Chapter 5 Plasma Nitridation
This chapter deals with the nitridation of GaAs and porous GaAs for the formation of GaN using N2+H2 gas mixture as the plasma forming gas. High Speed M2 steel has also been nitrided to form Iron Nitride Chapter 5:Plasma Nitr. 108
Index
Introduction 109
5A Section A: Gallium Nitride (GaN) 109
5A.1. Background Literature 109 5A. 1.1 Importance of GaN 109 5A. 1.2 Surface chemistry 110 5A.1.3 Electrical properties 110 5A. 1.4 The applications of GaN 111 5A. 1.5 Different techniques of synthesizing GaN 112 i) Chemical process 112 ii) Physical process 112 iii) Nitridation of porous GaAs 112 5A.2 Present experimental techniques of synthesis 114 5A.2.1 Nitridation of GaAs 114 5A.2.2 Nitridation of Porous GaAs 114 5A.3 Results and discussions 115 5A.3.1 Nitridation of GaAs 116 5A.3.2 Nitridation of porous GaAs 119
5A.4 Conclusions 122
5B: Section B: Iron Nitride 122
5B.1 Background 122 5B. 1.1 Surface Hardening 122 5B. 1.2 Nitridation 123 5B.2 Experimental details 125 5B.3 Results and discussions 126 5B.4 Conclusions 132
References Chapter 5:PIasma Nitr. 109
CHAPTER 5 Plasma Nitridation
Introduction Present chapter deals with the process of nitridation and presents some of results obtained by plasma nitridation over two important classical materials, namely GaAs and Steel. The main intention of the work is to demonstrate the potential of ECR plasma in reducing the time of nitridation usually required in the conventional chemical methods. However, there are different techniques of producing surface nitrides which involve deposition of a nitride layer on the required surface. Few attempts have been made in which GaN is deposited on GaAs by CVD. The chapter is split into two sections. The first section refers to the nitridation of GaAs whereas the second section refers to the nitridation of steel. The purpose of obtaining the nitrides on these two different classes of materials is completely different. GaN is a semi conducting optical material whereas nitrided steel is a mechanically important material for tool steel industry. Importance of these two kinds of materials is described in details in the following section.
5A Section A: Gallium Nitride (GaN)
5A.1. Background literature
5A.1.1 Importance of GaN
GaN is an important III-V group semiconductor well known for its stability and as a blue light emitter '. Some people 2'3 have developed a process for obtaining p-type GaN to demonstrate first P-N junction light emitting diode. The nitrides of III-V semiconductors have gained much importance recently because of their tremendous applications in the field of opto electronics 4. GaN is one of the most important candidates of these nitrides because of its potential applications for a solid-state lasers and optical data storage devices ' . Modern researchers have used improved crystal growth and processing technology to overcome many of the difficulties reported by the earlier workers. It is a highly stable material against temperature variation & its wide band gap has made it attractive material for device operation in high Chapter 5:Plasma Nitr. 110
temperature and caustic environment. It is an excellent candidate for protective coatings due to its hardness. Blue and UV wavelengths are technologically important regions of the electromagnetic spectrum in which efforts to develop semiconductor device technology are being carried out7 . Current semiconductor components emit wavelength from IR to green wavelength. With the advancement in the blue wavelength semiconductor components, emission and detection of the three primary colors of the visible spectrum is possible, which would have a major impact on imaging and graphics applications. Another technologically significant band occurs in the 240- 280 nm range (~4.75eV) where absorption by ozone makes the earth's atmosphere nearly opaque. Space to space communication in this band would be secure from the earth, although vulnerable to satellite surveillance. On the dark side of the earth, shielded by the sun's radiation imaging detectors operating in this band would provide extremely sensitive surveillance of objects coming out of the atmosphere. Like most wide band gap semiconductors, the nitrides are expected to exhibit superior radiation hardness compared to GaAs and Si, which makes them attractive for space applications. GaN is by far the most studied of the III-V nitrides, yet compared to the more commonly studied Si and GaAs semiconductors, relatively little is known about GaN. Large background n-type carrier concentrations, the lack of a suitable substrate material, difficulties with GaN p-type doping, and processing difficulties have discouraged many workers in the past. Films of GaN have been prepared by various methods. There are many reports 8"" of synthesizing thin films of GaN by exposing the surface of GaAs to nitrogen ions generated by different resources. This chapter deals with the nitridation of bulk GaAs which yields a layer of GaN on its surface. 5A. 1.2 Surface chemistry
Though it is said that GaN is highly stable, it is noticed that the stability depends on its environment. It is less stable in HC1 & H2 environment but stable in N2 environment. It is the chemical stability at elevated temperatures combined with its wide band gap that has made GaN an attractive material for device operation in high temperature and caustic environments. GaN is also an excellent candidate for protective coatings due to its hardness . 5A. 1.3 Electrical properties
Unintentionally doped GaN has been observed to be n-type with the best samples with the electron concentration of ~4xl016/cm-3.The mobility was also found likewise & it was Chapter 5:Plasma Nitr. Ill
2 observed that it depends on temperature .At room temperature it is 600 cm /Vs while at 2 13 temperature of liquid N2 it is 1500 cm /Vs . Table 5.1 Structural Properties of GaN
Crystal Structure Wurtzite Band Gap 3.39 eV at 300 K 3.50 eV at 1.6 K Lattice Constant a = 3.189Au c = 5.185 Au Coefficient of thermal Aa/a=5.59* 10"b /K expansion Ac/c = 3.17 *10"6/Ka= 3.189 A0 Thermal conductivity K=1.3w/cmK a + Electron effective mass - m e ( 0.20 . 0.02 )m0 Refractive index at 300 K 2.29 n-type doping Substitutional O on N site and N vacancy P type doping C,Be,Mg,Zn,Cd
