Question

Examine the following abstract:

“In this study, we investigate the behavior of new nano ZrO2 and Al2O3-13 wt.% TiO2 thermal sprayed coatings on commercially pure (cp)-Ti (grade 4) and titanium alloy substrates. Friction and wear tests against Al2O3 balls showed that the wear resistance of Al2O3-13 wt.% TiO2 is better than that ZrO2 coating. Both plasma sprayings have similar abrasive wear behavior; however, the average friction coefficient is higher for alumina-titania coating. Electrochemical tests, open circuit potential monitoring and potentiodynamic polarization, were performed in simulated body conditions (Hank's solution, 37 degrees C). Results showed that corrosion resistance was appreciably higher for alumina-titania coating.” INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS 28(1):115-120 Jan 2010

a) What is the purpose (purposes) of these nano ZrO2 and Al2O3-13 wt% TiO2 coatings on CP Ti and Ti alloys?

b) If these materials were used as a femoral stem, would the results of wear tests be important? What if these were coating for the femoral head?

c) What would be the ideal, long-term outcome of this coated Ti when used as a femoral stem? Please describe the key features of the implant that are critical to its success. (Be sure your answer includes responses to the following questions: What is its physical shape, does it have porosity or roughness, how do you intend the tissue around the implant to interact with the implant?)

Answer 1

Alumina (Al₂O₃) and Titania (TiO₂) ceramics are the most popular materials used for plasma spray coating of machine components in polyester manufacturing sector. The selection of the coating material directly depends on the application. Al₂O₃ is corrosion resistant and is mostly used on mating surface to resist abrasive wear and adhesive wear. TiO₂ is being increasingly used as a thermal barrier coating especially in textile/polyester/man- made fiber applications. Effective ceramic coating exhibits low thermal diffusivity, strong adherent to the substrate, phase stability and thermal shock resistance during thermal cycling and provides oxidation wear and corrosion protection to the substrate. Al₂O₃ceramic is stable with less solubility and shows good corrosion resistance but possesses less toughness. Therefore, it is beneficial to choose ceramic composites rather than individual ceramics. The use of Al₂O₃ composite rather than individual Al₂O₃ has certain advantages. Ramachandran et al. [1] stated that TiO₂ has a lower melting point and effectively binds alumina grains, contributing to high density. Further, studies, particularly in optimizing the critical input on parameters of Al₂O₃/TiO₂ coatings in various applications were identified and analyzed in detail. The effects of various parameters and the final coated surface properties of some of the oxides were also studied in depth, for the last one decade.

SS304 has high chromium content, to the range of 18% to 20%, commonly supplied in the form of bar or rolled condition. It can be flame or induction hardened to produce a high surface hardness with excellent wear resistance for an alloy steel grade. Applying a harder material as a thin coating on an SS304 steel surface can provide superior protection against abrasive wear and can be used effectively in the case of bearing seating applications. Addition of TiO₂ in the range of 3%, 13% and 40% to alumina powder is widely used for ceramic coating applications using thermal spray process. Increasing the TiO₂ content in the sprayed powder leads to a decrease in the melting temperature of the Al₂O₃-TiO₂ coating and has a linear tendency to diminish the porosity and increasing the fracture toughness of coating. The percentage of porosity of the 40% TiO₂ mixtures is lower than other compositions like 97/3 and 87/13. This clearly justifies the use for Al₂O₃-40% TiO₂ for the current experiment.

Despite increased interest in the fundamentals of plasma spraying there is still a lack of reliable models that relate engineering properties of coatings, such as hardness or roughness, to variations in process parameters or deposition geometry. Due to extremely rapid cooling after coating, the surface properties of plasma sprayed oxides are not necessarily the same as those for non-sprayed items, which make the scenario more complex. This gap between the need to understand a process to optimize it and a growing demand for good plasma sprayed coatings can be filled temporarily by various engineering analysis approach.

