Partial Case Hardening of Stainless Steeletd.dtu.dk/thesis/201790/master.pdf · Partial Case...

Partial Case Hardening of StainlessSteel

Master ThesisSupervisor: Andy Horsewell

Department of Manufacturing Engineering and Management, Technical University of Denmark

Kristian Holm NielsenDepartment of Manufacturing Engineering and Management, Technical University of Denmark

August 15, 2006

Preface and Acknowledgments

This master thesis is submitted at the Technical University of Denmark (DTU) asa part of the engineering degree in Applied Physics. The work has been carriedout from September 2005 to August 2006 at Material Science at the Department ofManufacturing Engineering and Management, DTU with Docent Andy Horsewellas supervisor. I would like to thank my supervisor for his guidance and enthusiasmand for his great social intelligence.

I would like to thank Professor assistant Thomas Christiansen for a very harmoniccooperation which has brought on many useful discussions and for his help andguidance with the sample preparation.

I also acknowledge: Engineer assistant Steffen S. Munch for his overall guidancein the laboratory and expertise, Christian Raun for his guidance using photoresistand Professor Marcel A. J. Somers for several fruitful discussions.

This thesis is dedicated to the most important person in the world: my son TristanHolm Nielsen.

Lyngby, August 2006

Kristian Holm Nielsen

Contents

1 Introduction 1

2 Applying nickel in patterns 5

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . 5

2.2.2 Equipment and programs . . . . . . . . . . . . . . . . . . . 7

2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.1 Dimensions in the sample preparation steps . . . . . . . . . 8

2.3.2 Case hardening . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.3 Curved interface phenomenon . . . . . . . . . . . . . . . . 25

2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3 Thickness determination by EDS and simulations 29

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29



3.3 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 31

3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

iv CONTENTS

4 Removing nickel in patterns 35

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35



4.3 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 37

4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5 Conclusion 45

5.1 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Chapter 1

Introduction

Stainless steel is widely used all over the world due to its excellent corrosion resis-tance. It is well known that stainless steel achieves this resistance by the formationof a thin layer of chromium oxide on the surface forming a barrier so that the ironavoids contact with oxygen. Austenitic stainless steel, typically AISI 316 and 304, isductile and easily processed into tubes, sheets, etcetera. The ductility is a conse-quence of the face centered cubic unit cell. However the hardness of stainless steelis relatively low and because of the chromium oxide layer a simple hardening byinfusion of either carbon or nitrogen could until 1985 only be done at temperaturesabove 500◦C with precipitation of CrN and with a significant loss of corrosion resis-tance [1]. The chromium is withdrawn from the solid solution and when it reachesbelow 11 wt. %, the steel loses its self repairing ability of rebuilding the passive filmat the surface [2].

In 1985 the possibility of low temperatures surface engineering of stainless steel byuse of plasma or implantation techniques was recognized. It was discovered that ahard and wear resistant surface layer could be obtained without experiencing a lossof corrosion resistance when nitriding at temperature below 450-500◦C [3, 4].

The new phase responsible for the favorable properties associated with low tem-perature nitriding was by Ichii et al. [4] designated the S-phase. The term S-phaserelates to the unidentified peaks in the X-ray diffraction patterns arbitrarily namedS1-S5. The S phase is also called expanded austenite because of the interstitialdiffusion of nitrogen in the austenite. It is a metastable phase and its stability ishighly dependent on the applied process conditions. The S phase decomposeswhen thermally exposed to critical temperatures. The decomposing of stainlesssteel like AISI 316 used in this projects happens within minutes at 600◦C and withinyears at 300◦C [5].

Plasma and implantation technologies circumvent the problem of passive layer im-

2 Introduction

penetrability either by sputtering the surface in a vacuum chamber or with a chem-ical removal of the passive layer. The problem with a chemical removal of the pas-sive layer is that the surface starts rebuilding the moment it gets in contact withoxygen and thereby recreates the problem of passive layer impenetrability. How-ever Somers et al. [6] have developed a patterned process where nickel is appliedto the surface directly after the oxide removal, making the surface thermodynam-ically stable. The depassivation of the surface is carried out in a solution of 80ml 15% w/w hydrochloric acid + 1 ml 35% hydrogen peroxide for 20-30 secondsthereby removing the chromium oxid layer. Then the specimen, while still wet, isdirectly placed in a Wood’s nickel bath, which is an acidic halogenide containingelectrolyte, where a nickel layer ís grown in 90-120 seconds. Afterwards severalarticles have been published by the same group [7, 8, 9, 10, 11, 12, 13] and moreare being prepared for publication [14, 15, 16].

The topic of this master thesis is low temperature surface hardening of stainlesssteel in micro scale patterns using photoresist to create the desired patterns. Therebycreating functional surfaces for different uses. The method of surface hardening isdeveloped by Somers et al. [6] and the idea of creating a functional surface is sug-gested by A. Horsewell. The basic idea is that before case hardening of the surfacea micro scale pattern of nickel is applied to the surface instead of applying it to thewhole surface. This can either be done by protecting a pattern of the surface fromgetting in contact with the nickel bath or by protecting a pattern of nickel alreadyapplied to the whole surface.

The technological advantages of only case hardening parts of a surface could be:a) The part to be welded can be left non case-hardened. b) The hardened contactsurface area can be reduced, thereby allowing non-hardened surface area to beremoved during sliding wear. c) Patterned surfaces of hardened and non-hardenedareas respectively can be tailored to allow local reservoirs of oil or other lubricants.d) Stresses which could lead to surface cracking might be relaxed in untreatedareas. Furthermore, basic understanding and fundamental research studies oninterfaces hardened and non-hardened regions - stressed and non-stressed regions- are needed.