5A. 1.4 The applications of GaN
As stated previously that GaN is very much important for its ability of emitting in the blue region of the visible spectrum hence has been used for the optoelectronic and electronic devices. GaN is also used in high pressure annealing up to 1600°C for modification of the material microstructure & chemical composition. It is used largely for high mobility transistors, which have been used widely for display l4, data storage and power amplifications , and high power microwave devices . The explosive increase in the AlGalnN family of materials, in recent years has been fueled by the application of blue / green / UV light emitting diodes in the full-colour displays traffic lights, automatic lighting & general room lighting using the so called white light16'17. In addition blue / green laser diodes are currently being used in high storage 1 o capacity DVDs. GaN based photo detectors are also useful for solar blind UV detection & have applications as flame sensors 19. The high band gap & small lattice constant of GaN is fairly well- lattice matched to SiC substrates, which have the concomitant advantages of durability & high thermal conductivity relative to the more commonly used AI2O3 substrate . There is, currently, a lot of interest in the science & potential technical applications of spin transport electronics (or spintronics) in which the spin of charge carriers (electrons / holes) is exploited to provide new functionality for micro electronic devices 2J"23. The phenomenon of giant magneto resistance & tunneling magneto resistance have been exploited in all metal - insulator-metal magnetic sensors, magnetic systems for read & write heads in computer hard devices, magnetic sensors, and magnetic random access memories (MRAM) etc. . Chapter 5:Plasma Nitr. 112
5A.1.5 Different Techniques of synthesizing GaN i) Chemical process
There are many techniques to grow GaN thin films but the oldest reported technique to grow GaN was by Halide Vapor Phase Epitaxy (HVPE) and the nitrogen precursor was ammonia. The metallic Ga was converted into GaN keeping Ga in an NH3 stream at elevated temperatures. The reaction is as follows
2Ga + 2NH3 -» 2GaN + 3H2 ...(5.1)
According to the other reports, it is possible to make GaN by the reaction given below
GaCl + NH3 •* GaN + HC1 + H2. ... (5.2)
Using this method, the first single crystal GaN thin films were realized. The growth rate was quite high which allows making extremely thick films, whose properties were less influenced by thermal & lattice mismatches with the substrate 16. ii) Physical process
In this case, a GaAs film which has been cleaned with a proper sequential etching process is used for nitridation process. When this GaAs faces the nitrogen ions made within a plasma chamber, GaN is formed on the surface of the GaAs. The process can be represented as given below
GaxAsx+N2+H2 -» GaxAsx.y+yN+H+H -» GaN + GaAs ... (5.3)
The nitridation process occurs when the nitrogen ions are accelerated by the electric field towards the GaAs surface. iii) Nitridation of porous GaAs
Porous semiconductors (Si, Ge, InP, GaP, GaAs, GaN, etc.) have attracted much attention recently since they allow one to engineer optical properties in a relatively simple way ' . Such materials are usually formed by electrochemical etching of the nonporous semiconductors in some special electrolytes. The formation of the pores in GaAs is more complicated due to its binary nature as compared with that in elemental silicon27. It is generally agreed that the anodic etching of an electrode requires electronic holes which have to be supplied by the electrode itself Chapter 5:Plasma Nitr. 113
' . The basic conditions for electrochemical pore formation in a homogeneous electrode are a passive state of the pore walls and an active state, which promotes dissolution, at the pore tips. Anodization is carried out in an acidic medium for formation of porous GaAs. Acid can 29 30 31 be HC1 , H2S04 , HF and KOH . The process of formation is to be understood by the understanding of the semiconductor - electrolyte interface. In order to oxidize and dissolve the GaAs according to GaAs+6h+ -> Ga3++3As+ 32, a sufficient concentration of holes have to be provided at the surface 33. This concentration of holes is given by Cl— if porous GaAs is preapared in HC1 solution. Positive holes are necessary for that reaction, which means their concentration is important factor. According to J. Sabataityte et. al.34'35 ,XPS data have shown that porous layer composed predominantly of Ga203 , As203 and GaAs grains are formed on GaAs substrate during electrochemical etching. He also states that at the top of porous layer, the facetted AS2O3 microcrystals of micrometer size were found. Also, in the bulk of porous structure, GaAs is a dominating compound. On the basis of this electrochemical etching mechanism, it is reasonable to assume that the dispersed structures of oxides and gallium arsenide should be of a nanometric scale and the layer formed to be porous. For a small current density at the initial etching stage, a solution saturated by Ga3+ and As3+ ions, is formed at the interface between electrolyte and solid phase. In the solution, the dispersed particles of gallium oxide, gallium arsenide and arsenic oxide are present. To form stable finely dispersed particles, a stabilizer of dispersed system is needed which creates ionic layer between dispersed phase and electrolyte. Therefore, all positive ions are shifted into adsorption layer and combine into stable dispersed particles. The presence of nanopores makes possible a further solution of substrate and increase of layer thickness. If nitridation of the surface is used for preparing GaN on the surface of GaAs, it is usually seen that compared to the virgin surface of GaAs, a porous surface of GaAs is more reactive. We have therefore; chosen a method of nitridation wherein the p-GaAs is subjected to the nitrogen plasma. In this method, GaAs is first made porous by anodic reaction and then the surface of P- GaAs is exposed to ECR plasma consisting of Nitrogen and Hydrogen. The present section emphasizes the importance of GaAs and provides for necessary references.
