, provides an opportunity to improve the wearresistance of copper and copper alloys while leaving2 EXPERIMENTALthe bulk characteristics relatively unchanged. By2.1 Apparatusthis method, optimizing of the properties of theThe Xu-Tec Process is performed in a vacuumsurface and the bulk can be realized. However,there is limited information available on the surfacechamber. Fig. 1 indicates the working principle ofengineering of copper and its alloys. The doublethis process. In the vacuum chamber, there areglow plasma surface alloying technique, known asthree electrodes: the anode and two negativelythe Xu-Tec Processft.5], is a hybrid plasma surfacecharged members called as the cathode(Cu sample)treatment technique which involves both plasma ni-and source electrode(Ti sheet) respectively. Bothtriding and sputtering techniques, and has been de-the cathode and source electrode are surrounded byveloped to satisfy the need for high quality alloydouble glow discharge with two DC power sup-layer at the surface of less expensive materials.plithe Cu substrateThis technology employs low temperature plasmaw中国煤化Ieeterede "nTYHCNMHGD Foundation item: Proect(50371060) supported by the National Naturel Seience Foundation of ChinaReeived date: 2003 - 06 - 02; Accepted date: 2004 -01 -04Crrespadene YUAN Qinglong, 8sciate pofessor PhD candidate Tel: +86-351 60650898 E-mail; yunginglong@tyut. edu. enVol. 14 No. 3Surface alloying of Cu with Ti by double glow discharge process●517.Ti ions or atoms sputtered from the source elec-Based on the considerable experiment, the op-trode are accelerated toward the Cu substrate at aerating parameters of Xu-Tec process in the testshigher negative potential, then deposit and diffuseare gained and listed in Table 1 together with theinto the copper surface forming a Cu-Ti alloy. Thefinal thickness of Cu-Ti alloying layer.vacuum chamber was kept at 2 - 3 Pa, then back-The morphology and structure of Cu-Ti alloyfillel with argon to 20 - 40 Pa before double glowlayers were investigated by scanning electron mi-discharging. The cathode(Cu sample) was heatedcroscopy( SEM). The distributions of Ti elementby ion bombardment to 940 - 950 C and was main-were examined by SEM ( LEO-438VP) equippedtained at that temperature for several hours. Thewith EDS. The micro-hardness from surface toholding time depended on the thickness requiredsubstrate and the wearesistance of the alloyingand on the actual temperature. After titanizing op-layers were measured with micro-sclerometereration was completed, let the sample cool down in(M400- H1) and MM200 wear tester, respectively.the chamber under the protection of Ar. In currentThe surface phase structure of Cu-Ti alloy layerexperiment, the source electrode potential waswas measured with Rigaku D/max 2500 X-ray dif-more negative than the cathode potential.fractometer.3 RESULTS AND DISCUSSION3.1 Microstructure and formation mechanism ofCu-Ti alloy layerThe SEM micrographs of Cu-Ti alloy layer are↓shownin Fig. 2. From Fig. 2(a), it can be seenthat the Cu-Ti alloy layer consists of two parts:deposited layer and diffused layer and there is aboundary between them. Fig. 3 shows that Ti con-5tent decreases quickly from surface to about 20 μmdeep but thereafter decreases slowly. The changeFig. 1 Schematic diagram of experimental apparatusof the decrease rate of Ti content suggests that1- sShell (anode); 2- Source electrode;there are the deposited layer and diffused layer.3-Cathode (Cu sample); 4-Vacuum pump;Fig.2(b) shows the surface morphology of Cu-Ti5- Inert gas inlet; 6, 7- DC power suppliesalloy layer. The composition of the white particlesis, measured in mass fraction, Ti : Cu=43. 54 :2.2 Material and experimental conditions55.62, which is the TiCu compound according toTests were performed on rectangular(50 mmthe Cu-Ti binary phase diagram11,12]. This is in a-X30 mmX 3 mm) pure copper specimens ( Si <0.007%; Fe≤0. 072%; P≤0. 019%; Sn≤greement with the result measured by Rigaku D/max 2500 X-ray diffractometer(Fig. 4). From Fig0.028%; P≤0. 007%; Cu≥99. 8%，mass frac-4, the alloy layer consists of CuTi, CuTi and Cu .tion). Prior to testing, the specimen surface was(Ti) solid solution.finished to 1 200 μm with silicon carbide abrasiveThe process of surface alloying is usually con-paper, rinsed in de-ionized water, degreased in ac-sidered to be controlled mainly by alloying elementetone and dried in air. The source electrode (130diffusion. There are originally three or four differ-mmX 130 mmX6 mm) has a chemical compositionent models to explain the migration of these substi-of0.1% C, 0. 