Each of the chapters contains its own introduction, experimental, discussion andconclusion. The Chapters 2 and 4 deal with the two different types of micro scalepattern creation. Chapter 2 deals with the process of protecting a pattern of thesurface from getting into contact with the nickel bath i.e. applying nickel in patternsand chapter 4 deals with the process of creating a pattern of nickel by protectingparts of nickel already applied to the whole surface i.e. removing nickel in patterns.The discussions and conclusions in Chapter 4 are based on preliminary observa-tions as the sample preparation used is not final. Chapter 3 deals with a thicknessdetermination of the nickel layer by use of energy dispersive X-ray spectrometry(EDS) and Monte Carlo electron simulations. Conclusions are presented in chapter

3

5 along with an outlook.

4 Introduction

Chapter 2

Applying nickel in patterns

2.1 Introduction

This chapter examines the possibility of applying a pattern of nickel lines by theuse of photoresist followed by a case hardening of the patterned surface. Thismakes it impossible for the nickel in a Wood’s nickel bath to come in contact withthe surface. Despite that this process might not give the optimal nickel layer growthand thereby not the optimal diffusion conditions, it does give the advantage thateach of the steps can be examined well in a Light Optical Microscopy (LOM) asonly the nickel applied surface will be grain boundary etched. This results in anoptical difference of the two different parts of the surface. Therefore this process isoptimal for categorizing each of the steps in the sample preparation. However thephoto resist deposition interrupts the optimal process of applying a perfect nickellayer as the sample cannot go directly from the degreaser to the activation, whichis known as the optimal process. After the nickel treatment three different types ofcase hardening are studied and discussed.

2.2 Experimental

2.2.1 Sample Preparation

First a 12 mm rod of AISI 316 stainless steel is cut into discs with a weight below3.9 g and applied with a hole that was turned on a lathe. The maximum of 3.9g is due to that nitriding and carburization equipment has a maximum limit of 4.0g and the hole is used for a wire in the sample preparation. The specimens arethen polished with successively finer emery paper and diamond polishing until a

6 Applying nickel in patterns

mirror like finish is obtained which lower the deformation below the surface. As oneof the later processes needs a very clean surface the specimens are treated in acaustic degreaser. The clean specimens are then sprayed with a uniform layer ofphotoresist positive 20 and dried at a maximum temperature of 70◦C for about 20minutes [17]. Fingerprints should be avoided and the process should kept in thedark as much as possible.

Figure 2.1: LOM overview image of the applied mask. The black areas are the mask and the brownareas are where the mask is transparent and the visible colour is from the background.

After the drying of the photoresist the masks seen in figure 2.1 and 2.2 are takeninto use. Ultra violet light is applied to the photoresist through the mask for a fewseconds in order to make parts of it soluble in NaOH thereby leaving a patternof photoresist. The processing times are highly dependent of the ultra violet lightsource but the optimal time is found when the exposed photoresist is removed over120 s. The patterned specimen is then treated as described by Somers et. al[6]. First a depassivation of the surface is carried out in a solution 80 ml 15% w/whydrochloric acid + 1 ml 35% hydrogen peroxide for 20-30 s thereby removing thechromium oxide layer. Then the specimen is directly placed in a Wood’s nickel bath,which is an acidic halogenide containing electrolyte, where a nickel layer is grownfor 90-120 s. After the nickel growth the photoresist is removed with acetone. Thecase hardening is carried out in a furnace flushed with either 60% NH3, pure NH3

or CO. The nitridation is performed in mixtures of NH3 and H2 and the carburizationis performed in mixtures of CO and H2.

2.2 Experimental 7

Figure 2.2: LOM image of the dimensions of the mask. The black areas are the mask and the brownareas are where the mask is transparent and the visible colour is from the background.

2.2.2 Equipment and programs

Scanning Electron Microscopy (SEM) observations are carried out in a JEOL 5900SEM operated under high vacuum conditions. Case hardening is carried out in aNetzsch STA 449C thermal analyser. LOM is carried out in a Neophot 30, a LeicaMZ 125 and Olympus Altra 20 and micro hardness testing is carried out with aFuture Tech FM-700.


2.3 Results and Discussion

The following sections are divided into three parts. In the first part the dimensions ofthe different steps in the sample preparations are discussed. Closer analysis of thecase hardening is left out of this part as it is fully discussed in the second part whichis concentrated on the physical effects of the different case hardening processes.In the third part a curved interface phenomenon is discussed.

2.3.1 Dimensions in the sample preparation steps

First focus is directed at how the dimensions are affected by the different steps inthe sample preparation. At Figure 2.3 an overview of the nickel treated discs isseen. The dark areas are the untreated stainless steel and the light gray areas arethe nickel treated areas.

Figure 2.3: LOM image of nickel treated sample. Because of the photoresist mask only the light areashave been in contact with the wood’s nickel bath. In the light areas the grain boundaries are visible asa result of the activation of the surface with grain boundary etching. The activation is etching belowthe photoresist thereby leaving two parallel lines in the transition zone.

At Figure 2.4 a zoom of the two different areas is seen. Only in the light areas thegrain boundaries can be observed and this is a result of the depassivation of thesurface which etches the grain boundaries thereby making them visible in LOM.

2.3 Results and Discussion 9

Figure 2.4: LOM image of nickel treated sample as a result of the photoresist mask only the lightareas have been in contact with the Wood’s nickel bath, while the dark are untreated stainless steel.In the light areas the grain boundaries are visible as a result of the activation of the surface with grainboundary etching. The activation is etching below the photoresist thereby leaving two parallel lines inthe transition zone.


