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A new method for isolating plasma interactions from those of the laser beam during plasma nitriding
Materials Characterization 134 (2017) 143–151
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A new method for isolating plasma interactions from those of the laser beam during plasma nitriding
MARK
A.N. Blacka, S.M. Copleyb, J.A. Toddc,⁎ a b c
Los Alamos National Laboratory, Los Alamos, NM 87544, USA1 Applied Research Laboratory, The Pennsylvania State University, P.O. Box 30, N. Atherton Street, State College, PA 16804-0030, USA Department of Engineering Science and Mechanics, 212 Earth and Engineering Sciences Building, The Pennsylvania State University, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser-sustained plasma Titanium Titanium nitride Open atmosphere nitriding Melt pool characteristics Crystal nucleation and growth
A new method for separating the interactions of a laser plasma from those of a laser beam with a titanium substrate was developed. A laser-sustained plasma (LSP) was generated by striking the plasma on a titanium plate in a flowing nitrogen atmosphere at ambient pressure and subsequently removing the strike plate. The resultant nitrogen plasma was sustained indefinitely around the laser focal plane until the nitrogen gas flow and/ or the laser power were switched off. The laser-sustained nitrogen plasma was then used as a “reactor” to study the effects of a nitrogen plasma, independent of the laser beam, when the plasma was brought into close proximity, (2 to 3 mm), parallel to a titanium substrate. There was no interaction of the parallel laser beam with the substrate. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and substrate resulting in melting, nucleation and growth of faceted and dendritic TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. Dense TiN layers up to 300 μm thick formed within 5 s. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed very rapidly on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti6Al-4V.
1. Introduction Industries ranging from aerospace to biomedical utilize titanium and its alloys due to their excellent strength, high strength-to-weight ratio, low density, high melting point, excellent corrosion resistance, high fracture toughness, good heat transfer properties, and biocompatibility [1,2]. The poor tribological characteristics of these metals, however, reduce their applicability under severe wear conditions [3]. To improve these properties, titanium nitride (TiN) is a candidate coating material due to its extreme hardness, excellent corrosion and wear resistance, high thermal conductivity, transport properties, chemical inertness, biocompatibility, and gold appearance [4]. TiN is commonly used for protective coatings on cutting tools and drill bits, diffusion barriers in microelectronics, metal smelting crucibles, optical coatings, surgical instruments, and for decorative features [5]. Laser nitriding is a process in which laser irradiation melts a titanium surface, which is exposed to a nitrogen-containing atmosphere. Since the 1980s, it has been investigated for its potential as a fast, effective TiN synthesis technique, particularly relative to vapor
⁎
deposition techniques which may take up to 24 h [6]. During laser nitriding, the interaction of electrons and gaseous species excited by the laser produces near-surface plasma. The impact of this laser-induced plasma is a subject of much discussion; some consider it to enhance the coating process [7–9], others say it is detrimental to nitridation [10,11], while most ignore its potential effects [12–14]. This research separates the interactions of the plasma from those of a laser beam with a Ti surface by utilizing a laser-sustained plasma (LSP). LSP, originally referred to as a “continuous optical discharge” is plasma generated by a laser beam in a gaseous atmosphere that can be sustained indefinitely away from any potentially interacting surface [15]. While LSP has been successfully used to deposit diamond [16–18], its potential contributions to the formation of a broad range of hard coating compositions have not yet been systematically explored. Laser chemical-vapor deposition is another alternative method for large area deposition of TiN. Chen et al. [19] showed high deposition rates close to 1 μm/s. The method differs significantly from laser and LSP nitriding, as a vacuum is required and TiN is generated by the gaseous species and deposited on the substrate, rather than the titanium
Corresponding author. E-mail addresses: [email protected] (S.M. Copley), [email protected] (J.A. Todd). Research conducted while a graduate student in the Department of Engineering Science and Mechanics, 212 Earth and Engineering Sciences Building, The Pennsylvania State University, University Park, PA 16802, USA. 1
http://dx.doi.org/10.1016/j.matchar.2017.10.011 Received 20 May 2017; Received in revised form 6 October 2017; Accepted 7 October 2017 Available online 13 October 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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surface being the titanium source for the TiN creation. An important distinction needs to be made between the experimental process investigated here and conventional laser nitriding. In laser nitriding, the main heat source responsible for melting is the laser beam. In these experiments, LSP is the sole heat source responsible for melting the titanium substrate. The laser is used only to maintain the LSP and does not have any direct interaction with the material being nitrided. 2. Experimental Procedures Tests were completed on annealed and cold rolled ASTM grade 2, commercially pure (> 99.5%) titanium (CP Ti, McMaster-Carr). The asreceived material had an unpolished, milled finish and was sectioned into 25.4 mm (1 in.) × 50.8 mm (2 in.) × 3.175 mm (1/8 in.) plates. All plates used in the experiments received identical preparation and heat treatments. Prior to LSP processing, the surfaces were swabbed with acetone and isopropyl alcohol. A 5 kW CO2 laser (PRC Laser STS5000) with coaxially flowing nitrogen (N2) gas (99.995% purity) was used to generate the LSP. Energy to maintain the plasma was provided by the continuous laser beam at an output power of approximately 3.5 kW, measured prior to each set of tests. A Precitec laser cutting head with a 10 mm diameter nozzle directed the N2 gas flow at a rate of 10 SLPM. The laser focal plane was located 13 mm below the nozzle. The titanium substrate was positioned parallel to the laser beam and at the same height with respect to the focal plane in every test. All tests were performed in an open atmosphere work cell with linear stages that moved the substrate in the X and Y directions and the laser head in the Z direction, as shown in Fig. 1. The LSP was ignited by running the laser beam across a Ti strike plate at 90 mm/s with the N2 gas flowing and then removing the plate. Ignition took place between five and 30 s prior to interaction with the titanium substrate, after which the LSP was translated to a predetermined distance, d, from the substrate for the duration of the experiment. Fig. 1 illustrates the final position of the LSP with respect to the substrate, where the variable distance, d, represents the distance between the location of the substrate surface, which was determined by aligning a HeNe laser beam parallel to the surface, and the center line of the laser beam. A baseline laser test (without LSP) was then conducted with the high power (3.5 kW) laser beam held at the distance, d, from the substrate surface for 5 s while the N2 gas was flowing. For all distances, it was determined there was no effect of the laser beam on the substrate when the LSP was absent. The LSP was positioned and held stationary at distances, d = 2, 2.5, and 3 mm, respectively, for 5 s before it was extinguished. The temperature of the plasma at the surface varied with the respective positions, being highest for d = 2 mm and lowest for d = 3 mm. Each distance, d, was tested at least three times to determine reproducibility. Throughout the LSP interaction with the substrate, images were collected via a CCD camera (JAI TM-6740CL) equipped with filters to minimize oversaturation, and modified frame grabbing software. Excited atomic and ionic nitrogen species were detected using an 870 nm (10 nm FWHM) bandpass filter in conjunction with a 0.6 neutral density (ND) filter. Excited atomic and ionic titanium species were detected using a 430 nm (10 nm FWHM) bandpass filter and 0.6 plus 0.4 ND filters. Substrate surface examination was performed with a Philips XL30 field emission gun environmental scanning electron microscope (FEG ESEM). An EDAX energy-dispersive X-ray spectrometry (EDS) system was utilized to determine the compositions of selected areas on the surfaces and cross-sections. Transverse and longitudinal cross-sections were polished using standard metallography procedures, submersion etched with Kroll's reagent, and then gold-coated once initial EDS measurements were completed.
