Journal of Materials Processing Technology 164–165 (2005) 954–957

HVOF coating and laser treatment: three-point bending tests

B.S. Yilbasa, ∗, A.F.M. Arif a, M.A. Gondalb

a Mechanical Engineering Department, KFUPM, PO Box 1913, Dhahran 31261, Saudi Arabiab Physics Department, KFUPM, Dhahran 31261, Saudi Arabia

Abstract

HVOF coating is one of the protective coatings of metallic materials. The mechanical locking is the mechanism for bonding the splatsonto the substrate material surface. Thermal integration of coating to the base material through melting and re-solidification is short comingin HVOF thermal spraying process. In the present study, thermal integration of HVOF coating via laser melting is considered. Inconel 625powders are sprayed onto mild steel samples. The mechanical properties of laser treated and untreated-coatings are examined through three-point bending tests. It is found that elastic limit of coating reduces after the laser treatment. The defect sites at coating–base material interfaceare the origin of the crack sites. Moreover, some locally distributed splats with high oxygen content are also act as crack initiation centers.© 2005 Elsevier B.V. All rights reserved.

K

1

pstmccitmc

tiTopspT

erac-icro-d byrredef-rties

reduc-ilbasmaltance

l in-asernt theion.mineOFgnousctural

0d

eywords:HVOF; Coating; Laser; Heating; Three-point bending

. Introduction

HVOF coating is involved with mechanical locking ofowders sprayed onto the roughened substrate materialurface. Although powder leaving the gun is hot, there is nohermal integration between powders sprayed and the baseaterial. This, in turn, lowers the mechanical properties of

oating through discontinuity in interfacial properties acrossoating and the base material. Thermal integration in coat-ng can be achieved through laser processing of coating. Inhis case, care should be taken to prevent excessive ther-al stresses in coating. In such situation, thermally induced

racking and voids can be avoided.Considerable research studies were carried out to examine

he properties of HVOF coating. The component repair us-ng HVOF thermal spraying was carried out by Tan et al.[1].hey indicated that the appropriate thickness of coating couldnly be achieved if the substrate material and the repair hadhysical matching properties. The effects of HVOF thermalpray procedure and thermal fatigue on micro-structure androperties of coating were examined by Hidalgo et al.[2].

on both the maximum temperature attained and the inttion time between the powders and the jet ambient. Mstructural characteristics of HVOF coating were examineDent et al.[3]. They observed that the oxide phases occuin the form of either inter-splat lamella or globules. Thefects of heat treatment on the micro-structure and propeof HVOF coating were examined by Lee and Min[4]. Theyshowed that as the annealing temperature increased,tion in porosity and grain coarsening were observed. Yet al.[5] examined the corrosion properties of HVOF thersprayed coatings. They showed that the corrosion resisof coating significantly higher than the base material.

Laser treatment of surfaces improves the structurategrity in the region irradiated by a laser beam. During lprocessing, use of assisting gas is necessary to prevehigh temperature oxidation reactions in the irradiated regConsiderable research studies were carried out to exalaser processing of HVOF coating. Laser remelting of HVcoating was considered by Potzi[6]. He indicated that meltinof particles in coating was achieved and almost homogestructures were resulted after the laser treatment. Stru

hey showed that the oxide content of coating was depended

∗ Corresponding author. Fax: +966 3 860 2949.E-mail address:bsyilbas@kfupm.edu.sa (B.S. Yilbas).

characterization of laser treated-coating was carried out bySerra et al.[7]. They showed that the interconnected porositywas completely eliminated, except few shallow cracks wereobserved. Laser treatment of HVOF coating due to improve

d.

924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserveoi:10.1016/j.jmatprotec.2005.02.091

B.S. Yilbas et al. / Journal of Materials Processing Technology 164–165 (2005) 954–957 955

corrosion resistance was carried out by Tuominen et al.[8].They compared the properties of laser re-melted coatings withthe properties of as per sprayed HVOF coating. They indi-cated that it was possible to obtain excellent properties ofcoating after proper selection of laser treatment parameters.

