Laser surface treatment of A1203 coatings plasma sprayed E. Fernandez, J.M. Cuetos, M. Cadenas, R. Vijande, HJ. Monies Oviedo University, Mechanical Engineering Area, ETS Ingenieros Industrials, Ctra. Castiello s/n. 33204 Gijon, Spain Abstract Laser surface treatment of A12O3 plasma sprayed coatings is studied in this paper from an experimental viewpoint. A CO2 laser operating in continuous mode is used to refuse a thick layer of 0,3 mm thickness. The changes in microstructure, microhardness and adherence caused by the laser treatment allows a better wear behaviour of this coating after laser treatment. The initial porosity of A12O3 plasma sprayed coating is considerably reduced after laser treatment. However in this case few vertical cracks appear. This fact can be reduced by using a preheating of the specimens to decrease the thermal effects of the laser treatment. 1 Introduction Plasma spray is a technique to obtain ceramic coatings used in many engineering applications to improve wear and corrosion resistance. However, the protective properties of plasma ceramic coatings are strongly limited by microstructural defects at the surface [I]. To improve such properties, surface laser treatments are widely used. Although some papers have been written on the subject [2-3] few have concerned themselves, with the influence of laser treatments on the wear-behaviour of the coatings [4]. The present paper studies the effect of CO2 laser-treatment on tribological behaviour of A12O3 plasma-sprayed ceramic coatings. 2 Experimental procedure The A12O3 coatings were deposited by plasma spray onto AISI 1043 steel substrates that had been previously shot blasted with synthetic corundum to provide suitable roughness to ensure adequate adherence between coating and substrate. Before to the spraying of A12O3 coating, a bond layer was deposited
62 Surface Treatment Effects II by plasma spray to improve bond strength. The features of the coating and the bond layer are shown in Table 1. Ceramic coating Powder size Fusion temperature Composition 0.5% other oxides Thickness Bond layer Composition Thickness... -90 + 15 mm 1982 C 98.5% A12O3; 1% SiO2; 0.30 mm 89.5% Ni; 5.5% Al; 5% Mo 0.05 mm Table 1. Coatings characteristics. The surface of the coating was melted using a CO2 laser (Spectra Physics) of 5 kw operating in continuous mode with TEM 00. An integrator beam was used to obtain a homogeneous distribution of the energy. To decrease the thermal effects of the treatment on the coating, the specimens were preheated by putting them in a furnace for 4 minutes at 700 C. The laser treatment parameters were optimised to obtain the following result: A molten layer of about 0.1 mm (maximum recommended to avoid damage to the coating or substrate) [5], minimum cracking, maximum reduction of porosity in the molten zone and good adherence among coating and substrate. The best results were obtained using the parameters shown in Table 2. Power Speed Density Energy Beam Preheating Protective gas 2000 W 3500 mm / min 22.222 W/mm2 5.7J/mm2 15 x 6 mm integrated 700 T Argon Table 2. Laser surface treatment parameters. Two types of test specimens were used, block and ring, defined according to the ASTM G77 standard, whose geometry is described on figure 1. The block, whose structure is shown in Figure 2, consists of a steel
Surface Treatment Effects II 63 substratum, a plasma sprayed bondage layer and a ceramic coating, also plasma sprayed, with the foregoing characteristics. The ring was made of SAE 4620 steel. 025,65 ro.25 034,99+ 0,25 mm 10,16^0,13 mm,,<+u,uumm- - 635. o,25mm J_ 15,75 mr Figure 1. Geometry of the block and ring test specimens. CERAMIC COATING TREATED BY LASER CERAMIC COATING NON TREATED BOND LAYER SUBSTRATUM Figure 2. Block structure. The two test specimens were placed in a LWF-1 tribometer with lineal contact to carry out wear tests with steel-ceramic pairs, under dry, abrasion and lubrication conditions at different speed and temperature rates. Alumina powder dissolved in water was used as abrasive substance with a 1:5 weight ratio. The lubricant used was a type of oil with 107 and 11.3 centistocks viscosity at 38* and 100'C respectively, and 0.882 mg/cm^ density. The tests were carried out in environmental conditions according to the ASTM S2714, G65, G40 and G77 requirements.
