University of Groningen

Surface modification of titanium with lasersKloosterman, Annejan Bernard

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2 EXPERIMENTAL CONCEPTS

2.1 Lasers

2.1.1 Introduction

In 1917, Einstein laid the foundation for the laser by introducing thephenomenon of stimulated emission.1 This occurs by the transition of anatom or molecule to a lower energy state under influence of electromagneticradiation. Many years later (in 1960), Maiman demonstrated the first laser byusing a ruby crystal.2 The acronym laser stands for "Light Amplification byStimulated Emission of Radiation". The principle of amplification is to createa population inversion between the ground and an excited state, by pumpingatoms from the ground state to a higher energy level. Subsequently,stimulated emission takes place if an incoming photon, with an energycorresponding to the energy transition of the atom, causes the emission of acoherent photon. By using a resonant cavity there will be an avalanche ofstimulated emission of photons and even more amplification takes place. Thecreated beam can be transported from the cavity by using a semi-transparentmirror. Besides its coherence, the emitted light is highly monochromatic anddirectional which enables very efficient focusing.3

The aforementioned properties of the laser beam make it possible to obtainvery small spot sizes. As a consequence, very high intensities up to 1010

W/m2 can be attained. For surface modification both a high power densityand a high total power are necessary, which restricts the type of lasers to beused. Presently, two types of high-power infrared lasers are available on themarket.

The first type is the continuous wave CO2 laser with a maximum power inthe range of kilo-Watts. This laser contains a mixture of three gases: nitrogen,helium and carbon dioxide. Vibrational modes of the CO2 molecule give riseto the laser effect with a wavelength of 10.6 µm. The efficiency of the laser is~10 %, which is relative high in comparison to many other types of lasers.

The second type is the Nd-YAG laser. Only recently, it is possible to reachoutput powers in the range of kilo-Watts for this type of solid state laser. Theactive element Nd3+ gives rise to the laser effect with a wavelength of 1.06µm. This wavelength makes it possible to transport the beam by means of

4 CHAPTER 2

A

B

B

C

D

E

F

G

HI

B

J

Figure 2.1 The laser set-up for both the Spectra Physics 1.5 kW CO2 and Rofin Sinar 2 kWNd-YAG laser. A=CO2 laser column, B=Nd-YAG focus head, C=powder feeder, D=lasercontrol unit, E=gas control unit, F=x-y table, G=powder cyclone, H=powder nozzle, I=x-y-zpowder positioning system, J=specimen.

optical fibres. In principle the Nd-YAG laser operates in a pulse-mode but byusing high frequencies a nearly continuous mode can be achieved. The maindrawback of this laser type is its low efficiency, which is 2-3%.

Both laser types can be used for cutting and welding applications. Untilrecently, only the CO2 laser was used to perform surface modifications likehardening, alloying and cladding. By the advent of powerful, nearlycontinuous Nd-YAG laser systems, these type of surface modifications arenot restricted to CO2 lasers anymore. In industry, there is a tendency toreplace the high power CO2 lasers by the new generations of Nd-YAG lasers,unless there are material restrictions involved. Especially the flexible fibreoptical beam delivery of the latter laser type, plays a crucial role.Furthermore, for metals the absorption of the laser radiation is much higherin the case of the Nd-YAG laser, which compensates its low efficiency. Figure2.1 displays the actual set-up for both the CO2 and Nd-YAG laser.

2.1.2 Laser nitriding

The laser nitriding experiments are carried out with the use of the continuouswave 1.5 kW Spectra Physics 820 CO2 laser. This laser operates in a TEM00

mode which results in a nearly Gaussian intensity distribution. The diameterof the unfocused beam amounts to 19 mm. After downwards reflection by asilicon mirror the beam is focused by a water-cooled ZnSe lens with a focallength of 127 mm. The minimum diameter of the focused beam amounts to~0.2 mm. Between the lens and the specimen a nozzle is assembled to supplya shielding gas. The benefit of the gas is to protect the lens as well as toprevent oxidation of the specimen. The specimen movement is realised by anCNC (computer numerical controlled) X-Y table. For alignment procedures aHeNe laser beam can be transmitted along the optical axes.