Figure 2.4 illustrates the dimensions of the pattern after the nickel process and iscompared with Figure 2.2. Comparing these images shows that the width of thearea not covered with nickel is slimmer than the width of the mask. The distanceof ∼ 430µm can still be recognized after the nickel growth but a second distanceis also apparent. From the pictures it can be concluded that the depassivation isgetting beneath the photoresist or etching it away. However it is hard to determinethe exact length of this second distance because the boundary is a little blurry. Inorder to get further info on these dimensions the photoresist is examined throughLOM.

Figure 2.5: LOM image of a photoresist lane. The image shows that the photoresist edge is in layerswhich could leave it soluble of the depassivation.

In Figure 2.5 the edge of one of the photoresist lanes is seen and from this image itis clear the the photoresist does not interrupt at once but gets thinner and thinner.This could explain why the nickel layer is grown beneath the photoresist as a thinnerlayer must be more fragile than a thicker layer.

Another way to observe the dimensions is to look at the sample after the casehardening process. In Figure 2.6 a LOM image of a surface sample which is nitridedwith pure NH3 is seen. Once again two distances are seen, one that correlates withthe width of the mask and a second which correlates with the second one observedafter the application of the nickel layer. But it must be the same line as there are noother lines. However in this image the distances are very visible and clear.

By observing Figure 2.7, which is a LOM image of the cross section of a carborized


Figure 2.6: LOM image of the surface of the sample nitrided with pure NH3. The nitridation has startedin the whole surface and is lifting the grains out of the surface thereby leaving it visible. The two linedtransition is seen clearly.


sample, the distance of the ∼ 410µm is again visible if a line from the center of thecarbon circles is chosen. This selection of drawing a line from the center of thecircles and the physical significance of the circles will be discussed in Section 2.3.2.So it is clear that the etching beneath the photoresist must be taken into account ifa specific dimension is wanted in the design.

Figure 2.7: LOM image of the cross section of the sample case hardened with carbon. The distanceof ∼ 410 µm is taken from the center of the circles as explained in Section 2.3.2. This distancecorrelates with the distance found from observing the surface.


2.3.2 Case hardening

In this section focus is on the different types of case hardening and the physicalresults obtained by studying these. The first type of case hardening that is examinedis that of 60% NH3.

Case hardening with 60% NH3

In Figure 2.8 a very interesting thing occurs. In the boundaries between the nitridedstainless steel and the untreated stainless steel the grain boundaries continues intoin the stainless steel region. This was not seen in the sample before the nitridinglike that of Figure 2.4 and indicates that either the expansion of the lattice to theS-phase is lifting the grains positioned in both regions and thereby making themvisible in the stainless steel area or that a nitriding is going on in few regions of theuntreated region.

An explanation of what sets off the nitridation could be that the surface is not aust-enized and therefore still cold deformed from the cutting of the discs. As the grainboundaries visible region is close to the nickel treated region the expansion to theS-phase must have an influence on the untreated region and it is therefore mostlikely that it is a combination of the two possible explanations. The grain boundariesvisible regions in the untreated region are very uneven and in Figure 2.8 (bottom) avery large region compared to that of Figure 2.8 (top) is seen.

The corresponding cross section of the same sample is shown in Figure 2.9. Fromthe image it is clear that the nitriding is not optimal as it is very uneven and thetransition between the treated and untreated stainless steel is very diffuse. Thisalso makes it impossible to recognize any of the distances determined in the formersection.

A closer look at the transition is shown in Figure 2.10 where the diffuse transi-tion is even more visible. The figure also shows the thickness of the chrome layeronly in some places is approaching the obtained value by Christiansen et al. of13.5 (±0.8) µm [14]. So by using this sample preparation method it is possibleto case harden a part of a surface, but the diffuse transition zone shows that thismethod is not yet optimal.


Figure 2.8: LOM image of the partially nitrided surface. The middle region is the untreated stainlesssteel and the areas containing the grains are the 60% NH3 treated. There are grain bounderies con-tinuing from the treated region over in the untreated region and this either indicates that the expansionof the lattice is lifting the grain positioned in both regions and thereby making them visible or that thenitriding is not enterely selective and commences in the untreated region.


Figure 2.9: LOM image of the cross section of the partially case hardened with 60% NH3. Thetransition zone from the treated and untreated area is diffuse but it is clear that a selection is occuring.

Figure 2.10: LOM image of the cross section of the partially case hardened with 60% NH3 where acloser look at the diffuse transition zone from the treated and untreated area is seen. The transitionfrom the nitrided surface at the left to the untreded surface in the right consist of nitrited islands.


Case hardening with pure NH3

The case hardening with 60% NH3 showed that the layer thickness rarely was thatof the expected and the process was difficult to control. Therefore focus is putinto case hardening with pure NH3. This is chosen to see if a thicker layer can beobtained and to examine how the untreated areas react differently. In Figure 2.11 animage of the case hardened surface is seen. For the first time the grain bounderiesare visible in the whole area even those not exposed to the Wood’s nickel bath. Thisindicates that a case hardening is happening in even the shaded areas as the grainsare being lifted out of the surface and thereby becoming visible. Different hardnessmeasurements confirm that the case hardening is carried through as the hardnessis within the normal for case hardened steel 900 HV - 1700 HV [9].

Figure 2.11: LOM image of the surface of case hardened with pure NH3. The light areas are thosethat have been exposed in the Wood’s nickel bath.

