Experimental investigations on ColdSprayed Ti-based Coatings for bioImplant applications

Abstract

As per the published data, the orthopedic implant market is growing exponentially. The life of implants is expected to be up to 15-20 years, depending upon various factors such as gender, age, and patient's body mass index. Hence, it is crucial to enhance the service life of orthopedic implants, which may further enhance the comfort of the implant users by delaying the implant replacement surgeries. Corrosion and wear contribute significantly to the degradation of bio-implants. Released ions and worn-out material (wear debris) leave toxic effects on the human body and promote several diseases such as Alzheimer's and Neuropathy. Therefore, the manufacturing of bio-implants is crucial. SS316L is a biomedical steel having excellent mechanical, good corrosion, and wear properties and is widely used for bio-implants manufacturing. The manufacturing of implants is a two-step process that includes manufacturing base parts and surface modification of these parts to improve their surface characteristics. Corrosion and wear largely depend on the surface properties of the bio-implants. The Food and Drug Administration U.S. has approved thermal spray coatings for implant surface modifications. However, the high temperature involved in thermal spraying causes changes in the material's original characteristics. In this regard, the cold spray process has been considered very promising for depositing temperaturesensitive materials on SS316L. Cold spraying has many advantages over thermal spraying, such as no change in phase, no oxidation, and better mechanical properties due to low processing temperature. A plenty of research work has been published on surface preparation in cold spraying. There are studies regarding substrate roughness effect on bonding in cold spray, however most of these are for the cold spraying of softer materials on soft/hard substrates. Therefore, a need was realized to understand the effect of surface preparation for cold spraying hard particles on hard substrate. Cold spraying of titanium-based powders on the SS316L substrate with different level of surface roughness is unavailable. In the current work, single-pass Ti/20TiO2 composite powder was cold-sprayed on SS316L steel substrates having three different roughness levels to understand the effect of substrate roughness on the coating adhesion. Ti/20TiO2 coated substrates were analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and scratch tester. The mirror-polished (MP) was found to be the most suitable surface condition to deposit the chosen Ti-based powders on SS316L steels with a good adhesion. This may be attributed to adequate Ti-particle deformation, which led to proper jetting formation and afterward good adhesion. Therefore, titanium-based powders in distinct compositions have been deposited on MP SS316L using highpressure cold spray. Two types of reinforcement’s viz. TiO2 and baghdadite (BAG) were used. BAG is used to explore it as a replacement for mechanically weaker hydroxyapatite (HA). The deposited coatings were analyzed using SEM, EDS, and Xray diffractometer (XRD). Furthermore, the coated and uncoated SS316L steel were characterized for porosity analysis, density calculation, surface roughness measurement, microhardness measurement, wettability, and biocompatibility. Corrosion and sliding wear behavior of the cold spray coated and uncoated SS316L steels were studied under a dry and simulated body fluid environment. Among the cold-sprayed Ti/TiO2 coatings, Ti/20TiO2 coating exhibited a relatively rough, hard, and dense surface in comparison with the uncoated SS316L surface, the average surface roughness, microhardness, and density were found to decline with the increase in TiO2 content. Electrochemical corrosion studies of the Ti/TiO2 composite coatings revealed that all the coatings reduced the corrosion of the substrate. However, corrosion performance dropped with the increase in TiO2 content. Dry sliding wear results indicated that only Ti/20TiO2 composite coating was successful in reducing the wear losses of SS316L steel. Superior microhardness and better retention of TiO2 in Ti/TiO2 coating have been ascribed as the reasons for its superior performance. All the other Ti/TiO2 coatings failed to protect the substrate against sliding wear. In general, abrasion wear was recognized as the dominant mechanism of wear for these coatings. Signatures of adhesive wear mechanisms were also observed over the worn-out coating surfaces. From the combined results of corrosion and sliding wear, Ti/20TiO2 coating can be recommended as a potential candidate to reduce the corrosion and wear of SS316L steel. For the cold-sprayed titanium/baghdadite (Ti/BAG) coatings, the average density and surface roughness of the coatings were observed to reduce with the increase in BAG content. The average microhardness and scratch resistance of Ti/BAG coatings increased up to 15% of BAG content, and beyond that the both start declining. Coldsprayed Ti/BAG coatings were found hydrophilic in nature, and the hydrophilicity was seen improving with the increase in BAG content. The coatings showed good cell viability, which was found to enhance with BAG content. The electrochemical corrosion tests were performed for 2 hours (hr), 16 hr, and 40 hr of immersion times in simulated body fluid environment. All the Ti/BAG coatings successfully reduced the corrosion rates for all the cases of immersion times. Moreover, the corrosion resistance was found to increase with the increase in BAG content as well as immersion time. Formation of passive oxide (TiO2) and dissolution of BAG into CaO, SiO2, and ZrO2 have been ascribed as the reasons for their excellent corrosion performance in simulated body fluid environments. The corrosion rates of the Ti/BAG composite coatings were compared with Ti/HA coatings, which established the Ti/BAG as a better option for the given environment. Sliding wear tests were performed following ASTM G99 standards. It was observed that Ti/15BAG composite coating performed best among all the investigated coatings against sliding wear under the dry as well as the simulated body fluid environment. The superior performance of Ti/15BAG coating could be attributed to a better combination of its microhardness and scratch resistance. Signatures of micro-cutting, micro-cracks, delamination, and material transfer were observed on the worn-out Ti/BAG coatings, indicating abrasive and adhesive wear as the sliding wear mechanism. Once again, Ti/BAG coatings were found to be superior to the reported Ti/HA coatings in terms of wear rate. Based upon the overall results, Ti/15BAG composite coating can be recommended as the best choice to reduce the corrosion and wear of SS316L steel in dry as well as simulated body fluid environments. Moreover, Ti/15BAG composite coating was found to perform better than Ti/20TiO2 coating against corrosion and wear. Laser surface melting was performed to eliminate the pores and improve the mechanical properties of as-sprayed coatings. Laser treatment of the coatings led to the recrystallization of Ti in deposited coatings without any adverse effect on the substrate. Equiaxed grains formed in the top layers of laser-treated Ti/BAG coatings, improved the density and microhardness of the as-sprayed coatings. All the laser-treated coatings were subjected to electrochemical corrosion tests in the simulated body fluid environment for 2 hr of immersion time. It was observed that the laser treatment improved the corrosion resistance of Ti/BAG coatings as compared to the respective assprayed coatings. Reduction in porosity and improvement in density are believed to be the reason for their better corrosion behavior in simulated body fluid. The sliding wear performance of the laser-treated coatings was also better than these as-sprayed coated counterparts under the simulated body fluid environment. The enhanced microhardness of laser-treated Ti/BAG coatings has been ascribed as the reason for their better sliding wear resistance. The signatures of micro-cutting, delamination, and material transfer were seen on the surfaces of the worn-out laser-treated coatings, however, these were relatively less severe than that of the respective as-sprayed coatings. Abrasion and adhesion wear were recognized as the main mechanism of wear. Among the lasertreated coatings, the Ti/15BAG coating was found to be the best to control corrosion and wear. Moreover, for the tribological joints of the bio-implant, laser-treated coldsprayed Ti/15BAG coating can be recommended as a better choice since porosity is not a major concern at these joints.