In fact, the laser nitriding of titanium only requires the presence of a nitrogenatmosphere near the laser melt pool. The best way to realise this is by using aclosed chamber in which an overpressure is applied. In that case, the laserbeam is transmitted by using a ZnSe window. However, it appeared that theatmosphere close to the melt pool should be frequently refreshed, to preventplasma formation. This can be achieved by using a movable nozzle in theclosed chamber, which is hard to realise. Another possibility is to use amulti-nozzle system in air. The axial nozzle is used to supply the nitrogenatmosphere. The side flow is necessary to prevent plasma formation and thethird nozzle prevents the suck in of oxygen by the side nozzle. All nozzle

EXPERIMENTAL CONCEPTS 5

diameters at least amount to several times the melt pool diameter. In order toprevent mixing with air, the gas flow must never become turbulent. Theapplied total gas flow amounts to 10 l/min.

To increase the absorption of the laser radiation, the specimens aresandblasted before processing. Subsequently, they are ultrasonically cleanedto remove induced impurities. For laser surface modification, the outputpower of the laser ranges from 1 to 1.5 kW, while the scan velocities varyfrom 25 up to 200 mm/s. Further, the focal point of the laser is set 4 mmbelow the surface, resulting in a spot diameter of 0.6 mm. Both single tracksand overlapping tracks are applied.

2.1.3 Laser embedding

The laser particle injection process is performed with the use of a continuouswave 2kW Rofin Sinar Nd-YAG laser. This process is schematically depictedin figure 2.2. The laser beam is transported by means of fibre optics, resultingin a homogeneous intensity distribution. After leaving the fibre, the beam iscollimated prior to being focused to the desired spot size. The diameter of thefibre amounts to 0.8 mm, which is at the same time the minimum spot size.The lens system is water-cooled and has a focal length of 120 mm. The use ofa semi-transparent mirror in combination with a camera, allows the opticalalignment of the specimen along the optical axis. Between the lens and thespecimen a nozzle is assembled to supply a shielding gas. The benefit of thegas is to protect the lens as well as to prevent oxidation of the specimen. Thespecimen movement is realised by an CNC X-Y table.

To avoid harmful reflections at the surface of the specimen back into theoptical system, the beam delivery system is positioned under an angle of 11°with the surface normal. The focal point of the laser is set 10 mm out of focus,resulting in a spot diameter of approximately 3 mm. The output power variesfrom 1 up to 1.5 kW, while the scan velocity range from 6-12 mm/s.

To embed a new phase in a substrate by means of laser processing, twodifferent methods can be applied. In the first place, the new material can bepre-positioned on the substrate. However, to melt the substrate the heat hasto be transported through the pre-positioned powder slurry. If the meltpointof both materials does not differ to a large extent, a reasonable degree ofmixture may occur. If this is undesirable, the possibility of powder injection

6 CHAPTER 2

should be considered. In this research the latter mentioned method is usedfor injecting ceramic particles into liquid titanium.

The powder feeding apparatus (Metco 9MP) provides a constant transport ofthe ceramic particles. This is realised by making use of gravity, fluidization,pressure difference and a carrier gas. For laser processing, the disadvantageis that the carrier gas distorts the melt bath. Therefore, it is essential to drainaway the carrier gas before reaching the melt. This is accomplished by meansof a cyclone, in which the major gas flow escapes through an upper outlet. Atthe same time, centrifugal forces prevent the powder to escape by the upperoutlet. So the powder is fed through the outlet at the bottom of the cyclone,with an amount of gas depending on the ratio between the diameter of thetwo outlets.

EXPERIMENTAL CONCEPTS 7

shielding gas

laser beam

carrier gas outlet

cyclone

powder

specimen

x

y

z

Figure 2.2 Schematic set-up of the particle injection process using a cyclone. The specimenis moved underneath the laser beam in the x-direction. The entire surface is coated byproducing successive laser tracks, which might overlap each other.