It was not expected that a case hardening with pure NH3 would take effect at thewhole surface of the sample and thereby ignoring the selection, however this isthe case. The reason for this could be that the samples are not austenized andtherefore are still cold deformed after the initial cutting and a combination of colddeformed material with pure NH3 could have triggered the nitriding. This argumentis supported by the observation done by Christiansen et al. [9] where a surface notpolished and therefore deformed was observed to have a thicker nitrided layer. It isexpected by Christiansen [18] that a surface polished but not austenized can reactthe same way. The two parallel lines positioned in and around the interface between


the nickel treated part and the untreated stainless steel are both described in thesection of dimension section 2.3.1, and the explanation of the third and curved linepositioned in the untreated region are will be discussed in section 2.3.3.

In Figure 2.12 an image of the cross section is seen and as expected the wholesurface is case hardened.

Figure 2.12: LOM image of the cross section of case hardened with pure NH3. The image shows thatthe whole surface is nitrided so the selection is not working.

Figure 2.13 shows that the thickness of the nitrided layer is much thinner than thethickness of 21.7 (± 2.3) µm obtained by Christiansen et al. [14]. By comparingthe nitrided layer thickness for 60% and pure NH3 it is seen that the relative differ-ence from what was expected increases when using pure NH3. So it seems thatthe problem with a non austenized surface increases when using pure NH3.


Figure 2.13: LOM image of the cross section of case hardened with pure NH3.

It is not in every part of the surface that the case hardening has started. In Figure2.14 a close look at a white spot is seen. There are no visible grain boundariesin the white spot which indicates that a case hardening has not taken place in thisspot. This is confirmed by hardness measurements as the hardness is at a valuecommon for untreated stainless steel ∼ 225 HV [19]. A possible explanation for thiscould be that the surface is not cold deformed right in this spot due to a cut directlyin the boundary of two grains causing no nitriding.


Figure 2.14: LOM image of the surface of case hardened with pure NH3. The white spot is the onlypart of the image where a case hardening has not taken place.

Case hardening with carbon

After seeing different problems with using either 60% or pure NH3 as case harden-ing, focus is turned towards using carbon instead. It is expected that carbon reactsdiffently with nickel so the result could look a lot different. Figure 2.15 is a macro-scopic image of the sample case hardened with carbon. The dark areas are theuntreated lines and the almost black one is caused by carbon’s most stable formbeing graphite. The light areas are the nickel treated areas where the missing darkcolour shows that the case hardening is accomplished.

In Figures 2.16 and 2.17 a closer look at a sample is seen. The phenomenon ofthe double line described in Section 2.3.1 is still present when using carbon andthe transition from the carbonized to the untreated is affected by this. The innerline seems very sharp but the outer line is a little diffuse. However it is difficultto compare the diffuseness to that seen with nitriding as carbonization gives extrainformation in colouring the surface.

The cross section of the carbonized samples are shown in Figures 2.18 and 2.19.The image shows a very sharp transition between the treated and untreated areasand a good control with the partial case hardening. The layer is a little uneven butthis problem could probably be solved by the aforementioned austenization.


Figure 2.15: LOM macroscopic image of the sample case hardened with carbon. The untreated linesare black as the carbon has stayed on the surface and the light areas are the nickel treated areas.

Figure 2.16: LOM image of the sample case hardened with carbon. The untreated lines are black asthe carbon has stayed on the surface and the light areas are the nickel treated areas.


Figure 2.17: LOM image of the sample case hardened with carbon. The untreated lines are black asthe carbon has stayed on the surface and the light areas are the nickel treated areas.

Figure 2.18: LOM image of the cross section of the sample case hardened with carbon. The imageshows two case hardened islands and thereby great selective control.


Figure 2.19: LOM image of the cross section of the sample case hardened with carbon. The imageis a closer look at the region between two nitrided regions. The image shows that the carbon layeris thicker at the edge of the case hardening and that it is circular. This means that in one particularspot the diffusion of carbon is happening faster than in the rest of the nickel treated part and the onlyreproducible spot where the environment is unique in every transition is exactly in the transition spotwhere the surface goes from chromium oxide to nickel.


In figure 2.20 a closer look at the transition is shown. The image shows that thecarbon layer is thicker at the edge of the case hardening and that it is circular.This means that in one particular spot the diffusion of carbon is happening fasterthan in the rest of the nickel treated part and the only reproducible spot wherethe environment is unique in every transition is exactly in the transition spot wherethe surface goes from chromium oxide to nickel. The distances described in section2.3.1 also supports this argument even though it is more valid the other way around.

There are some physical results that can be drawn from this. As the diffusion ishappening faster in the transition spot, then the diffusion in the plain nickel layermust be fixed from the diffusion velocity in the nickel layer and the maximum dif-fusion velocity must be higher in stainless steel. It is not possible to explain fromthese images why the diffusion is happening faster in the transition spot as there isno information of the nano scale surface area. But even though an explanation ofthis is not in this context possible it is however possible to use this information in asample preparation. If a surface is applied with a string of small nickel dots insteadof a continuous surface of nickel then the corresponding case hardening layer willbe thicker.

Figure 2.20: LOM image of the cross section of the sample case hardened with carbon. A zoom at thetransition between the treated and untreated. The image shows that the carbon layer is thicker at theedge of the case hardening and that it is circular. This means that in one particular spot the diffusionof carbon is happening faster than in the rest of the nickel treated part and the only reproducible spotwhere the environment is unique in every transition is exactly in the transition spot where the surfacegoes from chromium oxide to nickel.


From Figure 2.21 additional information of the diffusion in stainless steel is obtained.The diffusion stops at the grain boundary and this offers some information abouthow grain boundaries are acting as diffusion drain. This is because the area aroundthe grain boundary is getting saturated before diffusion begins in the next grain asthe diffusion acts easier along boundaries.