Fig. 1. Side view of the experimental setup showing d, the distance between the laser centerline and the substrate surface. The size and shape of the LSP are shown schematically. Note: In all figures, a green arrow indicates the direction of the laser beam and a white, dashed line indicates the location of the laser's focal plane. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results 3.1. Process Images Following the procedure described by Nassar et al. [20], a CCD camera system and associated filters monitored the mass transport processes taking place throughout the LSP-substrate interaction, as shown in Fig. 2 for d = 2 mm and d = 2.5 mm. Experiments recorded with the 430 nm filter failed to detect excited atomic and ionic titanium species at any time in the reaction images, hence they are not shown here. (Note, when present, Nassar et al. [20] showed that excited titanium species appeared as a lighter area close to the sample surface, or as brightening of the LSP.) The images presented in Fig. 2, taken with
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Fig. 2. CCD images observed through an 870 nm filter at comparable times for d = 2 mm (top row) and d = 2.5 mm.
the 870 nm filter, clearly show excited nitrogen species adjacent to the substrate surface from 1.4 s through the conclusion of the 5 second tests. The surface melted and formed a protrusion below the focal plane during heating of the d = 2 mm and d = 2.5 mm samples, but not when d = 3 mm. The protrusions started to grow approximately 1.25 s after the LSP reached its position then grew quickly for approximately 2 s before remaining a constant size and shape for the remainder of the interaction time. The geometry of the raised area was similar for all plates tested at the same d value, but varied in size with d. Fig. 2 compares the growth progression of the raised area for d = 2 mm and d = 2.5 mm at intervals ranging from 0.5 s to 5 s. 3.2. Macroscopic Observations Macroscale images of the LSP treated substrates are shown in Fig. 3(a) to (c) for d = 2, 2.5 and 3 mm, respectively. The laser beam direction is indicated by the green arrow in Fig. 3a and the white dashed lines indicate the position of the laser focal plane. A higher magnification view of a d = 2 mm sample, showing five major regions designated A through E, is presented in Fig. 4. The colors and compositions associated with these regions, according to the literature, together with EDS analyses taken in this research are presented in Table 1. Brown-colored regions (region A and beyond), extending > 10 mm from the center and indicative of hypo-stoichiometric and near-stoichiometric TiN [21], were produced at all three distances. (It should be noted that the EDS analysis of region A in Table 1 suggests that the TiN layer is very thin and that the majority of the signal comes from the Ti substrate.) Okamoto [22] has shown that hypo-stoichiometric TiN may form with as little as 28 at.% nitrogen. According to Kato and Tamari [23], the lustrous, dark red-brown region (region C), seen in d = 2 mm and d = 2.5 mm, indicates hyperstoichiometric titanium nitride (i.e. TiNx where x > 1). Mumtaz and Class [24], Roquiny et al. [25], and Nah et al. [26] agree that the stoichiometry dependent colors of TiN, described for regions A through
Fig. 4. A d = 2 mm sample showing five major zones that are distinguishable by color and luster. The white circle indicates the location of the plateau.
E, are independent of the processing method. In the center of region C, and extending above the laser focal plane, is a lustrous gold region indicative of stoichiometric TiN [27]. This indicates that near and above the laser focal plane (closer to the nozzle), the LSP reached sufficient size and temperature to produce a thick TiN layer on the titanium substrate, as will be confirmed in Section 3.3. All three specimens exhibited large gray regions (regions B, D), a white arc below the red-brown and gold region (region E), blue and orange areas surrounding the entire modified region (outer area of region D), and a purple region around the red-brown and gold area (outer
Fig. 3. Macroscale images for the LSP parallel configuration after 5 s at: a) d = 2 mm; b) d = 2.5 mm; and c) d = 3 mm.
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perimeter of region C). The shape of each treated area is also similar, resembling an inverted tear drop – rounded at the top while narrowing at the bottom of the plate. As d increased from 2 to 3 mm, the sizes of the lustrous gold and red areas in region C decreased accordingly, indicating that the distance, d, was an important parameter controlling the amount of stoichiometric TiN formed during LSP processing. The white circles in Figs. 3a, b, and 4 indicate locations for the plateau, a region raised above the original surface of the titanium plate that corresponds to the protrusions seen in the CCD images of Fig. 2. Fig. 5a shows that a raised ridge formed above the position of the laser focal plane (dashed white line) running down into the plateau. For the distance, d = 3 mm, even though no plateau developed, a raised ridge was still observed. The formation of this ridge is clearly captured in the time sequence for d = 2 mm in Fig. 2, but appears not to extend as far above the focal plane when d = 2.5 mm.
Table 1 Compositions of the regions delineated in Fig. 4 according to the literature and validated by EDS analyses [22,26–29]. Region
A B C D E
Description
Brown “halo” Silver Lustrous red Lustrous gold Dull gray Lustrous white
Literature
TiN [1] – TiN [2] TiN [1] TiO2 [3] TiO2 [4]
EDS results (at.%)
Composition
Ti
N
O
89.98 99.8 47.27 49.58 44.42 44.68
10.08 0.2 52.73 50.42 0 0
0 0 0 0 55.58 55.32
TiN Ti TiN TiN TiO2 TiO2
Fig. 5. a: SEM image of the region delineated in the macroscopic image of the d = 2.5 mm sample. The boxes show the locations of the higher magnification views in Panels b, c, and d. The bracket shows the area considered to be the plateau; b: Morphology exhibited by the vast majority of the surface area; c: The raised trail which segues into a crystalline region; d: A location on the raised trail that displays hollow, rectangularshaped crystals.
