- Email: [email protected]
Ti composite coating on titanium
Author's Accepted Manuscript
Laser processing of in situ TiN/Ti composite coating on titanium Himanshu Sahasrabudhe, Julie Soderlind, Amit Bandyopadhyay
www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(15)00284-2 http://dx.doi.org/10.1016/j.jmbbm.2015.08.013 JMBBM1573
To appear in: Journal of the Mechanical Behavior of Biomedical Materials
Received date:8 May 2015 Revised date: 29 July 2015 Accepted date: 7 August 2015 Cite this article as: Himanshu Sahasrabudhe, Julie Soderlind, Amit Bandyopadhyay, Laser processing of in situ TiN/Ti composite coating on titanium, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi. org/10.1016/j.jmbbm.2015.08.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Laser Processing of in situ TiN/Ti Composite Coating on Titanium Himanshu Sahasrabudhe, Julie Soderlind and Amit Bandyopadhyay W. M. Keck Biomedical Materials Research Laboratory School of Mechanical and Materials Engineering Washington State University Pullman, Washington, 99164-2920, USA. E-mail: [email protected] Abstract Laser remelting of commercially pure titanium (CP-Ti) surface was done in a nitrogen rich inert atmosphere to form in situ TiN/Ti composite coating. Laser surface remelting was performed at two different laser powers of 425W and 475W. At each power, samples were fabricated with one or two laser scans. The resultant material was a nitride rich in situ coating that was created on the surface. The cross sections revealed a graded microstructure. There was presence of nitride rich dendrites dispersed in α-Ti matrix at the uppermost region. The structure gradually changed with lesser dendrites and more heat affected α-Ti phase maintaining a smooth interface. With increasing laser power, the dendrites appeared to be larger in size. Samples with two laser scans showed discontinuous dendrites and more α-Ti phase as compared to the samples with one laser scan. The resultant composite of TiN along with Ti2N in α-Ti showed substantially higher hardness and wear resistance than the untreated CP-Ti substrate. Coefficient of friction was also found to reduce due to surface nitridation. Leaching of Ti4+ ions during wear test in DI water medium was found to reduce due to laser surface nitriding.
Keywords – Laser processing, Ti nitride, wear resistance, hard coatings, surface modification 1. Introduction One of the most widely used material in specialized engineering and biomedical application is titanium. Titanium and its alloys like the Ti6Al4V alloy have immense applications in various branches of engineering due to their properties such as excellent corrosion resistance, high strength to weight ratio, biocompatibility and low coefficient of 1
friction (COF) (Welsch et al., 1993, Liu et al., 2004, Niinomi, 1998, Long and Rack, 1998). In the biomedical implant industry, Ti and its alloys are mainly used for load bearing applications. These are implants such as artificial hip replacement or artificial knee replacement consisting of articulating surfaces. Such articulating surfaces are subjected to heavy point loads during the implant life. Motion between these articulating surfaces under load causes wear and release of metal ions from the surfaces. Loss of material due to wear as well as the leaching of metal ion can cause problems such as aseptic loosening, osteolysis and metallosis (Unwin et al., 1996, Wooley and Schwarz, 2004, Korovessis et al., 2006). The entire wear and degradation regime is further accelerated due to the corrosive nature of the biological media in the human body. Due to poor wear resistance of Ti and its alloys, typically CoCrMo alloy is used in articulating surfaces. Unfortunately, concerns related to Co and Cr ion release in vivo is a bigger problem today due to their toxic nature. Hence, there is a need for surface modification to improve the wear resistance of Ti based implant materials (Kurella and Dahotre, 2005, Bandyopadhyay et al., 2011). In the past, various approaches have been researched upon to increase surface hardness and wear resistance of Ti. These include coatings of different metallic and ceramic materials such as nitrides, carbides, carbon, graphite and calcium phosphates based materials (Man et al., 2006, Selvan et al., 1999, Li and Wang, 2011, Samuel et al., 2008, Roy et al., 2008, Grögler et al., 1998) as well as simple remelting of the material by using focused laser radiation (Singh et al., 2006, Balla et al., 2014.). The essential idea behind all of those approaches is to improve the surface performance of the material without significantly altering the bulk chemistry and properties of the base material, keeping in mind that the modification approach should be simple as well as economically viable. Ceramics are a good choice for articulating surfaces of implant materials. They are biocompatible, have low wear rates and low coefficient of friction (COF). However, they are brittle and cannot be utilized in bulk forms. An ideal way to use ceramic based materials is to use them as coatings, either as complete ceramics or as composites with metallic materials, on metallic surfaces like titanium. Amongst the many available ceramic materials, titanium nitride is very popular for bio medical implant applications (Schaff, 2002, Cool and Jacquot, 1988, Zhecheva et al., 2005). Titanium nitride coatings have been processed by different techniques. The most popular techniques are the thin film methods of CVD and PVD (Narayan et al., 1992, Schintlmeister et al., 1976, Tobe et al., 2000), plasma and ion beam techniques (Yoshida et al., 2
1979, Raveh et al., 1990, Han et al., 1995) and gas nitriding (Man et al., 2002, Man et al., 2005, Abboud, 2013) as well as direct laser processing (Balla et al., 2012). CVD and PVD techniques can produce good quality coatings but coating on complex shapes and surfaces as well as selective area coating is not easily possible. The drawbacks of plasma processing are the same along with large heating of the substrate. Additive manufacturing techniques such as laser engineered net shaping (LENS™) can offer selective area coatings, strong interfaces and ability to form compositionally graded structures. In this method, addition of separate coating material onto a substrate is possible as well. The ideal scenario would be to develop a coating in the existing part without damaging the part shape and inherent properties of the bulk material. Laser gas nitriding can achieve this but to a limited extent. Maintaining low oxide levels, selective area modification, strong interface formation and homogeneous nitride formation are some of the main challenges associated with laser gas nitridation. Hence, a combination of the processing simplicity of laser gas nitridation and the features of additive manufacturing such as 3D motion control, environment control and reproducibility are utilized in this work to realize in situ titanium nitride coating on CP-Ti. We have used a commercial LENS™ system to improve the surface performance of titanium by laser remelting it in a nitrogen rich inert environment. Samples were characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques for microstructural and phase analysis. Tribological performance was measured by wear testing in DI water and 10%FBS media along with micro hardness testing. Release of Ti ions in DI water medium during wear was measured using an ICP-MS technique.
2. Experimental Procedure 2.1 In situ fabrication of nitride coating on titanium: A commercially pure titanium plate (3mm thick and 99.99% pure, President Titanium, Hanson, MA USA) was used as the substrate material. Samples were fabricated using LENSTM 750 (Optomec Inc. Albuquerque, NM USA) equipped with a 500 W continuous wave Nd: YAG laser. LENSTM operation is generally performed in a glove box containing argon atmosphere and very low level of oxygen (<10ppm). In the laser surface modification experiments, argon was replaced with nitrogen by purging the 3
chamber with nitrogen gas (99.996% pure) for 25 min at an inlet pressure of 1200 psi. The resultant environment in the glove box contained approximately 75% nitrogen and remainder argon. Oxygen was maintained below 10ppm and was continuously monitored using an oxygen sensor. Laser surface nitriding was carried out by raster scanning the CP-Ti metallic substrate in the nitrogen rich environment. Raster scanning was done at a speed of 56 cm/min. Raster scanning was executed from a CAD design to fabricate square shaped samples with side-length of 14 mm. Samples were made with one and two passes (raster scans on the surface) at both 425W and 475W laser power. While fabricating samples with two raster scans, the second scan was done at 90° angle to the first one to promote homogeneity in remelting. Samples treated once at 425W is labeled as 425W 1P whereas sample treated twice at 425W is labeled as 425W 2P. Similarly, for 475W 1P is the sample treated once at 475W and 475W 2P is the sample treated twice at 475W. 2.2 Characterization of in situ nitride coatings: Each sample was transversely cut to allow examination of the cross sectional microstructure, and measure the hardness profile. Cross sections of each sample were ground with silicon carbide grinding papers of succeeding grits from 220 to 1000, and polished with alumina abrasive suspension of 1, 0.30 and 0.05 microns. The top surfaces of the sample was ground and polished following the procedure to a make a mirror finish surface. This was done slowly to prevent removal of the laser affected area while still achieving the desired surface finish. Samples were then washed in a DI water-jet and cleaned in an ultrasonic bath containing 100% ethanol for 30 min at room temperature, and finally blow dried. To observe the microscopic feature in the cross sections, the samples were etched using Kroll’s reagent with specific composition of 92 ml DI water, 6 ml Nitric acid, 2 ml Hydrofluoric acid for 30 seconds etching time. The samples were characterized using a Field Emission-SEM (FEI Quanta 200, FEI Inc., OR, USA) at an operating voltage of 20kV. X-ray diffraction analysis was performed using a Siemens D-500 Kristalloflex Diffractometer (Siemens AG, Karlsruhe, Germany) with Cu Kα radiation and Ni filter at 20 kV between the 2θ range between 10 and 100 degrees, with a step size 0.02 degree and a dwell time of 0.5s per step. 2.3 Hardness and wear testing: Vickers microhardness (Shimadzu, HMV-2T) tests were performed on the samples with a standard diamond Vickers indenter. The load was 100 g (0.98 4
N) and dwell time was 15s for all samples. Variation of hardness with the depth of the coating was recorded by testing the cross sections of samples, maintaining equal spacing in between the consecutive hardness measurements. Each sample was indented with at least six depth profiles consisting of six indents each. Mechanical tests were done on the samples including linear reciprocating wear test using Nanovea tribometer (Nanovea Series, Nanovea, Irvine CA, USA). For pin on disk type linear reciprocating wear test, ASTM guideline from standard G133 was followed (ASTM International, 2010). The medium used was de-ionized (DI) water and 10% (by vol.) fetal bovine serum (FBS) solution in DI water (ATCC, VA). Wear testing in DI water medium was done at room temperature; however tests in FBS solution were done at body temperature of 370C. Wear tests were performed with a 3 mm diameter silicon nitride ball at 1200 mm/min speed using a linear reciprocating motion (stroke length) of 10 mm. Wear tests were done in DI water under 5N load for 1000 m distance and in 10% FBS solution under 7N load for 2000 m distance. In between tests, the tribometer was rinsed nine times with heated DI water to prevent cross contamination from previous wear tests. Our idea was to first understand wear degradation of these surfaces in DI, and then do longer runs in simulated body environment for data that are more representative to real world applications. After wear testing was complete, the tracks of each sample were transversely cut to expose the cross section underneath the wear track. These were polished to the same standard as before, and the microstructure of the area below the wear track was imaged for analysis. The release of Ti4+ ions during wear in the DI water medium was measured using Inductively Coupled Plasma-Mass Spectroscopy (Agilent model 7700 ICP-MS). Three standard solutions of 10ppb, 500ppb and 1000ppb concentration were used for calibration of the instrument. At least three samples were analyzed from each wear runs.