Figure 2.21: LOM image of the cross section of the sample case hardened with carbon. A zoom atthe transition between the treated and untreated. The diffusion stops at the grain boundary and thisoffers some information about how grain boundaries are acting as diffusion drain. This is because thearea around the grain boundary is getting saturated before diffusion begins in the next grain as thediffusion acts easier along boundaries.


2.3.3 Curved interface phenomenon

In Figure 2.11 a curved interface phenomenon first becomes visible. As the phe-nomenon is seen in the untreated area it can only be seen in LOM after the grainboundaries becomes visible by use of a case hardening with pure NH3. So to seeit before case hardening with pure NH3 SEM images are taking into use - Figure2.22.

Figure 2.22: Secondary electron SEM image of the partially nickel applied surface. The area withvisible grain boundaries is the nickel applied surface. In the image there are visible curved lines inboth the nickel treated region and the untreated region. In the nickel removed region the area withinthe curves is darker than the region outside. This means that the intensity of electrons is lower andthis might be because the low energy electrons are being stopped in some unwanted rest materialpositioned right there due to capillary forces. The curves in the untreated region could be from thesame unwanted material and the curves could be at small topographical plateaus.

The SEM image is measuring the intensity in secondary electrons. Secondary elec-trons are low energy electrons from within the sample that are emitted after a col-lision from the incident beam. The Everhart-Thornley SE detector is biased with apositive field of 50-200 V that even attracts the electrons that initially were movingaway from the detector. This results in higher intensities from edges and therebyleaving the grain boundaries lighter as the area of these are relatively bigger. Theblack dots with light edges are holes from the activation of the surface - called pit-ting. The holes are black at the bottom as no secondary electrons can get awayand the edges are light because of the relatively higher surface area which results


in the ’edge effect’ [20].

In the secondary image there are visible curved lines in both the nickel treatedregion and the untreated region. In the nickel removed region the area within thecurves is darker than the region outside. This means that the intensity of electronsis lower and this might be because the low energy electrons are being stoppedin some unwanted rest material positioned right there due to capillary forces. thecurves in the untreated region could be from the same unwanted material probablyorganic [21] and curves could be like the ones the sea creates with seaweed at thebeach which mean that the two curves could be at small topographical plateaus andthat the dark area are the last place from where the water has disappeared.

If these curves are unwanted materials absorbing the low energy secondary elec-trons then the corresponding backscattered electron image would not have thesecurves as the higher energy would not allow it to be absorbed. There are nocurves showing in the backscattered electron image in Figure 2.23 which supportsthe statement that is it some unwanted material creating the curved interface phe-nomenon.

Figure 2.23: Back scattered electron SEM image of the partially nickel applied surface. The area withvisible grain boundaries are the nickel applied surface. There are no curves showing which supportsthe statement that is it some unwanted material creating the curved interface phenomenon.

From an additional LOM image seen in Figure 2.24 The curve seems to follow theparticle that is positioned in the transition between the two areas. The way the curveis following this particle and the general shape of it also indicates that it is some kind


of capillary effect from the sample preparation.

Figure 2.24: LOM image of the surface showing how the curved interface phenomenon is beingaffected by a particle. The curve seems to follow the particle that is positioned in the transition betweenthe two areas. The way the curve is following this particle and the general shape of it indicates that itis some kind of capillary effect from the sample preparation.


2.4 Conclusion

From the examination of the dimensions in the sample preparation it was discussedthat the depassivation of the surface was getting beneath the mask of the photoresist. A possibly explanation was that the photoresist edge was in layers whichcould leave it soluble to the activation.

The case hardening with 60% NH3 showed that despite being selective there weresome problems with a diffuse region between the treated and untreated areas.Some areas of the untreated region were nitrided and a possible explanation ofthis was that the samples were still cold deformed from the cutting of the samples.The problem with nitriding of the untreated region was even bigger with the use ofpure nitrogen and only individual grains were not nitrided. In both cases an unevennitridation layer was observed.

The case hardening with carbon showed greater selectivity in the boundary be-tween the treated and untreated regions than that of nitrogen and the boundarywere shown to be relatively sharp. The images showed a circular shape of the casehardened layer that was thicker than the rest of the layer and from that it was con-cluded that the diffusion was happening faster right there than it was through therest of the nickel layer and also that the diffusion velocity in the procedure withoutpatterns was below its maximum in stainless steel. As a result of the conclusion itwas suggested that an extra layer thickness could be obtained by applying a surfacewith a string of small nickel dots instead of a continuous surface of nickel.

In all of the samples the case hardened layer was thinner than that obtained withcase hardening a complete surface but that was also expected because of the cre-ation of photoresist pattern interrupting the optimal sample preparation.

A curved interface phenomenon was examined and it was suggested that it wassome unwanted materials that accumulated in the surface by capillary forces.

Chapter 3

Thickness determination by EDSand simulations

3.1 Introduction

By exposing part of a stainless steel surface area for a nickel treatment in a Wood’snickel bath a thin film of nickel on top of a stainless steel bulk is obtained. Theselection of areas to be exposed is obtained by using photoresist and a preferentialmask. By use of a very few EDS measurements and Monte Carlo electron simula-tion the thickness of the film is determined. The Monte Carlo program used is verysimple and a newer program can be obtained [22]. However as this an academicdiscussion concerning principles a personal well known program is used.