In the present thesis the plasma consisting of combination of N2 and H2 (30:70) was used as a source of ions. This kind of combination gas is often used for nitridation on account of the easier dissociation and ionization of H2 as compared to N2. Moreover, the hydrogen ions apart Chapter 5:Plasma Nitr. 114 from dissociating the N2 molecules by bombardment also help in removing oxygen from the surface of GaAs. During the reaction over the surface of GaAs, the nitrogen ions replace the As atoms on the surface of the GaAs and form GaN.
5A.2 Present experimental techniques of synthesis
5A.2.1 Nitridation of GaAs
n-type GaAs [100] was obtained from Crystal growth centre, Anna University, Chennai. The samples were cut into 8 * 8 mm pieces. Each sample was initially degreased by a treatment with boiling methanol for about two minutes, and then rinsed with double distilled water. The samples were etched before any further treatment by two kinds of etchants. A strong etchant consisting of H20:H202:H2S04 in 1:1:4 proportion and a mild etchant H20:H202:H2S04 in 80:1:1 proportion were used. The etching time was 60 seconds for the strong etchant and 5 minutes for the mild etchant. After etching the samples were rinsed and dried under an IR lamp. These GaAs samples were then mounted in an ECR plasma chamber and the system was 5 evacuated up to a base pressure of 10" mbar.N2+H2 gas having a ratio of 30:70 was used for nitridation. Nitridation was carried out at 10"2 mbar. The samples were treated for two temperatures 400°C and 500°C for 10, 20 and 30 minutes. A negative bias of 300V was applied to the substrate.
The 30:70 ratio of nitrogen and hydrogen was known to produce a greater number of ions of nitrogen as compared to a pure nitrogen plasma. This is due to the faster ionization rate of hydrogen as compared to nitrogen, which in turn helps to ionize nitrogen to a larger extent. Elevated temperatures are required for nitridation purposes as high temperatures increase the solid solubility of the GaAs sample and this helps in diffusion of elemental nitrogen into the substrate, which helps the reaction of nitrogen with gallium. The sample was placed around 6 cm from the ECR zone where the energy of ions was found to be maximum as per the ion energy diagnosis carried out by Retarding field analyzer.
5A.2.2 Nitridation of Porous GaAs
Porous GaAs (P-GaAs) was formed by the electrochemical dissolution of GaAs in HC1 based solutions. The n-type (100) GaAs wafers were used. The schematic of the cell, which was used to anodize the GaAs wafers is shown in the Fig 5.1. An 8 x 8 mm GaAs wafer served as Chapter 5:Plasma Nitr. 115 the anode wherein the cathode was spectroscopic grade graphite. The entire cell was made up of Teflon, as it is a high acid resistant polymer. GaAs was anodized in a solution of 0.1 M HC1. Such technique has been reported in literature . A constant current source was used in the process of anodization. The etching time was 20, 40, 60 and 80 minutes at a constant current. The processing parameters are mentioned in Table 5.2. After etching, the samples were rinsed & dried under an IR lamp. The change in the surface morphology after etching can clearly be seen from the SEM analysis. These samples were mounted in the ECR plasma reactor and evacuated to a base pressure of 10"5 mbar. N2+H2 gas was passed through the system until the pressure stabilized at 10"2 mbar. The time for nitridation was 30 minutes at a temperature of 400°C.
mA GaAs in sample holder
Graphite Electrode
IMHC1 Solution
Fig 5.1: Schematic diagram showing the setup used for preparing porous GaAs.