2.2 Microscopy

2.2.1 Metallographic examination

In the process of metallographic examination three stages are involved,namely preparation, etching and microscopical examination. Although, thepreparation process influences all next stages, it is without any doubt themost neglected part. Abraded surface layers always contain a damage layerwhich may result in the appearance of false structures.4 In order to removethis damage layer, the surface is subjected to polish treatments. However,this treatment may result in the formation of a plastically deformed layer,better known as the Beilby layer. The reason for this is that only a portion ofthe abrasive particles remove material by cutting, while the remainder indentinto or rub across the surface. During preparation, all these phenomenashould be kept in mind.

Before microscopical examination, the following preparation and etchingprocedure is applied. First, the specimens are abraded with sandpaper (320and 600 grit). After each sandpaper treatment the surface is quite rough anddamaged. Further abrasive particles may be stuck in the surface layer. Inorder to remove the deformed layer, including the indented particles, thespecimen is etched with Keller reagens after each sandpaper step. Allsandpaper treatment must be short, because the paper degrades quickly,which is caused by a decrease of cutting particles and an increase in rubbingparticles. At this stage, only the scars caused by the abrasive particles arevisible. The procedure continues with two polish treatments with the use of aBuehler polish machine. Firstly, a polish cloth with 9 µm diamond particlesin an oil suspension is applied, again followed by etching. Secondly, thespecimen is subjected to a polish treatment with 0.06 µm SiO2 particles in awater suspension. Finally, the specimen is etched moderately to reveal themicrostructure. In between the successive steps the surface is evaluated witha standard optical microscope (Olympus Vanox-AHMT).

2.2.2 Scanning electron microscopy

One of the most versatile instruments to study the microstructure ofmaterials is the scanning electron microscope. The resolution of a standardSEM is in the range of 5-10 nm, whereas only recently a resolution of theorder of 1 nm has been achieved. Furthermore, the depth of focus is large incomparison with light microscopy. Besides the improved imaging qualities

8 CHAPTER 2

the potential of the SEM increases by the advent of additional researchequipment. Presently, element analyses by EDX and crystallographicresearch by EBSD5 can be performed simultaneously in the SEM.

To study the microstructure a Philips XL-30 FEG-SEM, provided with a fieldemission gun, was used. The advantage of the field emission gun is itsbrightness and its high spatial coherence, resulting in a higher resolution incomparison with LaB6 filaments. The schematic representation of the SEM isdepicted in figure 2.3. The emitted electrons are accelerated by an electricfield up to an energy of 1-30 keV. By means of electromagnetic lenses thebeam is focused at the surface of the specimen. The minimum spot sizeamounts to 1 nm. Deflecting coils enable the electron beam to scan a certainregion of the surface.

The interaction of the electron beam with the specimen initiates severalphenomena, as shown in figure 2.4. Firstly, the incident electrons can bebackscattered (BSE) by the interaction with the nucleus of an atom.Therefore, the number of BSE increases with increasing atomic number.Under the appropriate geometrical conditions these BSE can also provide

EXPERIMENTAL CONCEPTS 9

SE detector

scanningcoils

condensorlenses

electronsource

specimen

imaging

illumination

X-ray detector

Figure 2.3 Schematic representation of a scanning electron microscope.

crystallographic information by means of diffraction. This will be describedin more detail in chapter 5. Secondly, the primary electrons release secondaryelectrons (SE) from the specimen with energies ranging from 1-50 eV.Because of their low energy, only the SE which are generated close to thesurface (1-10 nm) will escape and contribute to the SE signal. This results in ahigh lateral resolution and makes the SE signal suitable for studying thesurface topography.

Other relevant phenomena are related to the generation of SE. When theseare removed from an inner atomic shell, they leave behind an excited state.Relaxation takes place by electrons from outer shells which will fill up theseinner shell holes. This process is accompanied by the generation of Augerelectrons or X-ray photons with a characteristic energy. This characteristicenergy can be translated to the chemical composition of the specimen. Foreach excited atom one or more specific energy transitions may occur witheach their own probability. In comparison with electrons, the mean free pathof X-rays is quite large. Therefore the X-ray signal comes from a relativelylarge volume resulting in a poor spatial resolution. In the presentexperiments, X-rays are detected by means of an EDAX Energy DispersiveX-ray spectrometer, containing a sapphire detector. In combination with thesuper-ultra thin window, this detector is also sensitive for the lighterelements. The incoming X-ray energy creates electron-hole pairs in anintrinsic semiconductor. These are collected and detected as a charge pulse.