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Fig. 6. a: SEM image of the region delineated in the macroscopic image of the d = 2 mm sample. The boxes show the locations for the higher magnification views in Panels b, c, and d. The bracket shows the area considered to be the plateau; b: Rough-textured growth in the white region under the nitrided area; c: Branch of the raised trail with adjacent crystalline region; d: A region of faceted crystals on the plateau.
surface exhibited the cauliflower-shaped morphology shown in Fig. 5b. The raised ridge represented a melt region, which flowed down the plate to form a protrusion at the plateau. Surrounding the melt trail was a region of faceted crystals, some with rounded edges and blunt corners, as seen in Fig. 5c. The average facet length was 7 μm. A transition region, varying in width from 10 to 50 μm, separated the smooth, raised ridge and plateau from the crystalline regions, as observed in Fig. 5c. Evidence of subsequent nucleation and growth on the crystal facets was consistent with the characteristics of vapor growth. The crystals shown in Fig. 5d exhibited a hollow, rectangular morphology, and appeared similar to TiN crystals grown in an RF furnace with a nitrogen atmosphere above 2000 °C [31]. These areas only occurred sporadically near the raised trail and plateau. Figs. 6a–d present SEM images of the d = 2 mm sample for comparison with the d = 2.5 mm (Figs. 5a–d). Fig. 6b shows rough,
According to Wu et al. [30], the narrow, deep purple band around region C, which was more prominent in the d = 2.5 and d = 3 mm samples, represents a transition region where the TiO2 and/or titanium oxynitride in regions D and E form a nanometer thick coating on the TiN at the edge of region C, creating the purple color. 3.3. Surface Morphologies Fig. 5a shows an overall SEM view of the center of the sample shown in Fig. 3b (insert red box), which was processed at d = 2.5 mm. Higher magnification views of three regions of interest are presented in Fig. 5b, c, and d. It should be noted that the area away from the raised ridge and plateau (magnified in Fig. 5b) had a much smoother profile than the areas immediately adjacent to the plateau and ridge, which appear textured due to crystal growth (Fig. 5c and d). A majority of the nitrided 147
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Fig. 7. a–d: SEM images of rod-like growth along the surface of crystal facets on the d = 2 mm (Panels a, b) and d = 2.5 mm (Panels c, d) samples.
their closer distance to the LSP. Magnified views of faceted crystals shown in Fig. 5c (d = 2.5 mm) and Fig. 6c (d = 2 mm) are shown in Fig. 7a–d. Both samples exhibited nucleation and growth on the crystal facets. Fig. 7a and b (d = 2 mm) show early stage nucleation of rod-like morphologies along orthogonal, preferred growth directions. More advanced growth and coalescence of such features, together with facet rounding, is shown in Fig. 7c and d (d = 2.5 mm). While these results first appeared contradictory, (i.e. a colder substrate further from the LSP is expected to have slower growth kinetics), it was determined that the crystal facets in Fig. 7c and d (d = 2.5 mm) were located below the focal plane in an area protruding further into the LSP (i.e. higher temperature) than the facets in the d = 2 mm sample, which were observed in a region 0.5 mm above the laser focal plane, further away from the LSP. The surface features on the crystal facets in both the d = 2 mm and d = 2.5 mm samples appeared iridescent to the eye. When the profiles of the crystal facets were examined, it was determined that the surface features were extremely thin – rising < 10 nm above the surface on the d = 2 mm sample and approximately twice as high on the d = 2.5 mm sample. While it was not possible to obtain accurate compositional information with EDS analyses, backscattered images suggested that these features may have a higher nitrogen content than the underlying crystal, consistent with their nucleation in a flowing nitrogen atmosphere.
3.4. Cross-sectional Microstructures Fig. 8. SEM image of a longitudinal cross-section of the plateau region showing four distinct layers.
Longitudinal cross-sections (parallel to the laser beam direction) through the plateau region revealed layered microstructures similar to those observed in laser nitrided samples [2,32–34]. The most important difference was the thickness of each of the four layers within the plateau, as shown in the SEM image in Fig. 8. In describing the respective layers, TiN denotes titanium nitride, which exists over a wide composition range, and Ti(N) refers to a solid solution of α-Ti containing nitrogen. These phases can be differentiated by their crystal structures, since TiN has a cubic structure, and α-Ti and Ti(N) have hexagonal structures.
textured, cauliflower-shaped growth in the white area (region E) below the plateau. Fig. 6c and d show faceted and rectangular crystal morphologies, respectively. While the faceted crystals and the transition region were approximately the same size on both samples (averaging 7 μm crystal facets and a 10–50 μm wide transition region), the hollow, rectangular crystal morphologies (Fig. 6d) were less angular and more rounded, reflecting the higher temperature resulting from 148
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Fig. 10. Solid TiN layer acting as a platform for crystal growth.
oriented in multiple directions. The SEM image shows a crack through the center of this area, which was introduced during metallography, as no cracks were observed prior to grinding. The melt pool exhibited a solid TiN layer with average thickness of 17.5 μm at the top, and a very thin underlying HAZ. Region 2 (Fig. 9b) was located approximately 1 mm below Region 1 and consisted of a thinner (175 μm deep) layer of dendrites, with a very shallow (1 to 15 μm thick), solid TiN layer at the surface. Again, deep cracks were observed to develop and propagate during metallographic preparation. Region 3 (Fig. 9c) sectioned the plateau and the area approaching the laser focal plane. This entire region consisted of a very thick layer (up to 300 μm) of solid TiN with a relatively thin layer of dendrites. The layers appeared solid prior to etching, which removed regions of Ti(N) giving the porous appearance observed in Fig. 10. Faceted crystals, indicated by the red arrows at the lower left of the image, grew at the top of this solid layer. Thick nitrided layers were only present above and on the plateau, with the region below the protrusion exhibiting fewer dendrites without a thick solid TiN layer at the surface. Nitrogen gas flowed parallel to and down the substrate surface (coaxial to the laser) until it hit the protrusion of the plateau. This allowed a deep solid TiN layer to build up on the windward side of the plateau, leaving the leeward side sheltered from the nitrogen gas. A transverse cross-section at the bottom of the plateau (red line in the right image) is shown in Fig. 11, revealing the transport of N into the Ti melt. A thick overlying layer of TiN was not observed in this location. The dark areas at the top and close to the orange arrows contained TiN dendrites, while the gray material in the majority of the image was Ti(N) transitioning to CP-Ti. The orange arrows indicate the direction of convective flow within the melt pool during the LSP