Table 5.2 Parameters for Porous GaAs Voltage(V) 5 Current (mA) 100 Duration of anodisation (minutes) 20,40,60,80 Solution used 1M HC1 solution in triple distilled water
5A.3 Results and Discussions
5A.3.1. Nitridation of GaAs The surface of the treated GaAs was found to turn bluish grey indicating the formation of an implanted nitrogen surface. Further crystallographic analysis by X-Ray diffraction was carried out in order to confirm the crystal structure on the surface. Peaks corresponding to the plane (101), (110) and (103) in GaN were found on the surface of the treated GaAs. The intensity of the peaks was found to increase as the time and temperature of the treatment. However, the Chapter 5:PlasmaNitr. 116
GaAs peaks were also seen indicating that the surface is not fully covered with a nitrided layer. Diffraction patterns of samples treated at different time and temperatures are shown in Fig 5.2. SEM micrographs of virgin GaAs and treated GaAs are shown in Fig 5.3. It is seen that the surface morphology modifies after treatment. The GaAs sample treated at 400°C 30 minutes showed a certain change in the surface morphology as seen in Fig 5.3 (b). An increase in temperature to 500°C showed a profound effect on the surface roughness as can be seen in Fig 5.3(c). This might be due to the higher surface reactivity within the plasma at elevated temperatures thereby accelerating the reaction process between Gallium and Nitrogen with the formation of GaN. Table 5.1 shows the nitrogen contents within the films wherein it is clearly seen that an increase in treatment time causes a higher atomic percentage of nitrogen to react with the surface. This enables the formation of better quality GaN films. This variation in atomic percentage of nitrogen with respect to time and temperature of nitridation is plotted in Fig 5.4 . Fig 5.5 shows the photoluminescence spectra for GaN synthesized at 400°C with three different times: 10 minutes, 20 minutes and 30 minutes keeping the other system parameters to be the same. The spectrum of plasma nitrided GaAs exhibited a low energy tail extending upto 570nm. The peak at 365 nm is closely related to the near edge band luminescence from hexagonal GaN, whereas the low energy tail seem to appear from the cubic phase of GaN 36 since hexagonal GaN has a band gap greater than that of cubic GaN. However it seems, the continuous background has facilitated the merging of both the peaks. The peak intensities at 365 nm seem to vary as the time of nitridation is changed. Chapter 5:Plasma Nitr. 117
2400- 2200- d) S00°C20 minutes
2000< z 1800- 3 _ 1600- a c 1400-
S
£ S J Intensit y (art ) s 1 f 8
200- 0- *-^>r*XJ 20 30 40 50 60 70 80 20
2400- 2200- _ f) 500°C 30 minutes 2000* 3 1800- 1600- 1400- 1200- 1000- 800- 1 600- 400- II Ml |j 200- 0- 1 1 I ~ —I • 1 <—T ' 1 ' r—'
Fig 5.2: XRD patterns for GaAs nitrided for a) 400°C 10 minutes b) 500°C 10 minutes c) 400°C 20 minutes d) 500°C 20 minutes e) 400°C 30 minutes f) 400°C 30 minutes . Chapter 5:Plasma Nitr. 118
a) Virgin GaAs
b) 400°C 30 minutes c) 500°C 30 minutes
Fig 5.3: SEM micrographs of (a) virgin GaAs (b) Nitrided at 400°C for 30 minutes and (c) Nitrided for 500°C for 30 minutes.
Table 5.3 EDS measurements for atomic percentage variation of nitrogen content wit i time and temperature. Samples Gallium% Arsenic% Oxygen% Nitrogen% Virgin 52.71 45.47 1.82 0 400° C lOmins 47.28 42.55 9.05 1.12 400° C 20mins 52.6 37.33 0.65 9.42 400° C 30mins 63.23 0.20 0 36.57 500° C lOmins 46.87 43.37 7.61 2.15 500° C 20mins 52.29 10.02 0.68 37.01 500° C 30mins 61.09 0.32 0 38.59 Chapter 5:PlasmaNitr. 119
GaN treated at 400°C
10 minutes 20 minutes 30 minutes
3-
»:
5
0 • i -5 1 1 • 1 • 1 ' 1 • 1 1 1 1 1 5 10 15 20 25 Time of treatment(mm) Wavelength(nm)
Fig 5.4: Variation of nitrogen concentration Fig 5.5: Photoluminescence spectra for with time and temperature for nitrided GaN formed at different times samples. showing variation in intensity.
5A.3.2 Nitridation of porous GaAs:
Before nitriding the porous GaAs was first investigated for its structural and morphological behavior. Some interesting growth over the surface was observed and this is discussed in the following section. X-ray diffraction analysis of porous GaAs exhibited several
species of Ga and As. The species that were found in literature were As203 at 20= 30.4°, Ga-O-
OH at 26= 54.5° and AsH3 at 20=28° and 46°. The X-ray diffraction pattern for a typical porous GaAs made porous for 40 minutes is shown in Fig 5.6(a) and after nitridation in Fig 5.6(b). Raman spectra for porous GaAs samples were recorded at a wavelength of 633 nm as described in Chapter 3 , the results of which are shown in Fig 5.7. None, some, or all of a number of Raman peaks at 85, 183, 203, 268, 289, 369 and 414 cm"1 were observed depending on the sample and the probe laser beam location on the sample. The Raman lines seen at 268cm"1 were visible for almost all the samples due to the strong transverse optic (TO) phonon which might be due to the result of sample orientation. The absorption by the longitudinal optic (LO) phonon of GaAs is visible with the peak at 295 cm"1 37. 268 cm"1 also matches with Raman peak for AS2O3 as seen in the SEM images. Scanning electron micrography revealed the formation of pores on the surface of the GaAs layer whose density increased with an increase in the time of anodization. This is seen in the Fig 5.8 wherein the porosity of GaAs is seen to increase with time of anodization. Post nitridation treatment exhibited a further change in GaAs. Surface morphology of the samples was Chapter 5:PlasmaNitr. 120 seen to degrade indicating extensive plasma enhanced modification. This can be seen in Fig. 5.9 wherein a higher degree of surface corrugation is seen. Photoluminescence studies, shown in Fig 5.10(a), resulted in a peak around 540 nm expected for porous GaAs. Fig 5.10(b) indicates the peak intensities for the emission lines corresponding to 360 nm for GaN to increase compared to the nitridation of simple GaAs as seen in Fig 5.5. This indicates higher surface reactivity of porous-GaAs attributed to the presence of pores. The peak at 540 nm for P-GaAs is not visible after nitridation indicating that the surface is being uniformly covered with GaN which confirms the X-ray diffraction studies.