10 CHAPTER 2

Incident electron beam

Auger electrons Secondary electrons

Backscattered electronsCharacteristic x-ray

Figure 2.4 Interaction of the primary electron beam with the specimen. The primaryelectrons generate secondary electrons and can be backscattered. The generation ofsecondary electrons induces both Auger electrons and characteristic x-rays.

The pulse height can be translated to the X-ray energy and on its turn to theoriginal element.

2.2.3 Transmission electron microscopy

Information on the local microstructure concerning phase, orientation anddefect structures, can be obtained by means of transmission electronmicroscopy (TEM).6,7,8 The presented research is done with the use of ananalytical JEOL 200 CX TEM and a JEOL 4000 EX/II high resolutiontransmission electron microscope (HRTEM). The latter mentioned is able toresolve the atomic structure. The highest resolution, whereby two latticeplanes can be observed separately, amounts to 0.17 nm. The schematicrepresentation of a transmission electron microscope is depicted in figure 2.5.An electrically heated LaB6 filament emits electrons, which are accelerated upto 200 keV or 400 keV for the two different microscopes, respectively.Subsequently, the electrons are aligned along the optical axis and focused bythe use of several electromagnetic lenses and electrical deflectors. A more

EXPERIMENTAL CONCEPTS 11

condensor

apertures

electronsource

specimen

objective

intermediate

projector

O apertureSA aperture

phosphorscreen

Figure 2.5 Schematic representation of a transmission electron microscope.

thorough description of the basic design of the TEM is given by Hirsch et al.9

The almost parallel electron beam will be scattered by the specimen. Forcrystalline materials, electron diffraction is likely to occur for the geometryfor which the Bragg law, , is satisfied. Here λ is the Broglienλ = 2dsinθwavelength, dhkl the interplanar spacing and θ is angle between the directionof propagation of the incident electron and the (hkl) crystal planes.

The formation of images and diffraction patterns

The first impression of the microstructure can be obtained by only using thetransmitted beam for image formation. This is called the bright field image.The detection of the transmitted electrons only, can be achieved by the use ofthe objective aperture. The contrast in this image is a result of scatteredelectrons into Bragg reflection directions from different parts of theilluminated area.

Besides in image mode, the TEM can also operate in diffraction mode. Thismeans that instead of the projection of an image, the differently diffractedbeams will be displayed on the phosphor screen, resulting in the formation ofa diffraction pattern. This can be achieved by adjusting the intermediate andprojector lenses. If the scattering direction of the electrons satisfies theaforementioned Bragg condition, diffraction is likely to occur. Moreover,constructive interference of the diffracted electrons, results in a diffractedbeam with an intensity proportional to the structure factor10

(2.1)Sg = Σ fje−2πi(g⋅r j )

here fj is the atomic form factor, which is a measure for the scattering powerof the atom in the unit cell positioned at . Further, represents ther j ghkl

reciprocal lattice vector and is equal to the difference between the diffractedand the incident beam: . By calculating the structure factor, thek0 − ki

occurrence of forbidden reflections in the diffraction pattern can bedetermined. Nevertheless, these spots still may be present in the diffractionpattern by the phenomenon of double diffraction.7

By means of selected area diffraction (SAD), information can be obtainedfrom a specific area of the specimen. This can be achieved by positioning anaperture at the back focal plane of the objective lens. In this manner, only thetransmitted and diffracted rays of the selected area are allowed to pass.

12 CHAPTER 2

When subsequently changing to diffraction mode, the diffraction pattern isonly formed by the selected area. In this way, the phase and orientation of asingle crystal or precipitate can be obtained. For polycrystalline materialswith grain sizes substantially smaller than the selected area, many differentlyoriented grains contribute to the diffraction pattern. This results in theformation of a ring pattern.