Fig. 9. a: SEM image of the d = 2.5 mm sample showing the locations of regions 1 (Panel b), 2 (Panel c), and 3 (Panel d). Solid, straight lines on Panel a indicate the locations of the cross-sectional images, Panels b–d. In Panels b–d, the lightest parts of the images are the Bakelite mount, the left side of the images are the surface shown in Panel a. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Four layers with distinctive microstructures were observed. Layer 1: A solid TiN coating with variable thickness up to 300 μm was observed at the outer surface of the plateau (Fig. 9d). Layer 2: TiN dendrites, with nitrogen concentrations > 30% and decreasing in size towards the center of the protrusion, in a Ti(N) matrix, which was removed by the etching process. When Ti is laserirradiated in a nitrogen atmosphere during laser nitriding, such dendrites typically grow perpendicular to the substrate surface [32]. However, in the LSP processed samples, the dendrites were observed to grow in multiple directions, for example those with cross-shapes grew perpendicular to the plane of the image. Layer 3: Martensitic Ti(N) with acicular morphology. Layer 4: Commercially pure Ti base metal with the largest grains in the heat affected zone (HAZ) adjacent to the martensitic layer. Fig. 9a shows the locations (red lines) of three longitudinal cross sections in Regions 1, 2, and 3 (white brackets) on the d = 2.5 mm sample. In the higher magnification views shown in Fig. 9b, c, and d, the white area on the left side of each image is the Bakelite mount. Fig. 9b (Region 1), closest to the laser head, contained a pool of dendrites 1.75 mm in length and > 1 mm thick at the deepest point. The dendrites in this region varied from 200 to 500 μm in length and were 149
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Fig. 11. SEM image of a transverse cross-section of the d = 2 mm sample at the location indicated by the solid red line on the image to the right. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
growth directions on the TiN dendrites that had grown together to form the solid layer. This layer provided a base for epitaxial growth leading to faceted and hollow, rectangular crystal growth, depending on the mobility of the titanium and nitrogen atoms on the surface (Figs. 5–7). As shown in Figs. 5b and 6b, much of the surface area within region C contained cauliflower-shaped deposits typical of growth from the vapor under conditions of low mobility of the depositing species. Nassar et al. [20] demonstrated that when a titanium substrate was interacted with an LSP in flowing nitrogen and an open atmosphere chamber, the excited nitrogen species gettered the surrounding oxygen to form nitric oxide, resulting in lustrous, stoichiometric TiN surfaces. In this research, when the LSP was extinguished, excited nitrogen species were no longer present, allowing particulate TiO2 to form a thin layer over the TiN as the specimen cooled. For example, the purple color at the outer edge of region C can be accounted for by a nanoscale layer of TiO2 and titanium oxynitride covering the TiN. Oxidation was observed: (a) in regions where the nitrogen flow was directed away from the surface protrusion; (b) when the laser power was increased such that Ti vapor emission forced the nitrogen LSP away from the surface; and, (c) as vapor-deposited oxide at the outer perimeters of the LSP interaction zone, where atomic N was not produced. These results indicate the importance of both the shielding gas flow and the proximity of the nitrogen LSP to the surface for the formation of TiN. As the deep melt pool cooled, the TiN dendrites formed within were finer than those that had nucleated and grown in the raised ridge before the plasma was extinguished. As the protrusion rapidly cooled, and the nitrogen solubility limit in molten titanium was exceeded, a nitrogen bubble nucleated and grew, enhancing the observed convective flow. Bubble formation is well known in conventional laser-nitrided trails formed, as confirmed by Kamat et al. [36,37]. Control of the distance, d, and the substrate exposure time to the LSP, is needed to eliminate such porosity. The most interesting aspects of this study were the findings that (a) after only five seconds exposure to the LSP it was possible to develop dense TiN surface layers up to 300 μm in thickness; and (b) the LSPsubstrate interactions could be used to elucidate crystal nucleation and growth from their earliest stages, through development of crystal morphologies and formation of dense layers. Further understanding of these phenomena, through control of LSP exposure time and distance, d, has the potential to lead to a rapid surface LSP nitriding process for titanium and its alloys.
interaction. The bubble at the center of this image most likely formed during cooling as the solubility of atomic nitrogen in the supersaturated Ti(N) solid solution decreased, resulting in the formation of nitrogen gas. 4. Discussion The microstructural observations confirmed that the nitrogen LSP resulted in significant heat and mass transfer to the titanium plate, agreeing with conclusions of Nassar et al. [20], who showed that particle impact and re-radiation in the UV were important mechanisms of heat transfer from nitrogen LSPs. Akarapu et al. [35] developed a steady state axi-symmetric model to predict the size, shape and temperature of LSP in flowing argon. The formation of the deep melt pool located above the focal plane (Fig. 9b) was consistent with the location of the hottest region of the LSP predicted by this model. Nitrogen was absorbed by and titanium was evaporated from the melt pool. As the local substrate volume increased upon melting, convection moved liquid parcels at the surface away from the hottest regions in the center towards the cooler outer edges of the melt pool. Assisted by gravity, the melt in region C flowed down the plate forming a raised ridge-like trail. As it flowed, the melt continued to absorb nitrogen until the solubility limit was reached and TiN dendrites nucleated and grew. This caused blockages to occur, changing the flow direction and accounting for the crooked flow path down the plate. Along the trail, continued nitrogen absorption caused the dendrites to grow together forming a solid layer of TiN, encapsulating pockets of Ti (N), which were etched away during subsequent metallographic preparation (Fig. 10). As the flow progressed below the laser focal plane, where it encountered decreasing surface temperatures, the flow solidified and accumulated forming a protrusion containing Ti(N) at its center and a thick nitride coating on the windward side. The plateau redirected the nitrogen gas flow over the protruding material and also back towards the nozzle. This created stagnation of the gas flow above the plateau, caused the LSP to become non-axisymmetric, shortened the core plasma region, forced the LSP to bend away from the molten surface, and opened up a region below the plateau where ingress of air caused oxidation of the substrate. Titanium atoms in the ground state evaporated from the melt pool, reacted with nitrogen to form TiN particulates, and covered the surface producing the brown halo at the edge of the inverted teardrop. Along the sides of the raised trail, conditions were favorable for titanium and nitrogen atoms to react and nucleate TiN along orthogonal, preferred 150
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5. Summary and Conclusions [11]
1. A novel method for studying the interaction of LSP with a titanium substrate, independent of any contribution from the sustaining laser beam, was developed in this research. 2. LSP processing accessed novel microstructures compared to those produced by a laser beam with or without an associated surfacestruck plasma during laser nitriding. 3. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and the substrate resulting in melting, the nucleation and growth of dendritic and faceted TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. 4. Dense, TiN layers up to 300 μm in thickness, depending on the LSP exposure time and distance, d, were formed on the windward side of the plateau. These microstructures represent some of the most rapid TiN growth rates yet observed for an LSP/laser nitriding method. 5. The location of a deep melt pool observed above the focal plane was consistent with the hottest region of an argon LSP, as predicted by numerical models developed by Akarapu et al. [35]. 6. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti-6Al-4V.
[12] [13]
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[17]
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Acknowledgements
[23]
The authors would like to acknowledge the assistance of Amar Kamat, Dr. Abdalla Nassar, and Dr. Ravindra Kumar Akarapu for discussions and feedback, and also Scott Kralik for assistance with metallography and imaging. The authors gratefully acknowledge research support from the Office of Naval Research, Grant No: N00014-07-10121, and the National Science Foundation (NSF), which awarded a NSF Graduate Research Fellowship to Amber Black.
