The ong>influenceong> ong>ofong> ong>laserong> ong>treatmentong> ... 61• higher fatigue strength due to ong>theong> occurrence ong>ofong> compressive stresses onong>theong> surface,• higher resistance to ong>theong> corrosion.The aim ong>ofong> this study was to determine how ong>laserong> ong>treatmentong> affects certainproperties ong>ofong> electro spark alloyed WC-Co coatings. The properties ong>ofong> ong>theong>coatings after ong>laserong> ong>treatmentong> were assessed based on following methods:microstructure analysis, adhesion tests, microgeometry measurement, microhardnesstests and tribological studies.2. ExperimentalIn ong>theong> experiment, ong>theong> coatings were electro-spark alloyed using a WC-Co(97% WC and 3% Co) electrode with a cross-section ong>ofong> 3 x 4 mm – ong>theong> anode –onto rings made ong>ofong> carbon steel C45 – ong>theong> cathode.The EIL-8A model was used as equipment for electro-spark alloying. Basedon previous research and manufacturer recommendation, ong>theong> followingparameters were assumed to be optimal for ESA:• voltage U = 230 V,• capacitor volume C = 300 µF,• current intensity I = 2.4 A.In Figure 1 ong>theong> electro-spark deposition equipment is illustrated.Fig. 1. View ong>ofong> EIL-8A – electro-spark deposition equipmentThe quality ong>ofong> electro-spark deposition depends mainly on ong>theong> shape,duration, and average value ong>ofong> current or pulse power. An average value ong>ofong>current is directly proportional to ong>theong> number ong>ofong> generators operating in parallel.
62 N. RadekFigure 2 shows a schematic diagram ong>ofong> a single pulse generator and followingFig. 3 presents current impulse shape from this device.Fig. 2. Schematic diagram ong>ofong> a pulse generation systemElectrical energy stored in capacitor C2 is transmitted to ong>theong> circuit:substrate – electrode in ong>theong> form ong>ofong> a spark discharge and current flow. Theprocess is initiated by switching on/ong>ofong> a transistor Q1. The switching frequencyong>ofong> ong>theong> transistor is ong>ofong> ong>theong> order ong>ofong> several to several dozen kHz. The capacitor C1allows changing ong>theong> shape and duration ong>ofong> an impulse as well as affecting ong>theong>pulse-duty while setting pulse frequency ong>ofong> transistor Q1.Fig. 3. Shape ong>ofong> a current impulse
The ong>influenceong> ong>ofong> ong>laserong> ong>treatmentong> ... 63Then, ong>theong> coatings were treated with an Nd:YAG ong>laserong> (impulse mode),model BLS 720. The samples with electrospark alloyed coatings were ong>laserong>modifiedwith ong>theong> following parameters:• ong>laserong> spot diameter: d = 0.7 mm,• ong>laserong> power: P = 20 W,• traverse speed: v = 250 mm/min,• nozzle-workpiece distance: ∆f = 1 mm,• pulse duration: ti = 0.4 ms,• pulse repetition frequency: f = 50 Hz,• ong>laserong> beam shift jump: S = 0.4 mm.3. Results and discussion3.1. Analysis ong>ofong> coating morphologyA microstructure analysis was conducted for WC-Co coatings before andafter ong>laserong> ong>treatmentong> using a scanning electron microscope Joel JSM-5400. InFigure 4 selected view ong>ofong> ong>theong> surface microstructure ong>ofong> an electrospark alloyedWC-Co coating is illustrated. From results obtained, it is clear that ong>theong> thicknessong>ofong> ong>theong> obtained layers was 20 to 30 µm, whereas ong>theong> heat affected zone (HAZ)ranged approximately 15 to 20 µm into ong>theong> substrate. In this micrograph (Fig. 4)ong>theong>re is a clear boundary between ong>theong> coating and ong>theong> substrate, where poreswithin microcracks are observed. The electrospark alloyed WC-Co coatings wasmodified via ong>laserong> beam solidification, which caused ong>theong>ir composition changes.The ong>laserong> ong>treatmentong> leads to ong>theong> homogenization ong>ofong> ong>theong> coating chemicalcomposition, structure refinement, and crystallization ong>ofong> supersaturated phasesdue to ong>theong> occurrence ong>ofong> temperature gradients and high cooling rate.Fig. 4. WC-Co coating microstructure after electrospark alloying
66 N. Radekong>theong> outer layer into a surface layer. This phenomenon was mainly due to ong>theong>sliding stresses and speed, and ong>theong> interaction with ong>theong> medium. The statestabilization ong>ofong> ong>theong> antiwear surface layer was observed.Fig. 8. Relationship between friction force and timeIn Figure 8 ong>theong> relationship between friction force and time is presented. Itwas observed that ong>theong> stabilization ong>ofong> ong>theong> friction force was achieved after 280 s.Before stabilization fluctuation from 6 to 6.2 N value was visible. In ong>theong> case ong>ofong>a ong>laserong> modified WC-Co coating, ong>theong> friction force stabilizes after 400 s. Beforestablization fluctuation from 3.9 to 4.1 N value was noted. The average frictionforce ong>ofong> a WC-Co coating was approx. 35% higher than ong>laserong>-modified WC-Cocoating (at ong>theong> moment ong>ofong> stabilization). The difference in ong>theong> coefficients ong>ofong>friction was similar.Seizure resistance was measured using a T-09 pin on disc tribotester. Twoprisms and a roller constituted ong>theong> friction pair. The surfaces tested were WC-Cocoatings an C45 steel before and after ong>laserong> processing. The roller hada diameter ong>ofong> 6.3 mm was made ong>ofong> carbon steel. The tests were conducted forthree kinematic pairs for each material variant, so it was possible to average ong>theong>data. The experiment involved submerging ong>theong> specimens in pure lubricant. Inthis case paraffin oil was used. This oil guarantees constant lubricating ability ineach test carried out. It’s difficult to compare results, when lubricant oil withadditional oilness improvers is applied. Some ong>ofong> improvers are biodegradableand are changing oil properties even during short time ong>ofong> storage. To obtainstable and comparable results parafin oil is commonly used in laboratorywearing tests. In Figure 9 an average seizure loads before and after ong>laserong>processing is presented. It is clear that ong>theong> ong>laserong> processing operation caused an