3.0 Results In situ nitridation was done on CP-Ti substrate using a Laser Engineered Net Shaping (LENS™) system under nitrogen environment, raster scanning a high powdered continuous wave Nd:YAG laser on the CP-Ti substrate. Effects of nitridation on phase transformations, microstructure, surface hardness and wear resistance were measured. 5
3.1 Surface morphology and microstructure: Before the laser treatment, the CP-Ti plate had a microstructure of equiaxed α-Ti grains. Etched microstructures of surface nitrided CP-Ti under SEM showed a graded microstructure. There was no sharp interface observed and the microstructure showed gradual change in morphology from dendritic-composite structure at the surface to equiaxed grains of the CP-Ti substrate inside. Figure 1 shows the SEM images of the etched cross section of the CP-Ti substrate nitrided at 425W with 2 laser scans. The cross section can be divided into three distinct zones – zone 1, zone 2 and zone 3. Zone 1 was the uppermost region of the sample and had a depth of approximately 200 µm with a variation of 50 µm between different samples. This zone consisted of mostly dendrites that formed after laser remelting and solidification. These dendrites seemed to be dispersed in a secondary phase. Zone 2 was the layer below the Zone 1. With increasing depth, the dendritic phase appeared to reduce in proportion while more secondary phase was observed. This zone was mostly reduced dendrites with extensive and continuous secondary phase. The secondary phase appeared to be acicular or needle like. Figure 2 shows this mixed phase microstructure. Finally in Zone 3, acicular needles from the Zone 2 became more ordered and were seen to grow in the direction of heat flow. This region was mostly comprised of the needle like structures. After the needle like structure ended, there was a region of around 200 µm of finer microstructure which had been affected by the heat of the melt pool above. This was the heat affected zone (HAZ). At a depth of 600 µm and beyond, the original untreated microstructure of CP-Ti was seen. The structure of the remelted and solidified region of the substrate (Zone 1) was dendritic and dispersed in a secondary phase. The laser power used to remelt the samples as well as the number of laser scans had significant effect on the evolution of microstructure in this region. In the samples that were scanned only once, i.e., samples 425W 1P or 475W 1P, the dendritic phase appeared more continuous. The dendrites were extensive and not all were able to be individually identified. In the case of samples that were scanned twice (samples 425W 2P or 475W 2P), dendrites were smaller, discontinuous and the secondary phase was more dispersed in between the dendrites. These features are shown in Figure 3. 3.2 Phase analysis: X-ray diffraction analysis was performed on the surface of the samples showing the formation of different nitrides of titanium upon laser surface melting in a nitrogen rich environment. Figure 4 shows the formation of TiN and Ti2N as well as peaks of the 6
α-Ti phase from the unreacted substrate. The XRD signal was stronger for the samples with two surface scans for both the 425W and 475W power levels. Samples scanned once at 425W (425W 1Pass) showed similar peak intensity as compared to the sample scanned once at 475W (475W 1Pass). The samples scanned twice at 425W and 475W were also similar in terms of peak intensity. 3.3 Microhardness testing: Surface hardness was improved with the laser surface treatment. Table 1 shows the hardness comparisons of all four laser treated samples, and the untreated CP-Ti substrate. The CP-Ti substrate had a hardness of 85±5 HV0.1.The sample remelted with the 425W laser with one laser pass showed a higher hardness (1266±344.3 HV0.1) than the sample melted under two laser passes at the same laser power (1134±331.7 HV0.1). The hardness of 425W 1P was also higher than the samples melted under higher laser power of 475W for both 1P and 2P. The sample treated at 425W with one laser pass showed almost 15 times increase in hardness while the sample melted under two laser passes showed over 12 times increase in hardness compared to untreated CP-Ti. The hardness of the CP-Ti substrate near the HAZ was also increased to ~ 200 HV0.1. Variations of the hardness of all the samples with depth were recorded and shown in Figure 5. The sample remelted once at 425W had the highest hardness at all depths when compared to the other three treated samples. The decrease in hardness for all the four samples from high values of over ~900 HV0.1 to the ~200 HV0.1 of the HAZ and further into the substrate was gradual. There was no sudden drop in the hardness when moving from nitride rich regions to the untreated regions. 3.4 Wear testing in DI water and 10% FBS solution: After wear testing in both DI water and FBS solution, wear damage on the polished surface was found to be less in the laser nitrided surfaces than the untreated CP-Ti surface. In DI water, the wear rate of the CP-Ti substrate was 11.63±0.581 x 10-4mm3/Nm. For the in situ nitrided samples, the lowest wear was for the sample treated at 425W with two passes. It was 0.701±0.035 x 10-4mm3/Nm. This corresponds to a decrease in the wear rate by more than 16 times. The highest wear amongst the treated samples was with the sample treated once at 425W and was 1.859±0.093 x 10-4mm3/Nm. This was 6 times less than the wear on the untreated CP-Ti substrate. In 10% FBS solutions, both the treated and untreated CP-Ti samples underwent even less wear. The wear rate of the CP-Ti substrate in FBS was 6.424±0.480 x 10-4mm3/Nm. The lowest 7
wear amongst the in situ nitrided samples was for the sample that was treated twice at 475W and was 0.020±0.001 x 10-4mm3/Nm. Such a reduction in the wear rate corresponds to more than 300 times due to laser surface treatment. The highest wear was for the sample treated twice at 425W and was 0.262±0.013 x 10-4mm3/Nm, still 24 times less compared to the untreated CP-Ti substrate. Table 1, and Figure 6a and 6b show the wear rates of the different samples in DI water and in 10% FBS medium. During wear testing the coefficient of friction (COF) was recorded by the tribometer. Figure 7a and 7b show the coefficient of friction of the CP-Ti substrate and the in situ nitrided samples. In DI water medium for a distance of 1km, the maximum COF for the CP-Ti substrate was ~1.2. This was reduced to the range of 0.70-0.90 for the treated samples when tested under the same conditions. Similarly in 10% FBS solution CP-Ti reached a maximum COF of ~1.10. For the treated samples the COF was in the range of ~0.20-0.60. Closer looks at the trend of the COF in DI water revealed that the CP-Ti substrate reached a steady state in wear at a distance of ~300m. For the treated samples when tested in DI water, the steady state came at a distance of ~100m whereas in 10% FBS solution such a steady state was not established within the testing zone. SEM imaging was carried out on the wear track of all the samples. Figure 8a shows the typical surface damage of 425W 1P sample. Similar surface damage was observed in all other samples in the central regions of the wear track. Towards the edges of the wear track, cracking and accumulation of particles was observed and shown in Figure 8b. Figure 9a shows the cross section of the wear track of one of the nitrided CP-Ti substrates. Figure 9b shows a magnified corner of the same wear track after etching. It can be seen from these images that the wear took place primarily in the Zone 2 of the nitrided samples. More importantly, observation of the cross section of the wear track confirmed that the nitrided layers were thick enough to accommodate the surface damage during wear and the damage was not carried down into the Zone 3 (HAZ) or into the unreacted CP-Ti substrate. The concentration of Ti4+ ions released in DI water after 1km of wear distance was measured using ICP-MS techniques and reported in Table 2. The concentration of Ti4+ from the wear media of the untreated CP-Ti substrate was 15.595±0.097 ppm. Laser surface nitriding at 425W and with one laser pass reduced the ion release to 3.549±0.013 ppm. The ion release from 8
the sample treated at the same laser power but twice was even less with a measured value of 1.988±0.029 ppm. The sample that was treated at 475W with one laser pass was similar to the sample treated at 425W with one laser pass with measured ion release value of 3.517±0.031 ppm. The lowest Ti4+ ion release was measured in the sample that was treated with two laser passes at 475W with measured value of 0.752±0.006 ppm. Thus laser surface remelting in a nitrogen environment reduced the Ti4+ ion leaching during wear by at least 4 times.
4.0 Discussions Titanium, although a very popular engineering material, is not a first choice material in applications that require high hardness or wear resistance. To counter this, coating of titanium nitride is often used. Laser surface nitriding is a simple method used to form in situ nitrides. This process suffers from disadvantages like oxidation, difficulty in coating large area, curved surfaces and complex geometries. Combining the features of laser surface nitriding along with the features of additive manufacturing, these problems may be solved. In additive manufacturing setups, controlled environments may help prevent oxidation and 3D motion control may be utilized to coat complex shapes and geometries. Nitride coatings may be formed in situ on areas of high wear. Selective area modification can be done to enhance the hardness and wear resistance of Ti based materials without compromising with the bulk properties or the structural integrity of the base material. As an example, wear prone articulating surfaces of Ti based load bearing implants can be strengthened with titanium nitride coating. This concept was verified in the current research. Laser surface melting was carried out of CP-Ti substrate in a nitrogen rich environment. Laser source was utilized for in situ creation of a composite coating of titanium nitride in the existing α-Ti matrix. Laser engineered net shaping (LENS™) used here, has been previously used to process different compositions of TiN in Ti6Al4V alloy composites for improved tribological performance (Balla et al., 2012). Similar to the in situ nitriding reported in the current study, in situ oxidation of LENS™ deposited zirconium was also carried out to form zirconia ceramic for the improvement of surface performance of CP-Ti (Balla et al., 2009) as well as LENS™ fabrication of TiO2 ceramics has also been accomplished (Balla et al., 2009).
