1

HIGH VELOCITY OXY-FUEL COATING AND LASER TREATMENT OF SURFACES WITH PRESENCE OF POWDER FILMS

Bekir Sami Yilbas

KFUPM Box 1913, Dhahran 31261, Saudi Arabia

High Velocity Oxy-Fuel Coating finds wide applications in power industry due to its superior properties, such as thermal and wear resistance, low processing cost, and precision of operation. The coating characteristics depend on the powder type, spraying gun, and spraying parameters. Although thermal spraying results in acceptable coating structures, secondary treatment, such as thermal integration of the splats, further improves the coating properties. During the spraying process, partially melted splats anchor the workpiece surface; however, the presence of oxygen in the spraying and coating environments initiates formation of oxidation layer in the vicinity of the splat surfaces. Since the anchoring process is random, oxide films may not cover the splat surfaces completely. This, in turn, appears as oxide inclusions in the form of stringers like particles in the coating. The thermal properties of these inclusions are different than those of splats resulting in stress centers in the coating. Consequently, the failure of coating becomes unavoidable under the mechanical loads. Laser control melting and thermal integration of splats may offer improved properties of the coating through structural refining. Moreover, laser treatment of surfaces through hard particle injections provides superior surface properties such as high wear and corrosion resistance. In this case, workpieces surfaces are pre-prepared using a carbon film hosting the hard particles prior to the laser treatment process. Since the microstructure at the surface region is modified through the controlled melting, the stress levels at surface vicinity influences the practical use of the treated surfaces. Consequently, investigation into the treatment process and the resulting microstructures are necessary. In the presentation, the processes involved in High Velocity Oxy-fuel Coating, microstructural analysis of the resulting coatings, and laser treatment of the surfaces towards thermal integration of the coatings will be presented. In addition, laser applications in powder injection into metallic surfaces in relation to improved surface properties will be enlightened.

Keywords: HVOF, Laser treatment, Metallurgy, Mechanical properties

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INTRODUCTION

High Velocity Oxy-Fuel (HVOF) coating of metallic surfaces finds wide applications in power

industry. This is because of the requirement of thermal and erosion protection of the surfaces

from the harsh environments. Thermal and corrosion resistant alloys are sprayed on to the

surfaces in powder forms using HVOF gun. Depending on the coating requirements powders can

be made from different materials such as Inconel 625/718, which are used for high temperature

protection while carbide blended particles are used for wear and corrosion protection

applications. The resulting coating properties, such as mechanical, metallurgical, and surface

roughness, depends on the powder properties and spraying parameters. Consequently,

investigation into mechanical and metallurgical properties of HVOF coating becomes essential.

In HVOF coating, the main mechanism forming the coating is associated with the anchoring of

semi-molten splats onto the metallic surfaces. Therefore, the formation of voids and pores in the

coating as well as at coating-base material interface becomes unavoidable. The voids and pores

influence the mechanical properties and reduce the corrosion resistance of the coating. One of

the methods for thermal integrity of the coating while minimizing the pores and voids is to apply

a control melting over the coating. Laser offers significant advantages over the conventional

methods for control melting operation. Some of the advantages laser treatment include precise of

operations, local treatment, and low cost. Since the melting process is complicated and involves

with high heating and cooling rates, metallurgical examination of melted regions in the coating

becomes essential from process optimization point of view.

Lasers can effectively be used to treat metallic surfaces for improved wear and corrosion

protection. In this case, metallic powders can be imbedded below the surface during the melting

process and hard facing can be formed after the controlled solidification of the treated surfaces.

Laser beam intensity can be kept high to achieve enough melt thickness below the surface.

However, high intensity laser beam initiates evaporation at the surface, which in turn produces

cavities while increasing surface roughness values of the treated workpiece. Therefore, a care

must be taken when selecting the laser treatment parameters. In addition, the use of assisting

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inert gas prevents high temperature oxidation reactions at the surface and lowers the surface

temperature through convection cooling at the surface. In the case of nitrogen assisting gas,

nitride compounds are possibly formed at the treated surface. This, in turn, modifies the

metallurgical changes in this region. The injection of particles during laser melting of the

surfaces contributes to the microstructural changes in the surface region. Consequently,

investigation into laser induced melting under the inert assisting gas and powder injection into

the melted surface becomes necessary for improved metallurgical changes in the treated layer.