In the present study, laser melting of HVOF sprayed In-conel 625 powders on mild steel workpieces is carried out.Nd:YAG laser is used to irradiate coating surfaces after theHVOF process. The mechanical properties of HVOF coat-ing prior and after laser treatment were examined throughthree-point bending tests. SEM is used for morphologicalexaminations of coating surface.

2. Experimental

The HVOF system was employed to spray Inconel pow-der on to the workpieces. Propane was used in the combustorand powder flow was centered by using axial powder feedingunit. Inconel 625 powder (NiCrMoNb, 21.4% Cr, 9.1% Mo,3.6% Nb, Ni balance) was used and the powder has a particlesize distribution between 20 and 50�m with almost spher-ical morphology. Mild steel workpiece surfaces was rough-ened trough grid blasting to secure surface roughness within40�m for good adhesion of the splats on to the surface of theworkpiece.

Thee ch isg delG adia-t werb e re-d liversa h ofa z. Ino singl ed.T m att

sedi ninge cro-g

3

ilds per-f rop-e

n inF ear-s rticlesT

Fig. 1. SEM micrograph of powder used in HVOF spraying.

Fig. 2shows load–displacement characteristics of the lasertreated-coating, untreated-coating and base material duringthe three-point bending tests. The bending tests were car-ried out at constant strain rate and tests were terminatedonce coating failed. The elastic behavior of the laser treated-coating and untreated-coating are similar, provided that theelastic limit of the laser treated-coating is less than that corre-sponding to untreated-coating. However, as the bending loadincreases, both coating behaves similar. The early termina-tion of the elastic limit of the workpiece suggests that duringthe laser treatment, coating thickness reduces and coatingbecomes more compact, which, in turn, reduces the elasticlimit. Although laser treatment gives rise to compact struc-ture and thermal integration of splats in coating, due to thescattered splats with high oxygen content results in brittlestructure, particularly in the surface region. When comparingthe load–displacement characteristics uncoated and coatedworkpieces, the holding load of workpiece with coating ismuch higher than that corresponding to uncoated workpiecefor a known displacement. This shows that small thickness

F -pointb

Nd:YAG laser was used to irradiate the workpieces.xcitation source is 355 nm high power laser beam, whienerated from the third harmonic of Nd:YAG laser, moCR 250 Spectra Physics. The selection of 355 nm r

ion is due to the amount of absorption of the incident poy the substrate material, since reflectivity of the surfacuces considerably as the wavelength reduces. Laser denominal output energy of 450 mJ within a pulse lengt

bout 8 ns. The repetition rate of the laser pulses is 10 Hrder to increase the laser output pulse intensity a focu

ens with a nominal focal length of 100 mm was employhe diameter of the heated spot was kept at about 2.5 m

he workpiece surface.A computer controlled INSTRON 300 instrument is u

n the three-point bending tests. JEOL JDX-3530 scanlectron microscope (SEM) is used to obtain photomiraphs of coating.

. Results and discussions

Laser treatment of HVOF coating of Inconel 625 on mteel is considered and three-point bending tests areormed to investigate the mechanical and metallurgical prties of coating prior and after laser treatment.

The powder sprayed is examined under SEM as showig. 1. It is observed that the majority of the particles are npherical and some small satellites are attached. The paize ranged from 20 to 50�m with an average size of 35�m.he satellite particles are typically 4–12�m in size.

ig. 2. Load–displacement characteristics of workpieces after threeending tests.

956 B.S. Yilbas et al. / Journal of Materials Processing Technology 164–165 (2005) 954–957

Fig. 3. SEM micrograph of: (a) untreated-coating surface after three-pointbending test; (b) laser treated-coating surface after three-point bending test.

(∼250�m) of Inconel 625 coating enhances the toughnessof the workpiece considerably.