9
The use of a solid state laser in a controlled environment allowed uniform properties across a broader surface area. Presence of a predominantly nitrogen with some argon in a very low oxygen environment was crucial. Titanium will readily form oxides as was pointed out in some earlier attempts (Man et al., 2006, Chen et al., 2007, Zhecheva et al., 2006), therefore maintaining the level below 10ppm worked in favor of forming mainly nitrides without any oxidation. The microstructure that evolved after laser surface melting was dendritic-composite in nature. Along with the dendritic phase, a secondary phase coexisted. Our hypothesis is that, the dendritic phase is predominantly TiN and Ti2N whereas the secondary phase is α (hcp) Ti. Similar dendritic structure and phases have been reported as nitrides of titanium and α-Ti phase elsewhere (Man et al., 2005, Man et al., 2006, Chen et al., 2007, Cui et al., 2005). All four samples showed similar dendritic structure but with some significant differences. There were some areas in the dendritic microstructure where the area between the dendrites was lost. This may have happened due to selective etching of the α-Ti phase by Kroll’s reagent, leaving behind the titanium nitride dendrites. The samples that were melted just once at 425W and 475W showed a dense network of dendrites with very little secondary phase in between them. These dendrites were nearly continuous. On the contrary, both the samples that were subjected to two laser scans, showed “broken” dendrites and relatively larger amount of secondary phase. This happened due to the fact that the first laser scan simply remelted and solidified the samples, and during solidification the dendritic phase evolved. This new phase contained nitrides of titanium having higher melting temperature than the earlier untreated α-Ti. However, during the second laser scan, the initially formed dendrites were only partially remelted due to the higher melting temperature of titanium nitrides and thus only broke down into smaller fragments. The acicular phase and the HAZ in all the samples were similar, irrespective of the power or the number of laser scans. This gives us an estimate of the effective depth of the laser. The nitride rich layers, the needle like phase and the heat affected zone appeared to blend gradually with the lower unaffected substrate. The laser surface nitriding thus essentially formed an in situ coating of TiN in Ti matrix. X-ray diffraction data shows the formation of different nitrides of titanium. TiN and Ti2N phases were abundant along with the residual α-Ti from the composite coating layer as well as from the CP-Ti substrate. In the titanium nitride coatings processed by other techniques, similar phases were found to have evolved (Han et al., 1995, Jiang et al., 2000, Man et al., 2006, Nolan 10
et al., 2006, Subramanian et al., 2010, Man et al., 2011). As pointed out by Man et al., (2006), the amount of nitride formed due to a gas phase reaction of the molten titanium with nitrogen would be dependent on the amount of time the melt pool remains in the liquid state. The longer the melt pool remains liquid, the more the formation of nitrides till the solubility limit of nitrogen in titanium is reached. But this long time would subsequently cause delayed solidification and larger grains. The LENS™ system is popular for exceptionally high cooling rate of 103 to 105 K/s and thus the reaction time is short. Yet, there was clear formation of nitrides and this may be due to the fact that in the LENS™ setup, smaller melt pool has a very high surface area over volume ratio. For both the power settings, 425W and 475W, the samples that had undergone melting once showed stronger intensity than the samples that had undergone melting twice. The disturbance of the initially acquired orientation of the phases may explain this. When the surface of the CP-Ti substrate is melted, the melt pool is highly reactive. At this point it would react with the nitrogen in the atmosphere and form nitrides of titanium. The melt pool would then solidify with the nitrides forming dendrites and the unreacted titanium forming the acicular phase of α-Ti. During the second laser scan however, the laser is melting not the initial CP-Ti substrate but a newly formed ceramic layer of titanium nitrides and some residual α-Ti. During this second remelting, the laser would only partially melt the nitrides and break down the dendrites. It is also possible that during this second laser scan, the initially acquired orientation of the phases is damaged, thereby giving a weaker peak intensity for the samples with two surface scans. The absence of a sharp and distinct interface separates the current nitride composite coatings from nitride coatings processed from other techniques. Plasma nitrided Ti coatings reported by Ali et al., (2010) showed increased hardness and improved wear behavior. However nitrided samples showed a very distinct interface with the substrate. TiN coatings reported by Jiang et al., (2000) and by Yue et al., (2000) showed similar bonding to the substrate as reported here. The increase in the hardness after in situ nitriding was at least 10 times as compared to the untreated CP-Ti substrate. This was clearly due to the formation of TiN and Ti2N phases in the samples. The hardness was more than the hardness of simply laser remelted Ti6Al4V at 200W and comparable to the hardness of the laser remelted Ti6Al4V at 400W reported by Balla 11
et al., (2014). Similar hardness values were also reported by Balla et al., (2012) of LENS™ based processing of Ti6Al4V-40%TiN composites. The hardness values measured in the current TiN/Ti composite in situ coating on Ti were also comparable, if not higher, to the titanium nitride produced by laser gas nitriding (Jiang et al., 2000, Man et al., 2005, Man et al., 2006, Zhecheva et al., 2006, Chen et al., 2007, Abboud, 2013,) and to those processed by PVD, CVD, plasma nitriding and ion techniques (Man et al., 1995, Wilson et al., 1999, Nolan et al., 2006, Cassar et al., 2011). However, the nitrides reported by Cui et al., (2005) had hardness of up to 2000HV. The hardness depth profile recorded in the current research showed gradual drop in the hardness as the depth varied from the nitride rich coating to the CP-Ti substrate. This was also in contrast to the values reported by Man et al., (2006) where the hardness number took an abrupt drop from ~700HV to ~350HV in the laser modified layer. Similar drop in hardness was also reported by Cui et al., (2005) from ~1100HV to ~200HV. Gradual drop in the hardness depth profile reported in the current work was similar to that reported by Zhecheva et al., (2006) but with the reported hardness values lesser than the ones reported here. Yilbas et al., (1996) also reported similar trend in the hardness depth profile of plasma nitrided Ti6Al4V alloy with comparable hardness values. The increase in hardness of the nitrided samples was an initial sign for the improvement in the wear resistance of the samples. Tribological characterization in the present study included wear testing in each of DI water medium and 10%FBS solution for a distance of 1km (50,000 cycles) and 2km (100,000 cycles), respectively. The wear rates in mm3/Nm units in DI water medium were higher than the wear rates reported by Chen et al., (2007). However the wear rates in 10% FBS solution were lower in the current TiN/Ti composite coatings. The wear rates in DI water and 10%FBS were both lower than the LENS™ processed Ti6Al4V alloy reinforced with 10%, 15% and 20% TiN and higher than the wear rates of Ti6Al4V reinforced with 40%TiN (Balla et al., 2012). The current wear rates were also comparable to the laser assisted ZrO2 coatings processed by LENS™ (Balla et al., 2009). The wear rates in the present study were also higher than those reported by Wilson et al., (1999) for TiN coatings produced by different techniques. However, those wear tests were done for fewer number of cycles (500 cycles) whereas in the current study, the samples were tested for larger number of linear reciprocating cycles. Comparison of wear damage was difficult with other reported nitride coatings due to the difference in wear testing methods as well as the different units used to report wear damage (Han 12
et al., 1995, Man et al., 2006, Nolan et al., 2006, Chen et al., 2007, Subramanian et al., 2010, Subramanian et al., 2011). The coefficient of friction was reduced due to the in situ nitriding of the substrate from an initial value of ~1.2 to 0.70-0.90 in DI water medium and even less in 10%FBS solution. These values were however higher than those reported earlier for nitrides processed by other techniques (Subramanian et al., 2010, Sathish et al., 2010, Subramanian et al., 2011). The SEM images of the wear tracks revealed the difference in the damage of the nitrided samples at the central regions of the wear track and at the edges. The central regions appeared to be plastically deformed with some flaking and peeling of material. The plastically deformed regions may be the α-Ti phase whereas the brittle nitrides may have been flaked and eroded. Wilson et al., (1999) and Nolan et al., (2006) also reported similar damage in plasma nitrided titanium coatings. The edges appeared to be the region where the wear debris was accumulated and also showed some distinct cracking damage. There was also bright charging under the electron microscope in this region specifically and this may have happened due to the accumulation of the non-conducting nitrides. This overall wear damage bears resemblance to the fretting wear observed in plasma nitrided titanium (Ali et al., 2010). During wear, the soft α-Ti phase deforms faster due to hard counter body material i.e., silicon nitride, and there is a constant compressive load. The harder nitride phase however, resists wear to a large extent. Due to the continuous damage to the α-Ti matrix around it, the nitride phase finally deforms and was swept to the sides of the wear track. The region from where the nitride phase broke off was now exposed and the softer matrix phase was again subjected to deformation and subsequently to wear. A similar type of wear mechanism was described by Liang et al., (1995) in SiC reinforced Al alloys. The analysis of the wear media in DI water wear tests after a distance of 1km showed that the Ti4+ ions leached during wear reduced after surface remelting of CP-Ti in a nitrogen rich environment. A maximum drop of more than 20 times was observed in the sample that was remelted twice at 475W with two laser scans on the surface. There was a minimum drop of 4 times with respect to the untreated CP-Ti substrate. This proved that laser surface nitriding of CP-Ti was not only successful in improving the hardness and reducing surface wear of the samples but also successful in reducing the metal ion leaching during wear. This drop in the 13
metal ion leaching is a good indication of the usefulness of the surface nitrided CP-Ti from the biological implants point of view. Such selective surface nitriding treatment can be applied to the regions of high wear on the articulating implant surfaces. This may be useful in improving the life of the implant by reducing the wear damage and may also alleviate medical complications such as metallosis arising out of metal ion release in the human body. The ions released in 10%FBS solution are expected to be even lower due to lower wear rates as well as the coefficient of friction due to the lubrication effect of the proteins in the FBS media. This also needs to be investigated by doing careful wear studies in 10%FBS solution along with ICP-MS analysis of the wear media after testing.
5.0 Conclusions In this work, an in situ processing of titanium nitride using laser gas nitriding with features of additive manufacturing was explored. Laser remelting of commercially pure titanium was done in a nitrogen rich environment using a finely focused and high powered continuous wave Nd:YAG laser using a laser engineered net shaping (LENS™) system. Laser remelting was carried out at two power levels of 425W and 475W, and with one and two laser surface scans each. XRD analysis confirmed the formation of TiN along with Ti2N and unreacted α-Ti at the surface. The nitride phases were present as dendrites in a matrix of unreacted α-Ti phase. Since there was no material deposited or coated, the laser surface remelting created an in situ TiN in αTi composite coating on CP-Ti. The cross-section microstructure showed three distinct zones. The uppermost zone had extensive nitride phase. The intermediate zone showed lesser dendrites and more α-Ti phase, and finally there was a heat affected zone along with the interface with the unaffected CP-Ti substrate. There was a smooth transition from the three zones of the composite coating microstructure to the CP-Ti substrate. Effect of laser power and number of surface scans was prominent on the microstructural evolution. The samples with one laser scan showed large and continuous dendrites whereas the samples with two laser scans showed dendrites that appeared to be discontinuous. Due to the formation of extensive titanium nitride ceramic phase, all samples showed multiple times increase in hardness and better resistance to surface wear in both DI water and 10% FBS solution. Analysis of the wear track showed damage that was typical to the fretting of a hard phase embedded in soft ductile matrix. The leaching of Ti4+ ions 14
as well as the coefficient of friction also reduced during wear. Laser surface nitriding was thus successful in creating an in situ coating to improve the surface properties of CP-Ti without compromising the inherent bulk properties of the material.