64 Surface Treatment Effects II 3 Results 3.1 Coating control results The adherence among coating and substrate was determined before and after laser treatment. Before laser treatment, the adherence ranged among 48 and 64 MPa. In this case the bond layer failed, so the failure can be referred to as adherence failure. After laser treatment, the adherence ranged among 56 and 63 MPa, i.e. the mean value increased. Failure occurred sometimes within the coating itself. The failure in this case can be referred as semicohesive. The porosity before the treatment was about 5 to 6 %; after treatment, in a surface depth between 0.05 and 0.1 mm, it almost became zero, and the coating contained just a few vertical cracks, reduced with the preheating. Then comes a 0.1 mm thick transition zone where porosity increases gradually. Figure 3, showing the cross-section of an A12O3 laser-treated coating, illustrates these effects. Figure 3. Cross-sectional optical micrograph of laser-treated A12O3 coating. The zone non melted by laser had an average of microhardness of 1161 HV0.3, whereas the mean microhardness in the melted zone was 1444 HV0.3, which implies an average increase of almost 25 %. This would also account for the better behaviour of the treated coating when some of the wear mechanisms, such as abrasion or adhesion, are applied.
Surface Treatment Effects II 65 3.2 Wear results Wear tests were carried-out using treated and non-treated coatings at different loads and speeds under dry, abrasive and lubricated conditions. 3.2.1 Wear of ceramic coating. The first results obtained showed that the wear behaviour of the ceramic coating, regardless of whether it is treated or nontreated, is good compared with that of steel, under any of the three conditions studied: dry, abrasive and lubricated. Once the extent to which load and speed affected wear was analysed, the conclusion was reached that an increase in the load results in higher wear and that, for light loads, there is a linear relation between load and wear. On the other hand, for heavy loads, wear increases quickly. The wear rate decreases as speed increases, although under some conditions, specially for the laser-treated coatings, there is a zone where wear is minimum. This fact corroborates the studies of Denape and Lamon [6]. In the present study, the optimum speed value is lower, usually raging among 100 and 150 rpm (0.162 and 0.243 m/sec). Figure 4 shows results under dry conditions. In this figure we can see that, the laser-treated coatings have less wear than the non-treated ones, and this improves even further as the load increases. No doubt, the increase in hardness deriving from the laser treatment is partly responsible for such better behaviour. Dry (Speed 150 rpm) Ceramic laser treated; load 136 N Ceramic non treated; load 136 N Ceramic laser treated; load 45 N Ceramic non treated; load 45 N 0 40000 80000 120000 N (cycles) Figure 4. Wear comparison between laser treated and non-treated ceramic coatings sliding against SAE 4620 steel under dry condition.
