Wear mechanisms analysis by Scanning Electron Microscopy of bone- AISI 304ss/Ti-Al-N tribological pairs

Transcription

1 Wear mechanisms analysis by Scanning Electron Microscopy of bone- AISI 304ss/Ti-Al-N tribological pairs A. Esguerra-Arce 1, C. Amaya 2, N. A. de Sánchez 3, J. Muñoz-Saldaña 4, Y. Aguilar 1 and L. Ipaz 1 1 TPMR, Universidad del Valle, Calle 13 No , Cali, Colombia 2 GIDEMP, Centro ASTIN, SENA, Calle 52 No.2 Bis-15, Cali, Colombia 3 GCIM, Universidad Autónoma de occidente, Calle 25 No , Cali, Colombia 4 CINVESTAV, Unidad Querétaro, Libramiento Norponiente 2000 Real De Juriquilla, Querétaro, México Ti 1-x Al x N films are utilized in industrial applications because of their excellent properties [1, 2]. Now, these coatings are calling the attention for biomedical applications. In this paper, friction and wear properties of Ti 1-x Al x N films having various aluminum contents, x, have been studied. Friction was carried out by a pin-on-disk tribometer using an animal bone-pin as counterpart and wear mechanisms were identified by SEM and EDS, which is a powerful tool that combines a high-powered microscope with an energy dispersive spectrometer, making it possible to analyze them. SEM-EDS is a tool for deposits and wear debris analysis, particle sizing and characterization, failure and contaminant analysis and metallurgical studies. It was found that the behavior of friction coefficient versus distance curves depends on the quantity of adhered bone on the disk and this depends on the hardness of the bone and aluminum content. Also, it was found that using Ti-Al-N coatings improved the tribological behavior of 304ss, preventing the wear produced by abrasion. The coating with x = 0.41 exhibited the lowest friction coefficient causing the lowest wear of the bone. Keywords: Biotribology; scanning electron microscopy; coatings; wear 1. Introduction Typical wear mechanisms are surface fatigue, which involves cracking after repeated stressing (it is evidenced by cracks); abrasion, defined as the loss of material due to hard particles that are forced against and move along a solid surface (it is evidenced by scratches, grooves and striations); adhesion which refers to attachment of wear debris and material compounds from one surface to another; and tribochemical wear, evidenced by films or particles produced by chemical reactions, typically oxides [1]. SEM-EDS is a powerful tool that combines a high-powered microscope with an energy dispersive spectrometer, making it possible to analyze them. SEM-EDS is a tool for deposits and wear debris analysis, particle sizing and characterization, failure and contaminant analysis and metallurgical studies. The SEM permits the observation of materials in macro and submicron ranges. The instrument is capable of generating three-dimensional images for analysis of topographic features. When used in conjunction with EDS, the analyst can perform an elemental analysis on microscopic sections of the material or contaminants that may be present. Taking into account that the surface properties govern the tribological material performance, the modification of the surface of biomedical alloys to improve tribological properties could be considered as a solution. Ti-Al-N coatings seem to be an excellent material for this field, due to its excellent properties [2-6]. Now, Considering that the application of this coating onto the surface of implants fixtures needs further investigation, the purpose of this work is to study the effect of atomic aluminum content, x, in Ti 1-x Al x N coatings deposited by reactive magnetron co-sputtering, in the tribological properties, such as friction coefficient and wear mechanisms, using an animal bone as counterpart. 2. Materials and Methods The value of x will be used to show the Al content in Ti 1-x Al x N coatings (x = Al/(Ti+Al)) and the EDS analysis is shown in Fig. 1. Fig. 2 shows that coatings contain B4-Wurtzite AlN, hexagonal Ti 2 AlN and tetragonal TiN Besides these phases, the presence of intermetallic phases is observed: Ti 3 Al for coatings with x = 0.24, TiAl for coatings with x = 0.41 and TiAl 2 for coatings with x = The presence of the B4-Wurtzite AlN phase is incremented by adding of aluminum to the coatings. The roughness and grain diameter (measured by Atomic Force Microscopy), thickness (measured by Scanning Electron Microscopy), hardness and Young s Modulus (measured by nanoindentation) of the coatings and substrate are shown in Table 1. A cortical bone specimen was prepared from a pig humerus obtained from a local market. The bone was immersed in water at 100 C for 2 h to remove soft tissue attached to the bone. In order to prevent changes in the bone s mechanical properties, the humerus was never allowed to touch the heat source while being slowly heated. After this process, the remnant soft tissue attached to the bone was mechanically removed by hand and then the bone was cleaned, dried in ambient air and mechanized to spherical form. To evaluate the tribological properties of the Ti 1-x Al x N coatings, sliding wear tests were carried out by using a MicroTest pin-on-disk tribometer (three replicates: R1, R2 and R3). The tests were performed at a normal load of 10 N using an animal-bone ball with 6 mm in diameter as the wear counterpart. The sliding linear speed and total sliding 936