Acknowledgements Authors acknowledge financial support from the Joint Center for Aerospace Technological Innovation (JCATI), WA. Authors also acknowledge the financial support from W. M. Keck Foundation and M. J. Murdock Charitable Trust to establish the Biomedical Materials Research Laboratory at WSU. Authors would also like to thank Dr. Scott Boroughs for help with the ICP-MS analysis.
References Abboud, J. H. (2013). Effect of processing parameters on titanium nitrided surface layers produced by laser gas nitriding. Surface and Coatings Technology, 214, 19-29. Ali, M. M., Raman, Set al., (2010). Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V. Tribology International, 43(1), 152-160. ASTM G133-05(2010), Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear, ASTM International, West Conshohocken, PA, 2010,www.astm.org Balla, V. K., et al., (2009). Laser-assisted Zr/ZrO2 coating on Ti for load-bearing implants. Acta biomaterialia, 5(7), 2800-2809. Balla, V. K., et al., (2009). Fabrication of compositionally and structurally graded Ti–TiO2 structures using laser engineered net shaping (LENS). Acta biomaterialia, 5(5), 1831-1837. Balla, V. K., et al., (2012). Laser processed TiN reinforced Ti6Al4V composite coatings. Journal of the mechanical behavior of biomedical materials, 6, 9-20. Balla, V. K., et al., (2014). Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy. Journal of the mechanical behavior of biomedical materials, 32, 335-344. Bandyopadhyay, A., et al., (2011). Laser surface modification of metallic biomaterials. JOM, 63(6), 94-99.
15
Cassar, G., et al., (2011). Tribological properties of duplex plasma oxidised, nitrided and PVD coated Ti–6Al–4V. Surface and Coatings Technology, 206(2), 395-404. Chen, X., et al., Laser nitriding of titanium alloy in the atmosphere environment. Surface and Coatings Technology, 201(9), 4843-4846. Coll, B. F. and Jacquot, P. (1988). Surface modification of medical implants and surgical devices using TiN layers. Surface and coatings technology, 36(3), 867-878. Cui, Z. D., et al., (2005). Microstructure and wear performance of gradient Ti/TiN metal matrix composite coating synthesized using a gas nitriding technology. Surface and Coatings Technology, 190(2), 309-313. Grögler, T., et al., (1998). CVD diamond films as protective coatings on titanium alloys. International journal of refractory metals and hard materials, 16(3), 217-222. Han, J. G., et al., (1995). Development and characterization of TiN coatings by ion beam assisted deposition process for improved wear resistance. Surface and Coatings Technology, 76, 437-449. Jiang, P., et al., (2000). Wear resistance of a laser surface alloyed Ti–6Al–4V alloy. Surface and Coatings Technology, 130(1), 24-28. Korovessis, P., et al., (2006). Metallosis after contemporary metal-on-metal total hip arthroplasty. The Journal of Bone & Joint Surgery, 88(6), 1183-1191. Kurella, A. and Dahotre, N. B. (2005). Review paper: surface modification for bioimplants: the role of laser surface engineering. Journal of biomaterials applications, 20(1), 5-50. Li, J., et al., (2011). Wear behaviors of an (TiB+ TiC)/Ti composite coating fabricated on Ti6Al4V by laser cladding. Thin Solid Films,519(15), 4804-4808. Liang, Y. N., Ma, Z. Y., Li, S. Z., Li, S., Su, G. Y., & Bi, J. (1995). Wear behavior of reinforced aluminum alloys in various impact loadings. Wear,189(1), 45-50. Liu, X., et al., (2004). Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Materials Science and Engineering: R: Reports, 47(3), 49-121. Long, M. and Rack, H. J. (1998). Titanium alloys in total joint replacement—a materials science perspective. Biomaterials, 19(18), 1621-1639. Man, H. C., et al., (2002). Analysis of laser gas nitrided titanium by X-ray photoelectron spectroscopy. Applied surface science, 199(1), 293-302. Man, H. C., et al., (2006). In situ formation of a TiN/Ti metal matrix composite gradient coating on NiTi by laser cladding and nitriding. Surface and Coatings Technology, 200(16), 4961-4966.
16
Man, H. C., et al., (2011). Laser diffusion nitriding of Ti–6Al–4V for improving hardness and wear resistance. Applied Surface Science,258(1), 436-441. Man, H. C., Bai, M., & Cheng, F. T. (2011). Laser diffusion nitriding of Ti–6Al–4V for improving hardness and wear resistance. Applied Surface Science,258(1), 436-441. Narayan, J., et al., (1992). Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition. Applied physics letters, 61(11), 1290-1292. Niinomi, M. (1998). Mechanical properties of biomedical titanium alloys. Materials Science and Engineering: A, 243(1), 231-236. Nolan, D., et al., (2006). Sliding wear of titanium nitride thin films deposited on Ti–6Al–4V alloy by PVD and plasma nitriding processes. Surface and Coatings Technology, 200(20), 56985705. Raveh, A., et al., (1990). RF plasma nitriding of Ti6A14V alloy. Thin Solid Films, 186(2), 241256. Roy, M., et al., (2008). Laser processing of bioactive tricalcium phosphate coating on titanium for load-bearing implants. Acta Biomaterialia, 4(2), 324-333. Samuel, S., et al., (2008). Wear resistance of laser-deposited boride reinforced Ti-Nb–Zr–Ta alloy composites for orthopedic implants. Materials Science and Engineering: C, 28(3), 414420. Sathish, S., et al., (2010). Studies on the corrosion and wear behavior of the laser nitrided biomedical titanium and its alloys. Materials Science and Engineering: C, 30(3), 376-382. Schaaf, P. (2002). Laser nitriding of metals. Progress in Materials Science,47(1), 1-161. Schintlmeister, W., et al., (1976). Structure and strength effects in CVD titanium carbide and titanium nitride coatings. Journal of The Electrochemical Society, 123(6), 924-929. Selvan, J. S., et al., (1999). Laser boronising of Ti–6Al–4V as a result of laser alloying with preplaced BN. Materials Science and Engineering: A,260(1), 178-187. Singh, R., et al., Laser Surface Modification of Ti—6Al—4V: Wear and Corrosion Characterization in Simulated Biofluid. Journal of biomaterials applications, 21(1), 49-73. Subramanian, B., et al., (2010). Microstructural, mechanical and electrochemical corrosion properties of sputtered titanium–aluminum–nitride films for bio-implants. Vacuum, 85(5), 601609.
17
Subramanian, B., et al., (2011). Structural and tribological properties of DC reactive magnetron sputtered titanium/titanium nitride (Ti/TiN) multilayered coatings. Surface and Coatings Technology,205(11), 3485-3492. Tobe, R., et al., (2000). U.S. Patent No. 6,080,446. Washington, DC: U.S. Patent and Trademark Office. Unwin, P. S., et al., (1996). Aseptic loosening in cemented custom-made prosthetic replacements for bone tumours of the lower limb. Journal of Bone & Joint Surgery, British Volume, 78(1), 513. Welsch, G., et al., (Eds.). (1993). Materials properties handbook: titanium alloys. ASM international. Wilson, A. D., et al., (1999). A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti–6Al–4V. Surface and coatings technology,114(1), 70-80. Wooley, P. H. and Schwarz, E. M. (2004). Aseptic loosening. Gene therapy,11(4), 402-407. Yilbaş, B. S., et al., (1996). Plasma nitriding of Ti6Al4V alloy to improve some tribological properties. Surface and Coatings Technology, 80(3), 287-292. Yoshida, T., et al., (1979). The synthesis of ultrafine titanium nitride in an rf plasma. Journal of Materials Science, 14(7), 1624-1630. Yue, T. M., et al., (2000). The effects of laser surface treatment on the corrosion properties of Ti6Al-4V alloy in Hank's solution. Journal of materials science letters, 19(3), 205-208. Zhecheva, A., et al., (2005). Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods. Surface and Coatings Technology, 200(7), 2192-2207. Zhecheva, A., et al., (2006). Titanium alloys after surface gas nitriding. Surface and Coatings Technology, 201(6), 2467-2474.
18
Hardness Sample
CP-Ti Untreated 425W 1Pass 425W 2Pass 475W 1Pass 475W 2Pass
Hardness
Wear Rate in DI
Wear Rate in FBS
Increase w.r.t (HV0.1)
CP-Ti
(mm3/Nm)*10-4
(m3/Nm)*10-4
85
-
11.63±0.581
6.424±0.480
1266±344.3
~15x
1.859±0.093
0.108±0.005
1134±331.7
~12x
0.701±0.035
0.262±0.013
962.5±116.4
~11x
1.646±0.082
0.120±0.006
962.5±280.6
~11x
1.018±0.051
0.020±0.001
Table 1: Hardness and Wear Rates results of in situ nitrided CP-Ti.
Sample
Ti4+Concentration in ppm
Ion release decrease w.r.t CP-Ti
CP-Ti
15.595±0.097
Control
425W 1P
3.549±0.013
~4x
425W 2P
1.988±0.029
~8x
475W 1P
3.517±0.031
~4x
475W 2P
0.752±0.006
~21x
Table 2: Ti4+ ion release during 1km wear test in DI water at room temperature measured using ICP-MS techniques. 19
Figure 1: SEM image showing typical composite microstructure and different zones of the in situ formed nitride coating on titanium.
Figure 2: SEM images of typical composite microstructure of dendritic and secondary phase. phase
20
Figure 3: In situ nitrided composite microstructural variation with changing laser power and changing the number of laser surface scans.
21
Figure 4: XRD analysis of in situ nitrided CP-Ti Ti showing the formation of different titanium nitrides.
22
Figure 5: Hardness depth profiles of in situ nitrided CP-Ti Ti samples.
23
(a)
(b) Figure 6: Wear rates of CP--Ti substrate and in situ nitride CP-Ti Ti in (a) DI water at room temperature and (b) in 10% FBS solution at 37°C.
24
(a)
(b) Figure 7: Coefficient of Friction (COF) of the untreated CP-Ti substrate and in situ nitrided CPTi in (a) DI Water at room temperature and (b) in 10% FBS solution at 37°C. 37°C
25
Figure 8: Wear track analysis: (a) typical surface wear damage on the in situ nitrided composite coatings and (b) damage amage at the edges of the wear track track.
Figure 9: Cross section micrograph of the wear tracks (a) Unetched, at low magnification. magnification (b) Etched microstructure showing the approximate microstructure underneath the wear track. track
26
• • •
Laser remelting of Ti in LENS allows for site specific surface modification. Metal ion release can be lowered by modifying Ti surface due to hard coating. Laser remelting resulted high hardness, excellent wear resistance and low COF.