3.2 Experimental


First the specimen is polished with successively finer emery paper and diamondpolishing until a mirror like finish is obtained. As one of the later processes needsa very clean surface the specimen is treated in a caustic degreaser. The cleanspecimen is then sprayed with a uniform layer of photoresist positive 20 and driedat a maximum temperature of 70◦C for about 20 minutes [17]. Fingerprints shouldbe avoided and the process should be kept in darkness as much as possible.

After drying the photoresist the masks seen in figure 2.1 and 2.2 are put into use.Ultra violet light is applied to the photoresist through the mask for a few seconds in

30 Thickness determination by EDS and simulations

order to make parts of it soluble in NaOH thereby leaving a pattern of photoresist.The processing time is highly dependent on the ultra violet light source but theoptimal time is found when the exposed photoresist is removed over 120 s. Thepatterned specimen is then treated as described by Somers et al. [6]. First adepassivation of the surface is carried out in a solution 80 ml 15% w/w hydrochloricacid + 1 ml 35% hydrogen peroxide for 20-30 s thereby removing the chromiumoxide layer. Then the specimen is directly placed in a Wood’s nickel bath, which isan acidic halogenide containing electrolyte, where a nickel layer is grown for 90-120s.


SEM observations and X-ray spectroscopy are carried out in a JEOL 5900 SEMoperated under high vacuum conditions. The EDS system is INCA 400 from OxfordInstruments using a Si detector crystal with 133 eV resolution measured at 5.9 keV.The program used for simulations are: "Electron Flight Simulator" Version 3.1 andthe program used for processing the data is "Origin Lab" version 7.5.

3.3 Result and Discussion 31

3.3 Result and Discussion

For examining the nickel pattern a SEM analysis is carried out including an EDSanalysis. Figure 3.1 shows the two different areas examined. The first area is thestainless steel without a nickel layer and the second is the area with a nickel layer.

Figure 3.1: SEM image of the areas examined with EDS - 1. The regular stainless steel surface 2.The nickel grown surface.

The obtained results with EDS are listed in table 3.1.

Spectrum Si Cr Fe Ni1 0.3 9.6 79.9 15.1

2 (nickel area) 0.3 7.7 56.5 35.6

Table 3.1: EDS analysis of nickel pattern at 10 kV - all results in Weight Percent

In order to calculate the thickness of the nickel layer the interaction and emissionspectrum are needed. This is determined by use of the program "Electron flightsimulator" and an image of the interaction volume is shown in image 3.2.

The program gives a spectrum with number of emitted nickel X-rays and these areplotted in Figure 3.3. The calculations must be based on the area below the curvess the spectrum decrease when the depth increases. A1 is the emission area ofpure nickel and A2 is the emission area of stainless steel. P1 is the EDS deter-mined weight percentage of nickel in stainless steel and P2 is the EDS determined


Figure 3.2: "Electron flight simulator" image of the electrons interaction volume in stainless steel.

0 50 100 150 2000

2

4

6

8

Em

itted

X-r

ay y

ield

/ [1

0-3]

Depht of sample / [nm]

Figure 3.3: Spectrum of the emitted nickel X-rays. The number of emitted electrons decreases as thedepth increases.


weight percentage of nickel in region where nickel is grown. The relation of the fourparameters is written in equation 3.1.

A1 +P1 ·A2 = P2(A1 +A2) (3.1)

A1 +P1(A2 +A1 −A1) = P2(A1 +A2) (3.2)

A1(1−P1) = (A1 +A2)(P2 −P1) (3.3)

A1 =(A1 +A2)(P2 −P1)

(1−P1)(3.4)

By inserting the data into equation 3.4:

A1 =(48,9∗10−3)(0,356−0,151)

(1−0,151)= 11,80∗10−3 (3.5)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 2000

10

20

30

40

50

Em

itted

X-r

ay y

ield

/ [1

0-3]

Depht of sample / [nm]

Figure 3.4: Plot of the area below the emission spectrum as function of the depth.


This area can then be transformed into a thickness of the nickel layer with the useof Figure 3.4 which is a plot of the area below the emission spectrum as function ofthe depth. The thickness is then found to be:

d[ni] = 13,5 nm (3.6)

3.4 Conclusion

EDS measurements were carried out in both the nickel grown area and the un-treated area. The obtained data was then used together with data from a MonteCarlo electron simulation to calculate the thickness of the nickel layer. The biggestadvantages of this method were that it was very time saving as only two measure-ments were needed.

Chapter 4

Removing nickel in patterns

4.1 Introduction

This chapter examines the possibility of applying a pattern of nickel lanes by firstapplying nickel to the entire surface and then afterwards removing it in patterns byuse of creating lanes of photo resist that protects part of the nickel surface from 25-30 % HN03. This method is expected to be the best when it comes to growing anoptimal nickel layer. It has the disadvantage of not being able to follow optically eachstep in LOM as the whole surface will be grain boundary etched. Thereby leaving nooptical difference in the two different parts of the surface. After the reversed creationof nickel lanes, different types of case hardening will be examined and discussed.

4.2 Experimental


First a 12 mm rod of of 316 stainless steel is cut into discs with a weight below 3.9g each and applied with a hole. The maximum of 3.9 g is due to the nitriding andcarburization equipment and the hole is for applying a wire. The specimens are pol-ished with successively finer emery paper and diamond polishing until a mirror likefinish is obtained. To remove the deformations in the surface after the cutting andpolishing, the samples are austenized in an helium atmosphere at a temperatureof 1060◦C. As a very clean surface is needed for the process of applying nickel thespecimens are first treated in a caustic degreaser. After cleaning the specimens aretreated described by Somers et al. [6].