Considerable research studies are carried out to examine HVOF coatings. Residual stresses in

HVOF metallic coatings was examined by Totemeier et al. [1]. Their findings revealed that for

coating applied to thick substrates, near the surface, residual stress was essentially constant with

increasing coating thickness. The residual stress formation in thermal spray coating was

examined by Ghafouri-Azad et al. [2]. They indicated that stresses were relieved by voids such

as pores and cracks in the coating. In addition, residual stresses increased with coating

temperature and could be decreased by pre-heating the workpiece. Mechanical and thermal

properties of HVOF sprayed nickel based alloys with carbide was examined by Hematani et al. [

3]. They showed that addition of 20 percent NiCr-Cr3C2 resulted in improved coating properties

such as high adhesion strength, high corrosion resistance, and thermal shock resistance. The

microstructure, phase and microhardness distribution of HVOF sprayed and conventional

coatings were studied by Aw and Tan [4]. They demonstrated that bimodal distribution of

Vickers microhardness values were obtained, which were attributed to the presence of melted

and un-melted phases in the resultant coatings. Residual stress formation in HVOF sprayed thick

coating was investigated by Stokes and Looney [5]. The analyzed the condition for changing the

stress-state from tensile to compressive while varying the coating thickness. HVOF coating and

residual stress measurement incorporating the modified layer removal method was examined by

Lima et al. [6]. They showed that the residual stresses were mainly influenced by the thermal

history regarding the quenching of individual splats and the plastic deformation of the ceramic

deposits. Measurement of residual stress in thermal spray coating through incremental hole

drilling method was carried out by Santana et al. [7]. They use the integral method for analyzing

the non-uniform trough-thickness stresses. The results revealed that the nature of the residual

stresses were tensile. The residual stress distribution in thermally sprayed alloy coatings was

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examined by Otsubo et al. [8]. They indicated that the trough-thickness residual stress

distribution of the HVOF coatings exhibited low values in the whole area of coating. Laser-

assisted spraying and laser treatment of thermally sprayed coating were investigated by Suutala

et al. [9]. They indicated that laser coating was free from cracks and coating corrosion resistance

was notably increased as compared with similar HVOF sprayed coatings. Laser re-melted

coating properties was examined by Totemeir et al. [10]. The findings revealed that laser melting

resulted in full homogenization of the sprayed structure. Laser treatment of HVOF coating and

characterization was carried out by Taha-Al et al. [11]. They showed that laser treatment resulted

cellular structure in the coatings with the presence of corrugated cells. The morphology and

phase modification of HVOF coating after electron beam re-melting was studied by Utu et al.

[12]. They showed that electron beam melting improved the corrosion resistance of the coating.

The optimization study for electron beam re-melting of HVOF cracking with the presence of

carbides in the coating was carried out by Hamatani and Miyazaki [13]. They found that re-

melting relatively low fusing speed and homogeneous heating were preferable for improved

coating properties. Adhesion testing of thermally sprayed and laser deposited coatings were

carried out by Hjornhede and Nylund [14].They indicated that delamination mechanism was

involved with initial formation of a residual crack in the coating after which the coating-substrate

interface came under an increased tension load. Corrosion behavior of HVOF-sprayed and ND:

YAG laser melted coating was studied by Tuominen et al. [15]. They showed that coating re-

melted with the highest transverse speed suffered from cracking due to rapid solidification

inherent to laser processing. Improved sealing treatments for thick thermal barrier coating was

examined by Ahmaniemi et al. [16]. They indicated that the porosity of the outer layer of the

laser glazed coating decreased leading to as increased microhardness of the surface. Corrosion

resistance of HVOF coating after laser re-melting was investigated by Tuominen et al. [17].

Their findings showed that laser treatment improved the net corrosion resistance and adhesion of

the coating. Laser treatment of HVOF coating was examined by Yilbas et al. [18]. They found

that the elastic limit of the coating reduced after the laser treatment process. Askandarani et al.

[19] investigated the influence of nanoparticles in thermally sprayed coatings. They

demonstrated that the presence of nanoparticles increased the coating hardness through grain

refining in the coating. Al-Shehri et al. [20] studied mechanical and metallurgical properties of

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two layered HVOF coatings. They indicated that, the coating properties were influenced by the

presence of the second layer; in which case, elastic modulus and fracture toughness of coating

reduced.