2 distance were set at 8 mm/s and 400 m, respectively. The wear tracks were studied by EDS and SEM (JEOL JSM-649 OLV). Fig. 1 EDS spectra of the coatings Intensity (A. U.) γ γ γ δ δ δ x = 0.60 η x = 0.41 ω δ x = 0.24 ρ δ 2 Theta Degrees : t-tin 0.61 γ: h-aln δ: h-ti 2 AlN η: h-tial 2 ω: h-alti ρ: h-alti 3 Fig. 2 XRD diffractograms of the coatings 937

3 Table 1 Characteristics of the samples Sample Hardness (GPa) Young s Modulus (GPa) Roughness (nm) Grain diameter (nm) Thickness (nm) 304ss 5.3 ± ± ± 2 x = ± ± ± 5 85 ± x = ± ± ± 2 53 ± x = ± ± ± 4 80 ± Bone 6.0 ± ± 2 3. Results 3.1 Friction coefficient A comparison of the coated specimens regarding the relationship between the friction coefficient (COF) and the travel distance is shown in Fig. 3. It can be seen that all coated specimens exhibit a behavior as shown in Fig. 4-b, formed by two stages: a maximum friction coefficient and a steady state, which value is 0.57 ± 0.03 (ANOVA, p < 0.05) and it is exhibited by all the specimens, except by replica 2 of the film with x = 0.60, which conserves its maximum COF value. From the curves it can be noted that the length of the region of maximum friction coefficient (stage 1) is higher for coating with x = 0.24, followed by coating with x = 0.41 and minimum for coating with x = 0.60 (except by replica 2). From the Fig. 5, it is evident that the length of this region, inside each group, is related to the hardness of the bone (remember, the bone is a heterogeneous material; therefore, the hardness of the pin depends on the face showed to the disk [7]): the harder the bone pin, the longer the stage 1. Besides this, from SEM images it can be noted that the quantity of adhered material is related to the length of stage 1: longer regions of maximum COF implies a lower amount of adhered bone. This would explains the behavior exhibited by replica 2 of the film with x = 0.60, since it does not reach the steady state at COF = 0.57 ± 0.03, but conserved the maximum COF, and the adhered bone on the surface is negligible. So, it is not unreasonable to suppose that the maximum value of COF (stage 1) corresponds to the friction coefficient between bone and coated substrates tribological pair, i.e. corresponds to the direct interaction between the asperities of both the pin and coated substrate. While the COF exhibited at steady state (stage 2) corresponds to the coefficient of friction between the bone attached to the disk surface and the bone pin (0.57 ± 0.03). Coating with x = 0.41exhibits the lower value of maximum COF (0.76 ± 0.01). The maximum COF values exhibited by the films with x = 0.24 and 0.60 are 0.84 ± 0.04 and 0.82 ± 0.03, respectively. It was found that there was no correlation between maximum COF and hardness of the counterparts. However, it was found a correlation with the roughness of the harder counterpart, i. e. the disks, and the initial contact area (Fig. 6): coatings with higher values of average roughness exhibit higher friction with the bone, and higher initial contact areas cause higher friction between the counterparts. It should be noted that initial contact area is function of normal load, radius of the bone pin and the stiffness of the counterparts, i. e. the reduced elastic modulus [8]. About the uncoated substrate, it is observed the presence of two maximum peaks before the steady state is reached, as is represented in Fig. 4-a. Observing that the uncoated substrate suffered abrasion wear (as can be seen from electronic microscopy in Fig. 7), it might think that the stage 1 corresponds to the asperity interaction between bone and uncoated substrate; the stage 2 corresponds to the abrasive wear and the stage 3 to the bone attaching. The steady stage coefficient of friction value is 0.55 ± 0.02, which is statistically equal to the coefficient of friction value found at the steady stage of coated substrate-bone tribological pairs. The minimum quantity of bone adhered to the surface of the replica 1 (Fig. 3) is also related to the more pronounced presence of stages 1 and 2, although there is no correlation between the length of these stages and the hardness of the replicas, because those values are statistically equal. The average COF related to the asperity interaction between bone and uncoated substrate (stage 1) is 0.69 ± 0.08 (it is observed more variability in data). Having into account that the friction coefficient is an important value in biomaterials designs, since the friction coefficient affects the relative micromotion in the biomaterial-bone interface (for example, increasing the friction coefficient from 0.5 to 0.9, the peak micromotion between cup-pelvis pair could be reduced by 20.8%), it is observed that covering the substrate with Ti-Al-N with x = 0.24 and 0.60 performs this property, rising slightly the COF from 0.69 to 0.84 ± 0.04 and 0.82 ± 0.03, respectively. Traditional titanium coated implants presents COF values ranging in the range of