27
Laser processing of in situ TiN/Ti composite coating on titanium Himanshu Sahasrabudhe, Julie Soderlind, Amit Bandyopadhyay
www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(15)00284-2 http://dx.doi.org/10.1016/j.jmbbm.2015.08.013 JMBBM1573
To appear in: Journal of the Mechanical Behavior of Biomedical Materials
Received date:8 May 2015 Revised date: 29 July 2015 Accepted date: 7 August 2015 Cite this article as: Himanshu Sahasrabudhe, Julie Soderlind, Amit Bandyopadhyay, Laser processing of in situ TiN/Ti composite coating on titanium, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi. org/10.1016/j.jmbbm.2015.08.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Laser Processing of in situ TiN/Ti Composite Coating on Titanium Himanshu Sahasrabudhe, Julie Soderlind and Amit Bandyopadhyay W. M. Keck Biomedical Materials Research Laboratory School of Mechanical and Materials Engineering Washington State University Pullman, Washington, 99164-2920, USA. E-mail: [email protected] Abstract Laser remelting of commercially pure titanium (CP-Ti) surface was done in a nitrogen rich inert atmosphere to form in situ TiN/Ti composite coating. Laser surface remelting was performed at two different laser powers of 425W and 475W. At each power, samples were fabricated with one or two laser scans. The resultant material was a nitride rich in situ coating that was created on the surface. The cross sections revealed a graded microstructure. There was presence of nitride rich dendrites dispersed in α-Ti matrix at the uppermost region. The structure gradually changed with lesser dendrites and more heat affected α-Ti phase maintaining a smooth interface. With increasing laser power, the dendrites appeared to be larger in size. Samples with two laser scans showed discontinuous dendrites and more α-Ti phase as compared to the samples with one laser scan. The resultant composite of TiN along with Ti2N in α-Ti showed substantially higher hardness and wear resistance than the untreated CP-Ti substrate. Coefficient of friction was also found to reduce due to surface nitridation. Leaching of Ti4+ ions during wear test in DI water medium was found to reduce due to laser surface nitriding.
Keywords – Laser processing, Ti nitride, wear resistance, hard coatings, surface modification 1. Introduction One of the most widely used material in specialized engineering and biomedical application is titanium. Titanium and its alloys like the Ti6Al4V alloy have immense applications in various branches of engineering due to their properties such as excellent corrosion resistance, high strength to weight ratio, biocompatibility and low coefficient of 1
friction (COF) (Welsch et al., 1993, Liu et al., 2004, Niinomi, 1998, Long and Rack, 1998). In the biomedical implant industry, Ti and its alloys are mainly used for load bearing applications. These are implants such as artificial hip replacement or artificial knee replacement consisting of articulating surfaces. Such articulating surfaces are subjected to heavy point loads during the implant life. Motion between these articulating surfaces under load causes wear and release of metal ions from the surfaces. Loss of material due to wear as well as the leaching of metal ion can cause problems such as aseptic loosening, osteolysis and metallosis (Unwin et al., 1996, Wooley and Schwarz, 2004, Korovessis et al., 2006). The entire wear and degradation regime is further accelerated due to the corrosive nature of the biological media in the human body. Due to poor wear resistance of Ti and its alloys, typically CoCrMo alloy is used in articulating surfaces. Unfortunately, concerns related to Co and Cr ion release in vivo is a bigger problem today due to their toxic nature. Hence, there is a need for surface modification to improve the wear resistance of Ti based implant materials (Kurella and Dahotre, 2005, Bandyopadhyay et al., 2011). In the past, various approaches have been researched upon to increase surface hardness and wear resistance of Ti. These include coatings of different metallic and ceramic materials such as nitrides, carbides, carbon, graphite and calcium phosphates based materials (Man et al., 2006, Selvan et al., 1999, Li and Wang, 2011, Samuel et al., 2008, Roy et al., 2008, Grögler et al., 1998) as well as simple remelting of the material by using focused laser radiation (Singh et al., 2006, Balla et al., 2014.). The essential idea behind all of those approaches is to improve the surface performance of the material without significantly altering the bulk chemistry and properties of the base material, keeping in mind that the modification approach should be simple as well as economically viable. Ceramics are a good choice for articulating surfaces of implant materials. They are biocompatible, have low wear rates and low coefficient of friction (COF). However, they are brittle and cannot be utilized in bulk forms. An ideal way to use ceramic based materials is to use them as coatings, either as complete ceramics or as composites with metallic materials, on metallic surfaces like titanium. Amongst the many available ceramic materials, titanium nitride is very popular for bio medical implant applications (Schaff, 2002, Cool and Jacquot, 1988, Zhecheva et al., 2005). Titanium nitride coatings have been processed by different techniques. The most popular techniques are the thin film methods of CVD and PVD (Narayan et al., 1992, Schintlmeister et al., 1976, Tobe et al., 2000), plasma and ion beam techniques (Yoshida et al., 2
1979, Raveh et al., 1990, Han et al., 1995) and gas nitriding (Man et al., 2002, Man et al., 2005, Abboud, 2013) as well as direct laser processing (Balla et al., 2012). CVD and PVD techniques can produce good quality coatings but coating on complex shapes and surfaces as well as selective area coating is not easily possible. The drawbacks of plasma processing are the same along with large heating of the substrate. Additive manufacturing techniques such as laser engineered net shaping (LENS™) can offer selective area coatings, strong interfaces and ability to form compositionally graded structures. In this method, addition of separate coating material onto a substrate is possible as well. The ideal scenario would be to develop a coating in the existing part without damaging the part shape and inherent properties of the bulk material. Laser gas nitriding can achieve this but to a limited extent. Maintaining low oxide levels, selective area modification, strong interface formation and homogeneous nitride formation are some of the main challenges associated with laser gas nitridation. Hence, a combination of the processing simplicity of laser gas nitridation and the features of additive manufacturing such as 3D motion control, environment control and reproducibility are utilized in this work to realize in situ titanium nitride coating on CP-Ti. We have used a commercial LENS™ system to improve the surface performance of titanium by laser remelting it in a nitrogen rich inert environment. Samples were characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques for microstructural and phase analysis. Tribological performance was measured by wear testing in DI water and 10%FBS media along with micro hardness testing. Release of Ti ions in DI water medium during wear was measured using an ICP-MS technique.
2. Experimental Procedure 2.1 In situ fabrication of nitride coating on titanium: A commercially pure titanium plate (3mm thick and 99.99% pure, President Titanium, Hanson, MA USA) was used as the substrate material. Samples were fabricated using LENSTM 750 (Optomec Inc. Albuquerque, NM USA) equipped with a 500 W continuous wave Nd: YAG laser. LENSTM operation is generally performed in a glove box containing argon atmosphere and very low level of oxygen (<10ppm). In the laser surface modification experiments, argon was replaced with nitrogen by purging the 3
chamber with nitrogen gas (99.996% pure) for 25 min at an inlet pressure of 1200 psi. The resultant environment in the glove box contained approximately 75% nitrogen and remainder argon. Oxygen was maintained below 10ppm and was continuously monitored using an oxygen sensor. Laser surface nitriding was carried out by raster scanning the CP-Ti metallic substrate in the nitrogen rich environment. Raster scanning was done at a speed of 56 cm/min. Raster scanning was executed from a CAD design to fabricate square shaped samples with side-length of 14 mm. Samples were made with one and two passes (raster scans on the surface) at both 425W and 475W laser power. While fabricating samples with two raster scans, the second scan was done at 90° angle to the first one to promote homogeneity in remelting. Samples treated once at 425W is labeled as 425W 1P whereas sample treated twice at 425W is labeled as 425W 2P. Similarly, for 475W 1P is the sample treated once at 475W and 475W 2P is the sample treated twice at 475W. 2.2 Characterization of in situ nitride coatings: Each sample was transversely cut to allow examination of the cross sectional microstructure, and measure the hardness profile. Cross sections of each sample were ground with silicon carbide grinding papers of succeeding grits from 220 to 1000, and polished with alumina abrasive suspension of 1, 0.30 and 0.05 microns. The top surfaces of the sample was ground and polished following the procedure to a make a mirror finish surface. This was done slowly to prevent removal of the laser affected area while still achieving the desired surface finish. Samples were then washed in a DI water-jet and cleaned in an ultrasonic bath containing 100% ethanol for 30 min at room temperature, and finally blow dried. To observe the microscopic feature in the cross sections, the samples were etched using Kroll’s reagent with specific composition of 92 ml DI water, 6 ml Nitric acid, 2 ml Hydrofluoric acid for 30 seconds etching time. The samples were characterized using a Field Emission-SEM (FEI Quanta 200, FEI Inc., OR, USA) at an operating voltage of 20kV. X-ray diffraction analysis was performed using a Siemens D-500 Kristalloflex Diffractometer (Siemens AG, Karlsruhe, Germany) with Cu Kα radiation and Ni filter at 20 kV between the 2θ range between 10 and 100 degrees, with a step size 0.02 degree and a dwell time of 0.5s per step. 2.3 Hardness and wear testing: Vickers microhardness (Shimadzu, HMV-2T) tests were performed on the samples with a standard diamond Vickers indenter. The load was 100 g (0.98 4
N) and dwell time was 15s for all samples. Variation of hardness with the depth of the coating was recorded by testing the cross sections of samples, maintaining equal spacing in between the consecutive hardness measurements. Each sample was indented with at least six depth profiles consisting of six indents each. Mechanical tests were done on the samples including linear reciprocating wear test using Nanovea tribometer (Nanovea Series, Nanovea, Irvine CA, USA). For pin on disk type linear reciprocating wear test, ASTM guideline from standard G133 was followed (ASTM International, 2010). The medium used was de-ionized (DI) water and 10% (by vol.) fetal bovine serum (FBS) solution in DI water (ATCC, VA). Wear testing in DI water medium was done at room temperature; however tests in FBS solution were done at body temperature of 370C. Wear tests were performed with a 3 mm diameter silicon nitride ball at 1200 mm/min speed using a linear reciprocating motion (stroke length) of 10 mm. Wear tests were done in DI water under 5N load for 1000 m distance and in 10% FBS solution under 7N load for 2000 m distance. In between tests, the tribometer was rinsed nine times with heated DI water to prevent cross contamination from previous wear tests. Our idea was to first understand wear degradation of these surfaces in DI, and then do longer runs in simulated body environment for data that are more representative to real world applications. After wear testing was complete, the tracks of each sample were transversely cut to expose the cross section underneath the wear track. These were polished to the same standard as before, and the microstructure of the area below the wear track was imaged for analysis. The release of Ti4+ ions during wear in the DI water medium was measured using Inductively Coupled Plasma-Mass Spectroscopy (Agilent model 7700 ICP-MS). Three standard solutions of 10ppb, 500ppb and 1000ppb concentration were used for calibration of the instrument. At least three samples were analyzed from each wear runs.
