Narendra B. Dahotre

Review paper: Surface Modification for Bioimplants: The Role of Laser Surface Engineering

Often hard implants undergo detachment from the host tissue due to inadequate biocompatibility and poor osteointegration. Changing surface chemistry and physical topography of the surface influences biocompatibility. At present, the understanding of biocompatibility of both virgin and modified surfaces of bioimplant materials is limited and a great deal of research is being dedicated to this aspect. In view of this, the current review casts new light on research related to the surface modification of biomaterials, especially materials for prosthetic applications. A brief overview of the major surface modification techniques has been presented, followed by an in-depth discussion on laser surface modifications that have been explored so far along with those that hold tremendous potential for bioimplant applications.

Elevated Temperature Coatings: Science and Technology IV

Oxidation resistance and thermal barrier coatings for components on the hot section of gas turbine engines are desired to have lifetimes on the order of tens of thousands of hours. This presents a problem in evaluating new coatings and modifications to existing coatings tests, which completely replicate the operating conditions, could take years to complete. Therefore, a reliable accelerated testing protocol is required. In this paper efforts directed toward developing a mechanism-based protocol for evaluating the life times of oxidation resistant coatings under thermal cyclic and hot corrosion conditions and thermal barrier coatings under thermal cyclic conditions is described. The cyclic lifetimes of oxidation resistant and thermal barrier coatings are determined by spalling behavior. Spallation is a function of oxide thickness and stress level, which control the elastic energy available to drive spallation, and the structures and morphologies of the various layers and interfaces in a given system, which control the fracture toughness at possible planes of weakness. Efforts to evaluate these quantities in relatively short duration tests are described. Specific techniques include acoustic emission studies, indentation techniques, and detailed metallographic observations. The extrapolation of results from high temperature tests, where failure can be achieved in relatively short times, to lower temperatures, which are characteristic of service conditions is also described. An approach to control these variables in a manner to produce accelerated failures under conditions, which allow estimation of lifetimes under typical operating conditions, are described and preliminary results are presented.

Laser surface engineered TiC coating on 6061 Al alloy: microstructure and wear

Abstract Hard and refractory TiC has been deposited on 6061 Al alloy by Laser Surface Engineering (LSE). A “composite” coating is obtained with TiC particles of various shapes and sizes embedded in Al alloy–Ti matrix. The coating is uniform, continuous and free of cracks. The various reactions occurring during laser processing were thermodynamically analyzed and related to the experimental observations. Microhardness measurements suggested high hardness values in the coating region and a strong bonding at the coating/substrate interface. Dry sliding wear tests were performed to measure the wear resistance and the coefficient of friction of the coating. Wear resistance of the coated surface was found to be high when compared to the substrate side. The coefficient of friction was found to be 0.64.

Laser surface engineering of steel for hard refractory ceramic composite coating

Laser surface modification technique is applied to deposit ultrahard ceramic (TiB2) coating on 1010 steel. A uniform, continuous and crack free coating with a metallurgically sound interface is obtained. Coating is ‘‘composite’’ in nature comprising TiB2 particles and Fe in the laser melt zone. Polygonal and needle shaped boride particles are uniformly distributed in the laser melted zone. The interfacial microstructure consists of cellular dendrites and fine equiaxed dendrites. Metastable phase(s) such as FexBy and TimBn are also observed which is a characteristic feature of non-equilibrium synthesis by laser energy. Laser melt zone has a high hardness. However, hardness is not uniform in the layer. ” 1999 Elsevier Science Ltd. All rights reserved.

Synthesis of Boride Coating on Steel using High Energy Density Processes: Comparative Study of Evolution of Microstructure

Abstract Ultrahard titanium diboride (TiB2) coatings are deposited on plain carbon steel substrate using two high energy density processes, viz. pulsed electrode surfacing (PES) and laser surface engineering (LSE). These two processes are entirely different in physical nature and hence result in dissimilar microstructures. In the present investigation, a comparative study has been made between the evolved microstructures. Both processes produced a surface layer which is adherent and metallurgically bonded to the substrate. PES produced relatively thinner and less uniform coating than LSE process. The PES coating was, however, homogeneous and very fine grained. The laser-assisted coating was “composite” in nature with TiB2 particles embedded in Fe matrix. Mechanical characterization of these coatings has been performed using microhardness measurements.

Pulse electrode deposition of superhard boride coatings on ferrous alloy

Abstract Pulse electrode surfacing (PES) is an effective process for depositing ultrahard, corrosion- and erosion-resistant ceramic coatings on metals. In the present work, a titanium diboride coating deposited on 1018 steel using PES technique has been investigated. An attempt has been made to correlate the thermodynamic predictions and experimental observations. The role of the ceramic–metal interface in the development of these coatings has been studied. A thick, adherent, metallurgically bonded and crack-free coating is obtained. It has been found that Fe intermixes with TiB 2 layer and acts as an excellent binder for TiB 2 . It promotes the composite effect of both hardness and toughness in these coatings and, hence, provides a crack-free coating with the sound interface.

