Delamination-fretting wear failure evaluation at HAp-Ti-6Al-4V interface of artificial hip implant

Osteoarthritis due to rapid aging population in Malaysia and developed countries leads to an extensive application of titanium artificial hip implants. However, titanium alloys (Ti-6Al-4V) cannot directly adhere with human bone due to bio-compatibility issue. Thus, Hydroxyapatite (HAp:Ca10(PO4)(OH)2) coating which consists of main composition of human bone is plasma sprayed on titanium implants to maintain fixations during bone in-growth process. HAp coatings are susceptible to fail due to brittle fractures (coating through thickness crack) to initiate delamination which promotes fretting wear behaviour. Fretting wear particles are concerned for activating inflammations at surrounding organs, which lead to loosening of implants or subsequent failures. Present research aims to develop a finite element model to examine delamination-fretting wear behaviours that can suitably mimic actual loading conditions at HAp-Ti-6Al-4V interface of hip implant femoral stem component to formulate maximum wear depth predictive equation as a novel and fast failure prediction tool. Three simple finite element contact configuration models subjected to different mechanical and tribological properties consist of contact pad (bone), HAp coating and Ti-6Al-4V substrate are developed using contact modelling, cohesive zone modelling (CZM) and adaptive wear modelling (UMESHMOTION) approaches to be examined under static simulation. The developed finite element models are validated and verified with modified Hertzian theoretical solution and reported literatures. The findings revealed that significant delamination-fretting wear is recorded at contact edge (leading edge) as a result of substantial contact pressure and contact slip driven by stress singularity effect. Tensile-compressive condition (R = -1 ) experiences most significant delamination-fretting wear behaviour (8 times higher) compared to stress ratio R = 0.1 and R = 10. Finally, maximum delamination-fretting wear depth predictive equations are successfully formulated with significant goodness of fit and reliability as a fast failure prediction tool

Formulating the plastic deformation expression on coated substrate as a coating selection tool

This study presents the investigation of elastic coating performance responses on elastic-plastic substrate of advanced alloys using Finite Element (FE) method with an explicit numerical algorithm under quasi-static condition. Cylinder-on-flat contact configuration subjected to a normal and tangential loading is examined. Two aeroengine materials, namely Ti-6Al-4V and Super CMV (Cr-Mo-V) alloys are employed. Coating mechanical properties which are applied load (500 N 1000 N), sliding displacement amplitude (0.005 mm - 0.12 mm), friction coefficient (0.3 - 0.9), coating elastic modulus (100 GPa - 400 GPa) and coating thickness (0.01 mm - 0.1 mm) are investigated. The effect of coating parameters on stress-strain distributions along with plastic deformation of the coated substrate are evaluated. The FE model is validated by comparing with theoretical Hertzian contact solution for homogeneous substrate, meanwhile coated substrate is validated by comparing with reported literature. The evolutions of contact pressure, von mises stress, equivalent plastic strain, tangential stress and surface profile are examined for various coating parameters. There is a clear increasing trend of development in stress-strain distributions along with plastic deformation, maximum pile-up and depth-in values with the increase in all coating parameters for both coated substrates ( except for coating thickness effect on Super CMV material, where the contradict trend is noted). Friction coefficient acts as the significant. coating parameter that leads to plastic deformation failure of coated substrate. The relatively higher stiffness and yield strength of the coated Super C.t\IV alloy registered limited plastic deformation compared to the coated Ti-6Al-4V allo_v. The coated substrate plastic defonnation marhernaJ.icc1l expressions are formulated according to the Lagrange mulr.ivariar.<" interpolation which can be used as an alternative method for FE approach. The weighted scoring method is practised in coating selection approach based 011 plastic deformation failure of coated substrate. The verified coating selection tool can contribute to knowledge in suitable material selection for coating based 0n its performanc

Contact slip prediction in HAp coated artificial hip implant using finite element analysis

The rapid age growth in most of the developed countries leads to application of artificial joints such as knee joints and hip joints. The properties of titanium alloy such as light weight, high strength and good biocompatibility make it a suitable material for wide usage as artificial joints. However, titanium alloy cannot directly adhered with human bone; thus, bonds or coating are required. Plasma-sprayed hydroxyapatite (HAp) is widely used as a coating to bond artificial Ti-6Al-4V implants with human bone. The contact slip mainly occurs at the HAp-Ti-6Al-4V interface which also known as possible delamination interface in hip joint artificial implant. The coating fretting fatigue delamination condition can lead to contact slip at HAp coating-Ti-6Al-4V interface which will accelerate HAp coating fretting wear behavior. This paper presents the influence of normal loading, fatigue loading and delamination length on contact slip distributions at HAp coating-Ti-6Al-4V interface through finite element based methodology. A simple FE contact configuration model consist of contact pad, HAp coating and Ti-6Al-4V substrate is examined under static simulation. The predicted results revealed that lower normal load with higher maximum fatigue loading condition could promote more contact slip distribution. The contact slip is also increased with increasing delamination length. The induced contact slip can accelerates fretting wear behavior of HAp coating