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
A new method for isolating plasma interactions from those of the laser beam during plasma nitriding
MARK
A.N. Blacka, S.M. Copleyb, J.A. Toddc,⁎ a b c
Los Alamos National Laboratory, Los Alamos, NM 87544, USA1 Applied Research Laboratory, The Pennsylvania State University, P.O. Box 30, N. Atherton Street, State College, PA 16804-0030, USA Department of Engineering Science and Mechanics, 212 Earth and Engineering Sciences Building, The Pennsylvania State University, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser-sustained plasma Titanium Titanium nitride Open atmosphere nitriding Melt pool characteristics Crystal nucleation and growth
A new method for separating the interactions of a laser plasma from those of a laser beam with a titanium substrate was developed. A laser-sustained plasma (LSP) was generated by striking the plasma on a titanium plate in a flowing nitrogen atmosphere at ambient pressure and subsequently removing the strike plate. The resultant nitrogen plasma was sustained indefinitely around the laser focal plane until the nitrogen gas flow and/ or the laser power were switched off. The laser-sustained nitrogen plasma was then used as a “reactor” to study the effects of a nitrogen plasma, independent of the laser beam, when the plasma was brought into close proximity, (2 to 3 mm), parallel to a titanium substrate. There was no interaction of the parallel laser beam with the substrate. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and substrate resulting in melting, nucleation and growth of faceted and dendritic TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. Dense TiN layers up to 300 μm thick formed within 5 s. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed very rapidly on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti6Al-4V.
1. Introduction Industries ranging from aerospace to biomedical utilize titanium and its alloys due to their excellent strength, high strength-to-weight ratio, low density, high melting point, excellent corrosion resistance, high fracture toughness, good heat transfer properties, and biocompatibility [1,2]. The poor tribological characteristics of these metals, however, reduce their applicability under severe wear conditions [3]. To improve these properties, titanium nitride (TiN) is a candidate coating material due to its extreme hardness, excellent corrosion and wear resistance, high thermal conductivity, transport properties, chemical inertness, biocompatibility, and gold appearance [4]. TiN is commonly used for protective coatings on cutting tools and drill bits, diffusion barriers in microelectronics, metal smelting crucibles, optical coatings, surgical instruments, and for decorative features [5]. Laser nitriding is a process in which laser irradiation melts a titanium surface, which is exposed to a nitrogen-containing atmosphere. Since the 1980s, it has been investigated for its potential as a fast, effective TiN synthesis technique, particularly relative to vapor
⁎
deposition techniques which may take up to 24 h [6]. During laser nitriding, the interaction of electrons and gaseous species excited by the laser produces near-surface plasma. The impact of this laser-induced plasma is a subject of much discussion; some consider it to enhance the coating process [7–9], others say it is detrimental to nitridation [10,11], while most ignore its potential effects [12–14]. This research separates the interactions of the plasma from those of a laser beam with a Ti surface by utilizing a laser-sustained plasma (LSP). LSP, originally referred to as a “continuous optical discharge” is plasma generated by a laser beam in a gaseous atmosphere that can be sustained indefinitely away from any potentially interacting surface [15]. While LSP has been successfully used to deposit diamond [16–18], its potential contributions to the formation of a broad range of hard coating compositions have not yet been systematically explored. Laser chemical-vapor deposition is another alternative method for large area deposition of TiN. Chen et al. [19] showed high deposition rates close to 1 μm/s. The method differs significantly from laser and LSP nitriding, as a vacuum is required and TiN is generated by the gaseous species and deposited on the substrate, rather than the titanium
Corresponding author. E-mail addresses: [email protected] (S.M. Copley), [email protected] (J.A. Todd). Research conducted while a graduate student in the Department of Engineering Science and Mechanics, 212 Earth and Engineering Sciences Building, The Pennsylvania State University, University Park, PA 16802, USA. 1
http://dx.doi.org/10.1016/j.matchar.2017.10.011 Received 20 May 2017; Received in revised form 6 October 2017; Accepted 7 October 2017 Available online 13 October 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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surface being the titanium source for the TiN creation. An important distinction needs to be made between the experimental process investigated here and conventional laser nitriding. In laser nitriding, the main heat source responsible for melting is the laser beam. In these experiments, LSP is the sole heat source responsible for melting the titanium substrate. The laser is used only to maintain the LSP and does not have any direct interaction with the material being nitrided. 2. Experimental Procedures Tests were completed on annealed and cold rolled ASTM grade 2, commercially pure (> 99.5%) titanium (CP Ti, McMaster-Carr). The asreceived material had an unpolished, milled finish and was sectioned into 25.4 mm (1 in.) × 50.8 mm (2 in.) × 3.175 mm (1/8 in.) plates. All plates used in the experiments received identical preparation and heat treatments. Prior to LSP processing, the surfaces were swabbed with acetone and isopropyl alcohol. A 5 kW CO2 laser (PRC Laser STS5000) with coaxially flowing nitrogen (N2) gas (99.995% purity) was used to generate the LSP. Energy to maintain the plasma was provided by the continuous laser beam at an output power of approximately 3.5 kW, measured prior to each set of tests. A Precitec laser cutting head with a 10 mm diameter nozzle directed the N2 gas flow at a rate of 10 SLPM. The laser focal plane was located 13 mm below the nozzle. The titanium substrate was positioned parallel to the laser beam and at the same height with respect to the focal plane in every test. All tests were performed in an open atmosphere work cell with linear stages that moved the substrate in the X and Y directions and the laser head in the Z direction, as shown in Fig. 1. The LSP was ignited by running the laser beam across a Ti strike plate at 90 mm/s with the N2 gas flowing and then removing the plate. Ignition took place between five and 30 s prior to interaction with the titanium substrate, after which the LSP was translated to a predetermined distance, d, from the substrate for the duration of the experiment. Fig. 1 illustrates the final position of the LSP with respect to the substrate, where the variable distance, d, represents the distance between the location of the substrate surface, which was determined by aligning a HeNe laser beam parallel to the surface, and the center line of the laser beam. A baseline laser test (without LSP) was then conducted with the high power (3.5 kW) laser beam held at the distance, d, from the substrate surface for 5 s while the N2 gas was flowing. For all distances, it was determined there was no effect of the laser beam on the substrate when the LSP was absent. The LSP was positioned and held stationary at distances, d = 2, 2.5, and 3 mm, respectively, for 5 s before it was extinguished. The temperature of the plasma at the surface varied with the respective positions, being highest for d = 2 mm and lowest for d = 3 mm. Each distance, d, was tested at least three times to determine reproducibility. Throughout the LSP interaction with the substrate, images were collected via a CCD camera (JAI TM-6740CL) equipped with filters to minimize oversaturation, and modified frame grabbing software. Excited atomic and ionic nitrogen species were detected using an 870 nm (10 nm FWHM) bandpass filter in conjunction with a 0.6 neutral density (ND) filter. Excited atomic and ionic titanium species were detected using a 430 nm (10 nm FWHM) bandpass filter and 0.6 plus 0.4 ND filters. Substrate surface examination was performed with a Philips XL30 field emission gun environmental scanning electron microscope (FEG ESEM). An EDAX energy-dispersive X-ray spectrometry (EDS) system was utilized to determine the compositions of selected areas on the surfaces and cross-sections. Transverse and longitudinal cross-sections were polished using standard metallography procedures, submersion etched with Kroll's reagent, and then gold-coated once initial EDS measurements were completed.