In the presence study, HVOF spraying and laser re-melting of the resulting coatings are

presented. The special emphasis is given on mechanical and metallurgical properties of the laser

treated coating. In addition, laser induced control melting and powder injection at the surface is

introduced. Mechanical and metallurgical properties of the laser treated surface are

demonstrated.

EXPERIMENTAL

A HVOF coating unit was employed to spray the powders. The spraying parameters selected

based on the optimum coating conditions are given in Table 1.

The powder had a particle distribution between 25-45 µm with spherical morphology with some

small and local satellite attachments.

Oxygen

Pressure

(kPA)

Fuel

Pressure

(kPA)

Air Pressure

(kPA)

Powder

Feed Rate

(m3/h)

Spray Rate

(Kg/h)

Spray

Distance

(m)

1023 600 750 0.8 6.6 0.30

Table 1. Process parameters of HVOF thermal spray.

JEOL JDX- 3530 scanning electron microscope (SEM) and EDS are accommodated to obtain

photomicrographs of the cross-section and surface of the workpieces after the tests. A computer

controlled INSTRON 300 instrument is used in the tensile, fatigue and three-point bending tests.

Indentation Tests for Young’s Modulus and Fracture Toughness:

Using the indentation tests, the Young’s modules can be formulated as [1]:

6

505150

16

9 ...* R.h.P)(E −−= (1)

where P is the applied load, h is the elastic penetration of the indenter, and R is the indenter

radius. The true modulus of elasticity can be determined using the indenter properties [21], i.e.:

i

2i

*

2

E

)1(

E

1

1E

υ−−

υ−= (2)

where Ei and νi are the Young’s modulus and Poisson’s ratio of indenter, respectively. In the

calculations Ei = 1141 GPa and νi = 0.07 (diamond indenter), and ν = 0.24 are taken.

The fracture toughness of the surface is measured using the indenter test data for microhardness

(Vickers) and crack inhibiting. The crack length (c) from the center of the indent is the sum of

individual crack lengths (∑l) and half the indent diagonal length. Therefore, c = a + ∑l [22].

However, depending upon the ratio of c

a, the equation proposed by Evans and Wilshaw [23] is

used to determine the fracture toughness (K), which is applicable for (0.6 4.5c

a≤ ≤ [3]), i.e.:

32

0.079 .log(4.5 )cP a

K Pa c

=

(3)

where P is the applied load on indenter, c is the crack length, and a is the half indent diagonal

length. Moreover, in order to visualize the cracks formed around the indention mark, top surface

of coating is grinded slightly.

Determination of Young’s Modulus from Three-point Bending Tests:

The Formulation of Young’s modulus is given in the previous study [24], therefore, only the

governing equations will be presented. After assuming the symmetry during the bending test, the

Young’s modulus can be written as [25]:

7

d48

PlIEIE

3

sscc =+ (4)

where Is and Ic are the moment of inertia of the substrate material and the coating, P is the

applied load, l is the distance between the supports, and d is the displacement of the substrate

material and coating during the bending tests. The moments of inertia are:

∫ ∫−

+

−

==2/h

2/h

h)2/h(

2/h

2c

2s

s

s

cc

c

bdyyIbdyyI and (5)

where b is the width of the substrate material and the coating.

The load-displacement data in the plastic region is used to compute the stress relaxation rate.

Fatigue Tests

The fatigue tested workpieces were categorized into four groups. These are as received, which is

a base substrate material, as received coated, which was HOVF coated, as received laser treated.

ASTM E-739 standard was employed to secure the reliable fatigue data, i.e., a replication of 85%

is accommodated in tests. Therefore, 80%, 70%, 65%, 40% of the ultimate tensile stress was

used as maximum alternating stress in the fatigue tests. In this case, the maximum stresses for as

received workpieces ranged 859 – 553 MPa, for as received and coated workpieces ranged 788 –

394 MPa, for as received heat treated ranged 805 – 537 MPa, and for coated and heat treated

ranged 493 – 394 MPa.

Laser Treatment Tests:

The CO2 laser (LC-ALPHAIII) delivering nominal output power of 2 kW was used to irradiate

the workpiece surface. The nominal focal length of the focusing lens was 127 mm. The laser

beam diameter focused at the workpiece surface was ∼ 0.9 mm. Nitrogen assisting gas emerging

from the conical nozzle and co-axially with the laser beam was used. Table 2 gives the laser

treatment parameters.