Instead of using the transmitted electron beam for image formation, it is alsopossible to use one of the diffracted beams. This technique is called dark field(DF) imaging. When selecting the diffracted beam by moving the apertureposition, the image quality is poor due to the deviation of the optical axis.This problem can be solved by deflection of the transmitted beam, in such away that the diffracted beam is aligned along the optical axis, instead of thetransmitted one. The DF imaging technique is very useful in analysingcomplicated diffraction patterns and in performing quantitative analysis ofcrystal defects.

Defect analysis

The way in which microstructural features appear in conventional TEM is aresult of differences in intensities of the diffracted beams. In order to obtainhigh contrast images of lattice defects, the specimen should be tilted into atwo-beam condition. As a consequence, only one specific set of crystal planessatisfies the Bragg condition and contributes to the image formation. Whendislocations are to be imaged, the resolution can be improved by tilting thespecimen away from the two-beam position. This should be done in such away that the exact Bragg reflection is only fulfilled within a small region nearthe dislocation. The aforementioned technique is called weak beamimaging.11 The deviation from the exact Bragg position is given by

. s = k0 − ( ki + g)

In practice, the optimum magnitude of can be determined by making use ofsthe Kikuchi pattern12, which will only appear when a specimen is reasonablythick and contains a low defect density. The latter could be a problem whenanalysing laser tracks, since a high cooling rate results in substantial stresses.The Kikuchi pattern is realised by diffracted electrons that have beenpreviously inelastically scattered13, and consists of parallel pairs of bright anddark lines representing crystallographic planes. The exact deviation froms

EXPERIMENTAL CONCEPTS 13

the Bragg condition can be determined by the position of the Kikuchi lineswith respect to the diffraction spots.

In order to determine the increase in resolution of defects by using the weakbeam condition, the extinction distance has to be considered. In case =0,ξg s

is given byξg

(2.2)ξg = π Vcosθλ sg

where V is the volume of the unit cell, θ the Bragg angle and λ the electronwavelength. The width of a dislocation can be described in terms of theextinction factor and is approximately 0.3ξg. For most metals, the magnitudeof ξg ranges from 15 to 200 nm. In weak beam condition, which means that

, the effective extinction distance is reduced tosg ≠ 0 ξgeff

(2.3)ξgeff =

ξg

1 + wg2

where wg=sgξg. With increasing deviation from the Bragg condition theeffective extinction distance will decrease. Therefore, the width of adislocation can be reduced to the order of 1 to 5 nm.

For the description of image contrast two theories are available, namely thekinematic and the dynamic theory. The most important difference, is that thelatter mentioned also takes into account the re-diffraction of the weakdiffracted beam. However, in most cases image contrast can be sufficientlydescribed by using the more simple kinematic theory. This theory assumesthat the incident wave amplitude is constant with depth. Further, it isassumed that the intensity of the diffracted beam is small compared to thetransmitted one. The amplitude of the diffracted beam due to the incidentamplitude φ0 is given by

(2.4)ϕg = πiξg

ϕ0∫0

t

e−2πisgzdz

where t is the thickness of the foil. Evaluation of the integral yields thefollowing expression for the diffracted intensity as a function of the foilthickness

14 CHAPTER 2

(2.5)I g = π2

ξg2

sin2πtsπ2s2

It can be seen that the intensity of the diffracted beam oscillates as a functionof depth, giving rise to thickness fringes, with spacing .∆t = 1/s (s >> 1/ξg)The periodicity of the fringes is given by aforementioned effective extinctiondistance and can be used to determine the foil thickness. However, careshould be taken if the specimen is bent, which gives rise to contours due tothe local variation of the excitation error .s

Contrast of lattice imperfections can be described by a modified version ofequation 2.414

(2.6)φg = πiξg

φ0∫0

t

e−2πisgz+gR

dz

where describes the displacement of a unit cell from its lattice position inRthe perfect crystal. The displacement can be calculated by using anisotropiclinear elasticity theory.15 Although a precise calculation of image contrast fordefects is quite complicated, the equation allows the prediction of thevisibility of a dislocation. In case of a pure screw dislocation, the direction of

corresponds to the direction of the Burger's vector, Therefore, theR b.