Fig. 3 shows SEM micrograph of laser treated-coatingand untreated-coating after the three-point bending tests. Thetensile stress field is developed in the coating region of theworkpiece while compression occurs at the top surface of theworkpiece, where bending indent is loaded. The maximumnormal and shear stresses take place at coating–base mate-rial interface due to large difference in stiffness of coating andbase material. The defects sites at the interface act as a sourcefor crack initiation and failure of coating. Moreover, locallyscattered splats with high oxygen content are also respon-sible crack initiation and propagation in coating. The cracksites in coating relieve the stress levels in this region. Con-sequently, crack size is small and closely spaced. The tensileshear deformation occurs in the coating in the direction ofbending. In the case of laser treated-coating, shear deforma-tion is larger due to compact structure and cracks initiated incoating elongates in the bending plane normal to the bendingaxis. Splitting separation of coating occurs in the crack siteswhile sharp edge cracks are resulted for laser treated-coating.

Fig. 4 shows SEM micrographs of laser treated-coating.The bubbles formed during laser melting coalesce and es-cape leaving small holes at the surface, which can be seen inFig. 4b. The bubble formation is due to high cooling rates; inwhich case, air trap in the melt zone might not escape dur-

Fig. 4. SEM micrograph of: (a) untreated-coating surface; (b) laser treated-coating surface.

ing the solidification as well as non-uniform cooling rates inthe melt pool could cause porous structure. The laser treatedlayer is almost perfectly compact and completely sealed, i.e.smooth surface with excellent sealing effect. However, melt-ing and re-solidification did not cause cracking in the surfaceregion where cooling rates are high. In addition, no spread-ing of droplets at the surface is observed, which indicates thatevaporation rates during melting are small. Some local hemi-spherical protuberances are observed after the laser treatmentin the region of outer surface of coating. This is because ofthe partial melting of large splats in this region.

4. Conclusions

HVOF spraying of Inconel 625 powders on mild steelworkpieces is considered. In order to achieve thermal inte-gration in and across coating, laser treatment of coating isaccommodated and Nd:YAG laser is used to irradiate coat-ing surface. The mechanical properties of laser treated anduntreated-coatings are examined through three-point bend-ing tests. It is found that the elastic limit of the workpieceafter laser treatment reduces slightly. In this case, the com-pact structure due to laser melting formed in coating low-ers the elastic limit of the workpiece. The shear stress at the

B.S. Yilbas et al. / Journal of Materials Processing Technology 164–165 (2005) 954–957 957

coating–base material interface triggers the crack initiation atthe locally defected sites. Moreover, splats with high oxygencontent also act as crack initiation centers due to structuralbrittleness. During laser treatment, high cooling rates resultsin trapping of the gas in the melt zone, which in turn enhancethe porosity in the surface region of coating. The presenceof hemispherical protuberances after the laser treatment inthe outer edges of coating is observed, which indicates theexistence of partially melted splats in this region.

Acknowledgements

Thanks are due to King Fahd University of Petroleum andMinerals in Saudi Arabia.

References

[1] K.S. Tan, R.J.K. Wood, J.A. Wharton, K.R. Stokes, Corrosion mon-itoring of high velocity oxy-fuel (HVOF) sprayed aluminium bronzecoatings, in: Proceedings of the Post Graduate Conference on Engi-neering Materials, 2001, pp. 33–34.

[2] V.H. Hidalgo, F.J. Belzunce, A. Carriles, S. Poveda, J. Mater. Sci. 37(2002) 649–654.

[3] A.H. Dent, D.G. Horlock, Mc Cartiney, S.J. Harris, J. Therm. SprayTechnol. 8 (1999) 339–404.

[4] C.H. Lee, K.O. Min, Surf. Coat. Technol. 132 (2000) 49–57.[5] B.S. Yilbas, M. Khaled, B.J. AbdulAleem, J. Therm. Spray Technol.

12 (2003) 572–575.[6] R. Potzi, Mater. Sci. Forum 163–165 (1994) 595–602.[7] P. Serra, J.M. Miguel, J.L. Morenza, J.M. Guilemany, J. Mater. Res.

16 (2001) 3416–3422.[8] J. Tuominen, P. Vuoristo, T. Mantyla, M. Kylmalahti, J. Vihinen, P.H.

Andersson, J. Therm. Spray Technol. 9 (2000) 513–519.