Table 2 Laser heating conditions used in the experiment

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Scanning

Speed

(cm/s)

Power

(W)

Frequency

(Hz)

Nozzle Gap

(mm)

Nozzle

Diameter

(mm)

Focus

setting

(mm)

N2

Pressure

(kPa)

10 140 1000 1.5 1.5 127 550

RESULTS AND DISCUSSION

HVOF coating process and laser treatment of the resulting coating are examined. The influence

of the presence of WC powders on the coating microstructure after the laser treatment process is

presented. Laser controlled melting and TiC particle injection in the melted surfaces is also

demonstrated. The stress formed at the surface region and microstructural changes in the laser

treated layer are presented. The details of the results and discussion presented in this section can

be found in [19], [20], and [26 - 28].

In general, powders used in spraying had a particle distribution between 20-35 µm with spherical

morphology with some small and local satellite attachments (figure (1)). It had excellent high

temperature oxidation and corrosion properties.

Figure 1. SEM micrograph of powders used in the spraying process.

Figure (2) shows SEM micrograph of HVOF coating cross-section. The coating thickness is in

the order of 250 µm. Lamella structure occurs in the coating due to multi-pass spraying process

and partially melted particles are evident appearing as rounded shapes. Moreover, small pores,

which are scattered across the coating cross-section, are observed and the porosity of the coating

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varies within 2%-4%. Oxidation of some splats occurs in-flight prior to impacting at surface,

which is associated with high temperature spraying process. The dark inclusions in the coating

are the oxide compounds, particularly stringers like oxides are observed in between the splats. In

coating, the scattered oxide particles de-bond through their interfaces in the coating. This

situation occurs mainly in chromium oxides. Consequently, elongated brittle oxides (stringers

like oxides) enhance stress concentration and crack initiation.

Figure 2. Optical photograph and SEM micrograph of splats and oxide formation around the

splats.

Laser Treatment of HVOF coating:

The development of transverse periodicity in the solidification process is attributed to the

instability during the super-cooling process; in which case, cellular sub-structures are formed

and then fine regular corrugated structures are developed extending along the regular cellular

boundary. The corrugations are roughly parallel to the direction of growth of the crystal (Figure

(3)). Furthermore, the formation of the cellular structure (Figure(3)) is because of the liquid,

which is rapidly decanted exposing the solid-liquid interface. The cell size increases with

decreasing rate of growth and the growth direction depends on the impurity content, speed of

Oxide layer Splat Splat boundary Splats

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growth, and the inclination of the dendrite direction to the growth direction. The heat flow and

cooling rate are related to the asymmetry in the shape of the cells, which in turn results in

anisotropy of the growth rate of the cells. The segregation occurs at the grain boundaries during

the process of solidification (Figure (3)), i.e. two crystals grow side-by-side and the boundary

between them forms a groove. It should be noted that cellular segregation occurs when super-

cooling takes place during the solidification. However, segregation degreases as a result of

diffusion during the cooling after the solidification. In the cooling process, if the temperature

gradient is reduced, then the zone of super-cooling extends. Consequently, the cells change to

characteristic of dendrites forming cellular dendrites as seen from Figure (4). This appearance is

distinct from the cellular structure and free dendritic growth. One of the causes for this type of

morphology is that the cellular dendritic type of growth occurs when the temperature gradient is

small in the liquid phase providing the heat rejection into the solid at a low rate. Alignment of

dendrites forms webs, which enhances conducting path for heat flow from the liquid to the

crystals. It should be noted that the cellular-dendritic growth differs from cellular growth; in

which case, the depth of super-cooled zone is greater for cellular growth.

Figure 3. SEM micrograph of laser HVOF coating cross-section after the laser treatment. The

cellular and corrugations of the cellular structure are observed.

Segregated

cellular

structure

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Figure 4. SEM micrograph of laser HVOF coating cross-section after the laser treatment. The

dendritic structure is observed in the surface region of the coating.