Porous GaAs . m Porous GaAs after nitridation i (a) (b) 2000- 1000- o z O 10 ] c 1500- nts ) 3 a I p ar b I < I c >, o °„ 9 " n < 0 "c 0 500- I g f / \ ^r^ 1 0- uJ U^w WWW
1 • 1 • 1 ' 1 ' 1 20 30 40 50 60 70 80 20 30 40 50 60 70 80 26 26
Fig 5.6: XRD Pattern for Porous GaAs (a) as etched and (b) after plasma nitridation .
25000 20 minutes 40 minutes 60 minutes 20000 80 minutes-1 Virgin 'I 15000 -d u. (0 .1? 10000- w c „ . virgin 5000- 60 minutes -80 minutes 40 minutes 20 minutes 0-
"T "T 50 100 150 200 250 300 350 400 450 Wavenumber(cm -1\ Fig 5.7: Raman spectra for P-GaAs Chapter 5:Plasma Nitr. 121
\*:'-.'-" '•'•;' ' ;' ,'
jWW :-v.>-. •.*•-* ,y:
201.-U ' xit., eee . t«-m IQ.Z? sef-W. §?•• 2Sk->*L_^'x^ki^e "^Mm # ie 28»!Sij/ ,1 , . . ; 20 minutes 40 minutes
60 minutes 80 minutes
Fig 5.8: Scanning electron micrographs of porous GaAs and their structural variation with respect to time, as indicated in the titles.
60 minutes 80 minutes Fig 5.9: Scanning electron micrograph of nitrided p-GaAs for samples made porous for the time duration as indicated in the title. Chapter 5:Plasma Nitr. 122
180-, Porous GaAs Porous GaAs treated in N,+H plasma at 400 C for 30 minutes 160- -60 minutes 140- -80 minutes -40 minutes 120- a 100- E 40 b) 80-
60- 1 I"
40-
20-
0- —I— 300 400 500 600 700 800 700 Wavelength(nm) Wavelength(nm)
Fig 5.10: PL spectrum for (a) P-GaAs for showing peaks at -540 nm (b) After nitridation showing peaks at -370 nm
5A.4 Conclusions
After nitridation of plain GaAs as well as porous GaAs in N2+H2 plasma, photoluminescence studies showed a higher intensity of emission in the porous GaN samples. Thus it can be concluded that due to the higher surface area of porous GaAs, the surface reactivity increases which enhances the rate of nitridation of the GaAs surface. The synthesis of epitaxial GaN using MOCVD technique could not be completed due to the difficulties encountered during the transportation and handling of Tri-Methyl Gallium within the present laboratory constraints.
5B : Section B : Iron nitride 5B.1 Background 5B.1.1 Surface Hardening Tool materials, subjected to high amount of friction and impact loads, have a fixed lifetime of operation. This lifetime depends on the wear rate of the particular material which is in turn dependent on the surface hardness related property. Adding a hard layer on the surface can increase this surface hardness. In addition modifying the current layer by plasma process can also increase the hardness. Surface hardening, is a process, which includes a wide variety of techniques which are used to improve the wear resistance of parts without affecting the softer, tough interior. This Chapter 5:Plasma Nitr. 123
combination of hard surfaces and resistance to breakage upon impact is useful in parts such as a ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. There are basically two distinctly different approaches to the various methods for surface hardening. 1) Methods that involve an intentional buildup or addition of a new layer. Thin films, coatings or weld overlays are some of the examples. 2) Methods that involve surface and subsurface modification without any intentional buildup or increase in the part dimensions. Nitriding, Carburizing, Boriding, Nitrocarburizing etc. are some of the examples of this technique. Both techniques have their advantages and their disadvantages. Increase in hardness by coating becomes less cost effective when it is required on a large scale as well as the whole surface area has to be coated. However, coating by this technique gives exceptionally hard coatings like those of diamond, TiN and AI2O3. Diffusion techniques modify the chemical composition of the surface with hardening species such as carbon, nitrogen or boron. Diffusion methods allow effective hardening of the entire surface of a part and are generally used when a large number of parts are to be surface hardened. The basic process used is thermochemical because some heat is needed to enhance the diffusion of hardening species into the surface and subsurface regions of a part. The depth of diffusion exhibits a time temperature dependence such that Case depth a K V time Where the diffusion constant, K, depends on temperature, the chemical composition of the steel, and the concentration gradient of a given hardening species. In terms of temperature, the diffusion constant increases exponentially as a function of absolute temperature. Concentration gradients depend on the surface kinetics and reactions of a particular process. We have studied both the techniques for hardening of a given material. Diamond coatings were carried out on Ni substrates in a microwave plasma with a mixture of hydrocarbon and nitrogen as the reacting gas. This is reported in Chapter 6. Nitridation of industrial grade high speed steel was carried out in an Electron Cyclotron Resonance plasma with an N2+H2 mixture as the reacting gas. Both treatments gave high hardness values as studied by microhardness method and nanoindentation technique.