4 Fig. 3 Friction coefficient curves and wear track of the samples Fig. 4 Behavior of friction coefficient versus sliding distance for uncoated substrate-bone tribological pair (a) and coated substratebone tribological pairs (b) 3.2 Wear mechanisms The SEM micrographs shown in Fig. 7 present the wear mechanisms of the disks. For 304ss, a bony film covers a great part of the entire surface (related to adhesion wear mechanism) and ploughing tracks (related to abrasion wear mechanism) are observed. Despite the lesser value of hardness exhibited by the bone compared with the 304ss, the ploughing lines could be explained by a sporadic stress concentration that could remove some particles of 304ss. This particle could initiate a cascade of wear, because this particle suffers work hardening as a consequence of plastic deformation. So, the hardened particle can wear the 304ss surface and produce more particles of 304ss and the ploughing lines. In the case of Ti 1-x Al x N covered 304ss the EDS measurements did not show delamination, and nor ploughing tracks are observed. Bone adhesion was observed in all systems. The bone pins wear is because the disks are harder than the bone and the adhesion to the disk is explained because the COF between the bone and the disks is higher than the COF between bone-bone. In the case of 304ss/(Ti 0.40 Al 0.60 )N-R2 system partial delamination is observed, which results from loading and unloading of the surface, at a stress level in the material that it can sustain once but not if repeated many times. This mechanism forms the origin for cracking and liberation of surface material to form wear debris. The 304ss/(Ti 0.75 Al 0.24 )N bone and 304ss/(Ti 0.40 Al 0.60 )N bone tribological pairs debris analysis, shown in Fig. 8, corroborate the presence of bony particles alone. No presence of 304ss or Ti-Al-N debris was observed. 939

5 60 Hardness (Vickers 1 kg) R1 R2 R3 R1 R2 R3 R1 R2 R1 R2 R3 304ss x = 0.25 x = 0.41 x = 0.60 Sample Fig. 5 Hardness of the pin used as counterpart of the samples Fig. 6 Relationship between maximum COF and the aluminum atomic content (x) compared to the relationship between coating roughness and initial contact area and the aluminum atomic content (x) The pins used as counterpart of the film with x = 0.41 present similar diameter values of the tracks (x = 0.41-R1, 2.26 ± 0.17 mm; x = 0.41-R2, 2.07 ± 0.17 mm). Nevertheless, in the case of uncovered substrate, the pin used as counterpart of the replica 2 presents a diameter track of 1.56 ± 0.12 mm, which is smaller than observed in replicas 1 and 3 (R1, 2.33 ± 0.21; replica 2, 2.51 ± 0.06 mm). This behavior is probably related to the fact that replica 2 of the substrate presents the shortest length of stages 1 and 2, related to a direct contact between bone and disk. A similar behavior is observed in 304ss/(Ti 0.40 Al 0.60 )N system, when the pin used as counterpart of replica 2 presents a higher diameter track, of 3.04 ± 0.14 mm, that the others replicas (R1, 2.10 ± 0.04 mm; R3, 2.25 ± 0.09 mm). In the case of 304ss/(Ti 0.75 Al 0.24 )N system, the pin used as counterpart of replica 2 presents a very much higher diameter track, 3.56 ± 0.20 mm, than the exhibited by the other two replicas (R1, 2.20 ± 0.13 mm; and R3, 2.13 ± 0.10 mm), behavior also related to the higher length of stage 1 observed in COF versus distance curve for replica