36 Removing nickel in patterns

First a depassivation of the surface is carried out in a solution 80 ml 15% w/whydrochloric acid + 1 ml 35% hydrogen peroxide for 20-30 s thereby removing thechromium oxide layer. Then the specimens are directly placed in a Wood’s nickelbath, which is an acidic halogenide containing electrolyte, where a nickel layer isgrown in 90-120 s. All these processes are performed without interruption andkeeping the sample wet througout the entire process. Afterwards the samples aresprayed with a uniform layer of photoresist positive 20 and dried at a maximumtemperature of 70◦C for about 20 minutes [17]. After the drying, the masks seen infigure 4.1 and 4.2 are taken into use. Ultra violet light is applied to the photoresistthrough the mask for a few seconds in order to make parts of it soluble in NaOHthereby leaving a pattern of photoresist. The processing times are highly dependentof the ultra violet light source but the optimal time is found when the removal of theexposed photo resist takes 120 s.

Figure 4.1: LOM macroscopic image of the applied mask. The black areas are the mask and thebrown areas are where the mask is transparent and the visible colour is from the background.

With a pattern of photoresist the nickel is removed by exposing the sample to 25-30% HN03 in various time windows. Finally the photoresist is removed with acetone.The case hardening is carried out in a furnace flushed with either 60% NH3, pureNH3 or CO. The nitridation is performed in mixtures of NH3 and H2 and the carbur-ization is performed in mixtures of CO and H2.


Figure 4.2: LOM image of the dimensions of the mask. The black areas are the mask and the brownareas are where the mask is transparent and the visible colour is from the background.


Case hardening is carried out in a Netzsch STA 449C thermal analyser. LOM iscarried out in a Neophot 30, a Leica MZ 125 and Olympus Altra 20.

4.3 Result and Discussion

When applying nickel to the whole surface before removing a pattern, the possibilityof examining each step with LOM disappears as the whole surface is grain boundaryetched. The only step to control with LOM is the most important one, the casehardening. The thickness of the nickel, using the method described in Section 3,is determined to be less than 20 nm so a nickel detection and thereby dimensionexamination as in Section 2.3.1 would for example require surface sensitive analysislike X-ray Photoelectron Spectroscopy (XPS). So instead of once again focusing onhow each step affects the sample, focus is directed at the final result and to the onenew step in the sample preparation - the nickel removal.

In the following, two different sample preparations are discussed. The samplepreparation is not yet final so these are preliminary observations for getting the


sample preparation under control. Additional sample preparation methods havebeen tested but these two are selected for providing information.

Figure 4.3 shows the 60% NH3 case hardened surface. The nickel removal timeis 10 s. As expected there are no visible lanes of any kind so for judging the casehardening a study of the cross section is needed.

Figure 4.3: LOM image of the partially nitrided surface. The black part in the left bottom is the appliedhole. Because of the removal of nickel lanes the whole surface has been grain boundary etched.

Figure 4.4 shows two images of the cross section of the 60% NH3 case hardenedsurface. There is a selection going on at the surface but it is not under control asthe dimensions are imposible to recognize. At the top image the selective areaseems completely random while at the bottom image it looks like that obtained withthe 60% NH3 case hardening the nickel applied surface. So it seems that a nickelremoving time of 10 s is usable, but further investigations are needed.

A closer look at the nitrided layer is seen in Figure 4.5. The thickness of the layeris only half the expected 13.5 (±0.8) µm [14] and this again shows that the samplepreparation is not yet optimal.


Figure 4.4: LOM image of the cross section of a partially nitrided sample. At the top image theselective areas seem completely random with a lot of small nitrided islands while the bottom imageshows that a selection is possible.


Figure 4.5: LOM image of the cross section of a partially nitrided sample. The image shows that thenitrided layer is very thin.

The second sample examined in this chapter is exposed to only 1 s of nickel removaland afterwards the sample is case hardened with carbon.

Figure 4.6 shows the surface of the carburized sample. The black colour dominatingthe right side of the image is graphite which indicates a nickel removal. However byno means is it as black as that observed when carburizing the nickel pattern appliedsurface seen in Section 2.3 and this indicates that a case hardening is happeningin both areas.

The indication of a carburization of the whole surface is confirmed by Figure 4.7.The cross section image shows that the whole surface is being carburized. Thereason for this must be that the nickel removal time is too short and that the nickeltherefore still present at the surface.

The thickness of the carburized layer is shown in Figure 4.8. The layer is almostas thick as that observed by Christiansen et al. [12] which means that the nickelremoval time of 1 s does not really affect the surface.


Figure 4.6: LOM image of the partially carburized surface. The black colour dominating the right sideof the image is graphite which indicates a nickel removal. However by no means is it as black as thatobserved when carburizing the nickel pattern applied surface seen in Section 2.3 and this indicatesthat a case hardening is happening in both areas.


Figure 4.7: LOM image of the cross section of the carborized surface. The image shows that thewhole surface is carburized and the reason for this must be that the nickel removal time is too shortand that the nickel therefore still present at the surface.


Figure 4.8: LOM image of the cross section of the carburized surface. The layer thickness is almostas thick as that observed by Christiansen et al. [12] which means that the nickel removal time of 1 sdoes not really affect the surface.


4.4 Conclusion

The nickel removal in patterns gave the expected problem of the inability to followeach step in the sample preparation easily. The only way to observe the processusing LOM was to observe the final result after the case hardening. When usingnitrogen only the cross section gave some information whereas using carbon boththe surface and the cross section gave the needed info.