66 Surface Treatment Effects II In abrasive conditions, the treated coating has a better behaviour than the nontreated coating, though here the improvement is not so remarkable as it is under dry conditions. The abrasive condition implies a certain amount of lubrication since the abrasive material consists of water-diluted alumina particles. Under lubrication, there is a good wear behaviour in both, the treated and the non-treated coatings. 3.2.2 Steel wear. The wear of steel that is in contact with treated ceramic coatings is lower than the wear of the steel that contact with non-treated ceramic coatings, as shows Figure 5. The reason for this lies in the microstructural changes undergone by the coating owing to the laser treatment, as section 4 indicates. The improvement is larger under dry than under abrasive conditions. 300 Wear (10-4%) 25C 20C Dry Steel-ceramic non treated Steel - ceramic laser treatecj, Speed 200 rpm; load 45 N 15C IOC 50 0 0 10000 20000 30000 40000 N (cycles) Figure 5. The wear of SAE 4620 steel sliding against laser treated and nontreated coatings. Under lubrication conditions the results are different and, although on the whole wear is small, it is higher for the ring that contacts the treated coating. These facts are relevant from the point of view of industry, because the laser treatment of ceramic coatings can not only improve their performance in some circumstances, but it also protects the materials that are in contact with them from wear. 4 Microstructure and fractography The effect of laser treatment on the coating microstructure was determined using
Surface Treatment Effects II 67 an optical microscope, which allowed us to see how deep laser melting had penetrated. In addition, by means of a SEM, the wear mechanism and the microstructural changes taking place in the coating were also found. The X-ray diffraction patterns indicate that plasma sprayed alumina is mainly y A12O3. The formation of phase y from a A12O3 is due to the cooling of the molten A12O3 particles. The rapid cooling caused by the sudden impact between the molten alumina particles and the cold surface lead to a predominantly phase y rather than phase a crystallisation. When the laser treatment is used phase y dominates, but if the test specimen is heated before the treatment, as was done here and Figure 6 shows, there is more phase a. : J Figure 6. X-ray diffraction patterns: A12O3 with laser-treatment. The main wear mechanisms affecting the A12O3 coating, with and without the treatment, lead to: first, particle plastic deformation due to high temperatures and pressures; second, cracking due to fatigue in the plastically deformed zones. Finally, the coating itself undergoes cracking and loosening of particles. The same sequence is repeated whenever the ceramic coating contacts new particles. On the whole, these effects agree with what other authors have stated for this type of ceramic coating [7]. 5 Conclusions 1. Surface laser treatment is a method that allows improvement, in some circumstances, of the tribological behaviour of plasma sprayed ceramic coatings, modifying their microstructure from y A12O3 to predominantly a A12O3 phase.
68 Surface Treatment Effects II 2. The surface treatment of plasma sprayed ceramic coatings with CO2 laser allows molten layer thicknesses of 0.1 mm to be obtained. Whenever preheating of the coating is carried out, the layers present minimum vertical cracking 3, The increase in hardness generated from the treatment cause the behaviour under dry and abrasive wear conditions to be better for the treated than for the non-treated coatings. Under lubrication the behaviour is very similar in both coatings. 4. Wear of the steel is lower when the steel contacts the laser-treated than the non-treated coating. Acknowledgements This work has been carried out within the frame of the CICYT project "Physical and Chemical Aspects of Wear in Moving and Surface Treated Mechanical Systems", which has been developed jointly by the CISIC Tribology Unit and the Tribology Group of the Mechanical Engineering Department of the E.T.S.I.I. in Gijon. References 1 Nicoll, A.R. Protective coatings and their processing thermal spray,, pp. 24-29, in: High Temperature Materials and Coatings, Finland, 1984. 2 Mordike, B.L. and Sivakumar, R. Laser surface melting of ZrO2 protective layers. Laser Treatment of Materials, pp. 373-381, in: European Conference onlaser Treatment of Materials, Germany, 1986. 3 Hatefeuille, L., Beylat, L., Carron, D. and Puig, T. Amelioration des propietes de surface par refusion laser, pp. 27-31, in: Conf. imser de puissance et traitements des materiaux. Ecole de Printemps, France, 1991. 4 Cuetos, J.M., Ferndndez, E., Vijande, R., Rincdn, A. and Perez, M.C. Plasmasprayed coatings treated with lasers: tribological behaviour of Cr2O3. Wear, 1993, 169, 173-179. 5 Mordike, B.L. and Sivakumar, R. Laser melting of plasma sprayed ceramic coatings. Surface Engineering, 1988, 4, 127-140. 6 Denape, J. and Lamon, J. Le comportement en frottement sec de ceramiques a hautes performances, pp. 1-5, in: 4eme Congres Europeende Tribologie. Soc. Francaise de Tribologie. Elsevier, Amsterdam, 1985. 7 Vijande, R., Belzunce, J., Fernandez, E., Rincon, A. and Perez, M.C. Wear and rnicrostructure infineceramic coatings. Wear, 1991,148, 221-233.