Advances and challenges in sputter deposition of biomaterial coatings from a loosely packed hydroxyapatite powder target

Abstract

Hydroxyapatite (HA) coatings have numerous applications in the orthopaedics field due to the good osteoconductive properties and excellent bioaffinity of HA. Usually, preparation of a HA sputtering target is costly and time-consuming, while many of the target manufacturing approaches are rather problematic. Solid pressed and sintered targets tend to crack or chip during sputtering. Plasma sprayed target preparation is costly and yields decomposed HA targets. Both targets can be produced from HA powder, which also can be applied as a target. In this work, an approach to depositing calcium phosphate (Ca-P) compounds by sputter deposition from HA in a form of loosely packed powder (LPP) was investigated. A deposition system arrangement suitable for LPP of HA sputtering has been manufactured and developed. Amorphous calcium phosphate (ACP) coatings were deposited on titanium, magnesium, silicon and polyurethane (PU) substrates from an electrode arrangement for deposition from an LPP target by radio frequency magnetron sputtering under an argon environment pressure of 0.67 - 6.67 Pa and discharge power of 30 - 504 W. Heat treatment of ACP coatings at 600 °C led to crystallisation. It is shown that HA coatings from LPP equivalent to those deposited from solid target material are achievable, which suggests that deposition of HA coatings from a powder target is more flexible and cost-efficient than the deposition from HA targets produced by other widely used preparation techniques. The target decomposition mechanism was closely investigated and its effects on the coating chemical phase composition, which varies throughout the coating thickness, is explored. Theoretical assessment of these coatings bio-compatibility for bone tissue applications regarding chemical phase composition and surface roughness features has been performed while the biocompatibility of PU coated with ACP was further evaluated in vitro. Furthermore, plasma sprayed medical grade coatings were analysed to demonstrate, advances and challenges of radio frequency (RF) magnetron sputter deposition for HA coatings. This showed that RF magnetron sputtering can rather complement the plasma spraying technique than totally replace it.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ping Xiao (Supervisor) & Allan Matthews (Supervisor)