A number of researchers worked on different types of special coatings on various types of substrates which are important to manufacturing processes. Yong et al. [2] studied the coating degradation mechanisms of AlSi coated boron steel after the hot bending process. It was concluded that the bending deformation affected the coating layer behavior the most. Parisa et al. [3] studied the erosion performance of laser cladded Ni-60% WC coatings subjected to a controllable Abrasive Water Jet (AWJ). The chemical composition of coatings was modified by nanocrystalline WC powder and the rare earth element (La₂O₃). The tribological evaluation of the erosion scars showed a log-linear relationship between coating hardness and volume loss under erosion. Zalnezhad et al. [4] had conducted an optimization study on the parameters of titanium nitride coating on aerospace Al7075-T6 alloy, using magnetron sputtering technique. The effects of the temperature, DC bias voltage, rate of nitrogen, and DC power on the surface hardness, adhesion, surface roughness, and microstructure of the coated samples were investigated. Taguchi optimization method was used with the L₁₆ orthogonal array.

Zalnezhad et al. [5] coated Titanium Nitride (TiN) on aerospace Al7075-T6 in different conditions using PVD magnetron sputtering technique, and the surface hardness of TiN-coated specimens was measured using a micro hardness machine. A fuzzy logic model is offered to predict the surface hardness of TiN coating on AL7075-T6 with respect to changes in input process parameters, Direct Current (DC) power, DC bias voltage, and nitrogen flow rate. Toko et al. [6] performed the sensitivity analysis in the process for Mg/Al cladding model in order to evaluate the influences of extrusion parameters on the coating thickness uniformity. The sensitivities of initial thickness of the coating material plate, extrusion temperature, ram speed, die angle, and ratio of simulated flow stress to the experimental one for pure Al were evaluated. Results had shown that the initial thickness of the coating material plate and die angle influence on the uniformity most. Amir et al. [7] studied by modeling the coating characteristics of yttria-stabilized zirconia. The properties such as deposition efficiency, adhesion strength, surface roughness, and hardness in plasma spray process are studied in detail. Binu et al. [8] examined the characteristics of multilayer Ti, TiN, and Diamond-Like Carbon (DLC) coatings deposited on standard tool substrates at varying sputtering parameters and conditions, such as power density, partial pressure, substrate temperature, and reactive gases. The results indicated that a graded multilayer coating showed better adhesion to the substrates. Bor et al. [9] proposed a method to develop a robust Partially Stabilized Zirconia (PSZ) performance for the plasma spraying process with applications of surface response methodology and fractional factorial experiment. Experimental results showed that a quadratic model with the proposed two-step design make it a simple, effective, and efficient way to a robust process. Weiming et al. [10] presented Computational Fluid Dynamics (CFD) simulation for analyzing fluid flow patterns and heat transfer stimulations of plasma spray gun. It was concluded that the optimal velocity and direction of cooling water, which efficiently cools the nozzle improves the service life of the plasma jet. Azarmi et al. [11] introduced an advanced production technique for manufacturing foam core sandwich structures with high temperature constituents. It is a rapid technique which eliminates joining process. It was concluded that there is a good adhesion between skin and core due to mechanical and metallurgical bonds.

The techniques of fixation of implants used in total hip arthroplasty and the factors governing their longevity have been a subject of controversy and discussion over the last few decades. The results of long term follow ups of cemented implants and the mid-term results of uncemented devices have brought into focus the factors that cause a total hip replacement to fail. Fracture, wear and corrosion serve as primary causes. ^[1] It is not possible however to predict the time course of these failures. Judicious design and implementation of prostheses can serve to extend the functional life of a joint even with ongoing wear and corrosion. With the population undergoing arthroplasty becoming younger, orthopaedic research has to focus on newer bearing alternatives and fixation options to provide a durable and longer lasting hip arthroplasty.

In this paper, a porous femoral stem with two prominent properties is introduced; lower modulus of elasticity and biological fixation. The femoral stem is designed by CATIA V5R19. CATIA is powerful software for industrial designing and tetrahedral 10 node element was used for analysis. It supports multiple stages of product development. In this research, all design parameters of ideal femoral stem are collected. These elements include: proper shape for prosthesis, femoral stem geometry and adaptable cross-section, optimum length range, determining relevant offset, and correct angles. All these data are given to the software to design ideal porous femoral stem.