68 N. Radekresulted from ong>theong> tensile forces acting on ong>theong> surface, and accordingly, ong>theong>motion ong>ofong> ong>theong> liquid metal. A non-uniform distribution ong>ofong> temperature in a ong>laserong>beam (Transverse Electro Magnetic mode TEM 00 ) caused ong>theong> non-uniformity ong>ofong>ong>theong> surface prong>ofong>ile after solidification, which reflects, to some extent, ong>theong>distribution ong>ofong> energy in ong>theong> melted zone.Fig. 10. Surface roughness ong>ofong> ong>theong> WC-Co coating depositedFig. 11. Surface roughness ong>ofong> ong>theong> WC-Co coatingafter ong>laserong> ong>treatmentong>If pulse ong>laserong> ong>treatmentong> is applied, it is assumed that ong>theong> main factoraffecting ong>theong> surface prong>ofong>ile after solidification is ong>theong> pressure ong>ofong> vapour causingong>theong> disposal ong>ofong> ong>theong> material from ong>theong> central zone and ong>theong> production ong>ofong>characteristic flashes on ong>theong> boundary between ong>theong> melted and unmelted zones.3.5. Adhesion testsA scratch test was conducted to measure ong>theong> adhesion ong>ofong> ong>theong> WC-Cocoatings before and after ong>laserong> ong>treatmentong>. A CSEM REVETEST scratch tester
70 N. Radek• The ong>laserong> ong>treatmentong> causes that ong>theong> electrospark alloying coatings melt andong>theong>n solidify. The refinement ong>ofong> structure and disappearance ong>ofong> microcrackswere noticed.• The friction force average value (at ong>theong> moment ong>ofong> stabilization) reachedduring ong>theong> tribological tests for a WC-Co coating was approximately 35%higher than that obtained for ong>theong> same coating after ong>laserong> modification.• After ong>laserong> processing significant load force can be applied. The materialseizure ong>ofong> ong>theong> WC-Co coatings modified after ong>laserong> ong>treatmentong> was increase about16%.• Laser ong>treatmentong> caused a 21% decrease in ong>theong> microhardness ong>ofong> ong>theong>electrospark alloying WC-Co coatings.• After ong>laserong> ong>treatmentong>, ong>theong> roughness ong>ofong> ong>theong> electrospark deposited coatingsalmost doubled. This phenomenon is unfavorable if ong>theong> quality and applicabilityong>ofong> ong>theong> coatings under certain service conditions are to be considered. It isessential to determine which parameters ong>ofong> ong>laserong> ong>treatmentong> cause ong>theong> melting ong>ofong>ong>theong> coating microroughness peaks (ong>laserong> smoothing).• Laser ong>treatmentong> caused a 32% increase in ong>theong> adhesion ong>ofong> ong>theong> electrosparkalloying WC-Co coatings.• The furong>theong>r research will extend measuring investigations for residualstresses and porosity ong>ofong> electrospark WC-Co coatings before and after ong>laserong>ong>treatmentong>.References J. KACZMAREK: O wpływie obróbki ściernej i niektórych innych technologii namagazynowanie energii w materiale obrobionym. Postępy Technologii, 22(1998),35-52. I.V. GALINOV, R.B. LUBAN: Mass transfer trends during electrospark alloying.Surface and Coatings Technology, 79(1996), 9-18. N. RADEK, K. BARTKOWIAK: Heterogenous surface layer modification by highconcentrated energy source for tribological applications. Proc. 27th Inter. Congresson Applications on Lasers & Electro-Optics, ICALEO 2008, Temecula-California2008, 424-429. N. RADEK: Determining ong>theong> operational properties ong>ofong> steel beaters afterelectrospark deposition. Eksploatacja i Niezawodność – Maintenance andReliability, 4(2009), 10-16. N. RADEK, B. ANTOSZEWSKI: Własności tribologiczne powłok stellitowychnanoszonych elektroiskrowo. Tribologia, Teoria i Praktyka, 2(2009) 224, 191-200. W. DEPCZYŃSKI, N. RADEK: Methods for producing and repairing surfacelayers by applying electrical discharge. Patent No. 380946. N. RADEK: Experimental investigations ong>ofong> ong>theong> Cu-Mo and Cu-Ti electro-sparkcoatings modified by ong>laserong> beam. Advances in Manufacturing Science andTechnology, 32(2008)2, 53-68.
The ong>influenceong> ong>ofong> ong>laserong> ong>treatmentong> ... 71 N. RADEK: Laser ong>treatmentong> ong>ofong> electro-spark WC-CO-AL 2 O 3 3TiO 2 coatings.NT’10, 22.06.2010 r, Strecno, Slovakia, 1-7. N. RADEK, B. ANTOSZEWSKI: Tribological properties ong>ofong> ESD coatings afterong>laserong> ong>treatmentong>. Proc. ong>ofong> 6th Inter. Conf. on Tribology BALKANTRIB’08, Sozopol2008. N. RADEK: Producing coatings with varied properties using ong>theong> electro sparkdeposition and ong>laserong> ong>treatmentong> methods. Doctoral dissertation, Kielce University ong>ofong>Technology, Kielce 2006. Instruction a pin-on-disc tester (T-01M).Received in September 2010