i Chapter 5:Plasma Nitr. 124
5B.1.2 Nitridation
Nitridation is a heat treatment based surface modification process that introduces nitrogen into the surface of steel at a temperature range (500- 550°C), while it is still in the ferritic condition. This process is particularly useful in engineering and component applications wherein the life of the steel tools and steel parts play a very important role in the stability of the overall machinery. These steels are subjected to impact and high friction based applications wherein they need to be replaced upon failure by cracking. Some applications of nitriding in various industries are shown in Table 5.4 Table 5.4: Use of nitriding in industries for various components. Industry Components Automobile Crankshafts, Valves, Camshafts, Bearings, Gears, Piston rings. Tool HSS tools like drill bits, hobs, taps, etc. hot working tools like dyes Forging and metallurgy Dies, Mantrel Powder metallurgy Gears, Bearings Plastics Extrusion screws Ball point pen Balls Pressure dye casting Sleeves, spurs, Dyes. The hardness of steel can be increased by various inclusions in the steel itself, hardness can be increased by increasing the percentage of carbon, hydrogen, nitrogen or oxygen/ nitrogen [oxy-nitriding], carbon/nitrogen [carbonitriding]. Mechanical stability of the steel is affected with the addition of hydrogen as it causes its surface to become brittle. Carbon and nitrogen, above 0.2 % substantially increase the hardness , which after quenching , leads to the subsequent formation of carbides and nitrides. In the process of nitridation, the nitrogen content on the surface of the steel should increase. So, depending on the medium used, nitriding process has been termed as gas nitriding, liquid nitriding, ion nitriding, molecular beam epitaxy based nitriding and plasma nitriding. Various attempts have been made in the past few decades to modify the surfaces of steels in order to increase the surface hardness having enhanced tribological properties 38"42 . Plasma- nitriding is one of the advanced treatments for improving the surface properties of the materials 43_45. The foremost advantages46'47 of plasma nitrided steel surfaces include; dimensional accuracy of the treated surface, higher ductility of nitrided layers, better mechanical properties, stress-free annealing, increased fatigue life, increased wear & corrosion resistance, decrease in white layer growth and reduction in the nitriding time compared to the conventional ion nitriding processes 48"50. In addition, microwave-assisted electron cyclotron resonance (ECR) plasma provides a high degree of ionization and high electron & ion temperature with ion energy Chapter 5.-Plasma Nitr. 125
between 10-25 eV. This leads to the high plasma reactivity due to effective production of active species ' , and a high degree of plasma uniformity over large areas compared to other techniques 53'54. Different varieties of steels are used in industries for various applications. M2, high speed steel is important steel, which retains its dominating position in the industrial tools where high ductility and good machinibility in soft annealed conditions are required54. It is also used in fabricating various tools like drills, taps, reamers, cutters and many others 55. Composition of high speed steel is giving in the Table 5.4 Although there are several reports56"58 wherein the ECR plasma has been used for nitriding the steel surfaces, there is no report of its used for M2 steel. Recently59, we have reported the results of nitridation, of EN-41B steel using ECR plasma for generation of hard layer having no white layer in much shorter time. In view of this, the objective of the present study is to understand the process of nitridation of M2 steel using ECR plasma over the normal plasma-ion-nitriding processes. In this chapter, pure M2 steel samples were nitrided at four different temperatures. Measurements of hardness over different depths reveal interesting properties which is augmented by X-ray diffraction and X-ray photoelectron spectroscopic measurements. TABLE 5.4: Composition of M2 high speed steel obtained from data sheet. NO ELEMENT % composition in Fe 1 Carbon 0.46 2 Silicon 0.264 3 Manganese 0.54 4 Sulphur 0.011 5 Phosphorus 0.015 6 Nickel 0.12 7 Chromium 1.59 8 Molybdenum 0.081 9 Vanadium 0.01 10 Titanium 0.01 11 Cobalt 0.023 12 Tungsten 0.024 13 Aluminium 0.9-1.3 5B.2 Experimental details The schematic diagram of the experimental setup is shown in Chapter 3, Fig 1. Sample was mounted on a heater assembly at a distance of 20 cm from the ECR zone. The plasma density was measured at this position by using two-probe method. Chapter 5:Plasma Nitr. 126