contrast of screw dislocation vanishes when , the so called invisibilityg ⋅ b = 0criterion. In case of edge dislocations, similar arguments are applicable,resulting for the isotropic case in the invisibility criterion . g ⋅ (b × ξ line) = 0

Element analysis

Electron diffraction only allows the possibility to obtain crystallographicinformation. Therefore, EDX and Electron Energy-Loss Spectroscopy (EELS)are used in conjunction with electron diffraction and imaging techniques toobtain complementary information on composition. EELS is the analysis ofthe energy distribution of electrons that have been inelastically scattered bythe specimen. The inelastic collisions provide information about theelectronic structure which in turn reveals the nature of the atom. The relativeenergy resolution of the EELS is high in comparison with EDX. However,EELS requires very thin specimen, in contrast to EDX. Another advantage isthat EELS is capable of detecting all elements, whereas EDX only detectselements with , with a relative lower sensitivity for the lighter elements.Z ≥ 5

EXPERIMENTAL CONCEPTS 15

High resolution electron microscopy

Despite the fact that the theoretical resolution limit of a 100 keV electronmicroscope reaches the order of 2 pm, the practically attainable resolution isin the range of atomic distances. This can only be achieved in the TEM modeby the combination of a carefully designed objective lens, a good electricaland mechanical stability and a relatively high accelerating voltage. Thesefacilities make the difference between a high resolution transmission electronmicroscope and a conventional TEM. The resolution for the HRTEM isproportional to , where Cs represents the spherical aberration constantCs

1/4λ3/4

and λ the wavelength of the electrons. Therefore, the resolution can beimproved by a decrease of both factors.

Besides differences in construction, the accomplishment of a high resolutionimage is rather different. The HRTEM image is formed by using a largenumber of diffracted beams. Further, the interpreted contrast is aconsequence of phase differences among the different scattered beams,whereas for conventional TEM it is mainly caused by diffraction. With theuse of phase contrast microscopy structural information can be derived at anatomic level.

In reality the HRTEM is not a perfect phase contrast microscope which iscaused by the influence of defocusing. The imperfect objective lensintroduces a phase shift, dependent on the deviation of the beam from theoptical axis. The way in which this phenomenon affects the phase of theimaging beams is described by the phase contrast transfer function.16 It wasfound that some amount of underfocus is required to counteract the effects ofspherical aberration. The optimum defocus was found to be .17∆fs = −(4

3Csλ)0.5

In order to relate the high-resolution image to the atomic structure, imagesimulations are essential. Important factors which influence the image are thealignment of the beam, the specimen thickness, the defocus of the objectivelens, chromatic aberration and the coherence of the beam. A complication isthat the image simulations are carried out for the elastically scatteredelectrons, while the contribution of the inelastically scattered electrons isignored. The performed simulations are based on the multi-slice method.18 Inthis approach the specimen is sectioned into many slices perpendicular to theincident beam. The intensity of the scattered beam is calculated afterpropagation through each successive slice.

16 CHAPTER 2

Specimen preparation

Because of the high-electron absorption, the maximum specimen thicknessshould be in the order of a few hundred nanometers. However, a furtherdecrease of the thickness results in an improved image quality. To obtainsuch a thin foil, which is also sufficiently rigid to be handled, the followingprocedure is required. First a thin sheet (0.5-1 mm thickness) of the materialto be examined, is cut. In order to examine a laser track, two different routescan be followed. The first manner is by taking a sheet plan parallel to thetreated surface. Another more complicated possibility is taking a crosssection. In that case two cross sections are glued together in such a way thatboth cross sections are positioned face-to-face in the middle of the connectedsheets. The sheets are obtained by using a low-speed diamond saw with lowmechanical distortion. Hereafter 3 mm diameter discs are produced by usinga spark cutter. Subsequently, the specimen are thinned from one or bothsides in such a way that in the centre of the disk the removal rate is higherthan outside the centre. As soon as a hole is optically detected in the centre ofthe disk, the specimen is ready to be checked for transparent regions in theTEM.