3.0 Results In situ nitridation was done on CP-Ti substrate using a Laser Engineered Net Shaping (LENS™) system under nitrogen environment, raster scanning a high powdered continuous wave Nd:YAG laser on the CP-Ti substrate. Effects of nitridation on phase transformations, microstructure, surface hardness and wear resistance were measured. 5
3.1 Surface morphology and microstructure: Before the laser treatment, the CP-Ti plate had a microstructure of equiaxed α-Ti grains. Etched microstructures of surface nitrided CP-Ti under SEM showed a graded microstructure. There was no sharp interface observed and the microstructure showed gradual change in morphology from dendritic-composite structure at the surface to equiaxed grains of the CP-Ti substrate inside. Figure 1 shows the SEM images of the etched cross section of the CP-Ti substrate nitrided at 425W with 2 laser scans. The cross section can be divided into three distinct zones – zone 1, zone 2 and zone 3. Zone 1 was the uppermost region of the sample and had a depth of approximately 200 µm with a variation of 50 µm between different samples. This zone consisted of mostly dendrites that formed after laser remelting and solidification. These dendrites seemed to be dispersed in a secondary phase. Zone 2 was the layer below the Zone 1. With increasing depth, the dendritic phase appeared to reduce in proportion while more secondary phase was observed. This zone was mostly reduced dendrites with extensive and continuous secondary phase. The secondary phase appeared to be acicular or needle like. Figure 2 shows this mixed phase microstructure. Finally in Zone 3, acicular needles from the Zone 2 became more ordered and were seen to grow in the direction of heat flow. This region was mostly comprised of the needle like structures. After the needle like structure ended, there was a region of around 200 µm of finer microstructure which had been affected by the heat of the melt pool above. This was the heat affected zone (HAZ). At a depth of 600 µm and beyond, the original untreated microstructure of CP-Ti was seen. The structure of the remelted and solidified region of the substrate (Zone 1) was dendritic and dispersed in a secondary phase. The laser power used to remelt the samples as well as the number of laser scans had significant effect on the evolution of microstructure in this region. In the samples that were scanned only once, i.e., samples 425W 1P or 475W 1P, the dendritic phase appeared more continuous. The dendrites were extensive and not all were able to be individually identified. In the case of samples that were scanned twice (samples 425W 2P or 475W 2P), dendrites were smaller, discontinuous and the secondary phase was more dispersed in between the dendrites. These features are shown in Figure 3. 3.2 Phase analysis: X-ray diffraction analysis was performed on the surface of the samples showing the formation of different nitrides of titanium upon laser surface melting in a nitrogen rich environment. Figure 4 shows the formation of TiN and Ti2N as well as peaks of the 6
α-Ti phase from the unreacted substrate. The XRD signal was stronger for the samples with two surface scans for both the 425W and 475W power levels. Samples scanned once at 425W (425W 1Pass) showed similar peak intensity as compared to the sample scanned once at 475W (475W 1Pass). The samples scanned twice at 425W and 475W were also similar in terms of peak intensity. 3.3 Microhardness testing: Surface hardness was improved with the laser surface treatment. Table 1 shows the hardness comparisons of all four laser treated samples, and the untreated CP-Ti substrate. The CP-Ti substrate had a hardness of 85±5 HV0.1.The sample remelted with the 425W laser with one laser pass showed a higher hardness (1266±344.3 HV0.1) than the sample melted under two laser passes at the same laser power (1134±331.7 HV0.1). The hardness of 425W 1P was also higher than the samples melted under higher laser power of 475W for both 1P and 2P. The sample treated at 425W with one laser pass showed almost 15 times increase in hardness while the sample melted under two laser passes showed over 12 times increase in hardness compared to untreated CP-Ti. The hardness of the CP-Ti substrate near the HAZ was also increased to ~ 200 HV0.1. Variations of the hardness of all the samples with depth were recorded and shown in Figure 5. The sample remelted once at 425W had the highest hardness at all depths when compared to the other three treated samples. The decrease in hardness for all the four samples from high values of over ~900 HV0.1 to the ~200 HV0.1 of the HAZ and further into the substrate was gradual. There was no sudden drop in the hardness when moving from nitride rich regions to the untreated regions. 3.4 Wear testing in DI water and 10% FBS solution: After wear testing in both DI water and FBS solution, wear damage on the polished surface was found to be less in the laser nitrided surfaces than the untreated CP-Ti surface. In DI water, the wear rate of the CP-Ti substrate was 11.63±0.581 x 10-4mm3/Nm. For the in situ nitrided samples, the lowest wear was for the sample treated at 425W with two passes. It was 0.701±0.035 x 10-4mm3/Nm. This corresponds to a decrease in the wear rate by more than 16 times. The highest wear amongst the treated samples was with the sample treated once at 425W and was 1.859±0.093 x 10-4mm3/Nm. This was 6 times less than the wear on the untreated CP-Ti substrate. In 10% FBS solutions, both the treated and untreated CP-Ti samples underwent even less wear. The wear rate of the CP-Ti substrate in FBS was 6.424±0.480 x 10-4mm3/Nm. The lowest 7
wear amongst the in situ nitrided samples was for the sample that was treated twice at 475W and was 0.020±0.001 x 10-4mm3/Nm. Such a reduction in the wear rate corresponds to more than 300 times due to laser surface treatment. The highest wear was for the sample treated twice at 425W and was 0.262±0.013 x 10-4mm3/Nm, still 24 times less compared to the untreated CP-Ti substrate. Table 1, and Figure 6a and 6b show the wear rates of the different samples in DI water and in 10% FBS medium. During wear testing the coefficient of friction (COF) was recorded by the tribometer. Figure 7a and 7b show the coefficient of friction of the CP-Ti substrate and the in situ nitrided samples. In DI water medium for a distance of 1km, the maximum COF for the CP-Ti substrate was ~1.2. This was reduced to the range of 0.70-0.90 for the treated samples when tested under the same conditions. Similarly in 10% FBS solution CP-Ti reached a maximum COF of ~1.10. For the treated samples the COF was in the range of ~0.20-0.60. Closer looks at the trend of the COF in DI water revealed that the CP-Ti substrate reached a steady state in wear at a distance of ~300m. For the treated samples when tested in DI water, the steady state came at a distance of ~100m whereas in 10% FBS solution such a steady state was not established within the testing zone. SEM imaging was carried out on the wear track of all the samples. Figure 8a shows the typical surface damage of 425W 1P sample. Similar surface damage was observed in all other samples in the central regions of the wear track. Towards the edges of the wear track, cracking and accumulation of particles was observed and shown in Figure 8b. Figure 9a shows the cross section of the wear track of one of the nitrided CP-Ti substrates. Figure 9b shows a magnified corner of the same wear track after etching. It can be seen from these images that the wear took place primarily in the Zone 2 of the nitrided samples. More importantly, observation of the cross section of the wear track confirmed that the nitrided layers were thick enough to accommodate the surface damage during wear and the damage was not carried down into the Zone 3 (HAZ) or into the unreacted CP-Ti substrate. The concentration of Ti4+ ions released in DI water after 1km of wear distance was measured using ICP-MS techniques and reported in Table 2. The concentration of Ti4+ from the wear media of the untreated CP-Ti substrate was 15.595±0.097 ppm. Laser surface nitriding at 425W and with one laser pass reduced the ion release to 3.549±0.013 ppm. The ion release from 8
the sample treated at the same laser power but twice was even less with a measured value of 1.988±0.029 ppm. The sample that was treated at 475W with one laser pass was similar to the sample treated at 425W with one laser pass with measured ion release value of 3.517±0.031 ppm. The lowest Ti4+ ion release was measured in the sample that was treated with two laser passes at 475W with measured value of 0.752±0.006 ppm. Thus laser surface remelting in a nitrogen environment reduced the Ti4+ ion leaching during wear by at least 4 times.
4.0 Discussions Titanium, although a very popular engineering material, is not a first choice material in applications that require high hardness or wear resistance. To counter this, coating of titanium nitride is often used. Laser surface nitriding is a simple method used to form in situ nitrides. This process suffers from disadvantages like oxidation, difficulty in coating large area, curved surfaces and complex geometries. Combining the features of laser surface nitriding along with the features of additive manufacturing, these problems may be solved. In additive manufacturing setups, controlled environments may help prevent oxidation and 3D motion control may be utilized to coat complex shapes and geometries. Nitride coatings may be formed in situ on areas of high wear. Selective area modification can be done to enhance the hardness and wear resistance of Ti based materials without compromising with the bulk properties or the structural integrity of the base material. As an example, wear prone articulating surfaces of Ti based load bearing implants can be strengthened with titanium nitride coating. This concept was verified in the current research. Laser surface melting was carried out of CP-Ti substrate in a nitrogen rich environment. Laser source was utilized for in situ creation of a composite coating of titanium nitride in the existing α-Ti matrix. Laser engineered net shaping (LENS™) used here, has been previously used to process different compositions of TiN in Ti6Al4V alloy composites for improved tribological performance (Balla et al., 2012). Similar to the in situ nitriding reported in the current study, in situ oxidation of LENS™ deposited zirconium was also carried out to form zirconia ceramic for the improvement of surface performance of CP-Ti (Balla et al., 2009) as well as LENS™ fabrication of TiO2 ceramics has also been accomplished (Balla et al., 2009).