Mechanical Properties of HVOF Coating:

Fracture Toughness

Table 3 gives the Young’s modulus of the coatings prior and after the laser treatment. The

Young’s modulus obtained from three-point bending tests for uncoated substrate and data

obtained from literature are in good agreement. Moreover, Young’s modulus determined from

three-point bending tests and indention tests are in good agreement. However, laser treatment

modifies the Young’s modulus, in which case it increases. In addition, thermal stresses

developed during the laser treatment process results in brittle structure in the surface region of

the coating due to high rate of oxygen diffusion taking place in this region. Consequently, the

crack formation initiates in the surface region and extends towards the coating. However, the

multiple crack initiation in the surface region of the coating relaxes the stresses in this region.

Once the crack formed extends in the coating, particularly, reaching the interface of coating-base

material, the coating integrity fails and it peels off from the base material surface, i.e., total

failure is resulted. The fracture toughness obtained for the coatings subjected to the heat

treatment is less than that of untreated coatings. This is because of the high oxide content in the

surface region of the coating, which in turn results in a brittle structure. In addition, low

Dendrite

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Young’s modulus of the coating is also responsible for less fracture toughness of heat treated

workpieces.

Three Point Bending Tests

(GPa)

Indention Tests

(GPa)

As Received 138 141

Laser Treated 250 265

Table 3. Young’s modulus obtained from three point bending and Indention tests.

Fatigue Test

Figure (5) shows S-N curve, alternating stress with number of cycles prior to fracture, for coated

and uncoated as well as heat-treated coated and uncoated workpieces. Annealing heat treatment

of Ti-6 Al-4V alloy results in low strength and high ductility and some improvement in fracture

toughness occurs after heat treatment. The life cycle of the heat-treated and uncoated workpiece

is less than its counterpart corresponding to as-received workpiece. In this case, oxide formation

in the surface vicinity of the annealed workpiece gives rise to brittle structure in the surface

region. In addition, due to differences in thermal expansion coefficient of coating and the base

substrate, thermally induced residual stress is formed at coating-base substrate interface. These

are the main sources for early failure of the annealed workpiece. In general, the coating on the

surface of the alloy enhances the fatigue life of the workpiece. Moreover, any cracking and

coating failure due to local delamination results in reduction in the area of load carrying

segments. This situation enhances the low fatigue life of the heat treated workpieces. As-

received and coated workpiece results in highest fatigue strengths. This may be attributed to

strong bonding between the coating and the substrate material. In this case, grit blasting modify

the substrate surface such that surface hardness improves and plastic deformation that takes place

in the substrate surface vicinity. Moreover, the particle impacting force and particle size most

likely result in locally distributed compressive residual stresses concentrations in the surface

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region. Since the substrate surface was cleaned prior to thermal spray process, impurities left

over after grit blasting were removed. Therefore, the effect of impurities, captured between the

coating and the substrate material, on the fatigue response of the coated workpiece is minimized.

The fatigue failure of laser treated workpiece is expected to be faster than the as-received

workpieces. In addition due to the differences in thermal expansion coefficient of the coating

and substrate material, high concentration of residual stresses develop at the interface of the

substrate material and coating after the heat treatment process. This gives rise to total failure of

coating through delaminating or peeling off from the substrate surface during fatigue testing.

100

1000

10000

1.0E+04 1.0E+05 1.0E+06

ALT

ER

NA

TIN

G S

TR

ES

S (M

Pa)

NUMBER OF CYCLES

As Received - Uncoated

As Received - Coated

Laser Treated Uncoated

Laser Treated Coated

Figure 5. Logarithmic scale of alternating stress with number of cycles for different fatigue

tested workpieces.

Three-Point Bending

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Three-point bending tests are conducted two layered HVOF coated surfaces. The findings are

given below.

Figure (6) shows the cross section of the coatings. There is no loose splats or surface asperities

such as cavities or cracks are observed at the top surface of the coating. The micrographs for

cross-section of the coating shows that lamella like structure is formed due to the presence of

molten state of splats on set of impacting the surface. However, locally scattered spherical splats

are also evident. This indicates the presence of semi-molten state of some splats. However, they

appear to be few in number. Moreover, the presence of the dark inclusions (stringers) in the

surface region of the coating is evident. This indicates the presence of oxide particles with small

size, which is attributed to oxidation of small particles during in-flight. However oxidation of the

splats at coating surface after the impacting also contributes to the oxidation state of the coatings.

The splat size has significant effect on the oxidation process, in which case, oxygen content

increases as the particle size reduces. The splat size changes for Diamalloy 2002 due to the

presence of WC, which is 12%, in the coating. The porosity of Diamalloy 2002 coatings is

higher than that of Diamalloy 4010. This is because of the presence of solid phase WC particles,

which does not integrate with neighboring splats in molten state.