Wettability and kinetics of hydroxyapatite precipitation on a laser-textured Ca-P bioceramic coating.

Surface-textured calcium phosphate coatings at four different length scales were synthesized on titanium-based alloys using a pulsed Nd:YAG laser system by a direct melting technique. The textures were obtained by varying the laser spot overlap with a change in laser traverse speed. Surface roughness measurements of the textured coatings carried out using a white light interferometer indicated a decrease in roughness with increasing laser scan speed. Wettability of the coated samples measured using a static sessile drop technique demonstrated an increased hydrophilicity with increasing laser scan speed. The influence of such textures and the associated surface roughness on the precipitation kinetics of hydroxyapatite (HA) during immersion in simulated body fluid (SBF) was the prime focus of the present paper. The mineralized samples obtained after immersion in SBF were characterized using X-ray diffraction, energy-dispersive spectroscopy and scanning electron microscopy to understand the kinetics of HA precipitation. The results thereafter confirmed that the precipitation kinetics of HA was strongly modulated by the varying surface roughness.

Crystallographic and morphological textures in laser surface modified alumina ceramic

Laser surface modification is an advanced technique for improving the surface performance of alumina ceramics in refractory and abrasive machining applications. Surface performance is expected to be greatly influenced by the crystallographic and morphological textures of surface grains generated during rapid solidification associated with laser processing. In this study, an investigation of the evolution of crystallographic and morphological textures during laser surface modifications of alumina ceramic was carried out using a 4kW Nd:YAG laser with fluences in the range of 458–726J∕cm2. In these regimes of laser surface processing, the formation of equilibrium α-alumina was found to be assisted by catalytic sites provided by the substrate. Microstructure evolution was explored in terms of the development of crystallographic and morphological (size and shape) textures of surface grains as a function of laser processing parameters. The interdependence of crystallographic and morphological textures of the su...

Laser Surface Modification of Ti—6Al—4V: Wear and Corrosion Characterization in Simulated Biofluid

Laser surface melting (LSM) of Ti—6Al—4V is performed in argon to improve its properties, such as microstructure, corrosion, and wear for biomedical applications. Corrosion behavior is investigated by conducting electrochemical polarization experiments in simulated body fluid (Ringers solution) at 37 C. Wear properties are evaluated in Ringers solution using pin-on-disc apparatus at a slow speed. Untreated Ti—6Al—4V contains α+β phase. After laser surface melting, it transforms to acicular α embedded in the prior β matrix. Grain growth in the range of 65—89 µm with increase in laser power from 800 to 1500 W due to increase in associated temperature is observed. The hardness of as-laserprocessed Ti—6Al—4V alloy is more (275—297 HV) than that of the untreated alloy (254 HV). Passivation currents are significantly reduced to <4.3 µA/cm2 after laser treatment compared to untreated Ti—6Al—4V (≈12 µA/cm2). The wear resistance of laser-treated Ti—6Al—4V in simulated body fluid is enhanced compared to that of the untreated one. It is the highest for the one that is processed at a laser power of 800 W. Typical micro-cutting features of abrasive wear is the prominent mechanism of wear in both untreated and as-laser-treated Ti—6Al—4V. Fragmentation of wear debris assisted by microcracking was responsible for mass loss during the wear of untreated Ti—6Al—4V in Ringers solution.

Variation of structure with input energy during laser surface engineering of ceramic coatings on aluminum alloys

Abstract Surface modification of metal alloys using laser has become a unique tool to reduce surface related failure mechanisms such as wear, corrosion, erosion or high temperature oxidation. Laser surface engineered (LSE) ceramic coatings have been proved to enhance surface properties of Al alloys such as hardness and wear resistance. This technique has been shown to be capable of producing a wide variety of interesting metallurgical microstructure in the coating as well as in the adjoining substrate. These microstructures provide novel properties, which cannot be produced by any conventional processing technique. In addition, these coatings are metallurgically bonded, thus providing a sound and adherent interface between the coating and the substrate. In this present investigation, laser surface engineering technique has been employed to deposit ceramic (TiC) coating on aluminum alloy substrate. TiC coating was deposited on two types of aluminum substrates, alloy 2024 and 6061 using an Nd-YAG laser beam. The effect of laser processing parameters, such as power intensity and speed on the thickness, microstructure and morphology of both the coating and the heat-affected zone have been evaluated using a scanning electron microscope (SEM). Results of experiments in this study show that by controlling the process parameters it is possible to produce varied microstructures according to the surface requirement of the application.