The thinning is accomplished in the following way. After creating a disc witha thickness of 0.5 mm the samples are treated with a mechanical dimpler(Gatan Model 656), to obtain a thin region of 10-30 µm in the centre of thedisc. This is realised by making shallow dimples on one or both sides of thespecimen. The sample is mounted on a rotating table, while a rotating wheelcovered with a diamond slurry removes small amounts of material. Thisprocess is carried out under a small load and with a low speed. The processcontinues with the ion mill (Gatan Dual Ion Mill 600). The specimen ismounted on a rotating sample holder in a vacuum chamber to preventcontamination. Two ion guns are pointed at the centre of the specimen undera low angle of incidence (5-15°), one gun from above and the other one frombelow. The thinning takes place by a focused ion beam. Because of theirinertness and low solubility in metals, Argon ions are used. The millingprocess is finished when a laser optical system detects a hole in the specimen.

In case of metal matrix composites problems arise due to different sputteringyields for the two phases. The harder component will remain thicker than thesofter one and thereby complicates the simultaneously imaging of bothphases in the TEM. Therefore 400 keV electrons are preferable above 200 keV

EXPERIMENTAL CONCEPTS 17

electrons. Another difficulty with laser tracks is that high residual stresses,due to high cooling rates, may result in bending during thinning.

2.3 Mechanical testing

2.3.1 Hardness determination

In this field of research, hardness measurements are frequently used toobtain information of the mechanical properties of laser modified material.Hardness measurements are easy to perform and provide accurate data.Nevertheless, these measurements reveal the strength in a rather complexmanner, because it is a combination of yield strength and strain hardening.For wedge shaped indenters, like the Vickers diamond indenter, the hardnessis related to the flow stress at 15% strain by .19 HV and C representHV = Cσthe Vickers hardness and a proportionality constant, respectively. C dependsslightly on the alloy composition and the temperature and is approximately0.34 (σ in MPa). Beside the hardness, all relevant properties of the basicmaterials used in this research are depicted in appendix 2.

The actual measurements are performed on polished and slightly etchedsurfaces and are carried out several times to minimise statistical variations.The applied force varied from 0.5 to 2 N depending on the hardness of thematerial. The reason for this type of measurement is that the small thicknessand the large residual stresses make it practically impossible to produce atensile test bar consisting of the surface treated material. Furthermore, theheterogeneity of the layers would make the interpretation of the resultsdifficult.

2.3.2 Wear measurements

The principal aim of the laser treatment of materials, is to improve the wearresistance and to increase their lifetime in application. In practice the wearprocess is often extremely complex, because of the simultaneous influence ofseveral wear mechanisms. Therefore, the solution of a wear problem dependsupon the exact identification of the nature of that wear process. In principle,four basic wear mechanisms are involved in wear processes: adhesive,abrasive, chemical reactive and surface fatigue wear.20 To solve a particularwear problem, laboratory tests can be used to examine one specific wearmechanism to simulate the exact system conditions as used in practice. In

18 CHAPTER 2

this research two different tests are performed to determine the wearresistance of the laser modified layers. These tests are carried out at TNOIndustrie Apeldoorn.

The first wear test explores the effect of penetration of hard particles in thelaser coatings, known as the abrasive wear test. The performed test is derivedfrom the ASTM G65 abrasive wear test, as shown in figure 2.6. SiO2 particles,with a grain size between 0.2-0.5 mm, are applied as the abrasive material.These particles are fed at a rate of 0.25 g/s in between a rotating rubberwheel and the specimen. The wheel rotates at a speed of 0.1 m/s and has thedimensions of mm. The test specimen which is more than 9 mm∅86× 9wide, is pressed against the rubber wheel with a constant load of 50 N. Aftertesting, the wear rate can be determined by weighting the mass loss causedby wear. The experiments take between 10 and 45 minutes time. In the caseof 45 minutes, the weight loss is measured at specific time intervals tomeasure any gradient in the coating properties. The tests are performed atroom temperature.

EXPERIMENTAL CONCEPTS 19

v

FN

Rubber Wheel

Specimen

Abrasive

Figure 2.6 Rubber wheel abrasive wear test. The specimen is pressed against the wheelwith an applied force FN, while the wheel rotates at velocity v. The abrasive is fed in betweenthe specimen and the wheel.