6 Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) Fig. 7 represents the specific wear rate of the animal-bone used as counterpart of the disks as function of x; this behavior is compared with the COF behavior as function of x. As it is observed, there is correlation between bone wear and COF: at higher COF greater wear rate. The pin wear rate reported by the uncovered substrate is mm3/n m, statistically equal to the value reported by the films with x = This value reported by the uncovered substrate, despite its lesser value of hardness, may be related to the steel abrasive particles, which cause a more pronounced wear to the bone-pin. Fig. 7 Wear mechanisms of the samples Fig. 10 shows SEM images of the wear tracks of the pins used as counterparts to the films with x = 0.24-R1, x = 0.41-R2 and x = 0.60-R2. It is observed the formation of cracks and their propagation. These propagated cracks unite with other cracks and liberates surface material [1]. It is observed a corrugated surface, which corroborates the fatigue as the mechanism of wear of the bone-pins. This corrugated surface is more pronounced in the pin used as counterparts to the films with x = 0.60-R2, which exhibits the greater mass loss and which COF versus distance curve exhibits the longer stage

7 Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) Fig. 8 Debris produced by 304ss/(Ti0.76Al0.24)N and 304ss/(Ti0.40Al0.60)N-bone system Fig. 9 Pins wear rate as a function of aluminum content in the coatings Fig. 10 Wear mechanisms of the pins 4. Conclusions From this study it can be concluded that tribological interaction between bone and a biomaterial depends on mechanical properties of the bone -which depends on its anisotropy-, roughness of the biomaterial and its mechanical properties. Bone is capable of abrade AISI 304 stainless steel, and using Ti-Al-N coatings deposited onto its surface it improves the 942

8 tribological behavior of 304ss, preventing the presence of particles produced by the abrasion wear mechanism. The coating with x = 0.41 exhibits the lowest coefficient of friction, the most prolonged bony adhesion stage and causes the lowest wear of the bone, been a good candidate for using as a biomaterial. References [1] Holmberg K and Matthews A. Coatings Tribology Properties, Mechanisms, Techniques and Applications in Surface Engineering. Tribology and Interface Engineering Series. 2a ed. Elsevier 2009; 56 [2] Wang L, Su JF, Nie X. Corrosion and tribological properties and impact fatigue behaviors of TiN- and DLC-coated stainless steels in a simulated body fluid environment. Surface and Coating Technology. 2010; 205: [3] Liu ZJ, Shum P, Shen Y. Hardening mechanisms of nanocrystalline Ti Al N solid solution films. Thin Solid Films. 2004; 468: [4] Xuechao Li, Changsheng Li, Ye Zhang, Hua Tang, Guowei Li, Chaochao Mo. Tribological properties of the Ti Al N thin films with different components fabricated by double-targeted co-sputtering. Applied Surface Science. 2010; 256: [5] Ipaz L, Caicedo JC, Esteve J, Espinoza-Beltrán FJ, Zambrabo G. Improvement of mechanical and tribological properties in steel surfaces by using titanium aluminum/titanium aluminum nitride multilayered system. Applied Surface Science. 2012; 238: [6] Ipaz L, Esguerra-Arce A, Aperador W, Espinoza F, Ruiz H. Nanofriction study using atomic force microscopy (AFM) of multilayers based in titanium, chromium and aluminum. In Méndez-Vilas A, editor. Current Microscopy Contributions to Advances in Science and Technology (Microscopy book series) p [7] Baumann AP, Deuerling JM, Rudy DJ, Niebur GL, Roeder RK. The relative influence of apatite crystal orientations and intracortical porosity on the elastic anisotropy of human cortical bone. Journal of Biomechanics. 2012; 45: [8] Stachowiak GW and Batchelor AW. Engineering tribology. 2nd ed. Butterworth Heinemann;