Fig. 1. Side view of the experimental setup showing d, the distance between the laser centerline and the substrate surface. The size and shape of the LSP are shown schematically. Note: In all figures, a green arrow indicates the direction of the laser beam and a white, dashed line indicates the location of the laser's focal plane. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results 3.1. Process Images Following the procedure described by Nassar et al. [20], a CCD camera system and associated filters monitored the mass transport processes taking place throughout the LSP-substrate interaction, as shown in Fig. 2 for d = 2 mm and d = 2.5 mm. Experiments recorded with the 430 nm filter failed to detect excited atomic and ionic titanium species at any time in the reaction images, hence they are not shown here. (Note, when present, Nassar et al. [20] showed that excited titanium species appeared as a lighter area close to the sample surface, or as brightening of the LSP.) The images presented in Fig. 2, taken with
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Fig. 2. CCD images observed through an 870 nm filter at comparable times for d = 2 mm (top row) and d = 2.5 mm.
the 870 nm filter, clearly show excited nitrogen species adjacent to the substrate surface from 1.4 s through the conclusion of the 5 second tests. The surface melted and formed a protrusion below the focal plane during heating of the d = 2 mm and d = 2.5 mm samples, but not when d = 3 mm. The protrusions started to grow approximately 1.25 s after the LSP reached its position then grew quickly for approximately 2 s before remaining a constant size and shape for the remainder of the interaction time. The geometry of the raised area was similar for all plates tested at the same d value, but varied in size with d. Fig. 2 compares the growth progression of the raised area for d = 2 mm and d = 2.5 mm at intervals ranging from 0.5 s to 5 s. 3.2. Macroscopic Observations Macroscale images of the LSP treated substrates are shown in Fig. 3(a) to (c) for d = 2, 2.5 and 3 mm, respectively. The laser beam direction is indicated by the green arrow in Fig. 3a and the white dashed lines indicate the position of the laser focal plane. A higher magnification view of a d = 2 mm sample, showing five major regions designated A through E, is presented in Fig. 4. The colors and compositions associated with these regions, according to the literature, together with EDS analyses taken in this research are presented in Table 1. Brown-colored regions (region A and beyond), extending > 10 mm from the center and indicative of hypo-stoichiometric and near-stoichiometric TiN [21], were produced at all three distances. (It should be noted that the EDS analysis of region A in Table 1 suggests that the TiN layer is very thin and that the majority of the signal comes from the Ti substrate.) Okamoto [22] has shown that hypo-stoichiometric TiN may form with as little as 28 at.% nitrogen. According to Kato and Tamari [23], the lustrous, dark red-brown region (region C), seen in d = 2 mm and d = 2.5 mm, indicates hyperstoichiometric titanium nitride (i.e. TiNx where x > 1). Mumtaz and Class [24], Roquiny et al. [25], and Nah et al. [26] agree that the stoichiometry dependent colors of TiN, described for regions A through
Fig. 4. A d = 2 mm sample showing five major zones that are distinguishable by color and luster. The white circle indicates the location of the plateau.
E, are independent of the processing method. In the center of region C, and extending above the laser focal plane, is a lustrous gold region indicative of stoichiometric TiN [27]. This indicates that near and above the laser focal plane (closer to the nozzle), the LSP reached sufficient size and temperature to produce a thick TiN layer on the titanium substrate, as will be confirmed in Section 3.3. All three specimens exhibited large gray regions (regions B, D), a white arc below the red-brown and gold region (region E), blue and orange areas surrounding the entire modified region (outer area of region D), and a purple region around the red-brown and gold area (outer
Fig. 3. Macroscale images for the LSP parallel configuration after 5 s at: a) d = 2 mm; b) d = 2.5 mm; and c) d = 3 mm.
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perimeter of region C). The shape of each treated area is also similar, resembling an inverted tear drop – rounded at the top while narrowing at the bottom of the plate. As d increased from 2 to 3 mm, the sizes of the lustrous gold and red areas in region C decreased accordingly, indicating that the distance, d, was an important parameter controlling the amount of stoichiometric TiN formed during LSP processing. The white circles in Figs. 3a, b, and 4 indicate locations for the plateau, a region raised above the original surface of the titanium plate that corresponds to the protrusions seen in the CCD images of Fig. 2. Fig. 5a shows that a raised ridge formed above the position of the laser focal plane (dashed white line) running down into the plateau. For the distance, d = 3 mm, even though no plateau developed, a raised ridge was still observed. The formation of this ridge is clearly captured in the time sequence for d = 2 mm in Fig. 2, but appears not to extend as far above the focal plane when d = 2.5 mm.
Table 1 Compositions of the regions delineated in Fig. 4 according to the literature and validated by EDS analyses [22,26–29]. Region
A B C D E
Description
Brown “halo” Silver Lustrous red Lustrous gold Dull gray Lustrous white
Literature
TiN [1] – TiN [2] TiN [1] TiO2 [3] TiO2 [4]
EDS results (at.%)
Composition
Ti
N
O
89.98 99.8 47.27 49.58 44.42 44.68
10.08 0.2 52.73 50.42 0 0
0 0 0 0 55.58 55.32
TiN Ti TiN TiN TiO2 TiO2
Fig. 5. a: SEM image of the region delineated in the macroscopic image of the d = 2.5 mm sample. The boxes show the locations of the higher magnification views in Panels b, c, and d. The bracket shows the area considered to be the plateau; b: Morphology exhibited by the vast majority of the surface area; c: The raised trail which segues into a crystalline region; d: A location on the raised trail that displays hollow, rectangularshaped crystals.