9
The use of a solid state laser in a controlled environment allowed uniform properties across a broader surface area. Presence of a predominantly nitrogen with some argon in a very low oxygen environment was crucial. Titanium will readily form oxides as was pointed out in some earlier attempts (Man et al., 2006, Chen et al., 2007, Zhecheva et al., 2006), therefore maintaining the level below 10ppm worked in favor of forming mainly nitrides without any oxidation. The microstructure that evolved after laser surface melting was dendritic-composite in nature. Along with the dendritic phase, a secondary phase coexisted. Our hypothesis is that, the dendritic phase is predominantly TiN and Ti2N whereas the secondary phase is α (hcp) Ti. Similar dendritic structure and phases have been reported as nitrides of titanium and α-Ti phase elsewhere (Man et al., 2005, Man et al., 2006, Chen et al., 2007, Cui et al., 2005). All four samples showed similar dendritic structure but with some significant differences. There were some areas in the dendritic microstructure where the area between the dendrites was lost. This may have happened due to selective etching of the α-Ti phase by Kroll’s reagent, leaving behind the titanium nitride dendrites. The samples that were melted just once at 425W and 475W showed a dense network of dendrites with very little secondary phase in between them. These dendrites were nearly continuous. On the contrary, both the samples that were subjected to two laser scans, showed “broken” dendrites and relatively larger amount of secondary phase. This happened due to the fact that the first laser scan simply remelted and solidified the samples, and during solidification the dendritic phase evolved. This new phase contained nitrides of titanium having higher melting temperature than the earlier untreated α-Ti. However, during the second laser scan, the initially formed dendrites were only partially remelted due to the higher melting temperature of titanium nitrides and thus only broke down into smaller fragments. The acicular phase and the HAZ in all the samples were similar, irrespective of the power or the number of laser scans. This gives us an estimate of the effective depth of the laser. The nitride rich layers, the needle like phase and the heat affected zone appeared to blend gradually with the lower unaffected substrate. The laser surface nitriding thus essentially formed an in situ coating of TiN in Ti matrix. X-ray diffraction data shows the formation of different nitrides of titanium. TiN and Ti2N phases were abundant along with the residual α-Ti from the composite coating layer as well as from the CP-Ti substrate. In the titanium nitride coatings processed by other techniques, similar phases were found to have evolved (Han et al., 1995, Jiang et al., 2000, Man et al., 2006, Nolan 10
et al., 2006, Subramanian et al., 2010, Man et al., 2011). As pointed out by Man et al., (2006), the amount of nitride formed due to a gas phase reaction of the molten titanium with nitrogen would be dependent on the amount of time the melt pool remains in the liquid state. The longer the melt pool remains liquid, the more the formation of nitrides till the solubility limit of nitrogen in titanium is reached. But this long time would subsequently cause delayed solidification and larger grains. The LENS™ system is popular for exceptionally high cooling rate of 103 to 105 K/s and thus the reaction time is short. Yet, there was clear formation of nitrides and this may be due to the fact that in the LENS™ setup, smaller melt pool has a very high surface area over volume ratio. For both the power settings, 425W and 475W, the samples that had undergone melting once showed stronger intensity than the samples that had undergone melting twice. The disturbance of the initially acquired orientation of the phases may explain this. When the surface of the CP-Ti substrate is melted, the melt pool is highly reactive. At this point it would react with the nitrogen in the atmosphere and form nitrides of titanium. The melt pool would then solidify with the nitrides forming dendrites and the unreacted titanium forming the acicular phase of α-Ti. During the second laser scan however, the laser is melting not the initial CP-Ti substrate but a newly formed ceramic layer of titanium nitrides and some residual α-Ti. During this second remelting, the laser would only partially melt the nitrides and break down the dendrites. It is also possible that during this second laser scan, the initially acquired orientation of the phases is damaged, thereby giving a weaker peak intensity for the samples with two surface scans. The absence of a sharp and distinct interface separates the current nitride composite coatings from nitride coatings processed from other techniques. Plasma nitrided Ti coatings reported by Ali et al., (2010) showed increased hardness and improved wear behavior. However nitrided samples showed a very distinct interface with the substrate. TiN coatings reported by Jiang et al., (2000) and by Yue et al., (2000) showed similar bonding to the substrate as reported here. The increase in the hardness after in situ nitriding was at least 10 times as compared to the untreated CP-Ti substrate. This was clearly due to the formation of TiN and Ti2N phases in the samples. The hardness was more than the hardness of simply laser remelted Ti6Al4V at 200W and comparable to the hardness of the laser remelted Ti6Al4V at 400W reported by Balla 11
et al., (2014). Similar hardness values were also reported by Balla et al., (2012) of LENS™ based processing of Ti6Al4V-40%TiN composites. The hardness values measured in the current TiN/Ti composite in situ coating on Ti were also comparable, if not higher, to the titanium nitride produced by laser gas nitriding (Jiang et al., 2000, Man et al., 2005, Man et al., 2006, Zhecheva et al., 2006, Chen et al., 2007, Abboud, 2013,) and to those processed by PVD, CVD, plasma nitriding and ion techniques (Man et al., 1995, Wilson et al., 1999, Nolan et al., 2006, Cassar et al., 2011). However, the nitrides reported by Cui et al., (2005) had hardness of up to 2000HV. The hardness depth profile recorded in the current research showed gradual drop in the hardness as the depth varied from the nitride rich coating to the CP-Ti substrate. This was also in contrast to the values reported by Man et al., (2006) where the hardness number took an abrupt drop from ~700HV to ~350HV in the laser modified layer. Similar drop in hardness was also reported by Cui et al., (2005) from ~1100HV to ~200HV. Gradual drop in the hardness depth profile reported in the current work was similar to that reported by Zhecheva et al., (2006) but with the reported hardness values lesser than the ones reported here. Yilbas et al., (1996) also reported similar trend in the hardness depth profile of plasma nitrided Ti6Al4V alloy with comparable hardness values. The increase in hardness of the nitrided samples was an initial sign for the improvement in the wear resistance of the samples. Tribological characterization in the present study included wear testing in each of DI water medium and 10%FBS solution for a distance of 1km (50,000 cycles) and 2km (100,000 cycles), respectively. The wear rates in mm3/Nm units in DI water medium were higher than the wear rates reported by Chen et al., (2007). However the wear rates in 10% FBS solution were lower in the current TiN/Ti composite coatings. The wear rates in DI water and 10%FBS were both lower than the LENS™ processed Ti6Al4V alloy reinforced with 10%, 15% and 20% TiN and higher than the wear rates of Ti6Al4V reinforced with 40%TiN (Balla et al., 2012). The current wear rates were also comparable to the laser assisted ZrO2 coatings processed by LENS™ (Balla et al., 2009). The wear rates in the present study were also higher than those reported by Wilson et al., (1999) for TiN coatings produced by different techniques. However, those wear tests were done for fewer number of cycles (500 cycles) whereas in the current study, the samples were tested for larger number of linear reciprocating cycles. Comparison of wear damage was difficult with other reported nitride coatings due to the difference in wear testing methods as well as the different units used to report wear damage (Han 12
et al., 1995, Man et al., 2006, Nolan et al., 2006, Chen et al., 2007, Subramanian et al., 2010, Subramanian et al., 2011). The coefficient of friction was reduced due to the in situ nitriding of the substrate from an initial value of ~1.2 to 0.70-0.90 in DI water medium and even less in 10%FBS solution. These values were however higher than those reported earlier for nitrides processed by other techniques (Subramanian et al., 2010, Sathish et al., 2010, Subramanian et al., 2011). The SEM images of the wear tracks revealed the difference in the damage of the nitrided samples at the central regions of the wear track and at the edges. The central regions appeared to be plastically deformed with some flaking and peeling of material. The plastically deformed regions may be the α-Ti phase whereas the brittle nitrides may have been flaked and eroded. Wilson et al., (1999) and Nolan et al., (2006) also reported similar damage in plasma nitrided titanium coatings. The edges appeared to be the region where the wear debris was accumulated and also showed some distinct cracking damage. There was also bright charging under the electron microscope in this region specifically and this may have happened due to the accumulation of the non-conducting nitrides. This overall wear damage bears resemblance to the fretting wear observed in plasma nitrided titanium (Ali et al., 2010). During wear, the soft α-Ti phase deforms faster due to hard counter body material i.e., silicon nitride, and there is a constant compressive load. The harder nitride phase however, resists wear to a large extent. Due to the continuous damage to the α-Ti matrix around it, the nitride phase finally deforms and was swept to the sides of the wear track. The region from where the nitride phase broke off was now exposed and the softer matrix phase was again subjected to deformation and subsequently to wear. A similar type of wear mechanism was described by Liang et al., (1995) in SiC reinforced Al alloys. The analysis of the wear media in DI water wear tests after a distance of 1km showed that the Ti4+ ions leached during wear reduced after surface remelting of CP-Ti in a nitrogen rich environment. A maximum drop of more than 20 times was observed in the sample that was remelted twice at 475W with two laser scans on the surface. There was a minimum drop of 4 times with respect to the untreated CP-Ti substrate. This proved that laser surface nitriding of CP-Ti was not only successful in improving the hardness and reducing surface wear of the samples but also successful in reducing the metal ion leaching during wear. This drop in the 13
metal ion leaching is a good indication of the usefulness of the surface nitrided CP-Ti from the biological implants point of view. Such selective surface nitriding treatment can be applied to the regions of high wear on the articulating implant surfaces. This may be useful in improving the life of the implant by reducing the wear damage and may also alleviate medical complications such as metallosis arising out of metal ion release in the human body. The ions released in 10%FBS solution are expected to be even lower due to lower wear rates as well as the coefficient of friction due to the lubrication effect of the proteins in the FBS media. This also needs to be investigated by doing careful wear studies in 10%FBS solution along with ICP-MS analysis of the wear media after testing.
5.0 Conclusions In this work, an in situ processing of titanium nitride using laser gas nitriding with features of additive manufacturing was explored. Laser remelting of commercially pure titanium was done in a nitrogen rich environment using a finely focused and high powered continuous wave Nd:YAG laser using a laser engineered net shaping (LENS™) system. Laser remelting was carried out at two power levels of 425W and 475W, and with one and two laser surface scans each. XRD analysis confirmed the formation of TiN along with Ti2N and unreacted α-Ti at the surface. The nitride phases were present as dendrites in a matrix of unreacted α-Ti phase. Since there was no material deposited or coated, the laser surface remelting created an in situ TiN in αTi composite coating on CP-Ti. The cross-section microstructure showed three distinct zones. The uppermost zone had extensive nitride phase. The intermediate zone showed lesser dendrites and more α-Ti phase, and finally there was a heat affected zone along with the interface with the unaffected CP-Ti substrate. There was a smooth transition from the three zones of the composite coating microstructure to the CP-Ti substrate. Effect of laser power and number of surface scans was prominent on the microstructural evolution. The samples with one laser scan showed large and continuous dendrites whereas the samples with two laser scans showed dendrites that appeared to be discontinuous. Due to the formation of extensive titanium nitride ceramic phase, all samples showed multiple times increase in hardness and better resistance to surface wear in both DI water and 10% FBS solution. Analysis of the wear track showed damage that was typical to the fretting of a hard phase embedded in soft ductile matrix. The leaching of Ti4+ ions 14
as well as the coefficient of friction also reduced during wear. Laser surface nitriding was thus successful in creating an in situ coating to improve the surface properties of CP-Ti without compromising the inherent bulk properties of the material.