Diamalloy 4010

Diamalloy2002

15

Figure 6. Cross-sections of the HVOF coated workpieces.

Figure (7) shows load-displacement curve obtained from the three-point bending tests. It should

be noted that three point bending is carried out at constant stress rate. The elastic-plastic behavior

of the coatings, due to Diamalloy 2002 and Diamalloy 4010 as well as two layered structure, are

different. The flexural displacement increases at low load levels for coating produced from

Diamalloy 4010 powder while it is low for Diamalloy 2002 coating. This is attributed to the

presence of WC content in the coatings, which makes the coating stiffer and harder. The sudden

drop in flexural displacement reveals the failure of the coating during the bending tests. This

occurs after the long flexural displacement for Diamalloy 2002 coating and two-layered

structure. This suggests that the gradual deformation takes place, which relieve the stress levels

in the coating while contributing to the stress relaxation during the bending tests. Table 4 gives

the elastic modulus determined form the three-point bending tests. It is evident that Diamalloy

2002 has the highest elastic modulus because of the presence of 12% WC in the coating.

Diamalloy 4010

(GPa)

Diamalloy 2002

(GPa)

135 240

Table 4. Elastic modulus determined from three-point bending tests.

First layer

Second layer Second layer

First layer

16

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6

DISPLACEMENT (mm)

LO

AD

(N)

Powder 2002

Powder 4010

Two-Layer: 2002/4010

Figure 7. Load-displacement characteristics obtained from three-point bending tests.

Figure (8) shows SEM micrograph of fractured surface after three-point bending tests. Since the

coating was applied at the top and bottom surface of the workpieces, coating failure is due to

compression and tensile-shear. The extended crack formation at the bottom surface, where

tensile-shear failure takes place, is evident. However, in some region, the delimitation of coating

occurs because of excessive shear stress developed in the coating during the bending test. In

addition, the presences of oxide particles contribute to the shear failure, particularly in the

surface region of the coating. The tensile–shearing force enhances the internal stresses while

creating local stress concentrations in the coating. This s more pronounced at defect sites such as

oxide particles in the coating. Consequently, stress concentrations at defect sites become higher

than that of the mean internal stress. As bending progresses, a critical stress levels is reached at

the defect sites. This triggers the large crack formation. However, the presence of defect sites at

coating-base material interface results in the total failure of the coating, i.e. coating peels of form

the base material surface. In the surface region, the crack propagation is limited with this region;

in which case, small cracks are formed during the bending. However, if the energy dissipated

during micro cracks formation, the crack propagation ceases and the microcracks are formed in

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the surface region. In addition the compressive stress developed at the top surface of the

workpiece resulted in partial peeling of the coating in the surface region.

Diamalloy 2002 Diamalloy 4010

Figure 8. SEM micrographs of coating surfaces after three-point bending tests.

Laser Treatment of Surfaces with Presence of TiC particles

Surface Treatment of Ziconia:

Laser treatment of pre-prepared t-zirconia surface is carried out at high pressure gas

environment. The surface prepared contains a carbon film of about 50 µm thickness with the

presence of 7% TiC particles.

Figure (9) shows optical photograph and SEM micrographs of the top surface of the laser treated

workpiece. The laser treated surface composes of regular laser scanning tracks with 200 µm

wide. The close examination of the scanning tracks reveals that the workpiece surface is melted

over the laser tracks and there is no evidence of excessive molten flow between the tracks.

However, some scattered TiC particles in the solid phase, particularly in between the laser tacks,

are observed. This is attributed to the high melting temperature of TiC particles and low laser

irradiated energy between the laser tracks. In addition, no cavities and pores are observed at the

surface.

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a) SEM micrograph of laser treated surface. b) Close view of laser treated surface.

Figure 9. Optical and SEM micrographs of the laser treated surface.

