Hydroxyapatite coating on stainless steel 316L using investment casting technique

Decades of research had found that the use of hydroxyapatite (HA) could promote osseointegration and increase the mechanical stability and strength of the metallic implant. Currently, medical implants are produced by machining, metal forming and casting particularly investment casting technique. Comparatively, the investment casting technique is more practical to mass produced implant due to its simplicity, superior surface finish, relatively cheap process, capability to produce complex and near-net shape implants. Investment casting technique provides a single stage processing technique in producing coated implant i.e. HA coated on the cast implant. In this respect, the possibility of applying HA layer during casting for implant substrate using investment casting was explored. Previous studies used paint brush in applying HA layer onto the internal cavity of the ceramic mould. However, the layer thickness was inconsistence which affects the quality of the HA layer coated on the casting and impractical for small and complex shapes components. In this study, medical grade 316L stainless steel (316L-SS) was coated with HA using investment casting technique by pouring molten 316L-SS into a HA coated ceramic mould at temperature of 1650?C in argon gas. The coated samples were sintered in a furnace at four different temperatures (600, 800, 1000 and 1200?C) for 1 hour. The as-cast and sintered HA coated samples were characterised using scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray diffraction analysis. Results showed that the as-cast samples produced good HA coating bond, formed amorphous phases and complex calcium-chromium oxide (CaCrO) layer at the interface between HA and 316L-SS. Sintering process was conducted to recrystallise and improve the properties of the as-cast HA coating. The results confirmed that crystallinity and purity of the coating increased with increasing sintering temperature whilst Ca/P ratio and porosity decreased. Increasing sintering temperature from 600 to 800ºC did not significantly alter the crystallinity and purity. The crystallinity and purity recorded at that temperature range were 57.35% - 58.55% and 61.80% - 63.21% respectively. This temperature range was considered insufficient to recrystallise and purify the coating to an acceptable value for implant applications. Increased of sintering temperature from 1000 to 1200ºC increased the crystallinity from 73.52% - 74.47% and purity from 61.80% - 81.8%. Simultaneously, the Ca/P ratio and porosity were reduced to 1.51 and 14.14% respectively which is acceptable to human body. Sintered as-cast specimen at 1000ºC immersed into Simulated Body Fluid (SBF) solution showed increased in Ca/P ratio with increasing immersion time indicating that the coating was bioactive

Fabrication of low-cost, cementless femoral stem 316L stainless steel using investment casting technique

Total hip arthroplasty is a flourishing orthopedic surgery, generating billions of dollars of revenue. The cost associated with the fabrication of implants has been increasing year by year, and this phenomenon has burdened the patient with extra charges. Consequently, this study will focus on designing an accurate implant via implementing the reverse engineering of three-dimentional morphological study based on a particular population. By using finite element analysis, this study will assist to predict the outcome and could become a useful tool for preclinical testing of newly designed implants. A prototype is then fabricated using 316L stainless steel by applying investment casting techniques that reduce manufacturing cost without jeopardizing implant quality. The finite element analysis showed that the maximum von Mises stress was 66.88MPa approximately with a safety factor of 2.39 against endosteal fracture, and micromotion was 4.3um, which promotes osseointegration. This method offers a fabrication process of comentless femoral stems with lower cost, subsequently helping patients, particularly those from nondeveloped countries

Piston ring assembly of a new symmetrical multi-stage wobble-plate compressor

A new compressor design for a natural gas Home Refueling Appliance (HRA) as an equipment for a slow fill of Natural Gas Vehicle (NGV) usage has been developed. It is known as the Symmetrical Multistage Wobble-plate Compressor. This compressor was designed for a high compression gas that requires oil-free piston ring assembly in the cylinder to minimise contamination in order to maintain the purity of the natural gas. The goals of this research were to develop, design, analyse, and test an oil-free Piston Ring Assembly (PRA) for this new compressor. Through this research, the material selection and process of designing the piston ring assembly and considerations taken for this new compressor were explained. To function as an oil-free piston ring material, Polytetraflouroethelyne (PTFE) and Polyetheretherketone (PEEK) as polymer material was selected. The performances of these materials were evaluated using a laboratory scale reciprocating sliding test. Finite Element Analysis (FEA) was used as an effective tooling to analyse the component design of the piston ring assembly numerically. Symmetrical multi-stage wobble-plate compressor prototype with the proposed piston ring assembly was developed and tested successfully on a test rig which was built for this purpose. The results met the design requirement with 3 bar of suction pressure, 260 bar of discharge pressure (more than the specified discharge pressure of 250 bar), 1 m3/hr of flow-rate, and 1000 rpm of rotational speed. The tests and results presented in this study were preliminary tests by using air as the working fluid to replace the natural gas in compression process. The results of this test in this study showed that the proposed piston ring assembly is able to produce and withstand the extremely high pressure of about 250bar. The real tests processes on natural gas were conducted utilising all the experiences and lesson learnt from that of air