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Fig. 6. a: SEM image of the region delineated in the macroscopic image of the d = 2 mm sample. The boxes show the locations for the higher magnification views in Panels b, c, and d. The bracket shows the area considered to be the plateau; b: Rough-textured growth in the white region under the nitrided area; c: Branch of the raised trail with adjacent crystalline region; d: A region of faceted crystals on the plateau.
surface exhibited the cauliflower-shaped morphology shown in Fig. 5b. The raised ridge represented a melt region, which flowed down the plate to form a protrusion at the plateau. Surrounding the melt trail was a region of faceted crystals, some with rounded edges and blunt corners, as seen in Fig. 5c. The average facet length was 7 μm. A transition region, varying in width from 10 to 50 μm, separated the smooth, raised ridge and plateau from the crystalline regions, as observed in Fig. 5c. Evidence of subsequent nucleation and growth on the crystal facets was consistent with the characteristics of vapor growth. The crystals shown in Fig. 5d exhibited a hollow, rectangular morphology, and appeared similar to TiN crystals grown in an RF furnace with a nitrogen atmosphere above 2000 °C [31]. These areas only occurred sporadically near the raised trail and plateau. Figs. 6a–d present SEM images of the d = 2 mm sample for comparison with the d = 2.5 mm (Figs. 5a–d). Fig. 6b shows rough,
According to Wu et al. [30], the narrow, deep purple band around region C, which was more prominent in the d = 2.5 and d = 3 mm samples, represents a transition region where the TiO2 and/or titanium oxynitride in regions D and E form a nanometer thick coating on the TiN at the edge of region C, creating the purple color. 3.3. Surface Morphologies Fig. 5a shows an overall SEM view of the center of the sample shown in Fig. 3b (insert red box), which was processed at d = 2.5 mm. Higher magnification views of three regions of interest are presented in Fig. 5b, c, and d. It should be noted that the area away from the raised ridge and plateau (magnified in Fig. 5b) had a much smoother profile than the areas immediately adjacent to the plateau and ridge, which appear textured due to crystal growth (Fig. 5c and d). A majority of the nitrided 147
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Fig. 7. a–d: SEM images of rod-like growth along the surface of crystal facets on the d = 2 mm (Panels a, b) and d = 2.5 mm (Panels c, d) samples.
their closer distance to the LSP. Magnified views of faceted crystals shown in Fig. 5c (d = 2.5 mm) and Fig. 6c (d = 2 mm) are shown in Fig. 7a–d. Both samples exhibited nucleation and growth on the crystal facets. Fig. 7a and b (d = 2 mm) show early stage nucleation of rod-like morphologies along orthogonal, preferred growth directions. More advanced growth and coalescence of such features, together with facet rounding, is shown in Fig. 7c and d (d = 2.5 mm). While these results first appeared contradictory, (i.e. a colder substrate further from the LSP is expected to have slower growth kinetics), it was determined that the crystal facets in Fig. 7c and d (d = 2.5 mm) were located below the focal plane in an area protruding further into the LSP (i.e. higher temperature) than the facets in the d = 2 mm sample, which were observed in a region 0.5 mm above the laser focal plane, further away from the LSP. The surface features on the crystal facets in both the d = 2 mm and d = 2.5 mm samples appeared iridescent to the eye. When the profiles of the crystal facets were examined, it was determined that the surface features were extremely thin – rising < 10 nm above the surface on the d = 2 mm sample and approximately twice as high on the d = 2.5 mm sample. While it was not possible to obtain accurate compositional information with EDS analyses, backscattered images suggested that these features may have a higher nitrogen content than the underlying crystal, consistent with their nucleation in a flowing nitrogen atmosphere.
3.4. Cross-sectional Microstructures Fig. 8. SEM image of a longitudinal cross-section of the plateau region showing four distinct layers.
Longitudinal cross-sections (parallel to the laser beam direction) through the plateau region revealed layered microstructures similar to those observed in laser nitrided samples [2,32–34]. The most important difference was the thickness of each of the four layers within the plateau, as shown in the SEM image in Fig. 8. In describing the respective layers, TiN denotes titanium nitride, which exists over a wide composition range, and Ti(N) refers to a solid solution of α-Ti containing nitrogen. These phases can be differentiated by their crystal structures, since TiN has a cubic structure, and α-Ti and Ti(N) have hexagonal structures.
textured, cauliflower-shaped growth in the white area (region E) below the plateau. Fig. 6c and d show faceted and rectangular crystal morphologies, respectively. While the faceted crystals and the transition region were approximately the same size on both samples (averaging 7 μm crystal facets and a 10–50 μm wide transition region), the hollow, rectangular crystal morphologies (Fig. 6d) were less angular and more rounded, reflecting the higher temperature resulting from 148
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Fig. 10. Solid TiN layer acting as a platform for crystal growth.
oriented in multiple directions. The SEM image shows a crack through the center of this area, which was introduced during metallography, as no cracks were observed prior to grinding. The melt pool exhibited a solid TiN layer with average thickness of 17.5 μm at the top, and a very thin underlying HAZ. Region 2 (Fig. 9b) was located approximately 1 mm below Region 1 and consisted of a thinner (175 μm deep) layer of dendrites, with a very shallow (1 to 15 μm thick), solid TiN layer at the surface. Again, deep cracks were observed to develop and propagate during metallographic preparation. Region 3 (Fig. 9c) sectioned the plateau and the area approaching the laser focal plane. This entire region consisted of a very thick layer (up to 300 μm) of solid TiN with a relatively thin layer of dendrites. The layers appeared solid prior to etching, which removed regions of Ti(N) giving the porous appearance observed in Fig. 10. Faceted crystals, indicated by the red arrows at the lower left of the image, grew at the top of this solid layer. Thick nitrided layers were only present above and on the plateau, with the region below the protrusion exhibiting fewer dendrites without a thick solid TiN layer at the surface. Nitrogen gas flowed parallel to and down the substrate surface (coaxial to the laser) until it hit the protrusion of the plateau. This allowed a deep solid TiN layer to build up on the windward side of the plateau, leaving the leeward side sheltered from the nitrogen gas. A transverse cross-section at the bottom of the plateau (red line in the right image) is shown in Fig. 11, revealing the transport of N into the Ti melt. A thick overlying layer of TiN was not observed in this location. The dark areas at the top and close to the orange arrows contained TiN dendrites, while the gray material in the majority of the image was Ti(N) transitioning to CP-Ti. The orange arrows indicate the direction of convective flow within the melt pool during the LSP
Fig. 9. a: SEM image of the d = 2.5 mm sample showing the locations of regions 1 (Panel b), 2 (Panel c), and 3 (Panel d). Solid, straight lines on Panel a indicate the locations of the cross-sectional images, Panels b–d. In Panels b–d, the lightest parts of the images are the Bakelite mount, the left side of the images are the surface shown in Panel a. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Four layers with distinctive microstructures were observed. Layer 1: A solid TiN coating with variable thickness up to 300 μm was observed at the outer surface of the plateau (Fig. 9d). Layer 2: TiN dendrites, with nitrogen concentrations > 30% and decreasing in size towards the center of the protrusion, in a Ti(N) matrix, which was removed by the etching process. When Ti is laserirradiated in a nitrogen atmosphere during laser nitriding, such dendrites typically grow perpendicular to the substrate surface [32]. However, in the LSP processed samples, the dendrites were observed to grow in multiple directions, for example those with cross-shapes grew perpendicular to the plane of the image. Layer 3: Martensitic Ti(N) with acicular morphology. Layer 4: Commercially pure Ti base metal with the largest grains in the heat affected zone (HAZ) adjacent to the martensitic layer. Fig. 9a shows the locations (red lines) of three longitudinal cross sections in Regions 1, 2, and 3 (white brackets) on the d = 2.5 mm sample. In the higher magnification views shown in Fig. 9b, c, and d, the white area on the left side of each image is the Bakelite mount. Fig. 9b (Region 1), closest to the laser head, contained a pool of dendrites 1.75 mm in length and > 1 mm thick at the deepest point. The dendrites in this region varied from 200 to 500 μm in length and were 149
