Investigating the effect of substrate thickness on the integrity of plasma sprayed hydroxyapatite coatings

Abstract

The demand for biomedical implants is on the rise for the maintenance and improvement of human health. These biomedical implants reinstate the function of living tissue and/or organs in the human body. Accidents, sports injuries as well as basic daily activities have caused the bone joints in the body to deteriorate at a fast pace. Thus, there has been an increase in the demand for more suitable and reliable ways to replace these joints and organs. A biomaterial is any material of synthetic or natural origin that has the ability to repair or replace a function or a portion of the body in a secure, reliable, economic and physiologically acceptable manner to improve the quality of life. They may be categorized according to their relative tissue responses into bioinert, bioresorbable and bioactive. The bioactive materials, such as hydroxyapatite (HAp), upon contact with the human body, interacts with the environment and grows to become part of the component/environment. HAp is the second most thermodynamically stable and the least soluble of the calcium phosphates after fluorapatite (FAp). HAp contains many mineral and chemical similarities with that of natural bone and therefore it has been widely studied for the use of biomedical application as bone implants. However, HAp contains weak mechanical properties such as brittleness. A metallic substrate, such as Ti-6Al-4V, with durable mechanical properties would therefore yield a desired result if the substrate were coated with HAp. The coated metallic substrate would contain the property to bond with the surrounding bone tissue and accelerate bone regeneration together with strong mechanical properties to withstand load-bearing applications. This study was conducted by plasma spraying a HAp powder on Ti-6Al-4V substrates of different thicknesses. The effect of coating HAp on substrates with different thicknesses was investigated before and after immersing the sample in simulated bodily fluid. Immersion in simulated bodily fluid (SBF) was for various periods (7, 28 and 56 days). The structural changes, phase composition and stress before and after immersion were analysed using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). SEM was used to study the surface morphology of the surface of the substrates before and after immersion in the SBF. The as-sprayed samples for both substrate thicknesses appeared to have molten splats, various crystals and glassy regions on the surface indicating the surface was smooth. The 25 mm thick substrate (cylinder sample) had more cracks and crystals as compared to the 5 mm thick substrate (flat disk sample) which had more spherical particles. The surfaces after immersion for 7 days appear rougher and there was agglomeration of the crystals. A precipitate layer started forming on the surface after immersion for 28 days with a few voids present. After 56 days of immersion, the layer was observed to cover the full surface leaving no voids. The results from the EDS were obtained before and after immersion in the SBF. Calcium and oxygen were the main elements present in both sample geometries. The flat disks had an increase in the calcium after 7 days of immersion and thereafter the calcium decreased with immersion time. The calcium in the cylinder substrates decreased from initial immersion. However, both sample geometries showed an increase of oxygen and a decrease of phosphorous after 56 days of immersion. Only the flat disks showed a presence of aluminium, chlorine and silicone after 56 days of immersion. The results from the XRD analysis showed that the coating deposited on cylinder substrates had a greater intensity in the HAp peaks as compared to those on the flat disks. XRD quantitative phase analysis depicted, for both sample geometries, that HAp was the dominant phase with the thermal product CaO disappearing after 7 days of immersion. For the cylinder samples the HAp was observed to increase within 7 days of immersion and thereafter stabilized linearly with further immersion whereas for the flat disk sample it increased after 7 days of immersion and thereafter decreased gradually with immersion time. After 7 days of immersion the thermal products TCP and TTCP decreased for both geometries. The TTCP for the flat disks showed a small increase thereafter with immersion time while the TTCP for the cylinder substrates decreased with immersion time. The TCP for both geometries increases slightly after 28 days of immersion. XRD residual stress investigation revealed a tensile stress state in the coatings deposited on both geometries. After air plasma spraying, the cylinder substrates had an average stress of 54.35 MPa which exceeded the ultimate tensile stress (UTS) of 38.0 – 48.0 MPa of the material. The average of the normal stresses thereafter decreased within 7 days of immersion to 51.38 MPa and thereafter continues to decrease with immersion time, eventually reaching an average stress of 35.57 MPa after 56 days. After air plasma spraying the flat disk, it had an average stress of 37.72 MPa. After immersion for 7 days, it increased to 40.10 MPa, reaching the UTS. It thereafter decreased with immersion time after 28 days and further decreased after 56 days of immersion to an average stress of 29.57 MPa. The shear components were found to be negligible not exceeding 12 MPa.