WEAR AND CORROSION STUDIES ON PLASMA SPRAYED A1203 REINFORCED HA COATINGS

Abstract

The implant and devices used in human body for repair or replacement of damaged organs are based on metals, ceramics and plastics. None of them (except bio-ceramics) make the natural union with bone and tissues in body environment, moreover release of metallic ions and plastic debris and subsequently their accumulation in tissues near implants cause sever pain and inflammation. This leads to requirement of revision surgery to replace the existing implant with new one. The wear rate of ultra high molecular weight polyethylene (UHMWPE) inner socket which articulates against metallic surfaces of cup and femur head in hip joint is as high as 100 pm/year. The poly-methyl-methacrylate (PMMA) bone cement used for joining implant to bone further restricts the service life of these implants in harsh body environment. The interest of researchers and medical practitioner is changing towards the development of new techniques and materials to overcome the problems associated with conventional metallic-UHMWE articulating surface, which lead to shielding the metallic implant from corrosive body environment by application of coating and use of alumina-on= alumina bearings. Currently plasma spraying process is the only technique approved by Food and Drug Administration (FDA), USA to coat bio-ceramic materials. Since 2002, marketing of alumina-on-alumina articulating hips had been approved in USA. In this view, after extensively studying the published literature, it is concluded that the possible, practical and reliable way to prevent corrosion of body implants in harsh body environment is the application of hydroxyapatite based composite coating with incorporation of intermediate layer of bond coat which have reliable resistance to wear and corrosion, adequate strength, ability to form natural bond with bone and surrounding tissues and allows no migration of metallic ions from substrate to coating. Very little literature is available regarding utilization of plasma sprayed hydroxyapatite-alumina composite coating and alumina-titania bond coat for biomedical applications is scarce in existing literature. The current study elucidate effect of A1203-13 wt% Ti02 bond coat in reduction of elemental migration from substrate to coating, enhancement in bonding strength of coating and effect of alumina addition (0-30 wt%) on wear, hardness, bonding strength and corrosion behavior of .composite coatings. The effect of post coating heat treatment on incorporation of reinforcement (alumina) and provision of bond coat. The adhesive wear resistance of coatings (wear between similar articulating coatings) decreased with increase in alumina contents of HA composite coatings. The maximum value of tensile bond strength was recorded for HA-30 wt% A1203 coating with A1203-13 wt% Ti02 bond coat and minimum for pure HA coating without incorporation of bond coat. The corrosion and precipitation/dissolution behavior of uncoated and coated specimens was studied in simulated body fluid. All the coatings offered better corrosion resistance as compared to bare substrates; moreover their corrosion currerit 'densities were nearly same irrespective of substrate, which suggested that coatings'.were protective and did not allow interaction of substrate with electrolyte_. Most of the amorphous phases got dissolved in simulated body fluid after immersion period of 1 day, dissolution/precipitation occurred up to immersion period of 5 days and thereafter, precipitation of apatite layer was seen on the coating surface. The coated specimens were subjected to post coating heat treatment at 500, 700 and 900°C in air for 2h in order to regain crystallinity of coatings. The crystallinity of coatings was found to increase with increase in post coating heat treatment temperature from 500°C to 900°C. Development of ultra-fine particles was observed after post coating heat treatment at 900°C for 2h. These ultra-fine particles have great affinity for bone bonding and bone in-growth.