3% Fe and balance Ti(mass frac-tutional atoms, but one, the vacancy model, whichtion). It was immersed in the hydrofluoric acid so-can explain most availably the experimental evi-lution (HF : HNO3 : H20=1 : 3 : 7, volume ra-dence, has become increasingly popular. In the vatio) for 5 min to remove the oxidizing film beforecancy model, the diffusion coefficient is easily de-testing.rived as follows[La].Table 1 Operating parameters of Xu- Tec processExper.Source electrodeCathodeDistance betweenhickress ofNo.source electrode中国煤化工V./V I,/AV./VI。/A and cathode/ mmlayer/ pm1501.33801.820TYHCNMHG752801. 4402.12C30-3565●518●Trans. Nonferrous Met. Soc. ChinaJun.2004●- -Cu4Ti .:-CuTiji3035455520/(°)Fig.4 XRD pattern of CurTi alloying layerIn double glow discharge process, however,the collision of high enevgy ions with the surface ofspecimen will produce a high concentration of va-cancies on the treated surfacel. In current inves-tigation, the Ti ions and atoms from source elec-trode can be free to diffuse into the Cu substrate.The solid FCC copper has a fairly large solubilityFig.2 SEM micrographs of Cu-Ti alloy layerof titanium. This speeds up the diffusion velocities(a)-Cross section; (b)- Surfaceand thickening of the diffusion layer.With the increase of Ti content, the surface of100 [diffused layer gets into liquid (Cu) solid solution●一Condition !region, according to Cu-Ti phase diagram. So Ti-Condition 2and Cu atoms will dissolve directly into liquid,which makes the thickness between the deposited: 60layer and diffused layer increase quickly. After-wards, as the Ti content increases further, the40surface of diffused layer comes to the single liquidregion. This makes Ti and Cu atoms easier to dis-20solve and diffuse, and the Ti content increasesmore rapidly.During the cooling course, a series of reac-609tions occur in the liquid;Distance from surface/umL→L+Lz, L→(Cu),Fig.3 Distribution of Ti content in Cu-TiL+(Cu)→TiCu, L→TiCu + TiCu2,alloying layer under different process conditionsLz→TiCu, L2 +TiCu-→TiCu,L2+TiCu→TiCuz, Lx→TiCuz + TiCusAfterwards the compound TiCu, forms a eu-D=D.xp[-Rrm7=D.exp(-&+)(1)tectic together with TiCu2[4. The eutectic reac-where Qr is the activation energy for vacancy for-tion occurs at 875 C with 27% Ti(mole fraction).mation,Qm the activation energy for vacancy mi-TiCu may thus be formed both from liquid andgration, and Qod the activation energy for self-dif-from solid phase. It should be noted that TiCu2 isfusion of solvent atoms. It should be noted that thepresent only at a narrow temperature range and notactivation energy for self-diffusion in a pure metalat room temperature. This explains why there doesshould be equal to the sum of the activation ener-not exist TiCuz and TiCu on the surface of alloygies for vacancy formation and migration. Theay I中国煤化工same arguments are applied for diffusion of substi-tutional solutes by the vacancy mechanism. In3.2MYHCNMHGyer.most cases, it gives an intrinsic diffusion activationThe curves of microhardness distribution fromenergy Qud similar to or a little lower than that forsurface to the Cu substrate are shown in Fig. 5. Itself-diffusion.has the same trend with Ti content distribution.Vol. 14 No. 3Surface alloying of Cu with Ti by double glow discharge process●519●ture of the alloying layer is composed of CuTi,700CuTi and Cu(Ti) solid solution. The solid solu-600 t●一Condition 1tion strengthening and precipitation strengthening1- Condition 2effects of Ti result in high surface hardness and500 twear resistance.400REFENRENCES3001] Nagarjuna s, Balasubramanian K, Sarma D s. Effect200of Ti additions on the electrical resistivity of copper100[J]. Materials Science and Engineering, 1997, A225;118- 124.2] Nagarjuna.S, Balasubramanian K. Effect of prior cold2040680work on mechanical properties and structure of an ageDistance from surface/umhardened Cu-1. 5wt% Ti aly[J]J. Journal of MaterialsFig. 5 Microhardness of Cu Ti alloying layerScience, 1997, 32: 3375 - 3385.[3] Nagarjuna S, Srinivas M, Balasubramanian K, et al.On the variation of mechanical properties with soluteThis shows that the microhardness of alloy layercontent in Cu-Ti alloys[J]. Materials Science and En-depends on the Ti content. 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