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Fig. 11. SEM image of a transverse cross-section of the d = 2 mm sample at the location indicated by the solid red line on the image to the right. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
growth directions on the TiN dendrites that had grown together to form the solid layer. This layer provided a base for epitaxial growth leading to faceted and hollow, rectangular crystal growth, depending on the mobility of the titanium and nitrogen atoms on the surface (Figs. 5–7). As shown in Figs. 5b and 6b, much of the surface area within region C contained cauliflower-shaped deposits typical of growth from the vapor under conditions of low mobility of the depositing species. Nassar et al. [20] demonstrated that when a titanium substrate was interacted with an LSP in flowing nitrogen and an open atmosphere chamber, the excited nitrogen species gettered the surrounding oxygen to form nitric oxide, resulting in lustrous, stoichiometric TiN surfaces. In this research, when the LSP was extinguished, excited nitrogen species were no longer present, allowing particulate TiO2 to form a thin layer over the TiN as the specimen cooled. For example, the purple color at the outer edge of region C can be accounted for by a nanoscale layer of TiO2 and titanium oxynitride covering the TiN. Oxidation was observed: (a) in regions where the nitrogen flow was directed away from the surface protrusion; (b) when the laser power was increased such that Ti vapor emission forced the nitrogen LSP away from the surface; and, (c) as vapor-deposited oxide at the outer perimeters of the LSP interaction zone, where atomic N was not produced. These results indicate the importance of both the shielding gas flow and the proximity of the nitrogen LSP to the surface for the formation of TiN. As the deep melt pool cooled, the TiN dendrites formed within were finer than those that had nucleated and grown in the raised ridge before the plasma was extinguished. As the protrusion rapidly cooled, and the nitrogen solubility limit in molten titanium was exceeded, a nitrogen bubble nucleated and grew, enhancing the observed convective flow. Bubble formation is well known in conventional laser-nitrided trails formed, as confirmed by Kamat et al. [36,37]. Control of the distance, d, and the substrate exposure time to the LSP, is needed to eliminate such porosity. The most interesting aspects of this study were the findings that (a) after only five seconds exposure to the LSP it was possible to develop dense TiN surface layers up to 300 μm in thickness; and (b) the LSPsubstrate interactions could be used to elucidate crystal nucleation and growth from their earliest stages, through development of crystal morphologies and formation of dense layers. Further understanding of these phenomena, through control of LSP exposure time and distance, d, has the potential to lead to a rapid surface LSP nitriding process for titanium and its alloys.
interaction. The bubble at the center of this image most likely formed during cooling as the solubility of atomic nitrogen in the supersaturated Ti(N) solid solution decreased, resulting in the formation of nitrogen gas. 4. Discussion The microstructural observations confirmed that the nitrogen LSP resulted in significant heat and mass transfer to the titanium plate, agreeing with conclusions of Nassar et al. [20], who showed that particle impact and re-radiation in the UV were important mechanisms of heat transfer from nitrogen LSPs. Akarapu et al. [35] developed a steady state axi-symmetric model to predict the size, shape and temperature of LSP in flowing argon. The formation of the deep melt pool located above the focal plane (Fig. 9b) was consistent with the location of the hottest region of the LSP predicted by this model. Nitrogen was absorbed by and titanium was evaporated from the melt pool. As the local substrate volume increased upon melting, convection moved liquid parcels at the surface away from the hottest regions in the center towards the cooler outer edges of the melt pool. Assisted by gravity, the melt in region C flowed down the plate forming a raised ridge-like trail. As it flowed, the melt continued to absorb nitrogen until the solubility limit was reached and TiN dendrites nucleated and grew. This caused blockages to occur, changing the flow direction and accounting for the crooked flow path down the plate. Along the trail, continued nitrogen absorption caused the dendrites to grow together forming a solid layer of TiN, encapsulating pockets of Ti (N), which were etched away during subsequent metallographic preparation (Fig. 10). As the flow progressed below the laser focal plane, where it encountered decreasing surface temperatures, the flow solidified and accumulated forming a protrusion containing Ti(N) at its center and a thick nitride coating on the windward side. The plateau redirected the nitrogen gas flow over the protruding material and also back towards the nozzle. This created stagnation of the gas flow above the plateau, caused the LSP to become non-axisymmetric, shortened the core plasma region, forced the LSP to bend away from the molten surface, and opened up a region below the plateau where ingress of air caused oxidation of the substrate. Titanium atoms in the ground state evaporated from the melt pool, reacted with nitrogen to form TiN particulates, and covered the surface producing the brown halo at the edge of the inverted teardrop. Along the sides of the raised trail, conditions were favorable for titanium and nitrogen atoms to react and nucleate TiN along orthogonal, preferred 150
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5. Summary and Conclusions [11]
1. A novel method for studying the interaction of LSP with a titanium substrate, independent of any contribution from the sustaining laser beam, was developed in this research. 2. LSP processing accessed novel microstructures compared to those produced by a laser beam with or without an associated surfacestruck plasma during laser nitriding. 3. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and the substrate resulting in melting, the nucleation and growth of dendritic and faceted TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. 4. Dense, TiN layers up to 300 μm in thickness, depending on the LSP exposure time and distance, d, were formed on the windward side of the plateau. These microstructures represent some of the most rapid TiN growth rates yet observed for an LSP/laser nitriding method. 5. The location of a deep melt pool observed above the focal plane was consistent with the hottest region of an argon LSP, as predicted by numerical models developed by Akarapu et al. [35]. 6. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti-6Al-4V.
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Acknowledgements
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The authors would like to acknowledge the assistance of Amar Kamat, Dr. Abdalla Nassar, and Dr. Ravindra Kumar Akarapu for discussions and feedback, and also Scott Kralik for assistance with metallography and imaging. The authors gratefully acknowledge research support from the Office of Naval Research, Grant No: N00014-07-10121, and the National Science Foundation (NSF), which awarded a NSF Graduate Research Fellowship to Amber Black.
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