The samples of M2 high speed steel (0.86% C, 6.0% W, 5.0% Mo, 4.1% Cr, 1.9% V, 0.5% Co, in wt%) were in the form of discs with 3mm thickness and 25mm diameter. These substrates had uniform hardness of ~ 580 HV. The samples were first polished using sandpapers of different grades varying from 800 to 1200 and were then immersed in ethyl alcohol and cleaned in an ultrasonic bath for 10 minutes. Later they were immersed in triple distilled water, cleaned ultrasonically for 10 minutes and finally dried under an IR lamp. These samples were then mounted on a suitable sample holder having provision for heater and thermocouple. This assembly was then placed in the ECR reactor, which was then evacuated to a base pressure of 10"6 mbar. Prior to nitridation, the samples were depassivated in an argon plasma for 15 minutes with a negative bias of 300V. A mixture of reactive gases consisting of N2 (30 vol. %) and H2 (70 vol. %) from a premixed gas cylinder was introduced into the ECR reactor through a mass flow controller at a rate of ~ 40sccm. The Langmuir double probe was used to estimate the plasma density (ne) and electron temperature (Te) for argon and a mixture of nitrogen and hydrogen gases produced in the ECR plasma as functions of gases pressure, distance from plasma zone and magnet current. Nitridation was carried out at four different temperatures namely 400, 450, 500 and 550 °C for four samples under same conditions having chamber pressure ~ 10"3 mbar, nitridation time ~ 90 minutes, microwave power ~ 150 W. The temperature was measured by chrornel-alumel thermocouple embedded in to the sample. 5B.3 Results and discussions Microhardness was measured using a Vickers microhardness tester (HMV, Shimadzu Japan) at different loads of 25, 50, 100, 200, 300, 500 and 1000 gms. These loads were then related to different depth of indentation into the sample. Fig. 5.11 shows the microhardness (HV) versus indenter load (gm) for virgin sample and samples nitrided at different temperatures (400, 450,500 and 550 °C). Chapter 5:Plasma Nitr. 127
Scanning electron microscope (JEOL, JSM, 6360A- JAPAN) was used to view the nitrided layer. For this purpose the sectioned sample was mounted into the polymer binder and then polished with diamond paste. Prior to loading, the samples were etched with 2 % Nital solution (a mixture of 2 % nitric acid and 98 % ethanol) in order to make the nitrided layer appear much sharp and clear. SEM photographs of the etched (with 2% Nital) cross sections of the samples nitrided at different temperature 400, 450, 500, 550 °C are shown in Fig 5.12 along with the virgin sample. Fig. 5.13 shows the variation of the nitrided layer with respect to temperature. X-ray diffraction patterns were recorded with X-ray Diffractometer (Philips PW 1710) for the phase analysis of the nitrided layers. The XRD patterns for the virgin sample and those nitrided at different temperatures are shown in Fig. 5.14. X-ray photoelectron spectroscopy (XPS, ESCA-3000-V.G. Microtech. England) was used to study the chemical composition in the nitrided surface region. Monochromatic X-ray beam of Al Ka (hv =1486.6 eV) and Mg Ka (hv = 1253.6eV) radiations were used as the excitation source. A hemispherical sector analyzer and multichannel detectors were used to detect the ejected photoelectrons as a function of their kinetic energies. XPS spectra were recorded at a pass energy of 50 eV , 5mm slit width and a take off angle of 55°. The spectrometer was calibrated by determining the binding energy values of the Au 4f7/4 (84.0eV), Ag 3d5/2 (368.4eV) and Cu 2P3/2 (932.6eV) levels using spectroscopically pure materials. The instrumental resolution under these conditions was 1.6eV full-width at half-maximum (FWHM) for Au 4f7/4 level. The C Is (285 eV) and Au 4f7/4 (84.0 Chapter 5:Plasma Nitr. 128
eV) were used as internal standards whenever needed. Fig. 5.15 shows the deconvoluted spectrum for Nls level for the sample nitrided at 400°C.
(e) Fig.5.12: SEM micrographs for cross-sections of nitrided layer of HSS for nitrided at (a) 400°C (b) 450°C (c) 500°C (d) 550°C (e) Virgin sample. In all the figures 1- Mould material, 2- Interface, 3- Nitrided layer, and 4-Bulk material Chapter 5:Plasma Nitr. 129
^50- E
|45- JO T3 (D 40 I ~ j 'E O 1 . Thicknes s " 25- ,
.-?nU --| . 1 1 1 1 1 1 | 1 | 1 | 1 | 1 | 1 | 1 380 400 420 440 460 480 500 520 540 560 Temperature Fig 5.13: Variation of nitrided layer thickness with respect to temperature
/ (1 1 1) e(1 01 )„(1 10) y* (300) \ \ ; y'P00)Y(210)
e(10 e(103) 500 °C e(101?