Acknowledgements Authors acknowledge financial support from the Joint Center for Aerospace Technological Innovation (JCATI), WA. Authors also acknowledge the financial support from W. M. Keck Foundation and M. J. Murdock Charitable Trust to establish the Biomedical Materials Research Laboratory at WSU. Authors would also like to thank Dr. Scott Boroughs for help with the ICP-MS analysis.
References Abboud, J. H. (2013). Effect of processing parameters on titanium nitrided surface layers produced by laser gas nitriding. Surface and Coatings Technology, 214, 19-29. Ali, M. M., Raman, Set al., (2010). Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V. Tribology International, 43(1), 152-160. ASTM G133-05(2010), Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear, ASTM International, West Conshohocken, PA, 2010,www.astm.org Balla, V. K., et al., (2009). Laser-assisted Zr/ZrO2 coating on Ti for load-bearing implants. Acta biomaterialia, 5(7), 2800-2809. Balla, V. K., et al., (2009). Fabrication of compositionally and structurally graded Ti–TiO2 structures using laser engineered net shaping (LENS). Acta biomaterialia, 5(5), 1831-1837. Balla, V. K., et al., (2012). Laser processed TiN reinforced Ti6Al4V composite coatings. Journal of the mechanical behavior of biomedical materials, 6, 9-20. Balla, V. K., et al., (2014). Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy. Journal of the mechanical behavior of biomedical materials, 32, 335-344. Bandyopadhyay, A., et al., (2011). Laser surface modification of metallic biomaterials. JOM, 63(6), 94-99.
15
Cassar, G., et al., (2011). Tribological properties of duplex plasma oxidised, nitrided and PVD coated Ti–6Al–4V. Surface and Coatings Technology, 206(2), 395-404. Chen, X., et al., Laser nitriding of titanium alloy in the atmosphere environment. Surface and Coatings Technology, 201(9), 4843-4846. Coll, B. F. and Jacquot, P. (1988). Surface modification of medical implants and surgical devices using TiN layers. Surface and coatings technology, 36(3), 867-878. Cui, Z. D., et al., (2005). Microstructure and wear performance of gradient Ti/TiN metal matrix composite coating synthesized using a gas nitriding technology. Surface and Coatings Technology, 190(2), 309-313. Grögler, T., et al., (1998). CVD diamond films as protective coatings on titanium alloys. International journal of refractory metals and hard materials, 16(3), 217-222. Han, J. G., et al., (1995). Development and characterization of TiN coatings by ion beam assisted deposition process for improved wear resistance. Surface and Coatings Technology, 76, 437-449. Jiang, P., et al., (2000). Wear resistance of a laser surface alloyed Ti–6Al–4V alloy. Surface and Coatings Technology, 130(1), 24-28. Korovessis, P., et al., (2006). Metallosis after contemporary metal-on-metal total hip arthroplasty. The Journal of Bone & Joint Surgery, 88(6), 1183-1191. Kurella, A. and Dahotre, N. B. (2005). Review paper: surface modification for bioimplants: the role of laser surface engineering. Journal of biomaterials applications, 20(1), 5-50. Li, J., et al., (2011). Wear behaviors of an (TiB+ TiC)/Ti composite coating fabricated on Ti6Al4V by laser cladding. Thin Solid Films,519(15), 4804-4808. Liang, Y. N., Ma, Z. Y., Li, S. Z., Li, S., Su, G. Y., & Bi, J. (1995). Wear behavior of reinforced aluminum alloys in various impact loadings. Wear,189(1), 45-50. Liu, X., et al., (2004). Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Materials Science and Engineering: R: Reports, 47(3), 49-121. Long, M. and Rack, H. J. (1998). Titanium alloys in total joint replacement—a materials science perspective. Biomaterials, 19(18), 1621-1639. Man, H. C., et al., (2002). Analysis of laser gas nitrided titanium by X-ray photoelectron spectroscopy. Applied surface science, 199(1), 293-302. Man, H. C., et al., (2006). In situ formation of a TiN/Ti metal matrix composite gradient coating on NiTi by laser cladding and nitriding. Surface and Coatings Technology, 200(16), 4961-4966.
16
Man, H. C., et al., (2011). Laser diffusion nitriding of Ti–6Al–4V for improving hardness and wear resistance. Applied Surface Science,258(1), 436-441. Man, H. C., Bai, M., & Cheng, F. T. (2011). Laser diffusion nitriding of Ti–6Al–4V for improving hardness and wear resistance. Applied Surface Science,258(1), 436-441. Narayan, J., et al., (1992). Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition. Applied physics letters, 61(11), 1290-1292. Niinomi, M. (1998). Mechanical properties of biomedical titanium alloys. Materials Science and Engineering: A, 243(1), 231-236. Nolan, D., et al., (2006). Sliding wear of titanium nitride thin films deposited on Ti–6Al–4V alloy by PVD and plasma nitriding processes. Surface and Coatings Technology, 200(20), 56985705. Raveh, A., et al., (1990). RF plasma nitriding of Ti6A14V alloy. Thin Solid Films, 186(2), 241256. Roy, M., et al., (2008). Laser processing of bioactive tricalcium phosphate coating on titanium for load-bearing implants. Acta Biomaterialia, 4(2), 324-333. Samuel, S., et al., (2008). Wear resistance of laser-deposited boride reinforced Ti-Nb–Zr–Ta alloy composites for orthopedic implants. Materials Science and Engineering: C, 28(3), 414420. Sathish, S., et al., (2010). Studies on the corrosion and wear behavior of the laser nitrided biomedical titanium and its alloys. Materials Science and Engineering: C, 30(3), 376-382. Schaaf, P. (2002). Laser nitriding of metals. Progress in Materials Science,47(1), 1-161. Schintlmeister, W., et al., (1976). Structure and strength effects in CVD titanium carbide and titanium nitride coatings. Journal of The Electrochemical Society, 123(6), 924-929. Selvan, J. S., et al., (1999). Laser boronising of Ti–6Al–4V as a result of laser alloying with preplaced BN. Materials Science and Engineering: A,260(1), 178-187. Singh, R., et al., Laser Surface Modification of Ti—6Al—4V: Wear and Corrosion Characterization in Simulated Biofluid. Journal of biomaterials applications, 21(1), 49-73. Subramanian, B., et al., (2010). Microstructural, mechanical and electrochemical corrosion properties of sputtered titanium–aluminum–nitride films for bio-implants. Vacuum, 85(5), 601609.
17
Subramanian, B., et al., (2011). Structural and tribological properties of DC reactive magnetron sputtered titanium/titanium nitride (Ti/TiN) multilayered coatings. Surface and Coatings Technology,205(11), 3485-3492. Tobe, R., et al., (2000). U.S. Patent No. 6,080,446. Washington, DC: U.S. Patent and Trademark Office. Unwin, P. S., et al., (1996). Aseptic loosening in cemented custom-made prosthetic replacements for bone tumours of the lower limb. Journal of Bone & Joint Surgery, British Volume, 78(1), 513. Welsch, G., et al., (Eds.). (1993). Materials properties handbook: titanium alloys. ASM international. Wilson, A. D., et al., (1999). A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti–6Al–4V. Surface and coatings technology,114(1), 70-80. Wooley, P. H. and Schwarz, E. M. (2004). Aseptic loosening. Gene therapy,11(4), 402-407. Yilbaş, B. S., et al., (1996). Plasma nitriding of Ti6Al4V alloy to improve some tribological properties. Surface and Coatings Technology, 80(3), 287-292. Yoshida, T., et al., (1979). The synthesis of ultrafine titanium nitride in an rf plasma. Journal of Materials Science, 14(7), 1624-1630. Yue, T. M., et al., (2000). The effects of laser surface treatment on the corrosion properties of Ti6Al-4V alloy in Hank's solution. Journal of materials science letters, 19(3), 205-208. Zhecheva, A., et al., (2005). Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods. Surface and Coatings Technology, 200(7), 2192-2207. Zhecheva, A., et al., (2006). Titanium alloys after surface gas nitriding. Surface and Coatings Technology, 201(6), 2467-2474.
18
Hardness Sample
CP-Ti Untreated 425W 1Pass 425W 2Pass 475W 1Pass 475W 2Pass
Hardness
Wear Rate in DI
Wear Rate in FBS
Increase w.r.t (HV0.1)
CP-Ti
(mm3/Nm)*10-4
(m3/Nm)*10-4
85
-
11.63±0.581
6.424±0.480
1266±344.3
~15x
1.859±0.093
0.108±0.005
1134±331.7
~12x
0.701±0.035
0.262±0.013
962.5±116.4
~11x
1.646±0.082
0.120±0.006
962.5±280.6
~11x
1.018±0.051
0.020±0.001
Table 1: Hardness and Wear Rates results of in situ nitrided CP-Ti.
Sample
Ti4+Concentration in ppm
Ion release decrease w.r.t CP-Ti
CP-Ti
15.595±0.097
Control
425W 1P
3.549±0.013
~4x
425W 2P
1.988±0.029
~8x
475W 1P
3.517±0.031
~4x
475W 2P
0.752±0.006
~21x
Table 2: Ti4+ ion release during 1km wear test in DI water at room temperature measured using ICP-MS techniques. 19
Figure 1: SEM image showing typical composite microstructure and different zones of the in situ formed nitride coating on titanium.
Figure 2: SEM images of typical composite microstructure of dendritic and secondary phase. phase
20
Figure 3: In situ nitrided composite microstructural variation with changing laser power and changing the number of laser surface scans.
21
Figure 4: XRD analysis of in situ nitrided CP-Ti Ti showing the formation of different titanium nitrides.
22
Figure 5: Hardness depth profiles of in situ nitrided CP-Ti Ti samples.
23
(a)
(b) Figure 6: Wear rates of CP--Ti substrate and in situ nitride CP-Ti Ti in (a) DI water at room temperature and (b) in 10% FBS solution at 37°C.
24
(a)
(b) Figure 7: Coefficient of Friction (COF) of the untreated CP-Ti substrate and in situ nitrided CPTi in (a) DI Water at room temperature and (b) in 10% FBS solution at 37°C. 37°C
25
Figure 8: Wear track analysis: (a) typical surface wear damage on the in situ nitrided composite coatings and (b) damage amage at the edges of the wear track track.
Figure 9: Cross section micrograph of the wear tracks (a) Unetched, at low magnification. magnification (b) Etched microstructure showing the approximate microstructure underneath the wear track. track
26
• • •
Laser remelting of Ti in LENS allows for site specific surface modification. Metal ion release can be lowered by modifying Ti surface due to hard coating. Laser remelting resulted high hardness, excellent wear resistance and low COF.
27