The second test explores the sliding wear of laser coatings under conditionsof boundary lubrication. These tests are performed on a tribometer of the pinon ring type, as shown in figure 2.7. The specimen (pin) is pneumaticallypressed against a rotating wheel (100Cr6) with a force of 2000 N. Thedimensions of the pin and the wheel amount to mm and 12× 12× 8 ∅73× 25mm, respectively. After abrading of the pins, the average surface roughnessRa amounts to ~0.5 ±0.1 µm. To realise pure boundary lubrication,experiments are performed at a low sliding speed of 0.01 m/s. The lubricantused is BP Transcal of 20 °C with a dynamic viscosity of 0.07 Ns/m2. After 20hours the test is terminated and the weight loss of both pin and ring aremeasured. During the test, the coefficient of friction is measuredcontinuously.

20 CHAPTER 2

FN

v

Figure 2.7 Pin on ring wear test. The pin is pressed against the ring with an applied force FN,while the ring rotates at velocity v.

References

1. A. Einstein, Phys. Z., 18, 121 (1917).2. T.H. Maiman, Nature, 187, 493 (1960).3. W.V. Smith and P.P. Sorokin, The Laser, McGraw-Hill, (1966).4. L.E. Samuels, Metallographic Polishing by Mechanical Methods, Pitman, (1967).5. J.A. Venables and C.J. Harland, Phil. Mag., 27, 1993 (1973).6. L. Reimer, Transmission Electron Microscopy, Springer-Verlag, (1984).7. J.E. Edington, Practical Electron Microscopy in Materials Science, 2, 33, Philips

Technical Library, (1975).8. D.B. Williams and C.B. Carter, Transmission Electron Microscopy, Plenum

Press, New York, (1996).9. P.B. Hirsch, Electron Microscopy of Thin Crystals, Butterworth, London, (1965).10. C. Kittel, Introduction to Solid State Physics, John Wiley & Sons inc., New York

(1986).11. D.J.H. Cockayne in Diffraction and Imaging Techniques in Materials Science

edited by S. Amelinckx, North Holland, 153 (1978).12. S. Kikuchi, Japanese J. Phys., 5, 83 (1928).13. G. Thomas in Diffraction and Imaging Techniques in Materials Science edited

by S. Amelinckx, North Holland, 217 (1978).14. J.Th.M. De Hosson in Handbook of Microscopy, Ed. S. Amelinckx, D. Van Dijk,

J. van Landuyt, G. van Tendeloo, VCH, vol. 3, 1-110 (1997).15. J.P. Hirth, J. Lothe, Theory of Dislocations, John Wiley & Sons inc., New York

(1986).16. D. van Dyck, in Diffraction and Imaging Techniques in Materials Science edited

by S. Amelinckx, North Holland, 217 (1978).17. O. Scherzer, J. Appl. Phys., 20, 20 (1949).18. A.F. Moodie, Z. Naturforsch., 27, 437 (1972).19. G.J.L. van der Wegen, Met. Trans., 12A, 2125 (1981).20. K.H. Zum Gahr, Microstructure and wear of materials, Elsevier, (1987).

EXPERIMENTAL CONCEPTS 21

APPENDIX 2: MATERIAL PROPERTIES

Physical Properties

(room temperature)

cp-Ti

(grade 2)

Ti-6Al-4V Mo TiN TiC SiC

General

Density (Mg/m3) 4.51 4.43 10.2 5.4 4.92 3.22

Thermal

Melting point (°C) 1670 1650 2617 3290 3067 2545

Thermal cond. (W/m·K) 15.57 6.7 105 28 22 145

Thermal expansion (10-6/K) 8.6 8.6 4.9 6.3 6.3 4.4

Specific heat (J/kg·K) 518 564 250 630 557 690

Mechanical

Young's modulus (GPa) 102 113 255 430 435 415

Poisson's ratio 0.36 0.36 0.32 0.23 0.24 0.15

Tensile strength (MPa) 172 827 480 295 355

Hardness (GPa) 0.6 1.2 8 20 28.5 29

Fracture toughness(MPa·m½) 60-120 55-123 20 2-3 2-3 3-5