Virgin I i l I I I -1— 20 30 40 50 60 70 80 90 26( Degree)
Fig. 5.14: XRD-pattern of the samples HSS nitrided at 400 °C, 450 °C, 500 °C, 550 °C for 90 min and unnitrided sample. Different (hkl) planes have been identified by arrows. Chapter 5:Plasma Nitr. 130
The indented load provides the proportional depth of penetration from the surface and the value of hardness measured at that depth. Microhardness measurements in Fig 5.11 were taken at three different locations for each load (from 25 to lOOOgms) and for each sample. The average value for each load is plotted with respect to the variation in the load. The constant value of hardness at different loads for the virgin sample indicates the phase uniformity of the sample. It can be seen that the value of the microhardness for surface at a load of 25 gms has improved from 580 HV for virgin sample to 1190 HV for the sample nitrided at 550 °C. The hardness is seen to decrease with increasing depth of penetration (i.e. load), however reaching a steady value at larger depth. The steady value of the hardness, corresponding to a load of 500 gms is seen to be higher for the samples nitrided at higher temperatures. Further, the hardness of the samples at a given load increases with increasing temperature of nitridation. Improvement in the hardness for lower value of the load indicates that the surface region is more affected than the bulk of a given temperature of nitridation. High hardness at the surface indicates the difference in composition of the surface layer and subsurface layer. These might be a gradient of nitrogen concentration as we move from the surface towards the bulk. Also, it could be connected with the precipitation of the nitrides of iron and alloy-element inside the base material leading to an increase in the compressive stress in the subsurface layers up to depth depending upon the temperature 58. From the SEM photographs shown in Fig 5.12 four different regions are clearly seen in the images of nitrided samples. Region 1 and 2 correspond to the mould material and the boundary/interface between mould and sample respectively. The region 3 and 4 indicate the nitrided area and the core of the sample respectively. The nitrided layer is clearly seen in all the photographs of different samples nitrided at various temperatures. However, no zone corresponding to the compound/white layer is observed in all the nitrided samples. This might be due to its absence or having thickness < 1 urn in nitrided zone compared to diffusion layer. This
CO is expected since the chosen ratio of the mixture of gasses inhibits the formation of white layer . From the Fig 5.6 it is clearly seen that the thickness of the nitride layer (region c) increases from 25 to 48 jim with increasing temperature from 400 to 550°C. It indicates the diffusional behavior of nitrogen during nitridation. The structural analysis of nitride layer was carried out by using X-Ray diffraction studies. It shows in Fig 5.14 two main XRD peaks at 45° and 65° observed in each case correspond to the base material a-Fe. This is so because the normal XRD spectrum provides the bulk feature and Chapter 5-.Plasma Nitr. 131 since the nitride layer is fairly thin (20-50 u.m) for Cu Ka radiations, the bulk M2 steel is seen. The information about the presence of the nitride layer is obtained from the peaks at 20=38° and
78° which arises from s phase (Fe2.3N) of the nitride. The peaks corresponding to the y'- phase (Fe4N) are very feeble and are mostly buried under the noise. The expected positions are marked for samples nitrided at 500° and 550°C. The results indicate that the nitrided surfaces mainly consist of e phase and the core materials consisting of Fe. This makes the nitrided layer look as a uniform region without giving any observable appearance of white layer. The chemical composition of the nitrided layer is also studied using X-Ray photoelectron spectroscopy. Fig 5.15 shows the deconvoluted spectrum for Nls level for the sample nitrided at 400°C. The peak at 397.6 eV is assigned to Nls from e (Fe2-3N) phase 60. The peaks at higher energies 400.2 and 402.5 eV are assigned to CN group 61. It is reported 62 that during the nitridation process of steel, carbon from the hexagonal close packed FeaC is probably incorporated into E nitride Fe2-3N and an ill-defined interface would result in the formation of carbo-nitrides of the type Fe2.3(C,N). This represents a case of slightly negative shifted nitrogen and slightly positive centered nitrogen, which are attached to the iron at the surface of plasma 63 50 nitrided sample . The peak at 405.4 eV is assigned to N02 , which might be trapped inside the surface. The atomic ratio of Fe:N was determined by normalizing the peak intensities of Fe 2P3/2 (not shown in Figure) and Nls levels in the spectra with respect to the photoelectron cross section. This ratio was estimated to be 3.3:1. This result indicates that the epsilon Fe2-3N is the predominant phase in the nitrided sample, which is in agreement with our XRD results. Chapter 5:Plasma Nitr. 132
400.2 eV(C,N)
CO 'c D 397.6 eV(FeMN) and (Fe4N) < CO •*-> c o O
T T -i—|—i—[— 394 396 398 400 402 404 406 408 410 Binding Energy (eV)
Fig.5.15: Shows the XPS spectra of N Is for plasma nitrided HSS for 90 min and 400° C. Solid line represents spectra as it is recorded and the dashed line after deconvolution.
5B.4 Conclusions The experiments have shown that the thickness of the nitrided layer of high speed steel varies between 24 urn to 48 urn as the temperature of the sample is elevated from 400 °C to 550
°C during the process of plasma nitridation using ECR plasma. The E (Fe2.3N) is the major component in the nitrided surface. The absence of the white layer makes the process suitable for growing the nitride layer appropriate for high-speed steel, which finds applications in the impact loads. Chapter 5: Plasma Nitr. 133
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