Figure 10. SEM micrographs of cross-section of laser treated regions: a - Fine grains forming a

dense layer, b - Next to surface vicinity with some voids and presence of TiC particles.

a b

TiC Particles

Voids

19

Figure (10) shows SEM micrographs of the cross-section of the laser treated workpiece. The

uniform thickness of the laser treated region is evident from the micrograph. The depth of the

laser treated region extends almost 50 µm below the surface. Three regions can be identified

from the SEM micrographs. The first region corresponds to the surface vicinity and dense

structures consist of fine grains and TiC particles are observed in this region. The presence of

fine grains is attributed to the high cooling rates in the surface region. This is more pronounced

at the surface vicinity as shown in the Figure 10a. However, the formation of nitride species and

the presence of TiC particles in the neighborhood of surface vicinity results in formation of

small sized voids in this region, which indicates the volume shrinkage due to the density

variation and high cooling rates in this region (Figure 10b). In addition, the carbonic gases

resulted during the formation of ZrC contributes to the formation of voids in this region. It

should be noted that the experiments were repeated for different laser parameters to avoid the

voids formed in the neighborhood of the laser treated surface. In this case, the local evaporation

of the surface forming the locally scattered cavities was resulted at high laser power intensities

and low laser scanning speeds. On the other hand, reducing laser power intensity while

increasing laser scanning speed lowers the depth of the laser treated layer. Nevertheless, the void

sizes are small and they are randomly distributed below the surface. The fine structures in the

close region of the surface vicinity reveals the possible formation of the nitride and carbonitride

compounds in this region, since the nitride species are associated with the fine structures or small

dendrites in this region.

Surface Treatment of Alumina:

Figure (11) shows SEM micrographs of cross-section of the laser treated layer. The laser treated

layer consists of three regions, which are distinctly observed along the depth of the laser treated

layer. In the first region, fine grains forming a dense structure are observed. In this region, the

formation of small grains is due to the fast cooling rates. In addition, the presence of scattered

TiC particles is evident. This is mainly situated in the surface vicinity. The dense structure also

contains the nitride compounds, which can be seen from the XRD diffractogram (figure (12)). In

the second region, columnar like growth, which is almost normal to the surface, is observed. This

indicates relatively slower cooling rates as compared to that corresponding to the surface. It

20

should be noted that temperature in this region is not as high as at the surface. In addition, the

thermal conductivity of the alumina suppresses the fast cooling rates in this region. In addition,

the impinging assisting gas enhances the cooling rates at the surface. Moreover, no voids, pores

or microcracks are observed, in which case, almost uniform structure is formed in this region. It

should be noted that AlN has low density than Al2O, which in turn results in volume shrinkage in

the surface region. Consequently, the volume shrinkage due to nitride compounds contributes to

the dense structure in the surface vicinity. This situation is also seen at the interface of the first

and the second regions, i.e. partially developed columnar structure is observed at the interface

region. In the third region, large grains of Al2O3 are observed. The formation of large grains in

this region is attributed to the slow cooling rates. This is associated with the heat dissipated

through conduction from the second region to the third region, which takes place at a slow rate

because of low thermal conductivity of alumina. The heat affected zone between the laser treated

layer and the base substrate is not clearly visible from the SEM micrographs. However, it is

expected that the heat affected zone could be shallow because of slow cooling rates in the third

region.

Figure 11. SEM micrographs of cross-section of laser treated region.

21

Figure 12. XRD diffractogram of as received and laser treated workpieces.

CONCLUSION

HVOF coating and laser treatment of coating surfaces are investigated. The microstructural and

morphological changes prior and after laser treatment process are examined. The residual stress

formed in the coating and fracture toughness of the coating surface are presented. In addition

laser controlled melting and powder injections at the surface of engineering alloys are also

examined and metallurgical changes in the treated region are demonstrated. It is found that laser

treatment of HVOF coating improves the coating surface roughness considerably and the void

size in the coating reduces significantly. The coating microstructure changes and cellular type

structure with varying sizes are formed in the coating. The fracture toughness of the coating is

influenced by the presence of the second layer in the coating. Laser surface treatment and TiC

particle injection result in nitride compounds formation at the surface. This, in turn, increases the

microhardness of the surface. The volume shrinkage at the laser treated surface due to the

formation of fine structures and nitride compound contributes to microhardness at the surface

22

region. Since the melting temperature of TiC particles is high, partially dissolved TiC particles

are observed in the surface of vicinity. The differences in thermal expansion and contraction of

TiC particles and the base material, stress centers are formed around the TiC particles. This

results in fine microcracks formation around TiC particles. Since the microcracks are locally

scattered, their affects on the toughness of the surface are minimal.

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Acknowledgements The authors acknowledge the support of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia for this work.