The nickel removal time was in one case 1 s, shown to be too short as the wholesurface was case hardened and in another case 10 s shown to be working but notoptimal as the selectivity of the surface was very limited. So the nickel removal timemust be greater than 1 s and around 10 s. The nitrided layer was only half thethickness of the optimal process so many different sample preparations still need tobe examined.

Chapter 5

Conclusion

In the chapters 2 and 4 two different types of micro scale pattern creation werediscussed and in both cases the case hardening using CO was the one most fa-vorable. In the applying of nickel in patterns case hardening with carbon showedthe best selective control and the preliminary observations of the creation of lanesremoving nickel showed that by using carbon, information could be retained fromusing LOM at the case hardened surface.

The applying of nickel in patterns showed the expected problem with an unevenand thin case hardened layer and the examination of the opposite process is not asprogressed to see if the solution is this sample preparation.

Chapter 2 provided some very interesting conclusions. The cross sectional imagesshowed a circular shape of the case hardened layer that was thicker than the restof the layer. Both the circular shape and the extra depth were a consequence ofthe diffusion happening faster in a single spot and therefore spreading deeper andspreading in every direction. From this it was concluded that the diffusion washappening faster right there than it was through the rest of the nickel layer and alsothat the diffusion velocity in the procedure without patterns was below its maximumin stainless steel.

As a result of the conclusion it was suggested that an extra layer thickness could beobtained by applying a surface with a string of small nickel dots instead of a contin-uous surface of nickel. The images also showed that the diffusion was happeningeasier in grain boundaries.

In Chapter 2 it was seen that with a cold deformed surface could initialize a nitridingin a area still covered with a chromium oxide layer and that this nitriding was liftingthe grain out of the surface thereby making them visible.

46 Conclusion

Chapter 3 provided a method for determinating the thickness of a layer with use ofEDS and electron simulations and it could very interesting to make an experimentaltest of the method using XPS sputtering.

5.1 Outlook

In the near future it would be very interesting to complete the LOM examination ofthe method of removing a pattern of nickel and to see if it is possibly to get betterresults in this way.

For getting extra analysis of the methods it could be very useful using X-ray analysis.From this the phase could be analysis and stress analysis is also very useful forcategorizing the methods.

Erosion analysis could be very useful for examining some of the application of apattern surface.

Bibliography

[1] T. Bell, “Surface engineering of austenitic stainless steel,” Surface Engineering18(6), 415–422 (2002).

[2] ASM-International, Stainless steels, Materials Park,Ohio (1994).

[3] Z. Zhang and T. Bell, “Structure and corrosion resistance of plasma nitridedstainless steel,” Surface Engineering 1(2), 131–136 (1985).

[4] K. Ichii, K. Fujimura, and T. Takase, “Structure of the ion-nitrided layer of18-8 stainless steel,” Technology Reports of Kansai University (27), 135–144(1986).

[5] Proceedings: Stainless steel 2000 - Thermochemical Surface Engineering ofStainless Steel; T. Bell, K. Akamatsu; (2001), ISBN 1-902653-49-1.

[6] M. A. J. Somers, T. Christiansen, and P. Møller, “Case Hardening of Stainlesssteel Danish Patent DK174707 B1 and PCT/DK03/00497,”

[7] T. Christiansen and M. A. Somers, “On the crystallographic structure of s-phase,” Scripta Materialia 50(1), 35–37 (2004).

[8] T. Christiansen and M. A. Somers, “Controlled dissolution of colossal quanti-ties of nitrogen in stainless steel,” Metallurgical and Materials Transactions A:Physical Metallurgy and Materials Science 37(3), 675–682 (2006).

[9] T. Christiansen and M. Somers, “Low temperature gaseous nitriding and car-burising of stainless steel,” Surface Engineering 21(5-6), 445–455 (2005).

[10] T. Christiansen and M. A. J. Somers, “Decomposition kinetics of expandedaustenite with high nitrogen contents,” Zeitschrift fuer Metallkunde/MaterialsResearch and Advanced Techniques 97(1), 79–88 (2006).

[11] R. Frandsen, T. Christiansen, and M. Somers, “Simultaneous surface engi-neering and bulk hardening of precipitation hardening stainless steel,” Surfaceamp; Coatings Technology 200(16-17), 5160–5169 (2006).

48 BIBLIOGRAPHY

[12] T. Christiansen and M. A. Somers, “Low temperature gaseous nitriding andcarburising of stainless steel,” Surface Engineering 21, 445–455 (2005).

[13] T. Christiansen and M. A. Somers, “Avoiding ghost stress on reconstruction ofstress- and composition-depth profiles from destructive x-ray diffraction depthprofiling,” Materials Science and Engineering A 421, 181–189 (2006).

[14] T. Christiansen and M. A. Somers, “Controlled gaseous nitriding of austeniticstainless steel aisi 316,” In Preparation for publication .

[15] T. Christiansen and M. A. Somers, “Determination of concentration dependentdiffusion coefficients of nitrogen in expanded austenite,” Submitted .

[16] T. Christiansen and M. A. Somers, “Simultaneous determination of stress- andcomposition profiles in expanded austenite by low temperature gaseous ther-mochemical treatment of aisi 316,” submitted to Metallurgical and MaterialsTransaction A .

[17] Fabricational instructions for Photoresist positive 20.

[18] T. Christiansen, Private communications.

[19] Software, CES EduPack 2005.

[20] P. Goodhew, L. Cartwright, F. Humphreys, and R. Beanland, Electron Mi-croscopy and Analysis, Taylor and Francis (2001).

[21] A. Horsewell, Private communications.

[22] URL, http://www.gel.usherbrooke.ca/casino/index.html, Casino, monte carlosimulation of electron trajectory in solids (2006).

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